Roll forming is the most cost-effective way to achieve continuous production of complex shapes with multiple bends. But even we admit it’s not a perfect system. There are two particular quirks of roll forming that engineers must understand and accept before committing to the process: end flare and springback.
End flare, or end distortion, is an issue unique to roll forming manufacturing, while springback is a more universal sheet metal issue. So, what are these two roll forming anomalies? How will they affect your components? And how do we minimize them?
The following roll forming design guide will address each question and describe how these quirks can be prevented or corrected during fabrication:
Roll Forming Design Guide for End Flare
This is a distortion that occurs at the ends of a roll formed section, or at any point where the section is cut. This includes large knockouts or notches in the middle of the part.
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 for Springback Compensation
(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? Just look at the roll form design process -- when 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 coiled metal you use.
Springback Compensation Methods
Springback in metal forming is something you can predict ... to an extent. Knowing how to make sound springback predictions will allow you to make better roll form tooling selections, especially for bends with intense radii.
Knowing how to overcome the springback effect is less about prevention and more about preparation. There's no true solution to the quandary of how to reduce springback in bending. Instead, by overforming, a designer can usually compensate for the inevitable.
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).
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 Roll Forming Defects Before They Start
End flare and springback 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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