How Do Gas & Electric Powder Coating Ovens Work?
To get professional quality results, powder curing ovens need to have specific performance features. This article discusses those features and describes the most common types of powder coating ovens used for industrial job shop applications. At Reliant Finishing Systems, we manufacture various ovens, including models specifically designed for painting and powder coating operations. Powder coating ovens are used to cure powder previously applied to a substrate, usually metal parts. Unlike conventional liquid paints, powder coatings consist of small particles of solid material. Because of their different characteristics, wet paint and powder coatings have different curing requirements.
The Basics of Powder Curing
When powder particles are heated, they melt and flow together. This creates a uniform, bonded finish once it has cooled. Typically, the finished coating is 2 to 4 mils thick. Thicker coatings are possible and are often created by applying multiple layers of powder. All ovens used for powder coating operate at elevated temperatures and rely on some type of heating system. Regardless of the technology used, powder curing ovens heat the surface of the parts being coated, usually to about 400° F.
Powder suppliers provide data sheets that include tables showing the required dwell time across a range of acceptable curing temperatures. At higher temperatures, the time required to cure parts will be less than at lower temperatures. These tables are based on the assumption that the entire area to be coated has reached the desired temperature and will maintain it for the duration of the curing process. When curing the powder, only the surface of the part it was applied to has to reach and maintain the curing temperature. Because of the way it acts as a heat sink and absorbs heat energy, a large, dense part, like an agricultural implement, will take longer to cure than a small, thin part, such as a sheet metal trim piece, because it will require more time to get the surface of the part to the appropriate temperature.
What Powers Powder Coating Ovens?
The most common powder coating oven designs rely on natural gas fuel to power their heat systems. In rural areas, liquid propane (LP) gas may be used instead. Although electric heating can be used, it isn’t practical for most batch or constant process ovens used in a production environment. Ovens larger than about 6’ x 6’ x 10’ are almost always heated by gas fuel.
How Do Ovens Heat The Parts? Convection Or Radiant Heat
Most curing ovens on the market rely on forced convection to heat parts placed inside them. Convection is usually the dominant form of heat transfer in gases, such as the air circulating inside a powder curing oven. Convection (or convective heat transfer) is the transfer of heat from one place to another inside the oven due to air movement. Although often discussed as a distinct heat transfer method, convection involves the combined processes of conduction (heat diffusion) and advection (heat transfer by bulk airflow).
Simply explained, convection heaters are indirect heaters. Air is heated by one or more burners attached to the curing oven. The heated air is circulated through the oven by a fan system, where it comes in contact with the parts. Heat energy is transferred to the parts, increasing their surface temperature. The majority of the air is then cycled back through the heat system, where it is reheated, typically via one or more burners that heat the air directly. A small amount of the heated air inside the oven is exhausted to meet safety codes, such as NFPA 86.
Radiant heating systems offer more direct heating. In some ovens, special emitters generate focused infrared (IR) rays that radiate heat directly onto the parts. These are very effective, but heating takes place only in the “line of sight” of the emitters. Hidden portions of the part are not effectively cured, so these heaters are a better fit for parts with simple profiles, like flat panels, than complex parts. With small, inexpensive ovens designed for DIY coaters, conventional heating elements radiate heat to the air and parts within range without circulating the air. The heating covers less area but provides relatively efficient targeted heating. These heaters are more directional than convection heaters, which makes them reasonably effective for small parts being coated in a home workshop.
Considerations When Choosing Convection or IR Heating
As mentioned above, electric heating is very poorly suited to large powder-coating ovens. The amperage needed to operate the heating elements suited for convection curing ovens can be staggering and the cost to operate them is significantly greater than with equivalent gas-fueled heaters. When talking about IR emitters, they can be either electric or gas catalytic. The latter are more common in larger coating operations.
The size/shape of the parts being heated is typically the most significant factor when choosing between IR and convection. Convection heating is well suited for complex parts. Since it heats circulating air, it can heat the entire surface of a detailed part with relative ease. IR heating is almost never used in a job shop environment or where the coating system will be used for parts of varying size and complexity.
The reliance on circulation means convection heating is most effective in a closed area, such as a batch curing oven. When dealing with conveyorized constant process powder coating lines, it is important that the curing oven has features to limit the escape of heated air. This can include ware openings that are only slightly larger than the parts entering and exiting the oven, air curtains to help isolate the oven environment, and heat abatement zones that exhaust only a small amount of heated air or pump it back into the oven.
With almost all ovens or parts heaters, one or more motorized gas valves react to the appliance’s control system, enabling the device to use only enough fuel to maintain the required curing temperature or emitter output. For shop environments where a conventional oven enclosure may require too much floor space, a radiant heat system, like freestanding IR emitters, may be a better option if the parts are well suited to IR heating. These are typically only activated once the part to be cured is in place. IR heaters reach temperature quickly, but heat energy is quickly dissipated once turned off. A convection heater takes more time and energy to achieve a target temperature than an IR heating system. However, the heat is easy for the oven to maintain once it reaches curing temperatures.
A hybrid approach may be used in sophisticated powder coating systems where a high throughput rate is required. IR emitters preheat exposed areas of the parts, causing the powder to begin to melt and flow. These are sometimes called pre-gel heaters. The parts are then exposed to forced convection heating for the remainder of the curing cycle. This approach is most common with conveyorized constant process coating lines. It is also sometimes used with parts that require a primer coat and a second top coat, or where multiple layers of coating are needed to reach the desired mil thickness. The parts are coated, processed using an IR oven or IR heating system, coated again, then cured fully in a convection oven.
What Does It Take To Get Professional Powder Curing Results?
Professional quality ovens have certain traits in common. They have a “curing zone” inside the oven cabin where airflow is most effectively directed onto the parts or IR energy efficiently reaches the parts’ surfaces. With convection curing ovens, this area also has relatively uniform air temperatures, although some variation is common. Professional curing ovens are engineered to reach curing temperatures relatively quickly and maintain uniform curing results cycle after cycle.
With lower quality radiant heat or forced convection ovens, like those commonly built or bought by hobbyists, performance issues can compromise finishing results. These issues can include temperature inaccuracy and temperature overshoot due to unresponsive burners/elements/emitters or unsophisticated controls, poor heat transfer due to inadequate airflow or poorly aligned IR emitters, and extended curing cycles due to underpowered components.
What Oven To Buy?
You don’t need to understand all the science and engineering of an oven to be able to use it. However, your finishing department should understand this article’s key points in order to get the most effective results from the powder coating equipment they purchase and operate. If you want to consistently achieve professional quality results, you’ll need to select well-engineered powder curing ovens. At Reliant, we build finishing equipment that outperforms and outlasts competing products–typically at lower prices because we sell factory-direct. Please contact us if you are looking for reliable, fuel efficient powder coating ovens that will provide the best performance and value for your finishing operation.