While hot runner systems can improve efficiency and product quality in injection molding, they can also cause air entrapment due to improper design or operation. Air entrapment typically manifests as defects such as scorching, bubbles, and missing material on the surface of the plastic part, and in severe cases, can even lead to mold damage. One of the main causes of air entrapment in hot runners is the entrainment of air by the melt as it flows within the runner. This is especially true when the hot runner’s gate is poorly designed, such as when the gate is too small or improperly positioned. This can cause the melt to flow too quickly, creating turbulence and entraining large amounts of air. For example, when a factory was producing ABS electrical housings, the hot runner gate had a diameter of only 1mm. The excessive melt flow rate caused a large amount of air to be entrained, resulting in multiple scorch marks on the surface of the plastic part and a pass rate of only 70%.
Poor exhaust in the hot runner system is also a major factor causing air entrapment. If the clearance between the hot runner plate and the mold is too small, the exhaust slots are clogged, or there is not enough exhaust structure, the air will not be discharged in time, accumulating in the cavity to form high pressure, which in turn causes defects in the plastic part. Especially when producing large and complex plastic parts, the cavity structure is complex, making it more difficult to exhaust the air, and the air entrapment problem is more prominent. When an auto parts factory was producing car bumpers, the exhaust slots of the hot runner system were blocked by plastic debris, resulting in the inability to exhaust the air in the cavity. Large areas of bubbles appeared on the surface of the bumper, and production had to be stopped for cleaning, causing large economic losses.
From a process parameter perspective, improper injection speed and melt temperature settings can also exacerbate hot runner-induced air entrapment. Excessively fast injection speeds generate significant shear forces as the melt flows through the hot runner, easily entraining air. Excessively high melt temperatures, on the other hand, can cause material degradation and generate gases, which mix with the entrained air to form trapped air. For example, a toy factory producing PP toys increased the injection speed by 30% to increase production speed while simultaneously setting the melt temperature to 230°C (higher than the normal processing temperature for PP). This resulted in numerous burns and bubbles on the surface of the plastic parts. Testing revealed that this was caused by the combined effects of gases generated by material degradation at high temperatures and the entrained air.
To address air entrapment caused by hot runners, the first step is to optimize the hot runner design. Properly design the gate size and location to avoid turbulent melt flow within the runner and reduce air entrapment. For example, the gate diameter can be appropriately increased to ensure smooth laminar melt flow. Additionally, auxiliary venting slots can be installed near the gate to help expel trapped air. An electronics factory reduced the defect rate due to air entrapment from 15% to 2% by increasing the gate diameter of the hot runner for mobile phone casings from 1.2mm to 1.5mm and adding 0.05mm deep venting slots around the gate.
Adjusting process parameters and strengthening system maintenance are also effective ways to address air entrapment. Appropriately reducing the injection speed can smooth the melt flow and reduce air entrapment; properly controlling the melt temperature can prevent material degradation and gas generation. Regularly cleaning the hot runner system’s vent slots and runners ensures unimpeded venting. A household appliance company, producing refrigerator drawers, effectively resolved the air entrapment issue by reducing the injection speed by 20%, lowering the melt temperature from 220°C to 200°C, and cleaning the hot runner vent slots weekly. This effectively increased the product qualification rate to over 98%.
