Injection Molding
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Flatness out of tolerance of homopolymer polypropylene injection products
Problem Phenomenon
Flatness of large flat products exceeds tolerance requirements, with surface undulation or bending.
Out-of-tolerance flatness affects product assembly and appearance, especially prominent in large-area thin-wall products.
Cause Analysis
(1) Homopolymer polypropylene crystalline material shrinks inconsistently in each direction, more evident on large flat surfaces
(2) Uneven mold cooling, cooling rate differences across large flat areas
(3) Uneven product wall thickness, asymmetric rib distribution
(4) Improper gate position; single melt flow direction causes anisotropic orientation shrinkage
(5) Insufficient cooling time, continued shrinkage deformation after demolding
Solution
(1) Optimize mold cooling system, cooling water channels evenly distributed in large flat areas, temperature difference not exceeding 5°C
(2) Optimize product design, ribs symmetrically arranged, uniform wall thickness
(3) Adopt multi-point gates or fan gates, make melt flow uniformly to reduce orientation differences
(4) Extend cooling time, ensure the product is fully cooled before demolding
(5) Appropriately increase mold temperature to the higher value within the 40-90°C range, reduce internal stress
Key Takeaway
Out-of-tolerance flatness is a manifestation of uneven shrinkage of large-area products; shrinkage uniformity in each direction needs to be controlled by optimizing cooling, gating, and product design.
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