Deep Drawing Process for Sheet Metal
Deep Drawing Process for Sheet Metal
Deep drawing is a sheet forming process in which in its simplest form, a cylindrical shape or alike is produced from a thin disc of sheet metal by subjecting it to a compressive force (while it is held between a die and blank holder) through a circular punch which mainly work on the blank thickness. Deep drawing process is used to produce containers from flat circular blanks. The central portion of sheet of blank is subjected to pressure applied by punch into a die opening to get a sheet metal of needed shape without folding the corners. This normally needs the use of presses normally having a double action for blank holding force and punch force.
Deep drawing process involves several types of forces and deformation modes, such as tension in the wall and the bottom, compression and friction in the flange, bending at the die radius, and straightening in the die wall. The process is capable of forming beverage cans, sinks, cooking pots, ammunition shell containers, pressure vessels, and auto body panels and parts etc.
Any steel which can be processed into sheet form by a cold-rolling process is to be sufficiently ductile to be capable of deep drawing. Both hot-rolled and cold-rolled sheet products are used in deep-drawing process. The cold-work effects introduced during processing of the sheet products for deep-drawing applications are to be removed (e.g., by annealing), and the as-delivered rolled steel coils are to be free of any aging. This implies that the aluminum-killed drawing quality steel, for example, is preferred over rimmed steel.
Deep drawing can be defined as the combined tensile and compression deformation of a sheet to form a hollow body, without an intentional change in sheet thickness. It is one of the frequently applied methods in sheet metal forming. Deep drawing operation is based on producing engineering parts with specific shapes through major plastic deformation of flat metal sheets. An external force on a metal sheet does this plastic deformation. This external force has to be large enough to place the material in the plastic zone and to ensure that after displacing the external force, the metal part does not spring back or elastic deform again. The final quality of the parts produced through this operation is based on the final wall thickness and being wrinkle-free and fracture-free.
The term deep drawing implies that some drawing-in of the flange metal occurs and that the formed parts are deeper than can be achieved by simply stretching the metal over a die. Clearance between the male punch and the female die is closely controlled to minimize the free span so that there is no wrinkling of the sidewall. This clearance is sufficient to prevent ironing of the metal being drawn into the side-wall of the drawn part. If ironing of the walls is to be part of the process, it is done in operations subsequent to deep drawing. Suitable radii in the punch bottom to side edge, as well as the approach to the die opening, are necessary to allow the sheet metal to be formed without tearing.
In the majority of the deep drawing operations, the part has a solid bottom to form a container and a retained flange which is trimmed later in the processing. In some cases, the cup shape is fully drawn into the female die cavity, and a straight-wall cup shape is ejected through the die opening. For controlling the flange area and to prevent wrinkling, a hold-down force is applied to the blank to keep it in contact with the upper surface of the die. Presses used for deep-drawing operations can be either hydraulic or mechanical, but hydraulic presses are preferred because of better control of the rate of punch travel.
Deep drawing of sheet metal is used to form parts by a process in which a flat blank is constrained by a blank-holder while the central portion of the sheet is pushed into a die opening with a punch to draw the metal into the desired shape without causing wrinkles or splits in the drawn part. This normally needs the use of presses having a double action for hold-down force and punch force. The mechanics of deep drawing of a conical cup are shown in Fig 1, which shows the complexity of the process.
Fig 1 Mechanics of deep drawing of a conical cup
After the deep-drawing operation, ductility can be returned to that of the original sheet by in-process annealing, if necessary. In several cases, however, steel which has been deep drawn in a first operation can be further reduced in cup diameter by additional drawing operations, without the need for intermediate annealing. The properties which are considered to be important in sheet products designed for deep drawing include (i) composition, with a minimum quantity of inclusions and residual elements contributing to better drawability, (ii) mechanical properties, of which the elongation as measured in a tension test, the plastic strain ratio ‘r’, and the strain-hardening exponent ‘n’ are of main importance (the strength of the final part as measured by yield strength is also to be considered, but this is more a function of the application than forming by deep drawing), and (iii) physical properties, including dimensions, die geometry, modulus of elasticity, and any special requirements for maintaining shape after forming.
Plastic strain ratio ‘r’ is a measure of the ability of a sheet metal to resist thinning or thickening when subjected to a tensile or compressive force. It is typically advantageous if the material reduces in area a minimal quantity when subject to this force, meaning a good drawing material has a high ‘r’ value. The strain hardening exponent ‘n’ determines how the metal behaves when it is being formed. Materials which have higher ‘n’ values have better formability than those with low ‘n’ values. As metals work harden, their remaining capacity for work hardening decreases.
Once a work-piece has been deep drawn into a suitable form, it can be further processed to develop additional shape. The first shape is normally a round cylinder, or a modification of this, e.g., a square box with rounded corners. This latter shape is related to the cylinder in which the four corners are basically quarter segments with straight walls between each segment. For small cylinders, a relationship between the diameter of a circular blank and the bottom diameter of the cup shape to be formed is sometimes used to measure the deep drawability. The deep drawing cup test is a sheet metal test method in which a round blank is punched out and then formed into a cup. The details of this test are given in ISO (International Organization for Standardization) standard ISO 11531:2022, Metallic materials – Sheet and strip – Earing test.
For ductile low-carbon steel sheets, a 100 mm circular blank can be formed in a single draw. Increased plastic strain ratio ‘r’ and ductility allow larger blanks to be drawn successfully, the limit is reached when the bottom punches out, rather than forming a cup shape. The blank diameter divided by the punch diameter gives the limiting draw ratio (LDR), the value of which for the given example is 2.
Of the normally formed metals, brass and austenitic stainless steels show high values of LDR (up to 2.25). A few samples have reached a value of 2.5 to produce a cup with a side-wall height of nearly 65 mm. This is possible, although the total length of the cup cross-section is 180 mm, which is more than the blank diameter because of the deep-drawing forces. The interstitial-free (IF) steels, which have average ‘r’ values of 2.5 or higher, can be deep drawn to LDR values near 2.5. With these extremely deep draws, side-wall delayed splitting is to be prevented by such means as stress-relief annealing immediately following the drawing operation.
The thickness of the work-piece does not change appreciably in deep drawing and hence, the surface area of the final part is around the same as that of the initial blank. As the metal in the flange area is drawn into the die opening over the approach radius, it is subjected to radial-tension and, simultaneously, to circumferential compression. This explains why a 125 mm diameter blank with a surface area of 126 square centimetre (sq-cm) can form a cup shape 65 mm deep which has a total surface area of around 121 sq-cm.
With proper balance among the punch force, the hold-down force, and the strength of the sheet metal being formed, a cup shape can be developed. At the start of this process, the metal in the free area between the punch bottom and the flange hold-down is stretched and wrapped over the nose of the punch and the approach radius of the die. During this stretching, strain hardening strengthens the metal. If it is not capable of such strengthening or if its strength at any location is exceeded at any time during the forming, the bottom of the cup shape breaks out. Contributing to this strengthening is a high ‘r’ value, which is a measure of resistance to through thickness changes in the sheet metal.
If the sheet metal has a high resistance to thinning and thickening, the bottom radius and the upper side-wall areas remain close to their original thickness, and the radial and circumferential strains achievable in the drawn-in flange are increased to accommodate the deep-drawing process. After the bottom has been formed, the clearance between the punch and die is such that the metal in the cup side is free to move without excessive rubbing on the die walls. It has been found that slight roughening of the punch radius and minimizing the lubrication of this area contribute to deeper drawability, however, the die opening is to be smooth and well lubricated with a suitable drawing compound.
Fundamentals of drawing – A flat blank is formed into a cup by forcing a punch against the centre portion of a blank which rests on the die ring. The progressive stages of metal flow in drawing a cup from a flat blank are shown schematically in Fig 2. During the first stage, the punch contacts the blank (Fig 2a), and metal section 1 is bent and wrapped around the punch nose (Fig 2b). At the same time and in sequence, the outer sections of the blank (2 and 3, Fig 2) move radially toward the centre of the blank until the remainder of the blank has bent around the punch nose and a straight-wall cup is formed (Fig 2c and Fig 2d). During drawing, the centre of the blank (punch area, Fig 2a) is basically unchanged as it forms the bottom of the drawn cup. The areas which become the side-wall of the cup (1, 2, and 3 in Fig 2) change from the shape of annular segments to longer parallel side cylindrical elements as they are drawn over the die radius.
Fig 2 Drawing of a cup from a flat blank
Metal flow can occur until all the metal has been drawn over the die radius, or a flange can be retained. A blank holder is used in a draw die to prevent the formation of wrinkles as compressive action rearranges the metal from flange to the side-wall. Wrinkling starts because of some lack of uniformity in the movement or because of the resistance to movement in the cross-section of the metal. This happens since metal under compression has a high resistance to flow into the die cavity. A blank holder force sufficient to resist or compensate for this non-uniform movement prevents wrinkling. Once a wrinkle starts, the blank holder is raised from the surface of the metal so that other wrinkles can form easily. The force needed to hold the blank flat during drawing of cylindrical shells varies from practically zero for relatively thick blanks to around one-third of the drawing load for a blank having 0.76 mm thickness. Thinner blanks frequently need proportionally higher blank holder force.
