The Aluminum Beverage

Home , Aluminum can

The Aluminum Beverage Can Produced by the hundreds of millions every day, the modern can— robust enough to support the weight of an average adult— is a tribute to precision design and engineering

by William F. Hosford and John L. Duncan

akers of beer and soft-drink inchÑthree times the pressure in an Colo., and introduced to the public by containers in the U.S. produce automobile tire. the Hawaiian brewery Primo, it was M300 million aluminum bever- Yet the can industry is not standing made from two pieces of aluminum. To age cans a day, 100 billion of them ev- pat on its achievement. Strong econom- produce such cans, Coors employed a ery year. The industryÕs output, the ic incentives motivate it toward further so-called impact-extrusion process. The equivalent of one can per American per improvements. Engineers are seeking method begins with a circular slug that day, outstrips even the production of ways to maintain the canÕs performance has a diameter equal to that of the can. nails and paper clips. If asked whether while continuing to trim the amount of A punch driven into the slug forces ma- the beverage can requires any more material needed. Reducing the canÕs terial to ßow backward around it, form- special care in its manufacture than do mass by 1 percent will save approxi- ing the can. The process thus made the those other homey objects, most of us mately $20 million a year in aluminum side walls and the bottom from one would probably answer negatively. In (and make still easier and even less piece. The top was added after Þlling. fact, manufacturers of aluminum cans meaningful the macho gesture of crush- This early technique proved inade- exercise the same attention and preci- ing an empty can with a bare hand). quate for mass manufacturing. Produc- sion as do makers of the metal in an Aside from the savings it yields, the tion was slow, and tooling problems aircraft wing. The engineers who press modern manufacturing process imparts plagued the process. Moreover, the re- the design of cans toward perfection a highly reßective surface to the canÕs sulting product could hold only seven apply the same analytical methods exterior, which acts as a superb base for ounces and was not eÛcient structural- used for space vehicles. decorative printing. This attribute adds ly: the base could not be made thinner As a result of these eÝorts, todayÕs to the enthusiasm for the aluminum can than 0.03 inch, which was much thick- can weighs about 0.48 ounce, down among those who market beverages. er than it needed to be to withstand from about 0.66 ounce in the 1960s, Indeed, that industry consumes about the internal forces. when such containers were Þrst con- a Þfth of all aluminum used in the U.S. Nevertheless, the popularity of the structed. The standard American alu- Consequently, beverage cans have product encouraged Coors and other minum can, which holds 12 ounces of emerged as the single most important companies to look for a better way to liquid, is not only light in weight and market for aluminum. Until 1985, most make the cans. A few years later Rey- rugged but is also about the same cans held beer, but now two thirds of nolds Metals pioneered the contempor- height and diameter as the traditional them store nonalcoholic drinks. ary method of production, fabricating drinking tumbler. Such a can, whose the Þrst commercial 12-ounce alumi- wall surfaces are thinner than two pag- he aluminum beverage can is a num can in 1963. Coors, in conjunction es from this magazine, withstands more direct descendant of the steel with Kaiser Aluminum & Chemical Cor- than 90 pounds of pressure per square T can. The Þrst of these vessels ap- poration, soon followed. But pressure peared in 1935, marketed by Kreuger from large can companies, which also Brewing Company, then in Richmond, purchased steel from Kaiser for three- Va. Similar to food cans, this early bev- piece cans, is said to have obliged Kai- WILLIAM F. HOSFORD and JOHN L. erage container comprised three pieces ser to withdraw temporarily from alu- DUNCAN have been active in research of steel: a rolled and seamed cylinder minum-can development. Apparently, on sheet-metal forming for more than and two end pieces. Some steel cans these steel-can makers feared the com- 30 years and act as consultants to alu- even had conical tops that were sealed petition of a new breed of container. minum producers. Hosford is professor by bottle caps. During World War II, the Hamms Brewery in St. Paul, Minn., be- of materials science and engineering at government shipped great quantities gan to sell beer in 12-ounce aluminum the University of Michigan. He received of beer in steel cans to servicemen over- cans in 1964. By 1967 Coca-Cola and his doctorate in metallurgical engineering from the Massachusetts Institute of seas. After the war, much of the produc- PepsiCo were using these cans. Technology and has written books on tion reverted to bottles. But veterans Today aluminum has virtually dis- metal forming and the plasticity of ma- retained a fondness for canned beer, so placed steel in all beverage containers. terials. Duncan, who received his Ph.D. manufacturers did not completely aban- The production of steel three-piece cans, in mechanical engineering from the Uni- don the technology even though the which are now rarely made, reached its versity of Manchester in England, is pro- three-piece cans were more expensive peak of 30 billion cans in 1973. The fessor of mechanical engineering at the to produce than the bottles. number of two-piece steel cans topped University of Auckland in New Zealand. Like Hosford, Duncan has written a text- The Þrst aluminum beverage can out at 10 billion in the late 1970s. This book on the forming of sheet metal. went on the market in 1958. Developed design now accounts for less than 1 per- by Adolph Coors Company in Golden, cent of the cans in the U.S. market (they

