CA1307727C - Method and apparatus for bonding frp members - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method and apparatus is disclosed for bonding fiber reinforced plastic automotive body parts to reinforcement members using dielectric heating. This is accomplished by applying a thermosetting adhesive between the mating surfaces of the fiber reinforced plastic skin and the reinforcement member and then administering a high frequency signal for a period of time sufficient to cure the adhesive without causing substantial adverse effects to the surface quality of the fiber reinforced plastic skin.

Description

130~'7Z7 METH~D AN~ APPARATUS FOR RO~DIN~, FRP MEM~R.S

1. Technical Field This invention relates to ~ondinq techniques and, more particularly, to methods and apparatus for bondinq automotive fiber reinforced plastic (FRP) bodv members toqether.

2. Discussion The recent trend in vehicle desiqn is to replace heavier metal parts with ~lastic parts. In ~articular, many advantaqes can result from usin~ fiber reinforced plastic (FR~) exterior automotive bodv assemblies such as hoods, deck~, ~oors and the like instead of their sheet metal counterpart~. However, it is often necessary to reinforce larqer exterior FRP Panels or members with a reinforcement member in order to provide the resultinq assembly with sufficient riqiditv and mechanical ~trenqth.
The Present invention i~ particularly directed to technique~ for expeditiously bon~inq a reinforcement member to an exterior ~RP member in a manner that will not unduly adver~elv affect the surface qualities of the exterior ~ember.

SU~IARY OF TH~: INVENTIO~
In accordance with the teachinqs of the preferre~
emboAiment, an adhesive is placed between matinq surfaces of a fi~er reinforced plastic member and an associated reinforcement member therefor. A hi~h frequency si~nal is applied to electrodes on either side of the asse~bly whi~h heats the adhesive to above its curing temperature to thereby bond the reinforcement member to the fiber reinforced plastic member. It has unexpectedly been found that this technique does not generate excessive heat which could otherwise create surface imperfections in the FRP
member.
Accordingly, a principal object of the present invention is to provide a bonding method and apparatus capable of bein~ used to expeditiously adhere a reinforcement member to an exterior fiber reinforced plastic body member.
Another object of the present invention is to provide such a bonding metho~ and apparatus which will not adversely affect the surface qualities of the exterior fiber reinforced plastic body member.
A further object of the present invention is to provide such a bonding method and apparatus ~hich i~
capable of providing even bonding of the reinforcement member to the exterior fiber reinforced plastic body member notwithstandin~ the ~ize of the body member and the reinforcement ~ember.
BRIEF DESCRIPTION OF THE DRAWINGS
The various advantaqes of the present invention will become apparent to one skilleA in the art upon a study of the followinq detailed description of one preferred embodi~ent with reference to the drawinqs in which:
FIG. 1 is an exploded perspective view of an _3_ assembly comprising an exterior automotive body me~ber and a reinforce~ent member therefor FIG. 2 ls a partial bottom plan view of the assembly of F~G. 1, FIG. 3 is a partial sectional view of the assemh~y of FIG. 1 taken along lines 3-3 of FIG. 2 FIG. 4 is a cross-seetional view taken al~nq lines 4-4 of FIG. 5, which illustrates in simplified f~rm apparatus for carrying out the preferred method of this invention:
FIG. 5 is a cros6-sectional view taken al~ng lines S-5 of FIG. 4, FIG. 6 is a froat view of the apparatus o FIG~S. 4 and 5: and FIG. 7 is a graph illustrating the relationship between time and heat generated during the bondinq process.

D~S~RIPTlON OF THE PREFERRED ~MBODIMENT
The present invention is particularly concerned with providing reinforced exterior automotive body assemblies such as hoods, door6 and bumper assemblies.
The invention provides an expeditious method for bonding a first, typically exterior, fiber reinforced plastic member and a ~econd reinforcing member, which is also preferably manufactured of a fiber reinforced plastic, although other plastic materials or even metal may be u6ed. In certain case~, ~uch a~ the bumper assembly, the first and second members are more appropriately de~cribed a~ mutually reinforcing ~ince the second member may actually define part of the exterior ~urface of the automotive assembly, e.g., to locate automobile features ~uch as signal lights or head light~.
The invention is described in detail below with reference to the ~anufact~re of an automotive hood assem~ly, however, it will ~e appreciate~ by those skilled in the art that the present inventio~, as defined by the a~pended claims, is clearly not limite~
to this particular application.
A hood assembly, qenerally desiqnated by the numeral 10, is illustrate~ in the drawinqs. Assembly 10 consists of a relatively thin (about .080 to .120 inch thick) outer skin member 12 and a reinforcement member 14. Outer skin member 12 is constructed of fiber reinforced plastic (FRP) and is preferably made from thermosettin~ sheet molding compound ~MC) material which has been compression molded under vacuum. U.S. Patent Nos. 4,488,862, 4,551,085, and i,612,149 owned by The Budd Company of Troy, Michican, relate to such techniques.

