The Processing of Metal Coatings
Consumers of coated metal, wood, paper, glass, plastic and textile products are demanding attractive, long-lasting, wear-resistant surfaces. At the same time, stricter safety and environmental standards are forcing manufacturers to look at new types of finishes, e.g., water- based and powder coatings, and at different technologies to process them. Among the technologies well suited to both drying and curing the newer finishes is electric infrared (IR) process heating.
Electric infrared has actually been used by industry since the mid 1930s, when the Ford Motor Company developed and adopted it for curing paint on auto bodies. However, early users often became disillusioned with electric IR because the technology was oversold. But heater design and process control have been improved to such an extent that there is now essentially a "new IR" technology. And, unlike many traditional coatings, the new coating systems being developed are extremely tolerant of high levels of IR energy. Innovative manufacturers are capitalizing on these developments to obtain higher quality products for a lower unit cost.
This case study discusses current and potential applications of electric IR, as well as the technical and economic factors to consider in selecting a processing unit.
Advantages / Drawbacks
Companies in the automotive, plastic, paper converting, graphics, packaging, home appliance, textile and wood products industries find numerous advantages in using electric IR to process coatings. Many of these advantages translate into better product quality, increased productivity and lower costs. Electric IR processing offers:
o Energy efficiency - In a properly designed system, radiant heat acts directly on the surface to be treated, resulting in faster product heating and lower energy costs.
o Space savings - Faster heating means shorter ovens, so equipment occupies less floor space. IR ovens can be added to an existing production line without difficulty, and, since units are modular, ovens can be easily enlarged or reconfigured.
o Clean products - Since IR heats the product directly, there is no need to blow hot air, and dust, through the treatment area.
o Precise control - Electric IR heaters respond quickly, can be controlled by microprocessors and can follow process changes with little lag.
o Flexibility - Heaters can be zoned and adjusted to suit product width and processing requirements.
o Low maintenance - Electric IR heaters have a very long life and require little routine maintenance.
o Adaptability - Any heat-treatable coating can be processed with IR.
IR does have some drawbacks. On poorly conducting substrates such as fiberboard or plastic, there must be a direct line of sight between the radiation source and the surface to be treated. This largely restricts its use to flat products like wood panels. With metal parts, which conduct heat well, enough energy is absorbed by the metal substrate that areas not directly in the IR beam are heated by conduction. Therefore, even complex parts like engine blocks can be properly treated.
A major use of IR processing is drying both water- and solvent-based paints and inks. In a conventional oven, paints must be dried slowly to prevent the formation of a surface skin. If a skin forms before the underlying paint has dried, the remaining carrying agent will produce surface blisters as it evaporates, resulting in a less attractive and less durable finish. IR radiation passes through the outer surface and dries the paint from the inside out, so skinning is not a problem. The result is a better- looking, more durable surface, produced in less time.
IR is also an ideal method for curing powder coatings, which are being used increasingly for consumer products such as venetian blinds and appliances, and for automotive parts like oil filters. Electrostatically-charged powder is sprayed over the work piece and heated until it melts. It flows over the surface and is cured in an even layer. A major problem in convection ovens is that moving air can blow the powder around before it melts, leading to uneven coating. With IR curing there is no need for air flow so this problem does not arise.
Another growing use of electric IR is for booster ovens in front of existing convection ovens. For example, on auto body production lines an IR oven at the start of the line rapidly heats the paint and sets the body finish. The car then moves into a forced air convection oven where the underparts, on which surface finish is less critical, are dried more slowly. Initial rapid setting of the topcoat eliminates concerns about dust damage in the convection oven. With a booster oven, conveyor speed is increased significantly without much increase in oven length.
IR processing has been used successfully in many applications including drying and/or curing:
o Paint on car bodies and home appliances
o Paint and powder coatings on light fixtures
o Paints and varnishes on sheets of hardboard, particleboard and chipboard
o Coatings on steel and aluminum coil
o Epoxy powder coatings on oil filters and irrigation pipes
o Polyvinyl chloride waterproofing on automobile rocker panels
o Printing ink on paper
A company that manufactures coated steel strap replaced 180 feet of gas-fired convection ovens with 17 feet of IR heating units. They increased their line speed from 800 to 1000 feet/min and decreased their energy costs from $9.92 to $6.92 per hour.
Adding a 7-foot IR booster oven to a convection oven enabled a light fixture manufacturer to double line speed, double production, reduce energy cost per part by 25% and improve product quality. Payback time was six months.
Electric IR competes with both traditional methods, such as air or gas-oven drying, and with gas IR. Other types of radiation, such as electron beam (EB) and ultraviolet (UV), are used to cure specially- formulated, non-heat-treatable coatings that cannot be treated with IR.
Air-drying is slow and the coated surface is exposed to dust, insects and other airborne contaminants. Gas-fired convection ovens are costly, slow, cumbersome and energy inefficient. A large volume of air is heated and the heat is then transferred to the coating. This process is slow and difficult to control since oven heating and cooling takes a long time. Therefore, if a conveyor stops, the product can be ruined by overheating.
Heating a ceramic surface with a gas flame also produces IR radiation. However, heat transfer is mainly by convection, and gas IR is limited to bulk water removal applications.
A number of factors have to be considered when tailoring IR to a specific situation. They include:
o Heaters - Electric IR sources produce primarily either short or medium wavelength IR. Short wave IR is intense and easy to focus, and penetrates coatings well. It is used where intense, directed heat is required, such as in curing thick coatings, or in high speed conveyor lines for curing coatings on steel straps and wood products.
