Process considerations arise out of the realisation that what appears at the
outset as simple process is actually a complicated procedure demanding careful
attention to a number of interdependent details.
Obviously the application of heat is required to melt the plastic in the mold. Electrically heated air
is convenient, but slow and expensive. Radiant heat from infra-red heating elements has been used, but
the systems are limited to simple shapes because recesses and shaded areas cannot be heated at the same
rate. The latter systems are generally not used these days. Most rotational molding machines today are
operated with gas heated forced hot air.
Open-flame units are still in use where the shapes are simple, and they have the advantage of a lower
initial cost, but the production time is greater in comparison with the enclosed oven, sometimes in the
order of three times the cook time, and therefore a much greater fuel cost. The simple "rock and roll"
machine (it rolls on one axis and only rocks backwards and forwards on the other) is still the machine of
choice for very long narrow products.
The temperature and cook times of the product varies from mold to mold. Experience will indicate the
when the correct time is being used. Too short a heating cycle and there may be unused powder in the
cavity with the corresponding reduction in thickness of the wall sides. Excessive cook times or temperatures
will result in the long molecular chains being damaged or compromised with a resultant reduction in strength
and useful life of the product.