Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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at both the entrance and exit ends of the washer. Heating System This can be steam, gas, oil, or electric, but steam and gas are the most common. Heat- ing surfaces should be periodically acid descaled; scale buildup tends to insulate the heating surfaces, increasing the energy required to achieve a given temperature. In some cases, heavy scale can make it impossible to maintain temperature. Besides the basic components just described, washers may utilize other ac- cessories, such as filtering systems. When cloth filters (bag filters) are used, the mesh size determines the degree of filtration. Ultrafiltration is another tech- nology being investigated for its effectiveness in pretreatment applications. This filtration method is designed to enhance tank life by removing undesirable soils from the solution. CHEMICAL SYSTEM To begin, one must first understand the true purpose behind cleaning plastic. The exterior parts that are eventually painted must first be cleaned to remove all soils. If the part is left with soils or contaminants on the surface, the paint will be unable to achieve an acceptable level of adhesion or wet-out characteristics. Paint provides both a decorative finish as well as a protective coating. The key el- ement in both a decorative finish and a good protective coating is the ability of the paint to adhere to the substrate. Simply stated, good adhesion requires good cleaning. IS THE PLASTIC PAINTABLE? Most plastics, after having been cleaned of all soils, are paintable with the right kind of paint systems; however, there are some plastics that even when the sur- face is completely clean, the surface will not accept paint. That is to say, the paint will not wet-out. The old standard in the metal industry for determining whether a part is clean for painting is when water sheets over the surface. This cannot be used as a cleanliness indicator for plastics. The surface tensions of most plastics, even when clean, will not allow water to spread and sheet. The only accurate way to predict paintability on plastic is to paint the plastic. PRETREATMENT FOR HARD-TO-PAINT PLASTICS Polyolefin-type plastics, such as polyethylene and polypropylene, have surfaces that are waxlike and are not readily painted. Special pretreatment systems are nec- essary to paint polyethylene. Some of these are flame treatment, acid treatment, corona discharge, and plasma discharge. The painting of polypropylene can be achieved with innovations in the painting systems. Adhesion-promoter coat- ings supplied by paint companies are used to prepare these surfaces for painting. Vapor etching with chlorinated solvent is also used on some plastics. This is essentially swelling and roughening of the polymer surface, which improves ad- hesion; however, distortion of the plastic can occur. Solvent vapor etching is rapid- ly diminishing because of its environmental limitations. CONTAMINANTS The specific contaminants vary depending on the molding process, the type of plastic and the handling process. Common applications include the painting 59

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