Metal Finishing Guide Book


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5. Where high pressures exist between the work and the buffs, a deficiency of compositions has often resulted in such a high frictional heat that the muslin buff catches fire. The spray method eliminates this hazard by keeping the buff properly coated at all times; however, a spray composition must be selected that does not constitute a fire hazard, which would be present if a liquid composition were composed of volatile, combustible fluids. When using bar compound on an automatic machine, wheel speeds must be maintained in the higher range to generate sufficient friction to exceed the melting point of the bar; however, much lower wheel speeds may be used when liquid compounds are used. The ability to slow down the surface feet enables more intricate parts to be buffed. The lower buffing wheel speeds with large buff faces and liquid 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 highly efficient method of applying liquid buffing compositions for automatic and semiautomatic 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 nozzles. 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 precautions. 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 production 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 29

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