Metal Finishing Guide Book


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that would otherwise interfere with cleaning and rinsing. In effect, cleaners with adequate sequestering ability obtain better surfactant performance. Deflocculation: A cleaning mechanism whereby soil is peptized or broken down into very fine particulates and maintained in a dispersive phase to prevent agglomeration (coming together). DETERMINING A CLEAN SURFACE A clean surface is one that is free of oil and other unwanted contaminants. The degree of cleanliness required is dependent on the operation or process to which the part or product must pass. Manufacturers utilizing the cell cleaning concept or workstation cleaning are typically cleaning between process steps. Situations like these usually do not require the degree of cleanliness needed for final prepaint preparation. A water break-free surface tells you that you have removed all organic soils. The parts exiting the last pretreatment or rinse stage prior to drying will show a uniform sheeting of the rinse water indicating an organically clean surface. The water break-free surface has been the long-standing test for cleanliness. The key to this test is using fresh uncontaminated rinsewater. Detergent additives or rinse aids used in a final rinse may hide poor cleaning. Additionally contaminated rinses due to poor overflow may also mask poor cleaning due to the surfactant's wetting ability. A water break surface tells you that you have not sufficiently cleaned and that organic soils are still present. The part will exhibit a surface that resembles a freshly waxed car surface after a good rain. There will not be uniform sheeting of the water but beading. Normally, poor cleaning is most often found on or near weldments, or in areas that receive poor spray impingement to the part. Another test of a clean surface is the white towel test. Wiping a white towel across clean and dry surfaces will indicate the effectiveness of inorganic soil removal. Check flat surfaces and those areas most likely not to receive direct spray impingement. In the tape pull test, apply scotch tape to a clean and dry surface, then remove the tape and place on a white piece of paper. This will also indicate the effectiveness of inorganic soil removal as the contrast allows for easy identification of remaining soils. The ultraviolet (UV) detection requires soiling with a fluorescent oil, cleaning, and inspecting under ultraviolet light. The degree of cleanliness can be quantified by a numbering system. This is accomplished through photoelectron emission or reflectance. The higher the reflectance, the cleaner the surface is. THE IMPORTANCE OF CLEANING Cleaning of metals and other finishing-related substrates is the single most important consideration to successful coating application. Achieving clean surfaces has applications throughout a manufacturing facility: for corrosion protection, for welding operations, for part handling, for part inspection, and for metal finishing. All of these cleaning applications can and should have a quantitative degree of cleanliness required. The degree can vary from gross soil removal to a high degree of cleanliness, which surpasses the standard water-break-free test of cleanliness. The ultimate requirement is dictated by the requirements of the part, the process, the chemical type, and control of process parameters. With today's new coatings, a greater emphasis is placed on achieving a totally clean surface. 93

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