WO2022129824A1

WO2022129824A1 - A structural panel and method and apparatus for manufacture

Info

Publication number: WO2022129824A1
Application number: PCT/GB2021/000142
Authority: WO
Inventors: Walter Boersma; Leslie MEADER; Jonathan Richards; John Weightman
Original assignee: Foresight Innovations Ltd
Priority date: 2020-12-18
Filing date: 2021-12-16
Publication date: 2022-06-23
Also published as: GB2602126A; GB202020167D0

Abstract

There is disclosed a structural panel (12', 52') formed from a plastics material with fibre reinforcement and having a folded profile based on a Miura-Ori fold to provide a repeated pattern of chevron shaped folds. The folded profile defines a core of the panel. A sheet extends in over at least one side of the core and spans the chevron shaped folds so as to provide a skin (42, 44) of the structural panel (12', 52'). The panel has a compressive strength of at least 2.5MPa measured in a direction out of a plane of the sheet. The panel may be formed by pressing the folded profile into a pre-formed sheet.

Description

Title - A Structural Panel and Method and Apparatus for Manufacture

The present invention relates to structural or reinforcing panels. In particular, the invention relates to a method of forming a structural or reinforcing panel and to an associated panel.

It is known to provide composite panels with a central reinforcing element, or core, to provide strength and/or rigidity to a panel. Various types of strengthening core are known in the art, including honeycomb matrices and corrugated or embossed sheet materials.

Honeycomb cells arranged at right-angles to the outer surfaces or skins of a panel typically provide the best strength for a given weight of panel, but the cores and the final panels are relatively complex and therefore expensive to manufacture.

Using corrugated, embossed or otherwise formed sheets as a core is simpler and more cost effective, but the strength of the resulting panels is typically lower for a given weight. Including reinforcing fibres to a polymer sheet forming the core can help to increase the strength with minimal impact on weight, but the reinforced polymer can then become difficult to process because of the additional stiffness introduced by the reinforcing fibres. As a result, it becomes difficult to create sufficient depth in the core profile to maximise its strength and rigidity.

The cost of manufacture, and the cost of the panel product itself, is a significant consideration for many panel applications. For example, in flooring applications, such as vehicle flooring, the conventional product that is widely adopted is a wood chip board due to its low cost. Such panel products are installed and subject to heavy wear and tear due, including bearing large weight in compression, impacts and shear forces. However boards of this kind are heavy and difficult to handle.

Flooring panels provide just one potential application. There are a variety of flooring and structural products in which conventional panels are heavy, unwieldy or relatively hight cost.

The present invention aims to provide a structural/reinforcing panel and associated method of manufacture that can mitigate one or more of these problems. It may be considered an additional/alternative aim of the invention to provide a low cost and lightweight panel that offers suitable structural strength/stiffness and/or impact resistance to be considered as a viable alternative to conventional panels.

According to a first aspect of the invention there is provided a structural panel a structural panel comprising a plastics material with fibre reinforcement and having a folded profile based on a Miura-Ori fold to provide a repeated pattern of chevron shaped folds. The panel has a compressive strength of at least 2.5MPa measured in a direction out of the plane of the panel.

The structural panel may be used in vehicle floors and/or body panels. The structural panel could be used in other flooring or wall panel applications.

For the purposes of this application, a “folded profile” should be understood to refer to a profile imparted to a sheet of material that gives the sheet the general appearance of a folded flat sheet, not necessarily to a sheet that has been folded to produce the profile.

The panel has a high compressive strength, making it particularly suitable for use in vehicle floors and other vehicle body panels.

The plastics material may be a thermoplastic such as Polypropylene (PP), Acrylonitrile butadiene styrene (ABS), VX (ABS/PVC), Polyethylene terephthalate (PET), or Polystyrene (PS).

The plastics material may comprise reclaimed or recycled plastics material. Alternatively, solely virgin material may be used.

The fibre reinforcement may comprise glass fibre and/or carbon fibre. Additionally, or alternatively, natural fibres may be used.

The fibre reinforced plastics material may comprise chopped/short or continuous/long fibres. The fibres could be haphazardly or randomly oriented within the plastics material. Alternatively, the fibres could be purposely oriented and/or arranged, e.g. in fibre bundles, according to a desired pattern.

The fibres may be formed into a textile, mat, net or webbing, e.g. a fibre reinforcement that runs substantially continuously through the panel and/or plastics material. The fibres may be arranged into a regular or repeating pattern.

The fibres may be formed into a woven, meshed or knitted matting.

The fibres may form a fibre layer, e.g. being substantially two-dimensional or planar in form in a macroscopic sense, than runs through the plastics material.

The fibres may be impregnated by and/or encapsulated within the plastics material.

The fibres are typically non-elastic in nature.

The fibre reinforcement may comprise recycled or reclaimed material.

The fibre reinforcement may comprise greater than or equal to 10%, 20% or 30% by weight of the composite making up the panel/core. The fibre reinforcement may comprise less than or equal to 60%, 50% or 40% by weight of the composite making up the panel/core. The fibre reinforcement may make up from 10% to 60% by weight of the panel, for example 20-50%, 20-40% or 30-40%.

