Abstract:
A method of forming a layered, generally planar panel structure including the steps of (a) utilizing applied heat and pressure, causing a melt of resin present in a thermoformable material layer to flow toward and into an adjacent non-thermoformable material layer, and (b) by such utilizing and causing, forming a thermal-compression, mechanical, inter-material-transition bond between the layers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation from currently copending U.S. patent application Ser. No. 11/881,907, filed Jul. 30, 2007, for “Layered Panel Structure Including Self-Bonded Thermoformable and Non-Thermoformable Layer Materials”, which application claims filing date priority to U.S. Provisional Patent Application Ser. No. 60/835,313, filed Aug. 2, 2006 for “Thermoform Layered Structure and Method”. The entire disclosure contents of these two, prior-filed applications are hereby incorporated herein by reference. 
     
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
       [0002]    The present invention relates to methodology for creating a thermo-compression-formed, layered panel structure, and in particular, for creating such a structure which features a special assembly of both thermoformable and non-thermoformable layer materials self-bondedly joined through what is referred to herein as being a differentiated-material, transition-discontinuity-style, thermally bonded interface, or thermal bond (also called a thermal-compression mechanical bond). 
         [0003]    According to this invention, a generally planar, layered panel structure is created which, while, as just mentioned, being generally planar in nature, may have certain topographical surface features that may, at least partially, be compression-thermoformed during assembly of the layer materials which make up the panel structure. The panel structure produced by practice of the invention features a relatively low-density, low-cost, non-thermoformable, principal, generally planar body having opposite faces, to at least one of which faces is thermally compressively bonded to a face in a higher-density, typically-higher-cost, much thinner, fibre-strand-reinforced, thermoformable plastic skin. The body, as measured transversely between its opposite faces (its transfacial thickness), is thicker than the skin as also so measured. 
         [0004]    The thermal compression, or thermal compressive, bond existing between these thermoformable and non-thermoformable materials, as created in the panel structure of the invention, is referred to variously herein as a self-bond to reflect the fact that, preferably, no additional bonding adhesive is employed. Rather molten plastic material from the thermoformable material per se flows to create the uniting bond between the materials, and a consequence of this structural bonding arrangement is that a finished panel structure performs throughout with just the two desired structural characteristics of only the selected thermoformable and non-thermoformable materials. This has proven, in many application settings, to be a functionally desirable characteristic of the structure created by the invention, in that the inter-material, transition bond region per se does not exhibit the otherwise expectable, and perhaps somewhat unpredictable, load-managing behavior of a foreign bonding substance. 
         [0005]    This layered assembly, i.e., the panel structure produced by the methodology of the present invention, possesses useful overall dimensional bulk (i.e., mainly thickness) which is contributed chiefly by the transfacial thickness of the lower-density body (layer), along with elevated load-bearing strength which is furnished principally by the appreciably higher-density (to be discussed later herein), significantly thinner, strand-reinforced skin (layer). This skin, in addition to offering elevated load-bearing strength, as just mentioned, also affords a high degree of abrasion resistance. 
         [0006]    These two different-thermal-characteristic materials may be surface-joined in different organizational ways in an overall panel structure made in accordance with the present invention. Two such surface joined ways are specifically illustrated herein, including one wherein a finished panel structure is formed with just one-each layer of each of these two materials, and another wherein there is, in a finished panel structure, a central, or core, layer formed of the non-thermoformable material, united with a pair of opposite-side-cladding skin layers formed of the thermoformable material. 
         [0007]    The self-bond union of these two materials, produced via a thermally compressively bonded interface described herein, as mentioned above, as being a differentiated-material, transition-discontinuity-style interface, produces a composite structural panel assembly which exhibits (a) the strength that one would typically and intuitively associate with a unitary, homogeneous structure having the thickness and bulk of the principal, non-thermoformable body material, and (b) the light-weightness that one would typically and intuitively associate with a unitary, homogeneous structure having the thinness and low, apparent bulk of the thermoformable skin material. These “intuitive” associations, of course, would most probably come about, at least in part, because of a lack of an initial understanding of the appreciably different layer-density and internal-strength characteristics that are correctly associated with the two different materials that have been chosen for use in the panel structure created by the invention. 
