GB2551514B - A method of manufacturing a metal cup using a draw die having a variable transition surface - Google Patents

Description

A METHOD OF MANUFACTURING A METAL CUP USING A DRAW DIE HAVING AVARIABLE TRANSITION SURFACE

Field of the Invention

The invention relates to draw dies for cup-making presses, methods of theirmanufacture and to methods of manufacturing metal cups and can bodies.

Background

Drawn and wall-ironed cans for food and beverages are commonly manufactured usingthe steps of drawing a circular cup from a sheet metal blank, re-drawing the cup toproduce an elongated cup with a smaller diameter, and then ironing the side-wall of theredrawn cup to reduce its thickness whilst substantially maintaining the diameter. A number of blanks are typically cut and drawn from a wide coil of metal in each strokeof a cup-making press having a number of toolsets arranged across its width. In eachtoolset, the cut blank is held against the clamping face of an annular draw die by aclamping force applied by an annular pad, and a punch is pushed against the blank todraw it through the draw die to form a cup. The clamping force prevents the metalwrinkling. The draw die is provided with a radius, a “draw radius”, between its clampingface and its inside diameter to allow the metal to flow in smooth contact with it. In otherwords, the transition from the clamping face to the inside diameter is curved. Theinside diameter of the draw die is substantially constant.

The cups formed in the cup-making press are then fed into a body-making machine or“bodymaker” in which one cup at a time is redrawn and ironed in each stroke. In atypical bodymaker, the perimeter of the base of the cup is held against the face of anannular redraw die by a clamping force applied by an annular redraw sleeve, and apunch is pushed against the cup base to draw the cup through the redraw die, reducingits diameter. Again, the clamping force prevents the metal wrinkling. The redrawsleeve is provided with a radius between its outer diameter and its face that is heldagainst the cup base to allow the metal to flow in smooth contact with it. The redrawdie is also provided with a radius between its face and its inside diameter to allow themetal to flow in smooth contact with it.

The can body is usually held on the punch after it has been drawn through the redrawdie, and the punch continues to travel through one or more ironing dies. Each ironingdie has an internal diameter that provides a gap between it and the punch that issmaller than the thickness of the re-drawn body, and so the wall of the can body isironed to reduce its thickness and increase its height. The can body is of unevenheight after ironing, as will be further discussed below, and it is normally trimmed to aneven height after the formation of features in the base of the can body.

During drawing and re-drawing, the metal towards the open end of the cup or re-drawncup usually becomes thickened because it has been compressed by hoop orcircumferential stresses during its circumferential reduction to a smaller diameter. Theclamping force usually only partially limits this thickening effect. Conversely, the metaltowards the closed end of the cup or re-drawn cup may become thinned slightly,because this metal is subject to the most radial and axial tension.

The metal in the sidewall of the cup may also be thinned as it is drawn around theradius of the draw die in the cup-making press, and it may also be thinned as it isdrawn around the radius of the re-draw die in the bodymaker. When the metal is drawnaround the draw radius it is firstly bent around the draw radius whilst subject to radialtension. Secondly, it is straightened whilst subject to axial tension as it leaves contactwith the radius. Bending and straightening under tension in this way stretches themetal and makes it thinner. It is important that there is even tension all around the cupas it is drawn around the draw radius, in order to ensure the metal is bent tightly aroundit. The smaller the draw radius is in relation to the metal thickness, then the greater willbe the amount of stretching and thinning. A small draw radius may be intentionallyused to contribute to the desired wall thickness reduction. Such a method is commonlyknown as stretch-draw or stretch-re-draw.

For example, it may be desirable to use stretch-re-draw to achieve some of the desiredsidewall thickness reduction when making a can body from sheet metal which is pre-coated with polymer such as polyethylene terephthalate (PET) or polypropylene. Thisis because the ironing process may degrade the integrity of the polymer coating byfriction or heat generation. When making can bodies from polymer-coated metal it istherefore desirable to limit the number of ironing dies used and this limits the overall amount of reduction that is achievable by wall-ironing. Stretch-re-draw can be used asan alternative method of thinning the can body sidewall.

In general, dies that are used to draw or to re-draw cups must be accuratelymanufactured, and in particular the draw radius must blend tangentially with theclamping face. If there is a slight “corner” where the radius meets the face, this willbehave as if there was a very small radius, and uncontrolled or unwanted thinning mayoccur. Dies with small draw radii tangential to the face are therefore more difficult tomanufacture with sufficient accuracy.

As discussed above, food and beverage cans are commonly manufactured from asheet metal blank. When tensile properties of a sample test-piece of sheet metal aremeasured, a co-efficient, known as the Lankford co-efficient or “r” value, can bedetermined, which is the measured proportional change in width (width strain) dividedby the measured proportional change in thickness (thickness strain). This co-efficienttypically varies depending on the direction in which the test-piece is cut from the sheetsample and pulled in the test, relative to the direction in which the sheet metal wasrolled. The yield stress of the metal sheet may also vary depending on the direction inwhich the test-piece is cut from the sheet sample and pulled in the test, relative to thedirection in which the sheet metal was rolled. Such variations are commonly known asanisotropy.

