BACKGROUND OF THE INVENTION
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This invention relates to a method for repairing a defective coating on a coated ferromagnetic substrate and in particular to a method of repairing a defect in an existing coating which has been previously applied to discrete metallic articles such as cans and can ends manufactured from ferromagnetic materials.
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Sheet metal which is to be utilized for producing various products, such as metal cans and ends and decorative metal pieces, usually has a coating applied to the metal for protective and/or decorative purposes. The metal may be in coil form or in the form of individual sheets. The protective coating is usually applied to the metal in liquid form by various techniques, such as a roller coaters, dipping, spraying, electrostatic application and the like, as the metal substrate is passed through the coater. Various coatings and inks can be applied which are well known to those skilled in the art, including for example, vinyls, epoxys, alkyds and phenolics. These coatings include various resins and pigments dissolved in a solvent. The solvent may be either a volatile organic solvent or may be an inorganic solvent, such as a water based solvent.
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Many times, flaws in the coating are not detected until after the discrete article, such as a can or end, is cut and fabricated from a precoated sheet or strip. Various types of defective coatings may be encountered. For example, the coating may not properly adhere to the substrate or during the fabrication process a score line may be formed in the metal such as when an easy open can end is formed and the coating does not cover the score line, but is is desirable to have the score line coated to avoid oxidation of the metal. A further defect may arise out of a failure to have all of the solvent vaporized or a failure to have meat release products contained in the coating come to the surface where they are usable for releasing product from the can. Other defects may also be encountered.
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With prior practice, it was too late in the manufacturing process to correct problems which might be revealed. The cans or ends are usually disposed of as scrap. It would be of significant economic advantage to be able to repair the coating on these articles in which significant additional manufacturing steps have been added to the coated metal.
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There are various causes for the defects in the coating. For example, the metal substrate may not have been properly etched. The chemical balance of the etching, washing and rinsing solutions may not have been properly maintained or the solution temperature may have been inadequate, contributing to an unclean surface for which the coating is intended to adhere. In many instances, the final rinse was not made properly, leaving mill oil on the substrate or too many lubricants and rust inhibitors are applied to the metal which can cause adhesion problems when the coating is applied to the metal substrate.
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Coating chemistry can also cause adhesion problems. In some coatings, curing inhibitors are added to prevent the cross-linking of the coating surface particles prior to release of the coating solvents. Sometimes these inhibitors prevent the coating from becoming properly cured which causes adhesion problems between the coating and the substrate.
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Most commonly, coating problems are related to the bake or curing cycle. Insufficient cure of the coating may occur because the oven temperature is too high or too low or there is a lack of air flow to provide proper solvent release. From time to time the metal or coating is either too hot or too cold to provide the proper bond between the coating and the substrate.
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These problems may not manifest themselves until the metal substrate has passed through various metal forming operations in the production of the discrete articles, such as the can or end. By the time these problems manifest themselves, prior practice has dictated that the metal article must be scrapped as there was no known method or technique of repairing the defective coating. If the coating could be repaired, then these partially or completely manufactured articles could be salvaged, thereby reducing the total cost of manufacture. This is particularly important given the high quantities of articles manufactured in the metal packaging field.
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It has been discovered that defects in the coating which has previously been applied to ferromagnetic material and articles manufactured from that coated material can be repaired by subjecting the article to the energy produced by an electromagnetic coil such as the type included in the apparatus which forms part of the inventions described and claimed in U.S. Patent Application Serial No. 07/686,961, filed April 18, 1991, entitled "Method and Apparatus for Coating a Metal Substrate and for Drying and Curing Said Coating".
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While there are many processes and apparatus for using an electromagnetic induction heating coil for curing coatings applied to metallic substrates, including, for example the system disclosed in U.S. Patent No. 3,068,119, issued December 11, 1962, for curing the coating applied to discrete sheets and the combination electromagnetic coil/convection oven system shown in the aforementioned patent application, and apparatus for curing coatings applied to a continuous web or coil, such as that shown in U.S. Patent No. 3,576,664, issued April 27, 1971, it is believed that this is the first time an electromagnetic induction coil has been used for repairing defects in previously applied coatings.
