WO1999021694A1

WO1999021694A1 - Method of making a part utilizing an electrically conductive material

Info

Publication number: WO1999021694A1
Application number: PCT/US1998/022859
Authority: WO
Inventors: Charles D. Ginman; Raymond Bohlinger; Gregory A. Laporte; Kenneth R. Parrish; Andrew F. Pinkos; Jack M. Van Ert
Original assignee: Lear Corporation
Priority date: 1997-10-28
Filing date: 1998-10-28
Publication date: 1999-05-06
Also published as: AU1283799A

Abstract

An assembly, such as a seat cushion assembly (10) having a cover material (16) joined to a substrate (14), is made by a process including an initial step of positioning at least one heat-activatable adhesive layer (18) and an energizable electrically conductive layer (20) between the cover material (16) and the substrate (14). The cover material (16) and the substrate (14) are joined together with the electrically conductive layer (20) and the at least one heat-activatable adhesive layer (18) therebetween such that the electrically conductive layer (20) is in sufficient heat transfer relationship when energized to cause the at least one heat-activatable adhesive layer (18) to facilitate the adherence of the cover material (16), the electrically conductive layer (20) and the substrate (14) to one another. The electrically conductive layer (20) is energized to generate heat during one or more of the positioning and joining steps. Such a process avoids exposing an appearance portion of the cover material to high temperatures and is thus suitable for virtually all types of cover materials. Other aspects of the process, including thermoforming and welding with electrically conductive material, are also disclosed.

Description

METHOD OF MAKING A PART UTILIZING AN ELECTRICALLY CONDUCTIVE MATERIAL

TECHNICAL FIELD

This invention relates to a method of making a part using electrically conductive material to generate resistive heat to bond and/or form the part.

BACKGROUND ART

Various techniques have been developed for bonding a cover material to a substrate, particularly for use in automotive seat fabrication. U.S. Patent No. 4,692,199, assigned to the assignee of the present invention, shows a method that utilizes steam to melt an adhesive film in order to bond a cloth fabric layer to a foam pad. In this method, the steam penetrates the fabric layer to activate the adhesive film.

U.S. Patent No. 5,534,097, assigned to the assignee of the present invention and hereby incorporated by reference, shows a method that utilizes a magnetic flux to melt an adhesive film containing ferromagnetic particles to bond a fabric or trim cover to a foam cushion. An oscillatory radio frequency generator is used to energize copper coil tubing to produce the magnetic flux. The magnetic flux induces eddy currents in the ferromagnetic particles in order to generate heat to activate the adhesive film.

Various methods have also been developed for thermoforming parts, such as a headliner for use with a motor vehicle. One such method includes heating at least one layer of formable material and a cover member in an oven, transferring the layer and the cover member to a compression mold, and thermoforming the layer and the cover member together via compression to form the headliner. The oven used in this method, however, is relatively expensive and requires a significant amount of energy to operate. Furthermore, because significant heat loss occurs during the transferring step, the materials must be heated well above the required thermoforming temperature.

U.S. Pat. No. 4,828,910 shows another method of thermoforming a headliner which includes introducing several layers into a heated mold and forming the layers together to achieve a desired shape. This method, however, involves significant tooling and equipment costs.

In addition, various methods have also been developed for welding plastic components together. These methods include ultrasonic welding, vibration welding, hot plate welding and electromagnetic welding. All of these methods, however, require the use of complicated and relatively costly equipment. Some of these methods also result in excessive heat being transferred to appearance surfaces or cover materials, thereby forming visual defects in such surfaces or cover materials.

DISCLOSURE OF INVENTION

The present invention overcomes the above-referenced shortcomings of the prior art by providing a method of utilizing electrically conductive material, such as an electrically conductive layer, to efficiently and cost-effectively transfer heat for joining adjacent components and/or thermoforming components.

Accordingly, it is an object of the present invention to provide a method of joining components and/or thermoforming components using electrically conductive material to efficiently transfer heat between the electrically conductive material and a target component or components.

Another object of the present invention is to provide a method of joining components and/or thermoforming components which does not involve significant tooling and/ or equipment costs. One aspect of the present invention is a method of joining a substrate to a cover material having an appearance portion and a concealable portion comprises positioning a heat-activatable adhesive layer and an energizable electrically conductive layer between the cover material and the substrate with the heat- activatable adhesive layer and the electrically conductive layer in heat transfer relationship to each other; joining the cover material and the substrate together with the electrically conductive layer and the heat-activatable adhesive layer therebetween such that the electrically conductive layer is in sufficient heat transfer relationship when energized to cause the heat-activatable adhesive layer to facilitate the adherence of the concealable portion of the cover material, the electrically conductive layer and the substrate to one another while exposing the appearance portion of the cover material; and energizing the electrically conductive layer to generate heat during one or more of the positioning and joining steps.

Advantageously, the method described above can be used to join virtually any type of cover material to a substrate without requiring additional finishing steps for the cover material and without causing a substantial reduction in the thickness of the substrate.

A more specific object of the invention is to provide a method of the type described above in which the electrically conductive layer is configured as a seat heater that can be used to heat a seat after the cover material, the electrically conductive layer and the substrate have been joined to one another.

Another more specific object of the present invention is to provide a method of fabricating a seat cushion assembly, the method comprising positioning a cover material over a contoured mold surface of a mold; positioning at least one heat-activatable adhesive layer, an electrically conductive layer, and a foam pad in the mold with the at least one adhesive layer and the electrically conductive layer in heat transfer relationship to each other; forcing the cover material and the foam pad together in the mold with the at least one heat-activatable adhesive layer and the, electrically conductive layer therebetween; and energizing the electrically conductive layer to generate heat to activate the at least one heat-activatable adhesive layer for adhering the cover material, the electrically conductive layer and the foam pad to one another.

Another aspect of the present invention is a method for thermoforming a part. Thermoforming as used in this specification means compressing and/ or forming heated material using any suitable means such as a compression mold or vacuum mold. The method comprises positioning at least one formable layer and an electrically conductive layer between first and second mold portions and in heat transfer relationship to each other, the at least one formable layer being formable when sufficiently heated; energizing the electrically conductive layer to generate heat for sufficiently heating the at least one formable layer; and moving the mold portions sufficiently together to thermoform the at least one formable layer and the electrically conductive layer.

A more specific object of the present invention is to provide a method for thermoforming a headliner which includes positioning at least one formable layer, an electrically conductive layer and a cover material between first and second mold portions with the electrically conductive layer and the at least one formable layer in heat transfer relationship to each other; energizing the electrically conductive layer to generate heat; and moving the mold portions sufficiently together to form and join together the formable layer, the electrically conductive layer and the cover material.

