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Electroforming
ELECTROFORMING by Tony Hart and Alec Watson The Nickel Development Institute, Toronto Electroforming can be defined as a process that employs technology similar to that used for electroplating but which is used for manufacturing metallic articles, rather than as a means of producing surface coatings. The principles of the electroforming process are demonstrated in Fig. l. This shows a suitable metal, such as nickel, being electrodeposited onto the surface of a "conical mandrel (the term mandrel is used to describe the former onto which an electroform is grown). When the deposit has attained a sufficient thickness it is then separated from the mandrel to become a metallic product with a totally independent existence. The importance of electroforming in the modern world and the degree to which it impinges on everyday life often is not appreciated, even by professional engineers and technologists. This contribution, therefore, sets out to describe the nature of the electroforming process and its associated industry and to provide basic information regarding production techniques, together with an outline of important commercial applications.
THE SIZE OF THE ELECTROFORMING INDUSTRY Electroforming is not a new technology; it has existed as long as electroplating, the first applications being recorded around 1840. With hindsight, however, it does seem to be a
Fig. 1. Schematic diagram showing production of simple nickel electrofonn.
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process that was well ahead of its time. Little use was made of it during the 19th century, and only limited applications appeared in the first half of the 20th. The process really has come in to its own during the last 40-50 years. This has been due to the inherent advantages of etectroforming that enabled engineers, designers, and technologists to solve the problems of manufacturing specific products required by a very wide range of applications and industries, including many high-technology ones. One very important aspect of the electroforming industry that is not widely appreciated is its size. The worldwide turnover during the mid-199Os has been estimated at $2 billion per year. This figure was derived from calculations based on the quantities of the two major metals used by this industry--80,000 metric tons/year of copper and 5,000 metric tons/year of nickel. The value of the primary metal used in each application was multiplied by a factor--supplied by companies in the particular market sector--relating selling price of the electroformed article to basic metal cost. Using those factors, together with normal geographical considerations, the worldwide size of the electroforming industry was calculated, yielding the figures just discussed. In addition to this very significant size the electroforming industry has also shown a consistent pattern of growth over a long period of time. This has been due to requirements from new technologies increasing at a greater,rate than the decline of outdated applications. Frequently, this has resulted in steady growth of the industry even in times of general recession. •W H Y AND WHERE E L E C T R O F O R M I N G IS USED
Electroforming cannot always be regarded as the most economical method of manufacturing metallic products. If it is possible to use conventional metal working techniques, such as pressing or forging, these generally will be employed on cost grounds. There is, however, a very wide range of situations in which electroforming is the preferred method of manufacture and many other instances where it is the only one that can be used to produce a particular product. Applications where it is employed successfully always depend on the exploitation of one or more of the advantages of the electroforming process, the principal ones being as follows.
Accuracy of Reproduction Electroforming is by far the most accurate method of manufacturing metallic articles. Both the dimensional tolerances that can be achieved and the ability of the process to reproduce surface detail with absolute fidelity are unmatched by any other metal-fabrication processes. An excellent example demonstrating this critical advantage of the technology is a very familiar one in the modem world--the manufacture of molds for compact disk (CD) production. Information is stored on a CD in the form of a continuous helical track consisting of shallow depressions. These are typically 0.1-micrometer deep and 0.2-micrometer wide, and they vary in length from 0.2 to 2.0 micrometers, with an intertrack spacing of 1.6 micrometers. There are frequently 3 × 10 m such depressions on a CD, all of which must be of the correct size and shape and in the correct position to achieve the required performance. Electroforming is the only technique that is sufficiently accurate to be used to produce molds for CD manufacture. This key advantage of electroforming is why the process is used to produce many other types of molds, dies, and other products requiring very high-dimensional accuracy such as wave guides. It also enables electroforming to be used to reproduce natural finishes, such as wood grain and leather, with total realism. Manufacture of Thin-Section Products It is, perhaps, logical that electroforming, which produces a metallic article by building up the structure of the material atom layer by atom layer, should be an excellent method for
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manufacturing thin-walled products. In conventional metal working processes the metal is normally cast as a massive billet or ingot, which needs to be progressively reduced in size to yield a thin-section final product. This involves multiple working processes and can, therefore, be very costly as well.as inefficient in terms of energy consumption. The production of foil and mesh products is by far the biggest industrial use of electroforming. With copper the most important product is plain unperforated foil whereas for nickel perforated mesh products, in particular screen-printing cylinders, predominate. Further details of a number of commercially important thin-walled products are given later in the section that describes applications.
Manufacture of Complex Shapes An increasingly wide range of metallic products required by modem technology is of an extremely complex shape. Many of these are manufactured by electroforming because it is by far the most economical method of production. In a large number of cases, the technique is the only method available for producing the shape required. A good example of this is the waveguides used extensively in radar and microwave technology. There are many other instances of aerospace and defense uses, some of which are discussed in the applications section.
Manufacture of Large Products or Molds For the manufacture of very large products and molds, particularly ones with thin walls, electroforming can be the most economical method of production. It avoids the very high cost and difficulty of fabricating and machining large objects and, at the same time, minimizes raw material usage.
Manufacture of Very Small Products or Molds In recent years, a radical new development of electroforming technology called the LIGA process has appeared. This enables extremely small components, sometimes with dimensions of only a fraction of millimeter, to be made with very high precision. This process is becoming established in a number of industries.
HOW ELECTROFORMING IS DONE Differences Between Electroforming and Electroplating Although the basic technology of electroforming is very closely related to that used for electroplating, there are a number of important differences.
Deposit Thickness Electroplated deposits used as coatings--in order to confer corrosion resistance, wear resistance, or simply aesthetic appeal~are generally 1- to 30-micrometers thick. Electroformed products, however, need sufficient mechanical strength to be able to exist independently of the mandrel onto which they are deposited. They are, therefore, normally much thicker, ranging from foils of 10-micron thickness to mold faces that can be up to 5-millimeters thick. Electroformed backings for molds have been produced as thick as 50 millimeters. Snbstrate Adhesion If an electroplated coating is to fulfill its function it is essential that the adhesion between the deposit and the substrate be good. Electroforming, however, requires exactly the opposite.
