Method of manufacturing a thermoplastic composite structure

Abstract

A method of manufacturing for forming a composite structure comprising a thermoplastic reinforced material in a closed press form (100), the method comprising: • - forming at least one first shell part (30.1) having a first intermediate shape; • - forming at least one second shell part (30.2) having a second intermediate shape; • - fusing the at least first and second shell parts together by using heat and pressure to form a hollow part having a final shape (300); wherein the final shape (300) is different from the respective intermediate shapes.

Description

[0001] THERMOPLASTIC COMPOSITE STRUCTURE

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a method of manufacturing a composite structure in particular a hollow composite structure.

[0004] BACKGROUND OF THE INVENTION

[0005] Hollow composite structures can be found various fields of technologies and / or industries and may be e.g. a bicycle component or a bicycle frame component or a lightweight sports product such as a tennis racket or a stick for hockey, baseball, cricket, etc. Also vanes of a rotor or a stator are increasingly being made as hollow composite structures, as well as a helicopter blade or a wind turbine blade. Many composite structures may be possible, and over industries and / or fields of technology there is a challenge on how to produce such hollow composite structure efficiently, effectively and possibly in an automated or semi-automated manner.

[0006] Multiple industries, such as mobility are looking into composite materials to reduce weight and / or to improve versatility in manufacturing. To reduce the weight of a component companies are considering to replace heavy substrates such as metallic substrates, for example aluminum or steel substrates with more lightweight alternatives while maintaining the mechanical properties associated with metallic substrates. Typical examples are composite based substrates. Accordingly, multiple thermoset and / or thermoplastic polymer matrices in combination with reinforcement fibers have been investigated and evaluated. In particular, combinations of a thermoset polymer matrix or a thermoplastic polymer matrix with carbon fibers, glass fibers, aramid fibers, polyethylene fibers, or any other polymer based reinforcement fibers have been considered. The use of a thermoset as polymer matrix has the advantage of providing a rehable composite structure, however it has the disadvantage of an expensive and a difficult to automate manufacturing process. Most of the manufacturing steps, such as laying of the thermoset material in the mold, is done manually.

[0007] At present, thermoplastic fiber reinforced composite material has the disadvantage of limiting the form freedom for example during thermoforming of a composite structure or otherwise forming of the composite structure. In particular, the lack of stretching capabilities of the reinforcement fibers may give limitations to the designers of composite structures. Closed form shapes like for example a triangular structure or a torus shape may impose difficulties for the designer when using the thermoplastic fiber reinforced material as to allow sufficient material available to provide for the shape. Furthermore, it appears to be a challenge to provide in thermoplastic fiber reinforced material regions with a small radius. Therefore, design engineers are forced to adapt their design as to the limitations of the thermoplastic material.

[0008] This may, in particular, pose limitations for designing lightweight products having high structural requirements, such as aerospace, automotive and sports articles, or bicycle parts or bicycle frames. Due to the limitations of the thermoplastic polymer material, variations in the shape of e.g. a bicycle frame component while maintaining structural integrity and / or mechanical properties are thus limited. This negatively affects the design choices of the design engineers.

[0009] Consequently, designers or engineers at present tend to avoid or exclude closed form shapes and if they use them, they tend to design them such that the aforementioned difficulties can be avoided. The present invention is aimed at an improved manufacturing method for a thermoplastic composite structure obviating at least one of the above mentioned problems. In particular, the invention is aimed at an improved manufacturing method for a thermoplastic composite structure, in particular a hollow thermoplastic composite structure, allowing an increased form freedom while preserving mechanical properties associated with applications of the composite structure.

[0010] SUMMARY

[0011] The present invention is aimed at expanding the form freedom of a thermoplastic composite structure such as a bicycle frame component or a sports product etc. Typical examples of restrictions in form freedom may refer to avoiding complex geometries such as small inner radii or multiclosed form composite structures. For example, a composite structure such as a bicycle frame may form a so-called closed structure in that the composite structure has an enclosure, or otherwise said, encloses an opening. In side view, the composite structure may be for example a more or less triangular shaped structure. The composite structure may also be a multi-closed form structure in that the enclosure itself is divided in multiple enclosures. In side view, such structure may thus have a multi-closed form.

[0012] The present invention is directed to a method of manufacturing a composite structure such as a bicycle frame component. Other complex geometry structures can also be envisaged and may become possible with the method according to the invention, considering these structures have a complex geometry, such as comprising an enclosure as in a polygonal shaped structure or a torus shaped structure. The composite structure comprises at least one thermoplastic reinforced material. Manufacturing the composite structure comprises providing at least one first shell part having a first intermediate shape in a first final mold half; providing at least one second shell part having a second intermediate shape in a second final mold half; closing the first final mold half and the second final mold half forming a final mold; fusing the at least one first and second shell parts together in the final mold by using heat and pressure to form the composite structure as a final hollow part having a final shape

[0013] The final shape is different from the respective intermediate shapes. In particular, the final shape has a different geometry versus the intermediate shape. Examples of such a different geometry may for example be: an enclosed angle which is in the final shape smaller than in the intermediate shape, or a radius which is in the final shape smaller than in the intermediate shape so that a relative sharp corner can be formed, or a cross-sectional shape which is more round in the final shape than in the intermediate shape. The final geometry may typically be not possible to form in a single deepforming step due to e.g. lack of matrix material in some regions and / or lack of fibers in some regions. Therefore, in industry, designers and / or manufacturers adapt the geometry of the composite structure such that any forming problems related to a specific geometry can be avoided. This certainly limits the design freedom for hollow composite structure. By providing such a two-step forming method according to the invention, a more complex geometry can be provided for the hollow composite structure. Also complex geometries comprising e.g. an undercut may become available, those may be formed in the fusing step for the final shape. A shape having an undercut cannot be formed in a molding or a forming process or any other process using a mandrel or a stamp. In particular a local radius and / or an enclosed radius of at least one of the intermediate shapes is larger than a respective associated local radius and / or enclosed radius of the associated final shape, wherein the local radius is defined as, in cross-section, a radius of curvature of a section of the cross-section of the shell part or the final hollow part, wherein the enclosed radius is defined as a radius, in side view, of an enclosed angular section between two adjacent legs of the shell part or the final hollow part. So, typically a radius of the final hollow part having the final shape can be relatively small, such radius either be seen in cross-section as the local radius or in side view, or longitudinal section, as the enclosed radius of the final shape. By having now the possibility to manufacture a hollow composite structure with a small radius, complex geometries, such as a triangular frame with a sharp enclosed angle or rather sharp edge along the circumference may become possible. Before, designers avoided such complex, difficult geometries which may impair the quality and / or versatility in design of hollow composite structures of thermoplastic material.

