The coatings and paint industries strive to provide high technology coatings while reducing volatile compounds and energy consumption to provide a finished coating. Conventional curing process uses convection ovens to provide necessary cure. Here the air being heated in a chamber is circulated by using blowers inside the chamber. This process is time consuming and has high power requirements.
Infrared curing, an alternative for convection curing is one of the fastest growing technologies in the coating industry. It applies infrared rays to the part surface by direct transmission from an emitter. The direct transfer of energy creates an immediate reaction in the coating and cross-linking begins rapidly. Since infrared rays are electromagnetic radiations they do not require a medium for transmission. Hence the need for air recirculation is eliminated. Moreover the infrared units can be powered to full power in under one second, which results in fast curing. Moreover this system requires less floor area.
Though infrared ovens has certain advantages over convection ovens the common practice in industries nowadays is to incorporate the features of both infrared and convection ovens into one unit. Thus the advantages of both the systems can be sapped.
1. INTRODUCTION 1
2. CURING 2
3. CONVECTION CURING 3
4. INFRARED CURING 5
5. COMBINING INFRARED AND CONVECTION OVENS 11
6. CASE STUDY 17
7. CONCLUSION 24
The coatings and paint industries strive to provide high technology coatings while reducing volatile organic compounds and energy consumption to produce a finished coating. Conventionally Convection ovens are used to cure the coatings. But this process which uses electric heaters is not an optimal process and is associated with various disadvantages. Improved technologies are available today, which can either replace or improve the convection curing process. Infrared Curing is such a technology which uses Infrared rays emitted by an Infrared emitter to provide the required cure. Infrared curing applies light energy to the part surface by direct transmission from an emitter. Some of the energy emitted will be reflected off the surface, some is absorbed into the polymer and some is transmitted into the substrate. This direct transfer of energy creates an immediate reaction in the polymer and cross linking begins quickly once the surface is exposed to the emitter. Infrared emitters are often custom manufactured to suit the production demand. The various aspects of Infrared curing and convection curing and the possibility of combining these two technologies into a singe system will be discussed in this seminars.
Curing is a process of baking surface coatings so as to dry them up quickly. Curing is a broad term which means all the techniques employed for the finishing operations incurred during part production. Curing essentially involves either the melting of the coating or evaporation of volatile fluids present in the coating by the application of heat energy.
Curing is given to a wide range of materials both organic and inorganic .Usually curing is given to materials like ,
• Powder coatings
• Epoxy coatings
• Acrylic coatings
• Primers Etc
Curing is also given to Rubber and Latex .The principle used for curing can also be used for drying rice and grains.
Convection ovens are usually used for curing purposes. Traditional convection ovens use heated forced air to provide the necessary cure. Convection ovens consist of a chamber lined on the inside with Electric heaters. The shape of the chamber will be in accordance to the shape or geometry of the part being cured. A series of blowers circulate the heated air around providing the required cure. This process depends on convection to transfer heat from hot air to body surface and conduction to transfer heat to the interior of the surface. The air being delivered is held at temperature using closed-loop control, which provides predictable, repeatable results. Typically a temperature of around 250-500 degree farenheit is required for paint or powder. Though convection ovens are widely used today they have certain disadvantages, which chokes the overall productivity of a company
Disadvantages of convection ovens :
• Fairly long heating times:-
Convection is a slow process. It takes a considerable amount of time for the heaters to heat up and raise the temperature of air to the required level. This causes a lag in the process and hence the curing time increases. Longer curing time spells reduced assembly line movement. This in turn reduces productivity.
• High energy consumption:-
A convection column dryer uses around 2000 BTU(British Thermal Unit) of energy to remove 1 pound of moisture. They use around 7.7 KW of electrical energy to dry a ton of rice. These are significantly larger figures for any company trying to bring energy consumption under control. The additional use of blowers and compressors further increases energy consumption.
• Large floor area required:-
Convection ovens are bulky in nature. Due to the presence of compressors and blowers, additional space is needed, which in turn increases the floor area requirement.
• Air circulation is required:-
Convection heating requires a medium for transmission of heat. Hence blowers are employed for good circulation of heated air. This increases the overall cost of the equipment.
The use of an alternative technology to convection curing is essential to neutralize it’s disadvantages. Infrared curing is such a technology which is rapidly attaining acceptance in the coating industry.
