Tooting the electroforming horn

Electroforming is the precision reproduction of shapes and surface textures with low tooling costs. Leslie Lewis, chairman of BJS explains

Electroforming has tremendous potential for the design engineer, yet it is a relatively unknown process. A more accurate name would be `electrolytic casting’. It has a common basis with electroplating. But whereas plating produces a thin coating on an existing object, electroforming is a heavy metal deposit on to the mould of an original item to form a free-standing object.

The process can replicate complicated shapes and surface textures. There is no practical limitation on thickness, between 0.25mm to 2.5mm thicknesses can be achieved, which is an advantage over stamping. Tooling costs are low and computer technology now enables the deposit thickness to be controlled to a high degree of accuracy-better than +/-2%.

It is this advance in technology which has transformed a Victorian art into a modern, cost effective production technique for small and medium runs (up to around the 1,000 mark) and prototyping. It will often solve the design engineer’s problems when other methods fail.

What’s it all about?

Electroforming is basically a three part operation. The first stage is to produce a mould or mandrel. These can be re-usable or expendable, depending on the application. Where internal surfaces and contours are paramount, for example in wave guides, moulds are usually machined out of aluminium. Where shapes are external and relatively simple-such as sieves, screen, or trumpet bells, Figure 1-moulds are generally made from stainless steel and have an extensive life.

Where less precision than that given by the machined metal mandrel is acceptable, disposable mandrels made from low melting point alloys, polymers or waxes, may be used. Plastic and wax mandrels are metallised to make them conductive. Fabrication of extremely complex shapes is possible, eg one-piece pipework manifolds. After the metal has been electro-deposited around the mandrel, it is melted or dissolved out. The method is less precise because of the loss of precision in the moulding or in the fabrication of the sacrificial mandrel.

Flexible, re-usable moulds made from an elastomeric material provide a simple and economic way to produce complex, re-entrant shapes and to copy existing items. Extremely fine surface details may be reproduced down to microscopic dimensions. However, the overall dimensional precision of the formed parts is limited by the mechanical stability of the shaped elastomer.

The elastomer is moulded round a master pattern which may be an original example of the article to be produced. This is the optimum technique in restoration and conservation. Non-metallic moulds must be metallised to allow electro-deposition to task place.

In applications where only external metal surfaces are important, metal may be deposited on formers of any shape and almost any material. The mandrels are simply left encapsulated inside the metal form. Because the deposited metal conforms so closely to the substrate material, the surface texture will be maintained up to metal thicknesses exceeding 0.25mm. The accuracy of the finished product is dependent on the accuracy of the former. If a stable filled epoxy is used, the tolerances may be held to within a few tens of microns.

As with electroplating, a low voltage dc current is applied to an anode and cathode (ie the workpiece) which are suspended in a complex aqueous solution containing the salt of the metal to be deposited. Metals suitable for electroforming are copper, silver, nickel and gold.

After deposition to the required thickness, the mould and deposited metal are separated. Mandrels made of wax are melted out and aluminium mandrels dissolved away. With stainless steel, the mandrel is pulled off. The resulting piece of metal is an electroform, a seamless replica of the master pattern no matter how complex the shape.

Electroforming may be used to manufacture end products or to make tools for manufacturing products in other materials – plastics, for example.

Which metal is best?

Copper accounts of the greatest tonnage of metal electroformed. It is used in printed circuit boards, spark erosion electrodes, wave guides for high tech communications, micro wave ovens, formers for domestic and protective gloves, specialised trays and boxes in the electronics industry, dies, templates, moulds for the food industry, printing plates and rollers and for rapid prototyping.

Silver, once thought to be too expensive and too precious a metal for industrial use, is now finding many applications for electroforming too. The price has been relatively low and stable for a long time and there is little evidence of any change.

The properties of the metal have great advantages. Its excellent throwing power means that it can be deposited accurately even into deep recesses and undercuts. It is an excellent conductor of heat and electricity. As a precious metal, it is resistant to most chemical attack. It is malleable, ductile and easily solderable allowing elements to be joined together to form complex fabrications. Silver also has one of the highest reflective indices.