For each of the aspects of the invention described above, the electrically conductive layer may be efficiently energized by applying current to the electrically conductive layer up to a first current level and over a first time period sufficient to allow the current to sufficiently disperse throughout the conductive lay- er; applying current from the first current level up to a second current level over a second time period, the second current level being greater than the first current level; and applying current to the conductive layer at substantially the second current level for a third time period, thereby heating the conductive layer.

Another aspect of the present invention is a method of thermoplastic welding. The method comprises positioning an electrically conductive material proximate a bond interface of joinable surfaces of at least one body; positioning a thermoplastic material proximate the bond interface; applying current to the electrically conductive material up to a first current level and over a first time period sufficient to allow the current to sufficiently disperse throughout the conductive material; applying current to the conductive material from the first current level up to a second current level over a second time period, the second current level being greater than the first current level; and applying current to the conductive material at substantially the second current level for a third time period, thereby heating the conductive material to cause the thermoplastic material to bond the joinable surfaces together.

A more specific object of the invention is to provide a method of the type described above in which at least one of the joinable surfaces includes the electrically conductive material.

Another more specific object of the invention is to provide a method of the type described above in which at least one of the joinable surfaces includes the thermoplastic material.

For each of the aspects of the invention described above, the electrically conductive layer or material may be configured as a heating device for providing heat to an interior of a motor vehicle.

For each of the aspects of the invention described above, the electrically conductive layer or material may be configured as a static electricity dissipation device.

These and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in conjunction with the accompanying drawings. BRffiF DESCRIPTION OF DRAWINGS

FIGURE 1 is a front perspective view of an assembly for practicing the method according to the invention;

FIGURE 2 is a schematic diagram showing a foam pad, an electrically conductive layer, an adhesive layer and a cover material in a mold;

FIGURE 3 is a schematic diagram showing the foam pad, the electrically conductive layer and the cover material compressed against one another for bonding by the adhesive layer;

FIGURE 4 is a schematic diagram of a first alternative embodiment of the assembly showing the adhesive layer integrally pre-formed with the foam pad and the electrically conductive layer integrally pre-formed with the cover material;

FIGURE 5 is a schematic diagram of a second alternative embodiment of the assembly showing the adhesive layer integrally pre-formed with the cover material and the electrically conductive layer integrally pre-formed with the foam pad;

FIGURE 6 is a schematic diagram of a third alternative embodiment of the assembly showing the adhesive layer integrally pre-formed with the electrically conductive layer;

FIGURE 7 is a schematic diagram of a fourth alternative embodiment of the assembly showing the electrically conductive layer between a pair of adhesive layers;

FIGURE 8 is a schematic diagram of a control system for energizing the electrically conductive layer; FIGURE 9 is a profile of current and temperature vs. time for an exemplary electrically conductive layer;

FIGURE 10 is a profile of current and temperature vs. time for another exemplary electrically conductive layer;

FIGURE 11 is a schematic diagram of an apparatus and assembly for practicing another aspect of the method according to the invention, which is a method of thermoforming a part such as a motor vehicle headliner;

FIGURE 12 is a schematic diagram of the apparatus and assembly of Figure 11 showing a formable layer, an electrically conductive layer, an adhesive layer and a cover material positioned between first and second mold portions;

FIGURE 13 is a schematic diagram of the apparatus and assembly of Figure 11 with the mold portions closed together to form a headliner according to the invention;

FIGURE 14 is a schematic diagram of yet another assembly for practicing yet another aspect of the method according to the invention, which is a method of resistance welding automotive assemblies; and

FIGURE 15 is a schematic diagram of an alternative embodiment of the assembly for practicing the method of resistance welding automotive assemblies.

BEST MODES FOR CARRYING OUT THE INVENTION

With reference to the drawings, the preferred embodiments of the present invention will be described. A method according to the present invention of fabricating a part, such as a cushion assembly 10, generally utilizes an apparatus 12 as illustrated in Figures 1-3. The apparatus 12 is utilized to produce the cushion, assembly 10 by bonding a substrate, such as a foam pad 14, to a trim cover material 16 with a heat-activatable adhesive layer 18 and an electrically conductive layer 20 therebetween. The cushion assembly 10 is typically utilized as a seat bottom or a seat back in seat assemblies for motor vehicles.

The apparatus 12 includes a mold 22 having a contoured mold surface 24. The mold 22 may be made of aluminum, ceramic, epoxy-resin or other suitable mold material. A plurality of apertures 26 are disposed in the mold surface 24 for establishing fluid communication within the mold 22. A housing 28 disposed generally below the mold surface 24 provides an air-tight chamber in fluid communication with the apertures 26. A vacuum source 30 connected to the housing 28 is adapted to develop a negative pressure in the housing 28 and at the apertures 26 of the mold 22. The vacuum source 30 typically includes a vacuum pump and tank connected by fluid lines to the housing 28, and is controlled by a control circuit 32.

An upper platen 34 is adapted to compress the foam pad 14 against the cover material 16 on the mold 22. The upper platen 34 is suspended from a horizontal support structure 36. A pneumatic cylinder 38 is mounted to the horizontal support structure 36 and effectuates vertical movement of the upper platen 34 relative to the mold 22 for compressing the foam pad 14 against the mold 22. Further details of the apparatus 12 are disclosed in U.S. Patent No. 4,692,199, assigned to the assignee of the present invention and hereby incorporated by reference.

The cover material 16 is placed on the mold surface 24 of the porous mold 22. The cover material 16 may be any type of permeable or non-permeable material used in the manufacture of seats or other automotive interior parts such as, but not limited to, cloth, fabric, vinyl or leather. Pile cloth, vinyl and leather are particularly suitable for this process since heat is not directly applied to the appearance portion of the cover material. The cover material 16 is preferably sewn or otherwise formed to the desired final seat shape before it is placed on the mold surface 24. The mold surface 24 is preferably of a suitable contour conforming to the contour of the seat surface. In the case of a non-permeable cover material 16, a negative pressure is developed by the vacuum source 30 for drawing air through the mold surface 24 to thereby draw the cover material 16 against the mold surface 24. Thereafter, any wrinkles in the cover material 16 may be manually or otherwise removed.

