Plaster casting

By: Steve Manik

Plaster casting is similar to sand moulding the difference being that plaster is used in place of sand. Plaster is 70-80% gypsum and 20-30% strengthener and water. Normally it takes 4-6 days to prepare after which a production rate of 1-10 units/hr. mould is possible with items as big as 45kg and as small as 30 grams having very high surface resolution and fine tolerances.

Once used and cracked, it cannot be easily recast. Plaster casting is generally utilized for nonferrous metals like aluminium, zinc or copper based alloys. It cannot be utilized to cast ferrous metals because sulfur in gypsum slowly reacts with iron. Before mould is prepared the pattern is sprayed with a thin film of parting compound to prevent the mould from sticking to pattern. The unit is shaken so that the plaster fills the small cavities around the pattern. The form is extracted after the plaster sets. Plaster casting means a step up in sophistication and needs skill. The automatic functions may be given to robots but the higher precision pattern designs need even greater level of direct human assistance.

In plaster mould casting, a plaster, usually gypsum or calcium sulfate, is mixed with talc, sand, asbestos, sodium silicate and water to form a slurry. This slurry is sprayed on the polished surfaces of pattern halves (generally brass). The slurry sets in less than 15 minutes to form a mould. The mould halves are removed carefully from the pattern and dried in oven.

The mould halves are carefully joined, along with the cases. The melted metal is poured in moulds. After the metal cools down, the plaster is broken and cores cleaned out.

Parts cast are generally small to medium size, having weight of 30 grams to 7kg. the section thickness maybe as small as 0.6mm and tolerances are 0.2% linear. The draft allowance is 0.5-0.1 degree. The surface finish is 1.25 μm to 3 μm (50 μm to 125 μm) rms.

Low temperature melting materials like aluminium, copper, magnesium and zinc may be cast utilizing this system. This system is utilized to make fast prototype components as well as limited production parts.

Plaster casting as a sculpture system is of three types. One uses a waste mould, another a piece mould (both Plaster Of Paris) and the third a gelatin mould, all remake the original clay or wax model made by the sculptor. The waste mould is broken to free the hardened cast, which was poured in as a liquid plaster. The gelatin mould being reusable may be sprung from the cast with care and extracted intact and utilized for replicas. The piece mould can also be utilized again, being so divided as to be easily drawn away from the undercutting of the cast without damaging either of them. Plaster casts are utilized not only for creations of new sculptures, but also for the many replicas of famous marble and stone statues. The ancient Egyptians utilized models of plaster made directly from human body. The Romans cast in plaster many thousands of copies of Greek Statues. In another sense of the term, plaster casting refers to the surgical process of encasting in a Plaster-Of-Pairs cast any part of body in which bones are broken so that the bones set well without interference of motion or jarring or physical shock.


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