Flow lines appear as a wavy pattern often of a slightly different color than the surrounding area and generally on narrower sections of the molded component. They may also appear as ring-shaped bands on a product’s surface near the entry points of the Mold parts, or “gates”, which the molten material flows through. Flow marks won’t typically impact the integrity of the component. But they can be unsightly and may be unacceptable if found in certain consumer products, such as high-end sunglasses.
Causes and remedies for flow lines
Flow lines are most often the result of variations in the cooling speed of the material as it flows in different directions throughout the mold. Differences in wall thickness can also cause the material to cool at different rates, leaving behind flow lines.injection molding defects For example, molten plastic, cools very quickly during the injection process and flow marks are evident when the injection speed is too slow. The plastic becomes partially solid and gummy while still filling the mold, causing the wave pattern to appear.
Here are some common remedies for flow lines in injection-molded products:
Increase the injection speed, pressure and material temperature to ensure the material fills the mold before cooling
Round the corners of the mold where wall thickness increases to help keep flow rate consistent and prevent flow lines
Relocate mold gates to create more distance between them and the mold coolant to help prevent the material from cooling too early during flow
Increase the nozzle diameter to raise flow speed and prevent early cooling
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