What Coaters Need To Know About Powder Coating Particle Size
When troubleshooting your powder coating, you may find specific issues caused by the powder coating particle size. Unfortunately, while a lot of information is available on this subject, most of it is very technical in nature and doesn’t explain the real-world production issues that are caused by improper powder coating particle dispersion. If you’re looking for practical knowledge to improve your results, these basic guidelines will help you identify finish problems related to particle size.
How Powder Manufacturing Creates Different Particle Sizes
When powder is manufactured, it is combined in a giant mixer, melted, cooled into a big sheet, and then broken up into pieces. It is then ground and sifted through a screen to the specific particle size the powder manufacturer has designated.
During the manufacturing process, there are three types of particles produced: standard, fine and large.
Standard Particle: This particle size is what the manufacturer wants to put in the box. The size range of the particle is very tightly controlled, Standard particles will produce the best possible finish and generate proven, heavily tested results.
Fine Particle: This particle is very small and could cause some finishing issues like picture-framing if there was too much allowed to be packaged. The fine powder particles build up at a different rate from the standard particles, especially in areas of “wrap”, and cause unacceptable variations in the finish.
An excessive amount of fine particles can also contribute to application guns spitting or sputtering because fine particles can be fluidized more easily than standard particles. During the grinding process, powder manufacturers have suction hoses above the grinders to catch a lot of these fine particles that would not otherwise be sifted out. Fine particles, by nature, float in the air more than standard particles. That’s why the suction hoses can catch most them.
For more information, visit this powder troubleshooting guide from Tiger Drylac. Specifically, on page 16 of the document, they show a good example of “picture framing” due to excessive build-up of fine powder. http://www.tiger-coatings.com/fileadmin/user_upload/downloads_us_new/technical-information-sheet/tiger-drylac/TIGER_Drylac_Troubleshooting_Guide.pdf
Large Particles: This particle is too big to pass through the final filter screen. Sometimes manufacturers will re-grind these large particles to get good powder out of them, but sometimes they just throw them away. If large particles wind up in a box of powder because of a manufacturing error or flaws in the screening equipment, they do not hold a charge as well as a standard particle and will most likely wind up on the floor of a spray booth. In addition to decreasing your transfer efficiency, an excessive number of large particles can cause the part surface to be more susceptible to powder loss during pre-cure handling, resulting in thin spots that can’t otherwise be explained. (to learn about Transfer Efficiency, read more here.)
Some fine particles and some large particles will wind up in the powder box regardless of the process being used. The powder coating manufacturer has a particle size distribution analysis machine to measure samples of powder and see how much variation the powder particles have. In a normal box of powder, there shouldn’t be enough large or fine particles to cause problems.
What Is The Powder Coating Particle Size Supposed To Be?
There is no standard answer, because each different type of powder has specific particle size requirements due to the special effects components or pigment used in its formulation. Regardless of size, the key to good powder is generally to have as tight a particle size spread as possible.
Typically, smaller particles carry better charge and fill voids in the coated surface better, but they do not penetrate Faraday Cage areas well. To get a powder that both holds a charge well and creates an even distribution, the manufacturer will usually decide on a happy medium for the average particle size based on laboratory and sample production testing.
Here is an example of a graph of a sample of powder:
As you can see, there is a curve to the distribution. That means that some particles are a little bigger and some are a little smaller. The more peaked the curve is, the more reliable your finish should be. If the curve is flattened and you have a lot of fine particles or large particles, you could have more finishing problems and/or worse transfer efficiency.
Powder Coating Issues Related To Powder Particle Size
Powder coating particle size discrepancies can create issues with the quality of your finish and can increase your applied cost. These problems can be especially troublesome in a production environment, especially if you use a reclaim system as opposed to spray-to-waste. If you don’t reclaim and reuse spent powder, the particle size in the box is all you have to worry about. If you reclaim your powder, then you need to consider powder particle size even more closely. It’s likely you will have some adjustments to make to achieve your best finish.
“Virgin” powder, straight out of the box, will typically be more conductive than reclaimed powder. The reason for this is that there are some large particles, as well as some inert particles that were part of the original powder formulation. These particles are most likely to fall off the part and get recycled back into the powder supply. As you use more reclaimed powder, this percentage grows and your powder does not adhere as well to the parts. Most powder experts recommend a ratio of at least 60% virgin powder to 40% or less reclaimed powder to get acceptable finish results.
Remember, the fine particles will usually adhere to the part or get stuck in the exhaust filters of the booth, so it’s mostly the heavy particles (that are the least attractive to the parts) that get recycled. Some particles, even those of ideal size, will stick together due to contamination or partial gelling in hot environments. These clumps of small and standard particles act like large particles and don’t stick to the parts during spraying. It is important to hand filter all of your reclaimed powder or use an automatic sieve to filter reclaimed powder being pumped back into your powder hopper.
What If I Think I Have A Powder Problem?
If you think you may be having a finish issue that’s linked to a problem with your powder, always start by contacting your powder supplier. Powder coating vendors have access to tools that the average powder coating shop or finish line manager does not. Listen to your powder supplier and follow their suggestions. Don’t be shy about asking them to look into potential powder problems, but remember that most coating defects are due to process issues, not defective powder coating media.
In a spray-to-waste environment, your supplier may elect to replace questionable powder or have it analyzed to confirm that it isn’t defective. It’s likely that they will want to review the way your using their product, and they may find a production issue that you’ve overlooked.
If you do a lot of reclaim, your powder supplier can test your virgin and reclaim samples to let you know what you can expect from your powder as far as average powder particle size. By keeping good ratios of virgin and reclaim, you can adjust your guns to the appropriate settings to get repeatable, high-quality finishes.
Need Expert Help?
Reliant Finishing Systems is a U.S. manufacturer of high-performance powder coating equipment. In addition to building powder spray booths and powder curing ovens, we manufacture blast rooms and wash stations. We also have experienced powder coating specialists on-staff and offer line audits and on-site troubleshooting. If you need powder coating equipment or expert advice, give us a call today.
