Composite sandwich panels

Abstract

Sandwich panels 174, 176, 178 are produced by simultaneously and continuously applying outer layers 122, 124 to the top 108 and bottom 110 faces of an elongate thermoplastic foam panel 102. The outer

Description

Field The present invention relates to a method of producing a composite sandwich panel, and a system for producing a composite sandwich panel. Background Composite sandwich panels are known. Composite sandwich panels consist of a lightweight core, with thin outer skin layers applied to each side of the core. Composite sandwich panels may find utility in applications where high strength, but low weight is required, such as for one or more elements of a wind turbine blade. For example, composite sandwich panels may be used to form a shear web of a wind turbine blade. One known way of applying the outer skin layers to the foam core includes laying up dry layers of glass or carbon fibres on top of the core. The dry layers are then infused with a liquid resin, which must be left to cure for a period. Alternatively, a “pre-preg” can be used for the outer skin layers, where the pre-preg comprises one or more dry layers pre-impregnated with resin. The pre-preg is laid on the core, then heated before being allowed to cure. The process of laying the dry layers or the pre-preg can be time consuming. Also, whether dry layers are infused with liquid resin or a pre-preg is used, the composite sandwich panel must be left to cure for a period after the outer skin layers have been applied to the core. This slows down the overall manufacturing process. The present inventors have attempted to address or at least mitigate the above-mentioned problems and have aimed to provide a method of producing a composite sandwich panel in a highly efficient manner. The present inventors have also aimed to provide a system for the production of the composite sandwich panel.. Summary Accordingly, in a first aspect the present invention provides a method of producing a composite sandwich panel, according to claim 1. In a second aspect the present invention provides a system for producing a composite sandwich panel, according to claim 30. Preferred features of these aspects of the present invention are defined in the respective dependent claims. Brief description of drawings Figure 1 schematically shows a process flow of continuously producing a composite sandwich panel, according to an embodiment of the present invention; Figure 2 schematically shows a multi-ply outer layer of a composite sandwich panel, according to an embodiment of the present invention; Figure 3 schematically shows a reinforcement fibre being applied to a foam panel at intersecting angles; according to an embodiment of the invention; Figure 4A schematically shows a reinforcement fibre and a molten thermoplastic polymer being fed out of a common outlet, according to an embodiment of the present invention; Figure 4B schematically shows a reinforcement fibre and a molten thermoplastic polymer being fed out of separate outlets, according to an embodiment of the present invention. Detailed description Figure 1 schematically shows a process flow of producing a composite sandwich panel, according to an embodiment of the present invention. In some examples, the process flow comprises continuous production of a composite sandwich panel. According to the embodiment of Figure 1, the process takes place in a system or facility 100 for producing sandwich panels. At step (a), the method comprises providing an elongate thermoplastic foam panel 102. In some examples the method comprises extruding the foam panel 102 from an extrusion die 103 of an extruder 104. The thermoplastic foam panel 102 is extruded continuously in a longitudinal direction L away from the extrusion die 103. Solid particles 106 of a thermoplastic resin may be fed in to the extruder 104. As is well known to those skilled in the art, within the extruder the thermoplastic resin is melted and combined with a blowing agent, which may be a physical or chemical blowing agent, to form a molten body of cellular foam. The molten body of cellular foam is then extruded through the extrusion die 103 to form an elongate panel having a desired width and height. Various combinations of thermoplastic resins and blowing agents are well known to those skilled in the art for producing structural cellular foams having desired structure, composition and mechanical and thermodynamic properties. In some examples, the thermoplastic resin, and consequently the thermoplastic foam, comprises polyethylene terephthalate (PET or PETE), which may be composed of virgin PET (vPET) and / or recycled PET (rPET). Virgin PET is typically supplied as pellets. Recycled PET may be supplied as flakes recovered directly from the recycling process, or as pre-processed pellets. In other examples the foam panel 102 may be formed in a different manner. For example, the foam panel 102 may be formed in a mould in which foaming ingredients are placed, and then expanded to the shape of the mould prior to removal of the foam panel from the