>> What is Powder Coating?
Powder coating is a commonly used surface finishing technique. It is a type of coating that is applied as a free-flowing, dry powder.
The powder coating process involves applying a finely ground resin (powder) to a substrate and subjecting this powder to heat. During the heating process, the powder melts and creates a uniform, continuous coating.
The final cured powder coating is the same as a 2-pack wet paint. The main difference between a powder coating and conventional wet paint is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form.
>> Two Types of Powder Coating
There are two types of powder available on the market.
1. Thermoplastic powder:
A thermoplastic powder coating is one that melts and flows with the application of heat, but maintains the same chemical composition when it solidified on cooling. Thermoplastic powders will remelt when heated.
Thermoplastic powders do not chemically react during application or curing, nor do they cross-link. Therefore, they can be reheated, enabling an entire coating to be reflowed. This is a useful property allowing minor flaws to be touched-up.
2. Thermosetting powders:
Most powder-coating materials are thermosetting powders. Thermosetting powders will not remelt when heated. The thermosetting powders incorporate a cross-linker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder polymer and increases the molecular weight and improves the performance properties.
The solid resins melt and flow chemically, and cross-link within themselves or with other reactive components forming a higher molecular-weight reaction product. The coating film formed by this reaction is heat stable and will not soften back to a liquid on further exposure to heat.
The types of resins commonly used in thermosetting powder include:
· several types of epoxies
· hydroxyl and carboxyl types of polyesters
· several types of acrylics
· several types of silicones
>> Brands of Powder Coat
Several brands of powder coat are available including Morton, Tiger Drylac, Spraylat, HB Fuller, Cardinal, O'brien, etc.
>> The Benefits of Powder Coating
Powder coating materials are shipped ready to use and are easy to apply, thus labor costs associated with training, setup, and processing are low when compared with liquid coating processes.
Although equipment and materials costs are similar in powder coating and liquid coating processes, powder coating process provides a number of advantages over other surface coating methods.
· Powder coatings provide excellent corrosion, impact, and abrasion resistance
· Powder coatings emit no or extremely low volatile organic compounds
· powder coatings produce much fewer rejects
· Powder coatings produce much less hazardous waste than liquid coatings
· Less floor space are required with powder coating line
· Powder coatings produce much thicker coatings without running or sagging
· Powder coatings have more uniform finishes
· A wide variety of special effects can be achieved with powder coatings which would be very hard or impossible to make with liquid coating process
>> Applications of Powder Coating
Manufacturers use powder coating processes in a wide variety of applications as they are versatile and present savings in labor, materials, and energy cost. Powder coats are also very durable.
Powder coatings are now used in hundreds of applications. One of the biggest powder-coating users is the appliance industry. The high-quality finish is both attractive and durable, and a viable alternative for porcelain enamel and liquid finishes on traditional appliance surfaces.
The automotive industry represents approximately an additional 15% of the North American powder coating market. Wheels, bumpers, roof racks, door handles, interior panels, and various "under-the-hood" parts are being powder coated.
The architectural and building market uses powder coating on file cabinets, shelving, aluminum extrusions for window frames, door frames, and modular office furniture.
>> The Powder Coating Process
Electrostatic spraying is the most widely used method of applying powder-coating materials. This process is the most efficient and effective means of applying powder coatings.
Five basic pieces of equipment are needed for the electrostatic-spray process.
1. Powder feeder unit
2. Electrostatic powder applicator
3. Control module for powder applicator
4. Powder spray booth
5. Powder recovery unit
The powder coating process involves three basic steps:
1. Part preparation or part pre-treatment
2. Powder application
3. Powder Curing
> Part Preparation or part pre-treatment process:
The part must be cleaned of oil, soil, lubrication greases, metal oxides, welding scales, etc before it can be powder coated. A variety of chemical and mechanical methods are used to do the cleaning job. The selection of the method depends on the size and material of the part to be powder coated, or the type of soil to be removed and the performance requirement of the finished product.
Chemical pre-treatment involves the use of phosphates or chromates in a submersion or spray application. These often occur in multiple stages and consist of degreasing, etching, de-smutting, various rinses and the final phosphating or chromating of the substrate. The pre-treatment both cleans and improves bonding of the powder to the metal.
> Powder Application Process
The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun. The gun imparts a positive electric charge on the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated toward the workpiece by the powerful electrostatic charge.
> Powder Curing Process
When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form higher molecular weight polymer in a network-like structure. This cure process requires a certain degree of temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders cure at 200°C (390°F) in 10 minutes.
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