[0014] The thermoplastic reinforced material may be reinforced using fibers. The fibers may e.g. be of carbon or steel, or any other reinforcement material. The fibers may be oriented unidirectional in the thermoplastic material, may be weaved or braided fibers or may be oriented randomly in the thermoplastic material. The fibers may be long fibers, or may be short fibers. The thermoplastic material may even be made using continuous fibers, e.g. in a tape on a drum. Then the material may be cut at the required length. The thermoplastic material may for example be provided as a tape having unidirectional fibers, or as a sheet having unidirectional and / or woven and / or braided and / or randomly oriented and / or multidirectional fibers. Also, a mix of fiber orientations may be provided in the thermoplastic material, e.g. regions with unidirectional fibers, while other regions may have randomly oriented fibers.

[0015] By providing different intermediate shapes of the at least one first and second shell part versus the final shape has the advantage that materials like for example a thermoplastic polymer matrix and / or the reinforcement fibers may be guided towards areas that require more material to form the final shape. Consequently the form freedom of the composite structure may be increased. By manufacturing the composite structure using such a two-step process has the advantage that the process may specifically be arranged to guide the required material, e.g. matrix material and / or reinforcement fibers, to the regions where they are needed most, e.g. in regions with a complex geometry such as a small radius, in cross or longitudinal section, which would otherwise not be within reach.

[0016] More specifically, such two-step manufacturing process can be advantageous when a ratio of the local radius of the final hollow part versus a depth of the associated final mold half, measured from a section line of the final mold, at the position of the local radius is smaller than 0,25. Such a ratio can be present when a cross-section of the final hollow part may have a complex circumference, e.g. a circumference with a protrusion, or a relatively sharp edge etc. By providing the two-step manufacturing method for a hollow part with a complex geometry, also automation of the manufacturing process may be possible to a great extent.

[0017] Additionally and / or alternatively, such two-step manufacturing process can be advantageous when a ratio of the enclosed radius of the final hollow part versus a circumference of one of the adjacent legs of the final hollow part is smaller than 0,15. Then, for example sharp angles between branches of a triangular structure surrounding an enclosure can become possible without impairing the strength and / or stiffness of the structure. This may greatly improve the design options for the designer, and, as such, more freedom of form can be obtained.

[0018] The inventors found that geometries with the aforementioned ratios could not be manufactured with the conventional manufacturing techniques, while more and more such complex geometries were solicited for. The inventors found that with the proposed two-step manufacturing method such complex geometries became possible to produce, also in a manner that allowed at least some automation.

[0019] The method may be preceded by trimming the at least one first and / or second shell parts.

[0020] Trimming the shell parts may be necessary to remove excess material and may facilitate the upcoming processing steps to manufacture a composite structure by fusing the shell parts. For example, outer edges of the shell parts may be trimmed to improve joining and / or fusing of the shell parts at the outer edges. For example, excess material may be cut off as to form a more neat outer edge. The shell parts may be placed butt-to-butt in the final mold, or may be placed partially overlapping in the final mold. By trimming the shell parts before placing them in the final mold, a more neat fusing can be obtained.

[0021] The shell parts have an intermediate shape which is understood to be a three-dimensional shape, as opposed to a two-dimensional shape of a tape or a blank. A sheet, a tape or a blank having a certain thickness is understood to be a two-dimensional shape, extending in a plane. As opposed to such a two-dimensional shape, a three-dimensional shape is understood to also extend in a third dimension with a certain value, beyond the thickness of such two-dimensional part. The three-dimensional shape can have a substantially U-shaped cross-section, allowing the shell parts to be placed in their respective final mold halves. When then closing the final mold halves to form the final mold, the shell parts may abut or may partially overlap forming a hollow interior between them.

[0022] The method may further comprise aligning the at least one first shell part and the at least one second shell part such that the at least one first shell part may overlap with the at least one second shell part. An amount of overlap may be for example in the range of 2 mm - 100 mm, preferably in the range of 5 mm - 50 mm. Alternatively and / or additionally, along some parts of the outer edges, or along the outer edges, the outer edges of the first and the second shell part may be abutting each other, instead of overlapping. By providing abutment of the shell parts, a wall thickness of a joined section may remain approximately the same as the wall thickness of the shell parts, whereas when joining may be done in a region of overlap between the shell parts, a wall thickness of the composite structure may be superimposed of wall thicknesses of the individual shell parts. However, any overlap may be engineered to accommodate any requirements of the composite structure. Typical examples may be variations in thickness at an overlap. The alignment may be done before fusing the at least one first and second shell parts together by lying them in aligned position in the final mold. Aligning two or more shell parts may be understood to provide the respective shell parts in the same position, seen axially, one shell part in the first final mold and another shell part in the second final mold such that, when the first and second mold are closed, the respective shell parts are at a corresponding position to allow fusing of at least part of their outer edges. The outer edges may thus be partially overlapping and / or abutting, when abutting a butt-to-butt fusing may be established, while when overlapping, fusing of the overlapping portions is provided. When viewed in a longitudinal direction of the respective shell parts, it may be said that the respective shell parts are aligned in axial direction, i.e. along the longitudinal direction. Partially overlapping may then be provided in circumferential direction. Alternatively and / or additionally, two or more shell parts may be aligned alongside each other in one of the molds. As such, multiple shell parts may be laid in one of the molds adjacent to each other and their respective ends are aligned, in an abutting or overlapping manner, such that, when fused, one half of the composite structure is formed. Then, alignment may be understood as to be positioned next to each other in a longitudinal direction. For example, shell parts may be positioned adjacent each other, in abutting and / or overlapping manner, in the final mold half.