mould. In some examples, an injection moulding process can be used to produce the foam panel. The thermoplastic foam panel 102 comprises a top face 108 and a bottom face 110. At step (b) the method comprises applying to the top and bottom faces 108, 110 of the foam panel 102 a respective top layer 122 and bottom layer 124 at an outer layer applying station 120. The outer layer applying station 120 is downstream of the extrusion die 103. In some examples, the top and bottom layers 122, 124 are applied whilst the foam panel 102 is continuously extruded from the extrusion die 103. Once applied, the top and bottom outer layers 122, 124 respectively cover the top and bottom faces 108, 110 of the foam panel 102, so that the foam panel 102 forms a foam core 105 which is sandwiched between the top and bottom outer layers 122, 124. In examples, the outer layer applying station 120 comprises an upper additive manufacturing machine 126 for applying the top outer layer 122 by an upper additive manufacturing process. In examples, the outer layer applying station 120 also comprises a lower additive manufacturing machine 128 for applying the bottom outer layer 124 by a lower additive manufacturing process. The upper additive manufacturing machine 126 has an applicator head 130, which may be referred to as an upper applicator head. The lower additive manufacturing machine 128 has an applicator head 132, which may be referred to as a lower applicator head. The upper and lower applicator heads 130, 132 are each configured to move in a respective x-y plane which is respectively aligned with the top or bottom face 108, 110 of the foam panel 102. As schematically shown in Figure 1, a reinforcement fibre 134 and a thermoplastic polymer 136 are supplied to each of the upper and lower additive manufacturing machines 126, 128. In each of the first and second additive manufacturing machines 126, 128 the thermoplastic polymer 136 is heated to form molten thermoplastic polymer. For example, the thermoplastic polymer 136 may be heated to a temperature in a range of 240 to 270°C.. Then, the reinforcement fibre 134 and the molten thermoplastic polymer 136 are simultaneously, and continuously or continually, extruded onto the respective top or bottom face 108, 110 from the respective applicator head 130, 132 which is moved in the respective x-y plane to apply the respective top and bottom outer layers 122, 124 over a surface area of the top and bottom faces respectively. In this respect, continuously may be considered to mean that the process of extruding the reinforcement fibre 134 and molten thermoplastic polymer 136 occurs without interruption, whereas continually means there may be some starting and stopping. In some examples movement of the applicator heads 130,132 to apply the top layer 122 and bottom layer 124 is synchronized or coordinated with the extruding of the elongate thermoplastic foam panel 102 from the extrusion die 103. In some examples, for example where the reinforcement fibre 134 and molten thermoplastic polymer 136 are continually extruded, there may be a short delay between successive scans of the applicator heads 130, 132 over the respective top and bottom faces 108, 110. Or, there may be a short delay to account for reloading of materials into the additive manufacturing machines 126, 128. These short delays may occur whilst the foam panel 102 is still being continuously extruded from the extrusion die 103. Therefore, in some examples one or both of the applicator heads 130, 132 may be caused to temporarily increase their speed of movement in the x and / or y plane if required to “catch-up” with the speed of extrusion of the foam panel 102. This may obviate a requirement to start / stop the process of extruding the thermoplastic foam panel from the extrusion die 103. Then, at step (c), the thermoplastic polymer cools and solidifies, thereby forming a thermoplastic polymer matrix 138 in which the reinforcement fibre 134 is embedded. In some examples, step (c) occurs whilst the foam panel is continuously extruded from the extrusion die 103. Together, steps (a), (b) and (c) result in formation of a composite sandwich panel 170. According to some examples, in the applying step (b) each applicator head 130, 132 is moved in the respective x-y plane in a preset scanning motion. The preset scanning motion may be controlled by computer numerical control (CNC). In Figure 1, a controller is schematically shown at 140, which comprises at least one memory 142 and at least one processor 144. For example, the memory 142 may store information of one or more preset scanning motions for the applicator heads 130, 132. According to some examples, in the applying step (b) the reinforcement fibre 134 is applied as a monolayer of reinforcement fibres in each of the respective top and bottom outer layers 122, 124. According to some examples, in the applying step (b) the reinforcement fibre 134 is applied as a stack comprising a plurality of plies or layers of reinforcement fibres. This is schematically shown in Figure 2, which is a sectional side view through a top