Powder coating can be done on large scale in an industrial shop or in a garage by the weekend project warrior. Obviously the needs of the professional powder coating shop will differ greatly from the needs of the weekend warrior, but the overall process and workflow of powder coating remains the same for both.
Powder Coating Equipment
To begin powder coating you will need:
- Electrostatic powder coating gun
- Powder coating of your choice
- Spray booth capable of collecting overspray powder (preferably with attached vacuum system)
- An electric or gas powered oven to cure your work piece
- Personal protective equipment (mask, eyewear…etc)
- Supply of pressurized air for powder gun and clean-up
This is a very basic list of must have requirements to get anyone started powder coating.
Selecting Your Powder Paint
Choosing the right powder coating for your piece is more complicated than simply choosing a color. With most reputable powder suppliers there are hundreds of regularly stocked colors to choose from in a variety of gloss levels and textures. Common textures include smooth, fine texture (like sandpaper) and rough textures (wavy like exaggerated orange peel)- there exists a great deal a variation within these classes too! Gloss is also a very important criteria with gloss ranging from flat matte to glossy and high gloss products. Some combinations are difficult if not impossible to come by such as a high gloss fine texture product- the irregularities in the coating surface makes high gloss impossible. Additionally some color options become less feasible and more expensive at various gloss level.
Another good question to ask yourself is what type of exposure is your powder going to face. Will your powder be exposed to seawater, sunlight or electrical current? Knowing the conditions your powder needs to endure will help guide you to the correct type of powder chemistry. The variety of polyester powder coatings out there affords a wide variety of mechanical, chemical, and weathering properties in a coating. Some polyester coatings are classified according to AAMA (American Architectural Manufacturers Association) standards which specify various levels of UV, corrosion and chemical resistance. There are many different powder coating chemistries to choose from and for a more thorough and complete overview of the types available and their properties click here.
Cleaning and Preparing Your Substrate
The best powder in the world will be worthless if the part you spray is fouled with dirt, scale or oils. Pretreatment is imperative to achieve a good coating. Pretreatment can range from a simple wipe with a rag to a multi-stage physical and chemical washing with a pretreatment coating to improve substrate adhesion. At the very least make sure that your part is clean. For additional information on various pretreatment protocols see here.
Grounding and Spraying your Substrate
After cleaning your part and pretreating as necessary it is time to spray you part with your powder. Things to consider when spraying a part with powder coating:
- Spraying powder is very different than spraying liquid paint. Spraying liquid paint is somewhat of an art form that takes many years to develop, spraying powder does not. Powder is very forgiving to spray but if you spray like you would spray a liquid paint you will encounter troubles. Powder paint is best sprayed in one slow continues movement and not in many smaller passes as is common with liquid paint. This is because powder can easily achieve the desired thickness with one pass whereas liquid paint often requires multiple passes to achieve the same thickness while avoiding sag in the final surface.
- Trigger the gun off the part. Even with newer model powder guns it is highly advised to begin spraying your powder when the nozzle of the gun is pointed away from your part. This is due to the initial burst of air often times will carry with is small clumps of powder that have either become lodged within the gun or at the nozzle tip. Clumps hitting your part will lead to an inconsistent and poor finish.
- Adjust your settings. Try using 70kV/50uA on your gun for flat pieces, and adjust if necessary. Powder coating guns made today can spray far more powder than they can efficiently charge so adjusting your airflow to a reasonable setting is also advisable. All guns are different and even the same model of gun can spray differently at the same settings depending upon its installation. Airflow and voltage settings are best decided by the powder coater after various trials, but the old adage “less is more” is often times the best advice for spraying panels.
- Spray trouble areas first. Faraday cages and deep recessed areas should always be sprayed first. Faraday cage areas should be sprayed at a lower voltage than flat areas.
- Use slow methodical strokes of the gun to powder coat. Liquid painters use multiple thin strokes to avoid runs while building film thickness but powder coating does not require this. Powder paint can be applied in a single pass while achieving desired film thickness (2-4 mils). Slower strokes of the powder gun also increase the charging efficiency of the powder leading to better powder pattern. Using slow methodical passes will powder coat your part quicker and in fewer strokes with less distortion.
