Method of manufacturing a thermoplast composite structure

Abstract

A hollow composite structure (1) comprising: at least one first final part of a thermoplastic reinforced material, the first final part having a U-shaped cross-section: at least one second final part of the thermoplastic reinforced material, the second final part having a U-shaped cross-section; wherein the first final part and the second final part are fused to each other to form a hollow part; at least one insert inside of the hollow part, wherein the insert is fused to an interior surface of the first final part and to an interior surface of the second final part, wherein the insert has is able to deform at the fusion temperature.

Description

[0001] THERMOPLAST COMPOSITE STRUCTURE

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a hollow composite structure and to a method of manufacturing a hollow composite structure. For example, the thermoplastic reinforced composite structure can be a bicycle frame or a part of a bicycle frame.

[0004] BACKGROUND OF THE INVENTION

[0005] Multiple industries, such as mobility are looking into composite materials to reduce weight of a vehicle. To reduce the weight of a vehicle companies are considering to replace relatively heavy substrates such as aluminum or steel substrates with more lightweight alternatives while maintaining the mechanical properties associated with steel or aluminum substrates. Typical examples are composite based substrates. Accordingly, multiple thermoset polymer matrices and / or thermoplastic polymer matrices in combination with reinforcement fibers have been deeply investigated and evaluated in the past years. In particular, a thermoset polymer matrix such as epoxies or a thermoplastic polymer matrix in combination with reinforcement fibers such as carbon fibers, glass fibers, aramid fibers, polyethylene fibers, or any other polymer based reinforcement fibers have been considered.

[0006] The use of a thermoset as polymer matrix may have the disadvantage of labour intensive and time consuming manufacturing and / or a reduced repairability. Additionally, the joints where individual parts may be interconnected to each other may be prone to defects, which may negatively affect the performance of the final composite structure. On the other hand, using a thermoplastic composite matrix may improve manufactur ability, but may give difficulties in reaching the required strength and / or stiffness.

[0007] Further, considering a thermoset and / or a thermoplastic reinforced composite material limits the form freedom to a designer in view of the material properties. For example, when using a thermoplastic material that is being fused by using heat and pressure, complex geometries are avoided when not sufficient pressure can be put on such geometries. Additionally, the combination of a thermoplastic material with reinforced fibers may limit the flexibility and therefore reducing the form freedom.

[0008] Hence, composite structure such as a bicycle frame are often limited in form freedom.

[0009] Manufacturing composite structures may involve multiple steps making the production and assembly process time consuming, complex and limiting a widespread acceptance in the market. A typical example of a time consuming assembly is the manufacturing of a bicycle frame, but also other frames or components for transport vehicles, such as wheelchairs, child pram, a step etc., can be envisaged.

[0010] WO2022 192355 Al describes a composite structure and a method of manufacturing said composite structure. The composite structure includes composite inserts, for example in the form of solid composite structures or block members, or in the form of braided sleeves. The braided sleeves are used in place of welding strips for welding the composite parts to each other and become part of the structural integrity of the composite structure. The solid inserts or block members are fused to the composite panels and become part of the structure as well. WO2022192355 Al does not disclose how the form freedom can be addressed.

[0011] The present invention is aimed at providing a solution to restrictions in form freedom while at least preserving the mechanical properties of a composite structure. An increase in form freedom may allow more complex structure that can be more tailored to the structural and / or mechanical requirements. It may simplify the production process of a composite structure.

[0012] SUMMARY

[0013] The present invention is aimed at increasing the form freedom of a composite structure such as lightweight sports products like for example (tennis) rackets, golf clubs, sport sticks or a bicycle frame component or a bicycle frame. Typical examples of restrictions in form freedom may refer to the exclusion of protrusions, avoiding complex geometries such as small inner radii, relatively sharp angles etc.

[0014] Accordingly, the present invention is aimed to a method of manufacturing a composite structure of a thermoplastic reinforced material. The composite structure comprises multiple shell parts that are fused together by using heat and pressure. The method comprising the steps: placing at least one first shell part in a first mold half, placing at least one insert, at predetermined position, in the at least one first shell part, placing at least one second shell part in a second mold half. The first and second mold halves are then closed to form a final mold. In the final mold the at least one first and second shell parts are fused together by using heat and pressure. The at least one insert, provided at a predetermined position, is able to deform at the fusion temperature, such that during fusing, pressure is distributed to an inner surface of at least one of the shell part at the predetermined position via the at least one insert. By providing such an at least one insert, more freedom of form and / or more freedom of design becomes available for the composite structure. By providing such an at least one insert, pressure can be optimally distributed to the shell parts in particular to positions where the pressure unit solely cannot provide sufficient pressure. By providing the insert that deforms at fusing temperature, the pressure unit can apply the pressure to the insert which further distributes the pressure to the shell parts allowing them to deform in the final mold.

[0015] Thereby, the method of manufacturing hollow composite structures may become more widely available for a wider variety of composite structures, as more complex geometries may now become possible in an efficient and effective manner, without impairment to strength and / or stiffness. The temperature to which the final mold is heated and at which the fusing of the shell parts is intended, is referred to as the fusing temperature.

