As a core load-bearing component of automobiles, aluminum wheels not only need sufficient mechanical strength to support the weight of the vehicle body, but also excellent toughness to withstand impacts during driving. In the field of aluminum wheel coating, the application of low-temperature curing powder coatings is gradually becoming an industry trend—it can not only further improve the toughness of the wheel, but also significantly reduce energy consumption in the production process, perfectly meeting the current industrial development needs of energy conservation and environmental protection. Today, we will comprehensively analyze the key points of Low-Cure Powder Coating on aluminum wheels from the dimensions of coating process, film formation mechanism, low-temperature achievement principle, and surface defect control.

1. Basic Process of Powder Coating for Aluminum Wheels

Whether it is conventional powder coating or low-temperature curing powder coating, the basic coating process for aluminum wheels must follow a rigorous pretreatment + spraying logic to ensure a strong bond between the coating and the substrate. The specific process is as follows: First, the aluminum wheel hub to be coated is hung on a line and undergoes multiple cleaning processes in sequence—alkaline washing to remove surface oil stains, followed by water washing for neutralization; then acid washing to peel off the surface oxide film, followed by deionized water washing; after that, passivation treatment is carried out to improve the substrate's oxidation resistance, and after completion, deionized water washing is performed again; finally, the surface moisture of the wheel hub is dried, and it is transferred to the bottom line for base powder spraying. Each pretreatment step directly affects the adhesion and durability of the subsequent coating, and is the basic guarantee for coating quality.

2. Breakdown of the Powder Coating Film Formation Process

After the powder coating is sprayed onto the surface of the aluminum wheel hub, it does not directly cure into a film, but undergoes four consecutive physicochemical changes to ultimately form a smooth and dense coating:

The first step is the softening process. After absorbing a certain amount of heat, the internal resin components of the powder particles gradually become sticky and soften, preparing for subsequent melting. The second step is the melting process. When the absorbed heat reaches a threshold, the active chemical bonds in the resin rapidly open, and the originally dispersed powder particles quickly melt into a liquid state, uniformly covering the wheel hub surface. This stage is extremely short. The third step is the leveling process. The molten coating slowly flows from the initially uneven surface to form a smooth and flat liquid film. Finally, after curing and cooling, a stable solid coating is formed, completing the entire film-forming process.

3. The Principle of Low-Temperature Curing Process

Conventional powder coatings undergo three core steps from application to curing: melting, leveling, and curing. First, the powder particles agglomerate to form an irregular film layer (melting stage); then, through flow, they transform into a smooth and flat liquid film (leveling stage); finally, after being baked in a furnace, the liquid coating undergoes a chemical reaction, its viscosity gradually increases, and after cooling, it solidifies into a solid coating film (curing stage). The core of low-temperature curing is to enable this series of processes to be completed smoothly at lower temperatures through technological improvements.

Currently, there are three main approaches to achieving low-temperature curing of aluminum wheel powder coatings, and these three methods must work synergistically:

First, resin improvement. By adjusting the type, quantity, distribution, and relative molecular mass of the resin's functional groups, the resin's melt viscosity and softening point can be reduced, allowing it to soften and melt smoothly at lower temperatures.

Second, curing agent improvement. Optimizing the functional group characteristics and relative molecular mass of the curing agent improves its compatibility with the resin, ensuring rapid cross-linking reaction with the resin at low temperatures.

Third, the rational application of curing accelerators. Selecting suitable accelerator types based on system characteristics and precisely controlling the addition amount accelerates the curing reaction process.

It is worth noting that low-temperature curing primers for aluminum wheels are mostly hybrid systems: they require the synthesis of saturated carboxyl-terminated polyester resins with suitable reactivity and viscosity, combined with bisphenol A type epoxy resins with suitable softening points and epoxy values. These two act as curing agents to form the film-forming matrix, and various functional additives are then added to achieve a balance between low-temperature curing and coating performance.

