Exhaust system design principles
The exhaust system is an essential component of injection molds, and the rationality of its design directly affects the molding quality and production efficiency of plastic products. During the injection molding process, if the air in the mold cavity and the volatiles generated by the heated plastic melt are not discharged in a timely manner, defects such as material shortages, bubbles, scorch marks, and silver streaks may occur in the product. It may even cause high-temperature burns on the product surface due to gas compression. Therefore, the design of the exhaust system must follow scientific principles to ensure smooth gas discharge while avoiding melt leakage and mold wear. A high-quality exhaust system should have characteristics such as efficient exhaust capacity, reliable structure, and easy processing and maintenance. It must be designed in combination with the product structure, plastic properties, and molding process.
The primary design principle of the exhaust system is “exhaust at the nearest location”, that is, the exhaust position should be as close as possible to the area where gas is most likely to gather. In the process of plastic melt filling the mold cavity, gas usually gathers at the last place the melt reaches, such as the end of the mold cavity, dead corners, the thinnest wall thickness, and the location where the weld mark is produced. For example, the four corners of box-shaped products and the roots of complex structures with ribs or bosses are all high-incidence areas for gas accumulation, and exhaust grooves need to be set here. For large products, exhaust grooves can be set at intervals along the direction of melt flow to gradually discharge the gas gathered along the way; for products with symmetrical structures, the exhaust grooves should be arranged symmetrically to ensure uniform exhaust in all areas. In addition, the parting surface is the most commonly used exhaust location because it is easy to process and has a large exhaust area, but care should be taken to maintain an appropriate distance from the edge of the mold cavity to prevent the molten material from overflowing from the parting surface.
The dimensional parameters of the vent groove must be strictly controlled to ensure smooth venting while preventing melt leakage. The depth of the vent groove is a critical parameter and should be determined based on the flowability of the plastic. For plastics with good flowability (such as PE and PP), the vent groove depth should be controlled at 0.02-0.03mm; for plastics with poor flowability (such as PC and ABS), the depth can be increased to 0.03-0.05mm. If the depth is too large, molten material can easily enter the vent groove, forming flash and even blocking the vent passage. If the depth is too small, venting efficiency is insufficient. The vent groove width is typically 3-5mm, and the length is determined by the cavity structure. It generally extends to the outside of the mold to ensure direct exhaust to the atmosphere. For precision molding requiring high vacuum, porous metal blocks or vent inserts can be used. Their microporous structure allows for efficient venting while preventing melt from passing through, making them suitable for micro-sized products or complex cavities. Furthermore, the vent groove should be designed with a trapezoidal or semicircular cross-section to facilitate processing and cleaning, and to avoid gas entrapment caused by right-angle structures.
The overall layout of the exhaust system needs to be coordinated with other structures of the mold to avoid interference with the ejection mechanism, guide mechanism, etc. When designing components such as ejectors and push rods, the clearance between them and the template can be used for auxiliary exhaust. The clearance should be controlled at 0.01-0.02mm, which can both exhaust and not affect the ejection accuracy. For moving parts such as sliders and lifters, exhaust grooves can be set on their mating surfaces to ensure that the gas generated during the core pulling process is smoothly discharged. The layout of the cooling water channel needs to avoid the exhaust groove to prevent the cooling water from communicating with the exhaust groove, causing the gas to condense into water and affect the exhaust effect. In addition, the exhaust system should be easy to clean and maintain. For exhaust grooves that are easy to clog, they can be designed as detachable insert structures, which should be removed regularly to clean plastic residues and oil stains to avoid a decrease in exhaust capacity after long-term use.
The design of the exhaust system needs to be dynamically optimized in combination with the specific molding process and plastic characteristics. For thermosetting plastics, since more volatiles are generated during the molding process, the size of the exhaust groove should be appropriately increased, and a special exhaust channel should be set up to lead the volatiles out of the mold to prevent them from accumulating in the cavity and affecting the quality of the product. For molding processes that use high-speed injection, the melt front flow rate is fast and the gas compression degree is high. It is necessary to increase the number and area of the exhaust grooves, or use a vacuum exhaust device to actively remove the gas in the cavity. During the mold trial stage, it is necessary to judge the exhaust effect by observing the defects of the product: if scorch marks appear, it means that the gas is compressed and produces high temperature, and the exhaust groove needs to be deepened or widened; if bubbles appear, it may be that the exhaust is not timely, and the exhaust point needs to be increased. In addition, for large molds or multi-cavity molds, a staged exhaust design can be adopted. The exhaust channels in different areas are opened in sequence according to the melt filling order to ensure efficient exhaust throughout the process.
