What is disvantage of powder coating or drawback of powder coating.?

Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a "skin".

The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as household appliances, aluminium extrusions, and automobile and bicycle parts.

Newer technologies allow other materials, such as MDF (medium-density fibreboard), to be powder coated using different methods. Powder coatings emit zero or near zero volatile organic compounds (VOC). Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.

Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating. Powder coating production lines produce less hazardous waste than conventional liquid coatings. Capital equipment and operating costs for a powder line are generally less than for conventional liquid lines.

Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items. A wide range of specialty effects is easily accomplished which would be impossible to achieve with other coating processes. While powder coatings have many advantages over other coating processes, there are some disadvantages to the technology.

While it is relatively easy to apply thick coatings which have smooth, texture-free surfaces, it is not as easy to apply smooth thin films. As the film thickness is reduced, the film becomes more and more orange peeled in texture due to the particle size and glass transition temperature (TG) of the powder. On smaller jobs, the cost of powder coating will be higher than spray painting.

For optimum material handling and ease of application, most powder coatings have a particle size in the range of 30 to 50? M and a TG around 200°Ccitation needed. For such powder coatings, film build-ups of greater than 50?

M may be required to obtain an acceptably smooth film. The surface texture which is considered desirable or acceptable depends on the end product. Many manufacturers actually prefer to have a certain degree of orange peel since it helps to hide metal defects that have occurred during manufacture, and the resulting coating is less prone to showing fingerprints.

There are very specialized operations where powder coatings of less than 30 micrometres or with a TG below 40°C are used in order to produce smooth thin films. One variation of the dry powder coating process, the Powder Slurry process, combines the advantages of powder coatings and liquid coatings by dispersing very fine powders of 1–5 micrometre particle size into water, which then allows very smooth, low film thickness coatings to be produced. Powder coatings have a major advantage in that the overspray can be recycled.

However, if multiple colors are being sprayed in a single spray booth, this may limit the ability to recycle the overspray. There are two main categories of powder coatings: thermosets and thermoplastics. The thermosetting variety incorporates a cross-linker into the formulation.

When the powder is baked, it reacts with other chemical groups in the powder to polymerize, improving the performance properties. The thermoplastic variety does not undergo any additionactions during the baking process, but rather only flows out into the final coating. The most common polymers used are polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy) and acrylics.

Removal of oil, soil, lubrication greases, metal oxides, welding scales etc. is essential prior to the powder coating process. It can be done by a variety of chemical and mechanical methods. The selection of the method depends on the size and the material of the part to be powder coated, the type of soil to be removed and the performance requirement of the finished product.

Chemical pre-treatments involve the use of phosphates or chromates in 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 process both cleans and improves bonding of the powder to the metal.

Recent additional processes have been developed that avoid the use of chromates, as these can be toxic to the environment. Titanium zirconium and silanes offer similar performance against corrosion and adhesion of the powder. In many high end applications, the part is electrocoated following the pretreatment process, and subsequent to the powder coating application.

This has been particularly useful in automotive and other applications requiring high end performance characteristics. Another method of preparing the surface prior to coating is known as abrasive blasting or sandblasting and shot blasting. Blast media and blasting abrasives are used to provide surface texturing and preparation, etching, finishing, and degreasing for products made of wood, plastic, or glass.

The most important properties to consider are chemical composition and density; particle shape and size; and impact resistance.