PBF CNC Stretch Forming Machine

Cause: Contour deviation refers to the mismatch between the stretch-formed extrusion and the inspection template or forming die. The main causes include unstable springback behavior, variations in material properties and hardness between extrusion batches, large changes in bending radius, and inconsistent section dimensions of the extrusion. These factors result in unpredictable springback, causing the final profile to deviate from the intended curvature.
Solution: The die curvature should be compensated based on theoretical springback combined with correction values from stretching tests. When material inconsistency is expected, “over-pulling” with proper pre-compensation can help counteract springback. For cases with large radius transitions, a vertical hydraulic pusher can be added at the front of the forming die to keep the extrusion fully in contact with the die surface. During design, proper section dimensions and thickness must be ensured. For deviations caused by asymmetric cross-sections, pre-stretching force can be appropriately increased to maintain balance.

Cause: During stretch forming, the area below the neutral axis is subjected to longitudinal, radial, and width-direction compressive stresses. These stresses cause material contraction and radial thickening, and once they exceed the material’s stability limit, internal wrinkles appear. Extrusions with a low thickness-to-width ratio are more prone to instability. Asymmetric cross-sections, thin webs, or insufficient stretching force also increase the likelihood of wrinkling. Cracks are mainly caused by excessive tensile force combined with insufficient flange-release stress. Edge wrinkling (similar to “ear-like” deformation) results from inadequate edge control. Hollow extrusions are more likely to dent due to the lack of internal support, while side scratches typically originate from improper die clearance or unsuitable die materials.
Solution: Wrinkling resistance can be improved by increasing web thickness, optimizing the cross-section design, and adjusting stretching force. Hollow extrusions should be internally supported before bending with materials such as spring steel strips, PTFE inserts, or sand filling to prevent dents. Side scratches can be eliminated by improving die material and die clearance. Cracks and edge wrinkling can be resolved by optimizing the extrusion cross-section or adjusting the tensile and flange-release forces to achieve balance.

Cause: Twisting after stretch forming is primarily caused by asymmetric cross-sections. Uneven release and contraction of flange stresses lead to nonlinear torsional deformation after bending. Poor extrusion stability and local hardness variations also aggravate twisting.
Solution: Twisting can be reduced by increasing the stretching force or adding a vertical pusher at the die entrance to balance left-right stresses. Holding the stretch force for a short period after forming also helps stabilize the shape. Minor extrusion-related deformation can be corrected during the post-forming straightening process.

Cause: Perpendicularity deviation also results from asymmetric sections, which create uneven flange stresses that shift the angle between the extrusion cross-section and the reference plane after bending. Small distortions caused by inconsistent extrusion quality also contribute to perpendicularity errors.
Solution: Die curvature compensation should be adjusted to better match the springback characteristics, and “over-pulling” can be used when necessary for pre-compensation. Any instability or deformation originating from extrusion can be corrected during the post-forming calibration process to ensure the final perpendicularity meets specifications.