Method Of Forming Can Bodies

Abstract

To produce a can body a sheet metal blank is formed into a short cup-shaped workpiece. The cylindrical wall of the workpiece is then thinned and axially elongated by an ironing operation and finally the bottom wall is squeezed to reduce its thickness and to extrude the material thereof in a manner to extend the length of the cylindrical wall.

Description

BACKGROUND OF THE INVENTION

Various fabrication procedures have been employed heretofore for the production of cans of the general type commonly used for beverages and food products. In the conventional fabrication of a tin coated steel can, for example, a cylindrical shell is first formed out of sheet metal and then a stamped sheet metal bottom is assembled to one end of the shell.

To avoid the necessity of separately fabricating a cylindrical shell and a bottom wall, it is highly desirable to form a can body with an integral bottom wall and such one-piece can bodies made of aluminum alloy have been produced heretofore.

One prior art method of producing a one-piece aluminum can body is to use progressive drawing dies to produce a cup-shaped aluminum body of somewhat less than the desired axial dimension and then to place the cup-shaped body on a mandrel for the purpose of "ironing" the cylindrical wall of the body by means of a hard metal or carbide ring to thin the cylindrical wall and to elongate the cylindrical wall to the desired axial dimension.

Another prior art method of producing a one-piece aluminum can body employs impact extrusion to produce an intermediate cup-shaped workpiece. It is not practical to extrude such an intermediate workpiece in a single impact stroke because it would be too severe on the dies and because slight defects in the metal and the presence of minute bodies of lubricant would result in too many rejects, and therefore repeated extrusion is employed. The product of the repeated extrusion is placed on a mandrel and is finished to the desired final dimension by using a hard metal ring for a simple ironing operation.

All of these prior art techniques result in a can body in which the thickness of the material is greater than necessary in some parts of the can and therefore there is a pressing need for a method of fabricating a one-piece can body with greater economy of material. This need may be appreciated by considering the ideal thickness dimensions for a one-piece can body made of an aluminum alloy.

It has been found that the bottom wall of an aluminum alloy can may be relatively thin and yet be of adequate strength by a liberal margin if the bottom wall is shaped to nonplanar configuration, for example, shaped to an inwardly bulged configuration. Thus an aluminum alloy can body of a common size of 2 11/16 inches inside diameter and an axial dimension of approximately 4 3/4 inches may have a shaped bottom wall of a thickness of only 0.010 inches. It is further possible to make the cylindrical wall of the can relatively thin in the intermediate region between the ends of the can without reducing the strength of the can below a safe margin. Thus the cylindrical wall of an aluminum alloy can may taper in thickness from approximately 0.010 inches near its bottom end to a minimum thickness of 0.006 - 0.0065 inches. It has also been found that if the upper open end of such an aluminum alloy can body is necked down to reduced diameter to permit the use of a top wall of reduced diameter, the necked down configuration locally reinforces the cylindrical wall to permit the local thickness of the cylindrical wall to be reduced. Thus, with the open end of the aluminum can body strengthened in this manner, the thickness of the cylindrical wall may be of a thickness of only 0.0060- 0.0065 inches throughout the major intermediate portion of its length with a thickness of approximately 0.010 near the bottom end and a thickness of approximately 0.085 inches near the top end.

If an aluminum alloy can body were fabricated with the above specified minimum thickness dimensions, less than 28 pounds of metal would be required to produce 1,000 can bodies with integral bottom walls instead of 38 to 40 pounds. The economic significance of this fact may be appreciated when it is considered that with aluminum alloy can bodies produced by automatic machinery at rates as high as 600- 700 cans per minute, 80 percent of the cost of the cans is in the aluminum alloy, only 20 percent of the cost being labor, overhead and profit. Thus 15 to 30 percent saving in the amount of aluminum alloy means a saving of 16 to 24 percent of the total cost of the can bodies. At the high rate of production made possible by automatic machinery, a saving of only 5 percent would pile up rapidly.

