Agriculture Reference
In-Depth Information
11
Polymeric Composite Resins in
Injection Moulding
11.1 Introduction to Injection Moulding
The plastics industry uses many processes to convert plastic resins into plastics
products. Some of the popular ones are extrusion, blow molding, casting, and
compression moulding but the most popular is injection moulding . The basics of
injection moulding are introduction of a plastic resin into a heated barrel that contains
a screw or a plunger (used in older machines). The hot melt is pushed forward into
a nozzle of the injecting barrel and then forced by pressure into a closed mould in
two halves. The mould may be single or multi-cavity and is designed in such a way
so that the hot plastic melt flows freely into the closed mould and fills all the cavities.
The mould is then cooled for a few seconds, the mould opens, and the moulded parts
ejected. The cycle is repeated manually or automatically. Most large injection-moulding
machines are operated horizontally but vertically operated machines for small parts
(especially with inserts) are also used.
In the conversion of polymeric composites with rice hulls (PCRH) to final products,
extrusion holds sway over other methods but injection moulders, realising the
potential of these new composite resins, have started to use them widely. Injection
moulding will soon be a vital sector using these composite resins, gradually replacing
traditional petroleum-based polymer resins, so sound knowledge of injection moulding
will greatly help in understanding how to use these composite resins to obtain the
maximum benefit.
11.2 Injection Moulding Machine
From the early days when a material called a 'plastic' was created, the machines that
processed these materials were injection machines that were hand-operated. The
moulding principle has remained more or less identical over a decade of processing
but from this humble operation injection-moulding machines have made great strides
with screw feeds (instead of a plunger), automatic or semi-automatic operation and
high-tech electronics. Modern machines operate on hydraulic systems or electric
power and hybrids are also an option. The availability of 'closed-loop' automatic
temperature and other controls on most machines is a big boost for operators and
moulders.
 
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