48 SCIENTIFIC AMERICAN September 1994 Copyright 1994 Scientific American, Inc. RIVET TAB Used to secure the tab to This separate piece of met- the can, this integral piece of al is held in place by the in- LID the lid is made by stretching tegral rivet. The lid may make up 25 per- the center of the lid upward cent of the total weight. It slightly. It is then drawn to consists of an alloy that con- form a rivet. tains less manganese but more magnesium than the body does, making it stronger. To save on the mass, manufacturers make the diameter of the lid smaller than that of the body.

NECK The body of the can is 0.005” narrowed here to accommodate the smaller lid. FLANGE After the top of the can is trimmed, it is bent and seamed to secure the lid after ﬁlling. SCORED OPENING The lid is scored so that the metal piece pushes in easily without detaching.

BODY This aluminum alloy typically incorporates by weight 1 percent magnesium, 1 per- 0.003” cent manganese, 0.4 percent iron, 0.2 percent silicon and 0.15 percent copper. It LABEL is ironed to dimensions with- The ironing process that thins in 0.0001 inch and is made the body of the can produces thicker at the bottom for a highly reﬂective surface added integrity. It withstands suitable for decoration. The an internal pressure of 90 mirrorlike ﬁnish may be one pounds per square inch and of the main reasons mar- can support 250 pounds. keters of beverages adopted the aluminum can.

0.006”

0.012”

BASE The bottom of the can as- sumes a dome shape in order to resist the internal pressure.

ANATOMY OF MODERN BEVERAGE CAN reveals the dimen- minum needed without sacriÞcing structural integrity. A can sions that design and engineering must achieve on a daily now weighs about 0.48 ounce; the industry hopes to reduce basis. The goal of can makers is to reduce the amount of alu- that weight by about 20 percent.

