GB2640445A

GB2640445A - Method of joining thermoset parts

Info

Publication number: GB2640445A
Application number: GB2405514.7A
Authority: GB
Inventors: Santoni Claudio; Sordon Alessandro
Original assignee: Silverstone Performance Technologies Ltd
Current assignee: Silverstone Performance Technologies Ltd
Priority date: 2024-04-19
Filing date: 2024-04-19
Publication date: 2025-10-22
Also published as: WO2025219413A1; GB202405514D0

Abstract

A method 100 of joining two or more thermoset parts 2, 4 comprises: applying a thermoplastic interface layer 8 to a surface of each of the two or more thermoset parts; and moulding a thermoplastic connection structure 6 onto the thermoplastic interface layer on each of the two or more thermoset parts. Variations may include joining a single thermoset part and a thermoplastic part. A further method comprises forming a thermoset part by a pultrusion process, applying a thermoplastic interface layer to the thermoset part and moulding a thermoplastic part on the interface layer. A further method comprises applying a powdered thermoplastic material to a thermoset part whilst the thermoset part is at least partially uncured; curing the thermoset part and the powdered material to form a thermoplastic interface layer on the thermoset part; and moulding a thermoplastic part onto the thermoplastic interface layer. The step of moulding the thermoplastic part may comprise injection moulding, compression moulding, resin transfer moulding, reaction injection moulding or an over moulding process. The thermoset parts may comprise a composite material, such as carbon fibre composites. The method may be used to form a crash structure for a vehicle configured to absorb an impact.

Description

METHOD OF JOINING THERMOSET PARTS FIELD OF THE INVENTION

The invention relates to manufacturing processes. In particular, the invention relates to manufacturing processes for parts comprising thermoset and thermoplastic materials.

BACKGROUND

Thermoset materials can be highly structurally efficient, or, in other words, have a good strength-to-weight ratio, that makes their usage desirable in many different scenarios. However, it can be difficult to join discrete thermoset pieces while avoiding structural weaknesses in the overall structure. As some methods of manufacturing thermoset structures efficiently can only produce thermoset parts of a limited size, there is a demand for techniques for joining thermoset parts both efficiently, securely, and conveniently.

Crash structures for vehicles are one type of structure for which thermoset materials have useful properties. Specifically, there is a general demand for structural efficiency in vehicles to maximise impact absorption (and therefore safety) while minimising weight (and therefore fuel-efficiency). Crash structures for larger vehicles, such as cars, vans, or the like, are often required to be larger than the size of thermoset parts that efficient methods of manufacturing thermoset parts can produce. Additionally, automotive manufacturing typically requires high production volumes. Therefore, there is a demand for improved methods of forming crash structures.

Similarly, for many applications, it would be desirable to produce parts that are structurally efficient but can also be manufactured both efficiently and cheaply while comprising complex shapes. Thermoset materials are known to be structurally efficient, while thermoplastics can be formed using a wide variety of efficient and flexible manufacturing techniques. Composite parts formed of both thermoset and thermoplastic parts would therefore be useful in many scenarios, such as for creating crash structures for vehicles.

However, it can be difficult to achieve a good bonding strength between the thermoset parts and thermoplastic parts. Some techniques for achieving a sufficient bonding strength can be slow and difficult to manage in a manufacturing environment. For example, some existing methods involve partially curing the thermoset part in close proximity to a plastic moulding tool and carefully managing the temperature of the thermoset part while moulding plastic onto the thermoset part. This creates an undesirable time-and condition-sensitive manufacturing step. Other methods require additional surface treatment steps that prolong production times. There is therefore a demand for improved methods of joining thermoset and thermoplastic parts.

It is an object of the present invention to address these demands. SUMMARY OF INVENTION According to a first aspect of the present invention, there is provided a method of joining two or more thermoset parts, comprising: applying a thermoplastic interface layer to a surface of each of the two or more thermoset parts; and moulding a thermoplastic connection structure onto the thermoplastic interface layer on each of the two or more thermoset parts.

In this way, the two or more thermoset parts can be joined both strongly and conveniently. The thermoplastic interface layer can be applied during the manufacturing of the thermoset parts, or shortly thereafter using convenient methods of applying films. The thermoplastic connection structure can be joined to the thermoplastic interface layer on each thermoset part, which makes the process less sensitive to interruptions because the thermoplastic interface layer does not need to be in an at least partially uncured state to bond to the thermoplastic connection structure.

The method may involve a step of mechanically or chemically treating a surface of the thermoset parts before applying the thermoplastic interface layer, using techniques known in the art, such as surface laser ablation.

The thermoset parts may be any kind of material comprising a thermosetting polymer. Preferably, the thermoset part comprises pre-preg, which may be formed of carbon fibre fabric and epoxy resin, in one example.

Preferably, the method comprises (a prior step of) forming the two or more thermoset parts using a pultrusion process. In this way, the thermoset parts can be formed using a highly automatable process. As pultrusion methods can typically only form thermoset parts of a limited size, the thermoplastic connection structure has particular utility in joining the two or more thermoset parts because thermoplastic parts can be moulded to much larger sizes.

