Knit lines are formed when two or more plastic flow fronts collide and solidify or “knit” together during the molding process.

Overall, injection molding is a relatively simple process. A thermoplastic resin is heated to its melting point and injected into the cavity of an injection mold to produce a specific part geometry. The part is cooled in the mold until it reaches a temperature where it is solid enough to be ejected.

Knit lines most commonly occur around holes or other obstructions to the melt flow such as bosses. A boss is a feature with a hole that designed for a threaded fastener. A gate is an area where the resin is injected into the cavity.

Some thermoplastic resins with lower flow rates such as ABS and filled resins are more prone to having knit line issues. There are approximately 85,000+ thermoplastics available in the marketplace. Within the vast material options available, there are approximately 40 polymer blends or families.

While the presence of knit lines does not always compromise the structural integrity of the plastic part, they are almost always a cosmetic issue.

Changing the injection profile parameters – modifying the fill time for instance – may cause the knit line to move to a more favorable location.

Material selection, part design, tool design, and process parameters all also affect knit lines.

How to eliminate Knit Lines
Select resins that are less susceptible to knit line formation.
Change the boss or gate locations.
Thicken part walls to slow down the resin cooling process however be careful not to make them too thick that it causes sink marks.
Place knit line causing features farther from the edge of parts when the design allows for it to do so.