[0016] The difference in viscosity between the material of the insert and the material of the shell part is preferably such that at the fusing temperature used in the final mold, the material of the insert flows less than the matrix material of the shell part. Preferably, the material of the insert has a viscosity that is about 1,3 times, preferably 1,5 times, to about 100 times higher than the viscosity of the matrix material of the shell part at the fusing temperature. In practice, it may be that the material of the shell parts, in particular the thermoplastic matrix material of the shell parts, melts at the fusing temperature. The material of the insert then may be wholly or partially flowing as long as the viscosity of the material of the insert is still higher than the viscosity of the matrix material of the shell part. The flowing condition of the material of the insert at the fusing temperature allows pressure to be distributed onto an inner surface of the shell part against which the insert is positioned, under influence of the pressure applied by the pressure unit.

[0017] A typical example of an insert that may comprise of an at least 1,3 times higher viscosity compared to the viscosity of the thermoplastic reinforced material may be when the insert comprises a thermoplastic matrix material that is being reinforced with e.g. fibers of any kind and / or in any configuration, e.g. wires of any kind and / or in any configuration. Fibers may comprise carbon fibers, glass fibers, aramid fibers, polyethylene fibers and many others. Wires may e.g. be a steel wire or any other metallic wire. Also, various configurations are possible, such as randomly oriented, unidirectionally oriented, multi-directionally oriented, woven, braided, etc. Other means of augmenting the viscosity of the material of the insert may be possible.

[0018] As such, when applying heat and pressure, the thermoplastic matrix of the thermoplastic material of the shell part may flow, but the insert material may not flow that much as the thermoplastic material of the shell part, thereby optimally distributing the pressure applied to it. Thus an evenly distributed hydrostatic pressure may be applied to the inner surface of the shell part at the position where the insert is located. Alternatively and / or additionally, the insert may thermally expand thereby distributing the pressure applied by the pressure unit to the inner surface of the shell parts.

[0019] The difference in viscosity between the material of the insert and the material of the shell part is preferably such that at the fusing temperature used in the final mold, the material of the insert flows less than the matrix material of the shell part, so that the insert deforms to distribute the pressure of the pressure unit to the shell part. Preferably, the material of the insert has a viscosity that is 1,5 times or more higher than the viscosity of the matrix material of the shell part at the fusing temperature. In practice, it may be that the material of the shell parts, in particular the thermoplastic matrix material of the shell parts, melts at the fusing temperature. The material of the insert then may at least partially flow to allow deformation, or may wholly flow, such that the insert still can distribute the pressure of the pressure unit onto the material of the shell part to push the shell part against the final mold to be formed. The flowing condition of the material of the insert at the fusing temperature allows pressure to be distributed onto an inner surface of the shell part against which the insert is positioned, under influence of the pressure applied by the pressure unit.

[0020] Alternatively and / or additionally, the material of the insert may be subject to thermal expansion at the fusing temperature without necessarily becoming into the flowing condition. In the scenario wherein the at least one insert, at the fusion temperature, is subjected to thermal expansion without necessarily flowing, the at least one insert may have an elongation at break, at the fusion temperature, of at least 100% such that the at least one insert may not break during expansion at the fusion temperature. The thermal expansion allows the insert to more evenly distributing the pressure, applied by the pressure unit onto the insert, towards an inner surface of the shell part contacting the insert. Such material may be an elastomer, for example be a silicon material or a rubbertype material. In any event, whether the material of the insert goes to flowing condition at the fusing temperature or not, the viscoelastic behavior of the material of the insert is different from the viscoelastic behavior of the matrix material of the shell part at the fusing temperature and is such that the insert deforms at fusing temperature to distribute the pressure from the pressure unit onto the shell part. Thus, the pressure may be distributed rather evenly onto the shell part allowing deformation of the shell part in complex geometries. At the fusing temperature, the matrix material of the shell part may typically be in melted condition, while the material of the insert may be in flowing condition, typically a rubbery flow condition, or may be in a condition where thermal expansion occurs without flowing, merely a rubbery condition. In the melted condition, the material has practically no stiffness anymore. In the rubbery flow condition, the material still has some stiffness and some coherence in that some molecules are still connected and not all molecules are flowing. In the rubbery condition, the material can deform elastically. In the rubbery flow condition or in the rubbery condition at the fusing temperature, the insert can deform to distribute pressure from the pressure unit onto the shell part, preferably rather evenly as a hydrostatic pressure. So, advantageously, the insert is provided from a material that allows deformation at fusing temperature. Such material may be an elastomer from a thermoplastic material and / or a thermoset material. Other materials may also be possible, as long as they allow deformation at the fusing temperature.