The plastics material may comprise a sheet material having a thickness from 1-2mm, for example 1.5mm or 1.2mm.

The internal angle of each fold, that is the internal angle of a cross-section taken at right angles to a fold line of either part of a chevron, may be from 50°-80°, or from 55°-70°, for example 57° or 70°.

The length of each part of each chevron, i.e. the major length of each half of a complete chevron, may be from 15-25mm, for example 20mm.

The internal angle of each chevron in plan may be from 75°-105°, or from 85°-95° for example 90°.

The folded profile of the panel may be created by pre-forming a pattern or profile into a sheet or material and subsequently processing the pre-formed sheet to form a final panel, or the folded profile may be formed in a single moulding or pressing step performed on a substantially uniform flat sheet material.

The panel may comprise regions of thinner material and/or cuts or discontinuities at the fold lines of the folded profile. Where a pre-formed sheet is provided, the regions of thinner material and/or cuts or discontinuities may act as living hinges.

In the example of a pre-formed sheet, the sheet may comprise one or more formation to enable stretching of the sheet in a linear direction, e.g. in a length and/or width direction in the plane of the sheet. The fibre reinforcement and/or plastics material of the pre-form sheet may follow an undulating, pleated or concertina path in a length and/or width direction, e.g. when viewed in plan or section.

The fibres of the fibre reinforcement in the pre-form sheet, i.e. prior to pressing or processing into the Miura-Ori folded form, may follow a tortuous or non-liner path, e.g. in a length and/or width direction. The fibres may be arranged such that they can be reoriented or straightened during forming of a Miura-Ori fold. The non-linear/tortuous path may be a path with or through the plastics material. That is to say, the plastics material sheet may or may not share the same path.

It has been found that formation of the desired folded structure can cause residual internal stress/strain within the final panel structure, particularly in light of the fibre reinforcement. Providing an extensible/expandable arrangement in the plastics material and/or fibre reinforcement before forming a Miura-Ori fold can counteract such internal stresses in the final product, allowing the panel to adapt to a desired folded structure in a relaxed state. The cooled panel once formed is thus in a non-stressed or at-rest state.

The plastics material and/or fibre reinforcement may be expandable in a width/lateral direction, e.g. relative to a longitudinal or feed direction through a press/rollers used to form the Miura-Ori fold.

The panel may comprise a skin covering either or both sides of the structural panel. The skin may provide an outer surface layer of the panel. The skin may be planar and/or flat in form. The skin may be continuous or discontinuous. The skin may be joined to the Miura-Ori folded structure. The skin may span adjacent folds of the Miura-Ori folded structure. The skin may be joined to the creases, ridges or fold lines of the folded structure, e.g. along the vertices of the folded structure.

The skin may be bonded or fused with the plastics material of the folded structure. The skin may be joined with the plastics material by ultrasonic welding.

The skin, e.g. a continuous skin, may be provided on only one side of the Miura-Ori folded structure. An opposing side may be devoid of a skin or may comprise a discontinuous skin, e.g. spanning some but not all of the surface area of the folded structure.

In any embodiments, the skin may have a thickness/depth of at least 0.2, 0.3, 0.4 or 0.5 mm.

The thickness may depend on the intended durability, strength or stiffness of the panel in use. The thickness of the skin may be in the region of 0.5 to 1 mm.

The thickness/depth of the skin may be less than or equal to 3mm, 2mm, 1 ,5mm or 1 mm. Greater thicknesses could be used if required.

A second aspect of the invention provides method for forming a panel as defined in the appended claim 10. Further optional features are recited in the associated dependent claims.

The method may comprise providing a plastics material with fibre reinforcement to form the fibre reinforced polymer sheet material. The method may comprise forming a fibrebased textile or mat, e.g. having a repeating pattern. The method may comprise providing the plastics material so as to cover and/or impregnate the fibre-based textile.

A second aspect of the invention provides method for forming a structural panel comprising steps of heating a fibre reinforced polymer sheet material to soften the material, imparting a folded profile based on a Miura-Ori fold to the sheet material and cooling the material to set the final profile of the structural panel.

The method may comprise a further initial step of forming a plurality of living hinges corresponding to the folded profile in the polymer sheet material. The living hinges may be formed as regions of thinner material, for example using hot wires, and/or may comprise intermittent cuts or discontinuities in the material.

Additionally, or alternatively, the sheet material may be pre-formed with a folded profile of low amplitude folds, corresponding to the desired final profile, to provide the living hinges.

The pre-formed sheet may be provided during an initial manufacturing, e.g. moulding, process as the sheet is formed, or in a subsequent embossing or pressing operation process performed on a substantially flat, or pre-folded/pleated, sheet of material.

The addition of a reinforcing component to a plastics material provides additional strength, but reduces the material’s ability to stretch and form without cracking, particularly where long/continuous fibres or matting are used, so can limit the degree of possible shaping of the material into a suitable panel during manufacture. Incorporating a living hinge in a pre-forming step allows a less pronounced shape or profile to be provided initially, with the final shape being formed by a compression/folding operation to increase the amplitude/height of the folded profile in a second processing stage. Relying on a folding action means that limited stretch by bending occurs only at the fold lines of the sheet material.

