Blank Holder Detachment for Crack-Free Press Forming
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Solution Overview
Problem
Press forming methods for metal sheets often result in cracks, especially when forming parts with complex shapes or high-strength materials, and existing solutions require costly and time-consuming modifications to dies or materials.
Innovation Solution
A press forming method that detaches the blank holder or punch from the metal sheet before the forming process is complete, using dies with a surface roughness of 7.5 μm or less and a lubricant with kinematic viscosity of 500 mm2/s or lower to improve sliding performance and prevent cracks, allowing for mass production without die reshaping or material changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a blank holding force is excessively large to prevent wrinkles, then wrinkle prevention is improved, but sliding is inhibited resulting in cracks
Solution Approach 1:
The blank holder is detached from the metal sheet periodically during the forming process. Specifically, after the punch first contacts the metal sheet and forming is started, and before the punch reaches a stroke end and the forming is completed, the blank holder is detached from the metal sheet. This periodic detachment allows the metal sheet to slide freely during critical forming stages, preventing crack appearance while maintaining wrinkle prevention when the blank holder is engaged.
Solution Approach 2:
The blank holder's contact state with the metal sheet is dynamically changed from static (continuous contact) to dynamic (intermittent contact). The blank holder is detached at specific stages of the forming process when sliding is needed, and re-engaged when wrinkle prevention is needed. This dynamic adjustment optimizes both wrinkle prevention and crack avoidance.
2Object-affected harmful factors
If the shape of dies is corrected or material is changed to prevent cracks, then crack prevention is improved, but manufacturing time and cost increase
Solution Approach 1:
Instead of correcting die shapes or changing materials after crack issues arise, the patent applies preliminary action by detaching the blank holder at strategically determined stages before cracks can occur. The detachment timing is predetermined based on the forming process stages, preventing cracks before they happen without requiring any die modification or material changes.
Solution Approach 2:
The patent changes the operational parameter of the blank holder (its contact state with the metal sheet) rather than changing die geometry or material properties. By controlling when the blank holder is detached and re-engaged, the forming process parameters are optimized to prevent cracks while maintaining standard dies and materials, thus avoiding costly and time-consuming modifications.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method enhances the forming limit and energy absorbability of metal sheets with tensile strengths of 400 MPa or higher, enabling cost-effective mass production of crack-resistant automotive frame parts with improved collision safety.
Implementation Method 1
a lubricant with kinematic viscosity of 500 mm2/s or lower is used to improve sliding performance
Implementation Method 2
a surface roughness of the punch, the upper die, and the blank holder is an arithmetical mean roughness Ra of 7.5 μm or smaller
Data Source
AI summary
A press forming method for a metal sheet uses dies, a surface roughness of which is an arithmetical mean roughness Ra of 7.5 μm or smaller, which are used as a punch, an upper die, and a blank holder. Fluid with a kinematic viscosity of 500 mm2/s or lower is supplied to a space between a metal sheet and the blank holder, a space between the metal sheet and the punch, and a space between the metal sheet and the upper die. A die is detached from a workpiece in the middle of forming, and resuming the forming.