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 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 shared choice for ceramics, plastics and already wood.
Research shows that powder coating is the fastest growing coating medium and with the environmental advantages coupled with its excellent finishing similarities, 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 similarities.
Thermoplastic types don’t change specifically nor have any additional responses, it simply flows out into the final coating.
Powder coating course of action
Stage 1 – Pre treatment
This is about preparing the part or part, and as any painting application, preparation is all important to unprotected to the best possible finish.
It is basic 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 include 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 ecosystem.
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 appropriate 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 part by the powerful electrostatic charge.
The part is heated, and the powder melts into a uniform film, 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 unprotected to a more uniform finish but can also create other problems, such as runs caused by excess powder.
Powder can also be applied using specifically alternation electrostatic discs.
Another method, known as the Fluidised Bed method, involves heating the substrate and then dipping it into an aerated, powder-filled bed.
The powder sticks and melts to the hot object, with further heating required to finish curing the coating. This method is generally used when the coating exceeds 300 micros.
Electrostatic Fluidised Bed Coating: Electrostatic fluidised bed application uses the same fluidising techniques as above but with much less powder thoroughness in the bed. Electrostatic charging occurs in the bed so that the powder becomes charged as the fluidising air lifts it up. Charged powder particles form a cloud of charged powder above the fluid bed. When a grounded part is passed by the charged cloud the particles will be attracted to its surface. The parts are not preheated.
Electrostatic Magnetic Brush (EMB) coating is a coating method for flat materials that applies powder coating with roller technique.
Stage 3 – Curing
When thermoset powders are exposed to high increases in temperature, (usually via a convection or infrared cure oven), they start to melt, flow out, and then react to form a higher molecular weight polymer. This cure course of action, called cross linking, requires a certain degree of temperature for a certain length of time in order to reach complete cure and establish the complete film similarities for which the material was designed.
What are the disadvantages of Powder Coating
- Very thin coatings can show pinholes
- Very frequent colour changes can be time consuming
- Inside corners can have low film thickness
- Can be tricky on sharp corners
- Needs skill to continue uniformity of thickness
- Colour matching and uniformity can be harder than with liquid paints
What are the advantages of Powder Coating?
- Environmentally friendly – no solvents required
- Finishes are tough and flexible making it harder to chip or crack
- Generally gives a 100% substantial coating
- Almost no waste produced
- enormous range of colours and finishes
- Rust free
- Can be applied over a wide range of thicknesses
- Can produce thick coatings without running or sagging
- Special effects are easily achieved
- Fast turnaround times
- Protection against external UV fading
- Exceptional colour retention
- Excellent electrical insulation capabilities
- Resistant to most chemicals and solvents
Powder Coating; whilst being an economical system; not only completes the desired aesthetic qualities of the product, but also offers excellent durability and resistance to scratches and chemical corrosion, already in the harshest environments.
By using this system, powders are closest ready for use and do not require mixing, which eliminates variables in finish, reduces processing time and results in superior film similarities.