Process for the manufacture of sandwich structures.

0001The present invention has for its object a process for the manufacture of sandwich structures.

0002It is known that in the manufacture of structural elements formed by fabrics made from glass fibres, aramidic fibres, carbon fibres and similar, impregnated with thermoplastic or thermosetting resins, use has been made for several years of panels of the type known as sandwich, that is to say panels formed by two external surface layers of a fabric made of fibers of resistant material applied to a central filling core.

0003For instance EP-A-286.058 shows a sandwich structure comprising an inner core (formed by polyurethane resin foam), two layers (formed of vegetable, mineral or synthetic fibres) disposed on opposite sides of the core and intermediate spacer elements inserted between the layers and embedded in the core.

0004The central filling core is often constituted by a light honeycomb structure with the axis of the cells arranged perpendicularly to the plane of lying of the panel, or by a filling made from foamed or expanded resin.

0005Panels with a filling of foam only do not exhibit a satisfactory ratio between weight and mechanical characteristics, as a result of which their use is mainly limited to the field of heat and sound insulation.

0006Panels with a central honeycomb core have three kinds of disadvantages.

0007Firstly, the honeycomb structure lends itself only within precise limits to matching a cylindrical surface while it is not capable of being deformed in order to assume the shape of surfaces having double curvature.

0008In order to obtain double curvature panels it is necessary to segment the honeycomb structure into a finite series of elements capable of matching the surface having double curvature or to hot-shape it, only in the event that the honeycomb structure is made from thermoplastic resins.

0009Secondly, when filling of the honeycomb structure is required, for example with expanded resin, said filling has to be carried out before the insertion of the honeycomb structure between the two external peripheral layers.

0010Thirdly, the very reduced contact surface area between the external edges of the honeycomb structure and the surface layers involves a considerable reduction in the shearing strength parallel to the layers with a consequent increase in the risk of delamination of the same.

0011More recently, for the manufacture of composite panels, instead of honeycomb structures, it has been attempet to use so-called "three-dimensional" webs or fabrics, formed by two parallel spaced layers of fabric, connected together by means of filaments arranged perpendicularly to the above mentioned layers.

0012These three-dimensional webs have been filled with expanded resins intended to rigidify them and to render their strength comparable to that of the honeycomb structures.

0013The use of such webs is, however, difficult and has a certain number of disadvantages.

0014Firstly, it is not easy to fill such three-dimensional webs with expanded resins because the two layers forming their external surface do not remain naturally equispaced, as would be desiderable, but have to be maintained in this position artificially.

0015Secondly, said three-dimensional webs cannot be bent according to cylindrical surfaces or, even less, according to surfaces having double curvature, while keeping the external layers equispaced.

0016It is therefore one object of the present invention that of overcoming such disadvantages of the known prior art, by forming sandwich-type panels starting from three-dimensional webs, by means a process which permits to maintain the external layers of said panels perfectly parallel and equispaced.

0017Further object of the present invention, is a process for the manufacture of sandwich panels of the kind previously mentioned which, thanks to the use of three-dimensional webs, the external layers of which are constituted by tricot or knitted fabric, may assume any desired shape having single or double curvature, while keeping the external layers equispaced.

0018The claimed process is summarized and systematically presented in the claims. Further characteristics and the advantages of the process according to the invention will be better evident from the following description of some embodiments of same, made with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Figure 1 is an exploded view of a composite element according to the invention made by two layers of covering fabric and a central core of a three-dimensional web the outer layers of which are formed by weft and warp fabric.</li><li>Figure 2 is a perspective view of a three-dimensional web the outer layers of which are made by tricot or knitted fabric.</li><li>Figure 3 shows diagrammatically a plant for the manufacture of the sandwich structures according to the invention, and</li><li>Figure 4 shows, in enlarged scale, the mold for the final hardening of the panels in the plant of Figure 3.</li></ul>

0019With reference to the drawings, and with particular reference to Figure 1, numeral 1 denotes a sheet of three-dimensional web, formed by two external parallel layers 101, 201 of fabric provided with crossing warp and weft filaments. The said layers 101 and 201 are connected together by a number of filaments 301 extending perpendicularly to the laying plane of the fabric layers 101, 201. Numerals 2 and 3 denotes two sheets of reinforcing fabric, which are intended to be connected by adhesion to the layers 101, 102 in order to form a very resistant structure, as will be described later.

