PA66 is a typical nylon engineering plastic with strong hygroscopicity, which is widely used in automotive, electronic and industrial parts due to its excellent mechanical strength, heat resistance and wear resistance. However, PA66 is very easy to absorb moisture from the air during storage, transportation and production. If the raw material is not fully dried before injection molding, the residual moisture inside will vaporize rapidly under the high temperature of the barrel, resulting in obvious defects such as silver streaks, bubbles, flow marks and surface fogging on the plastic part. In severe cases, moisture will cause hydrolytic degradation of the material, break the molecular chain of PA66, significantly reduce the mechanical strength and dimensional stability of the product, and even lead to a large number of scrapped parts. Therefore, the full-process moisture control from raw material drying to finished product storage is the core to ensure the molding quality of PA66 products.
Raw Material DryingRaw material drying is the most critical step to solve the problem of PA66 moisture absorption. It is recommended to use a hot air circulation dryer or a dehumidifying dryer. For mass production, a dehumidifying dryer is preferred, with a dew point controlled below -40℃, which can ensure a more thorough and stable drying effect. The drying temperature is controlled at 80-90℃, too high temperature will cause yellowing, oxidation and viscosity reduction of the raw material, while too low temperature will lead to incomplete drying and residual moisture.
The drying time is 3-4 hours for ordinary PA66, and 4-6 hours for glass fiber reinforced PA66, so as to ensure that the moisture content of the raw material is reduced to below 0.05%. After drying, the hopper holding temperature is set at 70-80℃ to prevent the dried raw material from absorbing moisture again in the hopper, and the staying time of the dried raw material in the hopper should not exceed 8 hours. In production, the drying effect can be judged by empty injection: if the melt has no bubbles and silver streaks, it means the drying is qualified.
Moisture Control During ProductionIn the process of formal production, it is necessary to focus on preventing the secondary moisture absorption of dried PA66 raw materials. First, control the humidity of the production workshop below 60%. In humid seasons such as rainy season and southern return to the south sky, close doors and windows and use a dehumidifier to reduce the air humidity. After the raw material package is opened, it should be used immediately, and the unused part must be sealed and stored in a moisture-proof container, and it is strictly prohibited to place it in the open air for a long time. The feeding hopper of the injection molding machine should be covered and sealed to reduce the contact between the raw material and humid air. If the machine is shut down for a long time, the remaining raw material in the hopper should be taken out in time, sealed and stored, and re-dried before reuse. In addition, the proportion of recycled material should be controlled not to be too high, because recycled PA66 absorbs moisture faster and has higher requirements for drying, and excessive use is more likely to cause product defects.
Injection Process AdjustmentAppropriate injection process adjustment can effectively improve the defects caused by trace residual moisture. Appropriately increase the back pressure of the injection molding machine to improve the compactness of the melt, facilitate the discharge of gas and trace moisture, and enhance the uniformity of melt plasticization. Reduce the screw speed to avoid excessive shear heat and prevent material degradation caused by local overheating. Adopt segmented slow injection to help the gas in the mold cavity be discharged smoothly, reduce the formation of bubbles and silver streaks. Avoid setting the barrel temperature too high to prevent further degradation of the material. Extend the cooling time appropriately to ensure the full shaping of the product, stabilize the product size, and avoid deformation and warpage caused by uneven shrinkage after demolding.
Finished Product Post-treatment and StorageFor PA66 products with high dimensional accuracy requirements, humidity conditioning treatment is required after molding. Put the finished product into boiling water or steam for 2-8 hours to let the product absorb balanced moisture in advance, so as to stabilize the size in the subsequent use process and avoid size change caused by later moisture absorption. After conditioning, the product should be cooled slowly to room temperature to avoid internal stress and deformation caused by rapid cooling. Finished products should be packaged with moisture-proof vacuum bags and equipped with desiccants. Especially for products that need long-term storage or export, strict moisture-proof protection must be done to avoid performance attenuation and dimensional deviation caused by moisture absorption during storage and transportation.
