Mould Making

Plaster moulds used for the reproduction of items in resin, wax or latex are either open moulds for straightforward pouring or enclosed (piece) moulds comprising two or more interlocking pieces used for swilling or casting.

Flexible moulds, usually backed with plaster supporting-cradles and known as ‘case’ moulds are used for laying up glass-fibre objects or for casting complicated items where there are problems connected with undercuts or detail that could not be withdrawn from an inflexible plaster mould.

How to make a two-piece mould

Place the master to be moulded on a flat board and surround half of it with clay. Finish the edge of the clay cleanly and uniformly around the master to provide a firm support for the top half of the mould.

Using a cup, pour the freshly mixed plaster over the item until a thin coat has completely covered the surface. Repeat this action several times until a thick and uniform coat has been built up over the master.

Endeavour to flatten the top of the mould with a tool as the pouring progresses, so that it can stand steadily when you turn it over for the next operation. Allow plaster to harden and clean tools and utensils.

Invert the mould and remove all the clay. Do not disturb the master embedded in the first half of the mould.

Using a sharp knife, cut three tapering ‘vee’ slots in the edge of the mould. These will act as registers when the two halves come together for moulding.

Using a diluted clay wash, paint the edge where the second half will meet the first.

Repeat the first pouring operation until the second half has built up.

Allow plenty of time for plaster to harden then separate the mould and remove the master item.

Cut a pouring hole in the edge of one of the halves where it will not mar the reproduction.

Keep the halves of the mould together with adhesive tape or string and write on the outside what it is.

MOULD MAKING

1. First half-mould
The master is embedded in a block of clay and plaster is poured over it to form one half of the mould, a, Plaster, b, Master, c, Clay.

2. Preparing the mould
The poured half of the plaster mould. The clay has been removed and the master now lies embedded in plaster. Location notches are cut, and a clay plug formed for the pouring hole, a, Clay plug, b, Location notches.

3. Second half-mould
Plaster is poured on to a clay wash on the first half and flows into the locator notches to form an interlocking second mould, a, Clay plug, b, Clay wash.