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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.

  • Shape, location /size of gate and dimensions of the part are such as to ensure balanced melt flow during the filling stage. FLOW BALANCE

  • Cooling system is designed to ensure the balance between heat transferred to mould during injection stroke and heat removed by cooling system from the mould. Cooling system include water flow rate, cooling channels dimension and layout etc. HEAT BALANCE

  • Cooling system has to ensure the melt freeze uniformly through out the part. UNIFORM FREEZE

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:

  • Unbalanced flow of melt in mould results in some overpacked region and some just packed regions.
  • Inadequate injection speed results in faster freezing, thereby demanding more filling pressure which causes overpacked moulding.
  • Incorrect switching over from injection speed to follow-up pressure resulting in overpaking. This is caused because the balance unfilled volume available (at switchover point, in the mould) to accommodate the relaxation of compressed melt less than required. This can be damaging the mould if the mould has slender parts.
  • High follow-up pressure and time resulting in overpacking. Melt transfer during pressure phase can reach only unfrozen regions, which is normally near the gate. Therefore, the pressure set should gradually reduce with time to prevent overpacking.
  • Poor cooling system giving rise to non uniform mould surface temperature which in tern resulting in overpacking in some regions of moulding.

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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