Cell-like structure manufacturing method, cell-like structure and corresponding equipment

Abstract

Method for manufacturing a cell-like honeycomb structure, formed from a plurality of thermoplastic sheets attached to each other, wherein contoured areas are provided in each sheet, and each free sheet is attached to an adjacent sheet of an intermediate block formed by the different sheets previously attached therebetween, welding at least some of the flat areas contacting this free sheet and this adjacent sheet with a laser source along a continuous line parallel to the contour axis.

Claims

1. A method for manufacturing a cell-like honeycomb structure formed with several sheets of a thermoplastic material assembled together, the method comprising: making in each sheet areas provided with embossments; assembling each free sheet with an adjacent sheet of an intermediate block formed by different sheets assembled together beforehand, welding with a laser source at least certain planar areas of free sheet and of the adjacent sheet that are in contact with each other, along a continuous line parallel to an axis of the embossments, carrying out the welding by a conduction welding process, where the laser source comprises a set of emitters having a wavelength between 800 and 1,200 nanometers; generating a laser beam and directing the laser beam toward a mirror provided with facets and performing a rotary movement so as to generate a laser segment, wherein a width of the laser segment is substantially close to a diameter of the laser beam and a length of the laser segment is much greater than the diameter of the laser beam; wherein this laser segment is moved along said continuous line; wherein the length of the laser segment is about twenty-five to seventy-five times greater than the diameter of the laser beam; and wherein the length of the laser segment extends in a direction parallel to the continuous line.

2. The method according to claim 1, wherein electromagnetic radiation from the laser source is transmitted through optical fibers.

3. The method according to claim 1, wherein areas provided with embossments are made by compressively marking lines alternately on a first face and on an opposite face of the sheet, and then by exerting on either side of these folding lines, pressure forces so as to cause folding of this sheet.

4. The method according to claim 3, wherein lines parallel to a width of the sheet equidistant from each other are compressively marked, and then pressure forces are exerted on either side of these folding lines in opposite directions so as to cause the folding of the sheet in the longitudinal direction according to a semi-hexagonal pattern and then this strip is cut out in order to form different sheets.

5. The method according to claim 1, wherein each free sheet is heated up, before welding it to the adjacent sheet of the intermediate block.

6. The method according to claim 1, wherein each free sheet is pressed against the adjacent sheet, during the laser welding step and immediately after this step along said continuous line.

7. The method according to claim 1, wherein the cell-like honeycomb structure has a density of less than 80 kg/m.sup.3 with cells for which a largest transverse dimension is less than 12 mm.

8. The method according to claim 1, wherein the sheets are monolayer sheets in 100% recycled material containing carbon black.

9. The method according to claim 1, wherein the conduction welding process comprises: moving the laser segment on the free sheet along said continuous line; absorbing the laser at the free sheet; raising a temperature of the free sheet by said absorbing; conducting heat from the first sheet to the adjacent sheet to effect a melting of at least one of the sheets.

10. The method according to claim 9, wherein said melting comprises imparting mobility to macromolecules of the at least one of the sheets so that the macromolecules move to the other sheet where the macromolecules bond said other sheet.

11. The method according to claim 9, wherein said absorbing the laser at the free sheet comprises absorbing the laser at carbon black particles disposed in the free sheet.

12. The method according to claim 1, further comprising: rotating the mirror at a high rate of speed; receiving at each facet of the mirror the laser beam; projecting the received laser beam from the facets to the sheet of thermoplastic material to from the laser segment having a substantially linear shape with a generally uniform energy distribution.

13. The method according to claim 1, wherein the laser segment is said moved along said continuous line so that temperatures of both the free sheet and the adjacent sheet at the interface of the free sheet and the adjacent sheet gradually rise to reach the melting point of the thermoplastic material to form an elongated extending, generally rectangular shaped weld line disposed upon and parallel with the continuous line.

14. The method according to claim 1, wherein the diameter of the laser beam is about two to four millimeters and the length of the laser segment is about ten to fifteen centimeters.