Infrared ovens, unlike convection ovens use Infrared emitters. These emitters give out Infrared rays, which is a form of electromagnetic radiation located on the electromagnetic spectrum between visible light and microwaves and measured in microns. The three wave lengths of infrared, low, medium and high, will emit energy that will be absorbed by the organic coating, reflected from the surface, or transmitted into the substrate. The actual behavior of IR energy depends on the wavelength, the distance between the substrate and the emitter, the mass of the part and the surface area. The surface of a part with a low mass/high surface area will heat up very quickly when exposed to IR, while a part with a lot of mass and not much surface area will react more slowly. Short wave IR provides high electromagnetic energy levels. This intense band of electromagnetic energy does not provide much flexibility in the cure cycle. Long wave IR has low electromagnetic energy levels, providing less surface heating than short wave or medium wave. The most efficient use of IR energy is absorption by the coating. Higher absorption rates are usually found in the medium wavelength, 2.3 to3.3 microns.
Typically, testing should be done to determine the best setting , measured in microns on the electromagnetic scale. A radiant preheat zone at the entrance end of the cure oven can be an excellent enhancement. Curing will begin immediately and can be accomplished faster than convection heating alone. In most cases where there is a lot of product variety, curing with radiant emitters alone is not practical. The variety of shapes and sizes require convection heating to ensure that all areas reach the cure window temperature and no area is overheated.
High purity (quartz lamp emitter) IR ovens are very effective for this purpose and the bring up, or response time, is instant. The emitters can be arranged in zones for different part geometries. Turning on different numbers of emitters within a zone can vary the energy levels. The zone control can be turned on by a remote controller or operated manually.
Gas fired radiant emitters work very well also. Like quartz lamps, they can be purchased in modular sections, they can be zoned, and they have a fast response time. The Btu’s required to operate a gas-fired IR emitter will typically cost less than the kilowatt-hours required for electric infrared emitters.
Infrared preheat zones for powder cure ovens are typically around 30 to 60 seconds. For exact process requirements, testing must be performed. The emitters operate at a higher temperature than the cure temperature -- typically 1,100 to 4,000oF (593 to 2,204oC) direct transfer of energy creates an immediate reaction in the polymer and reactions takes place rapidly. Infrared rays have a curing power of 4000 watts.
Car body being cured in Infrared oven
Advantages of infrared curing
• Short drying times :-
The drying time is significantly reduced and in some cases halved. It can be operated from cold to full power in less than 1 second. This is significantly faster than a convection oven. Hence production rate Is enhanced.
• Low energy consumption :-
It uses around 1500 BTU of thermal energy as compared to 2000 by a convection oven. It consumes only 1.6 KWH of electrical energy to dry 1 ton of rice as compared to 7.7 KWH by convection ovens. Thus from these figures, it is an energy efficient process.
• Improved productivity :-
Since curing time is faster, line speed can be maintained at a higher rate . This means that larger number of products can be processed in less amount of time.
• Less floor area required :-
An Infrared oven takes up only ¼ of the space required by a convection oven. Smaller floor area translates to greater manageability and ease of transportation and installation.
• Improved product quality :-
Since there is no circulation of air inside an Infrared oven powder coatings are not disturbed by air turbulence. This provides better finish to the parts.
Disadvantages of Infrared curing
• Inability to cure complex shapes :-
Although IR ovens can cure a coating much faster than convection, differences in the part structure and mass will affect the results.
Infrared energy is an electromagnetic radiation and travels in a straight line. It is dependent on a precise and consistent relationship to the product for even distribution of energy and uniform curing. Areas of apart that are obscured from the emitter by their geometry will not heat up at the same pace as areas that are flush to the emitter. The distance of the part surface to the emitter will also have a profound influence on the curing of the coating. Hence complex shapes are not cured effectively.
• High cost of implementing :-
Cost of purchasing an Infrared system today is twice as much as a tunnel oven. Infrared lamps have an average life of 2 years and are expensive to replace.
• Entrapment of volatiles :-
The evaporating fluids cannot be removed due to the lack of air flow inside the chamber. Hence these fluids get trapped underneath the coating itself. This is highly undesirable.
Applications of Infrared curing
Infrared curing can effectively perform all the functions of convection curing. Major applications include :-
• For curing of Powder coatings.
• Curing of Paints and epoxy materials in Automobile industry.
• Drying of rice and grains.
• Powder curing in satellite dish components.
• Drying water based coatings.
• Drying of Inks and Adhesives.
COMBINING INFRARED AND CONVECTION OVENS
Both Infrared and Convection ovens have their own advantages and disadvantages. Inorder to optimize a production line the realization of both these technologies is a necessity. By combining both technologies in the same oven, the advantages of both can be realized while minimizing the disadvantages.
