Page 302 - Plastics Engineering

Processing of Plastics 285

In the hydraulic system, oil under pressure is introduced behind a piston
connected to the moving platen of the machine. This causes the mould to close
and the clamp force can be adjusted so that there is no leakage of molten plastic
from the mould.
The toggle is a mechanical device used to amplify force. Toggle mechanisms
tend to be preferred for high speed machines and where the clamping force is
relatively small. The two main advantages of the toggle system are that it is
more economical to run the small hydraulic cylinder and since the toggle is
self locking it is not necessary to maintain the hydraulic pressure throughout
the moulding cycle. On the other hand the toggle system has the disadvantages
that there is no indication of the clamping force and the additional moving
parts increase maintenance costs.

4.3.3 Moulds
In the simplest case an injection mould (or ‘tool’) consists of two halves into
which the impression of the part to be moulded is cut. The mating surfaces
of the mould halves are accurately machined so that no leakage of plastic
can occur at the split line. If leakage does occur the flash on the moulding
is unsightly and expensive to remove. A typical injection mould is illustrated
in Fig. 4.35. It may by seen that in order to facilitate mounting the mould in
the machine and cooling and ejection of the moulding, several additions are
made to the basic mould halves. Firstly, backing plates permit the mould to
be bolted on to the machine platens. Secondly, channels are machined into the
mould to allow the mould temperature to be controlled. Thirdly, ejector pins
are included to that the moulded part can be freed from the mould. In most
cases the ejector pins are operated by the shoulder screw hitting a stop when
the mould opens. The mould cavity is joined to the machine nozzle by means of
the sprue. The sprue anchor pin then has the function of pulling the sprue away
from the nozzle and ensuring that the moulded part remains on the moving half
of the mould, when the mould opens. For multi-cavity moulds the impressions
are joined to the sprue by runners - channels cut in one or both halves of
the mould through which the plastic will flow without restriction. A narrow
constriction between the runner and the cavity allows the moulding to be easily
separated from the runner and sprue. This constriction is called the gate.
A production injection mould is a piece of high precision engineering manu-
factured to very close tolerances by skilled craftsmen. A typical mould can be
considered to consist of (i) the cavity and core and (ii) the remainder of the
mould (often referred to as the bolster). Of these two, the latter is the more
straightforward because although it needs to be accurately made, in general,
conventional machine tools can be used. The cavity and core, however, may
be quite complex in shape and so they often need special techniques. These
can include casting, electro-deposition, hobbing, pressure casting, spark erosion
and NC machining.