Injection molding machine adjustment method to solve air trapping in dead corners
Dead-corner air entrapment during the injection molding process occurs when the melt is unable to escape quickly from complex structures such as corners, deep cavities, and ribs during mold filling. This pressure builds up, leading to high pressure and defects such as scorching, missing material, and bubbles in the molded part. This problem can be addressed by controlling the injection speed in stages during machine tuning. For example, when filling a deep cavity (30mm deep) on a certain automotive instrument panel, the melt front easily entrapped air in the corners, creating scorch marks. This was addressed by dividing the injection speed into three stages: an initial stage of 30mm/s (filling to 50%) to prevent air from being entrained in the melt jet; a middle stage of 50mm/s (filling to 80%), with a faster injection speed to shorten dwell time; and a final stage of 20mm/s (filling to 100%), which slowly fills the dead-corner area, allowing sufficient time for air to escape through the venting slots. After this adjustment, the scorch mark defect rate was reduced from 15% to 0.5%. Furthermore, pressure curve monitoring revealed that the maximum pressure in the air-trapped area was reduced from 180MPa to 120MPa, preventing overheating and degradation of the material.
Improving melt fluidity is an effective way to address air trapping in dead zones. This can be achieved by reasonably increasing melt and mold temperatures, but within the material’s tolerances. Insufficient melt fluidity caused air pockets to form during filling in the ribs of an ABS electrical housing (2mm wide, 8mm high), resulting in missing material in the molded part. Increasing the melt temperature from 220°C to 240°C (still below ABS’s thermal decomposition temperature of 260°C) and simultaneously raising the mold temperature from 50°C to 70°C reduced the melt viscosity by 20%, shortened the filling time in the ribs by 0.8 seconds, and eliminated the air pockets. For crystalline plastics like PA6, increasing the mold temperature to 80-100°C can slow melt cooling. In the case of a PA6 gear with air trapping in the tooth root dead zone, increasing the mold temperature from 60°C to 90°C eliminated the air trapping defect and increased the tooth root strength by 10%. It should be noted that the temperature should not be too high. When the melt temperature of a certain POM plastic part exceeded 220℃, the trapped gas area showed degradation and blackening, and 200℃ was finally determined to be the optimal value.
Optimizing holding pressure parameters can effectively alleviate the pressure buildup caused by trapped air in dead corners. By combining lowering the holding pressure with extending the holding time, the impact of high-pressure gas on plastic parts can be reduced. In a PC lampshade, excessively high holding pressure (100 MPa) caused trapped air to compress and produce silver streaks in the dead corners. Reducing the holding pressure to 70 MPa and extending the holding time from 5 seconds to 8 seconds allowed the melt to slowly feed under low pressure, eliminating the silver streak defect. Furthermore, staged holding pressure control was implemented, with an initial holding pressure of 70 MPa for 3 seconds, followed by a reduction to 50 MPa for 5 seconds, to prevent the impact of continuous high pressure on the trapped air area. Test data shows that for every 10 MPa reduction in holding pressure, the probability of burning in the trapped air area decreases by approximately 8%. However, sufficient feeding is crucial to prevent sink marks. In one case, too low a holding pressure (50 MPa) resulted in sink marks as deep as 0.2 mm. Ultimately, balancing the holding pressure to 65 MPa eliminated both trapped air and sink marks.
Adjusting the process parameters for gate location and number can alter the melt filling path and prevent air accumulation in dead spots. This requires targeted settings based on the part structure. The camera notch (5mm diameter, 4mm depth) on a mobile phone case is a typical dead spot. The original single-point gate caused the melt to fill from one side, trapping air within the notch. By changing the gate to a two-point symmetrical design and adjusting the injection speed differential between the two sides—increasing the injection speed by 10% on the side closest to the notch—the melt reached the notch earlier, forcing air toward the venting grooves. Combined with fine-tuning the gate size (from 1.2mm to 1.5mm), the melt flow rate increased, completely eliminating air bubble defects caused by air entrapment in the notch. For large parts, multiple gates can be used for simultaneous filling. For a circumferential dead spot on a washing machine drum, three evenly spaced gates were used at the same injection speed (40mm/s). This ensured simultaneous melt flow into the dead spot, allowing air to escape through the circumferential venting grooves. The air entrapment defect rate was reduced from 12% to 1%.
Utilizing venting-assisted processes and equipment is a supplementary means of resolving stubborn air entrapment in dead zones. Optimizing parameters during machine setup is crucial for maximum effectiveness. For a connector made of high-viscosity PBT material, conventional venting slots were inadequate to completely eliminate air entrapment in dead zones. Vacuum-assisted injection molding was employed: a vacuum pump lowered the cavity vacuum to -0.08 MPa before filling to reduce air volume; the vacuum was maintained during filling, forcibly extracting air from the dead zones. By increasing the melt temperature to 250°C and the injection speed to 60 mm/s, the air-trapped material shortages were eliminated, and the production yield increased from 65% to 98%. For molds where a vacuum system is unavailable, a “hold-release-re-hold” cycle can be added during machine setup. A PP part undergoes two holding cycles (70 → 0 → 60 MPa) to release pressure in the trapped zones , eliminating any scorch marks. In addition, regular cleaning of the vent grooves (every 1,000 molds) and maintaining their depth at 0.015-0.02mm (adjusted according to the material) are key to ensuring long-term stability in machine adjustment. In one case, trapped air recurred due to clogged vent grooves. After cleaning, the defect rate dropped from 9% to 0.3%.
