Polymeric Composite Resins in Injection Moulding - Introduction to Polymeric Composites with Rice Hulls - page 137

11.7 Recommended Processing Guidelines
Injection-moulded finished products with these PCRH, if moulded appropriately
with correct temperatures, speeds and a non-resistant flow path, will exhibit minimal
stress, smooth surfaces, uniform colour distribution and no evidence of gassing. The
two most important principles to remember if moulding with these composite resins
are to avoid excessive heat and shear stress.
Some people may think that the rice hulls in composites will act as inhibitors but
tests have shown that actually the reverse is true. For example, rice hull-propylene
composites low very quickly at relatively low temperatures and pressures, as a result
of which injection moulders can achieve significant energy savings. An added feature
is shorter cycle times and higher productivity due to the reduced mould filling and
cooling times (less polymer content).
Temperature guidelines for moulding with PCRH resins are:
• Rear zone: 171.1-187.7 °C
• Middle zone: 182.2-198.8 °C
• Front zone: 193.3-210 °C
• Nozzle tip: 198.8-210 °C
Injection moulders have to adjust these parameters if using different grades to achieve
the best results. Moulding pressures will depend on the part design as well as the runner
system and gate design. In principle, moulding with these composites will require
less pressure than moulding with traditional materials. However, moulders should
be careful with mould-filling speeds. The hot material will tend to flow quickly, but
avoiding excessive shot fill times is important because these materials are sensitive
to shear stress. If streaking occurs, it can be remedied by slowing down the injection
rate. Given that lower temperatures are suitable for moulding, hold times are often
lower than for standard materials.
The nozzle tip used in processing these composites should have an orifice as close as
possible to the diameter of the sprue to minimise shearing. Smaller orifices may cause
increased shear as well as discolouration resulting from overheating of the material
as it enters the mould. Injection-moulded parts with PCRH resins are 'natural' in
colour with a very light-brown tone and uniform grain. However, they can be easily
coloured and also have a matt or high-gloss finish as desired.