The fundamental principle that makes electrocoating work is that opposite charges attract each other. The electrocoat process uses a DC rectifier to create a voltage potential between a conductive part and counter-charged electrodes that are immersed in the electrocoat paint tank. The electrocoat paint particles are also charged and are deposited out of a water suspension to coat the conductive part when the rectifier is turned on.
The voltage from the DC rectifier is used to control the amount of paint that is deposited onto the part. As the voltage is increased, the amount of paint deposited will also increase. The deposition is self-limiting and slows down as the applied coating electrically insulates the part. Electrocoat solids deposit initially in the part areas that are closest to the counter electrode and as these areas become insulated to the current, solids are forced into more recessed bare metal areas to provide complete coverage. This phenomenon is known as throwpower and is a critical aspect of the electrocoat process.
Depending on the polarity of the part and paint particles, electrocoating is classified as either anodic or cathodic.
In anodic electrocoating, the part to be coated is the anode with a positive electrical charge which attracts negatively charged paint particles in the paint bath. During the anodic process, a small amount of soluble iron can migrate away from the part and into the paint film which results in a reduction of the performance properties of these systems. Their main use is for products in interior or moderate exterior environments. Anodic coatings are economical systems and offer excellent color and gloss control.
In cathodic electrocoating, the part to be coated is the cathode with a negative electrical charge which attracts positively charged paint particles in the paint bath. By reversing the polarities used in the anodic process, the amount of soluble iron that enters the paint film during the cathodic process is greatly reduced and the corrosion resistance properties of the paint film are improved. Cathodic coatings are high-performing coatings with excellent corrosion resistance that can also be formulated to meet tough exterior durability requirements.
The electrocoat process can be divided into four distinct sections:
- Pretreatment: Parts are cleaned and pretreated with a conversion coating to prepare the part for electrocoating.
- Electrocoat Tank: Direct current is applied between the parts and a "counter" electrode. Paint is attracted by the electric field to the part where it is deposited.
- Post Rinses: Parts are rinsed to reclaim undeposited paint solids.
- Bake Oven: Paint is thermally cross-linked or cured.
The parts are transported through the system, hung from a continuous moving monorail. Alternatively, programmed hoists or similar indexing devices are used to move the parts from stage to stage and in and out of tanks. The type of transport method is dependent on the part size, type and coated surface area requirement per minute.