[0021] The pressure applied in the final mold is typically provided as internal pressure. For example, a fluid under pressure is applied in the hollow interior formed by the first shell part and the second shell part when the final mold is closed. The fluid under pressure may be pressurized air, the pressurized air may be applied directly into the hollow interior formed by the first shell part and the second shell part. A drawback of directly inserting the pressure fluid into the hollow interior is that there may be a risk on leakage. Alternatively, a bladder may be provided in the hollow interior formed by the first shell part and the second shell part, and this bladder may then be inflated, by a fluid such as air or water. By inflating the bladder pressure is being applied to an inner surface of the first shell part and of the second shell part thereby pushing the shell parts outwardly against the associated mold surfaces. In case of complex geometries, such as relatively sharp bends, relatively narrow protrusions or relatively small angles, it may be difficult to apply sufficient pressure inside such geometries by inflating the bladder. In or at such complex geometries it appears to be difficult to apply sufficient pressure to the shell parts by the pressure unit to form the shell parts in the final mold. Therefore, by providing an insert having a viscosity higher than the viscosity of the thermoplastic material of the composite structure, this insert may somewhat expand, e.g. due to - partially - flowing and / or thermal expansion, but not melt due to the difference in e.g. viscosity, or e.g. elongation at break, thereby providing for an equal pressure distribution to an inner surface of the shell part to which it is engaged. Providing an at least one insert having a viscosity higher than the viscosity of the thermoplastic material may have the advantage of restricting flowability to an enclosed area such that a pressure can be provided to the thermoplastic reinforced material of the shell part surrounding the at least one inserts. A too low viscosity may result in a too high flowability such that the insert does not sufficiently distribute the pressure anymore. Since the thermoplastic matrix may melt due to the applied heat, but the insert may not melt, the insert may not fuse to the composite structure, and may thus not become a structural part of the composite structure. This allows the insert to be removed from the composite structure after fusing the shell parts together, if possible.

[0022] Otherwise, the insert may remain inside of the composite structure, but may not add to the mechanical and structural properties of the composite structure. Alternatively, it is possible that the insert may be fused to the inner surface of the associated shell part, thereby becoming an integral part of the composite structure.

[0023] Advantageously, the predetermined position at which the insert is positioned is the position having a complex geometry, such as a small inner radius, or a protrusion, or a narrow bend etc. usually having a sudden or discrete change in geometry. Such complex geometry is typically a geometry in which there cannot be provided sufficient internal pressure, for example because a bladder cannot reach the geometry. Typically, the pressure distribution on an inner surface of the shell parts is preferably approximately even over the entire inner surface of the shell parts. However, when difficult or complex geometries are provided, such as a protrusion or a narrow bent or a small inner angle, etc. at these positions it may be difficult to reach the required pressure distribution without the provision of the insert according to the invention. Any other complex geometries for which no sufficient pressure can be applied by the pressure unit alone, may have the advantage of an insert provided thereto to allow for an improved pressure distribution. Based on the required pressure distribution for forming the shell parts in the final mold, it may be determined to provide a deformable insert. Typically such required pressure distribution may be known as the pressure distribution allowing, at fusing temperature, to push the shell part against the final mold to form the final hollow part. When in view of the complex geometry, the required pressure distribution may not be reached by the pressure unit alone, a deformable insert can be used. Alternatively and / or additionally, positions eligible for receiving an insert and / or for benefiting from such an insert may be defined by a minimal bend radius, or minimal cross-sectional dimensions, or minimal protruding depth, etc.

[0024] Advantageously, the predetermined position of the at least one insert may be determined at a position of an abrupt increase of a circumference of the final hollow part wherein the circumference increases at least 1,5 over a distance equal to the circumference. Typically, a position where an insert is required to provide for sufficient pressure distribution, is a position with an abrupt change in circumference of the final hollow part. Here, it is an abrupt change of the geometry of the final hollow part, as it is the forming of the final hollow part in the final mold that may be difficult with a complex geometry. Since the final part is hollow, it can be said to have in cross-section a circumference. This circumference may be approximately the same along a longitudinal direction of the final part. When the geometry has a sudden increase in circumference, e.g. at a protrusion, or an elbow or otherwise a geometry change, it may be difficult for the pressure unit to provide sufficient pressure onto the shell parts to push them into the final mold, so an insert that deforms at fusing temperature at such a position may distribute the pressure of the pressure unit onto the shell part. Also a sudden decrease in circumference may provide for an abrupt change in geometry requiring a deformable insert to provide for adequate pressure distribution. A sudden change in geometry can be understood to be an increase, or decrease, of at least 1,5 the circumference of the hollow part over a length equal to the circumference. The circumference here is understood to be the circumference of a crosssection of the hollow part at a position outside of the changed geometry. A gradual increase or decrease of the circumference along the longitudinal direction of the final hollow part may usually be accommodated by the pressure unit. Nevertheless, when the gradual increase becomes an abrupt increase, an insert may be required to provide for sufficient pressure distribution. Then, the circumference may be considered the mean circumference over a distance equal to the circumference of the cross-section adjacent to the abrupt or discrete change in circumference.

