Environmental stress cracking (esc) is the development of cracks and eventual catastrophic failure due to a combination of mechanical stress (from external loading or from moulding strain) and exposure to an ‘aggressive’ chemical (a ‘near solvent’ for the thermoplastic material).
Esc can appear in most unexpected scenarios and featured in the majority of troubleshooting consultancies I had to deal with as an academic. Esc suddenly became a problem when a supplier of assembly screws changed the type of grease used. In another case esc was accelerated because a moulder had pushed up the moulding temperature (contradicting the customer’s specification) causing incipient degradation. A change of mould release spray caused another problem, which did not manifest itself until the mouldings reached the customer three days later.
Predicting and avoiding esc is not easy but I would suggest that:
- end users and designers be aware of service loading and chemical exposure;
- component designers avoid stress raisers and include generous radii;
- designers consider esc as one of the criteria in the material selection process;
- tool designers aim to design moulds to minimise moulded-in strain;
- moulders optimise process conditions to minimise moulded-in strain and esc.
Written by Dr.Charlie Geddes for Hardie Polymers
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