A METHOD OF JOINING STRUCTURAL ELEMENTS BY HEATING OF A BINDER

Abstract: A method for joining structural elements by heating of a binder, e.g. to form panel assemblies (21) for automobiles, including applying to a first structural element a binder at locations thereof intended to form bonding contact with the corresponding locations on a second structural element, mating the second structural element with the first structural element, and hemming an outer edge of the first structural element about a corresponding edge of the second structural element to make the binder contact upper and lower faces of corresponding edges. The combination of the first and second structural elements is brought into contact with a plurality of heating elements (16,20) at selected, spaced apart spots along an outer edge of the combination. Heat energy is applied at the spots to form a spot bonding between the first and second struc­ tural elements thereat by curing the binder at the spots. The remainder of the binder is left uncured, and the the spot-bonded combination is then removed from the heating elements and subjected to subsequent treatment, including treatment in an atmosphere at such elevated temperatures that the spot bonds are post-cured and the remaining binder is fully cured.

Description: The present invention relates to a method for joining structural elements by heating of binder, e.g. to form panels for automobiles, by applying to a first structural element a binder at locations thereof intended to form bonding contact with corresponding locations on a second structural element, mating the second structural element with the first structural element, and hemming an outer edge of the first structural element about a corresponding edge of the second structural element to make the binder contact the faces of the corresponding edges. In recent years it has become more and more usual to interconnect structural elements by means of adhesive bonding instead of using conventional methods, such as spot welding. In industrial processes, the curing of the adhesive must be as rapid as possible. Heat is then used to accelerate the curing. The elements to be glued together are conventionally placed in an oven having a temperature in the range of 100-200°C, dependent on the temperature required for suitable curing. For structural materials which are electrically conductive, it is advantageous to heat the material about joints having adhesive, by way of induction, to thereby obtain quick heating. By designing the induction coils and using a suitable frequency, the heating is limited and the energy consumption is thereby also limited to areas where the joining is to take place. The car industry has, to a large extent, made use of processes including binders such as epoxy resins and other adhesives. Such binders are, e.g., used for door panels, hoods and hatches. These parts consist of an outer panel or structural element and an inner structural element (skeleton). The outer edges of the outer panel are hemmed over the outer edges of the inner structural element, whereafter the hem, according to the prior art, is spotwelded to interconnect the two structural elements. If the structural elements have any surface treatment prior to spot welding, e.g. such as that which provides a galvanized surface, the surface treatment at the spot weld will be deteriorated and be a potential rust-creation spot. In recent years, the hem has been filled with an adhesive, and curing of such adhesive takes place when the car panel passes through paint curing ovens. Spot welding is thereby avoided while adhesive binding simultaneously provides better protection against corrosion by preventing moisture from entering into the hem. In order that car panels to be joined by adhesive are to be stable in form and have sufficient strength to sustain the handling which the panels are subjected to in manufacturing, "in line" curing stations have been used. In these stations, the hems of the panels are heated along their entire length by means of induction heating to 15°-20°C within a period of 4-20 seconds. By this approach, it is intended to obtain a partial curing of the binder, sufficient to give the panels form stability and sufficient strength for handling and installation into the car before the final curing takes place in the process of applying paint to the car and curing the paint in appropriate ovens. In addition, the adhesive is not rubbed off upon touching. Such induction heating is presently used with an excellent result. However, in order to avoid deformations caused by thermal expansion of the locally-heated periphery of the panels, the panels are clamped firmly against a panel shape defining bed while heating and partial temperature equalization takes place. This prior art technique is known from the brochure "Olofström induction heating station for inline heating of adhesive bonded hems on automotive panel assemblies" and is also illustrated in the brochure "Heating without flames" by ELVA induksjon A/S, assignee of the present invention. The induction coil is located like a loop around the periphery of the car panel assembly. Sufficient uniform temperature distribution is obtained by adapting the location of the coil and distance to the hem of the panel assembly around the panel assembly. Correct design of coil, correct positioning of clamping means and correct temperature cycle are required to keep deformations within acceptable limits. Curing by induction also has a tendency to amplify small deformations which are introduced in preceeding stamping and hemming devices. As such, the conventional induction heating prosess for joining automotive panel assemblies requires several ingredients to function, such as a correct model or a structural panel assembly in order to design and construct a bed which the panel assembly can be clamped against. This is frequently required before the final tools for stamping and hemming are available. Further a coil which is adapted to a frequently complicated form must be made. The coil must be adjusted and readjusted by testing on the actual panel assembly such that required temperature distribution around the periphery thereof is obtained. Finally, positioning