Powder Coating 1996.

 
 

Powder Coating 1996.

Infrared heating has many benefits that make it an attractive curing method for powder coatings. This article discusses those benefits and the various factors that affect infrared (IR) curing efficiency, productivity, and finish quality. As a result, finishers will understand the options and opportunities offered by IR heating and, from a practical standpoint, how to get the most out of an IR heating system.

Although IR process ovens have been used for decades to cure powder coated parts, the technology is viewed by some as powder coating technology was a few years ago: a sophisticated technology with limited application and potential. Yet, like powder coating, the advantages of IR curing have simply overwhelmed its doubters. No other heating method offers the economic, environmental, and quality advantages that IR curing technology does. Some of its advantages follow:

  • Reduced energy consumption
  • Accelerated curing cycles
  • Improved finish quality
  • Reduced operating noise
  • Reduced space requirements
  • Decreased warm-up time

Two basic properties of IR heating systems make these advantages possible: (1) the high rate at which energy can be transferred by IR heat and (2) the fact that IR heat can be transferred directly from its source to the coated part without the use of an intermediate energy carrier, unlike convection heating, which uses air as its energy carrier.

All improvements IR curing brings to a finishing operation are the result of the proper application of one or both of these basic properties. Conversely, if one or both of these properties cannot be applied in an operation, it is not likely that IR curing will offer any advantages.

Infrared oven types. Regardless of the fuel or heating element used, IR curing systems generally are one of two distinct types: a pure IR system or a combination IR and convection system.

IR only. Pure IR oven systems transfer heat by radiation. No air movement occurs except as a byproduct of the heat transfer process (natural convection) or the oven exhaust system. Pure IR ovens generally are suitable for powder coating cure applications in which the sole requirement is the transfer of a specified amount of heat. Curing powder coated flat panels is an example.

In general, IR curing systems require higher capital costs than convection ovens of comparable capacity. The cost per unit of energy input for IR elements or burners is higher than that of convection elements or burners. In addition, to achieve accurate temperature control, more sophisticated sensors and temperature controllers are required with IR systems compared with convection systems. A well-designed IR oven will be divided into smaller zones of control, increasing the overall control system's cost. However, operating economies, quality improvements, and production increases offset the higher initial costs in most applications.

IR-convection combination. Simply replacing a convection oven with an IR unit is usually not the best approach to take to cure powder coated parts unless the parts are simply shaped. As many converts to IR heating have discovered, the best curing system for powder coated parts usually involves a combination of IR and convection heating techniques.

This can be attributed to the two steps required in the powder curing process. First, the coating must be raised to fusion1 temperature without being disturbed. Since powder particles that have been applied electrostatically begin to lose their charge during heat up, they can be disturbed by vibration or excessive airflow. Second, the powder must be raised to a specific cure temperature and held at that temperature to complete crosslinking. Although fusion temperature is unique to each powder, it often falls between 200°F and 300°F with crosslinking occurring between 300°F and 400°F.

Because of the dual steps involved in the powder curing process, a combination IR and convection oven is often the most suitable powder curing system. The IR stage rapidly raises the part temperature without air movement. The convection stage provides accurate temperature control on all part surfaces, without danger of over heating the most salient part surfaces. Combination systems are generally used in powder coat curing applications to augment IR heat transfer or to improve temperature uniformity on complex shapes.

Infrared and convection differences. There are several important differences between IR heating and convection heating. Finishers who understand these differences are better equipped to determine which heating method-IR alone, IR and convection combined, or convection alone is most suitable for their application.

Heat-transfer rates. Infrared heating is only suitable for processes that can use a high rate of heat transfer. Using the area of a part being heated as a basis, a standard convection oven is generally designed to transfer 500 to 2,000 BTU/hour-square foot. By contrast, IR ovens are generally designed to transfer from 3,000 to 25,000 BTU/hour-square foot.