[0023] The at least one shell part may have a U-shape cross-section. The U-shaped cross-section may have two outer legs connected by a bridge section thus forming a U-shape, or in some embodiments a V-shape. An end of the legs may be an outer edge of the shell part. By positioning two U- shaped shell parts opposite each other, such that the outer edges of their respective legs face each other, as to abut and / or partially overlap, a hollow shape can be formed when fusing the outer edges together. Alternatively and / or additionally, a J-shape may be considered, wherein two adjacent J- shaped parts may form a U-shaped part, wherein two adjacent U-shaped parts may form a hollow part. Alternatively, the shell parts may have a J- shape to be each positioned in a final mold half, such that when closing the final mold halves to the final mold, fusing of both shell parts is done in an asymmetric manner with respect to a center line of the final hollow part. Asymmetric final mold halves may be envisaged in such a situation, or symmetric final mold halves can be used as well which may then be filled in an asymmetric manner by the asymmetric J-shaped shell parts. Also, by providing an intermediate shape of the shell parts, the final shape may differ from the intermediate shape for example not only in size, but for example also in form. The final shape may for example in some regions and / or at some positions be larger, having a larger volume or a larger dimension, than the intermediate shape. Alternatively and / or additionally, the final shape may for example in some regions and / or at some positions have a smaller dimension than the intermediate shape, e.g. may have a smaller radius. The two-step manufacturing according to the invention may make that possible. In an example, the intermediate shape may be U- shaped with relatively long extending legs, whereas the final shape of fused two such intermediate shape shell parts may have a circular cross-section, or a more square cross-section. This gives the designer a significant degree of freedom in designing the final shape of the composite structure. A longitudinal direction of such a U, V or J-shaped shell part may extend in a direction transverse to the cross- section.

[0024] Aligning and / or engineering an overlap of the at least one first shell part and at least one second shell part may have the advantage of reducing defects during fusing and / or ensuring mechanical properties at the overlap.

[0025] The method may further comprise placing two or more intermediate shell parts in a first intermediate mold. These intermediate shell parts may in itself individually be formed in an intermediate mold having an intermediate shape. Then, fusing the two or more first parts together using heat and pressure provides for forming a first composite shell half having the first intermediate shape. Also, two or more second shell parts can be placed in a second intermediate mold, wherein fusing the two or more second shell parts together using heat and pressure provides for forming a second composite shell half having the second intermediate shape. As such, this step may be considered an additional step in between the forming of a shell part with an intermediate shape and the forming of the complete composite structure with the final shape. This step is optional, as the individual shell parts having an intermediate shape may be positioned directly in a final mold half.

[0026] The method may further comprise forming the at least one first and / or second shell part utilizing a thermoforming process, e.g. deep forming or press forming. Such thermoforming process may comprise providing a blank of thermoplastic reinforced material. Further, the blank may be heated to a temperature above the softening point. Then, the heated blank may be positioned in an intermediate mold, or may be laid in the intermediate mold. Then, a stamp may be brought towards and into the intermediate mold to form the shell part having an intermediate shape. Thus a three-dimensional intermediate shape is being formed. Advantageously, the intermediate mold is heated at a temperature lower than the softening temperature of the thermoplastic matrix of the thermoplastic reinforced material. By providing the temperature of the mold lower than the softening temperature of the thermoplastic material, the cooling time of the thus formed shell part may be shorter, thereby reducing the cycle time of the forming process. It is noted that other thermoforming processes can be used to form the three-dimensional shell parts having an intermediate shape.

[0027] By providing a method for forming the composite structure comprising at least two steps, wherein first a shell part having a three- dimensional intermediate shape is formed and thereafter a composite structure having a final shape of two or more such shell parts is formed, it becomes possible to provide more complex geometries. By providing such an intermediate step of forming shell parts from a two-dimensional blank to a three-dimensional intermediate shape, sufficient material can be provided and / or can be directed towards a region of the structure having the final shape having the complex geometry. For example, the structure having the final shape may comprise a final inside edge section having a first leg with a first inside edge and a second leg having a second inside edge enclosing an angle therebetween, wherein the enclosed inside edge angle can be smaller than 45 degrees, preferably smaller than 30 degrees, more preferably smaller than 20 degrees. Such enclosed angle advantageously may be larger in the intermediate part than in the final part. In the shell part having the intermediate shape, the inside edge section may have an enclosed radius, the radius formed between the first inside edge section and the second inside edge section. By using the two-step forming process, the enclosed radius of the final shape can be made smaller than the enclosed radius of the intermediate shape. In particular when a ratio of the enclosed radius of the final hollow part versus a circumference of one of the legs of the final hollow part is smaller than 0,15, using the two-step method allows the forming of a small enclosed radius. Then, in the intermediate shape, at the edge section having the enclosed radius, sufficient material, or even excess material, both matrix material and reinforced fiber material can be present at the inside edge section. Then, by the second step forming, placing the intermediate shaped shell parts in the final mold, and closing the final mold to fuse the parts to the final hollow part, the sufficient material or even excess material at the edge section may flow towards the areas of the edge section having the complex geometry, allowing a small enclosed radius between the legs and / or a sharp enclosed angle vis-a-vis a circumference of the final hollow part and / or a depth of the final mold. Due to the two-step forming process, additional material can be directed towards areas of complex geometry from the blank to the intermediate shape and then to the final shape. The intermediate shape can thus be engineered to contain sufficient material at areas of complex geometry to be formed in the final shape. Also when a ratio of a local radius of the final hollow part versus a depth of the associated final mold half is smaller than 0,25 it may be advantageous to form the final hollow part by using the two-step method according to the invention. Then, sufficient material may be directed to the area of the to be formed complex geometry first in the intermediate shape which thereafter can be used to form the final shape. Such an at least two- step method allows forming of more complex three-dimensional geometries, while meeting the mechanical and structural requirements of the composite structure.

