Air entrapment in the injection mold cavity is a common problem that affects product quality. Its location distribution is closely related to the mold structure, product shape, and melt flow path. Accurately determining the location of trapped air is a prerequisite for solving the problem. First, the dead corners or deep cavities of the product are high-incidence areas for trapped air. When the plastic melt is filled into these areas, the air is difficult to discharge due to the narrow space and fast melt flow rate, which easily forms bubbles or scorch marks. For example, when molding products with deep holes or grooves, the melt will squeeze the air to the bottom of the hole or groove during the filling process. If the exhaust structure is not designed properly, trapped air will occur. Trapped air in such locations is often accompanied by local burning, because the temperature of the trapped air increases after being compressed, which may cause the plastic to degrade.
Secondly, the area where the melt is last filled is very prone to air entrapment, which is directly related to the flow characteristics of the melt. During the injection molding process, the melt advances in the form of a fountain flow. The time it takes for the melt at different positions to reach the end of the cavity is different. The last filled area is usually where multiple melt fronts converge, and the air is wrapped here and cannot be discharged. For example, the edge corners of large flat products and the cross-connections of complex structural products may become the last filling area, forming obvious signs of air entrapment, such as silver streaks, bubbles or material shortages. By simulating the melt flow path, the last filling area can be predicted in advance, and the gate position can be optimized or venting grooves can be added.
The parting surface of the mold and the fitting point of the insert are also common locations for air entrapment. The parting surface is the joint surface between the movable and fixed molds of the mold. If the fitting clearance is too small or there are burrs or foreign matter, it will hinder the exhaust of air. The fitting point between the insert and the template may also form a closed space due to unreasonable clearance, causing air to be trapped. For example, when the fitting clearance between the insert and the cavity is less than 0.01mm, it is difficult for air to be discharged through the gap. Under the action of melt pressure, it is compressed around the insert, forming a ring-shaped bubble. In addition, if the fitting surfaces of movable parts such as sliders and core pulls are sealed too tightly, local air entrapment will also occur because the air cannot escape. This type of air entrapment often manifests as fine white marks distributed along the fitting seam.
Air entrapment is also prone to occur near the gate and at the point where the melt flow turns. The gate is the entrance for the melt to enter the mold cavity. If the gate size is too small or the position is improper, turbulence will form when the melt enters the mold cavity, and air will be drawn in. At locations where the melt flow direction changes sharply, such as right-angle turns, the melt will form a low-pressure area on the inside of the corner due to inertia, and air will gather here. For example, when molding products with curved channels, the melt flows faster on the outside of the turn and slower on the inside, and air is easily trapped on the inside, resulting in local bubbles or insufficient filling. Air entrapment in such locations is often related to unstable melt flow and needs to be improved by adjusting the injection speed or optimizing the runner structure.
Finally, areas with unreasonable venting groove design are bound to have air trapping problems. The venting groove is the main channel for exhausting air from the cavity. If the venting groove position deviates from the air trapping point, the depth is insufficient or it is blocked, it will cause air stagnation. For example, although some molds are equipped with venting grooves, the position does not correspond to the last filling area, or the venting grooves are blocked by plastic debris accumulated during long-term production, making it impossible for air to be discharged, resulting in continuous air entrapment. In addition, when the depth of the venting groove exceeds the overflow value of the plastic, it may cause melt overflow, but if the depth is insufficient (such as less than 0.02mm), it cannot be effectively vented, especially for plastics with poor fluidity (such as PC, PMMA), which are more likely to cause air entrapment due to poor venting.