SUMMARY OF THE INVENTION
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Because of the diverse type of defects in the coating which may be encountered, it is first necessary to analyze the article and the defect in the coating to determine the nature of the defect. Following this analysis, a determination will be made whether or not additional coating must be added to the article in order to repair the defect. For example, when voids in the existing coating are large compared to coating thickness and area, additional coating must be applied in order to cover the void. If the voids are small, there may be sufficient coating on the article to fill the void when subsequent steps of the process are performed and the addition of coating is not required. Once it is determined whether additional coating is required, the method and means for applying this additional coating can be determined. For example, if the article is a flat article, it may be possible to pass the article through a roller coater. If the article is a formed article, such as a metal can or end, it may be appropriate to utilize either a spray application or an electrostatic device for applying additional coating. Other methods and techniques of applying additional coating will be apparent to those skilled in the art. The additional coating to be applied must be chemically and physically compatible with the existing coating.
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Following the step of analyzing the defective coating and if necessary the application of additional coating, the articles are passed through an electromagnetic induction coil. The induction coil produces eddy currents in the metal substrate causing the metal substrate to rise in temperature, thereby heating the coating from the inside towards the outer surface. As set forth in the above referenced prior U.S. patent application, by curing the coating from the inside out, a skin is not formed over the surface of the coating so that solvents contained within the coating can be released to atmosphere. With the present invention, the existing coating is softened by the heat generated and, because it is heated from the inside out, the coating does not skin over as would be the case if a convection oven were used and with the present invention the coating becomes sufficiently fluid so that any solvents which may have remained in the existing coating due to incomplete curing in the prior manufacturing process will be released or volatilized. In addition, solvents contained in the newly applied coating, if any, will be released to thereby dry and cure the coating. Solvents which may be released to atmosphere must be collected in a suitable collection device. If there are adequate solvents to be volatilized, means may be provided for condensing the volatilized solvents and collection of the same.
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Other types of defects which can be corrected by the present invention will be described below, including for example, voids in the coating or brittle coating.
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Following the heating and curing of the coating on the article, the article must be cooled for further handling.
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It is, therefore, the principle object of this invention to provide a method for repairing a defect in an existing coating on a metal article.
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It is a still further object of this invention to provide a method of repairing defects in a coating which has been applied to discrete metal articles by rapidly heating the coated article to a temperature sufficient to soften the coating, but not to a temperature that burns the coating.
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The foregoing and other objects will be carried out by providing a method of repairing a defect in an existing coating which has previously been applied to a discrete metallic article comprising the steps of analyzing the defective coating to determine whether it is required to apply additional coating to repair observed defects in the existing coating; if required following the preceding step, applying additional coating compatible with the original coating; heating the article and the coating to a temperature sufficient to cause the existing coating to become sufficiently fluid to repair the defect and cure the existing coating after it has become fluid and to volatilize substantially all of the solvents which are contained in the additional coating which may have been applied to the article and to cure such additional coating and the existing coating; and cooling the article.
EXAMPLES
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The following are some of the defective coating situations which have been encountered and which may be corrected by the present invention.
Undercured Coating
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In this case, the coating is soft, has good adhesion to the substrate, but does not pass the hardness test required by the customer specifications. In this case, with the present invention it has been discovered that if the defective article is passed through an electromagnetic induction coil to provide additional heat compared to that originally used to attempt to cure the coating, the proper molecular cross-linking occurs resulting in a substantial cure so that the proper coating hardness is obtained. There is usually no reason to add additional coating or thinners to the defective part.
