J. L. McCLOUD Ford Motor Company, Dearborn, Mioh.
The use of infrared light in the baking of automobile finishes has been investigated. The relative merits of various sources of heat are discussed. Types of reflectors and tests made with them in combination with various types of lamps are described. The application of infrared light to the baking of colored finishes on a number of different production parts is discussed and illustrated.
H E development and expanded use in the Ford Motor Company of infrared “light” for the baking of finishes no‘ various automobile parts have aroused considerable interest. Part of this interest is natural because of the spectacular character of this method of finishing, but its real value is due to the unusual saving in time and general cost of baking that has been effected. Our use of this method of baking was born of necessity. For many years automobiles had been finished with oleoresinous varnishes. These had two outstanding faults which were sufficient to stop their use: Their durability was very short, measured almoet in terms of days; and to obtain a satsifactory finish coat on automobile bodies required days of application. The next step in the development of finishes was lacquers, of which nitrocellulose was the principal character-giving part of the film. The time to finish bodies was cut from days to hours, and the durability was measured in terms of months. In the constant search for new materials which continues in the automobile industry, it was realized that the new synthetic resin enamel of the alkyd type presented advantages over the lacquers then in use. We had employed these enamels for some time for other purposes. First we applied them to the wooden parts of truck bodies. Later we used them on wheels where their toughness was well demonstrated. The principal advantage that these synthetic resin enamels presented was that the surface durability was substantially better than that of the nitrocellulose lacquer. It is measured in years instead of days or months. I n other words, they did not become dull and chalk so rapidly as the lacquer. Another decided advantage in their favor was that by proper application they formed a surface which was smoother than lacquers then available. I n fact, they were sufficiently smooth so that it was no longer necessary to sand and polish the final coat. This eliminated one of the large cost factors in the finishing of an automobile body. Another advantage over nitrocellulose lacquers was that they were easy to prepare in such a fashion that their luster as applied was entirely satisfactory. In this connection, the smoothness with which a coat of organic material of the enamel type may be spread on the surface appears to be better if there is little luster in its surface. Consequently, in the early work it was supposed that highluster enamels showed the effect of application by spray gun and were not so smooth as the dull lusterless lacquer. Actual measurements, however, indicated that this was merely another effect of the optical condition of the surface. Thus in enamels we had a high-luster durable finish which could be applied smoothly enough so that it was unnecessary to remove any of the surface by sanding.
However, one obstacle still had to be surmounted in the successful application of these synthetic resin enamels to automobile finishes, particularly bodies. Lacquers dry by the mere evaporation of the solvent, and theoretically, at least, no further change occurs in the life of the lacquer. This, obviously, is a pronounced advantage since, if the surface is marred in any way by scratching or if repairing is required, i t is necessary only to add another layer of the lacquer on the spot to be repaired. Of course, such necessary precautions as feathering out scratches, proper sanding, and rubbing were necessary, but this very ease of repair acted to make those finishing the parts more careless of the necessity for proper handling.
Drying Repairs Now we come to the repair of an automobile body finished with synthetic resin enamel. The outstanding difference in the finishing cycles of lacquers or enamels is that the latter require a definite bake rather than mere air drying. Here we have a material which changes chemically and physically in the baking cycle. The coating polymerizes and oxidizes. The polymerized and oxidized surface is chemically different from that of the material freshly applied. Repairs must also be sent back through an oven to be properly dried. Thus some sections are single-baked and some double-baked.