Conditions for drawing without a blank-holder depend on the ratio of the supported length of the blank to its thickness, the quantity of reduction from blank diameter to cup diameter, and the ratio of blank diameter to the work-piece thickness. For thick sheets, the maximum reduction of blank diameter to cup diameter in drawing without a blank holder is around 25 %. This ratio approaches zero for thin sheet. If a blank holder is used, the maximum reduction is increased to around 50 % for metals of maximum drawability and 25 % to 30 % for metals of marginal drawability in the same equipment.
Drawability – In an idealized forming operation, i.e., one in which drawing is the only deformation process which occurs, the blank holder force is just sufficient to permit the sheet material to flow radially into the die cavity without wrinkling. Deformation takes place in the flange and over the lip of the die. No deformation occurs over the nose of the punch. The deep-drawing process can be thought to be as analogous to wire drawing in that a large cross section is drawn into a smaller cross section of higher length.
The drawability of a metal depends on two factors namely (i) the ability of the material in the flange region to flow easily in the plane of the sheet under shear, (ii) the ability of the side-wall material to resist deformation in the thickness direction.
The punch prevents side-wall material from changing dimensions in the circumferential direction, hence, the only way the side-wall material can flow is by elongation and thinning. Hence, the ability of the side-wall material to withstand the load imposed by drawing down the flange, is determined by its resistance to thinning. High flow strength in the thickness direction of the sheet is desirable.
Taking both of these factors into account, it is desirable in drawing operations to maximize material flow in the plane of the sheet and to maximize resistance to material flow in a direction perpendicular to the plane of the sheet. Low flow strength in the plane of the sheet is of little value, if the work material also has low flow strength in the thickness direction.
The flow strength of sheet metal in the thickness direction is difficult to measure, but the plastic-strain ratio ‘r’ compares strengths in the plane and thickness directions by determining true strains in these directions in a tension test. For a given metal strained in a particular direction, ‘r’ is a constant expressed by the equation 1 which is ‘r = ew/et’, where ‘ew’ is the true strain in the width direction, and ‘et’ is the true strain in the thickness direction.
Sheet metal is anisotropic, i.e., the properties of the sheet are different in different directions. It is, hence, necessary to use the average of the strain ratios measured parallel to, transverse to, and 45-degree to the rolling direction of the sheet to achieve an average strain ratio ‘ra’, which is expressed by the equation 2 which is ‘ra = rL + 2r45 + rT/4’, where ‘rL’ is the strain ratio in the longitudinal direction, ‘r45’ is the strain ratio measured at 45-degree to the rolling direction, and ‘rT’ is the strain ratio in the transverse direction. If flow strength is equal in the plane and thickness directions of the sheet, then ‘ra’ = 1. If strength in the thickness direction is higher than average strength in the directions in the plane of the sheet, then ‘ra’ is higher than 1. In this latter case, the material resists uniform thinning. Normally, the higher the ‘ra’ value, the deeper is the draw which can be achieved (Fig 3).
Fig 3 Effect of average strain value on drawability
Since the average strain ratio ‘ra’ gives the ratio of average flow strength in the plane of the sheet to average flow strength normal to the plane of the sheet, it is a measure of normal anisotropy. Variations of flow strength in the plane of the sheet are termed planar anisotropy. The variation in strain ratio in different directions in the plane of the sheet, ‘ra’ higher than 1, is a measure of planar anisotropy, and ‘ra’ can be expressed by the equation 3 which is ‘ra = rL + 2r45 = rT/4’, where rL is the variation in strain ratio, and the other terms are as defined in equation 2. A completely isotropic material has ‘ra’ = 1 and delta ‘r’ = 0. These two parameters are convenient measures of plastic anisotropy in sheet materials.
Drawing ratios – Drawability can also be expressed in terms of LDR or percentage of reduction based on results of deep-drawing cup testing. The LDR is the ratio of the diameter ‘D’ of the largest blank which can be successfully drawn to the diameter of the punch ‘d’. It is given by the equation 4 which is ‘LDR = D/d’. Percentage of reduction is then be defined by the equation 5 which is ‘Percentage of reduction = [100(D – d)]/D’.
Defects in drawing – A number of defects can occur in deep-drawn parts. Fig 4 shows the type of defects which can be found after drawing cylindrical cups. The description of such defects is given after the figure.
Fig 4 Defects in deep drawn cylindrical cups
Wrinkling in the flange – Wrinkling in deep drawn parts consists of a series of ridges which form radially in the flange because of compressive forces.
Wrinkling in the wall – It occurs when ridges in the flange are drawn into the vertical wall of the cup.
Tearing – it occurs near the base of the drawn cup and results from high stresses in the vertical wall which cause thinning and failure of the metal at that location.
Earing – It occurs in deep-drawn parts made from anisotropic materials. Because of planar anisotropy, the sheet metal can be stronger in one direction than in other directions in the plane of the sheet. This causes the formation of ears in the upper edge of a deep-drawn cup even when a circular blank is used. In practice, enough extra metal is left on the drawn cup so that the ears can be trimmed.
Surface scratches – These scratches occur in a drawn part. if the punch and die surfaces are not smooth or if lubrication is not enough.
Presses – Sheet metal is drawn in either hydraulic or mechanical presses. Double-action presses are needed for the majority of deep drawing operations, since a more uniform blank holding force can be maintained for the entire stroke than is possible with a spring-loaded blank holder. Double-action hydraulic presses with a die cushion are frequently preferred for deep drawing because of their constant drawing speed, stroke adjustment, and uniformity of clamping pressure.
Regardless of the source of power for the slides, double-action straight-side presses with die cushions are best suited for deep drawing operations. Straight-side presses provide a wide choice of tonnage capacity, bed size, stroke, and shut height.
Factors influencing press selection – Drawing force requirements, die space, and length of stroke are the most important considerations in selecting a press for deep drawing. The condition of the crank-shaft, connection bearings, and gibs (press components which guide the reciprocating motion of the ram to maintain superior squareness and parallelism throughout the stroke) is also a factor in press selection.
Drawing force – The needed drawing force, as well as its variation along the punch stroke, can be calculated from theoretical equations based on plasticity theory or from empirical equations. The maximum drawing force ‘Fdmax’ needed to form a round cup can be expressed by the empirical relation given by the equation 6 which is ‘Fdmax = n x pi x d x t x su’, where ‘su’ is the tensile strength of the blank material (in megapascal, MPa), ‘d’ is the punch diameter (in millimetres, mm), ‘t’ is the sheet thickness (mm), ‘pi’ is 3.1416, and ‘n’ = ‘sd’/’su’, the ratio of drawing stress to tensile strength of the work material. Equation 6 yields Fdmax in kilo-newton (kN), depending on the other units used.
The force needed to draw a rectangular cup can be calculated using equation 7 which is ‘Fdmax = t x su x [(2pi x R x ka) + (L x kb)], where R is the corner radius of the cup (in mm), L is the sum of the lengths of straight sections of the sides (in mm), ka and kb are constants, and the other quantities are as defined in equation 6. When blank holder cylinders are mounted on the main slide of the press, the blank holder force is to be added to the calculated drawing force. When a die cushion is used to eject work-pieces, the main slide works against this force and hence, such set-ups need more drawing force than is calculated using equation 6 or equation 7.
In toggle draw presses, the blank holder force is taken on the rocker shaft bearings in the press frame, so that the crank-shaft bearings sustain only the drawing load. In other types of presses, both the drawing and blank holding loads are on the crank-shaft, and allowances are made when computing press capacity. For round work, the allowance for blank holding is to be 30 % to 40 % of the drawing force. For large rectangular work, the drawing force is relatively lower than that for round work, but the blank holding force can be equal to the drawing force. Where stretching is involved and the blank is to be gripped tightly around the edge (and a draw bead is not permissible), the blank holding force can be two or three times the drawing force.
Blank size – It governs the size of the blank holder surfaces. Some presses with sufficient force cannot be considered for deep drawing, since the bed size and shut height are inadequate. Blank size is determined by calculating the surface area of the part and then converting this area into a flat blank. For a deep-drawn round cup, the surface area of the cup is converted into a flat blank diameter.
Draw depth – Both the length of stroke and the force needed at the beginning of the working portion of the stroke are important considerations. Parts which have straight walls can be frequently drawn through the die cavity and then stripped from the punch and ejected from the bottom of the press. Even under these ideal conditions, the minimum stroke is equal to the sum of the length of the drawn part, the radius of the draw die, the sheet thickness, and the depth of the die to the stripping point, in addition to some clearance for placing the blank in the die.
