Building a Better Powder Coating System: Oven Design
Posted on Wednesday, February 6, 2019
So, you have decided you are ready for the investment in a finishing system and you need to figure out the oven portion. Where do you start?
When reviewing what type of oven designs you want to consider, you first want to consider the heat technologies available and then select a qualified partner and optimum design.
Convection – An electric or gas fired burner is used to heat air which is recirculated throughout the oven. The flow of heated air is what heats the parts. Convection is the most commonly used form of heat; it works for the largest variety of parts and is the simplest heat technology to operate. The downside to convection is it requires the most amount of floor space, is the slowest heat technology and is the least energy efficient technology.
Infrared – You may not realize it, but you feel infrared energy every day. The radiated heat we feel while standing in sunshine or near a fire is radiated infrared energy.
Infrared ovens are either powered by electricity or gas; each of these have different construction methods and different heat ranges. Gas operates at lower heat temperatures and is a softer and more flexible infrared heat, while electric is more powerful and the fastest heat technology but loses flexibility as the temperature rises. Gas is typically chosen when operating costs are a primary driver, electric when speed is the primary goal.
When we review oven functions, we must first consider the operating goals for the system. Powder coating systems can vary greatly depending on three major considerations – the 3Ps of powder coating designs.
Part = What type of substrate will be processed? Light gauge vs. heavy, dense parts, aluminum vs. steel.
Profile = How are the parts shaped? A 3 pound part is very different than a 300 pound part. Sheet metal is different than a part with heavy weldments. How complexly shaped are the parts? Are there thick and thin areas to the parts?
Process = How will the line function? At what line speed and conveyor process? A 3 ft/min line speed is a very different design from a 20ft/min system. Same with monorail vs. power-and-free conveyors.
Each of these issues affect what type of heat technology is best used for each application. There is no one right answer, but a complex set of questions, best answered by an experienced design engineer, who can ensure your system is designed to function for your immediate needs and be capable of any growth needs you may want to accommodate.
To review your system functionality, there will typically be a minimum of two ovens within the powder coating system PCT design. These can be combined, but the functions will remain. We start with dry-off oven.
As the name implies, a dry-off oven dries parts after they exit the pretreatment process. It is imperative that parts be fully dry before entering the powder booth. If parts have hidden recesses, water from the washer can pool and can be a challenge to remove. The best designs require airflow to displace cupped water, and remove water vapor, as well as heat energy to actually dehydrate the water. The most energy efficient dry-off ovens operate at a lower heat than a cure oven, since water boils (turns to vapor) at 212 degrees Fahrenheit. Therefore, temperatures well above that waste energy. While some residual heat in the part from the dry-off oven can be beneficial to promote film build (increase coating thickness) of the powder, an overheated part can create quality problems in the booth. Dry-off ovens can be combined into the cure oven, though they should be functionally separated. Depending on part configurations, the oven design will almost always require air movement, which limits the designs to convection or catalytic infrared. Electric infrared can be used for simple parts but does not operate well with air flow, while catalytic infrared is enhanced by air movement. Air knives, typically mounted at the end of the pretreatment washer, can also be used to move any pooled water and reduce the load of the oven.
Once the parts have been coated, the final step is the cure process. Depending on the powder used, the cure temperature can vary some. However, depending on the heat technology used, the time to cure can vary greatly. Powder manufacturers specify a cure time once the part has been brought up to temperature, but those times are greatly overstated for cure times using infrared. As a general rule, catalytic infrared can be used to cure (even complex parts) in about a third of the time it takes to cure the same parts using convection. Line of sight issues can be a challenge (radiant energy must come in contact with the coating surface to be cured), but the right design can overcome most of these issues with the downside being that catalytic can be less flexible. Conduction (the direct heating by contact) is actually a powerful ally to any curing process, even infrared. Electric infrared can cure up to 10 times as fast as convection, but the downside to the speed is that conduction does not have much time to assist the curing process so that increases the line of sight issue. Electric can be used to cure, but typically only for a consistent range of parts and typically small parts where an oven can be designed for a specific part configuration. Utility costs to operate electric in a cure environment can also be a concern when larger parts are introduced. Catalytic infrared is a softer heat and can successfully cure even complex parts. Convection heat, however, is by far the most common heat technology used for cure. It will successfully cure most any parts with the exception being those that are heat sensitive (parts with internal components) or alternative substrates (such as MDF or hardwoods).
Heat Technology and System Design
So, what considerations must be made in order to determine a heat technology and then decide on a design? We must return to the 3Ps. The parts being presented and the required production (i.e., parts/hour) create the assumptions to consider the next design factors. The reason convection is the most ubiquitous technology available is because it can handle most any needs for dry-off and cure. If the primary concern is flexibility, then convection is typically the best choice. If space restrictions are a concern, all infrared technologies will take up significantly less floor space due to the ability to cure powder in a fraction of the time of convection heat. If cost per piece or operating costs are key drivers, then catalytic infrared should be considered as it is by far the lowest cost heating technology. However, if speed of production is the primary challenge, then electric infrared should be considered. If a two-coat operation is desired, adding infrared is the easiest and most efficient method to gel powder between coats. One other option to consider is a combination of these technologies. Either gas or electric infrared can be used in conjunction with convection in a gel or boost application. This provides some of the best of each technology without sacrificing the flexibility of convection.
This leads to your design consideration, which generally is a function of the experience of the equipment supplier with whom you are working. Suppliers will have differing design philosophies, so approaches may vary. A powder coating system is a complex project and will require a significant amount of engineering. If infrared is being considered, testing is almost always a required step to provide the proof of projected process times. Look for possible partners through referrals from your consumable suppliers (powder or chemicals), other equipment distributors or industry publications. A great website is not always an accurate indicator of experience or quality of construction but seeing proof of the depth of a company’s designs typically is. An experienced manufacturer should be able to show you many examples of designs with similarities to your needs, as well as customer testimonials. Seeing an installation first hand to inspect quality and durability is always the best plan, though between travel and proprietary concerns it may be not be practical. As long as you are working with a company with in-depth experience and a strong reputation within your industry and needs, you should have a successful outcome.
by Marty Sawyer, CEO of Trimac Industrial Systems, Inc.