The molding of thermosetting plastics is to gradually melt and plasticize the plastics on the plastic pipe production line. At the same time, a chemical reaction occurs. Under the continuous action of pressure and heat, the molding is filled and cross-linked to solidify into parts. The molding principles of various molding processing methods such as compression molding, transfer molding and injection molding of thermoset plastics belong to this category.
Two processes occur simultaneously when thermosetting plastic is plasticized by heat in the barrel. One is that the mobility of resin molecules increases due to heat, and the viscosity of the resin decreases. The second is that the chemical reaction speed of the resin molecules increases due to heat, forming cross-linked bonds between the molecules and increasing the viscosity. Which of these two processes dominates depends on the temperature and heating time.
When the temperature is too low, the plastic of the plastic pipe production line may not be in full contact, and of course it does not have the fluidity necessary for injection. After the plastic is heated and melted, the viscosity of the plastic will decrease significantly as the temperature increases or the heating time increases. After the viscosity drops to a certain minimum point, the plastic will increase the cross-linking reaction as the temperature continues to rise or the heating time continues to extend, so the viscosity will increase significantly, and the plasticizing force will rise beyond the minimum point.
In order to facilitate the operation, the plastic should maintain the flow behavior for a long time at the temperature of the barrel, so the temperature of the barrel should be selected as the temperature corresponding to the longer point of the minimum torque. That is to say, the temperature of the barrel is generally selected to be close to the lower limit on the premise that the plastic melt contact flow can be satisfied, so as to avoid the occurrence of the crosslinking degree of the plastic in the barrel exceeding a certain degree during the injection operation. One category makes injection molding difficult to carry out, and even the screw is blocked due to the high curing level.
For example, phenolic plastics often melt at around 90°C, and begin to exotherm by themselves at 100-120°C. The plastic has been deeply softened and reaches its maximum fluidity range at around 130-140°C. The barrel temperature of the plastic pipe production line is generally controlled at 80~90℃ in the high temperature section, and never exceed 95℃, and the low temperature heating section is usually 50~60℃. After the plastic is plasticized at this temperature, it is heated again due to high-speed friction as it flows through the nozzle, and the temperature rises to 110~125°C.