Work-pieces with flanges or tapered walls are to be removed from the top of the die. In drawing these work-pieces, the minimum press stroke is twice the length of the drawn work-piece, plus clearance for loading the die. In an automatic operation using progressive dies or transfer mechanisms, at least one-half the stroke is to be reserved for the work-piece feed, since the tooling is needed to clear the part before feeding begins for the next stroke. For automatic operation, it is a normal practice to allow a press stroke of four times the length of the drawn work-piece. Hence, some equipment is not suited for automatic operation, or it is necessary to use manual feed with an automatic unloader, or conversely, because of a shortage of suitable presses.
Slide velocity – When selecting a press, it is also necessary to check slide velocity through the working portion of the stroke.
Means of holding the blank – Double-action presses with a punch slide and a blank holder slide are preferred for deep drawing. Single action presses with die cushions (pneumatic or hydraulic) can be used but are less suitable for drawing complex parts. Draw beads are incorporated into the blank holder for drawing parts needing higher restraint of metal flow than can be achieved by using a plain blank holder or for diverting metal flow into or away from specific areas of the part.
Press selection versus availability – The ideal press equipment for a specific job is frequently not available. This makes it necessary to design tools and to choose product forms of work metal as per the available presses and supplementary equipment. For example, if available presses are not adequate for drawing large work-pieces, the sequence of production is to be completely changed. It can be necessary to draw two sections and weld them together. In addition, operations which can otherwise be combined, such as blanking, piercing, drawing, and trimming, are to be performed singly in separate presses.
On the other hand, some producers have placed more than one die in a single press because of the availability of a large press and the shortage of smaller presses. This procedure can cause lower production since all blanks are to be positioned before the press can be operated. However, storage of partly formed work-pieces and additional handling between press operations are eliminated. Where several small dies are used to reduce overall tool cost, there is economic justification for the use of small capacity presses. If small presses are not available, it is frequently more economical to use compound dies. This is particularly true if overall part production is likely to exceed original estimates.
The availability of auxiliary equipment can also influence the type of press and tooling needed. For example, if equipment is available for handling coils, plans are to be made accordingly. However, if coil-handling equipment is not available and straight lengths of sheet or strip are to be processed, a compatible tooling procedure is to be used, even though it cannot be the most economical procedure.
Dies – Dies used for drawing sheet metal are normally one of the basic types consisting of (i) single-action dies, (ii) double-action dies, (iii) compound dies, (iv) progressive dies, and (v) multiple dies with transfer mechanism. Dies used can also be some modification of these types. Selection of the die depends largely on part size, severity of draw, and quantity of parts to be produced. Fig 5 shows three types of simple dies and a six stations progressive die.
Fig 5 Types of dies
Single-action dies (Fig 5a) are the simplest of all drawing dies and have only a punch and a die. A nest or locator is provided to position the blank. The drawn part is pushed through the die and is stripped from the punch by the counter-bore in the bottom of the die. The rim of the cup expands slightly to make this possible. Single-action dies can be used only when the forming limit permits cupping without the use of a blank holder.
Double-action dies have a blank-holder. This permits higher reductions and the drawing of flanged parts. Fig 5b shows a double-action die of the type used in a double-action press. In this design, the die is mounted on the lower shoe, the punch is attached to the inner or punch slide, and the blank holder is attached to the outer slide. The pressure pad is used to hold the blank firmly against the punch nose during the drawing operation and to lift the drawn cup from the die. If a die cushion is not available, springs or air or hydraulic cylinders can be used. However, they are less effective than a die cushion, especially for deep draws.
Fig 5c shows an inverted type of double-action die, which is used in single-action presses. In this design, the punch is mounted on the lower shoe and the die on the upper shoe. A die cushion can supply the blank holding force, or springs, or air or hydraulic cylinders are incorporated into the die to supply the necessary blank holding force. The drawn cup is removed from the die on the upstroke of the ram, when the pin like extension of the knock-out strikes a stationary knock-out bar attached to the press frame.
Compound dies combine several operations in a single die. They are practical to be used when the initial cost is warranted by production demands. Blanking and drawing are two operations normally placed in compound dies. With compound dies, work-pieces can be produced several times as fast as by the simple dies shown in Fig 5.
Progressive dies are used when the initial cost and length of bed needed for dies normally limit their application to relatively small work-pieces. Fig 5 shows a typical six-station progression for making small shell-like work-pieces on a mass production basis. However, larger parts, such as liners for automobile head-lights, have been drawn in progressive dies. The total number of parts to be produced and the production rate frequently determine whether or not a progressive die is to be used when two or more operations are needed.
There are, however, some practical considerations which can rule against a progressive die, regardless of quantity namely (i) the work-piece is to remain attached to the scrap skeleton until the final station, without hindering the drawing operations, (ii) drawing operations are to be completed before the final station is reached, (iii) in deep drawing, it is sometimes difficult to move the work-piece to the next station, (iv) if the draw is relatively deep, stripping is frequently a problem, and (v) the length of press stroke is to be more than twice the depth of draw.
Assuming that a progressive die can be used to make acceptable drawn parts, cost per piece is normally the final consideration. Progressive-die drawing is normally considered to be economical if savings in material and man-power can pay for the die in one year. Normally, the savings achieved by the use of a progressive die results from decreased man-power.
Multiple dies, in conjunction with transfer mechanisms, are frequently used instead of progressive dies for the mass production of larger parts. Multiple dies and transfer mechanisms are practical for a wider range of work-piece sizes than progressive dies are. Although the eye-let type transfer method is the most widely used for making parts less than 25 mm in diameter, transfer dies are practical for much larger work-pieces. The seven-station operation for making the 165 mm outside diameter cylindrical shell shown in Fig 6 represents a typical sequence for the transfer-die method. The work-piece is mechanically transferred from one die to the next. One advantage of the transfer-die method, as opposed to the progressive-die method, is the higher flexibility permitted in processing procedure, mainly since in transfer dies the work-piece does not remain attached to the scrap skeleton during deep drawing. Because of this, pre-cut blanks can be drawn by the transfer method.
Fig 6 Seven-station drawing and piercing station and ways of reducing the needed drawing forces
Preforms can also be used as blanks. For example, oil pans for automobiles are blanked and partly drawn in a compound die and then finish formed, pierced, and trimmed by the transfer method. Dies for producing a given part normally cost more for the transfer-die method than for a separate-die operation but around the same as for a progressive-die operation. The cost of adapting the transfer unit to the part is not included in the die cost. Similarly, the production rate for the transfer method is normally higher than that for a single-die operation but 10 % to 25 % less than that for drawing in a progressive die. Several parts can be produced equally well by all of these methods. Under these conditions, tool cost, rate of production, and total quantity of parts to be drawn determine the choice of procedure.
The selection of material for dies and punches for drawing sheet metal depends on work metal composition, work-piece size, severity of the draw, quantity of parts to be drawn, and tolerances and surface finish specified for the drawn work-pieces. For meeting the wide range of needs, punch and die materials ranging from polyester, epoxy, phenolic, or nylon resins to highly alloyed tool steels with nitrided surfaces, and even carbide, are used.
Effects of process variables in deep drawing – The process parameters which affect the success or failure of a deep drawing operation include punch and die radii, punch-to-die clearance, press speed, lubrication, and type of restraint of metal flow used (if any). Material variables, such as sheet thickness and anisotropy, also affect deep drawing.
As the blank is struck by the punch at the start of drawing, it is wrapped around the punch and die radii. The stress and strain which develop in the work-piece are similar to those developed in bending, with an added stretching component. The bends, once formed, have the radii of the punch and die corners. The bend over the punch is stationary with reference to both punch and the shell wall. The bend over the die radius, however, is continuously displaced with reference to both the punch radius and the blank, and it also undergoes a gradual thickening as the shell is drawn.
The force needed to draw the shell at the intermediate position has a minimum of three components namely (i) the force needed for bending and unbending the metal flowing from the flange into the side-wall, (ii) the force needed for overcoming the frictional resistance of the metal passing under the blank holder and over the die radius, and (iii) the force needed for circumferential compression and radial stretching of the metal in the flange.
Because of the variation in metal volume and in resistance to metal flow, the punch force increases rapidly, passes through a maximum, and gradually decreases to zero as the edges of the flange approach and enter the die opening and pass into the shell wall. With the cup diameter remaining constant, the maximum press load and the length of stroke needed to draw the cup depend on the size of the blank. The punch force-stroke relations for drawing blanks of different diameters from brass sheet 1.5 mm thick, using a 50 mm diameter punch are shown in Fig 7.
Fig 7 Force-stroke relations and effect of die radius on punch force
Under the conditions shown in Fig 7, during cupping, the shell bottom is subjected to tensile stress in all directions, while the lower portions of the shell wall, particularly the radius portion connecting the bottom with the wall, are mainly subjected to longitudinal tension. The stress in the metal being drawn into the shell wall consists of combined compressive stress and tensile stress. Separation of the shell bottom from the wall is likely if a reduction is made which needs a force higher than the strength of the shell wall near the bottom (Fig 7).
