Method and apparatus for bonding a cured fiber reinforced plastic part to a reinforcement member

Abstract: A method and apparatus is disclosed for bonding fiber reinforced plastic exterior automotive body panels (12) to reinforcement members (14) using dielectric heating (46).

Description: The invention relates to a method and apparatus of bonding a cured fiber reinforced plastic part to a reinforcement member. This method comprises forming an assembly by placing an uncured thermosetting adhesive between mating surfaces of the part and the reinforcement member; placing the assembly between two electrodes; and applying a substantially constant high frequency signal having a frequency between 10 and 110 megahertz to the electrodes from a dielectric heater. The recent trend in vehicle design is to replace heavier metal parts by plastic parts. In particular, many advantages can result from using fiber reinforced plastic (FRP) exterior automotive body assemblies such as hoods, decks, doors and the like instead of their sheet metal counterparts. However, it is often necessary to reinforce larger exterior FRP panels or members with a reinforcement member in order to provide the resulting assembly with sufficient rigidity and mechanical strength. According to the method of the generic kind, as disclosed in FR-A-1 582 931, two parts composed of reinforced thermosetting resin are bonded by applying an adhesive between them. Bonding is achieved by locating this assembly between electrodes having small faces. The electrodes are supplied by a high voltage current of 10,000 to 80,000 volts at high or very high frequency, for instance 30 megahertz, and pressing the parts together. The adhesive will instantaneously be heated very locally on a face which strictly corresponds to that of the two electrodes and its curing will be extremely rapid between 1 and 10 seconds, and thus welding points will be realised. The object of the present invention is to provide a bonding method and apparatus capable of being used to expeditiously adhere a reinforcement member to an exterior fiber reinforced plastic body member without adversely affecting the surface qualities of the exterior fiber reinforced plastic body member and providing even bonding of the reinforcement member to the exterior fiber reinforced plastic body member notwithstanding the size of the body member and the reinforcement member. In accordance with the teachings of the invention, the high frequency signal is applied at a voltage of 300 to 8000 volts for a period of time sufficient to cure the adhesive without substantial adverse effect to the surface quality of the exterior of the part. It has unexpectedly been found that this technique does not generate excessive heat which could otherwise create surface imperfections in the FRP member. Preferably an exterior automotive body panel is used as the plastic part having a surface area exceeding 0,1 m² (one square foot) and a length in at least one dimension exceeding 1,2 m (47 inches). Advantageously, a reinforcement member is used which is generally hat shaped in cross section characterized by a recessed dome and a pair of flanges, with a bead of adhesive being applied between the flanges and the interior surface of the skin. Preferably, a reinforcement member is made of cured fiber reinforced plastic. Usually an adhesive is used which is a two part epoxy adhesive having a resin and a hardener. It is convenient that an exterior automotive body skin is used as the plastic part, that the skin and the reinforcement member are both formed by compression molding charges of thermosetting sheet molding compound, and that the alternating electrical signal is applied for a period of no more than two minutes to form a bonded reinforced panel assembly. Usually a skin is used having the shape of a hood and having a thickness of 0,2 to 0,3 cm (.080 to .120 inch). The apparatus for bonding a fiber reinforced plastic exterior automotive body skin to a reinforcement member made of fiber reinforced plastic and having a hat shaped cross section with a dome and pair of flanges comprises an assembly for supporting the skin, exterior side down, with the reinforcement member being lying thereon and a bead of chemically reactive adhesive being located between the interior surface of the skin and the reinforcement member; moving means for moving the assembly between two electrodes; and voltage source means for applying a substantially constant high frequency signal having a frequency of 10 to 110 megahertz, preferably 25 to 40 megahertz, at a voltage of 300 to 8000 volts from the dielectric heater of sufficient duration to cure the adhesive without adversely affecting the surface qualities of the exterior of the skin. Preferably the electrodes are at least 1,2 m (47 inches) in length. It is also advantageous to provide cooling means for maintaining the exterior surface of the skin at a substantially constant temperature. Conveniently the assembly includes a nest member made of metal and wherein the cooling means comprises means for circulating cooling medium through the nest member. According to the invention reinforced exterior automotive body assemblies are produced, such as hoods, doors and bumper assemblies. A first, typically exterior, fiber reinforced plastic member and a second reinforcing member are bonded, which is also preferably manufactured of a fiber reinforced plastic, although other plastic materials or even metal may be used. In certain cases, such as the bumper assembly, the first and second members are more appropriately described as mutually reinforcing since the second member may actually define part of the exterior surface of the automotive assembly, e.g., to locate automobile features such as signal lights or head lights. The method according to the invention is advantageous over other methods for bonding reinforcement members to thermosetting SMC skins. Conventional radiant heating methods are generally too slow and result in nonuniform bonds and/or excessive heat generated by the adhesive which can burn and show through the exterior skin. Induction heating generally requires the use of metallic particles in the adhesive or areas to be heated. This is not a practical approach for exterior automotive body panel assemblies. The apparatus