Metal stamping, stretching, and forming are one of the main stamping processes used widely.
Stretching technology can produce thin-walled parts with various shapes such as cylindrical, rectangular, stepped, spherical, conical, parabolic, and other irregular shapes. When combined with other stamping forming technologies, it can also produce parts with even more complex shapes.
There are 16 types of metal stamping and stretching processes available when using stamping equipment.
Stretching process for cylindrical products with flanges. The flanges and bottom are both flat, with the side wall of the cylinder being axisymmetric and deforming uniformly distributed on the same circumference. The flange experiences stretching deformation on the blank.
The deformation of the blank on the flange is stretching deformation, but the amount of deformation and deformation ratio change along with the contour shape. The larger the curvature, the greater the plastic deformation of the blank; conversely, the smaller the curvature, the smaller the plastic deformation of the blank.
The low rectangular part is formed by a single stretching process. During stretching, the stretching resistance at the rounded corner of the flange deformation area is greater than that at the straight edge, and the deformation degree at the rounded corner is greater than that at the straight edge.
When the side wall of the stamped part is sloped, the side wall is suspended and not in contact with the mold during the stamping process, but only adheres to the mold at the end of the forming process. The deformation characteristics of different parts of the side wall during the forming process are not completely the same.
The deformation process of the blank for the conical cover part during forming is not simply a stretching deformation but a compound forming of stretching and bulge deformation.
The deformation of the blank on the material-feeding surface is a stretching deformation (radial stress as stretching stress and tangential stress as pressing stress), while the deformation of the blank inside the profile (especially in the central area) is bulge deformation (both radially and tangentially are stretching stress).
Half-sphere stretching with flange:
During stretching of spherical parts, the blank partially contacts the spherical top of the convex mold, while the rest is in a suspended, unconstrained, and free state.
Therefore, the main technological issue for stretching of such spherical parts is severe thinning at the locally contacting part, or instability and wrinkling of the curved surface.
Shallow stretching process for the flange part of a stretched product. Its stress-strain situation is similar to that of compression flanging. Due to the tangential compressive stress, it is prone to wrinkling, so the forming limit is mainly limited by compression wrinkling.
Angular stretching process on the flange part of the stretched product from the previous process, which requires the material to have good plasticity.
A stretching product exceeding the stretching limit needs to be stretched two or more times before completion. The product processed by deep stretching in the previous station is further stretched in the depth direction.
For a wide-flanged stretching part, the flange diameter is stretched to the required diameter in the first stretch, and the flange diameter remains unchanged in subsequent stretches.
For deep tapered products with h/d>0.8 and α=10°~30°, due to the large depth and deformation of the blank, only the local contact area between the blank and convex mold can transfer the forming force, making it prone to thinning and even cracking in some parts of the blank. It needs to be formed gradually through multiple transitions.
The stepped stretching method is to first stretch the blank into a stepped transition piece, whose stepped shape is tangent to the inner shape of the tapered part, and then expand it into a tapered shape.
The number of stretches and the process for the stepped transition piece is the same as for the stepped cylindrical part.
A high rectangular part formed by multiple stretching processes, which has different deformations not only from those of deep cylindrical parts but also from those of low box-shaped parts. Figure 1-46 shows the variation in the shape and size of the workpiece with stretching height during multiple stretching processes when the high rectangular box part is processed by a multi-station automatic transfer press.
Curved stretching forming is a stamping process for producing hollow products with non-straight and non-flat bottom curved surfaces by shrinking the outer flange part and elongating the inner flange part of a metal flat blank.
The left-side first-stretched product is further stretched to form the stepped bottom on the right side. The deeper part has deformation in the early stage of stretching forming, while the shallower part has deformation in the later stage of stretching. Deformation due to shear stress is easy to occur on the side wall of the step-changing parts.
Reverse stretching of a product that has undergone stretching in a previous process is another stretching method. Reverse stretching can increase radial stretching stress and have a good effect on preventing wrinkling. It can also increase the stretching coefficient of re-stretching.
Thinning stretching is different from regular stretching, as it mainly changes the thickness of the cylindrical wall of the stretched part during stretching. When the gap between the convex-curved and concave-curved dies is smaller than the thickness of the blank, the straight wall of the blank is subject to a larger and uniform compressive stress during the passage through the gap.
During stretching, the wall thickness is reduced, the wall thickness deviation is eliminated, the surface smoothness of the container is enhanced, and the precision and strength of the container are improved.
Panel products are sheet metal stampings with complex surface shapes. During the stretching process, the deformation of the blank is complex, and its forming property is no longer simple stretching forming, but rather a composite forming of stretching and deep drawing.