Common Problems in Sheet Metal Deep Drawing + Bending Hybrid Forming Processes
Sheet metal deep drawing combined with bending (often called drawing–bending composite forming) is widely used for complex metal parts such as automotive brackets, electrical enclosures, appliance housings, and structural components. This process integrates stretching, plastic deformation, and angular forming in multiple regions of a single part or in consecutive operations.
Although it improves structural integration and reduces assembly steps, the process also introduces multiple interacting deformation modes, which makes defect control more challenging.
1. Cracking at Bending or Transition Zones
1.1 Main Causes
Cracking often occurs where deep drawing transitions into bending regions.
Key reasons include:
High tensile stress concentration at bend radius
Insufficient material ductility after drawing stage
Small bending radius design
Excessive local thinning during pre-drawing
1.2 Typical Defects
Corner cracking at bend edges
Fracture at draw–bend transition zones
Surface splitting in high-strain areas
2. Wrinkling in Flange or Compression Zones
2.1 Causes
Wrinkling appears when compressive stress is not properly controlled.
Main factors:
Insufficient blank holder force during drawing stage
Excess material accumulation before bending
Uneven material flow distribution
Excessive clearance in forming tools
2.2 Defect Characteristics
Flange wave deformation
Edge buckling after bending
Progressive wrinkling from drawing stage
3. Springback and Angle Deviation
3.1 Causes
Springback is especially significant in bending regions after drawing.
Influencing factors:
High-strength materials
Small bending radius
Uneven stress distribution after drawing
Insufficient calibration force
3.2 Typical Problems
Angle opening or closing deviation
Inconsistent bend angles
Shape distortion after unloading
4. Surface Defects (Scratches, Galling, Impressions)
4.1 Causes
Surface quality is affected by both drawing and bending contact.
Main causes include:
High friction between sheet and tooling
Insufficient lubrication
Tool wear or rough surface finish
Material sticking (especially stainless steel)
4.2 Defect Types
Linear scratches in forming direction
Die marks at bend radius
Surface peeling or galling
5. Thickness Reduction and Local Weakening
5.1 Causes
Deep drawing creates initial thinning, and bending further concentrates stress.
Key reasons:
Excessive draw ratio
Improper material flow control
Small bending radius
Uneven strain distribution
5.2 Consequences
Reduced load-bearing capacity
Early fatigue failure risk
Local structural weakness
6. Dimensional Instability and Cumulative Errors
6.1 Causes
Drawing and bending interactions amplify dimensional variation.
Main factors:
Springback accumulation
Tool misalignment
Inconsistent forming force distribution
Material property variation
6.2 Typical Defects
Hole position deviation
Warping or twisting
Overall shape distortion
7. Interference Between Drawing and Bending
7.1 Process Conflict
The biggest challenge is the interaction between two deformation modes:
Drawing requires material flow
Bending requires localized deformation restraint
Improper balance leads to instability.
7.2 Resulting Problems
Local tearing near bend radius
Wrinkling due to excessive material flow
Uncontrolled deformation transition zones
8. Tooling Design Problems
8.1 Improper Transition Radius Design
Sharp transitions between drawn and bent regions increase stress concentration.
8.2 Poor Clearance Matching
Inconsistent clearance affects both drawing and bending quality.
8.3 Insufficient Tool Rigidity
Elastic deformation of tooling leads to unstable forming behavior.
9. Process Parameter Instability
9.1 Blank Holder Force Mismatch
Too low → wrinkling
Too high → cracking at bending zones
9.2 Improper Forming Sequence
Incorrect sequence between drawing and bending increases defect risk.
9.3 Inconsistent Lubrication
Uneven friction causes unstable material flow and surface defects.
10. Comprehensive Solutions
10.1 Optimize Process Sequence
Separate drawing and bending stages when necessary
Use intermediate calibration steps
Control deformation gradually
10.2 Improve Tooling Design
Increase transition radii
Optimize die clearance
Strengthen tool rigidity
Improve surface finish and coating
10.3 Control Material Flow Precisely
Adjust blank holder force distribution
Use draw beads for flow regulation
Balance tensile and compressive stress
10.4 Optimize Bending Parameters
Increase bend radius appropriately
Use compensation for springback
Apply bottoming or coining when needed
10.5 Improve Lubrication System
Use high-pressure forming lubricants
Ensure uniform application
Reduce friction variation between stages
10.6 Use Simulation and Intelligent Control
Finite element analysis (FEA) helps predict:
Stress concentration zones
Wrinkling and cracking risk
Springback behavior
Material flow interference
Conclusion
Sheet metal drawing–bending composite forming involves complex interactions between tensile and compressive deformation modes. Common problems such as cracking, wrinkling, springback, surface defects, and dimensional instability are mainly caused by process imbalance, tooling limitations, and unstable material flow. Effective solutions require coordinated optimization of tooling design, process sequencing, lubrication control, and forming parameters, supported by advanced simulation and precision manufacturing technologies to ensure stable and high-quality production.
References
Altan, T., & Tekkaya, A. E. Sheet Metal Forming: Fundamentals. ASM International.
Hosford, W. F., & Caddell, R. M. Metal Forming: Mechanics and Metallurgy. Cambridge University Press.
Lange, K. Handbook of Metal Forming. McGraw-Hill.
Kalpakjian, S., & Schmid, S. R. Manufacturing Engineering and Technology. Pearson Education.
Keeler, S., Kimchi, M., & Mooney, P. Advanced Sheet Metal Forming. SAE International.
