TECHNICAL FIELD

This invention relates to the construction of fibre reinforced plastic panels and methods of constructing fibre reinforced plastic panels.

BACKGROUND ART In the prior art, panel constructions are known which have a sandwich type configuration. They have two fibre reinforced plastic sheets which have a substantially equal separation over their entire surface, and include between the two sheets and bonded to the surface of each of the sheets a core medium. This core medium is generally of much lower density than the fibre reinforced plastic panels. In some configurations for example a core medium such as expanded polyurethane foam is used. Isotropic materials such as expanded polyurethane foam have now been replaced in many instances by specially constructed anisotropic core materials, such as paper honeycomb, which have substantially higher strength and rigidity in a direction perpendicular to the plane ofthe plastic sheets than in directions parallel to the plane of the plastic sheets. This concentrates the strength of the material in directions in which it is more useful.

Anisotropic materials such as paper honeycomb however are difficult to manipulate to fit complex surfaces such as curves. These known core materials can be difficult to use in rudimentary fabrication environments. Furthermore in most instances the core materials have differing chemical and physical properties to the outer sheets, and these must be taken into consideration when designing a product, along with the chemical or physical properties ofthe outer sheets.

DISCLOSURE OF INVENTION It is therefore an object of the invention to provide a fibre reinforced plastic panel and/or methods of constructing a fibre reinforced plastic panel which go some way to wards overcoming the above disadvantages or to at least provide the public with a useful choice.

In a first aspect the invention consists in a fibre reinforced plastic panel comprising a first fibre reinforced plastic sheet

a second fibre reinforced plastic sheet coextensive with said first fibre reinforced plastic sheet and facially displaced therefrom, the opposed surface of said second panel having substantially equal separation from said first panel over the entire surface, and an intermediate fibre reinforced plastic sheet coextensive with one of said first or second sheets and disposed between said first sheet and said second sheet, said intermediate sheet having, at a plurality of first regions, one surface thereof contacting and bonded to said first sheet, and, at at least one second region, the other surface thereof bonded to said second sheet, said intermediate sheet not contacting said second sheet at any of said first regions, and not contacting said first sheet at any of said second regions.

In a second aspect the invention consists in a method of constructing a fibre reinforced plastic panel comprising: a) laying up a first fibre reinforced plastic panel on a first mould, said first mould having a surface contour which includes a plurality of regions which collectively fit the contour of a second mould and at least one second region which collectively fit the contour of a third mould, b) subsequent to step a) removing said first sheet, when it has cured, from said first mould, c) laying up a second fibre reinforced plastic sheet on either said second mould or said third mould, the surface contour of said mould representing one exterior surface of said panel, d) subsequent to steps a) and c) bonding the appropriate surface of said first sheet to the opposed surface of said second sheet, e) subsequent to step c) removing said second sheet from its respective mould once said second sheet has cured, f) laying up a third fibre reinforced plastic sheet on the other of said second mould or said third mould, said mould having a surface contour which represents the other exterior surface of said panel, g) subsequent to steps a), b) and f) bonding the exposed side of said first sheet to the opposed side of said third sheet, and h) subsequent to step f) removing said third sheet from said third sheet mould once said third sheet has cured.

To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications ofthe invention will suggest themselves without departing from the scope ofthe invention as defined in the

appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

BRIEF DESCRIPTION OF DRAWINGS Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which;

Figure 1 shows a plan view in partial cut-away of a panel according to one embodiment ofthe present invention,

Figure 2 shows a side view of a panel of the embodiment of the present invention depicted in Figure 1 ,

Figure 3 shows a plan view in partial cut-away of a panel according to a second embodiment ofthe present invention,

Figure 4 shows a side view in cross-section of the panel according to the embodiment ofthe invention as shown in Figure 3, Figure 5 shows a plan view in partial cut-away of a panel according toa third embodiment ofthe present invention, and

Figure 6 is a side view in cross-section of a panel according to the embodiment ofthe invention shown in Figure 5.

