Sheet Metal Stamping: The “Microscopic Art” of Precision Manufacturing
Sheet Metal Stamping Technology: Technological Evolution and Application Innovation in Bending, Drawing, Embossing, and Relief Forming.
As a core branch of plastic processing, sheet metal stamping uses the coordinated action of dies and presses to give metal sheets, strips, or tubes precise geometric shapes and functional characteristics. Its forming processes—bending, drawing, embossing, and relief forming—hold irreplaceable positions in fields like automotive, electronic, and aerospace manufacturing. This article delves into the technological essence, innovative challenges, and future trends of these four processes, complemented by engineering diagrams and data comparisons, to build a systematic cognitive framework for readers.
Bending in Sheet Metal Stamping
Definitions and Mechanisms: Bending uses dies to impose bending moments on sheets, causing materials to plastically deform around the neutral layer to achieve predetermined angles or curvatures.
The core challenge lies in springback—after stamping, the part may locally or globally deform due to unloading of forming forces.
Fig.1 illustrates how parts undergo elastic unloading and springback during the bending process.
During the elastic stage, the higher the material’s yield strength and elastic modulus, and the more severe the work hardening, the greater the springback in bending deformation. Conversely, materials with lower yield strength and elastic modulus have stronger resistance to elastic deformation and smaller springback. Common solutions to springback include:
- Compensation Method: Pre-setting the die with a slope equal to the springback value to compensate for post-forming springback.
- Stretch Bending Method: Applying tension during bending to improve stress distribution and reduce springback.
- Material Selection: Using materials with better formability.
Bending Process Procedure:
Fig.2 Schematic of Bending Die and Process.
Image source: China International Metal Forming Exhibition “The Secrets of Processes” Metal Stamping.
Fig.3 Bending Process Flow.
Key technological parameters:
Bending radius (R): The ratio of the bending radius (R) to the material thickness (t) determines the stress distribution in the deformation zone. To avoid cracks, it is necessary to ensure R/t is greater than or equal to the material’s minimum bending radius.
Springback compensation: Over-bending die design or hydraulic forming technology can suppress springback.
Industry application:
Automobile door hinges: V – shaped bending dies achieve high-strength corners.
Fig.4Car Door Hinges Under Bending Process
Electronic Product Bracket: Multi-station Progressive Die for Complex Bending Sequence.
The progressive die is a high-efficiency and precision stamping process. It’s designed for mass-producing complex metal parts, especially for components like electronic product brackets requiring multiple bending operations.
Its key feature lies in the multiple stations arranged in order within the die. As the material strip feeds continuously, operations like blanking, forming, and bending are progressively completed, ultimately achieving the one-time formation of complex structures.
Fig.5 Schematic Diagram of Multi-Station Progressive Die
Deep Drawing in Sheet Metal Stamping
Process Essence: Drawing involves pressing a flat blank into a die using a punch to form hollow axially symmetric parts (e.g., cans, housings). Its success hinges on uniform material flow and a balance between anti-wrinkling and anti-fracture.
Specific Process Steps:
- Blank Pre-treatment: Circular or shaped blanks (with diameter D₀) are obtained via blanking or laser cutting, with edges deburred. A lubricant (such as a phosphate coating plus soapy solution) is applied to the surface to reduce the friction coefficient.
- Pre-loading of the Binder Ring: The binder force is set within the material’s yield strength range. A variable binder force is achieved using nitrogen springs or a hydraulic system: high initial pressure to suppress wrinkling and lower pressure later to promote material flow.
- Punch Travel and Forming: Punch speed: variable-speed control is needed for complex parts. Material flow path: the flange area contracts, and the side walls are stretched.
Fig.6 Drawing Die
Image source: 陈 端, 赵长财, 陈晓沂, 等. 强制润滑拉深工艺及润滑效果量化评价 [ J]. 塑性工程学报, 2025, 32 (3): 64-70. CHEN Duan, ZHAO Changcai, CHEN Xiaoyi, et al. Forced lubrication deep drawing process and quantitative evaluation of lubrication effect [J]. Journal of Plasticity Engineering, 2025, 32 (3): 64-70.
- Multi-stage Drawing :
In multi-stage drawing, the total drawing deformation is divided into several steps. Each step causes partial deformation, and the part is finally formed after multiple drawing operations.
In this process, the continuous deformation and hardening of the sheet metal accumulate, enhancing strength. Also, direct forming of sheet metal ensures high material utilization, is suitable for mass production, and keeps costs relatively low.
The process is as follows:
- First drawing: Drawing ratio A.
- Subsequent stages: Reduction ratio B, with intermediate annealing to eliminate work hardening.
Fig.7 Schematic of Multi-stage Deep Drawing Process.
- Trimming and Shaping:
- A progressive die with a trim station removes the flange excess.
- A die for precise finishing corrects the sidewall perpendicularity.
Compared with the traditional drawing process, which struggles with friction during sheet metal drawing, researchers have introduced an externally pressurized forced-lubrication drawing process. The pressurized oil film formed in the contact clearance effectively reduces the real contact area between the sheet and the die, enhancing lubrication. Fig.8 illustrates this.
Fig.8 Schematic of Forced-Lubrication Drawing Process.
Image source: 陈 端, 赵长财, 陈晓沂, 等. 强制润滑拉深工艺及润滑效果量化评价 [ J]. 塑性工程学报, 2025, 32 (3): 64-70. CHEN Duan, ZHAO Changcai, CHEN Xiaoyi, et al. Forced lubrication deep drawing process and quantitative evaluation of lubrication effect [J]. Journal of Plasticity Engineering, 2025, 32 (3): 64-70.
