Metal Finishing Guide Book

2013

Issue link: https://metalfinishing.epubxp.com/i/218436

Contents of this Issue

Navigation

Page 211 of 843

(smaller) spray cleaner stage is used beforehand. A slight overflow of this tank results in decreased cleaner loss. Rinsing Plant water rinsing is employed to remove residual cleaner carried through on the workpieces (and racks). This is followed by a rinse with deionized water to remove any hard water salts. The effect of salt buildup over time results in gradually decreasing coating film thickness per unit of immersion time. Autodeposition Autodeposition occurs by the reactions given above. The control parameters are paint solids (gravimetric determination), acid level (free fluoride concentration), and oxidation/ reduction potential (in millivolts), which is proportional to the ratio of FeF3 to Fe+2. Postcoating Rinsing A plant water rinse (usually immersion, but low-pressure spray applications have been used) removes traces of unreacted latex. A final sealing rinse contains chemicals that react with any soluble iron in the wet film to eliminate porosity after the film is cured. Cure The drying of a polyvinylidene chloride autodeposited coating is simply the removal of water from a coalesced wet film. No solvents are present. Commonly practiced parameters are 15 to 30 minutes at convection oven temperatures of 210 to 230°F. Shorter times (5 to 10 minutes) may be achieved on simple (i.e., line-of-sight to all surfaces) parts by the use of medium-intensity infrared radiation. Acrylic coatings require a higher temperature (320–350°F) range for complete cross-linking. FEATURES OF AUTODEPOSITION A phosphate pretreatment process is not required for autodeposition, minimizing requirements for capital and floor space. Dragout is also minimized because of low paint bath viscosity. There is no coating buildup on hangers because a cured autodeposited film is inert to further reaction. Furthermore, since the coating process relies on chemical reaction, coating of all hidden or recessed areas occurs with even coverage. The coating does not pull away from sharp edges, coats evenly over machined surfaces (e.g., threaded fasteners), and is free from runs, sags, orange peel, and similar defects. This effect is enhanced by the low redispersibility of the wet film, which allows water rinsing to remove excess supernatant prior to oven cure. Very low maintenance is required and energy use is reduced because of the elimination of the phosphate pretreatment process. Finally, autodeposition is environmentally benign with low or no VOC emission or heavy metal effluent. No fire hazards are present. Since the autodeposition reaction is diffusion controlled, the supernatant film on freshly coated pieces lifted from the bath continues to deposit paint solids. As a result, there is minimal loss (i.e., transfer efficiency averaging 95%) of solids to the water rinse following the coating bath. This effect is further enhanced by the absence of any external force (e.g., electric current), which would increase the concentration of solids at the immersed surface of the work. Since bath solids are maintained at only 5% to 7% by weight, any dragout effects are minimal. Autodeposition is a versatile means of coating complex parts and assem206

Articles in this issue

view archives of Metal Finishing Guide Book - 2013