Abstract
A method and apparatus are disclosed which are suitable for use in the manufacture of two-piece metal containers. In particular, a way of making cups from metal sheet is disclosed using a combination of stretching and drawing operations. The resulting cups have the advantage of having a base thickness that is thinner relative to the ingoing gauge of the metal sheet.
Claims
1. An apparatus for manufacture of a metal cup for a two-piece food container, the apparatus comprising: a clamping tool adapted to clamp a metal sheet during a stretching operation, the clamping tool adapted to clamp an annular region on the sheet to define an enclosed portion; a stretch tool adapted to deform and stretch all or part of the enclosed portion in the stretching operation to thereby increase the surface area and reduce the thickness of the enclosed portion, the clamping tool further adapted to restrict or prevent metal flow from the clamped region into the enclosed portion during the stretching operation; and a drawing tool adapted to draw the metal sheet into the cup having a sidewall and an integral base, the base comprising material from the stretched and thinned enclosed portion, the drawing tool further adapted to pull and transfer outwardly material of the stretched and thinned enclosed portion into the sidewall in a drawing operation, whereby lightweighting of the cup is achievable in a cost-effective manner.
2. The apparatus as claimed in claim 1, wherein the clamping tool comprises a clamping element having a clamping face, the clamping face having a textured surface.
3. The apparatus as claimed in claim 1, wherein the clamping tool comprises a first clamping element and a second clamping element, the first and second clamping elements adapted to clamp opposing surfaces of the metal sheet, each of the first and second clamping elements having a clamping face having geometric discontinuities to thereby assist in disrupting the flow of the metal of the metal sheet between the first and second clamping elements as the stretching operation is performed.
4. The apparatus as claimed in claim 3, wherein the geometric discontinuities comprise any one of: i. the clamping face of the first clamping element having one or more beads, ridges or steps which, in use, urge metal of the clamped annular region within corresponding one or more relief features in the clamping face of the second clamping element; or ii. the clamping face of the second clamping element instead having one or more beads, ridges or steps which, in use, urge metal of the clamped annular region within corresponding one or more relief features instead in the clamping face of the first clamping element; or iii. a combination of (i) and (ii).
5. The apparatus as claimed in claim 4, wherein the first and second clamping elements are adapted such that, in use, the one or more beads, ridges or steps in the clamping face of the first or second clamping element urge metal of the clamped annular region so as to be wholly enclosed by and within the corresponding one or more relief features in the corresponding clamping face of the second or first clamping element.
6. The apparatus as claimed in claim 1, wherein the stretch tool comprises a stretch punch, the apparatus adapted to move either or both of the stretch punch and the metal sheet toward each other so that, in use, the stretch punch deforms and stretches all or part of the enclosed portion.
7. The apparatus as claimed in claim 6, wherein the stretch punch has an end face with a non-planar profile, the apparatus adapted to move either or both of the stretch punch and the metal sheet toward each other so that, in use, the stretch punch deforms and stretches all or part of the enclosed portion into a corresponding non-planar profile.
8. The apparatus as claimed in claim 6, wherein the stretch punch comprises an end face having one or more relief features.
9. The apparatus as claimed in claim 6, wherein the stretch punch comprises a punch assembly, the assembly comprising a first group of one or more punches opposing one surface of the enclosed portion and a second group of one or more punches opposing the opposite surface of the enclosed portion, the first and second groups moveable towards each other to, in use, deform and stretch all or part of the enclosed portion.
10. The apparatus as claimed in claim 1, wherein the drawing tool is adapted to first initially draw the sheet into a cup profile and to then subsequently re-draw the cup in one or more stages.
11. The apparatus as claimed in claim 1, further comprising a tool for ironing the cup.
12. An apparatus as claimed in claim 1, wherein the drawing tooling is adapted to reduce a height of a dome formed by the stretch tool by pulling and transferring material of the stretched and thinned base.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a side elevation view of a container body of the background art resulting from a conventional DWI process. It shows the distribution of material in the base and sidewall regions of the container body.
(2) FIG. 2 is a graph showing in general terms how the net overall cost of manufacturing a typical two-piece metal container varies with the ingoing gauge of the sheet metal. The graph shows how reducing the thickness of the sidewall region (e.g. by ironing) has the effect of driving down the net overall cost.