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FIGURE1. COMPARISON OF THE RELATIVE INTENSITYOF RADIANTENERGYWITH WAVE LENGTHDISTRIBUTION FOR TUNGSTEN AND CARBON FILAMENT LAMPS
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In the finishing of an automobile body the color is important. Black is still the principal finish, but color does hold the stage and must be maintained in a satisfactory degree of matching. The general effect of baking one section of a surface for one cycle and the other section of a surface for two cycles is to obtain a mottled and varied colored coat on the whole article. So it becomes necessary not to overbake either from a time or temperature standpoint. Attempts were made in the industry to use air-drying materials of the lacquer type for repair purposes in the finishing of parts with synthetic resin enamel. But i t was early found that the durability of the synthetic resin was so much greater than that of the lacquers used in the repairs that the repaired zone quickly showed up and became unsightly. The only solution was to bake repaired areas locally. “Repairs” here do not mean areas refinished after the automobile is in use, due to accidents that may occur on the road, but those small mars which may be made by a screw driver or a scratch from buttons on workmen’s clothes and which must be refinished before the article is sold. Some experimental work was done on various methods of local application of heat electrically without success, until Groven (.2) used infrared lamps as a means of supplying the heat to the repaired area. At that time it was customary t o bake the finishes for one hour a t 225’ F., and if necessary we would have been glad t o be
(Above, reading from top to bottom) LIGHTCAVE
FOR BAKING ENAMEL ON A COMPLETED AUTOMOBILE; VIEWS OF A BODYENTERING A CLAMSHELL OVEN, AND A BATTERY O F ORIGIN.4L
CLAMSHELL OVENS
( L e f t ) FIRSTTUNNEL-TYPE OVEN FOR BAKING PRINEON AUTOMOBILEBODIES
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able to repair mars in the same period. However, with these infrared carbon filament lamps the time to bake was reduced by 90 per cent if the color was such that the radiator or electric light could be placed close to the surface. Obviously, we had the missing link to the program of employing synthetic resin enamel. The use of synthetic resin enamels and of infrared lamps was described in 1937 (3). Since then several articles have appeared on the general subject of baking paint with infrared, and the experiences of the Ford Motor Company have been repeated elsewhere. The General Electric Company published an article on drying with infrared radiation early in 1940 (1).
Sources of Infrared Heat We investigated a number of the factors of infrared heating. A comparison of the relative intensity of radiant energy with the wave length distribution of commercial tungsten and carbon filament drying lamps is shown in Figure 1. The energy a t various wave lengths is influenced by the temperature of the lamp filament. The peaks of the energy curves are 1.150 microns for tungsten and 1.400 microns for carbon filament lamps. This differenceof 0.250 micron is enough to give some difference in the temperature rise in panels of steel irradiated by the two types of lamps. Another factor is the ability to focus the energy with a reflector. Carbon filament bulbs yield a uniform pattern while standard tungsten lamps give a horseshoe pattern
(Above, reading jrom top to bottom) BAKINGT H H ~ FINISH COAT ON AUTOMOBILEWHEELS, A TRACTOR CHASSIS,AND STEERINQ WHEELS (Right) BAKINGTHB PRIMER COATON AmoMOBILE FENDERS
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(Figure 2 ) . The tests were made with the same gold-plated parabolic reflectors. In each case the lamps were adjusted to give the same diameter of focal spot. An incorrectly focused bulb may account for 15 per cent lower temperature or more (Figure 3). A carbon filament lamp will heat a panel to 300' F. in the time that a tungsten filament lamp takes to heat the same panel to 280' F. (Figure 4).
F I G U R E 2. LIGHTDISTRIBUTION FROM A 240\vATT CARBOX INFRARED LAMP(left) AND A 2 5 0 - \ v A T T TUNGSTEN INFRARED LAMP(Tight)
Of the various ways of applying heat to an object, radiation is recognized as the most efficient, provided the path that the radiant energy has to travel can be kept short. Of the other two methods, conduction is usually out of the question in heating an object on which we wish to bake paint. The other method, convection, involves heating by transfer through air currents and is an inefficient method of application. Induction heating has been used also for symmetrical objects that can be surrounded. It is usable, but for local areas on bodies it is not feasible. This leaves us with radiation as the best method. I n general, the rate a t which energy is radiated from one surface to another is proportional to the temperature difference between the two articles; consequently, to supply heat from the radiator to the object rapidly, there should be the highest possible temperature difference. However, to be effective the baking light rays must be kept in the red end rather than in the visible end. Any electric light bulb emits infrared light ; however, in illumination, electrical engineers have strived to give us the greatest possible percentage of the light in the visible region. Here we are choosing a lamp which gives little visible light as compared with the ordinary bulb. The bulbs chosen were like the original carbon filament incandescent lamp which Thomas Edison invented at Menlo Park. In this case, however, they are operated so a8 to be a t a much lower temperature.