As a consequence of this directional variation in tensile properties, when a cup isdrawn from a sheet metal blank, the wall height and wall thickness around the drawncup may vary, creating a number of relatively thin protruding “ears” and relatively thick“troughs”. The same effect applies when re-drawing a cup. This effect is commonlyknown as “earing”. Typically four ears occur, each approximately at 45 degrees to thedirection in which the sheet of metal has been rolled, known as the rolling direction.The thicker troughs occur in line with and perpendicular to the rolling direction.Different types of metal sheet may have different crystalline structures and may havebeen subjected to different rolling conditions. These factors may give rise to a differentnumber of ears, and for example aluminium cups may have six ears.

The occurrence of ears contributes to the unevenness of the can body height. Asdiscussed above, the can body must normally be trimmed to an even height and soearing adds to the amount of metal which must be recycled after trimming.

The occurrence of ears also causes problems in the initial cup-making step, as theedge of the sheet metal blank is drawn around the draw radius and the clamping forcebecomes concentrated on the ears as they form. Likewise, as the upper part of the cupwall is drawn around the redraw sleeve radius in the body-making step, the clampingload from the redraw sleeve cannot be applied evenly i.e. the load may be appliedmore-so to the thicker trough regions in between the ears, and less-so at the ears.Wrinkling may therefore appear in the thinner regions of the metal. As the edge of thecup continues to be re-drawn around the re-draw radius the clamping force may thenbecome concentrated on the ears. In each case, the excessive clamping force may nipor tear the metal or its coating. It is difficult to remove the clamping force soon enoughto avoid nipping any ears, without the risk of allowing a wrinkle to form elsewhere.

One approach to the problem of earing is to limit the radial gap between the drawpunch and the draw die in the cup-making step, so that excessively thick regions areironed to a constant thickness around the open end of the cup. However, additionallyironing the cup wall in this way can cause further problems due to increasing theamount of work-hardening caused by ironing, and by altering the surface finish of themetal. The more work-hardened the metal becomes in the cup wall, the more difficult itis to work, and the more tension is required to re-draw it into the can body wall. Whereone region of a metal cup becomes more work-hardened than another, the body wallmay ultimately fracture in a region that is less work-hardened. Ironing the cup wall isparticularly undesirable where a can body is made of polymer-coated metal, because itmay damage or reduce the formability of the polymer-coating, and it may create hairsof polymer at the bare edge. Likewise it is desirable to avoid ironing the re-drawn cupwall in the gap between the punch and the re-draw die in the bodymaker machine.

Thickness variation due to earing also limits the ability to iron the sidewalls of the canbody without tearing. There is a limit to the percentage thickness reduction that can beachieved in an ironing step, and the sidewall can only be ironed to a thicknessdetermined by the thickest part of the sidewall before ironing. Also, when ironing asidewall of circumferentially varying thickness, the movement of un-ironed metal along the punch becomes uneven around its circumference. This causes uneven stressesand maybe ripples in the sidewall, which impair the ironing process.

Stretch-redraw processes as described above can also be affected by uneven stressesand ripples, because uneven clamping can affect the tension needed to hold the metalin tight contact with the draw radius. The thinner regions at the ears may becomethicker than the regions between the ears and vice versa after stretch-redraw, becausethe ear regions may not be clamped sufficiently to generate the tension required tokeep the metal in tight contact with the draw radius. Such loss of contact may alsocause ripples in the sidewall. The walls of the redrawn cup may have several thinnerregions towards the closed end corresponding to thicker regions towards the open end,which will limit the ability to subsequently iron the wall without tearing.

As a result of the problems caused by earing in all of the processes described above, itis common for can bodies and can ends to be manufactured from slightly non-circularblanks having a periphery of varying diameter. The diameter is smaller where the earsmight otherwise occur and larger where the troughs might occur. However,compensating for earing in this way only provides a constant cup height, and does notprovide a constant cup wall thickness around its circumference.

Earing is a problem in the manufacture of can bodies even where no wall-ironing or re-draw carried out. Earing can also adversely affect cups that are drawn to leave aflange. Earing may result in a sidewall of varying circumferential thickness, which isonly as strong as its thinnest part. Additionally, the occurrence of earing leads to morematerial being trimmed away to achieve an even can height, or an even flange width.

Ears that form in the initial cup-making step may also problematic in the manufacture ofcan ends, in which a first step is to draw a shallow circular cup, and in which the cupwall is subsequently formed into a peripheral curl for seaming the end onto a can body.An example of manufacturing an end by first drawing a shallow cup is illustrated in Fig3 of US 4,715,208. Variations in cup wall thickness affect the thickness of theperipheral curl, and this can lead to wrinkling of the curl during either its formation orduring seaming. Can ends could be manufactured from less metal if these thicknessvariations could be avoided.

