What is the principle of twin-screw extruder temperature and deceleration

Extrudable plastics are thermoplastics that melt when heated and solidify again when cooled. Therefore, heat is needed during the extrusion process to ensure that the plastic can reach the melting temperature. So where does the heat to melt the plastic come from? First of all, the preheating of the weighbridge feed and the cylinder/mold heater may work and are very important at startup. In addition, the motor input energy, that is, the frictional heat generated in the cylinder when the motor overcomes the resistance of the viscous melt to rotate the screw, is also The most important source of heat for all plastics, except of course small systems, slow screw speeds, high melt temperature plastics and extrusion coating applications. In operation, it is important to realize that the barrel heater is not actually the primary source of heat and that it contributes less to extrusion than we might expect. Rear barrel temperature is important because it affects toothing or the rate at which solids are conveyed in the feed. In general, die and mold temperatures should be at or close to the desired temperature of the melt, except for a specific purpose (such as glazing, fluid distribution, or pressure control). In most extruders, the change in screw speed is achieved by adjusting the speed of the motor. The drive motor usually rotates at a full speed of about 1750rpm, which is too fast for an extruder screw. If it is turned at such a fast speed, too much frictional heat will be generated, and a uniform, well-stirred melt cannot be prepared because the residence time of the plastic is too short. Typical reduction ratios should be between 10:1 and 20:1. Either gear or pulley block can be used for the first stage, but it is better to use gears and position the screw in the center of the last large gear for the second stage. For some slow-running machines (such as twin-screw for UPVC), there may be three deceleration stages, and the maximum speed may be as low as 30rpm or lower (ratio up to 60:1). On the other hand, some very long twin-screws for agitation can run at 600rpm or faster, thus requiring a very low reduction rate and more deep cooling. If the deceleration rate is wrongly matched to the work, too much energy will be wasted. At this time, it may be necessary to add a pulley block between the motor and the first deceleration stage that changes the maximum speed, which will either increase the screw speed or even exceed the previous limit, or reduce the maximum speed. This increases the energy available, reduces the current value and avoids motor failure, in both cases the output may increase due to the material and its cooling needs.