End distortion is a birthmark caused by the roll forming process. Roll forming-a process in which a flat strip is bent continuously and progressively in the transverse direction by tandem sets of roll tooling-induces residual stresses in compression on the inner surface, residual stresses in tension on the outer surface, and residual moments in the cross section. Cutting the formed part releases the residual stresses and moments, which can result in distortion at both ends of the part. Understanding the causes of end distortion can help fabricators prevent and correct these types of defects in roll formed parts. Keep in mind that roll forming has three forming regions. The first region is the noncontact forming (air bending) region. The second region is the contact forming (tool bending) region that ends at the central line of the rolls. The third region is the spring-back region that begins as the section exits the central line of the rolls. The profile transition region comprises both the noncontact forming region and the contact forming region.
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