CONTAINER CLOSURE WITH INCREASED STRENGTH

Abstract: A metal closure (10) for a container includes a center panel (12), a center-panel ring (18) with a convex curved surface, and an inner leg (20). The metal closure (10) is provided with a band formed by at least coining to increase the buckling pressure. The band is defined as one of intersecting strain fields. The coining cold-works a total uncoined curvilinear length that includes a portion of the center-panel ring (18), and that optionally includes a portion of the center panel (12) and/or a portion of the inner leg (20). In one embodiment, the coining produces two frustoconical coined surfaces. In another embodiment, the coining produces a curvilinear coined surface with a generally constant coin residual.

Description: The invention relates to a method for making a metal closure having increased strength, which method comprises cold forming a metal stock to provide a closure having an inner closure portion, an outer closure portion circumscribing that inner closure portion and being spaced outwardly therefrom, and a curved ring circumscribing said inner closure portion, said curved ring being interposed between and integral with said inner and outer closure portions, and cold-working the curved ring by a mechanical pressing operation to form a surface of reduced thickness in at least the portion of said curved ring. Furthermore the invention relates to a metal closure produced by such method. Particularly the invention relates to closures for metal beverage containers. More particularly the present invention relates to container closures having increased strength. Metal beverage containers are a very competitive product in the packaging industry since the annual production of these containers is well over 70 billion per year in the United States alone. Even a small reduction in the thickness of the metal used in the container closure can result in savings of millions of dollars annually. The closures for the containers typically include a center panel that is generally planar, a center-panel ring that is disposed annularly around the center panel and that curves downwardly therefrom, an inner leg that projects downwardly from the center-panel ring, a curved connecting portion that connects to the inner leg distal from the center-panel ring, an outer leg that connects to the curved connecting portion and that extends upwardly, and an outer curl that is used for double seaming to the container. One of the limitations in the strength of a container of this type is the internal pressure at which buckling of the closure occurs. The value of this pressure is defined as the buckle strength of said closure. Buckling refers to a permanent and objectionable deformation of the closure, including the inner leg, the outer leg, and the center panel, in which circular uniformity of the closure is destroyed by fluid pressure that is exerted inside the closure. The buckle strength of a given closure is a measure of the resistance of the closure to failure by buckling. Various attempts have been made to increase the buckle strength of container closures; and these attempts are represented by issued patents which are discussed below. US-A-3 774 801 teaches a complex doming of the center panels as a method of increasing the buckle strength of the closures. US-A-3 441 170 teaches coining of the inside of the center-panel ring as a method of allowing the center panel to dome under pressure without this doming exerting a full buckling force on the inner and outer legs of the closure, and thereby also preventing the buckling from breaking the seal between the container closure and the sidewall. The inventor states that the coined area functions as a hinge. US-A-4 031 837 teaches increasing the buckling strength by reforming the closure with a reduced radius in the curved-connecting portion that interconnects the inner and outer legs, by increasing the angle of the inner leg to substantially vertical, and by moving the curved-connecting portion downwardly from the center panel. US-A-4 217 843 teaches a reforming operation in which the inner and outer legs are positioned more nearly vertical, the inside radius of the center-panel ring is reduced, and the inside radius of the center-panel ring is coined to produce doming of the center panel by stretching the metal in the central-panel portion. Some doming of the center panel has been found to increase the buckle strength of the containers because it eliminates any excess metal that results from scoring for the pull-tab opener. The patent discloses that the doming removes all excess metal and in fact stretches the metal in the central-panel portion. The document EP-A-0 153 115 discloses a method and an apparatus for forming a reinforced pressure-resistant can end wherein a blank is deformed in a first deformation step to form a flanged cup-shaped configuration having a central portion, a radius, frusto-conical wall, and annular flange and then deforming it a second time to offset the central portion and flange towards a common plane, to transform the radius into a reinforcing bead, the two deformation steps being carried out using two pairs of coaxial relatively movable metal forming tools at the same work-station. Further the document GB-A-2 107 273 discloses a distortion-resistant end closure for food cans, which is made from high temper steel plate to withstand the high pressure developed during processing and thus avoid permanent distortion or buckling. The closure has a structure which resists warpage comprising an upwardly projecting annular bead concentric with a countersink groove which is disposed between an end flange and the annular bead, the groove comprising a planar shelf radially inwardly of which is a downwardly projecting annular bead. The end closure further includes a series of transitional steps inwardly of the annular bead leading to a central panel of the closure. A method of the type mentioned in the beginning for making a metal closure having increased strength and a metal closure made by this method are disclosed in EP-A-0 088 968 as well as in US-A-4 434 641 and US-A-4 577 774. These documents teach coining of the convex outside surface of the center-panel ring to increase the buckling strength of the container closures. As taught by these documents, coining is a local deformation, or cold-working of metal by reduction of thickness in a specified and limited, or predetermined, area through a single mechanical pressing operation, usually in the conversion press, that is preformed on the outside portion of the closure. The coining produces compression doming of the center panel. Optionally, this doming is limited by providing a hold-down pad, as taught in the aforesaid