One of the most common types of plastic extrusion waste is eccentricity, and various forms of bending of the wire core are one of the important causes of insulation eccentricity. In jacket extrusion, scratches on the jacket surface are also often caused by bending of the cable core.
The main forms of the double screw extruder straightening device are drum type (divided into horizontal and vertical), pulley type (divided into single pulley and pulley group), and other functions, pressure wheel type (divided into horizontal and vertical) and so on.
Core preheating is necessary for both insulation extrusion and jacket extrusion. Insulation layers, especially thin layers of insulation, cannot allow for the presence of air holes. The wire core is preheated at a high temperature before extrusion to completely remove the moisture and oil on the surface.
For sheath extrusion, its main function is to dry the cable core to prevent the possibility of pores appearing in the sheath due to moisture (or moisture surrounding the cushion). Preheating also prevents the effect of residual internal pressure due to quenching of the plastic during extrusion. During the extrusion process, preheating can eliminate the huge temperature difference formed when the cold wire enters the high-temperature die when it contacts with the plastic, so as to avoid the fluctuation of the extrusion pressure caused by the fluctuation of the plastic temperature, and finally stabilize the extrusion volume and guaranteed extrusion quality. The double screw extruder extrusion unit adopts electric core preheating device, which requires sufficient capacity and ensures rapid heating, so that the core preheating and cable core drying efficiency is high. The preheating temperature is restricted by the pay-off speed, and is generally similar to the temperature of the machine head.
After the molded plastic extrusion layer leaves the machine head, it should be cooled and shaped immediately, otherwise it will be deformed under the action of gravity. The cooling method is usually water cooling. It is divided into rapid cooling and slow cooling according to the water temperature. Rapid cooling is the direct cooling of cold water, which is beneficial to the shaping of the plastic extruded layer. However, for crystalline polymers, due to rapid thermal cooling, internal stress is likely to remain in the structure of the extruded layer, resulting in cracks during use. Generally, the PVC plastic layer is quenched.
The slow cooling of the double screw extruder is to reduce the internal stress of the product. Water of different temperatures is placed in the cooling water tank to gradually cool the product and shape it. The extrusion of PE and PP is carried out by slow cooling, it is cooled in three stages of hot water, warm water and cold water.
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