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ANALYSIS OF PLASTIC PART FAILURES by RAPRA

High failure cause to low failure cause- in descending order

The predominantly material causes involved in failure were: The predominating process faults were:
Environmental Stress Cracking, Excessive moulded-in stresses,
Cyclic Fatigue, Voiding,
Notch Sensitivity, Poor weld lines,
Chemical Attack and; Stress Cracking, Adverse Orientation,
UV Degradation, Material contamination,
Thermal Degradation, Longer Residence time in the machine,
Creep, Wrong material selection or poor part design,
Buckling, Wrong material selection or poor part design

Moulding inconsistency and failure due to moulded-in stress are the major problem observed in moulding shop. Moulding inconsistency means shot to shot variation in quality parameters.

I have observed that if the following three balances are taken care of during the design stage, then moulding consistency is generally ensured.

These THREE balances also prevent or reduce the moulded-in stress, thereby avoiding major cause of part failure in service.
Any unbalancing on the above three balances will result in moulded-in stress. This is the biggest culprit for over 50% part failure.
CAE software can predict the part design related problems and gives vital inputs for design of mould. It can also provide the processing parameters with a range.

It is observed that moulded-in stress is behind most of the problems. It could be due to following reasons:

It is observed that over packing increases the density of the moulding. If we measure the density at different region of the moulding, you may find the variation in density if the mould filling is unbalanced.

The unbalanced mould filling can be due to part geometry or non uniform cooling.

Over packing during pressure phase results in over packed region near the gate, which can be verified by checking the density of the regions near the gate.


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