SW Dec 2015 - Page 49

Although this method is quick and convenient for small scale production, it suffers from high labour and material costs and, depending on the solvent selected, can be a fire or health hazard. Solvent dip With this method the workpiece is immersed in a tank of solvent and after withdrawal, when the solvent has evaporated, all oil and grease should have been removed. This method remains effective until, like the solvent wipe, contamination has built up in the solvent dip tank and an equilibrium is reached whereby as much oil or grease is redeposited on the work as it takes off. The only difference between the two methods is that this oil is spread over the whole component. Better results can be obtained by having a number of tanks in line on a cascade principle, but this takes up considerable space and is expensive as solvent losses due to evaporation are high. Again, depending on the solvent used it can be a health or fire hazard. Neither the solvent wipe or solvent dip methods are recommended. Solvent vapour degreasing Using this technique the workpiece is suspended in the vapour of a chlorinated solvent such as trichloroethylene in a specially designed plant and the metal is degreased by the condensation of the vapour on its cold metal surface, which solubilises the oils and grease which run off the parts with the liquid as it returns to the sump. This is a much more efficient process because the solvent is continuously boiled up to replace the vapour that condenses. On its own this method will degrease effectively but any solid particles left on the surface may remain there after all the oil and grease has been removed. Improvements can be obtained by including a boiling liquor stage or by the use of ultrasonic agitation. In addition special additives can be put into the chlorinated solvent to improve efficiency. Detergent The workpiece can be dipped into or preferably sprayed with a solution of a suitable detergent in hot water and then rinsed and dried. This will effectively remove light contamination but will not deal with aged oil, grease or heavy soils. Emulsion cleaners Emulsion cleaners are usually preemulsified kerosene/water emulsions, or kerosene-based concentrates which emulsify when added to water. Like the alkali cleaners, emulsions are most efficient when used in spray equipment but can be quite effective as immersion cleaners in many instances. Emulsion cleaners normally operate at lower temperatures than the alkali type and in some cases can be used at ambient temperatures. Alkali cleaners – Again the workpiece can either be dipped or sprayed with a hot aqueous solution of a suitable alkali mixture and then rinsed twice and dried. Spray application is more effective than dipping and is cheaper as higher operating temperatures (70-90oC) and concentrations have to be used with the latter. Spray application varies in time from 5-60 seconds whereas dip takes from 1-5 minutes. Immersion cleaners can disperse the grease and oil by emulsifying them into the solution. Alternatively cleaners are available which separate the oil into a layer so that it can be floated off the cleaner surface over a suitable weir. Alkali cleaners can effectively remove oil, grease and soils and will cope readily with the heaviest contaminants. There is a wide variety of alkali cleaners whose properties can be adjusted to give effective cleaning from any set of contaminants. These cleaners often include grain refining agents to ensure that phosphate coatings subsequently applied to steel surfaces have a fine grained crystal structure. In addition to the alkali the mixtures contain detergents, emulsifiers, sequestering and chelating agents and occasionally watersoftening additives. It should be noted that only under controlled conditions are alkali cleaners suitable for light alloys, zinc, galvanised metal or aluminium which are all attacked by alkali. Acid cleaning Acid pickling using either inhibited sulphuric or hydrochloric acid can completely remove rust and scale and can also condition the surface. This method is usually confined to iron or steel surfaces. great care be taken to ensure that subsequent water rinsing is of high standard to ensure that the dried and cleaned components are not contaminated with acid, alkali or emulsion. Also if a conversion coating system does not follo