Injection Molding
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Out-of-Tolerance Flatness of Low-Density Polyethylene Injection Molded Products
Problem Phenomenon
The flatness of large flat products exceeds the tolerance requirement, and the surface shows undulation or bending.
Out-of-tolerance flatness affects product assembly and appearance, and is particularly prominent in large-area thin-walled products.
Cause Analysis
（1）The shrinkage of low-density polyethylene crystalline material is inconsistent in all directions, which is more pronounced on large flat surfaces
（2）The mold cooling is uneven, with differences in cooling rates between parts of the large flat surface
（3）The product wall thickness is uneven, and the reinforcing rib distribution is asymmetric
（4）The gate position is improper, and the single melt flow direction leads to anisotropy of orientation shrinkage
（5）The cooling time is insufficient, and the product continues to shrink and deform after demolding
Solution
（1）Optimize the mold cooling system, distribute the cooling water channels uniformly in the large flat area, with the temperature difference not exceeding 5°C
（2）Optimize the product design, arrange the reinforcing ribs symmetrically, and make the wall thickness uniform
（3）Adopt multi-point gates or fan gates to make the melt flow uniformly and reduce orientation differences
（4）Extend the cooling time to ensure the product is fully cooled before demolding
（5）Appropriately increase the mold temperature to a higher value within the 30-60°C range to reduce internal stress
Key Takeaway
Out-of-tolerance flatness is a manifestation of uneven shrinkage of large-area products, and the shrinkage uniformity in all directions needs to be controlled by optimizing cooling, gating, and product design.
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