Metal Finishing Guide Book

2012-2013

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imperfections from areas that may need further surface finishing. Plastics cut and machined generally need abrasive finishing to bring back their original luster using belt polishing and buffing. Plastic compounds are formulated to remove large amounts of stock without generating too much frictional heat between the part and the wheel (preventing crazing of the plastic). Some buffing compounds contain built-in antistatic materials so that the buffed surface resists the adhesion of airborne lint. When buffing plastic, the material becomes statically charged. On surfaces of plastic laminates, where fibrous fillers are completely covered with either a thermoplastic or thermosetting plastic, polishing and buffing recommendations are the same as those given for the particular plastic binder involved. Heavy flash removal, sprues, flat surfacing, and beveling on thermosetting and thermoplastic articles are usually done with wet belt sanding. Special waterproof abrasive belts are most generally used. The abrasive grit size is determined by the amount of flash that must be removed. For flexible polishing of thermosetting plastic articles, greaseless compound provides a dry and resilient abrading face for removal of light or residual flash, imperfections in the surface, and cutting tool marks, or for smoothing out irregularities on the contours left by the belting operation. Thermoplastic articles readily distort with frictional overheating. To avoid this problem minimum work pressure against the coated buff wheel and low peripheral speeds are needed. To assure low frictional heat development, grease sticks also can be applied to the coated buffing wheel. This gives added lubrication and lowers the amount of drag, which produces the heat buildup. BUFFING OF PLASTIC Buffing is usually divided into cutdown and luster or color buffing. Cutdown buffing produces a semigloss finish from the dull, sanded surface resulting from belt sanding or greaseless compound operations. This semigloss finish is adequate as a final finish in some cases. Where a higher luster is required, this cutdown buffing is the intermediate operation prior to the final high luster buffing. The most popular buffs used are full disk sewn 80/92 count cloth for cutdown and full disk loose, bias type, or ventilated 64/68 count for luster. Buffing pressure should be at a minimum and the buff speed slow to prevent ���burning��� the plastic. Keeping the buff well lubricated with buffing compound in the cutdown operation helps minimize the burning. MILL AND ARCHITECTURAL FINISHES (STAINLESS STEEL) The main concern of most fabricators of stainless steel is to remove welds and machining marks, and blend and simulate the final finish with the original mill finish or the sheet or coil stock. To refine the area of welds and machining marks, standard rough polishing procedures used are as those previously discussed. Note that the final surface finish must closely approximate the original mill finish. There are eight basic stainless steel mill finishes used in the industry by product designers and architects. Mill finish Nos. 3, 4, 6, 7, and 8 are produced mechanically using some type of abrasive media and buffing wheels. Finish Nos. 3 and 4 have proven to be the most popular among fabricators of dairy, kitchen, cafeteria, chemical equipment, and architectural and decorative structures. The simplest way to produce these blended finishes is with string wheels coated with greaseless abrasive compositions containing 80, 120, or 180 grit abrasive, operating at relatively low speeds. Narrow, flat, or curved areas can easily be blended with a portable power tool and a string wheel up to 8 inches in face width. Medium or very wide areas are finished 27

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