1.How can we ensure that the mechanical properties of materials meet the standards?
Elongation (A80, A50): This is a core indicator of material plasticity. Suppliers must provide guaranteed elongation values that meet or exceed processing requirements. For complex forming or near-flanging, higher-grade deep-drawing steel (such as DC04, DC05, DC06) should be selected.
Work Hardening Index (n-value): The higher the n-value, the stronger the material's ability to elongate uniformly during deformation, and the less prone it is to localized necking and cracking. Deep-drawing steels have higher n-values.
Plastic Strain Ratio (r-value): A high r-value indicates that the material is less prone to thinning in the thickness direction and flows easily within the sheet plane, which is beneficial for flanging. An r-value > 1.5 is generally required.
2.How to check the quality of material edges?
Slitting Burrs: Burrs generated during the slitting process are stress concentration points and one of the main causes of edge cracking. The burr height after slitting should be controlled to a very small range (e.g., ≤0.02mm), or a deburring process should be arranged.
Edge Cracks/Hardened Layer: Inspect the rolled edge for micro-cracks or shear hardened layers that are not visible to the naked eye. If necessary, light edge rolling or shot blasting can be used to eliminate the edge hardened layer and micro-cracks.
3.How can I confirm the condition of the materials?
Avoid using overly hard materials: Ensure the material has undergone sufficient annealing and is in a soft state (e.g., DC01, DC04). Overly hard materials (e.g., hard or semi-hard states) have poor plasticity and are extremely prone to cracking.
Pay attention to surface coatings/oil films: The adhesion of the zinc layer on galvanized sheets (GI/GA), the coating ductility of coated sheets, and the uniformity of the anti-rust oil all affect the forming process.
4.How to optimize mold design?
Increase the bending radius (R angle): This is the most direct and effective method. The inner R angle of the rolled edge should be at least 1 times the material thickness (t), with 1.5t-2t recommended. The smaller the R angle, the greater the local strain and the easier it is to crack.
Properly set the die clearance: The clearance of the punching or bending die should be moderate. If the clearance is too small, the material will be severely squeezed and scraped, leading to edge damage; if the clearance is too large, there will be large burrs and unstable forming. The clearance on one side is usually 8%-12% of the material thickness.
Improve the die surface finish: The working surface of the die (especially the rounded corners of the die cavity) must be highly polished to reduce the frictional resistance of material flow.
Ensure die alignment and perpendicularity: Prevent uneven material stress caused by off-center loading.
5.What are some things to keep in mind regarding the process and operation?
Use a high-quality lubricant:
Applying a specialized stamping lubricant to the crimped area effectively reduces friction between the material and the die, lowers forming forces, and promotes more uniform material flow. Lubrication is crucial for difficult-to-form materials such as stainless steel.
Control forming speed:
Appropriately reducing the stamping or bending speed to allow more material flow time improves formability and reduces excessive localized stress caused by inertia.
Adjust crimp direction:
Aim for a certain angle between the crimp direction and the rolling direction (ideally 45° or perpendicular). Since the elongation of the material along the rolling direction is usually greater than that perpendicular to it, changing the direction utilizes the material's anisotropy.
Control the processing environment:
Keep the material clean and prevent dust and particles from being pressed in, which can cause stress concentration.