Injection screw speed
The injection molding screw speed is another key process parameter in the injection molding process, directly affecting the plasticizing efficiency, melt quality, and production cycle. The screw speed determines the shear rate and plasticizing time of the screw on the plastic. Properly setting the screw speed ensures sufficient plasticization and uniform mixing of the plastic, while also improving production efficiency. Too low a speed will result in an excessively long plasticizing time, low production efficiency, and possibly inadequate melt plasticization. Too high a speed will generate excessive shear heat, leading to overheating and decomposition of the plastic, while also increasing energy consumption and equipment wear. Therefore, scientifically setting and controlling the screw speed is crucial to ensuring smooth injection molding production.
Screw speed settings are typically measured in revolutions per minute (r/min). The appropriate screw speed range varies significantly depending on the type of injection molding machine and plastic material. Generally speaking, small injection molding machines have higher screw speeds, ranging from 100 to 300 r/min, while large injection molding machines have lower screw speeds, typically between 30 and 100 r/min. For plastics with lower viscosity and better fluidity, such as PE and PP, the screw speed can be set higher to improve plasticizing efficiency. For example, the screw speed for PE is generally 80 to 150 r/min, and for PP it is 100 to 200 r/min. For plastics with higher viscosity and lower fluidity, such as PC and POM, to avoid excessive shear heat generation, the screw speed should be set lower, typically between 40 and 80 r/min for PC and 50 to 100 r/min for POM. For plastics with poor thermal stability, such as PVC, the screw speed must be strictly controlled, generally not exceeding 60 r/min, to prevent overheating and decomposition.
Screw speed significantly affects melt temperature and viscosity. Increasing screw speed accelerates the relative motion between the screw and barrel, intensifying shear and generating more shear heat, which in turn increases melt temperature and reduces viscosity. This increase in melt temperature due to the increased speed can, to a certain extent, compensate for insufficient barrel temperature and improve melt fluidity. However, if the speed is too high, excessive shear heat can cause the melt temperature to exceed its decomposition temperature, leading to plastic degradation and impacting product quality. For example, when the screw speed of PS exceeds 200 r/min, the melt temperature can rapidly rise to over 250°C, exceeding its decomposition temperature and causing defects such as discoloration and burn marks in the finished product. Therefore, when setting the screw speed, it is necessary to comprehensively consider the effects of barrel temperature and shear heat to ensure that the melt temperature remains within a reasonable range.
The selection of screw speed also needs to consider product quality requirements and production efficiency. For high-precision products, a lower screw speed is required to ensure melt uniformity and stability and reduce internal stress caused by uneven shearing. For example, when producing precision products such as optical lenses, the screw speed is typically controlled between 30 and 60 r/min. For products with general requirements, the screw speed can be appropriately increased to shorten the plasticizing time and improve production efficiency. Furthermore, the size and shape of the product will also affect the screw speed setting. Large, thick-walled products require longer cooling times, so the screw speed can be appropriately reduced to match the cooling cycle and avoid idle equipment. Small, thin-walled products, on the other hand, have shorter cooling times, so the screw speed needs to be increased to speed up the plasticizing process and meet the demands of rapid production.
In actual production, screw speed adjustment must be coordinated with parameters such as back pressure and barrel temperature to achieve the optimal process combination. When increasing screw speed, the barrel temperature can be appropriately lowered to prevent melt decomposition due to excessive shear heating. Simultaneously, to ensure plasticizing quality, the back pressure can be appropriately increased to improve melt density and uniformity. Conversely, when decreasing screw speed, the barrel temperature can be appropriately increased to compensate for insufficient plasticizing power, and the back pressure can be reduced to reduce energy consumption. Operators should dynamically adjust screw speed based on product quality (such as surface quality, dimensional accuracy, and mechanical properties). For example, if a product exhibits surface roughness or weld marks, this may be due to insufficient plasticization caused by too low a screw speed, and the speed can be appropriately increased. If a product exhibits excessive internal stress, such as warping or deformation, this may be due to excessive shear heating caused by too high a screw speed, and the speed should be reduced. By continuously optimizing screw speed and related process parameters, efficient and stable injection molding can be achieved, improving product quality and economic benefits.
