JOINING OF COMPOSITE MATERIALS

Abstract: The invention discloses a method of joining plastics materials by forming a contact between two parts (10, 11) to be joined, placing an electrical induction coil (16) adjacent the contact and electrically inductively heating the parts in the vicinity of the contact. A layer comprising multidirectional discontinuous carbon fibres in the presence of thermoplastics material is provided in the vicinity of the contact, at least some of the carbon fibers being oriented to provide resistive conducting paths for the flow of eddy currents. Electrically inducing eddy currents in the layer (via coil 16) heats the thermoplastics material and causes it to fuse at the contact and form a join between the two parts (10, 11).

Description: JOINING OF COMPOSITE MATERIALS Technical Field This invention relates to the joining of plastics materials by the use of inductive heating to effect melt¬ ing, fusing, bonding or welding of the materials at a contact between parts to be joined. There is a need to be able to join plastics materials and in particular carbon fibre reinforced composite mater¬ ials, such as thermopl stics, (for example polyethe ' r ether ketone (PEEK), polyether sulphone (PES) and polyamide i ide) without impairing the strength of the components and without adding parasitic weight. To this end many attempts have been made to induction weld plastics including carbon fibre reinforced plastics material, but so far with limited success. The term "thermoplastics" as used herein * is taken to include thermosetting thermoplastic materials which exhibit thermoplastic characteristics up to a threshold temperature and thereafter exhibit thermosetting properties. One example of such a material is a polyamide imide known by the trade mark "Torlon". Background of the Invention Prior known techniques have employed the use of inductively heated metal platens placed against the sur- faces of the components at the contact where it is desired to melt and fuse the materials to form a joint. The preheated platens are removed from the surfaces and the two surfaces urged together to form the joint. In general, it is difficult to achieve uniform heating with this method of heating unless the components are very thin and therefore joint strengths produced using this prior known technique are low. In another method of inductively heating the plastics materials, wires have been implanted in the plastics materials in the vicinity of the region to be melted. Whilst reasonable joints can be formed in this way, the use of wires is a severe disadvantage because the wires add parasitic weight, can cause corrosion, impair the safety of the component, for example by protruding through the component, can affect the electrical screening, of the component, and can cause problems due to altering the physical characteristics, such as the coefficient of thermal expansion, of the joint compared to the rest of the component. An object of the present invention is to use carbon fibres as an inductively heated source of heat. It has been found that the use of carbon fibres as an inductive heating source is not straightforward. The use of tows of continuous carbon fibres when laid up as a single unidirectional layer, or laid up as two or more contiguous layers with unidirectional fibres in one layer at an angle to those in the juxtaposed layers, will not produce a suitable join. Attempts have been made to use a layer of carbon fibre fabric which is woven from tows of continuous carbon fibres but again without achieving any satisfactory joins. The failure to produce satisfactory joins is presumably due to the inability to generate sufficient uniform heating to melt the plastics materials at the contact. Summary of the Invention After extensive experimentation it has been concluded that what is required is an electrically "lossy" structure fabricated in such a way as to enable circulating eddy currents to be generated in the structure. According to the invention, therefor the joining method is characterised by the steps of providing in the vicinity of the contact, a layer comprising multidirectional discontinuous carbon fibres in the presence of thermoplastics material, at least some of the carbon fibres being oriented to provide resis¬ tive conducting paths for the flow of eddy currents, and electrically inducing eddy currents in the layer thereby to heat the thermoplastics material and* cause it to fuse at the contact and form a join between the two parts. The invention as claimed thus provides a method of joining plastics materials, and in particular joining reinforced plastics materials, by inductively heating a combination of carbon fibres and thermoplastics material, e.g. a bundle thereof. The term "bundle" as used herein should be taken tc include tows, slivers or yarns spun from tows of fibres. In a preferred embodiment, the bundle of carbon fibres comprises discontinuous fibres which preferably