Injection Molding
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Flow Marks Appearing on the Surface of High-Density Polyethylene Injection-Molded Products
Problem Phenomenon
Concentric circular or wavy streaks centered on the gate appear on the product surface, namely flow marks.
The surface at the flow marks is rough and the color may be slightly different, affecting the appearance quality of the product; it is more common in high-gloss products.
Cause Analysis
(1) When the melt flows in the cavity, the temperature of the front rises and falls repeatedly, forming flow marks with alternating heat and cold
(2) The injection speed is too slow or unstable, and the melt front stagnates intermittently during mold filling
(3) The mold temperature is too low, and the melt cools rapidly after contacting the mold wall to form a cold material layer
(4) The barrel temperature is set improperly, and the nozzle temperature is too low, producing cold material
(5) The mold gate size is too small, and the melt generates shear heating and pressure loss when passing through the gate
Solution
(1) Increase the injection speed and adopt multi-stage injection to maintain a stable speed front during mold filling
(2) Increase the mold temperature to the higher value within the range of 30-80°C to reduce the melt temperature drop
(3) Increase the nozzle temperature to be consistent with the front-section barrel temperature or 5-10°C higher to prevent cold material
(4) Increase the gate size to reduce the shear and pressure loss of the melt when passing through the gate
(5) Appropriately increase the barrel temperature to the higher value within the range of 180-260°C
Key Takeaway
Flow marks are a manifestation of an unstable melt flow front; the flow front is stabilized by increasing the injection speed, mold temperature, and melt temperature.
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