15. The method according to claim 1, further comprising: a first pressing member moving the intermediate block into contact with the free sheet; a second pressing member penetrating into cells of the intermediate block and applying a pressure between the free sheet and the adjacent sheet along the continuous line.

16. The method according to claim 15, wherein the second pressing member comprises steel or protruded carbon profiles of a harrow; wherein said penetrating comprises driving the harrow by pneumatic actuators into the cells and entirely through the intermediate block; wherein said applying the pressure comprises flattening the harrow against areas of the adjacent sheet in contact with corresponding areas of the free sheet which are supported by guides, such that the respective areas of the adjacent sheet and the free sheet are compressed between the harrow and the guides; and wherein said laser segment is moved along the continuous line during said applying the pressure.

17. The method according to claim 16, further comprising following the movement of the laser segment along the continuous line with a rotary knurl to exert a point pressure on portions of the free and adjacent sheets softened by heat imparted thereon by the laser segment and transferred through the free and adjacent sheets non-transmissively by conduction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and other advantages thereof will become more clearly apparent upon reading the following description of a method according to the invention and of an installation for applying this method, only given as an example and with reference to the appended drawings wherein:

(2) FIG. 1 is a schematic partial longitudinal sectional view of a thermoforming installation with a rotary cylinder allowing a plastic sheet to reproduce crimps of semi-hexagonal shape.

(3) FIG. 2 describes thermoformed cells of the cylinder.

(4) FIG. 3 is a simplified schematic view of a sheet portion, once the latter has been thermoformed in the installation of FIG. 1.

(5) FIG. 4 is a schematic view of a machine for making honeycombs by laser welding of thermoformed plastic sheets.

(6) FIGS. 5, 6, 7, 8, 9 and 10 are sections in a horizontal plane inside the welder, which show the introduction of the sheet, its pressing against the block and welding by the laser rays.

(7) FIG. 11 explains how the energy segment is generated and by the means of which strong pressure is exerted on the melting welding line.

(8) FIG. 12 is a top view of the plate which bears the optical welding device.

(9) FIG. 13 is a isometric perspective view of a cell-like structure according to the invention.

(10) FIGS. 14-16 show a few examples of honeycomb cells which may be obtained according to the invention.

DETAILED DESCRIPTION

(11) The installation illustrated in FIG. 1 comprises, on the left part of this figure, a strip 1 of sheet material delivered from a spool 2. This airproof sheet material is made in a thermoplastic material which is thermoformable in vacuo, with a thickness comprised between 0.1 and 1 mm (these values are not limiting). Such a thermoformable material is selected for example from polyolefins (polypropylene, polyethylene), polyesters, polystyrenes, polycarbonate, acrylonitrile-butadiene-styrene, polyetherimide and polyetheretherketone. It may be reinforced with glass, carbon fibers or aramide fibers which enhance the thermomechanical performances of the final product.

(12) The strip is unfolded in the vertical position between two infrared radiating panels 3 which increase its temperature up to a few degrees below its melting point. By gravity, the hot strip is deposited at the surface of a slowly rotating thermoforming cylinder 4. A device not shown ensures the seal between the surface of the cylinder and the strip. The cylinder includes an aluminium sleeve with a thickness of about 30 mm, cooled by circulation of water. The surface has been machined in order to engrave recessed rectilinear splines, all identical and parallel to the axis of the cylinder. A set of flaps 5a and 5b firmly attached to the fixed axis 6 allows the interior of the cylinder to be divided into two sealed boxes. The lower box 7 is connected to a permanently operating vacuum pump. Thus when, in its movement of rotation, the cylinder brings the hot strip 1 at the box 7, the air comprised between the cylinder and the sheet is suddenly sucked up through vents 8 and the latter is flattened at the cavity bottom of the spline 9 (FIG. 2). It then perfectly reproduces the surface of the cylindrical mold which it will only leave after having sufficiently cooled by moving past the other end of the box 7.

