Among all the core components of an injection molding machine, the screw is the critical element that determines the effectiveness of plasticization, the quality of the molded product, and the service life of the machine itself. Many people focus solely on the machine's tonnage or clamping force while overlooking the selection of the screw; consequently, they encounter a host of issues such as flow marks, scorching, material degradation, color variations, uneven plasticization, and stagnant production output. In reality, while the mold dictates the external shape of the product, the screw governs the properties of the plastic material itself. Processing different raw materials or manufacturing different types of products necessitates the use of a corresponding specialized screw; thus, the proper selection of the screw is a fundamental prerequisite for ensuring stable injection molding production.
As the core component of an injection molding machine's injection unit, the screw primarily performs the functions of conveying plastic, heating and shear-melting the material, metering and accumulating the melt, and injecting the molten plastic under high pressure. After plastic pellets enter the barrel from the hopper, they are uniformly melted from a solid state into a fluid through the combined effects of the screw's rotational propulsion, heating from the barrel's outer wall, and the shear heat generated by the screw itself. The screw's diameter, length-to-diameter ratio, flight depth, material hardness, surface treatment, and melt-conveying structure directly influence plasticization efficiency, melt uniformity, and the material's thermal stability-as well as the susceptibility to issues such as thermal decomposition, carbonization, carbon buildup, and black streaks.
When selecting a screw for general daily production, the first step is to distinguish the properties of the raw materials. For common general-purpose plastics-such as PP, PE, and PS-which exhibit excellent flowability, low melting points, and strong thermal stability, a standard general-purpose screw is sufficient to meet production requirements. Such screws ensure smooth plasticization and offer a wide processing window, making them suitable for the vast majority of daily necessities, baskets, pipe fittings, and ordinary plastic products. ABS, on the other hand, is classified as a medium-grade engineering plastic; it features a moderate molding temperature range and low corrosiveness, meaning it can also be processed using a general-purpose screw. However, for long-term, high-volume production runs, it is advisable to opt for an upgraded, wear-resistant screw to minimize the generation of plastic powder impurities caused by abrasion.
When processing engineering plastics and specialty polymers-such as PC, PA (Nylon), POM, PMMA, and PVC-the selection of the appropriate screw entails rigorous requirements. PC is characterized by high transparency, high viscosity, and inherent difficulty in plasticization; furthermore, it cannot withstand prolonged exposure to high temperatures within the barrel. Consequently, it necessitates a specialized screw featuring a high compression ratio, high shear capabilities, and a highly polished surface finish to ensure a uniform and transparent melt, thereby preventing defects such as silver streaks, flow marks, black spots, and insufficient clarity. Nylon (PA) is highly hygroscopic, exhibiting significant changes in flow behavior upon melting; it also places extremely high demands on screw wear resistance, thereby requiring a screw designed to withstand both corrosion and abrasion. PVC is a heat-sensitive plastic that decomposes readily at high temperatures, generating corrosive gases. Therefore, it is imperative to employ specialized corrosion-resistant screws and barrels to prevent equipment degradation, while simultaneously maintaining strict control over shear-induced heat to avert plastic decomposition and discoloration.
Secondly, the screw diameter should be selected based on the product's production volume and the required shot weight. A larger screw diameter results in a greater single-shot volume and a faster plasticization and discharge rate, making it suitable for large products, thick-walled parts, and high-volume, high-speed production. Conversely, a smaller screw diameter offers higher metering precision, making it ideal for small, precision components, thin-walled parts, and high-accuracy electronic accessories. A general industry guideline dictates that the total weight of the finished product should fall within the range of 20% to 75% of the screw's theoretical injection capacity; using a shot weight that is too small causes the plastic to dwell in the barrel for prolonged periods, leading to scorching, yellowing, or decomposition, whereas using a shot weight that is too large results in insufficient plasticization, weak mold filling, and product defects such as short shots or shrinkage.
The screw's length-to-diameter (L/D) ratio is another critical factor in the selection process. A higher L/D ratio results in a longer transport path for the plastic within the barrel, facilitating more thorough heating and melting, as well as superior mixing efficiency; this makes it ideal for processing modified plastics, glass-fiber-reinforced plastics, and products requiring color masterbatch blending. Conversely, a screw with a lower L/D ratio offers faster plasticization speeds and shorter residence times, making it suitable for heat-sensitive plastics by minimizing the risk of high-temperature degradation. While the standard L/D ratios found on general-purpose machines are entirely adequate for typical consumer plastic products, processing glass-fiber-reinforced modified materials or products requiring intensive mixing necessitates an upgrade to a specialized screw with a higher L/D ratio.
In terms of material composition and wear resistance, standard nitrided screws are suitable for processing pure plastics-specifically those containing no additives or glass fibers. However, when producing glass-fiber-reinforced PA, glass-fiber-reinforced PP, or mineral-filled modified plastics, the extreme hardness of the internal glass fibers causes rapid abrasion of the screw flights. This leads to increased clearance, unstable injection, internal material backflow, and significant dimensional deviations in the finished products. Consequently, it is imperative to select a bimetallic screw and barrel assembly. Such assemblies feature superior overall hardness as well as enhanced resistance to both wear and corrosion, thereby significantly extending service life and reducing the production costs associated with frequent component replacements.
Furthermore, the screw's plasticizing structure, the accompanying non-return ring, and the injection nozzle must be selected as a cohesive, integrated system. Only when the plasticizing components are precisely matched to the screw specifications can one ensure accurate injection metering, prevent melt backflow, stabilize holding pressure, and minimize fluctuations in product weight. Many manufacturing facilities frequently encounter issues such as erratic product weight, inconsistent shrinkage, and unpredictable flash formation. Aside from potential issues with machine tuning parameters, the root causes of these problems typically stem from a mismatch of the screw, excessive wear exceeding tolerance limits, or an inappropriate selection of components.
In summary, the selection of an injection molding machine screw should never be treated as a matter of simply choosing a generic, off-the-shelf standard part. Instead, the raw material dictates the screw type; the product weight determines the screw diameter; production requirements define the length-to-diameter ratio; and the presence of fillers or modifiers determines the required material grade. Only when the original equipment manufacturer customizes the screw-based on the client's specific raw materials, product dimensions, production volume requirements, and whether fiber reinforcement is involved-can the system achieve uniform plasticization, stable discharge, and a high yield rate, all while remaining energy-efficient. Moreover, this tailored approach minimizes equipment malfunctions and reduces long-term maintenance costs, serving as the fundamental prerequisite for an injection molding machine to deliver its absolute best performance.
