RESISTANCE WELDING OF THERMOPLASTIC COMPOSITE COMPONENTS

Abstract

Apparatus (10) and associated method for joining thermoplastic composite components (66, 68) to one another. Firstly, an electrically-conductive carbon-fibre textile (74) is positioned between two pieces of thermoplastic composite (66, 68) to form a weldable assembly (64), and pressure is applied to the weldable assembly (64). A voltage is then applied across the carbon-fibre textile (74) to heat the carbon-fibre textile (74), thereby melting the thermoplastic (82) of a carbon-fibre textile facing surface (78, 80) of each thermoplastic composite (66, 68), wherein the melted thermoplastic (82) fluidly fills the inter-fibre space (84) of the carbon-fibre textile (74). Upon removing the voltage to allow the carbon-fibre textile (74) to cool, a weld (86) forms between the two thermoplastic composites (66, 68) as the thermoplastic sets.

Claims

1. A method for joining thermoplastic composite components to one another, comprising the steps of: a) positioning an electrically-conductive non-metal pliantly flexible membrane between two pieces of thermoplastic composite to form a weldable assembly; b) applying pressure to the weldable assembly; c) applying a voltage across the flexible membrane to heat the flexible membrane, thereby melting the thermoplastic of a flexible membrane facing surface of each thermoplastic composite, wherein the melted thermoplastic fluidly fills the inter-fibre space of the flexible membrane; and d) removing the voltage to allow the flexible membrane to cool, a weld forming between the two thermoplastic composites as the thermoplastic sets.

2. A method as claimed in claim 1, wherein the flexible membrane is a carbon-fibre textile.

3. A method as claimed in claim 1, wherein the flexible membrane is a non-woven textile.

4. A method as claimed in claim 2, wherein the carbon-fibre textile is a carbon tissue.

5. A method as claimed in claim 1, wherein at least one of the thermoplastic composite components is a continuous fibre-based laminate material.

6. A method as claimed in claim 5, wherein one of the thermoplastic composite components is one of: a discontinuous fibre-based laminate material; a powder-filled thermoplastic; and an unfilled thermoplastic.

7. (canceled)

8. (canceled)

9. A method as claimed in claim 1, wherein the thermoplastic of the thermoplastic composite components is Polyether Imide.

10. A method as claimed in claim 1, wherein the thermoplastic of the thermoplastic composite components is Poly Ether Ether Ketone.

11. A method as claimed in claim 1, wherein the thermoplastic is Polyphenylene Sulfide.

12. A method as claimed in claim 1, wherein, during step a) electrically-insulative layers are inserted between the thermoplastic composite components and the flexible membrane.

13. A method as claimed in claim 12, wherein the electrically-insulative layers are formed from single-ply glass thermoplastic composite.

14. (canceled)

15. (canceled)

16. A resistance welding apparatus for use in a method as claimed in claim 1, the apparatus comprising: first and second toolings, between which the weldable assembly is positionable; first and second electrodes; and a power supply; wherein at least one of first and second toolings is actuatable towards the other, actuation of the or each tooling towards the other applying pressure to the weldable assembly; and wherein first and second electrodes are spaced apart so as to contact with the flexible membrane of the weldable assembly, the first and second electrodes being in electrical communication with the power supply, thereby supplying a voltage across the flexible membrane to achieve a welding condition.

17. A resistance welding apparatus as claimed in claim 16, wherein the second tooling is positioned above the first tooling, the second tooling being actuatable towards the first tooling.

18. A resistance welding apparatus as claimed in claim 16, wherein the first and second electrodes are affixed to the first tooling.

19. A resistance welding apparatus as claimed in claim 18, wherein the first and second electrodes each comprise a rigid support electrode and a flexible foil electrode, the rigid support electrode being affixed to the first tooling, and the flexible foil electrode being attached to the rigid support electrode, the flexible foil electrode contacting with the flexible membrane.

20. A resistance welding apparatus as claimed in claim 16, further comprising a computer control device for controlling at least the pressure application of the or each actuatable tooling.

21. A resistance welding apparatus as claimed in claim 16, further comprising at least one electrical safety device to override the actuation of the or each tooling.

