STRUCTURE COMPRISING HONEYCOMB CORE AND OUTER SKIN

Abstract: A honeycomb core (2) is bonded to a planar outer skin (10) by forming a pool of adhesive (24) on a surface of the outer skin and placing the honeycomb core (2) within the adhesive pool (24). In the resulting structure, a continuous layer of adhesive of substantially uniform depth is formed within the cells (8) of the honeycomb core (2). The depth of the adhesive and the size of the cells may be chosen so that the adhesive provides a lens effect on light passing through. The structures may be used for light, heat or sound transmission/retardation. In particular, a transparent structure can be provided with optical consistency between the cells of the honeycomb core. This can be used to provide large panels of glass, with the honeycomb core increasing the panels' strength and hence allowing the panels to be made in sizes larger than previously possible and to be used for new applications.

Description: STRUCTURE COMPRISING HONEYCOMB CORE AND OUTER SKIN The present invention relates to a structure comprising a honeycomb core with an outer skin on at least one side, to a method of bonding the honeycomb core to the outer skin, and to uses thereof. Honeycomb core/outer skin structures are known in the art. To date, outer skins have been bonded to opposite sides of the honeycomb core to provide a rigid structure which has then been used in the aerospace industry for applications requiring high integrity structures. It has also been suggested to use the structures for transparent table tops, but a table top having a uniform appearance has not been produced. In a first known manufacturing technique, a reticulating film is placed on each side of the honeycomb core, and the film is punctured in the centre of each cell of the honeycomb core. The honeycomb core and film are then sandwiched between two outer skins and heated in a kiln while applying pressure to the outer skins to maintain sufficient bonding contact between the honeycomb core, films and outer skins. During the heating process, the film melts and retracts to the walls of the honeycomb core, forming a bond between these walls and the outer skin. This technique suffers from a number of problems. For example, firstly, it is necessary to maintain the assembly under pressure within the kiln, requiring expensive equipment. Secondly, the bonds between the walls of the honeycomb core and the outer skin are highly visible, irregular and inconsistent, being formed from a build-up of the film around the wall edges. This build-up is uneven between cells because its formation depends on many factors, including the position at which the film is punctured and the evenness of the heat applied (uneven heat results in uneven build-up). Accordingly, this technique does not produce structures consistently, and their uneven optical appearance make them unsuitable for the transparent table tops mentioned above. A second technique for producing honeycomb core/outer skin structures is also known. In this technique, an adhesive is brushed onto the surface of an outer skin and the honeycomb core is then applied and retained under pressure until the adhesive has set. Whilst this technique avoids the need for a kiln, the bonds between the cell walls of the honeycomb core and the outer skin are unreliable since a suitable strength bond is not always formed. Accordingly, the resulting structure can not be used for high integrity applications. Further, the adhesive dries in a very uneven way on the surface of the outer skin. In particular, during drying, the adhesive tends to accumulate in particular areas in an inconsistent way, leaving other areas within the cell clear of adhesive. This produces highly noticeable, uneven and inconsistent optical effects, sometimes a "fish-eye" lens effect, on light passing through. The inconsistency between the optical effects produced within each cell of the honeycomb core means that the structure can not be used for the transparent table tops mentioned above. The applications to which honeycomb core/outer skin structures have been put to date are also limited. The present invention aims to address one or more of the above problems . According to a first aspect of the invention, there is provided a method of bonding a honeycomb core to a planar outer skin using a pool of adhesive of substantially uniform depth. The invention also provides a honeycomb core/outer skin structure having a continuous layer of adhesive within the cells of the honeycomb core. Preferably, the layer has a substantially uniform depth. The invention further provides the use of a honeycomb core/outer skin structure, or a honeycomb core/outer skin structure adapted for use, as a heat collector or a heat insulator. The invention further provides the use of a honeycomb core/outer skin structure, or a honeycomb core/ outer skin structure adapted for use, as a sound insulator or a sound transmitter. The invention yet further provides first and second honeycomb cores separating first, second and third outer skins, and the use of such a structure as a heat collector and a heat insulator. Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Fig. 1 shows a honeycomb core being expanded from a compressed state to an open state; Fig. 2 shows an individual cell of the honeycomb core; Figs. 3 to 5 illustrate steps of bonding a honeycomb core to an outer skin in accordance with an embodiment of the invention; Fig. 6 shows a honeycomb core bonded to an outer skin in accordance with an embodiment of the invention; Fig. 7 shows a honeycomb core having first and second outer skins bonded on opposite sides thereof and side clips attached to form a panel; Fig. 8 shows a composite structure comprising first and second honeycomb cores separating first, second and third outer skins; and Fig. 9 shows an alternative arrangement to that illustrated in Fig. 3. Referring to Figs. 1 and 2, an aluminium honeycomb core 2 comprising a plurality of cells 8 of size "a" and depth "d" is expanded from its compressed, transport state 4 to an open state 6. Typically, "a" may be in the range 3 to 30mm or much larger and "d" in the range 5mm to a metre or more. Other sizes are, of course, possible. Other materials of honeycomb core may be used e.g. a plastics or fibre-reinforced plastics material or a paper-based material such as phenol resin-impregnated paper. The honeycomb core 2 is then cut to the required length and width for bonding to an outer skin. The dimensions are dependent on the application, but each may, for example, vary from a few centimetres to many metres. Each of the cells 8 of the honeycomb core is then inspected and adjusted by hand to remove unopened or deformed cells . The cells are then checked by eye and further adjustments made, if necessary, to achieve a consistent cell size and orientation of cell walls 5. The outer skin, for example a sheet of glass 10, to be bonded to the honeycomb core 2 is cleaned with acetone or similar grease remover in a dust-free environment. The glass may be float glass, toughened glass or laminated glass. Referring to Figs. 3 to 5, the glass outer skin 10 is placed in an enclosure within a dust-free environment that will enable a pool of adhesive to be contained on the surface of the glass. In this embodiment, the enclosure comprises a rubber bund 12 formed with vertical end walls 14 and a horizontal base 16. The bund 12 is placed on a flat surface 18 together with rubber supports 20 having the same thickness as the horizontal base of bund 12 to support the glass outer skin 10. A protective layer, such as a piece of grease-proof paper 22, is placed on the inner surface of bund 12 to prevent adhesive bonding thereto. Glass outer skin 10 is then placed on the grease-proof paper 22 within bund 12 as shown in Fig. 3. The horizontal bottom 16 of bund 12, together with the supports 20 and the uniformly flat surface 18 ensure that glass outer skin 10 is held in a substantially horizontal position. Referring to Fig. 4 , an adhesive suitable for bonding the honeycomb core 2 to the outer skin 10 is poured evenly over the surface of the outer skin 10 to form a pool 24 of substantially uniform depth. In this embodiment, Araldite 20/20 is used to bond the aluminium honeycomb core 2 to the glass outer skin 10. The Araldite 20/20 is mixed before pouring to ensure that it contains no bubbles or other visible impurities. The Araldite may be replaced with other epoxy resins or other types of resin that preferably set at ambient temperatures . The resin may be of clear transparent material which may be coloured with a dye or pigment. The pool of adhesive 24 is formed to a depth dependent upon the cell size "a" of the honeycomb core 2. It is preferred, but not essential, that the depth is sufficient to form a layer within each cell to cause a lens effect on light passing therethrough, at least in part, by the formation of a meniscus within each cell of the honeycomb core, as will be described later. This lens effect is particularly useful for creating different optical effects in many applications, for example where the resulting structure is used as a transparent panel. In practice, the following depths of adhesive pool 24 have found to be satisfactory in providing such a lens effect: CELL SIZE "a" (mm) DEPTH OF ADHESIVE POOL (mm) 3 1 5 2 6 2 13 2 19 3 It is preferred that the ratio of the depth of the adhesive pool 24 to the cell size "a" of the honeycomb core is greater than 1:10, and preferably greater than 1:7 