STRUCTURAL PANELS WITH A CORRUGATED CORE STRUCTURE AND A METHOD FOR MAKING THE SAME

Abstract: A method of fabricating an expanded structural panel is provided comprising the steps of coating one side of plastic sheets (5, 9) with spaced apart stripes (7) of material to which hot thermoplastic adheres poorly. Two sheets having their striped faces oriented towards one another, are positioned above and beneath a third thermoplastic sheet (11). The centerline of the spaces between the stripes on one sheet are positioned opposite the centerline of the stripes on the other sheet, with the stripes being wider than the opposing spaces. The layers are pressed together and the stack of sheets heated so that the third plastic sheet adheres to the first and second sheet where the coating is not present and so that it allows plastic deformation of the third sheet. The first and second sheets are then separated causing the portions of the third sheet with stripes above and beneath to separate from the first and second sheet and form a corrugated core

Description: Background of the Invention The present invention relates to expanded light-weight structural panels fabricated from thermoplastic sheet stock. In many of the structural applications currently being sought for glass reinforced and unreinforced thermoplastic materials, it is highly desirable to increase the bending stiffness of the plastic panels without substantially increasing the weight of the structure. One way of satisfying this criteria is to increase the cross sectional moment of inertia by moving mass away from the center of mass of the panel and thereby increase the bending stiffness of the panel. To increase the cross sectional moment of inertia without increasing the weight, requires that the core density of the structural panel decrease. This can be accomplished by fabricating a honeycomb structure. A plastic honeycomb panel and apparatus for producing the same are described in United States Patents 4,315,050, 4,164,389 and 4,113,909. The material of the honeycomb panel forms a monolithic structure which has a series of large periodic holes on its surface. It is possible to bond the plastic and metallic skins to the surface of the panel to form a stiff structure which has a smooth surface. In the process for making the honeycomb structure, monolithic plastic sheet is heated between two platens that have periodic arrays of holes on their surface. After heating the plastic to attain a sufficiently high temperature of the plastic to stick to the platens, the plastic is expanded into a honeycomb shape by moving the two platens apart. During the separation process, the plastic adheres to the metal plates everywhere except where the holes are located. In this manner, a complex honeycomb structure is formed once the plastic is cooled. Forming the honeycomb structure requires special molds or platens. It is an object of the present invention to provide a simple method of fabricating expanded structural panels which are stiff in bending and also lightweight. It is a further object of the present invention to provide a method of fabricating lightweight structural panels with glass reinforced outer skins. Summary of the Invention In one aspect of the present invention a method of fabricating an expanded structural panel is provided comprising the steps of coating one side of plastic sheets with spaced apart stripes of material to which hot thermoplastic adheres poorly. Two sheets having their striped faces oriented towards one another, are positioned above and beneath a third thermoplastic sheet. The centerline of the spaces between the stripes on one sheet are positioned opposite the centerline of the stripes on the other sheet, with the stripes being wider than the opposing spaces. The layers are pressed together and the third plastic sheet heated so that it adheres to the first and second sheet where the coating is not present and so that it allows plastic deformation of the third sheet. The first and second sheets are then separated causing the portions of the third sheet with stripes above and beneath to separate from the first and second sheet and form a corrugated core. Brief Description of the Drawing While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, objects and advantages of the invention can be more readily ascertained from the following description of preferred embodiments when used in conjunction with the accompanying drawing in which: Figure 1 is a partial isometric view of a composite sheet having stripes of material painted on one surface to which hot thermoplastic adheres poorly; Figure 2 is a partial side view of an unexpanded structural panel in accordance with the present invention; Figure 