An apparent bubble in an injection-molded part may be a gas pocket or a shrinkage void. Identify the mechanism before changing the process. Gas-related defects can involve moisture, entrained air, or degradation. A shrinkage void forms when internal contraction is not adequately supplied with material during solidification. The same visual description does not justify the same correction.
Preserve Evidence Before Adjusting Settings
Retain defective and acceptable samples, identify the cavity, and record the material lot, startup condition, and current settings. Photograph the location relative to the gate and thick sections. Note whether the defect is repeatable in one location or scattered across the part.
Transparent parts make internal defects easier to see, but a sound-looking exterior does not rule out an internal void. Where the application warrants it, use an approved inspection method such as controlled sectioning or suitable imaging. Do not heat unknown plastics with an open flame as an informal diagnostic test.
| Observation | Useful investigation | What it does not prove alone |
|---|---|---|
| Pocket in the center of a thick section | Packing effectiveness and cooling-related shrinkage | That moisture can be ruled out without other evidence |
| Scattered pockets with surface streaking | Resin condition, moisture, contamination, and melt history | That drying is the only cause |
| Defect near a flow-ending region | Fill pattern and vent condition | That a higher injection speed will solve it |
| Problem after a prolonged machine interruption | Residence time and material degradation | That every subsequent shot is acceptable |
BASF’s troubleshooting guidance distinguishes gas bubbles from voids and notes the connection between voids and thick, internally molten regions. Use such guidance as a diagnostic starting point, not a replacement for examining the actual part.
Check Resin Condition at the Point of Use
For moisture-sensitive grades, verify the material supplier’s drying requirements and the condition of material entering the machine. A dryer temperature display does not confirm the pellets have reached an acceptable moisture level. Review residence time in the dryer, loading practice, and exposure during transfer.
Keep resin grade, colorant, additives, and regrind proportion identifiable. If the defect appeared after a material change, retain samples from both conditions and check whether the change was approved. Drying an unfamiliar mixture more aggressively is not a substitute for establishing what material is being processed.
Review the Melt and Filling History
Inspect the approved processing window against actual operating conditions. Barrel settings, measured melt behavior, recovery conditions, and residence time must be considered together. A machine idle period or an unsuitable shot-to-barrel relationship can change the thermal history even when the temperature setpoints remain unchanged.
Air management also deserves a targeted review. Examine the filling sequence, vent condition, and melt preparation with the process engineer. Raising temperature or changing injection speed without identifying the mechanism may move the defect or introduce another one. The melt-temperature and material guide explains why processing advice must be grade-specific.
Document one controlled adjustment at a time when isolating a cause. Where variables interact, use an agreed experimental plan rather than informal simultaneous changes. Keep machine operation and mold intervention within trained personnel’s procedures.
Test Whether Packing Can Feed the Critical Region
If evidence indicates a shrinkage void, review whether the flow path remains capable of supplying the region during packing. A holding-pressure increase cannot compensate indefinitely for a gate that has already frozen. It can also create flash, residual stress, or dimensional changes elsewhere.
A qualified process study may examine part-weight response to hold time together with gate behavior and defect incidence. Record the stable process conditions and use the material supplier’s limits. Do not treat a single heavier part as proof that the internal defect has disappeared.
Address Geometry When the Process Window Is Too Narrow
Thick regions cool and shrink differently from adjacent thin sections. Protolabs’ moldability guidance identifies thick sections as a risk for sink and void formation. Evaluate whether a heavy boss or local mass can be cored while preserving its structural function. Review wall transitions, gate location, and the ability to feed the affected region with the tooling specialist.
Consider a hypothetical mounting boss attached to a thin housing wall. If the pocket repeatedly appears inside the boss, making the entire surrounding wall thicker may add material and create additional cooling problems. A revised boss and rib arrangement may be worth evaluating instead. That decision requires a new structural and molding review, not just a cosmetic repair.
Any machining of samples for diagnosis should preserve the defect evidence and sample identity. A machined prototype can help assess fit, but it cannot prove the molding process is free of internal voids.
Verify the Correction Beyond the First Good Shot
Define success before running the confirmation lot. Check the internal defect with the chosen method and verify that the correction has not created flash, sink, warpage, unacceptable dimensions, or a new cosmetic issue. Include all relevant cavities and agreed operating conditions.
- Compare samples against the original defect map.
- Record the final approved process and material condition.
- Repeat the critical functional and dimensional checks.
- Assess startup or interruption conditions where relevant.
- Establish lot disposition and a reaction plan if the defect returns.
When requesting help with mechanical interfaces or prototype components, send MTSPi the drawing, defect photographs, and acceptance criteria through the technical inquiry page. Keep the molding-process investigation with the responsible molder and material specialists.
Frequently Asked Questions
Will increasing pressure always remove bubbles?
No. Gas pockets and shrinkage voids have different mechanisms, and pressure changes may introduce other defects. Diagnose first.
Are internal bubbles only a cosmetic issue?
No. Depending on location and application, internal defects can affect strength, sealing, or durability even if the surface looks acceptable.
Should every resin receive the same drying cycle?
No. Use the specific grade supplier’s requirements and verify the condition of material at the machine.
Does one clear sample prove the fix?
No. Confirm the agreed batch, cavity coverage, inspection method, and process conditions before releasing production.
Technical reference basis: BASF Injection Molding Troubleshooter and engineering-thermoplastic defect guidance; Protolabs moldability guidance on thick sections. Diagnostic examples are illustrative and do not replace resin-specific processing instructions.
