Injection Molding
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Sink marks (shrinkage) on PP injection molded products
Fenómeno del problema
Surface depressions appear on the product at ribs, screw bosses, the back of snap fits, or thick-wall areas; they are especially glaring on high-gloss and appearance surfaces, and in severe cases may be accompanied by internal shrinkage cavities, affecting appearance and assembly.
Análisis de la causa
PP has large crystallization shrinkage; at the junction of ribs/bosses with the main wall, the material is locally thick, cools slowly, and shrinks a lot, while after the surface layer solidifies first, the internal melt continues to shrink but receives no holding pressure feeding, so the surface is pulled inward and dented; insufficient holding pressure or time, gate freezing too early, and an excessive rib thickness ratio are the three direct causes.
Solución
①Control the rib thickness to 50%~60% of the main wall thickness, with a root fillet to reduce material; ②increase the holding pressure and extend it until gate freeze-off (determined by conducting a freeze-off study); ③core out thick-wall areas to reduce material and avoid solid thick blocks; ④place the gate near the thick wall and enlarge it appropriately; ⑤add inserts or conformal water channels in the sink mark area for intensive cooling.
Conclusión clave
PP sink marks must be tackled with both 'structure + holding pressure': structurally reduce material and control rib thickness first, and in the process do enough holding pressure feeding and gate freeze-off management; only slamming on holding pressure at the machine without touching the structure often fails to cure the sink marks and instead introduces internal stress and flash.
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