3 is a partial side view of an expanded structural panel in accordance with the present invention; and Figure 4 is a partial isometric view of the expanded structural panel of Figure 3. Detailed Description of the Invention Referring now to the drawings wherein like reference numerals refer to like elements throughout and particularly Figure 1 thereof, a sheet of material 5, preferably a glass reinforced plastic composite, has a plurality of stripes of material 7, to which hot thermoplastic adheres poorly, applied thereto. The stripes do not have to be parallel to one another or of equal sizes as shown in Figure 1. The stripes of material 7 can comprise thin foils, oils, glossy paints, granular material, painted or silk-screened onto the surface. High temperature resistant aluminum paint, for example, sprayed on the surface in stripes has performed well. Two sheets, 5 and 9, each having the stripes 7 applied to one of their surfaces, are situated on either side of a sheet of monolithic thermoplastic sheet stock 11. With the striped faces of the composite sheet facing one another, the two sheets are aligned so that the centerline of the stripes 7 on one sheet 5 or 9 are aligned with the centerline of the spaces between the stripes 7 on the other sheet 9 or 5, respectively. The width of the striped portion on one sheet 5 or 9 is greater than the width of the spaces between the stripes on the other sheet 9 or 5 with which it is aligned. Each sheet 5 and 9 does not have to have all the stripes with equal width nor do the spaces between the stripes have to have the same width. The two sheets do not have to have the same width stripes or spaces. The monolithic core plastic sheet 11 can be of the same type of thermoplastic as used in the outer sheets 5 and 9, which are preferably glass reinforced. Thermoplastics which can be used are polycarbonate, polybutylene terephthalate, and a blend of bisphenol A polycarbonate and polybutylene terephthalate, available from the General Electric Company as Lexan, Valox and Xenoy, respectively, all registered trademarks of the General Electric Company. The materials chosen are selected so that during heating the stack of sheets of the core sheet becomes plastically deformable at a temperature equal to or below that of the outside sheets. When the core and outer sheets contain the same type of plastic resin and the outer sheets are glass reinforced composites, the composite sheets will advantageously have a higher range of working temperatures than the monolithic sheet. The three sheets shown in Figure 2 are heated to an elevated temperature, such as by radio frequency heating, and bonded by applying a low pressure to the stack of sheets. The core sheet is raised to or above its glass transition temperature. The bonding that takes place between the layers when the sheets are pressed together is periodic, with regions containing very good bonds adjacent to regions that are poorly bonded or not bonded at all, dependent on locations of the stripes. The bonded stack of plastic sheets can either be formed immediately after the bonding has been done and while the central core sheet is still hot or the layers can be allowed to cool and the sheet reheated and processed at a later time. In either case, the final structure having a corrugated core is formed by applying a force normal to the outer sheets such as by using negative air pressure or by an array of suction cups of the type used for handling plate glass. The final expanded structural panel is shown in Figures 3 and 4. The plastic core remains bonded to the upper and lower surface sheets only in those locations which do not contain the "no-stick" stripes. The portion of the monolithic core sheet which has stripes above and beneath deforms plastically when the sheets are pulled apart a predetermined distance. The panel has a high cross-sectional moment of inertia and thus is very stiff in terms of bending. If desired, the bonded structure shown in Figure 2 can be shaped by roller forming or stamping, for example, to provide a desired contour prior to expansion. The stripes 7 can alternatively be applied to either side of the monolithic core piece 11 rather than the outside sheets 5 and 9, with the centerline of the stripes on one side of the core piece 11 in the line with the centerline of the spaces between the stripes on the other side of the core piece, with the stripes being wider than the spaces with which they are aligned. The structure would appear as shown in Figure 2 prior to expansion and would appear as shown in Figures 3 and 4 after to expansion. The foregoing has described a simple method of fabricating expanded structural panels which are stiff in bending and also lightweight.

