What Causes Flow Marks Near the Gate in Injection Molded Parts?
Rigid plastic materials such as PS may develop dense ripple marks on the surface near the gate area. These marks are usually centered around the gate and are commonly referred to as flow marks.
This defect typically occurs when the melt viscosity is too high and the material flows into the mold cavity in an unstable or interrupted manner. When the front end of the melt contacts the colder mold surface, it cools and shrinks quickly. The following molten material then pushes and stretches the partially solidified material forward. As this process repeats, ripple-like marks form on the part surface in the flow direction.
Solutions:
- Increase the barrel temperature, especially the nozzle temperature, and raise the mold temperature appropriately.
- Increase injection pressure and injection speed so the cavity can be filled more quickly and smoothly.
- Optimize the size of the runner and gate to reduce flow resistance.
- Improve mold venting and provide a sufficiently large cold slug well.
- Avoid designing parts with excessively thin wall sections.