Coating Too Brittle
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This is usually manifested by the coating having pulled away from areas where the metal object has been subjected to a metal forming or scoring operation, such as when the coating has pulled away from scarf marks on the pull tab of a can end. In this case, depending on whether the coating has pulled away before the scarf marks were formed in the end or afterwards, the condition is the same. Bare metal substrate will come in contact with the packaged product which is usually a liquid or beverage for human consumption. If the base metal is on the outside of the container, oxidation can occur resulting in an unsightly article which cannot be sold. For these reasons, the bare metal is an undesirable situation.
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By the present invention, it has been determined that there are three methods to overcome these types of defects. In one condition, the coating is underbaked, i.e., the coating contains solvents which have not been released. As a result, the coating has not flowed properly to cover the scarf marks. By subjecting the article to heating in an electromagnetic induction coil, it has been found that the coating and the entrapped solvent in some instances will become sufficiently fluid to flow into the scarf mark. The coating which has become fluid and flows into the uncovered area is then cured by the heat induced as the metal substrate by the induction coil, thereby making the article usable.
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In a second condition, additional coating must be applied through a suitable applicator, such as a spray nozzle or coated roller, to apply solvent to the scarf area prior to subjecting the article to induction heating in the electromagnetic induction coil. Within the induction heating coil, the substrate is heated first so that the coating next to the substrate becomes hotter than the coating on the surface to which the solvent has been previously applied. As a result, the solvent has time to react with the existing coating causing the newly applied surface coating and solvent mixture to flow into the scarf marks completely covering the metal substrate before all of the solvent has evaporated or been volatilized by the heat. The newly applied coating and the softened existing coating cross link and are bonded together to form an uninterrupted coating on the surface of the metal article after cure.
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A third condition is when the coating has cured properly and has sufficient molecular cross-linking, but for some unknown reason, it did not cover the scarf mark completely. Applying induction heating to the article to heat the coating or applying additional solvent does not effect any change in the coating structure. To cover the scarf mark, additional coating and solvent must be applied by using an applicator. Usually the applicator will be a simple spray device or for more complex requirements an electrostatic deposition device can be used. The electrostatic deposition device tends to deposit most of the coating on the bare metal substrate, in this case the bare scarf marks, thereby increasing coating application efficiency. Once coated, the end is subjected to induction heating which causes the coating-solvent mixture to flow in the uncoated areas and scarf marks prior to solvent evaporation or volatilization. Once cooled, the article is salvaged and can be utilized for its intended purpose.
Excess Solvent
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If the coating on the metal was originally cured in a conventional convection oven, the coating cures from the outside surface down to the metal substrate. This inward cure cycle is usually an eight minute bake at a temperature sufficient to drive off the solvents. Since the coated article is heated from the outside inwardly, the outer coating surface hardens first. This condition causes a skinning-over effect which traps the solvents, meat release products and internal lubrication products in the coating.
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By subjecting the defective coated part to induction coil heating, the ferrous substrate is rapidly heated. Once hot, the substrate causes the existing coating to become sufficiently fluid to permit solvents, internal lubricants and meat release products to volatilize at the metal substrate/ coating interface to be released allowing for a better bond and better adhesion to take place between the coating and the metal substrate. The meat release and internal lubricant products are driven to the coating surface where they are needed to complete the manufacturing process. The meat release product is an additive blended into the coating to better facilitate emptying the container after it has been opened. With the present invention, these meat release products are no longer trapped beneath the surface of the dried coating, but are now located at the surface where they can perform their intended function.
BRIEF DESCRIPTION OF THE DRAWINGS
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The invention will be further described in connection with the annexed drawings wherein:
- Fig. 1 is a diagramatic view of the overall apparatus which is utilized to carry out the process of the present invention; and
- Fig. 2 is a sectional view of the induction coil which is utilized in the process of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
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Referring to Figs. 1 and 2, the apparatus for carrying out the process of the present invention is illustrated. The apparatus includes a source 1 of articles, such as cans or can ends, which have a defective coating. The article is illustrated in the form of a can end 2. The particular article to be covered is not intended to be a limiting factor of the present invention. An important criteria is that the article should have a metal (preferably ferromagnetic) substrate which is coated with a heat cured coating such as resins contained in a solvent.