prior art documents. Finally, documents US-A-4 254 890 and US-A-4 354 784 both relate to scorelines for providing a line or zone of weakness in a metal end closure for opening the same with a conventional can opener. These documents do not address the problem of increasing the strength and buckle resistance of the metal end closure. It is a principal object of the present invention to increase the buckling strength that can be achieved in a container closure using a given thickness of metal, or alternately, to achieve the same buckling strength with a thinner material or with materials of lower strength. It is an object of the present invention to increase the buckling strength of a container closure by cold-working portions thereof. It is an object of the present invention to cold-work a greater curvilinear length of metal than has heretofore been achieved. It is an object of the present invention to cold-work a greater cross-sectional area of material for a given coin residual than has heretofore been achieved. It is a further object of this invention to utilize stock that has heretofore been used primarily for the body stock and includes aluminum and steel alloys. It is an object of the present invention to cold-work a greater cross-sectional area for a given uncoined curvilinear length of metal than has heretofore been achieved. It is an object of the present invention to increase the buckle resistance of closures by subjecting an end having a central-panel ring by cold-working first and second portions of the arcuate length of the convex surface of the center-panel ring in first and second coining steps. It is an object of the present invention to increase the buckle resistance of an end closure by cold-working a first portion of the arcuate length of the center-panel ring and an adjoining portion of the center panel in one coining operation, and to cold-working another portion of the arcuate length of the center-panel ring and an adjoining portion of the inner leg in another coining operation. Finally, it is an object of the present invention to substantially enhance the strength characteristics of a metal closure by cold-working a first portion of the arcuate length of the center-panel ring and an adjoining portion of the center panel in one coining operation, to cold-work the remainder of the arcuate length of the center-panel ring and an adjoining portion of the inner leg in another coining operation, and to cold-work an arcuate portion of the center-panel ring in both coining operations. The aforementioned objects are achieved according to the present invention by providing a method of the type mentioned in the beginning for making a metal closure having increased strength, which method in accordance with the invention is characterized in that there are provided surfaces of reduced thicknesses defined by deformations in at least two different directions, said deformations forming a band of intersecting strain fields in said curved ring to provide a closure having increased buckle strength. The aforementioned objects are further achieved according to the present invention by providing a metal closure produced by such method, which metal closure in accordance with the invention is characterized in that the surfaces of reduced thicknesses in said curved ring portion provide at least two intersecting strain fields. In an embodiment of the present invention, improved strength is provided in a container closure of the type which includes a center panel being disposed orthogonally to a container axis and having an outer perimeter, a center-panel ring being disposed perimetrically around the center panel and having a convex outer surface with a curvature that bends downwardly and that includes an uncoined arcuate length, an inner leg that extends downwardly from the center-panel ring, a connecting portion that curves upwardly and that includes a concave radius on the outer side of the closure, an outer leg that extends upwardly from the connecting portion, and an outer curl that curls outwardly and downwardly and that is used for double seaming the closure to the sidewall of a container. In a preferred embodiment of the present invention, one portion of the convex surface of the center-panel ring is coined at one angle to the container axis, thereby cold-working one frustoconical coined surface having a first perimetrical area; and another portion of the convex surface is coined at at different angle to the container axis, thereby cold-working another frustoconical coined surface having a different perimetrical area. By controlling the coin angles, by controlling the difference in the coin angles between the first and second coins, and by controlling the thickness of residual metal after coining, a significant increase in buckling strength is achieved. This significant increase in buckling strength is thought to be as a result of the formation of a band of intersecting strain fields and also an increase in material hardness and tensile strength that is a result of cold-working. The present invention achieves greater buckling pressures than container closures that are not coined; and the present invention achieves greater buckling pressures than has been achieved by coining such as is taught by the prior art. This improvement in buckling pressures has been achieved by coining a radially-disposed total curvilinear length of the outer surface of the closure which is greater than can be achieved by coining a single frustoconical coined surface, as is done in EPA-0 088 968 as well as in US-A-4 434 641 and US-A-4 577 774. This larger curvilinear length may include a portion of the center panel and/or a portion of the inner leg, as well as including most, or all, of the center-panel ring. In EP-A-0 088 968, US-A-4 434 641 and US-A-4 577 774 the cross-sectional area of the material that has been cold-worked is defined by a chord that is disposed at a given distance from the inner radius of the center-panel ring. The present invention cold-works a volume of material whose cross-sectional area is greater than the cross-sectional area as defined by the aforesaid chord. It is believed that the present invention achieves greater buckling strength by forming a narrow band of intersecting strain fields in the metal between and beneath the two cold-worked surfaces. This narrow band results in a strengthening device encircling the center panel. The band itself is characterized by a zone of intersecting deformation developed by separate steps, either serially or concurrently, of cold-working at more than one angle or direction to the container axis, and which differ from the surrounding metal by orientation and configuration of the mechanical texture extant in metal stock that has been subjected