of the preferred embodiment is also designed to overcome problems which have been found to be present when attempting to use dielectric heating for bonding relatively large parts. It has been discovered that if a single elongated electrode is used to bond parts exceeding about 1,2 m (47 inches) at 34 MHz, uneven heating can unexpectectedly occur. Although the reason for this phenomena is not completely understood, it is believed that the uneven heating is caused because various areas of the elongated electrode will have certain peaks of energy radiating therefrom which may be due to the fact that the electrode length approaches 1/15 of a wavelength of the alternating voltage source 46 (thereby producing heat variations of greater than or equal to 10%). Thus, various voltage rises are created along the length of the two opposing electrodes and can result in uneven heating of the parts. The invention is described in detail below with reference to the manufacture of an automotive hood assembly, however, it will be appreciated by those skilled in the art that the present invention, as defined by the appended claims, is clearly not limited to this particular application. For example, the method and apparatus may be successively utilized to produce assemblies comprising three or more members, such as an automotive door manufactured of a first, outer fiber reinforced plastic member, a middle metal member, and a second, inner fiber reinforced plastic member. Embodiments of the invention are described with the aid of drawings in which Fig. 1 is an exploded perspective view of an assembly comprising an exterior automotive body member and a reinforcement member therefor; Fig. 2 is a partial bottom plan view of the assembly of Fig. 1 Fig. 3 is a partial sectional view of the assembly of Fig. 1 taken along lines 3-3 of Fig. 2; Fig. 4 is a cross-sectional view taken along lines 4-4 of Fig. 5, which illustrates in simplified form apparatus for carrying out the preferred method of this invention; Fig. 5 is a cross-sectional view taken along lines 5-5 of Fig. 4; Fig. 6 is a front view of the apparatus of Figs. 4 and 5; and Fig. 7 is a graph illustrating the relationship between time and heat generated during the bonding process. A hood assembly, generally designated by the numeral 10, is illustrated in the drawings. Assembly 10 consists of a relatively thin - 0,2 to 0,3 cm (about .080 to .120 inch thick) - outer skin member 12 and a reinforcement member 14. Outer skin member 12 is constructed fiber reinforced plastic (FRP) and is preferably made from thermosetting sheet molding compound (SMC) material which has been compression molded under vacuum. U.S.-A- 4,488,862; 4,551, 085; and 4,612,149 relate to such techniques and are hereby incorporated herein by reference. The present invention finds particular utility when member 12 is an automotive exterior body panel such as a hood, deck, or the like which has a relatively large surface area exceeding about 0,1 m² (one square foot) and having a length in at least one dimension of 1,2 m (47 inches). These types of panels usually require reinforcement in order to provide them with sufficient mechanical rigidity and it is important to maintain the smooth aesthetically pleasing outer surface of the skin typically required by the automobile manufacturers. This standard is generally referred to in the industry as a class A surface. By way of a nonlimiting specific example, assembly 10 is a hood for Ford Aerostar minivan and its out dimensions are approximately 1,42 m x 0,56 m (56 inches by 22 inches) and is about 4,8 mm (0.19 inch) thick - 2,3 mm (0.09 inch) for the reinforcement member plus 2,5 mm (0.10 inch) for the skin. Preferably, reinforcement member 14 is likewise constructed of FRP material. In the embodiment illustrated, member 14 is configured into a "figure 8" design in plan view and each leg is formed of a "hat" design in cross section as can be seen most clearly in Figures 3 and 4. The hat cross section is characterized by a raised dome portion 16 and a pair of oppositely directed flanges 18. To bond the reinforcement member 14 to the skin member 12, a bead of adhesive 22 is laid on the flanges 18, with the reinforcement member 14 then being turned over and laid onto the interior surface of the skin 12 as shown in Figures 3 to 6. The adhesive 22 is a thermosetting resin that exhibits a change in dielectric properties when cured. The adhesive bead preferably contains a resin consisting of uncrossedlinked polymers and monomers, with a second part consisting of a hardener or catalyst. Epoxy resin adhesives are presently preferred although polyurethanes should prove to be acceptable. These adhesives are to be distinguished from glues such as animal glues which do not rely upon a chemical change to effect their adhesive qualities. As will appear clear from the following discussion, the present invention utilizes this characteristic of certain adhesives to great advantage. By way of specific, but nonlimiting example, adhesive 22 is a Lord 320/322 epoxy adhesive available from Lord Corporation. Referring to Figure 6, the skin/adhesive/reinforcement member assembly is then moved into a chamber 27 of a dielectric heater 26. This can be accomplished in a variety of manners. In this embodiment, the assembly is laid on a plurality of fingers 21 which contact the outer edges of the underneath surface of the skin 12. Fingers 21 ride on rollers 23 to form a trolley which carries the assembly rightwardly in Figure 6 by way of a piston arrangement 25. The assembly is carried into the chamber 27 of the dielectric heater 26 to a position wherein the skin 12 is supported above a nest 20. As can he seen most clearly in Figure 4, the upper suface of nest 20 is contoured so that it corresponds to the contour of the exterior or class A surface of skin 12 to provide it with uniform support during the bonding process. Nest 20 should be made of material which exhibits low dielectric loss characteristics. In this example, nest 20 is made of aluminum but other materials such as brass could also be used. A cooling medium is circulated through a conduit 21 from an inlet 23 to an outlet 25 for maintaining the exterior surface of the skin 12 at a constant temperature of 44°C to 66°C (40 to 150°F) throughout the