[0028] Optionally, in the final forming step, of fusing the shell parts together in the final mold, internal pressure can be used to press the shell part against an internal wall of the final mold. The internal pressure provides pressure in an outward direction pressing the shell parts outwardly against the internal wall of the final mold such that the shell parts become shaped in the shape of the final mold thus becoming the final hollow part. The internal pressure can be provided by an inflatable bladder or any other pressurized medium.

[0029] The at least one first shell part and the at least one second shell part may have an approximately U-shaped cross-section having a bridge section from which walls extend. As such, in the final mold they can be positioned such that an internal space can be created between the shell parts and / or the walls of respective shell parts may be partially overlap to allow fusing. Alternatively, the walls of the respective shell parts may abut or may partially abut and partially overlap. The associated first and second shell parts may have an approximately U-shaped cross-section, for example when one of the first and second shell parts has a smaller width than the other one of the first and second shell parts allowing to fit into each other, at least at outer edges where overlap of the shell parts is envisaged. Of course, it is understood that the at least one first shell part and the associated second shell part may have otherwise different shapes.

[0030] Additionally, the present invention is aimed to provide a hollow composite structure such as a bicycle component, or a bicycle frame component or a hollow sports product, such as a racket or a stick, or such as vanes or blades of a helicopter or wind turbine, or any other hollow structural component that is preferably to be manufactured production an automated or semi-automated manner. By manufacturing the hollow composite structure according to the method of the invention, automation of such manufacturing method may become possible, thereby making such products available in larger quantities. The composite structure is preferably manufactured by a method as explained above. The composite structure is provided from a thermoplastic material. The composite structure such as a bicycle component according to the invention may have the advantage of increasing the form freedom of the composite structure while preserving the mechanical properties of the composite structure.

[0031] Advantageously, the composite structure, e.g. a bicycle frame, may comprise an inner radius at an intersection between a top tube and a down tube in the final shape that is smaller than an inner radius at an intersection a top tube and a down tube in an intermediate shape.

[0032] Further advantageous embodiments are presented in the dependent claims.

[0033] BRIEF DESCRIPTION OF THE FIGURES

[0034] Figure 1A depicts an embodiment of a first step of the manufacturing method according to the disclosure;

[0035] Figure IB illustrates a material flow as according to the step of figure 1A.

[0036] Figure 1C shows an embodiment of a shell part in perspective view having an intermediate shape;

[0037] Figure ID shows a possible flow of material of a shell part in a final mold;

[0038] Figure IE shows a cross-section of shell parts in a final mold;

[0039] Figure IF shows a cross-section of the hollow structure having the final shape in the final mold;

[0040] Figure 1G shows a perspective view of a final hollow part.

[0041] Figure 2 A shows an embodiment of a shell part in top view having an intermediate shape;

[0042] Figure 2B shows a top view of a hollow structure having a final shape, formed of shell parts of Fig. 2A; Figure 2C shows a cross-section of two shell parts arranged in overlapping manner in the final mold;

[0043] Figure 2D shows a cross-section of the hollow structure with the final shape formed of the two shell parts of Fig. 2C.

[0044] Figure 3A shows a cross-section at a shell part being processed to a final shape.

[0045] Figure 3B shows a cross-section of an alternative shell part shape being processed to a final shape.

[0046] Figure 3C shows a top view of a shell part having an enclosed radius being processed to a final shape.

[0047] DESCRIPTION OF EMBODIMENTS

[0048] For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combination of all or some of the features described.

[0049] Terminology used for describing particular embodiments is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and / or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise.

[0050] The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to the schematic and / or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

[0051] As used herein the term “shell part”, refers to a component, preferably from a thermoplastic reinforced composite material, subjected to a deformation using heat and pressure and having an intermediate shape, wherein the intermediate shape is a three-dimensional shape.

[0052] As used herein the term “pressure unit”, refers to a component, device or means capable of providing an internal pressure to the shell parts.

[0053] As used herein the term “thermoplastic or thermoplastic polymer matrix”, refers to any material that, during heating, softens and / or melts and hardens during cooling.

[0054] As used herein the term “reinforcement fibers”, refers to any type of fibers capable of modifying the strength and / or stiffness of the thermoplastic or thermoplastic polymer matrix.

[0055] As used herein the term “intermediate shape”, refers to any at least partial consolidated shape of a shell part before reaching the final form of a composite structure. The present invention is aimed at increasing the form freedom of a composite structure such as a bicycle frame component while maintaining mechanical properties and / or reducing possible defects in the composite structure. Typical examples of limitations in form freedom may arise during manufacturing of complex geometries such as closed form shell parts, multiclosed form shell parts and / or regions having a small radius, such as at sharp angles or protrusions or undercuts. Undercuts may hinder the easy removability of a shell part from a mold. By using the two-step method according to the invention, complex geometries, such as geometries comprising an undercut may become possible. In the fusing step, forming the final shape, a shape with an undercut can be manufactured, which is not available in the intermediate shape.

