Injection Molding
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Coaxiality Out of Tolerance of High-Density Polyethylene Injection-Molded Products
Problem Phenomenon
The axes of the inner and outer circles or different shaft sections of the product do not coincide, and the coaxiality exceeds the tolerance requirement.
Coaxiality out of tolerance affects the dynamic balance and fitting precision of rotating parts, and is common in tubular products and stepped shaft products.
Cause Analysis
(1) The mold core is eccentric or the core shifts during injection, resulting in uneven wall thickness
(2) The gate position is improper, and the lateral force generated by single-side feeding pushes the core off
(3) The mold temperature is uneven, and the large temperature difference between the core and the cavity leads to asymmetric shrinkage
(4) The holding pressure is uneven, and the shrinkage compensation on each side of the cavity is inconsistent
(5) The product structure is asymmetric, and the cooling shrinkage difference is large
Solution
(1) Check the positioning precision of the core, and control the coaxiality of the core and the cavity within 0.02mm
(2) Optimize the gate position and adopt symmetric multi-point gates to reduce the eccentric force of single-side feeding
(3) Control the temperature of the core and the cavity to be uniform, with the temperature difference not exceeding 5°C (range 30-80°C)
(4) Optimize the holding scheme to ensure uniform shrinkage compensation in all directions
(5) Make the product design as symmetric as possible and avoid eccentric structures
Key Takeaway
Coaxiality out of tolerance is caused by core shift or asymmetric shrinkage, and needs to be solved from mold precision, gate symmetry, and temperature uniformity.
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