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To produce thin walled products in a precise and stable manner, FANUC have developed electric injection moulding machines driven by a rotary or linear motor to meet the particular features required for moulding products such as wall thickness, or type of plastic material. In this paper, the features necessary for each type of injection unit are introduced together with moulding examples.21615
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Introduction
As a result of the increasing development of the IT
industry, the demand for small and light or ultra precision
products is increasing. This trend is also significant in the
plastic moulding industry, and the development of
thinner or higher precision products continues.
Typical plastic products include tiny memory chip
holders, or battery cases for mobile phones or digital
cameras. To produce such thin moulded products, a
‘total technology’ – combination of mould, material and
injection moulding machine technology – is required.
In particular, for the injection moulding machine, a high
speed and high acceleration capability is required to inject
the mould cavity with molten material in very short fill times.
Basic injection moulding machine
performance required to produce thin
walled products
In injection moulding, injected molten material forms a
skin along the mould surface by cooling, and this results
in blocking of the material flow.
In the case of thin walled products, the skin forms
quickly because of the narrow cavity, so it is necessary to
fill the cavity in a very short time. Furthermore, in the
latter phase of the injection process, high pressure is also
necessary to pack the cavity through the narrowing flow
channel in the cavity caused by skin formation.
Figure 1 shows injection speed and injection pressure
for the injection stage in the case of moulding of a
cellular phone battery case. As mentioned before, the
injection pressure increases rapidly with time in the
injection stage. Therefore, the injection moulding
machine has to have a high injection speed and a high
acceleration injection capability, so that shorter injec-
tion times can be achieved accurately. Furthermore, it
can achieve relatively easy filling by increased shear rate
and hence lower polymer viscosity.
One approach to attempt to achieve the required high
injection speed and cavity filling is to use a hydraulicinjection moulding machine with an accumulator, which has
explosive power generated by releasing accumulated hydrau-
lic pressure. However, this was found to have a problem with
repeatability because of inconsistency of speed or pressure
influenced by hydraulic oil temperature variation.
To resolve the issue, as a replacement for the
hydraulic machine, a high injection speed and high
acceleration injection capability electric injection mould-
ing machine is required. This has been the most popular
choice for the high precision moulding field.
Classification of thin walled parts and
injection performance requirement
In general, there is no specific definition of thin walled
moulding products by wall thickness. However, thin
walled processing can be classified in practice into two
groups, by recommended injection speed. Products such
as a cellular phone battery cells or light guide panels,
with 0.
2to0.
4 mm wall thickness can be classified in the
first group, where 500 to 800 mm s
–1
injection speed and
an injection pressure of 3000 kgf cm–2
or more are
required. The tiny memory or speaker cone with wall
thickness 0.
2 mm or less can be classified in the second
group. To produce such thin walled moulding products,
injection speeds of 1000 mm s