Composite material sandwich structure and method for manufacturing the same

Abstract

A method for forming a composite material sandwich structure by compression molding is disclosed. The method includes placing a stack of two composite material skins (13a, 13b) separated by a non-adhesive layer (13c) into a compression molding tool (10); compression molding the stack of the composite material skins and the non-adhesive layer while applying heat to the tool, wherein the compression molding of the stack is performed until each composite material skin conforms and adheres to the adjacent molding surface, but the thermosetting resin in each composite material skin is not fully cured; opening the compression molding tool, wherein each composite material skin remains adhered to the adjacent molding surface; removing the non-adhesive layer from the compression molding tool; placing a core layer (14) into the compression molding tool; and compression molding until the core layer is joined to the composite material skins, thereby forming a molded sandwich structure (15).

Description

Technical Field

[0001] The present disclosure generally relates to composite material sandwich structures and methods of manufacturing the same.

Brief Description of the Drawings

[0002]

Figure 1-8

Figure 9

Figure 10

Modes for Carrying Out the Invention

[0003] Composite material sandwich structures having a low-density core sandwiched between two outer skins are used in applications where weight reduction is an important factor. Typically, the outer skins are thin sheets of fiber-reinforced composite materials. Such sandwich structures provide strength and rigidity while minimizing the weight of the structure. In certain applications such as aerospace and automotive composite parts, the outer skins typically include reinforcing fibers impregnated with a thermosetting resin. The thermosetting resin used for the thermosetting outer skin typically needs to be cured at a high temperature (e.g., 120°C to 180°C).

[0004] Conventionally, composite material sandwich structures containing thermosetting outer skins are consolidated and cured using an autoclave or vacuum bag only (VBO) process. To form a composite material without voids after curing, consolidation of the outer skin is required. An autoclave is a pressure vessel that can apply high temperature and pressure to form a cured thermoset composite material without voids, but it uses a large amount of energy, has high operating costs, and is limited in the size of the products to be processed. In the VBO process, a stack assembly of the outer skin and core material is placed on the tool surface and then surrounded by a gas-impermeable flexible membrane (referred to as a "vacuum bag"). The volume surrounded by the flexible membrane is evacuated to apply pressure while heating the stack assembly. The autoclave and vacuum bag only (VBO) processes require long cycle times of several hours and are not suitable for mass production.

[0005] Press forming or compression molding is a more rapid process for forming a sandwich structure. In a standard compression molding process, a stack assembly of the outer skin and core material is placed within a mold cavity defined by at least two movable mold parts, and the stack assembly is compression molded by the mold parts. In the case of a stack assembly having a thermosetting outer skin, pressure and heat are applied during compression molding to enable consolidation and curing of the outer skin in addition to sufficient bonding of the outer skin to the core. However, in such compression molding, very high pressures are used to enable processing of the sandwich structure in a short time. This required high pressure tends to crush or collapse core materials formed from thermoplastic foams or honeycomb structures. Although the compression pressure can be reduced to prevent crushing or collapse of the core, if the compression pressure is low, consolidation of the skin becomes insufficient. If an adhesive film is applied between the skin and the core material, insufficient consolidation can cause the skin resin to penetrate the adhesive film, potentially weakening the bond line at the interface between the skin and the core material. In the case of a thermoplastic foam core, collapse can occur when high temperatures are applied to the mold. Although collapse of the thermoplastic foam core can be reduced by lowering the temperature applied during molding, a low temperature results in a longer cycle time. As a solution to core crushing or collapse, cores with a higher density or reinforced cores with a higher compressive strength can be used, but using such cores forces a compromise in terms of higher cost and weight penalty.

[0006] In this specification, a method for forming a thermosetting composite material sandwich structure at low cost with a relatively short cycle time (e.g., less than 1 hour) by compression molding (hereinafter referred to as the "sandwich press forming" method) is disclosed without disintegrating or crushing the core material. Further, such a sandwich press forming method does not require the use of a high-density core or a reinforced core with high compressive strength that is costly and / or increases the weight of the sandwich structure. Other advantages of the sandwich press forming method include optimal skin consolidation and an optimal bond line interface between the skin and the core material.

