Page 339 - Plastics Engineering

322 Processing of Plastics

Fig. 4.61 illustrates that the mould temperature is quite different from the set
oven temperature (330°C) or indeed the actual oven temperature, throughout the
moulding cycle. An even more important observation is that in order to control
the rotational moulding process it is desirable to monitor the temperature of
the air inside the mould. This is possible because there is normally a vent tube
through the mould wall in order to ensure equal pressures inside and outside the
mould. This vent tube provides an easy access for a thermocouple to measure
the internal air temperature.
The internal air temperature characteristic has a unique shape which shows
clearly what is happening at all stages throughout the process. Up to point
A in Fig. 4.61 there is simply powder tumbling about inside the mould. At
point A the mould has become sufficiently hot that plastic starts to melt and
stick to the mould. The melting process absorbs energy and so over the region
AB, the internal air temperature rises less quickly. It may also be seen that the
temperature of the mould now starts to rise less quickly. At B all the plastic has
melted and so a larger proportion of the thermal energy input goes to heating
the inner air. This temperature rises more rapidly again, at a rate similar to that
in the initial phase of the process.
Over the region BC the melt is effectively sintering because at B it is a
powdery mass loosely bonded together whereas at C it has become a uniform
melt. The value of temperature at C is very important because if the oven
period is too short, then the material will not have sintered properly and there
will be an excess of pin-holes. These are caused where the powder particles
have fused together and trapped a pocket of air. If the oven period is too long
then the pin-holes will all have disappeared but thermaYoxidative degradation
will have started at the inner surface of the moulding. Extensive tests have
shown that this is a source of brittleness in the mouldings and so the correct
choice of temperature at C is a very important quality control parameter. For
most grades of polyethylene the optimum temperature is in the region of 200°C
43°C.
Once the mould is removed from the oven the mould starts to cool at a rate
determined by the type of cooling - blown air (slow) or water spray (fast).
There may be a overshoot in the internal air temperature due to the thermal
momentum of the melt. This overshoot will depend on the wall thickness of
the plastic product. In Fig. 4.61 it may be seen that the inner air temperature
continues to rise for several minutes after the mould has been taken out of the
oven (at about 13.5 minutes).
During cooling, a point D is reached where the internal air temperature
decreases less quickly for a period. This represents the solidification of the
plastic and because this process is exothermic, the inner air cannot cool so
quickly. Once solidification is complete, the inner air cools more rapidly again.
Another kink (point E) may appear in this cooling curve and, if so, it represents
the point where the moulding has separated from the mould wall. In practice this
is an important point to keep consistent because it affects shrinkage, warpage,