Over the years, technology has made a lot of advancements and as a result we have been bestowed with a variety of power saving inventions. In the past, only the most complex power application could justify the use and cost of a toroidal transformation as the machine was extremely costly because of the winding toroidals used. Luckily, technological advancements in engineering have lowered the cost of production of winding toroidals, making them more accessible. The availability of toroidal transformers have become a sought out and cost effective alternative to other forms of laminated transformers in a wide variety of applications.

Benefits of Toroidal Transformer design

The highly effective design of the toroidal transformer is cost-effective and saves energy. Here are some of highlights of toroidal transformer’s design:

Efficiency

The unique and excellent toroidal design of transformer offers great efficiency for its given size and weight. The toroidal core comprises of a powerful magnetic circuit that eliminates inherent air gaps unlike those found inside conventional laminated-bobbin transformers. The toroidal transformer features higher flux density due to full utilization of the core area. This results in the use of smaller components that take up less space. The overall efficiency of the toroidals typically ranges from 90% to 95%, however, with a custom design; the results can be much more favorable. The high quality and tightly wound, grain oriented silicon steel core means maximum utilization resulting in very low core losses. In addition, off-load magnetizing currents are also achievable resulting in higher efficiency.

Low noise and low stray flux field

Because of the highly efficient magnetic circuit and uniform distribution of the windings over the core, the toroidal transformer is surprisingly quiet in operation. In fact the toroidal transformer results with low or even zero mechanical hum caused by magnetostriction. With toroidal transformers you no longer have to worry about noise-inducing stray magnetic fields as it produces waves that are 8 times lower compared to laminated stack type transformers. The toroidal transformer is thus the best choice for sensitive electronic systems including high-gain preamplifiers and instrumentation.

Good regulation

Toroidal transformers guarantee low leakage inductance due to winding configuration. The inductance is created when a certain percentage of the magnetic flux produced by the primary source is not consumed by the secondary source therefore not generating any voltage. This is considered as a major problem in laminated stack transformers. Because of the tight coupling, toroidal transformers are able to utilize the all flux. As a result toroidal transformers produce off-load secondary voltages that are lower to laminated stack type transformers. In addition, toroidal transformers prevent power from being wasted as heat due to lower copper loss.

Ease of mounting

Unlike conventional laminated steel stack type transformers, most toroidal transformers are fixed using only one central screw. This speeds up production time and lowers the number of parts required in mounting of hardware.  However, for custom designs, advanced mounting techniques are used to improve efficiency.

Packaging versatility

Unlike conventional laminated stack type transformers, toroidal transformers with specific characteristics may be varied in terms of height and diameter according to the product-design requirements.  This allows the transformer to meet enclosure space constraints. In order to meet retrofit space requirements tall cylinders or flat discs can be made for low profile applications.

Approvals

P&A International Standard Toroidal Transformers obey the world’s major safety standards and are built according to the construction files of UL and CSA recognized designs. Custom designs can be produced to meet other standards whether European, medical, commercial or military. All P&A Internationaltransformers are CE marked.

Guide to Transformer selection and specifications

P&A International provides standard general-purpose transformers that offer dual primary windings for operation at 50/60 hertz with either series or parallel connection. P&A International incorporates dual secondaries that may use series or parallel in ranges of voltage and VA ratings in order to suit common power and electronic applications.

The unique combination of windings results in one component to be specified for both domestic and international use i.e. exports to other countries that have different lines of voltage as well as for providing several secondary power configurations.

If your application requires a power supply configuration does not meet P&A’s standard range of transformers then they will happily customize their services to tend to your needs by designing a custom transformer that matches your requirements. This may range to a slight modification to a completely new custom design.

Below are general guidelines that throw light to the criteria that should be considered before specifying a custom transformer:

Power rating:

Transformers are regarded in Volt Amps (VA) which is the product of rms AC voltage and rms current for a principally resistive load.

Example 1: A heating appliance needs 4 amps at 24 V AC and is to be driven from a 115 or 230 voltage, 60 hertz mains supply.

A 115 + 115v to 24V step down transformer is required along with a VA rating of 96 VA (4x24) for this, 100VA will be suitable.

Example 2: Two Halogen Lamps of 50 watt are connected in parallel and require 12 v AC for maximum brightness. An 115v fan also runs at the same time, which draws 182 milliamps. This combination is to be driven from 115v 60 Hz supply.

For this an 115v step down transformer with 2 separate secondary windings is required.

First Secondary: Should be rated 12v, 100 VA (8.33 Amps)

Second Secondary: 115 x 0.182 = 20 VA

The total transformer rating thus adds up to 100 + 20 = 120 VA

A transformer that supplying reactive or rectifier loads needs to be rated according to the load characteristics. P&A’s engineers are always happy to offer their guidance when it comes to the ratings of any appliance. More can be found out on the section on rectifier transformers.

Duty cycle

In case the load is much shorter than the thermal time constant (the time required to reach a steady on-load temperature, this may take several hours) and is not continuous, a small transformer can thus be specified. The following formula is used to calculate the rating required:

Operating Frequency

The size and weight are two things in which the operating frequency of the transformer depends upon.  The higher the frequency, the smaller the transformer whereas the lower the frequency the bigger the transformer. Therefore a transformer designed for 60 Hz operation will be smaller and lighter as compared to a transformer that has been designed for 50/60 Hz operation, but in this case the size reduction is only very slight as there is only a difference of 10 Hz. However, for a transformer that has been designed for 400 Hz the size may be up to 80% smaller than a 50/60 Hz transformer. It is thus important to specify the minimum expected operating frequency of transformer as operation may be possible above the designed frequency but would fail to operate at a lower frequency.

Primary Voltage

The primary voltages that have been mentioned in the standard transformer specification (or the ones that have been specified in customer inquiries) are the nominal voltages. However, the supply of these voltages may vary when the line supplies are at heavy loads (this may be during dinnertime or other peak timings) or lighter loads (when you and your family are asleep) the drop varying in the rise and fall of voltages may vary as much as 10% of the nominal depending from country to country. The change of voltage is redirected on the secondary voltage (on and off load) by the standard transformation relationship.

Example:

A toroidal transformer is rated at 12v while the primary is rated at 117v 60 hertz. The line supply regulation is started as ± 6% by a utility company.  When the line is at the lowest, the secondary voltage will be:

Regulation

The transformation regulation is the measure of voltage rise on the secondary due to off-load or light load conditions with the primary input voltage remaining constant. This is measured as a percentage of the secondary voltage; for example a transformer consisting of a regulation of 10% and rated at 12 Volt at full load will have an off load voltage of 13.2 Volt.

The regulation is calculated as:

The design characteristics of transformers thus inversely depend on the power rating (VA) and are approximately linear for any given load on the secondary. In the above example, we can deduce that the load was 50% of the full load, thus the voltage would then be 5% higher or 12.6v. This regulation needs to be kept in mind in the process of designing rectifier power supplies etc. This would affect the rating of the reservoir capacitors, voltage regulators etc.

However, in this case the custom transformers can be designed with very low regulation figures; however, this depends on the expense on the size and weight of the transformer. Larger cores and wire gauges would be required.

Temperature rise

P&A’s  standard toroidal transformer are able to withstand a temperature rise of maximum 60 ̊C and have a material rating of Class A 105̊C  while the winding wire that has a rating of Class F (155 ̊C) for further reliability. The temperature rise may be above the ambient temperature which is 30-35 ̊C.

If the surrounding components are able to tolerate high temperatures then a reduction in transformer size may be advised. P&A Internationalis qualified enough to manufacture transformers up to most standard temperature classes, however, the cost of manufacturing the transformer may vary depending on the materials used. So, it is important to furnish expected ambient temperatures in a custom inquiry for any temperature class. The running temperature of the transformer of transformer will be the ambient temperature in addition to the temperature rise.

Some standards temperature classes

Y=90̊ C

A=105 ̊C

E=120 ̊C

B=130 ̊C

F=155 ̊C

H=180 ̊C

Capacitive shielding

By nature transformers are wide band devices when it comes to stray signal coupling. A shielding layer is interposed between the primary and secondary windings  if incase a transformer is required to operate  in an electrically noisy environment, this minimizes the capacity between the two windings, helping to eliminate common mode noises however its effectiveness highly depends upon the noise characteristic including the transformer’s overall surface area. The shield may at times be required to successfully satisfy safety regulations and circuit configurations. In addition capacitive screens are also needed, they add layers and cost to a toroid’s overall build. If capacities screens are required then a larger core may have to be specified which would require a larger inner diameter to support the windings.

This kind of sheading should supplement and not be used to replace the line filters and suppressor networks required to operate circuits or EMC compliance.

Magnetic Shielding

While toroidal transformers only emit minimal stray magnetic fields by nature, a certain amount is always found in all sorts of magnetic devices. Compared to most appliances toroidal emission is considered as very low in order to have any effect on circuit operation.  However, some appliances may prove to be more sensitive than others.

This may include high gain instrumentation, high-end audio, wideband, high resolution CRT circuits. However, for these cases the magnetic shielding may be applied around the toroid in the shape of a highly permeable metal band. In most cases Silicon Steel or Mu Metal are used for such sensitive operations. On the other hand, for even more sensitive applications, total encapsulation in steel can or case may be the only option. There are many ways in which you can reduce emission by altering the design that is before any protection is added to the transformer) if your circuit is this prone to magnetic interference then you should provide specifications when requesting a quote.

In rush currents

Toroidal cores create an excellent magnetic circuit and carry high remanence, which is a result if the high number of square hysteresis loops that these cores possess. The high inrush currents can be encountered when switching on large toroidal transformers. This produces better results when compared to conventional laminated stack transformers and can last for a few half cycles of the mains voltage. This is mainly caused as a result of the core saturation though for only a split second. This is considered very normal.

However, this means that larger toroidals which are of 1.5KVA or even higher should not be switched on without taking necessary precautions. In such cases it is recommended that slow-blow type T fuses be used while making primary circuits especially when using transformers that are over 100 VA. For even larger transformers, NTC thermistors or circuit breakers that have been designed for motors and transformers may be used.

In addition normal relay-switched resistor soft start circuits may also be used to produce efficient results, however, a delay may be caused of about 30 to 300 millisecond. Keep in mind that some relays have pull-in delays of approximately this time. Soft start circuits should be used in addition with and should never be used to replace the proper circuit protection provided by fuses or circuit breakers.

Mounting

The traditional method of mounting toroidal transformers to a chassis or by using a dished steel washer, the transformer needs to be interposed between two cushioning gaskets so that the hardware is held in place using a single bolt that is passed through the central hole of thee toroid. The mounting kit is supplied with standard transformers while other mounting options may also be considered, just do your research.

Warning: Keep in mind that the metal chassis should not be in contact with both ends of the mounting bolt. This may cause a shorted turn that would cause the transformer to overheat leading to its destruction.

Varnishing and vacuum impregnation

Full vacuum and pressure varnish impregnation are provided by Avel, along with envelope dipping with mold-resistant polymers and additional protective barriers. Keep in mind that the process of impregnation and dipping cannot be carried out with the help of either solvent based varnished or solventless epoxy varnishes. For further information you may contact Avel.

Thermal Protection

Thermal protection can be provided to toroidal transformers using thermal sensitive fuses and switches, commonly known as thermostats. These protectors are generally attached to the primary winding and are thus in close contact with the windings. Keep in mind that thermal fuses may not resettable can cannot be reused after blown. Thermal switches are designed to operate according to temperature change, the open at a set temperature and close when the temperature goes down, reforming the primary circuit. The protectors used are present to satisfy certain safety approvals. For additional thermal protection, you may contact P&A International however keep in mind that this may increase costs.

2Al  +3H₂O→Al₂O₃ +6H⁺ +6e^-

A dense even layer of oxide about 0.08 µm thick is formed rapidly, followed much more slowly with a more porous layer up to 25µm thick.

Before anodising the surface of the article must be thoroughly cleaned, normally using a detergent based process, and etched with a solution of sodium hydroxide.

After anodising the surface may be coloured with a dye or by an electrolytic method using appropriate metal cations, and then sealed by placing in boiling water, the pores in the oxide layer being closed off.

INTRODUCTION

Anodising is a process for producing decorative and protective films on articles made from aluminium and its alloys. It is essentially a process where a thick film of aluminium oxide is built up on the surface of the aluminium through the use of a direct current electrical supply. In the majority of anodising plants in New Zealand it is carried out in an electrolyte bath containing sulphuric acid with aluminium sheet cathodes and the work to be anodised attached to the anode (Figure 1).

Figure 1 - A typical anodising cell

(Articles have to be securely fastened to ensure electrical contact during the anodising)

When the current is flowing in the cell the following sequence of events is believed to occur. Sulphuric acid begins to decompose, the hydrogen ions moving to the cathode where they are reduced to hydrogen gas:

2H⁺ +   2e^- →  H₂ (1)

Simultaneously, negatively charged anions, i.e. hydroxide, sulphate and maybe oxide ions move to the anode. The electrical charge in the circuit causes positively charged aluminium ions (Al³⁺) to be generated in the anode and in turn move toward the cathode. At the anode surface they react with the oxide/hydroxide ions to form aluminium oxide (in the case of the hydroxide ion, hydrogen ions are released into the solution).

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The sulphate ions also play some part as the oxide coating contains 12 - 15% sulphate ions. It is suggested that the sulphate ions facilitate the movement of hydrogen ions reducing the cell voltages required.

THE DEVELOPMENT OF THE ALUMINIUM OXIDE LAYER

Fresh aluminium reacts readily with oxygen to produce aluminium oxide. Once formed the oxide remains firmly bonded to the surface forming an impenetrable layer. Consequently, further reaction ceases. The film is very thin (0.01 m), and despite its tenacity it can be removed by abrasion and chemical corrosion. In such instances the aluminium is subject to wear or the surface will mark or become pitted at the site of corrosion.

Anodising produces much thicker coatings (12 - 25 m) which, if properly sealed, can extend the life of the surface appreciably. Recent research in New Zealand has shown that pitting of

the surface can be reduced by up to 90% with a 12m coating, and by up to 93% with a 25m coating.

In the initial stages (i.e. first 60 s) of anodising the oxide layer formed is dense and of even consistency. It provides the greatest resistance to wear and corrosion and consequently is called the barrier layer. The growth of this layer ceases when the high electrical resistance of the oxide reduces the potential of the applied voltage in the electrolytic cell. The depth of the coating at this stage is about 0.08 m. Subsequent growth is very slow and competes with the acid reaction:

Al₂O₃ + 6H⁺ _2Al³⁺(aq)+3H₂O

which releases Al³⁺ ions into the solution. Note that the H⁺ can be at high concentration near the oxide layer due to one of the anode reactions above. See equation (5).

At low applied voltages only the barrier layer forms. However, the gradual production of Al³⁺ ions tends to smooth out the underlying metal surface and give a brightening effect to the article. Objects such as wheel trims and bumper bars are general treated in this way.

At higher voltages the growth of the layer continues beyond the barrier layer. Unlike the initial barrier layer this secondary layer, although constitutionally the same, has an open pore-like structure; a consequence of the competing anodising and acid solution processes. Electron photomicrographs reveal the structure of these anodised surfaces to be as shown.

The conditions required to produce coatings vary according to the concentration and nature of the electrolyte, the voltage - current density applied, the alloy being anodised and the temperature of the bath. In the majority of electrolytic plants articles are anodised at a potential of 15 - 20 V and a current density around 1.6 A d^-1 m^-2; the electrolyte is 3.5 mol L^-1 sulphuric acid maintained at temperatures between 20 and 23^oC. Under these conditions the quality of the coating is satisfactory for most applications. At higher electrolyte concentrations and temperatures, and at lower voltages or current densities, the acid solution process occurs earlier in the development producing thin, open oxide coatings. Conversely, hard dense coatings are produced at low temperatures and high current densities. The conditions established in each plant are determined by the type of application.

