An overview of a powder coating line for fabricators

More metal fabricating companies are adding powder coating capabilities as they look to expand their value-added offerings to customers. Images: Reliant Finishing Systems

In recent years, there has been a dramatic increase in metal fabricating companies, both small and large, wanting to bring powder coating operations in-house rather than rely on other companies to coat their products. By performing powder coating tasks at their facility, they won’t have to worry about excess handling of the parts, which can be damaged in transit between the shop and the coater. They also retain control of their production schedule by not having to rely on a contractor, and they can significantly reduce coating and handling costs.

Despite the increased interest, plenty of metal fabricators don’t know a lot about what goes into a powder coating operation. This overview provides a breakdown of the different components of a powder coating line and the options available for a company looking to install this type of operation on its shop floor.

Why Powder Coating?

Powder coating gets its name from the material that ultimately creates the finish on a metal part. The powder particles, in effect, are the paint.

In most systems, the powder coating material is electrostatically charged as it is sprayed. The charged powder particles are then attracted to the metal parts, which are grounded via the metal racks or hooks from which they are hung. When the coating is heated in an oven, the powder melts, flows together, and becomes a uniform, durable finish when it cools.

Powder coating is favored by many manufacturers and metal fabricators because of its resistance to chipping, fading, scratching, and general wear. Because of this durability, powder coatings often are the finish of choice for products that are consistently exposed to moisture, sunlight, or other environmental factors.

It’s important to understand some common terms used to describe powder coating systems. The most basic systems are set up for batch coating. In batch systems, the parts are usually hung from rolling racks or placed on carts. The parts are moved from appliance to appliance, either one large part at a time or in batches of several small or mid-sized parts. The process steps are usually performed by human operators.

A conveyorized system can be manual or automatic. A manual system would require human operators to prepare parts and apply powder coating to them as they pass by on a conveyor. With an automated line, there is powered transport of the parts, combined with automated pretreatment and powder application. Both manual and automated systems use specially sized ovens for drying and curing.

What Determines a Powder Coating Setup?

The product mix and throughput requirements help determine the type and size of equipment in a powder coating operation. As a general rule, the more similar the products are in size and shape, the more likely they are to be good candidates for an automated or conveyorized line. A manual batch coating system may be a better fit for very large or complex parts, as well as for situations where there is a big difference in the overall dimensions of the parts to be coated.

In a shop where only a limited number of parts are coated each week, a batch system is the obvious solution. For companies with greater throughput demands, it may make sense to consider more sophisticated systems. For a manufacturer with an overriding need for high throughput, a conveyorized manual or automated system may be the only practical answer.

When comparing acquisition costs, simple batch systems cost significantly less to purchase and install than other types of systems. An automated system typically costs between three and 12 times as much as a batch system that might accommodate the same parts. A manual conveyorized system is priced somewhere between the two.

Because the footprint of a powder coating system can be fairly large, thorough planning is required ensure everything fits and the proper amount of room is dedicated for operation.

Another consideration is the amount of floor space available. The equipment arrangement must allow for maintenance access, and the layout must provide enough room for parts to be moved safely from appliance to appliance. With batch systems, these areas often can be condensed to save space. A conveyorized system typically uses quite a bit more floor space than a batch setup, especially if there is a multistage pretreatment washer involved. With more elaborate systems, there may be sections of a power-and-free conveyor. This configuration allows parts to stop moving while the main conveyor remains in motion. A power-and-free conveyor is sometimes used to help reduce the size of the appliances or minimize the amount of space needed for parts to cool before being handled.

This touches upon another issue: labor costs. If a company has a demanding production environment and uses manual equipment, it’s going to need multiple people to prep parts, mask them, and spray the powder. People also are needed to load and unload parts from the racks or conveyor. With a batch system, workers also have to move the parts between appliances.