Preferably, the thermoplastic interface layer is bonded to the two or more thermoset parts during a curing step of the pultrusion process, which can be performed using a heated press or die. In this way, the thermoplastic interface layer can be co-bonded with the thermoset parts, which encourages chemical bonding and avoids the need for a separate curing or bonding step. The thermoplastic interface layer may be applied to the thermoset parts before a main or sole curing step of the pultrusion process to maximise efficiency. In one example, the thermoplastic interface layer can be sprayed onto the thermoset before curing of the thermoset.

Co-curing in the art generally refers to integrating an uncured or partially pre-cured ('b-staged') preform part (also called a 'wet' preform), such as prepreg, with another thermoset part. Co-bonding generally refers to integration of an already cured, or 'hard', preform, or a non-curable element (such as a thermoplastic layer or part) with a thermoset part. It should be understood that "bonding", or "co-bonding", as used herein would encompass any type of bonding, as well as curing and co-curing of the thermoplastic interface layer or thermoplastic parts to or with the thermoset parts, for instance where the thermoplastic interface layer comprises at least one curable material, even though this is not a typical approach.

Preferably, the thermoplastic interface layer is co-extruded with the two or more thermoset parts during the pultrusion process. In this way, the step of applying the thermoplastic interface layer can be integrated easily into the pultrusion process.

Preferably, applying the thermoplastic interface layer comprises co-bonding the thermoplastic interface layer with the two or more thermoset parts. In other words, applying the thermoplastic interface layer comprises bonding the thermoplastic interface layer with the two or more thermoset parts during curing of the two or more thermoset parts. In this way, the need for a separate step of curing the thermoplastic interface layer is avoided. Furthermore, chemical bonding between the thermoplastic interface layer and the thermoset parts can be encouraged.

Where the thermoplastic interface layer is applied to the thermoset parts in a pultrusion process, the co-bonding can be carried out in a heated die, in one example. However, the co-bonding can be carried out using any suitable alternative technique.

In some embodiments, applying the thermoplastic interface layer is performed by applying a powdered thermoplastic material to the thermoset part while the thermoset part is at least partially uncured, wherein preferably the thermoset part is partially cured. In this way, the thermoplastic interface layer can be applied to thermoset parts that have more intricate shapes that can be difficult to coat using other methods of applying a surface layer. In such embodiments, the thermoset parts may be formed using forged carbon techniques, as known in the art, or by manually arranging ("laying-up") layers of a thermoset precursor material, such as pre-preg. Applying the powdered thermoplastic material can be carried out using a fluidised powder bed or a spray powder coating process, in some specific examples. Partially curing the thermoset parts can make the parts easier to handle.

Preferably, the thermoplastic interface layer is applied to the two or more thermoset parts while at least one of the two or more thermoset parts are substantially or partially uncured. In this way, the thermoset parts may be less pliable and therefore handled more easily.

Preferably, the step of moulding the thermoplastic connection structure is performed using an injection moulding, compression moulding, resin transfer moulding, reaction injection moulding, or overmoulding process, also known as "back-injection", the likes of which would be familiar to persons skilled in the art.

In other embodiments, any other suitable process of applying the thermoplastic connection structure can be implemented.

Preferably, the thermoplastic interface layer and the thermoplastic connection structure have melting points that differ by 50°C or less, preferably by 40°C or less, more preferably by 30°C or less, yet more preferably by 20°C or less. Preferably, the thermoplastic interface layer and the thermoplastic connection structure comprise copolymers, wherein the copolymers of the thermoplastic interface layer and the thermoplastic connection structure are the same copolymers or comprise at least one polymer that is common to both copolymers. In these ways, the thermoplastic interface layer and the thermoplastic connection structure have compatible properties that help to form a strong bond.

The thermoplastic interface layer and the thermoplastic connection structure may be formed of the same material to ensure good bonding compatibility. The thermoplastic interface layer is preferably non-metallic. The thermoplastic interface layer may comprise a plurality of polymers.

In some specific examples, the thermoplastic interface layer may comprise polyamide 6 or polypropylene.

Preferably, the thermoplastic connection structure comprises one or more of polypropylene, polycarbonate, acrylonitrile butadiene styrene, or nylon, such as polyamide 6. Polyamide 6 may be particularly preferred when the thermoset parts comprise pre-preg formed of epoxy resin. The thermoplastic connection structure can also comprise a fibrous material, such as carbon fibre, to provide greater structural rigidity.

In some embodiments, the thermoplastic interface layer comprises a first interface layer and a second interface layer comprising a different material from the first interface layer. The first layer can be optimised for bonding to the thermoset parts whereas the second layer can be optimised for bonding to the thermoplastic connection structure. The first layer and the second layer can be configured to form a strong bond with one another. In this way, the thermoplastic interface layer can enable a stronger overall bond to be formed between the thermoplastic connection structure and the thermoset parts, using a chain of strongly bonded interfaces.

In one example, the first layer can comprise a polyamide layer (e.g., polyamide 6) provided on the thermoset parts, for maximum adhesion to the thermoset parts, and a polypropylene layer, or a co-polymer layer where the co-polymer comprises polypropylene, provided on the polyamide layer and in contact with the connection structure. In this example, the thermoplastic connection structure may also comprise polypropylene.

