US3583348A

US3583348A - Method of making an easy opening container wall

Info

Publication number: US3583348A
Application number: US736870*A
Authority: US
Inventors: Omar L Brown
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-03-29
Filing date: 1968-03-29
Publication date: 1971-06-08
Anticipated expiration: 1988-06-08

Abstract

This invention relates to easy-opening cans of the type commonly filled with beverages and is directed to a number of problems that relate to the fact that the tops of beverage-filled cans are inevitably of outwardly bowed or domed configuration.

Description

United States Patent H 13,ss3,34s

Inventor Omar I... Brown Dayton, Ohio Appl. No. 736,870

Filed Mar. 29, 1968 Patented June 8, 1971 Assignee Erma! C. Frau Dayton, Ohio Division of Ser. No. 565,538, July 15, 1966, Pat. No. 3,428,210.

METHOD OF MAKING AN EASY OPENING CONTAINER WALL 8 Claims, 23 Drawing Figs.

U.S. Cl 113/121, 113/15, 29/522 Int. Cl B2ld 51/00 [50] Field of Search 220/54; 113/121, 121 A, 116 FF, 116 CC, 1200; 29/509,

[56] References Cited UNITED STATES PATENTS 3,291,336 12/1966 Fraze 220/54 3,251,503 5/1966 Bozek..... 220/54 3,404,800 10/1968 Arfert..... 220/54 3,254,790 6/1966 Dunn 220/54 3,366,086 l/1968 Fraze 113/121 Primary ExaminerRichard J. Herbst Assistant ExaminerMichael J. Keenan Attorney-Smyth, Roston & Pavitt ABSTRACT: This invention relates to easy-opening cans of the type commonly filled with beverages and is directed to a number of problems that relate to the fact that the tops of beverage-filled cans are inevitably of outwardly bowed or domed configuration.

PATENTEUJUN slsm 3583,3 18

SHLET 1 BF 6 IA/VINTOA- I 0014'- A. drawn JITOR/VEFI PATENTED Jun 8l97l 35 3134 sum 5 OF 6 17% 2 fieep Jrare A K X METHOD OF MAKING AN EASY OPENING CONTAINER WALL This application is a division of Ser. No. 565,538, now US. Pat. No. 3,428,2lfiled July 15, 1966.

Some of the problems with which the invention deals stem from the fact that no part of the top of a filled can should protrude beyond the rim of the can to interfere with stacking and handling of filled cans. ln other words, all of the structure of the top end wall of a filled easy-opening can must be confined to the shallow head space that is defined by the cylindrical rim flange of the can top.

lfthe top of a filled can bows outward excessively, the depth of the rim flange must be increased accordingly with consequent increase in cost per can in a situation where an exceedingly small fraction of a cent per can looms large in a mass production total. If the top of a filled can bows outward moderately but a tab for severing a tear strip of the can top protrudes excessively or a hollow rivet for anchoring the tab to the tear strip protrudes excessively, again a costly increase in the head space becomes mandatory. lf it is possible to reduce the head space of a filled can, the cost of the can may be correspondingly reduced without reducing the capacity ofthe can or, instead, the capacity of the can may be increased without increasing the cost of the can.

Any measures that are taken to reduce the outward bowing of a filled easy-opening can must take into consideration the metal-spreading effect of certain fabrication steps. Notably, the forming of a hollow rivet in the tear strip has a spreading effect on the sheet metal which is quite pronounced and especially so if a highly desirable method of forming the rivet is employed which involves substantial radial extrusion of the metal in an annular zone around the hollow rivet. The scoring of the metal to form the tear strip also has a spreading effect because the penetration of the sheet metal by the scoring tool displaces the metal in opposite directions.

The spreading of the sheet metal by these two fabrication steps, in effect, increases the area of the can top and the excess metal causes the can top to buckle or warp in an irregular manner. Such a can top is bistable in that the buckling of the sheet metal out of the plane of the can top is predominately in one direction and may be reversed in direction with a snap action.

The cans are filled with the liquid with the cans in upsidedown position, the can top being lowermost, and the last step is the assembling of the can bottom to the cylindrical body. During the filling operation the buckle of the can top which is lowermost may suddenly reverse or flip with a snap action that causes some of the liquid to spill from the can with consequent reduction of the can content. Such troublesome cans are commonly termed flippers.

Later when substantial fluid pressure develops in the filled can, the pressure causes the can top to bow outward by taking up the slack that is available in the buckle of the sheet metal. Thus the extent to which the top of the filled can is bowed outward by internal pressure depends upon the extent to which the prior steps of fabricating the can top have made available excess metal to permit the outwardly bowed configuration.

The present invention reduces the outward bowing of a can top by reducing the extent to which the fabrication steps make excess metal available for the bowing. The invention also, in effect, reinforces the can top to resist the bulging of the sheet metal by the fluid pressure. In addition, the invention substan tially eliminates buckling or warp of the can top and thus eliminates flippers."