and less than 1:2. Referring to Fig. 5, honeycomb core 2, is placed in adhesive pool 24 so as to contact glass outer skin 10. The honeycomb core 2 is placed into the adhesive pool 24 vertically in order not to disturb the depth of adhesive, so that a substantially even depth can be achieved throughout all the cells 8 of the honeycomb core 2. As shown in Fig. 5, honeycomb core 2 is sized to leave a gap between the edges thereof and vertical sides 14 of bund 12. In this way, no force is exerted by bund 12 on the honeycomb core which could deform the cells 8. A suitable weight (not shown), that is, one which is not so heavy that it will crush the honeycomb core 2 or so light that the honeycomb core is not evenly maintained against glass outer skin 10, is then applied on top of the honeycomb core 2. In this embodiment, MDF (Medium Density Fibre) wood of 20mm thickness is used. The assembly is then left so that the adhesive can set in the dust-free environment. After the adhesive has set, the structure is inspected to ensure that the layer of adhesive 24 is of a consistent depth between the cells 8 of the honeycomb core 2. Any inconsistencies can be addressed by adding further adhesive to individual cells. In this embodiment, the further adhesive is added by pipetting it into the individual cells. Once any additional adhesive has dried or cured, then the resulting structure is removed from the bund 12 and the edges on the outer surface of glass outer skin 10 are cleaned of any excess adhesive. The resulting structure is illustrated in Fig. 6. A continuous layer of hard adhesive 24 at a substantially uniform depth is formed within the cells 8 of honeycomb core 2, bonding the cell walls 5 to the glass outer skin 10. At the edge of each cell wall 5, the adhesive 24 forms a meniscus, which may contribute to the adhesive 24 causing a lens effect on light passing therethrough. A second outer skin, such as a further glass outer skin 10, may be added on the opposite side of honeycomb core 2. To do this, the second outer skin is placed in bund 12 as described previously with reference to Fig. 3, a pool of adhesive 24 is formed on the second outer skin as described with reference to Fig. 4, and the honeycomb core 2 with the first glass outer skin 10 attached thereto is placed in the adhesive pool and retained therein until the adhesive is set as described previously with reference to Fig. 5. The resulting structure is shown in Fig. 7. The structure may be cut to a desired size or shape. Edging material, such as C-shaped clamps 26 or other connectors, are placed around some, or all, of the edges of the structure to form a panel. The clamps 26 may be arranged to allow the interconnection of panels . In the same way, further honeycomb cores 2 and further outer skins 10 may be added to form a composite assembly. Fig. 8 shows an assembly formed of three outer skins 10 separated by two honeycomb cores 2. Many modifications to the method described above can be made. For example, although a honeycomb core 2 having hexagonal shaped cells has been described, the method may be applied to a honeycomb core having cells with any number of sides. The honeycomb core 2 need not be made of aluminium and the outer skin 10 need not be of glass. Indeed, the method is applicable to honeycomb cores and outer skins of any material, the adhesive being selected as appropriate for the bonding. For example, the honeycomb core and/or outer skin may be made from different plastics, metals, woods, papers, stones, carbon fibres etc. In the above embodiment, Araldite 20/20 is used as the adhesive, which dries to a clear state. However, coloured adhesives may be used, or dyes/pigments may be added to the adhesive, to produce a coloured adhesive layer 24. Arrangements other than the bund 12 shown in Figs. 3 to 5 may be used to form an enclosure to allow the adhesive pool 24 to be formed. For example, as shown in Fig. 9, grease-proof sheet 22 may be placed directly on table 18 and an enclosure formed using a rigid structure 28, such as may be formed using wooden blocks. In the embodiment above, additional adhesive is added to individual cells to correct variations in adhesive thickness after the original pool of adhesive has set. However, as well, or instead, the additional adhesive may be added before the original pool has set. The depth of the adhesive pool 24 may be made shallow with respect to the cell size "a" of the honeycomb core, so that a structure having substantially no lens effect is formed. Applications of a structure comprising a honeycomb core and at least one outer skin will now be described. It should be noted that the applications below do not require the honeycomb core to be bonded to the outer skin using the method described