Injection molding bubbles are usually caused by trapped air, moisture in the resin, gas from overheated plastic, poor venting, insufficient packing, or thick wall sections that shrink internally. The right fix depends on whether the bubble is near the surface, inside a thick section, around a gate, or spread across the molded part.
For buyers, bubbles are not only a cosmetic defect. They can reduce strength, create leak risk, weaken bosses and clips, and signal that the material or molding process is not stable. A supplier should separate resin drying problems from mold venting, gate, packing and part design issues before changing tooling.
Common Causes of Injection Molding Bubbles
| Cause / material | Typical sign or use | Buyer or engineering action |
|---|---|---|
| Moisture in resin | Splay, bubbles, brittle areas or silver marks | Dry resin according to material datasheet and confirm moisture control |
| Trapped air | Bubbles near ribs, bosses, end of fill or weld areas | Improve venting, gate location, flow path or injection speed profile |
| Overheating | Gas marks, burning, black specks or random voids | Lower melt temperature, reduce residence time and check screw speed |
| Insufficient packing | Internal voids in thick sections | Increase packing pressure/time and review gate freeze |
| Part design | Repeated voids in heavy walls or isolated bosses | Reduce thick sections, core out bosses and improve wall transitions |
How to Diagnose Bubble Location
- Bubbles inside thick sections often point to shrinkage voids rather than external trapped air.
- Bubbles near the flow end usually indicate venting or injection speed problems.
- Bubbles with silver streaks often suggest moisture or material degradation.
- Bubbles around bosses and ribs may need DFM changes, not only process tuning.
- Bubbles that appear only after material changes should trigger a resin drying and grade review.
Material and Drying Checks
| Material / case | Main concern | Selection note |
|---|---|---|
| Nylon PA6 / PA66 | High moisture sensitivity | Drying is critical before molding loaded or glass-filled parts |
| ПК | Moisture and heat sensitivity | Poor drying can cause splay, bubbles and brittle parts |
| ABS | Moderate drying need | Check moisture, overheating and long residence time |
| POM | Thermal degradation risk | Avoid overheating and confirm safe processing range |
| PP / PE | Lower moisture concern | Focus more on venting, packing and wall design |
Buyer RFQ Details for Bubble Troubleshooting
For a useful review, send the 3D model or 2D drawing, resin grade or target material, part photos if the defect already exists, machine and mold information if available, expected annual volume, cosmetic requirements, critical dimensions, application environment and any inspection standard. Nylon Plastic can review the design, material and molding route before tooling or production changes are made.
Related Engineering Guides
- Injection molded parts DFM checklist
- Injection molding wall thickness guide
- Custom molded plastic parts guide
Request a Quote or Design Review
Share your plastic part drawing, material requirements, production quantity and defect or DFM concern. Nylon Plastic can help compare material choices, tooling changes, process adjustments and production risks for custom molded plastic parts.
ЧАСТО ЗАДАВАЕМЫЕ ВОПРОСЫ
What causes bubbles in injection molding?
Bubbles in injection molding are commonly caused by trapped air, resin moisture, overheated plastic, poor venting, insufficient packing or thick wall sections that shrink internally.
Are injection molding bubbles always a material problem?
No. Moisture-sensitive materials can create bubbles, but mold venting, gate design, packing pressure, wall thickness and process stability can also be responsible.
How can buyers help suppliers troubleshoot bubbles?
Buyers should send part drawings, material grade, defect photos, bubble location, production conditions if available, and the cosmetic or strength requirement for the part.
Can DFM reduce bubble defects before tooling?
Yes. DFM can reduce bubble risk by avoiding thick isolated sections, improving boss design, reviewing gate location and confirming whether the selected resin needs strict drying control.