Figure 2 - Structure of anode layer

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The suitability of common aluminium alloys to be anodised is summarised in Table 2 below.

PRE-TREATMENT

Step 1 - Cleaning

Correct and adequate cleaning of the aluminium object prior to anodising is essential if the finished work is to have a uniform and attractive appearance. When aluminium arrives from the rolling, casting or extrusion mills it may be soiled in one or more of the following ways:

Cleaning these 'soils' from the surface may prove difficult, especially if the requirements of the work do not allow etching of the surface. Most cleaning solutions used in New Zealand operations are detergent based. In addition to the detergent, a wetting agent and a complexing compound may be used. The complexing compound frequently used is sodium polyphosphate - a component of many soap formulations - which prevents ions, such as Fe³⁺, adhering to the surface of the work. If etching is not a problem, sodium hydroxide or sodium carbonates (Na₂CO₃ / NaHCO₃) may be added to increase the effectiveness of the solution.

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Step 2 - Etching

Etching is most often achieved by the use of a warm, 10 % (2.5 mol L^-1) sodium hydroxide solution. It gives the surface of the metal a light grey satin finish (through diffuse reflection of the incident light). The vast majority of work is pre-treated in this way. In theory the reactions occurring in the etching solution are:

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• Dehydration of the solid hydroxide: 2Al(OH)₃ → Al₂O₃ + 3H₂O

The rate of etching is dependent on the concentration of the sodium hydroxide solution, the temperature and the concentration of aluminium ions which are released into the solution. When high concentrations of aluminium ions are present the solution loses its effectiveness. Presence of other ions, some of which may be a component in the alloy, can also interfere in the process, causing blemishes to appear on the surface of the work.

The problem of ion contamination is overcome by employing etching solutions which suppress the action of the Al³⁺ and other metal ions released. The compositions of these solutions are the propriety of the companies that develop them, but generally contain sequestering agents which complex metal ions. Such solutions do not have an infinite capacity to do this but, due to the 'carryover' of solution by the etched work and periodic replacement by fresh etch solution, the etching batch is maintained in an effective condition (Figure 3).

Figure 3 - The etching solution cycle

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Foaming agents are also a constituent of the etching solution; their action is to reduce the pungent mists/fumes that result from the vigorous reactions that occur.

It is important to note that the appearance of the end result is determined at this stage. Work which is poorly etched will reveal scratches or blemishes no matter how well it is anodised or coloured.

POST TREATMENT

After cleaning and anodising the work is coloured and sealed. As all anodised work is sealed, sealing will be considered first, although if colouring is to be done it is carried out prior to sealing.

Sealing

Sealing is the process in which the pores at the surface of the oxide layer are closed off. It is affected by placing the anodised object in boiling water for a 15 - 20 minute period. During

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that time the water reacts with the aluminium oxide to produce the mineral Boehmite - Al₂O₃.H₂O or AlO.OH:

Al₂O₃ +H₂O →  2AlO.OH

Boehmite is a hard, transparent material with a greater volume than the aluminium oxide. As it forms it closes off the openings of the pores.

As would be expected, the durability of the anodised surface, especially in regard to chemical corrosion, is greatly influenced by the effectiveness of the sealing. If the duration of the sealing is too short the pores, although constricted, remain open for corrosion agents to be in close proximity to the aluminium surface. Corrosion studies have shown that anodised aluminium which has been sealed for 15 minutes or more has greatly increased resistance to pitting by chemical corrosion agents such as H⁺ and Cl^-.

Colouring

Colouring involves the absorption of a coloured dye into the pores of the oxide coating which becomes fixed after the sealing process has been completed. Dyestuffs which can bond to the oxide or metal ions in the anodised layer have better colour properties than those that do not.

Electrolytic colouring is the most important method of colouring anodised aluminium today. It produces attractive finishes of very great colour and heat fastness and is easy to perform. The anodised work is dipped in a tank containing coloured ions of other metals. Although the constitutions of the solutions are patent, typical examples of the processes are given in

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Table 3 below.

Table 3 - Typical conditions for colouring anodised aluminium (all at room temperature)

Under the influence of alternating current the colouring agents deposit rapidly at the very base of the pores and the take is even over the entire surface. Unlike the process of dye absorption, electrolytic colouring is easy to control and gives uniformity of colour from one run to the next. The success of this technique is evident in the widespread use of bronze coloured aluminium in joinery and house fittings. Approximately 66% of all bronze tinted aluminium is coloured by this technique.

To a much lesser extent coloured inorganic compounds can be used to colour the work. Ammonium ferric oxalate is a very common compound used to impart a goldy colour to the metal. Other colours can be imparted by treating the absorbed ferric ammonium oxalate withother compounds: for example, potassium ferrocyanide solution will react with the ferrioxalate compound to produce a blue colour. The technique used is to dip the work firstly in a solution of the ammonium ferrioxalate followed by dipping the work in the potassium ferrocyanide solution. This double dipping technique can be used with other compounds to produce a variety of colours: e.g. copper sulphate followed by ammonium sulphide gives green, and lead nitrate followed by potassium chromate gives yellow.

CONCLUSION

This is a brief overview of the chemistry of the anodising process. In industry the process can encounter many difficulties if care is not taken to ensure that solutions are controlled with regard to concentration and temperature. Thorough rinsing of the work is carried out after each stage to ensure that it enters the next process in the correct state. It also ensures that contamination of solutions from one stage by the preceding stage is kept to a minimum.

A further aspect of the industry not covered herein is that of quality control. Even in small plants chemists are employed to constantly check the conditions of the solutions and make recommendations / adjustments. In addition, frequent checks are made on the thickness of the film, its density and the colour quality.

About Titanium

1. Corrosion Resistance

Titanium is a material with excellent corrosion resistance. It is able to withstand attack by acids, chlorine elements in water and fluorine and its compounds. Titanium actually forms a passive oxide layer when it comes into contact with any oxygen a property which serves to enhance its corrosion resistance even further. It is thus understandable why such a material would be desired in such oxygen rich environments as the human body.

2. High Strength

Titanium is an extremely strong material. Its strength matches that of common low grade steel alloys and as a result titanium is extensively used in aerospace and industrial applications.

3. High Strength- to-Weight Ratio

This is the property which separates Titanium from other high strength materials such as Steel. Titanium has the amazing property of being very high strength whilst also being very light. Without labouring too much over the physics behind this phenomenon this is simply due to the low density nature of Titanium materials.

5. Biocompatibility

Important property to mention in this article is that the human body LIKES titanium. When inserting materials into the body it is vital to choose a material which can be fully integrated into the body. So far research has shown Titanium to be the only material able to undergo this process of osseointegration. Titanium implants heal with the jaw until they become part of the jaw itself with all the same elements you would expect from a functioning body part including links to the bodies nervous and circulation systems. This integration means that titanium dental implants are not just there to improve your mouth’s aesthetics or to act as a structural bridge – what titanium provides is the ability to recreate a fully functioning tooth and therein lies the beauty of this amazing material.

 

 

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Fabrication of Titanium and Titanium Alloys

Titanium and its alloys can be readily hot worked at temperatures generally somewhat lower than those used for steels. Techniques for press and hammer forging of titanium are essentially the same as for low-alloy steels. Good handling methods and plant layout will reduce the number of reheats necessary, minimizing contamination during forging.

Hot Working

Titanium and its alloys can be readily hot worked at temperatures generally somewhat lower than those used for steels. To minimize surface contamination, titanium should be held at high temperatures for only a short time before forging. The rate of contamination, relatively low up to 700°C, increases rapidly with increase of temperature.

All forging furnace atmospheres contain free or combined oxygen, and some absorption of this element inevitably occurs. In addition to visible scaling, diffusion of oxygen results in hardening of a relatively shallow underlying layer. The effect of nitrogen is not usually significant at preheating temperatures. Subsequent operations such as machining will remove the hardened surface layer, and the final product will have hardness similar to forging stock.

Hydrogen, however, diffuses more rapidly than oxygen and may penetrate the full section of the work piece, which can have a serious effect on properties. Such material can only be recovered by prolonged vacuum annealing. Hydrogen is absorbed from both reducing and oxidizing gas- and oil-fired furnaces, but at a tolerably slow rate under strongly oxidizing conditions.

The order of preference of preheating atmospheres is therefore dried air (electric heating), undried air (electric heating), oxidizing oil- or gas-fired furnaces. Direct flame impingement must be avoided.

Forging. Techniques for press and hammer forging of titanium are essentially the same as for low-alloy steels. Good handling methods and plant layout will reduce the number of reheats necessary, minimizing contamination during forging. Because of the rapid cooling and the fairly narrow hot working range, the chilling effect of tools should be reduced to a minimum by keeping contact time as short as possible. Preheating the tools also helps. Repeated light blows, or attempts to continue forging at too low a temperature, may promote internal cracking and should be avoided. Moreover, a large number of reheats with only a small amount of deformation between heats is also detrimental, because it leads to a coarsening of the microstructure and consequently poor mechanical properties.

In drop forging, die contours should have larger radii and fillets than those used for steel; the lower thermal contraction of titanium requires a smaller shrinkage allowance. Trimming should be carried out hot; furnace, drop hammer and trimming press should be as close together as possible to minimize preheating and avoid wasting time and heat. A final stress-relief anneal is recommended.

Forming

Annealed and solution treated sheet can be pressed, stretch-formed, spun and dimpled, but maximum deformation depends upon the load being applied slowly. Good results are achieved with hydraulic presses, the rubber-pad method being useful for forming light-gauge parts. Drop hammer forming, with heated blanks, is widely used for sheet metal parts of complex contour. Punch presses, which should be slowed down to half or one-third their normal speed, can also be used.

Blanks may be prepared for forming by shearing, sawing, nibbling or blanking, using slow cutting speeds. Edge condition is important, and edge cracking may be minimized by keeping the guillotine blade sharp and close fitting or by heating metal before shearing. All burrs must be removed and, for more difficult forming operations, cut edges may need filing or polishing.

Simple shapes can be formed at room temperature, deformation being limited by the strength and springiness of the material. Solid lubricants such as soap, molybdenum disulphide or graphite are preferred to mineral oils and greases. ICI "Trilac" coating and polythene sheeting have been found to effect considerable improvement in difficult pressing operations.

For more complicated designs, the work piece and, where possible, the dies should be heated to facilitate forming. The use of heat in forming increases ductility, which is reflected in lower minimum bend radii and reduces both the load required to effect deformation and subsequent spring-back, thus ensuring greater accuracy.

Furthermore, at elevated temperatures, the spread between yield and ultimate strengths is increased, which also aids forming. The temperature to select for hot forming depends upon the alloy and the severity of the shape to be produced. Good results can be expected using temperatures of about 200-300oC for commercially pure titanium and IMI Titanium 230, and 550-650oC for IMI Titanium 317 and 318.

 

 

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Heat Treatment

With heating in conventional furnaces there is always some surface contamination and a risk of hydrogen absorption. Vacuum treatment, though ideal, is rarely practicable, so it is customary to use ordinary electric furnaces; hydrogen pick-up is not usually excessive. Fuel-fired furnaces should be avoided if at all possible; titanium rapidly absorbs free or combined hydrogen from the surrounding atmosphere, and this can be serious, particularly with thin sections.

Superficial hardening by oxygen diffusion is almost inevitable at the higher annealing and preheating temperatures suggested for some titanium alloys. The hardening effect is insignificant at low annealing temperatures but above 600°C may lead to surface embrittlement. Both the oxide film and the underlying oxygen-rich layer should therefore be removed by one of the methods of surface treatment; this is particularly important for high-strength alloys.

Machining and Grinding

Titanium China suppliers stock and its alloys can be machined successfully on conventional machine tools provided that certain requirements are satisfied. In all machining operations rigidity of both work piece and cutting tool is desirable. Best results will be obtained if the cutting tools have a good surface finish. If the cutting tools are in good condition, it is no more difficult to machine titanium than an alloy steel of equivalent strength.

Titanium has a tendency to gall or smear on to other metals. Sliding contact between the work piece and its support should be avoided, and the use of roller steadies and running centres is recommended.

Turning. In general, cutting speeds should be low and feeds as coarse as practicable. A good surface finish can be obtained with very coarse feeds by using suitably shaped tools with a large nose radius. This will, however, be limited by work piece rigidity as a large nose radius causes increased tool loads and work piece deflection. Due to the lower elastic modulus of titanium, these deflections are greater than would occur on steel workplaces.

Tool materials may be high-speed steel, cast alloy, or tungsten carbide. The "super" grades of high-speed steel are satisfactory, giving good results in turning where large feeds can be employed, and particularly where the surface is rough or the cut intermittent. Tungsten carbide may be necessary for heavy work on certain harder alloys or for intermittent cutting, but in general its use is confined to lighter, more continuous cuts. For economic use of carbide tools it is essential to regrind before wear becomes excessive, and mechanically clamped tips are an obvious advantage.

Threading. Single-point screw cutting is preferable to threading with a die. Conventional methods of screw-cutting can be used, but success can also be achieved when increments of cut of 0.25-0.50 mm are applied at right angles to the axis of the component. Cuts of less than 0.13 mm should be avoided. Machine tapping with cutting speeds up to 6 m/min is preferable to hand manipulation. Tapping of full threads should be avoided: a thread of 80% depth is much easier to tap and loses little strength.

Planing. Shaping and planing of titanium are not difficult, provided that the foregoing requirements of rigidity, speed and feed are satisfied. Tungsten carbide tools with a large radius, producing a broad and relatively thin chip, are most successful. As in all cutting operations, it is essential to use sharp tools and replace them before appreciable wear occurs. For planing, clamped circular buttons of tungsten carbide have obvious advantages.

Milling. In milling, the chief problem arises from chips welding on to the teeth, resulting in cutter chipping and breakage. This is minimized with climb milling, in which the tooth finishes its cutting stroke when moving parallel to the feed. Absolute rigidity is necessary to avoid chatter, but the chip is only attached to the tooth by a thin sliver which is easily broken off.

Drilling. Titanium parts manufacturing process may involve drilling with short high-speed-steel drills; the holes should be as shallow as possible. A 140o point is best for sizes below 6-5 mm and a 90° or double-angle point for larger sizes. For holes of a depth greater than five diameters, it is helpful to retract the drill at intervals and clear the swarf. Flood lubrication with a heavily chlorinated cutting oil reduces frictional troubles.

Grinding. A reduction in wheel speed to a half or a third of the conventional speed, together with the use of a suitable coolant, will usually achieve an acceptable grinding ratio. Water-base soluble oils result in poor wheel life, but some chlorinated or sulphurised grinding oils, and solutions of vapour-phase rust inhibitors of the nitrite-amine type, are satisfactory.

Polishing. Titanium can be mechanically polished by techniques similar to those used for stainless steel; reductions in wheel or mop speeds are often beneficial. If a high polish is required, light pressures are necessary during the final operations. Good results have been obtained with a canvas wheel coated with 240E1 `Alundum` grit, which can be blended with stearic acid for a finer finish.

Descaling and Surface Treatment

When titanium and its alloys are heated in air, absorption of oxygen and, to a lesser extent, nitrogen, results in the formation of an outer layer of oxide and nitride and an underlying thin layer into which oxygen and nitrogen have diffused. Removal of this hardened metal layer is essential for optimum mechanical properties, and an integral part of any descaling process.

All types of scale can be removed in fused caustic soda, but use of an unmodified bath leads to hydrogen contamination and poor surface quality. The sodium hydride process results in good surfaces and efficient scale removal but, again, hydrogen contamination occurs. Consequently, neither process is suitable for thin sections.

Caustic soda with about 10% oxidizing additions can be used for slightly thicker material, descaling conditions being 20-30 minutes immersion (longer for very heavy scale) at 425°C. Reaction between titanium and any fused caustic soda bath may lead to a dangerous build-up of heat if a stack of thin sheet is descaled. Thin-gauge material should, therefore, be handled in small batches, at a temperature not exceeding 425°C.

