Selection of screws for several commonly used plastic injection molding
In plastic injection molding, the screw, as the core component of the injection molding machine, directly impacts the plasticizing quality, molding efficiency, and final product performance. Different types of plastics have varying physical and chemical properties, such as melting temperature, melt viscosity, fluidity, and thermal stability. Therefore, the right screw is essential to achieve optimal injection molding results.
Crystalline plastics like polyethylene (PE) have a wide melting range, low melt viscosity, and good flowability. When selecting a screw, a tapered screw with a long compression section, typically 20% to 30% of the total screw length, is generally preferred. This design allows the PE plastic to be gradually compressed and sheared during the plasticization process, preventing material decomposition due to excessive local shear forces. Furthermore, due to PE’s excellent thermal stability, the screw’s aspect ratio can be selected between 20:1 and 25:1 to ensure sufficient plasticization time and a uniform melt. Furthermore, the screw’s compression ratio is typically set between 3:1 and 4:1, which meets the PE’s melting requirements while avoiding energy waste caused by excessive plasticization.
Polypropylene (PP) and PE are both polyolefin plastics, but PP has a slightly higher melting temperature and a more sensitive melt viscosity to shear rate. Given the characteristics of PP, a stepped screw is a more suitable choice. Its compression section is shorter, typically only 5%-10% of the total screw length. This design allows for material compression over a shorter distance, utilizing higher shear rates to reduce PP’s melt viscosity and improve its fluidity. PP has excellent thermal stability, and a screw aspect ratio similar to PE can be used, ranging from 20:1 to 25:1. However, the compression ratio should be slightly higher than that of PE, typically between 3.5:1 and 4.5:1, to enhance plasticization. Furthermore, since PP is prone to weld marks during plasticization, the screw’s metering section should ensure sufficient melt pressure and uniformity to minimize product defects.
Polyvinyl chloride (PVC) is a heat-sensitive plastic with a narrow melting temperature range and easily decomposes at high temperatures, producing harmful gases. Therefore, when selecting a screw, special attention should be paid to minimizing the material’s residence time in the screw and avoiding localized overheating. A screw with a short compression section is ideal, with the compression section typically accounting for less than 10% of the total screw length. The screw should also have a low aspect ratio, typically between 16:1 and 20:1. Furthermore, the compression ratio should be kept to a minimum, typically between 2:1 and 3:1, to reduce shear forces and prevent PVC decomposition. To further enhance plasticization and reduce the risk of decomposition, the screw and barrel should be chrome-plated for enhanced wear and corrosion resistance.
Polycarbonate (PC), an amorphous engineering plastic, has a high melting temperature, high melt viscosity, poor fluidity, and is sensitive to temperature fluctuations. When selecting a screw, a tapered screw with a long compression section should be preferred. The compression section should account for 30% to 40% of the total screw length to ensure sufficient melting and plasticization of the PC material. Because PC requires high temperatures and a long time to melt, the screw’s aspect ratio should be large, generally between 22:1 and 28:1. Furthermore, the compression ratio should be set between 2.5:1 and 3.5:1 to provide sufficient shear force for uniform melt formation. Furthermore, to prevent moisture absorption during the plasticization process, which can lead to bubbles in the finished product, a cooling device should be installed in the screw’s feed section to prevent premature melting and affect feeding efficiency.