Summary

Described herein is a draw die for use in a cup-making press. The draw die comprisesa body defining an internal circular draw throat, a substantially flat clamping faceperpendicular to the circular draw throat, and a transition surface between the clampingface and the circular draw throat. The transition surface has a curved axial cross-section which varies around the circular draw throat to define a draw radius and acircular junction with the clamping face.

The variation in the axial cross-section of the transition surface may be periodic aroundthe circular draw throat. The periodic variation may comprise at least a pair ofdiametrically opposed minima and at least a pair of diametrically opposed maxima.

The transition surface may have a circular junction with the throat.

The curved axial cross-section may define an inlet radius adjacent to the clamping faceand a draw radius between the inlet radius and the draw throat, wherein the curvatureof the inlet radius is shallower than that of the draw radius. Both the inlet radius andthe draw radius may have an axial cross-section that varies periodically around thecircular throat, the variation being in-phase.

The curved axial cross-section may further define an exit radius between the drawradius and the draw throat. The exit radius may have a substantially circular junctionwith the draw throat.

The draw die may be configured to be coupled to a cup-making press.

According to a first aspect there is provided a method of manufacturing a metal cupusing the above draw die. The draw die comprises a body defining an internal circulardraw throat, a substantially flat clamping face perpendicular to the circular draw throat,and a transition surface between the clamping face and the circular draw throat, thetransition surface having a curved axial cross-section which varies around the circulardraw throat to define a draw radius. The method comprises orienting the draw diewithin a cup-making press such that a diameter extending between a pair of maxima or a pair of minima of the draw die is aligned with a rolling direction of a sheet of metalfrom which the circular metal blank is cut, clamping the circular metal blank betweenthe clamping face of a pressure pad and the substantially flat clamping face of the drawdie and pushing the circular metal blank through the draw die using a punch to producea cup.

The method may comprise only partially pushing the circular metal blank through thedraw die to produce a cup having a flanged open end.

According to a second aspect of the present invention there is provided a method ofmanufacturing a metal cup according to the above first aspect of the invention, andprocessing the metal cup to produce a can body.

Also described herein is a method of manufacturing the draw die described above, thedraw die being configured to manufacture metal cups having a desired, substantiallyconstant, cup height. The method comprises producing a first cup from a circular blankcut from a sheet of metal using a first draw die having a substantially constant drawradius; producing a second cup from a circular blank cut from the sheet of metal usinga second draw die, the second draw die having a constant draw radius different fromthat of the first draw die; determining and comparing the cup height variations aroundthe first and second cups in order to determine a variation in axial cross section of thetransition surface around the draw die to be manufactured, suitable for manufacturingmetal cups of the desired, constant, cup height; and manufacturing a draw die havingthe determined variation in axial cross section.

Also described herein is a method of manufacturing the draw die according to the firstaspect above. The method comprises manufacturing a draw die such that thetransition surface has a uniform axial cross-section around the draw die and removingmaterial from the transition surface in order to vary the axial cross-section and provideregions of enlarged draw radius whilst leaving the junctions between the transitionsurface and the clamping face and the draw throat unchanged.

Further described herein is a method of manufacturing the draw die described above.The method comprises determining using finite element analysis (FEA) an axial cross-section required at each position around the transition surface of the draw die in order to increase or decrease a cup height or cupwall thickness at each said position to that desired, and manufacturing a draw diehaving an axial cross-section of the transition surface according to the determination.

Brief Description of the Drawings

Figure 1 is a schematic cross-section of a known cup-drawing toolset with a sheet ofmetal in place;

Figure 2 is a schematic cross-section of the cup-drawing toolset of Figure 1, moved toa position with a blank cut from the sheet;

Figure 3 is a schematic cross-section of the cup-drawing toolset of Figure 1, moved toa position with a cup partially drawn;

Figure 4 is a schematic cross-section of the cup-drawing toolset of Figure 1, moved toa position with the cup fully drawn;

Figure 5A is a perspective view of a sheet-metal blank used to make a can body;

Figure 5B is a perspective view of a cup made from the sheet-metal blank of Figure 5A;Figure 6 is a perspective view of a first embodiment of a variable radius draw die;

Figure 7A and Figure 7B are cross-sectional views of the variable radius draw die ofFigure 6;

Figure 8 is a perspective view of a second embodiment of a variable radius draw die;

Figure 9A and Figure 9B are cross-sectional views of the variable radius draw die ofFigure 8;

Figure 10 is a perspective view of a third embodiment of a variable radius draw die;

Figure 11A and Figure 11B are cross-sectional views of the variable radius draw die ofFigure 10;

Figure 12 illustrates a method of manufacturing a metal cup using the draw die ofFigures 6 and 7, 8 and 9, or 10 and 11;

Figure 13 illustrates a method of determining an optimised draw die profile;

Figure 14 illustrates a typical effect of draw radius on cup height; and

Figure 15 illustrates a typical effect of draw radius on cup wall thickness.

