In injection mold design, when the location of trapped air in the cavity is hidden or traditional exhaust grooves are difficult to effectively exhaust, installing inserts for exhaust at the trapped air locations is an efficient and flexible solution. This method installs inserts with exhaust functions in areas where trapped air is likely to occur (such as the bottom of a deep cavity or the corners of a complex structure), and uses the fit clearance between the insert and the template or the exhaust channel of the insert itself to exhaust the accumulated air out of the cavity, thereby reducing defects such as bubbles and scorch marks. Compared with conventional exhaust grooves, insert exhaust can accurately exhaust specific trapped air points, and is especially suitable for special-shaped products or scenarios with limited mold space. For example, when molding gear products with deep blind holes, exhaust inserts are installed at the bottom of the hole to directly exhaust the trapped air in the hole, avoiding the problem of poor exhaust due to the distance of traditional exhaust grooves.
The design of vent inserts must balance venting functionality with molding accuracy. Common types include gap, slot, and porous. Gap vent inserts precisely mate with the mold plate to create a tiny gap (0.01-0.03mm), allowing air to escape while preventing the plastic melt from escaping due to its high viscosity. This design is suitable for applications with moderate venting volumes, such as corners and air traps in small and medium-sized products. Slot vent inserts feature shallow grooves (0.01-0.02mm deep, 3-5mm wide) machined into the side or bottom of the insert. One end of the groove connects to the air trap in the mold cavity, while the other end leads to the exterior of the mold, forming an vent channel. These are suitable for areas with high venting volumes, such as the final filling area of large products. Porous vent inserts are made of powder metallurgy and contain numerous tiny pores, allowing air to escape while preventing the plastic melt from penetrating. These pores are suitable for air traps on complex curved surfaces, but they are more expensive and require regular cleaning of plastic debris from the pores.
The installation location and fixing method of inserts directly affect the venting effect and mold stability. Inserts must be precisely positioned in areas where air entrapment is most severe, such as the corners where the melt is last to reach, or the joints between the core and the cavity. Three-dimensional flow simulation can predict the location of air entrapment in advance to ensure the accuracy of the insert installation point. Fixing methods typically use an interference fit or bolt connection. Interference fit (fit tolerance H7/r6) is suitable for small inserts, ensuring a close fit between the insert and the mold plate to prevent melt leakage. Bolted connections are suitable for large inserts and facilitate subsequent disassembly and maintenance. The mating surface between the insert and the mold plate must be polished to Ra0.8μm or less to ensure a good seal. In addition, the height of the insert must be flush with the cavity surface, with an error of no more than 0.01mm to prevent the formation of steps or ridges on the part surface.
The material selection for vent inserts must meet requirements for wear resistance, heat resistance, and processability. For common plastics (such as PP and ABS), inserts can be made of 45 steel or Cr12, achieving a hardness of 50-55 HRC after quenching to improve wear resistance. For corrosive plastics (such as PVC) or high-temperature plastics (such as PC and PA66), stainless steel (such as 304 and 316) or heat-resistant alloys should be used to prevent corrosion or softening at high temperatures. The machining accuracy of inserts must be strictly controlled, especially for gap and slot inserts. The gap or slot depth error must be ≤0.005mm, otherwise it will result in poor venting or melt overflow. For example, when machining a 0.02mm deep vent groove, slow wire cutting should be used to ensure a uniform groove depth and burr-free edges to avoid clogging the vent passage.
When using exhaust inserts, regular maintenance is required to ensure long-term exhaust effectiveness. During the production process, small molecules or additives in the plastic melt will gradually deposit in the exhaust channel, clogging pores or gaps, resulting in reduced exhaust efficiency. Therefore, the inserts need to be cleaned daily. A copper wire brush or compressed air can be used to remove impurities in the channel, and the inserts should be disassembled and thoroughly cleaned monthly. For porous inserts, ultrasonic cleaning (frequency 40kHz, time 30 minutes) can be used to remove residual plastic in the pores. In addition, the wear of the inserts needs to be checked regularly. When the fitting clearance exceeds 0.05mm due to wear, the insert should be replaced in a timely manner to prevent melt leakage. Through reasonable design, precise installation and standardized maintenance, exhaust inserts can effectively solve the problem of hidden air entrapment in the cavity and improve the qualified rate of finished products.
