Definition of punching

For the mold designer, the quality of the designed product section is critical. However, first let's talk about what is punching. Blanking is a stamping process that uses the blanking die to separate the sheets under the action of the press. In a broad sense, punching is a variety of separation processes such as punching, blanking, cutting, cutting, cutting, etc. The general term. But in general, punching mainly refers to the blanking and punching process.

Blanking is the basic process in the cold stamping process. It is extremely widely used. It can directly produce the required finished parts or prepare blanks for other cold stamping processes.

After the blank is punched, it is separated into two parts, that is, a flushing part and a holed part. If the purpose of the punching is to obtain a shape of the shape of the wheel gallery and the size of the flushing part; In the blanking process, the remaining perforated portion becomes waste. Conversely, if the purpose of the blanking is to obtain an inner hole of a certain shape and size, at this time, the flushed portion becomes waste, with holes, and the part is the workpiece. This punching process is called a punching process. See the table below.

Punching classification

According to the roughness of the cut surface, or the accuracy of the punch, the punching is divided into ordinary punching and precision punching.

Ordinary blanking is when the workpiece is separated, due to the pressure of the die. In addition to the shear deformation of the material between the convex and concave cutting edges, there are deformations such as stretching, bending, and lateral extrusion, and the material eventually tears. The form is separated. Therefore, the section of the ordinary blanking workpiece is rough and has a certain taper, and its precision is low. Precision punching uses a special die structure to make the material at the edge of the convex and concave die eventually plastic. Shear deformation form separation. Precision punched parts, smooth section and perpendicular to the board surface, high precision.

At present, some punching parts with high precision requirements, such as instruments, cameras, enamel watches, etc., are mostly processed by precision blanking. If the blanking is divided according to the requirements of the separation part and the base material part, it can be divided. Such as punching, blanking, cutting, cutting, half-cutting, etc.

Deformation characteristics

According to the principle of metal plastic deformation, the main way to cause metal material damage during the deformation process is to break and shear. This means tensile stress and tensile strain. The factors causing the fracture of metal materials are probably shear stress and shearing. Variables. Compressive stress and compressive strain can only cause deformation of plastic materials without causing damage to the material.

Although the blanking separation process is completed in a flash, the deformation separation is very complicated, and the stamping separation deformation is mainly divided into the following three stages:

1. Elastic deformation stage

Under the action of the punch pressure, the material at the cutting edge first produces elastic compression, stretching and other deformations. The punch is slightly squeezed into the interior of the material, and the underside of the sheet is slightly squeezed into the cavity of the die. The material below is slightly curved. The material above the die starts to curl up. If the gap between the die and the die is larger, the bending and upturn are more serious. However, the internal stress of the material does not reach the limit state. After the external force, the material can still be restored to its original state. This stage is called the elastic deformation stage (Figure 2.1-1).

2 , plastic deformation stage

As the punch decreases, the pressure on the sheet increases, and the stress inside the material increases. When the internal stress reaches the yield limit of the material, it begins to enter the plastic deformation stage, when the convex and concave mold further rushes into the material interior. Due to the existence of the gap between the edges of the convex and concave molds, the tensile stress and bending inside the material become larger, the compressive stress component is reduced, and the material is further bent and stretched, so that the hardening of the material in the deformation zone is intensified. The punching force also reaches a maximum when the material near the edge begins to produce microcracks. The appearance of microcracks (cracks) indicates that the material begins to break and the plastic deformation phase ends.

3. Separation stage.

The punch continues to descend, causing the upper and lower cracks of the sheet to expand continuously. It extends to the inside of the material, as shown in Figure 2.1-3. When the upper and lower cracks of the sheet coincide, the material fibers are completely torn and broken, and the section of the part begins to separate. When the punch is lowered again, the falling portion of the sheet is pushed out of the die hole, and the initially formed burr is further elongated, as shown in Fig. 2.1-4. At this point, the punch is lifted to complete the entire punching process.

Sectional features of ordinary punched parts

For the section analysis of ordinary blanking parts, we can find such a rule. The punched section of the part and the surface of the part are not perpendicular, but have a certain taper; except for a narrow part of the bright band, the flaws are rough and dull. There are burrs and sag angles. We refer to the areas on the section of the blanking part as the sag band (also known as the rounded band), the bright band (also known as the shear band), the broken band and the burr. 2.1-5)

A-rounded b-bright band c-break band d-burr band

The high-quality blanking section should be: the bright belt is wider, accounting for more than 1/3 of the entire section; the sag angle, the broken belt, the burr and the taper are small, and the entire punching part is straight without bending, but The factors affecting the quality of the punched section are very complicated, and it varies with the properties of the material, thickness, edge gap, die structure and punching speed.