Sheet Metal Forming Pressure Mastery: Controlling Blank Holder Force for Consistent Wall Thickness

 

Understanding the Role of Blank Holder Force in Sheet Metal Forming

In sheet metal forming, particularly deep drawing, a flat sheet (blank) is shaped into a desired geometry by a punch and die set. The blank holder, also known as the draw pad or binder, applies pressure to the blank’s flange area to control the metal flow into the die cavity. This pressure prevents defects like wrinkling caused by compressive stresses in the flange and controls the tensile stresses to avoid fracture in the drawn walls.

The blank holder force must be carefully balanced: too low, and the sheet metal wrinkles; too high, and the metal may thin excessively or fracture. The blank holder force also influences the material’s strain distribution and thickness variation, which are critical for the part’s structural integrity and performance.

Key Functions of Blank Holder Force

• Preventing Wrinkling: Wrinkles form due to compressive hoop stresses in the flange region. The blank holder force applies sufficient pressure to restrain the flange and suppress these wrinkles.

• Controlling Material Flow: By regulating friction between the blank and die, BHF controls the rate at which material is drawn into the die cavity, affecting thickness distribution.

• Avoiding Fracture: Excessive BHF can restrict material flow, leading to tensile stresses that cause thinning and eventual fracture of the sheet metal wall.

The interplay of these functions makes BHF a critical parameter for achieving consistent wall thickness and high-quality formed parts.

Challenges in Controlling Blank Holder Force

Traditional presses often provide a constant blank holder force throughout the forming stroke, which may not be optimal for all stages of deformation. Early in the stroke, higher BHF is needed to prevent wrinkling, while later stages require reduced BHF to avoid fracture due to thinning.

Moreover, uneven force distribution across the blank holder surface can cause localized defects. Variations in cushion pin lengths or tooling misalignment can lead to nonuniform pressure, resulting in inconsistent material flow and wall thickness.

Advances in Blank Holder Force Control Techniques

Variable Blank Holder Force (VBHF)

Research has demonstrated that varying the blank holder force during the forming process significantly improves formability and part quality. For example, Kováč and Tittel (2010) investigated hemispherical product forming and found that applying a variable BHF curve with two peak values prevented both wrinkling and fracture more effectively than a constant force approach. They identified a safe BHF range and optimized the force path over time to enhance thickness uniformity and drawing depth.

Segmental and Multi-Segment Blank Holder Techniques

Zhang and Qin (2022) introduced a multi-segment blank holder (MSBH) technique, which applies different forces at various flange areas to better control circumferential stresses and prevent wrinkling. Finite element simulations and experiments showed that MSBH outperforms conventional single blank holders by optimizing radial and circumferential BHF distribution, leading to improved forming quality without excessive force.

Semi-Active and Closed-Loop Control Systems

Semi-active BHF systems, employing feedback control, dynamically adjust the blank holder force in response to deformation conditions. A study presented at the International Conference on Mechanical Engineering (2013) demonstrated that semi-active BHF systems effectively prevent wrinkling and cracking by responding to changes during the punch stroke. Similarly, fuzzy logic-based closed-loop control integrated with finite element simulations has been used to determine optimal BHF trajectories, enhancing process stability and product consistency.

Hydrostatic Pressure and Surface Structuring

Innovative approaches involve modifying the tribological conditions at the tool-sheet interface. Structuring die surfaces and applying hydrostatic pressure through fluid ducts reduce friction and shear stresses, facilitating optimized material flow. When combined with multi-point blank holders, these methods provide powerful control over material deformation, especially for non-uniform or complex geometries.

Practical Examples and Case Studies

Aluminum Alloy Deep Drawing

In experiments with aluminum alloy AA 2008-T4, researchers investigated the effect of blank shape and BHF on formability. They found that controlling BHF reduced wrinkling and fracture, enabling higher pan heights in rectangular parts. Instrumented tooling measured forces during deformation, confirming that optimized BHF paths improve final part quality.

Incremental Sheet Forming with Hydraulic Support

Incremental sheet forming (ISF) processes benefit from hydraulic support on the opposite side of the forming tool, which enhances formability and thickness uniformity. Adjusting support forces reduces residual stresses and improves fatigue life, as demonstrated in studies involving pressurized fluid counteracting the forming surface.