to drawing or rolling. Mechanical texture (or fiber texture) is the observed effect of the alignment of inclusions, cavities, second phase constituent particles, and possible lattice bending and fragmentation due to alignment of crystallographic slip planes in the main direction of mechanical drawing or rolling. Texturing or fibering is an important factor in producing typical mechanical properties in such metals. It was surprising to discover the phenomenal resistance to buckling provided by the present invention over that of the closure structures of the prior art and, although a satisfactory reason for this is still to be fully elucidated and it is to be assumed that the subject invention is not to be restricted thereby, it is here postulated that the acts of creating the aforementioned band results in a mechanical strengthening device of major significance comprising a zone or zones of overlapping deformations of fundamentally different directions. Within said band the symmetry of the mechanical texture or continuity with respect to the surrounding metal has been altered. Referring to FIGURE 7, the region labelled X depicts mechanical texturing in a portion of the closure that has not been subjected to cold-work by coining, region Y depicts mechanical texture of that portion of the closure that has been cold-worked by coining in only one direction (or at only one angle to the container axis), and region Z shows the band wherein the symmetry of texture is altered by the strain fields created as a result of coining in more than one direction. This band is thought to afford different properties from the uncoined metal and from metal that has been cold-worked in only one direction when subjected to fluid pressures and, thus, confers resistance to buckling by impeding additional uniform deformation of the closure. This effect may be due to the elimination or reduction of metal anisotropy in the band in which the continuity of the usual mechanical texture has been significantly altered. The subject invention is found applicable to a wide range of metals, particularly those exhibiting mechanical texture. Additionally, the metal in the coined regions, including the band, i.e., the zone of intersecting strain fields, is though to be harder and to have a higher tensile strength than that in uncoined regions due to mechanisms of work-hardening. It is believed that this increase in strength offsets the corresponding reduction in material thickness and, thus, also contributes to the resistance to buckling obtained through coining. Thus, in a preferred embodiment of this invention applied to closures of an aluminum alloy (e.g., Aluminum Association Specification AA 5182), the amount of reduction in thickness by coining should range from about twenty-five to forty percent of the original material thickness. It should be understood that other metal alloys exhibiting different ductilities or different work-hardening characteristics may permit differing amounts of coining to achieve high strength without incurring unacceptable collateral effects. Preferably, two areas of the outer surface of the closure are coined in separate cold-working operations. In the first operation, a first frustoconical coined surface is formed that includes a portion of the arcuate length of the center-panel ring and a portion of the outer surface of either the center panel or the inner leg. In the second operation, a second frustoconical coined surface is formed that includes another portion of the arcuate length of the center-panel ring, and that may include a portion of the outer surface of the other adjoining portion. That is, if the first operation included a portion of the center panel, then the second operation may include a portion of the inner leg. When certain coin angles are chosen, the coined surfaces overlap, so that the second coining operation reforms a portion of the first frustoconical coined surface to be a part of the second frustoconical coined surface. This reformed portion of the second frustoconical surface is hereafter referred to as a twice cold-worked perimetrical portion. If widely varying coin angles are chosen, a portion of the uncoined center-panel ring remains between the two frustoconical coined surfaces. While using such coin angles does not achieve the maximum advantage of the twice cold-worked portion, a zone of intersecting strain fields is still observed in the metal beneath the coined surfaces and the strengthening advantages of such a zone or band are obtained. Furthermore, widely differing coin angles cold-work a greater portion of both the center panel and the inner leg, and achieve strength advantages thereby. In a second preferred embodiment of the present invention, the cold-working produces a curvilinear surface, rather than two frustoconical coined surfaces. In the curvilinear embodiment, the curvilinear cold-worked surface follows the general contour of the product side of the closure, or generally follows the uncoined contour of the public side of the closure, or more preferably, leaves a generally uniform coin residual. Curvilinear coining cold-works a cross-sectional area of material that is greater than that which is achieved, for a given coin residual, by either the prior art or the frustoconical coining embodiment of the present invention. Also, curvilinear coining cold-works a cross-sectional area of material that is greater than that which is achieved, for a given curvilinear length of uncoined material, by either Nyugen or the frustoconical coining embodiment of the present invention. It will be appreciated that such curvilinear coining in accordance with the subject invention is considered to create a zone or zones of intersecting strain fields. The curvilinear coining of the present invention may be done in one or more steps, to achieve twice cold-worked areas, or to reduce the required per step press capacity. A preferred embodiment of the invention consists in providing a metal closure with an inner closure portion, an outer closure portion circumscribing said inner closure portion and being spaced outwardly therefrom, and a curved ring circumscribing said inner closure portion, said curved ring being interposed between and integral with said inner and outer closure portions, coining a first face by forming a first planar surface in said curved ring, and coining a second face by forming a second planar surface juxtaposed with and overlapping an area on the first planar surface. From the outer point of view, the aforementioned objects are achieved by providing a metal closure with an inner closure portion, an outer closure portion circumscribing said inner closure portion and being spaced outwardly therefrom, and a curved ring circumscribing said inner