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Mechanical principle

　　The basic mechanism of extrusion is simple-a screw rotates in the barrel and pushes the plastic forward. The screw is actually an inclined plane or slope, wound around the center layer. The purpose is to increase pressure in order to overcome greater resistance.

　　For an extruder, there are three types of resistance to overcome: the friction of solid particles (feed) against the barrel wall and the mutual friction between the first few turns of the screw (feed zone); the melt Adhesion on the cylinder wall; internal flow resistance when the melt is pushed forward.

　　Newton explained that if an object does not move in a given direction, the forces on the object are balanced in that direction. The screw does not move axially, although it may turn laterally quickly around the circumference. As a result, the axial force on the screw is balanced, and if it applies a large forward thrust to the plastic melt, it also applies the same backward thrust to an object. Here, the thrust it exerts is on the bearing behind the feed port-the thrust bearing.

　　Most single screws are right-handed, like screws and bolts used in woodworking and machines. If viewed from the back, they are turning in the opposite direction, as they try their best to unscrew the barrel backwards. In some twin-screw extruders, the two screws rotate in opposite directions in the two barrels and cross each other, so one must be rightward and the other must be leftward. In other bite twin screws, the two screws turn in the same direction and must therefore have the same orientation. However, in either case there is a thrust bearing that absorbs backward forces, and Newton's principle still applies.

　　2. Thermal principle

　　Extruded plastics are thermoplastics-they melt when heated and solidify again when cooled. Where does the heat from melting plastic come from? Feed preheating and barrel / mold heaters may work and are very important at startup, but the motor inputs energy—the motor is generated when the screw is turned against the resistance of the viscous melt Friction heat in the barrel-is the most important heat source for all plastics, except small systems, low speed screws, high melt temperature plastics and extrusion coating applications.

　　For all other operations, it is important to realize that the cartridge heater is not the main heat source in operation, and therefore has a smaller effect on extrusion than we expected (see principle 11). The rear barrel temperature may still be important because it affects the rate of solids transport in the teeth or feed. Die and die temperatures should generally be the desired melt temperature or close to this temperature unless they are used for a specific purpose like varnishing, fluid distribution or pressure control.

　　3. Deceleration principle

　All ordinary plastics have a drop-in-shear characteristic, meaning that the viscosity becomes lower as the plastic moves faster and faster. This effect is particularly pronounced with some plastics. For example, some PVCs increase the flow rate by 10 times or more when the thrust is doubled. In contrast, the LLDPE shear force does not drop too much, and the flow rate only increases by 3 to 4 times when the reasoning is doubled. The reduced shear reduction effect means high viscosity under extrusion conditions, which in turn means more motor power is required.

　　This may explain why LLDPE runs at a higher temperature than LDPE. The flow rate is expressed by the shear rate, which is about 100s-1 in the screw channel, between 100 and 100s-1 in most die openings, and greater than 100s-1 in the gap between the thread and the cylinder wall and some small die clearances. Melt coefficient is a commonly used measurement of viscosity but is reversed (eg flow / thrust rather than thrust / flow). Unfortunately, its measurement may not be a true measurement in an extruder with a shear rate of 10 s-1 or less and a fast melt flow rate.

　　11. The motor and the cylinder are opposed to each other

　　Why is the control effect of the cylinder not always the same as expected, especially in the measurement area? If the cylinder is heated, the viscosity of the material layer at the cylinder wall becomes smaller, and the motor needs more energy to run in this smoother cylinder. less. The motor current (amperage) decreases. Conversely, if the cylinder is cooled, the melt viscosity at the cylinder wall increases, the motor must rotate more forcefully, the amperage increases, and some of the heat removed when passing through the cylinder is returned by the motor. Generally, the barrel regulator does have an effect on the melt, which is what we expect, but the effect is not as large as the regional variable. It is best to measure the melt temperature to really understand what is going on.

　　Principle 11 does not apply to die heads and dies because there is no screw rotation there. This is why external temperature changes are more effective there. However, these changes are not uniform from the inside to the outside, unless they are evenly mixed in a fixed mixer, which is an effective tool for melt temperature changes and stirring.