Choose different screws for different plastics
During the injection molding process, the screw is the core component for plasticizing, conveying, and injecting plastic. Its structural parameters (such as aspect ratio, compression ratio, and screw groove depth) must match the characteristics of the plastic to ensure uniform plasticization and stable conveying of the melt. Different plastics exhibit significant differences in melt temperature, viscosity, fluidity, and thermal stability. Improper screw selection can lead to problems such as poor plasticization, melt degradation, and low production efficiency. For example, crystalline and amorphous plastics have vastly different screw requirements, while glass-fiber reinforced plastics require specially designed screws to minimize glass fiber damage. Therefore, scientifically selecting screws for different plastics is key to ensuring smooth injection molding production and stable product quality.
For amorphous plastics (such as ABS, PS, and PC), screw design should focus on improving plasticization uniformity and avoiding excessive shear. These plastics lack a distinct melting point and gradually soften during melting. They are highly temperature-sensitive, and excessive shear can easily lead to excessive melt temperature and degradation. Therefore, screws suitable for amorphous plastics typically feature a medium aspect ratio (20-24) and a medium compression ratio (2.5-3.0), with a moderate screw groove depth and a smooth transition between the feed and compression sections to minimize shear heating. For example, when processing ABS plastic, the screw compression ratio should be controlled between 2.6-2.8 and an aspect ratio of 22-24. This ensures sufficient plasticization of the ABS while preventing rubber phase decomposition caused by excessive shear, thereby reducing surface defects such as silver streaks and black spots. Furthermore, the screw’s homogenizing section should be appropriately extended, accounting for 20%-30% of the total screw length, to promote uniform mixing of the melt.
Screw design for crystalline plastics (such as PE, PP, and PA) focuses on optimizing melting speed and melt temperature uniformity. These plastics have a well-defined melting point, and the melting process is concentrated within a narrow temperature range, requiring strong shear to generate the heat required for melting. Therefore, screws suitable for crystalline plastics typically feature a larger aspect ratio (24-28) and a higher compression ratio (3.0-4.0). The feed section features a deep groove, while the compression section is short and has a steep transition. This enhances shear and accelerates plastic melting. For PP plastic, for example, the screw’s compression ratio should be 3.5-4.0, the aspect ratio 25-26, and the feed section groove depth should be 3-4 times that of the homogenization section. This intense compression and shear ensures rapid melting and uniform mixing of the solid PP. Furthermore, to prevent crystallization and cooling caused by excessive residence time in the homogenization section, the homogenization section should be appropriately shortened, typically accounting for 15%-20% of the total screw length. This should be combined with a high barrel temperature to ensure a stable melt temperature when entering the mold.
For heat-sensitive plastics (such as PVC and POM), the key to screw design is to minimize residence time and shear heating during the plasticization process to prevent plastic degradation. PVC plastic easily decomposes at high temperatures, producing HCl gas. This not only corrodes equipment but also causes discoloration and performance degradation in finished products. Therefore, screws specifically designed for this purpose require a low-shear design, with a minimal aspect ratio (16-20), a compression ratio of 1.8-2.2, and a deep screw groove to reduce shear rates. Furthermore, the screw surface should be chrome-plated to enhance wear and corrosion resistance and prevent corrosion from PVC decomposition products. POM plastic is oxygen-sensitive and prone to oxidative degradation at high temperatures. For this purpose, screws should have a lower compression ratio (2.0-2.5) and incorporate reverse threads or pressure relief grooves in the feed section to minimize melt backflow and shorten residence time. Furthermore, screws processing heat-sensitive plastics should minimize dead space. For example, the clearance between the screw flight and the barrel should be strictly controlled to 0.1-0.2mm to prevent plastic from accumulating and decomposing within this gap.
Screw design for glass-fiber-reinforced plastics (such as PP-GF and PA-GF) requires a balanced approach to plasticization and fiber length retention. Glass fiber reinforces plastics, but excessive shear damage during the plasticization process can significantly reduce the mechanical properties of the finished product. Therefore, screws specifically designed for reinforced plastics typically feature a low compression ratio (2.0-2.5) and a larger screw channel depth to minimize shearing. At the same time, the aspect ratio should be appropriately increased (24-26) to ensure uniform mixing of the glass fiber and the resin. The compression section of the screw should adopt a gradual design to prevent severe shearing of the glass fiber during sudden compression. The edges of the screw flights can be rounded to minimize fiber cutting. For example, when processing 30% glass-fiber-reinforced PA, the screw compression ratio should be 2.2-2.4, and the screw channel depth should be 10%-15% deeper than that of standard PA screws. A mixing ring or barrier structure should be incorporated into the homogenization section to enhance mixing while minimizing fiber breakage. In addition, the material of the screw and barrel needs to be high-strength wear-resistant steel (such as 38CrMoAlA) and nitrided to cope with the abrasive effect of glass fiber and extend its service life.