Infrared's high heat-transfer rate can substantially reduce the time required to cure parts. Because powder coatings only require a rise in temperature to fuse, they can be cured in one-third to one-half of the time that it takes in a convection oven on production parts. Greater reductions can be seen on flat, light-gauge parts of simple geometry

Time-temperature relation. Another significant difference between IR and convection ovens is that convection ovens are generally designed to provide an ambient air temperature not far above the desired part temperature. Stated another way, convection ovens are generally designed to heat parts to the equilibrium temperature the part would reach if left in the oven forever.

Infrared ovens, on the other hand, operate with source temperatures ranging from 600°F to as high as 4000°F. The equilibrium temperature a part would reach, if left in the oven indefinitely, would be considerably above the actual part temperature desired for the specific coating cure process. Hence, IR ovens do not heat a part to equilibrium temperature. Instead, they heat a part to a transient temperature, which depends on the part's thermal capacity, its exposure time, and the net heat transfer between the part, its environment, and the IR sources in the oven.

Thus, exposure time in an IR oven must be accurately controlled to achieve uniform results. Variations in exposure time from part to part will produce far greater variations in final part temperature than the same variations will produce in a convection oven.

If very accurate temperature control is desired, it can be readily achieved in a convection oven by operating with a long oven residence time and a supply air temperature equal to or only slightly above desired part temperature. More sophisticated control strategies are required to achieve precise part temperatures in an IR oven. As a result, the control system on an IR oven is more complex and expensive.

Line-of-sight heat transfer. An IR source has what is called line-of-sight heat transfer. In other words, it transfers heat only to surfaces it can see. Hence, a pure IR oven cannot heat all surfaces of a complex shape uniformly. If no variation in temperature can be tolerated from surface to surface on a complex part, an IR oven cannot be used to heat the part or cure its coating. However, the more tolerance allowed by the characteristics of the coating used, the more complex a part shape can be and still cure properly in an IR oven.

Though no absolute rule can be formulated, pure IR ovens have been successfully applied to cure powder coatings on flanged panels, formed angles such as table legs or stiffening brackets, and tubular parts. Combination IR and convection ovens have been used for steel file drawers and rural mailboxes. Shapes to avoid are deep-drawn parts with hidden recesses or assemblies with hidden pockets or corners.

Convection oven designers assume that the exhaust temperature will be equal to the final part temperature. When designing an IR oven, this assumption must be discarded. Because the IR energy is transferred directly to the part being heated, the ambient air temperature in the oven has no direct relation to the part temperature. Actual exhaust air temperature will depend on the exhaust airflow rate, IR source efficiency, and heat absorbed by the part being cured. Exhaust air temperature is determined by a heat-balance calculation to properly size the exhaust and determine proper energy input for the entire curing system.

Finishing properties and curing mechanisms. An oven designer has to know exactly what the oven must accomplish to cure a customer's coating. Frequently, a coater will specify that the oven must maintain the part at a certain temperature for a certain amount of time, such as 5 minutes at 350°F. Without additional qualification, such a specification is useless. Usually, the specification is based on the time the part takes to travel through a production or a laboratory oven, and the reading is based on the oven's temperature controller, rather than the actual part temperature. More precise information is needed to design a cure oven properly.

The best way an engineer can get reliable design data is to ask the powder coating supplier. The supplier's chemist can provide accurate information on the coating's curing mechanism. The chemist can also provide actual curing cycle curves relating residence time and substrate temperature as mutually dependent variables under well-defined oven curing conditions.

The next best way to get this information is through laboratory testing. Then the successful test results can be accurately scaled up to production size with respect to heat intensity or heat-transfer rate at the part surface.

Role of testing. Tests are frequently used to design IR ovens to avoid the need for or to verify the results of lengthy calculations. As it is rarely possible to conduct tests in a full-sized oven system, it is usually necessary to scale up the results of laboratory or field tests with a temporary setup.