closure portion, said curved ring being interposed between and integral with said inner and outer closure portions, and forming a band of intersecting strain fields in said curved ring to provide strengthening member circumscribing said inner closure portion. According to another preferred embodiment of the method of the invention a metal closure is strengthened, said closure having a substantial textured structure in cross section, said metal closure being provided with a curved annular ring, said method of strengthening comprising cold working the curved annular ring of the closure in more than one direction to provide a band of intersecting deformations providing the above mentioned band of intersecting strain fields thereby altering the mechanical texture and continuity with respect to the surrounding metal within said band. In accordance with a preferred embodiment of the invention the article of manufacture of the subject invention is a metal closure comprising an inner closure portion, an outer closure portion circumscribing said inner closure portion and being spaced outwardly therefrom, a curved ring circumscribing said inner closure portion, said ring being interposed between and integral with said inner and outer closure portions, said curved ring having a band of intersecting strain fields. Brief Description of the Drawings FIGURE 1 is perspective view of a metal closure made in accordance with a first embodiment of the present invention; FIGURE 2 is an enlarged and partial cross sectional elevation of the metal closure of FIGURE 1 showing the two frustoconical coined surfaces in cross section; FIGURE 3 is an enlarged cross section of a portion of the center-panel ring of FIGURE 2, taken substantially the same as FIGURE 2, and showing the coined surfaces by phantom lines; FIGURE 4 is a duplication of the view of FIGURE 2, included herein to facilitate numbering and describing various features of the present invention; FIGURE 5 is another duplication of the center-panel ring of FIGURE 2, included herein to facilitate numbering and describing the present invention; FIGURE 6 is yet another duplication of the center-panel ring of FIGURE 2, included herein to facilitate numbering and describing the present invention; FIGURE 7 is an enlarged cross sectional elevation of the embodiment of FIGURE 1 showing a schematic representation of the texture of metal as well as the dimensions for use in describing mathematical calculations included herein; FIGURE 8 is an enlarged cross sectional elevation of an embodiment of the present invention in which curvilinear cold-working is provided; FIGURE 9 is a graph of buckle strength vs. dome depth where slope A is a plot of double coined metal closure and slope B is a single coined plot; and FIGURE 10 is a graph of buckle strength (psig) vs. amount of cold-work (square inches) when slope C is a plot of double coined metal closure (in accordance with the subject invention) and slope D is a single coined plot. Description of the Preferred Embodiments Referring now to the drawings, and more particularly to FIGURES 1 and 2, a container closure, or metal closure, 10 includes a center panel, or inner closure portion, 12 that is disposed orthogonally to a container axis 14 and that includes a circular perimeter 16, a center-panel ring, or curved ring 18 that is integral with the center panel 12 and that curves downward from the circular perimeter 16, a circular inner leg, or outer closure portion, 20 that is integral with the center-panel ring 18 and that depends downwardly therefrom, a curved connecting portion 22 that is integral with the inner leg 20 and that includes an inner radius 23, a circular outer leg 24 that is integral with the connecting portion 22 and that extends upwardly therefrom, and an outer curl 26 that is integral with the outer leg 24 and that includes a peripheral outer edge 28. Since portions of the container closure 10 have been named and numbered that are integral with one another, phantom lines 30 are included to show where individual ones of the above-named parts terminate and join to adjacent ones of the above-named parts. Referring now to FIGURES 2 and 3, the metal closure 10, including the center-panel ring 18 thereof, has an uncoined thickness 32; and the center-panel ring 18 thereof, has an uncoined thickness 32; and the center-panel ring 18 has an uncoined arcuate length 34 which includes all of an uncoined convex curved surface 36. Frustoconical coined surfaces, 37 and 38 are shown by phantom lines 30 in FIGURES 3-6. In the example of FIGURE 3, the two coining steps of the frustoconical coined surfaces 37 and 38 include a total uncoined curvilinear length 39 which is greater than the uncoined arcuate length 34 of the center-panel ring 18, although such is not the case for all combinations of coining angles. Referring now to FIGURE 6, the frustoconical coined surface 37 includes a perimetrical portion, or uncoined arcuate length, 40 of the center-panel ring 18, and a perimetrical portion, or uncoined length 41 of the center panel 12. The frustoconical coined surface 38 includes a perimetrical portion, or uncoined arcuate length, 42 of the center-panel ring 18, and a perimetrical portion, or uncoined length 43 of the inner leg 20. Referring now to FIGURES 1 and 2, the metal closure 10, including the center panel 12, the center-panel ring 18, the inner leg 20, the curved connecting portion 22, the outer leg 24, and the outer curl 26, along with all of the above-named portions thereof, includes a public side, or outside, 44, and a product side, or inside 45. The frustoconical coined surface 37 is disposed at a cone angle 46 with respect to both a parallel axis 48 and the container axis 14; and the frustoconical coined surface 38 is disposed at a cone angle 50 with respect to both the parallel axis 48 and the container axis 14. It can be seen in FIGURE 2 that both the cone angle 46 and the cone angle 50 intercept the axis 14 on the public side 44 of the closure 10. Referring again to FIGURE 3, the center-panel ring 18 is coined to a coin residual 52 which is the thickness of metal between the frustoconical coined surface 37 and a concave curved surface 54 of the center-panel ring 18; and the center-panel ring 18 is coined to a coin residual 56 which is the thickness of metal between the coined surface 38 and the concave curved surface 54. Referring now to FIGURES 2-4, and more particularly to FIGURE 4, the total uncoined curvilinear length 39 of the closure 10 which is coined into the surfaces 37 and 38 includes a first perimetrical portion 58, a second perimetrical portion 60, and, in the example shown, a third perimetrical portion, or twice cold-worked portion, 62. It can be appreciated that the twice