Powder coating basics for metal fabricators

FIGURE 1. In an inline automated system, washed parts hang before entering the drying oven. The less time they hang in ambient air, the better the mitigation of flash rust.

Editor’s Note: The following is based on the first half of “Batch versus Automated Finishing Systems,” presented at FABTECH 2023 in Chicago by Nick Dawson, sales account manager at George Koch Sons LLC. The second half, presented by Frank Mohar, Northeast sales manager at Nordson Corp., will be covered next month. rapid thermal annealing

Click here for Part II in this series.

Whether you outsource or bring powder coating in-house depends on myriad factors, including the customers you serve, their delivery requirements, and the custom coaters you have in your area. Regardless, if you do need to bring powder coating in-house, where do you start?

With the basics, of course. Before anything else, you need to consider what kind of finishing department—one with a batch style or an inline automated system (or both)—will best serve your needs.

First, consider the floor space you have now or could acquire for the finishing process. Within that, you need space to stage and load parts onto carts or a conveyor. From there, we need room to clean, coat, and cure your parts.

A dedicated area that’s, say, 100 ft. long by 100 ft. wide and 30 ft. high is a good starting point to accommodate a variety of batch-style processes and even some automated inline systems, such as those dedicated to small parts. When designating a space, you need to consider the building columns and supports, roof trusses, floor drains, and anything else that will interfere with the total working area.

Depending on the products you fabricate, you might need to integrate some kind of blasting system to prepare the surface for optimal paint adhesion. If an aqueous pretreatment (chemical) process is used, the parts need to be dried. That is followed by the actual coating process and, finally, curing. All that takes space, and the best system configurations help you use that space to its maximum potential.

First consider the throughput you need. Do you need 20 parts a day or 20,000? If you’re coating 20,000 parts, you probably won’t want to rack them on a cart and roll them into a batch booth. An automated line with a conveyor makes more sense (see Figure 1).

Next comes the product size and weight. It’s a lot easier to put large, heavy parts (think 40-ft. custom trailers) through a batch system, as long as you don’t have 20,000 of them to do a day. Part variety also plays a role. If you have a high-mix, high-volume environment, a coating system would likely be designed around the highest-volume jobs within the broad product mix. In this case, those jobs might benefit from an automated inline system. If the “high runner” is not the largest part to be coated, you might consider a separate batch system dedicated to the smaller run, largest parts. Or, you might choose to outsource the coating of those particular parts.

If parts are especially large, a batch system might be the best fit (see Figure 2). High-mix, low-volume environments might benefit from a batch system with racks designed to hold a variety of parts. Ultimately, it’s all a balancing act involving part geometry, size, available floor space, and (of course) your available budget.

FIGURE 2. A batch finishing system is installed on a factory floor. The manufacturer needed a batch system to coat its large workpieces at low volume.

Start with the width, height, and length of the largest part. These measurements determine the part window, or window of opportunity. For lower-quantity or larger parts going into a batch system, you need about 4 ft. of clearance so that an operator can walk around and work on the piece being coated.

Automated systems require about 6 in. of clearance around the workpiece (see Figure 3). They usually have overhead conveyors with hangers that carry parts through the different stages. These have a minimum chain distance between where the part hangs and the hanger attachment point; about 27 in. is common. That space prevents powder and pretreatment chemicals from building up on the chain itself. If you get powder on the chain and it goes into the cure oven, that powder will cure on the chain, which can lead to numerous issues.

The part weight determines the cure time in the oven, and the metal (or other substrate) thickness determines the time it takes to bring the part up to temperature (or bring-up time) for both drying after pretreatment and curing after powder coating. Thin stock, like 18-ga. sheet metal, usually has a short bring-up time, while 1-in. plate can take a half hour or more to bring up to temperature (depending, of course, on the oven temperature and application).

Some might choose to dry pieces in ambient air, but this can be a recipe for rust. Others use compressed air, either manual (operator with an air gun) or automated. That said, these systems require clean, dry air, which means you need an air dryer. Otherwise, you end up blowing moisture and oil onto the clean part surface.

Certain operations dry parts using infrared heaters, but the best option, if the budget and floor space are available, is to use true convection dry-off oven. This allows you to use a combination of temperature, air movement, and time to give you consistent results. Note that air movement is a critical part of the drying stage. A dry-off oven will have faster air turns (how often air is replaced) than a curing oven. For an analogy, think of a hair dryer: Turning it on the “high” setting dries your hair faster than on “low.”

Air movement is critical during every stage of finishing, and curing is no exception. The core design of a gas convection oven (most common) is simple. A natural gas or LP-fired burner blows a large flame into an insulated burner box located on the top or side of the oven. A recirculation fan then directs the heated air into discharge duct inside the oven. A return air opening in the burner box (or heater unit) facilitates airflow, and an exhaust stack carries away fumes and particulates (see Figure 4).

Many batch ovens today have high-temperature roll-up doors that save floor space and can be controlled remotely (see Figure 5). This means fork truck drivers delivering parts needn’t jump off the truck to open the door.

Some oven manufacturers incorporate louvers on the discharge duct that can be opened and closed to adjust the heat profile. Ovens with adjustable side-wall ducts allow you to balance the heat profile throughout the oven. For instance, if the temperate is lower at the bottom of the oven than at the top, louvers at the bottom should be opened fully while choking those near the top.