[0007] Generally, the sandwich press forming method of the present disclosure includes placing a stack of two thermosetting composite material skins separated by a deformable non-adhesive layer into a compression molding tool; applying a compression pressure and heat to the stack for a time sufficient to mold and partially cure the skins; removing the non-adhesive layer from the tool while keeping the skins adhered to the forming surface of the tool; placing a core layer into the tool such that the core layer comes between the forming surfaces to which the skins are adhered; applying a compression pressure and heat to the core layer and the skins in the tool to affect (or cause) the molding of the core layer and the bonding of the skins to the core. The thermosetting composite material skins are composed of reinforcing fibers impregnated with or injected with a curable thermosetting resin composition. A curable adhesive film may be applied to the surface of the core layer that contacts each skin in the tool before the compression molding of the core layer. The curable adhesive film is formed from a curable resin containing one or more thermosetting resins and a curing agent. During the second compression molding in which the core layer is compression molded, the skins are fully cured when compression pressure and heat are applied. The entire sandwich press forming method can be performed in less than 1 hour and, in some embodiments, can be performed in less than 30 minutes.

[0008] Figures 1 - 8 show various stages of the sandwich press forming method according to an embodiment of the present disclosure.

[0009] Referring to FIG. 1, a compression molding tool 10 having at least a first (upper) mold part 11 and a second (lower) mold part 12 is provided. As shown in the figure, the molding tool is in the open position. The first and second mold parts 11 and 12 are movable relative to each other and have opposing molding surfaces 11a and 12a that cooperate to define a mold gap or a mold cavity when the compression molding tool is in the closed position. The mold gap or the mold cavity is in the shape of the final structure to be molded.

[0010] Referring to FIG. 2, the compression molding tool is in the open position and the upper mold part 11 is separated from the lower mold part 12a. A stack 13 of two thermosetting composite material skins (13a, 13b) and a deformable non - adherent layer (13c) is disposed on the lower mold part 12. Each thermosetting composite material skin contains reinforcing fibers impregnated with or injected with a curable thermosetting resin composition. At this stage, since the resin composition in the composite material skin has not cured, the composite material skin is very sticky and flexible. The non - adherent layer (13c) is disposed between the composite material skins (13a, 13b) in the stack 13 to prevent the skins from sticking to each other.

[0011] Referring to FIG. 3, the compression molding tool is closed by moving the upper mold part 11 towards the lower mold part 12, and the stack 13 of the composite material skin and the non - adherent layer is compressed between the molding surfaces of the upper mold part and the lower mold part. During compression molding, sufficient time, compression pressure, and heat are applied for each composite material skin to conform and adhere to the adjacent molding surfaces, but the curable resin composition in the composite material skin does not fully cure. At this stage, the composite material skin has undergone minimal consolidation, and the resin composition therein is only partially cured. The expression "partially cured" refers to a material state where the degree of cure is less than 100% but greater than 0%. As an example, the degree of cure of the partially cured resin in the composite material skin may range from 30% to 90%. In some embodiments, the degree of cure of the partially cured resin ranges from 60% to 65%. The compression pressure applied during compression molding may range from 1,000 kPa to 10,000 kPa. The temperature applied during compression molding may range from 120 °C to 200 °C. The time for compression molding may be less than 20 minutes. In one embodiment, compression molding is performed at 180 °C for 7 to 10 minutes.

[0012] The degree of cure of the thermosetting resin can be determined by differential scanning calorimetry (DSC). The thermosetting resin composition undergoes an irreversible chemical reaction during curing. When the components in the resin composition cure, heat is generated by the resin and is monitored by the DSC apparatus. The heat of cure can be used to determine the percent cure rate of the resin. As an example, the following simple calculation: Cure % = [ΔH 硬化していない - ΔH 硬化した / [ΔH 硬化していない × 100% can be used to obtain the degree of cure.

