HEATING OF COMPONENTS The present invention relates to the heating of components and relates particularly, though not exclusively, to the production of composites and methods and apparatus for their production. In the aerospace industry the use of composites to form components are commonplace. Composites are strong and lightweight. They generally consist of a carbon, Kevlar ™, fibreglass or similar fibres which are bound together by a resin or resins. The fibres or sheet of fibres are laid up with or pre- impregnated with the resin, placed in a shaping mould and the mixture cured by heat. Typical cure temperatures are in the range of 1 20-1 80° C (250-350 ° F) . Curing is the process whereby the resin used to bind and hold the fibres in place in the components, is set or hardened, usually by applying heat to the resin so that a cure temperature is reached, thus causing the resin to chemically set. The process may also involve application of pressures of one atmosphere or more to the component in the mould whilst curing within an autoclave. To achieve even and complete curing of the resin in the components (ie ensuring that the resin within the component matrix is heated uniformly to the cure temperature), there have been a variety of production techniques used.
The moulds used to lay up the components have been heated, typically by passing high temperature oil or steam through pipes embedded in the mould. The major disadvantages of such heating is that the area of the mould/component directly under the pipes receives the most heat/highest temperature, with areas between the pipes not being as hot. This can lead to uneven curing and in extreme cases, to sections of the components not being properly cured. Finally, the differences in coefficients of expansion between the material used to build the mould and the pipes through which the heating fluid is transmitted eventually leads to delamination or structural break down of the mould.
In production there are two common systems used. Firstly, heat blankets are laid over small components allowing direct heating to one face of the component. Heat blankets are effective for small open components and for repairs. Secondly, the use of autoclaves, where the whole mould is placed inside an autoclave and 'heat saturated' to the required curing temperature. The autoclave system also allows application of pressures of one atmosphere or more to be applied to the component matrix during the curing process. The use of autoclaves has many disadvantages. The components require a lengthy period in the autoclave to ensure heat saturation of the entire mould, component and the autoclave to an even temperature. As the mould is heated to the same high temperature, some twisting or warpage can be introduced into the component by the mould moving as it heats up. Special consideration is required in the design of moulds so that the mould does not shield any section of the component lay-up from achieving the required cure temperature. Care must be taken to ensure that the curing of the resin is allowed to continue uniformly throughout the matrix, and that a partial cure does not preclude final cure from inner areas of the component, due to poor heat transference. As different resin systems have different cure cycles, it is difficult to ensure that all components in an autoclave session fully meet their individual cure cycles. Finally, the autoclave must be as large as the largest mould used in production on the factory floor. Thus, production may be delayed whilst a full load of smaller components is built up (to ensure the autoclave is full before it is turned on) or a very large autoclave may have to be turned on to cure a very small part, with the attendant energy inefficiencies. Such production systems are expensive to install and operate. Mould design is complicated as the design must allow for differential expansion and contraction ratios between the mould and the component. It may be necessary to build special autoclaves to fit the complete mould. Energy consumption is high as the complete mould/component lay-up plus the autoclave itself is heat soaked to the required cure temperature, and cycle times are lengthy as the assembly is left in the autoclave for extended periods to ensure an even and complete cure of all parts of the component. An object of the present invention is to provide a method of curing composite components without requiring an autoclave or a heat blanket.
A further object is to reduce the time involved in curing composite components. A still further object is to allow for direct heating to the composite components rather than through heat soaking of the mould.
With these objects in view the present invention in one aspect provides a method of heating an article by using magnetic induction heating and/or resistive heating.
Preferably said method is for heating said article having a matrix of fibres and resin and said article is formed by curing said resin through said magnetic induction heating and/or said resistive heating. Said article may include an electrically conductive constituent which is heated by said magnetic induction heating. Preferably said electrically conductive constituent includes carbon fibres. The natural conductivity of the fibres themselves will be used as the medium for generation of the induction heating currents. Heat build up in a mould using this method is secondary, and incidental to the cure process. The article may be heated via resistive heating coils of carbon fibre or graphite laid within the matrix of the mould and its enclosing lid with said resistive heating coils located as near as possible to the working surface of the mould and the lid. These coils will be laid up so that the absolute minimum of a gap exists between the resistive coils, thus obviating areas of high/low temperature in the article. Further, as the resistive heating coils are to be constructed of carbon fibre/graphite, there will be no differences in coefficients of expansion between the mould and the resistive coils, thus negating the problem of mould delamination experienced with metal or metal sheathed coils.
