Method of Manufacturing Reinforced Thermoplastic Honeycomb Structure

1. Field of the Invention

The present invention is directed to a simple and economic process for the manufacture of thermoplastic resin structures particularly, honeycomb structures and pre-pregs utilized in the formation of such structures.

2. Description of the Prior Art

Paper honeycomb was first made by the Chinese approximately two thousand years ago, but at that time it was used primarily as ornamentation and not as a structural material. The modern utilization of honeycomb structures began just after 1940, and today there are about ten companies manufacturing the various core types.

While the primary utilization of honeycomb structure is in the construction of sandwich panels, it has many other applications, such as energy absorption, air directionalization, light diffusion and radio frequency shielding.

U.S. Pat. No. 4,500,583, to Naul, discloses a honeycomb structure made of resin impregnated molded glass wool. In particular, glass wool blankets containing about 20 to 25 percent by weight of an uncured binder such as urea-phenol-formaldehyde resin are molded into corrugated sheets under heat and pressure in a two-part mold. A plurality of the corrugated sheets can then be adhesively bonded to one another to form a honeycomb structure.

U.S. Pat. No. 2,734,843, to Steele, discloses a method of producing honeycomb wherein longitudinally extending, spaced, parallel lines of adhesive are applied to the face surface of continuously moving web material, the web material is cut into separate flat sheets of uniform size, and said sheets are adhered to one another with the obverse side of each sheet adhered to an adjacent sheet by a plurality of spaced parallel lines of adhesive and the reverse side of each sheet adhered to an adjacent sheet by a plurality of spaced parallel lines of adhesive, which are in staggered parallel relationship to the lines of adhesive on the obverse side.

U.S. Pat. No. 3,032,458, to Duponte et al., discloses a method of making an expandable structural honeycomb material which comprises securing together a number of layers of flexible sheet material in a stack by means of an adhesive distributed between the layers in patches arranged in arrays of intersecting rows and columns and positioned such that the columns at the obverse face of each intermediate layer are staggered with respect to the columns at the reverse face of said layer while the rows at said faces are coincident, and slicing the stack by cutting it in the direction of the rows at position such that the contacting pairs of faces of the sheet material within the slices thus produced are secured together over a part only of their width by at least a part of a single row of patches.

U.S. Pat. No. 4,128,678, to Metcalfe et al., discloses a method and apparatus for the manufacture of a heat insulating material from an unsecured, strip-shaped felt of fibers containing a heat hardenable bonding substance. The felt is first formed into a serpentine array of corrugations extending across the entire width of the uncured felt and then cured. The felt is then cured and the cured felt is cut longitudinally into two partial felts, the corrugations being severed so as to form a succession of U-shaped arrays along each of the partial felts.

U.S. Statutory Invention Registration H47, to Monib, discloses a lightweight structural panel of an aramid honeycomb core faced with a resin-impregnated fiber layer, wherein peel strength between the core surface and the facing layer is improved by interpositioning of a spunlaced fabric, containing at least 50% aramid fibers pervaded with a curable resin, between the core surface and the facing layer.

U.S. Pat. No. 4,012,738, to Wright, discloses a microwave radiation absorber comprising a layer of dielectric material of relatively high dielectric constant and a layer of magnetic material having a relatively high coefficient of magnetic permeability.

U.S. Pat. 3,600,249, to Jackson et al., discloses a method and apparatus for the production of a reinforced plastic honeycomb comprising the steps of: (1) impregnating a fabric which distorts under its own weight, such as a fiber glass fabric, with a heat-curable resin in an amount sufficient to cause the fiber glass fabric to have sufficient body to prevent its distorting under its own weight while permitting expansion after curing; (2) applying adhesive lines on the impregnated fiber glass fabric, the adhesive being applied so as to avoid penetration to the opposite side of the fiber glass fabric, and allowing said adhesive lines to advance to a relatively non-tacky state; (3) stacking sheets of the so-produced fiber glass fabric with the lines of adhesive on one sheet in staggered relation to the lines of adhesive on adjacent sheets; (4) applying heat and pressure to the so-formed stack to cause the adhesive to flow and bond to the surface of the next adjacent sheet in the stack; (5) expanding the stack to form a honeycomb configuration; (6) applying heat and pressure to the expanded stack created in step (5) to fully cure the impregnated resin and the adhesive; (7) dipping the rigid honeycomb structure formed in step (6) into a mass of uncured resin; and (8) following the dipping, curing the resin so-coated onto the rigid honeycomb structure.

