Powder coating is a process of applying a lacquer coating to protect and decorate a metal surface such as aluminium extrusions. Dry thermo setting powder pigment is sprayed and attaches itself to the metal by means of electro-static force. The powder is then cured in a baking oven where it melts and spreads on the surface to form an even coat before hardening to a final coating, which is tough and resilient. The powders come in a wide range of colours. The powder manufacturer provides a comprehensive 15-year guarantee against deterioration provided that the controls, consistency and quality of the treatment are satisfactory.
The actual spraying of the powder is done in a powder spraying booth where the extrusions to be sprayed are hung on a conveyor system to facilitate easy transfer to the curing oven after powder is applied. The powder is forced out of an electrostatic spraying gun directly onto the surface to be coated. The powder is imparted a negative electrostatic charge by the gun. Meanwhile the extrusions have a positive charge that makes the powder attach itself to the metal. The electrostatic force causes the powder to adhere to the metal surface until it is later melted in the oven.
Key in the coating process is that the aluminium extrusions go through a pre-treatment process before spraying on powder. The main stages are:
The degreasing step using a detergent to remove oil, grease and other surface contaminations from the extrusion surface to obtain a clean surface. Another stage is etching done using Alu-Etch to remove the thin natural oxide coating on the aluminium surface. The smut which occurs after etching is removed by dipping material into a solution of nitric acid. To enhance the adhesion between the metal surface and the lacquer coating and also improve the corrosion resistance of the lacquered product a stage called chromate conversion is passed by dipping the extrusions into a solution of chromic acid. After chromate conversion the material is dipped in de-ironised water to avoid adhesion and corrosion problems. In this case the conductivity of the water should not exceed 100µs/cm. The extrusions are rinsed in water to remove chemicals from the surface. Prior to powder coating the extrusions have to be completely dry since any moisture on the surface will interfere with the later coating process. Drying is done by forced air from a fan.