The electrocoating (e-coat) process is an organic, water-based dip coating method that uses electrical current to deposit paint onto an electrically conductive part, similar to the plating process. Although its greatest application is coating parts requiring a high degree of corrosion protection, e-coat can also be a cost-effective solution for high production (>1 million square m2 per year) of parts that drain well, benefit from a high racking density and even film build and are limited with respect to number of colors. Other economic benefits include reduced labor (no painters required), reduced operating cost and environmental friendliness. Disadvantages of e-coat relative to other coating technologies include generally higher capital equipment cost and difficulty changing colors.
Although e-coat is available in many colors, changing the color in a tank can be time-consuming, typically as much as 6-8 hours. Paint must be transferred to a holding tank and the piping and filters completely flushed. If frequent color changes are needed, multiple tanks and ancillary equipment are considered.
Two types of e-coat paint are available: cathodic and anodic.
Cathodic paint is the more corrosion resistant type and works by grounding the parts and applying a positive charge to the paint particles with a rectifier supplying high DC voltage to series of anodes arranged along the tank walls. The electric charge migrates the particles to the part surface then continues to "pull" the water out of the interface between the paint and part essentially adhering the paint to the part surface. As the charge is emitted from the anodes acid is created must be removed with an anolyte system drawing paint from around the anodes.
Anodic systems use the rectifier to apply the charge in an opposite direction, i.e. a cathode is placed along the tank walls and migrates the paint to the positively charged part. Although not as corrosion resistant, anodic coatings are generally less expensive and do not require an anolyte system.
When the part is dipped, the cathodic or anodic coating thickness migrates to the most conductive part surfaces first. As the film builds the resistance increases and shifts the coating build to other areas of the part resulting in a very even film build generally in the 0.5-1.2 mil thickness range. The electrical charge may diminish and inhibit coating inside tubes or deep recesses and should be tested.
E-coat paints are considered 98%+/- transfer efficient, i.e. very little paint is lost in the process. Excess paint residue remaining on the parts as they leave the tank is rinsed off with permeate (post rinses) and returned to the paint tank. Permeate is essentially water generated with an ultrafilter used to separate the paint into solids and liquid. The paint solids return to the paint tank and the liquid portion is used as the permeate rinse.
Maintaining the e-coat tank temperature may be required using a chiller and heat exchanger to remove heat imparted from the rectifier during the coating process. In some areas, heat is also required to keep the bath warm in the wintertime. Most e-coat baths are maintained between 25°C and 32°C; the temperature will be specified by the coating supplier.
After e-coat also require a thermal cure in the range of 150°Cto 200°C. Depending upon the specific coating, the ovens may be divided into two or three zones. The first zone is generally a lower temperature "dehydration" zone designed to remove excess moisture before "skinning" the paint. The second, and some cases, third zones provide a gradual higher temperature cure. Some e-coats tend to emit smoke when over a threshold temperature so the ovens must be properly ventilated and parts may need to be enclosed in a ventilated cooling tunnel after they exit the oven.