GAR Electroforming Div

Main component for waveguide is electroformed in layers of nickel and cadmium. Aluminum grow-ons are added to achieve the configuration shown at left. ELECTROFORMING Commercially available for decades but still considered a space-age technology, this metalforming process provides shapes and accuracies unmatched by any other fabrication method. E. N. CASTELLANO and H. WOELLMER GAR Electroforming Div. MITE Corp. Danbury, Conn. UPPOSE you electroplated a metal onto a part, precise, lightweight metal components has put elec- then threw the part away and kept the plat- troforming in a more favorable light with respect S ing. This procedure, in essence, describes the to the common metalworking methods. Wave- process of electroforming. Of course, electroform- guides, reflective metal surfaces of optical quality, ing techniques are not nearly as simple as this and even aerospace structural components are description might indicate. The methods differ con- some of the parts electroformed on a production siderably from those of ordinary electroplating. Yet basis. And to some extent, electroforming is even the family resemblance remains, and the capabili- providing new capabilities for the more conven- ties and limitations of electroforming are essentially tional processes in that electroformed dies have governed by the basic chemistry and physics of lowered the cost of molding intricate surfaces in plating. plastics and metals. Since electroforming is so drastically different from the more conventional forming, joining, or An Old Technology Updated cutting methods used to shape metal, the design rules for parts to be electroformed are also quite Electroforming, as a process, is not new; it is different. It is almost as if the light weight of almost as old as plating itself. What is new is the sheet metal fabrication, the complexity possible application of advanced technology to the field. with castings, and the accuracy of precision ma- Formerly, the process was in large measure prac- chining have been made available in a single ticed as an art—and correspondingly the results process. But electroforming also has its share of were sometimes erratic. Electroforming, as some drawbacks related, for the most part, to limited have said, has had a lot to live down. But now a bet- selection of materials, and the basic limitations of ter understanding of electrochemistry, and especial- plating. ly knowledge about the role of additives in plating Recently, however, the increased emphasis on baths, permits close control of electroformed parts. Results are now as reproducible as those obtained tics and elastomers are also employed as mandrels. with welding, casting, forging, and other conven- These materials are first made conductive with a tional techniques. surface coating. Mandrels must be handled with In electroforming, as in plating, metal ions are extreme care. The smallest imperfection—even a transferred electrochemically through an electrolyte fine scratch—will reproduce on the electroformed from an anode to a surface where they are de- part. posited as atoms of plated metal. But in electro- Of these materials, the castable ones are par- forming, the surface that is to receive the plated ticularly useful where large numbers of electro- metal, called a mandrel, is conditioned so that forms must be produced with disposable mandrels. the plating does not adhere. Instead, the plated The mandrels can then be made economically in metal, or electroform, is lifted away and retains large numbers by casting them in reusable molds. its as-deposited shape as a discrete component. Because of the large number of available man- A part formed by this process has several un- drel materials and the special features of each, a usual characteristics: number of tradeoffs must be considered in design- ∙ It can have extremely thin walls—less than one mil. ing the electroform. The disposable mandrels gen- In fact, minimum thickness is generally limited only erally cannot provide accuracy or surface finish by the fact that a part requires a certain amount of as good as that provided by permanent mandrels. sturdiness to avoid being bent or broken by normal Every effort should therefore be made to avoid com- handling. bining requirements for high accuracy or surface ∙ Surface features of the mandrel are reproduced with finish with undercut shapes. Small numbers of extreme fidelity on the surface of the electroform. parts can be made economically—if functional re- High surface finish and intricate detail are easily obtained. quirements are not stringent—by using one of the low-cost mandrel materials such as wax or wood. • Complex contours are produced quite easily. A number of different materials and special bond- • Dimensional tolerances can be held to high accuracy. ing techniques are sometimes employed to build Accuracies of ±0.0001 in. are not unusual. up a mandrel where extreme accuracy must be ∙ Maximum size is limited only by the size of the combined with a complex shape. This type of man- available plating tank. Parts over 7 ft long have drel construction