cold-worked portion defines a band of intersecting strain fields in the metal between and beneath the two cold-worked surfaces. Referring now to FIGURE 5, considering for purposes of illustration that the frustoconical coined surface 37 is produced first, although the actual order of the coining steps may be selectively determined, then the material that is cold-worked in the first coining step includes a cold-worked perimetrical area, or perimetrical portion, 64 and a twice cold-worked perimetrical portion 66, which together form a perimetrical area, or perimetrical portion 67. The second cold-working step includes coining, or cold-working, a perimetrical portion 68, reforming, or recoining, the perimetrical portion 66 to be a part of the frustoconical coined surface 38, and forming a cold-worked perimetrical area, or perimetrical portion, 70 which includes both the perimetrical portion 68 and the perimetrical portion 66. Thus, if the frustoconical coined surface 37 is produced first, the perimetrical portion 66 is twice cold-worked originally being a part of the frustoconical coined surface 37, and being reformed to a part of the frustoconical coined surface 38. However, as the difference between the cone angles 46 and 50 is increased, the overlap between the perimetrical portions 67 and 70 will decrease, and the twice cold-worked portion 66 will decrease. It is obvious by studying the illustration of FIGURES 2 and 5 that if the difference between the cone angles 46 and 50 is increased sufficiently, there will be a portion, not shown, between the perimetrical portions 67 and 70 that is not coined. It will be appreciated that although the portions that are coined are separate, the associated strain fields extend outwardly and do intersect though the zone of the intersection decreases in size as the separation increases. Testing of the present invention included varying the cone angle 46 of the frustoconical coined surface 37 from 90 to 52 degrees, or varying a coin angle 72 from 0 to 38 degrees, as measured from the public side 44 of the center panel 12. Also, testing included varying the cone angle 50 of the frustoconical coined surface 38 from 30 to 75 degrees, or varying a coin angle 74 from 60 to 15 degrees, as measured from the public side 44. The thickness 32 of the metal used in the tests (AA 5182 aluminum alloy) was 0.287 mm (0.0113 inches); and the coin residuals, 52 and 56, varied from 0.114 mm (0.0045 inches) to 0.241 mm (0.0095 inches). Shells 78, or closures 10 without pull-tab openers 76, manufactured at one time and on one press and from the above-disclosed metal stock (0.287 mm or 0.0113 inch) were used for the tests; and the average buckling strength (measured using a Reynolds-type buckle testing apparatus) for these shells 78, without coining was 6.950 bar (100.8 pounds per square inch) with a standard deviation of 0.134 bar (1.95 pounds per square inch). Single coining made according to the teaching of documents EP-A-0 088 968, US-A-4 434 641 and US-A-4 577 774 (using the above-disclosed shells) produced an average buckling pressure of 7.74 bar (112.3 pounds per square inch) with a standard deviation of 0.127 bar (1.85 pounds per square inch). Double frustoconical coining (using the above-disclosed shells), with a coin angle 72 of either 10 to 17.5 degrees, and with a coin angle 74 of 25 to 60 degrees, produced an average buckling strength in 36 tests of 10 containers each of 8.230 bar (119.4 pounds per square inch) with an average standard deviation of 0.134 bar (1.95 pounds per square inch). Thus, the average gain in buckling pressure of containers with double frustoconical coining was 1.28 bar (18.6 pounds per square inch) over uncoined shells and 0.490 bar (7.1 pounds per square inch) over shells coined according to the teaching of documents EP-A-0 088 968, US-A-4 434 641 and US-A-4 577 774. These results also indicated that it is possible to obtain larger increases in buckle strength while cold-working less material through the use of the double coin as opposed to the use of a single coin. The increase in buckle strength obtained through coining is known to vary directly with the amount of cold-work applied. Such cold-work has been quantified by approximating the cross-sectional area of the metal displaced when the coined surface or surfaces are formed. FIGURE 10 is a plot of the results of least-squares linear regression for buckle strength as a function of the approximate amount of metal cold-worked by applying either a single coin (slope D) or a double coin (slope C) to closures, as disclosed above. It was found that, for equivalent amounts of cold-work, the increase in buckle strength obtained using the double frustoconical coin was 43% greater than that obtained using the single coin, and that this result is significant at a confidence level of 95%. A sample of the above disclosed closures were treated with two coins having the same coin angle 72. For such closures no increase in buckle strength was observed in comparison with identical closures treated with a single coin of the same coin angle and final coin residual 52. The above results indicate that the mechanisms by which the double coin provides strength benefits are fundamentally different than those of the prior art and that by coining at more than one angle (i.e. , in more than one direction) a synergistic and beneficial effect is obtained with respect to buckle strength. Another significant increase in strength as gained by the present invention is seen in the increase of buckling strength vs. the dome depth of the center panel 12. It is known that an increase in buckling strength can be achieved by increasing the dome depth. However, the amount of dome depth that is allowable is limited by a tab-over-chime problem. That is, there is a maximum allowable dome depth that can be used without the pull-tab opener 76 extending upwardly above the remainder of the container, thereby presenting problems in automation. With containers coined with two frustoconical coined surfaces, the ratio of increase of buckling strength to increase in dome depth was 26.7 percent greater than for containers cold-worked according to the teaching of documents EP-A-0 088 968, US-A-4 434 641 and US-A-4 577 774. These relations are illustrated in FIGURE 9, which is a plot of the results of a least-squares linear regression analysis of empirical data obtained for closures treated according to the teachings of the documents EP-A-0 088 968, US-A-4 434 641 and US-A-4 577 774 and for closures treated with the double frustoconical coin. Analysis of variance of these two sets of data indicates that the benefits obtained through the use of the double frustoconical coin over those obtained following the teachings of the documents