With reference to reducing the available excess metal that is created in a can top by a fabrication step, it is old in the art to offset the sheet metal of a can top in various ways to take up metal after the rivet forming and scoring operations are completed and thus contract to some degree the buckled sheet metal. Both inwardly protruding ribs and outwardly protruding ribs have been formed in can tops for this purpose of reducing the buckle. The present invention, however, is based on the discovery that superior end results may be obtained by offsetting the sheet metal simultaneously with any fabrication step that tends to spread the sheet metal. lt has been found that once a buckled condition is created in a can top, offsetting the sheet metal to form ribs or the like has limited corrective effect on the buckle, but on the other hand, offsetting the sheet metal concurrently with such a fabrication step is highly effective to counteract buckling ofthe metal.

One reason for it being difficult to reduce a previously created buckled state by offsetting the sheet metal is that the buckled configuration per se creates resistance to remedial shifting of the sheet metal. Apparently another reason is that the stressing of the metal by the fabrication steps stiffens the sheet metal by workhardening and by orientation of the grain of the metal. The most important difference, however, between remedial action to correct a buckling that is already formed and preventative action concurrent with a metalspreading fabrication step is that, in the first instance, the sheet metal is in a static state when the counteracting step is taken and, in the second instance, the sheet metal is in a favorable dynamic state.

Where an operation on the sheet metal of the can top pushes the metal in opposite directions from a processing zone, the invention takes advantage of this fact by simultaneously pulling the metal in the same opposite directions in the two corresponding offset zones. Thus two forces instead of one force act on a particle of metal simultaneously in the same direction, one force tending to buckle the sheet metal and the other concurrent force negating the tendency to buckle.

ln the fabrication of a prevailing type of easy opening cans, a rivet-forming operation is carried out first, and subsequently the can top is scored to form the tear strip. A feature of the invention is the manner in which the successive buckling tendencies of these two operations are met by successive simultaneous countermeasures.

In one practice of the invention offset ribs are formed in the sheet metal simultaneously with the rivetforming operation to counteract the concurrent buckling tendency and subsequently the same ribs are enlarged concurrently with the scoring operation to again counteract a buckling tendency. In a second practice of the invention, rib-forming dies act directly on the sheet metal to form ribs during the rivet-forming operation,just as in the first practice ofthe invention. During the subsequent scoring operation, however, the dies merely clear the previously formed ribs but the sheet metal is free to slide under guidance away from the scoring zone towards the ribs to enlarge the ribs.

The invention also teaches that advantage may be taken of the fact that if two laterally spaced ribs are formed simultaneously by offsetting the sheet metal, the sheet metal between the two ribs is placed under tension. in both practices of the invention the two ribs that are offset in the metal during the rivet-forming operation are orientated to straddle the intended location of the tear strip so that the area of the intended tear strip is placed under transverse tension and a part of the displacement of sheet metal by the subsequent scoring step serves merely to relieve this tension instead of causing the sheet metal to buckle.

The ribs that are formed in the can top to forestall buckling are advantageous in that they stiffen the sheet metal to resist subsequent outward bowing of the can top by fluid pressure. With the two ribs stiffening the sheet metal and straddling the location of the hollow rivet, the hollow rivet stays in the plane of the two ribs instead of moving outward in response to internal fluid pressure. The formation of the two ribs also increases the amount of metal immediately adjacent the score lines along the two sides of the tear strip and thus reduces the extent to which the residual webs of metal along these lines are stressed longitudinally by the internal fluid pressure. Another advantage in the preferred practice of the invention is that the two flanking ribs are dimensioned and located to serve as lip guards when the user drinks the beverage directly from the can.

In the preferred practice of the invention, the tab that facilitates the severance of the tear strip functions as a secondvantage is that the hollow rivet that attaches the tab to the tear 10 Strip is spaced radially away from the region of maximum outward bowing of the can top. Another important advantage is that the offcenter location of the hollow rivet provides more room for the tab. The greater amount of available space for the tab makes it possible to do one of two things, either to prol vide more access room for manipulation of a tab of conventional size or to employ a tab that is larger than conventional size. In the preferred practice of the invention, the tab is a ring-shaped member and the greater space that is available makes it possible to employ a relatively large tab in which the ring opening is large enough to receive a man's finger.

A further feature of the preferred practice of the invention relates to the desirability of making the tear strip as easy to move as possible without inviting spontaneous severance of the tear strip by fluid pressure. Spontaneous severance is a serious hazard because the abruptly released tear strip is projected away from the can at high velocity.

Manual severance ofthe tear strip is facilitated by a number of provisions including: making the tear strip of generally triangular configuration, the smaller end being the inner leading end; employing a relatively small rivet to attach the tab to the leading end of the tear strip thereby to make it possible to make the leading end of the tear strip relatively narrow; by coining or squeezing the metal to make the metal relatively thin in the region of the leading end of the tear; scoring the metal to maximum depth around the leading end of the tear strip to leave a residual web ofminimum thickness; scoring the metal to moderate depth along the two longitudinal sides of the tear strip and along the trailing edge of the tear strip.