(13) A splined strip 1a is therefore obtained, visible in a longitudinal sectional view in FIG. 3 and provided with crests 2a and of recesses 3a. These embossed areas 2a and 3a are planar. In other words, the thermoformed strip 1a looks like corrugated metal sheet. It passes through an accumulator 11 and then in front of a splined counting cylinder 12. The latter controls the guillotine 14 which will produce the strip 1a with the sought length after cutting edges with a pair of cutting disks 13. After the cutting of this strip 1a, different identical splined sheets are therefore obtained, bearing reference 101, which are deposited in a container 15, by means of a device not shown.

(14) It is important to note that if this thermoforming line allows rapid switching from plastic material to another, specifically machined cylinders should be available for a given polymer, the shrinkage level of the sheet varying from one polymer to the other. By a fast change of cylinder, the same dimensional characteristics are guaranteed for the obtained formats.

(15) The container 15 is positioned by centering devices on the ground in front of the laser welding machine, as indicated in FIG. 4. A handling arm 16 provided at its end with a gripping plate 17 successively picks up each sheet at the top of the stack, in order to present it to the infrared radiating panel 18. The purpose is to bring the temperature of the thermoformed sheet to the highest value possible while avoiding causing its deformation. Inside the welding machine, a device for heating the air (not shown) allows this temperature to be maintained. The purpose is to reduce the time required for the laser welding by starting at the highest possible point on the temperature rise curve.

(16) Next, the arm transfers the hot sheet to a set of rotating rollers 19 which carry it away into the inside of the welder. This sheet moves down by gravity between steel guides 20 and the vertical wall of the intermediate honeycomb block 21 until it arrives in abutment on a smooth stainless steel horizontal plate 22. This intermediate block 21 is formed by different sheets 101, assembled beforehand. Two vertically positioned belt conveyers 23 exert permanent pressure on the flanks of the block and impart to it forward and backward movements. Both of these conveyers are firmly attached to a gantry 24 which itself moves them transversely, and alternately from left to right, so as to obtain a shift equal to of the length of a hexagonal cell, which allows the block and the sheet to come in opposite phase.

(17) FIG. 5 is an enlargement of a section in a horizontal plane which shows the intermediate block, to which a last sheet 101a provided with its crests 2a and its recesses 3a will be added. FIG. 6 illustrates the simultaneous actions for moving forward the intermediate block 21 under the effect of the side conveyers 23 and of side translation of this block by of a cell length under the effect of the gantry, and then the insertion of an additional sheet, a so-called free sheet 101b.

(18) FIG. 7 illustrates the backward movement of the block 21, which puts the planar areas of the embossments 3a and 2b into contact belonging to the last sheet 101a of the block 21 and to the free sheet 101b respectively. Next, the steel or pultruded carbon profiles of a harrow 25 driven by pneumatic actuators are introduced inside the cells of the last row of the block 21, as shown in FIG. 8. As soon as it has crossed the whole height of the block 21, the harrow is flattened against the guides 20 so as to actually apply pressure to the planar areas of the embossments 3a and 2b (FIG. 9). The laser beams 29 will then melt these contact surfaces between the sheets 101a and 101b. Their passage is immediately followed by that of a rotary knurl 32 or 33 which exerts a strong point-like pressure on the very softened portions in order to cause interpenetration of materials between both sheets, while the harrow 25 ensures counter-pressure (FIG. 10).

(19) FIG. 11 shows a plate 26 driven by actuators moving upwards at a uniform speed from the bottom of the block in order to reach its top. During the next cycle, it will perform a reverse movement. On this plate, optical fibers 27 are mounted which transport the electromagnetic radiation generated by the laser diode sources 28. The optical fiber sends a laser beam 29 onto a return mirror 30 which transmits it to the multi-facet rotary mirror 31. The latter at the surface of the sheet 101b bearing upon the steel guides 20 by the pressure exerted by the block 21 and the harrow 25, generates an energy segment S with a width equal to the diameter of the spot, i.e. 2-4 mm, and with a length which may be varied between 10 and 15 cm.

