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Injection Molding: Crush Ribs

This post is part of our Injection Molding Design Guide series.

Press-fit connections can be tricky to manufacture via injection molding if you’ve never heard of crush ribs. These structures can help your production process run smoothly while ensuring your parts fit and hold together correctly after the process is done. In this article, we’ll discuss what crush ribs are, why they’re necessary, and the difference between pointed and rounded crush ribs.

What are Crush Ribs?

Crush ribs are protruding features that are added to an injection molding design to aid in the stability of a press-fit connection. They are small, usually around a tenth of an inch in diameter, and need an interference of 0.01 inches between the two parts being fitted together to work best. These structures are used in holes or other components into which another part must be press-fit.

For plastic crush rib design, you can make them either pointed or rounded. Their tips will deform during assembly to create a tight fit where they come into contact with another part. To produce a firm hold and correct alignment, a minimum of three crush ribs should be used on a single fitting, but more can be added for extra support.

Why Crush Ribs are Necessary

Crush ribs may seem unimportant because with other manufacturing methods, such as 3D printing, can produce press-fit components with few complications. However, this is not true of injection molding. With injection molding, the majority of a part’s components must be drafted for easy ejection from the mold. This includes holes, and a drafted hole will make for a loose and weak press-fit hold. Instead, crush ribs can be added to the inside of the hole to avoid either removing the hole’s draft or settling for a flimsy hold. Since crush ribs have a small surface area, they do not pose a large risk of resistance during ejection and do not need to be drafted. As such, you can retain the draft in the hole while successfully creating a secure hold for your press-fit parts.

Pointed vs. Round Crush Ribs

As stated above, crush ribs may be designed as either pointed or rounded. Both designs will produce a secure connection because the radius tip of the rounded ribs can deform just as the pointed tips can. The main difference between the two is cost. Pointed crush ribs have to be cut into the mold with Electrical Discharge Machining (EDM), which adds an extra step into the mold-making process and increases your cost. Rounded crush ribs, however, can be milled directly into the mold as all other components.

Design at 3 Space

Here at 3 Space, we offer multiple services, including manufacturing via 3D printing and injection molding and designing via 3D scanning and part inspection. Our engineers have years of experience in designing parts for a variety of manufacturing methods and are happy to assess your design and make suggestions. For more information, contact us here.