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separate and apart from the obvious economical advantages of not having too much heat loss. Heating by radiation avoids much of this loss. With good gold-plated reflectors the heat loss from the back is extremely low, and one can walk or stand beside a bank of these reflectors without feeling discomfort even on a summer day. One of the special advantages of these infrared ovens is that substantial ventilation is applied by the convection of hot air from the automobile bodies or objects reradiating heat through the air, and this creates drafts which accelerate the drying of the enamel. I n the operation of an ordinary air-drying oven, it is necessary to maintain the oven constantly a t heat since the cycles are generally periodic and work may turn up a t any moment to go into the ovens. Further, it is necessary that the temperature be brought to a peak in the ovens long enough before they are to be used so that they will be ready to bake the articles put into them. For the same reasons a substantial amount of stored heat is lost a t the end of the day if the ovens are maintained on an 8- or even a 16-hour cycle. With the infrared ovens, however, it is not necessary to turn on the light unless work is actually in front of them. Arrangements of automatic switches are made so that the lights are not on unless the conveyor is actually moving and work is in front of the lamps.
Applications We first used infrared lamps for baking enamel in 1932-33. The first large-scale application was a light cave completely studded with lamps. This cave was just large enough to take an automobile body after the doors were closed. The body was surrounded with lamp bulbs 12 to 18 inches away from i t a t all points. It was here demonstrated that prime paint can be baked on the bodies in an extremely short time. Previously the lamps had been used almost exclusively on alkyd resin enamel. These prime paints were of the oleoresinous type-a combination of natural gum resin with linseed and tung oil, pigmented to the customary low luster level of primers a t that time. They required an ordinary baking cycle of one hour a t about 250" F. for a satisfactory sanding surface. The drying of these finishes or primer was accelerated by the addition of small amounts of lead, manganese, and cobalt, and we were able to get the drying time of the primer on automobile bodies down to 7 minutes with lights.
Reflectors The first reflectors were made of aluminum. However, aluminum reflectors dull rapidly, and the lamps soon lost their effectiveness. Of the various metals, gold presented the best possibilities (Figure 5) ; gold is a good reflector in the infrared region, does not tarnish in air, and retains its brilliance even in severe industrial atmospheres. Since it is advantageous to place the infrared "ovens" as close as possible to the spray booth, paint spray occasionally accumulates on the reflectors. Strong alkali must be used t o clean off this spray, and gold is not affected by strong alkalies. Returning to the relative advantages of the application of heat by radiation rather than by conduction as in ordinary hot air ovens, it should be pointed out that one of the biggest costs in the installation of the conventional baking oven is the necessity for substantial insulation so that heat losses will be kept to a minimum. Heat losses are very objectionable in a working area since, especially in summer, ventilation is necessary to supply the workmen a decent atmosphere. This is
FIGURE
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" F I T l N G T l U E - MINUTES EFFECT OF Focus ON S T E E L
TEhlPERATURE
Subsequently a series of clamshell ovens was installed a t the Rouge plant, which moved parallel to a center conveyer line on which automobile bodies hung. As the bodies approached this so-called infrared lamp oven, two halves of the clamshell surrounded the automobile body, the three moved together
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as a unit for the required time, and the prime was baked on the surface. This proved to be an acceptable method of baking the prime, but if any change were made in the type of body hung on the conveyer, either as to style of body or length, it was necessary t o rebuild the conveyer. Consequently, the feasibility of using tunnels was investigated. Of course, in a tunnel certain parts of the automobile body are closer to the radiators than in the clamshell ovens. However, by taking a somewhat longer time and by properly constructing the tunnel ovens, we have used them successfully for the baking of prime on automobile bodies.
