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The Difference Between Stretch Forming and Roll Bending Machine

Stretch bending process
Many companies do not know how to distinguish between a stretch forming machine and a 3-roll bending machine(such as angle roll), and how to choose. In fact, the difference between the two is very large and the working principles are different.

The Working Principle of Stretch Forming and Bending

Stretch Forming Machine

Stretch forming is commonly used for parts with a larger bend radius, as the minimum bend radius is generally two to three times greater than other forming/bending methods.

Stretch Forming involves placing an extrusion along with a rounded, fixed bending die which is secured with clamps on each end. The machine will then rotate the clamped ends together, bending the extrusion to angles up to 180°. The extrusion takes its shape when it is bent around the bending die to reach its desired angle. Stretch forming is commonly used with components that require a larger bend radius.

profile stretch press

For bending parts that require high precision, large length and radius of curvature, and small transverse dimensions, they can be stretched and bent on a special stretch-bending machine. During bending, the entire thickness of the plate is subjected to tensile stress, so only elongation deformation occurs, and the deformation caused by a rebound after unloading is small, which is easy to ensure accuracy.
Stretch forming machine(aka, Stretch Bending Machine, Profile Stretch Press) forming is widely used in the manufacture of high-pressure vessels, boiler drums, boiler tubes, hull steel plates and ribs, various utensils, instrumentation components, and cabinet inserts.) forming is widely used in the manufacture of high-pressure vessels, boiler drums, boiler tubes, hull steel plates and ribs, various utensils, instrumentation components, and cabinet inserts.

3-roll bending machine

The three-axis rolling arc is used, and the principle of cold bending is used.

The relative position change of the workpiece (through the up and down movement of the upper roller shaft or the lower and upper roller shaft of the profile bending machine) and the pressure generated during the rotation movement in the direction perpendicular to the deformation of the profile itself cause the profile to produce continuous plastic deformation (make the outer layer of the profile Deformation) The fiber stretches, the inner fiber shrinks to produce bending deformation, the middle fiber does not change), and the workpiece with a predetermined shape is obtained at room temperature (we call it cold bending).

Three-Roll Bending directs an alloy extrusion along with three different metal rolls or points that are typically triangular in shape. The rolls are adjustable to form accurate angles that can roll either vertically or horizontally. The extrusion is usually fed into the motorized rollers, which then bend and curve the extrusion accordingly. Extrusions are limited to a single bend per cycle. A higher angle of the bend would require more time to accomplish the desired angle.

Symmetrical bending

Refers to the three rollers in a symmetrical shape (pyramid shape) during bending. The upper roller or the two lower rollers rotate in a symmetrical relative position at the center to apply pressure to the profile to produce plastic deformation
Disadvantages: It is not possible to pre-bend the profile. There are larger straight edges at the end of the workpiece after bending, which causes waste.

Asymmetrical bending

Refers to the asymmetrical shape of the three rollers of the section bending machine during bending. The upper roller and the two lower rollers move vertically at asymmetrical central positions, and the upper roller or the two lower rollers rotate to exert pressure on the profile. Produce plastic deformation. The end of the profile can be pre-bent.

Stretch Forming vs Roll Bending

  • Metal surface: After spraying, electrophoresis, electroplating, polishing, wire drawing, etc., the material can be directly bent by the stretch forming machine without damaging the surface of the material, and the roller of the roll bending machine will cause damage to the metal surface.
  • Profiles with cavities and complex cross-sections: can be formed in one pass. When the relative bending radius allows, the inner wall will not be wrinkled and the section will not be distorted. It is well known that roll bending machines and pipe bending machines need to insert a mandrel into the profile to ensure no deformation when bending this type of material.
  • Stretch forming can effectively eliminate the residual stress inside the material, and the product has good dimensional stability. Due to the cold work hardening of the metal material, the comprehensive mechanical properties of the material can be improved after the material is stretched and bent.
  • The bending ability is stronger than the roll bending: the length of one forming can reach more than ten meters, which can improve the installation efficiency, reduce the installation cost, save materials and improve the quality of the project for projects that require a long arc length.
  • Bending radius: The stretch bender is not limited when the mechanical properties (elongation) of the material allow, but it cannot be greater than 180 degrees, and the roll bender can bend 360 degrees.
  • 3D bending, the bending of the stretch bender can only be carried out in one plane, through the Z-axis and CNC control, the roll bending machine can perform 3D bending.