or bottom layer 122, 124. The stack 146 comprises a first ply 148 and a second ply 150. Of course, this is by way of example and more than two plies can be applied in other examples. In some examples, in each stack 146 the plurality of plies of reinforcement fibres 148, 150 are applied by a common applicator head 130, 132 of the respective upper or lower additive manufacturing machine 126, 128. The common applicator head is additionally moved in a z-direction orthogonal to the x-y plane to thereby progressively build up the stack of plies 148, 150 in each of the respective top and bottom outer layers. In some examples, in each stack the plurality of plies of reinforcement fibres are applied sequentially by a plurality of applicator heads of the respective upper or lower additive manufacturing machine. This progressively builds up the stack of plies 148, 150 in each of the respective top and bottom outer layers 122, 124. By way of schematic example in Figure 2, in a top outer layer 122 the first ply 148 may be applied by a first applicator head 130 of additive manufacturing machine 126, and the second ply 150 may be applied by second applicator head 131 of additive manufacturing machine 126. The same or a similar principle may apply to the application of multiple plies in the bottom outer layer 124. It may be considered that reference to upper and lower additive manufacturing machines covers machinery being located on each side (top and bottom faces) of the foam panel 102. Each machine can include one head, or each machine may comprise multiple heads which could be in a common machine or in multiple independent machine units of the overall machine. According to some examples, an orientation of the reinforcement fibres in each ply of the stack is different from the orientation of the reinforcement fibres in any ply adjacent thereto. For example, the upper additive manufacturing machine 126 and the lower additive manufacturing machine 128 may be configured to apply sequentially a first layer of the respective top and bottom outer layers, wherein in the first layer the reinforcement fibre material is applied diagonally across the foam panel at a first angle, and a second layer of the respective top and bottom outer layers is applied over the first layer. In the second layer the reinforcement fibre is applied diagonally across the foam panel at a second angle that intersects the first angle, so that together the first and second layers provide multi-axial top and bottom outer layers. This is schematically shown in Figure 3, which is a plan view of a portion of a top or bottom outer layer 122, 124. A first layer of the fibre reinforcement material 134 is applied diagonally across the foam panel 102 at a first angle Qi, and a second layer of the reinforcement fibre is applied diagonally across the foam panel at a second angle 02 that intersects the first angle. In some examples the first angle ch is approximately +45° or -45° to a longitudinal axis X-X of the elongate thermoplastic foam panel 102, and the second angle ch is the other of approximately +45° or -45° to the longitudinal axis X-X of the elongate thermoplastic foam panel 102. In some examples, the reinforcement fibre 134 is selected from glass fibre, carbon fibre, aramid fibre or natural fibre, thermoplastic fibres, or a mixture of any two or more thereof. In some examples the reinforcement fibre 134 comprises or consists of glass fibre. In some examples the reinforcement fibre 134 is fed to the applicator head 130 or 132 from a supply roll of the reinforcement fibre. For example, the reinforcement fibre 134 may be fed to the top applicator head 130 from a top supply roll 135, and the reinforcement fibre 134 may be fed to the bottom applicator head 132 from a bottom supply roll 137. In some examples it may be considered that the upper and lower additive manufacturing processes are fused filament fabrication processes, and the thermoplastic polymer 136 is supplied to the upper and lower additive manufacturing machines 126, 128 in the form of a thermoplastic polymer fibre 139. In some examples the thermoplastic polymer fibre is fed to the applicator head from a supply roll of the thermoplastic polymer fibre. For example the thermoplastic polymer fibre 139 may be fed to the top applicator head 130 from a top supply roll 141, and the thermoplastic polymer fibre 139 may be fed to the bottom applicator head 132 from a bottom supply roll 143. In some examples the upper and lower additive manufacturing processes are fused particle fabrication processes, and the thermoplastic polymer 136 is supplied to the upper and lower additive manufacturing machines 126, 128 in the form of particles 145 of the thermoplastic polymer, as schematically shown in Figure 1. According to some examples the thermoplastic foam 102 comprises polyethylene terephthalate. According to some examples the thermoplastic polymer 136 comprises a polyester, optionally polyethylene terephthalate. In some examples, in each applicator head the reinforcement fibre and the molten thermoplastic polymer are extruded from a