Applying Powder in the Oven

Question:

Great column!! We make large pumps for mining and dredge work, with gray, ductile and white iron castings. There is no finish requirement other than the gray color and the paint simply protects the metal from the environment. Currently, we use water-based epoxy due to solvent emission restrictions. Due to the weight (up to 40,000 lb castings) conventional ‘clean-coat-bake’ powder processing is out of the question. However, after mold shakeout and riser/gate removal, the parts go thru a secondary heat treat cycle in a controlled oven where they are taken to red-hot and allowed to cool on a time cycle. Could the powder be applied at a specific temperature range (with the parts in the oven) such that it would bake on during the remainder of the cooling cycle? Thanks. B. C.

Answer:

There is a powder coating process that seems to fit your needs perfectly. It is called Fusion Bonded Epoxy (FBE). The process is normally applied as a functional coating to products such as rebar, pipe, etc. Parts are usually grit blasted first to remove any soils and to prepare the substrate surface for good bonding. The parts are pre-heated to 500°F and powder coated while the surface is still hot. As the part cools, the powder coating achieves full cure. Sometimes cooling is accelerated using water quenching. The resultant coating is a high-build epoxy (15 to 30 mils) that is very durable and corrosion resistant.

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Your process would be a little different. You probably would not have to grit blast your parts, since they would have not had a chance to become soiled after “shake-out.” Additionally, your part surface will have sufficient “tooth” to provide good bond to the epoxy powder coating without grit blasting, since they are sand castings.

You would heat treat your parts as you do now. When they have cooled to 500°F they would be removed from your oven and powder coated using the FBE material in an approved powder coating booth. The coating will melt and flow on contact. Cure will be complete by the time the part cools to ambient conditions. The resultant coating will be a thick, smooth, epoxy coating that will be very suitable for your fielded conditions.