If you require a continuous oven to dry or cure a wet coating or powder, look into combining infrared and convection heating. Often, it can provide the same results as a conventional convection oven within a smaller equipment footprint.
There are two ways of combining infrared and convection technologies. The first is to combine infrared and convection heating inside the same oven chamber. In this design, infrared heaters are mounted inside the oven along with a recirculating air system. Heat is provided in radiant form, and the air system delivers heat to hidden areas that the infrared cannot reach. A supply plenum delivers the air in much the same way as in a traditional convection oven. In some cases, the recirculated air also is heated separately using burners or electric heaters to provide better temperature control within the heating chamber.
When curing paint or powder on aluminum castings, infrared heaters provide accelerated heating and convection air transfers heat to the hidden areas.
An ideal application for this type of combination technology is curing paint or powder on aluminum castings. Infrared heaters provide accelerated heating, and convection air transfers heat to the hidden areas (figure 2). Aluminum parts conduct heat well, which also assists in heating the areas not in the infrared heater's direct line of sight.
Airflow arrangement is an important design consideration. The air velocity impinging on the parts must be reduced at the oven entrance end to prevent the powder from being blown off. After the powder has gelled, higher velocity air can be used to increase the heat transfer.
Combining the technologies can pay other dividends as well. For example, a manufacturer of satellite dish components in California combined infrared and convection and reduced its powder cure time from 20 to 4 min. With a line speed of 2.5 ft/min, the oven floor space requirement was reduced from 50 to 10'.
Combination convection/infrared commonly is used for drying water-based coating on ceiling tiles. Convection helps speed drying by removing the microscopic layer of moist, saturated air that forms on the surface.
Combined infrared/convection heating also is particularly useful for drying water based coatings. As these coatings release their moisture during drying, a microscopic layer of moist, saturated air forms near the surface. This boundary layer forms a barrier to further drying. It must be penetrated for the coating to dry. Traditional infrared heating does not penetrate the boundry layer. With the addition of hot air convection, however, this layer of moisture is removed, greatly accelerating the drying process and allowing the full benefit of infrared heating to be realized.
An example of this approach is in the drying of water-based paint on ceiling tiles. The tiles are carried on a chain- or belt-style conveyor through an oven with hot air impingement nozzles, directing the air onto the tiles. Gas infrared burners located between the nozzles provide the heat. The advantages of both technologies are utilized; the infrared burners provide the heat and the forced air removes the moisture.
The second manner in which infrared and convection technologies are combined is through the use of infrared to preheat parts prior to entering a convection oven. After the parts are coated, they are passed through an infrared oven section before entering the convection oven. Infrared provides an initial heat boost to gel the powder or bring the wet coating to temperature. The convection oven then provides the additional heating time required.
This technique often is used to speed up an existing line, with minimal increase in floor space. A short infrared section is added at the entrance to an existing convection oven to provide 3 to 5 min of preheat. The convection oven then evens out the temperature and helps heat hidden part areas.
This approach also is used when a change from solvent-based to water-based or powder coatings becomes necessary due to air quality or other environmental concerns. Water-based coatings often require a longer cure time and higher temperature than solvent-based coatings. An electric or gas infrared booster can provide this without requiring the convection oven to be lengthened. Installation involves simply locating the infrared section in front of the entrance to the convection oven.
Where an existing line needs to be sped up but there is no additional floor space available, infrared heaters can be added inside the first several feet of the convection oven. This provides the additional advantage that any infrared heat not absorbed by the parts will be contained by the convection oven.
This combination convection/infrared oven cures powder on aluminum castings with complex shapes
Whether infrared heaters are to be used inside a convection oven or as a preheat booster, be sure to use the type of heater best suited for your process. Not every heater is right for every application. If possible, work with a supplier who sells several different types of heaters and who will not be biased towards a particular style.
If the part conveyor stops in an infrared oven, the parts can overheat, resulting in burned coating and loss of product. If this is a concern, use low thermal mass heaters such as medium-wave quartz tubes or short-wave T3 lamps interlocked to shut off in the event of a line stoppage. Lower thermal mass allows the heaters to cool quickly when shut off.
If durability is a major concern, use flat panel or ceramic heaters. They can survive moderate impact without failure. These heaters have a greater mass than quartz tube or short-wave heaters and take several minutes to cool down after being turned off.