Anti-galling Treatments. The tendency for titanium to gall when in sliding contact with itself or with other materials can be reduced by some form of surface treatment. This is particularly desirable for bearing surfaces and for threads of bolts. Both anodizing and `Sulfinuz` treatments reduce the galling tendency, while adherent nickel and chromium deposits provide good wear resistant surfaces. Cadmium plating or the use of anti-galling paints are effective in preventing seizure of bolt threads. Details of electro deposition and anodizing procedures are given in the following paragraphs.

Electrodeposition. Adherent metallic coatings can only be electrodeposited on to titanium if the surface is suitably prepared. A procedure which has been found successful for depositing nickel, chromium, zinc and cadmium on to some titanium alloys uses a pretreatment comprising: (1) Vapour degrease, (2) Hydrochloric acid etch, 5 min in concentrated HCl at 90-110°C, (3) Cold water rinse, (4) Nickel strike for 3 min, (5) Cold water rinse.

Anodizing. Surface properties of titanium and its alloys can be modified by anodic oxidation treatment, which covers the entire surface with a thin but compact oxide film. Almost any aqueous solution can be used, but immersion in a solution of 80% phosphoric acid, 10% sulphuric acid and 10% water gives a particularly coherent film. A potential increasing from 0 to 110 volts over ten minutes should be applied.

Anodized titanium has no affinity for dyestuffs, but the film itself shows interference colors, determined by the final anodizing potential.

 

 

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There are many test results floating around the internet talking about thermal conduction of heat sinks or other dedicated heat dissipating devices. This test focus on PCB’s thermal parameters that concern developers during the design stages of high power circuits. We are going to test 1.7mm Aluminium PCB, 4 layers PCB and 6 layers PCB.
The test device consists of power diode that dissipates 25Watt of heat and aluminium heat sink of fixed size and airflow. We measure temperature of the diode at fixed load and subtract ambient temperature from it to get ‘temperature rise above ambient’. To test PCB thermal conduction we sandwich fragment of PCB between diode and heat sink, therefore adding thermal resistance. By comparing ‘base line’ values to newly generated data, we can see how much thermal insulation each fragment of PCB have added to the system.

First we will create the ‘base line’ test that indicates test device native performance. This will allow us to subtract those values at the later stage from performance figures obtained during the Printed Circuit Boards tests. Data table below is representation of the raw data collected. PCB manufactures normally do not indicate thermal conduction values of the PCB’s, unless it’s aluminium or copper based printed circuit board that is designed with heat dissipation in mind. What we have found intriguing is that 6 layer PCB thermally is more conductive that 4 layer PCB. At a closer look this is due to more copper been present in the same volume of PCB material. Copper acts as a heat conductor and help heat to travel between the layers of the PCB, displacing space otherwise occupied by thermal insulator: fiberglass.
We have been pleasantly surprised by excellent thermal conductance of Aluminium PCB used for LED lighting solutions. It generated temperature rise of 33C after 25 minutes. This is excellent considering that 4 layer PCB caused power diode thermal runaway due to temperature rise exceeding 70C after just 4 minutes of operation. A second chart on the right indicates component temperature rise over the ‘base-line’.

The above data indicate that when designer have a concern about thermal loan on components placed on PCB, he may employ number of ways to reduce this load, including use of more layers of copper, assuming this do not contribute to total PCB thickness. Even if this layer play no other role but heat conduction. Attaching PCB to a chassis or heat sink can also be an effective method but have to be designed around it due to necessity to place fasteners and power components on the PCB above the area of contact with heat absorber.

Conclusion:
Many PCB manufacturers including suppliers of PCB from China, do not provide thermal performance parameters for the product. At above test results indicate, there is a great degree on performance difference between the boards with different number of layers. There will be also a great degree of difference in performance between different manufacturers using various compounds and materials in making the actual dielectric itself. Ideally, each PCB should come with certificate indicating it’s performance including thermal to allow the engineers to better harness additional effect of the thermal conductance offered by PCB itself.

By Jenny Paddy

Numerous patents had been published before the 1950’s, concerning the anodization of alu- minum for coloring [15]. Since the early years, anodic processes at DC or AC current based on either chromic, sulfuric or oxalic acid as electrolytes have been paid attention to [15]. Con- sequently, it was observed that additives such as metal salts like copper, nickel, silver, arsenic, antimony, bismuth, tellurium, selenium or tin lead to a change of the physical and mechani- cal properties as well as of the colors of the oxide. Bengough’s and Stuart’s patent in 1923 is recognized as the first patent for protecting Al and its alloys from corrosion by means of an anodic treatment [16]. In 1936, Caboni invented the famous coloring method consisting of two sequential processes: anodization in sulfuric acid, followed by the application of an alternating current in a metal salt solution [17].
The development of electron microscopy led to a deeper understanding of the porous alumina structures.  In 1953, Keller and his coworkers described a porous alumina model as a hexag- onally close-packed duplex structure consisting of porous and barrier layers [18].  Also, they demonstrated the relationship of an applied potential and the geometric features of the hexago- nal porous structures such as the interpore distance. This model was the basis for initial studies that aimed at better a understanding of the physical and chemical properties of porous alumina.
A review paper dealing with anodic oxide films on aluminum was already published in 1968 [19].  Structural features concerning anion incorporation and water content in the oxide and theoretical models of formation mechanisms of both the barrier-type oxide and the porous-type oxide were described in detail in this paper.
Between 1970 and 1990, studies by the Manchester group (led by Thompson and Wood) re- sulted in a deep insight in the growth mechanisms of alumina oxide films. This was possible by the uses of new techniques such as Transmission Electron Microscopy (TEM), marker methods and microtome sectioning.  [20, 21, 22, 23].  A corresponding publication by O’Sullivan and Wood is one of the most cited articles on anodization of aluminum to obtain porous alumina structures [24]. Efforts on theoretical modelling of porous oxide growth were carried out by several groups [25, 26, 27, 28, 29, 30, 31, 32, 33]. Fundamentally, an instability mechanism in terms of a field focusing phenomenon was attributed to create pores in the barrier oxide. In these papers, it is claimed that the theoretic modelling of the pore formation mechanism in alumina is analogous to that for other porous materials which can be obtained via an anodic treatment, for example, mciroporous silicon.

Based on a two-step replicating process, a self-ordered porous alumina membrane with 100 nm interpore distance was synthesized by Masuda and Fukuda in 1995 [34].  This discovery was a breakthrough in the preparation of 2D-polydomain porous alumina structures with a very narrow size distribution and extremely high aspect ratios. Two years later, they combined the aluminum anodization method with nanoimprint technologies, which allowed for the first time the preparation of a monodomain porous alumina structure [35]. Numerous other groups, not mentioned here specifically, have also contributed to an improve- ment of porous alumina structures. The purpose of this dissertation is to understand self-assembly of porous alumina under spe- cific conditions (Chapter 1 and Chapter 2).  In addition, nanoimprint methods are developed to obtain monodomain porous alumina structures (Chapter 3). These nanoimprint methods are further advanced to fabricate porous alumina arrays with various configurations (Chapter 4). Furthermore, the combination of nanoimprint and anodization will be applied to obtain porous oxides grown on titanium, which is also a valve metal1  (Chapter 5).  Finally, in the last two chapters, applications of the alumina templates will be discussed, for example, templates for monodisperse silver nanowires (Chapter 6) and 2D-photonic crystals (Chapter 7).

1.2     Electrochemistry of anodic alumina

1.2.1          Thermodynamics
The spontaneous reaction leading to the formation of aluminum oxide in air can be ascribed to the large negative Gibb’s free energy changes [36].
2Al(s) +2 O2(g) −→ αAl2O3(s)       ; ΔG◦        =  −1582kJ/mol          (1.1)

2Al(s) + 3H2 O(l) −→ αAl2 O3(s) + 3H2(g)     ; ΔG◦        =  −871kJ/mol                                                                            (1.2)

If aluminum is electrochemically anodized, an oxide grows at the anode electrode [37],
2Al(s) + 3H2 O(l) = Al2O3 (s) + 6H + + 6e− ,                                  (1.3)

and hydrogen evolves at the cathode

6H + + 6e−  = 3H2(g).                                                                  (1.4)

Assuming there are no complex anions, the Nernst equation reads

E = E0  − ( zF )ln( [ox] )                                                                 (1.5)

where R is the universal gas constant, T is the absolute temperature in Kelvin, z is the charge number of the electrode reaction, and F is the Faraday constant (96,500 C mol−1). The electrode potential E at the anode can be written as

E = −1.550 − 0.0591pH                                                                  (1.6)

Table 1.1: Alumina oxide forms [39]

This explains that the reaction at the anode electrode (Al) thermodynamically depends on the pH value, which is determined by electrolyte and temperature.

1.2.2 Kinetics
The current density passing across the oxide film can be written as [19, 37]
j = ja + jc + je                  (1.7)
where ja , jc  and je  are the anion-contributing, cation-contributing and electron-contributing cur- rent density, respectively. Since the electronic conductivity in the aluminum oxide is very low, the ionic current density (ji = ja + jc) is the predominant mode to transport the charges. The relationship between the ionic current, ji , and the electric field, E, can be expressed in terms of the Guntherschultze-Betz equation
ji = j0 exp(βE)                   (1.8)
where both j0 and β are temperature- and metal-dependent parameters. For the aluminum oxide, the electric field E, j0   and β are in the range of 106   to 107   V/cm, 1 × 10−16   to 3 × 10−2 mA/cm2  and 1 × 10−7  to 5.1 × 10−6  cm/V, respectively [38]. Based on the Guntherschultze- Betz equation, the rate-limiting steps of the film formation are determined by the ionic transport either at the metal/oxide interface, within the bulk oxide or at the oxide/electrolyte interface [19]. Nowadays, it is generally accepted that the oxides simultaneously grow at both interfaces, e.g., at the metal/oxide interface by Al3+   transport and at the oxide/electrolyte interface by oxygen ion transport [21, 24]. For example, the transport number2  of Al3+  anions, tAl3+ , and the cation transport number, tO2− , were reported as 0.45 and 0.55, respectively, for 5mA/cm2 [22].

1.3     Composition of the oxide
Due to a number of polymorphs, hydrates, and incorporated ions, anodic Al2O3  can exist in various forms, e.g., Al2O3 ·(H2 O)n where n = 0 to 3 [37, 40]. Six forms are mostly discussed (see Table 1.1). Gibbsite and Boehmite are converted to several transition alumina minerals such as γ series (e.g., γ, ρ, ξ) and δ series, (e.g., δ, κ, θ) by heating. Corundum, which thermodynamically is the most stable form among alumina oxides, is generated above 1100◦C, regardless of the transition course.   Boehmite heated at between 400 ∼ 500 ◦C  yields γ-Al2O3   consisting of irregular structures [40]. Generally speaking, anodic Al2 O3  was mostly reported as a form of X -ray amorphous solid [19, 24, 37].  For the barrier layer, the presence of nanocrystallites of γ0 -Al2O3  with sizes of 2-10 nm was demonstrated by several authors. γ0 -Al2O3  is considered as an intermediate form between amorphous and γ-crystalline Al2O3. Thompson and Wood suggested that aluminum oxides may consist of nanoocystallites, hydrated alumina, anions, and water moleculars[20, 24].

1.4     Barrier-type and porous-type alumina
Depending on several factors, in particular the electrolyte, two types of anodic films can be produced.  Barrier type films can be formed in completely insoluble electrolytes ( 5 < pH <7 ), e.g., neutral boric acid, ammonium borate, tartrate, and ammonium tetraborate in ethy- lene glycol.  Porous type films can be created in slightly soluble electrolytes such as sulfuric, phosphoric, chromic and oxalic acid [19, 37].

1.4.1     Inner and outer oxide layer
As shown in Fig.  1.1, both the barrier-type and the porous-type alumina films consist of an inner oxide of high purity alumina and an outer oxide layer comprised of alumina which has in- corporated anions [18, 20, 24, 41]. In fact, the nomenclature of the inner and the outer oxide are determined in terms of the interfaces. The inner oxide is adjacent to the oxide/metal interface, while the outer oxide is adjacent to the electrolyte/oxide interface.

Figure 1.1: Schematic diagram for barrier type alumina and porous type alumina. The aluminum metal, an inner oxide consisting of pure alumina and an outer oxide consisting of an anion-contaminated alu- mina are indicated.

According to the Manchester group [20, 21, 22, 23, 24], the degree of incorporation of elec- trolyte species in the outer oxide layer of barrier-type alumina strongly depends on the type of electrolyte, the concentration of adsorbed anions, and the faradaic efficiency of film growth. This group found that the electrolytes can be classified into three categories in terms of ionic mobility in the oxide; immobile, outwardly mobile, and inwardly mobile ions. Table 1.2 shows the direction of  the mobility of electrolyte species, which is determined by the type of the charge of electrolyte species in the film. For example, if the incorporated species are cations, they would move outwardly during the anodic reaction in Al. Immobility of electrolyte species can be attributed to effectively no charge in the film (compensated or low mobility). Therefore, the thickness of the outer oxide film is strongly influenced by the directionality of the electrolyte species.

1.4.2     Oxide thickness as a function of applied potential

It is generally accepted that the thickness of barrier-type alumina is mainly determined by the applied voltage, even though there is a small deviation depending on the electrolytes and tem- perature [19, 24, 42, 43]. The anodizing ratio, which is defined as oxide thickness formed per volt, demonstrates that the barrier type films are also strongly influenced by the type of metal which is anodized as shown in table 1.3. The maximum attainable thickness in the barrier-type alumina film was reported to be less than 1 µm, corresponding to breakdown voltages in the range of 500 ∼ 700 V. Above the limited voltage (or actually the breakdown electric fields, EB0), dielectric breakdown of the films occurs [19]. On the other hand, since the thickness in the porous alumina film is time-dependent, much thicker films than those in barrier-type films can be obtained [19].  Anodizing time, current density and electrolytes are important parameters in determining the film thickness of porous alumina. For instance, thick, compact, and hard porous films are formed at low temperatures (0 ◦  < T < 5 ◦, so-called hard anodizing conditions), whereas thin, soft, and non-protective films are produced at high temperature (60 ◦ < T < 75 ◦, so-called soft anodizing conditions). As the temperature increases, the corresponding current density also increases. This does not mean that a higher current density increases the film thickness since the rate of complex dis- solution at the electrolyte/oxide interface increases, too. If the temperature is too high so that the rate of dissolution is faster than that of oxide formation, the film even vanishes, resulting in electropolishing of aluminum [19, 37]. The thickness of the thin barrier layer at the bottom of the porous structure is only dependent on the anodizing voltage, regardless of anodizing time.  However, electrolyte effects on the anodizing ratio in the barrier films  have to be considered.  For example, 14A˚ /V, 8A˚ /V, and only 1A˚ /V were reported in a dilute, 40, and 90 % (w/w)3  sulfuric acid, respectively [19, 42]. Comparing the anodizing ratio of the barrier-type oxide formed in non-dissolving electrolytes, the electrolyte effect can be ascribed to the dissolution of the formed oxide in acidic electrolytes.

1.4.3   Pore formation mechanisms

The transient of the potentiostatic current density reflects the formation of barrier-type or porous- type porous alumina (see Fig. 1.2) [19, 36, 44, 45, 46]. At the beginning of the oxide formation, both transients have an identical behavior. However, for the barrier film formation, the current density jb  decays exponentially.  Eventually, the barrier film current is dominated by an ionic current ji .In the case of the formation of porous films, the following current density profiles are typically observed [19, 36, 44, 45, 46]. First, the current density jp  decreases rapidly (regime 1 in Fig.1.2). Then, it passes through minimum value (regime 2 in Fig. 1.2). It increases to arrive at a maximum value (regime 3 in Fig. 1.2). Subsequently, it slightly decreases again. Finally, a steady current density remains (regime 4 in Fig. 1.2). One can consider the current density jp as the sum of jb  and hypothetic current density jhp , which means the pure current density for creating pores. jb  is determined by the applied potential in terms of the anodizing ratio, while jhp  depends on the electrolyte and the temperature as well as on the applied potential.

