Injection Molding
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Weld lines appearing in random copolymer polypropylene injection molded products
Fenómeno del problema
Linear marks appear on the product surface where two or more melt streams meet, 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.
Análisis de la causa
(1) Multi-gate design or complex product structure causes melt streams to split and then merge; the melt temperature at the merging point is low and fusion is insufficient
(2) Injection speed is too slow; melt temperature drops too much during filling and cannot fully fuse when merging
(3) Barrel temperature is on the low side; melt fluidity is poor and pressure at the merging point is insufficient
(4) Mold temperature is too low; the melt cools rapidly after contacting the mold wall, and fusion strength at the merging interface is low
(5) Poor venting; gas at the merging point cannot escape, forming a barrier layer
Solución
(1) Increase melt temperature to the higher value within the 190-260°C range to improve melt fluidity
(2) Increase mold temperature to the higher value within the 40-80°C range to delay melt cooling
(3) Increase injection speed and shorten filling time to reduce melt temperature loss
(4) Adjust gate position and number to minimize melt splitting; add overflow wells if necessary to eliminate weld lines
(5) Improve mold venting, setting vent grooves or vent pins at weld line positions
Conclusión clave
Weld lines are a manifestation of poor fusion of merging melt streams; melt fusion quality is improved by increasing melt temperature, injection speed and mold temperature.
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