U.S. Pat. No. 3,321,355, to Holland, discloses a method of making a honeycomb structure from fabric reinforced plastic wherein the warp and woof of the fabric in the final honeycomb product are obliquely disposed to the longitudinal axis of the honeycomb cells. In particular, the method comprises: providing a plurality of non-rectangular parallelogram shaped cut sections of fabric reinforced plastic material of substantially the same pattern and size in which the warp of the fabric extends parallel and the woof of the fabric extends perpendicular to a first pair of parallel sides of each section and at acute angles in reference to a second pair of parallel sides of each section; superimposing such sections one upon the other in a stack; adhering such sections to one another along spaced apart parallel bonding lines extending perpendicular to said second pair of parallel sides, the bonding lines of successive superimposed sections staggered relative to one another to form a honeycomb structure.

U.S. Pat. No. 3,598,676, to Noble, is an improvement over the aforementioned Holland patent, to reduce waste material, i.e. to eliminate the step of trimming portions of the parallelogram shaped core to produce a rectangular shaped core. In particular, the improved method comprises: forming a plurality of non-rectangular parallelogram shaped sections of fabric reinforced plastic material in which a first and second side of the section are substantially parallel to each other and to the warp or woof of the fabric and in which a third and fourth side of the section are substantially parallel to each other and disposed at an oblique angle to the warp and woof of said fabric, the distance between the third and fourth sides of each section being substantially equal; joining the first and second sides of said sections together in serial relationship to form a web having a width equal to the distance between the third and fourth sides of one of said sections and a length approximately equal to the sum of the first sides of all sections which are joined together in serial relationship, the third and fourth sides of said joined sections forming the lateral edges of the web; cutting a plurality of equal rectangular shaped sections from said web, two sides of each rectangular section being cut perpendicular to the lateral edges of the web; superimposing a plurality of said rectangular sections one upon another in a stack; and adhering said plurality of rectangular sections to one another along spaced apart bonding lines which are substantially parallel to each other and perpendicular to two sides of said superimposed rectangular sections, the bonding lines of adjacent superimposed sections being staggered relative to one another to form a plurality of adjacent cells having longitudinal axes which are substantially parallel to each other and perpendicular to two sides of said superimposed rectangular sections, whereby a bias weave honeycomb core structure is formed in which the warp and the woof of said fabric are disposed at an oblique angle to the longitudinal axes of said cells.

U.S. Pat. No. 3,759,775, to Shepherd, discloses a method for producing an absorbent, high bulk, very low fiber density stabilized web. In particular, an air laid web of fibers is thoroughly impregnated with a volatile liquid. The volatile liquid may contain a small amount of heat-activatable binder, or the web of fibers may include the binder in the form of a small amount of thermoplastic fibers or powder dispersed throughout the web. The so-impregnated web is then heated, preferably, by dielectric heating or the like, so as to vaporize the liquid whereby the web is explosively puffed up and the small amount of binder secures interconnections of the fibers to maintain the web superstructure.

U.S. Pat. No. 3,366,525, to Jackson, discloses a method of making honeycomb or similar laminated structures from sheets of heat sealable plastic which are cohered together under heat and pressure at selected areas. In an example a polyethylene web 10 inches wide and 4 mils thick is cut into sheets of material 18 inches long. A release film is printed on the sheets in lines 0.441 inch wide which are spaced apart by exposed or release-film-free regions 0.135 inch wide. The sheets are stacked in a mold and then subjected to heat and pressure to seal adjacent sheets together in those regions free of release film. The mold is cooled, the pressure reduced, and the stack of heat sealed sheets removed from the mold. The stack is then heated and pulled to expanded condition and the so-formed honeycomb is then cooled.

As may be readily ascertained from the above-noted documents, the preparation of honeycomb structure from fiber-reinforced plastics requires numerous web handling steps including multiple impregnation and/or dipping steps. In the case of obtaining higher shear modulus and improved handleability, the cutting of woven webs on the bias and their re-orientation requires even more handling steps.

It is an object of the invention to overcome the aforementioned problems with the prior art techniques for honeycomb structure formation.

It is a further object of the invention to provide a process which can be operated in a continuous, high-speed manner to produce thermoplastic resin structures, especially honeycomb structures.

These and other objects of the invention, as will become apparent hereinafter, are achieved by the provision of a method of making a thermoplastic structure comprising: (A) providing a longitudinally extending fiber-reinforced web, said fiber-reinforced web comprising a thermoplastic resin; (B) forming said fiber-reinforced web into sheets having a predetermined configuration; (C) stacking a plurality of said sheets to form a preform; (D) forming a structure from said preform, said forming step including bonding of at least portions of said sheets together by fusion of at least a portion of said thermoplastic resin.

