Moulds & Mouldmaking

Moulds

A mould has to extract water from casting slip while holding the slip in a very precise shape until there is a thick enough layer of solidified slip so that the excess can be removed. It has to continue to absorb water until the slip is hard enough and then the formed piece can be separated.

This means that the mould must be made of a porous material, but the pores must be so small that the finest particles of clay in the slip cannot get into them. It has to be able to be formed into some very complex shapes. It has to be rigid and resistant to wear. Most importantly it must be possible to get the water that has gone into the mould out again.

Plaster Moulds

When water is added to plaster, it reacts to reform an original hydrated material. By carefully controlling the excess water (the "plaster to water ratio") and the amount of mixing, we can produce plaster moulds with the desired porosity.

Plaster mould sections are formed from a liquid slurry using "case" moulds and so can take on almost any desired shape. At this stage, the pores of the mould are totally saturated and it has to be dried out before it can be used.

Plaster Mould Limitations

Moulds can only absorb so much water before becoming saturated and so have to have lengthy drying periods. This is done by heating the casting shop overnight, but there is usually a build up of retained moisture as the week progresses which is removed during the weekend.

Moulds have a limited useful life, and are monitored carefully for roughness, this varies depending on the type of piece.

Moulds can break, particularly when manhandled on the benches.

We have some 10,500 moulds in use at any one time and many more in store ready to replace worn out items or customers different requirements. These are all made in our own mouldmaking department.