Detailed Description

Figures 1 to 4 illustrate a simple schematic arrangement of known tools used to draweither a partially-drawn cup or a fully-drawn cup, as discussed above. For simplicity, details including small clearances and variations in cup wall thickness are not shown inFigures 1 to 4. The tools should be understood in the context of a cup-making presssuch as has been described in general terms above.

Figure 1 is a schematic cross section of a cup-drawing toolset comprising a draw die 1,a pressure pad 2, a cutting die 3, a plate 4 and a punch 5. A sheet of metal 6 is placedon the plate 4 (it will be understood that the sheet of metal may extend across anumber of cup-making presses arranged in parallel). The draw die 1 has a clampingface 11, a draw radius 12 and a throat 13. The pressure pad 2 has a clamping face 14.

Figure 2 shows the tools of Figure 1 moved to a position where the cutting die 3 hasbeen moved past the face 11 of the draw die 1 to cut a circular blank 7 from the metalsheet 6, leaving a scrap piece 8. The plate 4 has been pushed downwards by thedownward movement of the cutting die 3. The blank 7 is clamped by a clamping forceF between the clamping face 14 of the pressure pad 2 and the clamping face 11 of thedraw die 1 in order to allow the metal of the blank to slide radially inwards withoutwrinkling as it is drawn.

Figure 3 shows the tools of Figure 1 moved further to a position where the blank 7 isbeing drawn through the draw die 1 by the punch 5 to produce a partially-drawn cup 9.The partially-drawn cup 9 has a wall 17 and a flange 16. The flange 16 remainsclamped between the clamping face 14 of the pressure pad 2 and the clamping face 11of the draw die 1.

Figure 4 shows the tools of Figure 1 moved further to a position where a fully drawncup 10 has been produced, the cup having a generally cylindrical wall 18. As the cup10 has been fully drawn, the flange 16 shown in Figure 3 has been pulled frombetween the clamping face 14 of the pressure pad 2 and the clamping face 11 of thedraw die 1, and these two clamping faces 14, 11 are now in contact with each other.The flange 16 now forms part of the wall 17 of the fully drawn cup 10.

With reference to Figure 5A there is illustrated the sheet metal blank 7 having adiameter D1 and a thickness T1. The blank 7 is substantially circular. The metal blank7 may comprise aluminium, aluminium alloy, steel, steel alloy or other suitable metalsor metal alloys.

With reference to Figure 5B there is illustrated the drawn sheet metal cup 10 having asidewall 18 of internal diameter D2, and a base 19, the sidewall 18 and base 19 beingjoined by a radius 20. The diameter D2 of the drawn cup 10 is of course smaller thanthe diameter D1 of the metal blank 7.

It will be understood that a can body can be manufactured using the cups producedusing the draw die described above. This process is conventional, for example using acan bodymaker to draw and wall iron the metal cup.

In the light of the discussion above, it will be appreciated that the sidewall 18 of the cup10 is of uneven height and comprises a number of ears 21 and troughs 22. In thisillustrated example, the cup 10 has four ears 21 and four troughs 22. The sidewall 18has a substantially greater height in the region of the ears 21 than in the region of thetroughs 22. The thickness of the sidewall 18 at the ears 21 is thinner than at thetroughs 22, and the sidewall 18 is generally thicker towards the open end of the cup 10than towards the base 19.

This variation in cup height and sidewall thickness caused by anisotropy may lead tosome or all of the problems discussed above, for example during subsequent re-draw,stretch-re-draw and ironing operations.

Figure 6 is a perspective view of an embodiment of a variable radius draw die 30. Thedraw die 30 comprises a body defining an internal circular draw throat 32, asubstantially flat (upper) clamping face 31 perpendicular to the circular draw throat, anda transition surface 39 between the clamping face 31 and the circular draw throat 32.The transition surface 39 has a curved axial cross-section which varies around thecircular draw throat 32. The clamping face 31 and the throat 32 carry out substantiallythe same functions as the clamping face 11 and throat 13 of the draw die 1 of Figures 1to 4. Such a variable radius draw die 30 is used instead of the conventional draw die 1in the cup-making press of Figures 1 to 4.

In the embodiment of Figure 6, the transition surface 39 comprises a draw radius 33,over which a metal blank is drawn. The draw radius 33 varies periodically around thecircular draw throat 32, and comprises pairs of diametrically opposed maxima 34 and pairs of diametrically opposed minima 35. The transition surface 39 further comprisesa first non-circular junction 36 between the clamping face 31 and the draw radius 33,and a second non-circular junction 37 between the draw radius 33 and the throat 32.