The convergent section of the apparatus, or chute, compresses the pre-formed sheet laterally to increase the amplitude or height of a profile provided in the pre-formed sheet. Additionally or alternatively, the convergent section may reduce the lateral width of the material passing into the pressing/folding apparatus such that it can subsequently expand during folding, e.g. to counteract internal residual stresses caused by the folding process.

The method may further comprise the step of compressing the polymer sheet material laterally to create or increase the amplitude of folds at the living hinges. Heat may be applied, either to the entire sheet or specifically at the living hinges, before the compressing step.

The method may further comprise the step of passing the sheet material through matched rollers with a surface profile corresponding to the final profile of the structural panel. The temperature of the rollers may be controlled to assist in setting the profile. The method may comprise passing the polymer sheet material through a processing envelope of apparatus in which the method steps are performed.

The processing envelope may provide the lateral compression, where required. The movement of the sheet material through the apparatus may be continuous and/or may include controlled dwell time in certain areas.

The method may further comprise a subsequent step of bonding, joining or fusing at least one generally flat skin to the panel, e.g. using ultrasonic welding. The skin may span the folds one a side of the panel structure. The skin may fix the spacing/orientation of the folds.

A further aspect of the invention provides apparatus for forming a structural panel from a pre-formed sheet as defined in the appended claim 16. Further optional features are recited in the associated dependent claims.

The apparatus comprises a processing envelope for receiving the pre-formed sheet, the processing envelope surrounding the pre-formed sheet and having a convergent section to apply a force to lateral edges of the pre-formed sheet as it passes through the convergent section.

The pre-formed sheet material may be as previously described. For example, the sheet may be provided with a folded profile of low amplitude folds, corresponding to the desired final profile, to living hinges.

Alternatively or additionally, the living hinges may be pre-formed as regions of thinner material, for example using hot wires, and/or may comprise intermittent cuts or discontinuities in the material.

The pre-formed sheet may be provided during an initial manufacturing, e.g. moulding, process as the sheet is formed, or in a subsequent embossing or pressing operation process performed on a substantially flat sheet of material.

The processing envelope may further comprise a generally parallel lead-in section before the convergent section. One or more heaters may be provided to provide heat to the generally parallel lead-in section. The processing envelope may further comprise a generally parallel lead-out section after the convergent section. Cooling may take place in the lead-out section, and temperature control such as heating or refrigeration may be provided to control the cooling. Matched rollers with a surface profile corresponding to the final profile of the structural panel may also be provided. The matched rollers may be provided immediately after the convergent section, and before the lead-out section, where present.

The upper and lower parts of the envelope may be defined by spaced metal plates, for example aluminium plates.

A release coating may be provided on at least some of the metal plates to discourage material adhesion. The coating may be provided on the entire interior of the envelope, or may be focussed in specific areas, for example in a heated lead-in section. A possible coating material is Polytetrafluoroethylene (PTFE).

Lateral sides of the envelope may be defined by motor driven belts. The motors may be synchronised, to help drive both sides of a pre-formed sheet evenly, or may be unsynchronised.

The invention also provides a pre-formed reinforced plastics sheet for use in an apparatus as described above.

The sheet incorporates a folded profile based on a Muira-Ori fold, wherein the fold lines are provided as living hinges.

The sheet may be a pre-formed sheet for further processing into a structural panel or reinforcing core element.

Any of the preferable features defined in relation to any one aspect of the invention may be applied to any other aspect, wherever practicable. For example, any method features or steps may find corresponding physical features in the panel structure and/or any structural features in the panel may be created by corresponding method steps or associated apparatus. Practicable embodiments of the invention are described in further detail below with reference to the accompanying drawings, of which:

Figure 1 shows a schematic plan view of a structural panel forming apparatus;

Figure 2 shows a schematic side view of the apparatus of Figure 1 ;

Figure 3 shows an example of a section of pre-formed sheet material for forming a structural panel using the apparatus of figures 1 and 2;

Figure 4 shows an example of a section of a structural panel;

Figure 5 shows the section of structural panel from Figure 4 with skins on either side;

Figure 6 shows a schematic side view of an alternative panel forming apparatus;

Figure 7 shows a schematic side view of a further alternative panel forming apparatus; and

Figure 8 shows a plan view of a section of structural panel with example dimensions indicated;

Figure 8A shows a cross-section view taken at the line A-A in Figure 8; and Figure 9 shows a schematic plan view of a panel folding step.

A schematic plan view of an illustrative structural panel forming apparatus 2 is shown in Figure 1 . The apparatus comprises three defined areas, namely a lead-in section 4, a chute 6 and a lead-out 8 section, through which a sheet material passes during processing. The three areas 4,6,8 are bounded on their lateral edges by a pair of conveyor belts 10 to drive movement of a pre-formed sheet 12 through the apparatus 2 in the direction of arrow 14.