         [0008]    Additionally, thermoformability of the skin, and thermal bonding of the skin to the body, uniquely allow for the fabrication, in several different ways, of a structural panel having, if desired, a complex surface topography (a) dictated by either or both of (1) pre-shaping of the bonding face of the non-thermoformable body, and/or (2) modest thermal-deformation-compression of the thermoformable skin material, and/or (3) a combination of these two approaches. Where body-material pre-shaping is employed, such pre-shaping telegraphs easily into the final panel configuration of the skin because of the skin&#39;s naturally offered thermo-configurational-deformability that is enabled and invoked during thermal-compression bonding of the body and the skin. 
         [0009]    Many applications exist for the structural panel produced by this invention, including within-building wall and door applications, such as garage-door and hurricane-door-and window-panel applications, truck and trailer bed applications, vehicle door-panel applications, boating-structure applications, and many others. 
         [0010]    These and other features which are offered by the layered, composite panel structure which results from practice of the present invention will become more fully apparent as the description of the invention which shortly follows this text, is read in conjunction with the accompanying drawings. 
     
    
     
       DESCRIPTIONS OF THE DRAWINGS 
         [0011]      FIG. 1  is a fragmentary, schematic, cross-sectional view of one embodiment of a panel structure made in accordance with practice of the present invention, which panel structure includes a principal body layer made of low-density, non-thermoformable material, to one face in which there is thermally, compressively bonded a fibre-strand-reinforced, higher-density, thermoformable plastic skin layer. 
           [0012]      FIG. 2  is a larger-scale, fragmentary, schematic plan illustration of an arrangement of reinforcing fibres included within the skin layer mentioned in  FIG. 1 , which fibres are deployed in a non-woven, substantially orthogonal pattern with respect to one another. 
           [0013]      FIG. 3  is similar to  FIG. 2 , except that this figure illustrates reinforcing fibres which are unidirectional and substantially parallel to one another in the mentioned skin layer. 
           [0014]      FIG. 4  is similar to  FIG. 1 , except that it illustrates a modified form of panel structure made in accordance with implementation of the present invention wherein the generally planar principal body layer has thermally and compressively bonded to each of its two, opposite, broad, generally planar faces a skin layer of the type mentioned in relation to  FIG. 1 . 
           [0015]      FIG. 5  is a fragmentary illustration, similar to a portion of  FIG. 1 , showing a surface topographical feature which has been created in a region of the illustrated surfacing skin layer by deformation-compression of the skin-layer material during thermal-bonding of the skin layer to the principal body layer. 
           [0016]      FIG. 6  is somewhat similar to  FIG. 5 , except that  FIG. 6  shows surface topographical features that have resulted from pre-surface shaping of one of the faces of the illustrated non-thermoformable principal body layer, which shaping telegraphs through to the overall surface of the panel structure through the thermally bonded, thermoformable surfacing skin layer. Such telegraphing takes place during bonding of the skin layer to the principal body layer.  FIG. 6  also illustrates a surface topographic feature which has been created in the manner of the topographic feature pictured in  FIG. 5 . 
           [0017]      FIG. 7  illustrates another modified panel form created by practice of the invention in which, on the outer surface of the illustrated skin layer, an auxiliary surfacing material, such as a decorative patterning material, has been attached to the skin layer by thermal-bonding. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Turning attention now to the drawings, and referring first of all to  FIGS. 1-3 , inclusive, indicated generally at  10  in  FIG. 1  is a fragmentary, schematic cross-sectional view of a generally planar, layered panel structure made in accordance with a preferred manner of practicing the present invention. Structure  10  includes a principal body layer, or body layer portion,  12  which has a thickness T herein, also referred to as a transfacial thickness, of about 3/4-inches, and which formed of a suitable low-density, light-weight, relatively low-cost, non-thermoformable material, such as balsa wood. Such wood has a density d typically of about 7-lbs/ft 3 . 
         [0019]    Other satisfactorily usable principal body materials include PVC foam, urethane foam, honeycomb plastics, honeycomb metals and corrugated wood. The preferred density (d) range for a chosen principal body material herein is about 4-lbs/ft 3  to about 30-lbs/ft 3 . 
         [0020]    Thermally compressively bonded to the upper surface  12   a  of body  12  in  FIG. 1 , through what is referred to herein as the earlier-mentioned differentiated-material, transition-discontinuity-style bonded interface, which interface is shown by a darkened and thickened line  14  in  FIG. 1 , is a fibre-strand-reinforced, thermoformable plastic resin skin, or skin layer,  16  (reinforcing strands, or fibres, embedded within thermoformable resin). Preferably, skin  16  is formed as a thermally bonded/integrated plurality of three, stacked, generally planar sub-layers, such as the three sub-layers schematically shown at  16   a  in  FIG. 1 , which sub-layers are formed each of individually fibre-strand-reinforced, thermoformable, plastic-resin material in sheet form. Each of these three sub-layer sheets has a transfacial thickness At lying in the range of about 0.015- to about 0.020-inches, with the stacked collection of all three of these sub-layers thus having a collective transfacial thickness t lying in the range of about 0.045- to about 0.060-inches. In panel structure  10 , each of sub-layer sheets  16   a  preferably has a thickness of about 0.016-inches, and the overall transfacial thickness of skin layer  16  therefore has a transfacial thickness of about 0.048-inches. 