Next, the heat-activatable adhesive layer 18 and the electrically conductive layer 20 are manually or otherwise placed in heat transfer relationship to each other and over the cover material 16. In the case of a permeable cover material 16, the adhesive layer 18 preferably comprises a relatively air impervious barrier that is placed sufficiently adjacent the cover material 16 such that when the negative pressure is developed by the vacuum source 30, the adhesive layer 18 is drawn toward the mold surface 24, which in turn draws the cover material 16 against the mold surface 24.

The adhesive layer 18 may be a separate layer or it may be integrally pre-formed with either the foam pad 14 as shown in Figure 4, the cover material 16 as shown in Figure 5, or the electrically conductive layer 20 as shown in Figure 6. Figures 4, 5 and 6 represent first, second and third alternative embodiments, respectively, of the cushion assembly 10. Furthermore, multiple adhesive layers 18 may be used to provide additional bonding between the foam pad 14, the electrically conductive layer 20 and the cover material 16. Figure 7, for example, shows a fourth alternative embodiment of the cushion assembly 10 with the electrically conductive layer 20 disposed between a pair of adhesive layers 18. The adhesive layer or layers 18 may comprise any suitable material including a thermosetting resin of the reactive type, such as Bostik 812 adhesive available from Bostik Inc. of Middleton, Massachusetts, and/or a thermoplastic resin, such as product code 4232 available from Bemis Corp. of Shirley, Massachusetts, or product code XUS 66113 available from The Dow Chemical Company of Midland, Michigan.

The electrically conductive layer 20 may be a separate layer or it may be pre-laminated or otherwise integrally pre-formed to either the cover material 16 as shown in Figure 4, the foam pad 14 as shown in Figure 5, or the adhesive layer 18 as shown in Figure 6. It is not necessary that the electrically conductive layer 20 be flush against the surface of the layer with which it is pre-formed. In other words, the electrically conductive layer 20 may project out from the surface of the layer with which it is pre-formed.

The electrically conductive layer 20 is preferably sufficiently flexible to form a contour similar to and complimenting the mold surface 24 and the foam pad 14. The electrically conductive layer 20 may be any type of material capable of being energized to at least the activation temperature of the heat-activatable adhesive layer 18. For example, the electrically conductive layer 20 may be made with metal coils or fibers, carbon fibers, an electrically conductive coating applied to a surface, electrically conductive polymers, electrically conductive textiles, or any combination thereof.

Metal coils or fibers may be woven together to form a mesh, woven into or otherwise combined with a fabric, or otherwise pre-formed in a layer such as available from I. G. Bauerhin GmbH of Gruendau, Germany. Carbon fibers, such as P25 2K ST fibers available from Amoco Polymers of Hampton, New Hampshire, may be woven together to form a mesh, woven into or otherwise combined with a fabric, such as by knitting fibers into a fabric, or formed into a carbon slurry paper. An electrically conductive coating, such as M574 conductive ink available from Engelhard Corporation of Carteret, New Jersey, may be applied to, for example, the foam pad 14, the cover material 16, the adhesive layer 18, and/or a separate member such as a textile or a plastic mesh. Electrically conductive polymers may be formed into a separate layer or added to, for example, the foam pad 14, the cover material 16 or the adhesive layer 18. Electrically conductive polymers include such products as inherently conductive polymers available from Ormecon Chemie GmbH and Co. KG of Ammersbek, Germany, Eeonomer™ KP20- 7DN and Eeonomer™ KPY20-7D available from Eeonyx Corporation of Pinole, California, electronically conductive polymer films available from the Department of Chemistry of Drexel University, located in Philadelphia, Pennsylvania, and electrically conductive plastic meshes such as available from Nalle Plastics, Inc. of Austin, Texas. Electrically conductive textiles include Contex™ conductive textiles available from MiUiken & Company of Spartanburg, South Carolina, and Gorix Electro-Conductive Textiles available from Gorix Ltd. of Rotherham, S Yorks, United Kingdom.

The electrically conductive layer 20 may also be configured as a seat heater which an occupant of the vehicle can activate to warm the seat cushion assembly 10 during use. The electrically conductive layer 20 in this arrangement may be made from any of the above described materials or it may be any suitable heating element capable of heating a vehicle seat. Such a heating element may comprise coiled copper wiring, insulated or non- insulated, and encapsulated in foam sheeting and/or woven or non- woven material, such as is commonly available from Forsheda, Inc. of MullesjO, Sweden.

Furthermore, the electrically conductive layer 20 may be configured as a weight sensor, or present person detector, for sensing weight and/or position of an occupant seated on the cushion assembly 10. Such a sensor can be used in conjunction with, for example, a safety restraint system, such as an air bag, to determine if a particular occupant meets set criteria to allow for deployment of the air bag. The electrically conductive layer 20 in this arrangement preferably has an electrical resistance that varies with the weight applied to the conductive layer 20. Gorix Electro-Conductive Textiles, for example, have such a characteristic. Such an arrangement also preferably includes a resistance meter for measuring the resistance of the electrically conductive layer 20, and a controller for determining the weight and/or position of the occupant based on the measured resistance.

Advantageously, the electrically conductive layer 20 may be configured as both a seat heater and a weight sensor, which may be wired as a single circuit and controlled with a single controller. In such an arrangement, if the resistance of the electrically conductive layer 20 also changes with temperature, a temperature sensing device, such as a thermocouple or thermistor, may be required to provide temperature input to the controller. The controller will then determine the weight and/or position of the occupant based on both the resistance and temperature of the electrically conductive layer 20. Alternatively, the electrically conductive layer 20 may configured as both a seat heater and a weight sensor which are wired as separate insulated circuits and controlled with separate controllers. In this particular arrangement, one or both of the seat heater portion and the weight sensor portion of the electrically conductive layer 20 may be energized during the bonding process to form the cushion assembly 10. As yet another alternative, a separate weight sensor, such as a Present Person Detection System available from International Electronics and Engineering of Luxemburg, may be placed adjacent to or integrated with a particular electrically conductive layer 20 which is configured as a seat heater, such as a parallel-circuit seat heater available from I. G. Bauerhin GmbH and including sinusoidal, stainless steel, non-insulated wires held in place by a polyester mesh. The weight sensor and the electrically conductive layer 20 in this arrangement may also be wired and controlled as one circuit or separate circuits.

