Injection Molding
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Flatness Out of Tolerance of High-Density Polyethylene Injection-Molded Products
Problem Phenomenon
The flatness of large flat products exceeds the tolerance requirement, and the surface shows undulations or bending.
Flatness out of tolerance affects product assembly and appearance, and is especially prominent in large-area thin-walled products.
Cause Analysis
(1) High-density polyethylene, a crystalline material, has inconsistent shrinkage in all directions, which is more easily manifested on large flat surfaces
(2) The mold cooling is uneven, and the cooling rates of various parts of the large flat surface differ
(3) The wall thickness of the product is uneven, and the reinforcing ribs are distributed asymmetrically
(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, with cooling water channels evenly distributed in the large flat area, and the temperature difference not exceeding 5°C
(2) Optimize the product design, with reinforcing ribs arranged symmetrically and uniform wall thickness
(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 the higher value within the range of 30-80°C to reduce internal stress
Key Takeaway
Flatness out of tolerance is a manifestation of uneven shrinkage of large-area products, and the shrinkage uniformity in all directions needs to be controlled by optimizing cooling, gates, and product design.
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