[0013] Referring to FIG. 4, the compression molding tool is opened by moving the upper mold part 11 away from the lower mold part 12, whereby the non-adhesive layer 13c can be removed from the tool while the composite skin 13a remains attached to the molding surface of the upper mold part 11 and the composite skin 13b remains attached to the molding surface of the lower mold part 12.

[0014] Referring to FIG. 5, while the tool is in the open position, the core layer 14 is placed on the lower mold part 12. Optionally, the core layer 14 has a first adhesive film applied to the first surface and a second adhesive film applied to the opposite surface. If present, one of the adhesive films contacts the composite skin 13b on the lower mold part 12 when the core layer 14 is placed thereon, and the other adhesive film contacts the composite skin 13a on the upper mold part 11 when the tool is closed. The core layer 14 may be a flat (or substantially flat) layer that can be molded into the desired three-dimensional (3D) configuration, or a pre-molded layer having the final desired 3D configuration.

[0015] Referring to FIG. 6, the compression molding tool is closed and the second compression molding is performed, causing the core layer 14 to be slightly compressed. If the core layer is initially flat, performing the second compression molding also causes the shape of the core layer 14 to conform to the shape of the cavity of the mold. Compression pressure and heat are applied during the second compression molding for a time sufficient to cause bonding of the core layer (14) to the composite skins (13a, 13b) and complete curing of the resin composition in each composite skin. The compression pressure applied at this stage (FIG. 6) is less than the compression pressure applied during the first compression molding of the stack (13) of the composite skin and the non-adhesive layer (FIG. 3). The compression pressure applied at this stage may be between 100 kPa and 1,000 kPa. The temperature applied during the second compression molding may be in the range of 120°C to 200°C. The time for the second compression molding may be less than 20 minutes. In one embodiment, the second compression molding is performed at 180°C for 8 to 10 minutes.

[0016] Referring to FIG. 7, the compression molding tool is open and the molded sandwich structure 15 is obtained. In FIG. 8, the molded sandwich structure 15 is removed from the compression molding tool.

[0017] Composite material skin As described above, each thermosetting composite material skin includes reinforcing fibers impregnated or injected with a curable thermosetting resin. As used herein, the term "impregnating" refers to the introduction of a curable resin into the reinforcing fibers to partially or completely encapsulate the fibers with the resin.

[0018] In one embodiment, the composite material skin is a sheet of composite material including a layer of reinforcing fibers embedded in a layer of curable matrix resin. As used herein, the term "matrix resin" refers to a mass of resin, and the expression "embedded in the matrix resin" means firmly fixed or disposed within the surrounding mass of resin.

[0019] The layer of reinforcing fibers may be in the form of continuous fibers aligned in one direction or a fabric. The sheet of composite material is also referred to as a ply of "prepreg" in the present disclosure. The skin may be a laminate of a plurality of plies of prepreg, also referred to as a "prepreg layup".

[0020] A curable thermosetting resin composition for impregnating / injecting into reinforcing fibers is a solidifiable or thermosettable resin containing one or more uncured thermosetting resins, including, but not limited to, epoxy resins, bismaleimides, vinyl ester resins, cyanate ester resins, isocyanate-modified epoxy resins, phenolic resins, furan resins, benzoxazines, formaldehyde condensation resins (such as those using urea, melamine, or phenol), polyesters, acrylates, their hybrids, blends, and combinations. When thermoset by applying heat, the thermosetting resin composition crosslinks and cures irreversibly, resulting in a cured material that cannot be reshaped by thermoforming (a process that includes heating the material and then shaping the heated material to obtain a desired shaped object).

[0021] As used herein, the terms "curing" or "cure" mean the solidification of a polymeric material by chemical crosslinking of polymer chains. The term "curable" with respect to a curable composition means that the composition can be subjected to conditions that will solidify or thermoset the composition.

