Technology Interchange - Manufacturing Powder Coatings PART TWO Milling and Sifting

Producing a powder coating is a multi-step process. It can be described as being semi-continuous because it begins as a batch process (weighing and pre-mixing) but evolves into a continuous process (extrusion and milling). In the last Technology Interchange article, we discussed the first steps in manufacturing a powder coating. This consists of weighing, premixing and extrusion. This edition covers what happens to the flakes generated from the extrusion process.

Pulverization

The next step in manufacturing a powder coating involves reducing the particle size of the chips or flakes that have been collected off the cooling belt. The goal here is to pulverize these flakes into a fine powder that possesses a particle size distribution usable on a customer’s application equipment. This is typically accomplished with an air-classified mill.

Air-classified mills operate on a system of continuous negative pressure created by a large fan associated with a baghouse. This negative pressure draws the flakes into a chamber that contains a rapidly rotating plate. This plate has pins or hammers situated on its perimeter. The pins or hammers impact the incoming flakes. In addition to these high energy collisions, the rotation of the plate propels the fractured flake into a corrugated liner that further reduces the particle size. The particles are continuously smashed in this pulverizing chamber until they are small enough to escape past the vanes of a high speed classifying wheel. The rotation of this wheel also creates a buffer of air which restricts the exit of powder until it is ground to a certain fine particle size. The classifier speed will effect the ultimate particle size distribution (PSD).

Formulation variables influence the grinding process. High molecular weight resins produce powder formulas that are more difficult to pulverize and may require more cooling of the mill. Furthermore, feed rates may have to be reduced to allow for more time in the grinding chamber to achieve targeted particle size. Resin softening point or glass transition temperature (Tg) also affects grinding. Low Tg resins can cause fusion in the mill and may require extra cooling to keep from melting. High Tg resins can be more difficult to grind as well. In this case, extra cooling makes the powder more friable and easier to fracture.

The incorporation of high hardness fillers make the formula more abrasive and causes erosion of internal parts in the mill. High concentrations of some organic pigments can make cleanup of a mill arduous. Organic reds and oranges are notorious for coating internal surfaces of a mill and make cleaning a chore.

Process factors have a significant influence on the ultimate particle size distribution (PSD). The feed rate of flakes into the mill affects PSD. Faster rates create less time in the grinding chamber and will produce coarser particles. The temperatureof the material, the incoming air and the mill itself will affect particle size. Higher temperature creates coarser particles and may run the risk of fusing material in the mill.

The speed of the rotor plate which affects the peripheral speed of the hammers or pins influences PSD. Higher speed causes more impacts of material and hence a finer particle size. The speed of the classifier wheel also affects particle size. Higher speed makes it more difficult for particles to exit the grinding chamber and therefore creates finer particles. The speed of the fan on the baghouse affects particle size as well. Slow speed causes material to pass through the grinder more slowly creating finer particles.

Sifting and Packaging

From the grinding chamber, the powder is pulled through a cyclone and drops into a sifting device. The sifting device eliminates a very small coarse fraction of the ground powder. This coarse material can be re-introduced into the grinding process to optimize the yield.

Sifting can be accomplished with either a rotary sifter or a flat deck screener. With a rotary sifter the powder is introduced into a chamber consisting of a rotating vanes affixed to a horizontal shaft. The vanes propel the powder against a cylindrical screen. The fine particles pass through the screen and the coarse particles are carried beyond the screen and collected as “overs.” These coarse particles can be reintroduced into the mill to improve yield.

A flat deck screener consists of a horizontally oriented circular screen that is vibrated mechanically and in some cases, ultrasonically as well. The fine particles pass through the sieve screen and the coarse particles are carried centripetally off the screen and collected for reintroduction into the mill.

The screens used in sifters/ sieves are rated either in opening size (microns) or mesh (holes per square inch). Screen size for typical powder operations range from 80 to 120 mesh (125 to 180 microns). Powder coatings are typically packaged into boxes, barrels or bulk bags/containers that are lined with a plastic bag. The plastic bag is typically 3.0 to 4.0 mils (75-100 microns). The box must possess adequate strength to withstand transportation, storage and handling. A good quality bag will keep the powder isolated from the environment keeping it dry and free-flowing.

Summary

The process of manufacturing a powder coating is a relatively complex semi-continuous endeavor. As the powder material enters the final steps of milling, sifting and packaging the formulation is essentially fixed and cannot be altered. Milling is a complicated process that is influenced by a number of factors including the nature of the material, and process conditions such as temperature, feed rates and the speed of grinding components. The final steps in manufacturing a powder coating have a critical influence on the particle size distribution and ultimately the application performance at the powder applicator.

Kevin Biller is technical editor of Powder Coated Tough magazine and president of The Powder Coating Research Group. He can be reached via email at kevinbiller@yahoo.com.