English subtitles for clip: File:The Ingenious Design of the Aluminum Beverage Can.webm

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Every year nearly a half trillion of these
cans are manufactured—that’s about 15,000

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per second — so many that we overlook the
can’s superb engineering. Let’s start

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with why the can is shaped like it is. Why
a cylinder? An engineer might like to make

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a spherical can: it has the smallest surface
area for a given volume and so it uses the least

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amount of material. And it also has no corners
and so no weak points because the pressure

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in the can uniformly stresses the walls. But
a sphere is not practical to manufacture.

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And, of course, it’ll roll off the table.
Also, when packed as closely as possible only

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74% of the total volume is taken up by the
product. The other 26% is void space, which

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goes unused when transporting the cans or
in a store display. An engineer could solve

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this problem by making a cuboid-shaped can.
It sits on a table, but it’s uncomfortable

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to hold and awkward to drink from. And while
easier to manufacture than a sphere, these

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edges are weak points and require very thick
walls. But the cuboid surpasses the sphere

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in packing efficiently: it has almost no wasted
space, although at the sacrifice of using

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more surface area to contain the same volume
as the sphere. So, to create a can engineers

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use a cylinder, which has elements of both
shapes. From the top, it’s like a sphere,

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and from the side, it’s like a cuboid .A
cylinder has a maximum packing factor of about

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91% -- not as good as the cuboid, but better
than the sphere. Most important of all: the

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cylinder can be rapidly manufactured. The
can begins as this disk —called a “blank”—

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punched from an aluminum sheet about three-tenths
of a mm thick. The first step starts with

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a “drawing die,” on which sits the blank
and then a “blank holder” that rests on

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top. We’ll look at a slice of the die so
we can see what’s happening. A cylindrical

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punch presses down on the die, forming the
blank into a cup. This process is called “drawing.”

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This cup is about 88 mm in diameter—larger
than the final can — so it’s re-drawn.

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That process starts with this wide cup, and
uses another cylindrical punch, and a “redrawing

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die.” The punch presses the cup through
the redrawing die and transforms it into a

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cup with a narrower diameter, which is a bit
taller. This redrawn cup is now the final

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diameter of the can—65 mm—but it’s not
yet tall enough. A punch pushes this redrawn

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cup through an ironing ring. The cup stays
the same diameter, as it becomes taller and

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the walls thinner. If we watch this process
again up close, you see the initial thick

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wall, and then the thinner wall after it’s
ironed. Ironing occurs in three stages, each

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progressively making the walls thinner and
the can taller. After the cup is ironed, the

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dome on the bottom is formed. This requires
a convex doming tool and a punch with a matching

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concave indentation. As the punch presses
the cup downward onto the doming tool: the

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cup bottom then deforms into a dome. That
dome reduces the amount of metal needed to

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manufacture the can. The dome bottom 
uses less material than if the bottom were

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flat. A dome is an arch, revolved around its
center. The curvature of the arch distributes

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some of the vertical load into horizontal
forces, allowing a dome to withstand greater

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pressure than a flat beam. On the dome you
might notice two large numbers. These debossed

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numbers are engraved on the doming tool. The
first number signifies the production line

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in the factory, and the second number signifies
the bodymaker number -- the bodymaker is the

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machine that performs the redrawing, ironing
and doming processes. These numbers help troubleshoot

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production problems in the factory. In that
factory the manufacturing of a can takes place

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at a tremendous rate: these last three steps—
re-drawing, ironing and doming—all happen

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in one continuous stroke and in only a seventh
of a second. The punch moves at a maximum

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velocity of 11 meters per second and experiences
a maximum acceleration of 45 Gs. This process

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runs continuously for 6 months or around 100
million cycles before the machine needs servicing.

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Now, if you look closely at the top of the
can body, you see that the edges are wavy

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and uneven. These irregularities occur during
the forming. To get a nice even edge, about

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6 mm is trimmed off of the top. With an
even top the can can now be sealed. But before

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that sealing occurs a colorful design is printed
on the outside—the term of art in the industry

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is “decoration.” The inside also gets
a treatment: a spray-coated epoxy lacquer

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separates the can’s contents from its aluminum
walls. This prevents the drink from acquiring

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a metallic taste, and also keeps acids in
the beverage from dissolving the aluminium.

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The next step forms the can’s neck — the
part of the can body that tapers inward. This

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“necking” requires eleven-stages. The
forming starts with a straight-walled can.

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The top is brought slightly inward. And then
this is repeated further up the can wall until

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the final diameter is reached. The change
in neck size at each stage is so subtle that

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you can barely tell a difference between one
stage and the next. Each one of these stages

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works by inserting an inner die into the can
body, then pushing an outer die—called the

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necking sleeve—around the outside. The necking
sleeve retracts, the inner die retracts, and

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the can moves to the next stage. The necking
is drawn out over many different stages to prevent wrinkling,

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or pleating, of the thin aluminum. Since the
1960’s, the diameter of the can end has

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become smaller by 6 mm — from 60 mm to 54
mm today. This seems a tiny amount, but the

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aluminum can industry produces over 100 billion
cans a year, so that 6 mm reduction saves

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at least 90 million kilograms of aluminum
annually. That amount would form a solid cube

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of aluminum 32 meters on a side—compare
that to a 787 dreamliner with a 60 meter wingspan.

