Electrocoating Q&A: Obtaining Class A Automotive Part Quality
Q. What are the key process and equipment elements required to produce electrocoated parts with consistent class A automotive quality? We are upgrading our line to satisfy a new customer and want to make sure we cover all necessary upgrades.
A. Class A electrocoated parts in the automotive and transportation industries are typically parts with high visibility exterior vehicle mounting positions above an imaginary line between the front and rear bumpers. In addition to the surface appearance and visual quality necessary to generate a finished part, automotive Class A electrocoated parts must also provide very high levels of corrosion protection. Oftentimes, Class A automotive electrocoated parts receive a topcoat.
Automotive parts with limited visibility located under the vehicle or under the hood that do not receive a topcoat are typically considered Class B or C, depending on each specific application, OEM and part function or end use.
To assess the capability of an existing process and equipment to produce Class A-quality electrocoated parts, evaluate the following key process and equipment areas:
Substrate. Investigate the kind of metal substrate and the type of post-treatments the metal parts may have received prior to electrocoating. Keep in mind that the capability to obtain Class A automotive parts starts with the quality of the part substrate itself.
The metal surface to be electrocoated must be sufficiently smooth and free of cracks, surface defects and internal porosity. Without a high-quality metal surface, the electrocoat system won’t be able to produce Class A corrosion and appearance quality levels. It would be very difficult and costly to obtain a Class A automotive part if the metal substrate, for example, is a forging, hot-rolled steel or an iron casting with surface imperfections and rugosity.
Oftentimes, Class A automotive parts are electrogalvanized or galvanneal steel. These dual-steel substrates present smooth surfaces that are ideal to accept a layer of electrocoat and provide additional anodic corrosion protection.
It is also very important to assess the type of heat treatments, anticorrosion treatments and processing chemicals used during the transportation, storage and manufacturing of the metal substrate from raw metal to a finished electrocoated part. Understanding the exposure conditions (temperature and relative humidity), as well as the residence times during and between each of the manufacturing process stages, is also a very important piece of information.
Processing oils and other organic contaminants and soils present on the metal surface can oxidize or heat polymerize during any of the manufacturing or storage stages (depending on specific exposure conditions) and render the metal surface unacceptable for further finishing without prior pretreatments. For example, welded assemblies with significant heat-affected areas would require additional equipment and process steps for acid cleaning.
Cleaning. Assess your cleaning system capability for temperature, concentration, time and combination spray/immersion stages to chemically clean the parts to very high cleaning levels.
Knowing the type, aging and environmental exposure conditions of the soils provides valuable information when analyzing and determining the best metal cleaning strategy. The better the cleaning, the better the appearance and performance of the parts.
It is important that the oils, lubes and other rust preventatives used during processing or storage are chemically compatible with the system´s specific type of cleaner, cleaning times and temperatures. The cleaning must be excellent to reach Class A performance and appearance levels.
Pretreatment. Automotive Class A metal parts require the application of surface pretreatment technologies prior to electrocoating. Cleaning the metal surface alone will not produce Class A automotive finished parts.
The surface pretreatments are designed to enhance electrocoat adhesion to the metal substrate, as well as provide a smooth surface for deposition. Zinc phosphating or thin metal pretreatments based on titanium, zicronium, silicone or others, must be used. No cleaner coaters or iron phosphates are capable of providing Class A automotive performance levels. If a sludge-producing pretreatment technology is used (like zinc phosphate), then adequate filtration and post-rinsing is critical to obtain good quality parts.
Film thickness. Automotive Class A parts require a film thickness control system capable of depositing predictable and even films throughout the part/rack at lower application voltages. For good film thickness control, the system equipment must incorporate enough anode surface area, supply a large enough rectifier and have sufficient time and a high-level paint solids percentage to keep up with the design workload demand.
The film thickness level required depends on each specific application. With smooth galvanized steel substrates 18 to 22 microns of electrocoat is sufficient. Other substrates or surfaces may require as high as 30 to 35 microns.
Rinsing. Rinse stages after chemical stages in the electrocoat system are a significant element required to obtain Class A automotive corrosion and appearance levels. Sufficient rinsing frees the metal surfaces of unused chemicals, trapped solids or phosphate sludge residues.
Counter cascade double rinsing after the cleaner and phosphate stages is necessary to obtain Class A automotive parts. Tap water rinsing is typical after the cleaner stages and deionized (DI) or reverse osmosis (RO) water after the phosphate chemical stages. Ultrafiltrate (UF) for post-elecrocoat rinses is also necessary. Oftentimes, the last UF post rinse prior to curing is an RO or DI rinse to minimize permeate streaks and stains and improve the appearance.
Racking. Class A parts typically require racking to eliminate puddles and air pockets and enable excellent grounding to all parts with unblocked access to process chemicals, anodes and electrocoat paint.
The racking system must afford a robust and fail-safe grounding system, as well as proper part spacing and orientation. Part draining patterns and locations must be clearly understood and considered during racking design to ensure that top-mounted parts do not abundantly drain or splash onto lower hanging parts. Enabling the top parts to excessively drain or drip onto lower parts can lead to streaks and stains in the electrocoat finish. When possible, use angled support members for better rack draining and lower rack maintenance.
Oven. The electrocoat cure oven is a key element of the overall process equipment and a key factor to the final visual appearance and corrosion performance of the electrocoated parts.
The electrocoat oven must provide sufficient length and temperature to obtain part cure profiles in the 90 to 110 percent cure range. Under-cured or over-cured electrocoated films cannot meet Class A corrosion performance, physical properties or visual appearance levels.
Longer oven times are usually necessary when processing thick and thin gauge metal parts at the same time. The longer oven residence time enables lower operating temperatures and, consequently, more similar cure profiles for all different gauge parts.
Lastly, the electrocoat oven must be regularly cleaned, including all walls and ceilings, as well as the conveyor, recirculation and exhaust fans and burner sections. Proper oven cleaning steps must include a blow-off stage, a vacuum clean stage and a damp-rag wipe stage to completely eliminate oxidation and dirt particles that accumulate and build-up in electrocoat ovens.
Originally published in the June 2016 issue.
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This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.