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Table I. Metals and Alloys Used in Electroforming Metals
Nickel Copper Iron Silver Gold Aluminum
Alloys
Nickel*cobalt
Gold alloys
It is vital that there is little or no adhesion between the deposit and the mandrel so that the product can be separated without damage when the forming process is complete. Whereas in electroplating the pretreatment processes are selected to maximize deposit adhesion, electroforming pretreatments are specifically chosen to reduce the adhesion between the deposit and the mandrel to the minimum required to prevent premature separation.
Internal Stress in the Electrodeposit Because electroforming requires that the adhesion between mandrel and electrodeposit should be minimal, it is vital that the internal stress in the deposit is controlled. Normally, this means maintaining it as low as possible. Stresses are usually present in electrodeposited metals and may be either tensile or compressive in nature. Both tensile and compressive internal stresses, if sufficiently high, can produce distortion of the deposit if it is not restrained by the substrate. In electroforming, where there is no such restraint, it is, therefore, essential to select deposition processes that give low internal stresses in order to avoid unacceptable dimensional changes or gross distortion of the product. It is for this reason that sulfamate-based solutions, rather than those employing the cheaper sulfate salt, often are preferred for nickel electroforming.
Metals and Associated Processes Used for Electroforming The metals and alloys that can be used in electroforming are shown in Table I. The choice of materials is limited not only by the normal constraints that apply to electrodeposition in general but also by the additional requirement for low internal deposit stress. Thus, a metal such as chromium, which would have many desirable properties as an electroform, particularly in relation to its hardness, cannot be used because of the extremely high internal tensile stresses generated in the electrodeposited metal.
Nickel Nickel can be considered the workhorse of the electroforming industry. Although a larger tonnage of copper than nickel is deposited, it is only used for a very limited range of products. It is probable that 95% or more of the products manufactured by electroforming are produced in nickel This is due to a favorable combination of properties, both of the electrodeposit and of the deposition processes. Electroformed nickel shows excellent strength, toughness, and ductility and is extremely resistant to corrosion in a wide range of environments. In addition it can readily be deposited in a low-stress condition, and its hardness and strength can be varied over a wide range by selecting appropriate deposition conditions. There are a number of deposition processes available that are suitable for electroforming of nickel They are generally highly reliable and relatively easy to control, being both chemically and electrochemically stable and environmentally friendly. The operating parameters for these processes are also well documented, which allows a substantial amount of control to be exercised over the properties of the deposits obtained.
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Table II. The Conventional Nickel Sulfamate Solution Ni (NH2SO3)~-4H~O NiCI2"7H20 H3BO3
350-450 g/L 5-30 g/L 35-40 g/L
There is a strong preference for nickel sulfamate-based solutions in the electroforming industry. The deposits that they produce not only have a low internal stress, which is essential for this application, but the electrolytes are stable, reliable, and simple to operate. Conventional nickel sulfamate solution has an uncomplicated makeup. A typical composition range is given in Table II. In most published formulations chloride is included. This has been conventional wisdom for many years and is based on the fact that this anion is required for good dissolution of nonactivated nickel anodes; however, if one of the sulfur-activated grades of nickel anode materials, which are readily available commercially, is used, then a chloride-free solution will operate without problem at normal anode current densities. This is a considerable advantage because the chloride ion increases tensile stress in the deposit so that operating a chloride-free solution approaches closer to the ideal stress-free condition. Nickel sulfamate solutions can be used for long periods of time with minimal chemical analysis. There is a slow, steady rise in the solution pH due to a small imbalance between the cathodic and anodic current efficiencies, but this can easily be controlled by sulfamic acid additions. The same imbalance, however, also results in a gradual rise in nickel salt concentration in situations where drag-out is low, which is frequently the case in electroforming. This may require periodic dilution of the solution to maintain the optimum concentration. Nickel sulfamate-based solutions can also be used with a number of organic addition agents. These provide a much wider range of deposit properties than can be achieved with - standard processes without additives. For example additive-free solutions will give deposits with hardnesses around 200 HV. This can be increased to 500-600 HV by the use of suitable additives. Nickel sulfate-based solutions are also used for electroforming but always require the presence of an addition agent, such as the sodium salt of saccharin, to reduce the internal tensile stress to a tolerable level. These solfitions are generally based on the conventional Watts-type composition given in Table III. These basis solutions are also extremely stable, requiring only additions of acid to counteract the natural pH rise; however, the organic additives generally require frequent and more sophisticated analysis in order to maintain the required properties in the deposit. They are, thus, less suitable than the conventional sulfamate process for use in situations where regular chemical analysis is not available. Nickel fluoborate solutions are also mentioned in the literature, but there are few commercial applications. One further advantage related to both sulfamate- and sulfate-based nickel electroforming processes is that there is a considerable body of information available in the literature regarding the use of additives, both organic and inorganic, to these solutions.
