Injection molding machine adjustment skills: fast-first-slow-later injection method and its application
During the injection molding process, injection speed control is one of the key factors affecting the quality of plastic parts. The fast-first, slow-later injection method, as a classic machine adjustment technique, plays an important role in solving the molding problems of complex structural plastic parts. This method involves using a higher injection speed in the initial stage of melt filling the mold cavity, allowing the melt to quickly fill most areas of the cavity. The injection speed is then reduced near the end of the filling process, and the remaining area is slowly filled. The core principle is to reduce the cooling loss of the melt during the flow process by quickly filling it in the early stage, thereby ensuring the fluidity of the melt; slow filling in the later stage can effectively avoid defects such as flash and bubbles caused by melt impact, while facilitating the discharge of air from the cavity and improving the density of the plastic part. The fast-first, slow-later injection method is not suitable for all plastic parts, but it can significantly improve product quality and production efficiency in the production of plastic parts with specific structures.
The fast-injection-then-slow injection method’s initial rapid injection phase is primarily targeted at plastic parts with large cavities or complex structures. Its purpose is to quickly occupy the cavity before the melt cools significantly, thereby reducing the drop in melt temperature caused by an excessively long flow path. Rapid injection is particularly important for thin-walled plastic parts because the mold cools quickly in thin-walled areas. If the injection speed is too slow, the melt may lose fluidity due to cooling before reaching the end of the cavity, resulting in material shortages or unclear contours. For example, when producing thin-walled plastic parts such as mobile phone casings, rapid early injection allows the melt to fill the entire cavity in a very short time, ensuring the dimensional accuracy and surface finish of the plastic part. Furthermore, rapid injection can reduce melt stratification in the cavity and avoid weld marks on the plastic part, as the fast-flowing melt can maintain a higher temperature when it meets, achieving good fusion.
The primary purpose of the late, slow injection stage is to optimize the final mold quality of the plastic part and avoid the negative effects of high-speed injection. When the melt fills approximately 90% of the cavity, continuing high-speed injection can cause the melt to overflow from the parting surface, forming flash. Furthermore, the high-speed melt flow can entrap a large amount of air, leading to bubbles or gas lines within the part. Reducing the injection speed at this stage allows the melt to complete the final filling process more smoothly, allowing air in the cavity ample time to escape through venting grooves, thus reducing gas defects. For parts with fine structures (such as ribs and bosses), slow injection allows the melt to more evenly fill these details, avoiding localized stress concentrations caused by high-speed impact and reducing the risk of part deformation or cracking. For example, when producing gear parts with complex patterns, slow injection in the late stages ensures complete molding of the teeth and improves part meshing accuracy.
The application of the fast-then-slow injection method requires parameter adjustment based on the structural characteristics and material properties of the plastic part to achieve a smooth speed transition. First, the speed transition point must be determined. Typically, the percentage of melt volume filling the cavity is used as a guide. For general parts, the transition can be made when the cavity is 80%-90% filled. For parts with complex structures, multiple mold trials are required to determine the optimal transition point. Second, the speed gradient must be carefully controlled to avoid sudden speed drops that disrupt melt flow and cause pressure fluctuations, which can affect part quality. For example, when switching from high to low speed, the speed reduction can be controlled within 30%-50%, and the holding pressure can be adjusted to ensure continuous melt filling. Furthermore, comprehensive adjustments must be made to the injection pressure and mold temperature. If high-speed injection in the early stages causes mold temperature to rise, the mold temperature can be appropriately lowered to prevent shrinkage cavities in the part. If the melt is not fluid enough during slow injection in the later stages, the barrel temperature can be appropriately increased to ensure filling.
The application effect of the fast-first, slow-later injection method in actual production is remarkable, but it also has certain limitations and needs to be used flexibly. For thick-walled plastic parts, if the initial injection speed is too fast, it may cause excessive shearing of the melt in the mold cavity, breaking the molecular chains and affecting the mechanical properties of the plastic part. At this time, the initial speed needs to be appropriately reduced; and for materials that are prone to internal stress (such as polycarbonate), the time of the later slow injection should be appropriately extended to cooperate with the pressure release in the pressure holding stage to reduce the generation of internal stress. With the development of injection molding technology, modern injection molding machines are mostly equipped with multi-stage injection functions, which can achieve more precise speed control. For example, after the initial rapid injection, a medium-speed transition stage is added, and then switching to slow injection can further optimize the filling process. Through continuous practice and parameter optimization, the fast-first, slow-later injection method can play a role in the production of more types of plastic parts, providing reliable technical support for improving the quality of injection molded products.
