Every year nearly a half trillion of these
cans are manufactured—that’s about 15,000
per second — so many that we overlook the
can’s superb engineering. Let’s start
with why the can is shaped like it is. Why
a cylinder? An engineer might like to make
a spherical can: it has the smallest surface
area for a given volume and so it uses the least
amount of material. And it also has no corners
and so no weak points because the pressure
in the can uniformly stresses the walls. But
a sphere is not practical to manufacture.
And, of course, it’ll roll off the table.
Also, when packed as closely as possible only
74% of the total volume is taken up by the
product. The other 26% is void space, which
goes unused when transporting the cans or
in a store display. An engineer could solve
this problem by making a cuboid-shaped can.
It sits on a table, but it’s uncomfortable
to hold and awkward to drink from. And while
easier to manufacture than a sphere, these
edges are weak points and require very thick
walls. But the cuboid surpasses the sphere
in packing efficiently: it has almost no wasted
space, although at the sacrifice of using
more surface area to contain the same volume
as the sphere. So, to create a can engineers
use a cylinder, which has elements of both
shapes. From the top, it’s like a sphere,
and from the side, it’s like a cuboid .A
cylinder has a maximum packing factor of about
91% -- not as good as the cuboid, but better
than the sphere. Most important of all: the
cylinder can be rapidly manufactured. The
can begins as this disk —called a “blank”—
punched from an aluminum sheet about three-tenths
of a mm thick. The first step starts with
a “drawing die,” on which sits the blank
and then a “blank holder” that rests on
top. We’ll look at a slice of the die so
we can see what’s happening. A cylindrical
punch presses down on the die, forming the
blank into a cup. This process is called “drawing.”
This cup is about 88 mm in diameter—larger
than the final can — so it’s re-drawn.
That process starts with this wide cup, and
uses another cylindrical punch, and a “redrawing
die.” The punch presses the cup through
the redrawing die and transforms it into a
cup with a narrower diameter, which is a bit
taller. This redrawn cup is now the final
diameter of the can—65 mm—but it’s not
yet tall enough. A punch pushes this redrawn
cup through an ironing ring. The cup stays
the same diameter, as it becomes taller and
the walls thinner. If we watch this process
again up close, you see the initial thick
wall, and then the thinner wall after it’s
ironed. Ironing occurs in three stages, each
progressively making the walls thinner and
the can taller. After the cup is ironed, the
dome on the bottom is formed. This requires
a convex doming tool and a punch with a matching
concave indentation. As the punch presses
the cup downward onto the doming tool: the
cup bottom then deforms into a dome. That
dome reduces the amount of metal needed to
manufacture the can. The dome bottom 
uses less material than if the bottom were
flat. A dome is an arch, revolved around its
center. The curvature of the arch distributes
some of the vertical load into horizontal
forces, allowing a dome to withstand greater
pressure than a flat beam. On the dome you
might notice two large numbers. These debossed
numbers are engraved on the doming tool. The
first number signifies the production line
in the factory, and the second number signifies
the bodymaker number -- the bodymaker is the
machine that performs the redrawing, ironing
and doming processes. These numbers help troubleshoot
production problems in the factory. In that
factory the manufacturing of a can takes place
at a tremendous rate: these last three steps—
re-drawing, ironing and doming—all happen
in one continuous stroke and in only a seventh
of a second. The punch moves at a maximum
velocity of 11 meters per second and experiences
a maximum acceleration of 45 Gs. This process
runs continuously for 6 months or around 100
million cycles before the machine needs servicing.
Now, if you look closely at the top of the
can body, you see that the edges are wavy
and uneven. These irregularities occur during
the forming. To get a nice even edge, about
6 mm is trimmed off of the top. With an
even top the can can now be sealed. But before
that sealing occurs a colorful design is printed
on the outside—the term of art in the industry
is “decoration.” The inside also gets
a treatment: a spray-coated epoxy lacquer
separates the can’s contents from its aluminum
walls. This prevents the drink from acquiring
a metallic taste, and also keeps acids in
the beverage from dissolving the aluminium.
The next step forms the can’s neck — the
part of the can body that tapers inward. This
“necking” requires eleven-stages. The
forming starts with a straight-walled can.
The top is brought slightly inward. And then
this is repeated further up the can wall until
the final diameter is reached. The change
in neck size at each stage is so subtle that
you can barely tell a difference between one
stage and the next. Each one of these stages
works by inserting an inner die into the can
body, then pushing an outer die—called the
necking sleeve—around the outside. The necking
sleeve retracts, the inner die retracts, and
the can moves to the next stage. The necking
is drawn out over many different stages to prevent wrinkling,
or pleating, of the thin aluminum. Since the
1960’s, the diameter of the can end has
become smaller by 6 mm — from 60 mm to 54
mm today. This seems a tiny amount, but the
aluminum can industry produces over 100 billion
cans a year, so that 6 mm reduction saves
at least 90 million kilograms of aluminum
annually. That amount would form a solid cube
of aluminum 32 meters on a side—compare
that to a 787 dreamliner with a 60 meter wingspan.