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installations of wheel baking ovens, a t the Rouge plant and at branch plants. So far I have referred to the application of infrared only on iron or steel objects. It is equally possible to use infrared heating when the objects are not steel. Ford steering wheels are made of rubber over an inner core of steel, and a highluster smooth finish is obtained when they are given a coating of special enamel. This enamel is a urea-formaldehyde modification of an alkyd enamel. The original ovens used to bake this finish on the wheels required 2 hours and had a temperature peak around 270' F. An infrared oven was constructed which was practically the same in general appearance as that used for baking the enamel on the car wheels. The time was cut down to about 12 minutes; with a subsequent modification of this alkyd enamel with urea the time has been cut further. The quality of the finish has been improved, especially in regard to the effect of perspiration from the hands on the finish. Infrared light has been used as a means of applying heat in other miscellaneous uses. One interesting application was subsequent to the Ohio River flood in 1937 when the Louisville branch of the Ford Motor Company was under several feet of water. One of the big problems was the drying of motors, armatures, eto. I n the general cleanup and reestablishment of the plant, the Electrical Maintenance Department made temporary installations around these electrical parts with infrared. They were able to dry out the parts to a usable condition in only a fraction of the time that would ordinarily have been required had they been dismantled and put into conventional drying ovens. The drying of latex sprayed on cloth as a cement, the drying of photographic film,the heating of aluminum pistons in order to expand them for the insertion of piston pins, and even the baking of bread might be mentioned as other applications.
Advantages The all-over baking of the finish coat on automobile bodies is much more difficult because color must be maintained, and many colors are susceptible to the temperature differences that are of no significance in the undercoat. Finished enamel on the bodies themselves is not baked on with infrared except for repairs, which can be handled as individual cases and given appropriate attention. I have been describing the application of infrared light to an object which is large and is not symmetrical. However, certain objects in automobile construction are symmetrical and lend themselves ideally to baking with infrared. For example, the steering post column is a shell of steel and is perfectly symmetrical with respect to its axis. An oven is directly connected to a conveyer which brings the steering columns through a spray booth. The conveyer has holders which turn the steering columns continually as they are being sprayed and, a t a lower rate of rotation, as they pass down between banks of infrared lamps. Prior to the installation of this oven, one-hour bake had been given these steering columns in a large oil-fired oven. First we baked 6 minutes the columns in infrared light and, by subsequent modification and improvement in the enamel, have been able to bring the time down to 3 minutes. Wheels are symmetrical, a t least in one axis, and lend themselves very well to baking with infrared. The oven takes the form of a long tube of light, and the various reflectors direct the energy of the lamps in toward the center. Thus the center of the tube receives radiation from all sides. Since the temperature attained varies inversely as the distance to the radiator, we might expect the rim of the wheels to get much hotter than the hubs. Actually the effect of multiple reflection is to bring the hub up to heat as fast as the rim. We have several
There are no special requirements for coatings to be used with infrared baking. Any paint whose drying is accelerated by heat may be dried in infrared light. It will include the acceleration in drying of a lacquer coat, the drying of an oleoresinous varnish, even the drying of moisture out of a paint or enamel film-for instance, after water sanding of a prime or surfacer coat. Our most important application is, of course, the baking of alkyd resin enamel. I n the case of the alkyd resin varnishes the polymerization effect, which is the most .~ REFLECTOR TESTS I
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FIQURE 5. REILATIVE INFRARED-REFLECTING POWER OF DIFFBRENT METALS
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pronounced one, is accelerated enormously by the infrared method of heating. The heating is really done from the inside out, since the heat rays penetrate the finish and heat the steel or iron, which in turn bakes the inner layers of finish as well as the outer layers. This is especially noted when a finish formulated with quite volatile thinner is employed; the bubbling or blistering often encountered is not so pronounced as in the case of drying with an ordinary oven. It might be pointed out that one difficult place to use infrared lightnamely, in the drying of a varnish on wood-further demonstrates that this drying from the inside out occurs naturally. The wood with a varnish coating on the surface which is dried by infrared is much more inclined to blister than when the same varnish is baked in an air oven. The moisture existing in the wood is driven out of the interior surface a t such a rate that blisters occur. When metallic driers such as cobalt, lead, or manganese are added to an alkyd resin varnish and are baked in a conventional oven, a severe tendency to wrinkle develops and the condition must be watched carefully. For instance, 1per cent of 2 per cent cobalt drier is the greatest amount tolerated in