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A conveyor belt 5 is provided for transporting the article having the defective coating through the various apparatus required for carrying the process of the present invention. The defective parts are placed in an orderly fashion on the insulated conveyor belt 5. A suitable feeder device (not shown) may be utilized for distributing the articles from the source 1 onto the belt 5. The particular feeder device required will be dependent upon the quantity of articles to be treated, the type of article to be treated and other factors which will be apparent to those having ordinary skill in the art.
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It is to be understood that although this invention refers to salvaging both can ends and cans, in actual production environments, a separate line design will be required for each article.
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In order to keep the article from sliding off the insulated conveyor belt, a vacuum hold down system generally indicated at 10 is provided. Since the parts will be heated in an electromagnetic induction coil, it is necessary to use an insulated conveyor belt to prevent arcing between the conveyor and the part. The vacuum hold down system 10 may include a vacuum pump 11 and suitable vacuum lines 12 leading to a plenum chamber 13 for applying a vacuum through the porous conveyor belt 14 which forms parts of the conveyor 5.
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The coated article to be repaired is conveyed through a coating application apparatus generally indicated at 20. As previously stated, this application apparatus while illustrated as a spray system may take the form of an electrostatic coating applicator of the type known to those skilled in the art, a bath or other suitable means for applying additional coating to the defective article. Of course, as previously stated, it may not be necessary to apply additional coating to the article in which case the particular apparatus may either have the coater 20 deactivated or removed entirely.
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The next step in the process is to rapidly heat the defective article by means of an electromagnetic induction coil generally indicated at 25. This induction coil may include an induction generator 26, power lines 27 and water cooled induction coil 28. The induction coil 28 produces an oscillating magnetic flux which induces eddy currents in the ferromagnetic metal substrate that heats the outside surface of the metal substrate. Ferrous metal as thin as 0.0035 inches can be accommodated without becoming saturated by the magnetic field flux.
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The induction generator contains suitable transformers and capacitors to match the electrical load to the induction coil 28 to achieve maximum power transfer to the defective part 2. Details of the control system are disclosed in the aforementioned U.S. Patent Application Serial No. 07/686,961, filed April 18, 1991.
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The conveyor 5 conveys the parts 2 through the induction coil where they are subjected to the rapid alternating of the flux field produced in the coil 28 to produce eddy currents in the articles 2, thereby producing rapid heating of the metal substrate to heat the coating from the inside out to drive off the unwanted materials, provide increased adhesion and a more homogeneous protective covering to the metal substrate. Thus, the heating of the metal substrate causes the existing coating to soften and become sufficiently fluid so that any solvents which remain in the existing coating are volatilized and solvents in any newly applied coatings are volatilized. The softened coating flows into uncovered areas of the metal substrate. The heat generated by the induction coil will result in better adhesion of the coating to the substrate. Solvents in the existing coating which are volatilized and solvents in the newly applied coating which are volatilized will be discharged to atmosphere where they are collected in a hood 30 and conveyed through suitable ducting 31 to an air pollution control device (not shown). The rapid heating of the metal substrate results in a drying and curing of the coating as well as the repair of the defective coating.
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A cooling hood 35 has also been illustrated in Fig. 1 and is used to distribute cooling air supplied through a duct 36 to cool the article to a temperature suitable further handling. In some applications, another cooling method or apparatus may be used or the cooling hood could be eliminated and the part allowed to cool by exposure to ambient temperature.
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Repaired articles are conveyed by conveyor 5 to a collection point 40 to complete the process.
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As can be seen from Fig. 2, the coated articles 2 may be conveyed through the electromagnetic induction coil in a manner similar to that illustrated in U.S. Patent Application Serial No. 07/686,961, filed April 18, 1991.