STEPS IN CAN MANUFACTURE begin with an aluminum alloy second machine then redraws the blank, irons the walls and sheet. Blanks 5.5 inches in diameter are cut from the sheet; a gives the base its domeÑall in approximately one Þfth of a punch draws the circle to form a 3.5-inch-diameter cup. A second. These procedures give the can wall its Þnal dimen- are, however, more popular in Europe). erations with walls whose top edges are be seen, impressed on the inside of a The process that Reynolds initiated wavy, or Òeared.Ó To ensure a ßat top, can). On the exterior of the can the is known as two-piece drawing and wall machinery must trim about a quarter shearing of the surface against the iron- ironing. Aluminum producers begin inch from the top. After trimming, the ing rings yields the desired mirror Þnish. with a molten alloy, composed mostly cup goes through a number of high- The side walls can be thinned with- of aluminum but also containing small speed operations, including washing, out loss of integrity because, structur- amounts of magnesium, manganese, printing and lacquering. Finally, the ally, the can is a Òpressure vessel.Ó That iron, silicon and copper. The alloy is can is automatically checked for cracks is, it relies for part of its strength on the cast into ingots. Rolling mills then ßat- and pinholes. Typically, about one can internal force exerted by carbon diox- ten the alloy into sheets. in 50,000 is defective. ide in beer and soft drinks or by the ni- The Þrst step in can making is cut- Ironing is perhaps the most critical trogen that is now infused into such ting circular blanks, 5.5 inches in diam- operation in making the body of the uncarbonated liquids as fruit juice. In- eter. Obviously, cutting circles from a can. The precisely dimensioned punch deed, most beers are pasteurized in the sheet produces scrap. The theoretical holds and pushes the cup through two can, a process that exerts nearly 90 loss for close-packed circles is 9 per- or three carbide ironing rings. To thin pounds per square inch on the materi- cent; in practice, the loss amounts to and elongate the can, the punch must al. Carbonated beverages in hot weath- 12 to 14 percent. To reduce this waste, move faster than the metal does in the er may also build up a similar pressure. sheets are made wide enough to incor- ironing zone. The clearance between Filling introduces a diÝerent kind of porate 14 cups laid out in two stag- the punch and each ring is less than stress on the can. During this stage, the gered rows. Each blank is drawn into a the thickness of the metal. The friction can (without its lid) is pressed tight- 3.5-inch-diameter cup. generated at the punch surface assists ly against a seat in a Þlling machine. It The next three forming operations in pushing the metal through the iron- must not buckle, either during Þlling and for the can body are done in one con- ing rings. To increase this friction, the sealing or when Þlled cans are stacked tinuous punch stroke by a second ma- punch may be slightly roughened with one on another. Hence, can makers spec- chineÑin about one Þfth of a second. a criss-cross scratch pattern (which can ify a minimum Òcolumn strengthÓ of First, the cup is redrawn to a Þnal inside diameter of about 2.6 inches, which 12 increases the height from 1.3 to 2.25 inches. Then, a sequence of three ironing operations thins and stretches the walls, so that the body reaches a height PUNCH SLEEVE of about Þve inches. In the last step, the ALUMINUM punch presses the base of the can body BLANK against a metal dome, giving the bottom of the can its inward bulge. This curve behaves like the arch of a bridge in that it helps to prevent the bottom from bulging out under pressure. For added integrity, the base of the can and the bottom of the side walls are made thicker than any other part of the can body. Because the alloy does not have the same properties in all directions, the DRAWING AND IRONING constitute the modern method of beverage can manufac- can body emerges from the forming op- ture. The initial draw transforms the blank into a small cup (1). The cup is trans-

