During the injection molding process of plastic battery base plates, the generation of internal stress is closely related to process parameter control. Internal stress can not only cause warping and cracking of the base plate, but also affect its assembly precision and long-term reliability within the battery pack. Optimizing process parameters requires comprehensive adjustments across multiple dimensions, including temperature, pressure, speed, time, and mold design, to reduce internal stress sources such as molecular orientation and uneven cooling shrinkage.
Barrel temperature is a key parameter influencing the flowability and molecular orientation of the plastic melt. Excessively low temperatures increase the melt viscosity, forcing molecular chains to align during mold filling and resulting in significant residual stress after cooling. Excessively high temperatures can lead to material degradation or insufficient cooling, resulting in deformation during demolding. Optimization involves gradually increasing the barrel temperature to the upper limit of the material's recommended range to ensure uniform melt plasticization and moderate flowability. For example, for reinforced plastics such as PA66 GF30, the barrel temperature should be controlled between 280-290°C to fully integrate the glass fibers into the matrix resin and reduce localized stress concentrations caused by fiber orientation.
Mold temperature has a crucial impact on cooling shrinkage and crystallization behavior. When the mold temperature is too low, the outer plastic layer cools and shrinks rapidly, while the inner layer remains molten, resulting in a stress distribution characterized by tension in the surface layer and compression in the core layer. Excessive temperatures may prolong cooling time and reduce production efficiency. Optimization strategies include adjusting the mold temperature based on the thickness of the base plate. In thick-walled areas, the mold temperature should be appropriately raised (e.g., 80-100°C) to promote dense grain packing and reduce crystallization stress. In thin-walled areas, the mold temperature can be lowered to shorten the cycle time. Furthermore, the mold cooling system should be evenly distributed to avoid localized overheating or undercooling that can cause stress differences.
Controlling injection and holding pressures requires balancing mold filling requirements with stress generation. High injection pressures increase melt shear stress, leading to increased molecular orientation. Excessive holding pressures force material backfill after the melt temperature drops, freezing in more orientation stress. Optimization strategies include appropriately reducing the injection pressure to a level that just fills the cavity and ensuring that part density meets specified standards by matching holding time with pressure. For example, a staged holding pressure system can be employed: high pressure (e.g., 80% of injection pressure) is used for rapid shrinkage in the initial stage, followed by low pressure (e.g., 50% of injection pressure) in the later stages to maintain cavity pressure and reduce stress caused by sudden pressure changes.
Optimizing injection speed and filling pattern can reduce melt flow resistance and orientation stress. While high-speed injection can shorten filling time, it can easily lead to melt jetting, air streaks, or weld lines; low-speed injection can cause stress concentration due to melt delamination. An optimization approach is to use variable injection speeds: low speed through the gate in the initial stage to reduce shear heating, high speed filling in the middle stages to improve efficiency, and low-speed holding pressure in the final stages to reduce orientation. For example, for baseplates with metal inserts, the injection speed should be reduced in the area surrounding the insert to avoid stress concentration around the insert due to differences in thermal expansion coefficients.
The matching of holding time and cooling time must ensure sufficient solidification of the part. Excessive holding time will cause the melt to be continuously compressed, resulting in greater elastic deformation; insufficient cooling time can lead to deformation during demolding due to incomplete hardening of the part. Optimization efforts involve experimentally determining the minimum holding time to ensure the mold is opened after the gate is sealed, while also extending the cooling time until the product temperature approaches room temperature. For example, for thick-walled battery base plates, a gradual cooling method can be employed: initially holding at a high temperature (60-80°C) for a period of time to balance shrinkage, followed by cooling at a low temperature (20-30°C) to stabilize the shape.
The design of the mold's gating system and ejection mechanism significantly influences internal stress. An undersized gate increases flow resistance and generates orientation stresses; uneven ejection force can force the part out of the mold, causing high elastic deformation. Optimization efforts include increasing the gate size (e.g., point gate diameter ≥ 1.5mm), shortening the sprue length, and employing a large-area ejection design. For example, for battery base plates with ribs, multiple small ejector pins should be placed around the ribs to prevent stress cracking caused by excessive localized ejection force.
Post-processing can further eliminate residual stress. Heat treatment, by heating the material below its glass transition temperature, allows the molecular chains to become mobile and relax frozen elastic deformation. The optimization approach is to place the product in a constant temperature chamber, controlling the temperature and time to avoid deformation caused by overheating. For example, for PC/ABS alloy baseplates, a heat treatment process of 80°C for 2 hours can reduce internal stress by more than 50%.
