Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 105 of 707

mally derived through product testing and interaction between suppliers and end users. It should be understood that satisfying all of these requirements is necessary for the success of an aque- ous system. Parts should be tested in large volumes to insure repeatable results. MECHANICAL ACTION Fig. 1. Belt spray system with wash/rinse/rust inhibit/recirculating hot air dryer, oil coalescor, and wash filtration. Since the most important part of the aqueous cleaning cycle is mechanical interaction, the following are some def- initions for the standard types of mechanical interaction available. Soak Soaking of parts in chemistry relies solely on the mechanical interaction of chemistry to attack, dissolve, and/or emul- sify soils. For soak to be successful it is imperative that chemistries be aggressive and that adequate time be utilized to ensure that the chemistry has the ability to interact effectively with soils. Even in soaking, some form of part movement nor- mally takes place as parts are either introduced to a soak tank or withdrawn. It is likely that this small amount of mechanical interaction or part movement will, in effect, do more work than the soak cycle itself. Spray Spray is a widely used form of mechanical interaction in aqueous systems. Spray is used on belt systems, monorail systems (see Figs. 1 and 2), cellular and centralized washers, and with cabinet systems that used low-, medium-, or high-pressure delivery of solution. Spray systems are designed to direct solution so that it impinges the parts from above, from the sides, and from below. The solution is normally directed by spray headers utilizing nozzles to generate overlapping spray patterns. Parts are typi- cally drawn through spray zones by some form of conveyance, such as belts, mono- rail, rotating turntable, or rotating baskets. The spray solution is designed to impinge the part surfaces at pressure and high volume. Spray impingement loosens and removes soils. It is necessary to understand that high volume is as important as high pressure, as it is the volume of solution that causes dirts and soils that are removed by the spray impingement to float away or be removed from the parts. The combination of pressure and volume, in conjunction with heated chemistry, is effective in removing soils, oils, dirt, chips, and other by-prod- ucts associated with manufacturing. In most cases spray can only be delivered from four axes, while most parts have a minimum of six geometric axes. Likewise, spray can be easily deflected. Spray is not well suited for cleaning blind holes or complex geometries. If parts are processed in baskets, parts positioned on the outside may mask effective clean- ing of other components in the middle of the basket, as parts on the surface of the basket will be the ones effectively engaged by spray while parts in the middle of the basket must rely on solution volume and drippage for effective cleaning. 104

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