uch of the success behind the consistent and precise produc- M tion lies in the strong yet form- able alloy sheet. The metallurgical properties responsible for the performance of modern can sheet have been propri- etary and therefore not well known. Only within the past decade has that situation changed. Through the eÝorts of Harish D. Merchant of Gould Elec- tronics in Eastlake, Ohio, James G. Mor- DECORATE NECK FLANGE FILL AND ris of the University of Kentucky and SEAM others, scientiÞc papers on the metal- lurgy of can sheet have become more widely published. sions. After the ÒearsÓ at the top of the walls are trimmed, the can is cleaned, deco- We now know that three basic factors rated and then ÒneckedÓ to accommodate the smaller lid. The top is ßanged to se- increase the strength of aluminum. We cure the lid. Once Þlled and seamed shut, the can is ready for sale. have already mentioned one of them: manganese and magnesium dissolved into the material. These atoms displace about 250 pounds for an empty can smaller than the diameter of the cylin- some of the aluminum ones in the sub- body. Thin-walled structures do not eas- der. Then they Òneck downÓ the top part stance. Because they are slightly diÝer- ily meet such a requirement. The slight- of the cylindrical wall, from 2.6 to 2.1 ent in size, the manganese and magne- est eccentricity of the loadÑeven a dent inches, to accommodate the lid. An in- sium atoms distort the crystal lattice. in the can wallÑcauses a catastrophic genious integral rivet connects the tab The distortions resist deformation, thus collapse. This crushing can be demon- to the lid. The lid is scored so that the adding strength to the sheet. strated by standing (carefully) on an up- can opens easily, but the piece of metal The second contribution comes from right, empty can. Manufacturers avoid that is pushed in remains connected. the presence of so-called intermetallic failures by using machines that hold In addition to clever design, making particles. Such particles, which form the cans precisely. billions of cans a year demands reliable during the processing of the sheet, con- The second piece of the can, the lid, production machinery. It has been said sist of a combination of diÝerent met- must be stiÝer than the body. That is that in order to prove himself, an ap- als in the alloy (mostly iron and man- because its ßat geometry is inherently prentice Swiss watchmaker was not re- ganese). They tend to be harder than less robust than a curved shape (dams, quired to make a watch but rather to the alloy itself, thus supplying strength. for instance, bow inward, presenting a make the tools to do so. That sentiment Perhaps the most important contri- convex surface to the waters they re- applies to can manufacturing. As one bution to sheet strength, however, is strain). Can makers strengthen the lid production manager remarked, ÒIf at the work hardening that occurs when by constructing it from an alloy that the end of a bad day, you are a half mil- the sheets are cold-rolled (ßattened at has less manganese and more magne- lion cans short, someone is sure to no- room temperature). During this shap- sium than that of the body. They also tice.Ó A contemporary set of ironing dies ing, dislocations, or imperfections, in make the lid thicker than the walls. In- can produce 250,000 cans before they the lattice materialize. As the metal de- deed, the lid constitutes about one require regrinding. That quantity is forms, the dislocations move about and fourth the total weight of the can. To equivalent to more than 20 miles of alu- increase in number. Eventually they be- save on the mass, can makers decrease minum stretched to tolerances of 0.0001 come entangled with one another, mak- the diameter of the lid so that it is inch. Die rings are replaced as soon as ing further deformation more diÛcult.

3 4 CAN WALL

IRONING RING

ferred to a second punch, which redraws the can; the sleeve pushes the can past ironing rings, which thin the walls (3 ). Fi- holds the can in place to prevent wrinkling (2). The punch nally, the bottom is shaped against a metal dome (4).

