Understanding bending, moving material in roll forming
Directing forces for optimal results
Roll forming is a matter of two processes: shaping material using localized deformation with a large amount of material movement (in other words, bending and moving the material). Localized deformation (bending) is a permanent bend with a slight thickness reduction at the bending line. Material movement is a matter of relocating or rotating a section, either formed or unformed, without changing its shape. Although roll forming engineers often address these processes at the same time, it can be helpful to consider forming and movement separately.
In simple terms, roll forming is a matter of shaping material using localized deformation with a large amount of material movement. Localized deformation is a permanent bend with a slight thickness reduction at the bending line. Material movement is a matter of relocating or rotating a section, either formed or unformed, without changing its shape. Although roll forming engineers often address these processes at the same time, it can be helpful to consider formation and movement separately.
Figure 1illustrates a formed box that was rotated and moved horizontally to its final position. Preventing distortion during material movement requires elastic deformation in any of four directions: horizontal, vertical, longitudinal, and rotational.
A primary consideration is the workpiece's form and how that form will resist force. To put the least stress on the tooling and the material, the process should move the material in a direction that has a low moment of inertia, whether linear or rotational.
Figure 2illustrates one piece of beam in two settings. A force F is directed downward. The moment inertia of the vertical beam is:1
The moment inertia of the horizontal beam is:
Because d is greater than b, the moment inertia of the vertical beam is bigger than the moment inertia of the horizontal beam. This means that the vertical beam is stronger than the horizontal beam is under the vertical force F. In construction applications, a vertical beam is useful because it is lightweight and strong. In roll forming applications, roll design engineers can take advantage of a horizontal beam feature to move a formed or unformed section to a different location easily without changing it or destroying its shape. A rule of thumb: Do not move a horizontal section horizontally, and do not move a vertical section vertically. If you do, compression will likely wrinkle the inner edge of the bend, and tension likely will stretch the outer edge to a thinner gauge. Eventually waves may develop along the stretched edge.
The axis at the beam's center has a smaller polar moment of inertia than the axis at the beam's bottom. This means the axis at the center has less rotational resistance than the axis at the bottom of the beam. Roll form designers can get better forming results by rotating a section about the center instead of the end.
Note: This illustrates one section, or leg, of a profile, not an entire profile.
Figure 3illustrates a section rotated on two axes, center and bottom. The polar moment of inertia with an axis at the center of the beam (Figure 3A) is:2
The polar moment of inertia with an axis at the bottom of the beam (Figure 3B) is:
Preventing Longitudinal Elongation. Elongation in the longitudinal direction is another consideration regarding rotational movement. Longitudinal elongation is not an intentional part of the linear roll forming process, in this case rotational movement. Therefore, it is necessary to minimize the longitudinal elongation.
The length of the section is L. When the section is rotated about the center axis (see Figure 3A), the process stretches the top and bottom edges; the increase in length is ∆D. When it is rotated about the bottom axis, the top edge elongates ∆B. For the same rotational angle,
∆B ≈ 2∆C
Springback Versus Permanent Deformation. For low-carbon steel, the elastic elongation ratio, ∆/L, is less than 0.2 percent.3In other words, any section of low-carbon steel elongated less than 0.2 percent will spring back when it exits the forming rolls; the deformation isn't permanent. This is the goal in longitudinal elongation. If it exceeds the material's elastic limit, it will develop permanent flaws—it will buckle, bow, twist, or wrinkle. Keep the longitudinal elongation ratio less than 0.2 percent to prevent defects.
Keep in mind that rotating a section about the center axis reduces the amount of elongation, helping to prevent forming flaws.
How can these lessons be applied to material movement? Let's look at several strategies for two profiles. The first profile is a common hat; the second is a wide profile that consists of four hats.
Hat Profile. Three flower diagrams show three distinct strategies for producing a hat profile (see Figure 4).
- Figure 4A:The sides rotate around the bottom axis and the horizontal flanges move horizontally. These movements are disadvantages; both violate the guidelines for rotational and lateral movement.
- Figure 4B:The horizontal flanges move horizontally and the sides rotate around an axis near the middle of the side section. This is better than the strategy in 4A, but it is not optimum.
- Figure 4C:The sides rotate around an axis close to the middle of the side sections, and the flanges, which are horizontal, do not move horizontally. Theoretically, 4C has less resistance than the processes illustrated in 4A and 4B.
- Figure 5A:The horizontal sections move horizontally. This section has a high moving resistance, which may cause edge waves. Another problem is that the unformed edges are very wide, and the edges will bounce up and down between forming stations. The bouncing motion may elongate the edges, causing edge waves.
- Figure 5B:The horizontal sections slope downward to reduce moving resistance (in other words, this strategy does not move the horizontal sections horizontally). The bouncing is still a problem in this design.
- Figure 5C:This approach begins to form the outside hats first. It does not have the long, unformed flanges of 5A and 5B; in its intermediate steps, 5C has formed sections (which are stronger than unformed sections) and unformed sections that are narrower than those in 5A and 5B. The unformed areas will move smoothly between forming stations. The formed edges are sloped down to make lateral movement easier.
Wide Profiles. Making a profile with four hats uses the same principle but different forming steps than those used to make a single hat.
In manufacturing a wide product, moving unformed edges laterally is a serious design issue (seeFigure 5).
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