Tests should determine the maximum IR heat intensity at the part surface to achieve a proper cure in the least amount of time. Any lesser intensity would yield a longer oven than necessary; any higher intensity would deteriorate the part or overbake the coating. It is more important to find the maximum IR heat intensity at the part surface than the distance a part should be from the IR source. The former can be used to scale up a full-sized oven, the latter cannot.

For example, test results of parts at a distance of 6 inches from an IR source in a laboratory oven cannot be duplicated in a full-sized oven with a large bank of the same IR sources. A large bank of IR sources would have to be farther from the part, or the intensity of the IR sources would have to be reduced. Source intensity can be reduced by increasing the space between the elements or by using lower intensity elements.

Builders of IR ovens usually use test equipment that is calibrated so the heat intensity is known at various distances between the part and the IR elements. If an IR source has not been calibrated, builders use a heat sink whose temperature is accurately measured after a specific exposure time, or the time it takes to evaporate a known weight of water in a shallow cup that is recessed in a block of insulation.

Oven system efficiency. Efficiency is a term frequently used and misused in connection with processing systems. A common definition will usually eliminate much of the disparity in claimed efficiencies by vendors. For example, in a typical powder-coat curing application, energy input is distributed among these factors:

  • Heat absorbed by the product and carried out
  • Heat absorbed and carried out by the conveyor and tooling
  • Heat absorbed by phase changes or chemical reactions within the oven (fusing of powders)
  • Heat carried out by airflow through the exhaust system
  • Heat lost through the oven walls, roof, and openings

A utility representative can claim 100 percent efficiency because all of the electrical energy consumed by the oven is converted into heat energy, useful and necessary for the process. The skeptic can claim that the efficiency is zero percent because none of the energy was really necessary: All of it is eventually thrown away to the atmosphere. The contention arises when, for example, one vendor considers the first three energy distribution factors just listed, and another vendor considers the first, second, third, and fifth factors. Without a common definition, both vendors are right. The point is, be sure you understand a vendor's definition of efficiency before accepting or considering an efficiency claim.

For any project, tests and calculations are the best ways to determine which heating system is suitable for an application. Then the final decision can be made by calculating annual owning and operating costs.

Infrared heating sources. Infrared heating sources for powder coatings may be electric or gas fired. They consist of an IR radiating source, or surface, operating between 500°F and 4000°F. The energy input per unit of radiating area determines the specific temperature. Some IR sources may require reflectors to redirect the majority of the IR heat emitted toward the parts within the oven.

Gas-fired sources use the energy of fuel combustion to heat metal or refractory surfaces to a temperature at which they will radiate. Combustion byproducts and convection to the air in the oven carry off a portion of the energy. As a result, the radiant efficiencies of gas-fired IR sources are generally from 30 to 60 percent.

Electric IR sources use the flow of current through a resistance heating element to raise the element itself or surrounding material to a temperature at which it will radiate. If none of the element's heat is lost by conduction or convection, all of the energy is radiated and the element has a 100 percent radiant efficiency. However, a portion of the energy is always lost by convection, depending on the accessibility of the element to air and its general construction. Hence, radiant efficiencies for electric sources generally range from 70 to 90 percent.

While it is tempting to conclude that electric elements are better than gas-fired burners for IR ovens, radiant efficiency alone does not necessarily limit the oven efficiency in an application. In fact, it is quite possible that a source with a high radiant efficiency will yield a lower oven efficiency than another source with a lower radiant efficiency but with other characteristics making it more suitable for the application.

Equally important, the overall design of the oven can significantly affect the curing system's efficiency. Factors such as proper insulation, airflow control, and accurate temperature control can improve efficiency markedly. In addition, the cost per BTU of electric energy is generally higher than that of gaseous fuels, making it necessary to examine annual owning and operating costs, rather than element efficiency alone, to arrive at an economical selection.