Unfortunately, this work hardening directions. They showed that a small a rivet and then ßattened to secure the dramatically reduces the ability of the window of strains exists that permits tab (which is a separate piece of metal). material to stretch. Tensile tests indi- forming without structural failure. Al- cate that the elongation capacity drops though work hardening greatly reduces esides making the can sheet from 30 percent to about 2 or 3 percent. the size of this window, a small slit stronger, manufacturers also Conventional wisdom had it that sheets nonetheless remains openÑenough to B sought to reduce the amount of can be formed only if the material has permit the doming of the base and aluminum needed by controlling the a high tensile elongation. Certainly in drawing and redrawing of the side walls. waviness, or earing, which as we have the automotive industry, body parts are The crucial advance that made the seen takes place at the top of the can formed from fully annealed sheets that aluminum can economical, however, after ironing. The eÝect derives from can elongate more than 40 percent. This came from Linton D. Bylund of Rey- the crystallographic texture of the alu- philosophy guided the early attempts nolds. He realized that cans could be minum sheet, that is, the orientation of to make two-piece aluminum cans. Re- made from a fully work-hardened sheet its crystal structure. Hence, earing is in- searchers concentrated on annealed or using a carefully designed process that evitable to some extent. Hans-Joachim partially work-hardened sheets, which speciÞed the placement of the ironing Bunge of the Technical University in sacriÞced strength for ductility. rings, the shape of the punch and dies, Clausthal, Germany, and Ryong-Joon The understanding of formability re- and many other parameters. The strong, Roe of Du Pont and others have devel- ceived a major boost from studies in fully work-hardened sheet made it pos- oped x-ray diÝraction techniques to de- the 1960s by Stuart P. Keeler and Wal- sible to use sheet that was thinner, sav- scribe qualitatively the textures that ter A. Backofen of the Massachusetts ing enough weight to make the cans cause earing. Laboratory technicians Institute of Technology and Zdzislaw economically competitive. prepare specimens by grinding away Marciniak of the Technical University in Nowhere is the technique of forming layers of the sheet to expose material Warsaw, among others. Looking at the work-hardened sheet more apparent at diÝerent depths. X-ray diÝraction behavior of various sheet metals, they than it is in the cleverly designed rivet coupled with elegant analytical tech- considered more than just the behavior that holds the tab on the can lid. The niques automatically produces three- under tension applied in one direction rivet is an integral piece of the lid. To dimensional diagrams that reveal the (as is done in the tension test). They make it, the center of the lid must be preferred orientation of crystals as a also looked at what happens when ten- stretched by bulging it upward a bit. function of depth in the sheet. sion is applied simultaneously in two This ÒextraÓ material is drawn to form Such diagnostic approaches have en- abled aluminum companies to produce sheet that yields much smaller ears. 100 Metallurgists balance the two predomi- nant crystallographic textures that exist in the aluminum. One kind of texture arises during annealing of the alloy after the alloy is hot-rolled from ingots. It TOTAL BEVERAGE causes four ears to appear every 90 de- 80 CANS grees (at 0, 90, 180 and 270 degrees) around the circumference of the can. The second kind of texture results from cold-rolling the sheet, which produces TWO-PIECE an ear at 45, 135, 225 and 315 degrees. CAN THREE-PIECE Proper control of annealing and rolling 60 CAN can lead to a combination of the two textures such that ears caused by one Þll the valleys caused by the other. The result is eight very low ears. The maxi- TWO-PIECE mum height of an ear is often less than ALUMINUM 1 percent of the height of the cup. Consistent processing of metal and 40 careful design have now made each part THREE-PIECE of the can about as strong as any other. STEEL It is not unusual to Þnd cans in which the opening on the lid fractures, and the bottom dome and lid bulge at nearly the same pressure, within the range NUMBER OF CANS PRODUCED IN THE U.S. (BILLIONS) 20 of 100 to 115 pounds per square inch. Despite the success of current design and manufacture, can makers are still TWO-PIECE STEEL searching for reÞnements. Much of the investigation focuses on ways to use aluminum more eÛciently, because the 0 metal represents half the cost of the 1965 1970 1975 1980 1985 1990 can. One possibility for saving would ANNUAL BEVERAGE CAN PRODUCTION in the U.S. has increased by several billion be to cast the molten alloy into thin over the past few years. The two-piece aluminum can overwhelmingly dominates slabs rather than into thick ingots, as is the market; steel cans constitute less than 1 percent. Three-piece steel cans, which currently done. A typical ingot may be are now rarely made, reached their peak production in the mid-1970s. 30 inches thick, which is rolled down