Figure 7A is a cross-sectional view of the variable radius draw die 30 of Figure 6, withthe cross-section taken in a plane corresponding to a pair of symmetrically opposedmaxima 34 of the die 30 (indicated in Figure 6 with the lines AA). Figure 7A includesan enlarged view of the draw radius 33, illustrating the maxima 34. Figure 7B shows across-sectional view of the variable radius draw die 30 of Figure 6, with the cross-section taken in a plane corresponding to a pair of symmetrically opposed minima 35 ofthe die 30 (indicated in Figure 6 with the lines BB). Figure 7B also includes anenlarged view of the draw radius 33, illustrating the minima 35.

It will be appreciated from Figures 6 and 7 that the maxima 34 occur where the width ofthe axial cross-section of the draw radius 33 of the draw die 30 is greatest. Theminima 35 occur where the width of the axial cross-section of the draw radius 33 of thedraw die 30 is smallest.

In use, the draw die 30 is oriented within a toolset of a cup-making press (i.e. replacingthe conventional draw die of Figures 1 to 4), in order to align with the rolling direction ofthe sheet metal. In other words, the maxima 34 of the variable radius 33 of the drawdie 30 are aligned with the directions in which ears would form if the draw radius wasconstant and did not have a variable axial cross-section. The minima 35 of the variableradius 33 of the draw die 30 are aligned with the directions where troughs would form ifthe draw radius was constant and did not have a variable axial cross-section. Thevariable radius 33 of the draw die 30 therefore seeks to compensate for the anisotropyof the metal, while the circular draw-throat 32 provides a drawn cup having asubstantially constant diameter.

Figure 8 is a perspective view of a second embodiment of a variable radius draw die40. The draw die 40 comprises a body defining an internal circular draw throat 42, asubstantially flat upper clamping face 41 perpendicular to the circular draw throat 42,and a transition surface 50 between the clamping face 41 and the circular draw throat42. The transition surface 50 has a curved axial cross-section which varies around thecircular draw throat 42. The clamping face 41 and the throat 42 carry out substantially the same functions as the clamping face 11 and throat 13 of the draw die 1 of Figures 1to 4. Such a variable radius draw die 40 is used instead of the conventional draw die 1in the cup-making press of Figures 1 to 4.

In the embodiment of Figure 8, the transition surface 50 comprises draw radius 43,over which a metal blank is drawn. The draw radius 43 varies periodically around thecircular draw throat 42, and comprises pairs of diametrically opposed maxima 44 andpairs of diametrically opposed minima 45. The transition surface 50 further comprisesan inlet radius 48 tangential to the clamping face 41 at a circular junction 46. Thetransition surface 50 further comprises a first non-circular junction 49 between the inletradius 48 and the draw radius 43, and a second non-circular junction 47 between thedraw radius 43 and the throat 42.

Figure 9A is a cross-sectional view of the second embodiment of the variable radiusdraw die 40, with the cross-section taken in a plane corresponding to a pair ofdiametrically opposed maxima 44 (indicated in Figure 8 with the lines A’Aj. Figure 9Aincludes an enlarged view of the variable draw radius 43 illustrating the maxima 44.Figure 9B is a cross-sectional view of the third embodiment of the variable radius drawdie 40, the cross-section taken in a plane corresponding to a pair of diametricallyopposed minima 45 (indicated in Figure 8 with the lines B’Bj. Figure 9B includes anenlarged view of the variable draw radius 43, illustrating the minima 45.

It will be appreciated from Figures 8 and 9 that the maxima 44 occur where the width ofthe axial cross-section of the draw radius 43 of the draw die 40 is greatest. Theminima 45 occur where the width of the axial cross-section of the draw radius 43 of thedraw die 40 is smallest.

In use, the draw die 40 is oriented within a toolset, such as the cup-making press ofFigures 1 to 4, to align with the rolling direction of the metal, such that the maxima 44of the variable draw radius 43 align with the directions where ears would form if thedraw radius was constant, and such that the minima 45 of the variable draw radius 43align with the directions where troughs would form if the draw radius was constant.

In contrast to the draw die 30 of Figures 6 and 7, a circular tangential junction 46leading to an inlet radius 48 is provided between the clamping face 41 and the first non-circular junction 49 of the draw die 40 of Figures 8 and 9. To facilitatemanufacture of the junctions 46, 49 and 47 to be tangential, the inlet radius 48 is equalto or larger than the maxima 44 of the variable radius 43. Where the inlet radius 48 isequal to the maxima 44 of the variable draw radius 43, the inlet radius 48 will tend tomerge with the maxima 44. It is therefore preferable that the inlet radius 48 is largerthan the maxima 44 of the variable draw radius 43, in order to facilitate manufacture ofthe circular tangential junction 46.

