Example 1; Ceramic strip element hangers.
These are normally pressed in very expensive and complex steel dies from a refractory clay based ceramic. The die costs about R25 000. This method necessitates sharp corners and edges, which are undesirable from an engineering design point of view. Two alternative routes were tried, and both worked well.
The first route was slip casting. In this process, a "slip" of aluminium oxide in water suspension was poured into Plaster of Paris moulds. The moulds suck the water out of the slip, and the casting is removed, dried and fired. The slip must have an extremely low shrinkage for this particular complex shape. Here the Multimould was used to make the master or "case" moulds, from which the plaster production moulds were cast.
The second route was to produce casting moulds from Multimould and the hangers were made directly in these with Keratab.
PREPARATION OF MODEL for both routes.
The sample of a pressed ceramic hanger was first covered in parts with automotive body putty to eliminate certain sharp corners. (This step is usually performed to eliminate chips and scratches on the sample.) The outer corners and sharp edges were then removed with a ceramic nail file. (manufactured by Keramicalia) It was then sanded smooth. A pencil line was drawn around the exact middle of the model, and the model was buried in Plasticine up to the centre line. Clay can be used instead of Plasticine. Natches were then cut into the plasticine, using a teaspoon for one and a knife for the other. These serve to locate the two halves of the mould in the correct position. Sides or "cottles" were added. A filling funnel was built up in Plasticine.
SLIP CASTING: The surface was painted with dissolved bath soap. The first coat was absorbed by the ceramic, but after a few coats the pores were blocked with soap. Plaster of Paris was mixed in the ratio 1,6 weight parts to 1 weight part water and "Blended" for 5 minutes. It was then poured over the model, covering the highest by 10mm. After half an hour it was tripped and turned over. The cottles were placed back and 3 coats of dissolved soap painted onto the surface. Plaster of Paris was poured onto the first side. Stripped, dried and the 2 sides assembled with an elastic band and slip poured in, dried, stripped.