Solutions to deformation problems of large-sized planar plastic parts
During the injection molding process, large, flat plastic parts are prone to deformation, such as warping, bending, and denting, due to their structural characteristics (such as large area, relatively uniform thickness, and low rigidity). These deformations not only affect the part’s appearance but also reduce its assembly precision and performance. Therefore, addressing deformation of large, flat plastic parts is a critical issue in injection molding production, requiring comprehensive consideration from multiple aspects, including mold design, material selection, and process parameter optimization.
Mold design is one of the key factors affecting the deformation of large-sized planar plastic parts. A reasonable mold structure can effectively reduce the internal stress of the plastic part, thereby reducing the possibility of deformation. First, the mold cavity and core should have sufficient rigidity and strength to avoid deformation under the action of injection pressure, which will affect the dimensional accuracy of the plastic part. Secondly, the design of the pouring system should ensure that the melt can evenly fill the cavity to avoid local filling that is too fast or too slow. For large-sized planar plastic parts, a multi-point pouring method is usually adopted, so that the melt enters the cavity from multiple gates at the same time, reducing the flow distance and pressure loss of the melt, and ensuring that all parts of the cavity can be evenly filled and cooled. In addition, the gate should be located as close to the center or symmetrical position of the plastic part as possible to reduce asymmetry during the melt flow process and reduce internal stress.
The design of the cooling system also significantly impacts the deformation of large, flat plastic parts. Uniform cooling ensures consistent shrinkage across the entire part, reducing warping and bending. When designing the cooling system, cooling channels should be evenly distributed around the cavity and core, maintaining an appropriate distance from the cavity surface (typically 10-15mm), and ensuring uniform cooling water flow rate and flow rate. For thicker areas, the number or diameter of cooling channels should be increased to improve cooling efficiency. For thinner areas, stress concentration caused by excessive cooling should be avoided. Furthermore, the use of conformal cooling channels (such as complex cooling channels produced using 3D printing technology) can better conform to the shape of the part, improve cooling uniformity, and effectively reduce deformation.
Material selection and pretreatment are also important measures to address the problem of deformation of large-sized planar plastic parts. Different plastic materials have different shrinkage rates, fluidity, and thermal stability. Selecting the right material can reduce the risk of deformation. For large-sized planar plastic parts, priority should be given to materials with low shrinkage rates and good dimensional stability, such as modified polypropylene, ABS, and polycarbonate. At the same time, the plastic raw materials should be thoroughly dried before injection molding to remove moisture from the raw materials to avoid bubbles or internal stress caused by volatilization of moisture at high temperatures, which can lead to deformation of the plastic parts. In addition, for some crystalline plastics (such as polyethylene and polypropylene), the volume changes during the crystallization process are large and are prone to deformation. Therefore, it is necessary to adjust the crystallinity and reduce deformation by controlling the mold temperature and cooling rate.
Optimizing process parameters plays a crucial role in minimizing deformation in large, planar plastic parts. Parameters such as injection pressure, injection speed, holding pressure, melt temperature, and mold temperature all affect the internal stress and shrinkage of the part, which in turn influences the amount of deformation. Generally speaking, appropriately increasing injection and holding pressures can reduce shrinkage, but excessive pressures can increase internal stress and lead to warping. Therefore, finding a balance between these two pressures is crucial. The injection speed should be adjusted based on the part’s thickness and material fluidity. For large, planar parts, a slower injection speed is typically used to ensure smooth melt filling of the mold cavity, minimizing turbulence and air entrapment, and reducing internal stress. Furthermore, appropriately increasing mold temperature can slow the melt’s cooling rate, ensuring uniform cooling across the part and minimizing deformation caused by temperature gradients. A reasonable holding time ensures adequate shrinkage compensation during cooling, minimizing shrinkage deformation.
For large-sized planar plastic parts that have already deformed, appropriate post-processing measures can be taken to correct them. Common post-processing methods include heat setting and mechanical correction. Heat setting is to heat the plastic part to a certain temperature (usually 10-20°C below the glass transition temperature of the plastic), and then apply a certain amount of pressure to keep it flat. After cooling, the pressure is removed to eliminate some internal stress and correct the deformation. Mechanical correction is to forcibly fix the deformed plastic part in a flat state through a fixture or mold, and restore its shape after a period of time. It should be noted that post-processing measures can only correct deformation to a certain extent, and cannot completely replace the early mold design and process parameter optimization. Therefore, solving the deformation problem of large-sized planar plastic parts should prioritize controlling it from the source.
