Method for producing a honeycomb core in thermofusible material, and device for implementing same

This process consists: PA1 in continuously extruding, with the aid of a multislot die, parallel sheets (31) of thermally fusible material inside a cooling chamber (4), with the creation of a seal between the longitudinal edges of the sheets and the walls of the chamber; and PA1 in creating, in this chamber and from the end located on the die side, successively in the various compartments located on both sides of each sheet (31), successively a vacuum and the delivery of a coolant, the two compartments located on the two sides of the same sheet being, in the case of one of them, subjected to a vacuum and, in the case of the other, subjected to the delivery of a coolant, and conversely during the following phase, so as to deform the sheets and weld them in pairs with the formation of cells (36).

The subject of the present invention is a process for manufacturing a
 honeycomb structure made of thermally fusible material, such as made of
 thermoplastic or made of rubber, and a plant for the implementation of
 this process.
 The honeycomb structure according to the invention makes it possible to
 obtain, for example, sheets in which the cells are perpendicular to the
 plane of the sheet, these sheets having a simple honeycomb structure or a
 sandwich structure with a honeycomb core.
 It is already known to produce honeycomb structures made of thermally
 fusible material, which are essentially obtained by four processes.
 The first process consists in adhesively bonding films to one another, with
 the aid of offset sheets of adhesive, and then in carrying out a drawing
 operation in order to obtain a honeycomb structure. This is a batch
 manufacturing technique, which is very expensive to implement.
 A second process consists in thermoforming a sheet of thermoplastic. This
 is a batch manufacturing technique, the cells necessarily being tapered
 and the thickness of the structure obtained being very greatly limited.
 Another process consists in injection moulding the structure inside a
 complex mould. In this case again, this is a batch technique, the size of
 the parts produced being limited.
 A fourth process consists in extruding a thermoplastic profile of greater
 or lesser complexity, having cut-outs making cells. The cells are
 therefore oriented longitudinally in the extrusion direction. It is
 necessary, after extrusion, to cut the profile into slices, to reorient
 these slices at 90.degree. with respect to the extrusion direction and to
 fasten the slices to each other, either by adhesive bonding or by thermal
 fusion, with or without added material, such as woven material,
 needle-punched material or a sheet of thermoplastic, in order to form a
 panel.
 This technique is therefore complex to implement, as it requires off-line
 operations in order to end up with a final product consisting of a panel.
 The honeycomb structures, such as those that have just been defined,
 possess very good intrinsic properties resulting, on the one hand, from
 the honeycomb structure and, on the other hand, from the nature of the
 thermally fusible material. These properties are, in particular, the
 mechanical strength in compression, a low weight, recyclability,
 thermoformability, a beneficial thermal insulation coefficient, their
 imputrescible character, as well as the permeability to most forms of
 radiation. These structures therefore find applications in many
 industries: automobile, naval, aeronautical and railway, as well as in
 buildings and public works, allowing the production of sandwich structures
 which advantageously replace cellular foams and elastomers of the rubber
 type, of conventional design.
 Document DE 1,779,330 describes a process for manufacturing a crosslinked
 tubular net, in which strands of thermoplastic are extruded parallel to
 each other and are deflected in order to be brought into point contact in
 pairs due to the effect of hot-air pressure. This technique is not
 suitable for the production of a honeycomb cellular structure.
 The object of the invention is to provide a process and a plant for
 manufacturing such a honeycomb structure, allowing continuous manufacture
 of a structure in the form of a panel, without requiring any off-line
 operations, this panel possibly consisting of a sandwich structure with a
 honeycomb core. Another object of the invention is to allow easy
 adjustment of the thickness of the structure obtained, during production,
 as well as of the density of this structure, with the possibility of
 varying the shape of the cells while the structure is being obtained.
