Roll forming is the most cost-effective way to achieve continuous bending of complex shapes with multiple bends. But even we admit it’s not perfect. There are two particular quirks in the roll forming process that engineers want answers to before moving forward: end flare and springback.
End flare is an issue unique to the roll forming manufacturing process. Springback is a more universal sheet metal forming issue; we field a lot of questions about potential springback compensation methods. So, what are these two issues, and how will they affect your components?
These are the two main elastic distortion phenomena that both the shape and tool designer must be aware of in the roll forming process. This roll forming design guide will explain each and how they can be prevented or corrected during metal fabrication:
Roll Forming Design Guide: End Flare
This is a distortion that occurs at the ends of a roll formed section or at any point where the section would be cut. This includes large cutouts or notches in the body of the section.
The strain roll forming puts on a metal workpiece is much more complex than with other bending methods. Residual stresses tend to be particularly apparent -- they cause greater distortion at the ends of your components than at any other point.
How to Smooth Out Your End Flare Issues
Flare can be minimized by roll design procedures. However, it can’t be completely eliminated except by subjecting the metal to stretch forming or to a stress-relieving anneal.
In most cases this is done by stretch forming, which involves laterally moving the material to stretch and compress the metal. First your roll former overforms the section, then underforms it, and finally finishes it.
Meanwhile, annealing is the treatment of a metal by heating it, then cooling it to room temperature. This process improves ductility and reduces brittleness. Most importantly, it relieves stresses absorbed by the metal during roll forming.
Your designer and manufacturing partner should always take end flare into consideration for roll formed product and tooling design!
Roll Forming Design Guide: Springback Compensation Methods
(Graphic courtesy The Fabricator)
Springback is the general distortion of a part after its removal from the forming pressure. Literally your component springs back in its original direction.
Why is springback in roll forming a thing? As your metal is bent, the inner region of the bend is compressed while the outer region is stretched. That makes the density greater on the inside of the bend than on the outer surface. The compressive forces are less than the tensile forces on the outside of the bend, making your metal want to return to its old self.
The amount of springback varies based on the piece of metal you use.
How to Fight Back Against Springback
Springback in metal forming is something you can predict ... to an extent. Knowing how to forecast it will allow you to make better roll form tooling selections, especially for bends with intense radii.
The main predictors of springback in your metal are yield point (the point at which a metal will stop reverting to its original shape) and elastic modulus (the change in stress with an applied strain).
By overforming, a designer can usually compensate for springback.
Use the following as a bending springback calculator of sorts. They assume there is a 1:1 relationship between the metal’s thickness and the inside radius:
- Cold-rolled steel: 0.75 to 1.0 degree
- Hot-rolled steel: 0.5 to 1.0 degree
- Mild aluminum: 1.5 to 2 degrees
- 304 stainless steel: 2 to 3 degrees
- Copper: 0.00 to 0.5 degrees
- Brass: 0.00 to 0.5 degrees
Stop Design Flaws Before They Start
Springback and end flare are pretty much always present in roll forming. It’s all about how your roll forming partner deals with them.
Once you know the rules of both issues, you can have your design 100% correct before your roll former even sees it!
(Editor's note: This article was originally published in September 2018 and was recently updated.)
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