[0056] Accordingly, the present invention is aimed at a method of manufacturing a composite structure such as a bicycle frame component or lightweight sports products like for example tennis rackets, golf clubs, sticks etc. or any other hollow composite structures such as a vane or a blade. The composite structure comprises a thermoplastic reinforced material. The composite structure may be a closed form in that the structure encloses an aperture or opening, as e.g. known from a donut-shape or toroid-shape or triangle-shape etc. Typically, in a plane of the aperture, the aperture is enclosed by the composite structure which may have any possible shape for enclosing the aperture. The thermoplastic reinforced material comprises a thermoplastic polymer matrix and an embedded structure of reinforcement fibers. The method comprising: forming at least one first shell part having a first intermediate shape, forming at least one second shell part having a second intermediate shape and fusing the at least first and second shell parts together by using heat and pressure to form a hollow part having a final shape. The final shape being different from the respective intermediate shapes. The difference in the final shape and the respective intermediate shapes may comprise a difference in form geometries like for example a difference in angle between adjacent legs of a structure, a difference in depth, a difference in contact area between associated legs, a difference in radius etc. Typically, a local radius and / or an enclosed radius of at least one of the intermediate shapes is larger than a respective associated local radius and / or enclosed radius of the associated final shape, wherein the local radius is defined as, in cross-section, a radius of curvature of a section of the cross-section of the shell part or the final hollow part, wherein the enclosed radius is defined as a radius, in side view, of an enclosed angular section between two adjacent legs of the shell part or the final hollow part. By providing the two-step method for manufacturing of the composite structure, complex geometries such as geometries have a small local radius and / or a small enclosed radius can become possible.

[0057] In order to increase the form freedom of a composite structure such as a bicycle frame component, a lightweight sports product or any other hollow composite structure, a two-step manufacturing method may be envisaged. The at least one first and second shell parts comprise an intermediate shape distinct from the final shape. The intermediate shape may allow to guide the thermoplastic polymer matrix and / or the reinforcement fibers towards a region of the part having a complex geometry and / or form freedom challenges. Typical examples may be closed form shell parts or regions having a small local or enclosed radius at which the material available to deform in one step may be restricted. By applying the two-step method according to the invention, material such as the thermoplastic polymer matrix and / or reinforcement fibers can be guided more towards areas having otherwise form freedom limitations which may increase the available material to deform and accordingly form freedom limitations may be alleviated. Consequently, an increase in form freedom may be realized. An example of how material can be guided to specific regions in the composite structure is shown in FIG. 1A - FIG. 1G.

[0058] FIG 1A depicts a thermoforming mold 1 and a thermoplastic reinforced blank 3. The mold 1 comprises an upper lid la forming a stamp and a lower lid lb being a base. The mold 1 further comprises a thermoforming geometry, in cross-section seen as two thermoforming geometries 11, 12 representing legs of a three-dimensional intermediate shape. A first geometry 11 has in this example a lower height hl compared to a second geometry 12 having a height h2, but it is understood that the geometries 11, 12 may have the same height or otherwise a different height and / or shape. An area with restrictions in form freedom can be recognized as an area 3a of the blank 3, which here corresponds with an area between the two geometries 11, 12 of the mold 1. The footprint area of the geometries of the mold 1 is indicated by the contour line 3b. The mold 1 may be heated to a temperature T1 while the thermoplastic reinforced blank 3 may be heated to a temperature T2. Advantageously T2 is higher than T1 and by heating the mold 1, the forming process may be expedited. Once heated, the mold 1 may be closed e.g. by lowering the upper lid la towards the lower lid lb with the blank 3 in between. Thereby, the upper lid la forces the material of the blank 3, which is positioned in between the upper and the lower lids la, lb, into the geometry as defined by the mold la, lb. The blank 3 may be provided in various manners, e.g. it may be made by laying thermoplastic reinforced tape material in predefined directions and / or layers on a table and melt them altogether. Then the blank may be transported to the mold and may be heated prior to entering into the mold. Alternatively, the blank may be formed by placing the reinforcement fibers and injecting matrix material thereto. Various manners of forming a blank are available and are known to the skilled person. It is to be noted that the two geometries 11, 12 here have a different height, but alternatively, they can be of the same height.

[0059] FIG. IB illustrates how, during the forming process, the material of the blank 3 may flow along the geometry defined by the upper and lower lids la, lb. For illustrative purposes, only the upper lid la is represented. Arrow 4 indicates how material of the blank 3 may flow and / or may be guided to areas with restricted form freedom. The material may be directed or guided from an outer area of the blank to the area with a restriction in form freedom. Such guidance may also be obtained when the geometries 11, 12 have the same height. FIG. 10 shows a perspective view of a shell part 30 being formed from the blank 3 in the mold 1. The shell part 30 has a three-dimensional intermediate shape. The two geometries 11, 12 of the mold 1 have formed corresponding legs 31, 32 and the area 3a with restrictions in form freedom has here become the region 30a between the legs 31, 32. This area 30a may be recognized by a relatively small inside angle alpha and / or a relatively small enclosed radius Ro. Deforming a thermoplastic reinforced blank 3 with a mold 1 comprising these two geometries 11, 12 comprising different heights hl, h2 may allow to direct an abundance of material from an outer circumference of the thermoplastic reinforced blank 3 to the areas comprising restrictions in form freedom such as the area 3a between the two thermoforming geometries 11, 12. In the blank 3, and further the intermediate shape 30, can be seen that the two geometries 11, 12 form adjacent legs 31, 32 enclosing an angle alpha. This enclosed angle alpha is relatively sharp, typically smaller than 45 degrees. With the press forming of the blank 3, by the stamp la in the base lb, an intermediate shell part 30 is formed. The intermediate shell part 30 has an intermediate geometry shape. In Fig. 1C a perspective view of the half shell part 30 having an intermediate shape is shown. There, the two legs 31, 32 enclose the angle alpha, which angle may be different, typically larger, than the enclosed angle in the final shape structure. Hence, the available material to deform the thermoplastic reinforced blank 3 can be more optimally used. The intermediate shape 30 as shown in Fig. 1C is then further formed into a final shape 300 in a final mold 100, such as for example illustrated in Fig. ID, Fig. IE and Fig. IF. Although not at all to scale and merely as a matter of principle, a geometry ratio of the final part to be formed, as illustrated in Fig. IE with respect to the initial shape of the blank 3, as illustrated in Fig. 1A can be explained. The geometry ratio is considered to be the ratio of the surface area of the formed part, illustrated by Fig. IE, divided by the footprint area of the part being formed, illustrated by the contour 3b drawn in the blank 3 of Fig. 1A. Advantageously, this ratio is preferably approximately 1 or larger than 1, to allow sufficient material to be available for the shape to be formed. However, complex shapes such as closed form shapes having an aperture, typically do not respect this ratio, making it difficult to form such shapes by conventional methods. By using the two-step method according to the invention, complex geometries such as geometries having an undercut, a sharp angle, or a small radius, or another complexity, can be formed.

