Electroforming in Copper, Silver and Gold

Electroforming in Copper, Silver and Gold

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Unique Manufacturing Capability

Electroforming offers the ability to make shapes that would be impossible or impractical to manufacture in metal by any other method. Electrodeposited metal is fully dense, with a properly developed grain structure, and physical / chemical properties indistinguishable from those of a conventional metal. In fact high-specification metals are often refined by dissolution followed by electrodeposition.

The moving-coil formers below are made of copper 0.125mm thick and weigh 0.2g.

Moving Coil Formers

The electrodeposited copper is much stronger than would be expected of such thin material because the metal is effectively work-hardened during deposition by the introduction of stacking faults in to the fine-grained structure. The coil formers are therefore handleable.

Electroforming always involves applying a coating, (of unlimited thickness), to a 3- dimensional shape. This enables items with very complex internal shapes, for instance tubing manifolds, bellows, and mould recesses, to be electroformed on to a machined or fabricated mandrel.

If it is geometrically possible to extract the mandrel from the finished electroform then materials such as stainless steel may be used.

A good example of this is the moulds used to make bars of chocolate. A single machined stainless steel master mandrel is used to produce a large number of electroformed moulds. These are linked to produce a continuous belt of moulds on the production line. The thin walls possible in an electroform allows good control over the cooling rate of the chocolate.

If it is not geometrically possible to extract the mandrel once the metal has been deposited on the surface, the mandrel can be simply made from wax and melted out after electroforming. For greater accuracy a machined aluminium mandrel can be used and dissolved out of the finished electroform.

The machined aluminium compressor rotor below is a mandrel used to produce a metal mould.

A thick deposit of copper is applied.

Copper on Aluminium Mandrel

Then the aluminium is dissolved out of the electroform as shown below.

Copper Electroformed Compressor Negative

The resulting cavity exactly reproduces the aluminium mandrel. This is then used as a master mould to produce unlimited numbers of rubber patterns for investment casting in the large-scale production of the compressors.

The same principle can be applied to making injection moulding tools, the electrodeposited metal can be as thick as is required to withstand pressure.

The electroforming process exactly replicates surface finishes. The example below is of an X-block. The material is silver, which has been deposited onto a silicone rubber mould produced from a machined master. The fine surface finish of the central 5 X 5 array, and the blasted finish of the “arms” of the X were applied to the master pattern.

No further finishing is required.

Silver Electroformed X Block

The X-block is used to rapidly thermally cycle small, (100ul), biological samples. It is heated by passing electrical currents directly through the block via the arms. This needs a closely controlled distribution of metal thickness over the area of the block and of the walls of the sample wells. The required thickness of the silver is only 0.4mm to produce the required electrical and thermal characteristics. This item would be impossible to make by any method other than electroforming.

Prototyping / Small Quantities

Electroforming is ideally suited to producing small quantities of complex metal items without expensive tooling, and to accurately reproducing hand-made special patterns.

8 air flow nozzles were required for a prototype. The constructor made a model of the required nozzle shape by starting off with a piece of copper tube joined to a brass block with a thick length of wire. This was bent to shape and the relative positions of the nozzle ends adjusted, in position, in the apparatus. Automotive body filler was used to fill in a smooth transition between the tube and the block and filed to shape. This became the master pattern.

Electroformed Nozzles

A rubber mould was then cast around the pattern and used to produce 8 wax replicas. The wax shapes were then metallised and an electrodeposit of copper applied.

The wax was then melted out leaving the finished nozzles.

The cost of tooling to produce these complex and shapes was minimal, and the constructor received exactly the parts he needed without even having to produce a drawing. The nozzles were nickel plated simply to improve their appearance.

In general, if a machined master pattern is available, or an example of the item in any material can be found, it can be reproduced by electroforming. Again, in general, if an item can be stamped from sheet metal, then it can be electroformed on a re-useable mould or mandrel in small quantities without the difficulty and cost of making a steel stamping tool. Stamping is generally more economical for large production runs, electroforming for small production, The economic switch-over point will depend on the nature of the part.

In the case of the large helmet starburst below, the original stamping tools used to make it were long lost.

Large Helmet Badge Starburst

So a PVC mould was made from an old example, and this enabled several dozen electroformed copies to be made for an historically accurate film production.

Clearly this was a more economic course of action than either making new stamping tools to the original design, or hand making the starbursts.

Accuracy

Electroforming inherently gives an extremely accurate reproduction of the mould or mandrel. At the detail level the accuracy is sub-micron. Electroforming was used to produce vinyl gramaphone record stamping tools, and is now used to produce tools for CD stampers, and to reproduce holographic plates.

