Preheating Device
Core preheating is essential for both insulation and sheath extrusion. For insulation layers, especially thin ones, porosity is unacceptable. High-temperature preheating of the core before extrusion thoroughly removes surface moisture and oil. For sheath extrusion, its main function is to dry the core, preventing porosity in the sheath due to moisture (or moisture from the wrapping layer). Preheating also prevents residual internal pressure caused by rapid cooling of the plastic during extrusion. During plastic extrusion, preheating eliminates the significant temperature difference that occurs when cold wire enters the high-temperature die head and comes into contact with the plastic at the die, preventing fluctuations in extrusion pressure caused by plastic temperature fluctuations, thus stabilizing the extrusion volume and ensuring extrusion quality. Extrusion units typically use electrically heated core preheating devices with sufficient capacity and rapid heating to ensure high efficiency in core preheating and drying. The preheating temperature is limited by the unloading speed and is generally similar to the die head temperature.
Cooling Device
After the extruded plastic layer leaves the die head, it should be cooled and shaped immediately; otherwise, it will deform under gravity. Cooling is usually done with water, and depending on the water temperature, it is divided into rapid cooling and slow cooling. Rapid cooling involves direct cooling with cold water. Rapid cooling is beneficial for shaping the extruded plastic layer, but for crystalline polymers, sudden heating and cooling can easily leave residual internal stress within the extruded layer structure, leading to cracking during use. Rapid cooling is generally used for PVC plastic layers. Slow cooling, on the other hand, reduces internal stress in the product by placing water of different temperatures in sections in the cooling water tank, allowing the product to gradually cool and shape. Slow cooling is used for PE and PP extrusion, involving three stages of cooling: hot water, warm water, and cold water.
