Stamping Die Piercing Design

The Conception of Stamping Die Piercing

• Blanking is a stamping process that uses dies to separate sheet metal, primarily including blanking (for producing parts) and piercing (for creating holes).
• The process of shearing a part (or blank) of the required shape from sheet metal is called blanking, while the process of creating a hole of the required shape in a workpiece is called piercing.
• The die used for blanking processes is referred to as a blanking and piercing die. It can be used to produce parts or to prepare blanks for subsequent steps such as bending, drawing, and forming.

Stamping Die Deformation Characteristics 

• The entire blanking process can be divided into three stages: the elastic deformation stage, the plastic deformation stage, and the shearing fracture stage.

Elastic Deformation Stage: Under the pressure applied by the punch, the material undergoes elastic bending and slightly intrudes into the die edge, as shown in the figure below.

Plastic Deformation Stage: The stress on the material has exceeded its elastic limit. At this point, the punch intrudes into the material while the material simultaneously intrudes into the die. Due to the material's resistance to the intrusion of both the punch and the die, a bending moment M is generated, as shown in the following diagram.

Shearing Fracture Stage: The stress near the edge of the die cavity first reaches the shear strength of the material, causing cracks to initiate at the die edge. At this point, the material near the punch edge remains in a plastic state, and the punch continues to intrude into the material (Figure a). Subsequently, cracks also develop in the material at the punch edge. When the unilateral clearance z/2 between the punch and die is appropriate, the shear cracks originating from both the punch and the die propagate and coincide, thereby separating the blanked part from the material (Figure b). After separation, the punch continues its movement to push the blanked part into the die cavity (Figure c).

Sheared Surface: The surface resulting from blanking, as shown in the figure below, exhibits three distinct zones on the punched hole: a slightly rounded corner at the top known as the roll-over (deformation zone), followed by a smooth, shiny surface formed by shearing called the burnished zone, and finally a rough surface created by tearing referred to as the fracture zone. On the scrap material, similar three zones appear, but in the reverse order of distribution.

Sheared Surface and Clearance on Stamping Die

Type I – Large clearance, large roll-over (R), large angle (a), severe burrs (tensile tearing).

Type II – Relatively large clearance, relatively large roll-over (R), normal angle (a), normal burrs.

Type III – Moderate clearance, reduced roll-over (R), normal angle (a), normal burrs.

Type IV – Small clearance, reduced roll-over (R), normal angle (a), burrs ranging from normal to increased.

Type V – Very small clearance, very small roll-over (R), very small angle (a), increased burrs (both tensile tearing and compressive extrusion).

• The figure on the previous page illustrates changes in the sheared surface under different clearances, with the clearance gradually decreasing from Type I to Type V. Type III represents an optimal clearance. This surface consists of a burnished zone and a fracture zone, accompanied by a roll-over (R) and burrs. Burrs caused by tensile stress are referred to as tensile tearing. The fracture zone exhibits an angle (a).

• In Type II, the roll-over (R) increases compared to Type III, while the angle (a) remains largely unchanged, and tensile tearing is still normal.

• If the clearance is further enlarged (Type I), the roll-over (R) and angle (a) increase significantly, tensile tearing worsens, and part deformation occurs.

• In Type IV, the roll-over (R) decreases compared to Type III, but the angle (a) is similar. Due to the reduced clearance, bright spots appear within the fracture zone, burrs are moderate, and compressive extrusion burrs emerge in addition to tensile tearing.

• Type V shows an extremely small roll-over (R), almost no angle (a), alternating layers of burnished and fracture zones, and increased burrs (both tensile tearing and compressive extrusion). The height of extrusion burrs relates to the sharpness of the die edge: the sharper the edge, the lower the extrusion burr height.

Through analysis of stamping process technology, parameter adjustment, and optimization of die-formed parts, trial mold cycles can be significantly shortened, maximizing profitability for the company and enhancing market competitiveness.