 For this purpose, the process to which the invention relates consists:
 in continuously extruding, with the aid of a multislot die, parallel sheets
 of thermally fusible material inside a cooling chamber, with the creation
 of a seal between the longitudinal edges of the sheets and the walls of
 the chamber, the various sheets defining, among themselves and with the
 walls of the chamber, compartments;
 in creating, in this chamber and from the end located on the die side, a
 vacuum in every other compartment so as to deform and attract, in pairs,
 the extruded sheets in order to carry out localized welding over their
 entire height;
 in filling, from the end located on the die side, every other compartment,
 these alternating with the above compartments, with the aid of a coolant;
 and
 in alternating, in each compartment, the creation of a vacuum and the
 filling with the aid of a coolant in order to obtain a solidified
 honeycomb structure in the cooling chamber, in which structure the cells
 are perpendicular to the extrusion direction.
 This technique is very beneficial insofar as the structure is output
 continuously, directly from the extrusion die, with cells that are
 perpendicular to the extrusion direction. It is therefore possible to
 output from the die honeycomb structures of very large dimensions, for
 example in the form of panels, which panels are obtained directly without
 any off-line operations.
 The creation of a vacuum in the compartments defined by two sheets ensures
 that they come together and are welded over their entire height. Feeding
 the neighbouring compartments with thermally regulated coolant makes it
 possible, just after welding, to ensure that the structure solidifies in
 the cooling chamber.
 One advantage over the technology of adhesively bonded or welded films is
 that, according to the invention, the welded part of the sheets may have
 approximately the same thickness as the non-welded part, thereby resulting
 in a reduction in the density of the structure, lightness being an
 important criterion. For this purpose, the process according to the
 invention consists in exerting on the structure leaving the cooling
 chamber a jerked pull so as to reduce the thickness of the sheets in those
 regions of the latter that have to be welded to each other.
 It is possible to vary the shape of the cells, which may be in the form of
 regular or irregular polygons or have an elliptical, circular or oval
 shape, with the same die, during operation, by adjusting various
 parameters, such as the extrusion rate, as well as the alternating
 vacuum-coolant-feed cycles applied between two neighbouring sheets.
 A plant for the implementation of this process comprises an extruder
 delivering the thermally fusible material in the viscous state to a
 coat-hanger die having several parallel slots each intended for the
 continuous formation of a sheet, each slot being defined by two
 cone-shaped pieces made of thermally insulating material and made in each
 of the cone-shaped pieces is a groove capable of being connected in
 succession to a vacuum source and to a coolant source, this plant also
 comprising a tubular cooling chamber, of rectangular cross section, having
 a height equal to the height of the structure to be obtained, in the
 direction of the cells of the latter, and having a width equal to that of
 the structure, a coolant tank from which the coolant is drawn off with the
 aid of a pump, a vacuum pump and a directional-control valve which, being
 linked to these two pumps as well as to a circuit connecting it to the
 various compartments located on both sides of the sheets, is intended to
 connect, in succession, each compartment with the vacuum source and with
 the coolant source.

FIG. 1 illustrates a plant comprising an extrusion device 2 allowing the
 delivery of a thermally fusible material, such as a thermoplastic of the
 polypropylene or rubber-elastomer type in a pasty state under pressure,
 distributing it over a great width, for example by means of a coat-hanger
 die 3. It goes without saying that the width of the die, as illustrated in
 the drawing and especially in FIG. 2 is restricted and serves simply to
 illustrate the process according to the invention, it being possible for
 the number of cells obtained to be much greater over the same width.
 Located downstream of the die 3 is a chamber 4 for shaping and cooling the
 honeycomb structure, which chamber is pressurized. In the embodiment
 illustrated, this chamber is at least partially immersed in a tank 5
 containing water 6. Located after the chamber 4 is a pulling unit 7
 consisting of two rolls 8 driven in opposite directions and bearing on the
 two faces of the honeycomb structure 9. Located downstream of the tank 5
 is a device 10 for the continuous welding of two covering sheets 12 paid
 out from two reels 13, so as to form skins on the two faces of the
 honeycomb structure 9. Finally, downstream of the device 10 is installed a
 cutting device 14 which cuts the honeycomb structure transversely to the
 extrusion direction so as to form slabs.
