Manufacturing defects in polymeric medical devices - Durability and Reliability of Medical Polymers - page 223

Close
mould

Inject

Remove
moulding

Open mould

Cool

10.2 Injection moulding cycle with cooling the largest component.

conductivity. Many polymers must be cooled slowly so as to relieve internal
strains and stresses that can result from quenching, otherwise, a seriously
defective product can be created. where holes are needed in a product, the
flow of the melt has to divide, creating the problem of weld lines where they
rejoin. Voids and sinks can act as stress concentrators if in the load path
when the product is in service. Moulding features can include:
∑ frozen-in stress and strain;
∑ poor fusion at weld lines;
∑ voids in the centre of thick sections; and
∑ sink marks at the surface.
Other features may be created when the polymer granules are not dried
correctly, leading to degradation at the high temperatures of moulding
molten polymer. Because thermoplastic polymers have to be processed well
above the glass transition temperature T g or melting point T m , the process
temperature is usually well above the boiling point of water. Any traces of
moisture therefore result in voids and surface 'splay marks'. Hydrolysis may
occur, thus causing a reduction in the molecular weight. whether a moulding
feature becomes a defect depends on its location on the product, its further
treatment and its final environment. Thus, a weld line in an unstressed part

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