Screw is an important component of injection molding machine. Its function is to transport, compact, melt, stir, and apply pressure to plastics. All of this is achieved through the rotation of the screw inside the barrel. When the screw rotates, plastic will generate friction and mutual movement on the inner wall of the barrel, the bottom surface of the screw groove, the advancing surface of the screw edge, and between plastic and plastic. The forward movement of plastic is the result of this combination of motion, and the heat generated by friction is also absorbed to increase the temperature of plastic and melt it. The structure of the screw will directly affect the degree of these effects.
Ordinary injection molding screw structures can also be designed with components such as release screws, barrier screws, or split screws to improve plasticization quality. The structure of the material barrel is actually a circular tube with a discharge port in the middle.
In the process of plastic plasticization, the driving force for its advancement and mixing comes from the relative rotation of the screw and the barrel. According to the different shapes of plastics in screw grooves, screws are generally divided into three sections: solid conveying section (also known as feeding section), melting section (also known as compression section), and homogenization section (also known as metering section).
In textbooks related to plastic plasticization, the solid conveying section of the screw is regarded as a solid bed where plastic particles do not move with each other. Then, the speed of plastic forward transportation is determined by calculating the ideal state of motion and friction between the solid bed and the barrel wall, the screw edge pushing surface, and the screw groove surface. There is a considerable gap between this and the actual situation, and it cannot be used as a basis to analyze the feeding situation of plastic particles with different shapes. If the plastic particles are not large, they will layer and roll when pulled forward by the inner wall of the material barrel, and gradually be compacted to form a solid plug. When the diameter of the material particles is similar to the thickness of the screw groove, their motion trajectory is basically a linear motion along the radial direction of the screw groove plus a linear motion at an angle. Due to the loose arrangement of plastic in the screw groove when the particles are large, their conveying speed is also slower. When the particles reach a certain size and enter the compression section with a diameter greater than the thickness of the screw groove, the plastic will get stuck between the screw and the barrel. If the forward pulling force is not enough to overcome the force required to flatten the plastic particles, the plastic will get stuck in the screw groove and not push forward.
