The twin-screw extruder was developed on the basis of a single-screw extruder. It has been widely used in extruded products due to its good feed performance, mixing and plasticizing performance, exhaust performance, and extrusion stability. Molding process. There are three auxiliary devices of twin screw extruder:
Straightening device:
The most common type of plastic extrusion waste is eccentricity, and the bending of various types of cores is one of the important reasons for eccentric insulation. In sheath extrusion, scratches on the sheath surface are also often caused by the bending of the cable core. Therefore, the straightening device in various extrusion molding units is essential. The main types of straightening devices are: roller type (divided into horizontal and vertical); pulley type (divided into single pulley and pulley group); winch type, which has multiple functions such as dragging, straightening, and stable tension; Wheel type (divided into horizontal type and vertical type).
Preheating device:
Core preheating is necessary for both insulation extrusion and sheath extrusion. For the insulation layer, especially the thin layer insulation, the existence of pores cannot be allowed. The core of the wire can be completely removed from the surface by oil and oil before being extruded. For sheath extrusion, its main function is to dry the cable core to prevent the possibility of air holes in the sheath due to the action of moisture (or moisture around the cushion layer). Preheating can also prevent the residual internal pressure of the plastic from being quenched during extrusion. In the process of extruding plastic, preheating can eliminate the cold wire entering the high-temperature head and the disparity in temperature formed when the die is in contact with the plastic. It can avoid the fluctuation of the extrusion pressure caused by the fluctuation of the plastic temperature. Extrusion quality. The extruder unit uses an electric core preheating device, which requires sufficient capacity and ensures rapid heating, so that the core preheating and cable core drying efficiency are high. The preheating temperature is restricted by the speed of the discharge line, which is generally similar to the temperature of the machine head.
Cooling device:
After leaving the machine head, the formed plastic extrusion cladding should be cooled and shaped immediately, otherwise it will be deformed by the force of gravity. The cooling method usually uses water cooling, and it is divided into rapid cooling and slow cooling according to different water temperatures. Quenching is the direct cooling of cold water. The quenching is beneficial to the shaping of the plastic extrusion cladding, but for crystalline polymers, due to the rapid cooling, it is easy to leave internal stress inside the extrusion cladding structure, which causes cracks during use. Generally PVC The plastic layer is quenched. Slow cooling is to reduce the internal stress of the product. Water of different temperatures is placed in sections in the cooling water tank to gradually cool the product. The extrusion of PE and PP is performed by slow cooling, that is, hot water, warm water, and cold water. Three-stage cooling.