(3) FIG. 3 is a graph corresponding to FIG. 2, but based on actual price data for UK-supplied tinplate.
(4) Embodiments of the invention are illustrated in the following drawings, with reference to the accompanying description:
(5) FIG. 4 is a graphical representation of the variation in thickness of the enclosed portion of a metal sheet that has been subjected to a stretching operation using a stretch punch having a domed profiled end face.
(6) FIG. 5a is a side elevation view of a stretch rig used to perform the stretching operation of the invention. The figure shows the stretch rig before the stretching operation has commenced.
(7) FIG. 5b shows the stretch rig of FIG. 5a, but on completion of the stretching operation.
(8) FIG. 6a shows a cross-section through a first embodiment of clamping means used to clamp the metal sheet during the stretching operation.
(9) FIG. 6b shows a cross-section through part of the metal sheet resulting from use of the clamping means shown in FIG. 6a.
(10) FIG. 7a shows a cross-section through a second embodiment of clamping means used to clamp the metal sheet during the stretching operation.
(11) FIG. 7b shows a cross-section through part of the metal sheet resulting from use of the clamping means shown in FIG. 7a.
(12) FIG. 8 shows an alternative embodiment of stretch punch to that shown in FIGS. 5a and 5b.
(13) FIG. 9 shows a further alternative embodiment of stretch punch to that shown in FIGS. 5a and 5b, where the end face of the stretch punch includes various relief features.
(14) FIG. 10 shows an expanse of metal sheet on which the stretching operation of the invention has been performed on a plurality of enclosed portions separated from each other and disposed across the area of the metal sheet.
(15) FIGS. 11a and 11b show how, when performing the stretching operation to provide the stretched sheet shown in FIG. 10, any simultaneous stretching of two or more of the enclosed portions may be staggered to reduce the loads imposed on the tooling used.
(16) FIG. 12a is a side elevation view of the tooling of a cupping press used to perform an initial drawing stage of the drawing operation to form a cup from the stretched sheet metal. The figure shows the tooling before this initial drawing stage has commenced.
(17) FIG. 12b corresponds to FIG. 12a, but on completion of the initial drawing stage.
(18) FIGS. 13a-d show perspective views of a bodymaker assembly used to re-draw the cup in a re-drawing stage of the drawing operation. The figures show the operation of the bodymaker from start to finish of the redrawing stage.
(19) FIG. 14 shows a detail view of the re-draw die used in the bodymaker assembly of FIGS. 13a-d.
(20) FIG. 15 shows a sheet metal blank at various stages during the method of the invention as it progresses from a planar sheet to a finished cup.
(21) FIG. 16 shows the use of the cup of the invention as part of a two-piece container.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
Mode(s) for Carrying Out the Invention
(22) Stretching Operation
(23) A flat section of metal sheet 10 is located within a stretch rig 20 (an example of which is illustrated in FIGS. 5a and 5b). Steel tin-plate (Temper 4) with an ingoing gauge thickness (t.sub.in-going) of 0.280 mm has been used for the metal sheet 10. However, the invention is not limited to particular gauges or metals. The section of metal sheet 10 is typically cut from a roll of metal sheet (not shown). The stretch rig 20 has two platens 21, 22 that are moveable relative to each other along parallel axes 23 under the action of loads applied through cylinders 24 (see FIGS. 5a and 5b). The loads may be applied by any conventional means, e.g. pneumatically, hydraulically or through high-pressure nitrogen cylinders.
(24) On platen 21 is mounted a stretch punch 25 and a clamping element in the form of a first clamp ring 26. The first clamp ring 26 is located radially outward of the stretch punch 25. The stretch punch 25 is provided with a domed end face (see FIGS. 5a and 5b).
(25) On platen 22 is mounted a second clamp ring 27. The second clamp ring 27 is a tubular insert having an annular end face 28 (see FIGS. 5a and 5b). In use, loads are applied via the cylinders 24 to move platens 21, 22 towards each other along the axes 23 until the flat section of metal sheet 10 is clamped firmly in an annular manner between the first and second clamp rings 26, 27 to define a clamped annular region 15 on the section of metal sheet. In this way, the first clamp ring 26 and the second clamp ring 27 each act as clamping elements. The clamped annular region 15 defines an enclosed portion 16 on the metal sheet 10.
