Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 209 of 331

mines how efficiently paint is transferred from the gun to the part. Together, production rate and transfer efficiency influence the choice of air flow. Material Being Sprayed The type of material being sprayed affects the choice of filtration or exhaust method, either dry filter or water wash, to remove overspray from the booth. A dry-filter or paint-arrestor booth traps airborne paint particles (overspray) in disposable filters. A dry-filter is used in the majority of applications. De- pending on the material being sprayed, removal efficiency ranges from 95% to 99%. If more than one type of material is being sprayed, be sure that the materials are compatible. The combination of incompatible materials in the dry filter can be a cause of spontaneous combustion. In a water-wash booth, air washing action traps the paint solids from overspray. Water-wash systems should be used for very heavy spray rates (over 20 gal/8-hr shift/10 ft of exhaust chamber width). Removal efficiency for a water-wash booth can be as high as 98% to 99%, depending on the type of material being sprayed. Finish Quality The quality of the finish on the completed part has become more critical as cus- tomer's expectation levels have increased. The total process must now be con- sidered in order to achieve first-time-through quality levels. The spray booth design is one key aspect. Air flow, direction, filtration, air ve- locity, and balance are critical to accomplishing the various desired quality lev- els. Unpressurized cross flow designs would be at the low end and pressurized downdrafts at the high end of quality potential. One key thing to consid- er is that a spray booth is only one part of the process. Many other phases of the process must be designed and controlled to achieve the desired quality level. That would include the prepara- tion and cleanliness of the object going into the booth, the maintenance of the booth and surrounding processes, the quality of com- pressed air to the tools (in- cluding spray gun), the qual- ity of clothing and equip- ment the painter uses, and the quality of the paint preparation activities. The finish quality can only be as good as the design and control of the process. Fig. 8. Booth sizing for automated lines. Determining the Booth Size Determining booth size is the second step in selecting the application. It is de- pendent on booth location and the type of operation (manual or automatic). Re- view of the facility layout and proposed booth location is recommended to de- termine whether the allotted space is adequate for the size and style booth. The type of finishing operation, either manual or automatic, also determines the size 208

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