Precision Glass Moulding - Substrate Materials

Substrate Materials

The mould material must have sufficient strength, hardness and accuracy at high temperature and pressure. Good oxidation resistance, low thermal expansion and high thermal conductivity are also required. The material of the mould has to be suitable to withstand the process temperatures without undergoing deforming processes. Therefore, the mould material choice depends critically on the transition temperature of the glass material. For low-Tg-glasses, steel moulds with a nickel alloy coating can be used. Since they cannot withstand the high temperatures required for regular optical glasses, heat-resistant materials such as carbide alloys have to be used instead in this case. In addition, mould materials include aluminium alloys, glasslike or vitreous carbon, silicon carbide, silicon nitride and a mixture of silicon carbide and carbon.

A commonly used material in mould making is tungsten carbide. The mould inserts are produced by means of powder metallurgy, i.e. a sintering process followed by post-machining processes and sophisticated grinding operations. Most commonly a metallic binder (usually cobalt) is added in liquid phase sintering. In this process, the metallic binder improves the toughness of the mould as well as the sintering quality in the liquid phase to fully dense material. Moulds made of hard materials have a typical lifetime of thousands of parts (size dependent) and are cost-effective for volumes of 200-1000+ (depending upon the size of the part).