[0025] Alternatively and / or additionally, such predetermined position may also be a position of the final hollow part having a cross-section with a relatively sharp enclosed angle. Such a cross-section may be for example a droplet shape cross-section, wherein in the point of the droplet shape the enclosed angle may be relatively small, e.g. smaller than 30 degrees. In such a small enclosed angle section it may be difficult for the pressure unit to provide sufficient pressure to push the shell parts into the final mold. Then, a deformable insert aids distributing the pressure exerted by the pressure unit onto the shell part to be formed in the final mold. By providing a deformable insert, complex geometries such as a final hollow part with a droplet shaped cross-section or a polygonal cross-section or otherwise sharp edges or protrusions or undercuts may become possible. In the half mold, multiple shell parts may be arranged in an abutting and / or overlapping manner, such that the multiple shell parts may be fused to each other to form, together with the shell parts of the other half mold, a composite structure. By arranging multiple shell parts in the half mold, the fusing of all these shell parts may be done in a single fusing step using heat and pressure, and possibly and / or preferably inserts at predetermined positions. Providing multiple shell parts also may give the advantage of a single fusing step for manufacturing while one half of the composite structure can be assembled of multiple parts instead of a single part. Optionally, it may be possible to manufacture a single shell part as the half part of the composite structure first before fusing the shell parts in the final mold.

[0026] The at least one first shell part and / or second shell part may be trimmed prior to be arranged in the respective first and / or second mold half. As such excess material can be removed from the shell parts and / or side edges may be cut to provide for a more neat abutting and / or overlapping. This may provide for a more smooth final hollow part formed in the final mold that may benefit from less or no final treatment.

[0027] After fusing the at least one insert can be removed from the fused composite structure and / or can remain in the composite structure. When it may be impossible to remove the insert, the insert may remain in the hollow composite structure, since it typically has no impact on mechanical and / or structural properties of the composite structure.

[0028] The insert may even be provided of thermoplastic material having the same thermoplastic matrix and / or the same reinforcement fibers as the shell part, as long as the viscosity of the insert is at least 1,3 times, optionally at least 1,5 times higher than the viscosity of the matrix material. Alternatively, the insert may be provided from a thermoset material having a melting temperature higher than the melting temperature of the thermoplastic fiber reinforced material of the shell parts, thus having a viscosity at least 1,3 times, optionally at least 1,5 times higher than the viscosity of the matrix material of the shell part. As such, the thermoset material may not melt, but may typically thermally expand at the fusing temperature, and may provide for a more even pressure distribution at the inner surfaces against which it engages. In an example, the insert may be a silicon insert.

[0029] Advantageously, the insert is arranged to obtain contact with an inner surface of the shell parts, such that at least during the fusing process, when heat and pressure are applied, the insert contacts the inner surface of the shell part for distributing pressure onto the inner surface. In any event, when heat and pressure is applied to the insert, it may deform such that it contacts the inner surface of at least one shell part and provides for an even pressure distribution. The at least one insert is arranged in the predetermined regions such that the pressure unit in combination with the at least one insert provides for an evenly distributed pressure. The pressure unit provides pressure onto the at least one insert, which insert, due to the difference in viscosity, transmits and / or distributes the pressure onto the inner surface of the shell parts to which it is engaged providing a hydrostatic pressure against an inner surface of the shell parts.

[0030] The pressure unit that is configured to pressurize the hollow interior between the shell parts and may comprise a bladder that is to be arranged inside of the shell parts. So, in one mold half the at least one shell part is arranged engaging the mold, then the bladder can be arranged engaging the shell part. In the other mold half, the at least one second shell part is arranged. When closing both mold halves, the bladder becomes enclosed by the at least one first and second shell parts in the interior volume enclosed between them. By inflating the bladder, pressure can be imposed on the inner surfaces of the shell parts pushing the shell parts against the inner surfaces of the molds such that the shell parts are being pressed in the form of the mold. Also, the at least one insert is subject to the pressure provided by the inflated bladder. Since the at least one insert is deformable at the fusing temperature, it may impose an approximately even pressure distribution on the inner surfaces of the regions of the shell parts that are in contact with the insert. The at least one insert thus transmits the pressure of the pressure unit to inner surfaces of the shell parts to which it is engaged.

[0031] Optionally, the at least one insert may have a thermal behavior differing from the thermal behavior of the mold at the fusing temperature. For example, the at least one insert may have a thermal expansion coefficient that is higher, preferably at least 20% higher, than the thermal expansion coefficient of the mold, such that the at least one insert expands more than the mold when heat and pressure is applied, to allow distribution of the pressure from the pressure unit onto the insert to the shell part, which is then being pushed against the final mold. The at least one insert may have an elongation at break, at the fusion temperature, of at least 100% such that the at least one insert may not break during expansion at the fusion temperature.

[0032] Further, there can be provided a method of manufacturing a composite structure, preferably a hollow composite part, such as a bicycle frame component, from a thermoplastic composite reinforced material; the method comprising: providing at least one first shell part of a thermoplastic reinforced material; the first shell part having at least two outer edges facing each other; placing the at least one first shell part in a first mold half such that the two outer edges are facing upwardly; providing at least one insert of a material that is able to deform at the fusion temperature such that during fusing, the at least one insert distributes the pressure from the pressure unit to an inner surface of at least one of the shell parts at a predetermined position; arranging the at least one insert in the at least one first shell part at the predetermined position requiring a predetermined pressure; engaging a pressure unit to the first mold half; providing at least one second shell part of a thermoplastic reinforced material; the second shell part having at least two outer edges facing each other; placing the at least one second shell part in a second mold half such that the two outer edges are facing upwardly; closing the first mold half with the second mold half to provide a final mold, such that the outer edges of the respective first shell part and the second shell part are adjacent each other; heating the final mold towards the fusing temperature and pressurizing the pressure unit to provide for adequate internal pressure to the first and second shell parts to fuse the first and second shell parts; cooling of the final mold to allow the first and second shell parts to join to a hollow part; removing the fused hollow part from the final mold.