[0022] In a preferred embodiment, the thermosetting resin composition contains one or more epoxy resins and one or more curing agents and / or catalysts.

[0023] Suitable epoxy resins include polyglycidyl derivatives of aromatic diamines, aromatic mono-primary amines, aminophenols, polyhydric phenols, polyhydric alcohols, and polycarboxylic acids. Examples of suitable epoxy resins include polyglycidyl ethers of bisphenols such as bisphenol A, bisphenol F, bisphenol S, and bisphenol K, as well as polyglycidyl ethers of cresol and phenolic novolacs.

[0024] Specific examples are tetraglycidyl derivatives of 4,4'-diaminodiphenylmethane (TGDDM), resorcinol diglycidyl ether, triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, bromobisphenol F diglycidyl ether, tetraglycidyl derivatives of diaminodiphenylmethane, triglycidyl ether of trihydroxyphenylmethane, polyglycidyl ether of phenol-formaldehyde novolak, polyglycidyl ether of o-cresol novolak or tetraglycidyl ether of tetraphenylethane.

[0025] Commercially available epoxy resins suitable for use in the base matrix resin include N,N,N’,N’-tetraglycidyldiaminodiphenylmethane (e.g., MY9663, MY720, and MY721 manufactured by Huntsman); N,N,N’,N’-tetraglycidyl-bis(4-aminophenyl)-1,4-diisopropylbenzene (e.g., EPON1071 manufactured by Momentive); N,N,N’,N’-tetraclycidyl-bis(4-amino-3,5-dimethylphenyl)-1,4-diisopropylbenzene (e.g., EPON1072 manufactured by Momentive); triglycidyl ether of p-aminophenol (e.g., MY0510 manufactured by Huntsman); triglycidyl ether of m-aminophenol (e.g., MY0610 manufactured by Huntsman); diglycidyl ethers of bisphenol A-based materials, such as 2,2-bis(4,4’-dihydroxyphenyl)propane (e.g., DER661 manufactured by Dow, or EPON828 manufactured by Momentive, and preferably novolak resins with a viscosity of 8 - 20 Pa·s at 25°C; glycidyl ethers of phenol novolak resins (e.g., DEN431 or DEN438 manufactured by Dow); dicyclopentadiene-based phenol novolak (e.g., Tactix556 manufactured by Huntsman); diglycidyl 1,2-phthalate (e.g., GLY CELA-100); diglycidyl derivatives of dihydroxydiphenylmethane (bisphenol F) (e.g., PY306 manufactured by Huntsman). Other epoxy resins include alicyclic compounds such as 3’,4’-epoxycyclohexyl-3,4-epoxycyclohexanecarboxylate (e.g., CY179 manufactured by Huntsman).

[0026] The addition of a curing agent and / or a catalyst to the curable matrix resin is optional, but the use of such can, if necessary, increase the curing rate and / or lower the curing temperature. The curing agent is suitably selected from known curing agents such as aromatic or aliphatic amines, or guanidine derivatives. Aromatic amine curing agents are preferred, preferably aromatic amines having at least two amino groups per molecule, for example, diamino diphenyl sulfone in which the amino group is in the meta or para position to the sulfone group is particularly preferred. Specific examples are 3,3'- and 4,4'-diaminodiphenyl sulfone (DDS); methylene dianiline; bis(4-amino-3,5-dimethylphenyl)-1,4-diisopropylbenzene; bis(4-aminophenyl)-1,4-diisopropylbenzene; 4,4'methylene bis-(2,6-diethyl)-aniline (MDEA manufactured by Lonza); 4,4'methylene bis-(3-chloro,2,6-diethyl)-aniline (MCDEA manufactured by Lonza); 4,4'methylene bis-(2,6-diisopropyl)-aniline (M-DIPA manufactured by Lonza); 3,5-diethyltoluene-2,4 / 2,6-diamine (D-ETDA80 manufactured by Lonza); 4,4'methylene bis-(2-isopropyl-6-methyl)-aniline (M-MIPA manufactured by Lonza); 4-chlorophenyl-N,N-dimethyl-urea (for example, Monuron); 3,4-dichlorophenyl-N,N-dimethyl-urea (for example, DIURON TM) and dicyandiamide (for example, AMICURE TM CG 1200 manufactured by Pacific Anchor Chemical).