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In laboratory tests on coated cans with defects due to cracks and voids, vinyl organisol coating was heated in a circular electromagnetic induction coil to the temperatures stated in Table 1 and because of the coating's thermoplastic characteristics (melts upon extreme temperature application), the coatings flowed into the cracks in the metal substrate and coating voids making the can usable. Approximately twenty cans each were run at 350° and 400°, respectively.
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In a second test, the circular induction coil was replaced by a flat coil (similar to Fig. 2). Two types of can ends were previously partially cured. Those cured in an infrared oven are designated as "PC" in Table 2. Ends designated "C" were dried at room temperature only. The number following these letter prefaces represents the maximum temperature the ends reached after being subjected to the induction magnetic field. "C-2-480" represents that the sample was passed twice through the induction coil and each time reached a maximum temperature of 480°F.
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Vapor comparisons revealed the previously cured (PC) articles combined with heating the article in the induction coil produced the most completely cured coating. Those articles which were previously cured only by oven cure (infrared) produced the most effluent signifying that the coating was only partially cured and that the present invention completed the curing process and would be suitable for repairing a defective coating.
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Induction coil power input of 35KHz to 300KHz is usable depending on the thickness of the metal substrate. A lower frequency gives greater metal penetration and a higher frequency results in more of a skin effect.
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The conveyor speeds shown in Tables 1 and 2 have demonstrated that the desired temperature range of about 300° to about 500°F are achieved within approximately 0.75 seconds. In some cases, it may be desirable to heat the article at a faster rate so that the desired temperature is reached in approximately 0.5 seconds.
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With the method of the present invention, it is necessary to analyze the article to determine the particular defect. Usually, there will be a large quantity of articles which have the same or a similar defect. Thus, if during the original metal coating process the metal was not properly surfaced treated prior to the coating, there will be a large quantity of articles manufactured from that improperly treated and coated metal. The process of the present invention can then be set up to either apply additional coating or operate without the necessity of additional coating. At this time, the temperature at which the particular article needs to be heated to correct the defect in the existing coating can be determined and based on this temperature, the power which needs to be applied to the electromagnetic induction coil and the speed at which the conveyor used to move the articles through the induction coil can be determined. The temperature selected must be sufficiently high to cause the existing coating to become at least partially fluid to flow into voids and cracks and allow unvaporized solvents to escape, but not exceed a temperature which will cause the coating to burn. These temperature extremes will normally be available from the coating manufacturer.
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If it is determined that it is necessary to apply additional coating, the preferred embodiment is that the coating to be applied would be the same as that originally applied, although it may be possible to select a physically and chemically compatible but not identical coating in order to effect the cure.
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From the foregoing, it is apparent that the objects of this invention have been carried out. It is intended that this invention be limited solely by that which is within the scope of the appended claims.
The invention may be summarized as follows:
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- 1. A method of repairing a defect in an existing coating which has previously been applied to a discrete metallic article comprising the steps of:
analyzing the defective coating to determine whether additional coating must be applied to the article to repair observed defects in the existing coating;
if required following the preceding step, applying additional coating compatible with the original coating; and
heating the article and the coating to a temperature sufficient to cause the existing coating to become sufficiently fluid to repair the defect and cure the existing coating after it has become fluid and to volatilize substantially all of the solvents which are contained in the additional coating which may have been applied to the article and to cure such addition coating and the existing coating; and
cooling the article.
- 2. A method of repairing a defect in an existing coating according to
1 wherein the step of heating the article is carried out by conveying the discrete metallic article through an electromagnetic induction coil.
- 3. A method of repairing a defect in an existing coating according to
1 further comprising the step of selecting the method of applying the additional coating if it is determined that it is necessary to apply additional coating.
- 4. A method of repairing a defect in an existing coating according to
2 further comprising the step of selecting the method of applying the additional coating if it is determined that it is necessary to apply additional coating.
