coating methods - CHAPTER 27 APPLICATION METHODS The...

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CHAPTER 27 APPLICATION METHODS The successful use of organic coatings depends to a great extent upon their correct application. The general requirement placed upon the various coating methods used is that they be capable of depositing a layer of the coating material approxi- mately a thousandths of an inch thick. This layer must then be distributed with maximum uniformity and complete continuity. The selection of an application method is determined by the type of coating material applied, the coating thickness and final properties required, the quantity of parts being coated, and the transfer efficiency of the application method. The transfer efficiency is the percentage of paint solids con- sumed that actually coats the merchandise. Trans- fer efficiencies vary from 30 to almost 100%; in other words, some application methods waste 7070 of the consumed paint, while others are nearly waste-free. The most widely used application methods are discussed subsequently; each has specific advan- tages and limitations. Being familiar with all these methods will assist the manufacturing engineer in selecting the proper method for a specific appli- cation. Although one process may be suitable for one installation, it may not be suitable for another. Table 27-1 summarizes the capabilities of the various application methods. SUBSTRATE PREPARATION The successful performance of a finish applied to any substrate is dependent upon the proper conditioning of the substrate surface. Failure to properly prepare the substrate can result in paint peeling, flaking, or blistering, underfilm rusting of ferrous metals, and underfilm corrosion of non- ferrous metals. 1 Although in theory each surface should be chemically and physically clean before painting, in practice the choice of an adequate surface preparation or pretreatment is often dic- tated by the economics of the particular situation. To gain full advantage of the coating character- istics, surface preparation should be part of the finishing process. in general, substrate preparation serves to(1) remove soils or imperfections from the substrate, (2) create a surface susceptible to bond- ing, and (3) establish a chemical coating on the surface that will slow corrosion in case the coating film is damaged. The soils to be removed may range from oil, grease, and wax-based soils to defects such as oxidation, rust, corrosion, heat scale, tarnish, and smut. Oils and greases are generally removed with alkaline or solvent-type cleaners, whereas natural conversion coatings are removed with acidic or chelated alkaline cleaners. For additional informa- tion on cleaning procedures and solvents, refer to Chapter 18, “Cleaning.” Mechanical cleaning methods are discussed in Chapter 16, “Mechanical and Abrasive Deburring and Finishing. With some cleaning techniques, a surface suit- able for bonding may be produced while removing soils or imperfections; with other techniques, it may be necessary to roughen the surface. A corrosion-inhibiting surface can be produced by phosphating, chromating, or anodizing.
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This note was uploaded on 05/27/2010 for the course KARAJ UNIV 12648 taught by Professor Dr.heydarzadehsohi during the Spring '10 term at Islamic Azad University.

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coating methods - CHAPTER 27 APPLICATION METHODS The...

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