INJECTION MOULDING
Heart of glass
UK engineers develop injection moulded, glass-fibre reinforced
plastic ‘electro-bike’ for commuters. Siobhan Wagner reports
A LIGHTWEIGHT commuter bicycle
with a difference has been developed in
the UK using injection moulded, glass-
fibre-reinforced plastic parts.
38
The GoCycle can be pedalled like a
traditional bicycle until the rider hits a
button that revs up a high-powered
electric motor in the front hub. It can
travel at full legal urban driving speed
for about 12 miles before needing a
re-charge.
It is based on a magnesium frame,
but vital mechanical components, such as
the rear suspension unit, are made from
nylon filled with long, glass fibres. The
strong, lightweight material, which is 60
per cent glass, was injection moulded by
Protomold of Shropshire.
Managing director John Tumelty said
it was the first time Protomold’s senior
engineers worked with a material that
was as much as 60 per cent glass.
The engineers were unsure how
the material would flow into the cavity of
the injection-moulding tool. ‘Glass-filled
resins are generally more prone to
warp problems,’ said Tumelty. ‘The
higher the glass content, the worse it
can be.’
Protomold worked with Karbon
Kinetics, the UK designer of the bike, and
studied the CAD geometry of the parts
that needed moulding. Protomold used
this information to create a visual predic-
tion of the injection moulding process
with its in-house flow analysis software
Protoflow.
The engineers experimented with
different locations for the gates, the
points where the plastic enters the
cavity, and they studied the various
temperature and pressure conditions
needed to mould the parts. Eventually
the company was able to develop 47
different injection-moulding tools to
create all the parts.
The bike was designed to be durable,
which meant the Protomold engineers
designed tools that would distribute the
flow of material evenly without creating
weak spots.
It was also designed to be sleek and
The bike was
designed to be
durable, so
Protomold’s
engineers
designed tools
that would
distribute the flow
of material evenly
without creating
weak spots
‘If you get the moulding process wrong with
very high glass content, glass fibres will
rise to the top of the part’ John Tumelty, Protomold
stylish — something that would attract
the urban commuter.
The majority of the bike’s
components are a shiny black, and
Protomold’s engineers had to discover
the exact speed to inject the material
into the cavity to maintain colour
consistency.
‘On aesthetically critical
components you could end up with
literally changes in colour or changes
in texture as a result of sudden
acceleration,’ said Tumelty.
‘If you get the moulding process
wrong with very high glass content,
glass fibres will rise to the top of the
part so you’re actually looking at black
plastic through glass fibre. It’s an
odd silvering effect that’s not very
attractive.’
While the UK teams were working
on the mechanical parts of GoCycle, the
magnesium frame was being manufac-
tured by a Canadian firm using a
process previously unknown to bicycle
production. Called thixomolding, a type
of injection moulding for metal, the
technique is typically used for automo-
tive parts, creating magnesium pieces
that are thin walled and very strong.
GoCycle is being assembled by Ideal
Bicycle Co of Taiwan which plans to
launch it commercially in Europe this
year.
Tumelty said the bike, which is
lightweight and folds up easily, could
be a big hit with town and city
commuters.
‘It’s not only fun, but it’s also hugely
practical,’ he said. ‘It’s got a range of
several miles and if you’ve got a
five-mile commute to work and you
don’t want to get too hot in your best
suit, it’s perfect.’
the EnGIneeR 21 APRIL–4 MAY 2008
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