The stretch-forming process is a stamping process that uses a die to form a flat blank into an open hollow part. Stretching is widely used as one of the main stamping processes. Thin-walled parts of cylindrical, rectangular, stepped, spherical, conical, parabolic, and other irregular shapes can be made using the stretching process. More complex shapes can be produced when combined with other stamping and forming processes.
Stamping machines stretch products, including stretching, redrawing, reverse, and thinning.
The following 16 stamping and stretching processes are carried out using stamping equipment
Stretching of cylindrical products with flanges. The flange and the bottom are flat shapes, the cylinder side walls are axisymmetric, the deformation is evenly distributed on the same circumference, and the blank on the flange produces a deep drawing deformation.
The deformation of the flange on the blank for stretching deformation, but the deformation and deformation ratio along the contour shape changes accordingly. The greater the curvature of the part, the greater the amount of plastic deformation of the blank; conversely, the smaller the curvature of the part, the smaller the plastic deformation of the blank.
A low rectangular part is formed by stretching. When drawing, the tensile resistance at the rounded corner of the flange deformation area is greater than the tensile resistance at the straight edge, and the degree of deformation at the rounded corner is greater than the degree of deformation at the straight edge.
When the side wall of a stamped part is beveled, the side wall is suspended during the stamping process and is not attached to the die until the end of the forming process. The deformation characteristics of different sidewall parts are not identical during forming.
The billet deformation during the forming process is not a simple stretching deformation but a compound forming in which stretching and expansion deformation exist simultaneously. The blank deformation on the pressed surface is tensile (tensile stress in the radial direction and compressive stress in the tangential direction). In contrast, the deformation of the blank inside the profile (especially in the central area) is distension deformation (tensile stress in the radial and tangential directions).
With flange hemisphere drawing
Spherical parts stretching, the blank and the top of the sphere of the convex mold local contact, the rest of the majority of the emotional free state in suspension. Therefore, the main process problem of such spherical parts stretching is the serious thinning of the local contact part or the surface part of the instability of wrinkling.
The flange part of the product will be stretched through shallow processing. The stress-strain situation is similar to compression flanging. Due to the tangential compressive stress, it is easy to wrinkle, so the compression wrinkle mainly limits the forming limit.
The flange part of the product stretched in the previous process is re-stretched in an angular shape.
Stretching products that exceed the limit of stretching processing need to go through more than two times to complete the stretching. After the former station’s depth direction of stretching processing products, in the depth direction of stretching processing again. Wide flange stretching parts, the first stretching into the required flange diameter, in the subsequent re-stretching, the flange diameter remains unchanged.
h / d> 0.8, α = 10 ° ~ 30 ° deep tapered parts, due to the depth of a large, large degree of deformation of the blank, only rely on the local area of contact between the blank and the convex die to transfer the forming force, it is easy to cause excessive local thinning of the blank or even rupture, it is necessary to go through several transitions gradually forming. The step stretching method first stretches the billet into a step-shaped transition part, whose step shape is tangential to the conical part’s inner shape and expands to form a cone. The number of stretches and the process of stepped transitional parts are the same as the stretches of stepped cylindrical parts.
Multiple stretching and forming of high rectangular parts, the deformation is not only different from the stretching of deep cylindrical parts, and the deformation of low box-shaped parts is also very different. Figure 1-46 shows the change in the shape and size of the part along with the height of stretching during the multiple stretching processes when a multi-station automatic transfer press is used to process high rectangular box parts.
Surface stretching and forming so that the outer flange part of the metal flat blank shrinks, the inner flange part elongates, becoming a non-straight wall non-flat bottom curved shape of the hollow products of the stamping and forming method.
The left side of the initial stretching product is re-stretched and shaped into the right side of the step-shaped bottom. The deeper part is deformed at the beginning of the stretching process, while the shallower part is deformed later. The side walls of the step change section are susceptible to induced shear stresses and deformation.
The former process of stretching the workpiece, reverse stretching, is a kind of redrawing. The reverse drawing method can increase the radial tensile stress, for the prevention of wrinkling can receive better results. It is also possible to improve the tensile coefficient of redrawing.
Different from regular stretching, thinning stretching is mainly used to change the thickness of the cylinder wall of the stretching parts. The gap between the convex and concave die is smaller than the thickness of the blank, the straight wall part of the blank is under a larger uniform compressive stress when passing through the gap, and the wall thickness becomes thinner during the stretching process while eliminating the deviation of the container wall thickness, increasing the smoothness of the container surface and improving the accuracy and strength.
Panel products are sheet metal stamping parts with complex surface shapes. In the drawing process, the blank deformation is complex, and its forming nature is not simply drawing but deep drawing and expanding the composite forming simultaneously.