The hazard of spontaneous severance is reduced by the two ribs inasmuch as the two ribs reduce the load in tension longitudinally of the lines of scoring along the two sides of the tear strip. The possibility of spontaneous severance is further reduced by scoring the metal relatively lightly at the two cor- 45 preclude spontaneous severance. On the other hand, it is easy 50 to initiate severance at the leading end of the tear strip and the momentum of manually peeling away the tear strip readily overcomes the relatively thick residual web at the two rearward corners of the tear strip.

The features and advantages of the invention may be un- 55 derstood from the following detailed description and the accompanying drawings.

In the drawings, which are to be regarded as merely illustrative:

FIG. 1 is a plan view of the completed can top illustrating a 60 selected practice of the invention;

FIG. 2 is a similar view of the can top prior to the final step of mounting a tab on the tear strip;

FIG. 3 is a greatly enlarged fragment of FIG. 2 showing how the sheet metal of the can top is squeezed in the process of 65 forming a hollow rivet, the squeezing operation resulting in an annular zone around the hollow rivet in which the outer surface of the sheet metal is depressed to a lower level with two inclined ramps providing transitions between the two levels;

FIG. 4 is a transverse section along the line 4-4 of FIG. 3;

FIG. 5 is a transverse section along the irregular line 5-5 of FIG. 3;

FIG. 6 is a section along the irregular line 6-6 of FIG. 3 showing how a ramp makes a transition between the two levels of the outer surface ofthe can top;

FIG. 7 is a fragmentary plan view of a can top with a newly formed hollow rivet therein showing a pair of ribs that are formed simultaneously with the rivet;

FIG. 8 is a greatly enlarged fragmentary cross section taken along the line 8-8 of FIG. 7 showing how a pair of dies cooperate to form the rivet and the two ribs simultaneously;

FIG. 9 is a similar sectional view showing how the same two dies cooperate to form other portions of the two ribs;

FIG. 10 is a view similar to FIG. 7 showing the state of the can top after a subsequent operation in whichthe can top is scored to form a tear strip and simultaneously the two previously formed ribs are reformed and enlarged;

FIG. 11 is a cross-sectional view taken along the line 11-11 of FIG. 10 and showing how a pair of dies cooperate to score the can top in the region of the previously formed hollow rivet and to reform adjacent portions of the two ribs;

FIG. 12 is an enlarged cross section along the line 12-12 of FIG. 10 showing how the two dies of FIG. 11 cooperate to reform other portions of the two ribs;

FIG. 13 is an enlarged section along the line 13-13 of FIG. 7 showing the configuration of one of the ribs after the initial forming operation that is carried out simultaneously with the forming ofthe rivet;

FIG. 14 is an enlarged section taken along the line 14-14 of FIG. 8 and the line 14-14 of FIG. 10 and showing the final longitudinal configuration of a sheet metal rib after the rib is reformed simultaneously with the scoring of the can top;

FIG. 15 is an enlarged portion of FIG. 2 showing how the groove or line of scoring that forms the tear strip extends down a ramp to the lower surface of the depressed area sur rounding the hollow rivet;

FIG. 16 is a greatly enlarged fragmentary sectional view showing how a pair of dies cooperate to form the groove or score line in each of the regions where the score line extends down a ramp to the lower surface area, the view showing the pair of dies open in preparation for the scoring operation;

FIG. 17 is a similar view showing the two dies closed against the material of the can top to carry out the scoring operation;

FIG. 18 is a similar view showing the can top released by the scoring dies;

FIG. 19 is a view similar to FIG. 8 showing how a pair of dies may be employed to form the hollow rivet and simultaneously form the two sheet metal ribs to final configuration in one operation;

FIG. 20 is a sectional view similar to FIG. 9 showing how the same pair of dies cooperate to form other portions of the two ribs;

FIG. 21 is a cross-sectional view similar to FIG. 12 showing how a pair of dies may be employed to score the can top after the two ribs have been formed simultaneously with the formation of the hollow rivet;

FIG. 22 is a plan view of the tear strip on an enlarged scale; and

FIG. 23 is a sectional view of a can end with the tab removed and the rivet unstaked.

In the selected embodiment of the invention shown in FIG. 1 the can top, generally designated T, has a tear strip formed by a continuous groove or score line 32, the tear strip being of generally triangular configuration with straight sides. The tear strip has an inner relatively narrow leading end that is blunt in plan configuration, the leading edge being substantially perpendicular to the longitudinal axis of the tear strip. It has been found that squaring the leading end in this manner to form two corners and to bring the score line close to the rivet at the leading end of the tear strip greatly facilitates initiation of severance of the tear. The tear strip has a relatively wide trailing end that is formed with rounded corners 34. A suitable sheet metal tab of a well-known type has a relatively large opening 36 to receive the users finger and is connected to the tear strip 30 by means ofa hollow rivet 38 that is formed in the tear strip, the rivet extending through an aperture in the tab and being headed or staked into overlapping engagement with the rim of the aperture. In the construction shown, the tab 35 has a sheet metal tongue 40 that serves as a pliable connection between the tab and the hollow rivet. The tab is in the form of a secondcclass lever that is of forked construction to provide two fulcrum end portions or fulcrum arms 42 which straddle the tear strip and are shaped and dimensioned to exert fulcrum force against the can top adjacent opposite sides of the tear strip and outside of the area of the tear strip.