The pore formation mechanism is displayed schematically in Fig. 1.3, corresponding to the four regimes of Fig.  1.2 [26]. At the beginning of the anodization, the barrier film, which consists of non-conductive oxide ( = 1010  ∼ 1012   Ω cm [47]), covers the entire surface of the aluminum (regime 1 in Fig. 1.2 and Fig. 1.3). The electric field is focused locally on fluctuations of the surface (regime 2 in Fig. 1.2 and Fig. 1.3). This leads to field-enhanced or/and temperature- enhanced dissolution in the formed oxide and thus to the growth of pores (regime 3 in Fig. 1.2 and Fig. 1.3). Since some pores begin to stop growing due to competition among the pores, the current decreases again as shown in regime 4 in Fig. 1.2. Finally, jp  maintains an equilibrated state.

Figure 1.3: Schematic diagram of the pore formation at the beginning of the anodization.  Regime 1: formation of barrier oxide on the entire area; regime 2: local field distributions caused by surface fluctuations; regime 3:  creation of pores by field-enhanced or/and temperature-enhanced dissolution; regime 4: stable pore growth.

In this stage, pores grow in a stable manner.  However, it is very often observed that during the stable pore growth, the current density continues to decrease slightly. This is due to diffusion limits in the long pore channels [45, 46].

1.5     Self-ordered porous alumina - current state-of-the-art
The method discovered by Masuda et al. relies on self-ordering of pores at the bottom of porous alumina channels after a long first-anodization step [34]. The driving force for self-assembly has been attributed to mechanical stress caused by the repulsive forces between neighboring pores during anodization [48, 49].4 Several previous studies revealed that self-ordered porous alumina structures can only be ob- tained under specific conditions. For example, structures with pore spacing of 50, 65, 100, 420 and 500 nm are fabricated at 19 V and 25 V in sulfuric acid, at 40 V in oxalic acid, and at 160 V and 195 V in phosphoric acid, respectively [46, 50, 51, 52], as seen in Fig. 1.4.

1.5.1     Experimental set-up
Fig. 1.5 shows an apparatus of our electrochemical experiments (anodization and electrochem- ical deposition) [44, 45]. The electrochemical cell consists of a two-electrode system, e.g., the Platinum (Pt) mesh acting as the counter electrode (B in Fig. 1.5) and Al sheet acting as the working electrode (F in Fig.  1.5).  The Al sheet is inserted between the electrolyte container D in Fig. 1.5) and brass plate (G in Fig. 1.5) and fixed by screws (E in Fig. 1.5). To stir the electrolytes vigorously, a motor-controlled agitator is used (A in Fig.  1.5).  The apparatus is operated by a computer (I in Fig. 1.5), using home-made software. The cell was clad with sty- rofoam acting as the thermal insulator (C in Fig. 1.5). For cooling the apparatus, a combination of water-cooling and a Peltier element is employed (J in Fig. 1.5). The potentiostat/galvanostat (Keithley 2400) has a working range of either 0 ∼ 210 V (potentiostatic mode) or 0 ∼ 100 mA (galvanostatic mode) as shown in H of Fig. 1.5.

Figure 1.5: Schematic diagram of the apparatus used for the anodization. A: Motor-controlled rotator for agitating the electrolyte, B: Pt mesh working as counter electrode, C: isolator consisting of an outer styrofoam layer and inner brass layer, D: electrolyte container made of teflon, E: screw for fixing the electrolyte container to the brass plate, F: aluminum sheet, G: brass plate working as electric conductor connected with a positive electric source, H : potentiostat/galvanostat (Keithley), and I: computer to operate the potentiostat/galvanostat and the cooling, J: Peltier cooling element.

Figure 1.6: Stages of the formation of self-ordered alumina: a) Annealing at 500◦C for 3h; b) electropol- ishing in a solution of 1/4 HClO4  + 3/4 C2H5OH for 4 min at 8 V with agitation; c) first anodization (typically 1 ∼ 2 days); d) selective dissolution of the formed oxide layer; e) second anodization under the same conditions as the first anodization; and f) isotropic etching in 1 M phosphoric acid at 30◦C to widen the pores.

Experimental procedures are described in detail in Fig.  1.6 [44, 45].  Firstly, aluminum with a high purity (99.99 %) is cleaned with acetone in an ultrasonic bath. Then, it is immersed in 100 ml of a mixture containing HF/HNO3 /HCl/H2O at a ratio of 1:10:20:69 in order to remove impurities on the surface. After cleaning with deionized (DI) water (Ωm2  > 106), Al is annealed for 3 h at 500 ◦C in N2  to obtain large single cystalline grains. As a matter of fact, the larger the grains are, the larger are the domains of self-ordered porous alumina (see Fig. 1.6 (a)). To reduce surface roughness, electropolishing is carried out in a mixture consisting of 1/4 HClO4 + 3/4 C2H5OH (Fig.  1.6 (b)).  Electropolishing is a prerequisite for the formation of self-ordered porous alumina with large domain size. Note that caution is needed when perchloric acid/ethanol is used due to its explosiveness at moderate temperatures. After the pretreatment, anodization is performed either at 19 V in 2 M H2SO4, at 25 V in 0.3M H2SO4, at 40 V in 0.3 M (COOH)2, at 160 V in 1 M H3PO4, or at 195 V in 0.1 M H3PO4 for more than 1 day (Fig. 1.6 (c)). Since pores are randomly created on the surface, the initial pore arrangement is very irregular (Fig. 1.7 (a)). However, due to the repulsive forces between neighboring pores during the long-anodization, self-organization occurs. As a result, hexago- nally close-packed arrays are obtained at the interface between the porous alumina layer and the aluminum substrate ((Fig. 1.7 (b))). Then, the porous alumina film is selectively dissolved in a solution containing chromic acid (Fig. 1.6 (d)) [53]. Patterns that are replicas of the hexagonal pore array are preserved on the fresh aluminium surface. This allows the preparation of pores with a high regularity by a subsequent second anodization under the same conditions as the first anodization (Fig. 1.6 (e)). If needed, the resulting pores can be isotropically widened by chemical etching with 0.5 ∼ 1 M phosphoric acid (Fig. 1.6 (f)).

Figure 1.7: Scanning Electron Microscopy (SEM) images of a porous alumina sample produced by a first anodization (in 0.1 M phosphoric acid at 195 V). (a) the surface, and (b) the bottom of the membrane after selective removal of Al, which correspond to T and B in Fig. 1.6 (c), respectively.

1.5.3     The mechanical stress model
Jessensky et al. and Li et al. proposed a mechanical stress model to explain the formation of hexagonally-ordered pore arrays [48, 52]. They considered the following factors;

1. The oxidation takes place at the entire metal/oxide interface mainly by the migration of oxygen containing ions (O2−  or OH− ) from the electrolyte.

2. The dissolution and thinning of the oxide layer is mainly due to the hydration reaction of the formed oxide layer.

3. In the case of barrier oxide growth without pore formation, all Al3+  ions reaching the electrolyte/oxide interface contribute to oxide formation. On the other hand, porous alu- mina is formed when Al3+  ions drift through the oxide layer. Some of them are ejected into the electrolyte without contributing to the oxide formation.

4. Pores grow perpendicular to the surface when the field-enhanced dissolution at the elec- trolyte/oxide interface is equilibrated with oxide growth at the oxide/metal interface.

5. The formed alumina is assumed to be Al2O3 . Therefore, the atomic density of aluminium in alumina is by a factor of 2 lower than in metallic aluminium.  This means that the volume of the anodized alumina expands to about twice the original volume.

6. This volume expansion leads to compressive stress during the oxide formation in the ox- ide/metal interface. The expansion in the vertical direction pushes the pore walls upwards.

According to their studies, the degree of the volume expansion of aluminum, the volume ex- pansion coefficient ξ (see Eq. 2.5), varies with the applied potential and determines either the formation of self-ordered pores or the formation of disordered pores. If the stress is maximal (ξmax   ∼ 2), no pores are generated. If the stress is too small (ξ < 1.2), the force promoting ordering is too small and a disordered porous alumina array is formed. In the case of moderate promoting forces (ξ ' 1.2), ordered pore growth occurs. If 1.3 < ξ < ξmax , the size of domains of self-ordered porous alumina will decrease and finally disappear due to the strong repulsive interactions. They concluded that self-ordered porous alumina is best formed when the volume expansion coefficient ξ ≈ 1.2.

1.5.4     Anodization parameters influencing self-ordering

Potential
The applied potential, U , is one of the most important factors to adjust self-assembly of porous alumina. As seen in Fig. 1.4, the interpore distance, Dint , is linearly proportional to the applied potential with a proportionality constant k of approximately 2.5 ≤ k(nm/V ) ≤ 2.8 [18, 24]. Dint =kU (1.9) In addition, the thickness of the barrier layer can be approximately estimated as half of the interpore distance (Dint = 2DB , where DB   is the barrier-layer thickness).

Type and concentration of the electrolyte
The type and the concentration of the electrolyte for a given potential has to be selected properly to obtain  self-ordered pore growth.  In other words, the choice of the type of electrolyte is restricted. Usually, the anodization of aluminium is carried out in sulfuric acid in low potential ranges (ca.  5 ∼ 40 V), oxalic acid is used for medium potential ranges (ca.  30 ∼ 120 V) and phosphoric acid for high potential ranges (ca. 80 ∼ 200 V). This restriction is due to the conductivity and pH value of the electrolyte. For example, if aluminum is anodized in sulfuric acid at a high potential (note that sulfuric acid has a very high conductivity), breakdown of the oxide layer takes place very often. In addition, the pH-value of the electrolyte determines the size of the pores. The lower the pH value, the lower the potential threshold for field-enhanced dissolution at the pore tip.   This leads to a smaller size of the pores.  Therefore, large pore diameters are formed by using the phosphoric acid, and small pore diameter are obtained by using sulfuric acid [24, 41].

Temperature dependence
During the anodization, temperature should be kept lower than room temperature to prevent the formed oxide structure from being dissolved in acidic electrolytes. For instance, anodization at 40 V in oxalic acid is performed at 5 ∼ 18◦C and at 0 ∼ 2◦C in the case of anodization at

195 V in phosphoric acid.  A second reason to keep the temperature as low as possible is to avoid a local heating at the bottom of the pores during the course of anodization (specially, in the case of anodization at a high potential). The local heat causes an inhomogeneous electric field distribution at the bottom, leading to local electrical breakdown of the oxide (see Fig. 1.8). In fact, cracks and bursts of the oxide film are generated if porous alumina is formed without temperature controlling.  If the temperature is too low (just below zero  degree) and diluted electrolytes are used, the electrolyte may freeze. In addition, the speed of the growth of porous alumina is affected by the temperature. The lower the temperature, the lower is the growth rate.

Figure 1.8: (a) SEM image of porous oxide after electrical breakdown caused by local heating. Enlarged the image is shown in Fig. 1.8 (b).

Influence of impurity
Aluminum with a high purity (≥ 99.99 %) is usually recommended to obtain self-ordered porous alumina.   It is reasonable to expect that anodization of aluminum having impurities leads to defects since impurities have a different volume expansion coefficient than aluminum. To my knowledge there are no investigations about the effect of impurities on the self-ordering. In the next chapter (see chapter 3), we will discuss these effects.

Other important parameters
Additionally, annealing, electro-polishing, agitating, and the first anodizing time are important parameters to be considered to fabricate self-ordered porous alumina structures with large do- main sizes.

• The aluminum substrate is annealed below the melting point to obtain a large grain size (see Fig.1.6 (a)).  Usually, an annealing process is performed at 500 ◦ C for 3h (note that the melting point of Al is 680 ◦C).

• The surface roughness is smoothed by electropolishing (see Fig.1.6 (b)) [49].  Usually, the electropolishing is performed for 1 ∼ 4 min.

• During the anodization, the electrolyte should be vigorously stirred to effectively remove hydrogen bubbles and local heat on the surface, and to allow a homogenous diffusion of anions into pore channels [49].

• The first anodizing time affects the size of domains of self-ordered porous alumina.  It turned out that the domain size increases with the first anodizing time [54]. However, it was also found that the average of domain sizes decreased again if Al is anodized over a critical time.

P&A International

Introduction
Heat sink optimization involves many different considerations: thermal, mechanical, fluid, and system environment all play a role, as does manufacturability. This paper explores these aspects of heat sink design in the context of a practical application, and shows the many limits on what is otherwise considered “optimization”. In reality, the design process incorporates many different limits in the search for a suitable thermal solution that is easy to manufacture and works in the target system.

Analysis
This design task is carried out using thermal and pressure drop performance calculations that include developing flow, fin efficiency, and heat spreading in the base of the heat sink. More specifiically, the pressure drop for any given velocity between the fins is calculated using fully-developed flow correlations [1] in rectangular channels, varying with aspect ratio. Heat transfer calculations use developing flow correlations between parallel plates [1], corrected for channel aspect ratio [3], for the heat transfer coefficient. Air temperature rise is taken into account using the same log-mean temperature difference method as for heat exchangers [1]. Fin efficiency calculations are included to obtain the total fin-side thermal resistance. To obtain the total thermal resistance, an additional spreading resistance [4] is added to the finside thermal resistance. This spreading resistance captures the competing effects of heat spreading and conduction through the thickness of the base and is well documented in [4]. The fan operating point will be at the intersection of the heat sink pressure drop characteristic [5]. Among the many assumptions are a few key parameters: that the fan curve is valid even when the fan is directly adjacent to a parallel array of fins. The technology of choice for this application is convoluted or folded fin attached to an extruded base. Although there are other inexpensive heat sink manufacturing technologies like CNC machined, most of them carry geometric constraints that limit the minimum thickness of the fin. For example, a typical fin thickness in an extruded heat sink is 1.5 to 2 mm. This dimension along with the fin height limit of about 50 to 75 mm gives a thermally thick fin. It also limits the number of fins, in conjunction with the aspect ratio of the inter-fin space, that it is possible to fit on a given width of heat sink base. In a bonded-fin heat sink, the fin is typically 1.3 mm thick and must have a stable insertion. This insertion constrains the heat sink base to a minimum of about four times the fin thickness. For convoluted fin technology, the fins can be made in nearly any thickness up to 1 mm, and all the fins are made from a single piece of material. This strategy minimizes the number of parts in the heat sink assembly. The overall physical envelope for this particular application, including the fan, is an 89 mm cube. The fan is mounted outside the fin pack such that the air flow direction is parallel to the fins and to the heat sink base. The flow is confined to the fin area by a cover plate on the top. Were the fin envelope to be smaller than the fan, the flow would be shrouded to the front face of the fins, rather than allowing some of the air to bypass the fins. The thermal load is 20mm x 20mm and dissipates 75 W. The table below shows the parameters to select to specify the heat sink design and the manufacturable limits of each parameter. The constraints on fin pitch are related to the fin folding machinery and come directly from the manufacturer of the apparatus. The fin spacing (gap) must be greater than a certain multiple of the fin material thickness. For fins up to 75 mm long and 25 to 50 mm high, the multiple was 3.0. This results in the constraints on maximum fins per inch illustrated in Table 2 and Figure 1.

The thermal performance target for the total solution was 0.27 C/W including the effect of heat source spreading, air heating, and fin performance. The flow rate resulting from the fan is obtained from the intersection of the heat sink pressure characteristic and the fan characteristic curve. While some deviations from the fan curve are inevitable in practical applications, the local optima are not expected to shift dramatically.

