Including Crush Ribs in Injection Molded Parts

Draft angles are required to help your part be removed from the mold, but what do you do when you need a tight fit? Crush ribs help create a piece with the tight fit you require without eliminating necessary draft angles.

What Are Crush Ribs and Why Are They Necessary?

To explain crush ribs, it’s first necessary to talk about draft angles.

Plastic shrinks as it melts, which could cause perfectly straight part walls to stick to the mold. The process of removing each part can damage the part or even the mold itself. 

Draft angles are tapers that help reduce the friction between the part and the mold, helping it release in one piece. Commonly, you’ll need 1 degree of draft angle per 1 inch of mold depth.

However, the same draft that makes a part easier to eject from a mold can prevent your part from providing a tight fit with another part. The compromise is crush ribs.

While the main hole of a part is drafted, it will include 3 or more small undrafted ribs. These undrafted ribs provide the tight fit you need for your part without impeding its ability to be ejected from the mold. When you press the two parts together, the small ribs are crushed down creating a tight fit between the two parts.

Since the ribs have only a small surface area in the mold, their lack of draft doesn’t create much resistance to ejection, leading to less risk of damage to the part or mold.

Contact UPM for More Information

For more than 50 years, Universal Plastic Mold (UPM) has been the large part injection mold manufacturer of choice for businesses across the United States. We have all the equipment necessary to handle all of your injection molding needs and reduce overhead, save money, speed up the time to market, and increase your overall product quality.

Contact us today to find out more about how UPM can be your injection molding partner or get a quote by clicking here or calling 1-888-893-1587.