The extrusion process includes both the insulation and sheath production process. The basic method of producing the plastic insulation layer and sheath layer of wires and cables is to use a single-screw extruder for continuous extrusion. Since the extruder has the characteristics of continuous extrusion, the production process of plastic insulation and sheath is also continuous.
The extrusion machine is generally a single screw extruder. Before extruding the plastic, check whether it is wet or has other debris, and then preheat the screw and add it to the hopper. During the extrusion process, the plastic loaded into the hopper enters the barrel with the help of gravity or the feeding screw, and is continuously pushed forward under the thrust of the rotating screw, gradually moving from the preheating section to the homogenization section; at the same time, the plastic is stirred and squeezed by the screw, and is transformed into a viscous flow state under the action of the external heat of the barrel and the shear friction between the plastic and the equipment, forming a continuous and uniform material flow in the screw groove. Under the action of the temperature specified by the process, the plastic is transformed from a solid state to a molten plastic object, and then the fully plasticized plastic is pushed into the die head through the push or stirring of the screw; the material flow reaching the die head is extruded from the die sleeve mouth through the annular gap between the mold core and the mold sleeve, and is extruded around the conductor or wire core to form a continuous and dense insulation layer or sheath layer, and then cooled and solidified to make wire and cable products.
Three stages of the extrusion process
The main basis for plastic extrusion is the plastic state of the plastic. The plastic molding process of plastic in the extruder is a complex physical process, which includes mixing, crushing, melting, plasticizing, exhausting, compacting and finally molding. People often divide the continuous process of extrusion into different stages according to the different reactions of plastics, namely.
1 Plasticization stage (mixing, melting and homogenization of plastics)
This stage is completed in the barrel of the extruder. Through the rotation of the screw, the plastic is transformed from a granular solid to a plastic viscous fluid. There are two sources of heat for plastics in the plasticization stage: one is the electric heating outside the barrel; the other is the friction heat generated by the rotation of the screw.
2 Molding stage (plastic extrusion molding)
This stage is carried out in the die. Due to the rotation of the screw and the pressure, the viscous fluid is pushed to the die. Through the mold inside the die, the viscous fluid is formed into the required extruded material of various sizes and shapes, and then wrapped around the wire core or conductor.
3. Forming stage (cooling and solidification of plastic layer)
This stage is carried out in the cooling water tank or cooling pipe. After cooling, the plastic extruded layer changes from an amorphous plastic state to a solid state.
Extrusion quality
Extrusion quality mainly refers to whether the plasticization of the plastic is good and whether the geometric dimensions are uniform, that is, whether the radial thickness is consistent and whether the axial outer diameter is uniform. In addition to the plastic itself, the factors that determine the plasticization are mainly temperature, shear strain rate and action time. Excessive extrusion temperature not only causes fluctuations in extrusion pressure, but also leads to the decomposition of plastics and may even cause equipment accidents. Although reducing the depth of the screw groove and increasing the aspect ratio of the screw are beneficial to the heat exchange of the plastic and prolonging the heating time to meet the requirements of uniform plasticization, it will affect the extrusion volume and cause difficulties for screw manufacturing and assembly. Therefore, the important factor to ensure plasticization should be to increase the shear strain rate generated by the screw rotation on the plastic, so as to achieve uniform mechanical mixing and balanced extrusion heat exchange, thereby providing a guarantee for uniform plasticization. The size of this strain rate is determined by the shear strain force between the screw and the barrel. It can be seen that under the requirement of ensuring the extrusion volume, the depth of the screw groove can be increased while increasing the speed. In addition, the gap between the screw and the barrel also affects the extrusion quality. When the gap is too large, the backflow and leakage of the plastic will increase, which will not only cause extrusion pressure fluctuations and affect the extrusion volume, but also increase the backflow, causing the plastic to overheat and cause the plastic to burn or difficult to form.