Optionally, a third intermediate layer can be provided between the first and second layers. Continuing the above example of a polyamide layer and the layer comprising polypropylene, the third intermediate layer can be configured to perform various functions, such as increase surface gloss, increasing moisture resistance or resistance to other chemicals, or any other suitable function. The particular material of the intermediate layer can be selected based on the function required.

The first layer and the second layer may have different thicknesses. The second layer can be thicker than the first layer, to provide more material that can melt and join with the thermoplastic connection structure. This can also be useful to avoid damage to the first layer during the moulding onto the thermoplastic interface layer.

Preferably, the two or more thermoset parts comprise a fibrous material, such as carbon fibre. In this way, the thermoset parts can provide a strong structure. The fibrous material may be impregnated by any suitable resin material, such as epoxy, vinylester, or polyester Preferably, the two or more thermoset parts are joined to form a crash structure for a vehicle, the crash structure configured to absorb an impact. In this way, the crash structure can benefit from the cost-effective and flexible manufacturing of the thermoplastic connection structure and the structurally efficient thermoset material. In particular, the thermoset parts may be formed from a pultrusion process, in order to manufacture the crash structure as efficiently as possible, which is important in automotive applications where high-volume manufacturing is often necessary.

Preferably the crash structure comprises an array of parallel thermoset parts, 10 which may be pultruded thermoset parts. The thermoset parts may be substantially tubular or cylindrical. The crash structure may be configured to absorb an impact along a particular axis, which may be a longitudinal axis.

Preferably, the crash structure is for housing a battery. The thermoplastic connection structure may comprise one or more surface features that enable attachment of the battery to the crash structure, such as a seat configured to receive the battery. Some methods may comprise arranging a battery within the crash structure.

Preferably, at least one of the two or more thermoset parts are configured to provide a cross member of the crash structure, the cross member arranged perpendicularly to a longitudinal axis of the crash structure. In this way, the thermoset part can provide structural support to the crash structure.

Preferably, the step of moulding the thermoplastic connection structure is performed at a temperature that is higher than a glass transition temperature of the two or more thermoset parts. Said step may be performed by positioning at least one of the thermoset parts in the mould for the connection structure. In this way, the thermoset parts can conform to the shape of the mould, even if they had been fully cured previously. This avoids creating cracks or structural weaknesses in the thermoset without the need for a mould fitted precisely to the shape of the thermoset part.

Preferably, the thermoplastic interface layer is a film. The thermoplastic interface layer may also be described as a conformal layer, e.g. conforming to the surface of the thermoset part. The thermoplastic interface layer may have a maximum thickness of 5 mm, 1 mm, 0.5 mm, 0.1 mm, or less than 0.1 mm. The thermoplastic interface layer may have a substantially constant thickness across the thermoset part.

According to a second aspect of the present invention, there is provided a method of joining a thermoplastic part to a thermoset part, comprising: pultruding the thermoset part using a pultrusion process; applying a thermoplastic interface layer to the thermoset part; and moulding the thermoplastic part onto the thermoplastic interface layer.

In this way, the thermoplastic part and the thermoset part can be joined both conveniently and strongly. Application of the thermoplastic interface layer can be integrated easily in the pultrusion process in various different ways, for example using co-extrusion, or by spraying a coating onto the thermoset before curing.

Subsequently, the thermoplastic part can be moulded onto the thermoplastic interface layer. As creating a strong join between the thermoplastic interface layer and the thermoplastic part is not dependent on maintaining an uncured or partially uncured state, the step of moulding the thermoplastic part is less sensitive to conditions and interruptions in the process. This enables the parts to be joined more conveniently.

Preferably, applying the thermoplastic interface layer comprises co-bonding the thermoplastic interface layer with the thermoset part. In other words, the thermoplastic interface layer is bonded to the thermoset part during curing of the thermoset part. This can be achieved by applying the thermoplastic interface layer to the thermoset part before a curing step of the pultrusion process, preferably when the thermoset part is fully uncured. In this way, the need for a separate curing step is avoided and chemical bonding can be encouraged. The co-bonding can be performed in a heated die or press, as described above.

Preferably, the thermoplastic interface layer is co-extruded with the thermoset part during the pultrusion process. In this way, the thermoplastic interface layer can be conveniently and efficiently applied to the thermoset part.

In some embodiments, the thermoplastic interface layer comprises a first interface layer and a second interface layer that comprises a different material from the first interface layer. The first layer can be optimised for bonding to the thermoset part whereas the second layer can be optimised for bonding to the thermoplastic part. The first layer and the second layer can be configured to form a strong bond with one another. In this way, the thermoplastic interface layer can enable a stronger overall bond to be formed between the thermoplastic part and the thermoset part, using a chain of strongly bonded interfaces. The first layer and the second layer may have different thicknesses. The second layer can be thicker than the first layer, to provide more material that can melt and join with the thermoplastic connection structure as well as protect the first layer from damage. Each layer may be co-extruded in a pultrusion process.

In one example, the first layer can comprise a polyamide layer (e.g., polyamide 6) provided on the thermoset part, for maximum adhesion to the thermoset part, and the second layer can comprise polypropylene, or can be a co-polymer layer where the co-polymer comprises polypropylene, in contact with the thermoplastic part.

