First, heat the extruder to the predetermined temperature, then start the screw and feed materials simultaneously. Extrusion molding is a continuous process, with the key being the adjustment of initial extrusion parameters. It is necessary to adjust process parameters such as the temperature of each heating zone in the extruder barrel, the temperature of the die head, and the screw rotation speed according to the extrusion process properties of the plastic and the structural characteristics of the extruder die head, so as to control the temperature and pressure distribution of the material within the barrel. According to the requirements for the shape and dimensions of the product, adjust the die size, concentricity, traction equipment, and other devices to control the exit swelling and shape stability of the extrudate, thereby ultimately controlling the output and quality of the extrudate until the extrusion reaches normal operation for continuous production.
Different types of plastics require different screw characteristics and process conditions. The process conditions during extrusion have a significant impact on product quality, particularly the plasticization state, which directly affects the product's appearance and physical-mechanical properties. The main factors influencing plasticization effects are temperature and shear force.
The material's temperature primarily comes from the external heating of the barrel, followed by the shear force of the screw on the material and friction between the material particles. After normal operation is achieved, the heat generated by shear and friction even becomes more critical.
As the temperature increases, the material's viscosity decreases, which is conducive to plasticization. Meanwhile, it reduces the melt pressure, leading to faster extrusion. However, if the temperature of the die head and mold is too high, the shape stability of the extrudate will be poor, the product shrinkage will increase, and the product may even turn yellow or develop bubbles, causing the molding process to fail.
When the temperature decreases, the material's viscosity increases, the pressure in the die head and mold rises, the product density becomes higher, and the shape stability improves, but the exit swelling becomes more severe. In this case, the traction speed can be appropriately increased to reduce the dimensional changes of the product caused by swelling. However, the temperature must not be too low, as this will result in poor plasticization and excessively high melt viscosity, increasing power consumption. The temperatures of the mold and core must be consistent; if there is a significant difference, the product may bend inward or outward, or even twist. Increasing the screw rotation speed can strengthen the shear force on the plastic, which is beneficial for mixing and plasticization. Moreover, the melt viscosity of most plastics decreases with an increase in screw rotation speed.
