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The Extrusion Process Conditions For Plastic Tube
Oct 19, 2017

During plastic tube production, the extrusion process could be controlled from temperature, screw cooling, haul off speed, air pressure, vacuum pressure, and cooling water etc aspects: 

(1) temperature: temperature is the main factor which affects plasticization and product quality. The temperature required for extrusion molding is the barrel temperature and the neck temperature and the die temperature. The temperature is too low, the plasticizing is not good, the pipe appearance is not smooth, and the mechanical properties are poor. When the temperature is too high, the material is easy to decompose and discolor. The temperature control shall be determined according to the raw material, formula, extruder and head structure, screw speed, etc.. Under normal circumstances, the temperature of the neck and body is lower than the die temperature, and the molding temperature of the powder is lower than that of the granule material.

(2) screw cooling: when extruding soft PVC and polyethylene pipe, because the material flow performance is good, the screw produces less friction heat, and the screw does not need cooling. For the hard PVC pipe, because of the high viscosity of the melt, the heat produced during extrusion is large, which leads to the decomposition or the rough wall of the pipe, thus water cooling to screw is needed. But giving cooling water to screw will reduce the amount of extrusion and influence the plasticizing effect, if the screw temperature drops too much, material anti pressure increases, resulting in decreased production, and even cause material not output, which damage screw or bearing. Therefore, the cooling water temperature should be strictly controlled, and the normal output water temperature should be about 70 - 80 degrees celsius.

(3) screw speed and extrusion speed: the screw speed depends on the size of the extruder and the size of the pipe. The speed increases, the pressure of the material in the barrel increases, extrusion rate increases, extrusion volume increases, and can let the material get strong shear impact and produce more inner friction heat, which is good for material mixing and uniform plasticizing, thus improving the mechanical properties of plastic products. But if the speed of the screw is too high, extrusion rate is too fast, it will make plastic get too strong shear impact, produce too much friction heat, so that the temperature of plastic in the center of the barrel too high, the extrudate "bloom" and expand, product quality comes down, and may occur bending or deformation problem due to the cooling time is too short, In addition, too high screw speed would make melt in the barrel get too much flow leakage and counter flow, increase energy consumption, and accelerate the screw wear; 

Therefore, the screw speed can be adjusted according to the screw structure and the processed material, the shape of the product and the cooling rate of the auxiliary machine. It is generally controlled at lO to 35r/min. Screw diameter increases, screw speed decreases; the same extruder, the processing pipe diameter increases, then decrease screw speed.

(4) Haul off/pull speed: in the extrusion operation, the adjustment of haul off speed is very important. After the material is melted and plasticized by the extruder, it is drawn from the machine head continuously and then drawn into cooling calibration water tank,haul off unit etc.. The haul off speed has impact on tube's wall thickness, size tolerances and out-looking. If the speed is too fast, the wall is too thin, bursting strength of tube wall goes down, the tube is easy to get deformation and bending. If haul off speed is too slow, the wall is too thick, the material is easy to accumulate between die and machine head. It must be stable and match with the extrusion speed. If haul off speed is unstable, then the pipe diameter is uneven. 

(5) compressed air pressure: compressed air can calibrate the pipe and maintain its roundness, the pressure is greater than atmospheric pressure. The air pressure depends on pipe diameter, wall thickness and material viscosity, generally take 0.02 - 0.05MPa, when it meet the requirements of roundness, control the pressure as small as possible. If the pressure is too great, the mandrel is cooled, the inner wall of the pipe is split, the quality of the pipe is decreased; If the pressure is too small, the pipe is not round, and the pressure should be stable. Producing hose does not need compressed air, but the air inlet entrance should communicate with the atmosphere, otherwise the pipe is not round, will be flat and stuck together.

(6) vacuum degree: If the degree of vacuum is too high, the adsorption force is too big, it is not necessary for the forming of the tube, and it also makes the haul off unit get too much load, and sometimes it will cause haul off unit shake, and make the haul off speed uneven and produce "blocking material"". And it will increase resistance to product running, which makes rough surface. If the vacuum is too low, the pipe can not be completely absorbed on the surface of the inner chamber of the calibration sleeve, thus the roundness of the pipe is not enough. 

(7) cooling water temperature: the cooling water temperature should be controlled between 10~20 degrees Celsius.

Above as reported by PVC123.