In plastic machinery and equipment, the screw, as a core component, directly determines the efficiency of material melting and plasticization, thereby impacting product quality and production stability. Screw design requires comprehensive consideration of geometric parameters, structural form, and process compatibility. By optimizing shearing, compression, and conveying functions, efficient and uniform plasticization can be achieved.
The screw's aspect ratio is the primary parameter influencing plasticization efficiency. A larger aspect ratio increases the material's residence time in the screw, allowing for more effective heating and shearing, significantly improving plasticization uniformity. For thermally stable plastics, such as polyethylene and polypropylene, a screw with a larger aspect ratio can enhance mixing and reduce unmelted particles. Heat-sensitive materials, such as polyvinyl chloride, require a shorter aspect ratio to prevent decomposition. Furthermore, a long screw with a compression section can gradually compress the material, expel gases, and improve product density.
The screw's compression ratio and flute depth directly influence material compaction and melting. The compression ratio is defined as the ratio of the flute depth in the feed section to the flute depth in the metering section, and its magnitude determines the material's density and shear strength. A higher compression ratio can increase the bonding between material molecules and reduce air ingestion, but an excessively high compression ratio can lead to excessive shear heating and material degradation. The gradual design of the screw channel depth must be tailored to the material's characteristics. For example, high-viscosity materials require a deeper channel in the feed section to reduce conveying resistance, while a shallower channel in the metering section can increase melt pressure and ensure plasticizing quality.
The screw's flight structure and number of threads are key to optimizing plasticizing efficiency. Single-threaded screws offer a simple structure and are suitable for fast conveying of low-viscosity materials. Twin-thread or triple-threaded screws significantly improve shear efficiency by increasing the number of meshing points, making them particularly suitable for compounding highly filled and high-viscosity materials. For example, a triple-threaded extruder, through the synergistic action of multiple screws, subjects the material to intense shear and extension at multiple meshing zones. This significantly increases melt speed compared to a twin-screw extruder and provides superior dispersion and mixing, making it suitable for the production of highly filled systems such as carbon black masterbatch and calcium carbonate modification.
The screw's helix angle and clearance are crucial for melt efficiency and leakage control. Increasing the helix angle can extend the material's residence time in the screw channel, promoting melting. However, an excessively large helix angle can reduce the channel width and hinder material flow. The clearance design between the screw and barrel must balance shear efficiency and leakage control: too small a clearance will increase wear and energy consumption, while too large a clearance may cause leakage and reduce extrusion output. For example, when injection molding nylon, the screw-barrel clearance must be strictly controlled within a reasonable range to prevent overheating and ensure melt uniformity.
The screw head structure directly affects melt venting and molding stability. Vented screws feature vents in the compression section to effectively remove moisture and volatiles from the material, preventing bubbles and silver streaks in the finished product. For high-precision products, the screw head should be equipped with a check ring to prevent melt backflow and ensure metering accuracy. Furthermore, the screw head can be designed with a pointed cone or blunt rounded shape to optimize melt flow and reduce shear stress damage to molecular chains, depending on the characteristics of the material.
The design of the screw's temperature control system is a key component in ensuring plasticizing quality. By using zoned heating and cooling devices, the screw can precisely control the temperature distribution in each section to meet the plasticizing requirements of different materials. For example, when processing polycarbonate, the feeding section must be kept at a low temperature to prevent agglomeration, while the compression and metering sections must be gradually heated to the melt temperature to ensure sufficient plasticization and prevent decomposition. Advanced screw designs also integrate temperature sensors and feedback control systems to enable dynamic temperature adjustment and enhance process stability.
Screw selection must be optimized in conjunction with the overall design of plastic machinery and equipment. For example, in an extruder, the screw must be matched with components such as the barrel and mold to ensure stable melt pressure and flow rate. In an injection molding machine, the screw's plasticizing capacity must be coordinated with parameters such as injection speed and dwell time to avoid product defects caused by insufficient plasticization. Furthermore, for specialized processes such as compounding and reactive extrusion, the screw design must incorporate static mixers and pin elements to enhance dispersive and distributive mixing, thereby expanding the application range of plastic machinery and equipment.
