TIN MILL PRODUCTS

Tin-coated vessels are known to have existed as early as 23 A.D., but the tin was apparently only used for decoration. The first tinplate appears to have been made in Bavaria in the fourteenth century, and by the sixteenth century, a thriving tinplate industry existed in Saxony and Bohemia. Tinplate manufacture spread to England in the late seventeenth and early eighteenth centuries.

The tinplate industry as we know it today is founded on the invention of the process of preserving sterilized food for long periods of time by Nicolas Appert in 1810. By 1812, Appert was successfully packaging a variety of products in glass containers for a growing market.

The Appert process was adapted to the preservation of food in tinplate containers by John Hall in 1812 in London. By the 1820’s, canned foods were widely sold in England and France, and by 1839, foods were being canned in the United States. The American can industry grew explosively as a result of the Civil War, the subsequent settling of the west, and the growth of the oil industry.

Starting in the 1880’s, a series of technical innovations transformed the tinplate industry. These included the replacement of wrought iron with steel for the black plate in about 1880; the development of continuous cold reduction in 1927, that eliminated hot-pack rolling; the introduction of continuous electro-tinning on a small scale in Germany in 1934 and on a commercial scale in the United States in 1937, which replaced the hot dip process; the invention of double cold reduction in 1960; and the invention of TFS in the early 1960’s in Japan and the United States.

Concurrently, many advances occurred in canmaking technology that exploited the improvements in the quality of tin mill products. Tin mill products are also used by the automotive, building, appliance, and furniture industries, all of which take advantage of the unique properties of these light-gauge steel products.

• Charging the Coil – The black plate coil is placed on an uncoiling mandrel and fed into the unit. The charged coil is then welded to the preceding coil in order to run the facility continuously. Looping towers accumulate a length of material that is varied to enable the welding of two coils while not stopping the operation.
• Side Trimming – The material is side trimmed to the final width. The scrap is automatically baled, removed, and recycled to the steelmaking furnaces.
• Clean & Pickle – The strip is next electrolytically cleaned, pickled, and rinsed by running through a series of tanks to ensure the surface is clean and suitable to accept the electro-deposition of tin.
• Electroplating – The strip passes through tanks containing tin anodes on either side of the strip and in electrolyte. Tin dissolves from the anode and is transported through the electrolyte to deposit on the strip. The quantity of tin that deposits on the strip is determined by the quantity of electric current flowing between the anode and the strip. To produce differential coatings, different current settings are used for each of the anode banks. The dull as-plated tinplate is sometimes referred to as having a matte finish.
• Melting Tower (Tin Reflow) – The electrolytic tinplate next passes through a melting tower. In this section, the temperature of the strip is raised by resistance or induction heating to just above the melting point of tin, then is immediately quenched in cold water. Tin Plating LineThe tin begins to melt and reflows uniformly across the strip. The product now takes on the more typical bright or shiny surface appearance. Should a customer require a matte or unmelted tin finish, the melting tower can be turned off. When differentially coated tinplate is being produced, an identifying mark may be placed on either side just prior to melting.
• Chemical Treatment – A sodium dichromate solution is next applied to the electrolytic tinplate to create a light protective chromium oxide film. This passivation process protects the surface from the formation of tin oxides, which will decrease lacquerability. Two commonly used chemical treatments for electrolytic tinplate that have been adopted by the industry are:
• Sodium Dichromate Dip (Abbr: SDCD) Has a moderate resistance to tin oxide formation with limited storage stability and is used where a highly passivated tin surface is not required or is detrimental to the end use. Sodium Dichromate Dip has an aim not to exceed 0.15 milligrams (mg) of chromium/sq. ft. of surface.
• Cathodic Sodium Dichromate (Abbr:CDC) With the addition of a cathodic electric current, a highly passivated surface against the formation of tin oxide is provided. Cathodic Sodium Dichromate treatment normally has an aim of 0.5 milligrams (mg) of chromium/sq. ft. of surface.
• Oiling – A very thin film of oil is then electrostatically applied to the finished tinplate. The oil is applied to minimize scratching the tin surface in transit and handling. The primary function of oiling is not prohibiting rust formation as it is with uncoated products. The oil applied to electrolytic tinplate, almost exclusively, is acetyl tributyl citrate or ATBC. The oil is applied uniformly to both sides.
• Recoiling – The finished tinplate product is finally recoiled at the exit end of the electrolytic tin line on a take up reel that most commonly has a 16-1/2 inch inside diameter (ID).
• Quality Inspection – The entire process is monitored automatically and manually to ensure that the material conforms to specification and meets customer expectations.