Injection Molding Knit Lines

The term “knit lines” sounds like a harmonious union of two lines ready to mold into something wonderful. On the contrary, injection molding knit lines are a problem in the process and should be avoided. The experts at The Toolroom want you to know what a knit line is, how they form, why they are a problem, and how they can be avoided,

What is a knit line?

A knit line is formed when two or more plastic flows collide and solidify or knit together in the middle of a part during the injection molding process. A knit can be visible or not visible. Depending on design and material, a knit line could be no problem at all, merely a cosmetic issue, or result in a serious structural problem.

How do knit lines form?

Plastic injection molding is a process where a thermoplastic resin is heated to its melting point and injected into the cavity of an injection mold. Each mold produces as specific part. The part is cooled in the mold until it is solid enough to be ejected. Knit lines generally occur around holes or other obstructions to the melt flow. This might be an area where you have a core or shutoff feature that the plastic needs to flow around. Many thermoplastic resins with a low flow rate or filled resins are more likely to experience knit line issues.

Why are knit lines a problem?

Most issues with knit lines are cosmetic in nature and not related to the integrity of the final part. However, knit lines can result in structural issues behind a boss. A boss is a feature with a hole to accommodate a threaded fastener. The boss is created by a raised core pin inside of the injection mold that the melted resin flows around. When the resin reaches the back side of the pin, it forms a knit line. If the boss is near the edge of the molded part, the knit line will be short and leave little surface holding the two faces together. Add to this the effect of a screw being driven into the boss and that knit line can turn into a crack.

A knit line might also form between the gates of a part. Gates are the areas where the resin is injected. Knit lines are also sometimes present when utilizing filled resins. As the resin front moves through the mold, the fill material sits behind the front. When the two fronts meet and cool, there might be little or no fill material crossing the meeting line. That may not make the knit line weak, but it could affect reinforcement down the road.

How can you prevent knit lines?

A professional injection molding provider can take several steps to prevent the formation of knit lines. First, utilize resins that are known to be less susceptible to knit line formation. If knit lines from boss or gate locations has been an issue, the process can be re-engineered to relocated boss and gate location. You can also thicken part walls to slow the resin cooling process. This is an exacting process because walls that are too thick are susceptible to sink marks. If knit line causing features are unavoidable, place them farther from the edge of parts when the part design allows for it.

Injection molding knit lines are not desirable, but sometimes they are unavoidable. Any successful injection molding process starts with quality engineering, knowledgeable designers and the best technology and machinery available. The Toolroom provides plastic injection molding that is optimal right for your business to achieve a low cost-per-part for high volumes of production. From the start of your project, you can count on The Toolroom to exceed your expectations for all of your plastic injection molding needs. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. Find out more by calling 573-437-4154 today or email us.

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