Table III. Watts-Type Nickel Solution NiSO4.6H20 NiC12-7HzO H3BO3
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250-400 g/L 25-60 g/L 30-40 g/L
Table IV. Acid Copper Solution
CuSO4"5H20 H2SO4
70~50 g/L 50-200 g/L
Copper In the region of 80,000 metric tons of copper is used for electroforming each year. The vast majority of this is, however, for one single application--the production of thin foil used for the construction of printed circuit boards. Other uses of copper are mostly limited to those where electrical conductivity is also the property of prime ilnportance, for example waveguides and spark erosion electrodes. There are also a few miscellaneous uses, such a s backing of nickel mold faces, production of loudspeaker cones, and parts for musical instruments. Copper, although cheaper than nickel, is not used for the majority of electroforming applications because its mechanical properties are not as good as those of nickel, and it has less resistance to corrosion in many environments. In addition, the processes for depositing copper are, overall, not so attractive for general purpose electroforming. The solution most commonly used for copper electroforming is the simple acid copper sulfate. This can operate over a wide composition range, as shown in Table IV. This solution is stable and simple to operate and produces deposits with very low stress levels; however, owing to its very high sulfuric acid concentration, this formulation is very corrosive both to equipment and its surrounding environment. Acid copper sulfate solutions are used for the production of electroformed copper foil using an inert anode system and chemical dissolution of scrap copper to maintain the solution balance. This enables the gap between cathode and anode to be small and accurately controlled, thus minimizing power consumption. Acid copper solutions are also used with organic additives developed for decorative bright acid copper plating. In contrast to additives for nickel processes, however, there is relatively little information available on the effects that these materials have on deposit stress or hardness. Copper cyanide solutions can also be used for electroforming, but the internal deposit stress is considerably greater than that produced by acid sulfate solutions. This, combined with the toxicity and effluent problems associated with cyanide solutions plus the relatively complex control required of the solution chemistry, has limited its applications. Cyanide processes have, however, been used with periodic current reversal. This produces very uniform metal distribution and is used for thick deposits for backing electroformed nickel mold faces. The use of copper fluoborate solutions for electroforming appears to be very limited, although they are mentioned in the literature. The deposit properties, however, appear to offer few advantages over those from acid copper formulations. The use of copper pyrophosphate solutions seems to be confined to electroforming of spark erosion electrodes. Iron Electrotbrming of iron is technically possible, but despite the low cost of the metal, it has never found any successful commercial application. This is due to its very high susceptibility to corrosion and also to the difficulty in controlling iron deposition processes as a result of the inherent instability of the ferrous ion. A process was developed some years ago in the United Kingdom for the continuous deposition of iron foil but has never achieved commercial acceptability.
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Silver Silver is employed to a limited extent in applications in which the cost can be justified. These can be functional uses, generally for electronics, in which the high electrical conductivity of silver is important, or for purely decorative ones, such as jewelry. Only cyanide-based processes have been used successfully for silver electroforming. These produce deposits with good physical properties together with acceptable levels of internal stress. Bright silver can be deposited from proprietary solutions and is used in the jewelry and silverware industry. Gold The major use of gold electroforming is for jewelry and other decorative products; however, there are also a number of functional applications of gold electroforms, for example, prostheses for medical purposes and crowns and bridges in dentistry. Most of the gold electroforming solutions in commercial use are cyanide-based; many proprietary formulations are available. Aluminum Aluminum cannot be electrodeposited from aqueous solutions, although nonaqueous processes do exist, but these require the complete exclusion of both oxygen and water from the system. A process has been developed in Germany for electroforming aluminum, but the equipment is very expensive. There have been few successful applications, use being confined to the aerospace industry where the weight saving can justify the expense. Electroformed Alloys There is no commercially viable system for copper alloy electroforming, and the only one used commercially for nickel alloys is that for the deposition of binary alloys with cobalt. These can be produced from a sulfamate-based solution, which gives deposits with a hardness in the region of 300-350 HV. This is much harder than pure nickel (200-220 HV) but not as hard as organically hardened nickel deposits (500-600 HV). Nickel hardened with cobalt has the advantage that it is sulfur free. The organic compounds used as hardening agents for nickel all contain sulfur, and invariably small amounts of this element are codeposited. If the metal is then subsequently heated above approximately 200°C, the sulfur forms nickel sulfide at the grain boundaries of the deposit, giving rise to serious embrittlement. Cobalt-hardened deposits avoid this disadvantage. The only other alloys used for etectroforming are based on gold. These are for jewelry applications where it is essential to control the alloy composition closely so that the products will be acceptable for hallmarking. To maintain tight composition, a special computerized system is necessary, which can be quite expensive to install.
Mandrel Technology For the successful production of any electroformed product, the design and construction of the mandrel are of utmost importance. An exhaustive .coverage of this topic is beyond the scope of this review; so this section must necessarily be confined to a description of the principles. There are basically two type of mandrels: those that can be removed intact from the electroform, generally referred to as rigid or permanent mandrels, and those that when removed from the electroform are in some way distorted or destroyed, called temporary mandrels. If an electroform is to have reentrant shapes or angles in its geometry, a temporary mandrel must be used. If not, a rigid mandrel can be used. Rigid Mandrels A number of materials, both metallic and nonmetallic, are used as rigid mandrels. The material chosen must be capable of being formed into the correct shape and dimensions and,
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in many cases, needs to be resistant to damage and corrosion. The material must also not contaminate the process solution in which the electroform is deposited. Suitable metallic materials include stainless steel (preferably an austenitic grade), copper, brass, mild steel, aluminum and its alloys, and electroformed or electrodeposited nickel. Stainless steel can be used directly as a mandrel material following cleaning and suitable surface treatment. Copper and brass, after suitable surface preparation treatments, can also be used directly; however, they may be electroplated with nickel or nickel plus chromium to give a more corrosion-resistant surface. Mild steel mandrels are almost always treated in this way. Electroforming of nickel onto nickel mandrels is a widely used technique, enabling sequential replication to be carried out. It is the basis of production for audio records, CDs, and holograms. Nonmetallic materials used as rigid mandrels include thermoplastic resins~ thermosetting plastic resins, waxes, and photosensitive resins. Photosensitive resins are commercially important mandrel materials that are used to reproduce the highly accurate surface detail necessary in production of CDs and holograms.
