Profile bending also called section bending is the curving of various lengths metal profiles (steel, aluminum, brass and various other metals) into specific profile shapes. The metal profile bending process is characterized by stretching and compression, which may cause the complex cross-section to deform and important functions to be lost. The profile bending process must ensure that this deformation is kept to a minimum and that the functionalities are retained, even in the bend. Engineers use bent and formed metal profiles for a variety of applications, from electric vehicles up to demanding design spacecraft and buildings. Profile bending aims to bend the workpiece in as few passes as possible, it has two approaches asymmetrical or symmetrical bending: Fully plastic bending must be considered as asymmetrical whenever the axis of an applied bending moment is not parallel or perpendicular to an axis of symmetry of the cross-section. Read More: Best Beginner’s Guide to Metal Profile Bending 2023 Edition
Works principle of 3 roll profile banding
The most common profile roll bending process is the 3-roll roll bending process:
The bending takes place between three points. As the diagram shows, no bending takes place until the section touches all three points or rollers. Cold bending takes the material past its yield point which strain hardens the steel to some degree. Some of the yield plateaux have to be used, so in general plastic, design is not recommended. The toughness of the steel can also change, particularly at small radii. It is worth emphasizing that at most radii found in structural applications, the changes are modest. For normal low-carbon steel and including structural steel, the strain induced during the bending process produces no real problems, as the material exerts the same elastic characteristics in the elastic range.
Affect the bending effect of 4 parameters of metal profiles
The following four basic parameters of metal profiles affect the effect of profile bending
Parameters 1#: Strain curve
As mentioned, during the profile roll bending process, the material must have exerted on it stress greater than its yield strength or elastic limit. This is the maximum stress that the material can be subjected to and still spring back, or return to its original length. The yield point or elastic limits is shown as point ‘A’ on the figure below, a typical stress-strain curve. Stress less than the yield strength will not permanently bend the material. The amount of stress to apply to the material being bent is in area ‘C’, which is the plastic region. These lines show how, when the stress is removed, the material will spring back to a length somewhat smaller than when the stress was being applied.
Material suppliers often provide the yield stress and ultimate tensile strength for sheet materials. They don't, however, always provide the true stress-true strain (flow stress) curve. This curve is one of the most important variables for calculating input data for the finite element (FE) and analytical methods used to predict metal flow and defects.
Parameters 2#: Ductility
The steel sections become work-hardened when using the cold bending process. The amount of work hardening is dependent on the radius required and the geometry of the section. The results in a ‘flattened-out’ stress-strain curve as shown in the diagram above. A tensile test on a sample of steel that has been cold-roller bent will show a small loss in ductility, but a higher Ultimate Tensile Strength, which results in a loss of some ductility. Even though there is a loss of some ductility, for normal structural applications, the effect is minimal and can be ignored.
Ductility is defined as a measure of how much a material can be deformed before it reaches a breaking point. In mechanical engineering, ductility can be represented as a percent elongation or percent area reduction based on a tensile test. Another way of expressing ductility is by the material’s ability to be drawn into a wire.
In metalworking, ductility is frequently an important consideration. Because metals must often be shaped through various means such as forging, rolling, extruding or drawing, it is necessary to know whether the metal will break or otherwise become deformed under stress.
Parameters 3#: Visible distortion
Usually, the most important effect of the bending process is aesthetics, not structure. The steel on the outside of the curve tends to stretch (and therefore thinner), while the steel on the inside of the curve tends to become thicker.
The bending process may produce some visible deformation in the section.
Springback is the general distortion of a part after its removal from the forming pressure, 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 the metal want to return to its old self. The pringback is pretty much always present in roll bending, knowing how to make sound Springback predictions will allow you to make better roll form tooling selections, especially for bends with intense radii.
When a tube, beam, or open section bends, compression builds on the inside radius and tension builds on the outside. Left uncontrolled, especially on thin-walled workpieces, these forces create localized distortion like wrinkling or buckling on the inside radius, wall thinning and shrinkage on the outside radius, and distortion and ovality of the overall profile shape. Read More: Sheet Metal Bending Process: Basic Knowledge In Cold Bending & Forming
Parameters 4#: Minimum Radius And Tolerance
The minimum radius to which a section can be bent without any meaningful distortion depends on the section properties and bending methods being used. As the years have gone by these minimum radii have been reduced as new techniques have been developed, so the minimum has continued to get smaller Normal bending tolerances for single radius bends are in line with those specified in the National Structural Steelwork Specification.
List of the BIT profile bending machine bend radii: It is not easy to provide a definitive and comprehensive list of the radii to which every section can be curved. There are large numbers or standard sections (each with different bending characteristics), there are different methods of bending (hot and cold), and the end-uses vary widely. Also, with continuing technical developments, ‘minimum radii’ also change. The minimum radius you need to bend is best to view our bending machine specifications before purchase. Read More: View our Specification page
That the harder and thicker the plate is, the greater the minimum bend radius. The minimum inside bend radius is even larger when bending with the grain. In steel between 0.5 and 0.8 in. thick, grade 350 and 400 may have a minimum bend radius of 2.5 times the material thickness when transverse bending, while longitudinal bending may require a minimum bend radius that’s 3.75 times the material thickness.
But actual minimum radius values vary by grade. For steel, aluminum, and stainless, you will find a variety of minimum bend radii-to-thickness ratios, and you will need to research these values in data provided by your material supplier. Read More: Minimum Recommended Bend Radius Chart
Sections radius curves
Parabolic, elliptical, and other non-circular bends are variable-radius bends, also known as multi-radius bends. The funicular form for a member resisting a uniform gravity load is defined by a parabola; therefore, many arches are shaped to a parabolic curve. Elliptical bends can be required where a curved plane interfaces with a skewed plane.
When bending materials to an elliptical or parabolic configuration the radius of the bent section will be changing throughout the arc (or bent section) and stay in one plane.
Complex geometries such as spirals, s-bends, ellipses, napoleon bends, and custom curve and tangent combinations can be created with partial or fully automatic operation with CNC profile bending machines. They can also, within limits, be curved in two places or form spirals, they are especially useful for manufacturing OEM Parts. Read More: View Tolerance Information page