22. A heating element for use with a resistance welding apparatus as claimed in claim 16, the heating element comprising the flexible membrane.

23. A heating element as claimed in claim 22, the heating element further comprising two electrically-insulative layers laminated onto the flexible membrane.

24. A heating element as claimed in claim 23, wherein the electrically-insulative layers are formed from single-ply glass thermoplastic composite.

Description

[0038] The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

[0039] FIG. 1 shows a diagrammatic cross-sectional view through a first embodiment of the apparatus in accordance with the second aspect of the invention;

[0040] FIG. 2 shows a diagrammatic side-view representation of a welding assembly being welded in accordance with the first aspect of the invention, prior to heating;

[0041] FIG. 3 shows the welding assembly of FIG. 2, during heating; and

[0042] FIG. 4 shows the welding assembly of FIG. 3, following cooling.

[0043] Referring firstly to FIG. 1 there is shown an apparatus for resistance welding of thermoplastic composite components, indicated globally at 10. The apparatus comprises a cuboidal first tooling 12 having top and bottom planar faces 14, 16, and a complementarily sized second tooling 18, also having top and bottom planar faces 20, 22. Both first and second toolings 12, 18 are substantially elongate along a horizontal axis. The first and second toolings 12, 18 are positioned in a stacked arrangement relative to one another, the second tooling 18 being positioned above the first tooling 12.

[0044] Towards a first end 24 of the first tooling 12 is positioned a first electrode 26, projecting upwardly from the top face 14 of the first tooling 12. A second electrode 28, also projecting upwardly from the top face 14 of the first tooling 12, is positioned at the opposing end 30. Each electrode 26, 28 comprises a rigid electrically-conductive support 32 which is affixed to the first tooling 12, and a flexible electrically-conductive foil 34.

[0045] Electrical power is supplied to each of the first and second electrodes 26, 28 via connection to an electrical connector 36 within the body 38 of the first tooling 12.

[0046] Each flexible electrically-conductive foil 34 extends upwardly from its rigid electrically-conductive support 36 and has a distal tang 40 which is aligned towards the distal tang of the opposing electrode. The tangs 40 are spaced apart from one another so as to be able to receive a heating element 42 for use in the apparatus 10.

[0047] The second tooling 18 is affixed to a plurality of plungers or guides 44, at least one plunger 44 being positioned at each longitudinal end 46, 48 of the second tooling 18, and the plungers 44 being vertically actuatable. Each plunger 44 extends through the body 50 of the second tooling 18, and a projecting shaft 52 of each plunger 44 extends from the bottom planar face 22 of the second tooling 18 towards the first tooling 12.

[0048] The plungers 44 may be hydraulically operated pistons, or any other similarly actuatable devices, such as screw-threaded bits and/or rams. The plungers may alternatively be simply guides along which the second tooling is movable, with one or more rams or other suitable actuator, such as a hydraulic, pneumatic or electrically-drivable piston, being utilised to move the second tooling. A computer controller may be provided to allow a user to control and monitor the or each plungers or actuator, at least.

[0049] The first tooling 12 is connected with a power supply 54 which provides electrical power to the apparatus 10 via a series of electrical connectors 36 embedded within the body 38 of the first tooling 12. The power supply 54 is preferably an AC power supply, supplying a controlled low voltage, and the electrical connectors 36 connect to the electrically operable components of the apparatus 10. The computer controller may also be utilised to control and monitor the voltage and/or temperature of the heating element 42.

[0050] The plungers 44 enable the second tooling 18 to be actuated and thus moved towards the first tooling 12. In the top face 14 of the first tooling 12 is provided a series of complementary recesses 56 for receiving the projecting shafts 52 of the plungers 44. As a safety measure, inside the body 38 of the first tooling 12 at the innermost ends 58 of the recesses 56 are provided one or more electrical safety devices 60, which are activatable upon contact with the projecting shafts 52.

[0051] The area between the first and second toolings 12, 18 and between the first and second electrodes 26, 28 may therefore be termed the weldable assembly receiving area 62. This may or may not be demarcated or otherwise indicated on either the first tooling 12, second tooling 18 or both.