(26) The stretch punch 25 is then moved axially through the first clamp ring 26 to progressively deform and stretch (thin) the metal of the enclosed portion 16 into a domed profile 17 (see FIG. 5b).
(27) Ideally, the clamping loads applied during this stretching operation are sufficient to ensure that little or no material from the clamped annular region 15 (or from outside of the clamped region) flows into the enclosed portion 16 during stretching. This helps to maximise the amount of stretching and thinning that occurs in the enclosed portion 16. However, as indicated above in the general description of the invention, it has been found that stretching and thinning of the metal of the enclosed portion 16 can still occur when permitting a limited amount of flow of metal from the clamped annular region 15 (or from outside of the clamped region) into the enclosed portion.
(28) FIGS. 6a & 7a show detail views of two embodiments of the first clamp ring 26 and second clamp ring 27 used to clamp the metal sheet 10 during the stretching operation.
(29) FIG. 6a shows the face of the first clamp ring 26 provided with an annular step 261 having a width w that opens out to the radial interior edge of the first clamp ring. A corresponding annular cut-out 271 is provided in the face of the second clamp ring 27. In the embodiment shown, the step 261 and cut-out 271 have a height h of 1 mm and radii R.sub.261, 271 of 0.5 mm. The axially extending sides S.sub.261, 271 of the step 261 and cutout 271 are radially offset from each other by a distance greater than the thickness t of the metal sheet they are intended to clamp (see distance in FIG. 6a). This avoids the metal sheet being pinched or coined during clamping and thereby helps to minimize the formation of a weakened region that would be vulnerable to tearing during the subsequent drawing operation (or any subsequent ironing operation).
(30) FIG. 6b shows a partial view of the metal sheet that results from use of the clamping arrangement shown in FIG. 6a.
(31) FIG. 7a shows the face of the first clamp ring 26 provided with an annular bead 261 located away from the radial interior and exterior edges of the first clamp ring. A corresponding annular recess 271 is provided in the face of the second clamp ring 27. In this alternative embodiment, the bead 261 is capable of being wholly enclosed by and within the recess 271in contrast to the embodiment in FIG. 6a. Expressed another way, in use, the bead 261 of FIG. 7a urges metal of the clamped annular region 15 so as to be wholly enclosed by and within the recess 271. In this embodiment, the bead 261 has a height h of around 0.5 mm, with radii R261, 271 of around 0.3 mm and 0.75 mm respectively. As can be seen from FIG. 7a, in common with the embodiment in FIG. 6a, the bead 261 and recess 271 are profiled to avoid the metal sheet being pinched or coined during clamping.
(32) FIG. 7b shows a partial view of the metal sheet that results from use of the clamping arrangement shown in FIG. 7a.
(33) Both clamping embodiments have been used on 0.277 mm and 0.310 mm gauge metal sheet. However, this statement is not intended to limit the scope or applicability of the method or apparatus of the invention.
(34) Table 1 below shows for both clamping embodiments (FIGS. 6a and 7a) the axial clamping loads required during the stretching operation to achieve a given amount of stretching. Note that the data in Table 1 was based upon clamping and stretching the planar base of a cup (as shown in FIGS. 7a, 7b, 8a and 8b of application PCT/EP11/051666 (CROWN Packaging Technology, Inc); however, the data is equally applicable to the present invention because the region being clamped and stretched is planar in both cases. Table 1 clearly show that having the bead 261 adapted to be wholly enclosed by and within the recess 271 (as in the embodiment of FIG. 7a) drastically reduces the clamping loads required by almost 50% relative to the loads required when using the clamping arrangement of FIG. 6a. The reason for this difference in required axial clamping loads is that having the bead 261 capable of extending wholly within the corresponding recess 271 provides greater disruption to metal flow during the stretching operation and thereby provides an improved clamping effect. The disruption to metal flow is greater for the embodiment of FIG. 7a because the metal flow is disrupted by both axially extending sides S.sub.261 of the bead 261, whereas for the embodiment of FIG. 6a the metal flow is only disrupted by a single axially extending side S261 of its bead.