The present invention finds particular utility when member 12 is an automotive exterior ~ody panel such as a hood, deck, or the like which has a relatively large surface area exceedinq about one square foot and having a length in at }east one dimension of 47 inches. These types of panels usually require reinforcement in order to provide them with sufficient mechanical rigidity and it is important to ma~ntain the smooth aesthetically pleasin~ outer surface of the skin typically required by the automobile manufacturers. This standard is ~enerally referred to in the industry as a class A surface. By way of a nonlimiting specific example, assembly 10 is a hood for a Ford Aerostar minivan and its outer dimensions are approximately 56 inches by 22 inches and is about 0.19 inch thick (0.09 inch for the reinforcement member plus 0.~0 inch for the skin).
Preferably, reinforcement member 14 is likewise * Trade Mark ~l~

- s -constructea of FRP material. In the embodiment illustrated, memher 14 is configure~ into a "fiaure 8"
desiqn in plan view and each le~ is formed of a "hat"
design in cross section as can he seen most clearly in Fi~ures 3 and 4. The hat cross section is characterized by a raised ~ome portion 16 and a ~air of oppositely directed flan~es 18.
To bond the reinforcement member 14 to the skin member 12, a bead of adhesive 22 is laid on the flanqes 18, with the reinforcement memher 14 then being turned over and laid onto the interior surface of the sXin 12 as shown in Figures 3-6. The adhesive 22 is a thermosettin~ resin that exhibits a change in dielectric properties when cured. The adhesive bead preferably contains a resin consistinq of uncrossedlin~ed polymers and monomers, with a second part consisting of a hardener or catalyst. EPOXY resin adhesives are presently preferred althou~h polyurethanes should prove to be acceptable. These adhesives are to be distinguished from glues ~uch as animal glues which do not rely upon a chemical change to effect their adhesive qualities. As will appear clear from the fo~lowing discussion, the present invention utilizes this characteristic of certain adhesives to great advantaqe. By way of specific, but nonlim~ting example, adhesive 22 is a Lord 320/322 epoxy adhesive available from Lord Corporation.
Referring to Figure 6, the skin/adhesive/reinforce-ment 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 emhodiment, the assembly is laid on a plurality of fingers 21 which contact the outer edges of the underneath ~urface of the skin 12.
Fingers 21 riae on rollers 23 to form a trolley which carries the assembly righ~wardly in Fiqure 6 by way of * Trade Mark a piston arrangement 25. The assembly is carrie~ into the chamber 2~ of the dielectric heater 26 to a position wherein the skin 1~ is sup~orted ahove nest 20. As can be seen most clearly in Figure 4, the upper suface of nest 20 is contoured so th~t it correspon~s to the contour of the exterior or class A surface of skin 12 to provide it with uniform support durinq the bondinq process. ~est 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 coolinq medium is circulated through a conduit 210 from inlet 23 to outlet 25 for maintaining the exterior surf~ce of s~in 12 at a constant temperature of 40-150-F
throu~hout 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 sup~ortinq 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 seqmented or electrically isolated concentrator members 38 held together in a fixed position by a frame 40 (see Figures 4 ~nd 5). Concentrator members 38 each have a generally U-shaped lower section havinq 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 do~e 16 of reinforcement 130~727 member 14. Concentrator memher~ 38 serv~ to concentrAte energy from a hiqh frequency voltage sour~e 46 in selected areas containing the a~hesive 22.
Electrode assembly 36 further includes a pluralitv of electrode plates 4~ which generally overly the concentrator members 3~. In Fiqure 4 there are three such plates 48, 50 and 52 shown. However, it shoul~ 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 anA they are each connected to high frequency voltage source 46 as illustrated by upper electrode 54 and air qaps 49, 51, and 53, which exist between electrode 54 and electro~e plates 48, 50 and S2, 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 enerqy applied to the material to be heated. In this embodiment, by adjustinq 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 adhe~ive. Thus, it becomes po~sible to further en~ure that even heatinq occurs thereby resulting in a uniform bond ~trength.
Independent adjustment of the heat generated unAer each concentrator member 38 in effect ~llows for the application of substantially even heat acros~ a~sembly 10 notwithstanding its size and the inequalities qenerated thereby. The adjustment can be accomplished in many ways. In Figure 4, this function ~s provided by way of a telescoping tube 56 connected between each electrode plate 48, 50 and 52 and it~ res~ective ~ 1307727 concentrator member 38. Dependinq upon the size of concentrator member 38 and its horizontal area to be heated, ~ore than one electrode plate may be attacheA
thereto via multiple telescoping tubes 56.
Voltage source 46 generates a hi~h freguency electrostatic field between the electrode assemhly 36 and the lower electrode plate 30. The applied RMS