(20) The rotary knurls 32 and 33 have a width equal to the one which would have been selected for the segment. They are provided with a rubber flange, or a smooth TEFLON crown in the case when gradual fouling would have been noticed. There is one knurl for each welding line, and each line substantially corresponds to the centre of the planar area delimited by the embossments in contact 2b and 3a. When the movement of the plate 26 is upward, a set of pneumatic actuators 34 causes the knurls 32 to press against the sheet at the embossments 2b and 3a while the casters 33 remain set back. Conversely, when the plate moves downwards, the knurls 33 are the ones which will roll on the welding lines under the pressure of the actuators 35 while the casters 32 are set back.

(21) FIG. 12 is a top (partial) view of the plate showing the positioning of the optical fibers 27, of the laser rays 29, of the return mirror 30, of the rotary mirror 31. The knurls 32, 33 and the actuators 34, 35 have not been illustrated for better understanding. As soon as the plate 26 has arrived at the end of travel, the side conveyers 23 move the block 21 forwards and a new free sheet, i.e. not yet assembled, is introduced from the top of the machine. The cycle begins again.

(22) FIG. 13 illustrates a cell-like honeycomb structure, according to the invention, once it is made according to the method described above. This structure is formed with different sheets assembled together, the last two adjacent sheets bearing the references 101m and 101n. Further, this FIG. 13 illustrates the continuous welding lines, parallel to the axis of the embossments and noted as L, which allow both of these adjacent sheets to be assembled at their planar areas in contact.

(23) Each line L substantially extends over the whole of these planar areas in contact, in other words over the whole height of the structure. As indicated earlier, certain planar areas in contact may not be welded, notably in the case when very high mechanical strength is not sought.

(24) With the installation used within the scope of the invention, it is possible to make cells of the honeycomb structure, the section of which is not a regular hexagon, provided that a thermoforming cylinder 4 and corresponding steel guides 20 are machined. FIGS. 14-16 give a few examples of cell shapes which may be used within the scope of the invention, the last two allowing deformation of the honeycomb plate in at least one given direction.

(25) The honeycomb blocks obtained with the method described earlier will advantageously be cut out into plates by a hot set of wires. The melting causes a slight accumulation of material on the surfaces, which increases the contact surface area with the skins of the sandwich structure and therefore increases the cohesion of the latter. The skins are decorative (laminate, wood) in the case of the making of furniture and partitions, notably for the interiors of pleasure boats. They are structural (composite, steel sheet) in the case of automotive and railway parts. They are both structural and decorative (lacquered aluminium, composite coated with a gelcoat) when the question is of making walls for truck trailers, campers, wind turbine nacelles . . . . But the honeycomb plate may also be used as a hollow core for a composite structure. In this case, it is coated with a sealant film and a non-woven for adhering the resin which will be applied by contact or infusion, a resin itself reinforced by one or more glass mat layers deposited at the surface of the honeycomb plate.

(26) Further, blocks with large hexagonal mesh, manufactured from rot-proof polymer, will advantageously be used by the building industry for retaining rainwater.

(27) Whatever the industrial application, the laser-welded plastic honeycomb provides remarkable resistance to compression, to flexure and to shearing while lightening the existing structures. With the manufacturing method, it is possible to use most thermoplastic polymers and to thereby obtain blocks with lightness and robustness which were inaccessible previously. It also allows manufacturing of parts of very great length in a single piece.

(28) Finally, with the small bulk and the lightness of the welding installation and of the hot-wire cutting station it is possible to install these means as close as possible to large users. Consequently, only sheets provided with embossments, stacked recesswise, in specific containers will be transported, which considerably reduces logistics cost. In other words, in a first location it is possible to perform splining and cutting operations. On the other hand, the following operations, notably for welding the different sheets, may be performed in a different location, possibly at a great distance from the first location mentioned above.

(29) In the embodiment described and illustrated with reference to the appended figures, vacuum thermoforming of an airproof thermoplastic material is applied, a material which therefore lends itself to such a thermoforming operation. However, as a non-illustrated alternative, it is possible to use a non-airproof thermoplastic material, such as for example a non-woven fabric, a grid or a perforated sheet. In this case, as described earlier, areas provided with embossments may be made within each sheet, by a method different from vacuum thermoforming. This alternative method may in a non-limiting way, be in accordance with the teaching of FR 1532509.