Page 298 - Plastics Engineering

Processing of Plastics 28 1

However, nowadays this type of machine is seldom used because it is
considerably more complicated and more expensive than necessary. One area
of application where it is still in use is for large mouldings because a large
volume of plastic can be plasticised prior to injection using the primary cylinder
plunger.
For normal injection moulding, however, the market is now dominated by
the reciprocating screw type of injection moulding machine. This was a major
breakthrough in machine design and yet the principle is simple. An extruder
type screw in a heated barrel performs a dual role. On the one hand it rotates
in the normal way to transport, melt and pressurize the material in the barrel
but it is also capable, whilst not rotating, of moving forward like a plunger

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Fig. 4.31 Spica1 cycle in reciprocating screw injection moulding machine