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Blanking Deformation Process

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Blanking Deformation Process --- Aheadmold

A blanking process involves placing the blank on the die, moving the punch downward to deform and separate the blank with the edges of the punch and die. A clearance Z is existed between the punch and die. The forces of the punch and die applying on the blank are mainly concentrating on the edges of the punch and die.
Blanking deformation process is shown in Fig.1-2. Under the actions of the punch and die with sharp cutting edges and an appropriate clearance, deformation process undergoes three stages, namely, elastic deformation, plastic deformation and fracture separating stages.
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Fig.1-2   Deformation process of stamping

1.    Elastic Deformation Stage
When the punch contacts the blank, the material is compressed, resulting in tensile and bending elastic deformation. In this stage, the inner stress hasn’t exceeded the elastic limit of the blank yet. The deformation would recover if unloading is occurred.
2.    Plastic Deformation Stage
When the punch presses further downward on the blank, the inner stress of the blank reaches its yield strength, the plastic flow and sliding deformation begin to occur. Under the pressure of the punch and die, the surface of the blank is subjected to compression, due to the clearance between the punch and die, the blank is subjected to the actions of bending and tension simultaneously, the material beneath the punch is bended, and that above the die is curled
upwards. Circular angles are formed in regions ① and ② due to bending and tension, and
indentations appear in regions ③ and ④. While the punch squeeze further into the blank, the plastic deformation and the work hardening in the deformation zone increase further. When the inner stress of the blank near the cutting edge reaches the strength limit of the material, the blanking force reaches its maximum and the cracks occur in the blank, resulting in the damage of the material and the end of the plastic deformation stage (see Fig.1-2).

3.    Fracture Separating Stage
With the punch squeezing into the blank continuously, the cracks at the top and bottom extend to the inner layer of the sheet metal gradually, when the two cracks meet, the blank is cut, and then the process of fracture is ended.
Equilibrium of forces in the shearing zone during blanking is shown in Fig.1-3; where Fl and F2 are the acting forces of the punch and die perpendicular to the blank respectively; F3 and F4 are the lateral pressures of the punch and die exerting on the blank respectively; μF1, μF2 are the frictions on the end surfaces of the punch and die acting on the blank respectively; μF3, μF4 are the frictions on the lateral surfaces of the punch and die acting on the blank respectively. The directions of μF1 and μF2 vary with the clearance between the punch and die.
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Fig.1-3   Diagram of the blanking force

1-punch   2-blank   3-die

Analysis of the blanking forces shows that the lateral pressures F3 and F4  must be smaller than the perpendicular pressures F1 and F2; and that the cracks occur and extend more easily in the area of small pressure. Therefore, the initial crack occurs on the side surface of the die in blanking. Observation on crack initiating and developing with scanning electronic microscope shows that when the depth of punch squeezing downward into the material reaches 20% of the blank thickness, the crack occurs on the side surface of the punch and die edges, and then the cracks at the top and bottom extend rapidly. When the two cracks meet, the blank is sheared and the process of fracture is ended.

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