All About Bosses

If your final injection molded part requires screws or other assembly methods, you likely need to incorporate bosses into your mold design. What are bosses, and what do you need to know about them to successfully use them in your mold design without causing issues like sink or knit lines? Read on to find out everything you need to know about bosses.

What Are Bosses?

A boss is a feature raised above the surface of a part to assist in assembly, as the locator for a mating pin on another part, or as a receptacle for a threaded insert or screw. Most bosses are open-topped cylinders – essentially round ribs. You can learn more about ribs here – many of the rules for ribs apply to bosses, as well.

Boss Design Guidelines

To successfully incorporate bosses into your part design, you need to follow most, if not all, of these design guidelines. If your part design won’t accommodate one or more of these guidelines, contact Universal Plastic Mold and talk to our experts, who can help you figure out a way to make everything work.

Minimum Radius at Base of Boss

You should include a fillet with a radius of 0.25 to 0.5 times the nominal wall thickness at the base of a boss to reduce stress. This radius provides strength to the boss as well as plenty of draft to help the part release from the mold.

Radius at Base of Hole in Boss

Avoid sharp corners by using a radius on the core pin at the base of the hole in the boss that is 0.25 to 0.5 times the nominal wall thickness. This helps to reduce stress concentration and release the part from the mold.

Minimum Radius at Tip of Boss

To reduce stress, you should include a fillet of a certain minimum radius value at the tip of a boss.

Chamfer at Top of Boss

Bosses should have a chamfer on the top as a good lead-in for fasteners.

Spacing Between Bosses

You should have spacing between bosses that is at least 2 times the nominal wall thickness. Bosses placed too close to each other result in thin areas that are hard to cool, which affects productivity and part quality. Additionally, thin mold walls are difficult to manufacture and cause a shorter life for the mold.

Minimum Draft for Boss Outer Diameter

A minimum draft angle on the outer surface of the boss that’s at least 0.5 degrees helps make it easier for the part to be ejected from the mold.

Minimum Draft for Boss Inner Diameter

You should include a minimum draft on the hole in the boss of at least 0.25 degrees to help release the part from the mold.

Boss Height to Outer Diameter Ratio

Keep the height of a boss to less than 3 times the outer diameter because tall bosses generate a material mass and thick section at the base. Additionally, the core pin will be difficult to cool, affecting the cored hole dimensionally and extending cycle times.

Wall Thickness of Boss

To minimize voids and sink marks, wall thickness for bosses should be around 60 percent of the nominal wall. Bosses in areas of the part that won’t be visible can have an increased wall thickness to allow for increased stresses imposed by self-tapping screws.

Standalone Boss

Bosses should be cored and connected to the nearest sidewall with ribs or gussets for improved material flow and rigidity.

Counterbore Design

Since screws go into holes without threads, plastic is displaced as you thread the screw into the boss. A counterbore is additional space designed into the part to accommodate that extra material.


For molded parts that must accept screws, softer, less brittle materials work best.

Contact UPM for More Information

Universal Plastic Mold (UPM) has been the large part injection mold manufacturer that businesses across the United States have trusted for over 50 years. We have the tools and techniques to handle all your injection molding needs while saving money, reducing overhead, speeding up the time to market, and increasing product quality.

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

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