DETAILED DESCRIPTION OF DRAWINGS

Referring to Figures 1 to 6, panels are shown having a first fibre reinforced plastic sheet 1 and a coextensive second fibre reinforced plastic sheet 2, the opposed surfaces of the first and second fibre reinforced plastic sheets maintaining an approximately constant separation 3 over their entire surface. A core comprising an intermediate fibre reinforced plastic sheet 4, 5 or 6 maintains this separation. The intermediate fibre reinforced plastic sheet 4, 5 or 6 has, at plurality of first regions 12, 22, 36, one side 7 thereof contacting and bonded to the first sheet 1. At at least one second region 11, 21, 37 of the intermediate sheet 4, 5 or 6 the other side 8 of the intermediate sheet contacts and is bonded to the second fibre reinforced plastic sheet 2. The intermediate sheet 4 also includes transition regions 13, 23, 38 which cross the separation 3 between first sheet 1 and second sheet 2 and connect the first and second regions ofthe intermediate fibre reinforced plastic sheets 4, 5, or 6. The intermediate sheets 4, 5 or 6 do not contact the first sheet 1 at any ofthe second regions 11, 21, 37 or the second sheet 2 at any ofthe first regions. Referring to Figures 1 and 2, according to one embodiment of the present

invention the intermediate fibre reinforced plastic sheet 4 is shaped to have a series of parallel corrugations. In the drawings corrugations are shown as being substantially linear, however it should be appreciated that these corrugations could have for example a curved or wavy top profile. In this form ofthe invention, to maximise bonded surface area ofthe intermediate panel, the intermediate sheet 4 has first regions 12, being the peaks of the corrugations on one side of the panel, extending fully across the sheet. Second regions 11 being the peaks ofthe corrugations on the other side of the panel, extend fully across the sheet in the same direction as the first regions 12. Transition regions 13 connect the corrugation peaks 12, 11. The transition regions 13 have a steep inclination relative to the first and second reinforced plastic sheets 1 , 2. The steeper the inclination of transition regions 13, the larger the bonded area ofthe first and second regions. However, to maintain geometrical strength of the substantially trapezoidal enclosed spaces, this inclination should not approach too closely to a right angle. In this regard a suitable value for the inclination ofthe transition region would be 60 to 80°. This will be of particular relevance depending on the loading conditions ofthe panel. Loading conditions causing shearing forces between panels in directions perpendicular to the corrugations will necessitate lower inclination angles to avoid skewing.

It will be appreciated in respect of the embodiment of the invention shown in Figures 1 and 2 that the configuration ofthe corrugations in the intermediate sheet does not need to be linear across the sheet. The corrugations could for example be of curved configuration, such as wave or sinusoidal, or could be of closed curved configurations such as concentric ovals or circles, or parts of ovals or circles. Such differing configurations may be readily applicable when complex sheet geometries are involved to maximise the strength ofthe panel.

In particular, if loading conditions are well known, the direction of the corrugations should be substantially peφendicular to the axis of bending at all regions ofthe sheet.

A panel ofthe construction shown in Figures 1 and 2 has significant advantages over prior art panel constructions. The construction is lightweight and has substantial inherent strength, particularly in directions perpendicular to the direction of the corrugations. Furthermore the construction is of a uniform material, giving simple construction techniques which allow easy, separate, moulding of the individual components and later bonding. The core material panel is no more fragile or brittle than the fibre reinforced plastic sheets and will have the same inherent chemical or physical

characteristics. Judgements based on chemical and physical properties such as chemical resistance or inflammability need only be made for one material. The materials are easy to use in primitive fabrication environments.

The moulded intermediate FRP sheet makes the panel construction particularly durable. The panel construcion is expected to show a high resistance to delamination of the sandwich under extreme loading or fatigue loading conditions. This gives the panel construction a particular application on the transport sector, where rough terrain or road conditions, lead to unpredictable or unusual load patterns.

Referring to the Figures, the construction of a panel involves the mating of three moulded components. The intermediate sheet is laid up on a first mould, with the mould surface corresponding to the desired contours ofthe intermediate sheet. As will be described shortly the intermediate sheet mould may be formed corresponding to either surface ofthe intermediate sheet, as both faces ofthe sheet are enclosed within the panel. The first sheet 1 is laid up on a second mould with the mould surface corresponding to the desired outer surface ofthe first sheet. The second sheet 2 is laid up on a third mould, the mould surface corresponding to the desired outer surface of the second sheet. The mould surface ofthe intermediate sheet mould includes regions which correspond to the surface of the second mould surface, and regions which correspond to the surface ofthe third mould surface. In producing the panel each of the sheets is laid up on its respective mould before being mated and bonded to its adjacent sheet or sheets. In most cases the mating ofthe sheets can be achievd with one or more ofthe moulds still in place and/or with one or more ofthe fibre reinforced plastic sheets not fully cured. The order ofthe steps of moulding, curing, removing and bonding can vary considerably from application to application, and may depend on the complexity ofthe sheets, the size ofthe sheets, and the degree of automation that is used in the production process. The following description is given by way of an example of a possible process suitable for manual production.

