Effect of injection speed on melt filling
Injection speed is a key parameter in determining melt filling quality during injection molding. It directly influences the melt’s flow pattern, pressure distribution, and temperature changes within the mold cavity, significantly impacting the product’s appearance, mechanical properties, and molding efficiency. When the injection speed is low, the melt flows more smoothly and laminarly within the runners and cavity. Heat exchange between the melt and the mold cavity surface is sufficient, leading to rapid cooling. This can cause the melt to solidify before it completely fills the cavity, resulting in defects such as material shortages or cold spots. For example, when molding thin-walled parts, if the injection speed is too slow, the melt will be partially solidified by the time it reaches the end of the cavity, failing to fill the entire cavity and resulting in defective edges. However, lower injection speeds also have advantages: they reduce shear heat generated during melt flow, preventing plastic degradation due to overheating. This is particularly suitable for molding heat-sensitive plastics such as PVC and POM.
Higher injection speeds significantly improve melt fluidity, allowing the melt to fill the mold cavity more quickly, making it particularly suitable for molding complex structures or thin-walled products. During high-speed injection, the melt flows turbulently within the runner, intensifying intermolecular friction and generating significant shear heat. This increases the melt temperature, reduces viscosity, and further enhances fluidity, enabling it to successfully fill fine structures in the mold cavity, such as narrow gaps and small holes. For example, when molding decorative parts with intricate patterns, high-speed injection allows the melt to quickly fill the gaps between the patterns, ensuring clear reproduction of the pattern. However, excessively high injection speeds can also have negative consequences. Turbulent flow can easily entrain air, leading to defects such as bubbles and silver streaks in the product. Furthermore, high-speed injection increases the impact of the melt on the mold cavity, potentially causing overflow at the parting surface. This overflow is particularly pronounced when the mold clamping force is insufficient.
Injection speed significantly influences pressure loss during mold filling. Within a certain range, increasing injection speed increases the flow resistance of the melt within the runner and cavity, leading to increased pressure loss. This is because high-speed flow intensifies friction between the melt and the runner walls, while simultaneously increasing shear stress within the melt, leading to increased pressure consumption. For example, for products with long flow paths, high-speed injection requires higher injection pressure to ensure the melt fills the cavity. Insufficient injection pressure will result in underfill. Conversely, lower injection speeds minimize pressure loss and require lower injection pressure, reducing energy consumption and mold stress. However, this increases filling time and reduces production efficiency. Therefore, in actual production, it is necessary to balance injection speed and pressure based on the product’s structural characteristics and the plastic’s flow properties to achieve optimal mold filling.
The choice of injection speed is also closely related to the characteristics of the plastic. Different plastics have different sensitivities to injection speed. For plastics with good fluidity, such as PE and PP, a lower injection speed can meet the mold filling requirements. Too high a speed can easily lead to overflow and flash. For plastics with poor fluidity, such as PC and PMMA, a higher injection speed is required to improve the melt fluidity and ensure that the mold cavity is full. In addition, for crystalline plastics, such as PA and POM, the injection speed will affect their crystallization process. A higher injection speed can make the melt fill the mold cavity quickly, reducing the cooling time, thereby controlling the crystallinity and crystal morphology, and improving the mechanical properties of the product. For non-crystalline plastics, such as PS and ABS, the injection speed mainly affects the distribution of internal stress. Too fast a speed will lead to increased internal stress, making the product prone to warping or cracking during use.
Properly setting the injection speed in stages is an effective method for optimizing the melt filling process. Based on the structural characteristics of the cavity, the filling process is divided into multiple stages, each with a different injection speed to accommodate the melt flow requirements at different locations. For example, a lower speed is used immediately after the melt enters the cavity to prevent vortexes and air entrainment caused by the melt impacting the cavity walls. Once the majority of the cavity is filled, the injection speed is increased to expedite the filling process. Near the end of the cavity, the speed is reduced again to minimize overflow and flash. This segmented speed regulation ensures product filling quality while improving production efficiency and reducing defects. Furthermore, the injection speed should be considered when setting the mold’s venting capacity. If the mold vents well, the injection speed can be increased appropriately. If the venting is poor, the speed should be reduced to allow sufficient time for the gas to exit the cavity and avoid trapped air defects.