Gas-fired IR burner. These IR sources generally rely on heating a metal or ceramic to an incandescent temperature in the range of 700°F to 2000°F. Fuel and air are mixed at correct ratios for combustion and burned. If the mixture reaches ignition temperature in the piping or manifold before reaching the burner, pre-ignition or flashback will occur. This will destroy the burner.

Impingement burners. Made in a variety of shapes and sizes, impingement burners include a gas-burner flame firing on refractory ceramics of various shapes. The surface of the refractory ceramics radiates the IR. Impingement burners are generally not susceptible to pre-ignition or flashback, resist physical damage well, but have relatively low radiant efficiencies. They are useful in caring systems requiring combined convection- IR heating as a result of part geometry. They resist damage from falling parts and work well in multipass ovens in which cooling air cannot be provided to the rear of the burner. Life expectancies are measured in years, unless the burner is subjected to overfiring.

Porous matrix burners. These burners have porous or perforated refractory plates mounted on cast iron or formed steel plenum chambers. The refractory material may be porous ceramic, refractory blanket, ported ceramic, stainless steel, or metallic screens. The fuel-air mixture is supplied under pressure to the plenum chamber. It passes through the porous matrix to burn on the surface facing the load. Combustion occurs evenly on the exposed surface, heating it to incandescence. As the surface heats up, the flame recedes into the matrix, which adds radiant energy to the flame.

Such burners typically operate at surface temperatures approaching 1850F. Cooling of the plenum chamber on the rear of the burners must be done to prevent pre-ignition of the combustible mixture. Airflow as a result of natural convection usually accomplishes this. Porous matrix burners have the highest radiant efficiency of the gas-fired IR sources. Modulating the input fuel provides about a 3:1 turndown capability in oven heating intensity

Catalytic burners. Catalytic burners consist of a porous ceramic or blanket material impregnated with a catalyst, such as platinum black, through which a combustible air-gas mixture is fed. They are similar to the porous matrix units in construction, appearance, and operation, but the refractory material is usually glass or ceramic wool. The combustible air-gas mixture oxidizes within the matrix at temperatures below those normally required for combustion. No visible flame is produced. These burners provide low-to-moderate intensities. They must include an alternate heat source to preheat the catalyst before production. Usually, electric heating elements are the alternate heat source.

Radiant tube or panel sources. These sources are internally fired metal tubes or panels. Radiant tubes have a burner at one end firing down the tube. They typically operate at surface temperatures up to 1200°F. Radiant panel systems surround the parts to be heated with a metal enclosure. Radiation and hot combustion byproducts scrubbing the enclosure's surface heat the enclosures exterior. Infrared emission heats the parts in the enclosure's interior. The combustion byproducts can be vented or ducted to the convection portion of the curing oven.

Electric IR sources. Electric IR sources use heat produced by current flowing in a resistance wire or ribbon causing the wire and the IR source to reach an incandescent temperature.

Quartz tube IR sources. These IR sources contain a coiled nickel-chrome wire lying unsupported within a to 3/8 to 5/8 inch fused quartz tube. Quartz is used because it is much more transparent to IR rays than other materials. As a result, higher heat intensities are achieved. Porcelain or metal terminal blocks cap the tube. Because quartz tubes are not sealed or filled with inert gas, the oxidation temperature in air limits the operating temperature of the resistance wire. This also limits how closely they can be mounted in a curing oven to achieve intense heating. Normal operating temperatures range from 1300°F to 1800°F for the coil and about 1200°F for the tube.

Although impact or vibration can easily damage these units, they stand up well to thermal shock. They must be mounted horizontally or the internal coil will sag and short-circuit. Because the element radiates in all directions, they are usually mounted in a fixture that contains a reflector. Life expectancy depends primarily on how close the element operating temperature approaches its oxidation temperature.

Quartz lamps. Quartz lamps (T-3 lamps) are generally made as 3/8 inch-diameter tubes of various lengths. They include a fused quartz tube containing an inert gas and a coiled tungsten filament held straight and away from the tube by spacers made of tantalum. Filament ends are embedded in sealing material at the tube ends. The tube is pinched to hold each of the tantalum spacers in place to help prevent element sag. Standard lamps must be mounted horizontally, or nearly so, to minimize filament sag and overheating of the sealed ends. A modified design is available for vertical mounting.

