During the extrusion process, due to the resistance of the material flow, the change of the screw groove depth, and the obstruction of the filter screen, filter plate and die, etc., a certain pressure is generated inside the plastic along the axis of the barrel. This pressure is one of the important conditions for the plastic to become a uniform melt and obtain a dense plastic part.
Increasing the head pressure can improve the mixing uniformity and stability of the extruded melt and increase the density of the product. However, excessive head pressure will affect the output.
Like temperature, the pressure of the extrusion molding of the PVC pipe production line will fluctuate periodically with time. Such fluctuations also have an adverse effect on the quality of plastic parts. The change of screw speed and the instability of the heating and cooling systems are all causes of pressure fluctuations. In order to reduce pressure fluctuations, the screw speed should be controlled reasonably to ensure the temperature control accuracy of the heating and cooling device.
Temperature is one of the significant conditions for the smooth progress of extrusion molding.
Starting from powder or granular solid materials, high-temperature products are extruded from the die and undergo a complex temperature change process. Strictly speaking, the extrusion molding temperature of the PVC pipe production line should refer to the temperature of the plastic melt, but this temperature largely depends on the temperature of the barrel and screw. A small part comes from the frictional heat generated during mixing in the charging barrel, So the barrel temperature is often used to approximate the molding temperature.
Since the temperature of the charging barrel and the plastic are different in each section of the screw, the key issue is to control the temperature well in order to make the process of conveying, melting, homogenizing, and extruding the plastic in the barrel smoothly, so as to efficiently produce high-quality parts. The adjustment of the charging barrel temperature is realized by the heating and cooling system and temperature control system of the PVC extruder machine.
The head temperature must be controlled below the thermal decomposition temperature of the plastic. While the temperature at the die can be slightly lower than the head temperature, but the plastic melt should be guaranteed to have good fluidity.
In addition, the temperature fluctuation and temperature difference during the extrusion molding process of the PVC pipe extrusion line will cause defects such as residual stress, uneven strength at each point, and a dull surface of the plastic part. There are many factors that produce such fluctuations and temperature differences, such as unstable heating and cooling systems and changes in screw speed, but the quality of the screw design and selection has a large impact.