Bubbles around inserts

Bubbles or voids appear near the inserts. Bubbles can be classified by shape: spherical bubbles with smooth and glossy inner walls (moisture or decomposition gas), irregular cavities with rough inner walls (vacuum shrinkage cavities), slender stringy gas marks (entrapped air). The distribution position of bubbles can provide important diagnostic clues——near the gate it is mostly screw air entrapment, at the end of the product it is mostly poor venting, and at the center of thick walls it is mostly vacuum shrinkage cavities. The mechanical strength at the bubble decreases by more than 50%, and it becomes a fatigue crack source under alternating loads.

1. Material factors: the insert blocks gas discharge, poor wetting between the insert and the material, material flow obstructed at the insert. 2. Preheating factor: The preheating temperature and time of the material (especially thermosetting molding compounds) directly affect its flowability and curing behavior; improper preheating will lead to poor filling or uneven curing, and it is a key control point unique to compression molding. 3. Molding pressure factor: In compression molding, pressure is not only used to overcome flow resistance, but also directly affects the compactness of the product and the degree of fiber impregnation; insufficient pressure is an important cause of product looseness and insufficient strength.

1. Process parameter optimization and adjustment: add venting grooves around the insert, preheat the insert to improve wetting, optimize the insert position and material flow path. 2. Systematic troubleshooting method: Troubleshooting of compression molding problems follows the order of 'material -> temperature -> pressure -> mold'. First check the preheating and flowability of the material, then verify the mold temperature distribution and curing time, then adjust the molding pressure and mold closing speed, and finally check the mold venting and demolding conditions. 3. Preventive management: Establish standardized process parameter cards and quality control limits for each set of mold/product, and regularly conduct process capability index (CPK) evaluations. For recurring problems, use fishbone diagrams or 5Why analysis to trace the root cause, and solidify the countermeasures as PFMEA control points.

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