Now, after the neck has been formed the top
is flanged; that is, it flares out slightly
and allows the end to be secured to the body,
which brings us to the next brilliant design
feature: the double seam. On older steel cans
manufactures welded or soldered on the ends.
This often contaminated the can’s contents.
In contrast, today’s cans use a hygienic
“double seam,” which can also be made
faster. This can is cut in half so you can
see the cross-section of the double seam.
To create this seam, a machine uses two basic
operations. The first curls the end of the
can cover around the flange of the can body.
The second operation presses the folds of
metal together to form an air-tight seal.
While the operations themselves are simple,
they require high precision. Parts misaligned
by a small fraction of a millimeter cause
the seam to fail. In addition to the clamping
of the end and can body, a sealing compound
ensures that no gas escapes through the double
seam. The compound is applied as a liquid,
then hardens to a form a gasket. The end,
attached immediately after the cans is filled,
traps gases inside the can to create pressures
of about 30 psi or 2 times atmospheric pressure.
In soda, carbon dioxide produces the pressure;
in non-carbonated drinks, like juices, nitrogen
is added. So why is a beverage can pressurized?
Because the internal pressure creates a strong
can despite its thin walls. Squeeze a closed,
pressurized can—it barely gives. Then squeeze
an empty can—it flexes easily. The cans
walls are thin—only 75 microns thick—and
they are flimsy, but the internal pressure
of a sealed can pushes outwards equally, and
so keeps the wall in tension. This tension
is key: the thin wall acts like a chain — in
compression it has no strength, but in tension
it’s very strong. The internal pressure
strengthens the cans so that they can be safely stacked
—a pressurized can easily supports
the weight of an average human adult. It also
adds enough strength so that the can doesn’t
need the corrugations like in this unpressurized
steel food can. While initially pressurized
to about 2 atmospheres, a can may experience
up to 4 atmospheres of internal pressure in
its lifetime due to elevated temperatures;
and so the can is designed to withstand up
to 6 atmospheres or 90 psi before the dome
or the end will buckle. Why is there a tab
on the end of the can? It seems a silly question—how
else would you open it? But originally cans
didn’t have tabs. Very early steel cans
were called flat tops, for pretty obvious
reasons. You use a special opener to puncture
a hole to drink from, and a hole to vent.
In the 1960’s, the pull-tab was invented
so that no opener was needed. The tab worked
like this: you lift up this ring to vent the
can, and pull the tab to create the opening.
Easy enough, but now you’ve got this loose
tab. The cans ask you to “Please don’t
litter” but sadly, these pull tabs got tossed
on the ground, where the sharp edges of the
tabs cut the barefeet of beachgoers—or they
harmed wildlife. So, the beverage can industry
responded by inventing the modern stay-on
tab. This little tab involved clever engineering.
The tab starts as a second class lever; this
is like a wheelbarrow because tip of the tap
is the fulcrum and the rivet the load — the
effort is being applied on the end. But here’s
the genius part: the moment the can vents
the tab switches to a first class lever which
is like a seesaw: where the load is now at
the tip and the fulcrum is the rivet. You
can see clearly how the tab, when working
as a wheelbarrow, lifts the rivet. In fact,
part of the reason this clever design works
is because the pressure inside the can helps
to force the rivet up, which in turn depresses
the outer edge of the top until it vents the
can and then the tab changes to a seesaw lever.
Looking from the inside of the can, you can
see how the tab first opens near the rivet.
If you tried to simply force the scored metal
section into the can using the tab as a first
class lever with the rivet as the fulcrum
throughout you'd be fighting the pressure
inside the can: the tab would be enormous,
and expensive. If you’d like to learn more
about the entire lifecycle of the aluminum
can, watch this animated video by Rexam that
describes can manufacturing and recycling.
A typical aluminum can today contains about
70% recycled material. Also, Discovery’s
How It’s Made has some great footage of
the manufacturing machinery. Here are two
different stepwise animations of the entire
can forming process. And lastly, these are
two detailed animations of the cup drawing
and redrawing processes. The aluminum beverage
can is so ubiquitous that it’s easy to take
for granted. But the next time you take a
sip from one, consider the decades of ingenious
design required to create this modern engineering
marvel. I’m Bill Hammack, the engineer guy.
Thanks to Rexam for providing us with aluminum
cans in various stages of production. And
thank you very much to the advanced viewers
who sent detailed and useful responses for
this video. We read every single comment.
If you’d like you to help out as an advanced
viewer check out www.engineerguy.com/preview.
You can see upcoming projects and behind-the-scene
footage. For example, you can see a early
drafts of this beverage can video. And you
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Thanks again.