In one embodiment of the invention, the fiber-reinforced web includes thermoplastic resin as staple fibers.

In a further embodiment of the invention, the fiber-reinforced web may comprise up to about 80% by weight, based on the total weight of said fiber-reinforced web, of a reinforcing fiber; and, in a preferred form of this embodiment, comprises an admixture of staple fibers of said thermoplastic resin and said reinforcing fiber.

In a still further embodiment, the fibrous web comprises at least one first fibrous non-woven web material having it fibers substantially aligned in a first direction and at least one second fibrous non-woven web material having its fibers substantially aligned in a second direction.

FIG. 1A illustrates a method of forming a dry-laid non-woven web of fibrous material according to the present invention.

FIGS. 1B-1D illustrate methods of forming two-layer dry-laid non-woven webs of fibrous material according to the present invention.

FIG. 2A illustrates a method of consolidating a web of fibrous material according to the present invention.

FIG. 2B illustrates a method of consolidating a web of fibrous material with a melt extruded or hot melt coated thermoplastic resin according to the present invention.

FIG. 2C illustrates a method of consolidating a web of fibrous material with at least one preformed sheet of a thermoplastic resin according to the present invention.

FIG. 3 illustrates a flat die method for corrugating a web of fibrous material according to the present invention.

FIG. 4 illustrates a vacuum forming method for corrugating a web of fibrous material according to the present invention.

FIG. 5 illustrates a roller die method for corrugating a web of fibrous material according to the present invention.

FIG. 6 illustrates an apparatus for printing release layers on said web of fibrous material, transverse to the direction of travel of said web, according to the present invention.

FIGS. 7A and 7B illustrate an apparatus for printing release layers on said web of fibrous material, parallel to the direction of travel of said web, according to the present invention.

FIG. 8 illustrates a square corrugation pattern, according to the present invention.

FIG. 9 illustrates a curved corrugation pattern, according to the present invention.

FIG. 10 illustrates a hexagonal corrugation pattern, according to the present invention.

FIG. 11 illustrates a stack of sheets to be bonded together to form a honeycomb structure, after expansion, according to the present invention.

FIG. 12A is a top view of a structural element prepared using the present invention.

FIG. 12B is a cross-section along line B--B of the structural element of FIG. 12A.

The present invention utilizes a longitudinally extending fiber-reinforced web as a base material. The base material is typically formed as a dry-laid, non-woven web. That is, staple fibers-short lengths of crimped thermoplastic or thermosetting, organic or inorganic materials -- are distributed onto a moving conveyor via a modified cotton carding mechanism, as is known in the art. When a series of such cards are placed in line, a highly oriented assemblage of fibers (known as "laps" in the non-woven industry) is formed. If additional laps are added to the machine direction (direction of conveyor movement) laps in a cross-layered fashion, then significant cross-directional fiber orientation is also possible. Of course, additional layers may be built up in this manner, and any angular orientation between adjacent laps may be utilized. For example, in the case of honeycomb structures, orientations relative to the intended cell axis may vary from 0.degree. to 90.degree. or from -45.degree. to +45.degree..

Alternatively, the laps may be laid down on a preformed woven substrate, e.g., fiberglass cloth, so that in addition to the warp and weft of the woven fabric running at relative angles of 0.degree. and 90.degree., the non-woven substrate may be oriented at any desired angle, e.g., at -45.degree. and +45.degree., with respect to the woven substrate.

The assemblage of laps (and/or the assemblage of laps and preformed woven substrate) may be held together by a number of techniques, e.g., by mechanically interlocking the fibers as by needlepunching or water entanglement, by chemical bonding as by an acrylic adhesive latex emulsion, or by thermal bonding as by the use of blended thermoplastic fibers in the web and subsequent heating of the web to cause those fibers to soften and act as an adhesive.

Subsequent densification of the web can may be effected, e.g., by a calendering operation, e.g., the assemblage of laps is passed through at least one set of pressure nip rollers, whereby the thickness of the web is reduced to between about 0.001 and about 0.015 inch.

Preferably, the resin or matrix material, i.e. the thermoplastic resin, is incorporated into the web as thermoplastic resin fibers during the production of the laps. In contrast, current technology, as previously described, utilizes several impregnation steps, as well as several dipping steps (of the assembled honeycomb core) to incorporate thermosetting resin or matrix material into the product. Such repetitive steps are obviously time-consuming and costly.