Challenges and Solutions in Drawing Technology:
Wrinkling: Insufficient blank-holder force can destabilize the flange area. This is addressed by variable blank-holder force control or optimizing local lubrication to manage material flow.
Fig.9 Schematic of Wrinkle Defect.
Fig.10 Schematic of Contact Between Workpiece and Die After Local Lubrication Optimization.
Image source:陈端, 赵长财, 陈晓沂, 等. 强制润滑拉深工艺及润滑效果量化评价 [J]. 塑性工程学报, 2025, 32 (3): 64-70. CHEN Duan, ZHAO Changcai, CHEN Xiaoyi, et al. Forced lubrication deep drawing process and quantitative evaluation of lubrication effect [J]. Journal of Plasticity Engineering, 2025, 32 (3): 64-70.
Cracking:
Cracking occurs when the Drawing Ratio (LDR = Blank Diameter / Punch Diameter) exceeds the material’s limit, causing localized strain concentration. LDR indicates the max deformation a material can withstand in a single drawing operation. Using highly ductile materials can increase LDR above 2.2, showing excellent ductility and the ability to endure more plastic deformation without cracking.
Common solutions involve optimizing materials or process parameters like lubrication and blank-holder force. Fig.8 illustrates this.
A high LDR reduces the risk of wrinkling or cracking from localized strain concentration, improving product quality and yield.
Typical Case:
- Fuel Tank Shell: Multiple drawing stages combined with intermediate annealing achieve the complex structure.
Fig.11 Drawing of Fuel Tank Shell
- Battery Enclosure: A progressive die (see Figure 5 for the multi-station progressive die) integrates blanking and drawing, achieving an efficiency of 1,200 parts per hour.
Fig.12 Deep-Drawing of Battery Enclosure.
Coining in Stamping: Micron-Level Precision Forming for Anti-Counterfeit & Gear Correction
Process Definition and Core Mechanism: Coining is a process where high pressure is applied to cause localized micro-plastic flow of metal materials in a closed die. Its essence lies in the complete closure of the upper and lower dies. Using triaxial compressive stresses (radial, tangential, and axial), the material is forced to fill the micro-structures of the die cavity, achieving high-precision surface texture or geometric feature formation.
Specific Process Steps:
- Pre-loading of the Die: The upper and lower dies are made of hard alloy or titanium-plated steel. The cavity is designed with the target pattern (e.g., anti-counterfeit patterns, functional grooves). The metal sheet is fixed on the die table.
Fig.13 Schematic of Coining Die and Pre – loading Process.
- Material Positioning: The metal blank is precisely positioned at the center of the lower die using guide pins or a robotic arm.
- High-Pressure Closing: The upper punch moves vertically downward. The metal sheet is formed under the force within the die. The press applies an instantaneous impact load, causing the metal to complete micro-flow within 0.01 – 0.5 seconds.
Figure 14: High-Pressure Closing in Coining Process.
- Ejection and Inspection: The punch moves upward, and the ejection mechanism separates the die. A white – light interferometer checks the feature depth. Then, the part is removed for the next trimming and grinding operations.
Figure 15: Ejection Process in Coining.
Technical Highlights and Innovations:
- Material Selection: High-ductility materials like pure copper or 5052 aluminum alloy are optimal to prevent micro – cracks.
- Die Life Extension: Nano-crystalline coatings (e.g., CrAlN) can enhance die life to over 500,000 cycles.
- Dynamic Pressure Control: Servo presses modulate pressure waveforms, reducing residual stresses.
Typical Applications:
- Anti-counterfeit Features on Coins: Multi-layer coining creates Moiré fringe effects.
Figure 16: Anti – counterfeiting Patterns on Coins.
- Fine Gear Tooth Correction: Localized coining eliminates distortion caused by heat treatment.
Figure 17: Localized Coining to Eliminate Heat-Treatment Distortion.
Embossing Technology: Surface Texture Design for Electronics & Automotive Interiors
Embossing forms raised or indented surface textures on metal sheets using single or double-sided molds, while keeping the overall material thickness constant. Its deformation mechanism is local tension – compression coupling. It achieves morphological reconstruction through material redistribution, not volume compression.
Specific Process Steps:
- Die Design The upper die features a raised pattern (e.g., a logo or decorative texture), while the lower die is flat or has a complementary recess.
Figure 18: Die for a Mobile Phone Case.
- Progressive Pressurization: Multi – stage pressure control is used to prevent material cracking from sudden force application.
- Elastic Medium Assistance: A polyurethane pad is added between the die and sheet to evenly distribute pressure and enhance texture clarity.
- Springback Compensation: The mold surface is pre-adjusted based on the material’s elastic modulus to account for predicted deformation rebound.
Typical Applications:
- Decorative Surfaces for Electronic Products: Micro-scale radial textures on iPhone stainless steel bezels.
Figure 19: iPhone’s Stainless Steel Frame with Micron – Level Radial Texture.
- 3D Stereoscopic Embossing for Automotive Interiors: Simulates wood or carbon – fiber textures on metal trim panels.
Figure 20: Card Produced by Embossing.
Convergence of Technologies and Future Trends:
- Smart Process Chains: Virtual try-out based on finite element analysis.
- Green Manufacturing: Lubrication – free stamping and closed-loop scrap recycling.
- Hybrid Process Innovation: Laser-assisted local heating for drawing.
From “Forming” to “Advantage”
Metal stamping is not just about physical shape change. It’s a multi dimensional coordination of material properties, process parameters, and design intent. With additive manufacturing and AI driven process optimization, this traditional process is moving into a new era of “precision, intelligence, and sustainability. “