above. More particularly. unless an optically acceptable structure is required, the structure may be formed using one of the prior art techniques described previously, or indeed the honeycomb core may be trapped between the outer skins ( for example using suitable end clamps) and retained therein without any adhesive bonding. The structures formed using the method above have optical consistency between the cells of the honeycomb core. Accordingly, they have particular application as transparent panels etc. More specifically, large panels with glass outer skins can be manufactured with a uniform optical appearance, the honeycomb core increasing the panels' strength and allowing panels to be made with a greater size than previously (standard glass panels suffering from the problem that they bow due to the self- weight of the glass). Accordingly, panels strengthened by the honeycomb core can be made using the technique described above without losing the uniform optical appearance, allowing use in many new applications. For example, using a honeycomb core between two optically transparent outer skins, the resulting structure could be used for transparent floors, facade elements, beams, columns, windows etc, with the honeycomb core increasing the strength of the outer skins against breakage. Also, the honeycomb core could be used for security and/or safety purposes to provide a structure with integrity against deliberate or accidental breakage of an outer skin. The cell size "a" and depth "d" of the honeycomb core would be selected to provide the required mechanical properties . Different materials may be used for each outer skin. For example, since glass is good in compression but bad in tension, a glass outer skin could be put on the top surface of, for example, a floor and metal or plastic put on the bottom surface, so that the glass acts in compression and the metal or plastic acts in tension. The cell walls of the honeycomb core (5 in the embodiment described above) and the end connectors (26 in the embodiment described above ) can be provided with holes therein to allow the passage of fluid, that is liquid or gas, through the panels. In this way, different coloured gases or different densities of gas could be passed through windows or other optical panels to change the colour and/or optical transparency. Such panels could therefore be used to act as blinds or curtains etc. The strength of the structure could also be increased by passing fluid through the honeycomb core cells under pressure. Similarly, partial or complete vacuums could be formed within the cells. In particular, panels could be used for internal or external building walls, allowing air supply or extraction through the honeycomb core. In addition to controlling the passage of light, the propagation of heat or sound can be controlled by appropriate selection of the materials for the honeycomb core and outer skins. For example, the use of a metallic honeycomb core could be used to increase heat transfer between the outer skins. A plastic or wooden honeycomb core could be used to reduce the transfer of heat or sound between outer skins . Increased insulation could be provided by forming a vacuum within the cells of the honeycomb core, or by filling the cells with an insulating fluid or material. A structure could be used as a solar panel. For example, an aluminium or copper honeycomb core could be used to collect heat passing through a glass outer skin. An insulating, heat reflecting or heat absorbing outer skin may be provided on the opposite surface, such as an opaque wall to facilitate heat collection. Fluid having suitable thermal properties could then be passed through the cells of the honeycomb core to remove the heat for central storage. The structure shown in Fig. 8 could be used in both a heat collection and heat insulation capacity. For example, if used in a window type arrangement, the outer honeycomb core would be selected to have heat collection properties to act as a solar collector as described above, and the inner honeycomb core would be formed of a heat insulating material to prevent heat passing through the structure for example to the inside of the room. By using a honeycomb core to increase structural strength, structures may be provided for fluid retention. For example, a structure with glass outer skins could be used as a fish tank, swimming pools, aquariums etc. The cells within the honeycomb core can be filled, for example with small heat retarding balls for fire protection, or with glass balls/tubes to change the light transmitting characteristics of an optical panel . Similarly, a fire-protection film, such as a film which expands on exposure to heat filling the core of the panel, could be placed on the honeycomb core.

