Metal Finishing Guide Book


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coating application. (Transfer efficiency refers to the percentage of solids applied to the part.) In California the minimum acceptable transfer efficiency is 65%. Conventional air atomized, airless, and in some cases air-assisted airless spray equipment are not approved for use in some parts of California. High volume low pressure (HVLP), electrostatic, dip, flow, brush, and roller applications are considered to be "approved." This does not imply that these methods necessarily meet the 65% minimum; rather, they are considered to be more efficient than the nonapproved devices. The most extensively used coating techniques include spray, dip, electrodeposition or electrocoating, coil coating, and powder coating. Each of these techniques is described in one of the following sections of this Guidebook. Other methods are described briefly below. Brush Brush techniques are well known to the homeowner and the public at large. This is a versatile method with high transfer efficiency, but is slow, labor intensive, and not readily automated. It is commonly used for application of maintenance coatings, for touch-up, and in masking for a variety of finishing operations. Flow Coating In flow coating, the part is suspended and the coating is poured over it. The excess is collected for subsequent reuse. This technique is useful for large or oddly shaped parts, which may be difficult or impossible to dip. Nozzles may be directed over the part, but they are not of the atomizing type used for spray application. This technique is also useful for paints without longterm stability, which could not be used for dipping. The same control factors noted under dipping also apply in this case. Similarly, as with dipping, there is minimal control of film thickness, appearance, and film properties. Both dip and flow coating generally require little space, are low in cost, and require minimal operator skill. Curtain Coating Curtain coating consists of the rapid horizontal movement of flat or slightly curved parts through a curtain of falling paint. Since the volume of paint can be carefully controlled by the slot width, it is possible to apply either thin or heavy build films. The process is readily automated. Direct Roll Coating In this method the coating is applied by roller. It is limited to fixed shapes such as sheets and is used for continuous coating of steel or aluminum in the container industry. The coatings can be patterned or embossed to add decorative effects. CURING PROCESSES Drying is the process by which the solvents and/or water in the coating evaporate to allow the film to achieve a "dry-to-touch" or "dry-to-recoat" state. Curing, on the other hand, implies that the coating resin undergoes a chemical reaction, rendering the cured film hard, abrasion resistant, and relatively inert to the environment (chemicals, solvents, sunlight, etc.). Each resin type (alkyd, epoxy, polyurethane, etc.) undergoes its own type of curing mechanism. Some coatings, such as nitrocellulose lacquers, need only dry by solvent evaporation in order to achieve their final finish. Most other coatings require both drying and curing 157

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