In addition, the electrically conductive layer 20 may be configured as a static electricity dissipation device to effectively remove static electricity from the cushion assembly 10 during use. The electrically conductive layer 20 in this arrangement is preferably integrally preformed or otherwise integrated with the cover material 16 such that the conductive layer 20 is in sufficient electrical contact with the cover material 16. When the cushion assembly 10 is installed in the vehicle, the electrically conductive layer 20 may be grounded to the vehicle such as by connecting the conductive layer 20 to a metal frame or sheet metal body of the vehicle. As static charge builds up on the cover material, the charge will be transferred to the electrically conductive layer 20, which in turn will transfer the charge to ground. The removal of static charge will reduce the potential for damage to such items as computers and other electronic equipment. Furthermore, removal of static charge will enable dirt particles to be removed more easily from the cover material 16.

Next, the electrically conductive layer 20 is connected to a power source 40, which may be an alternating current (AC) or direct current (DC) power, source, by engaging opposed electrical circuit clamps 42 against the electrically conductive layer 20. Each electric circuit clamp 42 includes a bus bar 44 that engages substantially an entire edge of the electrically conductive layer 20. Alternatively, the electrically conductive layer 20 may be connected to the power source 40 by utilizing alligator clips, a quick disconnect, or by any other suitable means.

The foam pad 14 is then placed over the electrically conductive layer

20 on the mold 22. The foam pad 14 may comprise any suitable cushion material, such as polyester and/or polyurethane. Other substrate materials now utilized in the manufacture of other automotive interior parts may also be utilized in the method according to the present invention to produce seat cushions or automotive interior parts such as headliners, door panels, trunk liners and floor covers or flooring systems.

The electrically conductive layer 20 may be energized by the power source 40 to generate heat before, during and/or after placing the foam pad 14 thereover. In any case, the electrically conductive layer 20 is energized to at least the activation temperature of the heat-activatable adhesive layer 18. For a thermoplastic adhesive layer, such temperature is usually higher than the normal operating temperature of the electrically conductive layer 20 when used as a seat heater. For example, the activation temperature of a thermoplastic adhesive layer may be 220° F (104.4 °C) or greater, and the normal operating temperature of the electrically conductive layer 20 as a seat heater may be 100°F (37.8°C). For a thermosetting adhesive layer, the activation temperature may be lower than the operating temperature of the electrically conductive layer 20 when used as a seat heater.

As the electrically conductive layer 20 is being energized, the upper platen 34 is preferably moved downwardly against the foam pad 14 to force or compress the foam pad against the cover material 16 with the adhesive layer 18 and the electrically conductive layer 20 therebetween. The compressive force applied by the upper platen 34 may be from about 0 to about 10 pounds per square inch (0 to 0.7 kilograms per square centimeter), and is preferably in the range of 0 to 5 pounds per square inch (0 to 0.35 kilograms per square centimeter). Additional testing, however, may result in changes to the preferable compressive force range.

Alternatively, the foam pad 14 may be compressed against the cover material 16 prior to, during and/or after energizing the electrically conductive layer 20. Once the foam pad 14 is compressed against the cover material 16 with the adhesive layer 18 and the electrically conductive layer 20 therebetween, the negative pressure developed by the vacuum source 30 can be removed from the mold surface 24.

Next, the heat from the electrically conductive layer 20 melts the adhesive layer 18 such that at least a sufficient portion of the adhesive layer 18 is diffused into the foam pad 14, the electrically conductive layer 20 and the cover material 16. The electrically conductive layer 20 is, therefore, preferably configured to allow a sufficient portion of the adhesive layer 18 to pass therethrough. Alternatively, as previously mentioned, multiple adhesive layers 18 may be used. After the heat is removed and the adhesive solidifies, the foam pad 14, the electrically conductive layer 20 and the cover material 16 are bonded to one another to produce the finished cushion assembly 10.

It is desirable to heat the electrically conductive layer 20 to within a predetermined temperature range as quickly and as efficiently as possible in order to reduce cycle time and energy consumption. This is preferably accomplished by using a programmable current controller 46, as shown in Figure 8, to control the amount of current passing through the electrically conductive layer 20. The controller 46 is connected to a controller power source 48, a current monitoring device such as a current sense resistor 50, and a switch 52 for regulating the voltage across the electrically conductive layer 20 to thereby regulate the current passing through the electrically conductive layer. The current sense resistor 50, which is in series with the electrically conductive layer 20, monitors the amount of current passing through the electrically conductive layer 20, and transmits a signal to the controller 46 corresponding to the amount of current detected. A filter 54 and amplifier 56 may also be connected between the current sense resistor 50 and the controller 46 to improve the strength and/ or quality of the signal sent by the current sense resistor to the controller. Based on the signal sent by the current sense resistor 50, the controller 46 adjusts the switch 52 as necessary to achieve a desired amount of current passing through the electrically conductive layer 20. The controller 46 also includes a user interface 57 for programming and monitoring operation of the controller 46. This technique of controlling voltage and current is referred to as pulse- width-modulation. Alternatively, a controllable AC or DC power source may be used to control the amount of current passing through the electrically conductive layer 20.

Figure 9 is a graph or profile of current and temperature vs. time for an exemplary electrically conductive layer 20 in combination with a particular assembly 10 and associated tooling. The graph is representative of how current may be applied to the electrically conductive layer 20 so as to optimize cycle time and efficiency of the method according to the invention. Curve 58 represents current vs. time, and curves 60, 60' , 60" , and 60'" each represent temperature vs. time for one of four locations on the conductive layer 20. Obviously, temperature may be monitored at more or less locations on the conductive layer 20 as necessary to establish a sufficient representation of temperature variation across the conductive layer 20.

During time period t_, current is applied to the electrically conductive layer 20 up to a first current level 62 as shown in Figure 9. The current is typically ramped up relatively slowly during time t, in order to allow the current to sufficiently disperse through the conductive layer 20 without short circuiting and thereby damaging the conductive layer 20. Time period t_ preferably commences as the upper platen 34 is being moved downwardly toward the foam pad 14. During time period t_, current is ramped up more quickly to a second current level 64 which is greater than the first current level 62. During time period t₃, current is preferably maintained generally at the second current level 64 in order to effectively heat the conductive layer 20. Time t₃. represents the time at which the assembly 10 transitions from a heat conductor to a heat insulator. After time t₃, current through the conductive layer 20 is ramped down relatively quickly. Time t₄ represents the time required, after ramping down of the current has commenced, for a sufficient portion of the conductive layer 20 to reach a desired minimum temperature 66 for melting the adhesive layer or layers 18. During time period t₄, heat from localized, relatively hot sections of the conductive layer 20 and/or the assembly 10 is transferred to localized, relatively cool sections of the conductive layer, thereby reducing temperature differentials across the conductive layer 20. The length of time t₄ is dependent upon such factors as the thermal inertia of the assembly 10 after time t₃, and how uniformly a particular conductive layer 20 can be heated during time t₃. Time t₅ represents the cooling period for the assembly 10.

