Flame retardant plastics, both halogenated and halogen-free types, are highly prone to yellowing, blackening, burning, gas marks and material degradation during injection molding. These defects are mainly caused by excessive heat generation, prolonged material residence time, poor exhaust and improper processing parameters. Reasonable control of raw material management, mould structure and process conditions can effectively avoid such problems and ensure stable forming quality.
Raw Material Control at the SourceStrict raw material management is the primary measure to prevent burning. The drying process must be carried out according to the parameters specified by the material manufacturer to eliminate surface and internal moisture, as residual moisture will vaporize and increase local temperature during high-temperature processing, accelerating decomposition. Over-drying must be strictly prohibited, as prolonged heating will cause premature degradation of flame retardants and resins, reducing thermal stability. Mixed use of different batches or different types of materials should also be avoided, as incompatible additives will react under high temperature, releasing corrosive gases and promoting material decomposition. Before production, the material barrel must be cleaned thoroughly with clean PP or HDPE resin to remove residual materials and carbon deposits from previous batches, preventing cross-contamination.
Mould Structure Optimization and Exhaust Improvement
Poor exhaust is one of the main causes of burning defects. The mould should be designed with sufficient exhaust grooves at the filling ends, weld lines and deep cavity positions, with the depth controlled between 0.012mm and 0.02mm to ensure smooth gas discharge without flash. For complex products with long flow paths, multiple exhaust points can be set up to shorten the gas discharge distance and reduce the chance of air trapping. The mould surface should be polished smoothly to minimize flow resistance, and sharp corners or sudden changes in cross-section should be avoided, as these areas are prone to melt stagnation and local overheating. Adding auxiliary exhaust pins or breathable inserts in deep cavity areas can effectively solve exhaust dead angle problems, ensuring complete gas discharge before the melt solidifies.
Injection Process Parameter AdjustmentAdjusting processing parameters is the core means to prevent burning. The barrel temperature should be set to the minimum range that ensures good melt fluidity, avoiding excessive temperature which accelerates the decomposition of flame retardants. The nozzle temperature should be controlled slightly lower than the front zone of the barrel to prevent material stagnation and degradation at the nozzle. The injection speed should be optimized with a segmented control method: slow speed at the beginning to avoid trapping air, medium speed for the main filling, and slow speed near the end to prevent excessive shear heat generation. The back pressure should be reduced appropriately to avoid excessive shear heat and melt temperature rise. The screw speed should also be controlled within a reasonable range to reduce frictional heat generation during plasticization.
Production Process and Equipment Maintenance
Regular maintenance of equipment is essential to prevent burning. The screw and barrel should be checked periodically for wear, as worn parts will increase shear heat and cause material stagnation. The check valve should be inspected for leakage to prevent material backflow and prolonged residence time. The mould cooling system should be kept in good condition to ensure uniform temperature distribution, avoiding local overheating. During production, the residence time of the material in the barrel should be minimized by setting appropriate cycle times and avoiding long pauses. If production is suspended for more than 15 minutes, the barrel should be purged with clean resin to remove stagnant flame retardant materials. The hopper dryer should be checked regularly to ensure stable temperature and airflow, preventing moisture reabsorption by the material.
By implementing these comprehensive measures, the burning and degradation problems of flame retardant plastics during injection molding can be effectively controlled, ensuring product appearance quality and mechanical performance while extending the service life of moulds and equipment.