Conveyor belts 10 are synchronously driven in this example by a pair of motors 16 controlled by a motor controller 18, for example a variable-frequency drive (VFD).

The pre-formed sheet 12 is dimensioned so that its lateral edges engage, and are driven by, the conveyor belts 10 at the lateral edges of the lead-in section 4. The lateral edges of both the lead-in section 4 and lead-out section 8 are parallel, but the lateral edges of the chute 6 converge from the end of the lead-in section 4, so that the width of the lead-out section 8 is less than the width of the lead-in section 4. Thus, as the sheet 12 moved through the apparatus 2, it is compressed between its lateral edges. A schematic side view of the same apparatus 2 is shown in Figure 2. The side view shows that a process envelope 20 for receiving the pre-formed sheet 12 is defined between aluminium plates 22. In this case, 10mm aluminium gauge plates, keyed together at their seams, are used to provide the envelope 20. The plates 22 diverge over the length of the chute 6 so that the height of the envelope 20 increases as the width of the chute 6 reduces. This increased height is maintained until the end 24 of the lead-out section at which the processed sheet 12 emerges.

Heating elements 26 are provided on the aluminium plates 22 above and below the lead- in section 4 of the apparatus 2 to apply heat to that region of the envelope 20. In an embodiment, eight plate heaters 26 are provided, four on either side of the envelope 20, to allow heating across the entire lead-in section 4 to be controlled as desired. A controller 28, in this case an eight-channel temperature controller, is provided to control the output of the plate heaters 26.

The pre-formed sheet 12 is formed of a thermoplastic material with fibre reinforcement, for example a glass fibre reinforced polypropylene (GF/PP), and is provided with a folded profile during pre-forming. The folded profile includes areas of thinned or otherwise weakened material to provide living hinges in the sheet 12. A thinned region 30 at the apex of a ridge of a pre-formed fold 32 is illustrated in Figure 3 as an example of a living hinge. The hinges in the example are formed using hot wires applied to regions of the sheet material in the pre-forming step.

It should be understood that the folded profile of the pre-formed sheet 12 could be provided as part of an initial moulding process or as a subsequent embossing or pressing operation performed on a substantially flat sheet of material. The height or amplitude of the illustrated fold 32 shown in Figure 3 is relatively small, and it will be understood that this small amplitude is desirable for several pre-forming operations that might be employed to provide the sheet 12, as well as for the storage of a number of pre-formed sheets. However, a larger amplitude fold is often desirable to provide maximum strength to a reinforcing panel/core or a structural panel element. Further processing of the preformed sheet 12, using the apparatus shown in Figures 1 and 2, allows this larger amplitude to be provided.

In use, a pre-formed sheet 12 is inserted into the left-hand end of the envelope 20 as shown in Figures 1 and 2, and is drawn into the envelope 20 by the conveyor belts 10 acting on opposite lateral edges of the sheet 12. Control of the motors 16 in the apparatus 2 then allows the conveyor belts to be stopped and the pre-formed sheet 12 to be held static in the lead-in section 4, where it is heated to a suitable temperature to soften the thermoplastic material, at least at the elevated and/or depressed living hinges 30.

The temperature at various points within the lead-in section 4 can be monitored and the temperature controller 28 can adjust the output of one or more plate heaters 26 as required to ensure uniform heating and avoid damage to parts or all of the pre-formed sheet 12 during heating. Having a pre-formed folded profile in the sheet 12 also helps to ensure that the heating is focussed at the living hinges 30, maintaining integrity of the remainder of the sheet 12.

Heating the sheet 12 until ‘melt’ is approached ensures the foldability of the sheet 12 at the living hinges as it is moved out of the lead-in section 4 and through the chute 6 of the apparatus 2 by reactivation of the motors 16 and conveyor belts 10 in the direction of arrow 14. A release coating is applied to the aluminium plates 22 in at least the lead-in portion 4 to discourage material adhesion. The narrowing of the chute 6 applies pressure to the lateral edges of the panel 12, which encourages the living hinges 30 to fold and increase the amplitude of the folded profile as the sheet 12 passes through the chute 6. The increasing height of the envelope 20 in the chute 6 accommodates this increase in amplitude.

The conveyor belts 10 continue to move the sheet 12 through the lead-out section 8 of the apparatus 2, and apply a constant force to the lateral edges to maintain the sheet 12 in its laterally compressed state while the sheet 12 continues to cool, thus setting the final shape of the sheet 12 with increased amplitude in its folded profile. If necessary, the motor controller 18 could stop or pause the motors so that the part formed sheet could be held in the lead-out section 8 to provide more time for the sheet 12 to cool and for the final form to set before it is removed from the end 24 of the lead-out section 8.

All components of the apparatus 2 are provided on an aluminium frame to ensure stability and consistency of position of the various components defining the envelope 20 through which the sheet 12 passes during processing. The apparatus 2 can be dimensioned as required, but typically measures approximately 1 ,3m wide by 1 ,7m long. The envelope 20 provided by the conveyor belts 10 and aluminium plates 22 should be dimensioned to constrain the sheet 12 during processing. In other words, the spacing of the plates 22 in the lead-in section 4 should be just sufficient to receive the thickness of the pre-formed panel 12, and the spacing should increase through the chute 4 just enough to accommodate the additional thickness caused by the increased amplitude of the folded profile as the lateral edges of the sheet 12 are pressed together in that section. Maintaining close contact of the sheet 12 with the aluminium plates 22 throughout the envelope 20 helps to avoid buckling and ensure that bends or curves are not imparted to the sheet 12 during processing.