The articles being electrically conductive may alternatively be formed in moulds or parts of moulds where magnetic induction heating is used to directly induce heat into the articles themselves. The moulds may incorporate a solid large plug of carbon fibre or graphite to assist "scanning" of the article using the magnetic induction heating process. As well, carbon fibre/graphite induction heating coils may be built into the moulds where appropriate for cure of certain articles. The articles may be laid up in polymer/ceramic or between metal/metal surfaced moulds to contain the magnetic induction heating directly into the article. In such a process, the mould does get hot, but does not approach the curing temperature of the article, thus reducing twisting or warpage. Temperature control can be accurate and quick, thus allowing much more rapid re-cycling of moulds and a lower energy usage to effect a cure. With electronic control of temperature it is easy to cope with exothermic heat build up as the resin cures. The heating process is even over the whole surface area and internal volume of the article, ensuring even and complete curing. As the heat is induced into the article or radiated from near tool surface mounted resistive heating coils, the method avoids a progressive 4
cure of the resin within the matrix, which may lead to localised partially cured pockets. The method is adaptable to accommodate vacuum bagging/pressurisation of the article during production. The electronic control over the heating ensures that each article is cured within its exact specified resin cure cycle.
In a further aspect of the invention there is provided a magnetic induction heating coil including a non-conductive sheath which surrounds a conductive inner core. In a preferred aspect of the invention said coil is flexible or bendable. Said core may comprise carbon fibre filaments or woven metal wire rope.
In a further aspect of the invention in situ repairs can be made to damaged composite components using temporary moulds placed thereon.
In order that the invention may be clearly understood there shall now be described by way of non-limitative examples only preferred constructions of the invention incorporating the principal features of the present invention. The description is with reference to the accompanying illustrated drawings in which: Fig. 1 is a perspective view of a first embodiment of a mould made in accordance with the invention;
Fig. 2 is a perspective view of a second embodiment of a further mould;
Fig. 3 is a perspective view of a third embodiment of yet another mould; Fig. 4 are views of a coil strap used in the embodiment shown in Fig.3;
Fig. 5 are views of an alternate coil strap used in the embodiment shown in Fig.3;
Fig. 6 is a perspective view of a fourth embodiment of a mould; 4
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Fig. 7 are views of a partial section of the mould shown in Fig. 6;
Fig. 8 is a cross-sectional view of the bottom half of the mould shown in Fig. 6; Fig. 9 is perspective view showing operation of the mould shown in Fig. 6;
In Fig. 1 there is shown a mould 10 comprising a top half 12 and a bottom half 14. The halves 12,14 have internal surfaces or inserts/tools contoured to the desired final shape of a component or components inserted therein. In this embodiment a carbon fibre matrix fabric is used. Such fabrics are used in the aerospace industry (where most heat cured composite components are now used) and generally comprise a carbon fibre fabric, sometimes laid up with other fabrics such as fibreglass and Kevlar™ . In most cases a resin system is pre-impregnated onto the fabric. At other times film plies of resin are applied in a lay-up when plain fabrics (not pre-impregnated with resin) are to be used. Mould 10 is not subjected to direct heat and has no embedded pipes for flow of hot oil or steam. Mould 10 may be formed from any suitable material eg. metallic, metallic surfaced or non- metallic. A plurality of magnetic induction heating coils 16, 18,20,22 are placed on the outside of top half 12 and linked in either a parallel or series connection. The number and shape of the magnetic induction heating coils 16,18,20,22 may vary depending on the component(s) being produced. In this embodiment each coil is a flat plate or pancake shape. The use of magnetic induction heating coils are commonplace in metallurgy for surface hardening of steel but different types of coils are used. A high frequency alternating current is passed through the coils 16,18,20,22 to provide a magnetic field, the intensity of which varies periodically in magnitude and direction. The frequencies to 4
- 7 - be used in the process may encompass all frequencies at which induction heating is effected. Heating is accomplished by the thermal effect of current induced in the article being heated. As the temperature to cure the resin is substantially less than that to heat steel, the time to heat the components is in the order of seconds. This is a substantial reduction in time compared with the hours required for autoclaving. Coils used in the process may need magnetic field concentrators where they are coupled together to ensure that effective heating of the component is achieved. Such concentrators may include connectors which also include a small inductor. If required, the flat plate coils 16, 18,20,22 may also take the form of a fixed "scanning" coil (not shown) that is robotically moved continuously to and fro across the surface of moulds 10, thus inducing the heating currents into the component.