The punch and die radii and percentage of reduction determine the load at which the bottom of the shell is torn out. Drawing is promoted by increasing punch and die radii. For a given drawing condition, the punch force needed to move the metal into the die decreases as the die radius increases, as shown in Fig 7. The reduction of drawing force in a double action die by modification of the effective die radius can be accomplished in two convenient ways, as shown in Fig 6. In the conical lead-in die (Fig 6a), the cut-out is effective in reducing the frictional loads by removal of the portions of the die surface which are normally heavily loaded and increase friction. In Fig 6b, the sheet metal is formed into a conical shape before appreciable drawing begins. This has the effect of reducing the area of contact over the die radius by a quantity proportional to A/90 (where ‘A’ is the angle to declination of the hold-down surface to the horizontal, as shown in Fig 6b).
If the punch nose radius can be increased from one to five times metal thickness, the load in the side-wall of the shell decreases so that the reduction in blank diameter increases from 35 % to around 50 % (for steel). The shell can hence be drawn deeper before the side-wall tears. If the shell bottom radius is less than four times the sheet thickness, it is normally desirable to form it with a larger-radius punch and then to restrike to develop the specified radius. This minimizes bottom failures. However, the bottom corner radius normally cannot be increased beyond ten times the sheet thickness without the likelihood of wrinkling. The metal in dome-shaped parts is likely to pucker in the unconfined area between the punch nose and die radius. High blank holding forces or draw beads are frequently used to induce combined stretching and drawing of the metal when forming dome shapes.
The deep drawing of stainless steel or high strength alloy steel boxes with sides longer than 50 times sheet thickness can result in a stability problem called oil canning. The deflection of the sides by snap action can be eliminated by drawing the part in two operations with slightly different punches and an intermediate anneal. The first-draw punch has a larger nose radius than the second, hence, in the second drawing operation, the metal can be stretched to eliminate the oil-canning effect. Stretching of the metal in parts with long side-walls can be improved by gradually increasing the punch nose radius from the corner toward the centre. A constant nose radius is used on the second-draw punch.
Effect of punch-to-die clearance – The selection of punch-to-die clearance depends on the needs of the drawn part and on the work metal. Since there is a decrease and then a gradual increase in the thickness of the metal as it is drawn over the die radius, clearance per side of 7 % to 15 % higher than sheet thickness ‘t’ (1.07t to 1.15t) helps prevent burnishing of the side-wall and punching out of the cup bottom.
The drawing force is minimal when the clearance per side is 15 % to 20 % higher than the sheet thickness (1.15t to 1.2t) and the cupped portions of the part are not in contact with the walls of the punch and die. The force increases as the clearance decreases, and a secondary peak occurs on the force-stroke curve where the metal thickness is slightly higher than the clearance and where ironing starts.
Re-drawing operations need higher clearance, in relation to blank thickness, than the first draw in order to compensate for the increase in metal thickness during cupping. A sizing re-draw is used where the diameter or wall thickness is important or where it is necessary to improve surface finish to reduce finishing costs. The clearance used is less than that for the first draw. Tab 1 lists clearances for cupping, re-drawing, and sizing draws of cylindrical parts from metal of various thicknesses. As the tensile strength of the sheet decreases, the clearance is to be increased.
|Tab 1 Punch-to-die clearance for drawing operations|
|Metal thickness (t)||Clearance-to-metal-thickness relationship for|
|Up to 0.38||1.07t to 1.09t||1.08t to 1.1t||1.04t to 1.05 t|
|0.41 to 1.27||1.08t to 1.1t||1.09t to 1.12t||1.05t to 1.06t|
|1.29 to 3.18||1.1t to 1.12t||1.12t to 1.14t||1.07t to 1.09t|
|3.2 and higher||1.12t to 1.14t||1.15t to 1.2t||1.08t to 1.1t|
Clearance between the punch and die for a rectangular shell, at the side-walls and ends, is around the same as, or slightly less than, that for a circular shell. Clearance at the corners can be as much as 50 % higher than stock thickness to avoid ironing in these areas and to increase drawability.
Restraint of metal flow – Even in the simplest drawing operation, as shown in Fig 5a, the thickness of the work metal and the die radius offer some restraint to the flow of metal into the die. For drawing all but the simplest of shapes, some added restraint is normally needed in order to control the flow of metal. This additional restraint is normally achieved by the use of a blank holder, as shown in Fig 5b and Fig 5c. The purpose of the blank holder is to suppress wrinkling and puckering and to control the flow of the work metal into the die.
Drawing without a blank holder – A blank is not susceptible to wrinkling, and a blank holder need not be used, if the ratio of supported length to sheet thickness is within certain limits. The ratio of supported length ‘l’ to sheet thickness ‘t’ determines whether or not a blank holder is needed for deep drawing. The supported length ‘l’ is the length from the edge of the blank to the die cavity (point of tangency). The ‘l/t’ ratio is influenced little by other geometrical conditions, and it differs little for the different metals normally drawn. When the ‘l/t’ ratio does not exceed 3 to 1, a cup can be drawn from annealed low-carbon steel without a blank holder. For slightly harder work metals, this ratio is not to exceed 2.5 to 1.
An elliptical or conical die opening, such as that shown in Fig 6a, can be used where the die radius needed to draw the part reduces the length of the blank-supporting surface to less than three times stock thickness. The distance between the die opening and the punch is not to exceed ten times sheet thickness.
A 30-degree elliptical radius derived from a circle created by a given draw radius increases the strain on the metal being drawn by 4.2 %, but it decreases the metal out of control by 47 % of the length of the original draw radius. This shape has been helpful in the drawing of tapered shells from a flat blank. For these draws, it is desirable to increase the strain slightly to prevent puckers and to reduce the metal out of control for the same reason.
A 45-degree elliptical radius derived as stated previously reduces the strain on the metal being drawn by 1.03 % and reduces the metal out of control by 33 % of the length of the original draw radius. The 45-degree ellipse is useful only when a large radius draws the part, but produces wrinkles. A smaller radius does not permit the draw.
A 60-degree elliptical radius does not measurably reduce drawing strain and accounts for only a 9 % reduction of metal out of control. Its use on draw dies is not economically feasible when the small gains derived are considered in relation to the cost of producing the contour. The drawing of thick metal without a blank holder is frequently done when the blank diameter is not higher than 20 times the sheet thickness.
Blank holders – A blank holder, or binder, is used to control metal flow into the die cavity and to prevent wrinkles from forming in the flange of a deep-drawn part. The formation of wrinkles interferes with, or prevents, the compressive action which rearranges the metal from flange to side-wall. Considerably higher reductions are possible when a blank holder is used.
Blank holders can be used in double-action and single-action presses. In a double-action press, the blank holder advances slightly ahead of the punch and resides at the bottom of its stroke through-out the drawing phase of the punch cycle. The blank holder residence normally extends to a point on the punch up-stroke at which positive stripping of the shell is ensured. By using a die cushion and an inverted die, similar action can be achieved in a single-action press. A die cushion in a double action press supports the blank and holds it against the punch during the drawing operation. It then lifts the finished part out of the die.
A blank holder is needed to allow the work metal to thicken as the edge of the blank moves inward toward the working edge of the die. The quantity of thickening is expressed by the equation 8 which is ‘t1/t = square root(D/D1)’, where ‘t’ is the blank thickness, ‘t1’ is the thickness of the flange at any instant during the drawing operation, ‘D’ is the blank diameter, and ‘D1’ is the diameter of the flange at any instant during the drawing operation (or the mean diameter of the work-piece without the flange). As the metal flows, paths of least resistance are taken. Hence, the actual value of ‘t1’ is less than that calculated from the formula.
Types of blank holders – Blank holders can be flat, or they can have a combination of fixed draw-beads and constant blank holder displacements. In flat binders, restraining forces in the drawn sheet are provided by friction between the binder plates and the sheet metal. In fixed draw-bead binders, restraining forces result from a combination of plastic deformation and friction. The simplest type of blank holder is fixed to the die block and has a flat hold-down surface, as shown in Fig 8a. A disadvantage of this type of blank holder is that maintenance of the optimal gap between the die surface and the flat hold-down surface needs careful adjustment. As shown in Fig 8, the blank holder does not quite contact the work metal as drawing begins, and restraint begins and increases as the flange portion thickens. A gap which is either too small or too large increases force and reduces drawability. For optimal results, the gap is to be slightly smaller than the flange thickness, allowing 50 % to 75 % of the final thickening before the work metal contacts the blank holder.
Fig 8 Two types of blank holder and rigid sheet metal forming system
The flat controlled-pressure blank holder shown in Fig 8b is normally preferred in production operations since it can be adjusted to a pre-determined and closely controlled value by hydraulic or pneumatic pressure. Springs, unless extremely long, are not suitable for supplying pressure to a blank holder during deep drawing, since the force exerted by a spring increase rapidly as it is compressed. The force on hydraulic or pneumatic die cushions increases around 20 % when compressed the full stroke length. Some hydraulic systems have pressure control valves which supply a more nearly constant pressure during the entire stroke.