[0052] A weldable assembly 64 comprises first and second thermoplastic composite components 66, 68, first and second electrically-insulative layers 70, 72 and an electrically-conductive non-metal pliantly flexible membrane, in this case being a carbon-fibre textile 74. The assembly 64 is formed in layers, from the lowest level upwards: the first thermoplastic composite component 66; the first electrically-insulative layer 70; the electrically-conductive carbon-fibre textile 74; the second electrically-insulative layer 72; and the second thermoplastic composite component 68.

[0053] The electrically-conductive carbon-fibre textile 74 is a non-woven textile, preferably a carbon tissue. Such an electrically-conductive carbon-fibre textile 74 is used in preference to a continuous carbon-fibre, as is presently used in the industry. Continuous carbon-fibres encourage the formation of voids in the weld; areas in which there is no thermoplastic material. As such, the weld is significantly weakened. When utilising an electrically-conductive carbon-fibre textile 74, such voids are not formed.

[0054] The thermoplastic composite components 66, 68 are preferably both continuous fibre-based laminate materials, having continuous carbon or glass fibres embedded within a thermoplastic material, preferably Polyether Imide (PEI), Poly Ether Ether Ketone (PEEK) or Polyphenylene Sulfide (PPS). The strongest welds are achievable for components 66, 68 which are continuous fibre-based laminate materials; however, the present welding technique can be applied to any of discontinuous fibre-based materials, powder-filled thermoplastic or unfilled thermoplastics, provided an outermost layer of the material is thermoplastic.

[0055] The first and second electrically-insulative layers 70, 72 are preferably formed from a single-ply glass thermoplastic composite.

[0056] To weld the thermoplastic composite components 66, 68 of the weldable assembly 64 together, the layers are assembled as detailed above inside the weldable assembly receiving area 62. The distal tangs 40 of the flexible electrically-conductive foils 34 of the first and second electrodes 26, 28 are contacted with respective ends of the electrically-conductive carbon-fibre textile 74. In this case, the electrically-conductive carbon-fibre textile 74 is the standard heating element 42, and therefore the tangs 40 are separated by a distance approximately equal to the length of the electrically-conductive carbon-fibre textile 74.

[0057] Once the weldable assembly 64 is in place, the plungers 44 are activated to lower the second tooling 18 towards the first tooling 12. The bottom face 22 of the second tooling 18 will come into contact with an upper surface 76 of the second thermoplastic composite component 68, thereby applying a downward force to the weldable assembly 64, compressing the layers together.

[0058] Once the desired force is reached, the power supply 54 can be activated to supply a voltage across the electrically-conductive carbon-fibre textile 74. This will result in heating of the electrically-conductive carbon-fibre textile 74, which will initiate the welding of the two thermoplastic composite components 66, 68.

[0059] Both the plungers 44, and therefore pressure application, and the power supply 54, and therefore welding voltage, may be controlled by a single computer control device. This advantageously enables a single control unit to be installed, allowing control of the welding process as a whole. Such computer control device is known, and therefore will not be described in further detail.

[0060] The welding process is controlled by the temperature of the electrically-conductive carbon-fibre textile 74. This is depicted in FIGS. 2 to 4. As the temperature of the electrically-conductive carbon-fibre textile 74 rises, the thermoplastic at the welding interfaces 78, 80 of the first and second thermoplastic composite components 66, 68 will begin to melt.

[0061] The voltage to the electrically-conductive carbon-fibre textile 74 is carefully controlled to ensure that its temperature is at or is close to the melting point of the thermoplastic, the thermoplastic composite components 66, 68 being electrically insulated by the first and second electrically-insulative layers 70, 72. This ensures that only the thermoplastic at the welding interfaces 78, 80 melts, rather than the entire thermoplastic composite component 66, 68.

[0062] As the thermoplastic at the welding interfaces 78, 80 melts, the pressure supplied by second tooling 18 on the second thermoplastic composite component 68 forces the first and second components 66, 68 together. The melted layer of thermoplastic 82 then fills the interstitial spaces 84 in the conductive carbon-fibre textile 74, wetting out the tissue and thereby forming a liquid join between the first and second components 66, 68.