Gas catalytic heaters and gas burners often provide reduced operating cost in comparison to electric but have low turndown ability, which can lead to difficulty controlling the temperature. They offer durability and either high or low thermal mass, depending on the specific heater selected. One important feature of gas infrared is that it typically has less line-of-sight problems than electric due to the air added for combustion. Combustion air provides some agitation within the heating chamber and helps heat the hidden areas by providing a slight convection effect.
Due to the nature of infrared heating, testing prior to oven design always is recommended to confirm the performance of a specific type of heater with your coating and substrate. This is true whether infrared heaters are to be used inside a convection oven or as a preheat booster.
1 ) COMPANY : Progressive Powder Coating in Mentor, Ohio, USA
Like most metal finishing plants, Progressive Powder Coating in Mentor, Ohio, uses a convection oven in its manufacturing process. Progressive Powder was experiencing bottlenecks in its production process because of the time required to cure thicker pieces of metal in the convection oven. Curing thicker metal pieces forced the plant to slow the conveyor line speed, which reduced productivity. In an effort to save energy and improve production, Progressive Powder installed an infrared (IR) oven in between the powder coating booth and the convection oven on its production line. The IR oven allowed the plant to increase its conveyor line speed and increase production by 50%. In addition, the plant was able to reduce its natural gas consumption, yielding annual energy savings of approximately $54,000. With a total project cost of $136,000, the simple payback is 2.5 years.
Progressive Powder Coating is a subsidiary of Buyers Products Company, a manufacturer of products for the mobile equipment industry, including truck and trailer components. Buyers Products is a vertically integrated company whose manufacturing capabilities include forging, stamping, laser cutting, computer numerically controlled machining, robotic welding, powder coating, assembly, and retail-oriented packaging. Progressive Powder performs most of the powdered metal coating for Buyers Products in its 25,000-square-footfacility. In addition to the convection oven, Progressive Powder’s plant contains a five-stage product washing unit, a dedicated cleaning stage, an iron phosphate stage, and an anticorrosion sealer stage. Before the IR booster oven was installed, Progressive Powder depended on a traditional convection oven to cure the powder coats. This curing oven is approximately 128 feet long, with two 60-foot zones and an 8-foot wrap-around section. This length is typical for a facility such as Progressive’s plant and is necessary so that the plant’s products will have sufficient time to absorb heat at the proper curing temperature.
Progressive Powder Coating worked with Dominion Power1 to evaluate the convection oven to determine how to eliminate the production delays. Despite the convection oven’s length, it was unable to fully cure the thickest pieces at normal conveyor line speeds. The conveyor line speed had to be slowed down to 4 feet per minute (fpm) for the powder coating on thick pieces to become fully cured, which lowered the plant’s productivity, causing production bottlenecks and scheduling problems. Because the plant’s gas usage was consistently between 1.8 to2.4 million cubic feet per hour of operation, regardless of the mix of products that were cured, the thinner pieces were more costly to treat on a per-unit basis than thicker ones.
Although the line speed could be increased for thinner pieces and slowed when thicker pieces entered the oven, plant personnel determined that this was not optimal because the convection oven needs time for any new temperature to stabilize before it can be used effectively. If thinner pieces are in the oven and there is a change to thicker products, spaces must be left on the conveyor belt to allow for the temperature in the oven to rise and the thinner products to exit the oven. If not for the space, the thinner pieces would be over cured or burnt and the first few thicker pieces under cured.
In addition, the entrance to the convection oven was more than 100 feet away from the end of the powder-coating booth. This caused powder loss because of the conveyor belt vibrations and convection oven turbulence, and reduced product quality.
Working with Dominion Power consultants, Progressive Powder personnel analyzed the possibility that an IR booster oven could alleviate the plant’s problems. After reviewing the plant’s production needs and various types of IR ovens, Progressive Powder purchased and installed a 40-foot catalytic IR oven. The plant placed the new oven between the end of the powder coating booth and the convection oven. To minimize powder loss from the conveyor belt vibrations, Progressive placed the IR oven just 12 feet beyond the powder-coating booth. Because the IR oven can reach temperatures of up to 475°F, plant personnel realized that it could be used to pre-gel and cure powder coatings before the pieces enter the convection oven, thereby minimizing the amount of time they need to be in the convection oven.
The installation of the IR oven has allowed Progressive Powder’s plant to increase production and improve product quality, while decreasing energy consumption. The IR oven can pre-gel or partially cure thicker pieces before they enter the convection oven and can fully cure many thinner pieces.
