Injection Molding
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Low density polyethylene injection product coaxiality out of tolerance
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.
Out-of-tolerance coaxiality affects the dynamic balance and fitting accuracy of rotating parts, and is common in tubular products and stepped shaft products.
Cause Analysis
(1) Mold core is eccentric or the core shifts during injection, causing uneven wall thickness
(2) Improper gate position; single-side feeding produces lateral force that pushes the core off-center
(3) Uneven mold temperature; large temperature difference between core and cavity causes asymmetric shrinkage
(4) Uneven holding pressure; inconsistent shrinkage compensation on each side of the cavity
(5) Asymmetric product structure, large differences in cooling shrinkage
Solution
(1) Check core positioning accuracy; control the coaxiality of core and cavity within 0.02mm
(2) Optimize gate position, adopt symmetrical multi-point gates to reduce eccentric force from single-side feeding
(3) Control uniform temperature of core and cavity, temperature difference not exceeding 5°C (30-60°C range)
(4) Optimize the holding pressure scheme to ensure uniform shrinkage compensation in all directions
(5) Make product design as symmetrical as possible, avoid eccentric structures
Key Takeaway
Out-of-tolerance coaxiality is caused by core shift or asymmetric shrinkage, and needs to be solved from mold accuracy, gate symmetry, and temperature uniformity.
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