Figure 10 is a profile of current and temperature vs. time for another exemplary electrically conductive layer 20 in combination with a particular assembly 10 and associated tooling. Curve 68 represents current vs. time, and curve 70 represents average temperature of the conductive layer 20 vs. time. During time period t_, current is applied up to a first current level 72. Similar to the previous example, time period t, preferably commences as the upper platen 34 is being moved downwardly toward the foam pad 14. During time period t_, current is preferably maintained generally at the first current level 72 until the foam pad 14 is sufficiently compressed against the cover material 16, and the negative pressure developed by the vacuum source 30 is removed from the mold surface 24. During time period t₃, current is applied up to a second current level 74 which is greater than the first current level 72. During time period t₄, current is preferably maintained generally at the second current level 74 in order to effectively heat the conductive layer 20 up to a desired minimum temperature 76. After time t₄, current through the conductive layer 20 is ramped down relatively quickly to a third current level 78 which is less than the second current level 74 and typically greater than the first current level 72, but not necessarily. The third current level 78 is preferably the amount of current required to maintain the conductive layer 20 generally at the desired minimum temperature 76. During time period t₅, current is preferably maintained generally at the third current level 78 so as to sufficiently heat the adhesive layer 18. After time period t₅, current through the conductive layer 20 is ramped down relatively quickly to 0 .amps. Time period t₆ represents the cooling period for the assembly 10. Alternatively, current may be applied to the electrically conductive layer 20 in any suitable manner, such as by gradually applying current over a predetermined time period up to a maximum level.

Profiles of current and temperature vs. time are preferably predetermined for each different type of electrically conductive layer 20 in combination with a particular assembly 10 and associated tooling. The controller 46 also preferably has sufficient memory for storing such profiles. Consequently, the controller 46 can be programmed to regulate current passing through a particular electrically conductive layer 20 to thereby heat the conductive layer to within a predetermined temperature range based on the general profile for the particular type of electrically conductive layer 20, and without monitoring temperature during the process.

Because the current passing through the electrically conductive layer 20 is closely monitored and controlled, rather than the temperature of the conductive layer 20 which is a function of current, the electrically conductive layer 20 can be heated quickly and efficiently. Furthermore, system variables, such as thermal coefficients and resistances of the various components of the assembly 10 and tooling, are automatically compensated for by measuring and adjusting the current passing through a particular electrically conductive layer 20. In addition, because the electrically conductive layer 20 is adjacent to the adhesive layer or layers 18, the heat transfer efficiency therebetween is maximized. As a result, the heating time and the amount of heat required for this method are minimized.

Any other suitable means of controlling the energizing or heating of the electrically conductive layer 20 may also be utilized. For example, the resistance of the electrically conductive layer 20 may be periodically measured, and the current passing therethrough and/or the voltage potential across the conductive layer 20 may be adjusted as necessary to achieve a desired amount of power emanating from the conductive layer 20. The power required to sufficiently heat a particular type of electrically conductive layer 20 may be determined by establishing a power density and temperature vs. time profile, or power density profile, for the particular type of electrically conductive layer 20 in combination with a particular type of assembly 10 and associated tooling. Power density as used in this specification means power per unit of surface area of a particular component. Consequently, a desired temperature or temperatures of the electrically conductive layer 20 can be achieved by monitoring and controlling the amount of power applied to the conductive layer 20 based on the surface area of the conductive layer 20 and the power density profile. The measured resistance of the conductive layer 20 at a particular time may also be compared to a given resistance vs. temperature profile for the conductive layer 20 to determine whether the conductive layer 20 is at the proper temperature for the particular time as established by the power density profile. Furthermore, power density profiles may be predetermined for each different type of electrically conductive layer 20 in combination with a particular assembly 10 and associated tooling. Alternatively, the power required to sufficiently heat the electrically conductive layer 20 may be determined based on the thermal resistances of the assembly 10 and associated tooling, such as the mold 22 and the upper platen 34. For example, the power P_τ required to raise the temperature of the assembly 10 as shown in Figure 7 up to a desired temperature Tj may be determined by solving the following equations, where T₂ is the ambient temperature immediately adjacent to the upper platen 34, T₃ is the ambient temperature immediately adjacent the mold 22, P_Top is the power required to raise the foam pad 14 and the top adhesive layer 18 to the desired temperature T_j, P_Bottom i^s the power required to raise the bottom adhesive layer 18 and the cover material 16 to the desired temperature T R_UP is the thermal resistance of the upper platen 34, R_FP is the thermal resistance of the foam pad 14, R_AT is the thermal resistance of the top adhesive layer 18, R_AB is the thermal resistance of the bottom adhesive layer 18, R M ^IS ^e thermal resistance of the cover material 16, and R_M is the thermal resistance of the mold 22:

T, - T₂ = P_Top x (R_UP +Rpp + R_AT), Tj - T₃ = P_Bottom x (R_AB + Rc_M +R_M) and _τ = _{Top +} r_Bottom = Lι___L_{2 +} -Lι_---T₃

R_UP + R_FP + R_AT R_AB + R_CM + R_M The thermal resistance R for a particular component is equal to the length H of the component in the direction of thermal flow, divided by the product of the thermal conductivity k of the specific component and the cross-sectional area A perpendicular to the direction of thermal flow. Other means of controlling the heating of the electrically conductive layer 20 include using a thermocouple to measure the temperature of the electrically conductive layer 20, and adjusting the current and/or voltage applied to the electrically conductive layer 20 as necessary to achieve a desired temperature or temperatures of the electrically conductive layer 20.

After the seat cushion assembly 10 has been bonded together, the electrically conductive layer 20 may be re-energized to debond the assembly in order to remove any wrinkles in the cover material 16. A cross-linking adhesive medium may be utilized in order to minimize the amount of thermal energy needed to bond or debond the assembly 10. Cooling means, such as the vacuum source 30, may also be applied to assist the cooling and curing of the bonded assembly 10. For example, a negative pressure may be established at the mold surface 24 and/or the upper platen 34 to draw relatively cooler air through the assembly 10. Depending on the thermal inertia of the assembly 10, however, active cooling means may not be required.

The Figures indicate that the adhesive bond is disposed over entire adjacent surfaces of the foam pad 14, the electrically conductive layer 20 and the cover material 16. However, in accordance with the subject application, the adhesive bonds between the foam pad 14, the electrically conductive layer 20 and the cover material 16 may constitute only a portion of the adjacent surfaces, as commonly known in the art.