[0060] Fig. IE illustrates two final mold halves 100a, 100b in which two shell parts 30, namely part 30.1 and part 30.2, are arranged in a partially overlapping manner. Fig. ID illustrates a possible flow 400 of material during the fusing process in the final mold 100, of which in Fig. ID only a single mold half 100a is shown.

[0061] In a further step, the final shape is formed. Thereto two half shell parts 30.1, 30.2 are laid and aligned in their respective final mold halves 100a, 100b. Then the mold halves 100a, 100b are closed, as shown in crosssection in Fig. IF. Fig. IE shows the mold halves 100a, 100b open with arrows Ta, Tb illustrating a closing direction. The half shell parts 30.1 and 30.2 partly overlap at the edges E, providing for an overlapping region E, to allow firm fusing of the two shell parts 30.1 and 30.2. The shell parts 30.1, 30.2 are in this example approximately mirror-imaged, allowing an approximately symmetrical structure while one shell part 30.1 may fit into the other shell part 30.2 allowing the overlap E where the two shell parts may be fused to each other.

[0062] The shape of the final mold halves 100a and 100b is different than the intermediate shape of the shell parts 30.1 and 30.2, and thus, different from the shape of the press forming mold la, lb. The final mold 100 is being heated to soften the thermoplastic matrix in the thermoplastic material and pressure is being applied to press the material into the mold 100. Here, internal pressure is applied, as indicated by the arrows P. Here, the internal pressure P is applied using an inflatable tube 5. The internal pressure can be applied by fluid under pressure, e.g. pressurized air and / or by an inflatable body inside of the shell parts 30.1 and 30.2 and / or by a mandrel of a material expanding under heat, etc. Various ways of applying pressure are possible. The composite structure having a final shape 300, as shown in cross-section in fig. IF and in perspective view in Fig. 1G, has a different shape than the intermediate shape of the shell parts 30.1, 30.2. The flow of the material towards regions requiring more material, such as e.g. the enclosed angle alpha between legs 31, 32 during thermoforming in the final mold may be as illustrated in Fig. ID by the flow 400. It may be understood that the final shape may be asymmetrical and / or that the intermediate shapes of the intermediate shell parts may be different from each other as well, contrary to the example shown in which the both shell parts 30.1, 30.2 are approximately symmetric. By applying heat and internal pressure P in the final mold 100 onto the shell parts 30.1 and 30.2 the material of the shell parts 30.1 and 30.2 is being pushed into the geometry of the molds 100a, 100b and at the overlapping regions E is being fused to each other, thereby the final hollow part 300 is formed. Fig. 2 A shows a top view of a half shell part 30 having an intermediate shape, here with an enclosed angle alpha, and FIG 2B shows a top view of a final shape 300, then with an enclosed angle beta. The final shape 300 differs from the intermediate shape 30 in that the enclosed angle beta is smaller than the enclosed angle alpha. By providing a two-step process, relatively sharp angles beta can become possible, typically angles smaller than 40 degrees, preferably smaller than 30 degrees, may then be realized. Fig. 2C shows two shell parts 30.1 and 30.2 in cross-section having an intermediate shape being arranged with partly overlapping edges E. The overlapping edges E are then fused together to form the hollow structure 300, in cross-section in Fig. 2D, having a final shape. It is to be noted that the illustrations in Fig. 2C and Fig. 2D, as well as in the other figures, are certainly not to scale. The hollow part 300 having the final shape has in this example a relatively rectangular cross-section with relatively sharp corners, an inner radius Rf of such corners being rather small. Such a small inner radius may not be possible with a conventional forming method. The shell parts 30.1, 30.2 have in cross-section an inner radius of curvature Ri. The mold half 100a has in their form the inner radius of curvature Rf with which the final shape 300 has to be formed. Further, the mold half 100a has a depth d. The ratio of the local radius Rf of the final hollow part versus the depth d of the associated final mold half is here smaller than or equal to 0,25 which makes it beneficial to use the two-step forming method of the invention.

[0063] FIG. 3A shows an example of a shell part 30 being placed in a half final mold 100b and having a local radius Ri. When applying heat T and pressure P, in a closed final mold (not shown) of which the mold half 100b has a depth h3 and a local radius Rf, the final shape 300 can be formed. The ratio of the local radius Rf versus the depth h3 of the final mold half is smaller than or equal to 0,25, which makes a two-step process as according to the invention beneficial to achieve such small radius.

[0064] Fig. 3B shows a further example of a shell part 30 in a final mold half 100b, wherein the shell part has a local radius Ri. Here, the final mold half has a final local radius Rfl and a final local radius Rf2 corresponding respectively with a depth dl and a depth d2 of the final mold half. When the ratio of Rfl vs dl is smaller than or equal to 0,25 the two-step process as according to the invention is beneficial, as well as when the ratio of Rf2 vs d2 is smaller than or equal to 0,25.

