Investment casting is a tricky business with many variables. No two manufacturers use the same set of materials. If you change any variable, it is likely to cause a problem somewhere down the line.
Start with your mould. It can be metal, silicone or Multimould. How accurate do you need to be? How many wax patterns do you intend to make? For prototyping use Multimould wherever possible, because it is re-usable. (R273/kg + VAT) If you get into massive production, use metal for its chill ability and accuracy.
Next select your wax. Excellent hard waxes are available but expensive. Cheap waxes tend to expand and break the ceramic before melting. For artworks, particularly for origination, plastic waxes are used, and each artist has his own formulation to suit his style and methods.
Now for the casting materials: Large statues mostly use a Plaster of Paris stucco mixed with ash , grass or sand. Medium sized technical castings use a dipping technique. Small jewellery castings are attached to a runner called a “tree” and placed in a “flask” and a plaster/cristobalite slurry is poured over them. For medium sized castings, the wax model is first dipped in a fine slurry of mineral powder bonded with colloidal silica or ethyl silicate. Ethyl silicate can be rapidly hardened by exposing to an amine gas or ammonia. All the smaller, less automated processes use colloidal silica. Keramicalia’s trade name is “Super Binder L”, packed 6kg in 5 litre bottles @ R58/kg. Adhesion to the wax pattern is sometimes a problem, an addition of Plasticast acrylic resin solves it.
The minerals used in dip casting vary. Generally the best is considered to be zircon flour, but chamotte or “Molochite” is also common, and some manufacturers use fused silica for its low thermal expansion. Keramicalia makes a “Zircon coating” which contains some chamotte and suspension agents. After dipping, the first coat is dried. It is then dipped again and then immediately dipped into a fluidized bed with a suspension of grit. We sell chamotte 0-1mm for this, @ R18/kg + VAT. The dipping into slurry and grit is repeated between drying until a 5mm coat is built up.
The wax is then melted out and collected for re-use. The moulds are heated to about 700 ̊C and the molten metal poured into them.