In this example, the thermoplastic part may also comprise polypropylene.

Preferably, the thermoplastic part comprises one or more of polypropylene, polycarbonate, or acrylonitrile butadiene styrene, or nylon, such as polyamide 6.

Polyamide 6 may be particularly preferred when the thermoset parts comprise pre-preg formed of epoxy resin. The thermoplastic part can also comprise a fibrous material, such as carbon fibre, to provide greater structural rigidity.

The thermoplastic interface layer and the thermoplastic part may be formed of the same material to ensure good bonding compatibility.

As described above, the thermoplastic interface layer may comprise polypropylene or nylon, such as polyamide 6.

Preferably, moulding a thermoplastic part onto the thermoplastic interface layer comprises injection moulding, compression moulding, resin transfer moulding, reaction injection moulding or overmoulding the thermoplastic part. Any other suitable technique may be used in other embodiments.

Preferably, the thermoset part and the thermoplastic part are joined to form a crash structure for a vehicle. In this way, the crash structure can benefit from the cost effectiveness and design flexibility of the thermoplastic part and the structural efficiency of the thermoset part, as well as the manufacturing efficiency of the pultrusion process.

Preferably, the crash structure is for housing a battery. The thermoplastic part may comprise one or more surface features that enable attachment of the battery to the crash structure, such as a seat configured to receive the battery.

The thermoset part may be configured to provide a cross-member of the crash 20 structure.

According to a third aspect of the invention, there is provided a method of joining a thermoplastic part to a thermoset part, comprising: applying a powdered thermoplastic material to the thermoset part while the thermoset part is at least partially uncured; curing the thermoset part and the powdered thermoplastic material to form a thermoplastic interface layer on the thermoset part; and moulding a thermoplastic part onto the thermoplastic interface layer.

In this way, the thermoplastic part and the thermoset part can be joined both conveniently and strongly. The thermoplastic interface layer can be applied easily to the thermoset part even if the thermoset part is of an intricate shape comprising crevices or recesses that makes it difficult to cover with a pre-formed film.

The thermoset part may be any kind of material comprising a thermosetting polymer. Preferably, the thermoset part comprises pre-preg, which may be formed of carbon fibre fabric and epoxy resin. The powder may be applied when the pre-preg is in a partially cured state, rather than in a completely uncured state, which can make the thermoset part easier to handle.

In one embodiment, applying a powdered thermoplastic material to the thermoset part is performed using a fluidised powder bed. In this way, the entire surface, or a large surface area, of the thermoset part can be easily coated with the thermoplastic interface layer.

Alternatively, applying a powdered thermoplastic material to the thermoset part is performed using a spray powder coating process, which may be quicker or cheaper than using a fluidised powder bed where the entire surface, or a large proportion, of the thermoset part need not be covered with the thermoplastic interface layer Preferably, moulding a thermoplastic part onto the thermoplastic interface layer comprises injection moulding, compression moulding, resin transfer moulding, reaction injection moulding or overmoulding the thermoplastic part. Any other suitable technique may be used in other embodiments.

Preferably, the thermoplastic part comprises one or more of nylon, such as polyamide 6, polypropylene, polycarbonate, or acrylonitrile butadiene styrene. Polyamide 6 may be particularly preferred when the thermoset parts comprise prepreg formed of epoxy resin. The thermoplastic part can also comprise a fibrous material, such as carbon fibre, to provide greater structural rigidity.

As described above, the thermoplastic interface layer may comprise polypropylene, or nylon, such as polyamide 6.

In some embodiments, the thermoplastic interface layer comprises a first interface layer and a second interface layer that comprises a different material from the first interface layer. The first layer can be optimised for bonding to the thermoset part whereas the second layer can be optimised for bonding to the thermoplastic part. The first layer and the second layer can be configured to form a strong bond with one another. In this way, the thermoplastic interface layer can enable a stronger overall bond to be formed between the thermoplastic part and the thermoset part, using a chain of strongly bonded interfaces. The first layer and the second layer may have different thicknesses. The second layer can be thicker than the first layer, to provide more material that can melt and join with the thermoplastic connection structure.

In one example, the first layer can comprise a polyamide layer (e.g., polyamide 6) provided on the thermoset part, for maximum adhesion to the thermoset part, and the second layer can comprise polypropylene, or can be a co-polymer layer where the co-polymer comprises polypropylene, in contact with the thermoplastic part. In this example, the thermoplastic part may also comprise polypropylene.

Preferably, the method further comprises treating a surface of the thermoset part before applying the powdered thermoplastic material, wherein the powdered thermoplastic material is applied to the treated surface. In this way, a stronger bond can be formed between the thermoplastic interface layer and the thermoset part. Any suitable kind of chemical or physical surface treatment known in the art can be applied, such as sand blasting, chemical etching, manual abrasion, or plasma treatment.

Preferably, the thermoset part and the thermoplastic part are joined to form a crash structure for a vehicle, the crash structure configured to absorb an impact. In this way, the crash structure can benefit from the cost effectiveness and design flexibility of the thermoplastic part and the structural efficiency of the thermoset part. The thermoplastic interface layer can be applied to the thermoset part even if the thermoset part is of a complex shape that is both common to crash structures and difficult to coat using a pre-formed film.

