Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 27 of 707

uid compound allow the slowly rotating work to be pushed up into or "mushed" into the buff wheel. Although the amount of work per unit of time might be lowered, this is compensated by increasing the buff contact time on the work by using wide-faced buffs. Airless spray systems provide a significant breakthrough in developing a high- ly efficient method of applying liquid buffing compositions for automatic and semi- automatic buffing operations. Such a system uses high fluid pressures in the range of 600 to 1,800 psi. Specially designed, air-activated drum pumps generate such high fluid pressures and deliver custom-formulated, heavy viscosity liquid buffing compounds to special automatic spray guns with tungsten carbide insert noz- zles. Much like the action of a watering hose, the high fluid pressures force the heavy liquid buffing compounds through the orifice of the spray gun for controlled fracturing of the compound. This high velocity spray is capable of penetrating not only the wind barrier around a rotating buff, but has enough force behind it to impregnate the cloth buff up to a 1.5-in. depth, depending upon the construction and speed of the buff. Overspray, so common to regular external atomizing spray systems, is practically eliminated. Deep saturation of the buff with the compounds provides more consistent and uniform finishes, with reduced compound consumption up to 35%. Extended buff life also reduces changeover downtime. Operating costs are further reduced with lower compressed air consumption because airless spray guns do not require atomizing air to apply the compounds. Airless spray buffing systems presently in operation limit applications to custom- formulated, heavy viscosity liquid buffing compounds containing tripolis and unfused aluminum oxides. Properly designed drum pumping systems must be used. High pressure fluid hose and fittings are also necessary. The high fluid pressures generated in airless spray buffing systems make it necessary to exercise certain pre- cautions. When adjusting the spray guns, operators must be careful not to allow the force of the spray to come in contact with exposed skin, since the force of compound is strong enough to break the skin. Liquid abrasive compounds offer so many recognized advantages that their use is now accepted by the finishing industry as standard procedure for high pro- duction buffing. POLISHING AND BUFFING OF PLASTICS Due to the dies used to mold plastic, little buffing or polishing is required. Some do require removal of flash, parting lines, sprue, projections, gates, and 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 adhe- sion 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 recom- mendations 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. 26

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