Number of fins and fin height variation
The effect of varying the number of fins in a constant 75 mm flow width is shown in Figure 2 below. The chart shows the fin-side thermal resistance. The effect of changing the fin height is shown in the different curves. For this comparison, we assumed 0.8 mm fins. Varying the number of fins implies that the fin spacing varies, so that the overall flow width is fixed. For each fin spacing, the heat transfer coefficient and pressure drop is re-calculated. The results are shown in Figure 2. Note that as the number of fins increases, the space between the fins decreases. Smaller inter-fin spacing increases the pressure drop for a given volume flow rate. Coupled with the fan curve (in this case a typical 60x25 mm fan), this impedance increase gives a decrease in the total volume flow rate supplied by the fan. At the high end of the range of number of fins, the volume flow rate through the small spaces is so low that air heating outweighs the heat transfer coefficient advantage of the smaller spacing, and dominates the thermal resistance. At the low end of the range of number of fins, there is simply not enough fin area to achieve high performance. These two competing effects produce the minimal in the curves shown in Figure 2. Performance factors aside, there are physical limitations to producing a viable heat sink. Comparing the performance curves in Figure 2 to the manufacturing limits in Figure 1 and Table 2 results in the conclusion that the only manufacturable combination for 25 mm or higher is the lowest number of fins (24). There are a few conclusions to note at this point. Most significant is that the sensitivity to number of fins decreases as the fin height increases. The larger flow space has lower pressure drop. Making the fin space narrower by adding more fins produces less increase in pressure drop for a tall fin space than for a short one. Keep in mind that the fan responds to higher pressure drop by deliving less volume flow rate. The lower flow rate translates to a higher air temperature rise contribution to the thermal resistance. Secondarily, the addition of fin height brings less and less performance return as the fin height increases. This effect stems from the decrease in air speed (and thus heat transfer coefficient) and from the decrease in fin efficiency. It is worth noting at this point that experience shows that the thermal resistance is typically not quite as sensitive to the number of short fins as the calculations indicate. However, low flow rates at high pressure drops should be avoided to minimize acoustic noise in the system.

Base thickness variation
Figure 3 shows the relative contribution of fin-side and spreading resistance. The spreading resistance shows only a weak dependence on the number of fins, and is a significant contributor to the total thermal resistance. Thus, it is worth exploring the sensitivity of that resistance to base parameters. Figure 4 shows the effect of varying the base thickness for a constant total height of 50 mm with 0.3 mm thick fins and a 24 mm square source, again using a 60 mm fan. The total thermal resistance is the sum of the spreading resistance and the fin-side resistance. The spreading resistance in reality is probably somewhat less than calculated here because some spreading already occurs in the chip package. However, one may still conclude here that the spreading resistance is nearly independent of number of fins, since all the curves are parallel. It is also apparent that increasing the base thickness beyond 7.5 mm results in negligible performance improvement.

Fin thickness variation
The effect of varying the fin thickness is illustrated below for 43 mm tall fins and a 60x25 mm fan. It is interesting to note that the minimum fin-side thermal resistance can be obtained by more than one theoretical fin construction. However, because of manufacturing constraints such as diecasting, not all the possibilities shown in Figure 5 are realistic. The selection therefore tends toward a greater number of thinner fins, since with the thick fins maximum number of fins is not enough to optimize the thermal performance. Table 3 shows the feasible optimum combinations. This feasibility stems from the manufacturing limitations shown in Figure 1 and Table 2. Compare these combinations to Figure 5: many thin fins (48 at 0.3 mm thick) give better performance (approx. 0.22 C/W) than fewer, thicker fins (36 at 0.8 mm thick, approx. 0.25 C/W), given a specific air moving device. The parameters of the air mover govern the location of the minimum thermal resistance.

Fan characteristics
Many of the Figures above show sharp performance decreases as the fin array spacing decreases, most likely as a result of air heating (insufficient air volume). Figure 6 illustrates this phenomenon with 36 fins at 0.8 mm thick and 43 mm tall. Two performance curves are shown: one for the original 60 x 25 mm fan, and the other for a 60 x 15 mm fan. This suggests that a larger, more powerful fan could benefit the thermal performance. Flow length variation

Since the fin height, number and thickness now is constrained by manufacturability, it is worth checking whether the flow length has any effect. It is possible, as the upper curve in Figure 6 shows, that an increase in the flow length (and thus fin area) could paradoxically increase the thermal resistance. This performance degradation is caused by the higher pressure drop of the longer flow length and the flow  delivery characteristic of the fan.

The graph below is for 50 mm tall fins and a larger, 80 mm fan. Since the fan mounts directly at the inlet face of the heat sink, the fan thickness must be included in the overall size envelope. A 75 mm envelope and a 25 mm thick fan leaves 50 mm flow length, which has been the standard up to now. It is evident that a slightly longer envelope may bring small improvements in thermal performance with a sufficiently powerful fan, but that even that strategy has its limitations. This limit can be seen in the flattening of the performance curve in Figure 7 as the flow length increases.

Conclusions

Practical design considerations often place significant limitations on heat sink “optimization”. Moreover, using a fan curve-based design method sometimes results in performance effects that run counter to conventional wisdom. However, these real phenomena cannot be ignored in the development of a suitable system thermal solution.

Jenny Paddy

Bending is a flexible process by which a variety of different shapes can be produced though the use of standard die sets or bend brakes. The material is placed on the die, and positioned in place with stops and/or gages. It is held in place with hold-downs. The upper part of the press, the ram with the appropriately shaped punch descends and forms the v-shaped bend.

Bending is done using Press Brakes. Press Brakes can normally have a capacity of 20 to 200 tons to accommodate stock from 1m to 4.5m (3 feet to 15 feet). Larger and smaller presses are used for diverse specialized applications. Programmable back gages, and multiple die sets currently available can make bending a very economical process.

BEND ALLOWANCES

During sheet metal fabrication, when material is bent, the inside surface of the bend is compressed and the outer surface of the bend is stretched. Somewhere within the thickness of the metal lies its Neutral Axis, which is a line in the metal that is neither compressed nor stretched. What this means in practical terms is that if we want a work piece with a 90 degree bend in which one leg measures A, and the other measures B, then the total length of the flat piece is NOT A + B as one might first assume. To work out what the length of the flat piece of metal needs to be, we need to calculate the Bend Allowance or Bend Deduction that tells us how much we need to add or subtract to our leg lengths to get exactly what we want.

The location of the neutral line varies depending on the material itself, the radius of the bend, the ambient temperature, direction of material grain, and the method by which it is being bent, etc. The location of this line is often referred to as the K factor. K-factor is a ratio that represents the location of the neutral sheet with respect to the thickness of the sheet metal part.

The only truly effective way of working out the correct bend allowance is to reverse engineer it by taking a measured strip of material, bending it, and then measuring it to calculate the bend allowance. These bend allowance can be measured for many materials and scenarios and then tabulated so that the table can be used by CAD programs to produce accurate sheet-metal work. Many CAD programs, however, also work out bend allowances automatically by using K-factor calculations. (Or Y-factor in the case of Pro-E where Y-factor = K-factor * ð / 2). Bend allowances are calculated using a K-factor as follows:

This works extremely well and is pretty straight forward, providing we know the correct K-factor to use. Once again, the most accurate way of finding the correct K-factor to use in your CAD program is by using the reverse engineering method described above, and calculating the K-factor to use as described in the following section.

REVERSE ENGINEERING THE K-FACTOR

First, cut a strip of material and measure its length and thickness as accurately as possible. The width of the strip is not that critical but generally somewhere around 100mm (4 inches) or so usually does the trick. Then, bend the strip to 90 degrees, and measure its Length X and Length Y as shown in the diagram below. Note that it is very important to bend the sample piece in exactly the same manner as you plan to bend your real pieces, so that whatever you measure now becomes reproducible later.

The bend radius can be extremely difficult to measure accurately but, in this case, is not critical (within reasonable limits, of course!). The reason it is not critical is that what we are interested in is a number to use in our Chinese version of CAD program that, with the bend radius used in our CAD program, will produce the results you are measuring in real life. In other words the K-factor you calculate now will take into account any small inaccuracies in the bend radius measurement and compensate for it. If, for example, we are using a Bend radius of 0.5 in our CAD program, it does not matter if our real tooling radius is actually 0.4, as the K-factor, which was worked out from our real tooling, corrects for this. The only implication of this is that we may occasionally get a K-factor that seems odd (higher than 0.5, for example) if our real radius is very different from our CAD program radius. Remember though that most CAD programs such as Solidworks only accept K-factor values from 0 to 1, so if the calculated K-factor is outside these limits, then you may need to double-check your numbers. The correct K-factor to use in your CAD program can now be calculated as follows:

BendDeduction = Length X + Length Y - Total Flat Length

OutSideSetBback = (Tan(BendAngle / 2)) * (thickness + BendRadius)

BendAllowance = (2 * OutSideSetBback) - BendDeduction

K-Factor = (-BendRadius + (BendAllowance / (pi * BendAngle / 180))) / thickness

Using this method will produce the most acceptable results other than by using a bend table. There are however also some general rules of thumb that can be used for K-factors that will generally give results that are within acceptable tolerances for non-precision sheet metal work. Some of these sample K-factors are given in the methods of bending section below.

AIR BENDING

The amount of spring back depends on the material, thickness, grain and temper. The spring back will usually range from 5 to 10 degrees. The same angle is usually used in both the punch and the die to minimize set-up time. The inner radius of the bend is the same as the radius on the punch.Air Bending is a bending process in which the punch touches the work piece and the work piece does not bottom in the lower cavity. As the punch is released, the work piece springs back a little and ends up with less bend than that on the punch (greater included angle). This is called spring-back. In air bending, there is no need to change any equipment or dies to obtain different bending angles because the bend angles are determined by the punch stroke. The forces required to form the parts are relatively small, but accurate control of the punch stroke is necessary to obtain the desired bend angle.

BOTTOMING

Bottoming is a bending process where the punch and the work piece bottom on the die. This makes for a controlled angle with very little spring back. The tonnage required on this type of press is more than in air bending. The inner radius of the work piece should be a minimum of 1 material thickness. In bottom bending, spring-back is reduced by setting the final position of the punch such that the clearance between the punch and die surface is less than the blank thickness. As a result, the material yields slightly and reduces the spring-back. Bottom bending requires considerably more force (about 50%~60% more) than air bending.

COINING

Coining is a bending process in which the punch and the work piece bottom on the die and compressive stress is applied to the bending region to increase the amount of plastic deformation. This reduces the amount of spring-back. The inner radius of the work piece should be up to 0.75 of the material thickness.

TIPS AND TRICKS

General bending guidelines are as follows:

• The bend radius should, if possible, be kept the same for all radiuses in the part to minimize set up changes.
• For most materials, the ideal minimum inner radius should be at least 1 material thickness.
• As a general rule, bending perpendicular to the rolling direction is easier than bending parallel to the rolling direction. Bending parallel to the rolling direction can often lead to fracture in hard materials. Thus bending parallel to the rolling direction is not recommended for cold rolled steel > Rb 70. And no bending is acceptable for cold rolled steel > Rb 85. Hot rolled steel can however be bent parallel to the rolling direction.
• The minimum flange width should be at least 4 times the stock thickness plus the bending radius. Violating this rule could cause distortions in the part or damage to tooling or operator due to slippage.

Slots or holes too close to the bend can cause distortion of these holes. Holes or slots should be located a minimum of 3 stock thickness plus the bend radius. If it is necessary to have holes closer, then the hole or slot should de extended beyond the bend line.

Dimensioning of the part should take into account the stack up of dimensions that can happen and mounting holes that can be made oblong should be.

Parts should be inspected in a restrained position, so that the natural flexure of the parts does not affect measurements. Similarly inside dimensions in an inside bend should be measured close to the bend.

OTHER COMMON TYPES OF BENDING

In V-bending, the clearance between punch and die is constant (equal to the thickness of sheet blank). It is used widely. The thickness of the sheet ranges from approximately 0.5 mm to 25 mm.

U DIE BENDING

U-die bending is performed when two parallel bending axes are produced in the same operation. A backing pad is used to force the sheet contacting with the punch bottom. It requires about 30% of the bending force for the pad to press the sheet contacting the punch.

WIPING DIE BENDING

Wiping die bending is also known as flanging. One edge of the sheet is bent to 90 while the other end is restrained by the material itself and by the force of blank-holder and pad. The flange length can be easily changed and the bend angle can be controlled by the stroke position of the punch.

DOUBLE DIE BENDING

Double die bending can be seen as two wiping operations acting on the work piece one after another. Double bending can enhance strain hardening to reduce spring-back.

ROTARY BENDING

Rotary bending is a bending process using a rocker instead of the punch. The advantages of rotary bending are:

• Needs no blank-holder
• Compensates for spring-back by over-bending
• Requires less force
• More than 90 degree bending angle is available

REFERENCES

• P&A International - Sheet Metal Fabrication
• Efunda Design Standards of Bending
• Design and Engineering Data on Sheet metal
• ASMA Chronicle, Advanced Sheet metal Applications

Test rig consist of 70 Watt thermal load applied by the centrally mounted resistor. The heat sink size is 100mm wide and 250mm long. Thickness of the fabricated aluminium plate is 4.0mm and extruded part is 5mm base and 15mm fins.

The chart represents temperature rise (above ambient) of the aluminium in degrees centigrade over the time measured in minutes. You can see that not just aluminium extrusion stabilised temperature been over thirty degree lower, but it also took five time longer to reach say 40C. This is caused by both, added mass of the heatsink (it’s twice as heavy) and  improved heat dissipation via fins. Added base thickness of the extruded part also contributor to this factor allowing improved heat dissipation. The thermal inertia of the finned part can be a bonus to system with duty cycle less than 100%.

The interesting part of the chart is performance of the flat aluminium plate with the added forced cooling by the fan. While this is questionable method due to limited lifespan of the fan, the performance of the cooling device been really good. It quickly got up to saturation temperature of about 40c and stay there. That is a good example of efficiency of fan forced cooling and it’s place in the industry. Where possible, it will allow equipment designer to use much smaller and lighter heatsink with the addition off the cooling fan while achieving same of better result over the larger finned cooler.

Comparing extruded heat sink design to bonded fins type assembly, extrusions will outperform by large margin due to bonded assembly thermal transfer is somewhat reduced by the bonding agent added thermal resistance. There are four common techniques that are used to manufacture heat sinks: Die Casting, Machining, Extrusion and Forging.

Let’s discuss performance of each of those types of heat sinks, excluding Machined parts. This is due to high CNC machining costs of machined heat sinks making it economically feasible only for prototyping and special low volume and high cost products.

Thermal Conductivity

Diecasting is a method for manufacturing complex shapes in high volumes and low cost. Initial tooling cost outlay is high, but the parts cost during the production run is relatively low. Drawback is porous structure as the molten aluminium alloy cools in the die-set it expands creating voids. The porous structure weakens the part and adds to thermal resistance to the heat sink.

Extruded aluminium alloy heat sinks are most cost effective way for manufacturing linear shapes. The drawback is the limitation in the freedom in design, but this method requires low cost tooling. During the extrusion, alloy is heated below melting point and force-feed thru the forming die-set. However the grain structure cannot be controlled evenly and shape of the heatsink cannot be optimized completely thus somewhat reduce thermal performance.

Forging is the effective method to produce complex shapes in high quantity and also offers interesting thermal advantages due to processing alloys at room temperature. Due to the nature of the forging process, part is formed at high pressure and low temperature, process that allow better control of the grain structure. Making heat sinks stronger and better conductors of heat.

Below chart compares die casted, formed and extruded heatsink performance. 

The cold forging process produces heat sink with 13% better thermal conductivity over extruded and 60% over die-cast part. Note that in this test average samples been tested. Best examples of each heat sink likely to produce better results somewhat dampening large outperformance by forged part.

Heatsink Surface Area

Increased surface area of the heat sink will yield lower thermal resistance and better component cooling, but only if boundary layer is not formed and fins close proximity do not prohibit flow of air.

When designing extruded part, fins must be tapered so that the aluminium alloy will pass through the tool without breaking it. The number of fins in an extrusion limited by strength of the die set and the size of the extrusion. These restrictions will have impact on the surface area.