 The die 3 comprises a block 15 illustrated in greater detail in FIGS. 2 and
 3. Made in the central part of this block 15, into which the material is
 delivered in a viscous state, are parallel vertical slots 16, each defined
 by two neighbouring cone-shaped pieces 17, the apex of which is facing
 upstream. These pieces 17 are made of thermally insulating material,
 having good mechanical properties, such as a polyimide, which is covered
 in the zone preceding the slots with a heat-transfer material, for example
 one based on gold, silver or copper. This heat-transfer device or each
 cone-shaped piece 17 could optionally be combined with heating means. This
 allows heat recovered upstream to be very rapidly delivered into the
 thermally fusible material, just at the point of shaping the latter so as
 to shorten the thermal gradient with respect to the cooling zone and to
 allow this thermally fusible material not to have reached its
 solidification temperature before it is shaped. Given the fact that the
 slots 16 are parallel, these slots will allow the formation of parallel
 sheets 31. It should be noted that the spacing between two neighbouring
 slots 16 corresponds to half the width of the cells of the honeycomb
 structure that will be formed. The honeycomb structure is shaped by means
 of two exit shaping assemblies 18, having conical parts 19 complementary
 to those 17 of the die, so as to allow imbrication of the latter parts and
 to ensure that the hydraulic circuit is sealed, whatever the vertical
 position of the parts 18 with respect to the die. This is because each
 part 18 is mounted so as to be vertically adjustable on the die 3, that is
 to say in the direction of the cells of the structure that has to be
 formed, so as to allow the thickness of the structure to be adjusted. The
 two assemblies 18 define, by their facing surfaces 20, 22, the bearing
 zones of the two faces of the honeycomb structure perpendicular to the
 cells.
 It should be noted that the surfaces 20 converge from the exit of the die
 towards the shaping and cooling chamber 4, the upper and lower walls of
 which consist of the surfaces 22 of the two exit assemblies 18. Insofar as
 one sheet has a height on exiting a slot 16 of 32 mm for example, the
 surfaces 22 are separated by 30 mm. The passage from 32 mm to 30 mm is
 made by means of the inclined surfaces 20. This makes it possible to
 benefit from excellent contact between the longitudinal edges of the
 sheets output by the die and the upper and lower walls of the chamber 22,
 creating a perfect seal between the compartments defined by the various
 sheets. This configuration makes it possible to obtain a simple honeycomb
 structure, as shown in FIG. 7.
 In the embodiment illustrated in FIG. 4, the inclined surface 20 of each
 element 18 has a concave surface 23 facing the slot 16 for forming each
 sheet. It is thus possible to produce sheets with a substantially greater
 height than the separation between the two surfaces 22, causing the upper
 and lower edges to be folded over against the surfaces 22. In the conical
 parts 17, located on each side of a sheet leaving a slot 16, there are
 longitudinal grooves 24 which open downstream. Each groove is connected
 via two ducts 25 or 26 to a manifold 27 or 28, respectively. If a groove
 is connected via a duct 25 to a manifold 27, each neighbouring groove is
 connected via a duct 26 to a manifold 28. Each manifold 27, 28 may be
 connected in succession via a directional-control valve 29 to a vacuum
 pump 30 and to a coolant feed pump 32, this coolant consisting of the
 water 6 in the tank 5, which is thermoregulated by means of a device 33.
 It is therefore possible, depending on the position of the
 directional-control valve, and for a given groove 24, either to connect
 the latter to the vacuum pump 30, and thus create a vacuum between the two
 sheets defining the compartment into which this groove 24 opens, or to
 feed coolant into this groove 24 from the tank 5 via the pump 32.
 The value of the coolant feed pressure corresponds approximately to the
 value of the pressure drops in the feed ducts and may be of the order of 1
 bar (1 bar=10.sup.5 Pa). The value of the vacuum may be as high as
 possible depending on the conditions under which the plant is
 used--altitude, temperature, etc., without however reaching the surface
 tension limit of the coolant. This vacuum may be about 0.6 bar.
 FIGS. 5 and 6 illustrate, by way of example, a six-way directional-control
 valve in two positions.