EP-A-0 088 968, US-A-4 434 641 and US-A-4 577 774 are significant at a confidence level of 97.5 %. Also associated with the tab-over-chime problem is a limitation in the amount of bulging of the center-panel area when the closure is subjected to fluid pressure on the product side. Such bulging is quantified by double-seaming a closure onto a typical metal container, pressurizing said container, and measuring the displacement of the pull tab opener 76 as a function of internal pressure. In order to avoid problems in conveying it is desirable that the pressure at which the critical amount of bulging is reached be as high as possible. In tests conducted using closures 10 with pull tab openers 76 and other opening features and manufactured of 5182 aluminum alloy the double frustoconical coin was found to confer resistance to bulging superior to that obtained by the prior art. If, for example, 2.54 mm (0.100 inches) is chosen as the maximum allowable displacement of the pull-tab, closures treated according to the teachings of the aforementioned documents EP-A-0 088 968, US-A-4 434 641 and US-A-4 577 774 were found to exceed this value at 0.69 bar gauge pressure (10 psig) less than identical closures treated using the double frustoconical coin. Additionally, closures 10 with pull tab openers 76 and other opening features were manufactured from two samples of 5182 metal stock having thicknesses of 0.254 mm (0.0100") and 0.264 mm (0.0104") resp., using standard production presses to add the opening features. A portion of these closures were treated with a double frustoconical coin according to the present invention, with one cone angle of 80°, or a coin angle of 10°, and another cone angle of 52°, or a coin angle of 38°, each coining having coin residuals 52 and 56 of approximately 0.178 mm (0.0070"). Another portion of the above disclosed closures were not treated by coining. Closures treated with the above disclosed double frustoconical coin exhibited buckling strengths an average of 1.08 bar gauge pressure (15.6 psig) (with a standard deviation of 0.15 bar gauge pressure (2.2 psig)) greater than those of uncoined closures manufactured of like material thickness. Closures treated with a single coin according to the teachings of the documents EP-A-0 088 968, US-A-4 434 641 and US-A-4 577 774 are known to exhibit an increase of buckling strength not in excess of 0.34 to 0.48 bar (5 to 7 psig) over uncoined closures. Therefore, even though the testing thus far has been insufficient to optimize the increases in buckling pressures, the increases that have been achieved thus far, together with the small standard deviations which are involved, demonstrate that a significant improvement in buckling pressures, and/or a decrease in metal thickness can be achieved by the present invention. The material most commonly used in the manufacture of metal beverage container closures is Aluminum Association Specification AA 5XXX (where X represents integer, zero to nine) series of aluminum alloys. This series of alloys is characterized by a solid solution of alloying elements (primarily magnesium) which confers a strength higher than that of unalloyed aluminum. The AA 5XXX series alloys are high-strength alloys and exhibit high work-hardening rates. The aluminum alloys most commonly used for the manufacture of drawn and ironed beverage containers are of the AA 3XXX series. These alloys contain manganese and are strengthened primarily by the formation of second phase precipitate particles. Alloys of this series are, in general, less strong but more formable than those of the AA 5XXX series and generally exhibit lower rates of work hardening. Various steel alloys have been used to manufacture both drawn and ironed containers and closures for such containers. Steel is solid solution strengthened through the addition of carbon to iron and is characterized by a wide range of mechanical properties, depending on the composition of the alloy and the thermal and mechanical treatment to which it is subjected. The test results disclosed above involving both solid solution and precipitation strengthened alloys indicate that the present invention is applicable to each category of such alloys. Referring now to FIGURE 7, for a better understanding of the various mathematical relationships that are involved, the angle, 80 or 82, that is subtended in one frustoconical cold-worked surface is: α = 2cos⁻¹([R o -h]/R o ) where: R o = uncoined outer radius 84 of the center-panel ring h = max. depth of cold working, or chord height, 86 or 88 The angle of overlap, or double coining, 90 of two frustoconical cold-worked surfaces 37 and 38 is: α d = ϑ₁ - ϑ₂ + α₁/2 + α₂/2 where: ϑ₁ = the smaller coin angle 72 ϑ₂ = the larger coin angle 74 α₁ = angle subtended by coin angle 72 α₂ = angle subtended by coin angle 74 If α₁ and α₂ overlap, the total angle 92 that is subtended by the two frustoconical coined surfaces 37 and 38 is approximately: α t = ϑ₂ - ϑ₁ + α₂/2 + α₁/2 The total uncoined curvilinear length 39 of the closure 10 that is cold-worked is very nearly equal to: L = R o α t where α t is the total angle 92, in radians, that is subtended by cold-working. The cross-sectional area, 94 or 96, of a single frustoconical cold-worked surface, 37 or 38, is: A f = R o ²cos⁻¹([R o -h]/R o ) - (R o -h)√ 2R o h-h² where the angle of the arc cosine is in radians The overlapped, or double coined, area 98 of the cross-sectional areas 94 and 96 is: A d = R o cos⁻¹(R o -h o /R o ) - (R o -h o )√ 2R o h o -h o ² + [(Rsinα d /2)] [2tan(ϑ₂/2-ϑ₁/2)] where: h o = R o (1-cosα d /2) , and the angle of the arc cosine is in radians. And, it can be seen by inspection of FIGURE 7 that the total, or net, cross-sectional area 100 that is coined by the cross-sectional areas 94 and 96 is equal to the sum of the cross-sectional areas 94 and 96 subtracted by the overlapped area 98. Using the formulas given above the total uncoined curvilinear length 39 that is produced by two frustoconical coined surfaces, 37 and 38, is 23.9 percent greater than is produced by a single frustoconical coined surface, 37 or 38, for a given coin residual, 52 or 56, when the coin angles, 72 and 74, differ by only fifteen degrees. Thus, more of the material can be cold-worked than can be achieved by a single frustoconical coin, even with such a small difference in the coin angles, 72 and 74. Of even greater significance, the total cross-sectional area 100 that is cold-worked by two frustoconical coined surfaces, 37 and 38, is 33.9 percent greater than is produced by a single frustoconical coined surface, 37 or 38, when the coin angles, 72 and 74, differ by only fifteen degrees. Referring finally to FIGURE 7, the inner leg 20 bends downward by an angle 102, the angle 104 