bonding process. The nest 20 is attached to an electrically grounded 34 electrode plate 30. The nest 20 and electrode plate 30 subassembly is driven by a hydraulic piston 32. After the skin/adhesive/reinforcement member assembly has been located above nest 20, hydraulic piston 32 is actuated to cause the nest to rise upwardly and lift the assembly from its supporting fingers 21 thereby depositing the skin 12 in the nest 20. Piston 32 continues to carry the nest 20 upwardly to an upper electrode assembly generally designated by the numeral 36. Electrode assembly 36 includes a series of segmented or electrically isolated concentrator members 38 held together in a fixed position by a frame 40 (see Figures 4 and 5). Concentrator members 38 each have a generally U-shaped lower section having a pair of depending legs 42. Legs 42 are configured so that they generally correspond to the flanges 18 of reinforcement member 14 as shown most clearly in Figure 4. Recess 44 provides clearance for raised done 16 of reinforcement member 14. Concentrator members 38 serve to concentrate energy from a high frequency voltage source 46 in selected areas containing the adhesive 22. Electrode assembly 36 further includes a plurality of electrode plates 48 which generally overly the concentrator members 38. In Figure 4 there are three such plates 48, 50 and 52 shown. However, it should be understood that in practice that many more such plates are employed, e.g., see Figure 5. Plates 48, 50 and 52 are also constructed of electrically conductive material such as aluminum or brass and they are each connected to high frequency voltage source 46 as illustrated by upper electrode 54 and air gaps 49, 51, and 53, which exist between electrode 54 and electrode plates 48, 50 and 52, respectively. Provision is made for adjusting the relative air gaps or spacing between the electrode plates 48, 50, and 52, and upper electrode 54 (and thus the area below the electrode plates, via members 38, to be heated). The spacing between the electrode plates and the upper electrode will affect the amount of energy applied to the material to be heated. In this embodiment, by adjusting the size of air gaps 49, 51 and 53 it is possible to fine tune the amount of heat that is actually generated in the underlying adhesive. Thus, it becomes possible to further ensure that even heating occurs thereby resulting in a uniform bond strength. Independent adjustment of the heat generated under each concentrator member 38 in effect allows for the application of substantially even heat across assembly 10 notwithstanding its size and the inequalities generated thereby. The adjustment can be accomplished in many ways. In Figure 4, this function is provided by way of a telescoping tube 56 connected between each electrode plate 48, 50 and 52 and its respective concentrator member 38. Depending upon the size of concentrator member 38 and its horizontal area to be heated, more than one electrode plate may be attached thereto via multiple telescoping tubes 56. Voltage source 46 generates a high frequency electrostatic field between the electrode assembly 36 and the lower electrode plate 30. The applied RMS voltage from source 46 can, for example, be between 300 volts and 8,000 volts. A voltage of less than 300 is disadvantageous because it is insufficient to significantly heat adhesive 22. Conversely, an applied value in excess of 8,000 volts is disadvantageous because serious electrical insulation problems will begin to occur between electrode plates 48, 50, 52, etc., and ground. Voltage source 46 applies oscillating or alternating current frequencies in the range of between 25 megahertz and 40 megahertz. Frequencies less than 10 megahertz are disadvantageous because voltages required to heat the adhesive 22 are too large to achieve an identical heating time and there is a greater probability of arcing occuring. Frequencies in excess of 110 megahertz are disadvantageous because the number of required individual electrode plates 48, 50 and 52 becomes excessively large, compounding the difficulty of obtaining an even heating of the adhesive. Further, coupling between concentrator members 38 becomes more pronounced with frequencies above 110 megahertz, thereby making adjustments of individual electrode plates 48, 50 and 52 more interrelated. The optimum frequency is one that will allow a sufficiently fast heating time with a minimum of arcing in the electrode assembly. Also, with too high a frequency, it should be noted that it is possible to heat adhesive 22 so fast that the chemical reaction therein can not keep pace with the temperature rise. The present invention is particularly well suited for high volume production. By way of a nonlimiting example, after the nest 20 has been raised into the position shown in Figure 4, good bond strength is achieved by applying the electric field at a constant frequency and constant voltage for a period of between 15 and 120 seconds. The term "constant" as used herein refers to the fact that frequency and voltage are not purposely varied during the heating cycle to achieve a significant dual heating rate. It has unexpectedly been found that this process has a built in "fail safe" mechanism that prevents overheating which could destroy the smooth exterior surface of skin 12. With reference to Figure 7, it is believed that by using a chemically reactive thermosetting adhesive to bond together two previously cured thermosetting FRP parts that the heat generated in the adhesive will be sufficient to bring the adhesive just past its curing temperature (Th) relatively quickly. However, once the adhesive cures, the heat generated therein tends to decrease over time even though the applied field is constant. Although this phenomena is not totally understood, it is believed that it is due to a change in dielectric properties of the adhesive between its cured and uncured states. In this specific example, the voltage source 46 is energized for a period of between 30 to 40 seconds at a frequency of 34 MHz and a voltage of approximately 4400 volts. This provides sufficient time to ensure uniform curing of the adhesive while at the same time is quick enough to provide manufacturing economy. Then, piston 32 is lowered and the bonded assembly is removed from the heater 26.

