High Tech Steel Wheel E-coating Line

E-coating or Electrophoretic coating is an advanced metal surface treatment technology, particularly suitable for anti-corrosion coatings of automotive parts such as steel wheel hubs.

Steel wheel e-coating line general workflow: Load -> hot water rinse -> pre-degrease -> degrease -> rinse -> rinse -> surface conditioning -> phosphate -> rinse -> rinse -> pure water rinse -> e-coating -> UF1 rinse -> UF2 rinse -> pure water rinse -> drying -> powder coating/liquid coating -> curing -> unload

Basic Principles of Electrophoretic Coating Process

Electrophoretic coating is a process in which a coating material is deposited on the surface of a workpiece under the action of an electric field. Its core principle includes four key electrochemical reactions:

– Electrophoresis: Under the action of an electric field, positively charged coating particles (resin and pigment) move towards the cathode (workpiece). These particles range in size from 10^-7 to 10^-9 meters and remain dispersed without settling.

– Electrodeposition: When positively charged coating particles reach the surface of the workpiece (highly alkaline interface layer), they gain electrons and react with hydroxide ions, transforming into water-insoluble substances that deposit onto the workpiece.

– Electrolysis: In an ionically conductive solution, anions move to the anode and cations move to the cathode, undergoing oxidation-reduction reactions at the respective electrodes.

– Electro-osmosis: The freshly deposited wet coating film acts as a semi-permeable membrane. Under continuous electric field action, internal moisture will migrate toward the bath solution, turning the hydrophilic coating into a hydrophobic one.

Electrophoretic coating can be divided into anodic electrophoresis (coating particles carry negative charge) and cathodic electrophoresis (coating particles carry positive charge), with cathodic electrophoresis becoming the mainstream process due to its excellent corrosion resistance.

Process Flow of Steel Wheel Hub Electrophoretic Coating

The typical process flow for electrophoretic coating of steel wheel hubs is as follows:

– Pre-treatment:

• Degreasing: Remove surface oil
• Rinsing: Clean residual degreasing agents
• Rust removal: Remove surface rust
• Surface adjustment: Adjust microscopic surface structure
• Phosphating: Form a phosphate conversion film to improve coating adhesion

– Electrophoretic Coating:

• Immerse the workpiece in the electrophoretic bath
• Apply DC voltage (40-70V on steel surface)
• Coating particles deposit under the electric field (1-4 minutes)

– Post-treatment:

• Recovery wash: Remove excess surface coating
• Baking: Cure the coating (temperature and time depend on the coating)
• Fast cooling: Quickly cool the workpiece

Understanding the technologies

There are two specific electrocoat processes, anionic and cationic, both of which are commonly used. The anionic process involves placing a positive charge on the part while the paint bath is negatively charged. This process is commonly used in the general metal industry where low cost, color control and ease of operation are the driving forces. Many parts that are in noncorrosive environments are processed through this type of system.

The cationic E-coat process is used to provide a more corrosion-resistant film. The part to be coated has a negative charge; the paint bath, a positive charge.

The process involves driving charged particles out of a water suspension to a part capable of conducting a charge. It is a rather simple electrical process of positive and negative charges being attracted to each other while like charges repel.

The electrical charge seeks out the path of least resistance and coats the exterior portions of the part or parts nearest to the counter electrode. As the process continues, the charged particles resume their search for uncoated portions of the part and begin coating areas that are not as easily reached. This ability to coat hard-to-reach areas of the part is known as the paint’s throw power.

During the deposition process, the part’s electrical resistance begins to build as the E-coat film is deposited, driving the coating process to another portion of the part or another part on the rack. The film build is controlled by the amount of voltage applied and is self-limiting. After a short dwell time, all conductive areas of the part have been coated.

There are many process advantages of electrocoating, including total coverage of densely loaded racks and complex parts with a uniform film build; transfer efficiency routinely in the 95 to 99% range; highly automated systems with high throughput and low operating costs; environmental compliance for air and wastewater emissions; high line speeds; heavy-metal-free formulas; and workplace safety.

E-coat formulas are typically based on either epoxy or acrylic chemistry. Epoxy chemistry is used in environments where corrosion protection is paramount. It inherently provides superior results in salt-spray and cycle-corrosion testing.

Acrylic systems are used in applications requiring outstanding durability or color control. Recently, coating requirements have focused on providing both superior corrosion protection and durability. These hybrid systems are finding more use in the industrial market as coating requirements change.