In addition to the cylinder temperature is the key process parameters to ensure the quality of plasticization, plasticization pressure (back pressure), screw speed, molding cycle are also the main factors affecting the plasticization:
(1) Plasticizing pressure (back pressure)
When the plastic granulator is plasticizing, the screw retreat is subject to a certain resistance, and the screw end melt material establishes a certain pressure. This pressure is called plasticizing pressure, also called back pressure. The pressure can be adjusted by a relief valve (back pressure valve). In the plasticization process, the size of the back pressure depends on the structure of the screw, the quality requirements of the plastic parts, and the type and characteristics of the plastic. Under normal circumstances, increasing the plasticizing pressure will increase the temperature and uniformity of the molten material, and will facilitate the discharge of gas in the melt and the uniformity of the color, but it will reduce the plasticization rate and even increase the molding cycle.
Heat-sensitive plastics, such as HPVC, POM, etc., with high back pressure, can lead to overheated degradation of the frit, which can be used with lower screw speed and back pressure; plastics with higher viscosity such as PC, PSF, PPO, HPVC, etc. Pressure and speed should not be too high to prevent overloading of the screw rotation system; melt viscosity is low, and melt viscosity is sensitive to shear rate, temperature sensitive plastics. Use low back pressure and rotation speed, otherwise the plasticization speed will decrease. Large, easy to produce "casting" phenomenon, overflow flash easily when filling.
(2) Screw speed
Screw speed has a great influence on the quality of plasticization. Adjusting the speed of the screw can change the residence time of the raw material in the barrel. The degree to which the melt is sheared also changes. For heat-sensitive or high-viscosity plastics, such as HPVC, PC, PPO, etc., the screw speed should be reduced. The rotational speed of the screw is an important process parameter that determines the magnitude of the shear rate in the melt and how much the internal friction heat is. The heat generated by the shear friction in the melt is related to the viscosity and the shear rate of the melt. In general, the rotational speed and the plasticizing capacity have an approximately proportional relationship, that is, the plasticizing ability increases as the rotational speed increases.
(3) Extrusion cycle
The extrusion cycle also affects the plasticization process. This is because the length of the molding cycle will change the total residence time of the plastic in the barrel. Prolonging this time is beneficial to improve the uniformity of the melt temperature, but it is subject to the reduction of production efficiency and the allowable residence time of the melt at the barrel temperature. Two factors limit.
From the above analysis, we can see that in order to obtain a high-quality plasticized melt, without optimizing the extrusion cycle, the best working conditions are high rotational speed and high back pressure pre-molding at low temperatures. The conditions are conducive to the conversion of more mechanical work to the heat energy required for melting and plasticizing, and it is also possible to ensure that the screw has a relatively good plasticizing ability. Of course, whether or not this plasticizing condition can be used is largely determined by the thermal stability of the plastic.