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What are the types and process requirements of metal stretching process?

November 26, 2022

The metal stretching process shall be combined with the actual situation, and a reasonable process plan shall be selected from the aspects of quality, strength, environment and production, so as to reduce the process investment as much as possible on the basis of ensuring that the production meets the requirements of the drawings.
Type and process requirements of metal stretching process:

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1、 Type of metal drawing process
(1) Cylinder stretching: stretching of cylinder with flange. The flange and the bottom are straight, and the cylinder is axisymmetric. The deformation is uniform on the same circumference, and the blank on the flange has deep drawing deformation.
(2) Elliptical stretching: the blank on the flange is stretched and deformed, but the deformation amount and deformation ratio change accordingly. The greater the curvature, the greater the deformation of the part of the blank; The deformation of the part with smaller curvature is smaller.

(3) Rectangular stretch: a low rectangular piece formed by one stretch. When stretching, the tensile resistance at the fillet of the flange deformation area is greater than that at the straight edge, and the deformation degree at the fillet is greater than that at the straight edge.
(4) Hilly stretching: the side wall is suspended in the process, and it will not stick to the mold until the end of forming. The deformation characteristics of different parts of the side wall are not completely the same during forming.
(5) Dune shaped drawing: the blank deformation of the mound shaped cover plate in the forming process is not a simple tensile deformation, but a composite forming in which both tensile and bulging deformation exist.
(6) Hemispherical drawing with flange: when the spherical part is stretched, the blank partially contacts the spherical top of the punch, and most of the rest is in a suspended state.

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(7) Flange stretching: stretch the flange slightly. The stress and strain situation is similar to that of compression flanging.
(8) Edge stretching: angle re stretching is carried out for the flange, and the material is required to have good deformation.
(9) Deep stretching: it can be completed after more than two times of stretching. The wide flange stretch piece is stretched to the required flange diameter at the first stretch, and the flange diameter remains unchanged at the subsequent stretch.
(10) Conical drawing: due to the large degree of deep deformation, the deep conical parts are very easy to cause local excessive thinning and even cracking of the blank, and need to be gradually formed through multiple transitions.


(11) Rectangular re drawing: the deformation of high rectangular parts formed by multiple drawing is not only different from that of deep cylindrical parts, but also different from that of low box parts.
(12) Curved surface forming: Curved surface stretch forming, which makes the outer flange part of the metal plate blank shrink and the inner flange part extend, becoming a stamping forming method of curved surface shape with non straight wall and non flat bottom.
(13) Step stretch: re stretch the initial stretch to form a stepped bottom. The deeper part will deform at the initial stage of stretch forming, and the shallower part will deform at the later stage of stretch forming.
(14) Reverse stretching: reverse stretching is a kind of re stretching for the workpieces stretched in the previous process. The reverse stretching method can increase the radial tensile stress, and has a good effect on preventing wrinkling. It is also possible to increase the tensile coefficient of re stretching.


(15) Thinning stretching: different from ordinary stretching, thinning stretching mainly changes the thickness of the cylinder wall of the stretching piece during the stretching process.
(16) Panel stretching: the surface shape of the panel is complex. In the drawing process, the blank deformation is complex, and its forming property is not simple drawing forming, but the compound forming of deep drawing and bulging.

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2、 Scheme of metal drawing process
(1) According to the workpiece drawing, analyze the shape characteristics, size, accuracy requirements, raw material size and mechanical properties of the workpiece, and combine the available equipment, batch and other factors. Good drawing process shall ensure less material consumption, fewer processes and less equipment occupied.
(2) The calculation of main process parameters is based on the analysis of stamping process, to find out the characteristics and difficulties of the process, and to propose various possible stretching process schemes according to the actual situation, including process nature, process number, process sequence and combination mode. Sometimes there may be multiple feasible process schemes for the same workpiece. Generally, each scheme has its advantages and disadvantages. Comprehensive analysis and comparison shall be carried out to determine the best suitable scheme.

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(3) Process parameters refer to the data on which the process plan is based, such as various forming coefficients (drawing coefficient, bulging coefficient, etc.), part development dimensions, and various stresses. There are two cases of calculation. The first is that the process parameters can be calculated more accurately, such as the material utilization rate of the part layout, the workpiece area, etc; The second is that the process parameters can only be calculated approximately, such as general bending or deep drawing forming force, blank development size of complex parts, etc. The determination of such process parameters is generally based on the rough calculation of empirical formulas or charts, and some need to be adjusted through tests.
(4) The stretching equipment shall be selected according to the nature of the process to be completed and the force and energy characteristics of various equipment, the required deformation force, size and other major factors, and the equipment type and tonnage shall be reasonably selected in combination with the existing equipment.


3、 Selection method of stretching oil
(1) As silicon steel plate is a material that is easy to stretch, low viscosity stamping oil will be used to prevent scratches on the premise of easy cleaning of the finished workpiece.
(2) When selecting drawing oil for carbon steel plate, the better viscosity should be determined according to the process difficulty and degreasing conditions.
(3) Because of the chemical reaction with chlorine additives, it should be noted that chlorine type drawing oil may cause white rust when selecting drawing oil for galvanized steel plate, while sulfur type drawing oil can avoid rust.
(4) Stainless steel is easy to harden, so it is required to use drawing oil with high oil film strength and good sintering resistance. Generally, the drawing oil containing sulfur and chlorine compound additives is used to ensure the extreme pressure performance and avoid problems such as burrs and cracks on the workpiece.