Considering plastic shrinkage in injection mold design is a core requirement to ensure dimensional accuracy, functionality and appearance quality of the final product. Here are the key reasons why it is important to focus on shrinkage:
1. The inherent characteristics of plastic materials
Cooling phase transition: When the molten plastic is cooled and cured in the mold, the molecular chain changes from a disordered state to an orderly arrangement (especially for crystalline plastics), resulting in a reduction in volume. This physical property is the root cause of plastic shrinkage.
Material difference: the shrinkage rate of different plastics is significantly different (such as PA66 shrinkage rate of about 1.5-2.5%, while ABS is only 0.4-0.7%), and the mold design needs to be compensated according to the material characteristics.
2. Avoid product size deviation
Direct failure: If the mold cavity is not enlarged according to the shrinkage rate, the actual size of the product will be less than the design value. For example, the design of 100mm parts, if the material shrinkage rate is 1%, the actual size is only 99mm, resulting in assembly failure or functional defects.
Tolerance requirements: Precision parts (such as gears, connectors) have strict dimensional tolerances (within ±0.05mm), and shrinkage compensation is a necessary step to achieve accuracy.
3. Prevent appearance defects and structural problems
Sink Marks: thick-walled areas cool slowly, and uneven shrinkage will form depressions on the surface, which need to be alleviated by pressure preservation and mold design optimization.
Warping deformation: Anisotropic shrinkage (such as fiber reinforced materials contracting less in the direction of flow) can cause the product to bend and twist, affecting flatness or assembly.
Stress cracking: The internal stress caused by uneven shrinkage may cause micro-cracks and reduce the mechanical strength of the product.
4. Synergy between process and mold design
Pressure transfer: Mold gate and runner design should ensure that the melt in the pressure holding stage can effectively supplement the shrinkage space and avoid internal cavities (shrinkage holes).
Cooling system impact: Differences in cooling rates can lead to local shrinkage changes. For example, thick-walled areas cool more slowly, shrink more, and need to balance the temperature through the mold channel layout.
5. Economy and production efficiency
Reduce mold trial costs: Ignoring shrinkage can lead to multiple mold repairs (such as welding, polishing), increasing time and expense.
Avoid batch scrapping: If the size is abnormal due to shrinkage during mass production, the whole batch of products may be unqualified, resulting in a waste of resources.
6. Special materials and structural challenges
Fiber reinforced materials: glass fiber reinforced plastics (such as GF-PA) have low shrinkage, but significant anisotropy and need to be compensated separately.
Complex geometric features: the shrinkage rate of local structures such as reinforcement bars and clasp is different from that of the main body, and the mold size needs to be adjusted separately.