Metal stamping is a plastic forming method in which the sheet material is separated or deformed by the equipment (punch) mounted on the punch press. Sheet metal stamping is now a very mature and widely used metal plastic forming method. The metal products manufactured by this process can be used in automobiles, machinery, household appliances, electrical machinery, aerospace, daily necessities and other fields.
The main advantages of mature stamping technology and its products are:
1. Good surface quality; the appearance is smooth and clean without burr.
2. High dimensional precision; such as hole cover, cable ring and other categories, high precision requirements for products, stamping process can well achieve the precision requirements.
3. High strength, good rigidity and light weight;
4. can be made into more complex shape products, and less waste, high material utilization rate.
5. Easy operation and high productivity.
But in the reality of processing, due to a variety of problems, by the Chinese metal stamping technology processing products will also produce the corresponding defects, seriously affect the stamping parts and precision, mechanical properties and surface quality. Two of the most common - and most serious - problems in stamping are cracking and wrinkling.
A. To break the crack
In the tensile test, with the deepening of the deformation, the bearing area of the material decreases, and the hardening effect also increases. When the increase of the hardening effect can compensate for the reduction of the bearing area, the deformation is stable. When a certain limit is crossed, the material will first neck in the position of the weakest bearing capacity, and finally be pulled. For the sheet material, the process of material deformation is basically the same as the tensile experiment. When the strain exceeds a certain limit value, the sheet will be broken.
It is one of the important defects that seriously affect the quality of stamping parts because the crack phenomenon will lead to the scrap material directly. The rupture occurs in the following situations: the small radius area of the deep drawing process, the rounded corner of the punch, the center of the side wall, and the area where the material enters the die through the drawing bar and causes the flow obstruction.
Since the rupture is caused by the strain in the local area exceeding its limit value, the principle to eliminate the rupture phenomenon should be to change the distribution of normal contact force and tangential friction force, so as to reduce the tensile strain value in the rupture area. Engineering practices are generally:
1. Choose reasonable blank size and shape
In the process of sheet metal forming, the size and shape of blank will affect the final forming quality. For example, when drawing a square cylinder, square blank is first used for drawing. If there is a crack, the four corners of blank can be cut with appropriate size to eliminate the crack.
2. Increase auxiliary process (change product arc or slope, increase shaping or process incision)
On the premise of meeting the functional requirements of the parts, appropriately increasing the round corner or decreasing the inclination of the die can reduce the flow resistance of the material in the forming process, so as to avoid the rupture. Punch cutting process incision in the appropriate part of the plate, so that the easily broken area from the adjacent area to get material supplement, in order to improve the deformation of the area, also can avoid the generation of rupture.
3. Adjust drawing bar parameters or blank holder force
Although the use of drawing bar can prevent the flange part of the fold, 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 sheet cracking.
4. Improve lubrication conditions
The relationship between stamping quality and lubricant is very important. Poor lubrication conditions or improper choice of lubricant may lead to sheet cracking.
2. Wrinkling
Wrinkling is also a typical quality defect in the stamping forming process, which directly affects the quality of the product surface, and is more serious. Sometimes there will be wrinkling and then be ironed by the mold phenomenon, damaging the workpiece and even scratching the mold, bringing great loss to the production.
The cause of wrinkling is opposite to the cause of cracking. Wrinkling is caused by the instability of sheet thickness direction caused by excessive local compressive stress. When the local compressive stress of the material is too large, it is easy to lead to wrinkling, especially when the material is under the action of two stress states, so the processing principle of eliminating wrinkling is to accurately predict the flow of the material and increase the normal contact force at the wrinkling place. Engineering practices are generally:
1. Increase the blank holder force
The blank holder 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 deep drawing bars or increase the height
Drawing bars are divided into round bars, square bars and drawing bars, and the feeding resistance increases in turn. Which drawing bars to use should be considered from many aspects, such as drawing depth, material properties and product shape. The WRINKLING DEFECTS CAN BE EFFECTIVELY IMPROVED BY SETTING THE DRAWING BAR REASONABLY, CONTROLLING THE FEEDING RESISTANCE SCIENTIFICALLY, CHANGING THE STRESS STATE INSIDE THE MATERIAL AND ADJUSTING THE FLOW DIRECTION OF THE MATERIAL.
3. Modify product and mold shape to absorb excess material
In addition to pressure and material problems, product and mold design is not reasonable is also one of the reasons for wrinkling. Exclude product reasons, it is necessary to mold blanking edge size, punching design, concave and convex die structure design, mold pressure center and mold materials and other aspects of the modification and determination.
Stamping forming quality related process parameters is numerous, the correlation between factors, possible problems in the practical application is much more than fracture and wrinkling, more resilient, size not jing, distortion, burrs, CARDS, big and small problems, such as, some directly make sheet metal scrap, some sheet still can continue to use after to fix the problem.
The metal processing technology is complicated and interrelated. It seems simple but actually needs accurate cooperation of all aspects. Therefore, behind each high-quality hardware product, there is a high level and high degree of cooperation between the original material and each process.