Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Flow lines (also known as flow marks) are off-color lines, streaks, or wavy patterns that appear on the surface of a molded plastic part. They typically form when the molten plastic cools and solidifies unevenly as it flows through the mold cavity, often near the gate or in areas with complex geometry.

Primary Causes of Flow Lines

Flow lines can result from a combination of process parameters, material properties, and mold design issues. Below is a summary of the most common causes, based on industry insights:

1.Inadequate Injection Speed or Pressure

If the injection speed is too slow, the molten plastic begins to cool before fully filling the mold, leading to premature solidification and visible lines where flow fronts meet. Conversely, excessively high speeds can cause "jetting," where the material shoots out unevenly and cools rapidly upon hitting mold walls. Low injection pressure exacerbates this by failing to push the material uniformly into the cavity.

2.Uneven Cooling Rates

Variations in cooling speed across the mold—due to inconsistent mold temperatures, poor cooling channel design, or the plastic flowing in multiple directions—cause different sections of the material to solidify at different rates. This is one of the most frequent culprits, as the outer layers cool faster than the inner flow.

3.Improper Melt or Mold Temperature

A melt temperature that's too high can make the plastic too viscous or prone to shear heating, promoting uneven flow. If too low (or if the mold is too cold), the material solidifies too quickly. Optimal temperatures depend on the resin type (e.g., higher for crystalline polymers like nylon).

4.Gate and Runner Design Issues

Small or improperly sized gates/runners restrict flow, causing turbulence or hesitation in the melt front. This often leads to flow lines near the gate. Inadequate cold slug wells (to trap early-cooling material) can also contribute.

5.Material-Related Factors

High-viscosity resins, moisture in the material, or incorrect residence time in the barrel (leading to degradation) can amplify flow inconsistencies. Some plastics are more susceptible due to their flow characteristics.

6.Other Process Variables  Holding 

Pressure: Too low prevents proper packing, allowing shrinkage and line formation.  

Nozzle or Screw Issues: A too-small nozzle or improper screw speed can introduce shear heat unevenly.  

Mold Complexity: Sharp corners or thin walls increase the risk of flow hesitation.

Prevention Tips

To minimize flow lines, optimize process parameters iteratively (e.g., increase injection speed gradually while monitoring temperature). Simulate the process with software like Moldflow for design tweaks. Always dry hygroscopic materials and ensure uniform mold cooling.