The circular tangential junction 46 between the clamping face 41 and the inlet radius 48may provide the variable radius draw die 40 of Figures 8 and 9 with advantages overthe variable radius draw die 30 of Figures 6 and 7. A first advantage is that when the edge of a cup having walls of even height is drawnaround the inlet radius 48, the clamping force (F in Figures 1 and 2) provided by thepressure pad of a toolset (for example the pressure pad 2 shown in Figure 1) will bereleased simultaneously all around the cup edge, as the cup edge is pulled frombetween the pressure pad clamping face (14 in Figure 1) and the clamping face 41 ofthe draw die 40. This is due to the circular nature of the junction 46 between theclamping face 41 and the circular inlet radius 48. In the embodiment of Figures 6 and7, the junction 36 between the draw die clamping face 31 and the variable draw radius33 is non-circular and so as the edge of the cup is drawn over this junction theclamping force will not be released simultaneously all around the cup diameter.

The simultaneous removal of the clamping force F in the embodiment of Figures 8 and9 will reduce any tendency for the edge of the cup to be nipped by the clamping facesof the pressure pad 14 and the draw die 41, or for any coatings on the metal cup to bedamaged, but without risk of removing the clamping force too soon from any region ofthe cup that might wrinkle. A second advantage of the variable draw die 40 of Figures 8 and 9 is that it is easier tomanufacture a smoothly blended draw die 40 wherein the clamping face 41 is locatedtangentially to a larger and non-variable inlet radius 48 at a circular junction 46, ratherthan to a smaller and variable radius 33 at a non-circular junction 36.

Whilst it is advantageous that the junctions 49 and 47 of the draw die 40 of Figures 8and 9 are blended smoothly, they need not be perfectly tangential, provided that noneof the profile presents a corner radius smaller than that of the variable radius 43 (whichmight lead to unexpected thinning). The junctions 49, 47 may therefore be smoothedby polishing, providing that the majority of the variable draw radius 43 remains true toits intended shape. Tool-making tolerances are therefore less critical for the variableradius draw die 40 of Figures 8 and 9 than for the variable radius draw die 30 ofFigures 6 and 7, due to the inclusion of a non-variable inlet radius 48 at a circularjunction 46. A third advantage of the variable draw die 40 of Figures 8 and 9 is that the metal of thecup will need to flow through an angle of 90 degrees around a minima 35 of thevariable radius draw die 30 of Figures 6 and 7, but will only need to flow through asmaller angle around a minima 45 of the variable radius draw die 40 of Figures 8 and 9. This is best illustrated by comparing the enlarged sections of Figures 7B and 9B.The addition of a circular junction 46 and an inlet radius 48 in the embodiment of Figure9B provides a shallower, more gradual transition between the clamping face 41 and theminima 45 than is found in the draw die 30 of Figure 7B. Since the metal only needs tobe bent once against the variable radius 33, 43 and straightened once as it leavescontact with it, the preferred variable radius draw die 40 of Figures 8 and 9 willgenerate the same amount of thickness reduction as discussed above with regard tostretch-draw, but with less drag than the variable radius draw die 30 of Figures 6 and 7.This is particularly beneficial where the cup being drawn is made from pre-coated metalsheet.

Figure 10 is a perspective view of a third embodiment of a variable radius draw die 60.The draw die 60 comprises a body defining an internal circular draw throat 69, asubstantially flat upper clamping face 70 perpendicular to the circular draw throat 69,and a transition surface 61 between the clamping face 70 and the circular draw throat69. The transition surface 61 has a curved axial cross-section which varies around thecircular draw throat 69. The clamping face 70 and the throat 69 carry out substantiallythe same functions as the clamping face 11 and throat 13 of the draw die 1 of Figures 1to 4. Such a variable radius draw die 60 is used instead of the conventional draw die 1in the cup-making press of Figures 1 to 4.

In the embodiment of Figure 10, the transition surface 61 comprises draw radius 65,over which a metal blank is drawn. The draw radius 65 varies periodically around thecircular draw throat 69, and comprises pairs of diametrically opposed maxima 71 andpairs of diametrically opposed minima 72. The transition surface 61 further comprisesan inlet radius 63 tangential to the clamping face 70 at a first circular junction 62. Thetransition surface 61 further comprises a first non-circular tangential junction 64between the inlet radius 63 and the draw radius 65, a second non-circular tangentialjunction 66 between the draw radius 65 and an exit radius 67, and a second circulartangential junction 68 between the exit radius 67 and the throat 69.

Figure 11A is a cross-sectional view of the third embodiment of the variable radiusdraw die 60, with the cross-section taken in a plane corresponding to a pair ofdiametrically opposed maxima 71 (indicated in Figure 10 with the lines A”A”). Figure11A includes an enlarged view of the variable draw radius 65 illustrating the maxima71. Figure 11B is a cross-sectional view of the third embodiment of the variable radiusdraw die 60, the cross-section taken in a plane corresponding to a pair of diametricallyopposed minima 72 (indicated in Figure 10 with the lines Figure 11B includes an enlarged view of the variable draw radius 65, illustrating the minima 72.

It will be appreciated from Figures 10 and 11 that the maxima 71 occur where the widthof the axial cross-section of the draw radius 65 of the draw die 60 is greatest. Theminima 72 occur where the width of the axial cross-section of the draw radius 65 of thedraw die 60 is smallest.

