SHEET METAL MILL FINISHES Sheet Metal Mill Finishes Machinery's Handbook, 31st Edition
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Mill finish is the natural surface texture of sheet metal after it exits a rolling mill, with no external mechanical or chemical finishing. This surface can be rough and lack sheen. It also may show oxidation or contamination with mill oil. Most mill finish surfaces are treated with polishing, industrial etching, or some other surface finishing process before being considered complete. Typical sheet metal mill finishes can be identified as: Unspecified: This is the natural finish provided by hot-rolling or cold-rolling using un - polished rolls. Ranging from dull to bright, the finished surface may retain stains or a light film of oil. Specular as Fabricated: Produced on one or both sides of a sheet by passing it through highly polished cold rolls, this is a mirror-like finish. Non-Specular as Fabricated: Refers to a finish that is more uniform than the “unspeci - fied” process but without the reflective properties of a “specular as fabricated” finish. The quality and characteristics of various mill finishing processes can vary widely from one mill to another and even between lots (sets of similar parts, processed consecutively or in a short time). Hot-rolled sheets usually have dark, dull surfaces, oxidized from being hot-worked. But it is possible for a mill to influence the finish of produced stock. Carefully maintained and polished rollers can increase surface smoothness and luster; some mills will follow rolling with an annealing process to give stock a matte finish. Surface Finishes of Aluminum Sheets.— A light metal, aluminum has good tensile strength over a wide range of temperatures and excellent corrosion resistance. These properties and aluminum’s appearance can be improved by alloying it with other metals. Mill finish is the natural state of the aluminum sheet as it comes from the rolling mill, with no additional finishing. When aluminum is exposed to air, a thin, glass-hard oxidized film forms on the surface. This film clings tenaciously, presenting an effective surface barrier against the elements. Generally, a rolled sheet develops a thinner oxide film than hot extruded aluminum. With little care, aluminum sheets in mill finish state will maintain their original appear - ance for a long time. Surface coating can increase the effectiveness of aluminum’s corro- sion resistance. Types of aluminum surface treatment are shown in Table 18. Table 18. Types of Surface Treatment Type of Treatment Technique Characteristics
Produces a flat, even surface with a silk or matte appearance. Leaves fine lines in direction of grinding. Reduces or removes grinding lines, die lines, tool marks, surface blemishes, and “stress risers.” Yields a matte-to-shiny surface. Suited to small batches and mass-produced parts. Creates a mirror-like finish. Usually followed by bright anodizing. Removes surface to desired depth. Can be selectively applied for sign writing and other etched products. Leaves a matte or silky appearance. Usually a pretreatment for other finishing processes. Different etchants leave different textures that may show in anodizing sheen.
Grinding/brushing
Polishing and buffing
Mechanical
Vibration polishing High-finish polishing
Milling
Etching
Brightening Removes surface texture for a mirror-like finish. Residual film is usually less than 0.000591 in. or 15 µ m. Chromating/phosphating Pretreatment applied prior to powder coating or other coating processes. Electrochemically plating with copper, tin, nickel, or silver Improves corrosion resistance. Can provide a good soldering surface, conductivity, and reflectivity.
Chemical
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