Metal Finishing Guide Book


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PROCESS ALTERNATIVES The method or combination of methods selected to clean parts is critical and will depend upon the type of final finish (plated, painted, anodized, etc.) and whether it is an in-process cleaning operation, or just a final cleaning. And, of course, the other paramount factor to be considered is the volume of work processed per hour. The cleaning methods given may be used independently or in conjunction with one another, as in a preplate cleaning cycle, where three to four modes of cleaning are used. Immersion (Rack or Barrel) Rack immersion may utilize air agitation or work agitation to improve or shorten the cleaning time. In barrel lines the movement of the barrel will provide the necessary agitation to flush cleaner solution through the parts. Power, Spray Cleaning Spray washers (spiral spray, belt washer, spray strip line, cabinet, and monorailtype washer) provide reduced cleaning time by utilizing impingement to clean parts that may not respond to conventional soak cleaning. Spray pressures may range from 14 to 200 psi, depending upon the type of machine used. Some continuous strip spray washers may also utilize rotating brushes along with the spray cleaning solution. Ultrasonic Cleaning Ultrasonic energy is utilized in conjunction with aqueous cleaners or hydrocarbon chlorinated solvents to clean parts. Areas of application are small precision parts, parts with complex configurations, removal of tightly adhering or embedded particles from parts, or cleaning parts for hermetically sealed units. One of the key factors in a successful ultrasonic cleaning operation, besides selecting the proper cleaner, is proper racking. Alkaline Electrocleaning Alkaline electrocleaners should not be used as the initial cleaner to remove the bulk of soils such as drawing compounds, stamping oils, buffing compounds, machining oils, heavy rust, and weld scales. This function should be reserved for an alkaline soak cleaner, emulsion cleaner, spray cleaner, acids or combinations thereof, which precede the electrocleaner. The electrocleaner is the last alkaline cleaning process performed on the metal surface prior to electroplating. At the point entering the electrocleaner the only soils present should consist of smuts (carbon or iron oxides), light flash rust, light oxides, and residues of soils and cleaners left on the metal surface from the prior cleaning operations. Alkaline electrocleaning, prior to electroplating, is one of the most reliable cleaning methods available. The cleaning action not only depends upon the cleaner formulation but also utilizes the liberation of oxygen or hydrogen (depending on polarity) formed during electrolysis to scrub residues from the surface. Of the three electrocleaning methods anodic cleaning is most frequently used, especially for steels and brass and zinc die castings. Periodic reverse cleaning is usually confined to cleaning ferrous metals, which have as their soils heat scales (weld scale), rust, and smuts. Cathodic cleaning must be used when electrocleaning nickel, nickel alloys, pewter, lead and lead alloys, and stainless steels. Maintaining the proper current density, besides operating concentrations and temperature, is a paramount factor that must be maintained in order to obtain 56

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