Air Entrapment / Bubble

Bubbles appear inside or on the surface of the part. Bubbles can be classified by morphology: spherical bubbles with smooth and glossy inner walls (moisture or decomposition gas), irregular voids with rough inner walls (vacuum shrinkage cavities), and slender drawn streak-like air marks (entrained air). The distribution position of bubbles can provide important diagnostic clues — near the gate is mostly screw air entrainment, at the end of the part is mostly poor venting, and in the center of thick walls is mostly vacuum shrinkage cavities. The mechanical strength at the bubble location drops by more than 50%, and it becomes a fatigue crack source under alternating loads.

1. Process parameter factors: poor mold venting, air entrainment due to excessive injection speed, dissolved gas in raw material, air entrainment during mixing. 2. Temperature control factors: raw material temperature affects component viscosity and reaction rate, mold temperature affects the curing process and part surface quality, and both need to be precisely controlled within a narrow window.

1. Process parameter optimization and adjustment: add mold vent grooves, reduce injection speed, vacuum degas the raw material, improve the mixing head anti-air-entrainment design. 2. Systematic troubleshooting method: RIM problem troubleshooting follows the order of "formulation -> mixing -> temperature control -> equipment". First check the two-component ratio and catalyst dosage, then check the mixing head and self-cleaning function, then check the raw material and mold temperature control, and finally check the accuracy of the metering pump and hydraulic system. 3. Preventive management: establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluation. For recurring problems, use fishbone diagram or 5Why analysis to trace the root cause, and fix the countermeasures as PFMEA control points.

The three major causes of bubbles (moisture, air entrainment, vacuum shrinkage cavities) need to be treated differently: moisture type — full drying; air entrainment type — optimize plasticizing and injection parameters; vacuum shrinkage cavity type — strengthen holding pressure feeding. Accurate cause diagnosis is the prerequisite for effective resolution.