A mandrel is the original that is copied to make an electroform. Mandrels can be machined out of metals such as steel, copper and aluminum or produced out of non-conductors such as plastics or silicon. Non-conductive mandrels are used when the patterns can only be generated photolithographically or by etching silicon or glass, as for holograms or microfluidic devices. To be used as originals in electroforming, non-conductive mandrels are coated with a thin conductive (seed) layer, usually by vacuum deposition. In less demanding applications, conductive silver and copper paints can be used instead
Mandrels are either expendable or reusable. Reusable, or permanent, mandrels are often made of stainless steel, cupro-nickel alloy, brass or nickel, all chemically resistant and durable metals. Expendable mandrels are destroyed during separation from the electroform. This approach is commonly used when the mandrel shape makes it impossible to mechanically separate the electroform. For instance, bellows are usually electroformed on expendable aluminum mandrels that are chemically dissolved after electroforming.