common outlet of the applicator head. This is schematically shown in Figure 4A, which shows a common outlet 152 of a respective applicator head. The molten thermoplastic polymer is schematically shown with shading at 154, and the reinforcement fibre is schematically shown in cross section at 134. In some examples, in each applicator head the reinforcement fibre and the molten thermoplastic polymer are extruded from respective different outlets of the applicator head. This is schematically shown in Figure 4B, which shows reinforcement fibre 134 being fed out from a first outlet 156 and molten thermoplastic polymer 154 being fed out from a second outlet 158 of a respective applicator head. In some examples, at the outer layer applying station 120 the applicator head 130 of the upper additive manufacturing machine 126 is suspended from, and movable in a horizontal plane relative to, an upper gantry 160 extending over the foam panel 102. In some examples, at the outer layer applying station 120 the applicator head 134 of the lower additive manufacturing machine 128 is mounted on, and movable in a horizontal plane relative to, a lower supporting frame 162 extending beneath the foam panel 102. In some examples, each of the top and bottom outer layers 122, 124 comprise 30 to 90 wt% of the reinforcement fibre and from 10 to 70 wt% of the thermoplastic polymer. In some examples, each of the top and bottom outer layers comprise a reinforcement fibre volume of 30 to 90% in the thermoplastic polymer matrix in which the reinforcement fibre is embedded. According to some examples the reinforcement fibre is circular in cross section. In some examples the method further comprises a step (i) which occurs after step (a) and before step (b), of trimming the top and bottom faces 108, 110 of the foam panel 102 at a trimming station 164. In some examples the trimming station 164 comprise an upper trimming machine 166 for trimming the top face 108 and a lower trimming machine 168 for trimming the lower face 110 of the foam panel 102. Trimming the foam panel 102 may help provide a consistent thickness of the foam panel 102. In some examples, positions of the upper and / or lower trimming machines 166 and 168 may be adjusted, to allow fine control over the thickness of the foam panel 102. In some examples the method further comprises a step (d) which occurs after step (c), of cutting the composite sandwich panel 170 with a cutting machine 172 into one or more composite sandwich panel sections 174, 176, 178. In some examples it may be considered that each sandwich panel section has a length Lsp and a width Wsp . In some examples, the method comprises controlling the cutting machine 172 to control the length Lsp and / or width Wsp . In some examples, the cutting machine 172 comprises two or more cutting units, which may be in close proximity to each other or may be spaced apart. For example, one cutting unit may be used to control length Lsp and another cutting unit may be used to control width Wspof each sandwich panel section 174, 176, 178. According to some examples the method comprises a step (e) which occurs after step (d), of supplying the one or more composite sandwich panel sections 174, 176, 178 to an assembly station 180 for assembling one or more parts of a wind turbine blade. By way of non-limiting example, shear webs for a wind turbine blade may be assembled at assembly station 180. That the composite sandwich panel 170 may be “continuously” or “continually” produced will be understood to mean that the composite sandwich panel can be produced to any length so long as the constituent raw materials are supplied and the machinery such as extruder 104 and the outer layer applying station 120 are operational. It will therefore be understood that the phrase “continuously produced” refers to an extending length of composite sandwich panel being produced as the foam panel 102 is extruded and the top and bottom outer layers 122, 124 are applied. The term “continuous” (or similar) or phrase “in a continuous manner” (or similar) does not preclude that the process of producing the extending length of composite sandwich panel can be started and stopped (for example for maintenance or at the start / end of a shift). In practice, the main restriction to the overall length of composite sandwich panel 170 that can be produced in the continuous manner is a length of the production facility. The examples described herein are to be understood as illustrative examples of embodiments of the invention. Further embodiments and examples are envisaged. Any feature described in relation to any one example or embodiment may be used alone or in combination with other features. In addition, any feature described in relation to any one example or embodiment may also be used in combination with one or more features of any other of the examples or embodiments, or any combination of any other of the examples or embodiments. Various modifications to the preferred embodiments of the present invention, as defined by the appended claims, will be apparent to those skilled in the art.