Even with a highly efficient automated line that performs pretreatment and powder coating automatically, a company needs at least one touch-up/quality control person and additional personnel to maintain the equipment and monitor pretreatment chemistry titration and appliance settings. This is in addition to the people who perform masking and parts handling. Manufacturers and metal fabricators can’t get away from the fact they will need additional employees dedicated to their new powder coating operation.

Step 1: Pretreatment

The first, and arguably the most important, step to a good finish is getting the metal parts ready to accept powder coating. This is commonly known as pretreatment or parts prep. This stage involves cleaning the parts and preparing their surfaces so the powder coating adheres properly, providing a long-lasting finish and a protective layer to prevent corrosion. Inadequate or improper pretreatment practices cause the vast majority of finish defects.

The metal substrate drives the pretreatment approach. Is it steel? Is it aluminum? Is it a mix of both? Different materials typically require different pretreatment chemistries.

The next factor to consider is the amount of cleaning required. Is there a certain standard the finish must meet to be acceptable? Many common finish standards include strict guidelines for pretreatment.

The condition of the parts also dictates what type of preparation is required. Did a part come straight from manufacturing, or was it left in a bin near the welding department for weeks? Did larger parts briefly sit in the yard, or have they been outside for a while and gotten rusty? Does scale or welding spatter need to be removed from the surface? How much grease or machine oil is on the part? With material that is relatively free of rust and debris, a single chemistry may be all that’s needed to achieve a clean part.

Although pretreatment chemicals can address many surface issues, some parts might require mechanical preparation first. Media blasting is often used to remove surface rust and address imperfections. Blasting is frequently isolated from the rest of the coating operation, but some systems include blast rooms or automated blasting machines installed adjacent to the other finishing equipment.

Parts cleaning usually is followed by a conversion coating, which provides the metal surface with some tooth, enabling the powder to better affix itself to the surface during curing for maximum adhesion. This ensures a more durable finish.

Some shops require only a simple setup where operators manually apply a cleaner or pretreatment chemical with a pump sprayer or a low-pressure spray wand. This type of equipment can be used to apply cleaners and chemistries that work at ambient temperatures.

A manual pretreatment booth has to be durable enough to stand up to the chemicals sprayed onto metal parts to prepare them to accept the powder coating.

From there, a company might consider stepping up to a heated spray wand or a steam unit. These can be configured to work with more than one pretreatment product at a time and are fairly simple to use. Dry steam units generate less water as a byproduct when compared to other spraying methods but won’t provide the forceful cleaning of conventional spray wand systems.

Manual pretreatment is commonly done in a wash station that features a metal enclosure or curtain walls to prevent the spray from getting into the shop environment. The wash station usually includes a fan system to vent steam or mist that might have a negative impact on nearby production equipment or parts.

These wash stations usually rely on a metal or masonry basin to capture residual liquid, which is pumped into storage containers for disposal, given time to evaporate, or put down the drain when allowed by local codes. Most manual systems use a spray-to-waste design so operators don’t have to contend with storage and recycling. Others have limited recycling, but that approach might restrict which pretreatment chemistries a company can consider. It also requires more strenuous chemistry monitoring and adjustment. That’s why most manual systems are set up so spent chemistry is not reused, and operators are always spraying fresh chemistry.

When throughput demands are high and acquisition cost isn’t a key consideration, an automated multistage pretreatment system might be the best solution. Most automated pretreatment systems have between three and nine stages, although more elaborate systems exist. In a simple system, there’s usually a heated treatment stage, an ambient rinse, and an ambient final rinse or sealant. In a more complex system, there can be multiple heated stages, redundant rinses, and stages that involve specially treated water and chemical sealants.

A multistage pretreatment system often is the single most expensive component in a powder coating line. Price is dictated by the size and number of stages involved. During the design phase, the equipment is scaled to meet throughput demands. The higher the throughput rate, the faster the conveyor runs and the longer the pretreatment system must be to allow for adequate dwell times for the various chemistries. So, if the parts sizing, chemistry, and number of stages remain the same, a system designed for a 2-FPM conveyor rate will be smaller and less expensive than one designed for 10 FPM.