It has been found that progressive drawing alone, progressive drawing combined with an ironing operation, and repeated extrusion combined with ironing are all inherently incapable of producing such an ideally dimensioned aluminum alloy can body largely because in each instance the bottom wall is thicker than necessary. For example, if a can body is produced by first drawing a cup-shaped workpiece from sheet stock and then ironing the cylindrical wall of the workpiece to elongate and simultaneously thin the cylindrical wall, the body of the can body may be as thin as desired but the bottom wall will be uneconomically thick. If the initial sheet stock must be at least approximately 0.0145 inches thick to supply enough metal for the thin cylindrical wall of the finished can body, the bottom wall of the finished can body will be approximately 0.0145 inches thick instead of the desired thickness of 0.010 inches because the drawing operation does not significantly reduce the thickness of the bottom wall.

One solution heretofore proposed for the production of a can body with an economically thin bottom wall is to draw a cup-shaped workpiece and then squeeze the bottom wall of the workpiece to thin it to the desired degree by impact force. The difficulty, however, is that the squeezing of the bottom wall extrudes the material radially in all directions to produce a bead or rib of surplus metal around the bottom of the can body and this surplus metal must be displaced in the subsequent ironing operation. The surplus metal must be displaced all the way to the opposite end of the can body and therefore the surplus metal places a heavy burden on the ironing operation. An important object of the present invention is to avoid this disadvantage in a fabrication technique for producing one-piece can bodies of highly economical thickness dimension.

It is to be borne in mind that the economical thickness dimensions specified above for aluminum alloy are by way of example only, it being understood that other thickness dimensions may be specified for other metals such as steel and for other materials such as ductile plastics.

SUMMARY OF THE INVENTION

The method of fabricating an aluminum alloy can starts in the usual manner with an aluminum alloy sheet blank having a nominal thickness substantially in excess of the maximum thickness desired either in the cylindrical wall or in the bottom wall of the can body. The blank sheet metal is formed into a cup-shaped workpiece of substantially the inside dimensions desired in the can body, the thickness of the walls of the workpiece being substantially the thickness of the starting blank and the length or axial dimensions of the starting piece being substantially less than the length desired for the can body and preferable less than half of the desired length. The cup-shaped workpiece may be produced in a single draw but preferably is drawn in two stages.

The next step is to telescope the cup-shaped workpiece over a mandrel and to spread the metal of the circumferential wall longitudinally by an ironing operation employing a ring of hard material having an inside diameter equal to the desired outside diameter of the finished can body. The desired taper in thickness of the cylindrical wall from both ends towards the intermediate region of the can is achieved by varying the diameter of the mandrel along its length, the mandrel being somewhat barrel shaped with a maximum diameter in the intermediate region. The ironing operation by means of the hard metal ring thins the cylindrical wall of the cup-shaped workpiece and at the same time spreads the surplus metal longitudinally to increase the length of the cylindrical wall.

The amount of metal that is available in the cup-shaped blank is carefully determined to equal the amount of metal required for a can of the previously specified ideal dimensions. At the time the ironing operation is carried out, however, the bottom wall of the cup-shaped receptacle is substantially the starting thickness of 0.0145 inches and, therefore, there is insufficient metal in the cylindrical wall of the cup-shaped workpiece to extend the length of the cylindrical wall to the desired dimension. Therefore, when the material in the cylindrical wall of the cup-shaped workpiece is spread longitudinally with thinning action the resulting cylindrical wall falls short of the desired final length.

Since the metal required to achieve the necessary final increment of length of the cylindrical wall is in the relatively thick bottom wall of the workpiece, the next step is to displace this metal in a manner to add to the cylindrical wall adjacent the bottom wall.

To carry out this step suitable dies are employed to squeeze the bottom wall of the semifinished can body to thin the bottom wall of the desired final thickness with consequent extrustion of the material of the bottom wall in all radial directions. The dies to carry out this final step are precisely dimensioned to confine the radially extruded metal and to shape the extruded metal to match the cylindrical body and thus add the final increment of length to the can body.