[0063] As the voltage is removed from the apparatus 10, the electrically-conductive carbon-fibre textile 74 will cool, and the liquid thermoplastic 82 will begin to set. As the thermoplastic sets, it will form a solid, contiguous weld 86 between the first and second thermoplastic composite components 66, 68 with the carbon tissue 74 sandwiched therebetween. The plungers 44 can then be retracted to release the pressure on the weldable assembly 64, and the now-welded assembly can be removed from the apparatus 10.

[0064] The electrically-insulative layers 70, 72 may be formed from a glass thermoplastic composite, whereby the thermoplastic will also melt during the welding process.

[0065] The apparatus 10 is preferably further provided with an override mechanism in the form of the electrical safety devices 60. If too much pressure is applied through the plungers 44 and the force on the weldable assembly 64 becomes too great, then the projecting shafts 52 of the plungers 44 locate in the complementary recesses 56, thereby activating the safety devices 60. This will cause the apparatus 10 to shut down or the toolings to separate, in order to prevent damage to the components of the weldable assembly 64.

[0066] It will be appreciated that a large proportion of the method of welding thermoplastic composite components as described is dependent not only upon the provision of both the first and second thermoplastic composite components 66, 68 to be welded, but also upon the electrically-conductive carbon-fibre textile 74.

[0067] Since the electrically-conductive carbon-fibre textile 74 must fit into the apparatus 10 between the first and second electrodes 26, 28 during normal operation, it is an intention of the present invention to provide a heating element 42 compatible with the apparatus 10 which comprises such an electrically-conductive carbon-fibre textile 74.

[0068] Using such a replaceable heating element 42 allows for many different thermoplastic composite components to be welded together about a single type of electrically-conductive carbon-fibre textile 74. This increases the versatility of the apparatus 10.

[0069] Although a carbon-fibre textile, in the form of a tissue, is suggested, any suitable, preferably non-metal and/or pliantly flexible, electrically conductive sheet, membrane or layer may be utilised.

[0070] More advantageously, the electrically-conductive carbon-fibre textile 74 could be provided in combination with both first and second electrically-insulative layers 70, 72 as a single heating element 42. Provision of such a heating element 42 therefore removes the need to assemble five layers in the weldable assembly 64, which will speed up the welding process, and also reduce the probability of incorrect layering of the various components of the weldable assembly 64.

[0071] Thermoplastic composite components formed as a result of the method can be used in a variety of industries. In particular, the present method is intended for use in the aerospace industry, advantageously providing the necessary strength to welded components through use of the electrically-conductive non-metal pliantly flexible membrane. However, the method described is widely applicable.

[0072] It will be appreciated that it may be desirable to weld thermoplastic composite components together of different volumes. It may therefore be advantageous to provide electrodes in the first tooling of the apparatus which can accommodate a plurality of component sizes and shapes. This may be achieved by providing movable first and second electrodes, thereby being able to accommodate differently sized heating elements.

[0073] The welding process is also described as being utilised to combine two thermoplastic composite components; however, the process could conceivably be used to combine more than two components together at a single joint, if desired. Additionally or alternatively, other kinds of thermoplastic may be utilised, and/or a thermoplastic without glass may be utilised.

[0074] Whilst the plungers are described as being affixed to the second tooling of the apparatus, it will be understood that the important feature is the application of pressure to the weldable assembly during the resistance welding process, and therefore, the first tooling could be constructed in a similar manner so as to provide said pressure.

[0075] It will be apparent to the skilled person that there are numerous additional features known in the art which could be added to the apparatus in order to improve its performance. In particular, safety features such as additional pressure, temperature or voltage overrides could be included, in order to comply with regulatory guidelines.

[0076] It is therefore possible to provide a method of resistively welding thermoplastic composite components to one another, using a carbon-fibre textile as a heating element. The carbon-fibre textile heats and subsequently melts the thermoplastic interfaces of the composite components, wetting out the textile with molten thermoplastic. Upon setting of the thermoplastic, the two thermoplastic composite components will be joined. The utilisation of the textile results in the formation of a void-free weld, resulting in a strong bond between the two components.

[0077] The words comprises/comprising and the words having/including when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0078] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

[0079] The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.