of the clamping means must be tested in order to restrict deformations within acceptable limits. Such factors render the conventional curing process by induction heating expensive and the work involved in redesigning the equipment to adapt to new car models is also quite expensive. US-A-3,756,881 discloses a method of bonding together two structural elements using a thermo-plastic strip provided with an electrically-conductive core which serves as heater element to cure the thermo-plastic strip. Many of the cost inducing and complicated elements are avoided, according to the invention, when only spots or short portions of the periphery are heated and cured, sufficient to give the panel assembly adequate form-stability and handling strength. By exposing only a small portion of the periphery, suitably in the range of 10-30%, to heating, the thermal expansion of the panel assembly is substantially reduced. The inherent strength of the panel assembly is then sufficient to prevent permanent deformations. Thereby, the need for a tailor-made bed and clamping means is eliminated. Instead of having a continuous coil along the periphery of the panel assembly, such coil is replaced by a plurality of smaller coils having a general form, each of which is adjusted into position at a selected spot against the panel assembly. Such an approach is easy to implement when starting a production plant for the manufacturing of such panel assemblies. Further, the inventive method offers great possibilities for convenient adjustment to adapt to new car models. By heating a lesser portion of the panel assembly, the power requirement is reduced and the panel assembly is not supplied with more energy that will cause the temperature, shortly after termination of heat application, to drop to below 50°C, such that handling by humans can be accomplished without any precautions. The method according to the invention, thus comprises the steps of: providing first and second structural elements each having first and second faces and a perimetrical edge; applying to one of said first and second faces of said first structural element at at least one predetermined location, at least one band of uncured, heat-curable binder; and mating said first and second structural elements so that said at least one band of uncured, heat-curable binder is juxtaposed with a corresponding predetermined location on one of said first and second faces of said second structural element; characterised in that the method comprises the further steps of: inducing heat energy to at least one spot of said at least one band of uncured, heat-curable binder through only one of said first and second structural elements using at least one heating element, thereby curing said uncured, heat-curable binder at said at least one spot for spot-bonding said first and second structural elements together, while leaving a remainder of said uncured, heat-curable binder in said at least one band substantially uncured; and thereafter subjecting said spot bonded first and second structural elements to an elevated temperature treatment wherein said cured binder at said at least one spot is post-cured and said remainder of said uncured, heat-curable binder is substantially fully cured thereby further bonding said first and second structural elements together. According to further features of the invention, the method comprises the step of: hemming an outer edge portion of one of said first and second structural elements along at least one portion of said perimetrical edge thereof about a corresponding portion of said perimetrical edge of a corresponding other one of said first and second structural elements after applying said at least one band of uncured, heat-curable binder. According to a preferred embodiment of the method, said step of inducing heat energy is effected by using a respective inductive heating coil served by a respective power transformer at each said heating element. The respective primary windings of the heating element transformers are suitably connected in series. According to an alternative embodiment of the method, said step of inducing heat energy is effected by using a respective infrared radiation device at each heating element. Each such infrared radiation device is suitably a quartz lamp provided with an elliptical reflector, and each elliptical reflector is used in said step of inducing heat energy for focusing heat energy on a respective said spot. The step of inducing heat energy suitably is carried out for a maximum of 40 seconds. The step of inducing heat energy is carried out at approximately 200°C. Further, the method according to the invention suitably also comprises the step of: hemming an edge margin of one of said first and second structural elements along at least one portion of said perimetrical edge thereof about a corresponding portion of said perimetrical edge of a corresponding other one of said first and second structural elements after mating said first and second structural elements. The invention is now to be further explained with reference to the accompanying drawings illustrating non-limitative, preferred examples of the invention. Figure 1 illustrates prior art apparatus of Olofström-ELVA induction heating station; Figure 2 also illustrates prior art apparatus of Figure 1 in more detail with parts of the apparatus broken away for clarity; Figure 3 is a prior art heat temperature versus heating time diagram; Figure 4 illustrates the apparatus utilized in the method according to the invention for spot bonding of automotive panel assamblies; Figure 5 illustrates a way to interconnect induction coil transformers; Figure 6 illustrates a typical infrared radiation device; Figure 7a, 7b and 7c are side, plan and front views, respectively, of a coil unit for spot bonding; and Figure 8 illustrates a transformer unit with water-cooled power supply cables. The prior art device illustrated in Figure 1 is an Olofström-ELVA induction heating station which is designed for integration into automated panel assembly lines with a super compact under-the-table power source 14 and provides a fast, precise heating cycle. In such a conventional process, an outer panel 9 is supplied, adhesive is applied to