Page 46 - Plastics News October 2018

FEAturES

Why understanding processing is vital for success

Andrew Hulme

Injection moulding is a process which accounts for a huge range of different plastic components which are
manufactured globally. As a process suited to automated mass production, understanding the materials,
process and set-up are key to its success.

ith many plastic products a vast amount of time and may be that the component is of low value and is simply
Wresources go into the design and development phase, discarded by the consumer when it fails. However, for
to ensure that the product will meet its requirements and high value and safety critical components the potential
perform for its expected life. However all of this effort can costs of failure can be very large in terms of recalls and
be wasted when the product moves to the manufacturing loss of market share.
stage.
Processing and Material Behaviour

Unfortunately, with injection moulding there is a distinct
lack of understanding surrounding the effects of processing
on material behaviour. The fundamental behaviour of
thermoplastics is commonly overlooked due to this lack
of knowledge. Simply passing the manufacturing of a
component over to a moulder can be a recipe for disaster.
In some cases corners are cut to increase profitability,
other times the moulder may be inexperienced with the
material and not have the required equipment. However,
with some basic knowledge and independent assistance
a more robust product can be manufactured. Drying of
granules prior to moulding often seems unnecessary,
but excess moisture will create steam within the melt
leading to voids or silver streaking. For some polymers
the effects of moisture during moulding is far more
serious as hydrolytic degradation occurs, resulting in a
brittle moulding. In the case of hydrolytically sensitive
polymers the drying regime recommended by material
suppliers should be adhered to e.g. using desiccant or
vacuum driers.

Excessive heating of a polymer melt increases the rate
of thermal degradation, which again will lead to a brittle
moulding. Wrong machine sizing can give an over-long
residence time in the barrel, stabilisers within the
polymer are consumed and the polymer degrades When
From experience, poor processing has been a significant a moulding appears difficult to fill due to a requirement
factor in at least 40% of all product failures. This may cause for high pressure, many fall into the common trap of
immediate problems with components that are identified increasing temperatures to reduce the melt viscosity
at an early stage, before they are released on the market. and injecting more slowly. Consequently the part now
In the worst case the potential for failure remains unseen appears easier to fill but the polymer had degraded. The
and occurs after a period of time and during service. It viscosity of a polymer melt is more reactive to shear than

48
Plastics News October 2018ews October 2018
Plastics N 46