The fixed-type blank holder (Fig 8a) draws a cup without a flange and ejects it through the bottom of the die. The blank holder shown in Fig 5b and Fig 5c, and in Fig 8b can be used for drawing a cup with or without a flange. Cups without a flange can be pushed through the die if a pressure pad is not needed to support the blank.
Flexible blank holder – Conventional stamping with rigid binders does not allow local control of sheet metal flow into the die cavity, as shown in Fig 8. This produces a forming system with limited application and leads to an elaborate and costly die try-out process to produce a product. New tooling concepts and an advanced binder control system have been developed and implemented in Germany. The new development produces flexible binders with individually controlled hydraulic cylinders. Individual control of local binder areas allows the right quantity of metal to flow into the die cavity. Unlike rigid binders, a flexible binder produces the right quantity of elastic deformation in the binder area, and this produces the necessary friction condition for metal flow.
The cone-shaped segments localize the applied pressure in a specific area without influencing the pressure in adjacent areas. Trials on an automotive front fender have been conducted using a ten-point cushion system in a hydraulic press. Results from the trials have shown that flexible binder technology improves the quality of fenders by allowing the manipulation of pressure settings in binder areas which control metal flow to the defect location. Flexible binder control technology is presently being used to successfully deep draw stainless steel double sinks.
Blank holder force – Compressive forces on the metal in the area beyond the edge of the die cause the work metal to buckle. If this buckled or wrinkled metal is pulled into the die during the drawing operation, it increases the strain in the area of the punch nose to the point at which the work metal fractures soon after the beginning of the draw. Blank holder force is used to prevent this buckling and subsequent failure. The quantity of blank holder force needed is normally around one-third of the force which is needed for drawing. Thickness of the work metal is also to be considered when simple shapes are being drawn. The thinner is the work metal, the more is the blank holder force which is needed.
There are no absolute rules for calculating blank holder force for a given drawing operation with the majority of the blank holder force values are found empirically. Blank holder force is to be just sufficient to prevent wrinkling, and it depends on draw reduction, work metal thickness and properties, the type of lubrication used, and other factors. For a particular application, blank holder force is best determined experimentally.
Draw beads – They help prevent wrinkles and control the flow of metal in the drawing process. The use of draw beads increases the cost of tools, product development, and tool maintenance. However, they are frequently the only means of controlling metal flow in the drawing of odd shapes. Draw beads are normally used for the first draw only. Hence, production rates are the same as when conventional blank holders are used. For low production, draw beads are frequently made by laying a weld bead on the die after the optimal location has been determined.
Restraint of the metal flow, to the extreme of locking the flange of the blank to prevent motion, is needed for some draws. A deep shell with sloping walls can be made by drawing, followed by several re-draws. This results in a stepped work-piece. The final sizing draw is a stretching operation which is done with the flange secured by a locking bead in the blank holder. This kind of blank holder is also used in making shallow drawn panels.
Effect of press speed – Drawing speed is normally expressed in meters per minute (m/min). Under ideal conditions, press speeds as high as 23 m/min are used for the deep drawing of low-carbon steel. However, 6 m/min to 17 m/min is the normal range i.e., up to 17 m/min for single-action presses and 11 m/min to 15 m/min for double-action presses. Ideal conditions include (i) use of a drawing-quality work metal, (ii) symmetrical work-pieces of relatively mild severity, (iii) adequate lubrication, (iv) precision carbide tools, (v) carefully controlled blank holding pressure, and (vi) presses which are maintained to a high level of accuracy.
When one or more of the afore-mentioned conditions is less than ideal, some reduction in press speed is needed. If all, or nearly all, are substantially less than ideal, press speed is to be reduced to 6 m/min. When the operation includes ironing, the drawing speed is normally reduced to around 7.5 m/min regardless of other factors.
The punch speed in hydraulic presses is relatively constant throughout the stroke. In mechanical presses, punch speed is that which is at mid-stroke, since the velocity changes in a characteristic manner through-out the drawing stroke from maximum velocity to zero. The only adjustment in speed which can be made is to decrease fly-wheel speed or to use a press with a shorter stroke which operates at the same number of strokes per minute. This proportionately decreases maximum punch speed.
Speed is of higher significance in drawing stainless steels and heat-resistant alloy steels than in drawing softer, more ductile metals. Excessive press speeds cause cracking and excessive wall thinning in drawing these stronger, less ductile metals. At high speeds, the metal thins since it cannot react to the impact speed of the punch. Reducing the speed reduces the stretching and gives the metal enough time to flow plastically.
Effect of lubrication – When two metals are in sliding contact under pressure, as with the dies and the work metal in drawing, galling (pressure welding) of the tools and the work metal is likely to take place. When extreme galling occurs, drawing force increases and becomes unevenly distributed, causing fracture of the work-piece. The likelihood of pressure welding depends on the quantity of force and the work metal composition. Some work metals are more ‘sticky’ than others. For example, austenitic stainless steel is more likely to adhere to steel tools than low carbon steel is.
Lubricants are used in the majority of the drawing operations. They range from ordinary machine oil to pigmented compounds. Selection of lubricant is mainly based on the ability to prevent galling, wrinkling, or tearing during deep drawing. It is also influenced by ease of application and removal, corrosivity, and other factors. If a lubricant cannot be applied uniformly by ordinary shop methods, its purpose is defeated, regardless of its ability to prevent pressure welding. In general, as the effectiveness of a lubricant increases, the difficulty of removing it also increases. For example, grease or oil can be easily removed, but special procedures (frequently including some hand scrubbing) are needed for removing lubricants which contain zinc oxide, lithopone, white lead, molybdenum di-sulphide, or graphite.
A lubricant is sometimes too corrosive for use on certain metals. For example, copper alloys are susceptible to staining by lubricants which contain large quantities of sulphur or chlorine compounds. Lubricants containing lead or zinc compounds are not recommended for drawing stainless steel or heat-resistant alloy steels, since the compounds, if not thoroughly removed, can cause inter-granular attack when the work-pieces are heat treated or placed in high-temperature service.
Suitable safety precautions are necessary with toxic or flammable lubricants. Some metals, such as magnesium and titanium, are drawn at high temperature, which complicates selection of the lubricant. Majority of the oil-base and soap-base lubricants can be successfully used to 120 deg C, but above this temperature, the choice narrows rapidly. Some special soap-base lubricants can be used on work metals up to 230 deg C. Molybdenum di-sulphide and graphite can be used at higher temperatures. A lubricant is required to remain stable, without becoming rancid, when stored for a period of several months at different temperatures.
The cost of application and removal of the lubricant, as well as its initial cost, is to be considered since all of these items can add substantially to the cost of the drawn work-pieces. In some plants, when a new application is started, a heavily pigmented drawing lubricant is used, regardless of the difficulty of applying and removing it. Lubricant is then down-graded as much as possible to simplify the operation and to reduce costs. In other plants, the reverse of this practice is used, i.e., a simple lubricant, such as machine oil, is used at first, and lubricant is then up-graded when necessary.
The difficulty of removing drawing lubricants is an important consideration in deep drawing operations. In a number of applications, changes in drawing techniques (such as increasing the number of draws) or in work-piece design (e.g., larger radii) have been made solely to permit the use of an easier-to-remove drawing lubricant. Zinc phosphate conversion coating of the steel to be drawn is helpful for any drawing operation, and the importance of phosphate coating increases as the severity of the draw increases.
Materials for deep drawing – Sheet steels and other sheet metals with higher strengths and better formability have recently become available. Developments such as vacuum processing and inclusion shape control have been especially beneficial in increasing the drawability of steels. Other steels which can be deep drawn include some stainless steels. Low-carbon sheet steels are the materials which are mostly deep drawn and are normally used, for example, in the automotive industry.
Materials such as 1006 and 1008 steel grades have typical yield strengths in the range of 170 MPa to 240 MPa and elongations of 35 % to 45 % in 50 mm. These materials have very good formability and are available cold finished or hot finished in different quality levels and a wide range of thicknesses. Tab 2 lists mechanical properties of the different qualities of low carbon steel sheet.
|Tab 2 Typical mechanical properties of low carbon sheet steels|
|Quality level||Tensile strength||Yield strength||% Elongation||Plastic strain ratio||Strain hardening exponent||Hardness|
|MPa||MPa||in 50 mm||‘r’||‘n’||HRB|
|Drawing quality, Aluminum killed||358||234||38||1||0.19||54|
|Cold rolled, batch annealed|
|Drawing quality, Aluminum killed||303||193||42||1.5||0.22||42|
Other low-carbon steels which are normally deep drawn are steel grades 1010 and 1012. These materials are slightly stronger than 1006 and 1008 steel grades and are slightly less formable. They are frequently specified when drawing is not severe and strength of the finished part is of some concern. Grain size affects the drawability of these materials, and it can affect the selection of a grade. Grain sizes of ASTM 5 or coarser can result in excessive surface roughness as well as reduced drawability. Surface finish also influences drawability. The dull finish normally supplied on steel for drawing is designed to hold lubricants and to improve drawability. Brighter finishes can be needed if, for example, parts are to be electro-plated.
