Injection Molding
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PE '2079851475616104452 Sink Marks (Shrinkage)
Problem Phenomenon
The product surface is dented at ribs, posts and thick-wall areas, obvious on appearance surfaces, frequent in thick-wall containers and structural parts, and may be accompanied by internal shrinkage cavities.
Cause Analysis
PE has large crystallization shrinkage; thick walls and rib/post intersections have locally thick material and uneven cooling; after the surface layer solidifies first, the interior continues to shrink without holding pressure feeding, and the surface is pulled in; insufficient holding pressure, premature gate freeze-off and excessive rib thickness ratio are direct causes.
Solution
①Control the rib thickness to 50%~60% of the main wall thickness, with root fillets to reduce material; ②increase the holding pressure and extend it until gate freeze-off; ③core out the thick-wall areas to reduce material; ④place the gate near the thick wall and enlarge it appropriately; ⑤strengthen local cooling in the sink mark area.
Key Takeaway
PE sink marks require both 'material reduction + feeding': control the wall thickness and rib thickness structurally, and do sufficient holding and freeze-off management in the process; thick-wall PE parts especially must avoid solid thick blocks, otherwise no matter how good the holding is, it cannot fill them.
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