Figure 4 shows a section of a structural panel 12’ for forming the central reinforcing or core element of a composite vehicle body panel. The folded profile of the panel 12’ is based on a Miura-Ori fold, and has been formed by compressing a pre-formed sheet 12 with a lower amplitude folded profile. The fold design allows the sheet 12 to be compressed from its lateral edges 34,36 to increase the amplitude of all folds in the final panel 12’. As illustrated in Figure 4, the panel 12’ has already passed through the apparatus 2 shown in Figures 1 and 2, such that the amplitude of the folds 38 has increased compared to the example shown in Figure 3.

The design shown in Figure 4 includes folds 38 that are neither parallel nor normal to the lateral edges 34, 36 of the panel 12’. Lateral compression of the design nonetheless increases the amplitude of all folds 38 across a sheet 12 to provide the profiled panel 12’ as shown in Figure 4.

A final internal fold angle of 70° for the folds 38 forming the ridges 40 has been found to be suitable following processing of a pre-formed sheet 12. The resulting panel 12’ has high compressive strength and good resistance to point loads, particularly when bounded by suitable skins. The design of the folds 38 also allows bending or curving of the panel 12 perpendicular to the lateral edges without compromising the compressive strength provided by the folded profile. This is not the case with honeycomb reinforcing components. The described design thus makes the panels 12’ suitable for use not only in vehicle floors, but also where a bent or curved profile needs to be followed. In addition, the profile leaves clear passages through which air can pass. Again, this is in contrast to a typical honeycomb reinforcing structure, and may be of benefit where cooling is relevant, for example in packaging materials. In the example, the sheet 12, and thus the final panel 12’, is formed from a glass-fibre reinforced Polypropylene (PP-GF), e.g. with 20-40% glass-fibre.

The Polypropylene component is at least partly reclaimed/recycled, and the glass-fibre used in the reinforcement is in the form of chopped fibres. This provides a relatively strong, formable and cost-effective material suitable for producing the pre-formed sheet 12 as described. However, other materials could also be used. The reinforcing materials may include reclaimed and/or new/virgin glass or carbon fibre, and may be chopped or continuous fibres, or even provided woven or matting form. The typical drawbacks of processing reinforced materials with longer reinforcing fibres or matting are mitigated because the profile of the pre-formed panel 12 need only have a relatively small amplitude.

Skins may be applied to one or both sides of the core panel 12’ as described above to form a complete panel. The skin is typically a sheet of polymer material, that can be laid flat over a side of the core panel 12’. The skin may be a complete layer. The skin may comprise one or more layers, or example if an outer layer is required to have different properties from a layer that faces the core panel.

Figure 5 illustrates the section of core panel 12’ from Figure 3 with thermoplastic skins 42,44 schematically shown on both sides. The zig-zag design of the folds 38 in the core panel 12’ provides an arrangement of ridges 40 and troughs 46 that improves the joining of skins 42,44 to the core panel 12’ compared to straight corrugations, for example. The ridges 40 and troughs 46 provide contact areas/lines that extend in a variety of directions, minimising the possibility of shear at the joins between the core panel 12’ and the skins. This, along with the rigidity of the core panel 12’, allows the use of alternative joining methods that were not previously considered possible.

For example, ultrasonic welding can be used to join thermoplastic skins 42,44 to the core panel 12’ as described. It has been found that the panel design allows successful ultrasonic welding of relatively thick films, in excess of 0.7mm, to the core panel 12’.

One benefit of ultrasonic welding is that it avoids the use of adhesives in panel construction, which provides both cost savings and environmental benefits. The ultrasonic energy can be applied universally to a skin 42 and is effectively focussed at the peaks 40 of the core panel 12’ to achieve the bonding at those regions. A network of linear welds is thus provided, avoiding the need to employ spot-welding or other focussed/targeted welding techniques. The ultrasonic welding could therefore take place in line with the apparatus 2 as already described, potentially even while the panel 12’ and an associated skin/film 42 move through an extended processing envelope 20. If necessary, the profile of the core panel 12’ could be supported from beneath to remove any flex that might lessen the effect of the ultrasonic bonding, but this is unlikely to be necessary in many cases. A second skin 44 can be joined similarly, with the ultrasonic energy being effectively focussed by the troughs 46 in the core panel 12’. The increased stability provided by the presence of the first skin 42 further simplifies and improves the bonding operation for the second skin 44.

Skins need not necessarily be applied to both sides of a core panel 12’. In some applications a single skin 42 may provide enough rigidity, or skins may not be required at all, if the final component does not require a flat surface on one or either face. The panel 12’ may thus be considered a structural panel in isolation Leaving the panel 12’ exposed on at least one side reduces overall weight, simplifies curving of forming of the final panel to follow a contour, and also provides sound reduction properties. This may have benefits where the panel 12’ is used in vehicle lining, for example in place of plywood or similar.