Although the cost is relatively high, gold also has a place in industrial electroforming. Electroformed tubes, for example, with a diamond honed internal finish are used for scientific apparatus to weigh air. Gold formed thermometer pockets give temperature protection to sensors and transmitters. Gold electroforms are used in satellite equipment and grids for electron microscopes.

Silver electroforms, on the other hand, are now frequently used to carry current in complex breakers and switchgear. It was common practice to plate nickel formed waveguides internally with silver. However, it can be significantly cheaper, and give greater long term operational reliability, to electroform the whole waveguide assembly in silver. The signal is contained in a uniform silver cavity, regardless of the complexity of the overall shape.

Silver’s high coefficient of thermal conductivity and the seamless, crevice free benefits of electroforming are harnessed in bio chemical equipment. In the replication of DNA, for example, which requires rapid ramping of temperature, the quest is for the greatest number of cycles/time as well as the accurate control of temperature.

Silver electroforms are used in the heat exchange apparatus to give precise control of heating and cooling rates-with the added benefit of easy sterilisation. The conductivity of silver ensures that the heat will be rapidly and evenly distributed through a component. The low specific heat capacity means that little heat energy is wasted in changing the temperature of the component. Because of this, more of the heat will be transferred.

HEAT EXCHANGE

Heat exchange apparatus is becoming increasingly important in medical technology. Great precision in temperature control and heating and cooling rates is required. One-piece silver electroforms provide a cost-effective means of making complex shapes with all the high performance thermal properties of silver. Additionally they are free from crevices and seams associated with normal fabrication methods and are easily cleaned and sterilised.

Another medical application is in prostheses, where scanning, stereo lithography and electroforming come together to enable surgeons to replace bone structure. At BJS we produced, as a donation to medical science, an electroformed eye socket for a survivor of the Piper Alpha oil rig disaster. The victim suffered chemical burns which then developed into bone cancer and was given three months to live.

A scanner replicated a resin block of his eye socket contours and from this a silicon rubber moulding was cast. A silver electroform was produced to the exact shape, drilled and tapped for fixing screws and then heavily plated with pure gold. Surgeons cut away the diseased bone and successfully replaced it with the electroform.

In hybrid circuit packaging, silver electroforms are another solution to the problem of heat dissipation. A major problem with hybrid circuit packaging is that the nickel-iron alloys, which must expand to match the ceramic substrates, have an inherently low thermal conductivity. This makes heat dissipation from within the package difficult.

Conventional solutions include the use of copper-tungsten alloy blocks, but this increases the number of manufacturing stages as well as components. Silver electroforms are a less expensive solution. A further benefit is that the electroforming can, in one step, form the package and incorporate both RF and dc leads into the sides and bases.

Electroforming tooling can cut the cost and time of conventional methods significantly. For example, human hands are electroformed in copper for the mass production of industrial gloves. The cost is less than half that of casting and machining a two part aluminium mould. Production time for the tooling is three weeks compared to six. Apart from the initial savings, the lower cost of tooling enables the manufacturer to up-date models more frequently as fashions and regulations change.

Every plating solution has its own characteristics and there are some basic rules to get the best and most consistent results. The first property to consider is the throwing power or penetration of the metal being deposited. The best performing solution is silver, followed by gold, copper and then nickel.

Deep recesses can be a problem. However, this can be overcome by the use of plastic maskings and pumping the solution down into the recess or applying ancillary anodes. The wider the opening to a recess the better, as the agitation in the tank will keep freshening the solution. Sharp corners are a weakness and should be avoided if possible, for example, at the bottom of boxes. Radius corners should be specified, however small the radius.

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Electroforming

Electroforming is an inherently slow process-it may take 48 hours to form a heavy component. As a consequence, the process was, until recently, highly labour-intensive and therefore expensive. However, by developing and installing computer-controlled, fully-automated plant which automatically tracks each component, the cost of engineering electroforms can be substantially reduced.

When an article is received for electroforming its surface area is calculated. Combined with details of the deposit thickness required this information is used to calculate the optimum current for the deposition and the time that the item or mandrel should remain in the solution.

The scheduling computer allocates a unique job number to this work, so that when the plating technician enters the job number into the plating control computer, the correct current for the job is automatically applied and the current monitored.