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Sheet metal stamping is a very important metal plastic forming method, which is widely used in aerospace, automotive and locomotives, electrical appliances, food packaging, daily hardware, construction, packaging, and other industrial fields.
Various forming defects that often appear in the actual stamping production process seriously affect the geometric accuracy, mechanical properties, and surface quality of the stamping parts. As there are many process parameters related to the quality of stamping and the factors are related to each other, this brings great difficulties and challenges to the mold engineers on-site to repair and try out the mold. This article will analyze the causes of the three common quality defects in the stamping process: cracks, wrinkles, and rebound, and introduce the general solutions respectively.
1. Punching Fracture
Sheet thinning is the result of sheet stretching. From an engineering perspective, the thickness of the sheet is reduced by 4% to 20%, which is generally acceptable; however, if the thinning is too much, it will not only weaken The rigidity of the parts, in severe cases, may even directly cause the sheet to rupture and become a waste product. Therefore, the cracking phenomenon is one of the important defects that seriously affect the quality of stamping and forming parts.
In the tensile test of the material, as the deformation deepens, the bearing area of the material is continuously reduced, and its hardening effect is also continuously enhanced. When the increase in the hardening effect can compensate for the reduction in the bearing area, the deformation is stable; After a limit value, the material will first neck down at the weakest position and eventually be broken. For sheet materials, the process of material deformation is basically the same as the tensile test. When the strain exceeds a certain limit value, it will cause the sheet to break.
According to the different degrees of rupture, rupture can be divided into two types: microscopic rupture and macroscopic rupture. Microscopic cracking refers to the generation of cracks in the sheet that are difficult to see with the naked eye. Although the crack depth is very shallow, some of the materials have actually failed. Macro cracking refers to the appearance of cracks and fractures visible to the naked eye in the sheet. Macroscopic rupture is usually caused by excessive swelling in the plane of the thin plate, while microscopic rupture can be caused by pure swelling or simple bending. In the final analysis, both microscopic and macroscopic ruptures are caused by a local tensile strain of the material.
The occasions of cracking generally include a small radius area in the deep drawing process, the corner of the punch, the center of the sidewall, and the area where the material enters the cavity and causes the flow to be blocked.
Since the rupture is caused by the strain in the local area exceeding its limit value, the principle to eliminate the rupture phenomenon is to change the distribution of the normal contact force and the tangential friction force to reduce the tensile strain value in the rupture area. The methods generally include:
- Choose reasonable blank size and shape
During the sheet forming process, the size and shape of the blank will affect the final forming quality. For example, when the square tube is stretched, the square blank is first used for stretching. If cracks occur, the four corners of the blank can be cut with appropriate sizes. Treatment can eliminate the rupture.
- Add auxiliary processes (change the arc or slope of the product, increase the shaping or process incision)
On the premise of meeting the functional requirements of the parts, appropriately increasing the fillet of the mold or reducing the inclination can reduce the flow resistance of the material during the forming process, thereby avoiding breakage. The punching process incisions at appropriate parts of the plate, so that the easily ruptured area can be supplemented with material from the adjacent area, so as to improve the deformation of the area, and also to avoid the occurrence of rupture.
- Adjust the fracture parameters or blank holder force
Although the use of fracture can prevent wrinkles in the flange part, its side effect is to increase the flow resistance of the material into the die. Therefore, inappropriate fracture parameters may cause excessive flow resistance, resulting in sheet cracks.
- Improve lubrication conditions
The relationship between stamping quality and lubricant is extremely important. Poor lubrication conditions or improper selection of lubricants may cause sheet cracks.
2. Punching Wrinkle
Wrinkling is also a typical quality defect in the stamping process, which directly affects the surface quality of the product; more serious, sometimes there will be wrinkling and then be ironed by the mold, damaging the workpiece or even scratching the mold, which brings extreme results to the big loss.
The cause of wrinkling is opposite to the cause of cracking, which is caused by the instability of the thickness direction of the sheet due to excessive local compressive stress. This form of instability is called compression instability. When wrinkles occur, the direction of wrinkles is perpendicular to the compressive stress, but it cannot be simply considered that any wrinkles are caused by compressive stress.
There are various wrinkles during sheet metal stamping and forming. According to the different causes, they can be divided into material accumulation wrinkles and instability wrinkles. Material accumulation wrinkles are caused by too much material entering the cavity of the cavity. Wrinkles are caused by instability, and instability wrinkles refer to the wrinkles caused by the instability of the compression flange with weak binding force in the thickness direction of the sheet and the instability of the uneven stretched part. Although wrinkling does not weaken the strength and rigidity of the part like tearing, it affects the accuracy and beauty of the part. If wrinkles occur in the intermediate process, they may also affect the normal progress of the next process.
