The quality of plasticization has a great impact on the quality of plastic products, therefore, we should strive to improve the quality of plasticization, in general, the main factors affecting plasticization are the following.
(1) Barrel temperature
Barrel temperature is one of the key processes to ensure plasticizing quality, and its selection is related to the characteristics of the processed plastic. For amorphous plastics, the barrel temperature should be controlled above its viscous flow temperature, for crystalline plastics, it should be controlled above the melting point temperature, but the maximum barrel temperature of both must be controlled below their decomposition temperature. Due to the different types and grades of plastics, the viscous flow temperature, decomposition temperature, melting temperature and other characteristic parameters are also very different. For plastics with a small range from the viscous flow temperature (melting point temperature) to the decomposition temperature, the barrel temperature is slightly higher than the viscous flow temperature (or melting point temperature), and on the contrary, the barrel temperature can be slightly lower.
The temperature distribution of the barrel is generally low near the end (back end) of the hopper, and the temperature gradually rises from the back end to the front end, so that the material is gradually plasticized in the barrel. However, when the moisture content of the raw material is high, the back-end temperature can also be appropriately increased to facilitate the advance discharge of water vapor. For shear-sensitive plastics, because the shear friction heat of the screw helps plasticize, the temperature of the front section of the barrel can be appropriately lower than that of the middle section to prevent the overheating decomposition of the molten material.
(2) Plasticizing pressure
When the injection molding machine is plasticized, the screw backward is subjected to a certain resistance, and the melt at the end of the screw establishes a certain pressure, which is called the plasticizing pressure, and the size of the pressure can be adjusted by the relief valve (back pressure valve). In the plasticizing process, the size of the back pressure depends on the screw structure, the quality requirements of plastic parts, the type and characteristics of plastics. Under normal circumstances, increasing the plasticizing pressure will increase the temperature and uniformity of the molten material, which is conducive to the uniform gas discharge and color in the molten material, but it will reduce the plasticization rate and even extend the molding cycle.
For heat-sensitive plastics, such as HPVC, POM, etc., the back pressure is too high, which will lead to overheating degradation of molten material, and lower screw speed and back pressure can be used. For plastics with high viscosity, such as PC, PSF, PPO, HPVC, etc., the back pressure and speed should not be too high to prevent the overload of the screw rotation system; For plastics with low melt viscosity and melt viscosity sensitive to shear rate and temperature, lower back pressure and speed should also be selected, otherwise the plasticization speed is greatly reduced, easy to produce salivation phenomenon, and easy to produce overflow flash when filling the mold.
(3) Screw speed
The speed of the screw has a great influence on the plasticizing quality. Adjusting the speed of the screw can change the residence time of the raw material in the barrel, and the degree of the molten material being cut also changes. For heat-sensitive or high-viscosity plastics, such as HPVC, PC, PPO, etc., the screw speed should be reduced. The speed of the screw is an important process parameter that determines the size of the shear rate in the molten material and the amount of internal friction heat. The heat generated by the shear friction in the molten material is related to the viscosity and shear rate of the molten material. Generally, the speed and plasticizing capacity have an approximate direct proportional relationship, that is, the plasticizing capacity increases with the speed.
(4) Molding cycle
The molding cycle also has an effect on the plasticizing process. This is because the length of the molding cycle time will change the total residence time of the plastic in the cylinder, and extending this time is conducive to improving the uniformity of the molten temperature, but it is limited by the two factors of reducing the production efficiency and the permitted residence time of the molten material at the temperature of the cylinder. From the above analysis, it can be seen that in order to obtain high-quality plasticizer, the best working conditions are high speed and high back pressure preplasticizing at low barrel temperature without extending the molding cycle, because this condition is conducive to more mechanical work into the heat energy required for melting plasticizing, and can ensure that the screw has a higher plasticizing ability. Of course, whether this plasticizing condition can be used is largely determined by the thermal stability of this plastic.