The application of a skin to at least one side of the folded core structure can serve to ‘lock’ the spacing of adjacent folds. That is to say, the skin 42/44 provides a flat/planar layer of defined linear dimensions such that the fold lines of the panel cannot move relative to the skin once bonded thereto. The resistance to tension/compression of the skin thus acts in combination with the stiffness of the folded core panel to provide greater resilience in the panel, e.g. compression strength for loading applied normal to the plane of the skin 42/44. It has been found that a single skin 42/44 on one side of the folded core is sufficient to provide sufficient strength for most flooring applications, e.g. commercial vehicle flooring and the like, whilst retaining a light weight and low cost panel product.

The skin on one side of the folded core can provide an upper/outer protective surface for the panel. Furthermore, the avoidance of a skin on one side of the composite panel can provide further benefits, such as a reduced/profiled outer contact area on the opposing face of the panel.

In some examples, a partial skin could be provided on one side of the panel to match an opposing surface or formation against which the panel is to be located for use. For example, the partial skin could match the form of an undulating floor or other floor pattern against which the panel is to be seated. The panel can thus mechanically key to an opposing surface, where beneficial.

The application of a skin over the core panel creates voids between the skin and core panel. These internal voids help ensure the complete panel is low weight, i.e with the majority of the internal volume of the panel beneath the skin being are filled. The voids are channel-like in form, i.e. following the form of the folds in the core. As can be seen from Figure 5, the voids follow a zig-zig or chevron-shaped path through the panel.

Various changes could be made to the apparatus and process without departing from the invention. For example, it is also possible that with suitable speed control of the motors 18 and careful design consideration the sheet 12 could be suitably heated without being held static in the lead-in section 4 of the apparatus 2. This may require a variable speed control for conveyor belts 10 in separate sections 4,6,8. External or additional cooling could also be provided, perhaps in the lead-out section 8, to ensure that a continuously moving sheet 12 is sufficiently cooled to hold its processed shape before the end 24 of the lead-out section 8.

In some circumstances, a folded panel (for example a panel as shown in Figure 4) could be formed using a more straightforward method. For example, if a sheet material were provided with only short or chopped fibres as a reinforcing component, then a suitable profile could be provided without the need to pre-form living hinges into a sheet of material in an initial manufacturing step.

An alternative apparatus 48 for forming a folded panel is illustrated in Figure 6. The apparatus 48 comprises an infrared (I R) heater 50 arranged to heat and soften the sheet a flat sheet 52 formed of a thermoplastic material with short/chopped fibre reinforcement and a pair of matched rollers 54 to impart the folded profile to the panel. The matched rollers 54 have complementary surface profiles to emboss a design based on a Miura-Ori fold into the sheet 52.

Optionally, an array of metal strips or slats 51 may be provided between the IR heater 50 and the sheet 52. The strips 51 work as local blinds so the IR energy from the heater 50 gets dispersed and the area under each strip 51 does not get as hot as areas not covered by a strip 51 . Although shown in Figure 6 extending across the processing direction, the strips 51 are preferably aligned with the processing direction of the apparatus so that they can be slid under the IR heater 50 in this direction and hence adjust to local heating. A uniform temperature profile can be maintained over the whole processing width, which is beneficial for the process to work evenly.

A temperature controller 56, together with appropriate temperature sensors, ensures that the sheet 52 is heated to an appropriate degree to soften the polymer material and allow forming, for example to 180-185°C. Controlled heating 58 may also be provided to maintain the rollers 54 at an elevated temperature of around 90°C to manage the cooling of the formed sheet as it moves through the rollers 54, or they could be left at ambient temperature. The finished panel 52’ has a profile similar to the panel 12’ shown in Figure 4, and thus has similar properties.

Figure 7 illustrates a further alternative apparatus 102. Briefly, the apparatus is a modified version of that shown in Figures 1 and 2, incorporating matched rollers 54 as described above. Elements corresponding to those shown in Figures 1 and 2 are indicated with reference numerals differentiated by 100. The matched rollers 54 are provided between the chute 106 and lead out section 108, and are used in the apparatus 102 of Figure 7 to help set the final shape of the panel 12’, formed in the chute 106, before it passes in the lead-out section 108 for cooling. As above, heating control 58 can be provided to the rollers to manage the cooling/setting of the final panel 12’.

It has been found that folding of the sheet to achieve the desired final profile, e.g. using opposing roller, can introduce residual stress within the panel. This is because the folding process causes contorting of the thermoplastic with the fibre-reinforcement therein. The thermoplastic, when heated, can undergo deformation to resolve the forces acting on it during folding (i.e. differing regions of tension and compression). However the fibre reinforcement is generally inelastic and so regions of tension and compression cannot be readily resolved by extension/contraction of the fibres.