[0065] FIG. 3C shows a schematic top view of a shell part 30 having an enclosed radius Ro between legs 31 and 32. When applying heat T and pressure P in a final mold, the final hollow part 300 can be obtained, the final hollow part 300 having an enclosed radius Rfo. A circumference C of one of the adjacent legs 301, 302 is here illustrated as a circumference C of a corner section 303 between the legs 301, 302, thus assuming that the circumference of the final hollow part is constant. Of course, the circumference C may differ between the legs and the corner section 303. Then, the largest circumference can be referred to. When a ratio of the enclosed radius Rfo of the final hollow part 300 versus a circumference C of one of the adjacent legs 301, 302 of the final hollow part is smaller than or equal to 0,15 it is beneficial to use the two-step method according to the invention to obtain the complex geometry.

[0066] Using a thermoplastic or a thermoplastic polymer matrix may have the advantage of allowing improved repairability over a thermoset or a thermoset polymer matrix. Additionally, the reinforcement fibers have the advantage of modifying mechanical properties such as tensile strength, fatigue resistance, impact resistance, etc. to the desired levels associated with a specific application and specific requirements. The thermoplastic or thermoplastic polymer matrix may comprise any known thermoplastic polymer matrix known to date like for example polypropylene, polyethylene, polyamide, polycarbonate, polymethyl methacrylate, polyvinyl chloride, polyester, polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyetheraryl ketone (PEAK), polybutylene succinate (PBS), polyethylene imide (PEI) or any combination thereof.

[0067] The reinforced fibers may comprise any fiber known to date such as carbon fibers, glass fibers, ultra-high molecular weight polyethylene fibers such as Dyneema®, polyaramid fibers like for example Kevlar®, synthetic fibers, natural fibers, metal fibers such as steel fibers or any combination thereof. The fibers may be long or short, may be unidirectional or bidirectional, multi-directional or even randomly oriented.

[0068] The at least one first and second shell parts comprising an intermediate shape may be formed by any process involving heat and pressure. Classic examples of forming (intermediate) shell parts are e.g. press forming, thermoforming, injection molding or blow molding.

[0069] Thermoforming is a manufacturing process that applies a force to stretch a sheet or blank of heated thermoplastic material over or in an engineered mold to create a three-dimensional shape or part. Typical forces to stretch and / or form a sheet or blank of heated thermoplastic material may be vacuum forming, pressure forming, mechanical mold forming or twin sheet forming.

[0070] In a thermoforming process, generally the process may be preceded by the production of prepreg materials. The prepreg materials may comprise a thermoplastic polymer matrix and at least one reinforcement fibers. The prepreg material may be provided in the form of a tape or a sheet.

[0071] Injection molding is a manufacturing process wherein a molten material is injected into a mold. For a thermoplastic reinforced material, a mold may be covered and / or inlayed with the reinforcement fibers before injecting the molten material into the mold.

[0072] Blow molding is a manufacturing process to create (hollow) plastic products made from thermoplastic materials. The process involves heating and inflating a plastic tube known as a parison or preform.

[0073] The heat and pressure needed to form the a final shape and / or the at least one first and second shell parts comprising an intermediate shape may depend on the type of thermoplastic polymer matrix and / or reinforcement fibers used in the at least one first and second shell parts. In general the temperature may be in a range of 80°C to 350°C, preferably in a range of 120°C to 250°C, more preferably in the range of 180°C to 240°C. Overall the pressure to deform a shell part may be in the range of 0.3 bar to 300 bar depending on the type of thermoforming. For example, vacuum forming may have a pressure in the range of 0.3 bar - 1 bar while pressure forming may have a pressure in the range of 3 bar - 300 bar. A thermoforming process is illustrated in FIG 1A.

[0074] A two-step thermoforming process comprising a first thermoforming process to an intermediate shape and a second thermoforming process from the intermediate shape to a final shape may be determined by providing a cross-section at an inner radius or an outer radius wherein either a first ratio may be determined between the enclosed radius of the final shape and a circumference of the final shape or by a second ratio between an inner radius and a height of the final shape. Typically, the first ratio may have a value smaller than or equal to 0,15 and the second ratio may have a value smaller than or equal to 0,25. Both situations comprising an inner or an outer radius are illustrated in figures 3A and 3B and 3C.

[0075] In an embodiment the method of placing, forming and fusing may comprise heating a thermoplastic reinforced blank 3 to a temperature T1 above a softening point like for example a melting temperature of the thermoplastic polymer matrix followed by forming the at least one first and / or second shell parts / part in a first and / or second intermediate molds / mold as illustrated e.g. in Fig. IE. The first and / or second intermediate molds / mold may be heated to a temperature T2 below the softening point of the thermoplastic polymer matrix forming the at least one first and / or second shell parts / part. The at least one first and / or second shell part may have an intermediate shape.

[0076] A benefit of lowering the mold temperature below the softening point of the thermoplastic polymer matrix may be a reduced cycling time. Accordingly, the cycle time for forming a shell part may be reduced and thus, more parts may be produced in a certain time frame.

[0077] After forming the at least one first and second shell parts, the shell parts may be trimmed to remove excess materials.

[0078] The method for manufacturing a hollow composite structure such as a bicycle frame component may comprise a step to align the at least one first shell part and the at least one second shell part. Preferably the at least one first shell part and at least one second shell part are aligned such that an overlap with the at least one second shell part may be achieved. An amount of overlap may be in the range of 2 mm - 100 mm, preferably in the range of 5 mm - 50 mm. The alignment of the overlap may be done before fusing the at least one first and second shell parts. Additionally, the overlap may be further engineered to optimize the overlap including e.g. certain thickness increments.

