Technological Solutions for Quality in Injection Moulding of Plastics by Prabodh C. Bolur-

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TECHNOLOGICAL SOLUTIONS FOR
TOTAL QUALITY
IN INJECTION MOULDING OF PLASTICS

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Things to Know in Injection Moulding of Plastics

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A Guide to Injection Moulding of Plastics

Foreword
Preface
Contents
Summary

Archives of Articles by Prabodh C. Bolur

Selection of IM Machine

Technological Solutions

Technological Tools

Engineering Basics for Injection Moulding Machine

PlastIndia Photos

Useful Links

Author

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Answers to YOUR Questions

Continued from previous page.

Let us first list the problems of moulding.

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.

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Technical Papers by Prabodh C. Bolur

Understanding Energy Consumption in Injection Moulding Machine(1994)
Understanding Heat Exchange in Injection Moulds (1994)
Ideal Moulding Shop (2001)
Understanding Selection of Injection Moulding MachineThis paper was part of authors lectures at CIPET since 1980. It has been regularly updated.
Technological Solution to Injection Moulding of Plastics (1999)
Technological Tools for Part Design, Mould Design and Mould Fabrication (1999)
Extrusion of Thermoplastics(1998)
Archives.

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