Injection Molding Troubleshooting: How to Avoid Knit Lines

Injection molding is an efficient, often cost-effective way to manufacture custom plastics that are strong, durable, and long-lasting. However, with any practice, when the proper care and planning aren't taken, problems can occur, and with injection molding, knit lines are one of the most common problems. Our injection molding company in North Carolina is looking at what knit lines are, what causes them, and how to avoid them.

What Are Knit Lines?

When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. A knit line, also called a weld line, occurs when the liquid plastic moves through the mold and hits more melted plastic and they don't flow or fuse together. Instead, a line that looks like a seam forms.

This typically happens when two flows of plastic meet together from two separate injectors flowing into one tool or when the plastic flows around a shape like a circle and it meets on both sides.

The Problem with Knit Lines

In most cases, the plastic does fully fuse, but a line still appears, in which case the knit line is purely a cosmetic problem. However, when the two flows meet but don't fuse, the line can be a weak area of the component. When the component is a moving part or must take on weight, this can be a serious concern that can cause a malfunction or equipment failure.

What Causes Weld Lines?

As we mentioned, weld lines are caused by two flows of plastic not fusing together, but looking deeper, what causes the plastic to not fuse in the first place?


If the plastic begins to cool while it flows through the mold, it may solidify just enough to not join with the other flow.


When the injector doesn't exert enough pressure to move the plastic into the tool to push the proper flow fronts together, the flow line will form.

Mold Design

A mold in which the gates aren't placed in the proper location or a runner isn't the right thickness can block the proper fusion of flow lines.

Tips for Avoiding Knit Lines in Injection Molding

Knowing what causes the knit lines is key for troubleshooting and avoiding them.

  • Keeping the proper temperature in the injection molding machine, runners, and within the mold are all key to ensuring the plastic flows at a speed that allows it to fuse with other flows as well as to reach all the spaces within the mold itself. Heating the mold or using hot runners can keep the plastic in a liquid state.
  • Setting the proper pressure on the injectors to push the plastic into the mold with enough speed that the plastic will coat the mold and fuse with other flow lines.
  • Taking proper care to design the mold properly, avoiding bottlenecks, and changing the number or location of gates can improve performance.
  • Using a thermoplastic that stays in a liquid state for a longer period may be beneficial to avoid the knit lines.

Get a Free Quote for Injection Molded Plastics Today

For durable, high-quality custom plastics, reach out to Advanced Plastiform, Inc. We specialize in both thermoforming and injection molding for all types of industries across the Southeast and Mid-Atlantic,  including seven states, including North Carolina,  SouthCarolina,  Pennsylvania,  MarylandTennesseeGeorgia, and Virginia. providing low per-unit prices and fast turnaround times.

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