In use, the draw die 60 is oriented within a toolset, such as the cup-making press ofFigures 1 to 4, to align with the rolling direction of the metal, such that the maxima 71of the variable radius 65 align with the directions where ears would form if the drawradius was constant, and such that the minima 72 of the variable radius 65 align withthe directions where troughs would form if the draw radius was constant.

Figure 11 illustrates how the overall dimensions of the transition surface 61 and theradii of curvature of the transition surface 61 may be chosen in proportion to oneanother. It will be appreciated that the transition surface 61 in the example of Figures10 and 11 comprises an interior surface of the draw die 60 between the clamping face70 and the throat 69 (i.e. a first circular junction 62, an inlet radius 63, a first non- circular tangential junction 64, a draw radius 65, a second non-circular tangentialjunction 66, an exit radius 67, and a second circular tangential junction 68).

In the example shown in Figures 11A and 11B, an overall width W and an overall depthD of the transition surface 61 are both approximately 5mm. A radius R10 of the inletradius 63 and a radius R10 of the exit radius 67 are both approximately 10mm. Aradius R2, R4 of the draw radius 65 varies periodically from a maximum ofapproximately 4mm (R4) to a minimum of approximately 2mm (R2). These radii maybe scaled or altered in relation to one another according to the thickness of the metalbeing used and its earing characteristics and according to the desired dimensions ofthe drawn cup. The ability to manufacture the draw die 60 with tangential junctionsbetween surfaces is facilitated by the inlet 63 and exit 67 radii being larger than themaxima 71 of the draw radius 65.

The second circular tangential junction 68, which is between the exit radius 67 and thethroat 69, may provide the variable radius draw die 60 of Figures 10 and 11 with furtheradvantages over the variable radius draw die 30 of Figures 6 and 7 and the variableradius draw die 40 of Figures 8 and 9. A first advantage of the variable draw die 60 of Figures 10 and 11 is that it is easier tomanufacture a smoothly blended draw die 60 wherein the throat 69 is locatedtangentially to a larger and non-variable exit radius 67 at a circular junction 68, ratherthan to a smaller and variable radius 43 at a non-circular junction 47. A second advantage of the variable draw die 60 of Figures 10 and 11 is that it ispossible to manufacture the variable draw die in two stages. In the first stage ofmanufacture, a draw die is produced wherein the draw radius does not vary, and issubstantially equal to the draw radius intended at the minima 72. In the second stageof manufacture, material is removed from the draw die to enlarge regions of the internaldraw die surface and create the maxima 71, without affecting either of the first 62 or thesecond 68 circular tangential junctions. This method of manufacture enablesprogressive removal of material from the draw die 60 until cups produced using thedraw die 60 do not exhibit earing.

Figure 12 illustrates an exemplary method of manufacturing a metal cup using a drawdie described with reference to Figures 6 to 11 above. The method comprises thesteps of: S1, cutting a circular metal blank from a sheet of metal using a cutting die; S2,clamping the circular metal blank between the clamping face of a pressure pad and theclamping face of the draw die; and S3, pushing the metal blank through the draw dieusing a punch to produce a cup.

It will be appreciated that the dimensions of the transition surface 39, 50, 61 or variableradius 33, 43, 65 can be varied around the draw die 30, 40, 60 to create an optimised“profile” that minimises or prevents the effect of anisotropy, to achieve a drawn metalcup having sidewalls of a more even height and/or a more even thickness all aroundthe cup diameter. The optimised “profile” for any cup specification will depend onseveral factors, including the required height and diameter of the drawn cup, the metaltype, thickness and mechanical and metallurgical properties.

Figure 13 illustrates an exemplary method of determining an optimised draw die profilei.e. determining the axial cross-section of the transition surface between the clampingface and the draw throat of a draw die, to minimise or prevent the effect of anisotropyand to manufacture metal cups having a desired, substantially constant, cup height.The method comprises the following steps:

Step 1: Producing two sets of cup samples using two draw dies having substantiallyconstant draw radii, the second draw die having a different e.g. smaller draw radiusthan the first draw die. The cup samples are produced from circular blanks cut fromthe same sheet of metal.

Step 2: Determining and comparing the cup height variations around the first andsecond cups in order to determine a variation in axial cross section of the transitionsurface around the draw die to be manufactured, suitable for manufacturing metal cupsof the desired, constant, cup height.

Step 3: Based on the determination of step 2, manufacturing a variable radius draw diehaving the determined variation in axial cross section.

Step 4: Manufacturing and measuring a set of cup samples using the manufacturedvariable radius draw die, and determining whether any further adjustments arerequired.

