One of the most common types of plastic extrusion waste is eccentricity, and various types of bending of the 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. Therefore, straightening devices in various extrusion units are essential.
The main types of straightening devices for twin screw extruder are : roller type (divided into horizontal type and vertical type) ; pulley type (divided into single pulley and pulley group) ; winch type, which can also be used for dragging, straightening and stabilizing tension 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. For the insulation layer, especially the thin layer insulation, the existence of pores cannot be allowed, and the wire core can be completely removed from the surface moisture and oil by preheating at high temperature before extrusion.
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.
In the process of plastic extrusion, preheating can eliminate the huge temperature difference formed when the cold wire enters the high temperature die and contact with the plastic at the die mouth, avoiding the fluctuation of the extrusion pressure caused by the fluctuation of the plastic temperature, thereby stabilizing the extrusion volume and ensuring Extrusion quality.
The electric heating wire core preheating device is used in the extrusion unit, which requires sufficient capacity and ensures rapid temperature rise, so that the wire 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, and according to the water temperature, it is divided into rapid cooling and slow cooling. Rapid cooling is the direct cooling of cold water. Rapid cooling is beneficial to the shaping of the plastic extrusion coating. However, for crystalline polymers, due to rapid heat cooling, internal stress is likely to remain in the structure of the extrusion coating, resulting in cracks during use. Generally, PVC The plastic layer is quenched.
Slow cooling is to reduce the internal stress of the product. Water with different temperatures is placed in the cooling water tank in sections to gradually cool the product and shape it. The extrusion of PE and PP is carried out by slow cooling, that is, through three-stage cooling of hot water, warm water and cold water.
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