         [0021]    The plastic resin material in skin layer  16 , illustrated generally at  16   b,  is substantially continuous and homogeneous because of the thermoforming procedure which has been used to form panel structure  10 . This thermoforming procedure causes a resin melt and flow to occur within layer  16 , and this melt and flow create the homogeneity characteristic just described. It should be understood that, while horizontal sub-layer division lines appear in  FIG. 1  (and in two other drawing figures herein), these lines have been included just to clarify visually the presences of three sub-layer sheets used in the formation of panel structure  10 . 
         [0022]    The reinforcing fibre strands in skin layer  16 , represented generally by dashed lines  16   c,  lie (float) essentially in three, spaced, substantially parallel planes indicated at  17  in  FIG. 1 . 
         [0023]    Turning attention for a moment to  FIGS. 2 and 3 , and understanding that each of sub-layers is essentially the same in internal construction,  FIG. 2  illustrates, for one of sub-layers  16   a,  a preferred organization of therein-embedded strands  16   c,  with these strands being non-woven, and, as just above suggested, “floating” within the sub-layer with generally orthogonal dispositions relative to one another in resin  16   b.  In  FIG. 3 , strands  16   c  in the sub-layers are generally unidirectional and parallel one another. 
         [0024]    Not specifically illustrated in the drawings, but understood to be a possible organization within the skin sub-layers, is an organization wherein the reinforcing strands are woven in a mesh form. Also not specifically shown, it will be understood that sub-layers  16   a  may be stacked so as to have their included reinforcing strands, from a sub-layer to sub-layer point of view, oriented at angles relative to one another. Further, there may be applications wherein it is desirable to use a greater or lesser number of sub-layers than three. 
         [0025]    While many different specific skin materials may be used appropriately in the structure created by the present invention, preferably the thermoformable resin which is employed in this skin material—a thermoplastic resin—is selected from the group including polypropylene, polyester, polyethylene and the material known as PET, and preferably is chosen to be a polypropylene resin, such as that which is employed in a commercial product sold under the trademark Polystrand® made by a company of that same name, located in Montrose, Calif. This preferred-resin skin material has a density D of about 130-lbs/ft 3 . The fibre reinforcing strands embedded within the skin-material resin are preferably selected from the group including glass, E-glass, S-glass and carbon fibre, and from this group, are preferably made of S-glass. 
         [0026]    Within the skin material per se, it is preferable that the ratio, by weight, of strand material to resin lie in the range of about 60:40 to about 80:20, and a preferred ratio is 70:30. With such a preferred ratio, the density D of the preferred polypropylene skin material is, as was mentioned above, about 130-lbs/ft 3 . Additionally, it is preferred that an appropriate thermoforming temperature for the chosen resin, i.e., a suitable melt/flow temperature therefore, be about 340° F. This thermoforming temperature is what is associated with preferred polypropylene resin  16   b  herein. 
         [0027]    In accordance with the invention, with the principal body and skin materials  12 ,  16  chosen, panel structure  10 , as shown in  FIG. 1 , is formed from a layer stack of these two materials by compressively thermally bonding skin  16  to surface  12   a  in body  12  utilizing the appropriate thermal-bonding temperature associated with the particular resin employed in the skin material. An appropriate bonding application pressure of about 30-psi is employed, and when this is done at the appropriate temperature, interface bond  14  is formed by a melt and flow of the skin-material resin material into engagement with body surface  12   a  and body  12 . Where body  12  is formed of a material such as balsa wood, resin melt from the skin material usually enters the pores within such body material. This interfacial resin melt is what directly forms interface bond  14 , and is why this bond is referred to herein as a “self-bond”. 
         [0028]    What results in this procedure is a panel structure ( 10 ) having all of the important lightweightness and strength features set forth earlier herein. Appropriate panel bulk is furnished chiefly by the principal body material included in structure  10 , and appropriate rigidity and load-bearing strength are contributed principally by the fibre-reinforced thermally-bonded surfacing skin layer. Interface bond  14 , formed as it is by resin material melt and flow coming from the skin material during thermal compressive bonding, contributes to such load-bearing strength. 