Alternatively, the resin or matrix material, i.e. the thermoplastic resin, may be incorporated into the web as a thermoplastic film during the calendering operation. In this technique, the assemblage of laps and a film of thermoplastic resin are simultaneously fed through the calender rollers whereby, if the thermoplastic is heated so as to soften it, the thermoplastic film becomes bonded to and/or may interpenetrate the fibrous assemblage.

As a further alternative, the resin or matrix material, i.e. the thermoplastic resin, may be incorporated into the web as both thermoplastic resin fibers during the production of the laps and as a thermoplastic film during the calendering operation. The same or different thermoplastic resins can be utilized in each case.

Suitable thermoplastic resins for incorporation by either technique include any of the engineering grade thermoplastic resins such as polyethersulfone, polyphenylenesulphide, polyetherimide, nylon--4,6, polyamideimide, polyarylate, polyarylsulfone, polycarbonate, polyetherketone, polyimidesulfone, polysulfone, and polyether-ethersulfone, as well as such liquid crystal polymers as Vectra.RTM. and Xydar.RTM., and mixtures thereof.

The advantage of the present approach is that whether the resin or matrix material is added as a fibrous entity or added in the calendering process as a film material, all of the subsequent web handling, impregnation and core dipping steps have been eliminated from the manufacturing process.

Additionally, if the resin or matrix material is incorporated into the web as a film material, it can be precisely pre-coated with an "active" electrical and/or magnetic material so that the assembled web, and the honeycomb structure produced therefrom, will incorporate radar absorbing (i.e., microwave absorbing) capabilities into its properties.

Furthermore, if the resin or matrix material is incorporated into the web as a film material, then woven materials such as glass cloth, Kevlar.RTM. (DuPont, polyaramid fabric) or graphite cloth, as well as other non-woven materials such as Nomex.RTM. (DuPont, meta-phenylenediamine/isophthaloyl chloride copolymer fiber) or paper may be utilized.

Moreover, the present approach allows the shear modulus and handleability of the web to be varied by controlling the amount of "cross-lapping" that occurs during formation of the non-woven web. Currently, as previously noted, when woven glass is used, the web must be cut on a bias in order to achieve the correct fiber orientation in the core. This causes a tremendous waste of raw material. With the present non-woven approach, material properties relative to the machine direction of the web are easily varied due to the ability to cross-lap as required. The net result is little or no material waste and more designability for the final honeycomb product.

Also, if a radar absorbing core has been desired, its performance has been very sensitive to the direction of the woven glass in the final honeycomb structure. This is because the "active" materials have typically been introduced into the honeycomb or the web in solution form and the fibers tend to "wick up" the material in a very oriented fashion. The result of this "wicking" is a honeycomb which is very polarization dependent (dependent on the direction of the electric field for performance). The present approach eliminates the directionality aspect of the "active" material since the "active" material will remain uniform in distribution when applied with the resin or matrix film during calendering, i.e. it will not align itself with the fibers.

The fibrous web may comprise thermoplastic resin fibers, in toto, however, it is preferred to incorporate a reinforcing fiber in an amount of about 20% by weight to about 80% by weight, preferably, about 30% to 70% by weight, most preferably, about 60% to 70% by weight, based on the total weight of the fibrous web. These reinforcing fibers may be organic or inorganic. Preferred organic fibers include cellulosic fibers, polyaramid fibers and polyamide fibers. Preferred inorganic fibers include carbon fibers and glass fibers, most preferably cardable glass fibers (Owens-Corning Fiberglass).

Regardless, of the nature of the reinforcing fiber, it has been found desirable to use reinforcing fibers of a length of from about 1/2 inch up to several inches, e.g., 6 inches, preferably 3 inches. Similar fiber lengths for the thermoplastic resin fibers allow easy orientation when admixed with the reinforcing fibers.

There are two basic techniques for the manufacture of honeycomb, the expansion method and the corrugation method. The expansion method consists of printing adhesive lines or release areas on the web; cutting and stacking sheets of the web with the adhesive lines or release areas in staggered relation; bonding the stack along the adhesive lines or the non-release areas; cutting slices from the stack; and finally expanding the slice to form the honeycomb structure.

The corrugation method consists of cutting sheets of the web; corrugating the cut sheets to form a substantially sinusoidal pattern of alternating nodes and antinodes; stacking the corrugated layer with the antinodes of a lower layer in contact with the nodes of the sheet immediately thereabove; and bonding the nodes and antinodes which are in contact to one another.

