Page 338 - Plastics Engineering

Processing of Plastics 321

Fig. 4.61 illustrates typical temperature profiles during the rotational moulding
of polyethylene. With typical values of oven temperatures and data for an
aluminium mould
To = 300"C, Ti = 30°C, T, = 20°C

h = 22 W/m2K C, = 917 Jkg K, p = 2700 kg/m3

350

300 T, - Inside surface of mould
T, - Air inside mould
250

E 200
E
I- ls0
100
50

0
0 5 10 15 20 25 30 35 40 45
Time (min)

Fig. 4.61 Temperature profiles during rotational moulding

then for an aluminium cube mould 330 mm side and 6 mm thick, as was used
to produce Fig. 4.61 then
To - Tt -2700 x 917 330 - 220
r=*loge{-}= To - Ti lo00 x 22 loge { 330 - 30 }
Bh
r = 1.9 minutes
For a steel mould of the same dimensions and thickness, a quick calculation
(h = 11 W/m2K, C, = 480 Jkg K and p = 7850 kg/m3) shows that the steel
mould would take three times longer to heat up. However, in practice, steel
moulds are less than a third of the thickness of aluminium. Therefore, although
aluminium has a better thermal conductivity, steel moulds tend to heat up more
quickly because they are thinner.
It is important to note that the above calculation is an approximation for
the time taken to heat the mould to any desired temperature. Fig. 4.61 shows
that in practice it takes considerably longer for the mould temperature to get
to 220°C. This is because although initially the mould temperature is rising at
the rate predicted in the above calculation, once the plastic starts to melt, it
absorbs a significant amount of the thermal energy input.