Electroforming is inherently a one faced process. It will very accurately reproduce the geometry of a mould or mandrel on the surface of the electroform which is in contact with the tool.

The thickness of metal deposited can be controlled very accurately in terms of the total mass of metal deposited on the tool surface. However, the thickness of metal deposited any particular point is dependent on the geometry of the item as the electric field around a complex shape will not produce the same current density at all points on the surface of the mould. The electric field may be adjusted using various secondary electrodes, baffles, and masks to compensate. Basically every job is different, so consultation with our technical team is required if metal thickness distribution is critical.

An electroform will be an exact reproduction of the mould only at the time the electroforming process is carried out. The stability, and particularly, the thermal expansion coefficiant of the mould material are the factors that most influence the practical accuracy of the process.

The electroforming solutions are operated at controlled temperatures. A glass mould or mandrel made to be the required size at the solution temperature would give almost perfect accuracy. More practical mould/mandrel materials are;

Stainless Steel. A re-useable stainless steel mould is only useable where there are no re-entrant shapes to obstruct removal of the electroform, and a draft angle may be required. Stainless steel will give the best accuracy, with a practical linear tolerance of typically 0.00005 metres/metre.

Aluminium. An aluminium mould/mandrel can be readily dissolved away from an electroform. This means that re-entrant shapes are not a problem and an electroform can be made with a very complex internal structure. However this can be an expensive option using machined mandrels, less so in larger scale production where die cast mandrels can be used. Linear tolerances of 0.0001m/m are possible.

Low Melting Point Alloys. These materials can be easily cast to shape, and special alloys are available that give zero shrinkage. The melting points can be low enough that they can be removed from the electroform in boiling water. Linear tolerances of 0.0003m/m are possible. However, unless pressure die-cast in an expensive tool, they show all the normal defects present in a cast metal, particularly, surface porosity. These will be reproduced as blemishes and pin-holes in the electroform.

Engineering Thermoplastics. These materials can be selected to have relatively low thermal expansion coefficients and can be machined or injection moulded to close tolerances. Generally these materials are rigid and will not work with re-entrant shapes, however in some cases it is possible to heat the electroform to above the Tg of the plastic to enable removal of the mould. Linear tolerances of 0.0005m/m can be maintained.

RTV Silicones. A re-useable mould can be made either from a machined master, or from an example of the item to be produced. Since the rubbers are cured at room temperature, (Room Temperature Vulcanising), there is no significant shrinkage on curing. The rubber is very elastic, and so some re-entrant shapes may be accommodated, and there is no need for a draft angle. The same flexibility however means that to maintain a stable shape, the mould must be well supported throughout processing. The material does however have a high thermal expansion coefficient and low thermal conductivity. Linear tolerances of 0.0025m/m can be expected.

Hot Cured PVC’s. These materials are easily cast to shape and can be used to mould from existing items. They are flexible enough to cope with limited re-entrant shapes, and zero draft angles. They are a cheap way to make re-useable moulds. However they tend to shrink around 3% on curing and have a high thermal expansion coefficient. The detail reproduction can be very good, but the overall linear tolerances will not be expected to be better than 0.005m/m, as even if the shrinkage is allowed for in the design, the shrinkage is often unpredictably anisotropic.

Waxes. These materials are the easiest to use in creating a complex shape by casting, machining, fabricating, even moulding by hand. The wax may be melted out or off of an electroform at comfortably low temperatures. The tendency to sag when handled, or even just left in a warm place, is very dependent on the geometry of the item. The thermal expansion coefficients are large, as is the shrinkage if cast. With care, linear tolerances of better than 0.01m/m can be maintained.

Wood, Expanded Polystyrene, Polyurethane Foam, Easily Carved Materials. Any material can be used for electroforming if it is filled and painted so that the surface is no longer porous. Foam materials can generally be removed by a suitable solvent.

Designing for Electroforming

It is strongly recommended that our technical team is consulted as early as possible in the design process. We offer an unrivalled breadth of experience in the design of moulds and mandrels for electroforming.

The basic principles are simple in that any electroform can be regarded a sheet of metal, (however complex the shape), and so standard sheet metal engineering can be applied.

For instance where a threaded boss would be used in a casting, substitute a recess or other feature to accurately locate a threaded insert.

Two design aspects less familiar to conventional engineering are the effects of edges and corners. Again consultation with our experts is recommended.

The nature of electrodeposition is to produce higher current densities, (and therefore deposit more metal), at the edges and external corners of a form, and lower current densities in internal recesses. In particular, internal corners in a mould must have a small radius to avoid the current density falling to zero at the apex of the corner. It is important to remember that the internal corner in the mould is an external corner in the working face of the form, and vice versa.

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