Traditionally, high volume metal and plastic or rubber component
assemblies used in products such as pneumatic valves and cylinders, pumps
and motors, have been constructed from a combination of materials that
are bonded, welded or riveted together, writes Bob Parker of seal
solutions specialist, Ceetak. While these construction techniques are
effective, they can add considerably to component cost and weight.
Moreover, the bonding process can create a number of problems: during
assembly, with the need to purge all air pockets from the adhesives, and
when in use where sudden changes in operating temperatures or pressures
can result in differential expansion of the various materials.
New developments in multi-shot plastic moulding techniques, however, have
overcome these technical issues, at the same time improving component
performance and quality, and reducing unit price. Multi-shot injection
moulding techniques optimise the co-polymerisation characteristics of
hard and soft plastics, with two or more thermoplastic and TPE
(thermoplastic elastomer) rubber compounds being molecularly bonded using
conventional injection moulding machines.
There have been significant improvements in both mould tolerances and the
level of control over process conditions. As a result, it is now possible
to produce components by injecting hard plastic materials, then rotating
the mould tool or using multiple tools or mould cavities to make further
or simultaneous injections of TPEs to create single, integrated piece
parts. These parts can be made in large volumes, to consistent quality
standards and tight tolerances.
A typical example is replacement parts for pneumatic valves, which are
often produced from brass and rubber; by re-engineering the part using
thermoplastic and TPE rubber compounds it is possible to cut costs by as
much as 50%-80%, while maintaining identical standards of quality and
producing better mechanical properties.