Nonrigid Mandrels There are various types of nonrigid mandrel employing different principles to permit subsequent removal from complex-shaped electroforms with reentrant angles. They include the following: Fusible materials: Both low melting point alloys, Usually bismuth-based, and waxes can be used for mandrels, which are removed by heating above the melting point of the material. Soluble materials: Aluminum, and to a lesser extent zinc, alloys can readily be dissolved in sodium hydroxide solution. This enables them to be removed from within nickel electroforms because nickel is highly resistant to strong alkaline solutions. Flexible materials: Readily deformed materials, such as plasticized polyvinylchloride, are used for mandrels that are mechanically collapsed into the eleCtroform to facilitate their removal.
Mandrel Preparation Techniques Both electrically conductive and insulating materials are used to produce mandrels. With the conductive ones it is necessary that, following thorough cleaning, the surface be treated so as to ensure that the electroform does not adhere, thus allowing separation. The treatment used depends on the mandrel material. One of the simplest is the use of sodium dichromate solution to give a passive parting film on the surface of stainless steel or nickel. With insulating mandrel materials, it is necessary to provide an electrically conductive layer on the mandrel surface onto which the electrodeposition process can initiate. This can be done in a number of ways, but two of the most popular are the use of a chemically reduced silver film or silver paint.
APPLICATIONS OF ELECTROFORMING Although electroforming is widely used in many industries, its importance and range of application is rarely appreciated, even by qualified engineers, scientists, and technologists. Similarly the Ordinary "person in the street" has no idea of the extent to which they interface with electroformed goods in everyday life. When the average person wakes up in the morning, he or she may step out of bed onto a carpet printed using electroformed screen-printing cylinders, look at wallpaper, and put on clothes made from fabric similarly patterned. That first cup of coffee may be filtered through an electroformed mesh, the first morning cigarette will have been manufactured using a number of electroformed products, and many electric razors use electroformed foils. Any electronic device--radio, television, tape deck--switched on to aid the waking-up process will depend on electroformed copper foil used in the circuitry. In addition, any record or CD 395
Fig. 2. Electroformed nickel stamper and platinum-coated polycarbonate user-write optical read-out disk. (Photo courtesy of Plasmon Data Systems, Royston, England.) will have been made with an electroformed master. A wallet or purse picked up before leaving home will probably contain bank notes printed from electroformed plates, as well as credit cards with security holograms made using-electroformed masters. On entering most modem cars, the occupant may well be looking at a fascia panel molded into an electroformed mold. These examples make it obvious how relevant this technology is to modern living. The whole range of uses of electroforming is far too large to cover in .this summary of the process, so just a few of the more important and interesting examples have been selected.
Audio and Visual Disks The audio record and, more recently, the CD are two of the most commonly recognized products where electroforming is a vital part of the manufacturing process. This technique has been used to produce sound-recording devices since the beginning of the present century--first; shellac disks; followed by polyvinylchloride-molded, "long-playing" records; and most recently CDs. It has been necessary to refine the precision of the electroforming process in order to cope with the extreme accuracy of surface detail necessary for CD manufacture. As noted earlier, a C D bears a helical track with 30 billion depressions only 0.1 micrometer deep, 0 . 4 0 . 6 micrometer wide, and from 0.5 to 2.0 micrometers long. For good reproduction quality, these all need to be of the correct size and in precisely the right place on the track. The latest developments have increased the disk capacity eightfold to enable full-length color video recordings to be produced. Similar technology is used to produce computer "user-write" optical read-out disks of the type shown in Fig. 2. Electroformed Foil Products As noted earlier, copper foil production represents by far the biggest application of electroforming, consuming about 80,000 metric tons of metal per year. It used for the manufacture of printed circuit boards and has, therefore, formed the backbone of the whole electronics industry for over 30 years.
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Fig. 3. Printing with electroformed nickel screen-printing cylinders. (Photo courtesy of Nickehnesh AG, Rudolfstetten, Switzerland.) Nickel foil is also electroformed and used in resistance elements, printed circuit boards with welded connections, asbestos-free gaskets, and fire-resistant membranes. A specially blackened nickel foil is used to-make solar energy absorbers. Both nickel and copper foils are manufactured on cylindrical mandrels continuously rotating within a conforming anode arrangement. Electroforming represents the only economic method of producing thin, wide foils to the dimensional precision required for these applications.
Mesh Products Electroformed mesh products--that is, foils with regular patterns of perforations--comprise the largest single use of nickel in electroforming. There is a range of applications such as coffee and sugar filters, electric razor foils, tobacco tapes, and powder sieves. By far the single most commercially important use is the production of screen-printing cylinders. These consist of thin-walled (typically 100 micrometer) electroformed tubes, which are up to 250 m m in diameter and 8 meters in length. The pattern of holes formed in the surface dictates the design, which can be printed by ink forced from the inside to the outside of the rolls using doctor blades. Fig. 3 shows electroformed cylinders on a printing machine where they are used sequentially to apply the various component colors of the full design. Electroformed screen-printing cylinders are used extensively all over the world to produce wall papers, fabrics, and carpets.
Large Electroforms Very large products can be made by electroforming. Molds used for the manufacture of plastics components formed by low-pressure processes, for example, glass-fiber-reinforced
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Fig. 4. Electroformed mold for wing fairings of the Airbus 320. (Photo courtesy of Galvanoform GmbH, Lahr, Germany.)
thermoset parts, are a good example. These are employed widely in the aerospace industry. A typical example is the mold for a large wing fairing being shown in Fig. 4. (The building
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Fig. 5. Gear wheel produced by L1GA process. (Photo courtesy of CLRC Laborato13', Daresbut3,, England.)
in the background demonstrates the scale of the component.) Large electroformed molds are also used in the automotive industry for the production of one-piece moldings for the drivers' cabs of trucks.
