Injection Molding
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PP '2079851475616104452 Shear Mark (Melt Fracture Mark)
Problem Phenomenon
During high-speed filling of high-flow PP, misty or streak-like shear marks appear on part surfaces, mostly near gates and thin-wall areas.
Cause Analysis
Excessive shear rate destabilizes surface melt flow, forming visible shear marks; concentrated shear at the gate area, excessive injection speed, and low mold temperature all aggravate it—especially common in thin-wall high-speed processes.
Solution
①Reduce injection speed when passing the gate (speed up after the gate); ②appropriately enlarge the gate to disperse shear; ③raise mold temperature to improve surface conformity; ④optimize the speed profile to avoid prolonged full-speed filling; ⑤evaluate whether wall thickness and flow length of thin-walled parts are reasonable.
Key Takeaway
PP high-speed processes must prevent shear marks—the gate is a 'buffer valve': reducing through-gate speed and enlarging the gate eliminates marks while preserving capacity better than overall speed reduction.
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