As shown in FIG. I, the can top is of a conventional crosssectional configuration being formed with an outer circumferential groove 44 and an upstanding peripheral flange 45 which in the completed can isjoined to the cylindrical body of the can to form the rim or chime ofthe can. It is to be noted in FIG. I that the tear strip is substantially shorter than the radius of the can as measured inside the circumferential groove 44 and that the inner or leading end of the tear strip is spaced substantially from the center of the can top, the can top center being indicated by the numeral 46. By virtue of the tear strip 30 being dimensioned and located in this manner, the hollow rivet 38 is spaced a substantial distance from the center 46 of the can top.

It is apparent in FIG. 1 that the offcenter location of the hollow rivet 38 also provides more room for the tab within the circumference defined by the peripheral flange 45. The inven tion takes advantage of this fact by making the tab 35 larger than would otherwise be possible, the tab being unique in this respect in that the opening 36 is large enough to receive a man's finger. It is also to be noted that even though the tab is relatively large, the offcenter positioning of the hollow rivet makes available so much space for the tab that, with the tab located on the same diameter as the tear strip, there is a liberal clearance between the handle end of the tab and the upstanding flange to provide room for manual access to the underside ofthe tab.

An important feature of the invention is the provision of a pair of outwardly protruding ribs 48 that are formed in the sheet metal of the can top along opposite sides of the tear strip 30 just outside the area of the tear strip. The two ribs 48 are in convergent positions to conform to the triangular configuration of the tear strip and extend well beyond the leading end of the tear strip into the central area of the can top.

It may be seen in FIG. 1 that the two fulcrum arms 42 of the tab 35 rest on the two ribs 48 respectively, the ribs strengthening the sheet metal to resist inward flexure of the can top in response to the fulcrum force exerted by manipulation of the tab. Preferably, the inner end portions of the two ribs 48 that lie under the fulcrum arms 42 are partially flattened, the flattened zones being shaded and designated by numerals 50 in FIGS. 2 and 10. FIG. 14 is a longitudinal section through one of the ribs and shows how the flattened portion 50 of a rib 48 is less elevation than the remainder of the rib, the partial flattening of the rib reducing the extent to which the rib elevates the corresponding fulcrum arms 42 of the tab.

The hollow rivet 38 is initially formed by an operation which involves thinning the sheet metal of the can top in an annular zone 52 around the rivet, the annular zone being formed by a squeezing operation which displaces the metal radially inward to form the hollow rivet. The squeezing opera tion also displaces the sheet metal radially outward and it is this radial outward displacement of metal that tends to increase the extent to which the can top bulges into the head space. The initial configuration of the hollow rivet that results from the radially inward displacement of the metal is indicated by numeral 54 in FIGS. 2-8, 10, 11,15, 19 and 22.

The operation of squeezing the metal to form the hollow rivet to its initial configuration is preferably but not necessarily carried out in such manner as to depress the level of the annular zone 52 relative to the remainder of the can top. In the preferred practice of the invention the squeezing of the metal also forms two ramps 55 on opposite sides of the hollow rivet and as shown in FIG. 6, each ramp 55 provides a transition between the lower level of the depressed annular zone 52 and the higher level of the surrounding metal ofthe can top.

It is contemplated that the hollow rivet in the can top will be formed to its initial configuration in one operation and that the can top will be scored to form the tear strip 30 in a separate and subsequent operation. In one practice of the invention the two ribs 48 are formed in two stages, the ribs being partially formed in one stage simultaneously with the formation of the hollow rivet and being completely formed in a second stage simultaneously with the scoring operation. FIGS. 7 and 13 show the configuration of the ribs at the end of the first stage and FIGS 10 and 14 show the ribs at the end of the second stage. This particular practice of the invention will now be described.

FIGS. 8 and 9 show how an upper die 56 and a lower die 58 cooperate to form the hollow rivet to its initial configuration 54 and simultaneously partially form the two sheet metal ribs, i.e. form the two sheet metal ribs to an initial configuration. The upper die 56 has a central cavity 60 which is surrounded by an annular land 62. In addition the under face of the upper die 56 is formed with two channels 64 in which the two sheet metal ribs of the can top are to be formed to their initial configuration.

The lower die 58 has a shallow boss 65 that registers with the cavity 60 and promotes the formation of the hollow rivet in the cavity. The lower die 58 is further formed with elongated projections 66 which conform to the configuration in plan of the channels 64 of the upper die and which offset the sheet metal into the channels to form the partially completed ribs which are designated 68 in FIGS. 7, 8, 9 and I3.

