Automotive Coverings Process Issues

The surface quality of the automobile covering parts will directly affect the quality of the body, so there are high requirements for the dimensional accuracy and surface quality of the covering parts. Body coverings are not allowed to have surface defects such as wrinkles and scratches, and at the same time, they should have enough stable rigidity.

1.Dark pit

Dark pit is a slight depression or bulge that occurs locally on the continuous projecting surface on the covering part. As this defect is difficult to find by direct visual observation, it can only be shown by light after body painting, so it is called dark pit.

Cause: In the forming process, the deformation of the sheet is composed of elastic deformation and plastic deformation. When the external force is withdrawn, the elastic deformation is limited by the plastic deformation that has been generated.

The residual stress is formed in the stamped part because it cannot fully recover from the plastic deformation formed, and the shape and size of the stamped part is the comprehensive reflection of the elastic deformation and plastic deformation of the sheet. The elastic deformation of bending then causes the change of the radius of curvature of the made part at that place, forming a dark pit.

2. Waves

Waves are very slight wrinkles, similar to the continuous appearance of dark pits in a certain area, mainly generated near the edge of the part, such as above the wheel cover of the wing, near the flap line of the door. Causes: During the forming process, the plate material, which is wrinkled due to instability in the flange surface part under the action of tangential compressive stress, is retained on the surface of the stamped part through the gap between the convex die and the concave die, forming a surface wave. In addition, due to the bending deformation of the material during the flanging process, the outer part of the bent part is subjected to tensile stress and the inner part to compressive stress, and the slight warpage deformation produced after unloading due to the moment effect of these stresses is also reflected as surface waves. The generation of waves is influenced by the product shape, material properties, mold structure and other factors. In particular, factors such as unreasonable design of crimp circle and mold clearance during drawing can directly lead to the generation of waves.

3. The prism is not clear

The unclear prism is mainly caused by the mold shape, poor dimensional accuracy, the mold is not pressed to the bottom, inaccurate positioning of the workpiece, etc., while the hair pulling is mainly caused by the poor state of the rounded corners of the concave mold and poor lubrication or insufficient hardness of the mold.

4. Rebound

Overcome the rebound, through the use of reasonable technology to improve the stress state of the plate in the forming process, but also through the selection of reasonable rebound angle, rebound radius, mold clearance and other mold process parameters to solve.

5. Insufficient stiffness

Body coverings have certain stiffness requirements, if the stiffness is not enough, the car will produce resonance phenomenon during the driving process, causing a lot of noise, and will make the coverings damaged prematurely. Under the condition that the shape and structure size of the cover part are unchanged, the greater the plastic deformation of the car cover part in the stamping and forming process, the better its rigidity. In general, the plastic deformation of the blank of the automobile covering part should be greater than 2%. The main countermeasures to overcome the lack of stiffness are: when designing the product, the aesthetic decorative strip and decorative rib should be designed; when designing the tensile parts, the depth of the tensile parts should be increased appropriately. Appropriately increase the depth of the stretching parts, or use the process to supplement the depth of the stretching parts; in the mold design, use the stretching bars, stretching thresholds, etc.; increase the crimping force during the stamping process, etc. All these measures are helpful to increase the plastic deformation of the blank and improve the rigidity of the stamping parts.

6. Wrinkling

The wrinkles can be eliminated by increasing the radial tensile stress in the plate. The crimping force can be increased by increasing the tensile tendons, decreasing the radius of the concave die fillet, and adjusting the gap between the pressing surface and the tensile gap to avoid the occurrence of wrinkles.

7. Rupture

For the stamping parts with internal partial forming, since the deformation is two-way elongation deformation, if the external material cannot be replenished to the internal, the rupture will occur when the plastic deformation is too large. The root cause is that the tensile deformation force is greater than the actual effective tensile strength of the material at the sidewall cracking. The adjustment methods to overcome rupture are as follows.

(1) Methods to overcome edge rupture: improve the state of the billet edge, use the method of punching to undercut the billet, so as to ensure the quality of the billet edge section; for the determination of the inner concave flange billet size, use the appropriate increase

For the determination of the inner concave flanged blank size, methods such as increasing the material size are used. Blank size is too large or improper shape, the quality of the plate material and poor lubrication when stamping can also make the parts pulling crack, which should change the size or shape of the blank, adjust the stamping process.

(2) to overcome the shear rupture method: the convex, concave die clearance is adjusted evenly, appropriately increase the convex, concave die rounding, increase the deep drawing process or shaping process, control the material everywhere into the concave die resistance as much as possible to block the uniformity.

(3) Methods to overcome expansion rupture: reduce the degree of deformation, expand the range of deformation; open process incision or process solitaire in the connection part to reduce the radial tensile stress; use multiple forming processes.

(4) Methods to overcome tensile rupture: reduce the deformation resistance of flange deformation area; increase the radius of convex and concave die rounding; adjust the relative positions of convex and concave dies. Increase the number of drawing processes, increase the shaping process; adjust the crimping force appropriately. Under the condition of not affecting the use function, appropriately reduce the local forming depth, increase the corner, and increase the process cutout or process hole if allowed, in order to increase the flow of material inside the local shape and reduce the plastic deformation in the deformation area, etc.

In addition, the stretching temperature and lubrication state also have influence on the forming quality. If the temperature is low, the sheet forming performance is poor, which is not conducive to pulling out the ideal stretching parts. In the drawing process, the metal material is in direct contact with the surface of the die, and the interaction force is great, so that the material slides on the surface of the concave die with a large frictional force, which increases the pressure required for drawing and the tensile stress within the material, which is one of the reasons for the rupture of the workpiece. There are many reasons for the cracking of the part, and the cracking condition should be carefully checked during the adjustment, the part produced, the location of the stretching stroke that produces cracking should be determined, and the causes of cracking should be analyzed and judged according to the specific situation, so that a specific plan to solve the cracking can be formulated.

8. Indentation, pockmark, bump, scratch Indentation, pockmark, bump, scratch are the surface quality defects that are easy to occur in the body covering parts. Indentation, pockmark is because there are defects on the surface of the mold and the surface of the press material or blank of the mold is not clean, there are sand particles or other foreign matter, forming abnormal traces on the surface of the covering parts during the stamping process, reflecting bright spots and bright marks after polishing the surface by oil stone; bump and scratch may occur in every link from the steel sheet to the welding assembly, especially easy to occur in the manual operation of these links. operation process. In terms of probability, the more times the parts are turned around in the process, the more chances of bumping and scratching will occur. In addition, the choice of covering material also plays an important role in the quality of covering parts.