Step 2: Powder Application

Once the parts have dried after the pretreatment stage, powder application takes place. In the simplest setup, a shop has an open-faced enclosure with a filtered exhaust system where the powder is applied manually. The powder application gun is typically on a small cart and is either fed from a compact metal hopper or draws the powder directly from its box.

With hopper-fed powder guns, the powder is isolated from the shop environment, which helps prevent contamination issues. This approach works extremely well if only a few different powders are being sprayed. The coater uses different hoppers for different products, and they are rotated depending on what powder is needed. Larger hopper systems allow one or more guns to feed from a much greater reserve of powder, so less production time is wasted swapping from box to box or hopper to hopper. If numerous colors and textures are routinely sprayed, it is more common to see box-fed guns in use or a combination of hopper-fed guns for standard powders and one or two box-fed guns for custom jobs.

Most spray booths are designed to accommodate either one or two operators, depending on the size of the enclosure and the throughput requirements. Booths can include specialized lighting, intake filtration, observation windows, and solid or filtered doors to help isolate the application process from the rest of the shop environment. Sometimes the entire application area is contained in an environmentally controlled room.

Spray booths typically feature filtration systems that capture powder overspray so it can be discarded. In the most affordable booth configurations, disposable filters collect the overspray and are routinely replaced.

Most operators prefer self-cleaning cartridge filtration units because they eliminate the downtime associated with frequent filter changes. The cartridge filters are cleaned by pulses of compressed air that dislodge the powder, which falls into a collection area or bins. This powder can be discarded or reused if a reclamation system is mated to the booth.

Pretreatment chemicals can be applied via steam as well.

The most advanced setups may include off-board collectors, which allow spent powder to be drawn away from the booth or even outside the building. Booths that are designed for use where extensive overspray is anticipated may include cyclonic exhaust modules, not unlike those used for other industrial applications.

Automated powder application booths vary in complexity. The simplest models have an array of hopper-fed guns mounted on adjustable stands. The guns are oriented to ensure the spray pattern properly covers the incoming parts as they pass through on the conveyor.

Movement can be added to the spray gun arrays for greater versatility. Powered arms move the guns up and down or side to side, or both. Some systems also allow the guns to move in and out or to have their orientation changed without manual adjustment.

Higher-end systems can be programmed with unique routines for different parts. This allows the guns to move in specific patterns, spray at precise intervals, and adjust their settings to achieve optimum results with a variety of parts. With many automated systems, the operator also can create specialized job logs to track production.

Color changing is another part of the powder application discussion. With a manual box-fed gun, it’s as easy as removing the pick-up tube from the box of powder, cleaning the hose and gun, then swapping to a different box. Hopper-fed systems require more extensive cleaning during powder changes. With automated systems, the operators usually have to spend more time changing from one powder to the next. Rapid color-change systems have been developed, but they represent a significant investment.

In addition to the need for clean hoppers and application equipment when swapping between powders, different powders can’t be collected in the same set of filters and then reused. If a booth is designed for use with more than one powder that is going to be reclaimed, it is common to see filter modules that can be detached from the booth and swapped for different modules designated for particular powders. This prevents contamination and ensures that the captured powder is suitable for reuse. If custom work is anticipated, one or more filter modules are reserved for spray-to-waste use, and the powder collected from those modules is discarded.

Step 3: Curing

Curing ovens come in many shapes and sizes. Some companies, particularly those with lower-throughput batch coating operations, use the same oven for drying parts after pretreatment and curing the powder-coated finish. Most companies choose to maintain separate ovens for drying and curing, not only to maximize productivity but also to have redundant capabilities in case one oven is out of operation for repair or maintenance. Companies with conveyorized coating lines typically have separate dry-off and curing ovens to allow for constant throughput.

