Flexible Moulds

Although fibreglass mouldings and plaster casts can be produced from plaster piece-moulds, it is more convenient to cast them in moulds that are flexible. These need fewer sections and allow the casting to be withdrawn without danger of breakage or the trapping of undercuts. Because they are flexible each section must be encased in a thick layer of plaster, which will act as a support. Thereafter the two elements should never be separated.

Most articles will remove cleanly from a two-piece mould, but complicated shapes may demand more.

Hot melt

A proprietary material (Vinamold) which requires to be melted in special temperature-controlled heating equipment is poured over an object and allowed to cool. If the object is made of plaster it must be thoroughly dried and coated with a sealant.

Vinamold retains its shape and flexibility for a very long time and can be used to cast dozens of objects without loss of detail. After use it can be re-melted.

Small amounts of Vinamold may be very slowly heated in a saucepan, but the operation must be closely monitored.

Silicone rubber

This material, which cannot be reused, consists of a two-part mix in which a catalyst is added to the body material prior to pouring. Slowly the catalyst works and the liquid turns to a rubber-like substance. It retains its shape and detail and can be stored for long periods.

When poured (as with similar moulding materials) it should not be allowed to flow over the master, but should be introduced in such a fashion that it rises upwards, enveloping the master from below. This ensures that the material will enter undercuts without trapping air. Deep undercuts may require the master and its case to be tilted.

Silicone rubber sections must be encased in plaster.

Latex

The tough white latex (page 44) used for the production of flexible props may be employed for the making of moulds, but is less versatile than silicone rubber.

Registration

The plaster cases which stabilise flexible moulds should have ‘vees’ and ‘notches’ along their edges to provide a register.

FLEXIBLE MOULDS

Flexible rubber or other materials used for casting must be supported by plaster case moulds. Although a corner is shown being lifted here, it is unwise ever to fully separate the two.