Injection Molding
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PVC Injection Burnt (Gate Burnt Marks)
Problem Phenomenon
Brown/black burnt marks appear near the gate of PVC injection molded parts, may be accompanied by odor, positions concentrated in the gate area.
Cause Analysis
PVC melt viscosity is high and heat sensitivity is strong, high shear heat generation at small gates exceeds the thermal stability limit; overheating or residence degradation in the gate area; local overheating of hot runner also causes burning; PVC has extremely low tolerance to shear heat.
Solution
①Enlarge the gate to reduce shear; ②Reduce melt temperature and injection speed; ③Homogenize hot runner temperature to prevent local overheating; ④Check stabilizer remaining amount; ⑤Shorten the melt residence time in the barrel to reduce heat accumulation degradation.
Key Takeaway
PVC injection burnt = 'shear heat exceeding limit': the gate is the key protection area; PVC is a heat-sensitive material, gate shear heat is most likely to 'burn' it — large gate + low temperature + slow speed is the basic setup.
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