0020In Figure 2 a three-dimensional web sheet 4 is shown, which is similar to the sheet 1 of Figure 1. However, the layers 104 and 204 of the sheet 4 are made from knitted or tricot fabric, for the purposes which will be better described later. The said layers 104 and 204 are connected together by means of a plurality of filaments 304 in the same manner and for the same purposes as the filaments 301 of the sheet 1.

0021In Figure 3 a plant for the manufacture of the composite panels according to the invention is shown.

0022With reference to Figure 3, B1 is a bobbin from which a sheet 1 of three-dimensional web is unwound, and B2 and B3 are two bobbins from which the reinforcing fabric sheets 2 and 3 are unwound. Both the sheet 1 and the sheets 2 and 3 can be made from any suitable material, and for instance from fiber glass fabric, carbon fiber fabric, or even a fabric made from vegetal textile fibers.

0023The sheets 1, 2 and 3 are fed to a resin applying station, comprising the two opposed applying cylinders 5, 5' and the transfer rollers 6,6' dipped into the tanks 7, 7' containing the fluid resin 8. The resin-impregnated sandwich S thus formed is transferred, for instance by means of the conveyor belt 9, through the furnace 10, where the resin 8 is pre-hardened or pre-cured.

0024The pre-hardened, but still plastic sandwich S which comes out from the furnace 10, is cut into single panels P at the cutting station T, and the panels P are fed to the molds 11, which are best shown in Figure 4.

0025Each mold 11 is provided with heating means (not shown) and is provided with a mold cavity 111 which, in the example shown, is shaped in the form of a rectangular prismatic cavity, but which could be shaped at will with single or double curvatures, according to the requests, as it will be explained later.

0026The panel P is introduced into the mold cavity 111, so as to fill it completely. Numeral 12 denotes a feed duct which opens at the level of the filaments 301 of the fabric 1. Through the duct 12 a suitable expansion fluid is injected into the space confined between the two sheets 101 and 201. The said fluid may be, according to the requirements, an expandible resin, or a compressed fluid (compressed gas or air), or also a composition which is apt to develop a pressurizzed gas when the mold is heated to the curing temperature of the resin soaking the panel S. Thanks to the fact that the impregnating resin was already pre-cured, the fabric layers 101, 201, 2 and 3 are impervious to the gases, so that the gas pressure which is developed in the internal space of the three-dimensional web compresses the fabric layers 2,3 and 101, 201 toward opposite directions against the inner walls of the mold, so that once completed the polimerisation of the resin, the panel P which is extracted from the mold 11 has the exact outline of said mold, and the surfaces 101, 201 are equispaced through all the surface of the panel P.

0027By using a three-dimensional web 4 of the knitted fabric type, instead of the web 1, it is possible to confer to the final panel any single or double curvature, by simply using a suitably shaped mold 11.

0028As impregnating resins it is possible to use any thermoplastic or thermosetting resin. Phenolic resins are particularly suitable.

0029Although the composite structures according to the invention have been described as formed by a layer of a three-dimensional web 1 provided with two resistant outer layers 2 and 3, it will be evident that the composite panel may be formed with only one outer resistant layer or without any outer resistant layer, by transferring the function of resistant structure to the outer layers of the three-dimensional web, suitably impregnated with resin, or to an internal filling of expanded resin.

0030Further, by varying the density of the filaments in the direction perpendicular to the layers and by disposing them according to a pre-fixed pattern, it is possible to form within the three-dimensional web hollow passages for cables, ducts, and the like.

0031By introducing into the thus formed hollow passages solid or tubular bars of metallic or plastic material, before the injection of the expanded resin, it is possible to form in the composite panel cavities or passages, after extraction (once completed the polimerisation) of the previously introduced bars, if solid, whilst whenever tubular bars are used, same can be also left inside the panel.