[0079] The aligning and / or engineering of the overlap may be done to minimize potential defects that may arise during a fusing step. For example, a bad alignment or low amount of overlap may result in a reduction of mechanical properties in said overlap areas. A thick overlap may result in an imperfection of the composite structure which in turn may initiate a defect and consequently a possible failure.

[0080] The at least one first shell part may be interconnected forming a first composite shell half, such as a first bicycle frame shell half. Additionally, the at least one second shell part may be interconnected forming a second composite shell half, such as a second bicycle frame shell half. The method according to the invention may comprise the steps of: placing two or more first shell parts in a first intermediate mold, fusing the two or more first parts together using heat and pressure to form a first composite shell half. Followed by placing two or more second shell parts in a second intermediate mold and fusing the two or more second shell parts together using heat and pressure to form a second composite shell half. In a further step, these two composite shell halves may then be fused together in a final mold forming the final hollow composite structure.

[0081] Additionally, the invention may also be directed to a composite structure such as a hollow composite part, e.g. a bicycle frame component or a bicycle frame. The composite structure may be hollow and may comprise an aperture. In a preferred embodiment, the composite structure may be manufactured according to a method of the invention. Furthermore, the composite structure may have an enclosed radius at an intersection between a top tube and a down tube in the final shape that may be smaller than an enclosed radius at an intersection between a top tube and a down tube in an intermediate shape.

[0082] Producing a composite structure with a method of at least two steps of first providing a shell part with an intermediate shape, and then a hollow structure having a final shape, may result in an increased form freedom such that closed shell forms, multiple closed shell forms or areas with a small inner radius can be produced. Many variants are possible, and are understood to fall within the scope of the appended claims.

Claims

CLAIMS1. A method of manufacturing a composite structure comprising a thermoplastic reinforced material, the method comprising:- providing at least one first shell part having a first intermediate shape in a first final mold half;- providing at least one second shell part having a second intermediate shape in a second final mold half;- closing the first final mold half and the second final mold half forming a final mold;- fusing the at least one first shell part and at least one second shell part together in the final mold by using heat and pressure to form the composite structure as a final hollow part having a final shape; wherein a local radius and / or an enclosed radius of at least one of the intermediate shapes is larger than a respective associated local radius and / or enclosed radius of the associated final shape, wherein the local radius is defined as, in cross-section, a radius of curvature of a section of the cross-section of the shell part or the final hollow part, wherein the enclosed radius is defined as a radius, in side view, of an enclosed angular section between two adjacent legs of the shell part or the final hollow part.

2. The method according to claim 1, wherein a ratio of the local radius (Ri) of the final hollow part versus a depth (d) of the associated final mold half, measured from a section line of the final mold, at the position of the local radius is smaller than or equal to 0,25.

3. The method according to claim 1 or 2, wherein a ratio of the enclosed radius (Rfo) of the final hollow part versus a circumference (c) of one of the adjacent legs of the final hollow part is smaller than or equal to 0,15.

4. The method according to claim 3, wherein the ratio of the enclosed radius (Rfo) of the final hollow part versus the circumference (c) of the larger one of the adjacent legs of the final hollow part is smaller than or equal to 0,15.

5. The method according to any of the preceding claims, the method further comprises:- trimming the at least one first and / or second shell part prior to fusing.

6. The method according to any of the preceding claims, the method further comprising:- aligning the at least one first shell part and the at least one second shell part such that the at least one first shell part at least partially overlaps with the at least one second shell part, wherein the alignment is done in the final mold prior to the fusing.

7. The method according to claim 6, wherein the at least one first shell part and the at least one second shell part are aligned axially and / or partially overlap circumferentially.

8. The method according to any of the preceding claims, wherein providing the at least one first and / or second shell part comprises:- forming the at least one first and / or second shell parts in an intermediate mold, wherein the intermediate mold has a shape different from a final mold, in particular has at least one of an inner radius and of an enclosed radius that is larger than the associated one of the final mold.

9. The method according to any of the preceding claims, wherein forming the at least one first and / or second shell part comprises- providing a blank of thermoplastic reinforced material;- forming the at least one first shell part or second shell part from the blank in the intermediate mold.

10. The method according to claim 9, the method comprising:- heating the blank to a temperature above a softening temperature of the thermoplastic polymer matrix;- placing the heated blank in the intermediate mold;- wherein the intermediate mold having a temperature below the softening temperature of the thermoplastic polymer matrix for forming the at least one first shell part or second shell part.

11. The method according to any of the preceding claims, wherein the intermediate shape of the first shell part and of the second shell part is a three-dimensional shape.

12. The method according to any of the preceding claims, wherein the composite structure has a final inside edge section with a first inside edge and a second inside edge enclosing the enclosed angle therebetween.

13. The method according to any of the preceding claims, wherein a final inside edge section of the composite structure is formed by a first shell part and a second shell part each having a first intermediate inside edge and a second intermediate inside edge with an enclosed angle therebetween that is larger than the enclosed angle of the final inside edge section of the hollow part.

14. The method according to any of the preceding claims, wherein during fusing, the at least one first shell part and at least one second shell part are pressed into the final shape of the hollow part in the final mold by using internal pressure.

15. The method according to any of the preceding claims, wherein the at least one first shell part and the at least one second shell part have an approximately U-shaped cross-section.

16. The method according to any of the preceding claims, comprising providing multiple first shell parts at least partly overlapping in the first final mold half and providing multiple second shell parts at least partly overlapping in the second final mold half prior to fusing.

17. The method according to any of the preceding claims, wherein forming the final hollow part comprises forming an undercut in the final shape.

18. Method of manufacturing a composite structure of a thermoplastic reinforced material, wherein the composite structure comprises a final hollow part that is formed by fusing at least two shell parts having a three-dimensional intermediate shape in a final mold havinga final shape, wherein at least one shell part with the intermediate shape has a radius that is larger than an associated radius of the final hollow part.

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