Rotational casting is basically a method for manufacturing hollow products, although the final product need not be hollow. Some final operations may reform the product, and the method therefore competes effectively with injection moulding and blow moulding.
The technique was initially developed for commercial use with liquid PVC in the 1940s. In the beginning the growth of this process was hampered by the perception that it was a relatively slow casting method with a limited range of materials and casting opportunities. The great breakthrough did not occur until the end of the 1950s, when various types of polyethylene were developed for rotational casting.
Throughout the 1960s and 1970s, more types of polyethylene became available and it was discovered that also other types of plastic were "rotational casting friendly". This led to a stable growth in the rotational process, and interest has further increased over the past decade.
Designers manufacture moulds that are increasingly sophisticated, suppliers of materials have developed types that are specially adapted to the process and machine developers have improved efficiency of the equipment and developed modern process control systems. Currently, the growth rate is conservatively estimated at about 6-8% per annum.
Rotational casting differs from other process methods in that the heating, melting, moulding and cooling stages take place after the plastic has been placed in the mould. Also, no pressure is applied during the casting apart from gravitation.
The advantages of this are:
- Rotationally cast products are virtually without tensions.
- There are no welding seams.
- Large products can be manufactured at low costs.
- Tool expenses are relatively low.
- Designers have great freedom relating to design.
- The process is under-automated - often manual work when feeding materials, de-moulding and fashioning.
- It is a relatively labour-intensive process with higher material costs due to the fact that
- Pellets must be ground to a powder
- Better stabilising is required due to a longer exposure to heat in the furnace.