Powder & Paint
The Powder Coating Process
The powder coating process involves three basic steps:
- Part preparation or the Pre treatment
- The powder application
Powder coating developed in the 1950’s as 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.”
There are two main types of powder available to the surface finisher:
- Thermoplastic powders that will remelt when heated, and
- Thermosetting powders that will not remelt upon reheating. During the curing process (in the oven) a chemical cross-linking reaction is triggered at the curing temperature and it is this chemical reaction which gives the powder coating many of its desirable properties.
The most common polymers used are polyester, polyester-epoxy (known as hybrid), straight epoxy (Fusion bonded epoxy) and acrylics.
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 “whiteware”, 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.
The polymer granules are mixed with hardener, pigments and other powder ingredients in a mixer The mixture is heated in an extruder The extruded mixture is rolled flat, cooled and broken into small chips The chips are milled to make a fine powder The powder Coating Process The powder coating process involves three basic steps: Part preparation or the Pre treatment The powder application Curing
Part Preparation Processes & Equipment
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.
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.
Silicon carbide grit blast media is brittle, sharp, and suitable for grinding metals and low-tensile strength, non-metallic materials. Plastic media blast equipment uses plastic abrasives that are sensitive to substrates such as aluminum, but still suitable for de-coating and surface finishing. Sand blast media uses high-purity crystals that have low-metal content. Glass bead blast media contains glass beads of various sizes.
Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting recycles the media and is environmentally friendly. This method of preparation is highly efficient on steel parts such as I-beams, angles, pipes, tubes and large fabricated pieces.
Different powder coating applications can require alternative methods of preparation such as abrasive blasting prior to coating. The online consumer market typically offers media blasting services coupled with their coating services at additional costs.
Powder Application Processes
The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun, or Corona gun.
Before the powder is sent to the gun it is fluidised:
- To separate the individual grains of powder and so improve the electrostatic charge that can be applied to the powder.
- And so that the powder flows more easily to the 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.
Because the powder particles are electrostatically charged, the powder wraps around to the back of the part as it passes by towards the air off-take system. By collecting the powder, which passes by the job, and filtering it, the efficiency of the process can be increased to 95% material usage.
The powder will remain attached to the part as long as some of the electrostatic charge remains on the powder. To obtain the final solid, tough, abrasion resistant coating the powder coated items are placed in an oven and heated to temperatures that range from 160 to 210 degrees C (depending on the powder).
Under the influence of heat a thermosetting powder goes through 4 stages to full cure.
MELT, FLOW, GEL, CURE
The final coating is continuous and will vary from high gloss to flat matt depending on the design of the powder by the supplier.
Powder can also be applied using specifically adapted electrostatic discs.
Another method of applying powder coating, called the Fluidized Bed method, is by heating the substrate and then dipping it into an aerated, powder-filled bed. The powder sticks and melts to the hot object. Further heating is usually required to finish curing the coating. This method is generally used when the desired thickness of coating is to exceed 300 micrometres. This is how most dishwasher racks are coated.
Electrostatic Fluidized Bed Coating: Electrostatic fluidized bed application uses the same fluidizing technique and the conventional fluidized bed dip process but with much less powder depth in the bed. An electrostatic charging media is placed inside the bed so that the powder material becomes charged as the fluidizing air lifts it up. Charged particles of powder move upward and form a cloud of charged powder above the fluid bed. When a grounded part is passed through the charged cloud the particles will be attracted to its surface. The parts are not preheated as they are for the conventional fluidized bed dip process.
Electrostatic magnetic Brush (EMB) Coating: an innovative coating method for flat materials that applies powder coating with roller technique, enabling relative high speeds and a very accurate layer thickness between 5 and 100 micrometre. The base for this process is conventional copier technology . Currently in use in some high- tech coating applications and very promising for commercial powder coating on flat substrates ( steel, Aluminium, MDF, paper, board) as well in sheet to sheet and/or roll to roll processes. This process can potentially be integrated in any existing coating line.
When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This cure process, called crosslinking, 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. The curing schedule could vary according to the manufacturer’s specifications.
The application of energy to the product to be cured can be accomplished by convection cure ovens or infrared cure ovens.
Specifications Powder coating produces a high specification coating which is relatively hard, abrasion resistant (depending on the specification) and tough. Thin powder coatings can be bent but this is not recommended for exterior applications.
The choice of colours and finishes is almost limitless, if you have the time and money to have the powder produced by the powder manufacturer.
Powder coatings can be applied over a wide range of thickness. By way of example a 25 micron minimum for mild interior applications and up to 60 micron minimum for exterior applications might be applied. However, care must be exercised when quoting minimum thickness because some powder will not give “coverage” below 60 or even 80 micron. “Coverage” is the ability to cover the colour of the metal with the powder. Some of the white colours require about 75 micron to give full “coverage”. One of the orange colours must be applied at 80 micron.
Colour matching is quite acceptable batch to batch.
The term “Powder Coated Finish” has in some ways become a selling point for products that feature its durability and perceived attractiveness. Since its development in the 1950s, powder coating has remained a dynamic and constantly-changing procedure. In both Europe and North America, independent companies are now producing wood grain finishes with powder coating that can appear as good as any veneer. Rough texture powder coats that can be used to hide surface imperfections and can save finishing steps on metal are also available. When powder coating is used in abrasive or high wear conditions, the color will typically remain brilliant as the surface wears away.
As a finish, there are some downsides to powder coating. The color in powder coat is determined by the plastics ground down to make the coating, and is done in minimum size consignments by the manufacturer. When custom colors are needed for coating small batches of parts, this may not prove to be cost effective. This will also affect lead-time in getting the custom colors from the manufacturer, which can take weeks.
Installations and Maintenance
During installations, the powder coating should be protected from damage due to abrasion and materials of construction such as mortar and brick cleaning chemicals. Once installed, maintaining the initial appearance of a powder coating is a simple matter. The soot and grime which builds up on surfaces from time to time contains moisture and salts which will adversely affect the powder coating and must be removed. Powder coatings should be washed down regularly (at least once each 6 months in less severe applications and more often in marine and industrial environments). The coating should be washed down with soapy water — use a neutral detergent — and rinsed off with clean water.
When powder coated items are installed without damage to the powder coating and they are maintained regularly, they should be relatively permanent. The correctly applied coating, although not metallurgically bonded to the metal will not crack, chip or peel as with conventional paint films. Advantages and disadvantages of powder coating .
There are several advantages of powder coating over conventional liquid coatings:
- 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 limitations 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 TG (glass transition temperature) of the powder.
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.