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A part recently taken out of a fluid bed coater with about 10 mils of functionalized polyethylene applied. Photo courtesy of Wright Coating Company, Kalamazoo, MI.
In fluidized bed powder coating, heated parts are either dipped directly into a container of fluidized powder or passed through an electrically charged cloud of powder, which is created above a container of fluidized powder. Although it seems like a relatively simple process, questions abound when it comes to fluidized bed powder coating, its applications, benefits and costs. Compiling a number of inquiries that have come in from our readers over the course of the last several months, Products Finishing turned to a pair of industry experts — Ross Rector and Lloyd Long of Innotek Powder Coatings, LLC — for a better understanding of the process.
Q: How do I know if fluidized bed powder coating is a good fit for my products?
Rector: There are several questions that need to be asked. First, since fluidized bed coating generally applies a thicker coating, can the end part withstand the dimensional changes? Unlike electrostatic coating, fluid bed coating will generally smooth over any small details in the parts, such as embossed serial numbers, metal imperfections, etc. This can be extremely beneficial for parts where Faraday Cage effects are problematic. Welded wire products are good examples. Electrostatic spray has a hard time getting into the depths of the wire cross welds, whereas fluidized bed coating, due to the larger heat mass of the overlapping wires, encapsulates the entire weld.
Long: Also, part complexity and geometry should be considered. If you have the right configuration, fluidized bed coating can be the way to go. Parts that will allow the excess powder to fall off the part work great. Others—with areas where trapped air or excess powder that will be difficult to displace—should be avoided.
Q: What are the key differences between “fluidized bed” and “electrostatic fluidized bed”?
Rector: With fluid bed coating, the part has to be preheated, immersed in the coating and then flowed out.
Long: Electrostatically assisted fluidized bed coating does not dip the part into the powder, it generates a cloud of charged particles (much like a conventional electrostatic gun) through which a heated or an unheated part passes. It generally applies a thin coat vs. the thicker coat from fluidized bed coating.
Q: Can I use the fluidized bed method to coat products that do not require such a thick coating?
Rector: Not in most cases. It is very difficult to achieve a thin coating consistently. Film build generally occurs rapidly over the first five mils. It’s very difficult to get the part in and out of the fluidized bed coating tank quickly and still produce a consistent coating over the entire part.
Long: It is possible, but I suggest revisiting electrostatic spray coating if you are looking for thin coating thickness under three mils.
Q: Do I need to pretreat my parts any differently for fluidized bed powder coating than I would for electrostatic spray powder coating?
Rector: No. The same cleaners and pretreatments work for both coating methods. To achieve maximum performance, the part needs to be cleaned and pretreated just as in conventional electrostatic coating.
Long: If you are using a conversion coating, check with your supplier to make sure the preheat temperature you are using are not detrimental to the conversion chemistry.
Q: Are there any drawbacks associated with the fluidized bed process?
Long: Color changes can be difficult. Where practical, one should use a dedicated bed for each color. You will need to initially charge the bed with enough powder to completely cover the part. For smaller parts such as dish drainers, 50 lbs may be sufficient. For larger parts such as park benches, 2,000 lbs is not unheard of. It can be a large initial purchase of powder. After that, you only need to add make up powder as the coating is used on the part.
Rector: The drawbacks stem from the differences between fluid bed and electrostatic coating. Building film by heating and melting the powder to the part can overcome a lot of problems, but it can create some problems as well. Smaller details of a part, such as stamped or raised lettering, will most likely be smoothed over. Also, if the part has details that are close together, the coating may bridge from one to the other.
Q: What is required to heat my parts for use in a fluidized bed powder coating operation?
Rector: Conventional gas fire convection ovens are most common. However, electric and gas IR and other methods work fine as well, depending upon part dimensions. The main goal is to preheat the part above the melting point of the coating so the film thickness will build when submerged in the coater.
Long: Induction heaters are used as well, especially on long, continuous parts.
Q: Does the pre-heating stage make fluidized bed a more expensive process than electrostatic spray?
Rector: Generally not. The overall thermal requirements for both coating methods are basically the same. Obviously, fluidized bed coating uses more heat prior to coating, and electrostatic spray uses more heat after coating, but these usually net out.
Long: There are some cases where lower overall heating is required with fluidized bed coating since thermoplastics (the primary coating used in fluidized bed processes) do not require a cure cycle, only enough time and temperature to flow out the coating. Some parts with enough thermal mass will hold their heat long enough that a post heat cycle is not required. The coating flows out without it.
Q: What are some other benefits of fluidized bed powder coating?
Long: There is generally less waste since it’s a 100% transfer efficient coating method. When compared to electrostatic spray, capital investment in equipment and ongoing maintenance is most always lower. It’s basically a low-tech coating method, and once coating parameters are established, there are not that many things that can go wrong. It is by far the most efficient method of applying thick film coatings.
Rector: Productivity increases are very common when switching to a fluidized bed coating process. The ability to coat an entire rack of parts by a single dip into the coating tank can be very efficient.
Q: Can the fluidized bed process be used with both thermoset and thermoplastic powder coatings?
Rector: Yes. However, thermosets are rarely used and much more complicated due to the heat buildup in the fluid bed coater, which can cause the powder to cross-link in the coater. Thermoplastics are much more forgiving in this manner.
Long: Applying thermosets to a pre-heated part can result in stringing—where cobweb-like strings form coming off the hot part. This can happen in both fluid bed and electrostatic spray coating processes. Further complicating matters, most thermosets are sold in a smaller particle size and will tend to dust out of the coater into the plant environment.
Q: What are common fluidized bed process parameters?
Long: There are no common parameters since it changes dramatically with part thickness. Two-inch thick bar stock can be coated with functionalized polyethylene by preheating to 250°F, dip coated and will most likely flow out without any post heating. Conversely, thin expanded metal may have to be preheated to 450°F to achieve the desired coating thickness, and then post heated at 350°F for four minutes to complete the flow out. We have never been able to come up with coating parameters that work for everyone. Ovens are different, and parts cool down at different rates. Substrates, line speeds and environmental conditions all vary as well.
Rector: Here are some nominal starting points—take an average part of fabricated wire such as a refrigerator rack. Preheat it for six min at 500°F and then (within 10 seconds of heating) dip it for six seconds. Post heat it for one-and-a-half min at 350°F. This will normally produce a film build between 10–12 mils. On applications such as bike racks, where 30 mils of coating are desired, heat the part for six min at 550°F, dip it for 30 seconds and post heat it for one-and-a-half min at 400°F.
Q: Please address the issue of over spray and 100% transfer efficiency vs. charging a fluid bed coater.
Rector: One of the upsides to fluidized bed coating is the fact that virtually 100% transfer efficiency is obtained. Powder either melts to the part or remains in the fluidized bed. The downside is in the charging of the tank. A tank large enough for the part must be charged with the coating which—for large parts—can be a substantial amount of powder.
Long: To boil it down, fluidized bed coating is a fairly inexpensive system that will give you virtually 100% transfer efficiency. You can achieve 100% transfer with electrostatic spray, but it takes a relatively expensive system to get you there. In many cases, the cost of the initial fluid bed charge is generally lower than the cost of the electrostatic spray equipment needed to achieve 100% efficiency.
About the Authors:
Lloyd Long is technical manager for Innotek Powder Coatings, LLC in Big Spring, TX. He has been in the powder coating and plastic process industries for more than 18 years and served in numerous capacities.
Ross Rector is an account executive with Innotek Powder Coatings, LLC. He has been in the powder coatings industry for more than 30 years in numerous capacities.