- Spray 6-8 inches from substrate. Spraying too close or too far from the work piece can adversely affect the corona of the gun and prevent even powder application. Some powder coaters will finish their piece by “dusting” their piece with a cloud of powder sprayed from a far distance in the belief that such practices can lead to even out any irregularities in the coating- this practice is not recommended.
- Ground your part. This cannot be more plain, but also is frequently overlooked and neglected by large powder coating shops. Improper grounding means improper adhesion of powder to your material….and leads to costly overspray. Keep hooks and hangers clean, develop a cleaning schedule for your hanging system. In a pinch a metal coat hanger can be quickly cut to fashion simple hooks that works as excellent grounding hooks.
- Spraying you substrate to desired thickness. This is often easier said than done, but some tricks exist to help you get it right the first time. First trick is experience. Working with the same powder on similar substrates will give you an intuitive feel for time on target and powder appearance. Checking your part for bare substrate is also the most effective indicator of thin film build. Having a bright light handy will make seeing thinly coated spots much easier if your substrate is lustrous. Looking for the light gleaming off your substrate will make thin areas obvious. Working with dark colored substrates that do not reflect as much light can be more problematic and spraying test pieces might be the best approach if the light trick does not work.
This should go without saying but cure your piece according to the technical data sheet provided with your powder. When the manufacturer specifies a time and temperature they mean time that the substrate is at the specified temperature. This is an important point that many people overlook so an example might be helpful. Say your powder has a curing profile of 10 minutes at 200°C and you spray two different parts and cure in the same oven for 10 minutes: one part is a 3”x6” piece of sheet metal about 1/8” thick and the other piece is 5/8” thick piece of steel and you cure both pieces in the same oven for the same 10 minutes at the same time. In all likelihood DSC experiments would show that the 1/8” panel’s powder is fully cured whereas the 5/8” panel is not fully cured. Why? The manufacturer specifies a temperature that the substrate must be at in order for the powder to cure properly, so the 1/8” steel panel will rapidly reach the desired 200°C but the much thicker 5/8” piece will take much longer to reach 200°C and thereby spend less time at this temperature. Two methods to overcome this problem are preheating your piece to get to temperature quicker, and adding additional time to reach cure temperature.
Trouble Shooting Your Finish
After every cure you should always check film thickness, gloss and color of your finished substrate. Film thickness is important since values that deviate from the manufacturer’s specifications can mean that the resulting coverage won’t have the corrosion resistance, hiding or many other physical properties that it would have at the proper film thickness.
Checking the gloss of your substrate is also a good idea. Not only does gloss has a dramatic effect on the visual appearance of your finished coating, large deviations in gloss from the powder’s TDS may suggest an error in either film thickness or curing. Remember that for any given color a higher gloss finish will appear darker than the same color with a lower gloss. For most gloss ranges and applications the standard is to measure gloss at 60°.
If you are matching an existing color it is best to have a standard template by which you can compare the color. When visually comparing colors it is advisable to first determine the lightness and darkness of your substrate vs your target. Then compare whether your specimen is either too blue or yellow and too red or green. Due to the receptor cells in our eyes our perception of color can be broken down into these three parameters: lightness/darkness, yellow or blue, green or red. This is obviously a simplification of visual perception but it is how professionals make visual matches. If you find yourself in a well equipped color lab try using the spectrophotometer to get exact color coordinates.
Another common problem that is commonly encountered with powder coating is “orange peel”. Orange peel refers to a wavy like finish on the coating that resembles that of an orange. This orange peel effect has varying degrees often denoted on a scale of 1 to 10 (see ACT Orange Peel Standards). There are a variety of factors that can contribute to orange peel effect including: powder quality, cure heat ramping, and powder thickness. Powder coatings are typically designed to minimize any orange peel effect at the recommended specifications so make sure you are using quality powder that has a standard that shows no orange peel. Additionally make sure you powder has been stored properly and doesn’t clump in the gun (a sign of moisture). If your powder is fine ensure that your film thickness is to specification, spraying too thick will dramatically increase the likelihood of orange peel, and even too thin.