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PVC Pipe Extrusion Process

May 08, 2018

PVC pipe extrusion process:
Production Process Raw Material + Additive Preparation → Mixing → Feeding and Feeding → Forced Feeding → Cone Twin-screw Extruder → Extrusion Molding → Sizing Set → Spray Vacuum Setting Box → Immersion Cooling Water Tank → Ink Printing Machine → Crawler Traction Machine→lift knife cutting machine→pipe stacking rack→finished product inspection packaging
The PVC pipe is formed by using SG-5 type PVC resin, and stabilizers, lubricants, fillers, pigments, etc. are added. These raw materials are kneaded according to the recipe after proper processing. If the extruder pipe is a single screw extruder, it should be The kneaded powder is granulated and then extruded: if a twin-screw extruder is used, the powder can be directly formed
In addition, in the production process can be different from the above shown, that is, take the powder directly extrusion pipe without granulation, but should pay attention to two points: First, the direct extrusion of the powder is best to use twin-screw extruder Because the powder and the pellets are less than one time, the shearing and plasticizing process is reduced. Therefore, the twin-screw extruder can strengthen the shear plasticization to achieve the desired effect. Second, the pellets are denser than the powder and heated. And poor thermal conductivity, so the processing temperature of the powder can be about 10 °C lower than the processing temperature of the corresponding pellets.
Extrusion process control:
    In the production process, since PVC is a heat-sensitive material, even if the heat stabilizer is added, the decomposition temperature can only be increased and the stabilization time can be prolonged without being decomposed. This requires that the molding temperature of the PVC should be strictly controlled. Especially RPVC, due to its processing temperature and decomposition temperature is very close, often due to improper temperature control caused by decomposition. Therefore, the extrusion temperature should be determined based on factors such as formula, extruder characteristics, head structure, screw speed, position of temperature measurement point, temperature measurement instrument error, and temperature measurement point depth.
(1) Temperature control Temperature is an important factor affecting the quality of plasticization and product quality. The temperature is too low, the plasticization is poor, and the appearance and mechanical properties of the pipe are poor. After the shunt holder, the weld marks are obvious or the welds have low strength. Due to the thermal stability of PVC
Poor performance. Decomposition occurs when the temperature is too high, resulting in discoloration and scorching, making operation impossible. The specific temperature should be determined according to the raw material formula, extruder and head structure, screw speed operation and other comprehensive conditions.
(2) Screw cooling Because RPVC melt has large viscosity and poor fluidity, in order to prevent the screw from heating up due to excessive frictional heat, causing the screw binder to decompose or make the inner wall of the pipe rough, the screw temperature must be reduced, so that the material can be plasticized. The inner surface of the tube is bright and improves the quality of the inside and outside of the tube. The screw temperature is generally controlled between 80 and 100°C. If the temperature is too low, the back pressure increases, the output decreases, and even accidents in which the material cannot be squeezed out and damage the screw bearing may occur. Therefore, the screw cooling should control the temperature of the outlet water not lower than 70~80°C. The cooling method is water cooling with a copper pipe inside the screw.
(3) Screw speed The speed of the screw speed is related to the quality and output of the pipe. The speed of the screw is adjusted according to the specifications of the extruder and the specifications of the pipe. In principle, the large machine squeezes the small pipe and the speed is low: the small machine squeezes the large pipe and the speed is high. General ф 45 single screw extruder, screw speed is 20~40r/min, ф90 single screw extruder, screw speed is 10~20r/min; twin screw extruder is 15~30r/min. Although increasing the screw speed can increase the output in a certain period of time, if the output is too high and the material and screw structure are not changed, the material will be plasticized, the pipe wall will be rough, and the pipe strength will be reduced.
(4) The sizing pressure and degree of vacuum When the tube blank is extruded out of the die, the temperature is still high. In order to obtain a lower roughness, the correct size and geometry of the tube, the tube must be immediately sized and cooled as it exits the die. RPVC pipe generally adopts the method of external pressure outer diameter. The compressed air in the pipe makes the outer surface of the pipe close to the inner wall of the sizing sleeve and maintain a certain degree of roundness. Generally, the compressed air pressure ranges from 0.02 to 0.05 MPa, and the pressure requirement is stable. A storage cylinder is provided to stabilize the compressed air pressure. The pressure is too small, the pipe is not round and the pressure is too high. First, the gas plug is easily damaged and causes air leakage. The second is the easy cooling of the mandrel, which affects the quality of the pipe. The pressure suddenly changes and the pipe becomes bamboo-shaped. If vacuum gauge is adopted, the vacuum degree is about 0.035~0.070Mpa.
(5) Traction rate Traction rate directly affects the output of pipe production, and at the same time it affects the wall thickness of pipe. Unsteady traction rate will cause the pipe diameter to appear large and small. The pulling speed should closely match the extrusion rate of the pipe. In normal production, the pulling speed should be slightly faster than the extrusion line speed by 1% to 10%. The slower the traction rate, the thicker the wall, the faster the traction rate, and the thinner the wall, the more the longitudinal shrinkage of the pipe increases and the internal stress increases, which affects the pipe size, pass rate and use effect. Adjusting the traction rate in production can be done by the following simple method. The extruded pipe is placed in the traction track, but the track does not clamp the pipe. The difference between the track and pipe line speed is observed. If the traction rate is slower than the extrusion rate, the speed should be adjusted to the wall. Thickness meets the requirements.

 

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