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MOULDING PROBLEMS- Mould, Machine and Material related

MOULDING PROBLEMS

MOULD related

MACHINE related

IN-CONSISTENT CRITICAL DIMENSIONS

  • Unequal shrinkage due to variation in mould surface temperature.

Cooling circuit design related.

  • Core and cavity dimension related. Correct shrinkage is not considered for core / cavity dimension.

Hold-on pressure and hold-on time influences the shrinkage. High pressure and time reduces the shrinkage. Lower pressure and time increases the shrinkage of the part.

shrinkage in the direction of orientation is more.

dimensions could be controlled by controlling cooling rate in different areas of the mould when wall thickness is not constant. Cooling rate could be increased by using Ba-Cu insert in the mould.

SINK MARKS

Part design related- non uniformity of wall thickness.

  • Injection speed profile and hold on pressure profile can minimise to some extent.
  • Gas injection Moulding technique can be considered.

WARP

  • Use technique to improve part stiffness with part design.
  • Random distribution of mould surface temperature. Cooling circuit design related.

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WEAK WELD LINES

Part design related.

Injection speed can influence.

DEGRADATION

Check dimensions of runner system for excessive shearing.

Excessive Residence time due to use of oversized injection unit.

Lowering barrel temperature and injection speed can have some influence

EJECTION DIFFICULTY

Part design related -Unbalanced melt flow. Use flow leader to reduce unbalance in flow.

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POOR IMPACT strength

Radius at projections and sharp corners as it act like notch from where crack propagates.

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IN-CONSISTENT WEIGHT.

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% age utilisation of shot capacity is closer to 90% resulting in inconsistent melt quality- if metering stroke is more than 3 times screw diameter. Borderline case for shot weight.

IN-CONSISTENT FILLING

Increase gate size to reduce pressure drop across the gate.

Available maximum injection rate is not adequate for flow ratio of part being moulded. Borderline case for injection rate.

STRESS CRACKING

Melt flow is unbalanced. Part design related.

Over packed part. Set correct pressure profile during pressure phase.

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MATERIAL RELATED PROBLEMS

SINK MARKS

Filled polymer can reduce sink mark.

POOR MECHANICAL STRENGTH

Consider high molecular weight polymers.

Consider filled polymers. Pre heat polymer in dehumidifier if required for polymer as recommended for polymer.

DIMENSIONAL VARIATION

Check shrinkage characteristics of material.

DEGRADATION OF MELT

Check thermal stability of polymer as well as pigments and other additives if any used with material.

DIFFICULTY IN FILLING

Consider polymer of higher MFI

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