When the local compressive stress of the material is too large, it is easy to cause wrinkles, especially when the material is under the action of tension and compression. Therefore, the principle of eliminating wrinkles is to accurately predict the flow of the material and increase the wrinkles. Normal contact force, engineering practices generally include:
- Increase the blank holder force
The blank holder force can increase the flow resistance of the material into the die and can alleviate the wrinkling of the flange edge.
- Increase the number of rebounds or increase the height
Rebounds are divided into round bars, square bars, and drawbars. The feed resistance increases in turn. Which rebound to use needs to be considered from many aspects, such as the drawing depth of the workpiece, material properties, and product shape. The reasonable setting of rebounds, scientific control of feed resistance, change of internal stress state of materials, and adjustment of material flow direction can effectively improve wrinkle defects.
- Modify the product and mold shape to absorb excess material
3. Punching Rebound
Sheet metal rebound refers to a phenomenon in which the shape and size of the material undergo a change in the opposite direction to the deformation when loaded due to the elastic recovery of the material after the external load is removed during the stamping and forming process. Rebound is an inevitable forming defect in the sheet metal forming process, especially the bending process.
The rebound of the sheet material has seriously affected the shape and dimensional accuracy of the formed parts. Especially in recent years, with the widespread use of high-strength steel plates, the rebound phenomenon has attracted more and more attention. Due to the high yield strength and tensile strength, the high-strength sheet material has greater rigidity and hardness and exhibits a more obvious rebound phenomenon after unloading at room temperature.
After an in-depth analysis of the rebound phenomenon, we learned that the main reason for the rebound phenomenon is that the deformation state of each part of the sheet material is not synchronized. In the deforming stage, when the mold is unloaded, each part of the material needs to be elastically restored, which leads to uneven residual stress distribution in the thickness direction or in-plane direction of the sheet material, and eventually rebound occurs.
There are many influencing factors that affect the amount of sheet metal rebound, such as the mechanical properties of the material itself, the mold fillet and the gap between the concave and convex molds, the blank holder force, and so on. For the designers of the sheet metal forming process, the easier way to reduce the geometric error of the workpiece caused by the rebound phenomenon is to reduce the rebound of the workpiece by adjusting the process parameters, so that the geometric size of the workpiece meets the design requirements.
- Material performance
The smaller the elastic modulus of the material, the higher the yield limit, the more serious the work hardening phenomenon (large n value), and the greater the rebound of bending deformation. The rebound of high-strength steel plates and aluminum alloy steel plates is larger than that of ordinary steel plates.
- Relative bending radius
The relative bending radius refers to the ratio of the bending radius to the material thickness when the sheet is bent. When the relative bending radius decreases, the total tangential deformation on the outer surface of the bent sheet increases, and the plastic deformation and elastic bending components also increase at the same time, but the proportion of elastic deformation in the total strain decreases, so The rebound also decreases; on the contrary, when the relative bending radius increases, as the proportion of elastic deformation in the total deformation increases, the rebound also increases.
- Gap between concave and convex molds
For the rebound problem, the gap between the concave and convex dies of the stamping die has an impact on the rebound and surface quality of the best part. The smaller the gap, the smaller the rebound angle, and the larger the gap, the larger the rebound angle. However, if the gap is too small, the surface of the workpiece will be scratched or the thickness will become thin; when the gap is smaller than the material thickness, the workpiece may have a negative rebound.
The size of the stroke also affects the stress state of the sheet metal during the stamping and forming process. For shallow drawn parts, the stroke is smaller, and the influence of bending stress is greater than that of tensile stress, so the tendency of rebound is more obvious; for deep drawn parts, the stroke is larger, and the tensile stress during the stamping process As a result, the upper and lower surfaces of the sheet material form a two-way stretch state, the rebound tendency is partially offset, and the rebound amount is small.
- Blank holder force
Increasing the blank holder force can reduce the rebound of the sheet, but increasing the blank holder force is based on the premise that the part has no other forming defects. The blank holder force can usually be increased by increasing the blank holder force or by setting a rebound.
- Friction coefficient
The friction between the surface of the curved sheet and the surface of the mold can change the stress state of each part of the curved sheet. It is generally believed that friction can increase the tensile stress in the deformation zone, and can make the shape of the part close to the shape of the mold, thereby reducing the rebound of sheet metal stamping.