An alternative to the method of Figure 13 is to use finite element analysis (FEA) inplace of steps 1-3 described with reference to Figure 13 above. The graph of Figure14 shows FEA predictions of cup height in millimetres versus draw radius in millimetresusing two slightly different friction coefficients (f = 0.05 and f = 0.1) for a typical tin-plated steel cup. The draw radius described in the graph of Figure 14 is equivalent tothe draw radius 65 illustrated in Figures 11A and 11B above. The friction coefficient isan indication of the force that resists sliding between the die surface and the sheet ofmetal when one is moving relative to the other. From the graph in the example ofFigure 14, it can be seen that varying the draw radius of the draw die from 2.25mm to1.25mm could compensate for ears approximately 1mm taller than troughs.

The graph of Figure 15 shows FEA predictions of cup wall thickness, at a height of20mm up the cup wall from the closed end, versus draw radius, using two slightlydifferent friction coefficients (f = 0.05 and f = 0.1), for a typical aluminium cup. Thedraw radius described in the graph of Figure 15 is equivalent to the draw radius 65illustrated in Figures 11A and 11B above. From the graph in the example of Figure 15it can be seen that varying the draw radius of the draw die from 2.75mm to 1.75mmcould compensate for thickness variation of 3 microns (0.003mm) at a height of 20mmup the cup wall from the closed end.

The variable radius draw die 30, 40, 60 described herein reduces both thickness andheight variation in cup walls caused by anisotropy. It reduces or removes the need toiron excessively thick regions caused by earing in the initial cup-making process. Thisis especially beneficial where the cup is being drawn from sheet metal pre-coated withpolymer or the like. It also reduces the amount of metal which must be recycled due tothe need to trim off ears. The variable radius draw die 30, 40, 60 may be used in oneor more of a draw, re-draw, stretch-draw, stretch-re-draw and ironing process in orderto prevent or reduce the formation of ears and troughs.

The variable radius draw die 30, 40, 60 described herein may be used to draw acontainer wherein the edge is to be subsequently formed into a curl, without the needfor a trimming operation prior to forming the curl.

As an alternative to preventing the formation of ears and troughs in a cup, it may bepreferable to over-compensate for the earing that would otherwise occur in the cupwhen drawing it. In other words, a variable draw die profile can be formulated that willcreate troughs in place of ears and ears in place of troughs as the cup is drawn in thecup-making step. This may be beneficial when a can is manufactured by drawing acup and subsequently re-drawing that cup. Then, when subsequently re-drawing thecup using a conventional redraw die in the body-making step, the earing that wouldoccur during the re-draw could already have been compensated for, and a more even-walled can body may be manufactured.

Alternatively or additionally, it is possible to vary the inlet radii 48, 63 or the exit radius67, in addition to varying the draw radius around the circular throat 42, 69.

Cups made using a variable radius draw die 30, 40, 60 as described above may befully drawn through the draw die 30, 40, 60 to produce a generally cylindrical wall, ormay be partially drawn through the draw die 30, 40, 60 to produce a cup with a flangesurrounding the open end of the cup.

The variable radius draw die 30, 40, 60 may preferably be manufactured fromhardened tool-steel, more preferably from tungsten carbide, and more preferably froman engineering-grade ceramic. Tungsten carbide or ceramic may be inserted into atool-steel holder.

The variable radius draw die 30, 40, 60 may preferably be manufactured usingcomputerised numerically controlled (CNC) grinding, hard-turning, hard milling, orelectro-discharge machining. Electro-discharge machining using a wire (commonlyknown as wire-erosion) is further preferred for the manufacturing of the variable radiusfrom tool-steel or tungsten carbide.

Cups manufactured using the variable radius draw die 30, 40, 60 described above,may be used in the manufacture of can bodies, can ends or closures for cans, bottles or jars, using any of the following materials: steel, tinplate or aluminium. Any of thesematerials may be pre-coated with organic coatings, or polymer coatings such aspolypropylene or polyethylene terephthalate.

The invention has been defined in this description and the accompanying claims usingthe term “radius” in order to communicate the invention as clearly as possible.However, the scope of the invention includes curved geometry deviating from perfectlytrue geometric radii wherein the principles of the invention still apply.

Claims (3)

CLAIMS:

1. A method of manufacturing a metal cup using a draw die, the draw diecomprising a body defining an internal circular draw throat, a substantially flat clampingface perpendicular to the circular draw throat, and a transition surface between theclamping face and the circular draw throat, the transition surface having a curved axialcross-section which varies around the circular draw throat to define a draw radius; the method comprising: orienting the draw die within a cup-making press such that a diameter extendingbetween a pair of maxima or a pair of minima of the draw die is aligned with arolling direction of a sheet of metal from which the circular metal blank is cut; clamping the circular metal blank between the clamping face of a pressure padand the substantially flat clamping face of the draw die; and pushing the circular metal blank through the draw die using a punch to producea cup.

2. A method according to claim 1 and comprising only partially pushing the circularmetal blank through the draw die to produce a cup having a flanged open end.

3. A method of manufacturing a can body comprising manufacturing a metal cupaccording to the method of claims 1 or 2 and processing the metal cup to produce acan body.