(35) TABLE-US-00001 TABLE 1 Clamping Axial Clamping Slippage Embodiment Force (kN) (mm) FIG. 6a 46-53 0.85-1.3 FIG. 7a 25-29 0.05
(36) In an alternative embodiment, the single stretch punch 25 is replaced by a punch assembly 250 (as shown in FIG. 8). The punch assembly 250 has:
(37) i) a first group 251 of an annular punch element 251 a surrounding a central core punch element 251b; and
(38) ii) a second group 252 of an annular punch elements 252a.
(39) For ease of understanding, FIG. 8 only shows the punch assembly 250 and the section of metal sheet 10. Although not shown on FIG. 8, in use, an annular region 15 of the metal sheet 10 would be clamped during the stretching operation in a similar annular manner to the embodiment shown in FIGS. 5a and 5b.
(40) In use, the first and second groups of punch elements 251, 252 face opposing surfaces of the enclosed portion 16 of the metal sheet 10. The stretching operation is performed by moving both first and second groups of punch elements 251, 252 towards each other to deform and stretch (thin) the metal of the enclosed portion 16. The enclosed portion 16 is deformed into an undulating profile 170 (see FIG. 8).
(41) In a further embodiment, a single stretch punch 25 has a number of relief features in the form of recesses/cut-outs 253 provided in its end face (see FIG. 9). In the embodiment shown in FIG. 9, there is a central recess/cut-out surrounded by a single annular recess/cut-out. However, alternative configurations of recess/cut-out may be used.
(42) The embodiment in FIGS. 5a, 5b is shown punching a single enclosed portion in a section of metal sheet 10. However, the apparatus shown in FIGS. 5a, 5b can used to stretch and thin a plurality of enclosed portions 16 separated from each other and disposed across the area of the metal sheet 10. FIG. 10 shows the section of metal sheet 10 having undergone such a stretching operation to define a number of stretched and thinned domed enclosed portions 16, 17 disposed across the area of the sheet. Whilst this be done using a single stretch punch performing a number of successive stretching operations across the area of the metal sheet 10, it is preferred that the apparatus includes a plurality of stretch punches which allow simultaneous stretching operations to be performed on a corresponding number of enclosed portions disposed across the area of the metal sheet. However, to reduce the loads imposed on the tooling used for stretching, it is beneficial to stagger any simultaneous stretching operations so that not all of the enclosed portions across the sheet are stretched at the same time. FIGS. 11a and 11b indicate six groups of enclosed portionsa, b, c, d, e and f. In use, all the enclosed portions in each group would be stretched simultaneously. In the embodiment shown in FIG. 11a, the stretching would progress radially outwardly from group a, to group b, to group c, to group d, to group e, to group f. In the alternative embodiment shown in FIG. 11b, the stretching would progress radially inwardly from group f, group e, to group d, to group c, to group b, to group a. On completion of the stretching, separate blanks would be cut from the stretched metal sheet for subsequent drawing.
(43) Note that FIGS. 10, 11a and 11b are illustrative only and are not intended to be to scale.
(44) Initial Drawing Stage of Drawing Operation
(45) On completion of the stretching operation, the metal sheet 10 with its stretched and thinned domed enclosed portion 16, 17 is moved to a cupping press 30. The cupping press 30 has a draw pad 31 and a draw die 32 (see FIGS. 12a and 12b). A draw punch 33 is co-axial with the draw die 32, as indicated by common axis 34. The draw punch 33 is provided with a recess 35. A circumferential cutting element 36 surrounds the draw pad 31.
(46) In use, the section of metal sheet 10 is held in position between opposing surfaces of the draw pad 31 and the draw die 32. The sheet 10 is located so that the domed enclosed portion 16, 17 is centrally located above the bore of the draw die 32. After the metal sheet 10 has been positioned, the circumferential cutting element 36 is moved downwards to cut a blank 11 out from the metal sheet 10 (see FIG. 12a). The excess material is indicated by 12 on FIG. 12a.