Medium wave radiation is less intense, so heating with medium wave heaters takes longer. This can be an advantage for treating materials not tolerant to high heat levels. Medium wave IR is usually used where a lower temperature, more diffuse source of heat is required, such as in drying water from metal or plastic surfaces or curing inks on paper or screen-printed fabrics.
The number of heaters required will depend on product size, line speed and exposure time. Since heaters usually come in modular units, it is not difficult to add or remove them as necessary.
o Reflectors - Heaters radiate in all directions, so reflectors are placed behind them to redirect as much of the radiation as possible onto the product. Appropriately placed reflectors can radiate the whole product uniformly, or focus radiation on a section that needs extra heating.
Reflector materials include ceramics, polished aluminum or stainless steel, and gold-plated aluminum or steel. Gold is the most efficient reflector material. It reflects approximately 98% of the IR energy, whereas polished metals reflect only 70-75%. These values apply only to clean reflectors. Dirt severely degrades their performance, so reflectors should be cleaned regularly.
Reflectors come in different forms. Often, they are separate panels mounted behind the heaters. However, one heater design uses a twin bore tube with the filament in one bore and a thin layer of gold plated on the inside of the second bore. Thus, the reflector is an integral part of the heater.
It may be necessary to cool reflectors to protect them and the lamps and wiring they shield. Usually, air is blown over them (from behind to eliminate blowing dust). The hot air can be recycled to heat other parts of the work area, or to augment a convection oven if the IR unit is being used as a booster oven.
A number of factors have to be considered when tailoring IR to a specific situation. They include:
o Coating material - The color, thickness, reflectivity, and absorption chemistry of the coating being treated all affect the amount of heat required. For example, it may take longer to cure an automobile panel if the finish is a highly reflective metallic silver than if it is matte black.
o Carrying agents - Some organic solvent vapors become explosive at certain concentrations. Thus, with solvents like toluene, hexane, or methanol it is essential that there be adequate air flow in the oven to prevent vapor accumulation. (This applies to any type of oven in which organics are treated, not just IR ovens.) The contaminated air must then be properly treated. With water- based coatings, the accumulated water vapor must be removed.
o Substrates - IR can treat coatings on a wide variety of substrates including metals, wood products, most plastics, fabrics, ceramics, glass and paper. If you think that IR may be appropriate for treating coatings you use, talk to manufacturers about doing pilot tests. This service, which is often free, helps you determine the size and type of equipment you need. It also gives you an opportunity to evaluate both a company and its equipment before making any commitments.
When deciding on the economic feasibility of IR heating, the following items should be considered.
o Capital costs - IR ovens are custom-designed, so it is difficult to give exact costs. However, an oven with a heating area about 10 feet long and 4 feet wide would likely cost between $50,000 and $100,000. About half the cost is for the oven and reflectors and the other half is for the rather sophisticated control equipment. Although the control system is complex, it is designed to be easy to use, so highly skilled operators are not necessary.
o Operating costs - This category includes costs for energy, maintenance, and replacement heaters. Short wave heaters require more electric power (the filaments are heated to a higher temperature) but conversion to radiant energy is more efficient - 85-90% compared with 60-65% for medium wave lamps. Generally, 50-70% of the radiant energy is actually absorbed by the product. In gas IR ovens only about 25% of the input energy reaches the product, and in gas convection ovens it is only 15%. Thus, electric IR is significantly more efficient.
Maintenance is limited to cleaning the reflectors regularly and replacing burned out heaters. Heater life is long. Short wave heaters should last 5,000 hours, or about two years with one shift a day, when operated at full power. Decreasing the power input to 80-90% of maximum extends heater life to more than 30,000 hours. Medium wave heaters last even longer. Replacement heaters cost between $15 and $300, depending on size and whether or not they have an integral reflector.
o Payback - Electric IR ovens are about four times as efficient as convection ovens. Therefore, if electricity costs less than four times as much as gas, electric IR may be justified in terms of energy costs alone. However, with electric IR ovens there is also faster product throughput, a higher quality finish, and less wastage than with convection ovens. IR ovens also save space, which can be a major advantage. With all these savings, the payback period for electric IR equipment is often less than a year.
Electric infrared heating is an excellent method for treating many kinds of coatings, both new and traditional, on a wide variety of substrates. The process is easy to implement and control and is much more efficient than gas convection ovens and gas IR.
European manufacturers have been aware of the benefits of electric IR process heating for many years and the technology is well established there. The number of IR processing systems in the US is likely to grow substantially in the coming years as manufacturers here, too, begin to recognize the advantages this technology offers for producing long-lasting, durable and high-quality finishes on a variety of products.
Curing involves heating or irradiating a polymeric material so that it forms a new, three-dimensional network structure with improved physical and chemical properties. As a result of the structural changes, the coating usually bonds better to the substrate and is more durable.
Drying involves removing the carrying agent, either an organic solvent or water, from a liquid-coating mixture. The structure of the coating materials is not changed.
Heat treating involves heating a work piece for any of a variety of reasons including drying, curing, hardening, tempering, etc.
Powder coating involves spraying an electrostatically-charged powdered polymer onto an oppositely-charged substrate. There is no solvent - the spray is 100% coating. The coating is heated until it melts and flows over the substrate and is cured in an even layer.
Substrate is the material to which the coating is applied.
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