is sometimes required for wave- been successfully electroformed. guides and other electronic hardware having numerous cavities. How the Process Affects Design Most electroforms are produced over a positive, or male, mandrel. But sometimes accuracy or As with most metalworking processes, the only smooth surface finish is critical on the outside sur- meaningful way to organize design guidelines for face of the electroform, rather than on the inside electroforming is to relate these guidelines to the surface. In such cases a negative, or female, man- details of the process itself. Also important is how drel is built. design affects the cost of the electroform. Wall Thickness: Since plated metal is deposited Mandrels: Probably the most interesting aspect more or less uniformly, electroforms are essentially of electroforming is the ease with which complex parts of constant wall thickness. Parts are produced shapes are produced. The mandrel and electroform with walls as thin as 0.005 in. One-half inch is gen- bear the same geometric relationship that exists be- erally considered a practical maximum. Most elec- tween a mold and cast metal. Internal features of troforms are in the range of 0.010 to 0.050 in. thick. the electroform, therefore, are simply machined as Uniformity of the deposit is subject to the usual negative-image external features on the mandrel. variations encountered in electroplating. Deposits Two types of mandrels are used: permanent and build to greatest depth in areas of high plating- disposable. Permanent mandrels are used where current concentration at sharp edges or on con- the electroform has no undercut surfaces and can vex surfaces. Deposits are thinnest at low-current thus be lifted directly from the mandrel. Where areas within recesses or on concave surfaces. undercuts are required on the electroform, the man- The easiest way to avoid either excess or insuf- drel must be dissolved or melted away, or in some ficient current density is to provide adequate radii other way destroyed to be removed. at all edges and corners. Holes or slots should Stainless steel and aluminum are the materials also be at least as wide as they are deep. If the most often used for mandrels. Stainless steel pol- electroform cannot be designed to these rules, the ishes easily, providing high surface finish and high plater can use shields and "thieves" to reduce cur- dimensional accuracy in the electroform. Internal rent density, or he can use conforming anodes to (mandrel-facing) surfaces can be electroformed to boost current density. Masks that prevent deposi- 2 micro-in. rms. External surfaces are generally tion of metal in a given region are also used to similar to that of a diecasting. Aluminum mandrels produce variations in plating thickness. can be machined more easily, but do not provide the service life of stainless steel. Aluminum can be Inserts and Grow-Ons: One aspect of the proc- dissolved away and can thus serve as a disposable ess that makes electroforms particularly interest- mandrel. Invar, low-melting bismuth alloys, and ing is that the part need not be made entirely cast alloys of nickel or brass are used occasionally. from deposited metal. Other materials, even non- Plaster, glass, quartz, wax, wood, and various plas- conductors, can be incorporated into the compo- Waveguide electroformed in nickel has numerous complex internal cavities. The rectangular flange at left and the three brackets are grow-ons. Length of the part is 18 inches. Faithful reproduction of minute surface detail is illustrated by the barely discernible groove in the mirror-like, electroformed surface of this reflector. Conforming anode and electroforming technology were required to apply an 0.2-inch copper cladding to the inner diameter of the hole in the 750-lb component for a shake table. Depth of cladding is about twice that which normally can be achieved with close tolerances by ordinary plating. nent by plating onto, over, or around separate thermal or electrical conductivity. Gold, silver, pieces attached to the mandrel. Threaded inserts, and rhodium are used where unusually high resist- bearing surfaces, shafts, and other such inserts— ance to corrosion or oxidation are required. Iron called grow-ons—are often incorporated in elec- can be electroformed, but surface corrosion and the troforms by this technique, a patented proprietary corrosive nature of the electrolyte are some of the process. The nondeposited metal, in fact, often problems that restrict the use of electroformed iron. constitutes a larger portion of the final part than Handbook property values can generally be used the electroformed metal. Some waveguides are com- for preliminary design work, but an electroformer prised primarily of machined wrought pieces, joined should be consulted sometime during design be- by a relatively small amount of electrodeposited cause the mechanical properties of the formed part metal. are determined by plating conditions and composi- The joining aspect of electroforming is an im- tion of the plating solution.

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