Polymeric Composite Resins in Injection Moulding - Introduction to Polymeric Composites with Rice Hulls - page 133

Some of the common polymer resins used by injection moulders are: polyethylene
(PE); polypropylene (PP); polyvinyl chloride (PVC); polystyrene (PS); acrylonitrile-
butadiene-styrene; nylon and many others, including recycled materials. However,
PCRH are offered as composite resins and the technology is quite new, so this chapter
will discuss only composite resins made with PE and PP as the most popular composite
productions on the market Later on in this chapter I will present a practical model
with specifics on injection moulding with polymeric composite resins based on PE or
PP as the matrix and rice hulls as the constituent reinforcement.
11.4 Injection Moulding Process
Injection moulding is primarily done with thermoforming (soften on heating and
can be reused) polymers. However, smaller volumes of thermosetting (harden on
heating and cannot be re-used) polymers, biodegradable plastics, and elastomers are
also subjected to this process. There are several advantages in the injection-moulding
process to produce a plastic part: high production volumes; little need to finish
moulded parts; costs are lower than other processes despite the initial high cost of
tooling. Injection moulding can be used to produce very complex parts and is often
used as the preferred method if other processes are not practical. Other advantages of
the injection-moulding process include efficient use of materials and energy, precision-
formed parts and a negligible amount of waste generated that can go back into the
production cycle.
Once the mould in two halves is mounted carefully on the platens on opposite sides, the
set-up technician will test and synchronise its opening and closing by slow movement
of the large operating door that controls this movement. The injection-moulding
machine hopper is then filled with the polymer resin. The hopper will have a 'throat-
gate' or valve that will give access to the screw barrel on which it is mounted. The
hopper is designed to allow gravity feed of the polymer pellets or granules freely into
the barrel. However, sometimes it is important to have a uniformity of resin pellet size
and absence of contaminants to prevent constriction at the 'throat' of the hopper to
ensure a smooth and continuous flow of material into the heated barrel, which has
zonal heating for precise control of temperature. Modern machines have 'closed-loop'
operations (i.e., the machine works to pre-set parameters and fluctuations will be
negated within pre-set tolerances).
Plastic pellets will be subjected to heat for softening and the screw produces a hot
homogenous melt that is carried along the screw and injected under pressure into an
opening in the closed mould via an injection nozzle. The quantity of plastic material
to be injected into the mould is pre-calculated and controlled by a 'cushion' (pre-set
gap between the tip of the nozzle opening and the tip-end of the screw). This ensures