The basic cell shapes of honeycomb structures are "hexagonal", "over-expanded" and "flex-core". "Hexagonal" is the basic shape wherein the cross-section of the cell is substantially a regular hexagon. "Over-expanded" is just the standard hexagon over-expanded to a substantially rectangular shape. (This allows the core to be easily formed into a cylinder in the direction of the continuous sheets, i.e. the "ribbon" direction.) "Flex-core" is used when the honeycomb must be formed with compound curves, e.g., as described in U.S. Pat. No. 3,032,458, to Daponte et al. Other configurations are also possible, for instance, "reinforced core" has an extra flat sheet interposed between each node and antinode to be bonded together so as to increase the density and corresponding mechanical properties and "tube core" is manufactured by spirally wrapping a corrugated sheet and a flat sheet around a mandrel, with the nodes and antinodes of the corrugated sheet to be bonded to the flat sheet.

Turning now to the drawing figures, FIG. 1A illustrates a method of forming a dry-laid non-woven web of fibrous material wherein a foraminous belt 1 is supported by a pair of rollers 3, 3' for rotation in the direction indicated by the arrow. In the apparatus 5, a fibrous web material 7 is laid down on the belt 1 by carding or by passing an airborne stream of fiber through the foraminous belt 1.

In an alternative embodiment (as shown in dotted lines), the fibrous web material 7 may be laid down on a preformed substrate 8, e.g., a woven substrate such as fiberglass cloth, graphite cloth, Kevlar.RTM. cloth or a non-woven substrate such as paper or Nomex.RTM..

FIG. 1B illustrates a method of forming a two-layer dry-laid non-woven web of fibrous material wherein a foraminous belt 1b is supported by a pair of rollers 3b, 3b' for rotation in the direction indicated by the arrow. In the apparatus 5b, a fibrous web material 7b is laid down on the belt 1b by carding, so that the fibers of the web material 7b are aligned substantially in the direction of the belt 1b. As the belt 1b rotates about rollers 3b, 3b', the web material 7b, supported on belt 1b, passes through apparatus 9 wherein a second fibrous web material 11 is laid down on top of the first fibrous web material 7b by carding, so that the fibers of the second web material 11 are aligned substantially transverse to the fibers of the first web material 7b. The fibers utilized in the formation of the first web material 7b and the second web material 11 may be all thermoplastic fibers, although up to 80% of reinforcing fibers may be included. The fibrous web 13 formed by the first web material 7b overlaid with the second web material 11 is passed through an oven 15 wherein the fibrous web 13 is heated to a temperature sufficient to soften the thermoplastic resin fibers therein to cause adherence of the first and second web materials to each other.

FIG. 1C illustrates a method of forming a two-layer dry-laid non-woven web of fibrous material wherein a foraminous belt 1c is supported by a pair of rollers 3c, 3c' for rotation in the direction indicated by the arrow. In the apparatus 5c, a fibrous web material 7c is laid down on the belt 1c by carding, so that the fibers of the web material 7c are aligned substantially in the direction of the belt 1c. As the belt 1c rotates about rollers 3c, 3c', the web material 7c, supported on belt 1c, passes through apparatus 9c wherein a second fibrous web material 11c is laid down on the top of the first fibrous web material 7c, by carding, so that the fibers of the second web material 11c are aligned substantially transverse to the fibers of the first web material 7c. The fibrous web 13c formed by the first web material 7c overlaid with the second web material 11c is passed under needlepunch 17 whereby fibrous web 13c is pierced by a plurality of needles which reciprocate into and out of the fibrous web 13c to cause mechanical interlocking of web material 7c and web material 11c. The needles may be singly or doubly barbed. Singly barbed needles have barbs that catch fibers when they are moving in one direction and carry them along with the needle and then release the fibers when they are moving in the opposite direction. Doubly barbed needles have barbs that catch fibers as for the singly barbed needles and barbs that are reverse oriented so that when the first barbs are release fibers the second barbs are catching fibers, and vice versa.

FIG. 1D illustrates a method of forming a two-layer dry-laid non-woven web of fibrous material wherein a foraminous belt 1d is supported by a pair of rollers 3d, 3d' for rotation in the direction indicated by the arrow. In the apparatus 5d, a fibrous web material 7d is laid down on the belt 1d by carding, so that the fibers of the web material 7d are aligned substantially in the direction of the belt 1d. As the belt 1d rotates about rollers 3d, 3d', the web material 7d, supported on belt 1d, passes through apparatus 9d wherein a second fibrous web material 1d is laid down on the top of the first fibrous web material 7d, by carding, so that the fibers of the second web material lid are aligned substantially transverse to the fibers of the first web material 7d. The fibrous web 13d formed by the first web material 7d overlaid with the second web material 11d is passed under hopper 19, which contains an acrylate binder in an aqueous emulsion, which applies the acrylate bonder emulsion to the top surface of fibrous web 13d. The so-coated web is then passed over suction box 21 by which the aqueous binder is drawn through the fibrous web 13d and uniformly distributed therethrough. As the water evaporates, either naturally or through application of heat (not shown) the acrylate binder adhesively bonds the fibers of the fibrous web 13d together.

