Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 263 of 331

conveyor layout. These new, in-line systems are also available with an integral "batch stripping" pass line that allows small lots of rejected coated parts to be stripped for salvage and recoating in the same salt bath without interfering with normal rack stripping operations. (See Fig. 3.) Because salt baths are fully enclosed and ventilated, they afford operators a high degree of safety. Most systems include viewing windows to monitor stripping and can incorporate automation instead of manual load/unload. PART CARRIER DESIGN Parts can be processed through a molten salt bath by a variety of part carriers including racks, hooks, and baskets; however, part fixtures and racks used in a salt bath differ slightly from conventional part carriers. Strip- ping open tubular components can be a problem from the standpoint of salt drag out and rinsing. Sealed tubing should not be used because internal pressure buildup may cause tube or weld failure. When water- quenched and rinsed, a vacuum is formed and may draw water into the internal tube cavity. If the tubing was reprocessed in molten salt, the confined water would flash to steam and possibly result in an explosion. Fig. 2. Salt bath system designed for in-line rack plating. PERFORMANCE AND ECONOMICS Performance, not cost, often is the only consideration in specifying a molten salt bath paint stripping system. This would be the case when coatings to be stripped are tough-to-remove hybrid paints and powders, which cannot effi- ciently be stripped by any other process. When several different paint-stripping methods need to be compared, issues to be factored into a cost analysis should include: Rack Damage Costs: Stripping methods, like burn-off ovens, operate at tempera- tures that can cause rack distortion. Shot blasting methods often contribute to premature weld failures. As a result, frequent rack replacement is required. Salt baths operate at low temperatures and do not attack the base metal or integrity of hooks and racks. Rack Inventory Costs: With most stripping processes, part carriers must be re- moved from the line for stripping. This necessitates an equal number of racks, hooks, or fixtures to be kept in inventory to support production, which represents a significant investment. Continuous, in-line salt bath systems eliminate the need for additional rack inventory. Secondary Cleaning Costs: Conventional rack stripping methods often leave ash and residue, which must be removed by secondary cleaning operations. A salt bath removes 100% of coatings with no ash or residue, eliminating secondary clean- ing costs. Part Reject Costs: In electrostatic applications, paint, ash, or residues remaining 262

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