The thermoset part can provide a cross member of the crash structure, as described with respect to the first aspect of the invention.

Preferably, the powdered thermoplastic material is applied to the thermoset part when the thermoset part is partially cured, which can make the thermoset part less pliable and hence easier to handle.

Embodiments of the first aspect of the invention may generally be combined with the second and third aspects of the invention.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention are now described, by way of example, with reference to the drawings, in which: Figure 1 shows a schematic cross-sectional diagram of a composite structure manufactured according to an embodiment of the invention; Figure 2 shows a flowchart of a method according to an embodiment of the invention; Figure 3 shows a schematic diagram of a pultrusion apparatus that can be used to perform an embodiment of the invention; Figure 4 shows a schematic cross-sectional diagram of a fluidised powder bed apparatus that can be used to perform an embodiment of the invention; and Figure 5 shows a schematic cross-sectional diagram of a spray powder coating apparatus that can be used to perform an embodiment of the invention.

DETAILED DESCRIPTION

Figure 1 shows a schematic cross-sectional illustration of two thermoset parts joined by a thermoplastic connection structure using methods of the invention.

As shown, a composite structure 1 is formed from a thermoset part 2 joined to a thermoset part 4 by a thermoplastic connection structure 6. In this example, the composite structure 1 is configured to provide at least part of a crash structure that is suitable for use in any kind of vehicle. The thermoplastic connection structure 6 (which may also be referred to as the "connection structure") may comprise surface features 7 produced during moulding of the connection structure 6 for various purposes. A thermoplastic interface layer 8 is provided separately on a surface of the thermoset part 2 and the thermoset part 4. The thermoplastic connection structure 6 is attached to the outer face of the thermoplastic interface layer 8 at each thermoset part 2, 4.

The thermoplastic interface layer 8 enables the thermoset parts 2, 4 and the connection structure 6 to be joined efficiently and conveniently. Prior approaches to joining thermoset to thermoplastic involve carefully controlling a partial cure state of thermoset parts while plastic moulding thermoplastic onto the partially cured thermoset. In practical scenarios, the thermoset and thermoplastic parts often must be formed in close proximity and joined quickly after being formed. This can be cumbersome and sensitive to interruptions in the process, which means that previously it would be prohibitively inconvenient to join two thermoset parts with a thermoplastic structure.

The thermoplastic interface layer 8 can be applied more easily compared to a larger thermoplastic piece. That is, coating processes are typically easier to integrate within, or perform immediately after, thermoset part manufacturing processes. Subsequently, the connection structure 6 can be moulded onto the thermoplastic interface layer 8. This process is less sensitive to interruptions because the thermoplastic interface layer 8 and the connection structure 6 do not need to be in a partially cured state to be joined strongly. The thermoplastic interface layer 8 is preferably applied before curing of the thermoset parts 2, 4 so that it can be co-cured with the thermoset parts, reducing the number of separate curing or processing steps required.

As thermoplastic moulds can be made of a much larger size than some methods of forming thermoset, such as pultrusion, it can be more convenient to join two thermoset parts using a thermoplastic connection structure compared to a thermoset connection structure. Additionally, joining several pieces of thermoset separately formed can introduce structural weaknesses in the composite structure.

The composite structure 1 is a part of or a whole crash structure for a vehicle in this example. Specifically, the crash structure is a battery casing for housing a battery, wherein the thermoset parts 2, 4 each provide cross-members of the battery casing for structural support. The connection structure 6 comprises various surface features 7 that can enable attachment of a vehicle battery to the composite structure 1.

In some examples. the thermoset parts 2, 4 may be substantially parallel along an axis. This axis can be a main impact absorbing axis along which the crash structure is designed to crumple to absorb an impact. The thermoset parts 2, 4 can be substantially cylindrical or tubular, where the longitudinal axis of the thermoset parts 2, 4 is the main impact absorbing axis. One of the thermoset parts 2, 4 can form a cross member of the crash structure, or, in other words a structural part that is transverse to a longitudinal axis of the crash structure.

In other examples, the composite structure 1 can be for any other suitable type of device or structure, for a vehicle or otherwise.

The thermoset parts 2, 4 comprise carbon fibre impregnated with an epoxy resin.

In other examples, the thermoset parts can comprise any other suitable matrix or fibrous material and resin material, such as vinylester or polyester. More generally, the thermoset parts 2, 4 can be any material comprising a thermosetting polymer.

The thermoset parts 2, 4 may be formed using any suitable process, such as by manually laying-up layers of pre-preg or using forged carbon techniques. In this example, however, the thermoset parts 2, 4 are formed from a pultrusion process, described in more detail below, which is a particularly efficient and less labour intensive.

The thermoset parts 2, 4 can have any suitable shape or structure, such as a substantially flat panel shape, an L-shape, hollow shape, a thin-walled hollow shape, an open profile section, or an irregular shape comprising curves or bends. In the examples herein it is assumed the thermoset parts 2, 4 are made from the same process. However, it would be appreciated that each thermoset part can be made from a different process.

