TIPS WITH TONY: Ribs and Radii

Designing ribs and radii into your injection-molded part is not only important to increase strength and stability — it improves material flow, eliminates thick areas that create issues like sink, and ultimately, enhances the cosmetic appearance of your part.

A thick part will have several issues with sink or voids creating cosmetic, functional or molding concerns. The addition of ribs reduces the amount of volume in part thickness while still providing the part with the same overall height. Some believe that eliminating the volume/part thickness can decrease the strength of a part. This is not true — adding ribs can actually improve stability, strength and cosmetics depending on the material selection and part geometry.

Watch rib-to-wall thickness ratios. To prevent sink, the thickness of the rib should be about half of the thickness of the wall.

A major design consideration that is often overlooked is rib-to-wall thickness ratios. If you have a rib feature that is too thick on a wall, you’ll create thick areas that can result in unsightly sink or shadowing on the opposing surface. Avoid this by following a guide of 40 to 60 percent wall thickness for any rib.



Coring Out
Coring out is a technique where you remove material from a plastic part, leaving distinct walls and ribs that provide enough strength and mating surfaces for other parts in the assembly. It is necessary to make the part moldable and also saves cost and weight. Leave ribs in the right location and size to maintain strength, particularly in bending, and retain surfaces and features that interface with other parts in the assembly.

Ramps and Gussets
Continuing the discussion on strength and resin flow improvement, ramps and gussets are important features that you can build into your design. Sharp corners create high stress points whereas gussets and ramps are stress relievers, working to improve overall part quality.

Adding radii — edges or vertexes that have been rounded — to your part will improve how resin fills the mold as plastic can flow poorly around sharp corners. When you have sharp corners, resin can create stresses that cause the part to warp or bend, and may provide a location that can break since this is a weaker transition point. Resolve this by adding a generous angle to both the outside and inside corners.

Ideally, you should model radii so the inside and outside radii use the same center resulting in larger radii on the outside curves. This will help you retain a consistent wall thickness throughout your part.

Proto Labs employs an automated CNC machining process to mill the mold for you parts so some part corners will have a radius rather than a sharp edge. Part model (A), actual corner radius (B), desired corner geometry (C) and tool (D).


Resulting Radii
Since aluminum molds are machined using end mills, it is difficult for an outside corner that is designed sharp to actually be sharp. You won’t need to model in this radius as it is a result of the Proto Labs process, but feel free to model a large enough radius so that a consistent radius can be produced with our smallest tooling diameter for the depth of feature you have modeled. You can see this in our design for manufacturing (DFM) feedback found in our interactive quotes.

Crush Ribs
Crush ribs provide the ability to accept an interference fit based on size with less interference over the entire surface. When two parts are forced together using a crush rib, they are mating the two parts together that form a tight bond. These are a very important features in part design and can be accomplished using Proto Labs’ CNC milling capabilities as long as you take the sharp point out of the equation. Ideally, we would prefer you to model the part using full rounds slightly larger than our minimum tooling requirements so they are fully formed.

To learn more about ribs, radii and other design considerations that improve strength, stability and cosmetics, download our free Designing for Moldability white paper.

Tips with Tony is a weekly feature focused on improving the manufacturability of your parts. 

This entry was posted in Tips with Tony and tagged , , , by Tony Holtz. Bookmark the permalink.

About Tony Holtz

Tony is a technical specialist at Proto Labs with more than 10 years of experience ranging from CNC mill operator to mold designer to customer service engineer. While his formal education is in industrial machinery operations, he has extensive knowledge and experience in both traditional and advanced manufacturing processes and materials. Throughout his tenure at Proto Labs, Tony has worked with countless designers, engineers and product developers to improve the manufacturability of their parts.

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