Effects of material variables on deep drawing – The following material variables affect the deep drawing process.
Anisotropy – There are two types of anisotropy which are to be considered. These are (i) planar anisotropy, in which properties vary in the plane of the sheet, and (ii) normal anisotropy, in which the properties of the metal in the thickness direction differ from those in the plane of the sheet. Planar anisotropy (variations in normal anisotropy in the plane of the sheet) causes undesirable earing of the work metal during drawing. Between the ears of the cup are valleys in which the metal has thickened under compressive hoop stress rather than elongating under radial tensile stress. This thicker metal sometimes forces the die open against the blank holder pressure, allowing the metal in the relatively thin areas near the ears to wrinkle. Die design, draw reduction, and type of lubricant used all affect earing.
Sheet thickness – In deep drawing, the pressure on the dies increases proportionally to the square of sheet thickness. The pressure involved is concentrated on the draw radius, and increasing sheet thickness localizes wear in this area without similar effect on other surfaces of the die. Thick material has fewer tendencies to wrinkle than thin material. As a result, blank holder pressures used for the drawing of thick sheet can be no greater, and can even be less, than those used for thinner blanks.
Re-drawing operations – If the shape change needed by the part design is too severe (the drawing ratio is too high), a single drawing operation cannot produce complete forming of the part, and more than one drawing step is needed. Fig 9 shows direct and reverse re-drawing and set-ups using internal blank holders.
Fig 9 Direct and reverse re-drawing and set-ups using internal blank holders
Direct re-drawing – In direct re-drawing in a single-action die, the drawn cup is slipped over the punch and is loaded in the die, as shown in Fig 9. At first, the bottom of the cup is wrapped around the punch nose without reducing the diameter of the cylindrical section. The side-wall section then enters the die and is gradually reduced to its final diameter. Metal flow takes place as the cup is drawn into the die so that the wall of the re-drawn shell is parallel to, and deeper than, the wall of the cup at the start of the re-draw. At the beginning of re-drawing, the cup is to be supported and guided by a recess in the die or by a blank holder to prevent it from tipping, since tipping results in an uneven shell.
In a single-action re-draw, the metal is to be thick enough to withstand the compressive forces set up in reducing the cup diameter without wrinkling. Wrinkling can be prevented by the use of an internal blank holder and a double-action press (Fig 9), which normally permits a shell to be formed in fewer operations than by single-action drawing without the use of a blank holder. Internal blank holders (Fig 9) are slip fitted into drawn shells to provide support and to prevent wrinkling during direct re-drawing. The blank holder presses on the drawn shell at the working edge of the die before the punch contacts the bottom of the shell and begins the re-draw. It resides against the shell as the metal is drawn into the die by the punch, preventing wrinkles.
The bottom of the cup to be re-drawn can be tapered (Fig 9a) or radiused (Fig 9b), with the tip of the blank holder and the mouth of the die designed accordingly. An angle of 30-degree is used for metal thinner than 0.8 mm, and 45-degree is used for thicker work metal. A modification of the afore-mentioned is a blank holder fitted against an S-curve die (Fig 9b). The main disadvantage of an S-curve die is that it is more expensive to make and maintain. Near the bottom of a re-drawn shell, there is normally a narrow ring, caused by the bottom radius of the preceding shell, which is thinner and harder than the adjacent metal.
Re-drawing can be needed for reasons other than the severity of the drawn shape, e.g., to prevent thinning and bulging. Re-drawing can also be done in a progressive die while the part is still attached to the strip. Where space permits the extra stations, the quantity of work done in each station is less than that done in a single die. This reduces the severity of the draw and promotes high-speed operation.
Reverse re-drawing – In reverse re-drawing, the cupped work-piece is placed over a reversing ring and re-drawn in the direction opposite to that used for drawing the initial cup. As shown in the Fig 9, reverse re-drawing can be done with or without a blank holder. The blank holder serves the same purposes as in direct re-drawing. The advantages of reverse re-drawing as compared with direct re-drawing include (i) drawing and re-drawing can be accomplished in one stroke of a triple-action hydraulic press, or of a double-action mechanical press with a die cushion, which can eliminate the need for a second press, (ii) higher reductions per re-draw are possible with reverse re-drawing, (iii) one or more intermediate annealing operations can frequently be eliminated by using the reverse technique, and (iv) better distribution of metal can be achieved in a complex shape.
In border-line applications, annealing is needed between re-draws in direct re-drawing but is not needed in reverse re-drawing. The disadvantages of reverse re-drawing are (i) the technique is not practical for work metal thicker than 6 mm, and (ii) reverse re-drawing needs a longer stroke than direct re-drawing. Normally, metals which can be direct re-drawn can be reverse re-drawn. All of the carbon and low-alloy steels, and austenitic and ferritic stainless steels can be reverse re-drawn.
Reverse re-drawing needs more closely controlled processing than direct re-drawing does. This control is to begin with the blanks, which is to be free from nicks and scratches, especially at the edges. The restraint in reverse re-drawing is to be uniform and low. For low friction, polished dies and effective lubrication of the work are needed. Friction is also affected by hold-down pressure and by the shape of the reversing ring. Radii of tools is to be as large as practical, ten times the thickness of the work metal if possible. Reverse re-drawing can be done in a progressive die as well as in single-stage dies if the operations are divided to distribute the work and to reduce the severity of each stage.
Tooling for re-drawing – Tooling for re-drawing depends mainly on the number of parts to be re-drawn and on available equipment. In continuous high production, a complete die is used for each re-draw. The work-pieces are conveyed from press to press until completed. In low or medium production, it is a normal practice to use a die with replaceable draw rings and punches. A die of this type used for three re-drawing operations is shown in Fig 10. The three re-draws are made by changing to successively smaller draw rings and punches. The cup is drawn in a compound blank-and-draw die from a blank 1.7 mm thick and 170 mm in diameter.
Fig 10 Die for producing three successive re-draws and drawing of a hemi-sphere
Ironing – Ironing is an operation used to increase the length of a tube or cup by reducing wall thickness and outside diameter, while the inner diameter remains unchanged. Wall thickness is reduced by pulling tubes or shells through tight dies. The process of ironing is similar to tube drawing with a moving mandrel, whereby drawing is carried out by using several drawing dies located in tandem. In a typical application, a relatively thick-wall cup is first produced by extrusion or deep drawing. The wall thickness of this cup is then reduced by tandem ironing with a cylindrical punch, while the internal diameter remains unchanged.
Hot ironing and cold ironing produce parts with good dimensional accuracy while maintaining or improving concentricity. A very common application of ironing is the production of beverage cans by tandem drawing of steel. Ironing is done to (i) achieve a wall which is thin compared with the shell bottom, (ii) achieve a uniform wall, and (iii) achieve a tapered wall (as in cartridge cases), or merely to correct the natural wall thickening which occurs toward the top edge of a drawn shell.
The theoretical maximum reduction in wall thickness per operation because of ironing is around 50 %. In such a case, the cross-sectional area of the (unstrained) metal before ironing is around twice the cross-sectional area after ironing. Hence, the area which is being worked in compression and which yields is around equal to the strain-hardened area which is in tension and which does not yield. This indicates that the practical limit is apparently to be kept below 50 %, although slightly higher reductions are possible.
A shell ironed to a finished diameter of 100 mm, with a displacement of 0.25 mm of the total metal thickness, needs an ironing pressure of around 3.8 tons, assuming that the metal is spheroidized steel moderately strain hardened and hence offering a compressive resistance of around 620 MPa. This is to be added, of course, to the drawing or re-drawing load. The formula is ‘P = 1.2 x ‘pi’ x d x ‘i’ x S’, where ‘pi’ is 3.1416, ‘P’ is the approximate needed maximum pressure, ‘d’ is the outside diameter after ironing, ‘i’ is the reduction in wall thickness, and ‘S’ is the compressive resistance of the metal under existing conditions of strain hardening. There is included a 20 % allowance for surface friction in addition to the work of coining. This is an arbitrary figure which covers well-polished dies and suitable lubrication.
Lack of lubricant, tool-mark rings on the dies, or surface pick-up increases the friction load considerably. If the wall of a shell is ironed thinner by the same quantity for the entire length of the shell, then the work done is around the product of the length of the shell or of the ironed surface and the pressure needed for ironing. This is expressed as ‘W = P x l’ where ‘W’ is work, ‘P’ is pressure, and ‘l’ is length. If a reducing operation accompanies ironing, then the drawing pressure is to be added to the ironing pressure. If the ironing operation is done merely to correct the natural changes in wall thickness because of drawing and reduces the thickest portion near the top of the shell to equal the thinnest portion near the bottom, the average ironing pressure equals around half the maximum, and the formula is ‘W = 0.5 P x l’.