Small Electroforms In recent years, a new branch of electroforming has enabled extremely small components to be produced by electroforming. This technology, developed originally in Germany, is known as the LIGA process, its name being derived from the German acronym, Llthographie, Galvanoformung, und Abformung. The tiny products that can be made by this process are demonstrated by Fig. 5, which shows a perfectly formed, 12-tooth gear wheel small enough to be contained within the eye of an ordinary needle. The LIGA process is being developed as part of much larger programs of microengineering. Almost all major industries have potential uses for these miniature or microcomponents, with automotive, aerospace, and medical applications already in place. In the medical industry, for example, electroformed components are used to manufacture very small electric motors, which can be passed along arteries to scour the walls, removing deposits that cause blocking of the artery and restrict blood flow.
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THE SIZE OF THE ELECTROFORMING INDUSTRY Electroforming is not a new technology; it has existed as long as electroplating, the first applications being recorded around 1840. With hindsight, however, it does seem to be a
Fig. 1. Schematic diagram showing production of simple nickel electrofonn.
388
process that was well ahead of its time. Little use was made of it during the 19th century, and only limited applications appeared in the first half of the 20th. The process really has come in to its own during the last 40-50 years. This has been due to the inherent advantages of etectroforming that enabled engineers, designers, and technologists to solve the problems of manufacturing specific products required by a very wide range of applications and industries, including many high-technology ones. One very important aspect of the electroforming industry that is not widely appreciated is its size. The worldwide turnover during the mid-199Os has been estimated at $2 billion per year. This figure was derived from calculations based on the quantities of the two major metals used by this industry--80,000 metric tons/year of copper and 5,000 metric tons/year of nickel. The value of the primary metal used in each application was multiplied by a factor--supplied by companies in the particular market sector--relating selling price of the electroformed article to basic metal cost. Using those factors, together with normal geographical considerations, the worldwide size of the electroforming industry was calculated, yielding the figures just discussed. In addition to this very significant size the electroforming industry has also shown a consistent pattern of growth over a long period of time. This has been due to requirements from new technologies increasing at a greater,rate than the decline of outdated applications. Frequently, this has resulted in steady growth of the industry even in times of general recession. •W H Y AND WHERE E L E C T R O F O R M I N G IS USED
Electroforming cannot always be regarded as the most economical method of manufacturing metallic products. If it is possible to use conventional metal working techniques, such as pressing or forging, these generally will be employed on cost grounds. There is, however, a very wide range of situations in which electroforming is the preferred method of manufacture and many other instances where it is the only one that can be used to produce a particular product. Applications where it is employed successfully always depend on the exploitation of one or more of the advantages of the electroforming process, the principal ones being as follows.
Accuracy of Reproduction Electroforming is by far the most accurate method of manufacturing metallic articles. Both the dimensional tolerances that can be achieved and the ability of the process to reproduce surface detail with absolute fidelity are unmatched by any other metal-fabrication processes. An excellent example demonstrating this critical advantage of the technology is a very familiar one in the modem world--the manufacture of molds for compact disk (CD) production. Information is stored on a CD in the form of a continuous helical track consisting of shallow depressions. These are typically 0.1-micrometer deep and 0.2-micrometer wide, and they vary in length from 0.2 to 2.0 micrometers, with an intertrack spacing of 1.6 micrometers. There are frequently 3 × 10 m such depressions on a CD, all of which must be of the correct size and shape and in the correct position to achieve the required performance. Electroforming is the only technique that is sufficiently accurate to be used to produce molds for CD manufacture. This key advantage of electroforming is why the process is used to produce many other types of molds, dies, and other products requiring very high-dimensional accuracy such as wave guides. It also enables electroforming to be used to reproduce natural finishes, such as wood grain and leather, with total realism. Manufacture of Thin-Section Products It is, perhaps, logical that electroforming, which produces a metallic article by building up the structure of the material atom layer by atom layer, should be an excellent method for
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manufacturing thin-walled products. In conventional metal working processes the metal is normally cast as a massive billet or ingot, which needs to be progressively reduced in size to yield a thin-section final product. This involves multiple working processes and can, therefore, be very costly as well.as inefficient in terms of energy consumption. The production of foil and mesh products is by far the biggest industrial use of electroforming. With copper the most important product is plain unperforated foil whereas for nickel perforated mesh products, in particular screen-printing cylinders, predominate. Further details of a number of commercially important thin-walled products are given later in the section that describes applications.
Manufacture of Complex Shapes An increasingly wide range of metallic products required by modem technology is of an extremely complex shape. Many of these are manufactured by electroforming because it is by far the most economical method of production. In a large number of cases, the technique is the only method available for producing the shape required. A good example of this is the waveguides used extensively in radar and microwave technology. There are many other instances of aerospace and defense uses, some of which are discussed in the applications section.
Manufacture of Large Products or Molds For the manufacture of very large products and molds, particularly ones with thin walls, electroforming can be the most economical method of production. It avoids the very high cost and difficulty of fabricating and machining large objects and, at the same time, minimizes raw material usage.
Manufacture of Very Small Products or Molds In recent years, a radical new development of electroforming technology called the LIGA process has appeared. This enables extremely small components, sometimes with dimensions of only a fraction of millimeter, to be made with very high precision. This process is becoming established in a number of industries.
HOW ELECTROFORMING IS DONE Differences Between Electroforming and Electroplating Although the basic technology of electroforming is very closely related to that used for electroplating, there are a number of important differences.
Deposit Thickness Electroplated deposits used as coatings--in order to confer corrosion resistance, wear resistance, or simply aesthetic appeal~are generally 1- to 30-micrometers thick. Electroformed products, however, need sufficient mechanical strength to be able to exist independently of the mandrel onto which they are deposited. They are, therefore, normally much thicker, ranging from foils of 10-micron thickness to mold faces that can be up to 5-millimeters thick. Electroformed backings for molds have been produced as thick as 50 millimeters. Snbstrate Adhesion If an electroplated coating is to fulfill its function it is essential that the adhesion between the deposit and the substrate be good. Electroforming, however, requires exactly the opposite.