[0033] The first shell part and / or the second shell part may have a substantially U-shaped cross-section. The substantially U-shaped crosssection may comprise two opposite leg sides extending from a bridge section. The leg sides may be provided with outer edges. When positioning the first shell part in the first final mold half and the second shell part in the second final mold half, the respective outer edges of the shell parts are facing each other when the mold halves are closed to form the final mold. The outer edges may abut, or the respective leg sides may partially overlap.

[0034] Further, there may be provided a hollow composite structure such as a bicycle frame component or any other hollow composite structure, such as a hollow sports product, comprising: at least one first final part of a thermoplastic reinforced material, the first final part having a U-shaped cross-section: at least one second final part of the thermoplastic reinforced material, the second final part having a U-shaped cross-section; wherein the first final part and the second final part are fused to each other to form a hollow part; at least one insert inside of the hollow part, wherein the insert is fused to an interior surface of the first final part and to an interior surface of the second final part, wherein the insert is able to deform at the fusing temperature. By providing the at least one insert at predetermined positions, the composite structure may comprise more complex geometries, thereby providing more freedom of design to the designer of such a composite structure. More form freedom or more freedom of design allows to provide thermoplastic composite structures for more and / or more diverse applications, such as bicycle frame components, or hollow sport products such as tennis rackets, stick for hockey, cricket, baseball, golf etc., or otherwise hollow structures.

[0035] For example, a composite structure having an enclosed angle of less than 30 degrees may now become possible. The at least one insert may then be positioned at the region where the enclosed angle is. The enclosed angle may for example be between two branches of the composite structure extending from a base section, wherein the enclosed angle is then between the two branches, and the insert may be positioned at the base section. Alternatively, an enclosed angle may also be an angle in cross-section of a shell part between sides of the shell part, e.g. between legs of the U-shaped cross-section of the shell part. Other complex geometries may also become available to designers of hollow composite structures, such as protrusions, rather sharp elbow shapes or branches etc.

[0036] Further advantageous embodiments are represented in the sub claims. The disclosure will further be elaborated on the basis of the drawings.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 depicts a cross-section of a composite structure according to the disclosure; Figure 2 illustrates an embodiment of a longitudinal section of a composite structure comprising a protrusion.

[0039] Figure 3 displays an embodiment of manufacturing a final form of a longitudinal shell part comprising a protrusion.

[0040] Figure 4 illustrates an increase in circumference considering the total length of a shell part.

[0041] DESCRIPTION OF EMBODIMENTS

[0042] 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.

[0043] 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. As used herein the term “shell part”, refers to a part from a material, preferably a thermoplastic reinforced material subjected to an intermediate deformation using heat and pressure.

[0044] As used herein the term “viscosity”, refers to a measure of a fluid’s resistance to flow and hence describing the internal friction of a moving fluid. Viscosity and the measurement thereof are well known to the skilled person. Since for the invention only the difference in viscosity between the material of the insert and the matrix material of the shell part is of relevance at the fusing temperature, this difference can be determined by the skilled person irrespective of the measurement method for viscosity. The skilled person then understands that he has to use the same measurement method for measuring the viscosity of the insert and of the matrix material of the shell part.

[0045] As used herein the term “pressure unit”, refers to a component, device or means capable of providing a pressure, possibly an internal pressure, to the shell parts.

[0046] As used herein the term “thermoplastic or thermoplastic polymer matrix”, refers to any material that, during heating softens and hardens during cooling.

[0047] As used herein the term “reinforcement fibers”, refers to any type of fibers capable of modifying the strength of the thermoplastic or thermoplastic polymer matrix.

[0048] As used herein the term “protrusion or protrusions”, refers to an area that extends beyond or above a surface. The surface may be an outer surface of the shell part.

[0049] The present invention is aimed at increasing the form freedom of a composite structure such as lightweight sports products, e.g. a bicycle frame component, or another frame of a mobility instrument or a sports equipment, while maintaining mechanical properties and / or reducing possible defects in the composite structure. Typical examples of limitations in form freedom are acknowledged in manufacturing complex geometries like for example a composite structure with areas having a small inner radius or a closed form such as a triangle, or a composite structure comprising protrusions, or regions in need for specific mechanical properties, or regions subjected the specific aesthetic requirement such as areas visible for a consumer.

