Manufacturing defects in polymeric medical devices - Durability and Reliability of Medical Polymers - page 226

4

3

Polystyrene

2

Polyvinyl
acetate

1

0

Polyisobutylene

Polydimethyl
siloxane

-1

-2

2 3 4

Log N B

10.4 Effect of molecular weight or chain length on melt viscosity.

10.2.2 Extrusion
The process of injecting a stream of molten polymer through a die of constant
section is known as extrusion, and it is generally simpler than injection
moulding. Pipe, sheet and profiles are made using the method, but there are
several features in the final product that can be deleterious. Perhaps the most
important is the equivalent of weld lines, so-called 'spider lines', which are
aligned along the axis of a pipe, for example. They are formed by the internal
metal supports for the die head, where the melt divides before reforming.
if the melt is too cold, then reformation is poor, so leaving lines along the
extrudate. They can be seen clearly in the section of a pipe in Fig. 10.5.
Figure 10.5 also shows another problem encountered in all processes:
poor mixing of ingredients, in this case carbon black in polyethylene. To
achieve the best product strength, such fillers must be mixed to make a
uniform material, combining both high dispersion and an even distribution
of particles.
Owing to the simplicity of extrusion, polymers of higher molecular weight
can be used, so extruded products tend to be stronger than moulded products.
The problem of residual strain is also usually much reduced in extrudates.

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