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Table I. Metals and Alloys Used in Electroforming Metals
Nickel Copper Iron Silver Gold Aluminum
Alloys
Nickel*cobalt
Gold alloys
It is vital that there is little or no adhesion between the deposit and the mandrel so that the product can be separated without damage when the forming process is complete. Whereas in electroplating the pretreatment processes are selected to maximize deposit adhesion, electroforming pretreatments are specifically chosen to reduce the adhesion between the deposit and the mandrel to the minimum required to prevent premature separation.
Internal Stress in the Electrodeposit Because electroforming requires that the adhesion between mandrel and electrodeposit should be minimal, it is vital that the internal stress in the deposit is controlled. Normally, this means maintaining it as low as possible. Stresses are usually present in electrodeposited metals and may be either tensile or compressive in nature. Both tensile and compressive internal stresses, if sufficiently high, can produce distortion of the deposit if it is not restrained by the substrate. In electroforming, where there is no such restraint, it is, therefore, essential to select deposition processes that give low internal stresses in order to avoid unacceptable dimensional changes or gross distortion of the product. It is for this reason that sulfamate-based solutions, rather than those employing the cheaper sulfate salt, often are preferred for nickel electroforming.
Metals and Associated Processes Used for Electroforming The metals and alloys that can be used in electroforming are shown in Table I. The choice of materials is limited not only by the normal constraints that apply to electrodeposition in general but also by the additional requirement for low internal deposit stress. Thus, a metal such as chromium, which would have many desirable properties as an electroform, particularly in relation to its hardness, cannot be used because of the extremely high internal tensile stresses generated in the electrodeposited metal.
Nickel Nickel can be considered the workhorse of the electroforming industry. Although a larger tonnage of copper than nickel is deposited, it is only used for a very limited range of products. It is probable that 95% or more of the products manufactured by electroforming are produced in nickel This is due to a favorable combination of properties, both of the electrodeposit and of the deposition processes. Electroformed nickel shows excellent strength, toughness, and ductility and is extremely resistant to corrosion in a wide range of environments. In addition it can readily be deposited in a low-stress condition, and its hardness and strength can be varied over a wide range by selecting appropriate deposition conditions. There are a number of deposition processes available that are suitable for electroforming of nickel They are generally highly reliable and relatively easy to control, being both chemically and electrochemically stable and environmentally friendly. The operating parameters for these processes are also well documented, which allows a substantial amount of control to be exercised over the properties of the deposits obtained.
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Table II. The Conventional Nickel Sulfamate Solution Ni (NH2SO3)~-4H~O NiCI2"7H20 H3BO3
350-450 g/L 5-30 g/L 35-40 g/L
There is a strong preference for nickel sulfamate-based solutions in the electroforming industry. The deposits that they produce not only have a low internal stress, which is essential for this application, but the electrolytes are stable, reliable, and simple to operate. Conventional nickel sulfamate solution has an uncomplicated makeup. A typical composition range is given in Table II. In most published formulations chloride is included. This has been conventional wisdom for many years and is based on the fact that this anion is required for good dissolution of nonactivated nickel anodes; however, if one of the sulfur-activated grades of nickel anode materials, which are readily available commercially, is used, then a chloride-free solution will operate without problem at normal anode current densities. This is a considerable advantage because the chloride ion increases tensile stress in the deposit so that operating a chloride-free solution approaches closer to the ideal stress-free condition. Nickel sulfamate solutions can be used for long periods of time with minimal chemical analysis. There is a slow, steady rise in the solution pH due to a small imbalance between the cathodic and anodic current efficiencies, but this can easily be controlled by sulfamic acid additions. The same imbalance, however, also results in a gradual rise in nickel salt concentration in situations where drag-out is low, which is frequently the case in electroforming. This may require periodic dilution of the solution to maintain the optimum concentration. Nickel sulfamate-based solutions can also be used with a number of organic addition agents. These provide a much wider range of deposit properties than can be achieved with - standard processes without additives. For example additive-free solutions will give deposits with hardnesses around 200 HV. This can be increased to 500-600 HV by the use of suitable additives. Nickel sulfate-based solutions are also used for electroforming but always require the presence of an addition agent, such as the sodium salt of saccharin, to reduce the internal tensile stress to a tolerable level. These solfitions are generally based on the conventional Watts-type composition given in Table III. These basis solutions are also extremely stable, requiring only additions of acid to counteract the natural pH rise; however, the organic additives generally require frequent and more sophisticated analysis in order to maintain the required properties in the deposit. They are, thus, less suitable than the conventional sulfamate process for use in situations where regular chemical analysis is not available. Nickel fluoborate solutions are also mentioned in the literature, but there are few commercial applications. One further advantage related to both sulfamate- and sulfate-based nickel electroforming processes is that there is a considerable body of information available in the literature regarding the use of additives, both organic and inorganic, to these solutions.