[0050] A hollow composite structure is manufactured by a method of manufacturing a composite structure comprising at least one first thermoplastic reinforced material and an interior volume. The thermoplastic reinforced material comprises a thermoplastic polymer matrix and an embedded structure of reinforcement fibers. The method comprises placing at least one first shell part in a first mold half, placing at least one insert, at a predetermined position, in the at least one first shell part, placing at least one second shell part in a second mold half, combining the first and second mold halves forming a final mold. The first and the second mold halves in the final mold comprise the respective at least one shell parts and the at least one insert. At least one pressure unit may be arranged to at least one of the mold halves to provide internal pressure into the final mold when the final mold is closed. The at least one first and second shell parts are fused using heat and pressure. The at least one insert is able to deform at the fusion temperature such that the thermoplastic matrix material of the shell parts may melt to fuse to the shell parts, while the insert elastically deforms under the heat and / or pressure, thereby distributing the pressure applied by the pressure unit onto the shell parts.

[0051] Using a thermoplastic or a thermoplastic polymer matrix for the composite structure has the advantage over a thermoset or a thermoset polymer matrix that it improves recyclability, manufacturability and / or repairability of the composite structure. Additionally, the reinforcement fibers may have the advantage of modifying, in particular improving, the mechanical properties such as tensile strength, fatigue resistance, impact resistance to the desired levels associated with a specific application.

[0052] The thermoplastic or thermoplastic polymer matrix may comprise any known thermoplastic polymer matrix known such as 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.

[0053] The reinforced fibers may comprise any fiber known such as carbon 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.

[0054] The pressure unit may comprise any means to apply pressure to the shell parts, the pressure unit may provide internal pressure in the interior formed by the shell parts when the final mold is closed. A typical example may comprise a bladder. The pressure unit such as a bladder may typically be provided to ensure that a minimum pressure may be applied such that fusion of the at least first and / or second shell parts may be guaranteed at the fusing temperature. Accordingly, improper fusing may appear at areas where the minimum pressure required to properly fuse the shell part and form the final hollow part cannot be reached. Consequently, such areas may be more prone to defects and may benefit from the provision of a deformable insert. Also, this limits the form freedom and / or design freedom of the composite part.

[0055] The at least one first and second shell parts may comprise in cross-section a U-shape that may be formed by any process involving heat and pressure. Known examples of (thermo)forming such shell parts may be thermoforming, deep forming, injection molding or blow molding. Thermoforming is a manufacturing process that applies a force to stretch a sheet or blank of heated thermoplastic material over an engineered mold to create a three-dimensional shape or part. Typical processes to stretch / deform a sheet of heated thermoplastic material are vacuum forming, pressure forming, mechanical mold forming or twin sheet forming. 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 fiber.

[0056] 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 / inlayed with the reinforcement fibers before injecting the molten material into the mold.

[0057] 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.

[0058] The heat and pressure required to deform the at least one first and second shell parts into the final hollow part may depend on the type of thermoplastic polymer matrix and / or reinforcement fibers used in the at least one first and second shell parts. The skilled person is familiar with the process of fusing composite parts by using heat and pressure.

[0059] The method may comprise placing multiple first shell parts in the first mold half and placing multiple second shell parts in the second mold half.

[0060] The method may further comprise determining a predetermined position for the at least one insert based on a required pressure distribution. A predetermined position may be considered a position where there is a sudden or abrupt change in geometry, typically along a longitudinal direction of the hollow part. For example, such sudden change may be considered an increase or decrease of 1,5 or more of the circumference of the hollow part over a length equal to the said circumference, the circumference being the circumference of the hollow part outside of the increased or decreased region. Alternatively and / or additionally, determining the position for the insert may also comprise a position where the cross-section may have a relatively small enclosed angle or a relatively sharp enclosed edge.

[0061] Determining a position for the at least one insert beforehand, may have the advantage of increasing the form freedom of the composite structure. More than one insert can be used for forming a complex geometry of the composite part. Placing inserts at specific positions may allow to deform the shell parts to more complex geometries while preserving the mechanical properties in more challenging designs of the composite structure such as a bicycle frame.

[0062] Figure 1 illustrates a cross -section of a composite structure having an enclosed angle alpha a which is in this example relatively sharp and may be smaller than 30 degrees. Here, the cross-section of the final hollow part has a droplet shape with a relatively small enclosed angle.

[0063] Figure 1 displays a cross-section of a final hollow part of a composite structure 1. The angle alpha a is enclosed at an angular section 4 and is understood to be an angle formed between - in cross-section - inner sides of the hollow structure 1. As can be seen in this example, the angular section 4 here also includes a relatively small inner radius r2. However, the inner radius rl may as such be sufficient to qualify as a predetermined position for an insert. To provide pressure on the shell parts when in a closed final mold, a pressure unit 2, here a bladder 2 is provided in an interior volume 10 of the shell parts in the final mold and of the final hollow part 1. For reasons of simplicity, the shell parts are not separately shown, as in fused condition, they form the hollow part 1. The bladder 2 is inflated such that a pressure P may be provided. While the bladder provides enough pressure to a vast majority of the composite structure 1, the bladder 2 may not reach the angular section 4 as the bladder 1 may not be able to properly inflate into the angular section 4. Hence, not enough pressure may be provided to an interior surface 11 of the shell parts at the angular section 4. Consequently the angular section 4 may not be properly formed against the final mold (not shown here). Therefore, according to the invention, an insert 3 is provided at the angular section 4. The insert 3 is able to deform at the fusion temperature such that the pressure P applied by the pressure unit 2 can be distributed to the inner surface 11 at the angular section 4. The internal pressure P applied by the bladder 2 may then be applied onto the insert 3 such that insert 3 may due to the deformation at the fusing temperature contact an inner surface 11 of the angular section 4 of the shell parts of the composite structure 1. Thereby the pressure may be distributed more evenly onto the inner surface 11 of the angular section 4 against the final mold. This allows to form a composite structure 1 having more complex geometries, such as here with a small radius rl.