Forged fins require lesser taper for extraction from the tool, allowing for more fins per given heat sink size. Heat sink fins can also be formed in to elliptical shape if required.

The forged heat sink increases the surface area by 15% without increasing the size. The result is improved thermal performance.

Reduced Cost

Depending on design, forged heat sink process can simplify manufacturing process. In cases when secondary operation required when working with extruded or die-cast heat sinks in forging process this can be part of a single operation. That can greatly influence cost of the finished product.

The forging process has fewer limitations to forming heatsink shapes and fin designs. A forged part is formed in two dimensions within the stamping tool creating complex shapes without the need for secondary operations. Holes, chamfers, pins, elliptical fins and steps are created in the one tool in a single operation.

In the above example, cooper spreader plate is inserted in to the forging tool following by the aluminium alloy slab. When pressed in to the die, aluminium formed in to heat sink shape and creates tight fit over the copper base, with minimal added thermal resistance.

The forged heat sinks allow designers to creates unique shapes and fin designs that can be achieved in the forge tool in a single operation.

Conclusion

Forged heat sinks offer advantages over machined, die cast and extruded processes. The improved thermal performance due to aluminium grain structure coupled with the increased surface area without increasing the size of the heat sink and low process cost are main advantages.

Cold forging can also produce heat sinks of the complex shapes such as elliptical fins, staggered fins, round pin arrays, steps, all within the tool. Finally, precision forged heat sinks can often be manufactured at a lower cost because most operations can be performed in the tool and secondary operations are reduced.

Forging is also the most effective method for forming copper. Copper is difficult to extrude because it must be heated to high temperatures to soften the metal, making it challenging. Forging is a cold process, and copper heat sinks can be formed with minimal waste.

For you by P&A International your Heat Sink Specialist

(the units are in CGS system which is used throughout this catalog). Here le and Ae are the effective magnetic path length and cross sectional area of the core, . is the effective permeability of the material, and N is the number of turns. This formula may be used for any shape under all conditions provided the correct value of . is used and stray reactances are given proper consideration. In a toroidal core, this may be expressed as:

where OD, ID and H are the dimensions in inches. For low level conditions at comparatively low frequencies the formula may be simplified by using the Inductance Index, AL, listed in this catalog. Then;

The other value most frequently needed is peak flux density, which may be calculated from

Here E is the RMS voltage, 4.44 is a constant depending on the wave shape (use 4 when E is symmetrical square wave and 1 where E is a unipolar pulse), and f is the frequency in hertz.

LOW LEVEL INDUCTORS:

This section considers those applications where nonlinearity and losses due to hysteresis are negligible. Generally this means flux densities below a few hundred Gauss. The first material choice is the one having both the highest permeability and lowest loss factor, tan ./., at the operating frequency. Considering the space available, select a core from the table and, using its inductance index, AL, calculate the number of turns required to give the desired inductance. Now select the largest practical wire size that will fit on the core. This is somewhat difficult for a toroid, but generally the total wire cross section in the winding can be 30-60% of the window opening. If there are Q or loss requirements calculate the resistance of the winding, taking into consideration the skin effect if the frequency is high, and add it to the equivalent series resistance contributed by the core losses. Equation 5 shows the relationship between loss factor, Q and resistance.

If the calculated Q is inadequate you must reduce the total series resistance by selecting a larger core that will allow fewer turns of larger wire, select a less lossy material, or use Litz wire at high frequencies to minimize the skin effect.If losses are critical it is important to remember that hysteresis losses have been assumed to be negligible. Above a few 10’s of Gauss these losses are measurable and increase as approximately the 2.5 power of flux density. Also, remember that ferrites like other magnetic materials show variation in inductance from part to part, with temperature and with magnetizing force. Unlike powdered metals which have air gaps between the particles a ferrite toroid is a continuous magnetic material with variability effects undiluted by air gaps. This means that tight tolerances such as required for wave filters are not attainable in a toroid, but will generally require a gapped structure such as an E core, pot core, or slug.

POWER INDUCTORS: 
In this section we consider inductors where the design is limited by saturation or heating due to core or winding losses. Although there is no systematic connection between permeability and losses, below about 1 MHz relatively high permeability manganese-zinc ferrites have the most desirable combination of high saturation flux density and low hysteresis losses. The first step is to select one of those materials having the desired properties (usually B material) and select a core based on space limitations. Then select a suitable operating flux density. As a general rule, at room temperature materials may be operated to the knee of the BH loop when the frequency is 20 kHz or less. At higher frequencies hysteresis losses produce enough heat to require that the flux density be decreased. As a first approximation, the product of flux density and frequency can be held constant above 20 kHz. Knowing the voltage, frequency, flux density and area of the chosen core the minimum number of turns may be calculated from equation 4. The inductance can then be estimated from equation 3 or calculated more exactly from equations 1 or 2 by using the appropriate value of permeability under these operating conditions. If this inductance is less than the desired value, the number of turns can be adjusted upward provided there is sufficient space for the winding. If the inductance is too great it will be necessary to choose a larger core whose cross sectional area is greater but whose ratio of Ae/le is less, or a material with lower permeability. For inductors operating above 1 MHz the material choice becomes more difficult since other requirements such as return loss may be more important. The material choice and design procedure will depend on which factors predominate in your particular design. Inductors having dc current superimposed on the ac excitation must be given special treatment. The magnetizing force may be calculated using equation 6:

With this information it is possible to estimate from the BH curves how significant will be the effect of the dc current. Generally dc magnetizing forces less the coercive force will have only a small effect on permeability, moderate values will depress the permeability, and magnetizing forces approaching the knee of the BH loop will considerably reduce the permeability and severely limit the peak flux density available for ac excitation. In these cases, unless a higher inductance can be used it will be necessary to go to a core with a considerably longer magnetic path length or to provide an air gap such as by slotting the core.

In many power applications thermal considerations control the design. One rule of thumb that may be useful for first approximations is that core losses of 100 to 600 mW/cm3 produce an approximate 40° C temperature rise. The exact value depends on inductor geometry and thermodynamic considerations beyond the scope of this guide. You must also consider the power dissipated in the winding and its contribution to inductor heating. Heat sinking or coolants may be used to remove this heat, but the thermal conductivity of ferrite is relatively low, so the interior core temperature will be higher. Should a large temperature gradient develop, the core may crack from thermal stresses. Also, where considerable temperature excursions occur due either to self heating or ambient temperatures, the effect of these changes must also be considered with respect to changes in saturation flux density and inductance.

LOW LEVEL TRANSFORMERS:

The design procedure here is essentially the same as for low level inductors except, of course, that the winding space must be shared between the primary and secondary windings. Usually half the space is allotted to each. In selecting the inductance required it is easiest to envision the equivalent circuit as an ideal transformer (figure 1) with a primary self inductance shunting the transformer primary. When the impedance represented by this inductance is high compared to the primary and transformed secondary impedances it may be neglected and an ideal transformer results. Ordinarily this impedance is selected to be between 3 and 10 times the source impedance. At very high frequencies losses or winding capacitance and leakage inductance may predominate.

These situations are considered in the later paragraphs.

CURRENT TRANSFORMERS:

This special class of low level (and sometimes power) transformers includes ground fault interrupters (GFI) sensors. In this case it is simpler to design around the secondary. Because there are often few primary turns (usually one) and many secondary turns, the transformed source impedance (Zs) and primary winding resistance (Rwp) can not always be neglected. As shown in figure 2, these impedances are increased by 1/n2 (where n is the primary to secondary turns ratio) .

As n is decreased to raise the secondary voltage all four internal impedances shown in figure 2 increase. This limits the available load voltage, so a compromise must be made for optimum performance. Since the core losses of high permeability ferrites are small at audio frequencies, they may often be neglected. For this reason, ferrite toroids are usually selected for grounded neutral transformers in GFl’s-particularly when frequencies above 60 Hz are used for this test. Special manufacturing and test techniques can be used to enhance the properties of ferrite toroids for GFI differential fault transformers, as well.

POWER TRANSFORMERS:

Here we are considering the same kinds of situations we covered under power inductors, that is, those cases where the design is limited by saturation flux density or self heating due to core and winding losses. At low frequencies, say below 1 MHz, the design procedure is the same as that for power inductors except, of course, that winding space must be allowed for both windings. Ordinarily allot half each to the primary and secondary, or with a push-pull primary, slightly less than one third to each primary half. In most cases the voltage and frequency are known (use the lowest operating frequency for design purposes). Select a material and flux density in the same manner as for power inductors. Then using equation 4, calculate the product of Ae and N required. It is then a simple matter to go down the list of suitable core sizes substituting for Ae, calculating the minimum number of turns required and checking the fit of the winding in that core. Calculating the primary inductance from equations 1, 2, or 3, you will ordinarily find that the inductance will be large enough that the magnetizing current may be neglected under full load. (This is the current drawn by the primary inductance which shunts the ideal transformer.) The rest of the transformer design is fairly straight-forward and is covered in other publications. Most devices of this type are limited by either saturation or heat dissipation, temperature rise and efficiency. Often winding losses are greater than core losses below 50 kHz. In some cases other considerations such as regulation may take precedence, but the considerations described above must still be met. At higher frequencies in the MHz range other factors such as eddy currents influence the design. For this reason higher resistivity nickel zinc ferrites are ordinarily used. For example, the volume resistivity of G, J, K and P materials is typically 103 to 106 times greater than manganese zinc materials. Furthermore, winding design can be of major importance because of the critical nature of winding losses (including skin effects), leakage inductance and self capacitance. Again, cooling is often a major problem and increasing core size is limited by its effect on winding characteristics. It is sometimes helpful to assemble the core as a stack or two stacks of a number of smaller toroids since this facilitates cooling, and results in a compact winding. Occasionally oil cooling or heat sinking are used to improve heat transfer. Material triaselection is difficult because of the influence of several factors which do not lend themselves to analytical prediction. Lacking previous experience with a similar design, some guesses will have to be made. A good starting point is that material having the lowest loss factor at the minimum operating frequency. A trial design can be worked up using the same core selection criteria as at lower frequencies. Usually the flux density will have to be limited to a few hundred Gauss or less. Care should be taken to select a core which will allow a compact winding so that leakage inductance and winding self capacitance will be small. Winding design requires careful consideration also because skin effects will make the winding resistance (and, hence, loss) much greater than at low frequencies. A technique popular when one winding is a single turn is to use tubing. The wall thickness should be chosen to be slightly more than the current penetration depth, and the secondary winding can go within the tubing. Litz wire can also be used to reduce the effective resistance. A trial design and a few iterations are usually required to optimize RF power transformer designs.

WIDE BAND TRANSFORMERS:

The best starting point is with the equivalent circuit shown in figure 3.

Here Lp and Rp are the parallel inductance and resistance (loss) of the wound core, Rw is the winding resistance, Cd is the distributed self capacitance of the winding, Ll is the leakage inductance (representing flux that does not link the core), and Zs and ZL are the source and load impedances. At low frequencies the contribution of Ll and Cd are so small they may be neglected. The low frequency cut-off, where insertion loss, VSWR or source loading become unacceptable, is then determined by Lp, Rp, and Rw. Since the reactance of Lp (X = 2. f Lp) is proportional to frequency, it is usually the determining factor. The objective is then to choose a core, material and winding that will have the highest Lp and Rp at the lower frequency while keeping Rw small. To do this, select a material having high permeability and low loss at that frequency. Choosing a core with a high AL, it must be wound so that Lp and Rp are high enough and Rw low enough to meet the insertion loss, VSWR, return loss or loading requirements. At the high frequency cut-off, Lp can usually be neglected while Ll and Cd assume critical importance. These elements depend almost entirely on the winding and very little on the core. They can not be readily calculated, but are minimized by keeping the winding length and number of turns low. The optimum core is difficult to select since it must balance these considerations with winding space, ease of winding, integer turns, space limitations and core manufacturing constraints. Generally, it is best to choose a core with a large OD/ID ratio and the greatest practical height. For this reason high frequency wide band transformers are often wound on cores found in the BEAD and MULTIHOLE sections. There are also techniques covered in the literature on winding transformers with transmission lines such that at low frequencies the device operates conventionally as above. At higher frequencies coupling is via the transmission line enabling extension of the upper operating limit. In this section you will find curves o Xp, Rp and Z versus frequency for certain cores. This data simplifies material, core and winding selection. With the exception of the highest frequencies, these curves may be shifted upward or downward to fit a given application by the ratio of N2 of the new winding to N2 indicated on the graph.

PULSE TRANSFORMERS:

In many ways pulse transformers are a special case of wide band transformer because the pulse train can be represented by a number of sine waves of different frequencies. The turns ratio, though, is usually determined by voltage or current ratios rather than impedance matching, so the design approach is governed by pulse fidelity requirements rather than insertion or return loss. The equivalent circuit of figure 3 can help illustrate the elements influencing fidelity. Looking first at the flat top (low frequency) portion of a rectangular pulse (es), figure 4 shows some of the voltage and current wave shapes.

Neglecting the rise and fall portions (high frequencies),current through Rp is constant during the pulse and current through Lp flows according to equation 7.

If the voltage and inductance are constant the current will rise linearly with time. This produces a drop across Zs accounting for droop of the load voltage pulse (eL). In order to minimize droop, L must be made large. This can be accomplished by choosing the highest permeability material (typically B,T or V material) and largest core practical. To determine the AL value under pulse conditions, multiply the sine AL by 1.1. Lp can then be calculated from equation 3. Also, flux density must be considered. Equation 4 may be rewritten:

It can be seen that flux density rises linearly with time. As this approaches the knee of the hysteresis B-H) loop, permeability and inductance start to fall and the current begins to rise rapidly (figure 5).

From equation 3 and 8, you can see that increasing N will both raise L and diminish B. However, rise and fall time are limited by leakage inductance(Ll) and distributed self capacitance (Cd) in the same way as high frequency response in a wide band transformer. Therefore, the number of turns must be balanced between these conflicting requirements. The tools available are higher permeability and flux density material, and a larger core. High pulse repetition rate can have two effects. The dc level represented by averaging the pulses produces magnetizing force (H) to bias the starting point of each pulse to the right on the B-H loop (figure 5) . This can significantly reduce the available flux density. One possible solution is described under Slotted Toroids. Second, each pulse traverses a minor hysteresis loop producing an energy loss. This can cause core heating that will affect saturation flux density and permeability.

SLOTTED TOROIDS:

In a number of applications described earlier the design is limited either by dc current, excessive inductance, or variability effects of the ferrite. A slot cut through the cross section can sometimes be used to advantage. The effect of the gap is magnified by the material permeability according to:

Where lm and lg are the path length in the magnetic material and the gap respectively and, . is the material permeability. This can be used to reduce the effect of dc bias when the le calculated above is substituted into equation 6. For example 1 Adc flowing through 10 turns on a core with a path length of 2 cm produces a magnetizing force (H) of 6.28 Oe. This is enough to saturate most high permeability materials. Now if a .010" (.0254 cm) slot is cut and the material permeability is 5000, the effective path length (from equation 9) is 129 cm. The magnetizing force from the dc is reduced to .097 Oe and the effect of the dc bias is very small. In similar fashion a gap can be used to reduce inductance to the required value when the minimum turns are dictated by flux density considerations. The effective permeability of a gapped core can be calculated from:

This value of .can be used with equations 1 or 2 to calculate inductance. It is also apparent from equation 10 that as lg is increased relative lm changes in . will have a smaller effect on .e This can be used to reduce changes in inductance caused by permeability variations due to temperature, flux density, bias, stress, time, etc. For example, with 5000 permeability material and lg /lm = .01, a 20% change in .will result in only a 0.2% change en .e. Equations 9 and 10 are exact only when there is no flux fringing in the gap. This is a good assumption when A >>lg, but as the gap increases the actual .e will be greater than the calculated value and actual .e will be less. More elaborate equations can extend the range of accuracy somewhat, but with larger gaps some experimentation is necessary. A wide range of slot widths are available. Consult the factory regarding your application.

