Page 47 - Plastics News October 2018

FEAturES

it is to temperature. Therefore, a quicker fill rate is more is by undertaking injection moulding simulation. By doing
effective for filling and packing the cavity. this the correct configuration can be determined before
any metal is cut and minimising the trial and error of mould
Cycle times will always be shortened to increase
productivity. Cooling off the polymer melt quickly is one tool commissioning. For fibre reinforced engineering
way to achieve this. It is also an easy way to achieve materials the location of the gate is critical in terms of
dimensionally accurate parts, at least for a short period the orientation of fibres. Changes in geometry and wall
of time after de-moulding. Yet, thermoplastics do not like thickness disrupt the shear in the polymer melt and fibres
to be rushed and prefer to be cooled at their own rate. tend to align only in the direction of high shear stress.
Non-uniform cooling rates lead to differential shrinkage As a result fibres may not be aligned in the direction
which in turn can lead to dimensional inaccuracies or high required for maximum strength and shrinkage may not
residual stress. If the mould temperature is low and the be as expected.
gate size small then the gate will freeze-off very quickly Overall, by being considerate to the behaviour of a
leaving the melt in the cavity isolated and free to shrink. thermoplastic during injection moulding it is possible to
If this is the case and the moulding cycle has a long holding produce very high quality and durable components for
phase at high pressure, all that will be achieve is a nicely demanding applications. This can be achieved through
packed out runner and a lot of wasted energy. the use of moulding simulations, effective specification
sand quality checks and independent audits of the
Tool configurations can also introduce a wide variety
of features which can have a dramatic effect on part process. In the majority of cases the applications are
performance. The effect of gate location on the ability not so demanding that it is possible to get away with poor
to fill parts and weld line locations are often all that is quality mouldings. The downside can be that the optimally
considered. Gate sizing has a big influence on the shear moulded component may cost marginally more to produce.
rate of the polymer melt and its resulting viscosity. The But this may be a cost worth bearing in comparison with
best way to evaluate the effect of gate size and location the total cost of product failure due to shortcomings in
the moulding process.

October 2018 Plastics News 2018 Plastics News
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