Injection Molding
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General-purpose polyethylene injection molded products sink marks at thick-wall areas
Fenómeno do problema
Obvious depressions appear at thick-wall parts of the product （such as rib roots, the back of bosses, and abrupt wall thickness changes）, the surface is uneven, affecting appearance and dimensional accuracy.
The depression depth is usually 0.3-1.5mm, especially obvious under side light, and in severe cases the depression can be felt by fingers.
Análise das causas
（1） Holding pressure insufficient or holding time too short, the melt fails to fully feed shrinkage into the thick-wall area\
（2） General-purpose polyethylene has high crystallinity, with large volume shrinkage during cooling （shrinkage rate 1.5-3.0）\
（3） The mold gate size is too small or the position is improper; premature gate solidification prevents effective transmission of holding pressure\
（4） Product wall thickness is uneven; thick-wall parts cool slowly and shrink more\
（5） Mold temperature is too high （exceeding the 30-80°C upper limit）, and slow cooling rate aggravates shrinkage
Solução
（1） Increase holding pressure to 80-120MPa, extend holding time to 15-30s, ensure thick-wall areas are fully fed\
（2） Appropriately lower the mold temperature to the lower value within the 30-80°C range, speed up cooling to reduce shrinkage\
（3） Enlarge the gate cross-sectional area, gate thickness at least 60-70% of product wall thickness, extend gate freezing time\
（4） Optimize product design, keep wall thickness as uniform as possible, use slopes or arcs at thick-thin transitions, thick-to-thin ratio not exceeding 2:1\
（5） Appropriately lower the melt temperature to the lower value within the 160-260°C range to reduce volume shrinkage
Conclusão principal
Sink marks are a typical defect of crystalline materials in '2079851475616104452; the root cause is that local volume shrinkage is not effectively fed, and it needs to be solved coordinately from three aspects: holding pressure parameters, gate design and cooling conditions.
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