When it comes to powder coating, consideration of particle size is tantamount to achieving a superior finish. The particles making up the powder must be sized “just right” for the application. While it would be virtually impossible to make all particles identical in size, the distribution of these particles needs to fall within certain parameters to ensure a quality coating.
Basics of Powder Coating Particle Size
Powder coatings are used to color aluminum, steel, and other metals, offering tougher and longer-lasting results than liquid paint finishes and making finishes more aesthetically pleasing. To produce the powder for these coatings, materials are pre-mixed, then extruded into a large sheet. These sheets are broken into chips, ground to a specific size, and check-screened prior to packaging.
This process results in three basic particle types:
- Too Big: Larger particles are undesirable as they decrease transfer efficiency. When powder coating includes too many large particles, this causes non-uniform application thickness that leads to defects in the curing process and provides less surface protection for the component to which it's applied. Air classifying machines separate these larger particles to be reground.
- Too Small: Fine particles can also cause similar variations in coating thickness. When applying powder to a component, finer particles can build up more rapidly than correctly-sized particles, causing undesirable variations and visual defects in the finish. Excessive amounts of these fine particles also tend to fluidize more easily than standard sizes, contributing to problems such as clogging of powder coating application guns.
- Just Right: Standard particle sizes produce the most uniform finish, using proven methods to generate very tightly controlled particle distributions.
Importance of Powder Coating Particle Size Distribution
It is not only the size of particles that matters when applying a powder coating. Particle size distribution matters as well. The tighter the distribution, the better quality the finish. Material properties, along with the processing technology used, will affect this distribution.
Powder coating manufacturers use laser diffraction machinery to analyze powder coating particle size distribution, measuring samples to determine size variations. This equipment looks at percentages of particles within ascending or descending orders based on their size.
The machinery also provides statistical modeling of the particle distribution to determine deviations in particle size within the powder coating. Particle sizes that are all the same basic size are referred to as monodisperse, though more often, they will be polydisperse, referring to particles that vary in size. The efficiency of the grinding system used affects the quality of powder coatings as well.
Ensuring Proper Particle Sizes & Distributions
Air classifier mills are widely used in the manufacture of powder coating because they allow precise control of the upper and lower ends of the particle distribution, resulting in a narrower range of particle sizes that are most desirable for powder coatings.
Additionally, a properly sized pneumatic system will ensure the air classifying mill works most efficiently, reducing the chance that particles are overground and thus too small. Variations to design elements of the equipment can also affect particle size distribution. Altering the pitch between grinder blades or RPM of the grinding wheel will affect the prevalence and force used to impact the particles. Carefully selecting these parameters reduces the presence of finer particles within the distribution. Further, horsepower helps determine how much material a mill can produce, along with the speed with which air flows through it, so powder coating manufacturers should look to use air classifying mills that can handle their throughput.
Determining Correct Particle Sizes for Powder Coating Applications
Approximately 5-30 percent of particles will be reduced to below 10 microns after grinding and classification. A lower percentage of finer particles allows powder coatings to flow better and ensure a higher quality finish. The mechanical properties of the powder coating chips and the grinding technology used determine the fraction of fine material within the powdered product. Material properties depend on the extrusion process and components used for powder coating. These decide how brittle or ductile the end material is.
Within the automotive industry, new applications require powder coating that contains fewer finer particles. This can result in under 3 percent of particles that are 10 microns or smaller in size, even to a range where 1.5 percent of particles are below 8 microns. This lower percentage of fine material helps ensure a higher quality product.
How Powder Grinding Equipment Works
When working with applications involving powder coating, grinding systems primarily use an air classifying mill to reduce particle size. At the beginning of this process, a storage hopper dispenses extruded chips of powder coating via screw feeder into the grinding equipment. After grinding, the air classifying mill uses air classification to separate particles by size.
The air classifying mill uses fixed blades on a wheel to impact and accelerate particles outward. These particles collide against a specially designed, screenless grinding frame until they reach a size small enough to pass through the frame’s gap. Secondary air then circulates ground particles outside of the grinding chamber, fluidizing and cooling them.
The secondary air also pulls particles inward toward the classifying rotor, where they are separated by size. On-size particles pass through the classifier and are pneumatically conveyed to a filter receiver that collects the finished product, while oversize particles are returned to the grinding chamber for additional size reduction. Adjustments to particle size are precisely controlled simply by adjusting the independently driven classifier rotor’s speed.
Prater Equipment for Powder Coating
Prater Industries designs and builds an array of equipment used to process powder coating. Particle size distributions often need to vary according to application. Prater’s machinery enables easy customization to suit these specific needs. Along with its Air Classifying Mill, Prater can also assist with project management, system design and layout, scheduling, and other production requirements.
Additionally, Prater operates one of the largest test and toll facilities in North America, allowing our technicians to back their system calculations, properly scale the equipment and apply customizations necessary to best produce your product. Using our experience and knowledge gleaned from almost a century in business, Prater can meet the needs of any of our customers’ powder coating processing applications.