It may be noted that the elongated projections 66 of the lower die 58 are somewhat angular in cross section to make the corresponding ribs angular in cross section. FIG. 8 shows how portions of the two channels 64 in the upper die and corresponding portions of the elongated projections 66 in the lower die are relatively shallow to make the inner end portions of the two ribs 68 correspondingly shallow and FIG. 9 shows how the remainder of the channels 64 are deeper and the corresponding portions of the projections 66 are also deeper to make the outer end portions of the two ribs higher than the inner end portions.

As shown in FIG 8, the annular land 62 squeezes the sheet metal to a relatively thin thickness, the squeezing action causing the formation of the rivet to its initial configuration and causing the formation of the depressed annular zone 52. The land 62 is cut away in two opposite regions to cause the land to form the two previously mentioned ramps 55.

It may be readily appreciated that the squeezing of the metal to form the hollow rivet also has the effect of displacing the sheet metal radially outwardly to increase the bulging of the can top. The simultaneous production of the partially formed ribs 68 in the can top, however, takes up metal as the metal is being spread by the rivet-forming operation and does so to such extent that the formation of the rivet has substantially no bulging effect on the can top. In addition the formation of the two ribs by the first operation by virtue of drawing the sheet metal in opposite directions from the intended area of the tear places the sheet metal in the intended area under transverse tension.

The second operation in this first practice of the invention is to score the metal to form the tear strip 30 and to simultaneously deepen the two ribs in the can top to negate the bulging effect of the scoring operation. This second operation is carried out by a pair of dies of the character shown in FIGS. 11 and 12, the dies comprising an upper die 70 and a lower die 72. The upper die 70 has a central cavity 74 dimensioned to clear the initial configuration S4 of the hollow rivet. An annular land 75 surrounds the cavity 74 to make contact with the depressed annular zone 52 of the can top and the upper die is formed with a pair of channels 76 conforming to the plan configuration of the partially formed ribs 68. The upper die 70 is further provided with an integral scoring element 78 with a portion of the scoring element extending along the annular land 75.

It is to be noted that the portions of the channels 76 of the upper die 70 shown in FIG. 11 are relatively shallow to conform with the previously mentioned partially flattened portions 50 of the previously described completed sheet metal ribs 48. As shown in FIG. 12 the remaining portions of the two channels 76 are relatively deep to permit the corresponding portions of the completed can top ribs 48 to be relatively high.

The lower die 72 has a pair of projections 80 to mate with the two channels 76 of the upper die. As shown in FIG. 11 the two projections 80 are relatively shallow where the completed can top ribs are to be shallow and as shown in FIG. 12, the projections are of greater elevation where the cross sections of the completed ribs are to be higher.

It is apparent from a comparison of FIGS. 11 and 8 that the second stage of forming of the can top ribs 48 results in appreciably flattening and widening of the inner end portions 50 of the sheet metal ribs and a comparison of FIG. 12 with FIG. 9 shows that the second stage results in both broadening and deepening the remaining portions of the rivet. The relieving of the transverse tension of the sheet metal and the taking up of additional sheet metal by the ribs compensate for the spreading of the metal in the opposite lateral directions by the scoring element 78 and thus nullifies the tendency of the scoring operation to increase the bulging of the can top.

It may be seen in FIG. 11 that the scoring element 78 penetrates the metal in the depressed annular zone 52 to great depth to leave an exceedingly thin residual web but outside of the annular zone the scoring element 78 penetrates the sheet metal to lesser depth to leave a thicker residual web as shown in FIG. 12. As shown in FIGS. and 15, the groove that forms the score line extends down the two ramps 55 into the depressed annular zone 52 to loop around the initially formed rivet 54 and thereby form the leading end of the tear strip 30. The manner in which the upper die 70 and the lower die 72 cooperate to form this portion of the groove 32 is indicated by FIGS. 16-18.

As best shown in FIG. 16 the lower die 72 is formed with a plateau 82 that conforms with the area of the depressed annular zone 52 of the can top and the lower die is further formed with a planar working face 84 that conforms with the remainder of the can top FIG. 16 also shows how the lower die 72 is formed with a slope 85 which is a shoulder of the plateau, there being two shoulders 85 to conform to the two ramps 55 of the can top. Each slope 85 forms a transition from the level of the plateau 82 to the lower level of the planar working face 84 of the die in the same manner that the corresponding ramp 55 of the can top forms a transition from the level of the metal outside the annular zone 52 to the lower level in the annular zone. It is important to note, however, that the inclination of the slope 85 of the lower die is substantially less than the inclination of the corresponding ramp 55 of the can top. In this region around the leading end of the tear strip, the scoring element 78 of the upper die 70 conforms at its lower edge with a plane that is parallel to the working surface 84 of the lower die 72.

