Common defects and solutions in stamping forming

Sheet metal stamping is a very important metal plastic forming method, widely used in aerospace, automobile locomotive, electrical appliances, food packaging, daily hardware, construction, packaging and other industrial fields.

Various forming defects often appear in the actual stamping process, which seriously affect the geometrical accuracy, mechanical properties and surface quality of stamping parts. Due to the large number of process parameters related to the quality of stamping forming, and the correlation between each factor, it brings great difficulties and challenges to the on-site mold engineer. In this paper, three kinds of common quality defects in the process of stamping forming: rupture, wrinkle and rebound phenomenon are analyzed, and respectively introduced the general solution, only to find the root of the disease, the right medicine, can not blindly repair mold, time and money.

1. Rupture

Sheet thinning is the result of sheet stretching, from the practical point of view of engineering, the thickness of sheet material reduced by 4 % ~ 20 %, is generally acceptable, however, if the thinning is too much, it will not only weaken the stiffness of parts, serious, even directly lead to the fracture of sheet material, become waste products, therefore, Fracture phenomenon is one of the important defects that seriously affect the quality of stamping parts.

As we know, in the material tensile test, with the deepening of deformation, the bearing area of the material is reduced, and the hardening effect is also enhanced. When the hardening effect can compensate for the reduction of the bearing area, the deformation is stable; When a certain limit value is crossed, the material will first be necked at the position where the bearing capacity is weak and be pulled apart. For sheet metal, the process of material deformation is basically the same as that of tensile test. When the strain exceeds a certain limit value, the sheet will crack.

According to the different degree of fracture, the fracture can be divided into micro fracture and macro fracture. Microscopic cracking refers to the formation of cracks in the sheet material that are difficult to see by the naked eye. Although the cracks are shallow, part of the material has actually failed. Macroscopic fracture refers to the visible cracks and fractures in the sheet material. Metal processing wechat, the content is good, worthy of attention. Macro fracture is usually mainly caused by excessive tensile expansion in the plane of thin plate, while micro fracture can be caused by simple tensile expansion or simple bending. Both micro fracture and macro fracture are caused by excessive local tensile strain of materials in the final analysis.

The fracture occurs in the following occasions: the small radius area of deep drawing process, the rounded corner of punch, the center of side wall and the area where the material enters the die through the drawing bar and the flow is blocked.

As the rupture is caused by the strain in the local area exceeding its limit value, the principle for eliminating the rupture phenomenon should be to change the distribution of normal contact force and tangential friction force to reduce the tensile strain value in the fracture area. Engineering practices generally include:

1. Choose reasonable billet size and shape

In the process of sheet metal forming, the size and shape of the billet will affect the forming quality. For example, when stretching the square cylinder, the square billet is first used for stretching. If there is a rupture, the four corners of the billet can be properly cut to eliminate the rupture.

2, increase the auxiliary process (change the arc or slope of the product, increase the shaping or process incision)

On the premise of meeting the functional requirements of parts, proper die rounding or reducing the slope can reduce the flow resistance of materials in the forming process, so as to avoid cracking. Punching process incision in the appropriate part of the plate, so that the easy to crack area from the adjacent area to get material supplement, in order to improve the deformation of the area, also can avoid the generation of fracture.

3. Adjust drawing bar parameters or blank holding force

Although the use of drawing bar can prevent the flange from producing folds, its side effect is to increase the flow resistance of the material into the die. Therefore, inappropriate drawing bar parameters may lead to excessive flow resistance, resulting in plate cracking.

4, improve the lubrication conditions

The relationship between the quality of stamping and lubricant is very important. Poor lubrication conditions or improper lubricant selection may lead to plate cracking.

Two, wrinkle

Wrinkle is also a typical quality defect in the process of stamping forming, which directly affects the surface quality of the product. Like the poor forming quality of the domestic automobile appearance covering parts, one of the most important reasons is wrinkle problem; More serious, sometimes there will be wrinkles and ironing by the mold, damage the workpiece and even scratch the mold, to the production of losses.

The cause of wrinkling is opposite to the cause of fracture, which is caused by the instability of the thickness of sheet metal caused by excessive local compressive stress. This form of instability is called compressive instability. When wrinkling occurs, the direction of the wrinkle is perpendicular to the compressive stress, but it cannot be simply considered that any wrinkling is caused by compressive stress.

Wrinkles produced in sheet metal stamping forming are various, according to different causes can be divided into material accumulation wrinkle and instability wrinkle, material accumulation wrinkle is caused by too much material into the cavity of the die; The instability wrinkle refers to the wrinkle caused by the weak compression flange instability due to the binding force of the thickness direction of the sheet metal and the instability in the uneven tensile part. Although wrinkling does not weaken the strength and stiffness of a part like cracking, it affects the accuracy and aesthetics of the part. If wrinkles occur in the intermediate process, it may affect the normal progress of the next process.

When the local compressive stress of the material is too large, it is easy to lead to wrinkle, especially when the material is under the action of two stress states of tension and compression. Therefore, the treatment principle of eliminating wrinkle is to accurately predict the flow of the material and increase the normal contact force at the wrinkle. The engineering practices generally include:

1. Holding force

The blank holding force can increase the flow resistance of the material into the die and alleviate the wrinkling of the flange edge.

2. Increase the number of drawing ribs or increase the height

Drawing bar is divided into round bar, square bar and drawing bar, and the feeding resistance is large in turn. Which drawing bar to use needs to be considered from many aspects, such as the drawing depth of parts, material properties, product shape, etc. Wrinkling defects can be effectively improved by setting drawing bar reasonably, controlling feed resistance scientifically, changing stress state inside material and adjusting material flow direction.

3. Modify product and mold shape to absorb excess material