How to Avoid Knit Lines

Knit lines in injection molded parts can range from harmless to unsightly to a serious structural problem. What are knit lines, and how can you avoid them? Read on for tips on how to minimize or eliminate knit lines when designing your mold.

What Are Knit Lines?

Think about how a resin flows through a mold. If the final part is solid, the resin should flow smoothly through the whole mold. However, if there is a hole, boss, or another feature in the way, two resin flows will meet on the other side of that feature. A knit line is a line where those two resin flows meet.

Sometimes, knit lines are invisible and harmless. Other times, knit lines cause cosmetic issues but don’t affect the part strength. And occasionally, knit lines affect a product’s strength so much as to render it worthless.

How to Avoid Knit Lines

As you can imagine, it’s best to avoid knit lines when possible. Here are some things to keep in mind when designing a mold to help avoid knit lines.

Choose Resins Carefully

Some resins are more susceptible to knit lines than others. ABS plastic is notorious for causing knit lines, so it’s best to choose a different material if you can’t find other ways to eliminate knit lines from your design. With approximately 85,000+ thermoplastics available in the marketplace, you’re likely to find one that will work with your mold.

Increase Injection Speed and Pressure

Resin starts to cool as soon as it enters the mold. The faster the resin fills the mold, the less likely it is to cool enough to cause knit lines, so increasing the injection speed and pressure may help eliminate knit lines.

Thicken Part Walls

With thicker part walls, plastic can move more easily through the mold. However, walls that are too thick can cause sink, so it’s a fine line to design part walls that are thick enough to prevent knit lines while being thin enough to prevent sink.

Keep Knit Line-Causing Features Away from Edges

Holes and bosses near the edge of a part slow the resin down the most and are more likely to cause knit lines. When possible, keep features that are known to cause knit lines as far away from the edges of your part as possible.

Change Boss or Gate Locations

A boss is a feature with a hole designed to accommodate a threaded fastener, and it’s a common cause of knit lines. Changing the location of bosses may help reduce knit lines.

The gate is where the resin enters the mold. Changing its location could alter the flow of the plastic enough to minimize knit lines in your part.

Increase Material Temperature

Hotter plastic not only flows faster, but it cools slower, giving you more time to fill the mold before the resin cools enough to cause knit lines. Sometimes, that’s enough to prevent knit lines.

Contact Universal Plastic Mold Today

With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. To learn more about us or to get a quote, click here or call 1-888-893-1587 now.

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