The thermoplastic connection structure 6 can comprise any suitable thermoplastic material that is able to bond with the thermoplastic interface layer 8. In the example of Figure 1, the connection structure 6 comprises polyamide 6. The thermoplastic connection structure 6 may comprise polypropylene, polycarbonate, acrylonitrile butadiene styrene, or other type of nylon in other examples. The connection structure 6 can be moulded onto the thermoset parts at the thermoplastic interface layer 8 using injection moulding, compression moulding, resin transfer moulding, reaction injection moulding overmoulding, or any other suitable process. In the example of Figure 1, the connection structure 6 is formed using an overmoulding process, the likes of which would be familiar to those skilled in the art. The connection structure 6 may also comprise a fibrous material, such as carbon fibre, in some examples. The fibrous material can provide greater structural rigidity to the connection structure 6.

The thermoplastic connection structure 6 can have any suitable shape that can be formed by plastic moulding. The surface features 7 form a seat configured to receive a vehicle battery in this example but can be directed to any other suitable purpose in other examples. For instance, the surface features can comprise ribs or bosses for any suitable purpose.

The thermoplastic interface layer 8 can comprise any suitable material that is able to bond to the thermoset parts 2, 4 and the thermoplastic connection structure 6.

In some examples, the thermoplastic interface layer 8 comprises polypropylene, or nylon, such as polyamide 6. In the example of Figure 1, the thermoplastic interface layer 8 comprises a single layer of polyamide 6. The thermoplastic interface layer 8 may be provided as a film of negligible thickness compared to the thermoset parts 2, 4.

The thermoplastic interface layer 8 and the thermoplastic connection structure 8 may comprise the same material to ensure compatibility that generates a strong connection. Alternatively, the thermoplastic connection structure 6 and the thermoplastic interface layer 8 may comprise different materials with similar melting points that differ by less than 50°C. The thermoplastic connection structure 6 and the thermoplastic interface layer 8 may also comprise copolymers, e.g., copolymers that share a common polymer In other examples, the thermoplastic interface layer 8 can comprise a first layer and a second layer of different materials. The first layer can be optimised for bonding to the thermoset parts 2, 4, whereas the second layer can be optimised for bonding to the connection structure 6. In this case, each layer may be provided as a film of negligible thickness. Alternatively, one layer may be provided with a different thickness. For instance, the second layer can be thicker than the first layer to provide more thermoplastic material that can melt and join with the thermoplastic connection structure 6.

In one example, the first layer, provided on the thermoset parts 2, 4, can comprise polyamide and the second layer, in contact with the connection structure 6, can comprise polypropylene or a copolymer that comprises polypropylene. The thermoplastic connection structure 6 may also comprise polypropylene in this instance.

The thermoplastic interface layer 8 can completely cover the thermoset parts 2, 4, or alternatively may only be applied to specific locations where the connection structure 6 is to be formed on.

Figure 2 shows a flowchart of a method 100 of joining the thermoset part 2 and the thermoset part 4 to form the composite structure 1 according to an embodiment of the invention.

In step 102, the thermoset part 2 and the thermoset part 4 are formed using a pultrusion process. Figure 3 shows a schematic illustration of an example system 9 that can be used to perform the pultrusion, however any other suitable apparatus can be used. As shown, a sheet 10 dispensed from a roll 11 of unimpregnated carbon fibre fabric can be pulled through a tank 12 containing liquid epoxy resin 13 to impregnate the carbon fibre with uncured resin. The carbon fibre is pulled by a pulling mechanism 14 at an upstream location. The impregnated carbon fibre forms the thermoset part 2 and/or the thermoset part 4. It would be appreciated that the thermoset parts 2, 4 may be formed in parallel in separate pultrusion systems, each performing steps 102 to 106. Alternatively, the thermoset parts 2, 4 may be formed in sequence by repeating steps 102 to 106 in the same system, such as the system of Figure 3. It may also be possible to form both thermoset parts 2, 4 in the same heated die, then later separate the parts from a single discrete piece.

In step 104, the thermoplastic interface layer 8 is applied to the thermoset parts 2, 4. In this example, the thermoplastic interface layer 8 is co-extruded with the resin-impregnated thermoset parts 2, 4 before curing of the thermoset parts 2, 4, so that a thin film of the thermoplastic interface layer 8 is pulled from a roll onto the surface of the resin-impregnated thermoset parts 2, 4. As shown in Figure 3, the thermoplastic interface layer 8 is dispensed from two rolls 16 of polyamide 6 film to cover both sides of the impregnated carbon fibre. However, in other examples, only one side, or less than a whole side, may be covered by the thermoplastic interface layer 8. Where the thermoplastic interface layer 8 comprises multiple layers of different materials, each layer may be co-extruded in this way.

In step 106, the thermoset parts 2, 4 are cured. In this example, the thermoset parts 2, 4 are cured in a heated die 18 while in contact with the co-extruded thermoplastic interface layer 8. The heated die 18 comprises an internal mould (not shown) that applies pressure to the thermoset parts 2, 4 to press them into a desired shape, which in general can comprise curves or bends. The die is heated so that the resin impregnated in the thermoset parts 2, 4 can cross-link to form a solidified structure once cooled. At the same time, the thermoplastic interface layer 8 chemically bonds to the curing resin of the thermoset parts 2, 4 due to the heat, pressure, and cross-linking of the curing resin. Subsequently, the cured thermoset parts 2, 4 may be cut by a cutter 20 so that they become disconnected from the rolls of material from which they were extruded, as shown at the far right of Figure 3.