- 5. A method of repairing a defect in an existing coating according to
1 further comprising the step of removing volatilized coating to an air pollution control device.
- 6. A method of repairing a defect in an existing coating according to
2 wherein the article is heated to a temperature sufficient to cause the existing coating to flow into areas where the existing coating did not adequately cover the metal article.
- 7. A method of repairing a defect in an existing coating according to
2 wherein the article is heated to a temperature sufficient to result in an improved bonding of the existing coating to the metal article.
- 8. A method of repairing a defect in an existing coating according to
2 wherein the metal article is heated to a temperature sufficient to volatilize previously unvolatilized solvents which may be contained in the existing coating.
- 9. A method of repairing a defect in an existing coating according to
2 wherein the article is heated to a temperature of between 300 and 500°F.
- 10. A method of repairing a defect in an existing coating according to
2 wherein the article is heated for a duration of between approximately 0.5 and approximately 0.75 seconds.
- 11. A method of repairing a defect in an existing coating according to
9 wherein the article is heated for a duration of between approximately 0.5 and approximately 0.75 seconds.
- 12. A method of repairing a defect in an existing coating according to
1 wherein the article is heated for a duration of between approximately 0.5 and approximately 0.75 seconds.
- 13. A method of repairing a defect in an existing coating according to
2 wherein the article is conveyed at a rate of between approximately 20 and approximately 30 feet per minute and the power supplied to the electromagnetic induction coil is between 35 and 300KHz.
- 14. A process for repairing a defect in a coating on a ferromagnetic substrate, said coating including resins dissolved in a solvent comprising inductively heating the ferromagnetic substrate to a temperature sufficiently high to cause the coating to become sufficiently fluid to repair the defect in the coating and to a temperature less than that at which the coating and the metal substrate are damaged.
- 15. A process for repairing a defect in a coating on a ferromagnetic substrate according to 14 wherein the substrate is heated to a temperature of between about 300° and about 500°F in less than about 0.75 seconds.
- 16. A process of repairing a defect in a coating applied to a ferromagnetic substrate according to 14 further comprising the initial step of analyzing the coating to determine the type of defect encountered and to determine whether additional coating material needs to be applied to the ferromagnetic substrate.
- 17. A process for repairing a defect in a coating applied to a ferromagnetic substrate according to 14 further comprising the initial step of analyzing the defective coating to determine whether the defect is the result of one or more of: failure of the coating to adhere to the ferromagnetic substrate, failure to coat a portion of the ferromagnetic substrate, incomplete volatilization of the resin containing solvents and meat release products being trapped below the dried surface of the coating.
- 18. A process for repairing a defect in a coating applied to a ferromagnetic substrate according to 17 further comprising the step of determining whether additional coating material needs to be applied to the substrate and, if so, determining to apply additional coating which is physically and chemically compatible with the defective coating.
- 19. A process for repairing a defect in a coating applied to a ferromagnetic substrate according to 18 wherein the substrate is heated to a temperature of between about 300° and about 500°F in less than about 0.75 seconds.
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TABLE 1
| Conveyor Speed |
Temperature |
Power Input |
Remarks |
| 20 ft/min |
400°F |
65% | Single Pass | |
| 20 ft/min |
350°F |
55% |
Single Pass |
| 100% power input equals 7.5kw |
TABLE 2
| Conveyor Speed |
Temperature |
Power Input |
Remarks |
| 30 ft/min |
PC 420°F |
100% | Single Pass | |
| 30 ft/min |
C 420°F |
100% | Single Pass | |
| 25 ft/min |
PC 480°F |
100% | Single Pass | |
| 25 ft/min |
C 480°F |
100% |
Single Pass |
| 21 ft/min |
525°F |
100% |
Single Pass/Burnt Coating |
| 23 ft/min |
PC 500°F |
100% |
Single Pass |
| 23 ft/min |
C 500°F |
100% | Single Pass | |
| 25 ft/min |
C-2-480 |
100% |
Double Pass |
