The electronic door lock electrostatic coating line uses a pretreatment process to enhance the adhesion of the coating to the metal substrate. The first step is to thoroughly remove oil, dirt, and impurities from the substrate surface. During processing, the metal substrate of the electronic door lock is prone to residual substances such as cutting oil, stamping oil, and dust. These contaminants can form a barrier between the substrate and the coating, preventing the coating from adhering tightly. The pretreatment stage typically utilizes alkaline degreasing or environmentally friendly solvent degreasing, chemically dissolving the oil. Spraying or immersion is also used to ensure that the oil and dirt are fully removed from the substrate surface. After degreasing, multiple streams of running water are used to thoroughly rinse away any residual degreaser and oil, preventing any residual substances from affecting coating adhesion in subsequent processes.
The pretreatment process specifically removes oxide layers and rust on the metal substrate surface, creating a clean metal surface for coating adhesion. During storage or processing, oxide scale and light rust easily form on the metal substrate surface. These substances have a loose structure and cannot effectively bond with the coating. For steel substrates, pickling is often used to dissolve the oxide layer through a chemical reaction between the acidic solution and the metal. For more reactive metals like zinc alloys, a mild chemical polishing process is used to gently etch the surface to remove the oxide layer, leaving the substrate with a uniform metallic color and preventing blistering and peeling of the coating caused by the oxide layer.
Phosphating is a key step in the pretreatment process to enhance coating adhesion, forming a functional conversion film on the metal surface. This process involves immersing the substrate in a phosphating solution, which chemically reacts with the substrate surface to form a dense phosphate crystalline film. This film has a unique microscopic roughness that increases the substrate's surface area and provides an "anchoring effect" for the coating—allowing the coating to penetrate the film's tiny pores and, after curing, form a bond similar to a mechanical snap. Furthermore, the phosphate film's strong chemical stability isolates the substrate from the external environment, preventing secondary oxidation and ensuring a long-term bond between the coating and the substrate.
Passivation, a supplementary step after phosphating, further optimizes the substrate's surface condition and enhances coating adhesion reliability. Phosphate coatings have tiny pores on their surface. If the coating is applied directly, moisture or impurities from the air may penetrate these pores, affecting the adhesion between the coating and the phosphate coating. Passivation treatment involves immersing the substrate in an environmentally friendly passivating agent to form an extremely thin passivation film on the phosphate coating surface. This seals the pores and enhances the film's corrosion resistance and surface activity. This passivation film provides excellent compatibility with the phosphate coating and the coating, reducing adhesion issues caused by phosphate coating defects. It is particularly suitable for products such as electronic door locks, which require high coating durability.
While the surface conditioning process does not directly form a functional coating, it can enhance the subsequent phosphate treatment by optimizing the substrate's surface condition. Surface conditioning is typically performed after degreasing and before phosphate treatment. The substrate is immersed in a specialized surface conditioner to form uniform crystal nuclei on the surface. These crystal nuclei guide the subsequent phosphate crystallization process to smaller and denser sizes. Without surface conditioning, the phosphate coating may exhibit coarse and uneven crystal distribution, reducing the contact area between the coating and the substrate. Surface conditioning, however, creates a more regular structure, allowing for more complete contact between the coating and the substrate, significantly improving adhesion.
Proper control of washing and drying is crucial to preventing the ineffectiveness of pretreatment. Washing is required after each pretreatment step, adhering to the "countercurrent rinsing" principle. Residuals from the previous step are removed using overflow water from the subsequent wash, ensuring thorough cleaning while conserving water resources. Incomplete rinsing can cause residual chemicals to form spots on the substrate surface or react chemically with the coating, resulting in craters and pinholes. Hot air drying is necessary after rinsing to ensure the substrate surface is completely dry and free of any water marks. Moisture on the substrate surface can affect the atomization and curing of the coating and may also form bubbles within the coating, disrupting the bond between the coating and the substrate.
Parameter control and quality inspection of the pre-treatment process are the last line of defense to ensure that the coating adhesion is stable and meets the standards. The pre-treatment process of the electronic door lock electrostatic coating line mostly uses an automated control system to monitor the concentration, temperature, pH value and other parameters of the bath solution in real time, and make immediate adjustments when deviations occur to avoid manual operation errors. At the same time, samples need to be taken for testing from each batch of substrates: confirm by observation that the surface is free of oil and oxide layer; check under a microscope whether the phosphate film crystals are dense and uniform; and preliminarily test the coating adhesion strength through methods such as cross-cutting. Only substrates with qualified pre-treatment effects can enter the subsequent electrostatic coating process to ensure that the final coating is tightly bonded to the metal substrate to meet the long-term use requirements of electronic door locks.
