Manufacturing: Surface Finishing Coatings

Electrocoating

Parts Going Into Electrocoating Bath

Electrocoating, which is also referred to as electrodeposition, electrophoretic deposition, or electropainting, is an organic finishing process that uniformly applies thin-film primers and one-coat finishes to metallic substrates. Electrocoating resembles electroplating in that it utilizes an electrical current to deposit a coating onto substrates. However, electrocoating deposits waterborn paint onto substrates rather than metal ions. The overall process consists of four main process steps: pretreating, electrocoating, rinsing, and baking.

The electrocoating process may be anodic or cathodic, depending on the charge applied the substrate. Although the processes are virtually the same, properties of the resultant coating are dissimilar. Anodic systems, which were the first to be used for electrocoating, apply paint to positively charged substrates. The negatively charged pigment and resin particles deposit onto the substrate (anode). One disadvantage of this process is that substrate metals dissolve and become incorporated into the coating, which affects surface properties. Cathodic electrocoating deposits paint onto negatively charged substrates and offers several advantages over anodic electrocoating. For example, metal dissolution of the substrate does not occur, cathodic electrocoating has the ability to deposit over contaminants, corrosion resistance is improved, and a better color consistency occurs over welded areas.

The electrocoat process can be divided into four distinct steps:

1. Pretreatment
2. Electrocoat Bath
3. Post Rinses
4. Baking

Pretreatment - cleaning and phosphating the metal

The pretreatment zone cleans and phosphates the metal to prepare the surface for electrocoating. Cleaning and phosphating are essential to achieving the performance requirements desired by today's end user of the product. A high quality zinc phosphate system using the immersion method is primarily used where steel and iron parts are to be coated.

Electrocoat Bath - applying coating in bath

The electrocoat bath and ancillary equipment zone is where the coating is applied and the process control equipment operates. The electrocoat bath consists of 80-90% deionized water and 10-20% paint solids. The deionized water acts as the carrier for the paint solids which are under constant agitation. The solids consist of resin and pigment. Resin is the backbone of the final paint film and provides corrosion protection, durability and toughness. Pigments are used to provide color and gloss.

Post Rinses - rinsing off excess paint solids

The post rinses provide both quality and conservation. During the electrocoat process, paint is applied to a part at a certain film thickness, regulated by the amount of voltage applied. Once the coating reaches the desired film thickness, the part insulates and the coating process slows down. As the part exits the bath, paint solids cling to the surface and have to be rinsed off to maintain efficiency and aesthetics. The excess paint solids are called "drag out" or "cream coat." These excess paint solids are returned to the tank to create a coating application efficiency above 95%.

Bake Oven - thermally curing the paint film

The bake oven receives the parts after they exit the post rinses. The bake oven cross links and cures the paint film to assure maximum performance properties. The minimum bake schedule is 20 minutes with the part temperature at 375°F for most electrocoat technologies. However, there is also a "low temperature cure" electrocoat material. This material has a minimum cure of 20 minutes at a part temperature of 180°F so that many assemblies containing seals, bushings, bearings, or oil can use the electrocoat process.

Process Advantages -

E-coat provides an excellent base for a variety of topcoats. It provides an incredible "dual application advantage," creating a more decorative and durable finish. In addition, e-coat offers an "environomic" solution - no heavy metals, no HAPS and low VOCs.

It can be used on a wide variety of substrates including -

1. Cold Rolled/Hot Rolled Steel
2. Galvanized Steel
3. Iron Castings
4. Zinc Castings
5. Copper/Brass
6. Conductive Composites
7. Aluminum Extrusions and Castings
8. Rare Earth Magnets (NdFeB)
9. NiZn Plated Steel
10. Stainless Steel
11. Magnesium
12. Chrome Plate.

Electrocoating readily conforms to complex configurations and yet maintains engineered tolerances on parts ensuring intended operating functions. Some examples include: Internal Surfaces, Deep Recesses, Weldments, Fasteners, Small Parts, Large Parts, Uniform Coating- No Sags or Runs.