The non-woven fibrous web (7, 13, 13c, 13d) may then be consolidated by calendaring or any other method of applying heat and pressure. FIG. 2A illustrates a method of consolidating the fibrous web wherein a fibrous web 23 supported on a conveyor belt 25 is fed between two pressure rollers 27, 27' to form a consolidated web 29. Preferably, rollers 27, 27' are heated so as to cause softening of thermoplastic fibers contained in the fibrous web 23 whereby the consolidated web 29 is bonded together by the softened fibers.

FIG. 2B illustrates a method of consolidating the fibrous web wherein a fibrous web 23b supported on a conveyor belt 25b is hot melt coated with a layer of thermoplastic resin 31 delivered from extruder/coater 33 and then the so-coated web is fed between two pressure rollers 27b, 27b' to form a consolidated web 29b. Preferably, rollers 27b, 27b' are heated so as to cause softening of the coated thermoplastic resin layer and bonding thereof to the web 23b.

FIG. 2C illustrates a method of consolidating the fibrous web wherein a fibrous web 23c supported on a conveyor belt 25c is fed to two pressure rollers 27c, 27c', simultaneously with a preformed thermoplastic resin film 35, to form a consolidated web 29c. Preferably, rollers 27c, 27c' are heated so as to cause softening of the preformed thermoplastic resin film and bonding thereof to the web 23c. The preformed film 35 may contain or may be coated with "active" electrical and/or magnetic material to impart a radar absorbing capability into the ultimate honeycomb structure. Suitable electrical materials include those having a high dielectric constant such as barium titanate (BaTiO.sub.4) and also include particulate carbon such as carbon black, graphite, etc. Suitable magnetic materials include ferromagnetic materials such as iron, nickel, permalloy, ferrite, etc. The techniques illustrated in FIGS. 2B or 2C are particularly applicable to non-woven webs containing no thermoplastic fibers or woven webs such as glass cloth.

Alternatively, as shown in dotted lines in FIG. 2C, a second preformed thermoplastic resin film 35' may also be fed simultaneously to the two pressure rollers 27C, 27C' so as to "sandwich" the fibrous web 23C between films 35 and 35'. The films 35 and 35' may be the same or different thermoplastic resins, preferably, the same. Additionally, the films may be of the same or different thickness.

After the fibrous web has been consolidated it may be cut into sheets of predetermined size and corrugated. FIG. 3 illustrates a flat die method of corrugation wherein a sheet 37 of the consolidated web is placed between a pair of mating dies 39, 39' wherein the respective die faces 41, 41' are formed in the desired corrugation pattern. The dies may be heated so as to soften the thermoplastic resin to allow the sheet to be molded and, after removal from the dies, the sheet 37 will cool and set up in the corrugated shape. The dies 39, 39' may be mounted on shafts 42, 42' of a hydraulic press so as to allow the dies to be forced together.

FIG. 4 illustrates a vacuum forming method of corrugation wherein a sheet 37 of the consolidated web is heated to a temperature above the softening temperature of the thermoplastic resin and placed upon a foraminous die 43 shaped in the desired corrugated pattern. While maintaining the sheet at a temperature above the softening temperature of the thermoplastic resin, a vacuum is drawn in air box 45 by applying suction to pipe 47 (by means to shown). Ambient air pressure then forces the softened sheet into conformance with the corrugation pattern of the foraminous die 43. When suction is released from pipe 47, the now-corrugated sheet 37 may be removed from the die.

FIG. 5 illustrates a roller die method of corrugation wherein a sheet 37 of the consolidated web is passed between a pair of corrugating rollers 49, 49' which corrugate the sheet in the desired pattern. The rollers may be heated so as to soften the thermoplastic resin. In a particularly preferred embodiment, the corrugating rollers 49, 49' may be utilized in lieu of the pressure rollers 27, 27'; 27b; 27b'; 27c, and 27c' of the embodiments of FIGS. 2A, 2B and 2C, respectively, and sheets may then be cut from the corrugated strip exiting the rollers.

