How to produce PVC pipe
1、 Process flow
PVC resin is used for the forming of PVC pipe, and stabilizer, lubricant, filler, etc. are added. These raw materials are kneaded according to the formula after proper treatment. If single screw extruder is used for extruding pipe, the kneaded powder shall be made into particles and then extruded for forming; if twin screw extruder is used, powder can be directly used for forming.
In addition, powder can be used to extrude pipe directly without granulation in production, but two points should be noted:
(1) The twin-screw extruder is the best way to extrude the powder directly. Because there is less mixing shearing and plasticizing process between the powder and the granule, the twin-screw extruder can strengthen the shearing and plasticizing and achieve the expected effect;
(2) Because the granular material is more dense than the powder material, and the heating and heat conduction are poor, the processing temperature of the powder material can be about 10 ℃ lower than that of the corresponding granular material.
2、 Process conditions and control
In the production process, because PVC is a heat sensitive material, even adding heat stabilizer can only increase the decomposition temperature, and extend the stability time without decomposition, which requires that the molding and processing temperature of PVC should be strictly controlled.
1. Temperature control
Temperature control is an important control factor in extrusion operation. The temperature of the control factors needed for extrusion is barrel temperature, diameter temperature and die temperature. The temperature is too low, the plasticization is not good, the appearance of the pipe is not glossy, the mechanical properties are poor, the product quality can not meet the requirements: the temperature is too high, the material will decompose, the product discoloration and other phenomena appear.
2. Screw speed
With the increase of screw rotation speed, the extrusion volume will increase, so the output can be increased, but it is easy to produce the phenomenon of poor plasticization, resulting in the roughness of the inner wall of the pipe and the decrease of the strength. At this time, the head pressure should be adjusted to achieve the best output and quality. The temperature control of screw affects the conveying rate, plasticization and melting quality of materials. The extrusion pipe needs cooling water to reduce the screw temperature, which is conducive to improving the plasticizing quality. The screw cooling water temperature is about 50 ~ 70.
3. Traction speed
In the re extrusion operation, it is very important to adjust the traction speed. After extrusion, melting and plasticizing, the material is drawn after continuous extrusion from the head, thus entering the setting device, cooling device, traction device, etc. the traction speed should match the extrusion speed. Generally, in normal production, the pulling speed should be about 1% ~ 10% faster than the extrusion speed of pipe.
4. Compressed air and pressure
Compressed air can blow the pipe and keep the pipe roundness. It is required that the pressure should be appropriate. The pressure is too small, the pipe is not round, the pressure is too large, the core mold is cooled, the inner wall of the pipe cracks, is not smooth, and the quality of the pipe decreases. At the same time, the pressure is required to be stable. For example, the pressure varies from big to small, and the pipe is prone to produce the status quo of slub.
5. Temperature of sizing device and cooling device
Different sizing methods and cooling methods are used to extrude different plastic products. The cooling medium can be air, water or other liquids, and the temperature needs to be controlled, which is mainly related to production efficiency, product internal stress, etc.
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