First pioneered in the 1950s, powder coating has evolved as a popular dry finishing process used for functional (protective) and decorative finishes in manufacturing. Available in a wide range of colours and textures, powder coating is used on an increasing variety of products in nearly every major manufacturing industry today – from architecture and automotive to furniture and appliances.
Driven largely by the development of new materials, formulations and technological advancements, powder coating is fast becoming become the preferred finishing process over wet paint.
What is powder coating and how does it work?
To understand why powder coating’s more advantageous than wet paint, it’s important to first understand the process. Powder coating is the electrostatic application of organic powder to metal parts requiring protective or decorative coating. The powder is cured by heating or baking, resulting in a hard, continuous coating.
The powder coat process includes pre-treatment, powder application and high-temperature powder curing. The process begins with pre-treatment/surface preparation of the substrate. Each part is cleaned, removing grease, dirt and anything else that might interfere with the painting process. This may include abrasive/mechanical or chemical cleaning, though it usually consists simply of cleaning the metal surface and pre-treating it in some fashion ( e.g. phosphating) to prepare the surface to bond well with the powder during the curing process that follows as well as providing a degree of corrosion resistance. Without proper pre-treatment, the powder on a part will chip and corrode easily.
Following pre-treatment, the object must be completely dried before powder is applied. This can be accomplished using oven drying or air drying. If an oven is used for the drying process, the part usually must be cooled before the application of powder coating. Once completely dry, the part is ready to have powder applied using either spray techniques. The powder is electrically charged as it is applied to the part, giving each particle of the powder a negative charge. The part being powder coated is electrically grounded as a means of attracting and attaching the powder to the part’s surface. This electrostatic attraction is a key requirement of the process, aiding the coating evenness and the speed of applying the coating. The result is a uniform coating of dry powder clinging to the part. After the part is coated with powder, it is moved into a conveyorised curing oven. There the powder gels, flows and cures to produce a smooth, durable powder coat finish. During the curing process, a chemical reaction cross-linking between the particles of powder takes place. It is at this point that the part exits the oven, cooling in ambient air before being offloaded and inspected.
What are the advantages of powder coating over wet paint?
Wet painting is a traditional treatment method by which a pigment in any water-based or solvent-based paint is sprayed onto the surface of a part. Common applications of wet paint include automotive exteriors and extremely large or heavy items. When superior performance is required, powder coating as a finishing process can replace traditional wet painting.
Many concerns or issues associated with wet paint finishing are eliminated or minimized with the powder coating process. The advantages of powder coating over wet paint are many, and they generally fall under four categories:
Performance
Powder coatings are more durable and more resistant to corrosion, chemicals and weather than liquid coatings. Powder-coated surfaces are more resistant to chipping, scratching and other such wear due to the thermal bonding process during curing. Unlike wet paint, powders also don’t run or drip, providing a more uniform coating than liquid paint. The result of using powder coating is that the product will have the most attractive, durable, high- quality finish available.
Is powder coating expensive?
Powder coatings are cheaper and can cover more area than most other organic finishes, translating to lower material costs than wet painting processes. Because they lend themselves well to automation, powder coatings can also reduce costs related to operational labour, as reduced operator training and supervision are required for a powder line. Compared to wet paint processes, operational cost savings using powder coatings can also be realised in greater throughput, less waste produced, lower energy costs, reduced disposal costs and less rework due to lower reject rates.
Is powder coating safe?
Wet paints can be flammable, carcinogenic and full of volatile organic compounds (VOCs). These chemicals can be hazardous to plant/shop personnel if not handled properly. Powder coatings, on the other hand, are generally VOC-free materials and contain no solvents, meaning they do not involve the potentially harmful chemicals typically found in a wet spray project. Although powder coating materials can combust in a very narrow concentration of powder and air, their lack of solvents or VOCs means almost none of the short-term and long-term health or fire workplace threats associated with wet painting.
Is powder coating environmentally friendly?
VOCs in the wet painting process are also among industrial pollution concerns. Because powder coatings contain no solvents and emit negligible, if any, polluting VOCs into the atmosphere, they are more environmentally friendly than their wet paint counterparts. Moreover, powder coating is a clean process: powder overspray can be retrieved and reused, and unused powder can be reclaimed and returned to a hopper for recirculation through the system, with negligible waste. Plus, most powder coating materials are considered landfill/non-hazardous materials. Powder coatings meet all Environmental Protection Agency requirements for air and water pollution control.
When is wet painting preferred to powder coating?
Wet painting is ideal for products that cannot be exposed to the elevated temperatures required for powder coating. There are also air drying wet paints available that require no force drying in an oven. In addition much lower film thicknesses can be achieved with wet painting where reduced film thickness is important (e.g. automotive parts where a fastener applies a load pressure onto the coating). Wet painting is a much more economical finishing process for smaller batches where paint can be purchased in very low litre volumes , even when made to order, whereas powder coating is usually a minimum 20kg box purchase. Wet paints can also be more easily colour matched. In addition wet painting can create depths of field in the coatings such as a gloss black metallic whereby a combination of specialist primer, base coats and clear coats achieve the stunning colour finishes seen on premium automotive coatings.