Latent problems in INJECTION MOULDING OF PLASTICS

Logistics for understanding Quality problems

Injection Moulding of plastics involves quite a few steps before actual moulding operation.

The first step has to be

• PLASTIC PART DESIGN. There after
• MOULD DESIGN,
• MOULD FABRICATION,
• SELECTION OF CORRECT MACHINE AND PROCESS SETTING,
• POST MOULDING OPERATIONS,
• ASSEMBLY etc.

The root causes of quality and consistency problems may or may not lie in the present step - moulding process set-up. The mistakes of the earlier steps may not be possible to correct at the later steps. Some time it may be possible to correct partly. Such corrections are expensive. The problems originating from the present step can only be solved in the present step.

Mouldability

In mechanical engineering Part design has to be manufacturable. Similarly plastic part has to be mouldable to high degree. If moldability is poor than, you have all types of quality and production problems. Therefore part design has to be modified to make it more than 90% mouldable. General assumption that any geometry of the part is mouldable perfectly is not correct. General understanding- that once the mould is filled with the melt every thing has to be perfect- is not correct.

ROOT CAUSE for moulding inconsistency.

• Physical cause -normally very evident.
• Human cause - Human error
• LATENT cause - normally difficult to trace. It is recurring all the time. It requires deeper understanding of the process.

Physical cause is easily traced and trouble shooting is normally done at shop-floor level. All text books, polymer manufacturers literature provide these information in the form of trouble shooting table or chart.

Human errors are like wrong water line connection, usage of contaminated material, etc.

LATENT cause means existence of inherent problem at the part design or mould design stage. Basics of plastics part design lies in ensuring perfect mouldability for the part designed.

The most difficult to solve quality problems are

• Stress cracking,
• Dimensional inconsistency / inaccuracy
• Warpage,
• Sink-marks
• Weak Weld lines

Quality problems in injection moulding can be traced to

• Part design and mould design,
• Polymer / additive characteristics,
• Machine specification (Limitation) and
• Process setting.

Let us understand the factors influencing quality consistency in processing and quality in performance

Let us understand moulding problems.

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