Last year we ran an article titled “Understanding Fluidized Bed Powder Coating” featuring a couple of industry experts answering questions and lending their insights on this coating technique. At the request of many of our readers, we asked them to answer a few more questions about fluid bed coating techniques...
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Rector—Common product applications include coated wire products, refrigerator shelves, outdoor furniture, football helmet facemasks, snowplow blades, and exterior and interior automotive and truck parts.
Long—Many of the product applications Rector is mentioning are using fluid bed coating to overcome Faraday cage problems. Cross welds on wire parts are some of the most difficult for electrostatic spray processes to coat. With a fluid bed process, the added heat mass of the wire weld actually facilitates applying slightly higher film builds in the weld area, which provides higher corrosion protection in these trouble areas.
Long—Many of the problems arise from not choosing the correct coating process in the first place. Electrostatic spray coating is extremely good for applying relatively thin and consistent film thicknesses. Fluid bed coating is good for achieving relatively thick films and getting the coating into tight spots. This is not an ironclad rule and you can certainly achieve both of these results with either coating technique. But the further you get away from this principle, the more challenges you will run into. We try very hard to steer the customer to the correct coating technique up front.
Rector—One trick is to increase the preheat temperature of the entire part and lower the dip coating time. As an example, if your normal parameters call for a preheat temperature of 375°F and a dip time of eight seconds, try increasing the preheat temp to 450°F and decreasing the dip time to four seconds. The idea is to limit the film build with dip time as opposed to stopping the film build by letting the heat in the part fall below film building temperatures.
Long—Yes. Once the coating parameters are dialed in, the process is as repeatable as conventional electrostatic coating, if not more so. Many people find that a single fluidized bed coating process is much easier to maintain than a bank of electrostatic spray guns.
Rector—We have customers that use both electrostatic spray and fluid bed coating methods and while each has advantages and disadvantages, there seems to be far fewer technical problems with fluid bed coating. With electrostatic spray processes, one needs to monitor KVA, humidity, grounding issues, powder delivery issues, gun maintenance and replacement of wear parts. With fluid bed coating, one monitors much lower tech parameters such as fluidization level, the powder depth, preheat temperature and dip time.
Long—One can get caught up in terminology, but “decorative coating” can mean the coating is only decorative and not functional. If that’s the case, most fluid bed coatings are more than just decorative. The thicker film is used to provide high levels of corrosion and impact protection. People do not usually use the fluid bed process to apply a coating that is merely decorative.
Rector—While it’s true fluid bed coating is generally used to apply thicker film builds for corrosion and protection purposes, the finish is still a decorative quality finish.
Rector—Increased corrosion and impact protection tops the list but other somewhat less obvious reasons are quite common such as: using a thicker coating to make the steel appear thicker; overcoming Faraday cage problems; to cover up imperfections from welding, stamping and casting. A thin electrostatic coating follows the form of the metal—imperfections and all. A thicker coating can smooth over some of these problems.
Long—Since most all fluid bed coatings are thermoplastic, the thicker coating can provide other properties beyond the increased corrosion protection so often cited. Due to the higher tensile and elongation properties of thermoplastics, there are coatings available with extremely high wear resistance, flexibility and impact resistance. Foot traffic applications are quite common as well as tool handles where a soft grip is desired. Another point to keep in mind with thick film thermoplastics, since the specific gravity is up to 40% less than that of thermosets, applying twice the coating thickness can be accomplished with about the same amount of powder coating.
Rector—Air pockets can form in recessed areas of the part much like immersing a glass upside down into water. Moving the part while in the fluid bed helps to displace trapped air allowing the coating to reach these recessed areas.
Long—Large areas of flat, horizontal surfaces need to have the excess powder removed as it comes out of the fluid bed coater. This can be accomplished by shaking the part, tilting the part so the powder falls off, or even hitting the part with an air stream. However, many times this problem can be avoided in the first place by hanging the part so the flat surface is not facing up but on a sufficient angle for the excess powder to fall off without excessive shaking.
Rector—Sure. A couple of good examples are laboratory glassware and fluorescent light bulbs. If the proper thermoplastic chemistry is used, it bonds extremely tight to the glass making it shatter resistant. If the glass does break, you’re left with a plastic sack of glass instead of hazardous mess.
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.