Curls are features that are produced by rolling the edge of a stamped sheet metal part. These curls should generally have a diameter of about 10 to 20 times the thickness of the material being worked. Curls are produced in order to shape the material into a hollow ring. This is normally done to eliminate sharp edges. In some situations, these curls are used as hinge features.

Curls may also be used to increase the stiffness of the product. This is accomplished through increasing the moment of inertia at the ends. Curls may also function to act as stiffeners at the edges by increasing the effective thickness at the ends.
Curls are similar to hems. Hems are used for the same purpose as curls; however, their formation is slightly different. Hems are curls with a diameter often approaching zero. Hems are produced in a manner similar to curls except they are flattened at the last step.
Key Points:
(a) Curl diameter should be 10 to 20 times the thickness of the material stock.
(b) Prior to starting a curl or hem operation, the sheet metal should be notched first.
(c) Curl or hem operations should generally be preceded by a wiping operation.
(d) Three total operations should be required per curl or hem feature. Hems will be flattened following creation.
By following these key points, you ensure that curls and hems are satisfactorily formed. Otherwise, your sheet metal will not have proper rigidity.