 In the position illustrated in FIG. 5, the manifolds 28 are connected to
 the vacuum source 30 and the manifolds 27 are connected to the pump 32 and
 fed with coolant. In FIG. 6, the reverse situation applies.
 In order to limit the pressure drops, grooves 34 are made in the exit
 assembly 18, which interact with the grooves 24 in the main piece of the
 die in order to form channels large cross section which allow, in
 succession, the creation of a vacuum in and the introduction of coolant
 into the compartments defined by the sheets.
 In practice, the vacuum exerted in a compartment between two sheets causes
 the two sheets to come together and to be welded, the coolant consisting
 of water ensuring that the material on one face of each sheet is cooled
 and that the structure solidifies in the cooling chamber. The reverse of
 the vacuum and coolant-feed phenomena in succession in the grooves 24
 causes the cells to be formed and a structure as illustrated in FIG. 7 to
 be obtained.
 The cooling chamber 4 is also bounded laterally by two walls 35, creating a
 seal at the two outermost extruded sheets.
 The cooling chamber is therefore sealed and can be pressurized insofar as
 the water delivered by the grooves 24 cannot escape.
 Since the shaping and cooling chamber 4 is pressurized with water, this
 water contained in the cells 36 depressurizes and feeds the tank 5 on
 leaving the chamber 4. Once the process is running, it is possible to
 dispense with the pulling unit 7. This is because since the pressure of
 the coolant, delivered by the manifolds 27 and 28 depending on the cycle,
 is less than the pressure of the thermally fusible material coming from
 the extruder, a reaction force in the sealed chamber 4 forces the
 honeycomb structure formed to leave it and, consequently, to provide the
 pulling at the exit from the die. The advantage is that this exit is
 jerked because of the reversal at each cell-forming cycle of the vacuum
 and of the coolant feed. As a result, the welded parts of the sheets are
 drawn more, and are therefore thinner, than the non-welded parts, which in
 turn results in a weight saving of the structure thus formed. It is
 possible to conjugate the characteristics of the low-speed pulling unit
 and of the coolant feed, greater than that which would be necessary, in
 order to produce cells of a particular shape, such as domed cells or cells
 37 with a partial upper skin and a partial lower skin 38, as shown in FIG.
 8, such a configuration being obtained with the exit assembly illustrated
 in FIG. 4. The structure illustrated in FIG. 8 is a structure with a
 honeycomb core and permeable skins.
 The chamber 4 may be sealed by making the walls from a material having a
 good coefficient of friction, for example made of an elastomer foam.
 The coolant may be at a temperature of about 30.degree. C., for example in
 the case of polypropylene, thereby avoiding the phenomenon of quenching of
 the thermally fusible material on leaving the die, which would prevent it
 from deforming and being welded since it would be immediately frozen. In
 the process according to the invention, that face of the sheets on the
 vacuum side is not chilled sufficiently and is therefore able to be welded
 by contact and under the effect of very slight pressure against another
 sheet, because of the force exerted by the vacuum.
 The degree of chilling provided by the coolant must be sufficient to
 finally solidify the structure in the cooling chamber.
 With regard to the plant for the implementation of the process, it is
 possible to produce both the die and the exit and shaping assembly either
 by machining of solid pieces or by the juxtaposition of separate elements.
 As is apparent from the foregoing, the invention affords a great
 improvement over the existing technique by providing a honeycomb-structure
 manufacturing process which is of simple design and allows a structure to
 be obtained continuously, in which the cells are oriented so as to be
 perpendicular to the extrusion direction, thereby permitting large slabs
 to be obtained which can be used directly, after having been cut up
 downstream, or which can be immediately coated with covering sheets
 forming skins.
 As goes without saying, the invention is not limited to the single method
 of implementation of this process nor to the single embodiment of the
 plant which are described above by way of non-limiting example--on the
 contrary, it encompasses any variant thereof. Thus, in particular, the
 conical parts 17, 19 could be replaced with plane or semidome-shaped
 parts, the directional-control valve could be of a different type, for
 example a rotary one, or else the die having the slots could be
 dissociated from the coat-hanger die, without thereby departing from the
 scope of the invention.