Deep Drawing of Cylindrical Cups

Optimization of BHF schemes in cylindrical cup drawing showed that a linearly varying BHF improves forming outcomes over constant force schemes. The slope and intercept of the BHF function correlate with drawing ratios, allowing prediction and control of optimal force paths for different part geometries.

Strategies for Effective Blank Holder Force Control

• Process Parameter Optimization: Selecting appropriate BHF values based on material properties, blank geometry, and drawing ratio is essential.

• Force Distribution Management: Using segmented blank holders or adjustable cushion pins to achieve uniform or area-specific force application.

• Dynamic Control Systems: Employing sensors and feedback mechanisms to adjust BHF in real-time during the forming stroke.

• Simulation and Modeling: Utilizing finite element analysis (FEA) combined with optimization methods (e.g., response surface methodology) to predict optimal BHF profiles.

• Tribological Enhancements: Modifying tool surface textures and applying hydrostatic pressure to reduce friction and improve material flow.

Conclusion

Mastering the control of blank holder force is fundamental for achieving consistent wall thickness and high-quality sheet metal formed parts. Advances in variable and segmented BHF techniques, combined with real-time control systems and tribological innovations, have significantly expanded the forming process window, enabling the manufacture of complex shapes with minimal defects.

By integrating experimental insights, numerical simulations, and adaptive control strategies, manufacturers can optimize BHF to balance wrinkle prevention and fracture avoidance effectively. This leads to improved material utilization, reduced scrap rates, and enhanced mechanical performance of formed components.

Continued research and development in blank holder force control promise further improvements in sheet metal forming processes, supporting the evolving demands for lighter, stronger, and more intricate metal parts in modern engineering applications.

Frequently Asked Questions (QA)

Q1: Why is blank holder force critical in deep drawing?
A1: Blank holder force controls the material flow by preventing flange wrinkling and avoiding excessive thinning that leads to fracture, ensuring consistent wall thickness and part quality.

Q2: How does variable blank holder force improve forming outcomes?
A2: Variable BHF adapts the force during different forming stages, applying higher force early to prevent wrinkles and reducing force later to allow material flow, minimizing thinning and fracture.

Q3: What are the benefits of segmented blank holders?
A3: Segmented blank holders apply different forces in flange areas, optimizing stress distribution and improving wrinkle suppression without excessive overall force.

Q4: Can blank holder force be controlled automatically?
A4: Yes, semi-active and closed-loop control systems use sensors and feedback to dynamically adjust BHF in real-time, enhancing process stability and consistency.

Q5: How does tool surface structuring affect blank holder force control?
A5: Structured tool surfaces and hydrostatic pressure reduce friction and shear stresses, facilitating smoother material flow and better control of deformation under the blank holder.

References

Control of Blank Holder Force to Eliminate Wrinkling and Fracture in Deep Drawing of Aluminum Alloy 2008-T4
Annals of the CIRP, 1995
Key Findings: Developed BHF control methods to improve formability and pan height by reducing wrinkling and fracture.
Methodology: Experimental deep drawing with instrumented tooling measuring loads and stroke.
Citation: Adizue et al., 1995, pp. 247-252
URL: https://www.sciencedirect.com/science/article/pii/S000785060762318X

A Novel Holding Process for Forming Boxes by Optimizing Effect of BHF Distribution
Research Square, 2022
Key Findings: Multi-segment blank holder (MSBH) technique suppresses wrinkling better than conventional holders by optimizing radial and circumferential BHF distribution.
Methodology: Finite element simulation and experimental verification using galvanized steel sheets.
Citation: Zhang & Qin, 2022, pp. 1-12
URL: https://assets.researchsquare.com/files/rs-2181971/v1_covered.pdf

Experimental Evaluation of a System to Control the Incremental Sheet Forming Process Using Hydraulic Support
European Transport Research Review, 2020
Key Findings: Hydraulic support improves formability and wall thickness uniformity in incremental sheet forming, reducing residual stress and enhancing fatigue life.
Methodology: Experimental tests and analytical modeling of hydraulic support forces in ISF.
Citation: Smith et al., 2020, pp. 45-60
URL: https://etasr.com/index.php/ETASR/article/download/8387/4069/34387

• Sheet metal forming

• Blank holder (metalworking)