Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Bubbles (also known as voids or air traps) in injection-molded parts are typically caused by trapped air during filling, moisture in the resin creating steam, outgassing from the material, or poor venting. They can be distinguished from sinks by heating the part: bubbles swell due to expanding gas. To eliminate them, start with diagnostic steps and systematically adjust parameters using the "4M" approach (molding process, mold, machine, material).

Step-by-Step Troubleshooting and Prevention

Follow these methods in order, testing with short shots (partial fills) to visualize where bubbles form first—this helps isolate the issue.

1.Dry the Material Thoroughly

Moisture in the resin turns to steam during heating, creating bubbles. Use a dryer at the recommended temperature (e.g., 80–120°C for most resins) for 2–4 hours before processing. This is often the quickest fix for random bubbles.

2.Increase Back Pressure and Pack/Hold Pressure

Low back pressure allows gas to escape unevenly; raise it (e.g., 50–100 bar) to compress the melt and push out trapped air. Similarly, boost pack/hold pressure and time to densify the part. Ensure shot size is correct with adequate cushion (5–10% of barrel capacity).

Optimize Temperatures  Lower melt temperature (e.g., by 10–20°C) to reduce gas formation and viscosity, allowing better flow and less air entrapment.  

Increase mold temperature slightly (if venting allows) to improve flow in thick sections.

Avoid overheating hot runners, which can vaporize contaminants.

3.Improve Mold Venting

Poor venting traps air at the end of flow paths. Add or deepen vents (0.01–0.03 mm deep) at parting lines, ejector pins, and last-fill areas. Test by shooting with reduced clamp tonnage or inserting paper in vents to check for compression. For severe cases, use vacuum venting to pull air out before injection.

4.Adjust Injection Speed and Screw Settings

High injection or screw rotation speed can trap air; slow it down (e.g., reduce by 20–30%) to allow gas escape. Decrease decompression (screw pullback) to minimize gas release. Optimize screw design for better melting and mixing to avoid unmelts carrying air.

5.Check for Contamination and Machine Issues

Inspect for material contamination, water leaks in feed throats or water lines, or Venturi effects pulling in air. Clean the machine and use high-quality resin. For transparent materials, which are more bubble-prone, add extra drying and venting.