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 (101) attached to each other, wherein contoured areas (2a and 3a) are provided in each sheet, and each free sheet (101b) is attached to an adjacent sheet (101a) of an intermediate block (21) formed by the different sheets previously attached therebetween, welding at least some of the flat areas (3a and 2b) contacting this free sheet and this adjacent sheet with a laser source along a continuous line (L) parallel to the contour axis.

Description

TECHNICAL FIELD[0001]The invention relates to a method for manufacturing a cell-like honeycomb type structure in plastic material. The invention also relates to the thereby obtained structure, as well as to an installation allowing such a method to be applied.BACKGROUND[0002]Cell-like structures of the honeycomb type in thermofusible material have been produced industrially for about twenty years. The most used polymer is polypropylene (PP) because of its lightness, of its low cost, of the ease of its application, of its excellent resistance to humidity and to most chemicals. Its recycling is easy. Honeycombs in polycarbonate (PC) and in polyetherimide (PEI) occupy niche markets, and there exist productions, for the time being marginal productions, in polyvinylchloride (PVC), in polyethylene terephthalate (PET) and in cellulose triacetate (CTA).[0003]The main commercial outlet for these products is the sandwich panel, for which they make up the core. The upper and lower skins of the...

AI Technical Summary

Benefits of technology

[0016]Vacuum thermoforming of each sheet is accomplished with a rotating cylindrical mold, rather than with a planar mold, so as to be able to produce a variable length of thermoformed sheet.

[0020]The embodiment described in the previous paragraph is an alternative to vacuum thermoforming. As a non-limiting example, intended to illustrate this alternative embodiment, it is possible to fold the sheet under cold conditions, in particular according to the teaching of FR 1532509, by marking on its surfaces, by the pressure of the cylinders, lines parallel to the width of the sheet and equidistant from each other. A cylinder provided with thin rectilinear embossments located above the sheet will compressively mark two consecutive lines on the upper face of the sheet, and an identical cylinder placed under the sheet will then mark following both of these lines, two new lines on the lower face of the sheet. And the cycle is repeated endlessly. A set of actuators (compressed air nozzles, splined rollers) placed above and below the sheet causes the planar sheet to assume a three-dimensional shape, with a semi-hexagonal longitudinal profile, by folding along the lines scored beforehand on the faces of said sheet. Unlike vacuum thermoforming, folding does not cause material stretching. Therefore there is no shrinkage problem after cooling and the sheet has the same thickness everywhere. And unlike vacuum thermoforming, it is not required that the constitutive material of the sheet be airproof.

[0026]Indeed, the width of the beam, the high yield, the great durability and the low congestion associated with a moderate cost for example makes laser diodes an ideal tool for generating weld lines with a width from 2 to 4 mm. The principle of the welding is based on the heating of the material by electromagnetic radiation. The welding of two plastic sheets maintained against each other by pressure may be accomplished:

[0030]Each facet which receives the beam projects it on the plastic sheet by making it thereby cover a line segment. The rotation of the mirror is sufficiently fast so that an “energy line” is generated from the spot. The great uniformity of the energy distribution of the laser inside a segment with a width of 3-4 mm and with a length of about fifteen centimeters allows the temperature of the plastic sheet to be raised until it makes the macromolecules mobile in a more gradual and better controlled way than the spot directly produced by the laser beam would do at the surface of the sheet. For polymers of ordinary use such as polypropylene and polystyrene, a power of the order of 100 W per welding line is sufficient for guaranteeing a high production rate. Of course, with this device, it is also possible to weld bilayer sheets by transparence without it being necessary to provide the least modification to the installation.

Problems solved by technology

These methods have a certain number of significant drawbacks: the use of a hot melt adhesive with a low melting point limits the thermomechanical resistance of the cell structure to a level which is clearly inferior than that of the constitutive polymer of said structure.

This technical problem therefore prevents optimization of the weight and of the cost of the finished product.

Another consequence is that the cutting of the block into slices with a hot wire, which is more aesthetical than cutting with a band saw, can only be accomplished at a maximum rate of 10 cm per minute and with a risk of thermal degradation in the case of a polypropylene block with a density of 80 kg / m3, and it is therefore not used.

On the other hand, by using stacking containers and jaws it is not possible to obtain blocks with a length of greater than 3 meters which complicates and in certain cases excludes the manufacturing of panels of large dimensions.

There is also the problem posed by the extrusion screws, which are designed for a given type of polymer which prevents the manufacturer from proposing honeycombs in very different materials.

Finally, it is impossible to transport a compact semi-finished product and to assemble it as a honeycomb with lightweight industrial means as close as possible to large users.

As a conventional trailer truck can hardly take away more than 60 m3 of honeycomb, the transport costs quickly become prohibitive.

Now, the latter is limited by the complexity of the technique to about thirty millimeters.

Further it does not allow modulation of the thickness, which remains very limited because of the thermoforming on a cylinder.

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