Claims

1. A method of producing a composite sandwich panel comprising the steps of:(a) providing an elongate thermoplastic foam panel, the foam panel5 having top and bottom faces;(b) applying to the top and bottom faces of the foam panel a respective top and bottom outer layer at an outer layer applying station, the top and bottom outer layers respectively covering the top and bottom faces of the foam panel so that the foam panel forms a foam core which is sandwiched between the top and bottom outer layers,10 wherein the outer layer applying station comprises an upper additive manufacturingmachine for applying the top outer layer by an upper additive manufacturing process and a lower additive manufacturing machine for applying the bottom outer layer by a lower additive manufacturing process, each of the upper and lower additive manufacturing machines having a respective applicator head which is configured to move in a respective x-y plane LO 15 which is respectively aligned with the top or bottom face of the foam panel, CMwherein a reinforcement fibre and a thermoplastic polymer are supplied to each of 1— the upper and lower additive manufacturing machines, and in each of the first and second additive manufacturing machines the thermoplastic polymer is heated to form molten thermoplastic polymer, and20 wherein the reinforcement fibre and the molten thermoplastic polymer aresimultaneously, and continuously or continually, extruded onto the respective top or bottom face from the respective applicator head which is moved in the respective x-y plane to apply the respective top and bottom outer layer over a surface area of the top and bottom faces respectively; and25 (c) allowing the thermoplastic polymer to cool and solidify, thereby forming athermoplastic polymer matrix in which the reinforcement fibre is embedded.

2. A method according to claim 1, wherein in the applying step (b) each applicator head is moved in the respective x-y plane in a preset scanning motion controlled by computer numerical control (CNC).30 3. A method according to claim 1 or claim 2, wherein in the applying step (b) thereinforcement fibre is applied as a monolayer of reinforcement fibres in each of the respective top and bottom outer layers.

4. A method according to claim 1 or claim 2, wherein in the applying step (b) thereinforcement fibre is applied as a stack comprising a plurality of plies of reinforcement fibres.

5. A method according to claim 4, wherein in each stack the plurality of plies of5 reinforcement fibres are applied by a common applicator head of the respective upper or lower additive manufacturing machine, which common applicator head is additionally moved in a z-direction orthogonal to the x-y plane thereby progressively to build up the stack of plies in each of the respective top and bottom outer layers.

6. A method according to claim 4, wherein in each stack the plurality of plies of10 reinforcement fibres are applied sequentially by a plurality of applicator heads of the respective upper or lower additive manufacturing machine thereby progressively to build up the stack of plies in each of the respective top and bottom outer layers.

7. A method according to any one of claims 4 to 6, wherein an orientation of the reinforcement fibres in each ply of the stack is different from the orientation of the15 reinforcement fibres in any ply adjacent thereto.LO£\j 8. A method according to any one of claims 4 to 7, wherein the upper additivemanufacturing machine and the lower additive manufacturing machine are configured to aPPly sequentially a first layer of the respective top and bottom outer layers, wherein in the first layer the reinforcement fibre material is applied diagonally across the foam panel at a20 first angle, and a second layer of the respective top and bottom outer layers over the first layer, wherein in the second layer the reinforcement fibre is applied diagonally across the foam panel at a second angle that intersects the first angle, so that together the first and second layers provide multi-axial top and bottom outer layers.

9. A method according to claim 8, wherein the first angle is approximately +45°25 or -45° to a longitudinal axis of the elongate thermoplastic foam panel, and the second angle is the other of approximately +45° or -45° to the longitudinal axis of the elongate thermoplastic foam panel.

10. A method according to any one of claims 1 to 9, wherein the reinforcement fibre is selected from glass fibre, carbon fibre, thermoplastic fibre, aramid fibre or natural30 fibre, or a mixture of any two or more thereof.

11. A method according to claim 10, wherein the reinforcement fibre comprises or consists of glass fibre.

12. A method according to any one of claims 1 to 11, wherein the reinforcementfibre is fed to the applicator head from a supply roll of the reinforcement fibre.

13. A method according to any one of claims 1 to 12, wherein the upper and lower additive manufacturing processes are fused filament fabrication processes and the5 thermoplastic polymer is supplied to the upper and lower additive manufacturing machines in the form of a thermoplastic polymer fibre.

14. A method according to claim 13, wherein the thermoplastic polymer fibre is fed to the applicator head from a supply roll of the thermoplastic polymer fibre.

15. A method according to any one of claims 1 to 12, wherein the upper and10 lower additive manufacturing processes are fused particle fabrication processes and the thermoplastic polymer is supplied to the upper and lower additive manufacturing machines in the form of particles of the thermoplastic polymer.

16. A method according to any one of claims 1 to 15, wherein the thermoplastic foam comprises polyethylene terephthalate.15 17. A method according to any one of claims 1 to 16, wherein the thermoplasticpolymer comprises a polyester.

