Automotive Powder Coating & E-Coating
Automotive painting & coating is a technical system in automobile manufacturing involving surface treatment and coating application. It mainly includes pre-painting metal surface treatment and coating application, serving functions such as protecting the car body from corrosion and enhancing its appearance.
The role & functions of automotive painting and coating
- Protective Function: Since 90% of automobiles are made of steel, it’s easy to imagine the lifespan of a car would be significantly reduced without painting. Therefore, automotive painting is essential. The coating protects the car from corrosion, and with the advent of new materials, the protective function of the coating is becoming increasingly important, such as high scratch resistance, elasticity, and stain resistance.
- Aesthetic Function: The beautiful appearance, luxurious sheen, and diverse colors of paint films reflect current trends.
From the 1950s, automotive paint evolved from single colors to metallic paints and pearl paints, showcasing different perspectives and enhancing the car’s aesthetics. - Value Function: With the advent of the automobile, human demands for car colors have continuously developed, leading to value judgments. This has also resulted in significant improvements in painting technology and the variety of paint types available. For example, when buying a car at a 4S dealership, the price varies depending on the type of paint used for the same model. Generally, metallic paint and pearl paint are USD100-300 more expensive than solid color paint, and the price also depends on the car brand’s value.
- Identification Function: Broadly speaking, everything has a distinguishing function, and the colors of car bodies reflect different uses. This includes the international use of colors to differentiate things, such as military vehicles, engineering vehicles, and fire trucks.
Automotive painting & coating process principles and flow
A complete automotive body painting process typically includes: pretreatment (degreasing, washing, surface conditioning, phosphating) → electrophoresis (cathodic electrophoresis) → (intermediate coat) → topcoat (color paint + clear coat) → post-treatment (baking).
Pretreatment (Degreasing, Washing, Surface Conditioning, Phosphating) Modern automotive painting pretreatment has become a standardized core process, mainly including four steps: degreasing, washing, surface conditioning, and phosphating. The degreasing step uses alkaline cleaning agents to remove grease and dust from the car body surface; the washing step requires rinsing with deionized water; the surface conditioning step forms uniform “active points” on the car body surface; phosphating is the most critical step, where the car body is immersed in a phosphate solution, and a 2-3 μm thick phosphate film (mainly composed of zinc phosphate) is formed on the metal surface. This film significantly enhances coating adhesion and corrosion resistance.
- Pretreatment (Degreasing, Washing, Surface Conditioning, Phosphating)
Modern automotive painting pretreatment has become a standardized core process, mainly including four steps: degreasing, washing, surface conditioning, and phosphating. The degreasing step uses alkaline cleaning agents to remove grease and dust from the car body surface; the washing step requires rinsing with deionized water; the surface conditioning step forms uniform “active points” on the car body surface; phosphating is the most critical step, where the car body is immersed in a phosphate solution, and a 2-3 μm thick phosphate film (mainly composed of zinc phosphate) is formed on the metal surface. This film significantly enhances coating adhesion and corrosion resistance. - Electrophoresis (Cathode Electrophoresis)
Electrophoresis, also known as cathodic electrophoresis coating, works by immersing the car body as the cathode in positively charged electrophoretic paint (mainly epoxy resin). Under direct current, the electrophoretic paint particles are deposited on the surface, forming a uniform coating with a thickness of 18-25 μm. The advantage of electrophoresis is its “comprehensiveness and uniformity,” which can protect the car body from rusting for 1000 hours, providing comprehensive basic corrosion protection. - (Intermediate Coating)
The main function of the intermediate coating is to fill the tiny pores in the electrophoretic layer and enhance the coating’s resistance to stone impact (e.g., to withstand the impact of a 500g steel ball falling from a height of 1 meter). Traditionally, intermediate coatings need to be sprayed and dried separately, but in recent years, “intermediate coating-free processes” have become mainstream. By improving the compatibility between electrophoretic paint and topcoat, topcoat can be sprayed directly onto the electrophoretic layer, simplifying the process, reducing costs, and decreasing VOC emissions. - Topcoat (Color Paint + Clear Coat)
The topcoat is the finishing touch in the painting process, consisting of two layers: color paint and clear coat. The color paint determines the vehicle’s color; the clear coat is a transparent coating (35-45μm thick) covering the color paint, primarily composed of acrylic resin or polyurethane. Its function is to protect the color paint from UV rays and acid rain (weather resistance must reach at least 5 years), while also providing high gloss.
Automotive painting systems can be categorized by coating layers into two-coat systems (primer + topcoat), three-coat systems (primer + intermediate coat + topcoat or metallic paint/clear coat), and four-coat systems (adding a clear coat). - Post-treatment (Baking)
Based on drying conditions, post-treatment systems can be divided into oven-drying systems (suitable for mass production lines) and air-drying systems (suitable for small-batch or special vehicle painting).