Claims: CLAIMS 1. A method of bonding a honeycomb core to a outer skin in which a pool of adhesive is formed on a surface of the outer skin over the area to be occupied by the core and the honeycomb core is placed in the pool. 2. A method according to claim 1 , wherein the honeycomb core is held in the adhesive pool under pressure for at least part of the setting time of the adhesive. 3. A method according to any preceding claim, wherein the adhesive pool is at least 1mm deep. 4. A method according to any preceding claim, wherein the depth of the adhesive pool is such as to produce a lens effect within cells of the honeycomb core. 5. An method according to claim 4 , wherein the ratio of the depth of the adhesive pool to the cell size of the honeycomb core in a given direction is greater than 1:10. 6. A method according to claim 4, wherein the ratio of the depth of the adhesive pool to the cell size of the honeycomb core in a given direction is greater than 1:7. 7. A method according to claim 6 , wherein the ratio of the depth of the adhesive pool to the cell size of the honeycomb core in a given direction is between 1:7 and 1:2. 8. A method according to any preceding claim, further comprising the step of adding adhesive to a cell after the honeycomb core is placed in the adhesive pool. 9. A method according to any preceding claim, further comprising bonding a second outer skin to the honeycomb core on the opposite side to the first outer skin. 10. A method according to claim 9, wherein the second outer skin is bonded by forming a pool of adhesive on the second outer skin over the area to be occupied by the core, and placing the honeycomb core in the pool. 11. A structure comprising a honeycomb core bonded to a outer skin and having a continuous layer of adhesive on the inner surface of the outer skin within cells of the honeycomb core. 12. A structure according to claim 11, wherein the adhesive is at least 1mm deep. 13. A structure according to claim 11 or 12, wherein the depth of the adhesive is such as to produce a lens effect within cells of the honeycomb core. 14. A structure according to claim 13, wherein the ratio of the depth of the adhesive to the cell size of the honeycomb core in a given direction is greater than 1:10. 15. A structure according to claim 13, wherein the ratio of the depth of the adhesive to the cell size of the honeycomb core in a given direction is greater than 1:7. 16. A structure according to claim 15, wherein the ratio of the depth of the adhesive to the cell size of the honeycomb core in a given direction is between 1:7 and 1:2. 17. A structure according to any of claims 11 to 16, further comprising a second outer skin bonded to the honeycomb core on the opposite side to the first outer skin. 18. A structure according to claim 17, having a continuous layer of adhesive on the inner surface of the second outer skin within cells of the honeycomb core. 19. Use of a structure comprising a honeycomb core between first and second outer skins as a heat collector. 20. Use of a structure according to claim 19, wherein one of the outer skins is opaque . 21. Use of a structure comprising a honeycomb core between first and second outer skins as a heat insulator. 22. Use of a structure according to any of claims 19 to 21, wherein fluid is pumped between the outer skins. 23. A structure comprising a honeycomb core between first and second outer skins adapted for use as a heat collector. 24. A structure according to claim 23, wherein one of the outer skins is opaque. 25. A structure comprising a honeycomb core between first and second outer skins adapted for use as a heat insulator. 26. A structure according to any of claims 23 to 25 adapted so that fluid can be pumped between the outer skins . 27. Use of a structure comprising a honeycomb core between first and second outer skins as a sound insulator. 28. A outer skin comprising a honeycomb core between first and second outer skins adapted for use as a sound insulator. 29. Use of a structure comprising a honeycomb core between first and second outer skins as a sound transmitter. 30. A outer skin comprising a honeycomb core between first and second outer skins adapted for use as a sound transmitter. 31. Use of a structure comprising a first honeycomb core between first and second outer skins and a second honeycomb core between the first outer skin and a third outer skin as a heat collector and a heat insulator. 32. A structure comprising a first honeycomb core of a first material between first and second outer skins and a second honeycomb core of a second material between the first outer skin and a third outer skin. 33. A structure according to claim 32, wherein the first and second materials are such that the structure acts as a heat collector and a heat insulator. 34. A method of bonding a honeycomb core to a outer skin substantially as described herein with reference to, or as shown in, the accompanying drawings. 35. A structure comprising a honeycomb core bonded to a outer skin substantially as described herein with reference to, or as shown in, the accompanying drawings. 36. Use of a structure comprising a honeycomb core between first and second outer skins substantially as described herein with reference to, or as shown in, the accompanying drawings . 37. A structure comprising a honeycomb core between first and second outer skins adapted for use substantially as described herein with reference to, or as shown in, the accompanying drawings. 