Pretreatment Stage

1. Degreasing
   The surfaces of heavy workpieces often accumulate grease and oil during manufacturing or storage. Degreasing is the first crucial step to clean these contaminants. Commonly, alkaline degreasing agents are used. The workpieces can be immersed in a degreasing solution tank or sprayed with the solution. In the immersion method, the workpieces are left in the tank for a certain period to allow the degreasing agent to react with and dissolve the grease. During spraying, the solution is evenly sprayed onto the workpiece surfaces through spray nozzles at a specific pressure to remove the grease effectively. This step ensures a clean surface for subsequent coating operations and helps improve the adhesion of the paint.

2. Rust Removal
   Heavy workpieces may have rust due to exposure to the environment or long storage time. There are mainly two common methods for rust removal. One is shot blasting, where high-speed steel shots or grit are projected onto the workpiece surface by a shot blasting machine. The impact force of these abrasive materials can effectively remove rust layers, scale, and other impurities from the surface, while also roughening the surface slightly to enhance the adhesion of the paint. Another method is chemical rust removal, which involves using acidic solutions like phosphoric acid or hydrochloric acid mixtures. The workpieces are immersed in these chemical solutions, and the rust reacts with the acid to form soluble compounds that can be removed by rinsing. However, proper safety measures and waste treatment are required when using chemical rust removal methods.

3. Surface Conditioning
   After rust removal, surface conditioning is usually carried out. For example, phosphating treatment is often applied. The workpiece is immersed in a phosphating solution containing metal phosphates and accelerators. Through chemical reactions on the surface, a thin layer of phosphate film is formed. This film not only further improves the adhesion of the paint but also provides some corrosion resistance to the workpiece before painting. Another common surface conditioning method is passivation, which can make the metal surface more stable and less likely to react with the environment or the paint during the coating process.

Painting Stage

1. Primer Application
   The choice of primer depends on the material of the heavy workpiece and its intended application environment. Epoxy primers are widely used for their excellent adhesion and corrosion resistance properties. The application methods mainly include spraying and brushing. In spraying, airless spray guns are often employed. They can atomize the primer into fine droplets and evenly distribute them onto the workpiece surface at a certain spraying pressure and distance. Brushing is suitable for some areas that are difficult to reach by spraying or for workpieces with complex shapes. Workers use brushes to apply the primer carefully to ensure complete coverage of the surface.Paint coating line for heavy workpieces.

2. Intermediate Coat Application (if applicable)
   For some heavy workpieces that require higher coating quality or better protection, an intermediate coat may be applied between the primer and the topcoat. The intermediate coat can fill in small surface imperfections, further enhance the corrosion resistance, and improve the overall smoothness of the coating system. Similar to the primer application, spraying is the main method used, and the thickness of the intermediate coat needs to be controlled carefully according to the specific requirements.

3. Topcoat Application
   The topcoat determines the appearance and final protection performance of the workpiece. Polyurethane topcoats are commonly chosen for their good weather resistance and gloss retention. Spraying is the dominant method for topcoat application as well. During spraying, parameters such as the spray gun angle, spraying speed, and distance need to be adjusted properly to obtain a uniform and smooth topcoat with the desired thickness. Moreover, in some cases, multiple layers of topcoat may be applied to achieve better visual effects and durability.Paint coating line for heavy workpieces.

Drying and Curing Stage

After each layer of paint is applied, proper drying and curing are essential. For solvent-based paints, drying mainly involves the evaporation of the solvent. This can be achieved by natural drying in a well-ventilated area or by using drying ovens with controlled temperature and air circulation to accelerate the drying process. For some paints that require curing reactions, such as epoxy and polyurethane paints, specific curing conditions like certain temperature and humidity levels need to be maintained for a certain period. Curing ovens or UV curing equipment (if applicable for UV-curable paints) are used to ensure that the paint fully cures and forms a hard and durable coating film on the workpiece surface.

In summary, the paint coating production line for heavy workpieces involves a series of meticulous processes from pretreatment to painting and finally to drying and curing to ensure high-quality paint coatings on the workpieces.Paint coating line for heavy workpieces.