Some curing ovens rely on infrared (IR) technology to cure the powder. These ovens typically have a smaller footprint than convection ovens. Gas catalytic or electrically powered emitters provide radiant heating. When the IR radiation is directed at the part, the transfer of energy allows curing to begin very quickly. IR ovens work well for simple parts such as flat panels, tubes, and boxes but may not be a good fit for more complex parts. This is because the IR emitters need to be aligned so energy can be radiated directly onto the powder-coated surfaces.

Hybrid ovens are becoming increasingly popular for conveyorized applications. IR emitters are used to heat the parts quickly and pre-gel the powder before the parts pass through a convection oven to complete the curing process. The overall size of the convection oven can be reduced, freeing up floor space.

In most systems, however, the curing ovens are gas-fueled and rely solely on convection heating. One or several powerful fans cycle heated air through the oven, with only a small amount exhausted to ensure safety and meet code requirements. Convection ovens designed for the powder curing process typically need to reach temperatures between 350 degrees F and 450 degrees F. Most ovens are set to cure powder at around 400 degrees F.

A booth for manual application of powder coatings typically has filtration systems that capture powder overspray so it can be discarded.

Many mistakenly assume that a curing oven is expensive to operate because of the natural gas or liquified petroleum gas needed to run it. However, well-made ovens are surprisingly fuel-efficient. When in operation, these ovens use the most fuel during startup. Once an oven is operating at the desired curing temperature, it’s not using as much gas. Only a small amount of heated air is exhausted from the oven, and the rest is constantly recirculated and reheated. So the heat system is not at full output.

In terms of operational expense, a walk-in-sized batch curing oven with a single heater may cost as little as $10 per hour to run, and a much larger oven for a conveyorized system may cost around $100 per hour. With conveyorized lines that are constantly curing parts, oven operation is an ongoing expense. With batch coating, proper planning can reduce operating costs.

The more thoughtful a company is about batching parts for curing, the less expensive it is to operate a batch curing oven. For example, if a company has a part that it is going to be fabricated on a predictable basis, it can accumulate a large supply of those parts, coat them, and then fully load the curing oven, rather than stopping and starting the oven throughout the day to cure only a few parts at a time. If throughput demands allow it, a company might save money by running its batch oven only a few days per week, processing multiple loads of parts back to back instead of stopping and starting the oven repeatedly each day.

Other Factors to Consider

Companies considering a potential powder coating system layout shouldn’t overlook the total amount of floor space required to work efficiently. The last thing a company wants is a powder coating operation that is unsafe or presents space constraints for workers. Is there enough room between appliances to remove a part from the line if there’s a problem? In the booth, can the coaters efficiently access all sides of the parts being coated? Does the conveyor have adequate space to move parts safely through the entire coating process? If it’s a batch operation, is there enough room to accommodate the movement of rolling racks or forklifts? There are few things more frustrating than having to wrestle with a heavy batch of parts because the turning radius needed for the parts rack to enter the equipment wasn’t properly accounted for.

Another commonly overlooked space consideration is the need for staging areas. With batch systems, space is needed to accumulate parts as they are made ready for each stage of the powder coating process. Otherwise, the operator is running back and forth moving parts instead of staying focused on prep work and coating.

With conveyorized systems, the loading and unloading areas should be well thought out. There needs to be adequate time for coated parts to cool down after curing, so there is sometimes a section of conveyor added after the curing oven. It is sized to ensure parts can be handled safely during unloading and may include a series of zigzags or a straight section of track, sometimes with a turn so parts go down and come back before being unloaded.

A final consideration is planning for growth. After a system has been in operation for a while, it’s not uncommon to end up needing to process more parts per shift or coat new parts that are too large for the equipment to handle. Most batch coating systems have some degree of modularity, so existing appliances can be expanded or new appliances can be added to accommodate larger parts or greater throughput requirements. This can only be done if the system hasn’t been shoehorned into a cramped space. With conveyorized systems, enlarging and repositioning the equipment is much more difficult and much more expensive, so it's better to size the system with potential future needs in mind.

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