CLICK IMAGE TO ENLARGE (+)
Electrocoating is a process by which electrically charged particles are deposited out of a water suspension to coat a conductive part. During the electrocoat process, paint is applied to a part at a certain film thickness, which is regulated by the amount of voltage applied. The deposition is self-limiting and slows down as the applied coating electrically insulates the part.
Electrocoat solids initially deposit 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 throwpower and is a critical aspect of the electrocoat process.
- E-coat bath and ancillary equipment
- Post rinses
- Cure oven
In a typical E-coat process, parts are first cleaned and pretreated with a phosphate conversion coating to prep the part for electrocoating. Parts are then dipped into a paint bath where direct current is applied between the parts and a “counter” electrode. Paint is attracted by the electric field to the part and is deposited on the part. Parts are removed from the bath, rinsed to reclaim undeposited paint solids, and then baked to cure the paint.
Prior to paint film application, most metal surfaces receive pretreatment that usually involves a conversion coating.
- Cleaning (one or more stages)
- Conversion coating
- Deionized water rinsing.
Phosphating processes can be separated into two types: iron phosphate and zinc phosphate. Iron phosphate has been the process of choice for applications where overall cost considerations override performance needs.
All coatings, including electrocoatings, are made from 1) polymeric resin or binder, 2) pigments, and 3) solvents and diluents. The resin (typically 10–20 percent) is the backbone of the final paint film and provides properties such as corrosion protection and ultraviolet durability. Pigments provide color, gloss, and corrosion protection. Deionized water is the major component of an electrocoat bath, making up 80-90 percent of the electrocoat bath. The deionized water acts as the carrier for paint solids, which consists of resins, pigments, and small amounts of solvent. Solvents also help to ensure smooth film appearance and application.
Economics of Electrocoat
Electrocoat is normally the lowest cost finishing application. The key word is “application.” Anytime one evaluates the cost to paint parts, they need to look beyond just the material cost per square foot or the cost per gallon. Some of the major factors that enter into the equation for the application selection are:
- Part complexity. Parts come in all shapes and sizes. Electrocoat excels over other technologies in this category because all surfaces receive a consistent film thickness due to the electrical insulating effect of the electrocoat as it deposits onto the part.
- Production volume. As manufacturers increase production quantities above 2 million sq feet annually per shift, electrocoat becomes a more preferred application method. The dense rack loading achieved with electrocoat allows manufacturers to produce greater volumes of parts.
- Energy. All coatings operations require energy. Liquid and powder lines require daily rack cleaning and an additional dry-off oven after pretreatment. Powder lines normally require an environmental room to maintain air quality for consistent paint application to the part. Electrocoat has its share of energy requirements as well as requiring recirculating pumps, a chiller, and a rectifier. The energy differences are not normally great when all factors are considered.
- Repair and maintenance. There is normally more mechanical equipment associated with electrocoat; however, the labor requirements to maintain a liquid or powder system are usually greater.
- Paint material. Electrocoat is usually the most cost-effective when comparing the costs of applied paint material. This is due to the high transfer efficiency (95–98%) and the self-limiting ability of the electrocoating process resulting in a differential of only 0.2 mils deposited paint film.
- Capital. In most cases, electrocoat lines require more capital dollars. But after other cost variables (film thickness, transfer efficiency, and labor requirements) are taken into consideration, the electrocoat process produces the cheapest coating on an applied cost per square foot basis.
- Miscellaneous items. There will be additional overhead expenses such as insurance, building lease or mortgage, material handlers, shipping, etc. that the manufacturers needs to consider when determining the “final applied cost” for parts.
The choice of electrocoat over other coating technologies is usually driven by the lower total applied cost of electrocoat compared to technologies like liquid spray and powder coatings. The low total applied cost is achieved in part through more consistent deposition of paint film on parts (particularly complex geometries), high transfer efficiencies, and the lower costs associated with labor.
- Home appliances. Home laundry appliances have traditionally required a high performance coating system using a twocoat application. A cationic epoxy primer was used for corrosion and detergent resistance. This required a liquid or powder topcoat for appearance consistency. Now a detergent-resistant cationic acrylic electrocoat supplies all of the requirements of the previous two-coat system. With this improvement, performance and appearance are at all-time high levels. The advantages on cost, through automation and simplification, have kept this industry competitive in a price-conscious marketplace.
- Frames. The automotive industry standard for 20 years has been to coat the frames of cars and light trucks with hot wax, providing corrosion protection for one to three years. Hot wax required shielding from hot exhaust pipes and catalytic converters. The elimination of this shielding helped reduce weight and unwanted noise. New electrocoat products meet OEMs’ requirements for 10-year corrosion protection, demonstrating superior heat resistance and lowers applied cost. Applications include truck and trailer frames, engine cradles, trailer hitches and tow hooks, suspension components and underbody components.
- Decorative clears. The decorative-metal-finishing market has traditionally used spray and dip lacquer to achieve decorative clear finishes. New electrocoat systems designed to replace traditional methods provide higher transfer efficiency, reduced manual handling and fewer rejects. The products are formulated for application over a wide range of substrates, including plated gold, silver and brass, as well as copper, aluminum and steel base metals. Typical applications range from small components (cigarette lighters and jewelry) to larger household items (furniture and lighting fixtures).
- Decorative colors. Vibrant colors can now be achieved using decorative resins combined with specialty pigments. The unique pigment formulations allow for transparent colors which truly enhance the finished substrate. The colors can be used throughout the gloss range, high full) through matte. The matte finish can resemble typical anodized finishes. The ability to replicate an anodized appearance makes electrocoat an attractive replacement for anodizing. Applications include automotive aftermarket, decorative, sporting equipment, and lighting industries (Figure 9).
- Radiators. Radiators have historically been coated with a low-cost, low-performance, spray-applied black coating for cosmetic purposes. This poses several significant problems for heat exchangers. The lack of complete coverage leads to corrosion failure and core replacement. Cathodic epoxy electrocoat offered a promising solution to the corrosion problems encountered with heat exchangers. One of the advantages of electrocoat is its ability to provide complete surface coverage with unsurpassed film uniformity due to throwpower. Throwpower is the ability to throw paint into recessed areas. By being able to coat the entire core, corrosion resistance is greatly enhanced. Special epoxy electrocoat products, which have the ability to cover very sharp metal edges with a substantial film thickness of coating, further enhance corrosion performance over the fins in heat exchangers.