Generally powder coating may be regarded as an environmentally favoured method of applying a finish particularly as it avoids the use of solvent-based paints avoids overspray film thickness wastage, and any unused powder may be fully recycled and used again.
Although powder coating was conceived as a method of finishing metal, technology has evolved so that it is now a common choice for ceramics, plastics and even wood.
Research shows that powder coating is the fastest growing coating medium and with the environmental advantages coupled with its excellent finishing properties, it is a trend that is likely to continue.
Types of powder coating
There are two main types of powder coatings; thermosets and thermoplastics
With thermosetting variations, as the powder bakes, it reacts with chemicals in the powder polymer which increases molecular weight; improving the performance properties.
Thermoplastic types don't change specifically nor have any additional reactions, it simply flows out into the final coating.
Powder coating process
Stage 1 - Pre treatment
This is about preparing the component or part, and as any painting application, preparation is all important to achieve the best possible finish.
It is essential to remove oils and lubricants and metal oxides and this is performed usually by a variety of chemical and mechanical procedures, dependent also upon the material, size, and finish required.
The multiple stage chemical pre-treatments usually involve using phosphates or chromates in submersion or by spraying.
From an environmental perspective those offering phosphate preparations are my preferred option as chromates can be toxic to the environment.
Another method of preparation is sandblasting and shot-blasting, whereby blasting abrasives are used to give surface texture and preparation for wood, plastic or glass.
Silicone carbide is suitable for grinding metals and plastic media blasting uses plastic abrasives that are sensitive to substrates such as aluminium.
Stage2 - The powder application
The most used method is electrostatic spraying via a spray gun.
The object is grounded and the gun imparts a positive electric charge onto the powder which is then sprayed and accelerated toward the component by the powerful electrostatic charge.
The component is heated, and the powder melts into a uniform film film thickness, and cooled to form a hard coating. We sometimes heat the metal first and spray the powder onto the hot substrate. Preheating can help to achieve a more uniform finish but can also create other problems, such as runs caused by excess powder.
tag : film thickness