are twisted yarns. Ideally the bundle of carbon fibres comprises discontinuous carbon fibres co-mingled with thermoplastics fibres. Preferably the co-mingling of carbon fibres and thermoplastics fibres is achieved by stretch-breaking a tow of carbon fibres and thermoplastics fibres. The method of the invention enables one to achieve an acceptable join by inductively heating the multidirectional discontinuous carbon fibres by means of an electrical induction coi 1. Brief Description of the Drawings The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically, one embodiment of equipment used for joining two laminated composite plastics materials in accordance with the method of this invention, Figures 2 to 4 are graphs showing the results of bond strength tests on laminates joined on the equipment of Figure 1 , and Figure 5 illustrates how the equipment of Figure 1 can be used for bonding two alternative plastics materials together in accordance with the method of the invention. Description of Preferred Embodiments Referring to Figure 1, composite materials 10 and 11 to be joined each comprise a mul i-layered core 12 flanked oy two skins 13, 14. Each core comprises a plurality of layers of continuous carbon fibre reinforced PEEK iden¬ tified herein as "material X". The fibres in each layer of the core are parallel to each other (but need noτ, be) but layers can be assembled so that all fibres in the core are parallel or so that the fibres in the core are multidirec¬ tional . A number of laminates were constructed in which the skins 13, 14 were made of a stretch-oroken blended fibre yarn of carbon and PEEK fibres. This yarn is manufactured by Heltra Limited, a Courtaulds company and sold under their U.K. registered trade mark "Filmix". The "Filmix" yarn was fabricated into various materials having different structural forms, namely a woven fabric, a braid, knitted fabrics, or tied multidirectional matting. The "Filmix" materials comprised approximately 55 per cent by volume of carbon fibres and had as an essential feature the fact that the carbon fibres were multidirectional and discon¬ tinuous. Further the PEEK fibres were intimately inter¬ mingled with the carbon fibres in the "Filmix" yarn. Further laminate samples were constructed where the skins 13 r 14 were not made with "Fi mix" yarn but were made of random oriented carbon fibre mats or woven carbon fibres or a layer of material X. In these instances the plain carbon fibres and the random carbon mats were sandwiched between thin layers of PEEK resin prior to offering the skins up to the core 12. This was to provide enough resin to wet the carbon fibres. In some cases the laminates were coated with either PEEK or with polyether sulphone (PES) to give a resin-rich surface. The various laminates are set out in Table 1. Table 1 Key U.D. - unidirectional carbon fibres in the core. M.D. - multidirectional carbon fibres in the core, F-fabric - fabric made from "Filmix" yarn. F-braid - braid made from "Filmix yarn. F-U.D. - unidirectional oriented "Filmix" yarns. C-fabric - 5 harness satin carbon fabric. C-mat - Random carbon fibre mat. Two further laminates made without cores but comprising 2 layers of knitted carbon/PEEK "Filmix" yarn (L11), or 6 layers of woven carbon/PEEK (L1 ) were also manufactured. The laminates L1 to L12 were manufactured as follows The various layers were stacked and sandwiched between two stainless steel sheets, each of which was pre-coated with a release agent. The layers were then pressed to a thickness of about 1 mm between pre-heated platens for a " period of 10 minutes. The platens were then quickl.y cooled and the hardened laminate so produced removed from between the stainless steel sheets. Samples measuring 100 mm x 25 mm by 1 mm thick we ' re removed from the laminates L1 to L3 and L5 to L10 such that in each case the unidirectional fibres in the cere were aligned parallel to the longitudinal axis of the sample. Samples removed from L4 and L5 were oriented such that t*.e first layer cf material X in each was oriented parallel to the sample longitudinal axis. Those cut from L11 were oriented such that the major fibre direction was parallel to the sample longitudinal axis. Those removed from L12 were oriented such that the warp or weft ibres ran paral¬ lel to the sample longitudinal axis. Pairs of identical samples removed as described above were then joined together in accordance with the method of the present invention by forming a single lap joint as shown in Figure 1. Prior to making the lap joint, the sur¬ faces forming the contact of the proposed joint between the two parts were degreased using a grease solvent (e.g. Propan-2-ol ) . The overlap at the joint contact was 12.5 mm. The overlapping samples of materials 10 and 11 were joined using an induction welding machine 15. The specific machine used was a Stanelco 1000 Watt, 2.5 MHz unit with a flat coiled (pancake coil) induction coil 16. The induction