(47) After the blank 11 has been cut from the sheet 10, the draw punch 33 is moved axially downwards into contact with the blank 11 (see FIG. 12b). The draw punch 33 first contacts the blank 11 on an annular region 18a located adjacent and radially outward of the domed enclosed portion 16, 17 (see FIG. 12a). The recess 35 provided in the draw punch 33 avoids crushing of the domed enclosed portion 16, 17 during drawing. The draw punch 33 continues moving downwardly through the draw die 32 to progressively draw the blank 11 against the forming surface 37 of the die into the profile of a cup 19 having a sidewall 19.sub.sw and integral base 19b. However, the action of the draw punch 33 against the blank 11 also causes material of the domed enclosed portion 16, 17 to be pulled and transferred outwardly (as indicated by arrows A in FIG. 12b). This initial drawing stage results in a reduction in height of the domed region due to its material having been drawn outwardly. Dependent on the depth of the draw, the drawing may be sufficient to pull and transfer some of the stretched and thinned material of the domed enclosed portion 16, 17 into the sidewall 19.sub.sw during this initial drawing stage, rather than this stretched and thinned material remaining wholly within the base 19.sub.b.
(48) FIG. 12b includes a separate view of the drawn cup 19 that results from use of the cupping press 30, with the reduced height domed region in the base indicated by 17. A detail view is included in FIG. 12a of the radius R.sub.32 at the junction between the end face of the draw die 32 and its forming surface 37. As for conventional drawing operations, the radius R.sub.32 and the load applied by the draw pad 31 to the periphery of the blank 11 are selected to permit the blank to slide radially inwards between the opposing surfaces of the draw pad 31 and draw die 32 and along forming surface 37 as the draw punch 33 moves progressively downwards to draw the blank into the cup 19. This ensures that the blank 11 is predominantly drawn, rather than stretched (thinned) (or worse, torn about the junction between the end face of the draw die and the forming surface 37). Dependent on the size of radius R.sub.32 and, to a lesser extent, the severity of the clamping load applied by the draw pad 31, negligible stretching or thinning should occur during this initial drawing stage. However, in alternative embodiments of the invention, it is permissible for the load applied by the draw pad 31 to be sufficient that a combination of drawing and further stretching occurs under the action of the draw punch 33. The cup 19 that results from this initial drawing stage is also referred to the first stage cup.
(49) In an alternative embodiment of the invention not shown in FIGS. 12a and 12b, if the depth of draw were sufficient it would result in the domed enclosed portion 16, 17 being pulled essentially flat in this initial drawing stage to define a cup 19 having an essentially flat base 19.sub.b.
(50) Re-Drawing Stage of Drawing Operation
(51) The first stage cup 19 resulting from the cupping process shown in FIGS. 12a and 12b and described above is transferred to a bodymaker assembly 40 (see FIGS. 13a to 13d). The bodymaker assembly 40 comprises two halves 41, 42 (indicated by arrows in FIGS. 13a to 13d).
(52) The first half 41 of the bodymaker assembly 40 has a tubular re-draw punch 43 mounted on the same axis as circumferential clamp ring 44. As can be seen from FIGS. 13a to 13d, the clamp ring 44 circumferentially surrounds the re-draw punch 43 like a sleeve. As will be understood from the following description and looking at FIGS. 13a to 13d, the re-draw punch 43 is moveable through and independently of the circumferential clamp ring 44.
(53) The second half 42 of the bodymaker assembly 40 has a re-draw die 45. The re-draw die 45 has a tubular portion having an outer diameter corresponding to the internal diameter of the cup 19 (see FIGS. 13a to 13d). The re-draw die 45 has a forming surface 46 on its inner axial surface which terminates in an annular end face 47 (see FIGS. 13a to 13d).
(54) In use, the first stage cup 19 is first mounted on the re-draw die 45 (as shown on FIG. 13a). Then, as shown in FIG. 13b, the two halves 41, 42 of the bodymaker assembly 40 are moved axially relative to each other so that annular region 18b of the base of the cup 19 is clamped between the annular end face 47 of the re-draw die 45 and the surface of the circumferential clamp ring 44.
(55) Once clamped, the re-draw punch 43 is then forced axially through the clamp ring 44 and the re-draw die 45 (see arrow B on FIGS. 13c and 13d) to progressively re-draw the material of the cup 19 along the forming surface 46 of the re-draw die. The use of the re-draw punch 43 and die 45 has two effects:
(56) i) to cause material from the sidewall 19.sub.sw to be drawn radially inwards and then axially along the forming surface 46 of the re-draw die 45 (as indicated by arrows C on FIGS. 13c and 13d). In this way, the cup is reduced in diameter during this re-drawing stage (as indicated by comparing FIG. 13a with FIG. 13d).
