In the production process, since PVC is a kind of heat sensitive material, even if heat stabilizer is added, it can only increase the decomposition temperature and prolong the stable time without decomposition, which requires strict control of PVC molding temperature. Especially for RPVC, since its processing temperature is very close to the decomposition temperature, often resulting to decomposition due to improper temperature control. Therefore, the extrusion temperature should be determined according to the formula, extruder characteristics, die structure, screw speed, temperature measuring point position, thermometer error, temperature measuring point depth, etc.
I. Temperature control is an important factor in extrusion operation. The control factors of extrusion include barrel temperature, diameter temperature and die temperature. Low temperature, poor plasticization, dull pipe appearance, poor mechanical properties and product quality, all these can’t meet the requirements. If the temperature is too high, the material will decompose and the product will change color.
II. With the increase of screw speed, the extrusion quantity will increase, so as to increase the output. However, it is easy to produce poor plasticization, which will lead to rough inner wall and decrease of strength. For this problem, the head pressure should be adjusted to achieve the best output and quality. The temperature control of screw affects the conveying speed, plasticizing and melting quality of materials. The extrusion pipe needs cooling water, and reducing the screw temperature is conducive to improving the plasticizing quality. The temperature of the screw through the cooling water is about 50 ~ 70 ℃.
III. In the re-extrusion operation, it is very important to adjust the traction speed. After the material is extruded, melted and plasticized, it is continuously pulled out of the die head and into the setting device, cooling device and traction device. The traction speed should match the extrusion speed. Generally, in normal production, the pulling speed should be 1% ~ 10% faster than the extrusion speed of the pipe.
IV. Compressed air can blow up the pipe to keep it round. The pressure should be enough. If the pressure is too low, the pipe is not round, if the pressure is too high, the spindle cools down, the inner wall of the pipe cracks and is not smooth, and the quality of the pipe drops. At the same time, the pressure needs to be stable. If the pressure fluctuates, it is easy to produce crack in the pipe.
V. Different sizing and cooling methods are used to extrude different plastic products. The cooling medium can be air, water or other liquid, and the temperature needs to be controlled. The temperature is mainly related to the production efficiency and the internal stress of the product.