# Modified : Thickness to diameter ratio A third measure

Modified beverage cans

HAMZA PERVEZ                                15-ME-91

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Mujahid Hussain                                 15-ME-92

Raw material

Beverage can consists
mostly of aluminum, but it contains small amounts of other metals as well.
These are typically 1% magnesium, 1% manganese, 0.4% iron,
0.2% silicon, and 0.15% copper.

Before understanding the
manufacturing process for beverage cans, we will have to understand the
processes of drawing of sheet and plate.

DRAWING
OF SHEET AND PLATE

In drawing process, a
combination of a punch and a die is used which draw a circular blank of metal
sheet into a 3-D cylindrical cup. Basically the punch descends, pushing metal
through die, converting circular blank to a cylindrical cup. Height
of cup walls is determined by difference between the diameter of original blank
and diameter of punch .The drawing operation is done in multiple stages and is
not a one stage process. Wrinkles can appear in cup walls as circumference is
reduced, or punch can act as a piercing tool. If gap between punch and
die is less than thickness of incoming material, cup wall is thinned and
elongated. This process is often called ironing
or wall ironing.

The punch and die must have corner radii, given by Rp and Rd.

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The sides of the punch and die are separated by a clearance c. For Drawing, the clearance is
greater than the stock thickness as follows :

As the punch first begins to push into the work, the metal is subjected
to a bending operation. As the punch moves further down, a straightening action
occurs in the metal that was previously bent over the die radius. Holding force is critical for a successful
drawing operation. If too small, wrinkling occurs and if too large, it prevents
the metal from flowing properly toward the die cavity. This results in
stretching and possible tearing of the sheet metal.

Drawing Ratio

For a cylindrical shape the drawing ratio is the ratio of blank diameter
Db to punch diameter Dp.

The greater the ratio, the more severe the operation. An approximate
upper limit of drawing ratio=2.0

Reduction

For a given drawing operation, the reduction ‘r’ is also used as :

Thickness
to diameter ratio

A third measure in deep drawing is the thickness-to-diameter ratio,
which gives the tendency for wrinkling

It is desirable for the t/Db
ratio >1%. As t/Db decreases, tendency for wrinkling increases.In cases where
these limits on drawing ratio,
reduction, and t/Db
ratio are exceeded by the design of the drawn part, the blank must be drawn in
two or more steps, sometimes with annealing between the steps.

DRAWING FORCE

Force equation estimates the maximum force in the operation

Where, F=drawing force, N, t=original blank thickness, mm, TS=tensile
strength, MPa, and Db and Dp are the starting blank
diameter and punch diameter, mm. The constant 0.7 is a correction factor to
account for friction.

HOLDING FORCE

As a rough approximation, the holding pressure can be set at a
value=0.015 of the yield strength of the sheet metal. This value is then
multiplied by that portion of the starting area of the blank that is to be held
by the blank holder. In eq. form

Holding force is usually about one-third
the drawing force.

Ironing of sheet metal

If gap between punch and die is less than
thickness of incoming material, cup wall is thinned and elongated. This process
is often called ironing or wall ironing Ironing of sheet metal is a
manufacturing process that is mostly used to achieve a uniform wall thickness
in deep drawings.. Ironing of sheet metal can be incorporated into a deep
drawing process or can be performed separately. A punch and die pushes the part
through a clearance that will act to reduce the entire wall thickness to a
certain value. While reducing the entire wall thickness, ironing will cause the
part to lengthen. The percentage reduction in thickness for an ironing
operation is usually 40% to 60%. Percent
reduction can be measured
(ti – tf)/ti X
100%. With ti being
initial thickness and tf being
final thickness. Many products undergo two or more ironing operations. Beverage cans are a common product of
sheet metal ironing operations.

MANUFACTURING PROCESS

It consists of the following processes.

Cutting the blank

Initial drawing of the blank

Redrawing the cup formed via initial drawing

Trimming the ears

Cleaning and decorating

The lid

Filling and seaming

These processes will now be explained in detail .