Fig. 2 shows a mould 24 having a top half or lid 26 and a bottom half 28. The halves 26,28 can be made of plastics or ceramic material. Again the internal surface(s) of the halves 26,28 can be contoured or include tools/inserts to provide the desired end shape of the component(s) to be moulded. In this embodiment bottom half 28 has an magnetic induction heating coil 30 encased therein. It is preferable that moulds/tools/inserts to be used in the manufacture of composite components are made from materials with a similar coefficient of expansion to the material to be used in the component. Coil 30 may be formed of metallic or non-metallic material and may be enclosed within a non-conductive sheath. The sheath will accommodate any expansion or contraction of the coil as it heats and cools.
In a variation, heating coil 30 may be of a different configuration in that it is constructed of carbon fibre/graphite strips, placed very near to the tooling or working surface of the mould, with minimal gaps between the strips, and heated resistively to generate heat by convection into the component. A similar resistive heating coil would be inserted into the closing lid 26 of the mould 10 to ensure equal heat curing to both faces of the component.
Fig. 3 shows a mould 32 which may be of similar construction to that of mould 10 in Fig. 1 . In this embodiment a flexible or bendable magnetic induction heating coil 34 is used. Coil 34 may comprise a single wound coil or a plurality of coils connected together serially or in parallel. Coil 34 is wrapped around mould 32 in such a way that the coil forms a closed loop coil around the mould. These coils 34 may be manufactured from metal and non-metallic conductive materials that exhibit sufficient flexibility or ductility to be wound around compound curves and tight angles, and not be subject to fatigue fracture due to constant usage. Such materials may include carbon fibres themselves. Figs. 4 and 5 illustrate some examples of such materials.
Fig. 4 shows carbon fibres 36 laid in continuous filaments inside a non-conductive sheathing 38 to hold fibres 36 in alignment. Fig. 5 shows woven metallic wire ropes 40 inside non- conductive sheathing 42 to hold ropes in alignment on mould 32.
The use of moulds 10 and 32 of Figs. 1 and 3 can be extended to allow for in situ repairs of damaged composite components eg aircraft wings, without partial disassembly of the aircraft. The damaged section can be dressed and prepared for the installation of a patch repair. A temporary or disposable mould could be made of the outer face of the section to be repaired from another undamaged component. A flexible vacuum film is inserted at the back of the damaged section and sealed against the rear of the damaged component. The vacuum film will be left inside the component. The repair plies of carbon fibre (and core material, if necessary) are cut to fit the dressed damaged section and laid up to fill the damaged area. The mould is laid against the outer face of the component and a vacuum drawn through the mould to compress the repair plies. The magnetic induction heating coils are placed against the mould and the appropriate frequency and duration of cycle are used to induce heat in the all repair plies to effect a cure and bonding the repair material to the component.
Fig. 6 shows a variation of the moulds shown in Figs. 1 to 3. Mould 44 has a top half 46 and bottom half 48. The halves 46,48 can be sealingly attached to one another to ensure an airtight fit. An air pressurisation valve 50 is located in top half 46 to allow entry of pressurised air into mould 44. A pressure relief valve 52 is also provided as a safety device in case of over- pressurisation. A flexible membrane (not shown) can be moulded on top half 46 to conform to the profile of the component. Air pressure will be used to inflate the membrane, thus bearing down on the component within the mould to whatever pressure is required for that particular cure process. Air can be pumped into mould 44 via valve 50. Typical pressures would be one (1 ) atmosphere and above. Such moulds 44 may incorporate a solid near surface carbon fibre/graphite plug or core 63 to assist with scanning of the component by the induction heating coil and thus generation of the heating currents within the component.