(57) ii) to cause the stretched and thinned material that remains in the reduced height domed region 17 of the base 19.sub.b to be further progressively pulled out and transferred from the base into the reduced diameter sidewall (as indicated by arrows D on FIGS. 13c and 13d). This has the effect of flattening the base 19b (see especially FIG. 13d).
(58) FIG. 13d shows the final state of the re-drawn cup 19 when the re-draw punch 43 has reached the end of its stroke. It can clearly be seen that the formerly domed region 17 of the base 19.sub.b has now been pulled essentially flat, to provide a cup or container body 19 where the thickness of the base 19.sub.b is thinner than that of the ingoing metal sheet 10. As stated earlier, this reduced thickness in the base 19.sub.band the consequent weight reductionis enabled by the stretching operation performed previously.
(59) As shown in the detail view of the re-draw die 45 in FIG. 14, the junction between the forming surface 46 and the annular end face 47 of the re-draw die 45 is provided with a radius R45 in the range 1 to 3.2 mm. The provision of a radius R45 alleviates the otherwise sharp corner that would be present at the junction between the forming surface 46 and the annular end face 47, and thereby reduces the risk of the metal of the cup 19 tearing when being re-drawn around this junction.
(60) The re-drawing stage illustrated in FIGS. 13a to 13d may also be followed by one or more further re-drawing stages to induce a further reduction in diameter of the cup 19.
(61) Note that although FIGS. 13a to 13d show use of a tubular re-draw punch 43 having an annular end face, the punch may alternatively have a closed end face. The closed end face may be profiled to press a corresponding profile into the base of the cup.
(62) The drawing operation described above and illustrated in FIGS. 13a to 13d is known as reverse re-drawing. This is because the re-draw punch 43 is directed to invert the profile of the first stage cup. In effect, the re-draw punch reverses the direction of the material and turns the stretched cup inside out. This can be seen by comparing the cup profiles of FIGS. 13a and 13d. Reverse re-drawing the cup has the advantages of:
(63) i) preventing uncontrolled buckling of the reduced height domed region 17 of the base (especially when using a re-draw punch having a closed end face); and
(64) ii) maximises transfer of material from the domed region 17 to the sidewalls 19.sub.sw.
(65) Note that although the embodiment shown in FIGS. 13a to 13d illustrates reverse re-drawing, conventional re-drawing would also work; i.e. where the re-draw punch acts in the opposite direction to reverse re-drawing and does not turn the cup inside out.
(66) FIG. 15 shows the changes undergone by the metal sheet 10 from before any forming operations have been undertaken (view a), to after the stretching operation in the stretch rig 20 (view b), to after the initial drawing stage in the cupping press 30 (view c), and finally to after the re-drawing stage in the bodymaker assembly 40 (view d). The figures clearly show that the base of the final cup (t.sub.stretch) has a reduced thickness relative to the ingoing gauge of the metal sheet 10 (t.sub.in-going), i.e. t.sub.stretch<t.sub.in-going. As previously stated, this reduced thickness (relative to the ingoing gauge of the metal sheet) is enabled by the stretching process of the invention. The effect of the initial drawing stage in progressively pulling and transferring outward material of the domed enclosed portion 16, 17 is shown on views b and c of FIG. 15, with material at location X pulled and transferred outward to location X as a result of the initial drawing stage. The effect of the re-drawing stage is shown in view d of FIG. 15, with material at location X pulled and transferred to location X in the sidewall 19.sub.sw.
(67) To maximise the height of the sidewall 19.sub.sw of the cup with its thinned base, the cup may also undergo ironing of the sidewalls by being drawn through a succession of ironing dies (not shown) in an ironing operation. This ironing operation has the effect of increasing the height and decreasing the thickness of the sidewall.
(68) FIG. 16 shows a container 100 where the final resulting cup 19 has undergone such an ironing operation to form container body 110. The container body 110 is flared outwardly 111 at its access opening. Can end 120 is provided with a seaming panel 121, the seaming panel enabling the can end to be fastened to the container body by seaming to the flared portion 111.