It will be readily apparent to those skilled in the art that while preferably the squeezing of the bottom wall of the cup-shaped workpiece is carried out after the cylindrical wall is attenuated by the ironing operation, nevertheless the invention may be practiced by squeezing the bottom wall before carrying out the ironing operation. In this alternate sequence of operations the increment of length is added to the cylindrical wall of the can body in advance of the ironing operation and the ironing operation begins at the point where the extruded metal terminates and progresses from that point to increase the length of the cylindrical wall to the desired final length.

As heretofore stated, the bottom wall of the can may be as thin as 0.010 inches if the bottom wall is strengthened by offsetting the central portion of the bottom wall inwardly. The operation of reforming the bottom wall may be carried out either before or after the bottom wall is thinned by the squeezing operation but in the preferred practice of the invention it is carried out at the same time. In other words, the dies that squeeze the bottom wall are shaped and dimensioned to offset the bottom wall inwardly to the desired configuration.

The features and advantages of the invention may be understood by reference to the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 1 is a side elevational view partly in section showing, by way of example, a configuration that may be used for the finished can body before the top end is added;

FIG. 2 shows how a circular sheet metal blank of the required thickness may be first drawn to form a cup-shaped receptacle of substantially larger diameter than the desired diameter of the can body and having a relatively short cylindrical wall;

FIG. 3 shows a second cup-shaped workpiece which is produced by redrawing the workpiece shown in FIG. 2 but which, if desired, may be drawn in one operation directly from the original sheet metal blank;

FIG. 4 is a sectional view illustrating the ironing step which is carried out by telescoping the cup-shaped body of FIG. 3 over a mandrel and advancing a hard metal ring over the length of the cylindrical wall of the cup-shaped member;

FIG. 5 is a greatly simplified sectional view showing how die means may be employed to thin the bottom wall and at the same time to shape the bottom wall with an inward offset; and

FIG. 6 shows the configuration of the can body that results from the closing of the die shown in FIG. 5.

DESCRIPTION OF THE PREFERRED PRACTICE OF THE INVENTION

To produce a semifinished can body, designated "C" in FIG. 6 with an inside diameter of 2 11/16 inches and with substantially the various thickness dimensions heretofore specified, an aluminum alloy sheet blank is employed of a thickness of approximately 0.0145 inches. The sheet metal blank is processed to form the cup-shaped workpiece, designated 8, in FIG. 3 which has a cylindrical wall that is of a length much shorter than the desired final length, the thickness of both the cylindrical wall and the bottom wall being substantially the starting thickness of 0.0145 inches. The workpiece shown in FIG. 3 may be produced in a single drawing operation but preferably the sheet metal is drawn in two stages, the result of the first stage being an intermediate workpiece, designated 9 in FIG. 2 which has a larger diameter than the workpiece shown in FIG. 3 and has a substantially shorter cylindrical wall.

The next step which is illustrated in FIG. 4 is to place the workpiece 8 shown in FIG. 3 on a suitable mandrel, generally designated 10, and to advance a hard metal ring 12 over the length of the cylindrical wall of the workpiece to thin the cylindrical wall and simultaneously spread the metal to increase the length of the cylindrical wall. For this purpose the metal ring 12 may be removably mounted in a suitable holder 13 and actuated in a well known manner that need not be described.

It is obvious that the ironing operation carried out by the hard metal ring 12 results in the attenuated cylindrical wall of the can body having a uniform outside diameter. Since it is desired that the thickness of the cylindrical wall taper from both ends towards its intermediate region, the dimensions of the mandrel vary accordingly, the mandrel being reduced in diameter at both ends to give it a somewhat barrel shaped configuration to achieve the heretofore recited ideal dimensions of the cylindrical wall.

The outer end of the mandrel adjacent the bottom wall of the workpiece has an outside diameter that is sufficiently smaller than the inside diameter of the hard metal ring 12 to result in a wall thickness of approximately 0.010 inches and the wall thickness of the mandrel increases to make the major intermediate portion of the length of the cylindrical wall of the desired thickness of 0.0060 inches -0.0065 inch. As the region of the opposite end of the mandrel is approached, the mandrel again is reduced in thickness in accord with the requirement that the metal near the open end of the can body be of a thickness of approximately 0.085 inch.