coil 15 was coiled in a common plane to form a substantially square shape, when viewed in plan, measur¬ ing 25 mm x 25 mm. The alternating magnetic field produced by the induction coil 16 induced eddy currents in the carbon fibre reinforcement in the skins 13, 14. Two power settings namely 70 per cent and 80 per cent were used for laminates L1 , L2 and L3 and two different weld times at each power setting were used. To weld samples removed from laminate L4, 100 per cent power was applied for 240 seconds. Samples of laminate L5 were welded with a 30 second exposure at 80 per cent power. Samples from lamin¬ ates L7, L8 and L9 were heated using SO per cent power applied for approximately 60 seconds. To examine the possibility of joining laminates consisting of differing fibre arrangements, samples from laminate L10 were welded to samples from laminates L1 , L11 and L12 using SO per cent power for 75, 60 and 45 seconds respectively. A consolida¬ tion pressure of 0.66 MPa (90 psi ) was applied over the area of the joint contact during the welding period and also for a period of 30 seconds after the induction field was switched off by means of non-metallic platens, such as a high silicate glass (e.g. that known under the Trade Mark "Pyrex") (platen 17) and a phenolic/glass composition e.g. Paxolin (platen 18). The welded joints so formed were tested in accordance with ASTM D-2919 (D1002). Table 2 shows the results of these tests for the four laminates L4, L5, L7 and L3. Table 2 In Table 2 bond strength at the joint, expressed as an apparent shear strength, was obtained by dividing the load at failure by the nominal joint overlap area. Bond strength results obtained for the different conditions of time and power for laminates L1 , L2 and L3 are shown in Figures 2 to 4. Samples of laminate L6 did not heat sufficiently to allow welding to take place even when the welding .time was increased to 360 seconds and the power increased to 100 per cent. This was also the case for samples removed from laminate L9 which had unid rectional oriented "Filmix" yarn in the skins. The tests on these samples, L6 and L9 show that it is difficult to weld samples which contain predom¬ inantly unidirectional fibres. The results show that induction heating can be used to weld carbon fibre reinforced PEEK composite materials and give bond strength of the order of 50 MPa providing that the multidirectional electrically conductive fibres are present at the interface to be joined. This compares with a bond strength of about 15 to 25 MPa obtained when using epoxy adhesives to join the parts together. An important feature of the present invention is that laminates with un directional fibres are difficult if not impossible to join using the induction heating process. In fact such laminates were unable to be joined successfully. Referring to Figure 5 a further embodiment of the invention is illustrated. This embodiment differs from that described above with reference to Figure 1 in that the samples 10 and 11 are slightly different and a bonding insert 20 is employed. In other respects the apparatus is the same as that described above in respect of Figure 1. Specifically, the samples 10 and 11 do not necessarily comprise laminates which have skins as described above and are thus shown just as single component layers 12A. In this latter embodiment the components to be bonded are electrically non-conductive materials such as plas¬ tics, ceramics, or glass, and the bonding insert 20 com- prises one or more layers of multidirectional discontinuous carbon fibres in the presence of a plastics material. The preferred material for the insert 20 is the previously discussed yarn sold under the Trade Mark "Filmix" which is fabricated into different structural forms, namely a woven fabric, a braid, knitted fabrics, or multidirectional matting. The "Filmix" material may have additional ther¬ moplastics coatings or skins applied to it if desired. However the essential feature of the bonding insert 20 is that it includes multidirectional discontinuous carbon fibres which act as receptors and heaters when located in the field of an electrical induction coil. It is envisaged that the material for the bonding insert 20 could be produced as a long length which is cut into discrete lengths as required for insertion between the components to be bonded. It was found that with samples having unidirectional fibres at the interface and in the core there was a tenden¬ cy for the fibres to be displaced and the component to become bulged without forming good joins. An essential feature of the present invention is the use of a layer of multidirectionally oriented discontinuous carbon fibres at or near the surface of the plastics materials to be joined.