When mould 44 is formed from plastics or ceramic materials it may be necessary to provide additional strength to the halves 46,48 to prevent movement of the mould/tool when pressures in excess of one atmosphere are applied during the cure process. A solution to this problem is shown in Fig. 7. Fig. 7 shows a mould half 54 which has a waffle slab construction. When forming mould half 54 removable mould elements (not shown) are inserted in the base of the mould half 54 to be formed. When hardened the mould elements are removed to leave behind hollow ribs 58. Ribs 58 will substantially increase the strength of mould half 54. Such moulds will also solve the current production problem experienced with steel moulds where, under heat, they experience a positive coefficient of expansion, whereas carbon fibre has a zero or negative coefficient of expansion. Further, polymer/ceramic moulds involve a substantially reduced capital cost to build compared to steel moulds.
Turning to Fig. 8 there is shown a method to further assist in accurate conformance of the carbon fibre fabric 68 in mould 44 where a vacuum outlet 66 is provided which opens into lower half 48 of mould 44. A vacuum pump (not shown) can be coupled to outlet 66. A vacuum bag or membrane 70 is sealed to bottom half 48 by bead 72. A bleed cloth 74 lies between fabric 68 and membrane 70 which provides a passage for air to escape from surface of fabric 68 to vacuum outlet 66.
Fig. 9 illustrates a schematic drawing of the completed process for forming components. In order to avoid duplication of description, integers in Fig. 8, which correspond with integers in Figs. 1 to 8, will be given an 'A' suffix. To ensure that the correct resin cure temperature and component conformity with mould 44A are achieved, some means of measurement of induced heat and pressure in the component whilst undergoing cure is required. Mould half 48A has heat sensors 60 which face the component and in the membrane or adjacent the membrane from which measurements of temperatures may be recorded. Heat sensors 60 may include digital silicone sensors and heat sensitive fibre optic cabling and others. Heat sensors 60 should not be conductive themselves, so that the heat measurement will properly reflect the induced temperature in the component. Further, measurement of heat build-up in the magnet induction heating coil 34A itself may be used as the measure of the induced heat in the component. Coil 34A preferably has sufficient depth to allow mould 44A to be able to be slipped therebetween without interference from valves 50A,52A. Measurement of heat, elapsed time, air pressure, and variations thereof will be monitored by an electronic control unit or programmable processor 62 during the cure process. Unit/processor 62 may control temperature/current/frequency/pressure as necessary, including varying same during the cure process, so that the desired cure cycle required for components will be controlled. Temperature, pressure, and time measurements may be recorded by the unit/processor and may be printed out on completion to provide a record of the complete cure cycle. Cures are even and complete, and the induced heat achieves the required cure temperature within seconds rather than hours in an autoclave. Electronic control of the heat induction currents will allow the required cure temperature to be maintained accurately and allow the effect of the exothermic curing reaction to be factored in to the overall heating energy input. The chart on the next page shows the operating parameters and characteristics which may be used by unit/processor 62 for controlling operation of mould 44A.
Unit/Processor Flow Chart and Characteristics
To select current/frequency to suit on receiving input on component weight/volume.
To select on/off current/frequency and pressure/time to maintain cure profile, adjusted to suit required inputs as evidenced from monitoring of heat/pressure/time within component.
Cure Cycle Inputs:
weight/volume of component time of cure cycle and how parameters are to be ramped - heat levels to be achieved/maintained over time pressure levels to be achieved/maintained over time stepped progress of heating and cooling overall cycle.
Cure Monitoring Systems
time pressure in mould - on/off of air pressurisation pump, or on/off of pressure relief valve temperature of component or of induction coil current frequency - on/off of current, or adjustment of frequency The invention provides a simpler system with substantially reduced costs and production times compared with the prior art. The magnetic induction heating coils
16, 18,20,22,30,34, resistive heating coils, the design of the self pressurised moulds 44, and the electronic control system
62 of the invention may be used for production of many different components resulting in less inventory.
Whilst there has been described in the foregoing description preferred constructions of a system incorporating certain features of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications and details of design or construction may be made without departing from the essential features of the present invention.