A mandrel is the original from which a replica is electroformed. 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 often used when the patterns can be generated photolithographically or by etching silicon or glass as for holograms or nanofluidic 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 or nickel, both chemically resistant and durable metals. Expendable mandrels are destroyed during separation from the electroform. This latter approach is commonly used when the mandrel has features preventing it from being mechanically separated from the electroform. For instance, bellows are usually electroformed on expendable aluminum mandrels that are chemically dissolved after electroforming.