In the injection molding process of traffic light shells, poor mold venting is a key factor leading to air streaks. Air streaks typically manifest as flow marks or localized discoloration on the product surface. Their root cause lies in the failure of air and volatile plastic gases within the mold cavity to escape in time during injection molding, becoming trapped in the high-temperature molten metal and ultimately forming defects on the product surface. Solving this problem requires a comprehensive approach involving mold design optimization, process parameter adjustment, and material treatment.
Optimizing the mold venting system is crucial. Traditional molds for traffic light shell injection molding often rely on natural venting at the parting line. However, for products with complex structures and uneven wall thicknesses, such as traffic light shells, the parting line gap alone is insufficient for venting. Venting grooves need to be added to areas prone to gas accumulation, such as the parting line, cavity ends, and ribs. The depth of the venting grooves must be precisely controlled according to the material's flowability; too deep, and flash will occur; too shallow, and venting will be insufficient. For deep cavity structures or thin-walled areas, venting inserts or permeable steel can be embedded in the mold to enhance venting performance through a porous structure. In addition, the clearance between the ejector pin and the slide block can also serve as an auxiliary venting channel. By rationally designing the clearance dimensions, gas can be discharged while ensuring motion accuracy.
Precise adjustment of process parameters is crucial for reducing air marks. Injection speed and pressure in traffic light shell injection molding require multi-stage control: during the melt filling runner and gate stages, lower speeds and pressures are used to avoid air entrapment; as the melt enters the cavity, the speed and pressure are gradually increased to ensure rapid cavity filling. Excessive injection speed can lead to jetting, causing air to be trapped in the melt; insufficient speed prolongs the melt's residence time in the cavity, increasing the risk of gas absorption. Mold temperature must be matched to material properties. For PC or ABS materials commonly used in traffic light shells, appropriately increasing the mold temperature can reduce melt viscosity and promote gas escape, but excessively high temperatures must be avoided to prevent material decomposition. Barrel temperature control is equally critical, ensuring the plastic is fully melted without thermal degradation, reducing the generation of volatile gases.
Material pretreatment is fundamental to preventing air marks. Moisture in plastic raw materials vaporizes at high temperatures, forming water vapor. If not sufficiently dried, this water vapor condenses within the mold cavity and is reabsorbed by the melt, forming bubbles or gas streaks. Therefore, pre-drying is necessary based on material characteristics. For example, PC materials require drying at 120℃ for 3-4 hours, while ABS materials require drying at 80℃ for 4-6 hours, ensuring a moisture content below 0.05%. Furthermore, the presence of recycled materials or impurities in the raw materials may lead to decomposition and gas generation due to differences in thermal stability. Therefore, the proportion of recycled materials must be strictly controlled, and raw material screening must be strengthened.
The gating system design directly impacts venting efficiency. The gate location should avoid thin-walled areas of the product, prioritizing thick-walled sections or locations near the end of the mold cavity to reduce melt flow resistance and promote gas venting. For thin-walled shells, point gates or submarine gates can be used, with increased gate size reducing jetting. For thick-walled structures, side gates or fan gates can improve filling stability. Runner design should avoid right-angle turns and use rounded transitions to reduce pressure loss. Simultaneously, increasing the runner cross-section reduces melt velocity, providing more time for gas to escape.
Mold maintenance and cleaning are long-term measures to ensure venting performance. Venting channels are prone to failure during use due to plastic residue or cold material blockage, requiring regular cleaning and checks for unobstructed venting. For permeable steel inserts, avoid using corrosive cleaning agents to prevent damage to the material's oxide layer, which would reduce permeability. After mold shutdown, thoroughly clean any residue from the cavity to prevent cold material from hardening and clogging the venting structure.
The application of auxiliary venting technology can further improve venting efficiency. For molds with extremely high venting requirements, a vacuum suction system can be integrated. During mold closing, a vacuum valve extracts air from the cavity, creating a negative pressure environment to promote gas escape. Furthermore, the in-mold ejection mechanism can be combined with a piston-pin design to seal the molten material before it reaches the clearance position, preventing gas from being compressed within the cavity.
Solving the airflow defect in traffic light housings caused by poor mold venting requires a multi-dimensional collaborative approach, including venting system optimization, process parameter adjustment, material pretreatment, gating system improvement, mold maintenance, and the application of auxiliary technologies. A systematic solution can significantly reduce the occurrence rate of airflow defects, improve product surface quality and structural reliability, and meet the stringent requirements of traffic signal equipment for weather resistance and safety.
