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In the production process, since PVC is a heat-sensitive material, even adding a heat stabilizer can only increase the decomposition temperature and extend the stable time without decomposition, which requires strict control of the molding temperature of PVC. Especially RPVC, because its processing temperature is very close to the decomposition temperature, often decomposes due to improper temperature control. Therefore, the extrusion temperature should be determined according to the formula, the characteristics of the extruder, the structure of the head, the screw speed, the position of the temperature measuring point, the error of the temperature measuring instrument, the depth of the temperature measuring point, etc.
1. Temperature control. Temperature control is an important factor in extrusion operations. The controlling factors for extrusion are barrel temperature, diameter temperature and die temperature. Low temperature, poor plasticization, dull appearance of the pipe, poor mechanical properties, and poor product quality: the temperature is too high, the material will decompose, and the product will change color.
2. The screw speed. and screw speed increase, the amount of extrusion increases, thereby increasing the output, but it is easy to produce poor plasticization, resulting in rough tube inner wall and reduced strength. At this time, the head pressure should be adjusted to optimize the output and quality. The temperature control of the screw affects the conveying speed, plasticizing and melting quality of the material. The extrusion pipe needs cooling water, and reducing the screw temperature is beneficial to improve the quality of plasticization. The temperature of the screw through the cooling water is about 50 ~ 70 ℃.
3. Towing speed. In re-squeezing operation, it is very important to adjust the towing speed. After the material is squeezed, melted and plasticized, it will be continuously pulled out of the machine head and enter the setting device, cooling device and traction device. The traction speed should match the extrusion speed. Generally, in normal production, the traction speed should be about 1%~10% faster than the extrusion speed of the pipe.
4. Compressed air. Compressed air can blow up the pipe and keep the pipe round. The pressure should be sufficient. The pressure is too low, the tube is not round, the pressure is too large, the mandrel is cooled, the inner wall of the tube is cracked, not smooth, and the quality of the tube is reduced. At the same time, the pressure needs to be stable, such as pressure fluctuations, and bamboo is easily generated in the pipe.
5. The temperature of the sizing device and the cooling device. Extrude different plastic products, using different sizing and cooling methods. The cooling medium can be air, water or other liquids. The temperature needs to be controlled. The temperature is mainly related to the production efficiency and the internal stress of the product.
