Drawing, also known as deep drawing, cupping, or calendering, is one of the stamping processes. It refers to a stamping process in which a die is used to press a flat blank of a certain shape obtained after blanking into various open hollow parts, or to reduce the diameter and increase the height of an open hollow blank. The drawing process can be used to manufacture thin-walled parts in cylindrical, stepped, conical, spherical, box-type, and other irregular shapes. When combined with other stamping forming processes such as flanging, bulging, flaring, and necking, it can also produce parts with extremely complex shapes. This process has a very wide range of applications and plays a role in various automotive covers, panels, housings for electrical instruments, and numerous daily necessities. Therefore, in the production processes of the automotive, aircraft, communications, electrical, instrumentation, electronics, and other industrial sectors, the drawing process occupies a considerably important position.
However, the drawing process still differs considerably from other stamping processes. The drawing process places high technical and experience requirements on die designers, die fitters' experience and skills, product structure analysis, process parameter calculation, drawing process sequencing, die manufacturing precision, raw material performance analysis, die debugging, press selection, and lubricant selection. This is precisely why companies in the same industry that can fully excel in drawn products are, compared with other ordinary stamping companies, as rare as the feathers of a phoenix and can be counted on one's fingers.
I. Types of Metal Drawing Processes (1) Cylindrical drawing: drawing of a flanged cylinder. The flange and bottom are both flat surfaces, the cylinder is axisymmetric, deformation is uniform on the same circumference, and the blank on the flange undergoes drawing deformation. (2) Elliptical drawing: the blank on the flange undergoes drawing deformation, but the amount and ratio of deformation vary accordingly along the contour shape. The greater the curvature, the greater the deformation of the blank in that part; the smaller the curvature, the smaller the deformation of the blank in that part. (3) Rectangular drawing: low rectangular parts formed by a single drawing. During drawing, the drawing resistance at the rounded corners of the flange deformation zone is greater than that at the straight edges, and the degree of deformation at the rounded corners is greater than that at the straight edges. (4) Mountain-shaped drawing: the side walls are unsupported during the stamping process and only contact the die at the end of forming. During forming, the deformation characteristics of different parts of the side walls are not entirely the same. (5) Hill-shaped drawing: during the forming of hill-shaped cover parts, the blank deformation is not simple drawing deformation but a composite forming in which drawing and bulging deformation exist simultaneously. (6) Flanged hemispherical drawing: during the drawing of spherical parts, the blank makes local contact with the spherical top of the punch, while most of the remainder is in an unsupported state. (7) Flange disk drawing: shallow drawing of the flange disk portion. Its stress-strain condition is similar to that of compression flanging. (8) Edge drawing: angular redrawing of the flange portion, requiring the material to have good formability. (9) Deep drawing: requires more than two drawing operations to complete. For wide-flange drawn parts, the required flange diameter is drawn in the first drawing operation, and the flange diameter remains unchanged in subsequent drawing operations. (10) Conical drawing: deep conical parts, due to the large degree of depth deformation, are very likely to cause local excessive thinning or even fracture of the blank, and require multiple transitional forming steps to gradually take shape. (11) Rectangular redrawing: high rectangular parts formed by multiple drawing operations. Their deformation differs not only from that of deep cylindrical parts but also greatly from that of low box-shaped parts. (12) Curved surface forming: a stamping forming method for curved surface drawing in which the outer flange portion of a flat metal blank contracts and the inner flange portion elongates, resulting in a curved surface shape with non-straight walls and a non-flat bottom. (13) Stepped drawing: redrawing the initial draw into a stepped bottom. The deeper portion undergoes deformation in the early stage of drawing forming, while the shallower portion undergoes deformation in the later stage of drawing. (14) Reverse drawing: reverse drawing of a workpiece drawn in the previous operation is a type of redrawing. The reverse drawing method can increase radial tensile stress and can achieve better results in preventing wrinkling. It may also increase the redrawing coefficient. (15) Ironing: unlike ordinary drawing, ironing mainly changes the wall thickness of the drawn part's cylindrical wall during the drawing process. (16) Panel drawing: the surface shape of panels is complex. During the drawing process, blank deformation is complex, and its forming nature is no longer simple drawing forming but a composite forming in which drawing and bulging exist simultaneously.
II. Metal Drawing Process Plans (1) Design review: Our engineers review the part design in detail to ensure it is suitable for deep drawing stamping. This includes in-depth analysis of part dimensions, material, drawing ratio, and required tolerances. Based on the workpiece drawings, the shape characteristics, dimensions, precision requirements, raw material dimensions and specifications, and mechanical properties of the workpiece are analyzed, combined with factors such as available equipment specifications and production batch size. Good drawing manufacturability should ensure low material consumption, a small number of operations, few pieces of equipment occupied, simple die structure with long service life, stable quality, and simple operation. (2) DFM analysis: calculation of main process parameters. On the basis of stamping manufacturability analysis, the characteristics and difficulties of the process are identified, and various possible drawing process plans are proposed according to actual conditions, including operation nature, number of operations, operation sequence, and combination methods. Sometimes multiple feasible process plans may exist for the same workpiece, and each plan usually has its own advantages and disadvantages. A comprehensive analysis and comparison should be carried out from aspects such as product quality, production efficiency, equipment occupancy, ease of die manufacturing and service life, production cost, and ease and safety of operation, so as to determine the best plan suitable for existing production conditions. (3) Calculation of process parameters: process parameters refer to the data on which the formulation of the process plan is based, such as various forming coefficients (drawing coefficient, bulging coefficient, etc.), part unfolded dimensions, and various stresses. There are two cases in calculation. The first is that process parameters can be calculated relatively accurately, such as material utilization rate in part nesting and workpiece area. The second is that process parameters can only be calculated approximately, such as general bending or drawing forming force and the blank unfolded dimensions of complex parts. Determining such process parameters is generally based on empirical formulas or charts for rough calculation, and some need to be adjusted through testing. (4) CAE simulation analysis: CAE simulation analysis software is used to create a prototype of the part. Before the production process begins, the prototype is run through extensive operation simulations to identify any design issues. Blank unfolding for drawn and deep-drawn parts — schematic diagram of simulation analysis (5) Equipment selection: drawing equipment is selected according to the nature of the operations to be completed and the force and energy characteristics of various equipment, considering major factors such as the required deformation force and dimensions, and in combination with existing equipment conditions, so as to reasonably select the equipment type and tonnage.
III. Methods for Selecting Drawing Oils (1) Since silicon steel sheet is a material that is relatively easy to draw, low-viscosity stamping oil is selected to facilitate cleaning of the finished workpiece while preventing scratches. (2) For carbon steel sheet, the first thing to note when selecting drawing oil is the viscosity of the drawing oil. The optimal viscosity is determined according to process difficulty and degreasing conditions. (3) Because it reacts chemically with chlorine-based additives, when selecting drawing oil for galvanized steel sheet, attention should be paid to the issue that chlorine-type stamping oil may cause white rust, while the use of sulfur-type drawing oil can avoid the rust problem. (4) Stainless steel is a material prone to work hardening and requires drawing oil with high oil film strength and good anti-seizure performance. Drawing oil containing sulfur-chlorine composite additives is generally used to ensure extreme pressure performance while avoiding problems such as burrs and fracture on the workpiece.
The above are the common types of metal drawing processes. With the rapid development of precision drawing processes, fields such as machinery manufacturing, automotive R&D, and aerospace will benefit even more from them.