the edges of the outer panel by means of a robot, the inner panel is supplied and spot welding of elements thereto may be performed on a coordinate table, whereafter the inner panel is mated with the outer panel at a marriage point, followed by two-stage hemming of the edge of the outer panel about the edge of the inner panel. Thereafter, the panel combination is subjected to heating along the entire length of the edges thereof, whereafter the panel assemblies, now bonded together, are delivered for further processing and control. As clearly seen from Figures 1 and 2, there is required a working table 12' having bed supports 12 for the bed 3, 6, element 3 being a fixture frame and 6 being a cover plate. A cushion 1 is provided at the top of the fixture frame 3. The reference numeral 4 indicates a segment of the induction coil and the reference numeral 5 indicates a bolted joint of the induction coil. Coolant 2 flows through the induction coil 4. The structural element combination is now placed on the bed 3, 6 as shown in Figure 1 and is firmly clamped onto that bed by clamping devices 11 which are adjustably connected to posts 10. From Figure 1, it is seen that in the chosen example, there is required no less than nine clamping devices in order to firmly clamp the panel assembly with the hem 8 to the bed 3, 6. Such clamping devices 11 are, however, fairly expensive in addition to the cost of the tailor-made bed 3, 6. After application of an adhesive, mating and hemming, the panel assembly is placed outer panel side down and lowered onto the fixture having a plastic cushion 1 that supports it at the hemmed outer edges. Once in place, the panel assembly is secured by the clamping devices, which are suitably pneumatic clamps. The induction coil 4 under the cushion 1 is then energized to heat the hemmed flanges. Temperature and cycle time, as indicated in Figure 3, can be programmed according to curing process specifications and the stepping speed of the production line. At the conclusion of the heating cycle, the clamps are caused to and means are provided to facilitate transfer of the panel assembly to the next station. The bed is custom-made suitably from models supplied by the user. The fixture frame 3 has individually positionable and adjustable clamps, and load sensors are available to provide input to the line control system. The coil used in this prior art, supplied by ELVA induksjon A/S, consists of bolted sections of copper tubing with a built-in coolant circuit. Such a construction has, up to the present date, simplified optimum positioning relative to the workpiece at all points. The coil according to Figure 2 is fully enclosed. The prior art apparatus has a compact power source housed within two boxes which are small, to fit under the fixture table 12'. The power source makes use of a converter technology which has done away with the need for cumbersome closed-coolant systems and jacketed cables and, incidentally, has reduced the output of audible sound frequencies. A fully modulized freqency converter is one consisting of a controlled rectifier and bridge-connected transistor inverter. A matching unit thereto includes a transformer and compensation capacitor built into a single oil-filled, water-cooled unit. Ordinary plant water can be used as a coolant as consumption for cooling both the matching unit and the induction coil is fairly low. An interface is provided for connection to a station or line PLC. Operation can be monitored by an energy supervision system that measures the energy which is supplied to the panel assembly. This has proved to be highly precise and have better reliability than direct temperature-measuring systems. In the prior art system, any abnormality in coil setting, heating cycle or panel location will yield a different energy consumption reading that will trigger an alarm. Apart from the bed, including the cushion fixture frame and cover plate as well as the peripheral induction coil, all elements discussed above could be used in implementing the present invention. From prior art, Figure 2, it is noted that the outer panel 9 is closest to the heating element or induction coil 4, whereas the inner panel or structural element 7 is intermediate the outer panel 9 and its hem 8. From Figure 3, it can be seen that the temperature of the outer panel 9 at the edge thereof is higher than the temperature of the inner panel 7 which is higher than that of the hem 8 up to a temperature of approximately 170°C until approximately 13 seconds after the start of heating. Although the same maximum heating temperature could be used with the present invention, the example given by the prior art in Figure 3 is in no way to be construed as limiting to the present invention, as temperatures higher than 170°C or lower than that temperature could be used, dependent on the temperature of curing of the adhesive in question. However, with present types of binder, it is preferred that the temperature of the actual spots of the hem be brought rapidly to approximately 200°C and kept there over a period within the cycle time such that the adhesive is supplied with a sufficient amount of energy. For simplicity, the embodiment according to the invention as shown in Figure 4 does not include the power source 14, but it should be understood that such a power source can be located at any suitable point of the device according to that Figure. In Figure 4, the reference numeral 15 denotes a lifting means for lifting the panel assembly or structural element combination 21 to the desired level by means of heating elements 16 which support the structural element combination at the edges thereof. Each heating element consists of an induction coil, labelled 16, as well as an output transformer 17 for each coil 16. The heating elements 16 and output transformers 17 are supported by a mechanical fixing arrangement which is adjustable in at least three dimensions. Power is supplied to the transformers through flexible, water cooled cables 19. The reference numeral 20 denotes a connection box having capacitors 20'. Figures 7a through 7c illustrate a coil unit 16, having a coil connection member 16', a core 16'' and a Cu-tube 16‴. Figure 8 illustrates the transformer