[0064] Figure 2 depicts a longitudinal section of a composite structure 1 having a protrusion 5. Here, the protrusion 5 provides for a complex geometry. In fact, the protrusion 5 provides for a sudden increase in diameter, and consequently in circumference at a cross-section. A nominal section of the hollow part 1 has a diameter dl and an associated circumference cl. At the protrusion 5, there is a diameter d2 and an associated circumference c2. The change in circumference from cl to c2 is considered sudden, or abrupt, or discrete, as opposed to gradual or smooth, when within a length of the circumference cl the circumference increases at least 1,5 times. So, here the length f is equal to the circumference cl, within this length the circumference changes from cl to c2, wherein c2 is more than 1,5 times larger than cl. As can be seen in Figure 2, the pressure unit 2 may, when inflated, not reach the protrusion 5 to push the shell part into the protrusion section of the final mold (not shown). Consequently the pressure unit 2, here the bladder 2, may not properly inflate into the protrusion 5. To ensure proper fusing and / or deforming a deformable insert 3 is arranged at the position of the protrusion 5. The insert 3, during heating, may deform such that a pressure is provided on the shell part / parts surrounding the protrusion 5 to push them in the final mold for forming the final hollow part 1. The insert 3 may be removed from the protrusion 5 after fusing, or may remain, depending on the accessibility of the insert after fusing of the shell parts. The bladder 2 typically is removed after fusing.

[0065] Figure 3 displays a longitudinal section of a final mold 7 comprising a first half mold 7a and a second half mold 7b, when closed forming the final mold 7. In the mold halves 7a, 7b a first shell part 6a and a second shell part 6b is positioned. The second mold half 7b is here having a complex geometry. Not only is the mold half 7b provided with an abrupt increase in circumference resulting in a protrusion 5b, the protrusion 5b is also provided with a relatively small enclosed angle alpha a. The circumference c2, with associated diameter d2, is more than 1,5 larger than the circumference cl with associated diameter dl of a nominal section of the final hollow part. The final hollow part 1 is formed against an inner surface 12 of the final mold 7 and is for sake of simplicity not drawn here. Both aspects make it difficult for the pressure unit 2 to exert sufficient pressure on the shell part 6b to form it into the protrusion 5b. Consequently, the pressure unit 2 may not properly deform into protrusion 5b, hence to produce a composite structure having a protrusion 5b, as depicted in the final mold 7, an insert 3 may be positioned at the protrusion 5b. The insert 3 is deformable at the fusing temperature, while the matrix material of the thermoplastic composite material of the shell parts 6a, 6b is flowing. At the fusing temperature, the viscosity of the insert is higher than the viscosity of the matrix material of the shell part. Due to the deformation of the insert 3, the pressure P applied by the pressure unit 2 can be more evenly distributed to the inner surface 11 of the shell parts 6a, 6b to push the shell part against the final mold, in particular into the protrusion of the final mold. The insert 3 may be removed from the protrusion 5b after fusion or may remain in the protrusion 5b depending on the accessibility of the insert 3.

[0066] Figure 4 illustrates how a predetermined position may be determined based on an increase of the circumference cl over a distance f considering the overall length L of at least a part of the final composite structure 1. Typically, the predetermined position may be based on a sudden, or abrupt, or discrete change in circumference from cl to c2 as opposed to gradual or smooth increase. Generally, a change in diameter / circumference may be regarded as sudden when a complex geometry is involved. Overall in a complex geometry, the circumference cl may increase at least 1.5, preferably at least 2.0 times, to a circumference c2 within a distance f. The presence of said complex geometry in the at least one shell part (not shown) may result in positioning an insert (not shown) at the location where the complex geometry, here shown as a protrusion 5, is found. Consequently, the at least one shell part may properly deform by the pressure provided by the pressure unit combined with the deformation of the at least one insert. Typically, the distance f for determining the predetermined position is considered similar to the circumference cl measured at the nominal tube diameter dl. Additionally, the insert 3 may be removed from the protrusion 5 after fusion or may remain in the protrusion 5 depending on the accessibility of the insert.

[0067] Alternatively and / or additionally, an inner side of the at least one shell part in contact with the insert may be treated with a functional coating such that the at least one insert may not fuse with the at least one first or second shell part. A typical example of such a coating may be a hydrophobic or a superhydrophobic coating.

[0068] Many variants are possible, and fall within the scope of the following claims.

