Bending refers to the forming method of processing plate, bar, pipe or profile into parts with a certain angle and shape. It is a common process in sheet metal stamping. Generally speaking, the bending process needs to ensure the fillet radius of the bending part of the sheet metal part, the structure, shape and size of the part. In actual processing, the most common quality problem of sheet metal bending is springback, followed by crack, and others include warpage, distortion, size offset, hole offset and other problems. This paper mainly analyzes the springback and crack problems produced most in processing, and gives quality control measures.

Quality control of bending springback of sheet metal parts

In the process of bending forming, the blank will produce plastic and elastic deformation under the action of external load. Once the external load is removed, the elastic deformation will disappear immediately, while the plastic deformation can continue. Due to the existence of elastic deformation, the shape, position and size of parts will produce deformation opposite to the loading direction, which is rebound. The influencing factors of springback are complex. In order to ensure the quality control in the springback process, it is necessary to analyze its influencing factors.

factors affecting rebound and Control Countermeasures

1) Material selection

The material selection depends on the mechanical properties of the material, that is, the higher the yield limit of the material and the smaller the elastic modulus, the greater the springback after bending, the more serious the work hardening phenomenon and the greater the springback. Therefore, in order to prevent the influence of materials on springback, process designers should try to choose materials with small yield point, large elastic modulus and small hardening index.

2) Structure of sheet metal parts

When the value of R / T is small, the total elastic deformation of the inner surface of the part is relatively small, on the contrary, when the value of R / T is small, the total elastic deformation of the part is relatively small. Therefore, when bending sheet metal parts, the relative radius must exceed the scale factor, but the smaller coefficient value should be selected as far as possible. In addition, on the premise of ensuring the use of sheet metal parts, stiffeners can be pressed in the bending area, so as to increase the inertia moment of the part interface and inhibit the springback.

3) Process reasons of parts

When bending sheet metal parts, the larger the bending angle is, the longer the deformation zone is, and the larger the springback angle is; In addition, in actual production, the greater the applied bending force and the greater the deformation, the smaller the springback; Finally, when the bottomless die is used for free bending, the springback is large. On the contrary, when the corrected bending is used, the die is corrected with V-shaped parts, and the springback is small.

In view of the above factors, the process can use the law of rebound angle to calculate the compensation in the die design, and make the rebound of bending parts compensated after debugging and repairing the die. In addition, the following measures can be adopted in the structural design of the die: first, the root part of the punch of the bending die can be designed into a convex structure, which can better concentrate the force of the punch on the bending part after the bending, and the inner part of the extruded part will produce tangential deformation to alleviate the problem of rebound; Secondly, for the concave part of the bending die, soft rubber material or polyurethane material can be used instead of the original material, and then the press depth of the punch can be adjusted to ensure springback.

4) Other factors

The springback of bending parts is also affected by many factors, such as part shape, plate thickness deviation, plate performance fluctuation, die clearance and die fillet radius.

Problems and Countermeasures of bending cracks

In the bending process of sheet metal parts, if the thickness of the plate is thick, the bending radius is small, and the side with large burr in the plate is located outside the bending surface, if the plasticity of the plate itself is not high, the blank will produce too many cold work hardening layers during blanking, which will lead to cracks in the bending part of the part. To solve this problem, the die can be improved in the bending process, and the fillet radius of the punch can be increased appropriately. If it has no effect on the bending size, the burr side shall be used on the inner side of the bending as far as possible, and the material shall be heat treated to increase plasticity. In addition, the fiber direction of the material shall be marked, and the bending direction shall be at right angles to the fiber direction.

Bending quality control measures of two common materials sheet metal part

Stainless steel is a kind of material with high yield point and hardness, which is easy to produce cold work hardening effect. Therefore, it has the following characteristics when bending: first, because stainless steel belongs to high carbon steel, its thermal conductivity and ductility are relatively low, so the bending deformation process requires greater external force and greater rebound; Second, due to the low ductility, the bending radius r of stainless steel parts must be greater than that of ordinary carbon steel materials, otherwise the bending part is easy to produce cracks; Third, because the stainless steel material itself is prone to cold work hardening, the tool with high hardness should be used during bending. The tool material is generally tool steel, and quenching heat treatment should be carried out before use to ensure that the hardness of the tool is above 60HRC and the surface roughness should be RA3 two μ Within M.

Therefore, the key points of stainless steel plate bending quality control are as follows: first, under the unit size, the larger the thickness of stainless steel plate, the bending angle should also be increased as much as possible, the larger the bending force required, and the bending allowance on one side should also be increased, so as to prevent quality problems such as cracks in bending; Secondly, according to the material thickness, bending radius and bending angle given in the drawing, accurately calculate the unfolding size of stainless steel plate, especially pay attention to the thickness tolerance range of material, so as to ensure that the bent products can meet the final bending accuracy requirements and improve the one-time forming rate of bending; Third, due to the high yield strength of stainless steel, the rebound is also large. The rebound of stainless steel is indeed a difficult problem in production, which is related to many reasons. In view of this phenomenon, if the angle is in place in the bending process, the tool angle used in bending should be less than the actual bending angle. The second is the hardness of the material. The greater the hardness, the greater the rebound. In addition, it is also related to the ratio of bending radius to material thickness. The greater the ratio, the greater the springback. The springback of different grades of stainless steel is also different. Among them, the springback of 301 is larger than that of 304. Under the same premise, the springback of 304 is 2 degrees smaller than that of 301. Finally, the difference of forming methods will also lead to different springback. The springback of one-step forming is greater than that of multiple forming. The springback value can be corrected after the mold test. Note that each springback has angular springback and radius springback. The bending quality should be guaranteed in the actual production, and the specific situation should be analyzed.

Quality control of aluminum plate bending

The rebound problem of aluminum plate after bending will not be repeated, but if the cracking problem is obvious during bending, or the surface treatment through anodizing will burst after bending, it is first related to the hardness of aluminum plate. If the hardness of aluminum plate is too high, it is easy to crack, such as T-state aluminum plate commonly used by our company. Then it is necessary to heat treat to o state in advance. The heat treatment methods include annealing and quenching. The difference is that the quenched parts need to be bent within four hours, otherwise the aging in the air will restore the material to a hard state; The annealed material is always in soft state. In addition, in the processing technology, quenching is more difficult than annealing. Secondly, it is the thickness of aluminum plate. The thicker the material thickness is, the more difficult the bending processing is. Thinner aluminum plate should be used as much as possible. In addition, the size of bending R is also affected by it. The larger the R angle is, the less difficult it is to process. Therefore, on the premise of meeting the assembly requirements, the R value can be increased appropriately. Finally, the grain direction of the aluminum plate, that is, the fiber direction we usually call, the bending direction should be consistent with the fiber direction, not parallel. For aluminum parts requiring wire drawing, on the premise that the process allows, it is best to stretch after bending, otherwise the probability of bending cracking will be added.