The intermediate sheet is preferably laid up first, and the resin is allowed to cure sufficiently that the sheet will retain its integrity when removed from the mould, after which time the sheet is removed from the mould. Once the intermediate sheet has been produced, the first sheet 1 is laid up. While the first sheet 1 is still freshly laid and rolled, the appropriate surface of the intermediate sheet is applied thereto while the resin ofthe first sheet is still wet. The first sheet is then allowed to cure sufficiently to allow removal from its mould, and removed from its mould, with the intermediate sheet

bonded thereto. Once the first sheet 1 has been removed from its mould, the second sheet 2 is laid up, and rolled. While the second sheet 2 is still wet, the exposed surface ofthe intermediate sheet 3 is applied thereto, and the second sheet 2 is allowed to cure, thereby bonding the intermediate sheet (which has the first sheet bonded to its other surface) to the first sheet 1.

Referring now to Figures 3 and 4, an alternative intermediate panel configuration to that of Figures 1 and 2 is shown wherein rather than the substantially corrugated configuration of first and second regions, a two dimensional repeating array of discrete first regions 22 are arranged across the intermediate sheet 5. In the embodiment illustrated the discrete first regions 22 comprise a plurality of circular regions raised from the remainder of inteπnediate sheet 5 leaving a single second region 21 which surrounds each and all of the first regions 22, with a frusto-conical transition region 23 between the second region 21 and each ofthe circular regions 22.

Other configurations for the array of first regions are equally viable, such as differing shapes (squares, triangles) and differing repeating arrangements (square or triangle based tessellation). Also, while the embodiment shown has a single surrounding second region, other alternatives are possible such as discrete second regions arrayed with the discrete first regions in a checkerboard pattern.

The size and spacing ofthe first regions 22, and inclination ofthe transition 23 are preferably related so that overall the bonded surface ofthe intermediate sheet 5 to first sheet 1 is similar to that ofthe intermediate sheet 5 to the second sheet 2.

As with the corrugated embodiment described with reference to Figures 1 and 2 it has been found that the angle of the transition region can affect the overall characteristics ofthe panel. A steep transition region is prefered in that it maximises the bonded surface areas ofthe first and second regions, however a steeper transition region may be adversely affected by shearing forces between the two panels. In the panel of Figures 3 to 4 this is less marked than in the panel of Figures 1 and 2, where such shearing forces, in directions close to perpendicular to the corrugations, can have very marked effects at steep transition angles due to a low degree of triangulation. The embodiment described with reference to Figures 3 and 4 has improved performance when such shear forces between panels are experienced from multiple directions. The panel has generally anisotropic properties in such situations.

This embodiment ofthe invention has similar advantages and disadvantages as the first embodiment described above, except for the strength properties of the panel which are less dependent upon the direction of applied forces.

Referring now to Figures 5 and 6, a third variation is shown. In this variation the separation between first and second sheets 1 and 2 is relatively low in comparison to that of Figures 1 - 4, taking into account the thickness of the sheets 1 and 2. In this embodiment the intermediate sheet 6 is formed as a smooth curved wave, as it has been found that in small constructions such a wave is easier to produce in the intermediate sheet, yet when bonded to the first and second sheets retains sufficient contact area. The panel shown in Figures 5 and 6 is intended in particular for applications requiring high resistance to impact, and the first fibre reinforced plastic sheet 1 in this embodiment has an inner layer 33 of standard layup fibre reinforced plastic, and an outer layer of standard layup fibre reinforced plastic with a comparatively soft impact absorbing layer bonded there between. The impact absorbing layer could for example be a porous non- woven, non-brittle core material. An example of a suitable core material is FIBRECORE mat jointly developed by Toyobo Inc, Toyo Cloth Co, and Japan U Pica Co. The FIBRECORE mat absorbs a substantial quantity of plastic resin thereby providing a tough, non-brittle interface with the surrounding FRP sheets. The mat, including absorbed resin is still substantially lower in density than the FRP and is formed from generally non-brittle materials, being more like a non-woven cloth.

The provision of the impact absorbing layer allows the outer layer of fibre reinforced plastic to flex and distribute the force of impact loads over a broader area of the panel. This embodiment of the panel is particulalry suitable for applications where repeated surface impacts are likely, such as for floor type applications.

In manufacturing a panel of this embodiment, given that there is a relatively complex formation involved in the production ofthe first fibre reinforced plastic sheet, the first sheet 1 may be laid up in two distinct stages, with the outer and impact layers 31 and 32 laid up first, and allowed to cure, and inner layer 33 laid up later, at the time of bonding the intermediate panel thereto.