Figure 27.1 Schematic of the electrodeposition process; DIW = deionized water.
1. Conveyor
High-volume production uses an overhead conveyor that enters the tank at a 30° angle. To save energy and provide space for 90° vertical entry, free and powered conveyors are sometimes combined with bulk coating.
2. Metal preparation
Most electrocoating installations use grades seven to nine of zinc phosphate. Iron phosphate is also widely used. For non-ferrous metals, special pretreatments are applied.
3. Storage tank shell
Tank enclosures interlocked with power and conveyors provide safety. Any entry de-energizes the system.
4. Dipping tank
Tanks are run in a continuous manner (conveyor or coil) or entered in batches, in the latter case a "power and free" conveyor is used. The workplace is typically fully submerged 15 cm below the surface with a 15 cm clearance on each side of the surface. Tanks provide approximately 3 minutes of work, while coating of cans and coils is done in seconds.
5. Rectifier
Use a DC power supply with less than 5% ripple factor over the entire range.
6. Counter electrode
Most electrocoat tanks are lined with an epoxy coating approximately 10 mils thick. In this case, tanks, conveyor belts, merchandise, etc. are all grounded: only the counter electrode, inserted underneath the painted surface, has the opposite (thermal) polarity. The inserted electrodes are protected by a plastic grate. If counter ions (acids or bases) need to be collected, then the counter ions are separated from the filled tank by a process called electrodialysis through a membrane. It is desirable that the tank itself can be used as a counter electrode unless the counter electrode fluid is separated .
7. Stir
Electrophoretic baths with volumes of up to 500,000 liters contain 10 to 20 wt% solids, about the viscosity of water. Eductor nozzles and other agitation devices are used to move the entire bath for 6 to 30 minutes to prevent settling of solids. The residence time in the bath is described as "turnover". For example, a filled 100,000 liter volume of an average tank contains approximately 10,000 kg of paint solids. 10,000kg reaches a turnaround with solids (feed) added, which takes anywhere from 10 working days to 10 months, depending on commodity configuration and production rates. At this time, according to the law of probability, a lot of the original filling material still stays in the tank. Therefore, considerable pumping stability is required.
8. Temperature control
Most tanks run between 75 and 95°F. Almost all of the electrical and stirring energy required is converted to heat, which needs to be removed using a cooler.
9. Ultrafilter
Uses membranes to save paint and remove dissolved impurities permeable water, salts and substances with a molecular weight less than 300 by ultrafiltration. Ultrafiltrate (permeate) is typically used for 3 spray rinses, sometimes 1 dip rinse, followed by a DI water rinse.
10. Paint filter
Use filter bags, wound filters and graduated filters with a pore size of 5 to 50 μm.
11. Paint preparation
Expressed in Equations 27.1 and 27.2, paint is actually the final step in the manufacture of paint for water-based paint, and is done by mixing the contents of the can with all the required paint components.
12. Deionized water
Water at 75,000 Ω-cm is used to replace water that is evaporated or otherwise lost, and this water is also used as a final rinse.
13. Baking or curing
Most e-coats require a 20-minute bake at 350°F. However, lower bake materials can be used; even ambient cure materials are available.