[0027] Suitable curing agents also include acid anhydrides, especially polycarboxylic acid anhydrides such as nadic anhydride, methyl nadic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, endomethylenetetrahydrophthalic anhydride, and trimellitic anhydride.

[0028] The curable thermosetting resin composition can contain comonomers, rheology control agents, tackifiers, inorganic or organic fillers, thermoplastic and / or elastic polymers as reinforcing agents, stabilizers, inhibitors, pigments, dyes, flame retardants, reactive diluents, and other additives such as other additives well-known to those skilled in the art for modifying the properties of the matrix resin before or after curing.

[0029] Suitable reinforcing agents for the curable resin composition include, but are not limited to, polyamides, copolyamides, polyimides, aramids, polyketones, polyetherimides (PEI), polyetherketones (PEK), polyetherketoneketones (PEKK), polyetheretherketones (PEEK), polyethersulfones (PES), polyetherethersulfones (PEES), polyesters, polyurethanes, polysulfones, polysulfides, polyphenylene oxides (PPO) and modified PPO, poly(ethylene oxide) (PEO) and polypropylene oxide, polystyrene, polybutadiene, polyacrylate, polymethacrylate, polyacryl, polyphenylsulfone, high-performance hydrocarbon polymers, liquid crystal polymers, elastomers, and homopolymers or copolymers of any of these alone or in combination. When present, the total amount of the reinforcing agent in the resin composition is less than 25% (weight percent) based on the total weight of the resin composition.

[0030] Core material The core material can be any low-density material including a foam core and a honeycomb structure. As used herein, the term "foam" refers to a solidified material in which a large proportion of air bubbles are dispersed. Usually, a honeycomb structure has a plurality of open cells defined by cell walls. A suitable honeycomb structure is a thermoplastic honeycomb.

[0031] The foam core is 20 - 1000 kg / m 3 30 - 800 kg / m 3 35 - 500 kg / m 3 40 - 300 kg / m 3 or 45 - 200 kg / m3 may have a density. The density can be measured in accordance with ASTM D1622.

[0032] The foam core can be formed from a foamable composition comprising one or more polymers selected from thermoplastic polymers and synthetic polymers. By way of example, the foam core can be formed from a foamable composition comprising at least one polymer selected from poly(aryl ether sulfone) (PAES), particularly polyether sulfone (PES), polyether ether sulfone (PEES), poly(biphenyl ether sulfone) (PPSU), polyamide (PA), polyimide (PI), polyetherimide (PEI), polyvinyl chloride (PVC), polystyrene (PS), polyurethane (PU), polymethacrylamide, styrene acrylonitrile, and copolymers thereof. Generally, PAES polymers having a Tg in the range of 201 °C to 290 °C are suitable for the purposes disclosed herein. In some embodiments, the foam core is formed from a foamable composition comprising a combination of different thermoplastic polymers.

[0033] Non - adherent layer The non - adherent layer is formed from any material that does not form a permanent chemical bond with the curable matrix resin in the composite material skin. In one embodiment, the non - adherent layer is a thermally deformable layer. By way of example, the non - adherent layer is formed from a thermally deformable material selected from silicone, rubber, hydrophobic fluoropolymers (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), fluorinated ethylene - propylene (FEP), ethylene tetrafluoroethylene (ETFE), ethylene - chlorotrifluoro - ethylene (ECTFE), perfluoropolyether (PFPE), etc.), and combinations thereof. Alternatively, the non - adherent layer contains a rigid material and is pre - formed to have the shape or 3D configuration of the core layer of the ultimately formed sandwich structure. For example, the non - adherent layer may be composed of a metal layer or a composite material layer whose two opposing surfaces are coated with a polymer such as PTFE. The composite material layer may be a pre - cured composite material or prepreg composed of cured thermosetting resin, for example, fibers embedded in cured epoxy, such as carbon fibers. Such a composite material layer has a low coefficient of expansion so that it does not expand during compression molding.