Instead, it has been found that the ability to splay or ‘open out’ the fibre structure during folding can accommodate the folding process in a manner that leads to reduced residual stress in the final panel. In particular, the manner in which the chevron-like pattern of a Miura-Ori folded structure is formed causes tension or pulling apart of the sheet in a lateral direction. As an addition/alternative to the specific types of pre-formed folded profile described above, it has been found that providing the pre-formed sheet with the ability to expand in a lateral direction can lead to an improved final panel form. One example of a way to achieve this lateral expansion during folding is to provide longitudinally extending pleats or undulations in the sheet material prior to folding into the Miura-Ori form. The undulations/pleats can thus be pulled laterally to offer greater compliance of the thermoplastic and fibre reinforcement to the desired final folded form. Thus it will be appreciated that the profile applied to the pre-formed sheet need not correspond to the final form of the Miura-Ori fold but can instead take a different profile.

A schematic example of this is shown in Figure 9 showing the longitudinal pleats 130 from above leading into the opposing rollers 54. The rollers 54 splay/unfold the pleats 54 at the same time as applying the chevron-shaped folds. However this ability to splay outward is not limited to rollers and may be applied to static presses also.

Furthermore, since the thermoplastic may be readily deformable at elevated temperature to a greater extent than the fibre reinforcement, the fibre reinforcement of the pre-formed sheet structure may be provided with this ability to open out or expand even if the thermoplastic sheet has a flat form. That is to say the fibres may follow a tortuous path within the plane of the thermoplastic sheet material, or a path that is oblique/offset from the applied tension during folding. As the fibres enter the rollers/press, they may be pulled tight during the folding process such that they follow the shortest linear path through the folded panel, e.g. substantially straight lines though each face of the folded core turning only around the fold lines.

The fibres, prior to folding by the rollers/press, may follow a tortuous path in both the lateral and longitudinal directions, i.e. being able to accommodate extension in either or both of lateral and longitudinal directions during folding. The example of Figure 9 shows schematically the lateral expansion as the sheet 12 passes through rollers 54, although it will be appreciated that expansion/contraction in length may need to be accommodated in a press.

In examples where a woven textile provides the fibre reinforcement, the warp and weft of the woven textile may be angularly offset from the intended direction of expansion of the fibre reinforcement during folding, i.e. to provide give in the fibre reinforcement during pressing. An open woven structure or net-like fibre structure may be preferable in this regard. However, various other textile structures are possible for providing the required directional give in the textile. In some examples, long fibres may be laid down, rather than being woven or knitted, to provide inherent give in the resulting mat.

The handling of the stress/tension in the fibres during the pressing process, and in the resulting panel, has been found to be an important consideration. Whilst this is particularly important for long fibres, it may also have benefits for shorter/chopped fibres. As a result of the fibre handling details described above, the fibres in the resulting composite panel may be relaxed, less stressed and/or without residual tension.

Short or chopped fibres as referred to herein may refer to fibres having an average/mean length less than 1cm, where as long fibres may refer to fibres having an average/mean length of greater than 1 cm, e.g. greater than or equal to 2cm or 3cm.

It will be understood that the various apparatus 2,48,102 described above could produce panels with various different profiles as described above. However, a design as shown in Figure 4 has been found to have favourable properties. Testing has further determined some preferred dimensions of the folded sections of the final panel 12’, 52’ as illustrated in Figures 8 and 8A. Figure 8 shows a plan view of one element of the repeated fold pattern from the panel 12’, 52’, which for convenience will be referred to as a chevron 62. The dimensions and angles are based on a configuration where each flat section 60 of the chevron 62 has a length L of 20mm, where the internal angle X of each chevron 62 in plan is 90° (making angle Z 135°), and where the internal angle of the ridges (as shown in Figure 8A) is 70°. The width Wi of each folded section as defined for the cross section of Figure 8A is approximately 12mm and the width W₂ of each folded section at either end of the chevron is approximately 17mm. This results in a total width for each chevron 62 in plan, W₂, of approximately 28mm and an overall height of each ridge 40 (or depth of the panel) of 7.6mm. Through suitable variation of the fold angle, this depth may vary from 6mm-12mm. The base material for forming the example panel 12’52’ has a starting thickness of 1 ,4mm. As shown in Figure 8A, the final thickness T of the core material after processing is 1 ,2mm, and the internal radius Ri and external radius R₂ of each fold are 0.8mm and 1 ,5mm respectively.

The dimensions were selected specifically for an application in vehicle floors, where one consideration was providing reliable support and crush resistance to the load applied by standard castor wheel of 25mm width. The dimensions in Figures 8 and 8A provide sufficiently small peaks and troughs in the panel 12’, 52’ to avoid the risk of such a wheel running on any unsupported sections of an outer skin. It will be understood that the various angles and dimensions can be modified as required for a particular application, for example to account for larger or smaller wheels or other localised loads, or as manufacturing considerations require or allow. For example, providing a smaller internal angle Y for the ridges 40 would increase compressive strength but complicate manufacture and thus increase cost.