At the termination of the ironing operation shown in FIG. 4, the length of the cylindrical wall on the mandrel is less than the length of the cylindrical wall shown in FIG. 6 by an increment which is subsequently added by the squeezing operation.

When the ironing operation illustrated by FIG. 4 is completed to produce a semifinished can body designated 14, the mandrel 10 is longitudinally retracted through a set of radial stripper fingers 15 in a well known manner, the stripper fingers engaging the edge of the cylindrical wall to strip the can body from the mandrel by keeping the can body from following the retraction of the mandrel.

As illustrated in FIG. 5, the can body 14 that is stripped from the mandrel 10 is then telescoped over a punch, generally designated 16, that is shaped and dimensioned to cooperate with a die, generally designated 18, to squeeze the bottom wall 20 of the can to the desired final thickness of 0.010 inch and at the same time to accomplish the two additional purposes of reshaping the bottom wall and of adding the desired increment of length to the bottom end of the cylindrical wall. The can body that is designated 22 in FIG. 6 is the result of this die operation, the can body having a bottom wall 24 that is centrally offset inwardly for increased strength.

The punch 16 and the die 18 are precisely dimensioned to thin the bottom wall 20 to the desired degree and at the same time to serve as a mold for the radially extruded metal to shape the radially extruded metal into a configuration that is a continuation of the cylindrical wall 15. Thus the extruded metal matches both the adjacent 0.010 inch thick metal of the bottom wall 24 and the adjacent 0.010 inch thick metal of the cylindrical wall of the can body. In this manner the can body 14 shown in FIG. 5 is increased to the desired length in the can body 22 shown in FIG. 6. In effect, the metal added to the bottom end of the cylindrical wall by extrusion causes the cylindrical wall 14 in FIG. 5 to be displaced longitudinally towards the open end of the can body to result in the desired final length of the can body 22 in FIG. 6. Actually the can body 22 in FIG. 6 that is produced by the final extrusion step is forming longer than the desired length to permit the can body to be trimmed precisely to the desired length. The trimmed can body may then be necked down to the configuration shown in FIG. 1 by a well-known dimension operation which need not be described.

Our description in specific detail of the presently preferred practice of the invention will suggest various changes, substitutions and other departures from our disclosure within the spirit and scope of the invention.

Claims

We claim:

1. A method of fabricating from ductile sheet material of a given thickness a can body having a cylindrical wall of a desired length and having an integral bottom wall with the thickness of both walls substantially less than the given thickness, characterized by the following steps but not necessarily in the following order:

forming the ductile sheet material into a cup-shaped workpiece of a preliminary configuration having a cylindrical wall of substantially the desired inside diameter of the can body and having an axial dimension substantially less than the desired axial dimension of the can body with the cylindrical wall and the bottom wall of the workpiece both substantially thicker than the thickness desired in the can body;

telescoping the cup-shaped workpiece over the end of a mandrel having a cross section corresponding to the desired internal configuration of the finished can body;

providing hard ring means of an inside diameter exceeding the outside diameter of the mandrel by substantially twice the thickness desired for the cylindrical wall of the finished can body;

advancing the hard ring means axially over the telescoped workpiece from the bottom end thereof to thin the cylindrical wall and simultaneously to elongate the cylindrical wall by an ironing operation;

squeezing the bottom wall to reduce the thickness of the bottom wall to the desired final thickness with consequent extrusion of the material of the bottom wall radially in all directions; and

confining the extruded material to a configuration matching both the cylindrical wall and the adjacent bottom wall thereby adding to the cylindrical wall adjacent the bottom end thereof.

2. A method as set forth in claim 1 in which cooperative die means are used to squeeze the bottom wall and which the cooperative die means are shaped and dimensioned to offset the thinned bottom wall inwardly to strengthen the bottom wall.

3. A method as set forth in claim 1 in which the mandrel tapers in diameter towards its opposite ends for maximum thinning of the material of the cylindrical wall in the intermediate region of the length of the cylindrical wall.

4. A method as set forth in claim 1 in which the ironing operation is carried out in advance of the squeezing operation.

5. A method as set forth in claim 1 in which the squeezing operation is carried out in advance of the ironing operation.