Application Of Stretch Forming

Stretch forming provide better shape control and surface quality than roll bending parts.
  • Almost any shape that can be produced by other sheet-forming methods can be produced by stretch forming. Drawn shapes that involve metal flow, particularly straight cylindrical shells, and details that result from such compression operations as coining and embossing cannot be made. However, some embossing is done by the mating-die method of stretch draw forming
  • Stretch forming is used to form aerospace parts from steel, nickel, aluminum, and titanium alloys and other heat-resistant and refractory metals. Some of these parts are difficult or impossible to form by other methods, for example, the titanium alloy gas-turbine ring
  • Stretch forming is also used to shape automotive body panels, both inner and outer, and frame members that could be formed by other processes but at a higher cost.

Advantages Of Stretch Forming

Stretch forming has the following advantages over conventional press forming methods:

  • Approximately 70% less force is needed than that required for conventional press forming.
  • Stretch forming can reduce material costs by as much as 15%. Although allowance must be made on the stock for gripping, it is gripped on two ends only. The allowance for trimming is usually less than that in conventional press forming.
  • Because stretch forming is done on the entire area of the workpiece, there is little likelihood of buckles and wrinkles. Tensile strength is increased uniformly by approximately 10%.
  • Hardness is increased by approximately 2%.
  • Springback is greatly reduced. There is some springback, but it is easily controlled by over forming.
  • Residual stresses are low in stretch-formed parts.
  • Form blocks are made of inexpensive materials, such as wood, plastic, cast iron, or low carbon steel, and are approximately one-third the cost of conventional forming dies. If the workpiece is formed hot, the dies must be able to withstand the forming temperature. However, most stretch forming is done at room temperature.

Limitations Of Stretch Forming

Stretch forming is subject to the following limitations:

  • It is seldom suited to progressive or transfer operations.
  • It is limited in its ability to form sharp contours and reentrant angles. It is at its best in forming shallow or nearly flat contours.
  • If the piece is not pinched between mating dies, there is no opportunity to coin out or iron out slight irregularities in the surface of the metal.
  • In some applications, especially in stretch wrapping, the process is slower than competitive processes, and it is not suited to high-volume production. However, stretch draw forming with mating dies can be done as rapidly and automatically as conventional press operations. In fact, punch presses are used with dies incorporating draw beads or other means of gripping the blank in order to perform some stretch-forming operations.
  • Metals with yield strength and tensile strength very nearly the same, such as titanium, necessitate the use of automatic equipment for determining the amount of strain for uniform results

Top 9 Limitations Of Stretch Forming

  1. Material Properties: Stretch forming is most effective with ductile materials, such as aluminum and some other non-ferrous alloys. Brittle materials or those with limited ductility may not be suitable for stretch forming as they can crack or fracture during the process.
  2. Thickness Variation: Stretch forming may result in non-uniform thickness distribution across the formed part, especially in regions with tight radii or complex shapes. Thin sections may experience excessive thinning, potentially affecting structural integrity.
  3. Maximum Curvature: There’s a limit to how tight the radii or curvature can be achieved with stretch forming. Extremely tight radii can cause excessive strain on the material, leading to defects or even material failure.
  4. Springback: Like many metal forming processes, stretch forming can cause springback, where the formed part partially returns to its original shape after releasing the forming forces. This effect must be considered and compensated for in the design.
  5. Tooling Costs: Developing precise and complex tooling for stretch forming can be expensive, especially for one-off or low-volume production runs. This makes stretch forming more economical for larger production quantities.
  6. Surface Finish: Stretch forming can leave marks or imperfections on the material’s surface due to contact with the forming die. Careful tooling design and lubrication can mitigate this, but it’s important to consider the required surface finish.
  7. Limited to Sheet Metal: Stretch forming is primarily used for sheet metal or extruded profiles. It may not be suitable for forming solid or thicker components, which could require other forming methods like forging or machining.
  8. Complexity: While stretch forming is excellent for producing simple and moderately complex shapes, extremely intricate or highly contoured forms may be challenging to achieve with this process. Deep draws and tight radii can be problematic.
  9. Tolerances: Achieving tight tolerances across the entire formed part can be challenging, particularly in large or complex components. Dimensional variations should be considered in the design and quality control process.

Resource: Learn Mechanical Engineering