Claims: A method of bonding a cured fiber reinforced plastic part (12) to a reinforcement member (14), said method comprising forming an assembly (10) by placing an uncured thermosetting adhesive (22) between mating surfaces of the part (12) and the reinforcement member (14); placing the assembly (10) between two electrodes (30, 36); and applying a substantially constant high frequency signal (46) having a frequency between 10 and 110 megahertz to the electrodes (30, 36) from a dielectric heater (26), characterized by applying said signal (46) at a voltage between 300 and 8000 volts for a period of time sufficient to cure the adhesive (22) without substantial adverse effect to the surface quality of the exterior of the part (12). The method according to Claim 1 wherein the part (12) is an exterior automotive body panel having a surface area exceeding 0,1 m² and a length in at least one dimension exceeding 1,2 m. The method according to Claim 1 or 2 wherein said reinforcement member (14) is generally hat shaped in cross section having a recessed dome (16) and a pair of flanges (18), with a bead of adhesive (22) being applied between the flanges (18) and the interior surface of the part (12). The method according to Claim 3 wherein the reinforcement member (14) is made of cured fiber reinforced thermosetting plastic. The method according to one of the Claims 1 to 4 wherein the adhesive (22) is a two part epoxy adhesive having a resin and a hardener. Method according to one of the preceding claims, characterized in that the part (12) is an exterior automotive body skin, in that the skin and the reinforcement member (14) are both formed by compression molding charges of thermosetting sheet molding compound, and in that the alternating electrical signal (46) is applied for a period of no more than two minutes to form the bonded reinforced panel assembly. The method according to Claim 6 wherein said skin (12) is in the shape of a hood. The method according to Claim 6 or 7 wherein the skin (12) is about 2 to 3 mm thick. A method according to one of the preceding claims wherein an alternating electrical signal (46) is applied having a frequency between 25 and 40 megahertz. Apparatus for bonding a fiber reinforced plastic exterior automotive body skin (12) to a reinforcement member (14) made of fiber reinforced plastic and having a hat shaped cross section with a dome (16) and pair of flanges (18), said apparatus comprising: an assembly for supporting the skin (12), exterior side down, the reinforcement member (14) lying thereon and a bead of chemically reactive adhesive (22) being located between the interior surface of the skin (12) and the reinforcement member (14); moving means (23, 25) for moving the assembly between two electrodes (30, 36) of a dielectric heater (26); and voltage source means (46) for applying a substantially constant high frequency signal having a frequency between 10 and 110 megahertz at a voltage between 300 and 8000 volts from the dielectric heater (26) of sufficient duration to cure the adhesive without adversely affecting the surface qualities of the exterior of the skin (12). The apparatus according to Claim 10 wherein the electrodes (30, 36) are at least 1,2 m in length. The apparatus according to Claim 10 or 11 which further comprises cooling means (21, 23, 25) for maintaining the exterior surface of the skin (12) at a substantially constant temperature. The apparatus according to Claims 10 to 12 wherein the assembly includes a nest member (20) made of metal and wherein the cooling means (21, 23, 25) comprises means for circulating cooling medium through the nest member (20).

Inventors: ISELER KENNETH ALFRED, WILKINSON ROBERT EARL, ISELER, KENNETH ALFRED, WILKINSON, ROBERT EARL
Applicants: BUDD CO, THE BUDD COMPANY
Application year: 1989
Main classification: B29C65
IPC codes: B29C35, B29C65, B29K105, B29L31