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 process starts 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. Initially it is cut into
a circle, called a blank, which will
be used to form the bottom and sides of the can. Material is definitely lost
between each circle .Material loss will be minimum when the sheets contain 2 staggered rows of 7 blanks per row.

(staggered rows)

Only 12-14% of the sheet is
wasted.

Redrawing
of cup

After the initial draw the cup is redrawn to further decrease its
diameter and increase its height. Then ironing is done which will stretch the
can and reduce the wall thickness. All this process is done in one single punch
which takes 1/5th of a second. An another punch gives an inward
bulge in the base of the can and this is basically done to sustain the pressure
and also the bottom base and walls are also a little thicker than the upper
walls. Trimming the ears

After the above described processes, the can becomes slightly wavy at
the top and such ripples formed in the metal are called “ears.” Quarter inch of the metal will be trimmed from that portion to
straighten the upper walls.

CLEANING AND DECORATING

The above mentioned process leaves the outer wall of the can with a
smooth and shiny surface, thus it will not require any finishing such as
polishing. Then the next procedure is the trimming of the ears and finally the
can is cleaned and a label is imposed or printed over it.

FORMATION OF A NECK AT THE TOP

The can is squeezed at the top to make a neck and from the very top
portion of it, outward flanges are made which will be folded over the lid at
the time of application of the lid.

THE LID

The alloy used for the formation of lid is slightly different than that
used for making the base and walls of the can. At the bottom of the can an
inward bulge was provided so that the it can sustain the pressure of the liquid
filled inside but as the upper lid is flat it must be stronger to sustain the
pressure so it is made with aluminum with higher percentage of magnesium and
less percentage of manganese. Diameter of the lid is kept smaller than the
walls and after that with the help of a rivet a pull tab is attached on lid and
the lid is scored so that a proper opening can be made when the pull tab is
opened.

Filling and seaming

After the formation of the neck , the can is filled and thus it is
filled and then the lid is added and the upper flanges made during the neck
formation  are now bent around the lid
and closed.

After these processes the can is ready to be saled.