illustrates the material of the inner leg 20 that is coined, and the angle 106 illustrates the material of the center panel 12 that is coined. Referring now to FIGURE 8, in a second preferred embodiment of the present invention, a curvilinear coined surface, or cold-worked surface, 108 is produced on the public side 44 of a metal closure, or container closure, 109. The curvilinear coined surface 108 may be produced by one or more coining tools, such as the coining tools 110, 112, and 114. It is to be noted that in curvilinear coining as implied herein that the die tool surface or surfaces that is to be brought to bear on the curved ring portion of the metal closure is curved in design. The curvilinear coined surface 108 produces a coin residual 116 that is generally constant. A total uncoined curvilinear length 118 of the curvilinear coined surface 108 includes a curvilinear uncoined length, or radial portion, 120 in the center panel 12 and a curvilinear uncoined length, or radial portion, 122 in the inner leg 20 as well as including a curvilinear length, or portion, 124 in the center-panel ring 18. The curvilinear coined surface 108 includes a total cold-worked cross-sectional area 126 which includes a first cold-worked perimetrical portion, or first perimetrical area, 128 in the center panel 12, a second cold-worked perimetrical portion, or second perimetrical area 130 in the inner leg 20, and a third cold-worked perimetrical portion, or third perimetrical area, 132 in the center-panel ring 18. The total cold-worked cross-sectional area 126 that is displaced by the curvilinear coined surface 108 can be approximated by the following formula: A = 1/2 ϑ t (R o ²-R r ²) where: ϑ t is the total angle 134 subtended by curvilinear coining R r is the radius 136 of the curvilinear coined surface 108 Using the formula given above, and with the same coin residual 116 as used for the coin residuals 52 and 56 for the preceding calculations, the total cross-sectional area 126 of curvilinear coining is 61 percent greater than is achieved with a single frustoconical coined surface, 37 or 38, and is 49 percent greater than is achieved with two frustoconically coined surfaces, 37 and 38, when the surfaces 37 and 38 are separated by the same angle as used for the previous calculations. In summary, the first embodiment of FIGURES 1-7 provides first and second coined surfaces 37 and 38 by cold-working the surfaces. The depth of coining varies from a maximum at the depths 86 and 88, to zero at radially-spaced locations 138, 140, 142, and 144 where chords 148 and 150 intercept the outside 44. As noted above, the first embodiment of the present invention, achieves a significant increase in the buckling pressure, and achieves a significant increase in the ratio of increase in buckling strength vs. dome height. The first embodiment, with the frustoconical coined surfaces, 37 and 38, thereof, coins a significantly greater total uncoined curvilinear length 39 of the metal closure 10 than a single frustoconical coined surface, 37 and 38, that is defined by a chord, 148 or 150, that is spaced from the product side 45, and that intercepts the public side 44 at radially spaced locations, 138 and 140, or 142 and 144. And finally, the first embodiment of the present invention coins a significantly greater cross-sectional area 100 for a given coin residual, 52 or 56, than the cross-sectional area, 94 or 96, of a single frustoconical coined surface, 37 or 38. The initial deformation made on the curved ring portion is followed by or concurrent with a second deformation which is generally overlapping the initial one or may be slightly spaced therefrom. The upper coined angle may be, for example, from 0 to above 45°, the lower from above 5 to 90° as measured from the horizontal. The amount of overlap or contact between the coined surfaces can be from about 0 to 95%, preferably about 20 to 40%. The second embodiment of FIGURE 8 cold-works a curvilinear coined surface 108 which: has a greater curvilinear length 118 than can be achieved by coining a single frustoconical coined surface, 37 or 38, has a generally constant coin residual 116, has a generally constant depth of cold-working 152, has a total cold-worked cross-sectional area 126 that is considerably greater than the cross-sectional area, 94 or 96, that is produced by a single frustoconical coined surface, 37 or 38, and has a total cold-worked cross-sectional area 126 that is greater than the total cross-sectional area 100 that is produced by cold-working two frustoconical coined surfaces, 37 and 38. More importantly, the curved ring portion that has been cold-worked by curvilinear coining provides a wide zone or zones of intersecting strain fields. FIGURE 8 usually illustrates the fact that the total cold-worked cross-sectional area 126 for curvilinear coining, in the example quoted, is 61 percent greater than a cross-sectional area 154 that lies between the uncoined convex curved surface 36 and the chord 148 that intercepts the uncoined curved surface 36 at the radially-spaced locations 138 and 140. It is common practice to form the shells 78 in a shell press which blanks and forms the basic shape from sheet metal stock. The partially completed shell 78 is then transferred to a conversion press where the opening features, as well as the rivet which holds the pull-tab opener 76, are formed. The conversion press is a multi-station press. Each of the shells 78 is advanced progressively to new tooling wherein additional operations are performed. It is contemplated that as many as three coining operations, as shown in FIGURE 8, can be performed in the general area of the center-panel ring 18, and that the resultant strength can be greater than has resulted from tests that included only two coining operations. A preferred material for the closures 10 is aluminum alloy AA 5182; although other aluminum alloys, such as AA 3004 and other metals, such as steel, may be used with the process described herein. Preferably, the process is performed on a closure 10 for attachment to a container having sidewalls, however, it is equally suitable for use on an integral end of a container. While specific apparatus has been disclosed in the preceding description, it should be understood that these specifics have been given for the purpose of disclosing the principles of the present invention and that many variations thereof will become apparent to those who are versed in the art. Therefore, the scope of the present invention is to be determined by the appended claims. Industrial Applicability The present invention is applicable to metal closures for containers, and more particularly, the present invention is applicable to metal closures for containers, such as beverage containers.