Curing is a function of temperature and time, two factors that create your cure schedule. More time equals less heat, while less time equals more heat. Specifics depend on the application here, but just to give an illustrative example, a part cured at 350 degrees F in 20 min. might be cured within 8 min. at 425 degrees F.

You can’t fight physics here. Curing takes longer than drying. Also, dry-off ovens need to be set at lower temperatures, because higher temperatures can “cook off” the conversion coating, be it zirconium or iron phosphate, that the pretreatment process applied.

FIGURE 3. Automated systems are designed around your largest parts. You generally need at least 6 in. of clearance on all sides and at least a 27-in. distance between the part and hanger attachment point.

Using the same oven for both drying and curing can be a serious constraint on overall throughput in a batch setting, especially if you’re coating thick parts. That said, how bad a constraint this is will depend on your part mix, demand trends, available floor space, and overall business strategy.

Some fabricators have a batch processing oven with two areas separated by a wall—one for drying and another for curing—yet both share one heater box (see Figure 6). Vigilant operators make this work by closely monitoring the time parts spend in the ovens. They also might close off some ductwork in the dry-off side of the oven to cool the work area (though not in a precise way). This approach can sometimes work, but be sure to scrutinize your overall process first.

Ideally, the dry-off and cure ovens are set to different temperatures and specific air turns (level of air movement). This gives you superior process control, ensuring the parts are dried and cured properly. Scrapping parts in finishing gets expensive, especially considering all the value your upstream processes put into them.

Manual pretreatment options include steam cleaning, which involves lower pressure, a lower volume of pretreatment chemicals, and less chemical impingement. Spray wand cleaning, which is basically a pressure washer with a chemical added to it for better coating impingement. The latter has higher pressure, though it also produces more wastewater.

Automated pretreatment systems are designed with the largest part in mind. Your longest part will determine how long each pretreatment stage needs to be.

For instance, say you have a pretreatment line designed around a zirconium conversion coating process, and your longest part is 6 ft. We’ll assume the conveyor moves at 8 ft./min. You start with an alkaline cleaner heated to about 140 degrees F for about 90 sec. From there, parts move into two 30-second rinse stages at ambient temperature, then spend 60 sec. in a stage that applies the zirconium conversion coating before moving on to the final 30-sec. stage entailing a reverse-osmosis rinse at ambient temperature.

Note that an operator can accomplish all these pretreatment steps in one batch booth with a pressure wand. Of course, the process is entirely manual and much, much slower.

Whether your pretreatment process is manual or automated, be sure you minimize the time parts spend between the final rinse and the dry-off oven. Many pieces flash-rust very quickly, often within minutes. This is another reason monitoring oven capacity is so critical. The last thing you want are wet, clean, yet still uncoated (and unprotected) parts waiting in a long line in front of the dry-off oven.

Automated pretreatment systems come in a variety of configurations, including traditional stainless steel and polyurethane models. Stainless steel gives you a longer-lasting system, while polyurethane systems have lower upfront costs and offer some modularity in case more pretreatment stages are needed in the future.

Also, don’t forget to account for proper drainage. With parts that are racked or hung from an automated system, be sure pretreatment chemicals don’t pool in a particular area.

FIGURE 4. This side view of an oven interior shows the burner box, the discharge ducts running along the oven wall, return air opening, and exhaust stack.

Imagine you have a long flat piece with a bottom flange. On an automated line, if you hang it perfectly horizontal, chemicals will pool on top of that flange. Angling the piece downward, opposite the direction of travel, allows the chemicals to flow off behind the piece as it moves forward. If chemicals flow down in the direction of travel, you risk cross-contamination as your pretreatment chemicals drip into the next stage (see Figure 7).

In batch systems, operators often move pieces from stage to stage with carts, but parts also can hang from an overhead conveyor, either moved manually or powered with buttons. In these cases, operators need to keep their eyes on the process, making sure parts don’t crash and that they’re headed to the correct finishing stage.

Conveyors come in a variety of styles. Enclosed track systems are common in lightweight applications, while I-beams can be built to hold heavier pieces. The larger the I-beam, the greater its load-carrying capacity.

Another type is the power and free conveyor, which allows you to stop the part at any point in the line and break certain pieces off into separate lanes. If, say, you have two powder booths, one dedicated to black and another to green, two separate tracks can lead to each, allowing operators to feed parts simultaneously into each booth. For extremely heavy parts, forklift transfer might be the best option, though some operations use a tow-line conveyor, in which a truck system pulls extremely heavy parts through a finishing system.

Of course, no matter how advanced your powder coating line is, none of it matters if your parts aren’t grounded properly. Powder coat an improperly grounded part, and you can get back-ionization; the powder will start popping off the part surface, giving your finish very noticeable starburst patterns. Coating an ungrounded part can have more serious ramifications too, such as booth fires after a spark ignites the atomized powder.

In a manual batch processing setup with rolling racks, every rack needs to be connected to an electrical ground. Many connect a grounding clamp to a copper grounding rod installed near the enclosure. You need good metal-to-metal contact everywhere. Hooks covered with paint need to be either cleaned or replaced. If an ohmmeter shows you have more than 1 megohm of resistance to ground, quality issues can arise.

No matter how advanced your powder coating system is, you need to think of the system holistically, from the first surface prep to the final cure. This includes the cost of scrap. Think of all the value already added upstream. In truth, scrap in finishing is some of the most expensive scrap there is.

Powder coating basics for metal fabricators—Part I