Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 110 of 331

Pump wear can be reduced by a filter on the suction side. Self-cleaning filters reduce maintenance times. In some installations, the dip tank is complemented by an additional collec- tion tank (for return of the drippings) and a circulation tank (for isolated addi- tions of paint, solvent/water, and other additives). Paint flows from the collec- tion tank to the circulation tank (for adjustment) and then to the dip tank. Care must be taken not to introduce foam along the way. Covers (removable) on the collection and circulating tanks elevate humidity and help to reduce foam. Drip/Flash-Off Zone The area following the dip paint tank allows the recovery of paint by means of a pan that returns drippings to the tank. Depending on formulation, air-flow reg- ulation and temperature control may be required. Some waterborne paints require humidity control. Too rapid solvent loss will result in a rapid increase in coating viscosity on the surface that tends to "fix" runs or sags, with a resultant decrease in the quality of appearance. A minimum of 3 min is recommended for dwell time to allow 90% plus paint recovery and enable optimum flow/leveling. In general, high air velocities should be avoided. Some installations provide a controlled heat input, either from the cure zone or from some other related source (e.g., pretreatment) to prepare the film for the final cure. Curing Zone The time and temperature parameters for cure are dependent on the polymer and cross-linking polymer used. Although a forced-air convection oven is used most often, infrared technology has also been employed. Energy can be consumed by bottom entry and exit from the oven. The exhaust system should result in an oven under negative pressure compared to the shop. Forced air can be used following the oven to facilitate cooling. MAINTENANCE Floating residues must be removed from the paint tank to prevent clinging of the material on withdrawn workpieces. This is usually accomplished by periodic skimming. Overhead conveyor systems require lubrication for maximum life. However, any lubricants used should be pretested as potential contaminants in the paint because some materials can cause cratering and other defects in the final prod- uct. Racks must be periodically stripped of dried paint. Either thermal stripping (molten salt bath or high-temperature oven), cryogenic stripping (exposure to liq- uid nitrogen, followed by physical removal of the embrittled paint), or media blast- ing (sand, steel shot) can be used. Cleaning of the drain-off area must occur on a regular basis. Care must be tak- en to avoid getting dried paint into the circulation system, with resultant dam- age to filters, pumps, and nozzles. 109

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