OTHER APPLICATIONS: 
Most other uses for toroids are variations on the above classes. Toroids used for noise or RFI suppression are covered in the BEADS section. If you have a special problem, Ferronics engineers will be happy to assist you.

COATINGS:

Ferrites are hard, abrasive ceramic materials which can abrade wire insulation films during winding. Ferronics toroids are ordinarily tumbled so that sharp edges are rounded. However, if a higher level of insulation protection is desired, a smooth as well as an insulating coating can be provided. This coating should be soft to prevent stressing the core upon curing or during temperature cycling, have a low coefficient of friction, withstand normal environments (including cleaning solvents) and provide some additional insulation. We use two materials which admirably fill these requirements. Parylene® C is used for smaller cores. It is vapor deposited - a process well suited to bulk coating and produces an exceptionally uniform coating normally about .0007 inches thick. Epoxy is used on larger cores. It is sprayed producing a variable thickness of about .001-.005 inches, and has better physical and chemical properties than other choices. Standard minimum voltage breakdown for both Parylene®and epoxy coated cores is 500VAC. If a higher level of protection is required, please consult with our engineering department. Parylene is a registered trademark of Union Carbide

Introduction

Optimal design of a heatsink, meeting program targets for cost, weight, size, and performance, is one of the more challenging activities within most electronics engineering teams. Without a dedicated solver, designers or thermal engineers can be involved in a game of ‘opinioneering’, which typically involves overdesign, or initiate an expensive and time-consuming physical design of experiments that provides limited results. In the case study below, we worked to optimize an extruded aluminum heat sink for an IGBT module in a low-cost, high-volume design. Reliability requirements (10-year life), harsh environments (vibration, elevated temperature), limited ability to perform maintenance (consumer household), and cost constraints eliminated forced air cooling as a practical solution. The focus was instead to optimize the design within the dimensional constraints provided by the end-user.

IGBT Module

Optimization was requested because the IGBT module starts to behave intermittently at 60°C and shuts down completely at 65°C. The module shutdowns due to a thermal cutoff limit of 100°C at the internal thermistor. The data sheet for the IGBT module indicates that the cutoff temperature of the IGBT junction temperature is 150°C. The module thermal resistances are supplied by the manufacturer and are shown in Figure 1. The goal of this analysis was to reduce the thermistor temperature during operation such that the module achieves stable operation at 60°C and shutdown at or above 65°C. To achieve this goal, the case temperature of the IGBT must be decreased. This increase in the operating margin will be achieved by modifying the relevant heatsink parameters. The baseline heatsink dimensions are displayed in Figure 2.

Figure 1: Thermal resistance of IGBT module

Model Calibration

A thermal analysis of the original heatsink design was conducted to baseline the thermal model. A thermal image of the heatsink while the IGBT module is dissipating 40 watts at 20°C is shown in Figure 3 (left image). The maximum heat sink temperature is measured as 90.1C, or a 70.1C rise above ambient. The results of thermal simulation at 40 watts at 20C are also shown in Figure 3 (right image). The maximum temperature rise was predicted to be 75.7C. The difference between measured and predicted is within 6%, which is a reasonable margin of error.

Figure 3: Heatsink temperatures at 20°C ambient

Design of Experiments

The heatsink parameters assessed in this design optimization study are presented in Table 1. Number of fins, fin height, location of heat source, and surface treatment were all assessed in terms of their ability to lower IGBT case temperature.

Table 1: Heatsink DoE Parameters

Results

When the calibrated model is run at the desired use condition of 20 watts at 60°C ambient, the case temperature rise is 43.1°C over ambient (see Figure 4). This clearly explains the intermittent operation of the IGBT under these conditions and indicates that for the heatsink optimization to be successful, the new heatsink design must reduce case temperatures by at least 5 and preferably 10C.  Interesting finding within the DoE: At 20C and 40W, the optimum number of fins is six (6). However, when the ambient temperature is increased to 60C, but the power dissipation is lowered to 20W, the optimum number of fins drops to five (5).

Figure 4: Baseline heatsink analysis at 20 watts, 60°C ambient, five fins optimal, 103°C

Heat source location 
The effect of moving the heat source was simulated at 60°C ambient conditions. Moving the heat source to the heatsink center had little effect on the performance (<1°C temperature change), as shown in Figure 5. Moving the heat source above the centerline increased the temperature by almost 3°C, as shown in Figure 6.

Figure 5: Heat source located at center

Figure 6: Heat source located near top

Heatsink Fin Height

The effect of changing the fin height was modeled at 60°C ambient conditions. Increasing the fin height by 0.25” decreased the temperatures by 5.5°C, as shown in Figure 7. Increasing the fin height by 0.5” decreased the operating temperature by 10°C. This indicates that the module will be running at 93.6°C. Increasing the ambient temperature to 65°C has a minimal impact on the temperature rise. At 65°C the IGBT module will be operating at 98.7°C and should be at the limitations of its operating range. Further modifications of the heatsink should be done to increase this margin.

Figure 7: Heatsink fin height +0.25”

Figure 8: Heatsink fin height +0.50”

Effect of Anodized Surface

The effect of radiation heat transfer is very important in natural convection, as it can be responsible of up to 25% of the total heat dissipation2. The capability of a material to radiate heat is given by its emissivity. Extruded low-cost aluminum has a relatively low emissivity (0.02 to 0.2), which can impede its thermal performance. One way to improve emissivity of aluminum is through an anodization treatment. Anodization is an electrochemical treatment process that introduces a relatively thin layer of oxide. When the treatment is combined with a black dye3, it can increase emissivity to almost 0.9. To assess the impact of anodization, the emissivity was increased from 0.4 to 0.9 for the model with the as designed fin height. The model was then run with the 0.5” added to the fin height. The results are shown in Figure 9 and Figure 10. Increasing the emissivity alone on the heatsink is not sufficient to drop the operating temperature below 100°C when operating at 65°C and should be combined with additional fin height which should place the operating temperature at about 94.3°C at 65°C ambient.

Figure 10: Emissivity increase with +0.5” fins, 13.8°C drop

Conclusions

Using more standardized optimization techniques, it was determined that fin height and anodization had the greatest ability to drop case temperatures below 100C when the IGBT was dissipating 20 watts at 65°C ambient. Orientation of the heat source and fin count had minimal effects.

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Proper design and optimization of a thermoelectric (TE) heat sink has been a topic of some neglect in the design of the TE cooling systems. Collectively, TE material researchers have spent tens of millions of dollars to advance the performance level of TE materials only to have it dashed away by ineffective heat sink design. The combination of detailed thermal modeling and high-speed personal computers makes it no less effort to fully optimize a heat sink design then to just derive one that "works." Of course, derivation of a thermal model employing accurate calculations for fluid dynamics and heat transfer can be a significant investment of time and effort but, once completed, the pay-off can yield big dividends. The process of model development begins with examination of classical fluid dynamic theory but should be validated and verified by experiment. The model used by TE Technology, Inc. was developed over a period of over 30 years using feed-back test data from literally thousands of exchangers and exchanger configurations. Empirical corrections were applied to adjust the classical theory to better match "realworld" test results. As such, the details of this model are proprietary. However, the use and application of this model and the methods employed to optimize are the subject of this paper.

SYSTEM DESCRIPTION

Throughout this paper, the term, heat sink, shall refer to a metal (usually aluminum) exchanger with a fluid flowing through it. Aircooled heat sinks can consist of a finned area with either flat, louvered, wavy, perforated, "breathing effect," slotted or pin-type fins. These exchangers are usually combined with a centrifugal blower or tubeaxial-type fan as the air mover. The air direction can be vertical, parallel or radial. Although the method demonstration presented in this paper involves a simple flat fin exchanger together with a tubeaxial fan and centrifugal blower, the principles and methodology are generically the same as with other configurations, including liquid-cooled heat sinks. Of course, liquid cooled heat sinks can involve simpler tube-type configurations because of the higher heat transfer characteristics of water or other liquids. Properly applied, the methods described in this paper provide the means for optimizing the heat sink design and cold or hot extenders as described by Lau., et al (1). The result can maximize the effectiveness of the TE modules resulting in reducing size, weight, power-consumption and cost of the entire thermoelectric product, as described by Ritzer.

AIR MOVER (Coolong Fan) CHARACTERISTICS

After the general size of the heat exchanger has been scoped, selection of the potential air movers should be the first step in the design optimization process. This is because geometry and configuration of the air mover is fixed, once selected. Whereas, the mating finned exchanger is flexible and can be modified to accommodate the air mover characteristics. FIGURE 1. Free-flow rate comparison between fans and blowers of the same power consumption. FIGURE 2. Stall pressure head comparison between fans and blowers of the same power consumption.Figures 1 and 2 illustrate the typical maximum characteristics of tubeaxial fans versus centrifugal blowers as a function of the power required to drive them. Each point on the charts are different models of fans and blowers. These are specific data taken from a single manufacturers, but they represent the typical trends of all fans and blowers. That is, fans typically increase quite rapidly with free flow rate but blowers do not improve much in this respect as input power increases. The reverse of this situation is evident in Figure 2 with respect to maximum pressure head. Blowers are typically high pressure air movers and increase rapidly in this feature as the size (input power) increases. It is interesting to note that higher power fans actually deliver less pressure head even though their free flow rate has greatly increased. One might surmise from these graphs that the best way to capitalize on the combinations of these properties is to use a fan for low density (more open area) exchangers, where high flow rates can be achieved. Conversely, one might also expect that a high density and interruptive fin-type exchanger may be better for a blower which can more easily overcome the higher pressure head generated. Indeed, this trend is typical, but it can also be surprising how similar the optimum exchanger can be for both kinds of air movers. The complete characteristics for the selected 10 Watt and 20 Watt fans and blowers are given in Figures 3 and 4. These curves are quite typical for an equal-sized fan versus blower and appear to suggest quite different optimum heat exchanger designs. FIGURE 3. Performance characteristics of a 10 watt fan versus a 10 watt blower. FIGURE 4. Performance characteristics of a 20 watt fan versus a 20 watt blower.

EXCHANGER CHARACTERISTICS

The detailed configurations and maximum enveloped dimensions for the selected exchangers are given in Figure 5. FIGURE 5. Exchanger configurations selected to study the optimization process for each selected air mover. These two example systems were arbitrarily chosen in order to illustrate the nature of the modeling calculations and the heat sink design optimization process. The 10 Watt heat sink system is typical for small, portable TE refrigerators using one or two 12-volt TE modules. The 20 Watt heat sink system is typical for many laboratory or instrument cooling units where two to four TE modules are used. The air flow enters the ends of each exchanger through the open end indicating by the vertical fins and travels parallel with the fins exiting the opposite end. The performance of these two exchangers, using fin geometry consistent with standard finned extrusions, is illustrated in Figures 6 and 7. FIGURE 6. Fluid dynamic performance characteristics for the selected exchangers. 173 mm 64 mm 127 mm 38 mmThe 20 Watt extrusion, having more open area, exhibits a lower pressure drop for any given flow rate (Figure 6) and a lower (better) heat sink resistance (HSR) for any given flow rate by virtue of its larger overall finned surface area. FIGURE 7. Thermal performance characteristics for the selected exchangers

The optimization of the air mover-plus-exchanger consists of changing the fin density (number of fins over the indicated area) and thickness of the fins. This is evident
from Figures 8 and 9 for the 10 Watt case. FIGURE 8. Effect of fin density on fluid dynamic characteristics for the 10 watt system. FIGURE 9. Effect of fin density on thermal performance of the 10 watt system. The fin thickness was held constatnt while the number of fins was parametrically increased from 14 (typical for a standard extrusion withdimensions as given in Figure 5 for the 10 Watt exchanger) to 24, 34 and, finally to 44 fins. As one would expect, the pressure head needed to establish a given flow rate increased, primarily due to less and less open area. The performance curves for the fan and blower are also plotted in Figure 8 together with the exchanger curves. The points of intersection represent the operating points of the respective fan-plusexchanger system. For example, if the 14-fin exchanger were to be combined with the fan, the air flow would be approximately 72 CFM. Similarly, if combined with
the blower, it would only be 25 CFM. Clearly, the resulting HSR is much better for the fan system as indicated in Figure
8. The operating point for the blower yielded an HSR of approximately 0.32oC/Watt while the fan produced a much better HSR value of approximately 0.15oC/Watt. The intercepts for each exchanger design indicates improvement in HSR as the number of fins increase but eventually passes through a minimum and rises as the optimum fin density is surpassed. The finer detail of this optimization process is illustrated in Figure 10. FIGURE 10. Heat sink optimization calculations parametrically with fin thickness for 10 watt system using a fan. These set of curves are for the 10 Wattdesign combined with a fan. The case illustrated in Figures 8 & 9 for the fan correspond to the 1.63 mm thickness fins. Here, it is observed that the best HSR is achieved when the number of fins is approximately 34 as verified in Figure 9. The next step in the optimization process was to change the fin thickness and repeat the entire set of calculations. Figure 10 represents six separate steps in fin thickness and the determination of minimum HSR for each fin thickness. Note that the optimum fin density shifts for each selected fin thickness. Also, there will exist a single case of fin thickness and fin density that represents an optimum of the optima. This is further illustrated in Figure 11.These curves were constructed by connecting the minimum HSR points for each parametric value of fin thickness. In fact, the case marked "10 WATT FAN" corresponds to the data given in Figure 10. FIGURE 11. Summary of heat sink optimization calculations for 10 watt and 20 watt systems. The other curves represent the results for the other three cases studied. These four "OPTIMUM" designs are depicted in the summary barchart given in Figure 12 together with the original cases representing standard-extrusion-type fin geometry. FIGURE 12. Comparison of the best heat sink resistance obtainable from all systems studied.

CONCLUSIONS

The results given in Figure 12 indicate the impact of exchanger optimization on the overall HSR of the system. It also suggests some interesting trends:

1. A blower in combination with a standard extrusion appears to be an especially poor combination. This is quite interesting in view of the fact that many commercial TE products now being marketed employ that combination.

2. The optimization process appears to be well worth the effort wherever overall performance is important. In fact, it has been shown by Ritzer, et al (2) that the optimization process can also yield the lowest cost design, holding HSR constant.

3. According to the results given in Figure 11, it is interesting to note that the geometry of the optimum exchanger design is not so different for a fan versus blower of the same power input. This quite unexpected since the performance curves are quite different as shown in Figures 3 and 4. This is especially surprising for the 20 Watt case where these curves are so dramatically different.

4. At least for the cases studied, the use of a fan appears to be the best choice when input power is driving the selection. Furthermore, the heat exchanger design is much more "forgiving" when a fan is used.

Finally, the impact of an improved HSR is illustrated in Figure 13. This curve was extracted from the analysis presented by Nagy, et all (3). One can observe that over 30oC can be lost from ideal maximum cooling performance using a simple standard extrusion with a 10 Watt blower. Optimization and the use of a 10 Watt fan can recover up to 20oC of that loss. This corresponds to a TE material improvement in Z of more than 0.9 x 10-3oK-1. From 1960 to 1994 the thermoelectric cooling industry gained less than onethird that amount! Certainly, the effort put into heat sink optimization is well worth while when viewed on these terms. FIGURE 13. Effect of heat sink resistance on the maximum cooling performance of an example system consisting of two TE modules on the selected 10 watt system.

REFERENCES

1. Lau, P.G., Ritzer, T. M, and Buist, R. J., "Optimization of Hot/Cold Sink Extenders in Thermoelectric Cooling Assemblies" in Proceeding of the XIII International Conference on Thermoelectrics, Kansas City, Missouri, USA, 1994.

2. Ritzer, T. M, Nagy, M. J. and Lau, P.G., "Economic Optimization of Heat Sink Design" in Proceeding of the XIII International Conference on Thermoelectrics, Kansas City, Missouri, USA, 1994.