[0034] For the purposes disclosed herein, the non - adherent layer can have a thickness greater than 0 and up to 20 mm.

Examples

[0035] Example 1 Conventional method A multi-layer assembly with a configuration where a honeycomb core (C1-32-48 from Euro-Composites) with a thickness of 12.6 mm is sandwiched between two composite material skins was formed, and a control panel (Panel #1) was prepared by placing this multi-layer assembly into a compression molding tool with a two-part configuration. The tool included two movable molding parts made of steel. Each composite material skin was composed of two prepreg plies, and each prepreg ply was composed of unidirectional carbon fibers impregnated with Solvay's CYCOM® EP2750 (epoxy resin). The prepreg plies of each skin were stacked in a [0 / 90] configuration. The honeycomb core was coated on each surface in contact with the skin with Solvay's FM® 209-1 adhesive film. The compression molding of the multi-layer assembly in the tool was carried out at 3 bar and 180 °C for 20 minutes to obtain a cured sandwich structure. The compression molding of Panel #1 is a representative example of the conventional method.

[0036] Based on visual observation, the cured Panel #1 contained macrovoids on the outer surface of the skin. Based on microscopic observation of the thickness of Panel #1 (Figure 9), the following defects were found: voids in the skin, fiber distortion, and separation between the skin and the core due to resin penetration into the honeycomb core. The lower image in Figure 9 is a decomposition diagram of the skin in the upper image.

[0037] To determine the adhesive failure mode of the cured Panel #1, a tensile test in the plane direction was carried out. The test results showed adhesive failure at the adhesive interface between the skin and the honeycomb core.

[0038] Example 2 Method using a non-adhesive layer A multilayer assembly configured with a non-adhesive layer sandwiched between two composite material skins was placed within the two-part compression molding tool described in Example 1. Each composite material skin was composed of two prepreg plies as described in Example 1 for Panel #1. The non-adhesive layer was a press-formed and pre-cured prepreg with PTFE film coated on each opposing face. The pre-cured prepreg was formed by molding and curing a prepreg of the same composition as the uncured prepreg ply of the skin.

[0039] The first compression molding of the multilayer assembly with the non-adhesive layer was carried out at 30 bar and 180 °C for 11 minutes. The tool was opened, the non-adhesive layer was removed from the tool, and the skins were left adhered to the surface of the tool. A honeycomb core (the same as described in Example 1) was placed within the tool. The second compression molding was carried out at 3 bar and 180 °C for 9 minutes to obtain a cured sandwich structure (Panel #2). The total processing time was 20 minutes.

[0040] Based on visual observation, the outer surface of the skin of the cured Panel #2 did not contain voids. Based on microscopic observation of the thickness of Panel #2 (Figure 10), no voids were shown in the skin of the panel, there was no fiber distortion, and a clear separation was seen between the skin and the core. The lower image in Figure 10 is a decomposition view of the skin in the upper image.

[0041] To determine the adhesive failure mode of the cured Panel #2, a planar tensile test was carried out. The results did not show adhesive failure at the adhesive interface between the skin and the honeycomb core.