In this example, the thermoplastic interface layer 8 is applied to the thermoset parts 2, 4 while the thermoset parts 2, 4 are uncured, then the thermoset parts 2, 4 and thermoplastic interface layer 8 are co-bonded. However, in other embodiments, it would also be possible to apply the thermoplastic interface layer 8 to the thermoset parts 2, 4 after they are cured. In this case, the surface of the cured thermoset parts 2, 4 may be treated, e.g. by laser ablation, to allow the thermoplastic interface film 8 to disperse and diffuse into the surface to encourage bonding. Thus, in some embodiments, step 106 may comprise two separate stages of curing. Surface treatments, such as sand blasting, chemical etching, manual abrasion, or plasma treatment, may be used in situations where a particularly strong bond is required, but in general may not be necessary.

Nevertheless, applying the thermoplastic interface film 8 before curing the thermoset parts 2, 4 may be preferred as it can be adhered to the thermoset parts 2, 4 during curing of the thermoset parts 2, 4 themselves. This avoids additional surface treatment and curing steps.

In step 108, the thermoplastic connection structure 8 is moulded onto the parts of the thermoset parts 2, 4 covered by the thermoplastic interface layer 8 by overmoulding to form the composite structure 1. To facilitate this, the thermoset parts 2, 4 may be arranged in or sealed against a mould before molten thermoplastic material is injected onto the thermoplastic interface layer 8 present on each thermoset part. The mould may comprise surface features for forming the corresponding surface features 7 in the composite structure 1. Step 108 can be more conveniently performed at a different time and location than steps 102 to 106 because the cure-state of the thermoset parts 2, 4 does not need to be carefully controlled to create a bond between the thermoset parts 2, 4 and the thermoplastic connection structure 6.

In other examples, any other suitable thermoplastic moulding process can be used. The overmoulding (or thermoplastic moulding more generally) may be performed at a temperature at or above the glass transition temperature of the resin of the thermoset parts 2, 4 to enable the thermoset parts 2, 4 to conform to a part of a mould of the connection structure 6.

In optional step 110, which can occur once the composite structure 1 has cooled, other parts may be added to or removed from the composite structure 1 by machining or additive manufacturing to create features necessary for attachment to other parts.

Steps 102 to 106 of the method 100 are described above with respect to Figure 3 and a pultrusion process, which is a particularly efficient method of forming thermoset parts. However, these steps can also be performed in various other ways to meet other demands of the composite structure 1. For instance, it may be desirable to manipulate the thermoset parts into more curved or complicated shapes than can be achieved by heated die used in pultrusion processes, such that use of a pultrusion process is not feasible. Crash structures, in particular crash structures for vehicle batteries, are one example where this can be desirable. One alternative implementation of the method 100 that seeks to address this issue is described below with reference to Figures 2, 4 and 5.

In step 102, the thermoset parts 2, 4 are formed into a partially cured state using any suitable alternative method, such as by manually laying up pre-preg sheets or using forged carbon techniques and controlling the pre-preg's temperature to manage the cure-state. In other embodiments, the thermoset parts 2, 4 may be completely uncured at this point.

In step 104, a powdered form of the thermoplastic interface layer 8 is applied to the surface of the thermoset parts 2, 4. This can be carried out using a fluidised powder bed 22, as shown in Figure 4, which comprises a tank 22 containing a powder 26 of the material that is to form the thermoplastic interface layer 8, which in this example is polyamide 6. Jets 28 are provided at the base of the tank 24 to expel gas through the powder 26, causing the powder to act like a fluid. This allows the partially cured thermoset pad 2, for example, to be dipped easily into the tank 24 to become evenly coated by the powder 26.

Alternatively, as shown in Figure 5, the powder 26 can be applied using a powder spray device 30 that uses gas to expel powder through a nozzle 32 to coat the thermoset part 2 as desired.

Applying powder to the thermoset part 2 in these ways can be used to apply the thermoplastic interface layer 8 to the thermoset part 2 even if crevices, recesses, or shapes that are difficult to coat using a roll of film are present in the thermoset part 2.

Optionally, before applying powder 26 using the fluidised powder bed or spray coating techniques, the surface of the thermoset part 2 may be treated in any suitable manner, e.g. using laser ablation, to enable a stronger bond to form between the thermoset part 2 and the thermoplastic interface layer 8.

At step 106, the thermoset part 2 and the powder coating can be co-cured (i.e., cured simultaneously) to simultaneously melt the powder 26 coated on the thermoset part 2 and cure the thermoset part 2, thereby bonding the two materials.

This can be performed in any suitable manner, such as using heat and pressure in a mould.

Steps 102 to 106 can be repeated in this way to form and coat the thermoset part 4 in the same manner. In cases where the thermoplastic interface layer 8 comprises multiple layers, steps 104 and 106 may be repeated for each part to bond the additional layers onto the first layer.

Subsequently, steps 108 and 110 can be repeated as described above to join the thermoset part 2 and the thermoset part 4.

