Biomedical Engineering Reference
In-Depth Information
10
4
Polypropylene
copolymer (MFI 4) at 230 °C
10
3
PBT at 240 °C
Nylon 66 at 285 °C
10
2
10
1
Injection moulding range
10
0
10
1
10
2
10
3
10
4
10
5
Shear rate (
g
·
) s
-1
)
10.3
Effect of increasing shear rate on polymer melt viscosity.
The melt viscosity below entanglement is proportional to molecular
weight:
h
=
k
M
w
[10.5]
However, when the chains start to entangle with one another, the melt
viscosity rises very steeply according to a power law:
h
=
k M
w
3.4
[10.6]
The influence of the two equations is illustrated for some polymers by
Fig. 10.4, where the molecular weight is now plotted in terms of the number
of atoms in the backbone chain
N
B
. This is why injection moulding grades
of polymers tend to be chosen near to the entanglement molecular weight
in order to minimise melt viscosity. However, the tensile strength lies at the
lower end of expectations, and is sensitive to any mechanism that cuts or
degrades chains. it might only need a few single-chain scissions to lower
the molecular weight locally to below entanglement, thus causing the tensile
strength to drop dramatically and a brittle crack to be initiated.
injection moulding is the most sophisticated moulding technique, and
the tools are an expensive part of the process, their cost being determined
by their complexity. Production runs must be long to justify their cost, and
there are various ways of increasing the rate, by using multicavity tools for
example.
1, 2
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