Engineering plastics are widely used in precision injection molding, and most high-performance engineering plastics have strong hygroscopicity. Moisture remaining inside plastic particles will cause silver lines, bubbles, shrinkage cavities, weld line defects and mechanical performance degradation of finished products. Plastic drying is not simply surface drying but requires systematic control of temperature, drying time, dew point, air volume, material thickness and environmental moisture prevention to completely remove internal bound water and ensure molding quality stability.
Classification of Plastic Hygroscopic CharacteristicsEngineering plastics are divided into hygroscopic and non-hygroscopic types. Hygroscopic materials including PC, PA, PBT, PET, PES and PPSU can absorb a large amount of bound water in the air. Ordinary hot air drying cannot remove deep moisture, and special low-dew-point dehumidifying dryers are mandatory. Non-hygroscopic materials such as PP, PE and POM only absorb surface free moisture, which can be removed by conventional hot air drying. Excessively high temperature and long-time drying will cause molecular degradation, yellowing and brittleness of materials, so drying processes must be formulated according to material characteristics.
Core Drying Parameter Control
Drying temperature must be lower than the material thermal deformation temperature to avoid particle agglomeration and thermal decomposition. Effective drying time starts after the material reaches constant temperature, excluding heating-up time. High-precision plastic molding requires a dew point below -40℃ to ensure continuous removal of internal bound water. During production, the material level of the drying hopper is kept stable at two-thirds capacity. Excessively thin material layers cause uneven drying, while over-thick stacking leads to poor hot air circulation and incomplete drying at the bottom. Different materials are matched with standard parameters of temperature, time and dew point to unify on-site operation specifications.
Standardized Operation of Drying EquipmentOrdinary hot air ovens are only suitable for low-hygroscopic materials. Raw materials shall be laid flat with limited thickness and turned regularly to ensure uniform hot air penetration. The oven must be sealed to prevent damp air backflow. Dehumidifying dryers for high-end engineering plastics need regular inspection of molecular sieve regeneration effect to guarantee stable low dew point. The circulating air volume shall match the hopper volume to eliminate damp air dead corners. After drying, materials shall be kept warm in the injection machine hopper to prevent secondary moisture absorption. Especially in high-humidity workshops and rainy seasons, constant temperature insulation is an essential measure to stabilize molding quality.
On-site Rapid Drying Quality InspectionThree practical detection methods are adopted for on-site judgment. First, visual inspection of injection strips: qualified dried materials produce smooth strips without silver lines, bubbles and foggy marks, with dense and pore-free fracture surfaces. Second, high-temperature plate verification: no bubbles or water vapor overflow when particles are heated at high temperature, indicating qualified drying. Third, precision instrument detection: professional moisture testers are used for mass production, with qualified moisture content controlled below 0.02% for most engineering plastics. These methods can quickly screen unqualified materials and avoid batch defective products.
Environmental Moisture Prevention and Daily Management
Unpacked raw materials and recycled crushed materials are more susceptible to moisture and need sealed storage and prolonged drying time. Workshop ambient humidity shall be controlled below 65%. Frequent opening of drying hoppers is prohibited to reduce material contact with humid air. Different materials are strictly forbidden to share drying equipment to avoid parameter confusion and cross contamination. Regular cleaning of dryer filters and ventilation pipelines ensures smooth hot air circulation and stable drying efficiency.
Abnormal Analysis and Correction MeasuresIf silver lines and water patterns still appear after drying, the main reasons include unqualified dew point, blocked air ducts and failure of molecular sieve regeneration. If materials turn yellow and brittle, the drying temperature is too high or the drying time is excessive, and parameters shall be reduced immediately. Intermittent quality fluctuations are usually caused by unstable material level and non-standard operation. Standardizing drying parameters, equipment maintenance and on-site operation processes can completely solve moisture-related defects, ensure sufficient and stable drying of engineering plastics, and provide reliable guarantee for high-quality injection molding production.