FIG. 16 shows the upper and lower dies 70 and 72 retracted with a can top resting on the plateau 82 of the lower die in preparation for the scoring operation. It may be noted that there is a clearance space 86 between the lower surface of the can top and the lower planar surface 84 of the lower die 72. When the two dies 70 and 72 are operated for a scoring operation, the two dies close towards each other with consequent flexing of the can top as shown in FIG. 17, the flexed can top following the inclination of the die slopes 85 and making contact with the lower planar working face 84 ofthe lower diev FIG. 17 shows the two dies 70 and 72 at their minimum spacing in carrying out the scoring operation. It may be noted that by virtue of the inclination of each slope 85 being less than the inclination of the corresponding ramp 55, the penetration of the scoring element 78 into the metal progressively increases down the length of each ramp with the result that the thickness of the residual web 87 left by the scoring tool progressively decreases to a minimum thickness in the annular zone 52.

FIG. 18 shows how the sheet metal springs back from the planar working face 84 of the lower die when the two dies are retracted. FIG. 18 further clearly shows how the residual web 87 left by the scoring tool progressively decreases as the residual web approaches the annular zone 52.

In the preferred practice of the invention the residual web 87 formed by the score line 32 is of minimum thickness at the leading end ofthe tear strip and is of moderate thickness along the two longitudinal sides of the tear strip as well as along the trailing end of the tear strip. At each of the two corners 34 of the tear strip, however, the residual web is of maximum thickness. For example, referring to FIG. 22, if the nominal thickness of the sheet metal stock is 0.0145 inch the residual web in the depressed annular zone 52 at the leading end of the tear strip may be only 0.00350.0040 inch thick. The thickness of the residual web increases along each of the two ramps 55 to a thickness of 0.00550.0065 inch and this thickness dimension prevails along the two straight sides of the tear strip outside of the annular zone 52. Between the two

lines

88 and 90 in FIG. 22 that bound each ofthe two corners 34 of the tear strip, the residual web may have the relatively great thickness of 0.009 inch to 0.012 inch and between the two lines 90 at the trailing end of the tear strip the residual web may again be of the moderate thickness 0.00550.0065 inch.

As may be understood from the previous discussion of FIGS. 16I8, the edge of the scoring element 78 that forms the leading end of the tear strip and the two straight sides of the tear strip is parallel to the planar working face 84 of the lower die 72. In the region of each of the two corners 34, however, the striking surface of the scoring element is reduced in height to make the residual web correspondingly thick.

The fact that the residual web is exceedingly thin at the leading end of the tear strip greatly facilitates initiation of the severance of the tear strip by initial lifting of the ring portion of the tab 35. Once the leading end of the tear strip is severed the severance is continued along the two straight side edges of the tear strip by a single hand motion and the momentum of this hand motion easily overcomes the resistance of the thicker residual web portions at the two corners 34 of the tear strip. Thus the thickening of the residual web at the two corners 34 provides the required insurance against spontaneous severance of the tear strip by fluid pressure but at the same time does not offer undue resistance to the final manual separation of the tear strip from the can top.

After the hollow rivet is formed to the initial hollow configuration 54 and the can top is scored with the two sheet metal ribs formed in two stages as described, the can top is ready for the final operation of attaching the tab to the tear strip. This final operation consists first, of placing the tab on the can top with the hollow rivet extending through the aperture in the tongue 40 of the tab and, then, staking or spreading the outer end of the hollow rivet in a suitable manner to reform the hollow rivet into overlapping engagement with the tab.

The second practice of the invention illustrated by FIGS. 19, 20 and 21 differs from the first practice in that the two can top ribs 48 are substantially completely formed in one operation simultaneously with the forming ofthe hollow rivet.

FIGS. 19 and 20 show an upper die 92 and a lower die 94 that cooperate to form the hollow rivet and to form the two ribs simultaneously to their final configuration. The upper die 92 has the usual cavity 95 and the lower die 94 has the usual shallow boss 96 to cooperate for the forming of the hollow rivet. The upper die has the usual pair of channels 96 and the lower die has the usual corresponding elongated projections 97 for cooperation with the channels to form the two can top ribs 48 in one operation. As may be seen in FIG. 19, portions of the channels 96 and of the projections 97 are relatively shallow to cooperate to form the partially flattened portions 50 of the sheet metal ribs 48 and, as may be seen in FIG. 20, the remaining portions of the channels 96 and the projections 97 are deeper to deepen the remaining portions of the sheet metal ribs. v

The second step of scoring the sheet metal to form the tear strip is carried out by a pair of dies designated 100 and 102 in FIG. 21. The two dies 100 and 102 are similar to the previously described pair of scoring dies 70 and 72 except that the channels 103 in the upper die 100 are large enough to clear the previously formed sheet metal ribs 48. It is also to be noted that the two dies confine the sheet metal with freedom for the sheet metal to slip towards the two previously formed ribs 48.

Two factors cooperate to compensate for the spreading of the sheet metal by the scoring operation and thus prevent buckling of the can top. One factor is that the prior operation of forming the two ribs has drawn the sheet metal in opposite directions from the intended area of the tear strip to leave the sheet metal across the area under a state of transverse tension. The relief of this transverse tension by the scoring operation partially compensates for the tendency of the scoring operation to buckle the can top.

