Tin can
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- This article discusses sealed containers.
- For the American naval slang term, see Destroyer.
A tin can, also called a tin (especially in British English) or a can, is an air-tight container for the distribution or storage of goods, composed of thin metal, and requiring cutting or tearing of the metal as the means of opening. Cans hold diverse contents, but the overwhelming proportion preserve food by canning.
The modern tin can is an elaboration of the invention in the decade of the 1800s by Nicolas Francois Appert; it became a consumer standard late in the same century, primarily in industrialized countries but nearly universally known elsewhere.
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Classic shapes
Cans with the most common, three-piece, construction usually have identical and parallel round tops and bottoms, and vertical sides. (However, where the small volume to be contained and/or the shape of the contents suggests it, the top and bottom may be rounded-cornered rectangles or ovals. Other contents may justify a can that is overall somewhat conical shape.)
The fabrication of most cans results in at least one "rim", a narrow ring whose outside diameter is slightly larger than that of the rest of the can. The flat surfaces of rimmed cans are recessed from the edge of any rim (toward the middle of the can) by about the width of the rim; the inside diameter of a rim, adjacent to this recessed surface, is slightly smaller than the inside diameter of the rest of the can.
Three-piece can construction results in top and bottom "rims"; in two-piece construction, one piece is a flat top and the other a cup-shaped piece that combines the (at least roughly) cylindrical wall and the round base; the transition between the wall and base is usually somewhat gradual. Such cans have a single rim at the top.
In the mid-20th century, a few milk products were packaged in nearly rimless cans, reflecting different construction; in this case, one flat surface had a hole (for filling the nearly complete can) that was sealed after filling with a quickly solidifying drop of molten solder. Concern arose that the milk contained unsafe levels of lead leached from this solder plug.
Materials
No cans presently in wide use are composed primarily or wholly of tin; that term rather reflects the near-exclusive use in cans, until the last half of the 20th century, of tin-plated steel.
Use of aluminum in cans began in the 1960s; often the top is tin-plated steel and the rest of the can aluminum.
A can usually has a printed paper or plastic label glued to the outside of the curved surface, indicating its contents. Less commonly, a label is painted directly onto the metal.
Food that does not require complete sealing, and some non-food products like engine oil may be sold in can-like containers whose cardboard tube fills the role of the wall, with a metal top and bottom.
Fabrication of cans
Rimmed-can construction necessarily has three phases:
- Joining the bottom and wall (or forming the cup-shaped piece, for a two-piece can)
- Filling the can with the intended contents
- Joining the wall and top.
The rim or rims already discussed (which later serve a function in the opening of the can) are crucial to the joining of the wall to a top or bottom surface. An extremely tight fit between the pieces must be accomplished to prevent leakage; the process of accomplishing this radically deforms small areas of the parts. Part of the tube that forms the wall is bent, almost at its end, turning outward through 90 degrees, and then bent further, toward the middle of the tube, until it is parallel to the rest of the tube, a total bend of 180 degrees. The outer edge of the flat piece is bent against this toward the middle of the tubular wall, until parallel with the wall, turning inward through 90 degrees. The edge of bent portion is bent further through another 90 degrees, inward now toward the axis of the tube and parallel to the main portion of the flat piece, making a total bend of 180 degrees. It is bent far enough inward that its circular edge is now slightly smaller in diameter than the edge of the tube. Bending it yet further, until it is parallel with the tube's axis, gives it a total bend of 270 degrees. Outward from the axis of the tube, the first surface is the unbent portion of the tub. Slightly further out is a narrow portion of the top, including its edge. The outward-bent portion of the tube, including its edge, is slightly further out. Furthest out is the 90-degree-bent portion of the flat surface. The combined interacting forces, as the portion of the flat surface adjacent to the interior of the tube is indented toward the middle of the tube and then outward away from the axis of the tube, and the other bent portions of the flat piece and the tube are all forced toward the axis of the tube, drives these five thicknesses of metal against each other from inside and out, forming a "dry" joint so tight that welding or solder is not needed to strengthen it.
Opening cans
The outside of the rim (or either of the two rims) provides a strong surface useful in cutting into the nearby flat surface. The edge of the rim where it meets the outside surface of the wall at about a right angle is an anchorage used to pull a sharp edge on the can opener toward the middle of the can; using a can opener with moving parts, pressing teeth against that same edge provides traction for turning the can relative to that anchorage point.
Waste
Discarded tin cans are commonly used in crafts, and a number of simple toys can be made from them. Tin cans of the right size may also be used to make very effective Wi-Fi antennas. Discarded tin can lids tend to be sharp and care should be exercised when handling or disposing of them.
See also
External Links
- Dawn of the New Can (http://www.washingtonpost.com/ac2/wp-dyn/A37489-2004Oct16?language=printer) - article on the history of cans and how cans are evolving to become more competitive against other containers.