Table III. Watts-Type Nickel Solution NiSO4.6H20 NiC12-7HzO H3BO3
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250-400 g/L 25-60 g/L 30-40 g/L
Table IV. Acid Copper Solution
CuSO4"5H20 H2SO4
70~50 g/L 50-200 g/L
Copper In the region of 80,000 metric tons of copper is used for electroforming each year. The vast majority of this is, however, for one single application--the production of thin foil used for the construction of printed circuit boards. Other uses of copper are mostly limited to those where electrical conductivity is also the property of prime ilnportance, for example waveguides and spark erosion electrodes. There are also a few miscellaneous uses, such a s backing of nickel mold faces, production of loudspeaker cones, and parts for musical instruments. Copper, although cheaper than nickel, is not used for the majority of electroforming applications because its mechanical properties are not as good as those of nickel, and it has less resistance to corrosion in many environments. In addition, the processes for depositing copper are, overall, not so attractive for general purpose electroforming. The solution most commonly used for copper electroforming is the simple acid copper sulfate. This can operate over a wide composition range, as shown in Table IV. This solution is stable and simple to operate and produces deposits with very low stress levels; however, owing to its very high sulfuric acid concentration, this formulation is very corrosive both to equipment and its surrounding environment. Acid copper sulfate solutions are used for the production of electroformed copper foil using an inert anode system and chemical dissolution of scrap copper to maintain the solution balance. This enables the gap between cathode and anode to be small and accurately controlled, thus minimizing power consumption. Acid copper solutions are also used with organic additives developed for decorative bright acid copper plating. In contrast to additives for nickel processes, however, there is relatively little information available on the effects that these materials have on deposit stress or hardness. Copper cyanide solutions can also be used for electroforming, but the internal deposit stress is considerably greater than that produced by acid sulfate solutions. This, combined with the toxicity and effluent problems associated with cyanide solutions plus the relatively complex control required of the solution chemistry, has limited its applications. Cyanide processes have, however, been used with periodic current reversal. This produces very uniform metal distribution and is used for thick deposits for backing electroformed nickel mold faces. The use of copper fluoborate solutions for electroforming appears to be very limited, although they are mentioned in the literature. The deposit properties, however, appear to offer few advantages over those from acid copper formulations. The use of copper pyrophosphate solutions seems to be confined to electroforming of spark erosion electrodes. Iron Electrotbrming of iron is technically possible, but despite the low cost of the metal, it has never found any successful commercial application. This is due to its very high susceptibility to corrosion and also to the difficulty in controlling iron deposition processes as a result of the inherent instability of the ferrous ion. A process was developed some years ago in the United Kingdom for the continuous deposition of iron foil but has never achieved commercial acceptability.
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Silver Silver is employed to a limited extent in applications in which the cost can be justified. These can be functional uses, generally for electronics, in which the high electrical conductivity of silver is important, or for purely decorative ones, such as jewelry. Only cyanide-based processes have been used successfully for silver electroforming. These produce deposits with good physical properties together with acceptable levels of internal stress. Bright silver can be deposited from proprietary solutions and is used in the jewelry and silverware industry. Gold The major use of gold electroforming is for jewelry and other decorative products; however, there are also a number of functional applications of gold electroforms, for example, prostheses for medical purposes and crowns and bridges in dentistry. Most of the gold electroforming solutions in commercial use are cyanide-based; many proprietary formulations are available. Aluminum Aluminum cannot be electrodeposited from aqueous solutions, although nonaqueous processes do exist, but these require the complete exclusion of both oxygen and water from the system. A process has been developed in Germany for electroforming aluminum, but the equipment is very expensive. There have been few successful applications, use being confined to the aerospace industry where the weight saving can justify the expense. Electroformed Alloys There is no commercially viable system for copper alloy electroforming, and the only one used commercially for nickel alloys is that for the deposition of binary alloys with cobalt. These can be produced from a sulfamate-based solution, which gives deposits with a hardness in the region of 300-350 HV. This is much harder than pure nickel (200-220 HV) but not as hard as organically hardened nickel deposits (500-600 HV). Nickel hardened with cobalt has the advantage that it is sulfur free. The organic compounds used as hardening agents for nickel all contain sulfur, and invariably small amounts of this element are codeposited. If the metal is then subsequently heated above approximately 200°C, the sulfur forms nickel sulfide at the grain boundaries of the deposit, giving rise to serious embrittlement. Cobalt-hardened deposits avoid this disadvantage. The only other alloys used for etectroforming are based on gold. These are for jewelry applications where it is essential to control the alloy composition closely so that the products will be acceptable for hallmarking. To maintain tight composition, a special computerized system is necessary, which can be quite expensive to install.
Mandrel Technology For the successful production of any electroformed product, the design and construction of the mandrel are of utmost importance. An exhaustive .coverage of this topic is beyond the scope of this review; so this section must necessarily be confined to a description of the principles. There are basically two type of mandrels: those that can be removed intact from the electroform, generally referred to as rigid or permanent mandrels, and those that when removed from the electroform are in some way distorted or destroyed, called temporary mandrels. If an electroform is to have reentrant shapes or angles in its geometry, a temporary mandrel must be used. If not, a rigid mandrel can be used. Rigid Mandrels A number of materials, both metallic and nonmetallic, are used as rigid mandrels. The material chosen must be capable of being formed into the correct shape and dimensions and,
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in many cases, needs to be resistant to damage and corrosion. The material must also not contaminate the process solution in which the electroform is deposited. Suitable metallic materials include stainless steel (preferably an austenitic grade), copper, brass, mild steel, aluminum and its alloys, and electroformed or electrodeposited nickel. Stainless steel can be used directly as a mandrel material following cleaning and suitable surface treatment. Copper and brass, after suitable surface preparation treatments, can also be used directly; however, they may be electroplated with nickel or nickel plus chromium to give a more corrosion-resistant surface. Mild steel mandrels are almost always treated in this way. Electroforming of nickel onto nickel mandrels is a widely used technique, enabling sequential replication to be carried out. It is the basis of production for audio records, CDs, and holograms. Nonmetallic materials used as rigid mandrels include thermoplastic resins~ thermosetting plastic resins, waxes, and photosensitive resins. Photosensitive resins are commercially important mandrel materials that are used to reproduce the highly accurate surface detail necessary in production of CDs and holograms.
Nonrigid Mandrels There are various types of nonrigid mandrel employing different principles to permit subsequent removal from complex-shaped electroforms with reentrant angles. They include the following: Fusible materials: Both low melting point alloys, Usually bismuth-based, and waxes can be used for mandrels, which are removed by heating above the melting point of the material. Soluble materials: Aluminum, and to a lesser extent zinc, alloys can readily be dissolved in sodium hydroxide solution. This enables them to be removed from within nickel electroforms because nickel is highly resistant to strong alkaline solutions. Flexible materials: Readily deformed materials, such as plasticized polyvinylchloride, are used for mandrels that are mechanically collapsed into the eleCtroform to facilitate their removal.
