The design of a precision injection mold's venting structure is crucial for resolving the issue of product scorching. Its core goal is to scientifically plan gas escape paths, balance gas pressure within the mold cavity with melt flow characteristics, and prevent high-temperature gas stagnation from causing plastic decomposition and carbonization. Scorching occurs when air, water vapor, or gases generated by plastic decomposition within the mold cavity are unable to escape promptly during the precision injection molding process. Under high-pressure, high-speed filling conditions, the gases are compressed and generate high temperatures, leading to localized overheating at the melt front and, in turn, material degradation. Therefore, venting design requires a comprehensive approach encompassing seven key aspects: gas source analysis, channel layout optimization, structural innovation, machining precision control, material compatibility, dynamic adjustment mechanisms, and verification testing.
Accurate identification of gas sources is fundamental to venting design. During precision injection mold filling, gases within the mold cavity primarily originate from three sources: air trapped within the cavity when the mold is closed; water vapor generated by the evaporation of water contained in the plastic raw material at high temperatures; and volatile gases generated by the decomposition of the plastic under high-temperature shear. If these gases cannot be quickly discharged through the venting structure, they will form "gas locks" at the melt front, leading to sudden localized pressure and temperature increases, and ultimately, scorching. Therefore, venting design requires a multi-stage venting system tailored to the characteristics of different gas sources to ensure efficient escape of all types of gases.
The layout of the venting channels must be closely coordinated with the melt flow path. In precision injection molds, after the melt enters the cavity from the gate, it typically diffuses radially or laminarly. Gases tend to accumulate at the melt flow end, cavity corners, deep ribs, or thin-walled areas. Therefore, venting grooves should be prioritized at these gas accumulation "hotspots," forming a venting network covering critical cavity areas. For example, for parts with complex curved surfaces, mold flow analysis software is used to simulate gas accumulation locations and specifically design the venting grooves' orientation and depth to ensure the shortest possible gas escape path without dead ends.
Innovation in venting structure is key to improving venting efficiency. Traditional parting surface venting, while simple, can easily affect the product's appearance due to flash, making it particularly unacceptable in precision injection molding. Therefore, implicit venting structures are becoming mainstream. For example, cores or inserts made of permeable steel have internal microporous structures that allow gas permeation without producing noticeable flash. Alternatively, stepped venting grooves can be designed on side core pulls and sliders to reduce gas exhaust resistance through stepwise pressure reduction. For ultra-thin-walled parts, a vacuum venting system can be integrated to actively extract gas from the mold cavity via an external vacuum pump, completely eliminating the risk of gas entrapment.
Strict control of machining precision is key to ensuring venting performance. The depth, width, and surface roughness of the venting groove directly impact venting efficiency and product quality. If the venting groove is too deep, melt overflow will form flash; if it is too shallow, venting will be impeded. In precision injection molds, venting groove depth is typically controlled to the micron level. High-precision CNC milling or EDM techniques are employed during fabrication, accompanied by microscopic inspection to ensure dimensional accuracy. Furthermore, the venting groove surface must be polished to a mirror finish to prevent turbulence and localized overheating caused by gas flow.
The compatibility of the venting structure with the mold material is crucial. Under high-temperature and high-pressure conditions, the edges of vent grooves are prone to cracking due to concentrated thermal stress. Especially when there are chemical compatibility issues between the mold material and the plastic, the vent grooves may widen due to corrosion, leading to flash or venting failure. Therefore, the vent area of precision injection molds must be constructed from materials with excellent thermal fatigue and corrosion resistance, such as H13 steel, S136 stainless steel, or permeable steel. Surface treatments such as chrome plating and nitriding can be used to enhance the durability of the vent structure.
Dynamic adjustment of vent parameters is essential for managing complex operating conditions. Gas generation and venting requirements vary significantly depending on the plastic material, part structure, or injection molding process parameters (such as injection speed, melt temperature, and mold temperature). Therefore, the vent design of precision injection molds must be adjustable. For example, by replacing vent plugs of varying thicknesses, adjusting the vacuum pump's pumping rate, or optimizing the porosity of the permeable steel, venting capacity can be dynamically matched to actual requirements to avoid burns or flash caused by insufficient or excessive venting.
Validation testing of the vent structure is the ultimate guarantee of design reliability. During the design phase, mold flow analysis software is used to simulate the gas exhaust process and assess the feasibility of the exhaust solution. During the processing phase, an airtightness tester is used to check the connectivity of the exhaust slots. During the mold trial phase, the product surface is observed for defects such as scorch marks, silver streaks, or bubbles. Short-shot testing is also used to analyze the gas exhaust path and gradually optimize the exhaust structure parameters. Through multiple rounds of iterative verification, a venting design solution that meets the requirements of precision injection molding can be ultimately determined, achieving both improved product quality and improved production efficiency.
