Formulators Forum—How Are Powder Coatings Made?
Posted on Monday, March 21, 2022
The powder coating manufacturing process generally consists of six stages. Weigh up, mixing, extrusion, cooling/flaking, grinding, and packaging. It is typically considered both a batch and continuous process.
The first two stages, weigh up and mixing, occur in containers or mixing vessels, often called bowls. Each bowl is a unique batch. Typically, several of these bowls or “small” batches are made, then run one after the other continuously through the last four stages.
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The process begins by weighing up the ingredients specified in the formula. There are four main types of ingredients: resins, crosslinkers, pigments, and additives. After the ingredients are weighed up, they are put into a bowl or mixing vessel.
These dry ingredients are then blended to provide a uniform mixture. The mixing process can be done by low-speed, tumble, or ribbon-type blending, or by using a high intensity mixer. High intensity mixing is generally preferred because it provides more thorough mixing and begins the pigment dispersion process. This translates into more uniform color, better consistency throughout the product, higher gloss, and a smoother appearance.
The mixture is then fed into an extruder, which is basically a hot barrel with either one or two rotating screws inside. The rotating screws consist of machined elements.
Some elements have a screw-like profile to convey material, while others look like blocks or paddles for mixing. The screws produce shear and heat to melt the resin to a liquid taffy-like consistency, while incorporating all the ingredients into the resin matrix.
This is an important step as pigments and other agglomerates are further dispersed. The material is in the extruder for a very short period of time, about 5-10 seconds, and temperature is watched closely to avoid curing. Once the material goes through the extruder, the formula and color are set. When it is necessary to adjust the color or another property of extruded material, new components would need to be added to it and the entire material would be re-extruded to ensure everything is incorporated into the resin. This is a time-consuming and costly process. Typically, small samples are tested first, and adjustments made before extruding all the material.
The hot material, or extrudate, is cooled as it comes out of the extruder. This typically happens by running the material onto cooled, rotating drums, often called chill rolls.
When it goes between the chill rolls the material flattens out, increasing the surface area to cool it down more quickly.
Once the extrudate is cool, it becomes brittle and is broken into small flakes so it can be fed into a grinder.
The flakes are ground into small particles that can be sprayed. The size of the particles are tightly controlled. This final grinding process is critical to the quality of the powder coating. The particle size distribution affects the smoothness, how the material fluidizes and pumps, and transfer efficiency. Fluidizing and anti-caking agents are also added at this stage.
After grinding, the fine powder is run through a cyclone to remove any ultra-fine particles. The powder is also typically run through a sieve to prevent any oversized particles before packaging into the specified size container. At this point, samples are taken and tested to ensure the product meets the formula’s requirements for color, gloss, or other specifications. A good quality assurance program and sampling process are important to monitor the process and prevent contamination or off-spec material.
Manufacturing powder coatings is a complex batch, continuous process involving several steps. It is difficult and time consuming to adjust product after extruding. Good housekeeping and attention to detail are critical throughout the process to ensure high quality material is supplied to customers.
By Ross Lefever, assistant technical director industrial liquid and powder coatings for Diamond Vogel.