Powder Coating Huge Parts: How to Ensure Consistency
Posted on Thursday, March 19, 2020
Powder coating huge parts is not for the faint of heart. Just ask Jase Kaser of Kaser Blasting and Coatings located in sunny Lincoln, Nebraska. He recently told Powder Coated Tough that one of the biggest (pun intended) issues is being able to spray the entire part without stepping on/brushing/sweating on the areas that have already been sprayed. Indeed, coating and curing gigantic items can introduce monumental challenges to the custom coater.
The first consideration involves capacity. Can your facility accommodate a very large part? This involves more than simple length times width times height calculations. You may have a reasonably large finishing shop footprint; however, will your conveying infrastructure handle the weight? Are your hooks, hangers, racks, and hoists certified to manage the load? Do your hoppers hold enough powder to finish the job without having to reload? Is your booth exhaust sufficient to capture any overspray generated? Do you have adequate lighting to ensure clean surfaces and even powder deposition? Also, will the part not only fit in your oven, but do its burner(s) possess the necessary BTUs to heat the metal mass of the part within a reasonable amount of time? Editor’s Note: Readers can contact the author for an MS Excel file: Oven Energy Calculator for metal parts.
Prepping a large part is most often accomplished in a static batch process; that is, the part stands still while the operator(s) perform the cleaning and pretreatment steps. Getting a perfectly clean surface is essential to obtaining a durable coating. The subsequent processes of chemical pretreatment and powder coating application require the absence of dirt and contaminants to provide an excellent bond to the substrate and a tough barrier to the environment.
Jase Kaser explains that depending on the project specification and substrate, they blast in an 18 foot wide x 15 foot tall x 50 foot long blast booth then pretreat in their 16 foot wide x 12 foot tall x 30 foot long wash bay. All blasting and pretreatment is done manually with blast equipment and a pressure washer, respectively. On steel, they blast to SSPC-SP10, and pretreat with iron phosphate and a dry-in-place seal.
One of the most important challenges with finishing large parts is ensuring proper adhesion to welds. These junctions can harbor welding spatter and flux which are very difficult to clean and pretreat. Particular attention must be paid to the complete removal of these contaminates. Thorough blasting usually can get the job done; however, some finishers use an acid pickle on these areas to ensure a pristine surface for subsequent coating adhesion.
Another challenge deals with laser cut edges. Laser cutting leaves a hard metal oxide that is difficult to adhere to. Blasting is the most common method to ablate the tenacious oxide layer, although strong acid pickle can help as well. Regardless, laser cut edges must be cleaned to ensure good adhesion of a powder coating.
Single Coat vs. Primer/Topcoat
Meeting a customer’s coating specification is integral to a successful coating job, a satisfied customer and just as important, piece of mind. For indoor applications, usually a single coat of a high-quality powder coating is more than enough to meet requirements. However, a primer/topcoat system is recommended for outdoor applications requiring longevity and durability. The primer can be zinc-rich or alternately a high-quality zinc-free epoxy. New generations of zinc-free powders possessing excellent corrosion resistance have recently been introduced. Primers typically provide excellent corrosion resistance but poor UV durability. A superdurable polyester topcoat resists UV degradation and provides the durability needed to meet most outdoor specifications.
Achieving complete coverage and consistent film thickness on a large item can be a tedious process. It is nearly always a manual operation and can be very operator-dependent for the results. Jase tells us that coating the substrate in dim/poor lighting requires the use of flashlights, especially in shadow areas. Newer powder gun designs offer the option of an LED light positioned on the body of the spray gun to overcome issues with non-ideal lighting. He articulates that it is wise to keep a log on previous jobs’ film thickness data and oven conditions. In addition, pre-cure QC inspection with LED lights as well as post-cure inspection with LED lights will ensure consistent and in-specification film thickness.
A superdurable polyester topcoat resists UV degradation and provides the durability needed to meet most outdoor specifications.
One of the greatest challenges in finishing large parts is ensuring complete cure. Large, heavy parts take longer to heat up than lighter ones; in addition, an object comprised of both thin and thick gauge metal poses its own unique issues. Kaser quips, “We strive to ensure that the entire part is cured but avoid overbaking the thin portions of the part.” Indeed, this quandary requires the careful selection of a powder coating that possesses low temperature cure capability for the heavy components, while exhibiting adequate overbake resistance for the thinner gauge sections. Most high-quality polyesters, including superdurable ones, are up to the task.
Essential to complete cure is having a method to measure part temperature. As most coaters know, the recommended cure schedule for a particular powder can be found on the Product Data Sheet and is quoted as time at part temperature. Consequently, the clock only starts once the substrate reaches the recommended temperature. The best method to use is a temperature data logger (suppliers can be found at on the Powder Coating Institute website at: www.powdercoating.org/auxiliaryequipment). These instruments have temperature probes that can be attached to various locations on your parts including thin and heavy gauge metal sections in addition to the oven air. The downside is the probes have contact with the part and therefore will leave marks on the surface. Alternately you can use a non-contact digital temperature gun. This will require you to peek at the part while it is in the oven. Some clever coaters fashion access windows in their oven to allow temperature measurements without having to open the doors of a batch oven.
After curing the powder coated part, it’s time to remove it from the oven and experience the long wait for it to cool. Some coaters employ cooling fans to accelerate this process. During the wintertime, this event can help lower your heating bills; however, summertime poses its own environmental comfort challenges.
Confirming the adequate cure of a powder coating is essential to ensuring a durable coating. One of the easiest techniques is a simple solvent rub test. ASTM D-5402 “Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs” depicts a reasonable test for completeness of cure. It is best to consult your powder coating manufacturer for the amount of solvent resistance expected with a particular powder. As a rule of thumb, however, a 90/10 mix of xylene and MEK (methyl ethyl ketone) is a good blend to check the solvent resistance of a polyester powder coating. Straight MEK is acceptable for most polyurethanes and epoxy coatings. Usually 25 double rubs with a cloth soaked with solvent will demonstrate if a coating is sufficiently cured. A slight amount of softening and transfer of coating is usually acceptable, whereas significant coating removal denotes a lack of complete cure. It is then highly recommended to reintroduce the part back into the oven for more heat.
A careful analysis of part positioning techniques should help minimize touch-up requirements.
This is a touchy subject that requires explanation. Why would one take painstaking effort to select the best performing powder coating, the most effective cleaning/pretreatment materials, a highly controlled process including tight temperature control, and then leave a number of uncoated spots that require touching up? Hanger and hook marks are inevitable; however, the best strategy is to minimize places that require paint after the job has exited the oven. Easier said than done, but a careful analysis of part positioning techniques should help minimize touch-up requirements. When touch-up is needed, it is wise to engage a company that specifically formulates touch-up paint to meet your customer’s requirements. Using an off-the-shelf aerosol paint may hide the uncoated spots but will invariably miss the mark with durability. A simple internet search will yield a few high-quality producers of industrial grade touch-up paint.
Jase and his crew at Kaser Blasting and Coatings have been powder coating huge items at their facility in Lincoln for a number of years. He is a strong advocate of doing the job the right way, the first time. This involves careful preparation and analysis of the entire finishing process from hanging the part safely to precise spraying and film thickness control to complete cure with minimal of touch up. The operation isn’t for the faint of heart; are you up to the challenge?
Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.