Claims: A method for making a metal closure (10) having increased strength, which method comprises cold-forming a metal stock to provide a closure (10) having an inner closure portion (12), an outer closure portion (20) circumscribing that inner closure portion (12) and being spaced outwardly therefrom, and a curved ring (18) circumscribing said inner closure portion (12), said curved ring (18) being interposed between and integral with said inner and outer closure portions (12, 20), and cold working the curved ring (18) by a mechanical pressing operation to form a surface (37, 38, 108) of reduced thickness in at least the portion of said curved ring (18), characterized in that there are provided surfaces (37, 38, 108) of reduced thicknesses defined by deformations in at least two different directions, said deformations forming a band (Z) of intersecting strain fields in said curved ring (18) to provide a closure (10) having increased buckle strength. A method according to claim 1, characterized in that the band (Z) of intersecting strain fields is formed by separate coining operations. A method according to claim 2, characterized in that the separate coining operations are performed to cause the overlapping thereof. A method according to claim 1, 2 or 3, characterized in that the metal closure (10) is made from Aluminum Association Specification AA 3XXX or AA 5XXX series alloys. A metal closure (10) having an inner closure portion (12), an outer closure (20) circumscribing that inner closure portion (12) and being spaced outwardly therefrom, and a curved ring (18) circumscribing said inner closure portion (12), said curved ring (18) being interposed between and integral with said inner and outer closure portions (12,20), said curved ring (18) being cold worked by a mechanical pressing operation to form a surface (37, 38, 108) of reduced thickness in at least the portion of said curved ring (18), characterized in that the surfaces (37,38,108) of reduced thicknesses in said curved ring (18) portion provide at least two intersecting strain fields to provide a closure (10) having increased buckle strength. A metal closure (10) according to claim 5, characterized in that the surfaces (37, 38, 108) of reduced thicknesses which provide a band (Z) of intersecting strain fields are defined by at least two coined surfaces (37, 38). A metal closure (10) according to claim 6, characterized in that the at least two coined surfaces (37, 38) overlap to provide a zone of twice cold-worked metal. A metal closure (10) according to claim 5, 6 or 7, characterized in that the metal closure (10) is of aluminum alloy. A metal closure (10) according to any one of claims 5 to 8, characterized in that said surfaces (37, 38) of reduced thicknesses comprise: a first cold-worked perimetrical area (37) of said metal closure (10) which includes a first perimetrical portion of said curved ring (18); a second cold-worked perimetrical area (38) of said metal closure (10) which includes a second perimetrical portion of said curved ring (18); and one of said perimetrical areas (37, 38) including a twice cold-worked perimetrical portion defining said intersecting strain fields. A metal closure (10) according to claim 9, characterized in that the closure (10) defines a container axis (14) oriented transversely to the inner closure portion (12), wherein the inner and outer closure portions (12, 20) comprise respective marginal portions adjacent the curved ring (18), wherein the marginal portions are rectilinear when viewed in a section plane that includes the container axis (14), and wherein the curved ring (18) is curvilinear adjacent both marginal portions when viewed in the section plane. A metal closure (10) according to claim 10, characterized in that one of said cold-worked perimetrical areas (37, 38) includes a perimetrical portion of one of said closure portions (12, 20). A metal closure (10) according to claim 10, characterized in that said first cold-worked perimetrical area (37) includes a perimetrical portion of said inner closure portion (12). A metal closure (10) according to claim 10, characterized in that said second cold-worked perimetrical area (38) includes a perimetrical portion of said outer closure portion (20). A metal closure (10) according to claim 9, characterized in that said closure (10) includes outer and product sides (44, 45); said curved ring (18) includes a concave curved surface on said product side (45) of said closure (10); and one of said cold-worked perimetrical areas is on said outer side (44) of said closure (10). A metal closure (10) according to claim 9, characterized in that said closure includes outer and product sides (44, 45); said curved ring (18) includes a concave curved surface on said product side (45) of said closure (10); and said cold-worked perimetrical areas (37, 38) are on said outer side (44) of said closure (10). A metal closure (10) according to claim 14 or 15, characterized in that the closure (10) defines a container axis (14) oriented transversely to the inner closure portion (12), wherein the inner and outer closure portions (12, 20) comprise respective marginal portions adjacent the curved ring (18), wherein the marginal portions are rectilinear when viewed in a section plane that includes the container axis (14), and wherein the curved ring (18) is curvilinear adjacent both marginal portions when viewed in the section plane. A metal closure (10) according to claim 16, characterized in that one of said cold-worked perimetrical areas (37, 38) includes a perimetrical portion of one of said closure portions (12, 20). A metal closure (10) according to any one of claims 9 to 17, characterized in that said first cold-worked perimetrical area (37) includes a surface that is generally frustoconical in shape and that is disposed at a first coin angle (72); and said second cold-worked perimetrical area (38) includes a surface that is generally frustoconical in shape and that is coined at a second coin angle (74).

Inventors: DIERKING, STANLEY E., KALENAK, ROBERT D.
Applicants: BALL CORPORATION
Application year: 1988
Main classification: B65D17
IPC codes: B65D8, B21D51, B65D43, B65D41, B65D17