1. Structural principle
For the basic mechanism of the extrusion process, simply a screw rotates in the barrel and pushes the plastic forward. The screw structure is a bevel or a slope wound around the center layer for the purpose of increasing the pressure to overcome the large resistance. As far as the extruder is concerned, there are three kinds of resistances to be overcome during operation: one is the frictional force, which contains the friction between the solid particles (feed) and the wall of the cylinder and the number of turns before the screw is rotated (feeding zone) between them. There are two kinds of frictional forces; the second is the adhesion of the melt on the wall of the cylinder; the third is the internal flow resistance of the melt when it is pushed forward.
According to Newton's theorem, if an object is stationary in a certain direction, then the object is in equilibrium with the force balance in this direction. For a circumferentially moving screw, it has no axial movement, that is to say the axial force on the screw is in equilibrium. So if the screw exerts a large forward thrust on the plastic melt, it also applies the same magnitude but the same direction to the other object. Obviously, the thrust it exerts is on the thrust bearing behind the feed port. Most single-screws are right-handed threads, and if viewed from the back, they are rotated in the opposite direction, and they are rotated back out of the barrel by a rotary motion. In some twin-screw extruders, the two screws rotate in opposite directions in the two cylinders and cross each other, so they must be a right-handed, one-left, and the two screws are the same for the occlusal twin-screw. The direction of rotation is such that it must have the same orientation. However, in either case, the thrust bearing that bears the backward force still conforms to Newton's theorem.
2. Temperature principle
The extrudable plastics are thermoplastics that melt upon heating and solidify again upon cooling. Therefore, heat is required during the extrusion process to ensure that the plastic can reach the melting temperature. So where does the heat of the molten plastic come from? First, the pound feed preheating and the barrel/die heater may work and it is very important at start-up. In addition, the motor inputs energy, ie the resistance of the motor against the viscous melt. The frictional heat generated in the barrel when turning the screw is also the most important source of heat for all plastics, except for small systems, low speed screws, high melt temperature plastics and extrusion coating applications. In operation, it is important to recognize that the barrel heater is not the primary source of heat, and its effect on extrusion is less than we expected. The post-cylinder temperature is important because it affects the rate of solids transport in the teeth or in the feed. In general, in addition to being used for a specific purpose (such as light, fluid distribution or pressure control), the die and mold temperatures should reach or be close to the desired temperature of the melt.
3. Deceleration principle
In most extruders, the change in screw speed is achieved by adjusting the speed of the motor. The drive motor typically rotates at full speed of approximately 1750 rpm, which is too fast for an extruder screw. If it is rotated 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. A typical reduction ratio should be between 10:1 and 20:1. The first stage can be either gear or pulley block, but in the second stage it is best to use the gear and position the screw in the center of the last large gear. For some slow-running machines (such as twin-screws for UPVC), there may be three deceleration stages, and the maximum speed may be as low as 30 rpm or lower (ratio of 60:1). On the other hand, some very long twin-screws for agitation can run at 600 rpm or faster, thus requiring a very low deceleration rate and more deep cooling. If the deceleration rate is mismatched with the work, too much energy will be wasted. It may be necessary to add a pulley block between the motor and the first deceleration phase that changes the maximum speed, either to increase the screw speed even beyond the previous limit or to decrease the maximum speed. This increases the available energy, reduces the current value, and avoids motor failure. In both cases, the output may increase due to materials and their cooling needs.