Tin can
From Wikipedia, the free encyclopedia
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 majority preserve food by canning.
"Tin cans" are not made of tin but rather tin-coated steel.
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[edit] Description
Most cans have identical and parallel round tops and bottoms with 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-corner 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 "rim"; 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.
[edit] Materials
No cans currently 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 tinplate steel, which combined the physical strength and relatively low price of steel with the resistance to corrosion of tin.
Use of aluminium in cans began in the 1960s. Aluminum is less costly than tin-plated steel but offers the same resistance to corrosion in addition to greater malleability, resulting in ease of manufacture; this gave rise to the two-piece can, where all but the top of the can is simply stamped out of a single piece of aluminum, rather than laboriously constructed from two pieces of steel. 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, like nuts, and some non-food products like engine oil may be sold in can-like containers where a cardboard tube fills the role of the wall, with a metal top and bottom.
The insides of most food cans are coated with a resin containing the organic compound Bisphenol A, which is an endocrine disruptor. Since the early 21st century this has caused concern due to the findings of studies showing the presence of the chemical in canned food due to leaching.
[edit] Standard sizes
American can sizes have an assortment of designations and sizes. For example, size 1/4 contains one serving of half a cup with an estimated weight of 4 ounces; size 1 "picnic" has two or three servings totalling one and a quarter cups with an estimated weight of 10½ ounces; size 303 has four servings totalling 2 cups weighing 15½ ounces; and size 10 cans, most widely used by food services selling to cafeterias and restaurants, have twenty-five servings totaling 13 cups with an estimated weight of 103½ ounces (size of a roughly 3 pound coffee can). These are all "U.S. customary" cups, and not equivalent to the former Imperial standard of the British Empire or the later Commonwealth.
In the United States, cook books will sometimes reference cans by size. These sizes are currently published by the Can Manufacturers Institute and may be expressed in three-digit numbers, as measured in whole and sixteenths of an inch for the container's nominal outside dimensions: a 307 x 512 would thus measure 3 and 7/16" in diameter by 5 and 3/4" (12/16") in height. Notice that this is not in millimetres. Older can numbers are often expressed as single digits, their contents being calculated for room-temperature water as approximately eleven ounces (#1 "picnic" can), twenty ounces (#2), thirty-two ounces (#3) fifty-eight ounces (#5) and one-hundred-ten ounces (#10 "coffee" can).[1]
Canned goods are one of the main sticking points in the project of the European Union to have everything in "hard metric" units. Shortly, any canned goods shipped to Europe will have to be in standard European sizes.[citation needed]
[edit] 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 tube. 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.
[edit] Opening cans
The first tin cans were heavy-weight containers that required ingenuity to open, using knives, chisels or even rocks. Not until cans started using thinner metal about 50 years later were any dedicated can openers developed.
While beverage cans or cans of liquids such as soup merely need to be punctured to remove the product, solid or semisolid contents require access which is generally gained by removing the top (or bottom) of the can. Although this can be accomplished by brute force using something like a large, heavy knife, many more convenient can openers have been devised and marketed.
Some cans, such as those used for sardines, have a lid which is specially scored so that the metal can be broken apart by the leverage of winding it around a slotted church key.
The advent of pull tabs in beverage cans has also spread to the canning of various food products, such as pet food or nuts, allowing the convenience of opening without need for any tools or implements.
[edit] Waste
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Discarded tin cans are commonly used in crafts and a number of simple toys can be made from them. Tin can telephones are common craft projects. Tin cans of the correct size (diameters of 73 to 92 mm or 2.874 to 3.622 inches [2]) may also be used to make very effective Wi-Fi antennas. Discarded tin can lids tend to be sharp, so care should be exercised when handling or disposing of them.
A new style of can opener has recently emerged (U.S. Patent 5,946,811)that cuts the rim neatly in half in the plane of the flat end, leaving half of the rim attached to the can and the other half attached to the flat end. No sharp edges are produced on the lid. The driving teeth are very much finer than those of the classical can opener and reside at the bottom of a V-shaped groove which surrounds the rim on three sides at the point of action.
To prepare tin cans for collection, remove tops and bottoms and flatten the cans. (Flatten seamless cans like cat food, tuna fish cans, or some soup cans, as best as you are able.) When cans are flattened, the curbside collector is able to load more into the truck, thus saving the time it would take to drive the truck to the storage facility, unload it and resume the collection. Since costs of shipping the cans to detinning plants also are determined by truckload, loads of compacted, flattened cans are more economical to ship.
Steel cans, also called tin cans, are the most recycled packaging material.[3] Around 65% of steel cans are recycled. [4] In the US, 63% of steel cans are recycled, compared to 52% of aluminium cans.[5]
Aluminum cans have always been one of the most popular and cost-effective waste products to be recycled, due to the high energy costs of extracting aluminum from ore. Recycling old aluminum cans into new ones takes only 2%-4% of the energy that would be required to make them from ore. Each aluminum can represents energy equal to half that can's volume of gasoline, enough to light a 100 watt bulb for almost four hours or power a television for three hours. In 2003, 54 billion aluminum cans were recycled. This resulted in a savings of the equivalent of 15 million barrels of crude oil, equal to United States gasoline consumption for one day. [6]
[edit] References
- History of the Tin Can
- Brody, A. L., and Marsh, K, S., "Encyclopedia of Packaging Technology", John Wiley & Sons, 1997, ISBN 0-471-06397-5
- Soroka, W, "Fundamentals of Packaging Technology", IoPP, 2002, ISBN 1-930268-25-4