The second factor is that the scoring dies 100 and 102 provide ample clearance for increase in depth of the previously formed ribs 48 and at the same time the two dies confine the metal between the ribs to a plane with freedom for the sheet metal to shift towards the two ribs. Any tendency of the scoring operation to increase the buckle in the can top serves instead to displace metal into the two ribs.

My description in specific detail of the selected embodiments of the invention will suggest various changes, substitu' tions and other departures from my disclosure within the spirit and scope of the appended claims.

l claim: 1. A method of fabricating an easy'opening container ofthe character described wherein a sheet metal wall of the container is scored to form a tear strip and a tab is attached to the tear strip by a hollow rivet formed in the tear strip, said method being characterized by the steps of:

squeezing the sheet metal wall around the base of the rivet to reduce the thickness of the wall and to form a zone in which the outer face of the wall is depressed to lower level and to form two ramps on the opposite sides of the zone leading to the lower level; placing one face of the sheet metal wall against a first die having a plateau conforming to said zone and having an extensive planar working face at the base of the plateau, the plateau having slopes conforming to the two ramps respectively of lesser inclination than the ramps, each slope leading from the top of the plateau to said working face; positioning a second die against the other face of the sheet metal wall, the second die having a protruding scoring element to score the wall of the container to form the tear strip, said scoring element having a leading face in a plane substantially parallel to said working face, said scoring element being of a configuration to loop around the hollow rivet in the region of said zone with two spaced portions ofthe scoring element positioned to extend from the region of the plateau across the regions of the two slopes respectively to the region of said working face; and

causing relative movement of said dies to force said scoring element into the sheet metal of the can top with consequent resilient deflection of the sheet metal against the two slopes and the working face of the first die to cause the scoring element to form a relatively thin residual web in said zone with the residual web progressively increasing in thickness along the two ramps from said zone to the region outside ofthe zone.

2. A method of forming a hollow rivet in a container wall of sheet material comprising:

forming a hollow rivet of sheet material integral with the container wall;

said step of forming including squeezing a zone of sheet material surrounding the rivet to displace some of the material from said zone into the rivet and to displace other portions of the material from said zone radially outwardly; and

offsetting the sheet material simultaneously with said step of squeezing at a region radially outwardly of said zoneto take up at least some of said other portions of the material. 3. A method of forming a tear portion in a container wall of sheet material comprising:

offsetting regions of the container wall to form first and second ribs extending in the same general direction and having a section of the container wall therebetween with said step of offsetting placing said section in tension; substantially confining the sheet material of said section against relative movement in a direction generally transverse to the plane of said section and providing free space adjacent the ribs to allow enlargement of the dimensions thereof; and scoring said section along a predetermined line subsequent to the offsetting operation to form the tear portion with sheet material being displaced laterally of said predetermined line by the scoring operation to at least partially relieve said tension in said section, the free space adjacent the ribs allowing for enlargement of the dimensions of said ribs as a result of the displacement of sheet material during the scoring operation. 4. A method of fabricating an easy-opening can top of sheet material comprising:

forming a hollow rivet in the sheet material by an operation that spreads the sheet material; offsetting the sheet material simultaneously with the rivetforming operation to form two ribs on opposite sides respectively of the rivet to take up the sheet material as the sheet material is displaced by the rivet-forming operation and to place a section of the sheet material between the ribs in tension; and scoring the sheet material including said section of sheet material to form a tear strip and at least partially relieve said tension in said section of sheet material, said step of scoring being carried out subsequent to the rivet-forming operation, the two ribs straddling the tear strip and extending longitudinally thereof. 5. A method of fabricating an easy-opening can top of sheet material comprising:

providing a can top of sheet material having a peripheral flange defining a head space with the can top bowing outwardly into the head space and with the maximum outward bowing being at the center of the can top; forming a hollow rivet in the can top at a location spaced radially from the center of the can top to avoid locating the rivet in the region of maximum outward bowing of the can top, said rivet being adapted to attach a second class lever to the can top; offsetting the sheet material simultaneously with the rivetforming operation to form two ribs on opposite sides respectively of the rivet to take up the sheet material as the sheet material is displaced by the rivet-forming operation and to place a section of the sheet material between said ribs in tension; and scoring the sheet material including said section of sheet material to form a tear strip and at least partially relieve said tension in said section of sheet material, said step of scoring being carried out subsequent to the rivet-forming operation, the two ribs straddling the tear strip and extending longitudinally thereof, the two ribs being at locations to receive the fulcrum force of the lever and to stiffen the can top against flexure by the fulcrum force. 6. A method of forming an easyopening container wall of sheet material comprising:

forming a hollow rivet in the sheet material by an operation that spreads some of the sheet material outwardly of the rivet; offsetting the sheet material simultaneously with the rivetforming operation to form at least two ribs and to take up at least some of the material displaced in the rivet-forming operation, the formation of said ribs placing at least a section of the sheet material in tension;

formed simultaneously with the rivet-forming operation and an operation for completing the forming of the two ribs being carried out simultaneously with the scoring operation.

8. An improvement as set forth in claim 4 in which the sheet material of the two ribs is unrestrained during the scoring operation and metal displaced by the scoring operation is directed to the ribs to increase the ribs instead of buckling the can top.

