PLASTIC DEFECTSEliminating Bubbles, Voids, Sinks & Blisters in Injection-Molded PartsBy John Bozzelli

BUBBLES, VOIDS, SINKS & BLISTERS

These are some of the most significant causes for part rejects on an injection-molded part.

They can inhibit part performance and are problematic to solve.

BUBBLESCan be caused by:• Pockets of trapped gas• Vacuum voids

Test the part area to determine what kind of bubble it is.

TESTING TOOLS

To determine which type of bubble exists in your part, warm the part area containing the bubble. Try these tools:

• Hot air gun• Small lighter• Torch (use caution)

TRAPPED AIR/GAS BUBBLE

If there is air or gas in the bubble, the gas will expand the bubble as the part softens.Trapped air is a root cause of bubbles and may stem from:• Converging fronts• Jetting• Non-vented core pins• Poor tool venting• Resin degradation

BUBBLE ASSESSMENT

Melt flow pattern is a major cause of bubbles.

• Examine each part’s flow pattern• Note any race-tracking effect or jetting• Observe flow path for back flow or trapped air• If you find an issue, run a short shot molding sequence for

further analysis.

VOIDS AND SINKS• A void occurs during cooling, usually in thick section of

the part.• A sink is a depression that does not mimic the mold steel

surface.Both are signs of internal stress – a warning sign that the part may not perform as expected.

USING ENOUGH PLASTIC?

Insufficient plastic can be the main cause for voids or sinks.

• Pack more plastic into the cavity• Use a consistent cushion on the press• Make sure you don’t bottom out the screw

AVOIDING VOIDS AND SINKS

Possible solutions:

• Slow fill rates• Use gas counter-pressure• Increase back-pressure• Open the gate for longer gate seal times• Reduce melt temperature

BLISTERS

A blister is a thin film of plastic that bubbles up from the part’s surface.

BLISTER CAUSES

Blisters can be caused by:

• Gas traveling across the surface during fill or pack• Trapped air issues like inadequate venting or melt flow pattern

• Process problems• Resin or additive degradation• Excessively high injection rates

BLISTER SOLUTIONS

• Stick tape to the part and lift the layer off• Inject the melt at a slower rate• Use a lower steel temperature near the gate• Lower screw decompression so air doesn’t enter the melt

stream• Avoid resin or additive degradation by using new or virgin

material• Verify the resin melt temperature range recommendation

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