Cutting the blank

The modern method for making aluminum beverage cans is called two-piece
drawing and wall ironing. The process begins with an aluminum ingot (a material, usually metal, that is cast into a
shape suitable for further processing) which
is rolled into a thin sheet. The first step in the actual manufacture of the
can is to cut the sheet into a circle, called a blank , that will form the bottom and sides of the can. Some
material is necessarily lost between each circle, but manufacturers have
found that minimum aluminum is lost when the sheets are wide enough to hold two staggered rows of seven blanks
each.

(staggered rows)

About 12-14% of the sheet is
wasted, but can be reused as scrap. After the circular blank is cut, it is
“drawn” or pulled up to form a cup.

Redrawing
of cup

The small cup resulting from the initial draw is then transferred to a
second machine. A sleeve holds the cup precisely in place, and a punch lowered
swiftly into the cup redraws it to a further smaller diameter. The height of
the cup increases simultaneously .The punch then pushes the cup against three
rings called ironing rings, which stretch and thin the cup walls. This entire
operation—the drawing and ironing—is done in one continuous punch stroke, which
takes only one fifth of a second to complete. Then another punch presses up
against the base of the cup, causing the bottom to bulge inward. This shape
counteracts the pressure of the carbonated liquid the can will contain. The
bottom and lower walls of the can are also a little thicker than the upper

Trimming the ears

The drawing and ironing process leaves the can slightly wavy at the top.
These small ripples in the metal are called “ears.” “Earing” is an unavoidable effect of the
crystalline structure of the aluminum sheet. Aluminum companies have studied
this phenomenon extensively, and they have been able to influence the placement
and height of the ears by controlling the rolling of the aluminum sheet.
Nevertheless, some material is lost at this stage. About a quarter inch is trimmed from the top of the can, leaving the upper
walls straight and level.

CLEANING AND DECORATING

The drawing and ironing process leaves the outer wall of the can with a
smooth, shiny surface, so it does not require any further finishing such as
polishing. After the ears are trimmed, the can is cleaned and then imprinted
with its label.

FORMATION OF A NECK AT THE TOP

After the can is decorated, it is squeezed in slightly at the top to a
make a neck, and the neck is given an out-ward flange at the very top edge,
which will be folded over once the lid is added.

THE LID

The lid is made of a slightly different alloy than the aluminum for
the base and sides of the can. The inward bulge of the bottom of the can helps
it withstand the pressure exerted by the liquid inside it, but the flat lid
must be stiffer and stronger than the base, so it is made of aluminum with more
magnesium and less manganese than the rest of the can. This results in stronger
metal, and the lid is considerably thicker than the walls. The lid is cut to a
diameter smaller than the diameter of the walls. The center of the lid is stretched
upward slightly and drawn by a machine to form a rivet. The pull tab, a
separate piece of metal, is inserted under the rivet and secured by it. Then
the lid is scored so that when the tab is pulled by the consumer, the metal
will detach easily and leave the proper opening.

Filling and seaming

After the neck is formed, the can is ready to be filled. The can is held
tightly against the seat of a filling machine and a beverage is poured in. The
lid is added. The upper flange formed when the can was given its neck is then
bent around the lid and seamed shut. At this point, the can is ready for sale.

Checking criteria

To ensure that the cans are made properly, they are automatically
checked for cracks and pinholes. One in 50,000 cans is usually found to be defective.

Byproducts / waste

Some aluminum is lost at several points in the manufacturing
process—when the blanks are cut and the ears are trimmed—but this scrap can be
reused. Cans which have been used and discarded by consumers can also be
reused, and as mentioned above, recycled material makes up a significant
percentage of the aluminum used for beverage cans. The savings from recycling
are quite significant to the industry. The major expense of the beverage can is
in the energy needed to produce the aluminum, but recycling can save up to 95%
of the energy cost. Can producers also try to control waste by developing
stronger can sheet so that less aluminum goes into each can, and by carefully
controlling the manufacturing process to cut down on loss through earing. The
lid of the typical can is smaller in diameter than the walls in order to
conserve the amount of aluminum that goes into it, and as world-wide demand for
beverage cans continues to grow, the trend is to make the lid even smaller.