Mandrel Types and Selection

Mandrels are either permanent or expendable. Permanent mandrels are usually metallic, but they can also be made of a conductive plastic. They can be used repeatedly until surface wear or scratching renders them useless. The most widely used permanent mandrels are made of metals that are resistant to adherent bonding by the metal being electroformed. The 300-series stainless steels are the preferred materials for permanent mandrels because of the naturally passive surfaces. Substrates such as copper, brass, or steel may also be used, but these must be plated with chromium to provide a passive surface for ease of separation. It is also possible to use copper or brass for engravure mandrels if they are chemically passivated to prevent electroform bonding. Nickel is frequently employed for producing multiple first-generation replicas for mass production of second-generation electroforms. Adherence on nickel is unpredictable, so it is advisable to passivate the surfaces chemically.

Plastics are suitable for permanent mandrels where flat electroforms are involved and separation is relatively simple. Such mandrels are made conductive by the silver reduction method (Ref 1) or by use of silver-filled paint. Plastic mandrels are often used for the electroforming of Fresnel lenses. Glass plates can also be used as permanent mandrels containing holographic imagery.

Expendable mandrels may consist of cast fusible metals, plaster, plastics, waxes, soluble metals, or wood. Fusible metals are commonly alloys of tin, lead, bismuth, antimony, and cadmium. Aluminum is a popular expendable mandrel material because it is easily machined and polished to close surface and dimensional tolerances. It is also easy to dissolve in caustic solutions.

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