Metal Finishing Guide Book

2012-2013

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by the applied coating. Electrocoat solids deposit initially in the areas closest to the counter electrode and, as these areas become insulated to current, solids are forced into more recessed, bare metal areas to provide complete coverage. This phenomenon is known as throwing power and is a critical aspect of the electrocoat process and materials. Electrocoat bath solids are deposited electrically via a system that includes a number of components: the rectifier, which supplies a DC charge to the bath enabling deposition of ionic species; circulation pumps to maintain proper paint bath uniformity; a heat exchanger and chiller to control the temperature of the bath; filters, which remove dirt particles introduced into the systems; and ultrafilters that produce permeate for rinsing and allow for recovery of excess paint solids. Postrinsing As the part exits the bath, excess paint solids not deposited electrically cling to the part and must be rinsed off to maintain process efficiency and optimal aesthetics. Rinse material is supplied from the ultrafilters and is called permeate. The permeate, containing low molecular weight organics and some solvent, is used to rinse the dragout from the parts; the excess solids and permeate are returned to the bath in a counterflow fashion, affording superior levels of transfer efficiency. Baking After exiting the postrinses, the coated parts enter the bake oven for curing and cross-linking of the paint film, resulting in a high-quality finish void of runs, drips, and sags (see Fig. 3). Bake temperatures range from 180 to 375��F depending on the type of electrocoat applied. TYPES OF ELECTROCOAT PRODUCTS Electrocoat products are referred to as either anodic or cathodic, indicative of where coating deposition takes place (see Fig. 4). The first electrocoats developed in the late 50s and early 60s were anodic systems. Cathodic systems were developed later and were initially commercialized in the appliance industry in the late 60s and early 70s. Anodic Electrocoats Fig. 3. Typical electrocoat finish after baking. Anodic electrocoating involves the use of negatively charged paint particles, which are deposited onto positively charged metal substrates. The polymer species are acid functional and amine solubilized. Anodic electrocoats offer economical finishes with excellent color and gloss control. The anodic deposition process leads to some dissolution of metal ions from the parts being coated. These ions become trapped in the depo259

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