In other implementations of the method 100, the thermoset parts 2, 4 can be formed using any other suitable process. Equally, the thermoplastic interface layer 8 can be applied to the thermoset parts 2, 4 using any other suitable technique.

It would be appreciated the method 100 could be adjusted to join more than two thermoset parts by providing more than two thermoset parts to be joined by the connection structure 6 at step 108.

Variations of the method 100 can also be employed to join a single thermoset part and a thermoplastic part. In these cases, the steps and embodiments are the same as described above, except that steps 102 to 106 are performed only once, for the single thermoset part, and instead of a thermoplastic connection structure 6 a thermoplastic part is moulded onto the single thermoset part at step 108. This can produce a composite structure similar to the composite structure 1 but comprising a single thermoset part.

The thermoset part and the thermoplastic part can be produced using the pultrusion variation of the method 100 described above with respect to Figure 3.

In this case, the thermoplastic interface layer 8 can be co-extruded and co-cured with the thermoset part, as described previously. Alternatively, the thermoplastic interface layer 8 can be applied using a powdered thermoplastic material, as described above with respect to Figures 4 and 5. The thermoplastic part can be moulded onto the thermoplastic interface layer 8 by any suitable plastic moulding technique, such as overmoulding, resin transfer moulding, or any of the other techniques described previously.

The resulting composite part can be used in any suitable device or structure. In one example, the composite formed of the joined thermoplastic and thermoset part can form at least a part of a crash structure for a vehicle, where the thermoset part may provide a cross member of the crash structure. The crash structure can be configured for housing a battery of a vehicle, as described previously.

Claims

CLAIMS1. A method of joining two or more thermoset parts, comprising: applying a thermoplastic interface layer to a surface of each of the two or more thermoset pads; and moulding a thermoplastic connection structure onto the thermoplastic interface layer on each of the two or more thermoset parts.
2. The method of claim 1, further comprising forming the two or more thermoset parts using a pultrusion process.
3. The method of claim 2, wherein the thermoplastic interface layer is bonded to the two or more thermoset parts during a curing step of the pultrusion process.
4. The method of any of the preceding claims, wherein the thermoplastic interface layer is co-extruded with the two or more thermoset parts.
5. The method of any of the preceding claims, wherein applying the thermoplastic interface layer comprises bonding the thermoplastic interface layer with the two or more thermoset parts during curing of the two or more thermoset parts.
6. The method of claim 5, wherein applying the thermoplastic interface layer is performed by applying a powdered thermoplastic material to the thermoset part while the thermoset part is at least partially uncured, wherein preferably the thermoset part is partially cured.
7. The method of any of the preceding claims, wherein the thermoplastic interface layer is applied to the two or more thermoset parts while at least one of the two or more thermoset parts are substantially or partially uncured.
8. The method of any of the preceding claims, wherein the step of moulding the thermoplastic connection structure is performed using an injection moulding, compression moulding, resin transfer moulding, reaction injection moulding or overmoulding process.
9. The method of any of the preceding claims, wherein the thermoplastic interface layer and the thermoplastic connection structure have melting points that differ by 50°C or less, preferably by 40°C or less, more preferably by 30°C or less, more preferably by 20°C or less.
10. The method of any of the preceding claims, wherein the thermoplastic interface layer and the thermoplastic connection structure comprise copolymers, preferably wherein the thermoplastic interface layer and the thermoplastic connection structure comprise the same copolymer or comprise copolymers with at least one copolymer in common.
11. The method of any of the preceding claims, wherein the thermoplastic connection structure comprises polypropylene, polycarbonate, acrylonitrile butadiene styrene, or nylon, such as polyamide 6.
12. The method of any of the preceding claims, wherein the thermoplastic interface layer comprises polypropylene or nylon, such as polyamide 6.
13. The method of any of the preceding claims, wherein the thermoplastic interface layer comprises a first interface layer and a second interface layer that comprises a different material from the first interface layer
14. The method of any of the preceding claims, wherein the two or more thermoset parts comprise a fibrous material, such as carbon fibre.
15. The method of any of the preceding claims, wherein the two or more thermoset parts are joined to form a crash structure for a vehicle, the crash structure configured to absorb an impact.
16. The method of claim 15, wherein the crash structure is for housing a battery.
17. The method of claim 15 or claim 16, wherein at least one of the two or more thermoset parts are configured to provide a cross member of the crash structure, the cross member arranged perpendicularly to a longitudinal axis of the crash structure.
18. The method of any of the preceding claims, wherein the step of moulding the thermoplastic connection structure is performed at a temperature that is higher than a glass transition temperature of the two or more thermoset parts.
19. The method of any of the preceding claims, wherein the thermoplastic interface layer is a film.
20. A method of joining a thermoplastic part to a thermoset part, comprising: pultruding the thermoset part using a pultrusion process; applying a thermoplastic interface layer to the thermoset part; and moulding the thermoplastic part onto the thermoplastic interface layer
21. A method of joining a thermoplastic part to a thermoset part, comprising: applying a powdered thermoplastic material to the thermoset part while the thermoset part is at least partially uncured; curing the thermoset part and the powdered thermoplastic material to form a thermoplastic interface layer on the thermoset part; and moulding a thermoplastic part onto the thermoplastic interface layer