Different types of screws should be selected for different plastics
The injection molding machine screw is the core component of the plasticizing system. Its structural parameters (such as aspect ratio, compression ratio, and screw groove depth) must match the characteristics of the plastic. The melt temperature, viscosity, and fluidity of different plastics vary significantly, so the corresponding screw type must be selected to ensure plasticizing quality. The difference between screws for thermoplastics and thermosetting plastics is most significant. Thermoplastics must be melted and homogenized within the barrel before injection. The screw aspect ratio is typically 18-25, with a compression ratio of 2-3. Thermosetting plastics, on the other hand, undergo chemical crosslinking during the plasticizing process and require rapid plasticization and injection. The screw aspect ratio is smaller (10-15), the compression ratio is 1.5-2, and the screw groove is deeper to prevent the material from dwelling too long and causing premature solidification. For example, when processing thermosetting phenolic plastics, using a thermoplastic screw can cause the material to solidify within the barrel due to the prolonged plasticizing time, blocking the flow path.
Crystalline and amorphous plastics place different demands on screws, requiring screw parameters to be selected based on their crystallization characteristics. Crystalline plastics (such as PP, PE, and PA) absorb significant heat during melting, and their melt viscosity is sensitive to temperature. Therefore, the screw must possess strong plasticizing and mixing properties. A low-to-diameter ratio of 20-25, a compression ratio of 3-4, and a deep screw channel (the feed section should be 2.5-3 times deeper than the metering section) are recommended to enhance solid conveying and melting efficiency. Furthermore, a mixing ring or barrier section should be incorporated into the metering section to ensure uniform melt plasticization. Amorphous plastics (such as PS, PC, and ABS) have a wide melting temperature range and their melt viscosity is sensitive to shear rate. Screws with a low-to-diameter ratio of 18-22, a compression ratio of 2-3, and a shallow screw channel depth are recommended to generate heat through shear and promote melting. A long homogenizing section (accounting for 20-30% of the total screw length) is also recommended to ensure melt uniformity. For example, PC, due to its high viscosity and temperature sensitivity, requires a low compression ratio (2.5) and shallow screw channel to avoid degradation caused by shear overheating.
Screw designs for high-viscosity and low-viscosity plastics differ significantly and must be tailored to the material’s fluidity. High-viscosity plastics (such as PC, PMMA, and PVC) exhibit high viscosity after melting and high flow resistance. Therefore, the screw requires a lower compression ratio (1.8-2.5) and a deeper groove to reduce shear forces and frictional heat, thereby preventing material degradation. Furthermore, a check ring should be installed at the screw head to prevent backflow during melt reshaping, which could affect metering accuracy. Low-viscosity plastics (such as PE, PP, and PA) exhibit good fluidity and are prone to backflow. Therefore, the screw requires a higher compression ratio (3-4) and a shallower groove to enhance shear and homogenization. Furthermore, the clearance between the check ring and the barrel should be minimized (0.03-0.05mm) to prevent melt leakage during injection. For example, when processing PVC, using a high-compression screw can cause shear overheating, leading to PVC decomposition and the generation of HCl gas, which can corrode equipment and affect product quality.
Plastics containing additives require specialized screws to ensure even material dispersion. Glass-fiber-reinforced plastics (such as glass-fiber-reinforced PA and PC) experience significant screw wear, requiring a nitrided or carbide-coated screw for improved wear resistance. A shallow groove depth and a compression ratio of 2.5-3 are recommended to prevent excessive shearing and breakage of the glass fiber, which could affect the reinforcement effect. Furthermore, a reverse thread or mixing head should be incorporated into the homogenization section to ensure uniform mixing of the glass fiber and resin. Flame-retardant plastics (such as ABS containing brominated flame retardants) are prone to generating volatiles during processing. Therefore, the screw should be made of corrosion-resistant material (such as 316 stainless steel) and have a minimal aspect ratio (18-20) to minimize material residence time and prevent volatile accumulation. Foamed plastics require a specialized venting screw with a mid-screw venting section to expel gases from the material, ensuring uniform foaming. A compression ratio of 1.2-1.5 and a deep groove depth are recommended to reduce shear forces.
The selection of screws must be combined with actual production verification and cannot rely solely on theoretical parameters. When changing the type of plastic, it is necessary to check the plasticizing effect through trial molds, observe whether the melt is uniform, whether there are unmelted particles or degradation, and measure the melt temperature fluctuation (should be ≤±5°C). If defects such as silver streaks and discoloration are found on the product, it may be that the screw and the plastic are not compatible, and the screw parameters need to be adjusted or the screw type needs to be changed. For example, if a PS screw is used when processing PP, insufficient compression ratio will lead to poor plasticization, cold material spots will appear on the surface of the product, and a high compression ratio screw needs to be replaced. Check the screw wear regularly, and repair or replace it in time when the screw ridge wear exceeds 0.5mm to ensure the long-term stability of the matching between the screw and the plastic, and improve production efficiency and product quality.