Drawing of box-like shells – When there is no flange, square shells or rectangular shells can be formed by re-drawing circular shells. When flanges are needed, the difficulty of producing acceptable box-like shapes by drawing is increased. For deep-drawn square or rectangular shells (e.g., where the depth is higher than either length or width), the best approach for forming a narrow flange is to allow sufficient metal and to form the flange after re-drawing from a cylindrical shell.
Shallower box-like shapes can be drawn with a flange, which is then trimmed to the desired width. Calculations for the area of a blank used for a circular work-piece cannot be used for a square or rectangular box. These need metal in the bottom, ends, sides, and flange, when a box is unfolded (flat pattern). The excess metal at the corners is an issue. A seamless square or rectangular shell is made by drawing metal into the corners. The metal not needed for the corners is pushed into the walls adjacent to the corner radius and into ear-like extensions of the corners. The compressive stresses set up when the metal in the corners is rearranged and causes the metal to be thicker in the corners than in the side-wall or in the original blank.
The more difficult draws are made more easily by using a carefully developed blank. There are methods of developing the shape at the corners of a blank for a square or rectangular shell so that there is a minimum of excess metal. However, by cropping the corners and by using a blank holder, satisfactory parts can normally be made. Draw beads in the blank holding surface surrounding the die are frequently used.
Drawing of work-pieces with flanges – Regardless of whether the drawn work-piece is circular, rectangular, or asymmetrical, producing acceptable small-width flanges on work-pieces is rarely an issue. Flanged work-pieces are normally drawn in two or more operations, frequently with restriking as a final operation.
Cylindrical work-pieces with wide flanges are troublesome to draw because of excessive wrinkling or fracturing in the side-wall because of lack of metal flow. Even though the metal is restrained by a blank holder, it is difficult to achieve acceptable flatness without special procedures. Wide flanges on relatively large work-pieces can be made flat by coining after drawing. Another means of dealing with wrinkling, when design permits, is to provide ribs in the flange. This controls the wrinkling by allowing space for the excess metal. Ribs are normally spaced radially around the flange, although circular, concentric ribs also are effective.
Rectangular, box-like work-pieces which have flanges are difficult to re-draw in such a manner that the flange is unaffected in re-drawing operations. Hence, it is normal practice to draw the part first to a shallower depth and with larger bottom radii than needed for shaping the final contour. The part is then reformed in a final operation.
Asymmetrical work-pieces which have flanges are frequently difficult to draw, particularly when neither draw beads in the die nor ribs in the work-piece can be permitted. Under these conditions, considerable development is normally needed to determine the blank holder pressure which results in the desired metal flow without using a larger blank than necessary.
Drawing of hemi-spheres – In the drawing of a hemi-sphere, metal flow is to be closely controlled for balance between excessive thinning in one area and wrinkling in another. In the top diagram in Fig 10, the punch has begun to stretch the round blank, which is restrained by the blank holder, and the crown section of the hemi-sphere is being formed. At this stage, the crown section is subjected to bi-axial tension, which results in metal thinning. With correct pressure on the blank-holder, thinning is in the range of 10 % to 15 %. Higher than 15 % thinning is likely to result in fracture of the crown section. In the top diagram in Fig 10, the portion of the blank under the blank holder has not begun to move.
As the drawing operation continues, the metal begins to move from the blank holder, and a different issue develops (centre diagram of Fig 10). Here, the metal has been drawn into a partial hemi-sphere with unsupported metal in a tangential slope between the punch and the clamped surface. Unlike the drawing of straight sided shapes, the wide gap (wrinkling area of Fig 10) prevents the use of the draw ring bore as the means of forcing the metal against the punch surface. Hence, the probability of wrinkling increases. Since the metal cannot be confined between the punch and die, wrinkling is likely to occur in this area.
For preventing the wrinkles, the metal is needed to flow from the flange area and, at the same time, is to be securely held in tension. This needs an additional stretching force, derived from the portion of the blank which remains clamped. The area of metal between the clamping surfaces is gradually reduced as the punch advances, but the draw radius offers some resistance since the metal is to follow a sharper bend as it moves into the di
One means of controlling wrinkling is by the use of draw beads, as shown in the bottom diagram in Fig 10. Another means is by a sharp draw radius. Small radii are susceptible to metal pickup and, depending on sharpness, can produce undesirable circumferential grooves in the hemi-sphere if the punch does not move at a steady rate.
Reducing drawn shells – Necking and nosing are used for reducing the diameter of a drawn cup or shell for a part of its height.
Necking – By the die reduction method, the work metal is forced into compression, resulting in an increase in length and wall thickness. The thicknesses of a shell before and after necking are related by the equation 9 which is ‘t2 = t1 x square root(d1/d2)’ and heights before and after necking by the by the equation 10 which is ‘h2 = h1 x square root(d1/d2)’, where ‘t1’ is the shell thickness before necking, ‘t2’ is the shell thickness in the necked area after necking, ‘d1’ is the mean diameter of the shell before necking, ‘d2’ is the mean diameter after necking, ‘h1’ is the unit of height before necking, and ‘h2’ is the unit of height after necking. In a necking operation, as the metal flows, paths of least resistance are taken. Hence, the actual value for ‘t2’ is less, and for ‘h2’ is higher, than those calculated from equation 9 and equation 10.
Necking results are uniformly better if the work-piece has been slightly cold worked. This provides added strength to resist bulging in the column section and buckling in the section is reduced. The entry angle on the necking die is important since the probability which the metal collapses is decreased as the angle with the vertical becomes smaller. This angle is to be less than 45-degree. If the angle is higher than 45-degree, a series of reductions are necessary, with localized annealing between reducing operations. With a die entry angle less than 45-degree, thin-wall tubes can be reduced as much as 15 % in diameter and thick-wall tubes can be reduced as much as 20 %.
Nosing reduces the open end of a shell by tapering or rounding the end (normally by cold reduction) and is mainly used in making ammunition shells. Shells are frequently machined before, instead of after, nosing. Shells are normally cold reduced as much as 30 % of their original diameter by nosing.
Ironing is the intentional reduction in wall thickness of a shell by confining the metal between the punch and the die wall. When ironing occurs, the force needed to displace the punch frequently increases to a secondary maximum in the force-displacement curve. The second force maximum can be of such magnitude that the shell can break. However, after ironing has started and metal has been wrapped around the punch, the force is uniform and frequently less than that needed for the re-drawing operations.
Ironing is seldom used with re-drawing operations unless the quantity of wall thinning is relatively small, since it results in excessive die wear, causes work-piece breakage, and increases press tonnage requirements. If a shell with constant wall thickness is needed, however, it can be achieved only by ironing.
Expanding drawn work pieces – There are several methods for expanding portions of drawn work-pieces in a press. Since the wall thickness is reduced during expansion, it is not advisable to increase the diameter for ductile metal shells (such as low-carbon steel) more than 30 %. If the increase in a diameter by more than 30 % is needed, the operation is to be done in two or more stages, with annealing between stages.
Expanding with a punch – In expanding with a punch as shown in Fig 11, the portion to be expanded is first annealed. Localized annealing, instead of annealing the entire cup, helps retain strength in the remainder of the cup. Regardless of whether or not the strength is needed in the finished part, maximum column strength is desirable to prevent buckling as the punch enters the cup.
Fig 11 Expansion of the mouth of a drawn shell with a punch
After the cup has been placed in the die (Fig 11a), the punch moves downward and expands the top of the cup (Fig 11b). During the return stroke, the work-piece is stripped from the punch by the stripper ring and is ejected from the die by the ejection pad. In an expanding operation of this kind, die dimensions are pre-determined within reasonably close limits during the design stage. However, the possibility of later design changes is always to be considered. Depending on the shape and location of the expanded section, a height reduction of the cup can occur which needs some modification of the die and punch after try-out.
Expanding with segmented dies is frequently used for forming side-walls of drawn shells or sections of tubing. The forming segments are contracted by compression springs and expanded radially by a tapered punch. The die is made of two or more segments held apart by compression springs. As the press ram descends, cams move the die segments together. The punch then moves the inner segments outward, hence forming the contours in the side-wall. The presence of gaps between the forming segments is one of the disadvantages of this method and is the reason an alternative method, such as rubber-pad forming, is sometimes selected.
Deep drawing of pressure vessels – Different grades of steel, several of them high strength alloy steels, are deep drawn to make cylinders for compressed gases. Joints (when they are made) are around the girth of the vessel, rather than longitudinal. The integrity of the vessel is critical. Commercial-quality hot-rolled steels in the as-rolled condition are normally used. The work metal is normally induction heated or induction annealed to minimize scale. For propane gas, pressure tanks need to have high strength at minimum weight. In one application, the weight of such a tank has been reduced from 59 kilograms to 32 kilograms by changing from 1025 steel grade to a high-manganese deep-drawing steel grade (Fe – 0.2 % carbon – 1.6 % manganese – 0.025 % phosphorus – 0.3 % sulphur). Before drawing, the high-manganese steel had a minimum yield strength of 345 MPa and a minimum tensile strength of 485 MPa.