Purpose:

Design/methodology/approach: Light microscopy (LM), electron microscopy techniques (SEM and TEM)  in combination with X-ray analysis (SEM/EDS), and X-ray diffraction (XRD) were used.

Findings: The examinations of the as-cast alloy after slow solidification at a cooling rate 2°C/min reveal that the Si,Si, Al9Mn3(FeMn)3FeSi, α-Al15microstructure consisted a wide range of intermetallics phases, namely: ß-Al5 Si.2Si, Mg3Fe12α-Al.

Research limitations/implications: To facilitate confirmation of the achieved results it is recommended to execute supplementary analysis of the aluminium alloys, 6082 series in particular Practical implications: Since the, what involves changes of alloy properties, From a practical position it is important to understand formation conditions of the intermetallics in order to control final components of the alloy microstructure. The importance of this is due to the fact that morphology, crystallography and chemical composition of the intermetallics strongly affect the properties of the alloy.

Originality/value: This work has provided essential data about almost all possible intermetallic phases precipitating in 6000 series aluminium alloys

Full document: Intermetallic phase particles in 6082 aluminium alloy

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Many manufacturers now chase up factories in  China and send them drawings and  manufacturing files to quote. It all appear fairly  simple: locate the business in China you can  trust, send files for a quote, request samples,  order product as lower cost and enjoy the  benefit.

Here, I like to look in to the details of what is actually happening on the Ground in China. I will focus mainly on four areas: Culture, Language, Economy and Government Regulations. All in the light of outsourcing challenges.

Culture. Disregarded by many it is most influential driving force in your relationship with China. It is not the way they speak, dress, have fun and eat. It is a way of thinking. Way they see the world, based on thousand years of cultural history. Do you know that there are 56 ethnic groups in China that are officially recognized? Each and every one have it’s own distinct cultural heritage. It’s own Goods, jokes, language or dialect. Now you will ask why it is important?  It is, as behavior of individual you will be working with will be guided by his/her moral standards and ethnic background. This will impact on your communication quality a great deal. For example, generally speaking if the problem arises, westerners educated to share the issue with clients in order to work together towards a better solution. Chinese will do opposite, they hide the problem. They will go thru pain and effort in order to resolve it the way they think it is the best, based on limited knowledge of your product/business. It is a frustrating process for them and painful for you, resulting in delays and quality variations.

Language. Here I manly like to focus on Chinese using English for communication with customers abroad. It is important to know that English in China is a ‘dead’ language. 99.9% of Chinese never been overseas and study English from Chinese. They think Chinese and they translate Chinese in to English when they speak. The result is poor communication, as two languages structurally and culturally very different. You are lucky if you have known the individual for some time, then you can understand the fine meaning based on past experience. You can also tell if person on the other side is happy or frustrated with subject at hand. If your sentence in the email you have sent is longer than 5-6 words and more complex than simple command presented in “buletpoint” form, do you really think they get the sound meaning of what you are trying to tell?

Economy.  Well, think of China as west during booming times. Workers are lured by employees offering higher wages next door, factories been build everywhere, new generation of young people buying Iphones  and fashion clothing all on credit cards. All this adds to the turbulent mix of the fast economic growth. Your contact person in the company can change every 6 months, as a matter of fact 50% of factory staff members can change over 12 months as less than half of workers will come back to same factories after spending Festive season in hometowns. Question remains open, if you spend 3 months explaining a person specifics of your project and you get a new account manager in 6 months, where does it leave you?

Government regulations. Here I like to make a special note that it requires a license for a company in China to export. License allows company to sell only narrow range of goods/services. Only a minority of factories will go in to the trouble of getting it, limiting them to local market only. Trading companies on the other hand, have licenses allowing them to trade wide range of goods/services and they specialize in customs, freight and banking with overseas. You would want to work direct with factories for pricing, but with trading company for ability to communicate, solve problems and been able to deliver goods to you.

Summary. There are few ways you can effectively work with China. Unless you are big entity that can do an acquisition of the Chinese business and relocate part of its western management team to full time oversee the operation in China, your options are as follows:

-        Locate facility that deliver desired service and can export. Get to know them well, travel monthly to see them, make sure they are financially stable and reliable source. Become part of their business/team, stay close and learn the culture and basics of the language.

-        Do a Joint Venture. Your professionalism in doing business in China well above the basics covered here. You will not be reading it.

-        Contact some type of sourcing company. Find a reliable company, that is capable of coming over to meet you, speak fluent English and happy to meet you in China. Once your business volumes become high, you can look at other alternatives.

Email hacking. This is when a person or a group of people hack emails server of a company and watch it for some time. They select usually largest customer and redirect all the communication, presenting them self’s as original company. Aim of the game is to present invoice for prepayment of goods with new bank account details in order to steal the funds.

Fake Factory. Usually a single person show, when a copy of some Chinese company website is created and unsuspected customers are lured in to transferring money for goods and/or services. When large amount is transferred or some money been accumulated from smaller transactions website is liquidated.

Employee resignation. When a sales person leaves the company he carries his email address with him supporting the ongoing correspondence. He locates a factory which can supply simular product and takes the order presenting ‘updated’ invoice with bank details for payment. Occasionally this works out OK and sales person starts his own ‘export’ company. But in majority of cases this is a disaster due to quality, specifications and logistics issues.

Sourcing Company. A person registers himself on the number of major supplier’s searching websites such as Alibaba. Present himself as a supplier of particular service or product. Upon contact he will try to deliver the service he is not an expert at nor have access to supporting businesses such as freight, licensing for export, banking and technical knowledge of the subject at hand. This is one single most common and dangerous type of fraud. It will consume a lot of time, money and give China overall bad reputation.

So, you will ask, what can we do about China Sourcing Scams?

First and most effective: Deal with the company that have local representative, known in professional circles such as engineering association, LinkedIn (you can check profile and ask feedback from connections) and provides professional service from the start. If you are to spend thousands of dollars on the project and serious about ongoing future business, why not travel to meet the company? In one day, you will get fairly good idea who you are dealing with. Capability, personality, assets and credibility all can be assessed the same way you do it at home during your day to day bossiness. It’s not different in China.

Second, find someone to inspect factory or company: Pay a well known company few hundred dollars to go out there and do the inspection on your behalf. Provide them checklist or rely on the summary they will generate. The local guys can also check company registrations with Chinese Government Authority, bank’s manager feedback and supplier’s feedback.

What to pay attention to:

Do they have representative that can come and see you? If so, this is a good start.

Do they suddenly change bank account? If so, do whatever necessary to confirm you still deal with original supplier.

Do you know owner in person? If so, stay in touch. It will be useful to get quick and direct answer when needed.

Delivered to you by P&A International.

-Heavy piston

-Large piston volute (takes time to evacuate)

-Small vacuum line (Forge offers cap with larger fitting)

There are number of things you can do about it. I will start with heavy piston. As supplied by Forge, it’s weight is 118g or 4.2oz. Compare this to say Bosch valve fitted to AUDI/VW, membrane weight is barely 1/100th of that, making it much faster in operation.

By doing some stress analysis, I came up with reduced wall thicknesses that provide balance between mechanical stifles of the piston and its weight. I have CNC machined the sample for trials to evaluate its operation and wear/tear. Reduced weigh will reduce movement delay (AKA reaction delay time) and cycle time during piston oscillation. But may increase piston wear due to more dynamic piston operation (more movements as it reacts to smaller signal inputs) as well as reduce the piston seat wear at the same time, due to piston reduced weight.

Above image is part delivered from machine shop versus origional. The piston with thick wall section is as supplied by Forge, the one on the right is the one we machined out to achieve weight reduction of over 60%. 4.2oz for Forge part and 1.5oz for sample machined by CNC. That is 118gram vs 43gram for those who read metric.

This was tested in the application of a modified turbo engine running above 450HP. Immediate feedback is that it is more sensitive with accelerator input while on boost. You are more in control and it is easier to alternate boost level. The power fluctuation during sudden throttle drop by some 20-50% during high RPM is somewhat reduced. When combined with weaker spring supplied by Forge, it’s a very nice linear feel of accelerator VS power, much like on OEM turbo motors made by VW/AUDI.

Externally almost identical, newly machined piston is 60% lighter!

Next step is to reduce amount of air that that needs to be evacuated from the volute under the piston. Smaller the volume of air to be removed from the under the piston, the quicker the pressure in intake manifold and valve will balance. Thus reducing delay between vacuum signal and the piston movement. For a trial purpose we have machined a blanking plate and glued it with the Loctite. This modification reduced the working volume of air under the piston by 50%.

Next step was to updrade the vacuum feed line. A new cap was machined with enlarged vacuum feed nipple and more agressive hand grip for ease of use. Worth to note, that you can actually buy replacements caps with larger feed nipple from Forge direct as spare part.

The result of all the above changes is the improvement of the pedal linearity to the power output of the engine. The modification has dramatically improved power surge issue that was evident during throttle drop from 100% to 50% during boost operation near engine maximum RPM. There is no more delay in the pressure drop after throttle angle been reduced, it's instant like with good old BOSCH membrane valve. But with an added benefit of high flow FORGE Blow Off Valve.

Advantages of using copper over aluminium for heat sinks are often discussed. If you look at high end CPU coolers for personal computers market for example, you will see extended use of copper. Coper thermal conductance is 4.01 W/cmK and aluminium is 2.37 W/cmK. With copper thermal conductivity so much higher that aluminium one would expect significant performance difference in real world. Let’s run some thermal tests.

We have set up a test rig consisting of 200mm X 200mm X 2mm copper and aluminium plates. A heat source was chosen as power mosfet and was mounted in the middle of the plate. A continuos thermal load of 30 Watts was dissipated via heat sink. Temperature was measured at heat source (mosfet mounting pad). Ambient air temperature was measured at 24 Celsius and air was stationary.

During first 10 minutes there was no significant temperature difference between copper and aluminium heat sinks. We recorded 1 degree C and temperatures have reached 60C and 61C with cooper heatsink running cooler. Such a small difference can be accounted for the higher thermal inertia of the copper heat sink, as the mass is higher it takes longer to reach same temperature at fixed thermal input.

On 15^th minute, mosfet mounted on to the copper heatsink reached 65C while mounted on aluminium heatsink was running at 66C. At about this point advantage of copper heat sinking becomes evident.  By transmitting heat more efficiently, it allows components to operate cooler.

After 120 minutes, the 1 degree C thermal advantage remained valid. Temperatures reached 84C for coper and 85C for aluminium heatsink. There was no further improvement with coper versus aluminium, both plates have saturated at about 150 minutes maintaining same temperature difference.

Is the coper heatsink better than aluminium? Sure, thermal component will run cooler mounted on the coper. Is it wort additional cost as coper is so much more expensive and punishment of extra weigh? Maybe, depends on application. If you already exhausted all the possibilities of using cheap and light aluminium heatsink and chasing small improvement for the critical thermal system, then definitely coper is a great candidate.

Now, at the same time we had a third test rig with the extruded aluminium heat sink with finns and same mosfet dissipating 30 Watts of heat on it. It was cooled via conventional air flow. Heatsink was 1/6 the size of the plate, but had 50 mm fins on the back. It is old Pentium III cooler. Made out of extruded aluminium, about the same weight as our 200mm aluminium plate it performed great. After 120 min run, it reached only 66 degrees Celsius.  Compare this to the 85C of the copper plate.

So by using same amount of material, copper or aluminium, you can have a dramatic improvement in heat dissipation just by altering the design of the heatsink. This is the most important message from the test result. Only when heat sink design has exhausted itself for particular application, then material choices worth considering. Or fan forced cooling, as been activated on 121 minute and resulted in finned aluminium heat sink temperature drop of 17C.

As tested by engineers at P&A International

But for companies that still don't have a presence in China, there are a number of issues that must be considered in order to establish and operate manufacturing plant in that country. As a China based observer I like to share my observations on the cost of labor, form of ownership, plant location, building a plant, staffing a plant, patents and copyrights, and getting paid.

Cost of labor of most manufacturing workers in China are less than a 1/4 of what their counterparts in the Australia earn. But that advantage is quickly diminishing as wage rises regularly and businesses loaded up to provide benefits to the workers.

One factor is that China is in the focus of implementing a social safety system that will combine the functions performed by Social Security, Medicare. The Chinese Social Security would be financed by contributions from employers and employees alike.

The other factor is that production workers wages are rising faster in China than they are in the rest of the world. As expected, the most rapid growth in labor costs is in metropolitan areas.

Foreign company can own 100 percent of a venture it establishes in China. However majority of the businesses making their presence in China prefer to do so via a joint venture.  Joint ventures are attractive because the government gives them tax incentives. They require a minimum of investment (the foreign company need own only 25 percent of the business), and the Chinese partner often can help negotiate the maze of regulations the government imposes on business. It’s worth knowing that tax advantages might be revoked and careful selection of the joint venture partner are required like in any other country.

Checking out about the track record of Chinese companies or the background of their executives can be a tiresome task. There's no law requiring Chinese companies to report their financial results, and no incentive for them to do so because only a very small percentage of them have access to capital in the public markets as majority go to the bank. That's where Chinese companies usually go when they need capital. And the banks closely monitor the performance of the companies they lend to, and they are usually well aware of the background of these companies' managers.

Because labor costs are lower in the inner provinces of China, companies may be tempted to build plants in one of these remote areas.  There are at least two negative factors. It’s hard to lockate competent management in the inner provinces, and it's difficult and expensive to recruit the necessary expertise from the metropolitan areas in the coastal provinces. The other downside is that they don't provide the same degree of legal protection and government cooperation that is enjoyed by companies in the metropolitan areas of the coastal provinces. The provinces that are most hospitable are those along the coast: Shangdong, Jiangsu, Guangdon and Zhejiang.

Remember that foreign companies can't buy land anywhere in China; a plant site must be leased, for terms ranging from fifty to seventy years. Companies that want to manufacture in foreign countries usually prefer to have their plants build by the contractors they're accustomed to working with. But it may not be economical to bring a major western construction firm into China to build anything less than a large factory. That leaves a choice of contractors domiciled in mainland China and those based in Taiwan and Hong Kong. The better choice will usually be a contractor from Taiwan or Hong Kong that is licensed to build on the mainland. Some of these companies have good quality control, better technology and can get projects done on time, on budget. Here again, it will pay to do extensive due diligence to find a good contractor.

There are two positives in working with the Chinese. Most employees are willing to put in long hours, and there is only minor union interference.

On the negative side, Chinese workers in general have a lack of initiative. Typically, a foreign company finds it must put Chinese workers through a long training process before they are able to do their jobs and learn to come up with solutions to problems on their own.

Company can make all the right moves in building a plant in China, manufacturing its products and getting them into efficient distribution channels, but can they get paid for products sold and delivered?

It would be nice if China had a credit ratings agency, but no such entity exists. What, then, can a company do to avoid this problem?

First off companies must accept as a fact of life that the percentage of their receivables that are not collectible in China will almost always be higher.

The way to hold uncollectible receivables to a minimum: Check with the banks that are familiar with prospective customers. As noted earlier in the discussion on joint ventures, the banks know the financial condition and the creditworthiness of Chinese better than any other organization in the country. There will always be one or more banks that know any potential customer of a foreign company.

Company should also differentiate between different types of customers, and be leery of selling to state-owned companies. It's not that these companies deliberately cheat their suppliers; rather, they simply may not be able to pay their bills.

Extreme caution is warranted if a prospective customer is a state-owned manufacturer, because many such companies have obsolete technology and therefore have trouble selling their products.

China subscribes to international treaties and conventions on patents, copyrights and other intellectual property, and the government is sincere in its vow to catch and prosecute infringers. Still, the country is so huge and the profits to made from infringements so great that some infringement does go on.

This does not mean that companies should not manufacture any of its proprietary products in China. But they would be well advised to confine the manufacturing of components containing their core technologies in the country of origin, and have the less proprietary parts of their products made in China.