U.S. patent number 3,583,348 [Application Number 04/736,870] was granted by the patent office on 1971-06-08 for method of making an easy opening container wall.
This patent grant is currently assigned to Ermal C. Fraze. Invention is credited to Omar L. Brown.
United States Patent |
3,583,348 |
Brown |
June 8, 1971 |
METHOD OF MAKING AN EASY OPENING CONTAINER WALL
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.
Inventors: |
Brown; Omar L. (Dayton,
OH) |
Assignee: |
Ermal C. Fraze (Dayton,
OH)
|
Family
ID: |
24961652 |
Appl.
No.: |
04/736,870 |
Filed: |
March 29, 1968 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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565538 |
Jul 15, 1966 |
3428210 |
|
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Current U.S.
Class: |
413/14;
29/522.1 |
Current CPC
Class: |
B65D
17/4012 (20180101); B21D 51/383 (20130101); Y10T
29/49938 (20150115) |
Current International
Class: |
B21D
51/38 (20060101); B21d 051/00 () |
Field of
Search: |
;220/54
;113/121,121A,116FF,116CC,12Q ;29/509,522 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Herbst; Richard J.
Assistant Examiner: Keenan; Michael J.
Parent Case Text
This application is a division of Ser. No. 565,538, now U.S. Pat.
No. 3,428,210 filed July 15, 1966.
Claims
I claim:
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.
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.
Description
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. In 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.
If the 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. If 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 of the 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 substantially
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. It 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 of the
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 metal-spreading 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.
In 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 rivet-forming 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 of the 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
second-class lever and fulcrums against the sheet metal adjacent
opposite sides of the tear strip. In this regard, a further
advantage of the invention is that the two ribs are located to
receive the fulcrum pressure of the tab, the ribs serving to
reinforce the can top to resist inward yielding of the metal.
Other advantages of the preferred practice of the invention result
from the provision of a relatively short radially positioned tear
strip with the leading end of the tear strip spaced radially from
the center of the can top. One important advantage is that the
hollow rivet that attaches the tab to the tear 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 provide 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 of the 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 of
minimum 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 corners of the trailing end
of the tear strip to make the residual web of metal relatively
thick at these two points.
By virtue of the described features the tear strip readily
withstands the static pressure of the fluid in the container to
preclude spontaneous severance. On the other hand, it is easy 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 understood 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
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 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 of the 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 which the 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 of the 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 surrounding 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 30 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 35 of a well-known type has a relatively large
opening 36 to receive the user's finger and is connected to the
tear strip 30 by means of a 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 second-class
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. 1, the can top is of a conventional
cross-sectional configuration being formed with an outer
circumferential groove 44 and an upstanding peripheral flange 45
which in the completed can is joined to the cylindrical body of the
can to form the rim or chime of the can. It is to be noted in FIG.
1 that the tear strip 30 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 35 within the
circumference defined by the peripheral flange 45. The invention
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 45 to provide room for
manual access to the underside of the 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 operation 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 of the 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 13.
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
54 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. 10 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
of the lower die.
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 of the 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
0.0035-- 0.0040 inch thick. The thickness of the residual web
increases along each of the two ramps 55 to a thickness of 0.0055--
0.0065 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 of the
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 0.0055-- 0.0065
inch.
As may be understood from the previous discussion of FIGS. 16-- 18,
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 of the 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.
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, substitutions and other
departures from my disclosure within the spirit and scope of the
appended claims.
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