Application of screw in plastic machinery

Plastic raw materials are widely used in the packaging industry. In the manufacturing of blow molding, injection molding, extrusion molding, etc., the raw materials of plastic particles are heated and rolled, squeezed into the mold or injected into the mold, and then cooled to form.

    The plastic raw material is heated, and the screw is driven by the motor and the hardened gear box to rotate and roll the raw material in the barrel to change the shape of the plastic raw material. This process is called "plasticization". The plasticizing capacity and plasticizing quality determine the capacity of the equipment and the quality of the products. It can be seen that the extrusion system is very important in the equipment composition system. The screw is the main operating part in the plasticizing process. The material and design of the screw directly affect the plasticizing capacity and quality of the equipment.

   Let's analyze the application of screw in blow molding extrusion equipment. There are generally two kinds of screw materials: 38CrMoAlA and double alloy materials. The first one is commonly used, which is suitable for all new materials, and there is no problem with plasticization. Because plastic raw materials are difficult to degrade and other reasons, the world is looking for better solutions to use recycled materials to produce plastic packaging products. The screw suitable for full or partial return is made of bi-alloy material. There are also two cases: 1. The surface of the screw is coated with bi-alloy; 2. The screw is all made of bi-alloy material. The base alloy of the double-alloy screw is mainly SKD61, and the surface layer is a wear-resistant and corrosion-resistant alloy, mainly imported tungsten carbide (Wc/Co), nickel-based alloy (Ni+B/Cr), cobalt-based alloy (Co+Ni/Cr) ) and other self-fluxing alloys, etc. The surface hardness of the double-alloy screw can reach HV1180-1480 degrees, and it has the characteristics of high temperature resistance and corrosion resistance.

   The design of the screw is also very important, mainly reflected in: length-diameter ratio, compression ratio, pitch, barrier section, mixing section and other parameters. The aspect ratio is the ratio of screw length and diameter. Under normal circumstances, the larger the aspect ratio, the higher the plasticizing ability. The conventional aspect ratio of hollow extrusion blowing equipment is 23-30. The screw is divided into three sections: feeding section, compression section and homogenization section. The compression ratio is the ratio of the volume of the first groove in the feeding section to the volume of the last groove in the homogenization section. Generally, the larger the compression ratio, the more uniform the plasticization will be. But production will be lower.

The pitch is relatively intuitive, that is, the distance between threads. The smaller the pitch, the better the plasticization, but the plasticizing force will also be reduced.

   In general, the improvement of plasticization is mainly to comprehensively adjust and change the following parameters under the condition that the length-diameter ratio of the screw remains unchanged: the compression ratio of the screw, the proportion of the three segments of the screw, the pitch of the screw, and the separation type of the screw (BM type). ). Comprehensively change the structure of the screw, find the best solution for the respective plastic equipment, improve the plasticizing output and quality of the machine, and finally improve the quality of the product and increase the productivity!