How To Eliminate Knit Lines

Understanding Knit Lines in Injection Molding: Causes and Solutions

In the injection molding process, knit lines are a common occurrence when two or more plastic flow fronts collide and solidify, resulting in a visible seam or line on the molded part. While injection molding itself is a straightforward process involving melting thermoplastic resin and injecting it into a mold cavity, knit lines can pose challenges in terms of both aesthetics and, to a lesser extent, structural integrity.

Knit lines typically form around holes, bosses (features with holes for threaded fasteners), or other obstructions to the flow of the molten plastic. Certain thermoplastic resins with lower flow rates, such as ABS and filled resins, are more prone to developing knit lines. With a wide range of approximately 85,000 thermoplastics available, including various polymer blends or families, material selection plays a significant role in knit line formation.

To minimize or eliminate knit lines, several approaches can be employed:

  1. Select resins with lower susceptibility to knit line formation: Choosing thermoplastic resins with higher flow rates can reduce the likelihood of knit lines occurring during molding.
  2. Optimize boss or gate locations: Modifying the placement of bosses or gates can help redirect the flow of molten plastic, reducing the chances of knit line formation in critical areas.
  3. Adjust part wall thickness: Increasing the thickness of part walls can slow down the cooling process, giving the molten plastic more time to merge and solidify, potentially minimizing knit lines. However, it’s important to strike a balance, as excessively thick walls can lead to sink marks on the surface.
  4. Position knit line-causing features away from part edges: When feasible from a design perspective, relocating features that are prone to knit lines further away from part edges can help mitigate their visibility.

It’s worth noting that knit lines are primarily a cosmetic concern rather than a structural issue. However, their presence can affect the overall appearance and perceived quality of the molded parts.

To address any specific questions or concerns regarding knit lines in your design, we encourage you to reach out to our Technical Specialists. You can contact them at 931-766-7290 or email them at They will be able to review your design and provide tailored guidance based on your requirements.


For more information go to