MOULDING PROBLEMS

MOULD related

MACHINE related

SINK MARKS

• Part design related- non uniformity of wall thickness .

• Injection speed profile 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.

WEAK WELD LINES

• Part design related.

• injection speed can influence.

IN-CONSISTENT CRITICAL DIMENSIONS

∓ Fluctuating quality during long production run.

• Unequal shrinkage due to variation in mould surface temperature.
• Cooling circuit design related.
• Core ∓ cavity dimension related. Correct shrinkage is not considered for core / cavity dimension.

• Melt temperature, Injection speed ∓ hold-on pressure can have little influence.
• Unstabilised process due to unbalanced heat exchange in mould ∓ unbalanced melt flow ∓ high %age utilisation of shot capacity - more than 3D injection stroke.

DEGRADATION

• Check dimensions of runner system for excessive shearing.

• Excessive Residence time due to use of oversized injection unit.
• Lowering barrel temperature ∓ injection speed can have some influence

EJECTION DIFFICULTY

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

POOR IMPACT strength

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

IN-CONSISTENT WEIGHT.

• % 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.

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.