Theoretical Analysis of the Electroforming Process

Abstract

A theoretical model is developed for electroforming with a shaped, soluble anode-electrode. Cases are considered for which the electrode is either flat or conforms initially to the shape of the cathode-mandrel. The model is first analysed by a perturbation procedure for a case where the amplitude and slope of the surfaces of sinusoidally-shaped electrodes are small compared with the inter-electrode gap width. The analysis shows that for d.c. electroforming, when the soluble anode is initially flat, the upper surface of the electroformed metal can become fiat at some stage. After that stage has been passed, the amplitude of the metal layer increases indefinitely with time of electroforming. A uniform thickness for the electroformed metal layer cannot, therefore, be achieved, unlike the case for insoluble plane anodes. When a soluble anode which conforms initially to the cathode-mandrel is used, a fiat upper surface for the metal layer can be obtained. When periodic reversal of polarity is used with soluble anodes, a uniform thickness for the electroformed layer cannot be achieved. A numerical method of analysis of electroforming is proposed which removes the restrictions on the size of shape treated by the perturbation method. The latter method is then applied to the practical case of electroforming of a mould for a rubber ‘O’ ring. Conditions are investigated where the anode is (i) plane and insoluble, (ii) plane and soluble, (iii) and (iv) conforming initially to the mandrel-shape and either insoluble or soluble. The numerical method has revealed that a variation in the metal layer thickness occurs for all these types of anode, and that little marked improvement arises if the simplest type of anode—plane and insoluble—is replaced by any one of the others.