Detailed Explanation of Sheet Metal Bending Process
Bending is a core sheet metal process that uses pressure to induce plastic deformation in metal sheets to obtain the desired angles and shapes. The core is controlling springback and unfolding dimensions to ensure the accuracy of angles, radii, and positions.
Sheet Metal Bending Process
I. Core Principles
Plastic Deformation: When external force exceeds the yield strength, the metal undergoes permanent deformation.
Neutral Layer: The inner layer of metal whose length remains unchanged during bending, located approximately 40%–50% of the material thickness, used to calculate the unfolded length.
Springback: The material elastically recovers after unloading, with a slight opening of the angle (more pronounced in stainless steel/high-strength steel), requiring over-bending compensation.
Bending Radius R: The inner corner radius; too small a radius easily leads to cracking. Typically, R ≥ 0.5t (t is the material thickness). II. Mainstream Process Classification
1. V-Bending (Most Commonly Used)
Air Bending: The upper die does not press all the way down, only partial contact; the angle is controlled by the stroke, one die can make multiple angles; relatively large springback, suitable for 30°–150°, medium and thin plates.
Bottoming: The upper die presses the sheet completely into the V-groove; precise angle, small springback, requires matching die angle; suitable for 90° high-volume, high-precision parts.
V-shaped Bottom-Pressure Bending Process
Coining: High pressure presses the sheet against the die, eliminating springback; highest precision, high pressure, suitable for thin parts, precision small radius.
2. U-Bending
Using U-shaped upper and lower dies, pressing out a U-shape in one or more passes; suitable for channel-shaped, edge-sealed parts, pay attention to side wall springback and bottom arching.
U-shaped bending process
3. Roll bending: 3-4 rollers continuously feed material, progressively bending it into an arc/cylinder; suitable for large radii, long parts, and materials without sharp edges (such as ducts, cylinders), with good surface quality.
Roll bending process
4. Stretch bending: Applying axial tension while bending reduces springback and cross-sectional distortion; suitable for large cross-sections, high precision, and difficult-to-deform materials (such as aluminum profiles, high-strength steel).
Stretch bending process
III. Standard process flow
Drawing analysis: Clearly define material, thickness, angle, inner radius, and dimensional tolerances; indicate bending direction and sequence.
Material cutting and pretreatment: Laser/shear cutting; deburring, cleaning oil stains, and leveling.
Die selection:
V-die width: Low carbon steel V=6T–8T, stainless steel V=8T–12T (T = material thickness).
Upper die: Straight edge/gooseneck/pointed die, matching angle and radius.
Parameter Calculation:
Unfolded Length: L = A + B – BD (A/B are the straight edge lengths, BD is the bending deduction, related to T/R/angle).
Bending Force: F = (650 × T² × L) / V (kN, approximate formula for low carbon steel).
Trial Bending: Use scrap material for trial bending, measure angle, dimensions, and springback; adjust stroke, pressure, and back gauge; determine over-bending compensation (e.g., 90° target bend to 88°–89°).
Batch Bending: Follow the order of “inner to outer, smaller to larger, shorter to longer”; use positioning blocks/tooling to avoid offset; perform full inspection on the first piece, and random inspection during the process.
Post-Processing: Deburring, grinding, and shaping; surface treatment (powder coating/electroplating/brushing).
Inspection: Angle (angle ruler/2D measuring tool), dimensions (calipers/measuring tape), R (R gauge), appearance (no cracks/indentations). IV. Key Parameters and Common Problems
1. Material Properties (Affecting Springback and Pressure)
Low Carbon Steel (SPCC): Easy to bend, low springback, commonly used V=6T–8T.
Stainless Steel (SUS304): High strength, high springback, requires higher pressure + compensation angle, recommended V=8T–12T.
Aluminum/Aluminum Alloy: Soft, easily scratched, use polishing molds/nylon molds; pure aluminum has low springback, hard aluminum has high springback.
Copper/Brass: Good ductility, prone to stress cracking, increase R angle, annealing if necessary.
2. Common Defects and Countermeasures
Excessive Springback: Increase pressure, extend holding pressure; over-bending compensation; add ribs; select low springback materials.
Bending Cracking: Increase inner R (R≥0.5T); bend direction perpendicular to rolling texture; deburring; annealing.
Dimensional Deviation: Calibrate back gauge; correct unfolding calculations; control die wear; optimize bending sequence. Surface Indentation: Use wear-resistant/polished molds; apply protective film; adjust gaps; reduce pressure.
V. Equipment and Molds
Equipment: CNC bending machine (mainstream, accuracy ±0.1mm, automatic compensation), hydraulic bending machine, manual bending machine (small batch).
CNC Bending Machine
Molds: Upper die (straight edge die, gooseneck die, pointed die), lower die (V die, U die, R die); material Cr12MoV, quenched HRC58–62.
VI. Design and Production Considerations
Minimum Inner R: R≥0.5T (mild steel), R≥T (stainless steel/hard aluminum).
Minimum Side Height: H≥2T+R (avoid short sides that cannot be bent).
Hole Avoidance: Hole distance from bending edge ≥3T to prevent deformation; bend before drilling/lasering near holes.
Bending Sequence: Inner to outer, smaller to larger, shorter to longer to avoid interference.
