Injection Molding
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Weld lines on homopolymer polypropylene injection products
Problem Phenomenon
Linear marks appear on the product surface where two or more melt fronts converge, usually V-shaped grooves or color difference lines, with lower strength.
Weld lines not only affect appearance, but are also weak points of the product, easily cracking from there under stress.
Cause Analysis
(1) Multi-gate design or complex product structure causes melt to split and converge; low melt temperature at the convergence, insufficient fusion
(2) Injection speed too slow; melt temperature drops too much during mold filling, cannot fully fuse when converging
(3) Barrel temperature too low, poor melt flowability, insufficient pressure at convergence
(4) Mold temperature too low; melt rapidly cools after contacting the mold wall, low fusion strength at the convergence interface
(5) Poor venting; gas at the convergence cannot escape, forming a barrier layer
Solution
(1) Increase melt temperature to the higher value within the 200-280°C range, improve melt flowability
(2) Increase mold temperature to the higher value within the 40-90°C range, delay melt cooling
(3) Increase injection speed, shorten mold filling time, reduce melt temperature loss
(4) Adjust gate position and number, minimize melt splitting; add overflow wells when necessary to remove weld lines
(5) Improve mold venting, set vent grooves or vent pins at weld line positions
Key Takeaway
Weld lines are a manifestation of poor fusion at melt convergence; melt fusion quality is improved by increasing melt temperature, injection speed, and mold temperature.
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