At normal design voltages, quartz lamp filaments operate at about 4050°F, while the envelope operates at about 1100°F. At full voltage they have an average life of 5,000 hours and can withstand higher oven ambient temperatures than bulbs or tubes. This allows their use in more closely packed densities to provide higher output intensities. Reducing voltage even slightly increases lamp service life significantly.

Quartz lamps are usually mounted on banks of reflectors, which form the oven's sidewalls. Heat-up and cool down times are short because the mass of the filament is very low. The ovens are not always insulated; the lack of insulation helps keep lamp terminals and wiring cool. In some very high-intensity applications air-cooled reflectors, water-cooled reflectors, or both are used.

Metal sheath elements. These elements include resistance heating wire embedded in an electrically insulating ceramic material enclosed by a tube of steel or alloy. Tube diameters generally range from 3/8 to 5/8 inch. Similar elements are used in broilers of electric ranges.

The oxidation or scaling temperature of the resistance wire embedded in the tube limits operating temperatures. Metal sheath elements are quite rugged; have excellent resistance to thermal shock, vibration, and impact; and can be mounted in any position. At full voltage, the elements attain a sheath surface temperature between 1000°F and 1500°F. Higher radiant efficiency is achieved when these elements are shielded from direct airflow. The thermal storage of the element's filler insulation and sheath yield long heat-up and cool-down times. This can be a disadvantage. However, unlike most electric elements, inexpensive percentage timers can be used.

Radiant panels. Radiant panels include resistance heating wire grids or ribbons sandwiched between a thin plate of electrical insulation on the radiating side and thermal insulation on the back or cool side. Low- temperature panels often use thin ceramic papers and boards as the radiant surface. High-temperature panels often use Y4-inch-thick quartz or ceramic plates. Panels come in various dimensions ranging in widths from 10 to 30 inches and lengths from 12 to 96 inches.

Maximum heat generated is typically 12,000 BTU/hour-square foot. Normal operating temperatures of the radiating surface are 500°F to 1400°F. The maximum temperature the radiant surface can withstand and the oxidation temperature of the resistance wire limits operating temperatures. Because the entire surface of the element serves as a radiator, no reflectors are generally needed.

Since the entire surface emits IR radiation, relatively high IR intensities can be achieved at lower source temperatures compared with lamp or tubular sources. Like sheath elements, most panels heat up slowly but provide smooth heat control with contactors or percentage timers. Panel elements generally cost more per BTU/hour input than other elements. Life expectancies are quite long (5,000 to 10,000 hours), unless elements are overheated or damaged some other way.

Infrared curing is ideal for transmitting large amounts of energy to quickly bring a part up to cure temperature. A convection oven that circulates air at the desired part temperature is often more suitable for holding a uniform and constant part temperature. Thus, the most suitable heating system often includes an IR stage, followed by a convective stage.

For example, a good oven choice for a job shop that runs a large variety of parts in many shapes, sizes, and gauges of material on a monorail conveyor would be a zone of impingement or matrix burners followed by a forced-convection section. A tubular electric element could also be used because it resists damage and accommodates various coating colors.

A good oven choice for a job shop that powder coats similarly sized flat panels of the same metal thickness in a range of colors, such as an appliance manufacturer, would be a full-sized IR oven with quartz tubes or lamps. Quartz tubes would provide rapid control response and high energy delivery, minimizing overall oven length and providing accurate, repeatable temperature rise. Quartz lamps would be less likely to require adjustments to accommodate various coating colors.

To determine the most suitable oven, end users should compile a list of criteria unique to their parts and production requirements. They should also become familiar with the curing characteristics of the powder coatings they use. Then they can make the best oven choice for their application.