Another aspect of the invention is a method of thermoforming a part, such as a headliner assembly 110, and the method generally utilizes .an apparatus 112 as illustrated in Figures 11 through 13. The apparatus 112 is utilized to form the headliner assembly 110 by shaping and bonding a substrate, such as a formable layer 114, to a cover material 116 with a heat-activatable adhesive layer 118 and an electrically conductive layer 120 therebetween.

The apparatus 112 includes a press 122 having a pair of non-thermally regulated mold portions 124 and 126. Alternatively, the press 122 may include thermally regulated cold, warm or hot mold portions. An AC or DC power source

127 and a programmable current controller 128 are disposed adjacent the press 122 for energizing the electrically conductive layer 120. A safety light curtain 129 preferably surrounds the apparatus 112 to protect workers during the energizing of the electrically conductive layer 120 and the closing of the mold portions 124 and 126.

The formable layer 114, the cover material 116, the adhesive layer 118 and the electrically conductive layer 120 are robotically or manually positioned between the mold portion 124 and 126, with the electrically conductive layer 120 in heat transfer relationship with the formable layer 114 and the adhesive layer 118. The formable layer 114 may be any material which is sufficiently formable when sufficiently heated. Such materials include resin impregnated fiberglass, thermo- formable rigid urethane (TRU), or polyethylene terephthalate (PET). The cover material 116 preferably comprises non- woven PET, but it may comprise any suitable cover material such as cloth, fabric and/or vinyl. Alternatively, the cover material 116 may be eliminated if the characteristics of the formable layer 114 are aesthetically satisfactory. Furthermore, multiple formable layers 114 may be utilized to form a headliner.

The adhesive layer 118 may be a separate layer or it may be integrally pre-formed with either the formable layer 114, the cover material 116, or the electrically conductive layer 120. The adhesive layer 118 may comprise any suitable material including thermosetting resin of the reactive type, such as Bostik 812 adhesive available from Bostik Inc. of Middleton, Massachusetts, and/or thermoplastic resin, such as product code 4232 available from Bemis Corp. of Shirley, Massachusetts, or product code XUS 66113 available from The Dow Chemical Company of Midland, Michigan. Alternatively, multiple adhesive layers 118 may be used to improve bonding within the headliner assembly 110, or the adhesive layer 118 may be eliminated if, for example, not required to achieve suitable bonding.

The electrically conductive layer 120 may be a separate layer or it may be pre-laminated or otherwise integrally pre-formed to either the formable layer

114, the cover material 116, or the adhesive layer 118. Alternatively, multiple electrically conductive layers 120 may be used within the headliner assembly 110. The electrically conductive layer 120 is preferably sufficiently formable so that it can be easily formed into the final shape of the headliner assembly 110. The electrically conductive layer 120 may also be sufficiently rigid so that it can provide structural support to the headliner assembly 110. The electrically conductive layer 120 may comprise any suitable material capable of being energized to at least the necessary thermoforming temperature of the formable layer 114 and the activation temperature of the heat-activatable adhesive layer 118, if used. For example, the electrically conductive layer 120 may comprise any of the materials described with respect to the electrically conductive layer 20 of the cushion assembly 10.

The electrically conductive layer 120 may also be configured such that different sections extending between opposite ends of the conductive layer 120 have different electrical resistances. In other words, the electrically conductive layer 120 may be configured to function as multiple resistive elements connected in parallel, wherein at least two of the elements have different electrical resistances. Such a configuration enables different amounts of heat to be generated at different locations on the electrically conductive layer 120 to thereby selectively heat, for example, the formable layer 114. As a result, particular portions of the formable layer 114 requiring more shaping and/ or drawing than other portions may be heated to relatively higher temperatures to thereby increase the formability of the particular portions.

Next, the electrically conductive layer 120 is connected to the power source 127 and the programmable current controller 128 by using opposed electrical circuit clamps 130, as shown in Figure 12. Alternatively, the electrically conductive layer 120 may be connected to the power source 127 and current controller 128 using opposing bus bars, alligator clips, quick disconnects, or by any other suitable means.

The electrically conductive layer 120 is then energized by the power source 127 and current controller 128 to generate resistive heat, which is transferred to the formable layer or layers 114, the adhesive layer 118 and the cover material 116. After sufficient heating, the mold portions 124 and 126 are forced together to thermoform the headliner 110 as shown in Figure 13. During the thermoforming process, the formable layer 114, the adhesive layer 118, the electrically conductive layer 120 and the cover material 116 are bonded together and shaped into the desired contour of the headliner 110. Alternatively, the mold portions 124 and 126 may be forced together prior to energizing the electrically conductive layer 120.

It is desirable to heat the electrically conductive layer 120 to within a predetermined temperature range as quickly and as efficiently as possible in order to reduce cycle time and energy consumption. This is preferably accomplished by using the programmable current controller 128, which functions in a manner similar to that described with respect to the programmable current controller 46. Any other suitable means of controlling the heating of the electrically conductive layer 120 may also be utilized, such as the means described with respect to the electrically conductive layer 20.

Because the electrically conductive layer 120 is adjacent to the formable layer 114 and the adhesive layer 118, the heat transfer efficiency therebetween is maximized. As a result, the heating time and the amount of heat required for this method are minimized. Furthermore, the heat is provided without requiring expensive tooling and/or equipment.

Another aspect of the invention is a method of resistance welding an automotive assembly such as a door panel 210, a portion of which is shown in Figure 14. The door panel 210 includes at least two joinable bodies, such as a substrate layer 212 and a retainer 214, joinable together at respective joinable surfaces 216 and 218. The retainer 214 is configured to receive and retain a suitable fastener (not shown) which is used to fasten the substrate layer 212 to another member such as a sheet metal body of a motor vehicle. An energizable electrically conductive layer 220 is disposed at a bond interface between the surfaces 216 and 218 for facilitating the welding process. Alternatively, the method of resistance welding according to the invention may be used to bond joinable surfaces of the same body.

At least one of the substrate layer 212 and the retainer 214 preferably comprises a thermoplastic material, such as polypropylene, polyester, and/or nylon, which is sufficiently meltable and bondable to other materials when sufficiently heated. Alternatively, the thermoplastic material may be provided as a separate layer or layers. The electrically conductive layer 220 may be a separate layer or it may be pre-laminated or otherwise integrally pre-formed or integrated with either the substrate layer 212 or the retainer 214. The electrically conductive layer 220 is preferably sufficiently flexible so that it can easily conform to the bond interface between the substrate layer 212 and the retainer 214. Furthermore, a flexible electrically conductive layer is particularly useful in automotive assemblies that flex during use, such as flooring systems and trunk liners. Alternatively, the electrically conductive layer 220 may also be sufficiently rigid so that it can provide structural support to the door panel 212, or other automotive assembly.