unit 17 with the cables 19. Reference numeral 24 denotes connectors for the coil unit 16. When using induction heating, it is, as indicated schematically in Figure 5, desirable to use transformers having their primary respective windings connected in series. Such transformers may be directly connected to the coils 16 and the flexible power supply by means of the cables 19 thereby rendering individual adjustment possible to provide equal energy supplied to each spot and to regulate these transformers/heating elements to provide equal energy to each spot for different panel assemblies. By a series connection, equal primary current in all transformers is secured. By using a fixed distance between the coil and the panel assembly, like amounts of energy will also be secured. As an alternative, Figure 6 shows the use of a plurality of infrared quartz lamps 22 which may be substituted for the coil 16/transformer 17. Suitably, such lamps have an elliptical reflector 23 to cause the energy to be consentrated in small line pieces along the hem on the panel assembly. As seen from Figure 6, the binder 13' is contacting only one face of the structural element 7. However, the binder may be present, as in Figure 3, to also contact the other face of the structural element 7. The essential point is that a spot binding is performed between at least one face of the structural element 7 and either the hem 8 or the outer panel 9. It will be recalled that the binder material which is not spot bonded is cured in the processes at elevated temperatures that follow the spot bonding process. In summary, the present method provides for "in line" curing of the binder in the hems on panel assemblies for e.g. cars, such as panel assemblies of doors, hoods, boot-cover, hatch-backs, roof-hatches, by letting only small longitudinally spaced-apart portions of the hems be heated and cured to give the object in question sufficient form-stability and strength in order to enable handling through the further production steps. The heating, either by induction heating or IR-heating would suitably take place within a time space of 40 seconds. When using induction heating, there are transformers connected in series, as seen in Figure 5, with each having an induction coil arranged so that such induction coil is easy to move and adjust in order to obtain optimum heating of the pieces to be heated. The present invention offers a simpler control system, requires lower power consumption, has no requirement for a tailor-made bed, no clamping means are necessary, a simple readjustment for adaption to a different panel assembly is available, and higher temperatures are possible, such that a greater spectrum of adhesive/binder may be available.

Claims: A method for joining a first structural element to a second structural element to form a panel assembly, said method comprising the steps of: providing first and second structural elements each having first and second faces and a perimetrical edge; applying to one of said first and second faces of said first structural element at least one predetermined location, at least one band of uncured, heat-curable binder; and mating said first and second structural elements so that said at least one band of uncured, heat-curable binder is juxtaposed with a corresponding predetermined location on one of said first and second faces of said second structural element; characterised in that the method comprises the further steps of :- inducing heat energy to at least one spot of said at least one band of uncured, heat-curable binder through only one of said first and second structural elements using at least one heating element, thereby curing said uncured, heat-curable binder at said at least one spot for spot-bonding said first and second structural elements together, while leaving a remainder of said uncured, heat-curable binder in said at least one band substantially uncured; and thereafter subjecting said spot bonded first and second structural elements to an elevated temperature treatment wherein said cured binder at said at least one spot is post-cured and said remainder of said uncured, heat-curable binder is substantially fully cured thereby further bonding said first and second structural elements together. A method according to claim 1, and comprising the further step of: hemming an outer edge portion of one of said first and second structural elements along at least one portion of said perimetrical edge thereof about a corresponding portion of said perimetrical edge of a corresponding other one of said first and second structural elements after applying said at least one band of uncured, heat-curable binder. A method according to claim 1 or 2, wherein: said step of inducing heat energy is effected by using a respective inductive heating coil served by a respective power transformer at each said heating element. A method according to claim 3, wherein: said step of inducing heat energy is effected by using a respective infrared radiation device at each heating element. A method according to claim 4, wherein: each infrared radiation device is a quartz lamp provided with an elliptical reflector, and each elliptical reflector is used in said step of inducing heat energy for focusing heat energy on a respective said spot. A method according to any of the preceding claims, wherein: said step of inducing heat energy is carried out for a maximum of 40 seconds. A method according to any of the preceding claims, wherein: said step of inducing heat energy is carried out at approximately 200°C. A method according to claim 1 and comprising the further step of: hemming an edge margin of one of said first and second structural elements along at least one portion of said perimetrical edge thereof about a corresponding portion of said perimetrical edge of a corresponding other one of said first and second structural elements after mating said first and second structural elements A method according to claim 3, wherein respective primary windings of said respective power transformers are interconnected in series.

Inventors: LARSEN, JOHAN IVAR
Applicants: ELVA INDUKSJON A.S.
Application year: 1989
Main classification: F16B11
IPC codes: B29C65, B23K11, F16B11, B29C35, B29L7, C09J5