18. A method according to any of claims 1 to 17, wherein in each applicator head the reinforcement fibre and the molten thermoplastic polymer are extruded from a common outlet of the applicator head.20 19. A method according to any of claims 1 to 17, wherein in each applicator headthe reinforcement fibre and the molten thermoplastic polymer are extruded from respective different outlets of the applicator head.

20. A method according to any of claims 1 to 19, wherein at the outer layer applying station the applicator head of the upper additive manufacturing machine is25 suspended from, and movable in a horizontal plane relative to, an upper gantry extending over the foam panel.

21. A method according to any of claims 1 to 20, wherein at the outer layer applying station the applicator head of the lower additive manufacturing machine is mounted on, and movable in a horizontal plane relative to, a lower supporting frame extending30 beneath the foam panel.

22. A method according to any of claims 1 to 21, wherein each of the top andbottom outer layers comprise 30 to 90 wt% of the reinforcement fibre and from 70 to 10wt% of the thermoplastic polymer.

23. A method according to any of claims 1 to 22, wherein each of the top and5 bottom outer layers comprise a reinforcement fibre volume of 30 to 90% in the thermoplastic polymer matrix in which the reinforcement fibre is embedded.

24. A method according to any of claims 1 to 23, wherein the method further comprises a step (i) which occurs after step (a) and before step (b), of trimming the top and bottom faces of the foam panel at a trimming station.10 25. A method according to any of claim 1 to 24, further comprising a step (d)which occurs after step (c), of cutting the composite sandwich panel with a cutting machine into one or more composite sandwich panel sections.

26. A method according to claim 25, further comprising a step (e) which occurs after step (d), of supplying the one or more composite sandwich panel sections to an 15 assembly station for assembling one or more parts of a wind turbine blade.

27. A method according to any of claims 1 to 26, wherein step (a) of providing the thermoplastic foam panel comprises extruding the foam panel from an extrusion die of an extruder, the foam panel being extruded continuously in a longitudinal direction away from the extrusion die.20 28. A method according to claim 27, wherein the outer layer applying station isdownstream of the extrusion die, and the method comprises applying the top and bottom outer layers whilst the foam panel is continuously extruded from the extrusion die.

29. A method according to any of claims 27 to 28, wherein step (c) occurs whilst the foam panel is continuously extruded from the extrusion die.25 30. A system for producing a composite sandwich panel comprising:a supply of elongate thermoplastic foam panel having a top face and a bottom face;an outer layer applying station for applying to the top and bottom faces of the foam panel a respective top and bottom outer layer, the top and bottom outer layers30 respectively covering the top and bottom faces of the foam panel so that the foam panel forms a foam core which is sandwiched between the top and bottom outer layers;wherein the outer layer applying station comprises an upper additive manufacturing machine for applying the top outer layer by an upper additive manufacturing process and a lower additive manufacturing machine for applying the bottom outer layer by a lower additive manufacturing process, each of the upper and lower additive manufacturing machines5 having a respective applicator head which is configured to move in a respective x-y plane which is respectively aligned with the top or bottom face of the foam panel;a supply of a reinforcement fibre and a supply of a thermoplastic polymer which are arranged to be supplied to each of the upper and lower additive manufacturing machines, and each of the first and second additive manufacturing machines are configured to be10 heated so that the thermoplastic polymer is heated to form molten thermoplastic polymer, andwherein the upper and lower additive manufacturing machines are configured so that the reinforcement fibre and the molten thermoplastic polymer are simultaneously, and continuously or continually, extruded onto the respective top or bottom face from the15 respective applicator head which is moved in the respective x-y plane to apply the respective LO top and bottom outer layer over a surface area of the top and bottom faces respectively.CM31. A system according to claim 30, wherein each applicator head is configured to be moved in the respective x-y plane in a preset scanning motion controlled by computer numerical control (CNC).20 32. A system according to claim 30 or claim 31, wherein the upper and loweradditive manufacturing machines are configured to apply the reinforcement fibre as a monolayer of reinforcement fibres in each of the respective top and bottom outer layers.

33. A system according to any of claims 30 to 31, the upper and lower additive manufacturing machines are configured to apply the reinforcement fibre as a stack25 comprising a plurality of plies of reinforcement fibres.

34. A system according to claim 33, wherein in each stack the plurality of plies of reinforcement fibres are applied by a common applicator head of the respective upper or lower additive manufacturing machine, which common applicator head is additionally configured to be moved in a z-direction orthogonal to the x-y plane thereby progressively to 30 build up the stack of plies in each of the respective top and bottom outer layers.