Testing, conducted at the University of Exeter using a Lloyd-Ametek EZ20 Material Testing Machine, has shown that a panel 12’, 52’ having the design/configuration shown in Figures 8 and 8A and constructed from a glass reinforced Poly Propylene (PP-GF) with 30% by weight glass fibre demonstrates a compressive strength in excess of 4MPa. A similar panel formed from a thinner (1 mm) starting material still exhibits a compressive strength in excess of 2.5Mpa. For comparison, PP honeycomb/tubular core with 8mm tube diameter, commercially available from Tubus Waben, has a compressive strength of 1 ,5MPa at 80kgm^-3 density and 2.95MPa at 120kgm^-3 density.

The properties of the panel 12’, 52’ as described make it particularly suitable for transport applications, for example as floor or body panels of vehicles.

Various other modifications would also be apparent to a skilled reader. As such, it is emphasised that the forgoing description is provided by way of example only, and is not intended to limit the scope of protection as defined with reference to the appended claims.

Claims

Claims:

1 . A structural panel, the panel comprising: a plastics material with fibre reinforcement and having a folded profile based on a Miura-Ori fold to provide a repeated pattern of chevron shaped folds, said folded profile defining a core of the panel; and a sheet extending in over at least one side of the core and spanning the chevron shaped folds so as to provide a skin of the structural panel; wherein the panel has a compressive strength of at least 2.5MPa measured in a direction out of a plane of the sheet.

2. A structural panel according to claim 1 , wherein the plastics material is a thermoplastic such as Polypropylene (PP), Acrylonitrile butadiene styrene (ABS), VX (ABS/PVC), Polyethylene terephthalate (PET), or Polystyrene (PS).

3. A structural panel according to claim 1 or 2, wherein the plastics material comprises reclaimed or recycled plastics material.

4. A structural panel according to any preceding claim, wherein the fibre reinforcement comprises glass fibre and/or carbon fibre.

5. A structural panel according to any preceding claim, wherein the reinforcing material comprises recycled or reclaimed material.

6. A structural panel according to any preceding claim, wherein the plastics material has a thickness from 1 -1 ,5mm.

7. A structural panel according to any preceding claim wherein the internal angle of each fold is from 50°-80°.

8. A structural panel according to any preceding claim wherein the length of each part of each chevron is from 15-25mm.

9. A structural panel according to any preceding claim wherein the internal angle of each chevron in plan is from 75°-105°.

10. A structural panel according to any preceding claim wherein the fibre reinforcement comprises fibres of average length greater than 1cm in length.

11. A structural panel according to any preceding claim wherein the fibre reinforcement comprises a textile, such as a mat, net or webbing.

12. A structural panel according to any preceding claim wherein the skin is joined to the core along the ridges/vertices of the folds.

13. A method of forming a structural panel comprising: heating a fibre reinforced polymer sheet material to soften the material; imparting a folded profile based on a Miura-Ori fold to the sheet material; cooling the material to set the final profile of the structural panel; and applying a sheet extending in over at least one side of the core and spanning the chevron shaped folds so as to provide a skin of the structural panel.

14. A method of forming a structural panel according to claim 13, comprising the further initial step of forming a plurality of living hinges corresponding to the folded profile in the polymer sheet material.

15. A method of forming a structural panel according to claim 14, further comprising the step of compressing the polymer sheet material laterally to create or increase the amplitude of folds at the living hinges.

16. A method of forming a structural panel according to any of claims 13 to 15, further comprising the step of passing the sheet material through matched rollers with a surface profile corresponding to the final profile of the structural panel.

17. A method of forming a structural panel according to any of claims 13 to 16, wherein the method comprises passing the polymer sheet material through a processing envelope of apparatus in which the method steps are performed.

18. A method of forming a structural panel according to any of claims 13 to 17, further comprising bonding the skin to the panel along ridges of the folds, for example using ultrasonic welding.

19. A method of forming a structural panel according to any of claims 13 to 18 comprising providing the polymer sheet material in an expandable profile comprising undulations and/or pleats prior to imparting the folded profile.

20. A method of forming a structural panel according to any of claims 13 to 19 comprising providing an expandable fibre reinforcement structure within the polymer sheet material, such that the fibre reinforcement structure expands in one or more direction during the imparting of the folded profile.

21 . Apparatus for forming a structural panel from a pre-formed sheet of material, the apparatus comprising a processing envelope for receiving the pre-formed sheet, the processing envelope surrounding the pre-formed sheet and having a convergent section to apply a force to lateral edges of the pre-formed sheet as it passes through the convergent section.

22. Apparatus according to claim 16, wherein the processing envelope further comprises a generally parallel lead-in section before the convergent section.

23. Apparatus according to claim 21 , wherein a heater is provided in the generally parallel lead-in section.

24. Apparatus according to any of claims 21 to 23, wherein upper and lower parts of the envelope are defined by metal plates.

25. Apparatus according to claim 24, wherein a release coating is provided on at least some of the metal plates to discourage material adhesion.

26. Apparatus according to any of claims 21 to 25, wherein sides of the envelope defined by motor driven belts.

27. A floor or wall of a vehicle comprising the structural panel according to any one of claims 1 to 12 or formed according to the method of any one of claims 13 to 20.

SUBSTITUTE SHEET (RULE 26)