4. Surface Tension Issues and Solutions During Film Formation

Surface tension is a key factor affecting the film quality of powder coatings. Surface tension refers to the force acting on a liquid surface that causes the liquid's surface area to decrease. It arises from the sparse distribution of molecules in the surface layer where the liquid and gas come into contact, with intermolecular forces manifesting as attraction. The surface energy of molten powder coating particles is similar to that of ordinary liquids; therefore, changes in surface tension directly lead to common defects such as orange peel, pinholes, and craters. The causes and solutions for these three types of defects are analyzed below:

4.1 Orange Peel Defect: Surface Unevenness Caused by Eddy Effect

The essence of orange peel defects is the localized eddy effect generated by the melting and flow of powder coatings during film formation. Because the viscosity change during powder melting leads to a change in surface tension, the high-viscosity, low-surface-tension coating sinks to the center of the eddy, while the low-viscosity, high-surface-tension coating rises to the periphery of the eddy, ultimately forming an uneven texture similar to orange peel. The following measures can be taken to improve the situation:

4.1.1 Standardize spraying and baking parameters: The thickness of a single coating should ideally be controlled between 60-80 μm. Excessive thickness or rapid heating during baking will exacerbate orange peel effects. Appropriately extend the melt-leveling time to allow sufficient space for the coating to flow and level.

4.1.2 Adjust powder melt viscosity: Appropriately increasing the viscosity of the powder coating during melting can increase flow resistance and reduce eddying.

4.1.3 Precisely select leveling agents: High-quality leveling agents should possess both wetting and leveling effects—wetting is dominant at around 100℃, requiring a low surface tension; above 150℃, leveling is dominant, requiring an appropriately increased surface tension. Therefore, leveling agents are usually formulated from two or more materials.

4.2 Pinhole Defect: A Sinking Problem Caused by Low Surface Tension Points

Pinholes are a special defect caused by low surface tension points on the coating surface. Electron microscopy reveals that most pinholes are vortices formed by insufficiently wetted particles and surrounding incompatible resin. A small protruding point is often found at the center of larger sinking areas. Three main countermeasures are: first, strictly control the cleanliness of the processing environment to prevent impurities from introducing low surface tension points; second, select suitable wetting agents to improve the coating's wetting and dispersing ability for impurity particles; and third, appropriately increase the viscosity of the powder coating to suppress abnormal flow of the coating liquid.

4.3 Pinhole Defect: Penetrating Voids Formed by Unescaped Bubbles

Pinholes are penetrating voids formed when gases from the bottom layer of the powder fail to escape after passing through the nearly sealed elastic resin layer during the melting and solidification process. These gases mainly originate from low-molecular-weight volatiles in the raw materials or reaction products during the curing process of the powder coating. They form bubble clusters in the molten coating film, and if they cannot escape in time, pinholes will form. Improvement measures need to focus on source control: on the one hand, strictly control the surface treatment quality of the wheel hub to ensure that there are no small molecules such as stains or spots adhering to the surface; on the other hand, standardize the spraying process, the air compressor needs to be drained regularly to remove moisture, and the coating thickness during electrostatic spraying should not exceed 100μm to leave a channel for gas escape.

5. Conclusion

Practice has proven that the application of Low-Cure Powder Coating on aluminum wheels can fully meet industry standards and has significant advantages: it can reduce energy consumption in the production process, aligning with the industrial orientation of energy conservation and environmental protection, and can also improve the toughness and impact resistance of aluminum wheels, further ensuring driving safety.

However, there is still room for improvement in Low-Cure Powder Coating technology. For example, issues such as the balance between coating leveling and durability at low temperatures and performance stability under special working conditions still require continuous innovation and research from industry researchers. In the future, with breakthroughs in core technologies such as resin synthesis and curing system optimization, Low-Cure Powder Coating will surely achieve more perfect applications in the field of automotive aluminum wheel coating, driving the industry towards a lower carbon footprint, higher efficiency, and higher quality.