Claims: A method of fabricating an expanded structural panel comprising the steps of: coating one side of a first (5) and second (9) plastic sheet with stripes (7) of material to which hot thermoplastic (i.e. thermoplastic heated to or above its glass transition temperature) adheres poorly, the stripes being spaced apart from one another; positioning the two sheets with the sides having stripes facing one another and above and beneath a third sheet (11) comprising thermoplastic material, the centerline of the spaces between the stripes on one sheet being aligned opposite the centerline of the stripes on the other sheet, the stripes being wider than the opposing spaces; compressing the sheets together; heating the third sheet so that it adheres to the first and second sheets where the coating is not present and so that it allows plastic deformation of the third sheet; and separating the first and second sheets causing the portions of the third sheet situated above and beneath stripes to separate from the first and second sheet and elongate to form a corrugated core. The method of claim 1 wherein said first and second sheets comprise thermoplastic material. The method of claim 1 wherein said third sheet comprises a monolithic thermoplastic sheet. The method of claim 2 wherein said first and second sheets comprise glass fiber reinforced thermoplastic material. The method of claim 1 wherein the stripes are applied parallel to one another. The method of claim 5 wherein said stripes are wider than the spaces between the stripes on the same sheet and the width and spacing of the stripes on the first and second sheet are the same. An expanded structural panel prepared according to the method of claim 1. An unexpanded structural panel comprising: a first (5) and second (9) outer plastic sheet; material to which hot thermoplastic (i.e. thermoplastic heated to or above its glass transition temperature) adheres poorly, applied in stripes (7) to one surface of each of said outer sheets, said stripes being spaced apart; and a core sheet (11) of thermoplastic material having said outer sheets situated above and beneath said core sheet, the surfaces of said outer sheets having their stripes facing one another with the centerline of the spaces on one sheet situated opposite the centerline of the striped portions on the other sheet, the striped portions being wider than the opposing spaces between the stripes, said core sheet being periodically bonded to said first and second outer sheets where said material to which hot plastic adheres poorly is not present. A method of fabricating an expanded structural panel comprising the steps of: coating both sides of a first sheet (11) of thermoplastic material with stripes (7) of material to which hot thermoplastic (i.e. thermoplastic heated to or above its glass transition temperature) adheres poorly, the stripes being spaced apart from one another with the centerline of the spaces between the stripes on one side of the sheet, being aligned with the centerline of the stripes on the other side of the sheet, the stripes being wider than the spaces on the other side with which they are aligned; positioning a second (5) and third (9) plastic sheet on either side of the first sheet; compressing the sheets together; heating the first sheet so that it adheres to the second and third sheets where the coating is not present and so that it allows plastic deformation of the first sheet; and separating the second and third sheets causing portions of the first sheet situated between opposing stripes on opposite sides of the sheet, to elongate and form a corrugated core. The method of claim 9 wherein said second and third sheets comprise thermoplastic material. The method of claim 9 wherein said first sheet comprises a monolithic thermoplastic material. The method of claim 10 wherein said second and third sheets comprise glass fiber reinforced thermoplastic material. The method of claim 9 wherein the stripes are applied parallel to one another. The method of claim 9 wherein said stripes are wider than the spaces between the stripes on the same side of the sheet, and the width and spacing of the stripes on one side of the first sheet are the same as on the second side of the first sheet. An expanded structural panel prepared according to the method of claim 9. An unexpanded structural panel comprising: a first (5) and second (9) outer plastic sheet; a core sheet (11) of thermoplastic material, and material to which hot thermoplastic (i.e. thermoplastic heated to or above its glass transition temperature) adheres poorly, applied in stripes (7) to both sides of said core sheet, the stripes being spaced apart from another, the centerline of the spaces on one side of the sheet being aligned with the centerline of the stripes on the other side of the core sheet, the striped portions on one side of the sheet being wider than the spaces with which they are aligned on the opposite side of the sheet, said first and second outer sheets being periodically bonded on either side of said core sheet where said material to which hot plastic adheres poorly is not present.

Inventors: NIED, HERMAN FREDERICK
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1988
Main classification: B32B31
IPC codes: B29C65, B32B3, B32B17, B32B37