c. Types of Plating. Cans are made from sheets of steel that are tin-plated,
then enameled and labeled. There are several types of plating.
(1) Electrolytic plating. This is the most common method of plating. Tin is
the anode, the steel base plate is the cathode. Both are suspended in a concentrated
solution of a tin salt, and tin is deposited with large electric currents on the steel base
plate by ion transfer. The thickness of the coating is controlled by the length of time the
sheet is in the solution. After tinning, the sheets are heated and rolled to ensure a
uniform tin film. Electrolytic plating can be recognized by the presence of fine, straight,
parallel lines on the surface. This type of plating requires only about a third as much tin
as a comparable hot dipped sheet.
(2) Differential plating. In this process, more tin is deposited on one side of
the base plate than on the other. The heavier surface is used on the interior of the can.
(3) Hot dipped. In this rarely used method, sheets of steel base plate are
dipped into vats of molten tin as often as necessary to produce the desired thickness of
tin coating. Hot-dipping tin can be recognized by the presence of treeing (the
successive layers of tin plate form a crystalline pattern).
STEPS IN CAN FABRICATION
The tin has been applied to the steel base plate during the plating process to
make what is known as the tin plate.
a. Step A. Initial Sizing and Cutting. The first step is the slitting of the tin
plate into body shanks which yields can bodies with the desired dimensions and the
notching and beveling of the body blank. Notching and beveling reduces the layers of
metal incorporated in the can ends. Notching is cutting slits on one side 1/16-inch deep
and 1/8-inch from each edge (Figure 1-2A).
b. Step B. Preparing the Edges. The next step is the bending and hooking of
the notched and beveled sides. The notched and beveled sides are bent directions to
form hooks (Figure 1-2B).
c. Step C. Shaping the Can. Next the body is formed around a steel mandrel
of the desired shape and the hooks are interlocked (Figure 1-2C).
d. Step D. Preparing the Side Seam. During the next step, called bumping,
the hooked areas are hammered together to form the side seam (Figure 1-2D). This
bumping does not completely flatten the seam. This bumping is followed by fluxing and
soldering. Fluxing is done to remove any oil or excess enamel or other impurity from
this area. In the soldering operation, hot solder is allowed to run into the seam to coat
all the tin plate area therein. Following this, a second bumping occurs while the solder
is still hot to completely compress the side seam and form a hermetic seal (Figure 1-