The electrically conductive layer 220 may comprise any suitable material capable of being energized to at least the melting temperature of the thermoplastic material in the substrate layer 212 and/or the retainer 214. For example, the electrically conductive layer 220 may comprise any of the materials described with respect to the electrically conductive layer 20 of the cushion assembly 10. In addition, the electrically conductive layer 220 is preferably configured to allow the thermoplastic material in the substrate layer 212 and/or the retainer 214 to pass therethrough when the thermoplastic material is sufficiently melted.

The electrically conductive layer 220 may also be configured such that different sections extending between opposite ends of the conductive layer 220 have different electrical resistances. In other words, the electrically conductive layer 220 may be configured to function as multiple resistive elements connected in parallel, wherein at least two of the elements have different electrical resistances. Such a configuration enables different amounts of heat to be generated at different locations between the substrate layer 212 and the retainer 214 to thereby selectively weld the substrate layer 212 and the retainer together.

The method of resistance welding the substrate layer 212 to the retainer 214 includes positioning the electrically conductive layer 220 between the substrate layer 212 and the retainer 214 and in heat transfer relationship with the thermoplastic material in the substrate layer 212 and/or the retainer 214. Next, the electrically conductive layer 220 is connected to a power source 222 and a programmable current controller 224 by using opposed electrical circuit clamps 226, as shown in Figure 14. Alternatively, the electrically conductive layer 220 may be connected to the power source 222 and the current controller 224 using opposing bus bars, alligator clips, quick disconnects, or by any other suitable means.

The electrically conductive layer 220 is then energized by the power source 222 and current controller 224 to generate resistive heat, which is transferred to the substrate layer 212 and the retainer 214 to melt the thermoplastic material. Compressive force is preferably applied to the substrate layer 212 and/or the retainer 214 before, during and/or after the heating process to ensure that the thermoplastic material is in sufficient heat transfer relationship to the electrically conductive layer 220, and to assist in the bonding of the substrate layer 212 to the retainer 214. After sufficient heating, the thermoplastic material is allowed to pass through the conductive layer 220, thereby bonding the substrate layer 212 to the retainer 214. One or more adhesive layers may also be used to provide additional bonding between the substrate layer 212 and the retainer 214. Such adhesive layers may be separate layers or they may be integrally preformed with the substrate layer 212, the retainer 214 and/or the electrically conductive layer 220.

Alternatively, the electrically conductive layer 220 may be configured such that thermoplastic material is not able to pass therethrough. In such a case, both the substrate layer 212 and the retainer 214 preferably contain thermoplastic material which bonds to the electrically conductive layer 220 to thereby join the substrate layer 212 to the retainer 214. Furthermore, one or more adhesive layers may also be used to provide additional bonding. Advantageously, the use of a suitable adhesive layer or layers allows dissimilar materials to be bonded together, which is not possible with conventional plastic welding methods.

It is desirable to heat the electrically conductive layer 220 to within a predetermined temperature range as quickly and as efficiently as possible in order to reduce cycle time and energy consumption. This is preferably accomplished by using the programmable current controller 224, which functions in a manner similar to that described with respect to the programmable controller 46. Any other suitable means of controlling the heating of the electrically conductive layer 220 may also be utilized, such as the means described with respect to the electrically conductive layer 20.

Because the electrically conductive layer 220 is preferably adjacent to the meltable thermoplastic material and/or the adhesive layers, if used, the heat transfer efficiency therebetween is maximized. As a result, the heating time and the amount of heat required for this method are minimized. Furthermore, because heat from the electrically conductive layer 220 is localized, exterior surface defects in the joinable bodies caused by heat transfer through the bodies are not likely to occur. The lack of surface defects is particularly advantageous when one of the joinable bodies is a cover material.

After the retainer 214 has been bonded to the substrate layer 212, the electrically conductive layer 220 may be re-energized to debond the retainer 214 and the substrate layer 212. Such a feature enables defective welds to be corrected, thereby significantly reducing scrap rate. Furthermore, components can be easily separated for recycling.

Figure 15 shows an alternative embodiment 310 of the door panel for practicing the method of resistance welding according to the invention. The door panel 310 includes at least two joinable bodies, such as a substrate layer 312 and a retainer 314, joinable together at respective joinable surfaces 316 and 318. At least one of the substrate layer 312 and the retainer 314 is formed with a conductive additive or filler material, such as carbon powder and/or metal powder. Conductive polymers comprising conductive filler material are available from LNP Engineering Plastics of Exton, Pennsylvania, under the product name of Stat-kon. Because the substrate layer 312 and/or the retainer 314 comprises conductive filler material, the power source 222 and the programmable current controller 224 may be connected directly to the substrate layer 312 and/or the retainer 314, depending on which component or components comprise the conductive filler material. Electrical terminals 316 are preferably formed in the substrate layer 312 and/or the retainer 314 to facilitate electrical connection with the power source 222 and the current controller 224. Furthermore, at least one of the substrate layer 312 and the retainer 314 also preferably comprises a thermoplastic material for bonding the substrate layer 312 and the retainer 314 together. Alternatively, the thermoplastic material may be provided as a separate layer.

The method of resistance welding the door panel 310 includes energizing the conductive filler material in the substrate layer 312 and/or the retainer 314 to generate resistive heat. The conductive filler material is preferably concentrated near the bond interface between the substrate layer 312 and the retainer 314 in order to concentrate the resistive heat at the bond interface. The electrical terminals 316 are also preferably configured to effectively direct current to the conductive filler material. The programmable current controller 224 is used to control heating of the conductive material in a manner similar to that previously described. The heat from the conductive filler material then melts the thermoplastic material to thereby bond the substrate layer 312 and the retainer 314 together. Compressive force is preferably applied to the substrate layer 312 and/or the retainer 314 before, during and/or after the heating process to assist in the bonding of the substrate layer 312 to the retainer 314.

Because there is no separate electrically conductive layer disposed between the substrate layer 312 and the retainer 314, the bond therebetween is not affected by such a conductive layer. One or more adhesive layers may also be used between the substrate layer 312 and the retainer 314 to provide additional bonding.