S. G .
OR a long time Chrysler Corporation has been interested in the problem of paint recovery and what it would mean in terms of savings and more efficient plant operation. We have carried on extensive experiments, as a result of which a paint recovery system has been developed which gives satisfactory results. The problems which had to be overcome and the steps in the development of the recovery system will be described here. I n considering the feasibility of paint recovery from overspray in water-type spray booths, one question immediately came to mind. It was obvious that many constituents of paint solids are a t one time wetted with water, or water is given off or evaporated as a part of the resin cooking operation, Why, then, could not water be removed from the finished product and leave us with the same quality product as in the original manufacture? The process itself as regards lacquer was relatively simple. Water alone “kicked” the lacquer out of solution. The resulting sludge had no adhesion to walls and pipes and could be collected, dried in a centrifugal dryer, redissolved, and clarified, etc. After reduction with a thinner, from 2 to 3 per cent water remained. Then the thinner had to be altered slightly to obtain the same flow, evaporation rate, etc. Exposure of this material was better than on the new lacquers, but this may have been because of the darker color of recovered material. The next problem was, where could the recovered lacquer be used advantageously-i. e., to replace new lacquer materials. The following uses were developed: crankcase sealer, underneath fenders and hoods, ground coat where the color was satisfactory, and several jobs using reclaim as the finish coats. After recovering and using the lacquer for two years we changed t o alkyd resin enamel finishes on cars. These turned
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an ordinary baking cycle. Ten per cent may be added when the drying is in infrared light, and the drying is accelerated as in an ordinary steam- or gas-heated oven. The cost of baking with infrared is dependent primarily on the cost of electric power. Since this varies, a comparison of the cost of this method of heating with others will likewise vary. With a power cost more than 1.5 cents per kilowatt hour, the heating is expensive. Our cost for power is lower and the method is economical for us. The main saving to us with infrared heating is in time. In the first place, less space is used. In many instances we have been able t o install the infrared ovens in existing buildings without having to add t o them. We have been able t o increase the capacity of the painting departments in the same floor space. The number of parts or bodies in process are reduced by short finishing cycles. Literature Cited (1) Bennett and Haynes, Chem. & Met. Eng., 47,108 (1940). (2) Groven, F. J., U. S. Patents 1,998,615(1935); 2,057,776(1936): 2,186,067(1940). , 131-6T (1935). (3) McCloud, J. L.,S . A . E. J o u ~ n a l42,
SAUXDERS, Chrysler
Corporation, Detroit, Mich.
out to be even more troublesome, principally because enamel sludge is sticky. I n trying to keep sludge from sticking, over a period of several years, we have tried a t least two hundred different compounds in the wash water. Where recovery is not a factor, alkalies are commonly used in the water medium. Alkali is partially satisfactory from the standpoint of keeping the paint systems clean, but if used in high enough proportion t o do this, the sludge is useless from a recovery angle because the alkali saponifies the resins or fatty acids in the sludge. When enamel sludge was first reclaimed in our plants, water medium was not used. In its place was employed a product known as mineral seal oil, a type of light paraffin oil. This worked almost perfectly, as far as keeping the paint systems clean, but the capillary action of this oil was such that when it condensed in the ducts it tended t o drip back on the finished jobs. In addition, the baking schedules of the enamels were not sufficient to get rid of the oil entirely, so that the drying was impaired. From mineral seal oil we went t o soluble oil additions t o the water. This oil consisted of a naphthenic acid soap with paraffin oil. The concentration used was 3 per cent in water. Here again the oil left behind (when the sludge was reclaimed) hindered the drying of the enamels under regular baking schedules. Also, the lubrication was rather poor as compared to mineral seal oil and necessitated frequent cleaning of the paint systems. During another experiment a colloidal graphite dispersion in water was our reclaiming medium. This material was inert and exerted no influence on the resultant enamel reclaimed in regard t o drying or other properties. However, the lubricating properties were only fair, the graphite discolored reds, whites, etc., and much work was done trying to find a better product.