voltaqe from source 46 can, for example, be between 3 volts and 8, noo volts. A voltaqe of less than 30n is disadvantageous because it i8 believed insufficient to significantly heat adhesive 22. Conversely, an applied value in excess of 8,000 volts is believed disadvantageous because 6erious electrical insulation problems will begin to occur between electrode plates 48, 50, 52, etc., and qround. Voltaqe source 4~
applies oscillating or alternating current frequencies in the range of between 25 megahertz and 40 meqahertz.
Frequencies less than 10 meqahertz are believed disadvantageous because voltaqes required to heat the adhesive 22 are too large to achieve an identical heating time and there is a greater probability of arcinq occuring. Frequencies in excess of 110 megahertz are believed disadvantageous because the number of required individual electrode plates 48, 50 and 52 become~ excessively large, compoundinq 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 makinq ad~ustments of individual electrode plate~ 48, 50 and 52 more interrelated. lt i~ believed that the optimum frequency is one that will allow a sufficiently fast heatinq time with a minimum of arcinq in the electrode as~embl~. Also, with too hiqh a frequency, it hould be noted t~at it i~ po~sible to heat adhesive 22 80 9 1307~27 fast that the chemical reaction therein can not keep pace with the temperature rise.
The present invention is particularly well suited for hiqh volume production. ~y way of a nonlimitinq example, after the nest 20 has been raise~ into the position fihown in Figure 4, qood bonA strenqth is achieved by applyin~ the electric field at a constant frequency and constant voltaae for a period of hetween 15 and 120 seconds. The term "constant" as used herein refers to the fact that frequency and voltaqe are not purposely varied auring the heatinq cycle to achieve a significant dual heatinq rate. It has unexpectedly been found that this process has a built in "fail safe"
mechanism that prevents overheatinq which could destroy the smooth exterior surface of skin 12. With reference to Figure 7, it is believeA that by using a chemically reactive thermosetting adhesive to bond together two Dreviously cureA thermosettinq FRP parts that the heat generated in the adhesive will be sufficient to bring the adhesive just past its curing temperature (Th) relatively quickly. ~owever, once the adhesive cures, the heat ~,q,enerated therein tends to decrease over time even though the applied field is constant. Althouqh thi~ 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 ~pecific example, the voltaqe rource 46 i~ energized for a period of between 30 to 40 ~econds at a frequency of 34 MHz and a voltaqe of approximately 4400 volts. Thi~ provides lufficient time to ensure uniform curing of the adhesive while at the same time is quick enoug~ to provide manufacturinq economy. Then, piston 32 is lowered and the bonded assembly is removed from the heater 26.
The above technique i~ believed to be considerably more advantageous over other methods for bondinq reinforcement ~embers to thermosettinq S~C skins.
Conventional ra~iant heating metho~s are qenerally too slow and result in nonuniform bonds and/or excessive heat qenerate~ by the a~hesive which can ~urn and show throu~h the exterior skin. In~uc~ion heatinq qenerally requires the use of metallic particles in the adhesive or areas to be heated. This is not a ~ractical 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 useA
to bond parts exceeding about 47 inches at 34 MHz, uneven heating can unexpectectedly occur. Althouqh the reason for this phenomena is not completely understood, it is believed that the uneven heatinq is causeA
because various areas of the elongated electrode will have certain peaks of enerqy radiating therefrom which may be due to the fact that the electrode lenqth approaches 1/15 of a wavelength of the alternatinq voltage source 46 (thereby producin~ heat variations of greater than or equal to 10~). Thu~, various voltage ri~es are created along the length of the two oppoQinq electrodes and can result in uneven heating of the parts. The preferred embodiment of th$s invention is believed to solve many of these problems and provide the manufacturer with the needed flexibility to fine tune the apparatus to the particular parts being bonded. Still other advantages of the present invention will be apparent to those skilled in the art upon a study of the drawing~, specification and claims herein presented.
Although one detailed e~bodiment of the method and 13~)~77Z7 apparatus of this invention has been illustrated in the accompanying drawings and described in the foreqoinq detailed description, it will be understood that the invention is not limited to the particular embodiments described herein but is capable of numerous rearranqements, modifications and substitutions without departing from the scope of the invention. For example, the method and apparatus may be successivelv utilized to produce assemblies comprisinq 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. Other changes will sugqest themselves to those skilled in the art. ~he following claims are intended to encompass all such modifications.