52 SCIENTIFIC AMERICAN September 1994 Copyright 1994 Scientific American, Inc. by a factor of 2,500 to 0.011 or 0.012 inch. So much rolling requires expensive capital equipmentÑfurnaces and rolling millsÑand consumes a lot of energy. It is possible to cast aluminum contin- uously into slabs that are an inch thick or less. These thin slabs would require much less rolling to reach the desired Þnal sheet thickness. Continuous casting is used for some soft aluminum al- loysÑfor example, aluminum foil is made from material cast to a thickness of 0.1 inch. Unfortunately, production of satisfac- tory can stock from thin slabs thwarts the metallurgists. The faster cooling and decreased rolling inherent in continuous casting do not yield the desired metallurgical structure. Two main problems arise. First, crystallographic texture cannot be properly controlled to prevent large ears. Second, the faster cooling rate produces severe diÛculties in ironing the can walls. These ironing problems develop because of the nature of the intermetallic particles that form when the molten alloy solidiÞes. Intermetallic particles that develop during solidiÞcation are much larger than those that originate during processing (which as we have seen im- part strength to the sheet). Because of their size, they play a key role in iron- EASY-OPENING LIDS were introduced on three-piece steel cans in 1961. The original ing. During this procedure, aluminum caption reads: ÒHousewives of ancient Greece and the space age compare contain- tends to adhere to the ironing rings. ers for the kitchen at the press debut of the new canning innovation by the Can-Top Machinery Corp., Bala-Cynwyd, Pa.Ó Ordinarily, the intermetallic particles, which are about Þve microns in size, act like very Þne sandpaper and polish the ironing rings. The faster cooling the aluminum in one can. This value is wards of even small improvements, rates of continuous casting, however, equal to about the amount of energy however, are quite substantial. The de- produce intermetallic particles that are expended to keep a 100-watt bulb lit mand for aluminum beverage cans con- much smaller (about one micron). At for six hours, or about 1.7 percent of tinues to grow everywhere in the world; this size, the particles are not very ef- the energy of a gallon of gasoline. Al- their production increases by several fective in removing aluminum that though small, it represents the major billion every year. The success of the sticks to the ironing rings. As a result, expenditure of a can. can is an industrial lesson about what aluminum builds up on the rings and One way to reduce this expense is can be achieved when scientiÞc and en- eventually causes unsightly scoring on through recycling, which can save up gineering skills are combined with hu- the can walls. The problem of achiev- to 95 percent of the energy cost. Indeed, man perseverance. ing thin slabs with the desired interme- more than 63 percent of aluminum cans tallic particles may yet be solved, per- are now returned for remelting. Recy- haps by altering the composition of the cling also has an important part within FURTHER READING alloy or by shifting the rate of solidiÞ- the aluminum mill. For every ton of can A GOLDEN RESOURCE. Harold Sohn and cation from the materialÕs molten state. bodies made, a ton of scrap metal is Karen Kreig Clark. Ball Corporation, produced. This scrap is remelted and 1987. he control of casting epitomizes thus injected back into the manufactur- FROM MONOPOLY TO COMPETITION: THE a recurrent feature of the whole ing cycle. Developing simpler ways of TRANSFORMATIONS OF ALCOA, 1888Ð can story: one behavior is care- producing can sheet and Þnding strong- 1986. George David Smith. Cambridge T University Press, 1988. fully traded oÝ against another, from er materials that can lead to lighter cans THE MECHANICS OF SHEET METAL FORM- the control of earing and ironability to should save more money and energy. ING. Z. Marciniak and J. L. Duncan. Ed- economical sheet production, from can Meeting these goals presents a great ward Arnold, 1992. weight to structural integrity. Yet one challenge. Existing cans already use a ALUMINUM ALLOYS FOR PACKAGING. Ed- cost element eludes an easy balance: highly strengthened, well-controlled ited by J. G. Morris, H. D. Merchant, E. J. the energy needed to make cans. Most sheet. Their shape is Þnely engineered Westerman and P. L. Morris. Minerals, of this outlay lies in the aluminum it- for structural strength and minimum Metals and Materials Society, Warren- self. Taking into account ineÛciencies weight. And with little tool wear, the dale, Pa., 1993. METAL FORMING: MECHANICS AND MET- in electricity distribution and smelting, production machinery in a single plant ALLURGY. William F. Hosford and Rob- industry experts estimate that 2.3 mega- is capable of making many millions ert M. Caddell. Prentice Hall, 1993. joules of energy is needed to produce of cans a day with few defects. The re-