         [0029]    A modified layer form producible by the invention is illustrated in  FIG. 4 . Shown here, and employing substantially the same reference numerals utilized so far herein in  FIG. 1 , a modified panel structure  18  is formed having a principal body  12  which is thermally joined, in addition to a thermally-bonded skin  16  on body surface  12   a,  to a companion and like surfacing skin  20  that is similarly thermally bonded to the opposite face  12   b  in body  12 . 
         [0030]    One of the interesting features uniquely accommodated by the panel structure created by the methodology of the present invention, because of the thermoformable nature of the surfacing skin material which is employed, is illustrated in several different versions in  FIGS. 5 and 6  in the drawings. Specifically, these two drawing figures illustrate the possibility of forming subtle, and even complex, three dimensional surface topography features in the outwardly facing surface of the surfacing skin material layer employed in a produced panel structure. 
         [0031]    In  FIG. 5  a singular surface depression  16 A is illustrated in skin layer  16 , this depression, which exists only in layer  16 , having been formed principally by deformation-compression of layer  16  during thermal-bonding of the assembled panel-structure materials. Topographic shaping, as is here shown, may be accomplished by the use of an external device, such as a heated platen having the appropriate, complementary surface shape. With cooling after such a procedure, the surface of skin layer  16 , as shown in  FIG. 5  takes on with permanence the formed depression  16 A. 
         [0032]    In  FIG. 6 , pre-shaping in the surface topography of surface  12   a  in panel body  12  has taken place to create a pair of stepped depressions  12 A,  12 B. This kind of body-surface shaping may, of course, be performed in any appropriate manner. During thermal-bonding of skin  16  to body  12 , the thermoformable nature of the skin allows it, through what is referred to herein as telegraphing, to take on with great precision, the pre-shaped topography formed in the underlying body-layer surface (see depressions  16 B,  16 C which “follow” depressions  12 A,  12 B, respectively). 
         [0033]    It is of course possible to create even more complicated surface topographies through utilizing a combination of the surface shaping approaches so far discussed, respectively, with respect to in  FIGS. 5 and 6 . For example,  FIG. 6  shows, at  16 D,  16 E, two other kinds of depression-character surface topography which illustrate a certain quality of obtainable surface topographic complexity. 
         [0034]    Finally,  FIG. 7  illustrates yet a further modified panel-structure form creatable by the invention, which form may be chosen for use in certain applications. This modification includes the thermal-bonding, to the outside surface of a skin layer, such as skin layer  16 , of some form of appropriate “other” surfacing material, such as the layer material shown generally at  22  in  FIG. 7 . This other surfacing material ( 22 ) is bonded to skin layer  16  during thermal-bonding of skin layer  16  to principal body layer  12 , with the bond which thus develops between the skin layer and the added surfacing material developing through contact of that added material with thermally melted resin coming from the skin layer resin. 
         [0035]    Thus, production of a special composite panel structure has been shown and described herein in both preferred and modified forms with this resulting panel structure featuring the union, in different ways, of both thermoformable and non-thermoformable materials having the relative density and transfacial thickness characteristics mentioned above herein. The union of these two materials opens the door for the creation of a relatively wide-ranging family of generally planar panel structures which may be employed successfully in a number of applications, such as the several applications mentioned earlier herein. The load-bearing characteristic of the panel structure made in accordance with the present invention, with respect to the thermal bond which exists at a transition interface region between thermoformable and non-thermoformable materials relies entirely upon bonding between these materials which is achieved during thermoforming by a melt and flow of the resin employed in the thermoformable material. Accordingly, no additional adhesive material is employed and, as was also mentioned earlier herein, uncertainties about the load-bearing characteristics of such additional material play no role in the consideration of the design of a panel structure made in accordance with practice of the invention the invention. 
         [0036]    By various techniques and user choices, interesting and even quite complex surface topographies can be created in the manner generally described above, and a consequence of this is that a panel structure made according to the practice of the invention may be designed to fit in a number of different applications where surface topographical features are desired for various reasons. 
         [0037]    Accordingly, while preferred and modified manners of practicing, and embodiments of, the invention have been illustrated and described herein, it is appreciated that variations and modifications may be made without departing from the spirit of the invention, and it is intended that all such variations and modifications will come within the scope of the claims to invention presented herein.