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MOULDED IN STRESS
According to RAPRA Technology, who carried out post-mortems on thousands of industrial plastic parts, found that environmental stress cracking was the major cause of plastic part failures. Well, Stress cracking in service is due to the moulded-in (residual) stress in the moulding. There are three possibilities of inducing moulded-in stress in a plastic part. There are:
When melt is over packed (while moulding) it can cause moulded-in stress. This can be avoided by applying follow up pressure just enough to avoid sink marks.
Even unbalanced melt flow in the mould can cause overpack in some regions. It is not possible to avoid overpacking with unbalanced melt flow while processing. This problem should have been considered while designing the part itself.
Un-equal freezing of melt through out the part, can also cause moulded-in stress. Cooling system, which does not provide uniform mould surface temperature can be responsible for this cause. This is the responsibility of mould design.
It is assumed that every shape can be mouldable in plastics. It is partly not correct. For perfect moulding you should have 95 to 100% balanced melt flow. It means that melt front in different direction should reach the boundary of the mould at the same instant. Otherwise, you may have some region filled earlier and while filling unfilled region the previously filled region gets over packed.. To solve this problem flow balancing technique should be used to modify the part design. By this technique, flow leader or flow deflectors are used to balance the flow to avoid moulded-in stress.
Moulded-in stress also causes unequal shrinkage in the moulding resulting in dimensional problem.
To avoid moulded-in stress due to un-uniform freezing, we must ensure that melt flow velocity in the mould is uniform or constant through out the part during filling phase.
Stress cracking can be classified in to
It can occur in almost any plastic. However, it is more common with amorphous thermoplastics. The solvent actually replaces the polymer at the surface of the part. The bond between polymer and the solvent molecule is weaker than the original bond between the polymer chains and does not contribute to the strength of the material. In other words, the solvent weakens the strength of the polymer. If the stresses either moulded into the part or applied externally to the part exceed the strength of the weakened polymer, the material ruptures at the surface. Solvent then penetrates deeper and the cracks extend further into the part over a period of time.
How to overcome the failure of plastic part due to stress cracking?
It can be observed that this failure occurs when a stress (internal or applied) and particular chemical are present in an application simultaneously. If one of them - stress or chemical- is removed, then there would be no problem. We discussed earlier that moulded-in stresses are induced by unbalanced flow of melt, un-equal freezing of melt and over packing during follow-up pressure phase of moulding. This can be tackled at the part design and mould design stage of any plastic part development project. It should be possible to get minimum or even zero moulded-in stresses at design stage.
If the moulded-in stress exists, then the influence of chemical on particular plastic should be studied. Testing a plastic with test samples may not give practical result as samples are moulded with out moulded-in stress or with lower level of moulded-in stress. Therefore, performance of sample test piece for stress cracking test may not be very much helpful.
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