Injection molding product quality is closely related to the drying treatment of plastic raw materials. Most engineering plastics and modified plastics contain trace moisture. Under high-temperature melting conditions, excessive moisture will cause silver streaks, bubbles, cracking, and reduced mechanical properties of molded parts. Standardized drying parameter setting and standardized operation management are essential links to stabilize injection molding yield and improve product surface quality. Reasonable control of drying temperature, drying time, material storage and feeding management can fundamentally avoid molding defects caused by raw material moisture.
1. Raw Material Moisture Hazard and Drying PrincipleTrace moisture inside plastic particles will vaporize instantly after entering the high-temperature barrel. The generated water vapor cannot be discharged in time during the filling process, forming internal bubbles and surface silver streaks on the plastic parts. For hygroscopic materials such as PA, PC, and PEEK, long-term placement will absorb a large amount of ambient moisture, which will also lead to molecular chain degradation at high temperature, resulting in reduced toughness and easy brittle cracking of the product. The drying principle is to use hot air circulating heating to evaporate internal moisture of the raw materials, and continuously take away water vapor through circulating air flow, so as to control the raw material moisture within the safe forming range and ensure the stability of melt fluidity and physical properties.
2. Standard Drying Parameters of Common Injection Molding Materials
Different plastic materials have distinct heat resistance and hygroscopicity, and their matching drying temperature and time are also different. Conventional ABS materials adopt a drying temperature of 70 to 85 degrees Celsius with a constant drying time of 2 to 3 hours, which can meet the molding requirements of ordinary household parts. PC material requires a higher drying standard, with the temperature controlled at 110 to 125 degrees Celsius and the drying time maintained for 3 to 4 hours to completely remove internal bound water. Nylon series materials have strong hygroscopicity, with a drying temperature of 80 to 95 degrees Celsius and a drying time of more than 4 hours. Special high-performance materials such as PEEK need high-temperature drying at 150 to 160 degrees Celsius to avoid high-temperature hydrolysis and carbonization.
3. Selection and Use Specifications of Drying EquipmentHot air dryers are widely used for conventional plastic drying, suitable for small and medium batch production with simple structure and low cost. Dehumidifying dryers are adopted for high-precision and high-hygroscopic materials. They can reduce air dew point and achieve deep dehumidification and drying, which is not affected by ambient humidity. During equipment operation, the material loading capacity shall comply with the rated standard of the dryer, and excessive stacking shall be avoided to prevent uneven hot air circulation and incomplete internal drying. The air filter and heating pipeline shall be cleaned regularly to prevent dust accumulation from polluting raw materials and affecting drying efficiency.
4. Operation Management and Storage PrecautionsRaw materials shall be sealed and stored in a dry and ventilated warehouse after leaving the factory. Unused raw materials after opening shall be sealed in time to isolate humid air and prevent secondary moisture absorption. The dried raw materials shall be put into production immediately. If the machine needs to be shut down temporarily, the raw materials shall be kept warm and sealed in the dryer instead of being placed in the open air. Recycled materials shall be sorted and screened before use, and the drying time shall be appropriately extended on the basis of new materials to ensure that moisture meets the molding standard.
5. Common Defects and Drying Optimization Measures
Silver streaks and bubbles on the surface of plastic parts are mostly caused by incomplete drying of raw materials or excessive ambient humidity. It is necessary to appropriately increase the drying temperature or extend the constant temperature time, and replace the dehumidifying dryer for processing high-hygroscopic materials. Unstable melt fluidity and inconsistent product weight are related to uneven drying. The internal stacking amount of the dryer should be adjusted to ensure uniform circulation of hot air. Yellowing and carbonization of raw materials are caused by excessively high drying temperature or long-term constant temperature. The temperature parameter should be reduced reasonably and the drying timing should be strictly controlled.
6. Process SummaryRaw material drying is the basic guarantee for high-quality injection molding production. Enterprises must formulate targeted drying parameter standards according to material characteristics, standardize the use of drying equipment and raw material storage management procedures. Strictly controlling every link from warehousing, drying, feeding to temporary shutdown maintenance can effectively reduce defective rates such as silver streaks, bubbles and cracking, stabilize product dimensional accuracy and mechanical properties, and create basic conditions for efficient and stable mass production of injection molded parts.
