I. Key bending points of stainless steel plate
Due to its high yield strength, hardness and prominent cold working effect, stainless steel sheet bending has the following characteristics:
- Due to its lower thermal conductivity compared to normal low-carbon steel, it has a lower elongation rate and requires a higher deformation force.
- Stainless steel sheets have a stronger tendency to rebound when bent compared to carbon steel.
- The percentage of elongation of the stainless steel sheet is lower than that of carbon steel, leading to a greater bending angle of the part (R) or the possibility of cracking.
- Given the high hardness of the stainless steel sheet, which is significantly hardened by cold working, you should choose a punch die made of tool steel, with a hardness greater than 60 HRC after heat treatment. The surface roughness of bending tools will be higher than that of carbon steel bending tools.
According to the characteristics above, in general:
Under the same unit size, the thicker the plate, the greater the bending force required. The bending force increases as the plate thickness increases.
Under the same unit size, the greater the tensile strength, the lower the elongation rate, the greater the required bending force, and the larger the bending angle must be.
When designing the thickness of the plate in relation to the radius of curvature, according to experience, the unfolded size of the part with a bend should be calculated by adding the two right-angled sides and subtracting two thicknesses. This will fully meet the project accuracy requirements. Using an empirical formula to calculate quantity can simplify the calculation process and significantly improve production efficiency.
The greater the resistance to flow of the material, the greater the elastic return. Therefore, the angle of the punch die for the 90 degree bent part must be smaller.
Compared to carbon steel, for the same thickness of stainless steel, the bending angle is larger. It is important to pay special attention to this point, as cracks may appear due to flexion, affecting the strength of the piece.
II. The stainless steel rear spring
The return of stainless steel is really problematic and is due to several reasons:
- Hardness: The higher the hardness, the greater the elastic return. The last time I used 301-EH stainless steel, the rear spring was 14 degrees.
- Ratio between bending radius thickness and stainless steel thickness: The greater the ratio, the greater the recovery.
- Difference in steel types: The rear spring of SUS301 stainless steel is larger than that of SUS304 stainless steel. In the same situation, the back spring of 304 stainless steel is 2 degrees smaller than that of 301 stainless steel. Furthermore, the elasticity of Japanese 301 stainless steel is greater than that of Taiwanese stainless steel.
- Differences in shaping methods: The springback of a one-step forming process is greater than that of multiple forming processes.
To mitigate these issues, I make a habit of reviewing the mold after each test and making any necessary changes, taking into account both angle rebound and radius rebound. Of course, a qualified technician can make adjustments efficiently and will inform the designer of any necessary changes.
I usually try to move 5 pieces at a time, and efficiency largely depends on the skill and experience of the technician.