38. Use of a structure comprising a first honeycomb core between first and second outer skins and a second honeycomb core between the first outer skin and a third outer skin substantially as described herein with reference to, or as shown in, the accompanying drawings. 39. A structure comprising a first honeycomb core between first and second outer skins and a second honeycomb core between the first outer skin and a second outer skin substantially as described herein with reference to, or as shown in, the accompanying drawings. 40. A method of forming a structure in which a honeycomb frame or core is bonded directly to a substrate, in which a pool of adhesive is formed on a surface of the substrate over the area to be occupied by the frame or core and the honeycomb frame or core is placed in the pool, said substrate forming an external skin of the resulting structure. 41. A method according to claim 40, wherein the honeycomb frame or core is held in the adhesive pool under pressure for at least part of the setting time of the adhesive. 42. The method of claim 40 or 41, wherein reservoir walls are formed on or about the substrate and the adhesive is poured into the resulting reservoir and allowed to settle so that the surface of the adhesive becomes horizontal before the honeycomb frame or core is inserted . 43. A method according to claim 40, 41 or 42, wherein the adhesive pool is at least 1mm deep. 44. A method according to any preceding claim, wherein the depth of the adhesive pool is such as to produce a lens effect within cells of the honeycomb frame or core. 45. An method according to claim 44, wherein the ratio of the depth of the adhesive pool to the cell size of the honeycomb frame or core in a given direction is greater than 1:10. 46. A method according to claim 44, wherein the ratio of the depth of the adhesive pool to the cell size of the honeycomb frame or core in a given direction is greater than 1:7. 47. A method according to claim 46, wherein the ratio of the depth of the adhesive pool to the cell size of the honeycomb frame or core in a given direction is between 1 : 7 and 1:2. 48. A method according to any of claims 40 to 47, further comprising the step of adding adhesive to a cell after the honeycomb frame or core is placed in the adhesive pool. 49. A method according to any preceding claim, further comprising bonding a second substrate to the honeycomb frame or core on the opposite side to the first substrate. 50. The method of claim 49 wherein the second substrate forms an external skin of the resulting structure. 51. A method according to claim 49, wherein the second substrate is bonded by forming a pool of adhesive on the second substrate over the area to be occupied by the core, and placing the honeycomb frame or core in the pool . 52. The method of any of claims 40 to 51, wherein the first substrate, the second substrate or both the first and second substrates are of glass sheet. 53. The method of any of claims 40 to 52, wherein the honeycomb frame or core is of a metal, plastics or of paper. 54. The method of claim 53, wherein the honeycomb frame or core is of aluminium or phenol-impregnated paper. 55. A structure comprising a honeycomb frame or core bonded to a substrate and having a continuous layer of adhesive on the inner surface of the substrate within cells of the honeycomb frame or core, the honeycomb frame or core being bonded directly to the substrate, and the depth of adhesive within each cell of the honeycomb frame or core being substantially constant. 56. The structure of claim 55, wherein the substrate forms an external skin of the resulting structure. 57. A structure according to claim 55 or 56, wherein the adhesive is at least 1mm deep. 58. A structure according to any of claims 55 or 57, wherein the depth of the adhesive is such as to produce a lens effect within cells of the honeycomb frame or core. 59. A structure according to claim 58, wherein the ratio of the depth of the adhesive to the cell size of the honeycomb frame or core in a given direction is greater than 1:10. 60. A structure according to any of claims 55 to 59, further comprising a second substrate bonded to the honeycomb core on the opposite side to the first substrate. 61. The structure of claim 60, wherein the second substrate also forms an external skin of the resulting structure. 62. A structure according to claim 60 or 61, having a continuous layer of adhesive on the inner surface of the second outer skin within cells of the honeycomb core. 63. A structure according to any of claims 55 to 62 in the form of a panel . 64. A structure according to any of claims 55 to 63 in the form of a flat panel. 65. The structure of any of claims 55 to 64 wherein the first substrate, the second substrate or both of them are of glass sheet. 66. The structure of claim 65, wherein the glass sheet is selected from float glass, laminated glass and toughened glass. 67. The substrate of any of claims 55 to 66, wherein the adhesive is of an epoxy or other resin that cures at ambient temperatures . 68. The substrate of any of claims 55 to 67 wherein the resin is transparent. 69. The substrate of claim 68 wherein the resin contains a dye or pigment.

Inventors: MCMAHON NICHOLAS, MCMAHON, NICHOLAS
Applicants: MCMAHON NICHOLAS, MCMAHON, NICHOLAS
Application year: 1997
Main classification: B23B3
IPC codes: B31D3, E04C2