- Whilst it is not fully understood why multidirectional discontinuous bundles of carbon fibres work, and unidirec¬ tional fibres do not, it is believed that the multidirec¬ tional fibres -produce a much more uniform heating effect over the entire surface because unidirectional fibres tend to dissipate the eddy currents induced in them without producing beneficial heating effect at the surface. In general, the bundles of discontinuous carbon fibres produce a layer which is more "lossy" than that produced from continuous fibres because the resistivity along the length of a bundle of discontinuous fibres is greater than that for continuous fibres. Furthermore hairiness of the bundle (due to broken ends projecting from the bundle) may contribute by providing resistive conduction paths which enable a better circulation of eddy currents in the layer and between various adjacent layers. Certainly when discontinuous carbon fibres are laid up into a woven or non-woven fabric, the regions where the warp yarns cross the weft yarns form conductive paths (or capacitive cou¬ plings) that create small "cells" around which the eddy currents flow. Although the Examples based on Figure 1 used laminates with skins formed by materials such as "Filmix" materials, it will be understood that it is not necessary to provide separate skins if the assembled component of parts to be joined comprises one or more layers of multidirectional discontinuous carbon fibres adjacent to the contact between parts to be joined. It is not necessary to use an insert 20 if at least one of the parts at the contact embodies carbon fibres of the required type. Furthermore, it is the. inventor's belief that co- mingled carbon fibres and thermoplastics fibres produce the best results because there is more intimate contact between the carbon fibres in the yarn and between yarns than, for example between carbon fibre cloth woven solely from discontinuous carbon fibres with thin layers of thermoplas¬ tics film or sheet in contact with the carbon cloth. Furthermore the inventor believes that the use of twisted tows of discontinuous carbon fibres to make yarn is advantageous because it may bring about preferred orienta¬ tion of the fibres in directions which improve contact between yarns and also improve the formation of circulating eddy currents in the plane of each layer.

Claims: CLAIMS 1. A method of joining plastics materials by forming a contact between two parts (10, 11) to be joined,, placing an electrical induction coil (16) adjacent the contact and electrically inductively heating the parts in the vicinity of the contact, characterised by the steps of providing in the vicinity of the contact, a layer (13, 14; 20) compris¬ ing multidirectional discontinuous carbon fibres in the presence of thermoplastics material, at least some of the carbon fibres being oriented to provide resistive conduct¬ ing paths for the flow of eddy currents, and electrically inducing eddy currents in the layer thereby to heat the thermoplastics material and cause it to fuse at the contact and form a join between the two parts. 2. A method according to claim 1, further charac¬ terised in that the carbon fibres are in the form of a bundle of discontinuous fibres. 3. A method according to claim 2, characterised in that the carbon fibres comprise a bundle of staple fibres. 4. A method according to claim 2, characterised in that the carbon fibres are stretch-broken fibres. 5. A method according to claim 1, characterised in that the carbon fibres comprise a bundle of continuous fibres which have been texturised to produce discontinuous fibres in the bundle or loops of fibre protruding from the bundle in various directions. 5. A method according to claim 1, characterised in that the thermoplastics material is in the form of fibres. 7. A method according to claim 6, characterised in that the carbon fibres and thermoplastics fibres comprise a bundle of co-mingled fibres. 8. A method according to claim 7, characterised in that the bundle of co-mingled fibres is made by stretch- breaking the fibres. 9. A method according to claim 2, characterised in that the carbon fibres comprise a yarn formed by twisting a tow of discontinuous carbon fibres. 10. A method according to claim 2, characterised in that the bundle is woven to form a fabric. 11. A method according to claim 2, characterised in that the bundle is laid up as contiguous layers arranged with the carbon fibres in each layer lying at an angle to those in the adjacent layer or layers so that resistive conducting paths are formed between layers to allow the eddy currents to flow in each layer. 12. A method according to claim 2, characterised in that the bundle is woven to form a braid. 13. A method according to claim 2, characterised in that the bundle is laid up as a non-woven fabric. 14. A method according .to claim 1 , characterised in that the layer constitutes a discrete bonding insert which is placed at an interface defining the contact between the parts to be bonded.

Inventors: GREEN STUART, GREEN, STUART
Applicants: COURTAULDS PLC
Application year: 1990
Main classification: B29K105
IPC codes: B29C65, B32B5, C09J5