The Importance Of Radius Design

Sharp corners have their purpose – but they’re incredibly difficult to make with injection molding. That’s where radius design comes in when creating the mold for your part. So, what is a radius, and why is it important?

What Is A Radius?

A corner radius (corner radii if there is more than one) is a technical term for smoothing out or rounding any sharp corner in a design. Internal radii are used for inside corners and external radii are used on outside corners. The general rule is to use a radius equal to at least 0.5 times the adjacent wall thickness.

Fillet Radii

An internal radius is also called a fillet radius. They are designed into molds to improve the quality of the part, increase load-bearing strength, and improve the injection molding process. Fillet radii are usually found on inside corners or the bottom of a compartment, and they can be used between ribs, bosses, and gussets to connect them to walls for better strength.

Round Radii

An external radius is called a round radius. It’s the rounding of outside corners for the same purposes as fillet radii.

Why Is Radius Design Important?

While rounding out sharp corners makes a part easier to handle, incorporating radii into your design also helps improve the quality and stability of your part. Why?

Due to the way that molten plastic fills a mold, it’s incredibly difficult to fill sharp corners in a mold. External radii help make sure the whole mold is filled, ensuring better part quality.

Additionally, as a part cools, it shrinks. If it has to shrink around a sharp angle, it adds additional stress to the part that can lead to cracking.

For these reasons, it’s also important for sharp corners at the base of ribs, bosses, and gussets to have a radius design, since they help to offset stress points.


With more than 50 years of experience, Universal Plastic Mold (UPM) in California can help you streamline your manufacturing process, save money, and increase part quality. To learn more about the importance of radius design or why UPM is the perfect partner for your injection molding needs, click here or call 1-888-893-1587 today.

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