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Major Plastic Injection Molding Defects and How to Prevent Them

It is difficult to prevent moulding defects due to the complex structure of your prototype. Problems range from weld lines to sink marks and air pockets to jetting, which are expensive to correct. Here’s a list of significant defects, their causes and how to prevent them during plastic injection molding.

The defects with their preventive measures

  • Vacuum voids or air pockets

Air pockets trapped within or close to the surface of an injection moulded prototype due to uneven solidification between two surfaces. Two moulds incorrectly aligned can also lead to aformation of bubbles.

You are advised to use a less viscous plastic and ensure the moulded parts are perfectly aligned. You may also increase holding time and holding thepressure of thesemoulds. It ensures that less gas is trapped.

  • Weld lines

Weld lines are present where molten plastics meet each other. It is caused due to partial solidification of melted plastic during plastic injection molding. It occurs due to inadequate bonding of two or more front lines.

Raise the temperature of molten plastic to prevent anyformation of weld lines. Increase the injection speed and adjust the design of flow pattern.

  • Burn marks

They are usually rust colours that appear on a prototype of injection moulding. Degradation of plastic material due to excessive heating causes burn marks. Fast injection speeds combined with overheating of trapped air are significant signs of a burn mark.

Reduce injection speeds and optimise gas venting to avoid burn marks. It is beneficial to reduce mould and melt temperature to prevent rusting on your prototype.

  • Jetting

The highrate ofplastic injection molding prevents the molten plastic from sticking to the mould surface. Plastic solidification forms wavy folds on the surface of your prototype’s jet stream. When the melting temperature is too low or at times of high viscosity of molten plastic, the mouldis prevented from smooth flowing. Jetting causes scrape marks on the surface of your finished product.

Optimise gate design for proper contact between the mould and the molten plastic to prevent jetting. You may also increase the gate’s size for a smooth flow of mould.

  • Sink marks

Due to shrinkage of inner portions of your prototype, small craters or depressions develop on its surface. It occurs due to insufficient cooling time for the plastic while in the mould. The excessive temperature at the gate or low cavity pressure leads to sinking marks.

You may lower mould temperatures for adequate cooling time for your prototype’s plastic. Increase holding pressure and holding time for prevention of sink marks.

That’s it with the causes and prevention. Now you can go on with the process safely and efficiently.

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