35. A system according to claim 33, wherein in each stack the plurality of plies of reinforcement fibres are applied sequentially by a plurality of applicator heads of the respective upper or lower additive manufacturing machine thereby progressively to build up the stack of plies in each of the respective top and bottom outer layers.

36. A system according to any one of claims 33 to 35, wherein the upper andlower additive manufacturing machines are configured to arrange an orientation of the reinforcement fibres in each ply of the stack different from the orientation of the reinforcement fibres in any ply adjacent thereto.5 37. A system according to any one of claims 33 to 36, wherein the upper additivemanufacturing machine and the lower additive manufacturing machine are configured to apply sequentially a first layer of the respective top and bottom outer layers, wherein in the first layer the reinforcement fibre material is applied diagonally across the foam panel at a first angle, and a second layer of the respective top and bottom outer layers over the first10 layer, wherein in the second layer the reinforcement fibre is applied diagonally across the foam panel at a second angle that intersects the first angle, so that together the first and second layers provide multi-axial top and bottom outer layers.

38. A system according to claim 37, wherein the first angle is approximately +45° or -45° to a longitudinal axis of the elongate thermoplastic foam panel, and the second angle 15 is the other of approximately +45° or -45° to the longitudinal axis of the elongateLO thermoplastic foam panel.CM39. A system according to any one of claims 30 to 38, wherein the reinforcement fibre is selected from glass fibre, carbon fibre, thermoplastic fibre, aramid fibre or natural fibre, or a mixture of any two or more thereof.20 40. A system according to claim 39, wherein the reinforcement fibre comprises orconsists of glass fibre.

41. A method according to any one of claims 30 to 40, wherein the reinforcement fibre is configured to be fed to the applicator head from a supply roll of the reinforcement fibre.25 42. A system according to any one of claims 30 to 41, wherein the upper andlower additive manufacturing processes are fused filament fabrication processes and the thermoplastic polymer is supplied to the upper and lower additive manufacturing machines in the form of a thermoplastic polymer fibre.

43. A system according to claim 42, wherein the thermoplastic polymer fibre is30 configured to be fed to the applicator head from a supply roll of the thermoplastic polymer fibre.

44. A system according to any one of claims 30 to 43, wherein the upper and lower additive manufacturing processes are fused particle fabrication processes and the14 446069GBthermoplastic polymer is configured to be supplied to the upper and lower additive manufacturing machines in the form of particles of the thermoplastic polymer.

45. A system according to any one of claims 30 to 44, wherein the thermoplastic foam comprises polyethylene terephthalate.5 46. A system according to any one of claims 30 to 45, wherein the thermoplasticpolymer comprises polyethylene terephthalate.

47. A system according to any of claims 30 to 46, wherein each applicator head comprises a common outlet for extrusion of the reinforcement fibre and the molten thermoplastic polymer.10 48. A system according to any of claims 30 to 46, wherein each applicator headcomprises respective different outlets for extrusion of the reinforcement fibre and the molten thermoplastic polymer.

49. A system according to any of claims 30 to 48, wherein at the outer layer applying station the applicator head of the upper additive manufacturing machine is15 suspended from, and movable in a horizontal plane relative to, an upper gantry extending over the foam panel.

50. A system according to any of claims 30 to 49, wherein at the outer layer applying station the applicator head of the lower additive manufacturing machine is mounted on, and movable in a horizontal plane relative to, a lower supporting frame extending20 beneath the foam panel.

51. A system according to any of claims 30 to 50, comprising a trimming station for trimming the top and bottom faces of the foam panel.

52. A system according to any of claim 30 to 51, comprising a cutting machine for cutting the composite sandwich panel into one or more composite sandwich panel sections.25 53. A system according to claim 52, further comprising an assembly station forreceiving the one or more composite sandwich panel sections, the assembly station for assembling one or more parts of a wind turbine blade.

54. A system according to any of claims 30 to 53, comprising an extruder for continuously extruding the foam panel from an extrusion die of the extruder, in a longitudinal 30 direction away from the extrusion die.

55. A system according to claim 54, wherein the outer layer applying station is downstream of the extrusion die, and the outer layer applying station is configured to applythe top and bottom outer layers whilst the foam panel is continuously extruded from the extrusion die.

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