The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, the cushion assembly 10 described above may be used in seats for applications other than motor vehicles. Furthermore, the present invention can be utilized to manufacture other automotive parts which require a substrate being bonded to a cover material, including door panels, trim panels, trunk liners and flooring systems. The present invention can also be utilized to thermoform and/or resistance weld such automotive parts. The electrically conductive material or layers in such parts may be further configured as heating devices similar to the seat heater described with respect to the cushion assembly 10. Advanta- geously, a boundary heating system may therefore be established by providing heating devices in parts located at or near the perimeter of a vehicle, such as headliners, door panels, and floor systems. In addition, the electrically conductive material or layers in such parts may be configured as static electricity dissipation devices which may be properly grounded to a vehicle to effectively remove static electricity from the parts during use. For example, an electrically conductive layer provided in a carpet flooring system comprising strands of yarn may be configured such that the conductive layer is in contact with the strands of yarn. As static charge builds up on the strands, the charge will be transferred to the conductive layer, which in turn will transfer the charge to ground. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

WHAT IS CLAIMED IS:

1. A method of joining a substrate to a cover material having an appearance portion and a concealable portion, the method comprising: positioning at least one heat-activatable adhesive layer and an energizable electrically conductive layer between the cover material and the substrate with the at least one heat-activatable adhesive layer and the electrically conductive layer in heat transfer relationship to each other; joining the cover material and the substrate together with the electrically conductive layer and the at least one heat-activatable adhesive layer there- between such that the electrically conductive layer is in sufficient heat transfer relationship when energized to cause the at least one heat-activatable adhesive layer to facilitate the adherence of the concealable portion of the cover material, the electrically conductive layer and the substrate to one another while exposing the appearance portion of the cover material; and energizing the electrically conductive layer to generate heat during one or more of the positioning and joining.

2. The method of claim 1 wherein the substrate comprises a foam pad.

3. The method of claim 1 wherein the substrate comprises at least one formable layer which is formable when sufficiently heated.

4. The method of claim 3 further comprising thermoforming the at least one formable layer, the electrically conductive layer, and the cover material into a desired shape.

5. The method of claim 1 wherein the substrate comprises a thermoplastic material.

6. The method of claim 1 wherein the cover material comprises a fabric.

The method of claim 1 wherein the cover material comprises vinyl.

The method of claim 1 wherein the cover material comprises leather.

9. The method of claim 1 wherein the electrically conductive layer comprises a conductive coating.

10. The method of claim 1 wherein the electrically conductive layer comprises a conductive polymer.

11. The method of claim 1 wherein the electrically conductive layer has at least two sections with different electrical resistances.

12. The method of claim 1 further comprising configuring the electrically conductive layer as a heating device for providing heat to an interior of a motor vehicle.

13. The method of claim 1 further comprising configuring the electrically conductive layer as a seat heater for heating a seat.

14. The method of claim 1 further comprising configuring the electrically conductive layer as a static electricity dissipation device.

15. The method of claim 1 further comprising configuring the electrically conductive layer as a weight sensing device.

16. The method of claim 1 wherein the energizing step comprises: applying current to the electrically conductive layer up to a first current level and over a first time period sufficient to allow the current to sufficiently disperse throughout the conductive layer; applying current from the first current level up to a second current level over a second time period, the second current level being greater than the first current level; and applying current to the conductive layer at substantially the second current level for a third time period, thereby heating the conductive layer.

17. The method of claim 16 wherein the energizing step further comprises reducing the current applied to the conductive layer from substantially the second current level; and allowing heat transfer to occur throughout the conductive layer to reduce temperature differentials across the conductive layer, thereby enabling a sufficient portion of the conductive layer to achieve a minimum desired temperature for melting the adhesive layer.

18. The method of claim 1 wherein at least one of the cover material and the substrate comprises a thermoplastic material, and the electrically conductive layer is configured to allow a portion of the thermoplastic material to pass therethrough when the thermoplastic material is sufficiently heated to thereby further facilitate the adherence of the concealable portion of the cover material, the electrically conductive layer and the substrate to one another.

19. A method of fabricating a seat cushion assembly, the method comprising: positioning a cover material over a contoured mold surface of a mold; positioning at least one heat-activatable adhesive layer, an electrically conductive layer, and a substrate in the mold with the at least one adhesive layer and the electrically conductive layer in heat transfer relationship to each other;

forcing the cover material and the substrate together in the mold with the at least one heat-activatable adhesive layer and the electrically conductive layer therebetween; and energizing the electrically conductive layer to generate heat to activate the at least one heat-activatable adhesive layer for adhering the cover material, the electrically conductive layer and the substrate to one another.

20. The method of claim 19 wherein the energizing step com mences prior to the forcing step.

21. The method of claim 19 further comprising developing a negative pressure proximate the contoured mold surface to draw at least one of the cover material and the heat-activatable adhesive layer against the contoured mold surface and in contour therewith.

22. A method of thermoplastic welding, the method comprising: positioning an electrically conductive material proximate a bond interface of joinable surfaces of at least one body; positioning a thermoplastic material proximate the bond interface; applying current to the electrically conductive material up to a first current level and over a first time period sufficient to allow the current to sufficiently disperse throughout the conductive material; applying current to the conductive material from the first current level up to a second current level over a second time period, the second current level being greater than the first current level; and applying current to the conductive material at substantially the second current level for a third time period, thereby heating the conductive material to cause the thermoplastic material to bond the joinable surfaces together.

23. The method of claim 22 further comprising: reducing the current applied to the conductive material from substantially the second current level; and allowing heat transfer to occur throughout the conductive material to reduce temperature differentials throughout the conductive material, thereby enabling a sufficient portion of the conductive material to achieve a minimum desired temperature.

24. The method of claim 22 further comprising applying pressure to the bond line so as to increase the strength of a bond between the joinable surfaces.

25. The method of claim 22 wherein the electrically conductive material comprises a conductive coating.

26. The method of claim 22 wherein the electrically conductive material comprises a conductive polymer.

27. The method of claim 22 wherein the electrically conductive material is configured to allow a portion of the thermoplastic material to pass there- through.

28. The method of claim 22 wherein the electrically conductive material is configured as a layer including at least two sections having different electrical resistances.

29. The method of claim 22 wherein at least one of the joinable surfaces includes the electrically conductive material.

30. The method of claim 22 wherein at least one of the joinable surfaces includes the thermoplastic material.

31. The method of claim 22 further comprising positioning a heat- activatable adhesive layer between the joinable surfaces.