Mandrel Preparation Techniques Both electrically conductive and insulating materials are used to produce mandrels. With the conductive ones it is necessary that, following thorough cleaning, the surface be treated so as to ensure that the electroform does not adhere, thus allowing separation. The treatment used depends on the mandrel material. One of the simplest is the use of sodium dichromate solution to give a passive parting film on the surface of stainless steel or nickel. With insulating mandrel materials, it is necessary to provide an electrically conductive layer on the mandrel surface onto which the electrodeposition process can initiate. This can be done in a number of ways, but two of the most popular are the use of a chemically reduced silver film or silver paint.
APPLICATIONS OF ELECTROFORMING Although electroforming is widely used in many industries, its importance and range of application is rarely appreciated, even by qualified engineers, scientists, and technologists. Similarly the Ordinary "person in the street" has no idea of the extent to which they interface with electroformed goods in everyday life. When the average person wakes up in the morning, he or she may step out of bed onto a carpet printed using electroformed screen-printing cylinders, look at wallpaper, and put on clothes made from fabric similarly patterned. That first cup of coffee may be filtered through an electroformed mesh, the first morning cigarette will have been manufactured using a number of electroformed products, and many electric razors use electroformed foils. Any electronic device--radio, television, tape deck--switched on to aid the waking-up process will depend on electroformed copper foil used in the circuitry. In addition, any record or CD 395
Fig. 2. Electroformed nickel stamper and platinum-coated polycarbonate user-write optical read-out disk. (Photo courtesy of Plasmon Data Systems, Royston, England.) will have been made with an electroformed master. A wallet or purse picked up before leaving home will probably contain bank notes printed from electroformed plates, as well as credit cards with security holograms made using-electroformed masters. On entering most modem cars, the occupant may well be looking at a fascia panel molded into an electroformed mold. These examples make it obvious how relevant this technology is to modern living. The whole range of uses of electroforming is far too large to cover in .this summary of the process, so just a few of the more important and interesting examples have been selected.
Audio and Visual Disks The audio record and, more recently, the CD are two of the most commonly recognized products where electroforming is a vital part of the manufacturing process. This technique has been used to produce sound-recording devices since the beginning of the present century--first; shellac disks; followed by polyvinylchloride-molded, "long-playing" records; and most recently CDs. It has been necessary to refine the precision of the electroforming process in order to cope with the extreme accuracy of surface detail necessary for CD manufacture. As noted earlier, a C D bears a helical track with 30 billion depressions only 0.1 micrometer deep, 0 . 4 0 . 6 micrometer wide, and from 0.5 to 2.0 micrometers long. For good reproduction quality, these all need to be of the correct size and in precisely the right place on the track. The latest developments have increased the disk capacity eightfold to enable full-length color video recordings to be produced. Similar technology is used to produce computer "user-write" optical read-out disks of the type shown in Fig. 2. Electroformed Foil Products As noted earlier, copper foil production represents by far the biggest application of electroforming, consuming about 80,000 metric tons of metal per year. It used for the manufacture of printed circuit boards and has, therefore, formed the backbone of the whole electronics industry for over 30 years.
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Fig. 3. Printing with electroformed nickel screen-printing cylinders. (Photo courtesy of Nickehnesh AG, Rudolfstetten, Switzerland.) Nickel foil is also electroformed and used in resistance elements, printed circuit boards with welded connections, asbestos-free gaskets, and fire-resistant membranes. A specially blackened nickel foil is used to-make solar energy absorbers. Both nickel and copper foils are manufactured on cylindrical mandrels continuously rotating within a conforming anode arrangement. Electroforming represents the only economic method of producing thin, wide foils to the dimensional precision required for these applications.
Mesh Products Electroformed mesh products--that is, foils with regular patterns of perforations--comprise the largest single use of nickel in electroforming. There is a range of applications such as coffee and sugar filters, electric razor foils, tobacco tapes, and powder sieves. By far the single most commercially important use is the production of screen-printing cylinders. These consist of thin-walled (typically 100 micrometer) electroformed tubes, which are up to 250 m m in diameter and 8 meters in length. The pattern of holes formed in the surface dictates the design, which can be printed by ink forced from the inside to the outside of the rolls using doctor blades. Fig. 3 shows electroformed cylinders on a printing machine where they are used sequentially to apply the various component colors of the full design. Electroformed screen-printing cylinders are used extensively all over the world to produce wall papers, fabrics, and carpets.
Large Electroforms Very large products can be made by electroforming. Molds used for the manufacture of plastics components formed by low-pressure processes, for example, glass-fiber-reinforced
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Fig. 4. Electroformed mold for wing fairings of the Airbus 320. (Photo courtesy of Galvanoform GmbH, Lahr, Germany.)
thermoset parts, are a good example. These are employed widely in the aerospace industry. A typical example is the mold for a large wing fairing being shown in Fig. 4. (The building
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Fig. 5. Gear wheel produced by L1GA process. (Photo courtesy of CLRC Laborato13', Daresbut3,, England.)
in the background demonstrates the scale of the component.) Large electroformed molds are also used in the automotive industry for the production of one-piece moldings for the drivers' cabs of trucks.
Small Electroforms In recent years, a new branch of electroforming has enabled extremely small components to be produced by electroforming. This technology, developed originally in Germany, is known as the LIGA process, its name being derived from the German acronym, Llthographie, Galvanoformung, und Abformung. The tiny products that can be made by this process are demonstrated by Fig. 5, which shows a perfectly formed, 12-tooth gear wheel small enough to be contained within the eye of an ordinary needle. The LIGA process is being developed as part of much larger programs of microengineering. Almost all major industries have potential uses for these miniature or microcomponents, with automotive, aerospace, and medical applications already in place. In the medical industry, for example, electroformed components are used to manufacture very small electric motors, which can be passed along arteries to scour the walls, removing deposits that cause blocking of the artery and restrict blood flow.
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