Zinc Alloys

Zinc alloys have poor mechanical] strength and are unsuitable for production tooling. They are, however, often used for prototype tooling where only a relatively small number of parts are required for preproduction evaluation purposes.

They have a low casting temperature of around 400—450 °C and are therefore well suited to casting from patterns of plaster, wood, steel and aluminum. Zinc alloys can also be cast by sand molding as described above or by using ceramic molding techniques- The resulting castings reproduce intricate pattern detail extremely well, giving a smooth non porous surface that ultimately produces good-quality plastic moldings.

Premium-grade zinc alloy is also suitable for cold hobbling, usually with a rotating hob (for circular components), to impart undercut forms into the cavity insert. Many undercut forms can be replicated in this way (helical gears, for example), giving a cheap accurate method for producing accurate undercut forms*

In view of the poor mechanical strength of this material, normally only the cavity forms are made from it. The resulting casting or hobbed cavity would be inserted into a steel bolster to support it. Two of the best-known zinc alloys are Mdzak and Kirksite.

Beryllium-Copper Alloys

Pure copper is a very soft, ductile material and is unsuitable for use as cavities or cores, Copper is used in mold tools only as a heat exchanger, usually inside core pins. Its excellent thermal conductivity makes it ideal for conducting heat away from smaller core pins where direct water-cooling is not possible.

When copper is alloyed with beryllium, the resulting beryllium—copper alloy is a much stronger, tougher material and is used for cavity and punch applications With increasing beryllium content, the mechanical properties improve but the thermal properties worsen. At least 1.7% beryllium content is required for direct cavity and punch applications, with around 2% being the norm. Cavities made from this material may be heat-treated toaround 400 0C, giving a hardness of about 40 HRC, The surface hardness may also be increased by about 15% by the use of ion implantation techniques,Beryllium copper may be cast and machined and will accept chromium and nickel plating. Cold hobbing is used for shallow forms and hot hobbing at temperatures of 600-800 °C, followed by solution annealing.

Unlike steel, this material is totally resistant to thermal shock and therefore highly suitable for cavity inserts where large amounts of heat have to be removed. This can result in significantly shorter cooling periods, thereby reducing the overall molding cycle.