Claims

1. A method of fabricating an easy-opening container of the character described wherein a sheet metal wall of the container is scored to form a tear strip and a tab is attached to the tear strip by a hollow rivet formed in the tear strip, said method being characterized by the steps of: squeezing the sheet metal wall around the base of the rivet to reduce the thickness of the wall and to form a zone in which the outer face of the wall is depressed to lower level and to form two ramps on the opposite sides of the zone leading to the lower level; placing one face of the sheet metal wall against a first die having a plateau conforming to said zone and having an extensive planar working face at the base of the plateau, the plateau having slopes conforming to the two ramps respectively of lesser inclination than the ramps, each slope leading from the top of the plateau to said working face; positioning a second die against the other face of the sheet metal wall, the second die having a protruding scoring element to score the wall of the container to form the tear strip, said scoring element having a leading face in a plane substantially parallel to said working face, said scoring element being of a configuration to loop around the hollow rivet in the region of said zone with two spaced portions of the scoring element positioned to extend from the region of the plateau across the regions of the two slopes respectively to the region of said working face; and causing relative movement of said dies to force said scoring element into the sheet metal of the can top with consequent resilient deflection of the sheet metal against the two slopes and the working face of the first die to cause the scoring element to form a relatively thin residual web in said zone with the residual web progressively increasing in thickness along the two ramps from said zone to the region outside of the zone.

2. A method of forming a hollow rivet in a container wall of sheet material comprising: forming a hollow rivet of sheet material integral with the container wall; said step of forming including squeezing a zone of sheet material surrounding the rivet to displace some of the material from said zone into the rivet and to displace other portions of the material from said zone radially outwardly; and offsetting the sheet material simultaneously with said step of squeezing at a region radially outwardly of said zone to take up at least some of said other portions of the material.

3. A method of forming a tear portion in a container wall of sheet material comprising: offsetting regions of the container wall to form first and second ribs extending in the same general direction and having a section of the container wall therebetween with said step of offsetting placing said section in tension; substantially confining the sheet material of said section against relative movement in a direction generally transverse to the plane of said section and providing free space adjacent the ribs to allow enlargement of the dimensions therEof; and scoring said section along a predetermined line subsequent to the offsetting operation to form the tear portion with sheet material being displaced laterally of said predetermined line by the scoring operation to at least partially relieve said tension in said section, the free space adjacent the ribs allowing for enlargement of the dimensions of said ribs as a result of the displacement of sheet material during the scoring operation.

4. A method of fabricating an easy-opening can top of sheet material comprising: forming a hollow rivet in the sheet material by an operation that spreads the sheet material; offsetting the sheet material simultaneously with the rivet-forming operation to form two ribs on opposite sides respectively of the rivet to take up the sheet material as the sheet material is displaced by the rivet-forming operation and to place a section of the sheet material between the ribs in tension; and scoring the sheet material including said section of sheet material to form a tear strip and at least partially relieve said tension in said section of sheet material, said step of scoring being carried out subsequent to the rivet-forming operation, the two ribs straddling the tear strip and extending longitudinally thereof.

5. A method of fabricating an easy-opening can top of sheet material comprising: providing a can top of sheet material having a peripheral flange defining a head space with the can top bowing outwardly into the head space and with the maximum outward bowing being at the center of the can top; forming a hollow rivet in the can top at a location spaced radially from the center of the can top to avoid locating the rivet in the region of maximum outward bowing of the can top, said rivet being adapted to attach a second class lever to the can top; offsetting the sheet material simultaneously with the rivet-forming operation to form two ribs on opposite sides respectively of the rivet to take up the sheet material as the sheet material is displaced by the rivet-forming operation and to place a section of the sheet material between said ribs in tension; and scoring the sheet material including said section of sheet material to form a tear strip and at least partially relieve said tension in said section of sheet material, said step of scoring being carried out subsequent to the rivet-forming operation, the two ribs straddling the tear strip and extending longitudinally thereof, the two ribs being at locations to receive the fulcrum force of the lever and to stiffen the can top against flexure by the fulcrum force.

6. A method of forming an easy-opening container wall of sheet material comprising: forming a hollow rivet in the sheet material by an operation that spreads some of the sheet material outwardly of the rivet; offsetting the sheet material simultaneously with the rivet-forming operation to form at least two ribs and to take up at least some of the material displaced in the rivet-forming operation, the formation of said ribs placing at least a section of the sheet material in tension; scoring the sheet material along a predetermined line to form a tear portion of the desired configuration with the scoring operation displacing material generally laterally outwardly from the predetermined line to thereby at least partially relieve said tension, at least portions of said predetermined line being adjacent said ribs; and further offsetting said ribs to enlarge said ribs simultaneously with the scoring operation.

7. An improvement as set forth in claim 4 in which the two ribs are formed in two stages, the two ribs being partially formed simultaneously with the rivet-forming operation and an operation for completing the forming of the two ribs being carried out simultaneously with the scoring operation.