Claims

1. A method for forming a composite material sandwich structure by compression molding, (a) to prepare a compression molding tool having at least a first mold portion and a second mold portion, wherein the first and second mold portions have opposing molding surfaces that cooperate to define a gap or cavity in the mold when the compression molding tool is in a closed position; (b) Placing a stack of two composite material skins separated by a non-adhesive layer on the molding surface of the first or second mold portion while the tool is in the open position, wherein each composite material skin includes reinforcing fibers impregnated with or injected with a curable thermosetting resin; (c) Compressing the stack of composite material skin and non-adhesive layer between the molding surfaces of the first and second mold portions while closing the compression molding tool and applying heat to the tool, the compression of the stack until each composite material skin adheres to the adjacent molding surface in a matching shape, but the thermosetting resin in the composite material skin is not completely cured; (d) Opening the compression molding tool such that each composite material skin remains attached to the adjacent molding surface; (e) Removing the non-adhesive layer from the compression molding tool; (f) Positioning the core layer within the compression molding tool while the tool is in the open position such that the core layer rests on one of the composite material skins attached to the molding surface; (g) Compressing the core layer between the first and second mold portions by closing the compression molding tool while applying heat to the tool, the compression of the core layer is carried out until the core layer is bonded to the composite material skin and the thermosetting resin of each composite material skin is completely cured, thereby forming a molded sandwich structure; (h) Opening the compression molding tool; and (i) Removing the molded sandwich structure from the compression molding tool. A method that includes this.

2. The core layer has two opposing surfaces and an adhesive film applied to each of the opposing surfaces. The method according to claim 1, wherein, in (f), when the core layer is placed in the compression molding tool, one of the adhesive films faces one of the composite material skins.

3. The curable thermosetting resin in each composite material skin comprises one or more thermosetting resins and an optional curing agent. Preferably, the method according to claim 1, wherein the thermosetting resin is selected from epoxy resins and the curing agent is an amine-containing compound.

4. The method according to claim 1, wherein the compression of the stack of composite material skin and non-adherent layer in (c) is performed at a compression pressure in the range of 1,000 kPa to 10,000 kPa for a period of less than 20 minutes while applying heat at a temperature in the range of 120°C to 200°C.

5. The method according to claim 1, wherein the heating in (c) is performed to achieve a partially cured composite material skin having a degree of curing greater than 0% but less than 100%, preferably in the range of 30% to 90%, and in some embodiments, 60% to 65%.

6. The method according to claim 1, wherein the compression of the core layer in (g) is performed for less than 20 minutes at a compression pressure lower than the compression pressure in (c), preferably in the range of 100 kPa to 1,000 kPa, while applying heat at a temperature in the range of 120°C to 200°C.

7. The method according to claim 1, wherein the non-adherent layer is formed from a material that does not form permanent chemical bonds with the curable matrix resin in the composite material skin, and is a layer that can be peeled off from the composite material skin.

8. The method according to claim 1, wherein the non-adherent layer is formed from a heat-deformable material selected from silicone, rubber, hydrophobic fluoropolymers (such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), fluorinated ethylene-propylene (FEP), polyethylene tetrafluoroethylene (ETFE), polyethylene-chlorotrifluoroethylene (ECTFE), perfluoropolyether (PFPE), and combinations thereof).

9. The method according to claim 1, wherein the non-adherent layer is composed of a layer of metal or composite material in which two opposing surfaces are coated with a polymer.

10. The method according to claim 1, wherein the thickness of the non-adhesive layer is greater than 0 mm and up to 20 mm.

11. The method according to claim 1, wherein the core layer includes a foamed material or a honeycomb structure.

12. The method according to claim 1, wherein the core layer is a foamed material formed from a foamed composition containing one or more polymers selected from thermoplastic polymers and synthetic polymers.

13. The method according to claim 1, wherein the core layer is formed from a foaming composition containing one or more thermoplastic polymers.

14. The method according to claim 1, wherein the core layer is a thermoplastic honeycomb structure.

15. The method according to claim 1, wherein each composite material skin is a prepreg ply or a multilayer laminate comprising two or more prepreg prisms, and each prepreg ply comprises reinforcing fibers embedded in a layer of curable matrix resin.

16. The method according to claim 15, wherein the reinforcing fibers in each prepreg ply are in the form of continuous unidirectional fibers or woven fabric.

17. The method according to claim 15, wherein the reinforcing fibers in each prepreg ply are carbon fibers.

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