The
most obvious difference between micro moulding and injection moulding
is the size of the components and the accuracy with which they are
produced. Sovrin's new Battenfeld system has a maximum shot volume
of approximately one cubic centimetre of plastic and can produce
components weighing as little as 0.0008g with dimensional tolerances
of as little as a few microns. The system is specifically designed
to work with such a small amount of material, offering for example
a large reduction in sprue weight. Tool design is another area in
which micromoulding has its own set of demands. 'Part of the investment
programme is to re-equip the toolroom to make moulds for micromoulding,'
says Wigmore. 'We still need to understand the concepts of toolmaking
for micromoulding and have appointed a specific tool designer to
become a specialist in this area.'
Another important area of production in making micromouldings is
the inspection and handling of he finished components. The Battenfeld
system incorporates an automatic quality control system that optically
inspects and sorts the parts. 'This inspection and handling on the
machine is a big part of the process,' says Joiner. 'The checking
of each part, measurement and statistical analysis is very important
in ensuring the customer gets 100% correct parts.' Errors that might
be spotted with the naked eye on regular injection mouldings can
easily go undetected on micromouldings, such as a missing tooth
on a gear wheel. Careful handling of micromouldings is also important
to prevent the miniature details of the parts jamming or linking
together when packaged. The Battenfeld system overcomes this problem
with an automatic handling system that deposits individual parts,
each oriented in the same direction, in a kind of blister pack roll.
This could then be used for automatic delivery of parts during downstream
assembly.
At first glance, the material used for micro mouldings might seem
like the least significant part of the technology, with processing,
tooling and inspection all more specific to this new field. Certainly
the cost of the material is insignificant, given the amount used.
'The maximum shot on the machine is 1cc, so a 25kg bag of plastic
goes an awful long way,' says Wigmore. For this reason, Sovrin is
using mainly relatively expensive engineering polymers such as LCPs,
PPS, PS and so on at the moment. There are no commodity polymers
being used,' explains Wigmore. 'There are lots of good thermoplastics
around that produce very good regular products. The company hasn't
experienced any problems with the plastics it has used so far.'
That's not to say that micro mouldings couldn't throw up some problems
owing to their small size. There aren't any specifications or data
sheets available for materials when used in components with the
tiny dimensions of micromouldings, and it might be that the physical
properties of the plastics are somewhat different. Research is under
way at the IRC in Polymer Science at the University of Bradford,
which is also using a Battenfeld machine, to determine what happens
to polymer properties during the moulding process.
