Injection Molding
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Coaxiality out of tolerance of random copolymer polypropylene injection molded products
Problem Phenomenon
The axes of the inner and outer circles or different axial sections of the product do not coincide, and coaxiality exceeds tolerance requirements.
Coaxiality out of tolerance affects the dynamic balance and fitting accuracy of rotating parts, and is common in tubular products and stepped shaft products.
Cause Analysis
(1) The mold core is eccentric or shifts during injection, causing uneven wall thickness
(2) Improper gate position; single-side feeding produces lateral force that pushes the core off-center
(3) Mold temperature is uneven; large temperature difference between core and cavity causes asymmetric shrinkage
(4) Holding pressure is uneven; shrinkage compensation is inconsistent on all cavity sides
(5) The product structure is asymmetric, with large cooling shrinkage differences
Solution
(1) Check core positioning accuracy, controlling coaxiality of the core and cavity within 0.02mm
(2) Optimize gate position, using symmetric multi-point gates to reduce eccentric force from single-side feeding
(3) Control core and cavity temperature uniformity, with temperature difference not exceeding 5°C (40-80°C range)
(4) Optimize the holding pressure scheme to ensure uniform shrinkage compensation in all directions
(5) Design the product as symmetric as possible, avoiding eccentric structures
Key Takeaway
Coaxiality out of tolerance is caused by core shifting or asymmetric shrinkage and needs to be solved from mold precision, gate symmetry and temperature uniformity.
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