In any case, the consolidation (and/or corrugation) is effected at sufficient temperature and pressure as to allow the thermoplastic resin to melt and flow together in a proper manner to act as the matrix material. It has been found that a suitable temperature is 50.degree. F., above the heat deflection temperature (HDT) of the thermoplastic resin, preferably, 50.degree.-300.degree. F. above the HDT, and, most preferably, 100.degree.-200.degree. F. above the HDT. For the preferred "engineering grade" thermoplastic resins, this typically means temperatures of 550.degree.-650.degree. F. The HDT value for a number of these "engineering grade" thermoplastic resins is set forth in the following Table.

Suitable pressures are from about atmospheric to 1,000 psi or higher, preferably, about 200 psi to 600 psi, most preferably 300 psi to 500 psi.

As shown in FIG. 10, the so-formed "half-cell" corrugated sheets 37', 37", 37"' may then be stacked with nodes 51', 51" of an upper sheet 37', 37" in contact with the antinodes 53", 53"' of a lower sheet. The contacting nodes and antinodes are then bonded to one another either adhesively or by melt bonding of the thermoplastic resin by resistive, inductive, radiant or ultrasonic heating. As shown in FIG. 10, the electrodes 55a, 55b of an inductive (dielectric) heating device may be disposed on opposite sides of a node/antinode contact, and melt bonding may then be induced by application of a high frequency oscillating current to the electrodes.

Alternatively, the consolidated web (29, 29b, 29c) may be coated with stripes of a the release film, the so-coated web cut into sheets, which when stacked in staggered array and melt bonded, can be expanded to form the honeycomb structure.

FIG. 6 illustrates an apparatus for printing release layers on the surface of the consolidated web wherein the consolidated web 29' passes below printing roller 57 which has raised portions 59 and depressed portions 61 extending across (perpendicular to the plane of the drawing) its entire surface. The raised portions 59 contact an intermediate roller 63 while the depressed portions 61 do not contact the intermediate roller 63. Intermediate roller 63, in turn, contacts a pick-up roller 65 which is partially immersed in a dispersion 67 of a release film forming resin, e.g., cellulose acetate in ethylene glycol monomethyl ether acetate or an aqueous polyvinylalcohol suspension. As the rollers 65, 63, 57 rotate, the release layer dispersion is transferred from roller 65 to roller 63 to the raised portions 59 of roller 57. Since only the raised portions 59 of the roller 57 contact moving web 29', a striped pattern of release film dispersion, which upon drying forms a release film, is printed onto web 29'. The web 29' may then be cut into sheets of predetermined size (by means not shown).

FIGS. 7A and 7B illustrate an apparatus for printing release layers on the surface of the web, parallel to the direction of travel of the web, wherein the consolidated web 29' passes below printing roller 57' which has raised portions 59' and depressed portions 61' extending perpendicular to the axis of rotation 69 of the roller. The raised portions 59' contact an intermediate roller 63' while the depressed portions 61' do not contact the intermediate roller 63'. Intermediate roller 63', in turn, contacts a pick-up roller 65' which is partially immersed in a dispersion or solution 67' of a release film forming resin, as previously described. As the rollers 65', 63', 57' rotate, the release layer dispersion is transferred from roller 65' to roller 63' to the raised portions 59' of roller 57'. Since only the raised portions 59' of roller 57' contact moving web 29', stripes 59.increment. of release film dispersion, which upon drying form a release film, are printed onto web 29'. The web 29' can then be cut into sheets of predetermined size (by means not shown).

As shown in FIG. 11, the so-striped sheets of consolidated web 29' may then be stacked with the stripes 59" of release film in staggered array. Upon the application of pressure and heat (sufficient to soften the thermoplastic resin) to the stack, the adjacent sheets of web 29' are bonded to one another in the areas 71 where no release film is found.

Although the present invention has been discussed in terms of hexagonal cell structure, any substantially sinusoidal repeating pattern of nodes and antinodes may be utilized to form a honeycomb structure. FIG. 8 illustrates a square pattern; whereas FIG. 9 shows a generalized sinusoidal pattern with wavelength .gamma. (node-to-node or antinode-to-antinode distance) and node-to-antinode distance of .gamma./2. Any such pattern can be utilized in the present invention.

The thermoplastic resin utilized in the present invention may incorporate colorants, fillers, etc., as are conventional in the art, provided that they are stable under the processing conditions of the invention, in addition to the "active" electrical and/or magnetic materials previously noted. These additives may be incorporated whether the thermoplastic resin is in fiber or film form.

