Bubble / Void

Air holes or vacuum bubbles appear inside 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: insufficient drying of the material, air entrainment by the screw, shrinkage cavities caused by insufficient holding pressure, air entrainment caused by too fast injection speed. 2. Temperature factors: the settings of barrel temperature and mold temperature directly affect melt fluidity, cooling rate, and crystallization behavior; low temperature is a common cause of filling and molding quality problems, and precise control is required according to the material's recommended processing window. 3. Mold comprehensive factors: in addition to venting, mold design parameters such as cooling water channel layout, gate position and size, and demolding draft angle all have a systematic impact on part quality, and optimization is required from the source of mold design.

1. Process parameter optimization and adjustment: fully dry the material, reduce screw speed, increase back pressure for venting, adjust injection speed. 2. Systematic troubleshooting method: troubleshoot item by item in the order of "parameters -> material -> mold -> equipment". First adjust the parameters that are easiest to change, such as injection pressure and temperature, and observe the effect; if there is no improvement, check the drying and fluidity of the raw material; further check mold venting and gate design; finally check the mechanical condition of the equipment (check ring, screw). Change only one variable at a time to determine the root cause. 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.