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Now, after the neck has been formed the top
is flanged; that is, it flares out slightly

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and allows the end to be secured to the body,
which brings us to the next brilliant design

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feature: the double seam. On older steel cans
manufactures welded or soldered on the ends.

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This often contaminated the can’s contents.
In contrast, today’s cans use a hygienic

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“double seam,” which can also be made
faster. This can is cut in half so you can

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see the cross-section of the double seam.
To create this seam, a machine uses two basic

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operations. The first curls the end of the
can cover around the flange of the can body.

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The second operation presses the folds of
metal together to form an air-tight seal.

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While the operations themselves are simple,
they require high precision. Parts misaligned

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by a small fraction of a millimeter cause
the seam to fail. In addition to the clamping

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of the end and can body, a sealing compound
ensures that no gas escapes through the double

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seam. The compound is applied as a liquid,
then hardens to a form a gasket. The end,

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attached immediately after the cans is filled,
traps gases inside the can to create pressures

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of about 30 psi or 2 times atmospheric pressure.
In soda, carbon dioxide produces the pressure;

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in non-carbonated drinks, like juices, nitrogen
is added. So why is a beverage can pressurized?

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Because the internal pressure creates a strong
can despite its thin walls. Squeeze a closed,

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pressurized can—it barely gives. Then squeeze
an empty can—it flexes easily. The cans

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walls are thin—only 75 microns thick—and
they are flimsy, but the internal pressure

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of a sealed can pushes outwards equally, and
so keeps the wall in tension. This tension

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is key: the thin wall acts like a chain — in
compression it has no strength, but in tension

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it’s very strong. The internal pressure
strengthens the cans so that they can be safely stacked

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—a pressurized can easily supports
the weight of an average human adult. It also

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adds enough strength so that the can doesn’t
need the corrugations like in this unpressurized

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steel food can. While initially pressurized
to about 2 atmospheres, a can may experience

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up to 4 atmospheres of internal pressure in
its lifetime due to elevated temperatures;

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and so the can is designed to withstand up
to 6 atmospheres or 90 psi before the dome

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or the end will buckle. Why is there a tab
on the end of the can? It seems a silly question—how

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else would you open it? But originally cans
didn’t have tabs. Very early steel cans

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were called flat tops, for pretty obvious
reasons. You use a special opener to puncture

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a hole to drink from, and a hole to vent.
In the 1960’s, the pull-tab was invented

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so that no opener was needed. The tab worked
like this: you lift up this ring to vent the

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can, and pull the tab to create the opening.
Easy enough, but now you’ve got this loose

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tab. The cans ask you to “Please don’t
litter” but sadly, these pull tabs got tossed

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on the ground, where the sharp edges of the
tabs cut the barefeet of beachgoers—or they

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harmed wildlife. So, the beverage can industry
responded by inventing the modern stay-on

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tab. This little tab involved clever engineering.
The tab starts as a second class lever; this

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is like a wheelbarrow because tip of the tap
is the fulcrum and the rivet the load — the

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effort is being applied on the end. But here’s
the genius part: the moment the can vents

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the tab switches to a first class lever which
is like a seesaw: where the load is now at

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the tip and the fulcrum is the rivet. You
can see clearly how the tab, when working

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as a wheelbarrow, lifts the rivet. In fact,
part of the reason this clever design works

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is because the pressure inside the can helps
to force the rivet up, which in turn depresses

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the outer edge of the top until it vents the
can and then the tab changes to a seesaw lever.

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Looking from the inside of the can, you can
see how the tab first opens near the rivet.

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If you tried to simply force the scored metal
section into the can using the tab as a first

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class lever with the rivet as the fulcrum
throughout you'd be fighting the pressure

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inside the can: the tab would be enormous,
and expensive. If you’d like to learn more

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about the entire lifecycle of the aluminum
can, watch this animated video by Rexam that

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describes can manufacturing and recycling.
A typical aluminum can today contains about

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70% recycled material. Also, Discovery’s
How It’s Made has some great footage of

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the manufacturing machinery. Here are two
different stepwise animations of the entire

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can forming process. And lastly, these are
two detailed animations of the cup drawing

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and redrawing processes. The aluminum beverage
can is so ubiquitous that it’s easy to take

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for granted. But the next time you take a
sip from one, consider the decades of ingenious

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design required to create this modern engineering
marvel. I’m Bill Hammack, the engineer guy.

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Thanks to Rexam for providing us with aluminum
cans in various stages of production. And

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thank you very much to the advanced viewers
who sent detailed and useful responses for

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this video. We read every single comment.
If you’d like you to help out as an advanced

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viewer check out www.engineerguy.com/preview.
You can see upcoming projects and behind-the-scene

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footage. For example, you can see a early
drafts of this beverage can video. And you

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can sign up there to become an advance viewer.
Thanks again.