This allows the conveyor line speed to be maintained at 6 fpm instead of 4 fpm and has led to a production increase of just over 50%. In addition, because the IR oven can pre-gel the powder coating before the piece enters the convection oven, less powder is shaken off by the conveyor or blown off by convection oven turbulence before gel temperature is reached. This has led to more consistent product quality and fewer products that have to be rejected or reworked. With the IR oven’s curing ability and the improved product quality, Progressive Powder’s plant now has more flexibility in its production scheduling. Because some pieces can be fully cured in the IR oven, the convection oven uses less natural gas. The IR oven does not use blowers or fans, so the plant’s electricity consumption is unchanged from before the IR oven’s installation. However, natural gas usage is down 15.5% per piece and 6.8% per pound of product. Overall, the Mentor plant’s natural gas consumption has declined by 25%, yielding annual energy savings of $54,000. Because the project’s total cost was $136,000, the simple pay back is just over 2.5 years.
IR heating ovens can improve energy efficiency and productivity in industrial plants that perform process heating. In the case of Progressive Powder, the IR heat generated by the IR oven was able to pre-gel and cure many types of products before they entered the convection oven. This allowed the plant to increase its production and reduce its energy consumption because pieces did not have to spend as much time in the convection oven to become fully cured. By experimenting on thinner products plant personnel have found they can boost line speeds to as high as 12 fpm, and they are exploring additional ways to fully utilize the IR oven. By optimizing the proportion of IR heating versus convection heating that will deliver the highest efficiency, Progressive Powder Coatings has improved the effectiveness of its production process.
2) COMPANY: Ford motors
The Ford Motor Company manufactures the Lincoln LS vehicle, which has a welded seam in the middle of the C-pillar located between the rear door and the back light. In order to obtain a smooth surface suitable for painting, five manual grinding stages are required to finish the joint. After Ford’s body-in-white process, his seam is cosmetically covered by stitch MIG welds and a thermal-sprayed silicon bronze material in preparation for the paint process. Due to the high porosity in the thermal-spray coating, an additional step is taken in the paint shop to apply a gray glaze and anti-chip material to the vehicle in order to produce a high-quality finish. In addition to the operator-dependent, manual process of welding and painting this seam, the process is susceptible to quality concerns and production issues. In an effort to eliminate numerous steps and cut down on cost and production steps, Ford decided to investigate a one-component epoxy material to replace the thermal sprayed silicon bronze. Although the epoxy material is conductive to the e-coat, is fully paintable and sandable, and has very low material porosity, no efficient and robust in-line heating source was found to accomplish the curing process.
A Ford Motor Company researcher, contacted the Oak Ridge National Laboratory (ORNL) to discuss the potential development of improved joining/brazing and resin curing technologies for this application. Ford had been familiar with previous work performed at ORNL on rapid infrared processing sponsored by the U.S. Department of Energy’s Automotive Propulsion Materials Program.
A Metals Processing Laboratory User (MPLUS)project was established at ORNL to observe both rapid brazing of aluminum and rapid curing of epoxy joints. Although several technologies were investigated, a focused tungsten halogen lamp line heater showed the most promise. Developed in ORNL ’s Infrared Processing Center, the infrared line heater can be operated from cold to full power in less than one second, converts electrical power into radiant power at 90 percent efficiency, and targets the energy to only the area which needs curing. The lamp was found to provide sufficient curing in very short process cycles. Full commercial-scale process trials were completed by Ford Motor Company, and the durability test was passed. The curing of the epoxy material was performed in only 90 seconds with the use of the infrared lamp. Set up from a distance of6 inches and covering a 10-inch wide region, the lamp achieved a 98-percentcure, which is suitable for grinding and sanding. The cure is completed later in the e-coat oven.
The infrared lamp technology developed by ORNL has been adopted in one of Ford’s assembly plants. The Ford Motor Company has targeted this process for Job 1 production in the near future.
• Cost savings of $10 per vehicle over the traditional process
• Less porosity in the coating, which eliminates additional surface preparations
• Reduces initial cycle time by 30 seconds
• Reduces energy consumption as a result of eliminating several steps in the production process
The Infrared curing technology is a growing industry .One hundred percent reactive inks, coatings and composites produce high quality finished products for the consumer market. Manufactured items may be tested, packaged and shipped in the same day. There is no pollutants given off into the atmosphere, nor is high energy required for curing or drying.
This growing technology allows efficient manufacturing of coatings with energy efficiency, no Volatile Organic Compounds (V.O.C’s), improved health and safety considerations and a cure-in-place technology. This technology is sure to optimize coating processes in coming years.
• Powder Coater’s Manual.