Claims (11)

1. A method of bonding a cured fiber reinforced plastic skin to a reinforcement member, said method comprising:
forming an assembly by placing an uncured thermosetting adhesive between mating surfaces of the cured fiber reinforced plastic member and the reinforcement member;
placing the assembly between two electrodes, one of said electrodes including a plurality of electrically isolated concentrator members; and applying a high frequency signal having a frequency of between about 10 megahertz to about 110 megahertz at a sufficient voltage to the electrodes for a period of time sufficient to cure the adhesive, said concentrator members providing even concentration of energy from the high frequency signal at selected areas of the adhesive such that the fiber reinforced plastic skin is bonded to the reinforcement member without substantial adverse affect to the surface quality of the fiber reinforced plastic skin.

2. The method of claim 1, wherein said reinforcement member comprises a cured fiber reinforced plastic member.

3. The method of claim 1, wherein the adhesive is a two part epoxy adhesive having a resin and a hardener.

4. The method of claim 3 which further comprises the steps of:
placing the skin, exterior side down, into a nest contoured to correspond to the contour of the exterior skin surface:
applying the adhesive to the reinforcement member;
placing the reinforcement member onto the interior side of the skin; and raising the nest until the flanges are brought into position beneath the concentrator members.

5. The method of claim 4, wherein the high frequency signal voltage applied between the electrodes is between 300 and 8000 volts RMS.

6. The method of claim 5 wherein the high frequency signal has a substantially constant frequency and voltage, and is applied for no more than two minutes.

7. A method of bonding an exterior automotive body skin to a reinforcement member, said skin and reinforcement member being preformed by comparison molding charges of thermosetting sheet molding compound, said method comprising the steps of:
placing the outer skin, exterior side down, into a nest having a contour corresponding to the contour of the exterior surface of the skin;
applying a bead of heat curable, two-part epoxy adhesive containing a resin and hardener onto the reinforcement member;
placing the reinforcement member onto the skin to form an assembly;
inserting the assembly into a chamber of a dielectric heater;
surrounding the assembly with a grounded electrode plate and an upper electrode assembly, the upper electrode assembly including a series of fixed electrically isolated, concentrator members; and applying an alternatively electrical signal between the electrodes having a frequency of between 10 MHz and 110 MHz at a voltage of between 300 and 8000 volts RMS for a period of no more than 2 minutes to form a bonded reinforced panel assembly, whereby heat generated below the concentrator members serves to expeditiously spot cure the adhesive for subsequent handling of the assembly such that the skin is bonded to the reinforcement member without substantial adverse affect to the exterior surface quality of the skin.

8. The method of claim 7, which further comprises:
connecting the signal through a plurality of vertically adjustable electrode plates above the concentrator members; and adjusting the vertical distance between the plates and a spaced, overlying electrode.

9. Apparatus for bonding a fiber reinforced plastic exterior automotive body skin to a reinforcement member made of fiber reinforced plastic, said apparatus comprising:
nest means for supporting the skin, exterior side down, with the reinforcement member 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 nest means between two electrodes;
one of said electrodes comprising an upper electrode assembly, said upper electrode assembly including a series of fixed, electrically isolated concentrator members, said concentrator members being shaped to engage the reinforcement member; and voltage source means for applying a high frequency signal having a frequency of between about 10 megahertz to about 110 megahertz at a sufficient voltage between the electrodes of sufficient duration to cure the adhesive without adversely affecting the surface qualities of the exterior of the skin.

10. The apparatus of claim 9 wherein the moving means comprises:
means for lifting the nest means upwardly until the concentrator members substantially abut the reinforcement member.

11. The apparatus of claim 10 wherein the upper electrode assembly further comprises:
a plurality of electrode plates connected through an air gap to the voltage source, each of said electrode plates also connected to a concentrator member; and means for adjusting each air gap, whereby the strength of the applied signal to selected areas of the adhesive can be adjusted.