Bottles for dispensing small quantities of liquefied gases or gases under high pressure are normally made of drawing-quality low-carbon steel to take advantage of the improved mechanical properties produced by deep drawing. The bottles range in size from 12.5 mm in diameter and 32 mm long to 38 mm in diameter and 150 mm long.
Deep drawing using fluid-forming presses – Fluid forming (also termed hydro-forming) is a deep-drawing process which uses only one solid die half. Forming pressure is applied by the action of hydraulic fluid against a flexible membrane, which forces the blank to assume the shape of the rigid tool. Fig 12 shows the deep drawing process using the fluid forming press.
Fig 12 Deep drawing process using the fluid forming press
Fluid forming can be used for deep drawing and, in fact, offers advantages over other forming techniques. One of these is that the draw radius can be varied by changing the pressure of the hydraulic fluid during the forming operation. This makes it possible to have, e.g., a large draw radius at the start of the operation which decreases as the draw continues. Hence, reductions of up to 70 % in a single pass are possible when drawing cylindrical cups. For rectangular shaped parts, a height of six to eight times the corner radius can be achieved in a single operation. Presses for fluid forming sometimes use a telescoping ram system.
Ejection of work-pieces – In drawing operations, the drawn work-piece can adhere to either the punch or the die. Adherence is increased by depth of draw, straightness of work-piece walls, and viscosity of lubricant. The simplest means of ejecting a small work-piece is by compressed air through jets in the punch or the die. Timed air blast is widely used for ejecting relatively small work-pieces, e.g., where cup diameter is no higher than 100 mm to 125 mm. In some production drawing operations, the work-piece is ejected by compressed air, and another timed blast of air from the side removes the piece by sending it down a chute or into a container. However, for larger work-pieces or for those which are deep, some other means of ejection is needed. Fig 13 shows techniques of ejecting drawn work-piece.
Fig 13 Techniques of ejecting drawn work-piece
Mechanical techniques of ejection include (i) edge stripping by means of a lip on the draw ring (Fig 13a) or by a spring-actuated stripper (Fig 13b), (ii) the use of a blank holder in combination with an upper ejector (Fig 13), and the use of a lower ejector in combination with an upper stripper ring (Fig 13).
A number of other mechanical techniques using cams or links have been devised to meet specific needs. These techniques are normally modifications of those described previously. For example, thin shells are sometimes stripped from punches near the top of the press stroke by a cam actuated rod which extends through the punch. This technique is frequently used to avoid damage to the open end of the shell, which can occur when the piece is ejected by other techniques. The major factors influencing the technique of ejection are work-piece design (especially the presence or absence of a flange), work metal composition and thickness, and the type of equipment available.
Trimming – Trimming in a lathe (using a cutting tool), roll trimming in a lathe, rotary shearing, die trimming (regular and pinch), and trimming on special machines are the techniques normally used for trimming drawn work-pieces.
Techniques for specific shapes – Cylindrical work-pieces can be trimmed by at least four different techniques namely (i) in a lathe, with a cutting tool, but production cost is high, (ii) by roll trimming in a lathe or in a rotary shear where the production cost is lower than that for trimming with a cutting tool, but the finish of a rolled edge is poor and maintenance cost of the rolls is high, (iii) by pinch trimming in the press at the bottom of the drawing stroke, which involves almost no increase in production cost but needs a more expensive die (this technique produces a thinned edge at the trim line, which can be unacceptable), and (iv) in a shimmy die or trimming machine, but production quantities are to be high to warrant the investment.
Cylindrical workpieces with flanges can also be trimmed in a lathe, although certain shapes are ideal for trimming in a die and can be die trimmed for around 5 % of the cost of trimming in a lathe. Rotary shearing can also be used for trimming circular drawn parts with flanges if the dimensional tolerance is 0.76 mm or more. Drawn work-pieces with an irregular trim line can be trimmed in a die for low-production requirements, or with a shimmy die or trimming machine for high-production requirements. The cost of a trimming die is around half that for a special trimmer (excluding the cost of the original machine). However, the trimming cost per piece with the special trimmer is only around half the per-piece cost with multiple dies, and the trimmed edges are better.
Flanged work-pieces can be trimmed in a die for 5 % of the cost of trimming in a rotary shear. In low production, drawn work-pieces of such a shape are frequently trimmed in a nibbler and filed to conform to a template. This means of trimming costs up to 60 times as much as die trimming.
Developed blanks versus final trimming – The most important factor which influences a choice between using a developed blank or a final trimming operation is whether or not the shape of the drawn edge is acceptable. A semi-developed blank is sometimes necessary to draw an acceptable part, and the edge is to be trimmed to meet dimensional tolerances.
The next consideration is the cost of blanking versus final trimming. This includes the adaptability of the process to the available equipment, based on expected production needs. The principal advantage of a developed blank is that strip or coiled work metal can be used. The use of strip eliminates the need for shearing the work metal to a rough-blank shape, as is sometimes needed when final trimming is used. The developed-blank approach is normally more economical than final trimming since a blanking die is frequently less expensive than a final-trimming die.
When using developed blanks, the draw dies are made, and several blanks are drawn to select the optimal developed shape before the blanking die is made. This causes a delay in placing the draw die into production. However, with proper planning and scheduling, this is not an issue. Another disadvantage of developed blanks occurs when variations in work metal properties and thickness are sufficient to affect the uniformity of the drawn work-piece. Under these conditions, closer tolerances are achieved by final trimming. It is possible to develop blank contours accurately enough so that the outline of the drawn part is within the tolerance, hence avoiding a final-trimming operation.
Cleaning of work-pieces – Normally, the more effective is the lubricant, the more difficult it is to remove. Hence, an overly effective drawing lubricant is to be avoided. The cleaning method depends on the work metal composition, the lubricant, the degree of cleanliness needed, work-piece shape, and sometimes the length of time between application of lubricant and its removal. Some metals are attacked by cleaners which are not harmful to others, e.g., strong alkaline cleaners are suitable for cleaning steel and several other metals, but they are likely to attack aluminum alloys.
Unpigmented oils and greases can be removed from steel work-pieces by several simple shop techniques, including alkaline dipping, emulsion cleaning, and cold solvent dipping. These techniques are normally sufficient for in-process cleaning. However, if the work-pieces are to be painted, a more thorough cleaning by emulsion spray or vapour degreasing is needed. For plating, electrolytic cleaning plus etching in acid (immediately prior to plating) is needed. These latter techniques normally follow a rough cleaning operation.
Pigmented drawing lubricants and waxes greatly increase cleaning problems. At a minimum, in-process cleaning normally needs slushing in a hot emulsion or vapour degreasing. If the lubricant is not removed for several days after application, soaking in a hot alkaline cleaner or an emulsion cleaner is needed. Particularly for complex work-piece shapes, some hand or power brush scrubbing is needed. If the work-pieces are to be painted or plated, additional cleaning is needed.
Dimensional accuracy – Dimensional accuracy in deep drawing is affected by the variation in work metal thickness, variation in work metal condition (mainly hardness), drawing technique (mostly the number of operations), accuracy of the tools, rate of tool wear, and press condition. Control of dimensions begins with the purchase of sheet to closer-than-commercial thickness tolerance, which adds substantially to the cost. Close control of sheet hardness also costs more. In-process annealing is needed to minimize spring back or war-page, it is not needed if tolerances are more liberal. Annealing, handling, and cleaning operations are costly.
As tolerances become closer, it is frequently necessary to add more die stations to minimize the quantity of drawing in any one station. Close tolerances demand restriking operations which are not necessary for parts with more liberal tolerances. Additional operations increase tool costs and decrease productivity, hence increasing the cost per piece.
The initial cost of tools increases as tolerances become closer because of higher cost for precision machining and grinding or more costly tool materials. In addition, tool life before re-conditioning and total tool life decrease as tolerances become closer. Maintenance costs and downtime of presses are also higher.
When needed, extremely close tolerances trim can be maintained on some parts. In majority of the deep drawing operations, the accuracy is either impossible or impractical. The normal practice when dimensional accuracy is important, is to check critical dimensions at specified intervals during a production run and to plot the variation. Data from this method of quality control show the capabilities of the process under shop conditions and the magnitude of drift during a production run. When results (either initially or during a run) are unacceptable, one or more of the controls can be applied.
Safety – Deep drawing, like other press operations, involves potential hazards to operators and other personnel in the work area. No press, die, or auxiliary equipment can be considered ready for operation until these hazards are eliminated by the installation of necessary safety devices. Operators are to be properly instructed in safe operation of equipment.