During plastic injection molding, the influence of moisture on quality of plastic material is great, which not only may result in plastic cracks, whitening, air bubble, but also degrades the plastics and decreases the performance. Therefore, it’s necessary to control the moisture of plastic materials in injection molding.
There are many factors affecting the moisture of plastic materials in the process of drying that mainly include the drying temperature, airflow and time, dew-point temperature and environment. Drying temperature varies with different materials, which is critical for complete drying of the plastics. Insufficient temperature will lead to poor drying effect or prolong the drying time, while too high of the drying temperature will decompose the material and decrease the mechanical-physical properties. Although the greater the drying airflow, the better the drying effect, it requires large consumption, and there’s slight difference between actual minimum airflows required by different materials for drying. The common airflow is 1.5~3.5m3/ (kg.hr), and specific requirements please refer to the table of physical properties for different materials; Dew-point temperature ≤-20°C that can completely dry ordinary plastics, and the lower the dew-point temperature of the dry air, the better the drying effect.
As to control the moisture in plastic materials effectively, it is necessary to choose suitable drying equipment first of all. On the market, in the aspect of equipment, there are the common hot-air dryers, partial hot-air dehumidifying dryers and hot-air dehumidifying dryers; in the aspect of materials, the moisture required by different plastics is different due to the multiple varieties. Thus, the suitable choice of drying equipment should be made in accordance with the requirements of plastic materials and products. For plastics without strong hygroscopicity, such as: PP, PE, PS, PVC and PPS that have no requirement for the moisture, the common hot-air dryer can be adopted to reduce the cost. For medium hygroscopic plastics, such as ABS, PMMA, PC, POM and PBT that require the moisture ≤0.05%, the partial hot-air dehumidifying dryers can be adopted. For strong hygroscopic plastics, such as PA, PET and PES that require the moisture ≤0.02%, the hot-air dehumidifying dryers are required for complete drying.
Secondly, as to prevent plastics from moisture regain after drying. When the floor mount drying equipment is used, the dried materials require low dew-point drying hot air for the conveying, and shut-off suction box to avoid moisture regain. The materials left in the pipe with heat insulation, and the materials are not easy to become hygroscopic when it at high temperature state. When selecting the storage hopper to mount on the IMM, the hopper should be chosen based on the output, and don’t store the materials in the hopper too long as to avoid the moisture regain or mount the hot-air dryer to prevent moisture regain due to material temperature decrease. For occasions that require less moisture amount, the hot-air dryer still needs to option with the hot-air recycler or replenish the dehumidifying air for the drying.
In conclusion, suitable drying equipment should be chosen based on plastic variety and product to reach effective control of the moisture in the material.