Injection Molding
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Flow marks on the surface of homopolymer polypropylene injection products
Problem Phenomenon
Concentric circle or wavy stripes centered on the gate appear on the product surface, i.e., flow marks.
The surface at flow marks is rough, color may differ slightly, affecting product appearance quality, mostly seen in high-gloss products.
Cause Analysis
(1) The temperature of the flow front repeatedly rises and falls as the melt flows in the cavity, forming alternating hot-cold flow marks
(2) Injection speed too slow or unstable; melt front intermittently stagnates during mold filling
(3) Mold temperature too low; melt rapidly cools after contacting the mold wall, forming a cold material layer
(4) Improper barrel temperature setting; nozzle temperature too low producing cold material
(5) Mold gate size too small; melt generates shear heat and pressure loss when passing through the gate
Solution
(1) Increase injection speed, adopt multi-stage injection, maintain a stable speed front during mold filling
(2) Increase mold temperature to the higher value within the 40-90°C range, reduce melt temperature drop
(3) Increase nozzle temperature to match the front barrel temperature or 5-10°C higher, prevent cold material
(4) Enlarge gate size, reduce shear and pressure loss of melt passing through the gate
(5) Appropriately increase barrel temperature to the higher value within the 200-280°C range
Key Takeaway
Flow marks are a manifestation of unstable melt flow front; the flow front is stabilized by increasing injection speed, mold temperature, and melt temperature.
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