Additionally, microwave absorbent properties may also be achieved by incorporation of a minor proportion of electrically conductive fibers (e.g., graphite or metal fibers) of a length equal to one-half of the wavelength of the microwave radiation to be absorbed. Broadband microwave radiation obviously requiring a mix of fiber lengths across the wavelength spectrum.

The following examples are presented to illustrate the present invention, but, are not intended to be limitive thereof.

A style 104 woven glass web was impregnated with a curable resin to allow sufficient drapeability for subsequent forming operations. This produced a 50% by weight resin content, which if formed into a hexagonal honeycomb core (face length=1/8") would produce a honeycomb of 0.9 lb/cu. ft. density. Additional dipping in curable resin to produce a 4 lb/cu. ft. density would reduce the fiber content to about 10% by weight.

A mixture of 30% by weight Ryton.RTM. fiber (polyphenylene sulfide, Phillips), average length 1.5", 1.5 denier and 70% by weight fiberglass (Owens Corning), average length 3", 1.5 denier, was carded to produce a uniformly mixed web having approximately 80% of the fibers oriented in the machine direction. The web weight was approximately 0.25 oz/yd.sub.2.

A style 112 woven glass web was sandwiched between a 0.002" thick film of polyethersulfone (S-100, ICI Americas) and a 0.010" thick film of polyethersulfone (S-100, ICI Americas), in the manner illustrated in FIG. 2C, and then immediately corrugated to form hexagonal honeycomb half-cell (face length=1/8") by passage through a pair of rotary dies, as illustrated in FIG. 5, operating at about 550.degree. F. and 300 psi nip pressure. The so-formed honeycomb half-cell corresponds to a honeycomb density of 4.5 lb/cu. ft.

A style 112 woven glass web was consolidated with a single ply of 0.002" thick film of polyethersulfone (S-100, ICI Americas), in the manner illustrated in solid lines in FIG. 2C, and then immediately corrugated to form hexagonal honeycomb half cell (face length=1/8") as in Example 1. The so-formed honeycomb half cell corresponds to a honeycomb density of 4 lb/cu. ft. with a 70% glass content.

In the following examples, mixtures of Ryton.RTM. fiber (polyphenylene sulfide, Phillips), average length 1.5", 1.5 denier and fiberglass (Owens Corning), average length 3", 1.5 denier, were carded to produce uniformly mixed webs. Consolidation and/or corrugation was carried out at approximately 600.degree. F. and 500 psi nip pressure. Unless otherwise indicated webs were laid up by alternate layers at 0.degree. and 90.degree. to cell axis.

The web produced in Preparative Example 1 was laid up to equate to a material yielding a hexagonal honeycomb half cell (face length=1/8") corresponding to a honeycomb density of 1.5 lb/cu. ft. and then consolidated without corrugation.

The web produced in Preparative Example 1 was laid up to equate to a material yielding a hexagonal honeycomb half cell (face length=1/8") corresponding to a honeycomb density of 2 lb/cu. ft. and then simultaneously consolidated and corrugated.

A web having a 60% glass content was produced in the manner of Preparative Example 1. This web was laid up to equate to a material yielding a hexagonal honeycomb half cell (face length=1/8") corresponding to a honeycomb density of 4 lb/cu. ft. and then simultaneously consolidated and corrugated.

The web produced in Preparative Example 1 was laid up to equate to a material yielding a hexagonal honeycomb half cell (face length=3/8") corresponding to a honeycomb density of 1.1 lb/cu. ft. and then simultaneously consolidated and corrugated.

The web produced in Preparative Example 1 was laid up by alternate layers at -45.degree. and +45.degree. to the cell axis to equate to a material yielding a hexagonal honeycomb half cell (face length=1/8") corresponding to a honeycomb density of 7 lb/cu. ft. and then simultaneously consolidated and corrugated.

The web produced in Preparative Example 1 was used to fill a mold and a structural element 81, as illustrated in FIGS. 12A and 12B was prepared under pressure and temperature conditions, as above.

While the present invention has been generally described with respect to the preparation of honeycomb structures, Example 8 clearly indicates the far-reaching applicability of the fiber-reinforced non-woven web of the present invention in molding, in general.

In this regard, the present process in conjunction with the preferred thermoplastic-resin-fiber-containing, reinforced non-woven web, allows the fabrication of molded articles wherein long fiber reinforcement, i.e. fibers greater 1 inch in length, has traditionally been found to be difficult, i.e. in products having corners or folded edges, e.g., boxes, suitcases, etc., in products having complex contours.