Technologies and techniques for solving deformation problems of large flat plastic parts
Large flat plastic parts are prone to deformation after injection molding, manifesting as warping, bending, denting, or undulations. This not only affects product assembly precision but can also lead to functional failure. These parts typically have a surface area exceeding 500 cm² and a relatively thin thickness (1-3 mm), such as car dashboards, TV housings, and refrigerator door panels. Deformation primarily stems from uneven internal stress distribution, varying cooling rates, and anisotropic plastic shrinkage during the molding process. For example, when the melt flows through a large flat cavity, the edges and center cool at different rates, resulting in inconsistent shrinkage. This creates internal stress, ultimately causing the part to bend toward the faster-cooling side. Addressing deformation in large flat plastic parts requires a comprehensive approach encompassing mold design, process parameter optimization, and material selection to achieve stable production quality.
A rational mold structure design is fundamental to controlling deformation in large, planar plastic parts. First, the surface roughness of the cavity and core must be controlled below Ra0.8μm to ensure smooth melt flow and reduce internal stress caused by uneven flow resistance. For parts exceeding 1000 cm², a multi-gating method, such as fan gates or latent gates, should be employed. This allows the melt to evenly fill the cavity from multiple directions, avoiding the long flow path and uneven cooling caused by a single gate. For example, a television housing part (1200mm x 600mm) utilizes four symmetrically distributed fan gates, which reduces deformation by 40% compared to a single-gate design. Furthermore, the mold cavity must have ample venting slots, with a depth of 0.01-0.02mm and a width of 5-10mm, to ensure smooth evacuation of gases within the cavity and prevent localized pressure buildup and uneven shrinkage caused by trapped gases. For thin-walled, large-plane plastic parts, a 0.5-1° demoulding angle can be added to the mold cavity surface to reduce frictional resistance during demoulding and prevent deformation caused by forced demoulding.
Optimizing the cooling system is crucial for minimizing deformation in large, flat plastic parts. Due to the large surface area of these parts, uneven cooling is a major cause of deformation, necessitating the design of evenly distributed cooling channels. Channel diameters are typically 8-12mm, with spacing controlled between 30-50mm and a distance of 15-25mm from the cavity surface to ensure consistent cooling rates across all areas. For complex, large, flat parts, conformal cooling channels can be employed—channel shape aligned with the part’s surface contours. 3D printing can be used to create custom-shaped channel inserts for more uniform cooling. For example, using conformal cooling in a mold for an automotive instrument panel reduced the temperature difference between various areas of the part from ±8°C to ±2°C, while keeping deformation within 0.5mm/m. The cooling medium temperature should be tailored to the type of plastic. For crystalline plastics (such as PP and PA), the mold temperature should be controlled 10-20°C above the glass transition temperature to promote uniform crystallization. For amorphous plastics (such as PC and ABS), the mold temperature can be lowered to accelerate cooling. At the same time, it is necessary to ensure that the water flow in the cooling water channel is turbulent (Reynolds number > 4000) and improve the heat exchange efficiency by adjusting the water pump flow (5-10L/min).
Precise control of process parameters is crucial for preventing deformation in large flat parts. During the injection phase, a low-speed, high-pressure filling method should be employed to avoid turbulence and excessive shearing of the melt in the mold cavity. For example, reducing the injection speed from 50 mm/s to 30 mm/s and increasing the injection pressure by 10%-15% can ensure smooth melt filling. The holding phase is crucial for minimizing shrinkage deformation. The holding pressure should be set to 60%-80% of the injection pressure, and the holding time should be determined based on the part thickness (typically 5-15 seconds/mm) to ensure adequate melt shrinkage compensation. For example, a large flat PP part (2mm thick) with a holding pressure of 60 MPa and a holding time of 10 seconds reduced shrinkage from 2.5% to 1.8%, significantly reducing deformation. The melt temperature must be controlled within the plastic’s optimal processing temperature range. Excessively high temperatures can lead to molecular chain degradation and internal stress, while excessively low temperatures can increase flow resistance. For example, the melt temperature for ABS should be controlled between 220-240°C, and for PC between 280-300°C. In addition, the cooling time must be long enough to ensure that the temperature of the plastic part is lower than its glass transition temperature when it is demolded. For example, the demolding temperature of ABS plastic parts should be lower than 80°C to avoid deformation due to continued shrinkage after demolding.
Material selection and post-processing can further improve deformation issues in large flat plastic parts. Plastics with low shrinkage and good dimensional stability should be prioritized. For example, glass fiber-reinforced plastics (e.g., PP + 30% GF, PA66 + 30% GF) can reduce shrinkage from 1.5%-2% to 0.3%-0.8%, significantly reducing deformation. For parts requiring less-demanding strength, plastics with low crystallinity, such as high-impact ABS, can be used, as they exhibit less anisotropic shrinkage. After molding, internal stresses can be eliminated through annealing. This involves placing the part in an oven at 10-20°C above its glass transition temperature for 2-4 hours, followed by slow cooling to room temperature. For example, the annealing temperature for PC parts is 120-130°C. After 3 hours of annealing, internal stress can be reduced by over 50%. For plastic parts with minimal deformation, mechanical correction can be used. Fixing the parts to a fixture consistent with the design shape and maintaining them at a constant temperature for 1-2 hours allows the deformation to recover. Furthermore, during the product design phase, locally thick walls or sharp corners should be avoided on large surfaces. Adding reinforcing ribs (with a height of 2-3 times the wall thickness and a thickness of 0.5-0.7 times the wall thickness) can increase the rigidity of the plastic parts and reduce their tendency to deform.