Claims

CLAIMS1. A method of manufacturing a composite structure, such as a bicycle frame component, of a thermoplastic reinforced material, the method comprising: placing at least one first shell part in a first mold half; placing at least one insert, at predetermined position, in the at least one first shell part; placing at least one second shell part in a second mold half; combining the first and second mold halves forming a final mold, arranging at least one pressure unit to at least one mold half; fusing the at least one first and second shell parts in the final mold by heat and pressure to a final hollow part; wherein the at least one insert deforms at the fusing temperature, such that during fusing, pressure is distributed to an inner surface of at least one of the shell parts at the predetermined position via the at least one insert.

2. The method according to claim 1, wherein the viscosity of the at least one insert at the fusion temperature is at least 1,3 -100 times higher than the viscosity of a matrix material of the thermoplastic reinforced material of the at least one shell parts.

3. The method according to claim 1, wherein the at least one insert has an elongation at break at the fusion temperature of at least 100%.

4. The method according to any of claims 1-3, further comprising: determining the predetermined position of the at least one insert at a position of an abrupt change of a circumference of the final hollow partwherein the circumference increases at least 1.5 over a distance equal to the circumference.

5. The method according to any of claims 1 - 4, further comprising: determining the predetermined position of the at least one insert based on a pressure distribution on the inner surface of the at least one shell part required for forming the final hollow part in the final mold.

6. The method according to any of the claims 1 - 5, wherein determining the predetermined position of the at least one insert is at a position where the pressure applied by pressure unit on the inner surface of the at least one shell part is insufficient to form the final hollow part in the final mold.

7. The method according to any of claims 1-6, further comprising placing multiple first shell parts in the first mold half and placing multiple second shell parts in the second mold half.

8. The method according to any of the preceding claims, comprising: providing at least one first trimmed shell part and at least one second trimmed shell part.

9. The method according to any of the preceding claims, further comprising: removing one or more of the at least one insert from the hollow part.

10. The method according to any of the preceding claims, wherein at least one insert is fused to the inner surface of the final hollow part.

11. The method according to any of the preceding claims 1 - 10, wherein the insert is provided of a thermoset material having a melting temperature higher than the melting temperature of the thermoplastic fiber reinforced material of the shell parts.

12. The method according to any of the preceding claims, wherein placing the pressure unit comprises placing a bladder in at least one of the first and second shell part.

13. The method according to any of the preceding claims, wherein the method further comprises: placing the at least one insert at the predefined position such that the pressure unit in combination with the at least one insert provides the required pressure in the final mold to form the final hollow part in the final mold.

14. The method according to any of the preceding claims, further comprising: placing the at least one insert such that the pressure unit in combination with the at least one insert provides pressure by thermal expansion of the at least one insert, wherein the thermal expansion of the at least one insert is at least 20% higher than the thermal expansion of the mold half at the fusing temperature.

15. The method according to any of the preceding claims, further comprising: placing the at least one insert such that the pressure unit in combination with the at least one insert provides hydrostatic pressure by flowing of the insert, wherein the viscosity of the insert is at least 1,3 timeshigher than the viscosity of the matrix material of the shell parts at the fusing temperature.

16. The method according to any of the preceding claims, further comprising: placing the at least one insert, wherein the at least one insert is a silicon insert.

17. A method of manufacturing a hollow composite part, such as a bicycle frame part, from a thermoplastic composite reinforced material; the method comprising: providing at least one first shell part of a thermoplastic reinforced material; the first shell part having a substantially U-shaped cross-section having outer edges; placing the at least one first shell part in a first mold half; providing at least one insert of a material that is able to deform at the fusion temperature; arranging the at least one insert in the at least one first shell part at a position requiring a predetermined pressure, where the required pressure cannot be provided solely by a pressure unit; engaging a pressure unit to the first mold half; providing at least one second shell part of a thermoplastic reinforced material; the second shell part having a substantially U-shaped cross-section having outer edges; placing the at least one second shell part in a second mold half; closing the first mold half and the second mold half to provide a final mold, such that the outer edges of the respective first shell part and the second shell part are adjacent each other;heating the final mold and pressurizing the pressure unit to provide for adequate internal pressure to the first and second shell parts to form and fuse the first and second shell parts; coohng of the final mold to allow the first and second shell parts to join to a final hollow part; removing the final hollow part from the final mold.

18. A composite structure, such as a bicycle frame, comprising: at least one first final part of a thermoplastic reinforced material, the first final part having a U-shaped cross-section: at least one second final part of the thermoplastic reinforced material, the second final part having a U-shaped cross-section; wherein the first final part and the second final part are fused to each other to form a final hollow part; at least one insert inside of the hollow part, wherein the insert is in contact to an interior surface of the first final part and / or to an interior surface of the second final part, wherein the insert is able to deform at the fusion temperature.

19. The composite structure according to claim 18, wherein the final hollow part comprises a protrusion, wherein the insert is positioned in the protrusion, wherein the protrusion provides a position of an abrupt increase of a circumference of the final hollow part wherein the circumference changes at least 1.5 over a distance equal to the circumference.

20. The composite structure according to any of the claims 18-19, wherein the one or more inserts comprise at least one thermoplastic polymer matrix.

21. The composite structure according to any of the claims 18-20, wherein the one or more inserts comprise a silicon insert.

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