The majority of moulds for rotational moulding are currently made from metal, usually sheet steel or aluminium or coast aluminium. The moulds are quite thin, shell like structures which are designed to allow the heat source to quickly transfer heat through the metal into the powder.
Large, simple moulds are generally made from fabricated metals. For highly detailed parts where a high quality finish is necessary, either cast metals or nickel electroformed moulds are used.
The process doesn't use pressure to form the part, so the mould itself needs only to have enough strength to support itself. As the material cools, it naturally shrinks away from the surface of the mould and mould design must take this into account to create an accurately size final part.
Most moulds are manufactured in two pieces, however for complex designs three and four piece molds can be used. The area where the pieces of the mould connect to each other is known as the parting line and it can include a complex curvature. The parting line is vital because the mould sections must remain tightly clamped together during the heating and cooling of the part. Careful handling of the mould is necessary to ensure the parting line is not damaged.
Moulds are usually mounted to frames in halves to allow them to be placed on the machine. In some cases multiple moulds will be placed on the same frame, known as a spider. This can provide substantial savings in demoulding and charging the moulds, reducing cycle times.