Page 336 - Plastics Engineering

Processing of Plastics 319
independent so that if cooling is completed then that arm can leave the cooling
bay whilst the other arms remain in position.
It is important to realise that rotational moulding is not a centrifugal casting
technique. The rotational speeds are generally below 20 rev/min with the ratio
of speeds about the major and minor axes being typically 4 to 1. Also since all
mould surfaces are not equidistant from the centre of rotation any centrifugal
forces generated would tend to cause large variations in wall thickness. In fact
in order to ensure uniformity of all thickness it is normal design practice to
arrange that the point of intersection of the major and minor axis does not
coincide with the centroid of the mould.
The heating of rotational moulds may be achieved using infra-red, hot liquid,
open gas flame or hot-air convection. However, the latter method is the most
common. The oven temperature is usually in the range 250-450°C and since
the mould is cool when it enters the oven it takes a certain time to get up to a
temperature which will melt the plastic. This time may be estimated as follows.
When the mould is placed in the heated oven, the heat input (or loss) per
unit time must be equal to the change in internal energy of the material (in this
case the mould).
(4.42)

where h is the convective heat transfer coefficient
A is the surface area of mould
To is the temperature of the oven
T, is the temperature of the mould at time t
p is the density of the mould material
C, is the specific heat of the mould material
V is the volume of the walls of the mould
and t is time
Rearranging this equation and integrating then

(4.43)

where Ti is the initial temperature of the mould and fi is the surface area to
volume ratio (AIV).
This equation suggests that there is an exponential rise in mould temperature
when it enters the oven, and in practice this is often found to be the case.