"Tray-shaped mould made of a vulcanized composite material11 Description The present invention relates to a tray-shaped mould made with a new rubber- based composite material. Although such type of mould can be used in different industrial applications, a: specific reference is here made to the manufacture of slabs of conglomerate stone material. In recent years technologies for the manufacture of thin and large-size articles in the form of slabs (up to 3.10 x 1.40 metres) have become more established on an industrial level, wherein a mix consisting of a natural stone material in the form of granulate or sand, or a ceramic material, also in the form of granulate of predefined particle size, and a binder, is deposited in a tray-shaped mould which is subjected to a compaction step accompanied by application of a vibrating movement and finally to a hardening step which in some cases envisages the baking of the "raw" article contained in the tray-shaped mould and in other cases hardening of the binder, for example in the case of a synthetic resin such as a polyester resin, through the action of heat and/ or a catalyst. As a rule, the said steps are performed in different stations, which causes the mould to be transferred, by means of translational motions, along the plant for the manufacture of the slabs. The tray-shaped moulds used hitherto are made of a composite material in the form of a sandwich consisting of several layers of rubber and one or more layers of fabric arranged internally and so as to line the external surface of the mould and in particular the bottom surface of the mould. As regards the rubber, it is preferably a synthetic rubber chosen from among the rubbers resistant to temperatures utilized in order to bake or harden the slab formed in the mould. These rubbers must also be chemically resistant to the components of the mix which is poured into the mould. Preferably the rubber used hitherto for the manufacture of tray-shaped moulds has been either a rubber called EPM (namely a terpolymer obtained from ethylene and propylene monomers) or an EPDM (namely a terpolymer obtained from monomers of ethylene, polypropylene and at least one third monomer consisting of an unconjugated diene). As regards on the other hand the fabrics used together with the rubber in order to form the bottom of the tray-shaped mould - said fabrics in addition to forming a kind of reinforcement also favouring sliding of the mould during transfer thereof along the plant - they have hitherto been made of polyester fibre and/or polyamide fibre. During manufacture of the composite materials mentioned above, the main problem consisted in the low adhesiveness of the EPDM rubber ■with respect to the underlying fabrics. In order to overcome this drawback, different measures have been adopted such as the application of primer based on resorcinol and formaldehyde to the interface between rubber and fabric, with the arrangement in between of a so-called adhesive "liner" based on butadiene rubber, which has a high degree of adhesiveness. However this latter solution, which ensures a good adhesion between EPDM rubber and fabrics, also has further drawbacks, namely firstly the low resistance of the butadiene rubber forming the liner to high temperatures and secondly the large degree of shrinkage of the polyester fabrics since prolonged use of the mould at a high temperature results in a not insignificant dimensional variation thereof. The main object of the present invention is that of solving the abovementioned problems and drawbacks and in particular providing a tray-shaped mould made with a composite material of an elastomeric nature which is formed with components which are resistant to the high process temperatures, is dimensionally and geometrically stable and has at least one surface, preferably the outer surface, with a very low coefficient of friction. More specifically, the composite material comprises a layer based on gkssfibre which is arranged in between two layers of synthetic rubber, the said layer of based on gkssfibre having a structure permitting the rubber to flow and therefore to mix up. Similar composite materials are per se already known from the following patents : GB-A-I 382 486, where it is provided for preliminarily treating the gkssfibre with agents suitable to improve its bonding to the rubber; GB-A-I 467 470, where the gkssfibre is a network of strands arranged side by side and in a zig-zag form; FR-A-2 657 293, where the strands of gkssfibre are carded and consequently in alignment. No one of said patents, however, the object is using the composite materials for making a tray-shaped mould. This object and other objects of the present invention are achieved by a tray- shaped mould as defined in the appended claims. In a preferred embodiment, the composite material with which the mould is made also comprises a cotton gauze which is applied onto the outer surface of bottom wall of the mould. The number of rubber layers of a glassfibre-based structure arranged in between is chosen depending on other desirable characteristics of the mould, consequently of final composite material, which is vulcanized, mainly depending on the desired rigidity of the end product. In particular, an optimum composite material was found to be one comprising three layers of rubber with two layers of a glassfibre-based structure arranged in between in an alternating arrangement, the said glassfibre-based structure being either an open-mesh netting or a mat. Actually, in case of a tray-shaped mould utilized in the manufacturing process of a conglomerate slab, a mould made with a composite material comprising only two layers of rubber with a glassfibre-based structure arranged in between would be of a much too low stiffness with the result that, particularly as a consequence of the compaction steps with application of a vibratory movement, the slab would have an undulate surface or at any rate not perfectly flat, so as to requite additional sizing and/ or smoothing operations. These drawbacks do not arise if the vulcanized composite material is formed by three layers of EPM or EPDM rubber with the alternating arrangement in between of two layers of either a mat or an open-mesh netting of glassfibre. As already mentioned, the tray-shaped moulds of the present invention which are intended to resist high temperatures and to the action of any corrosive components, are made with a composite material comprising rubbers of the EPM or EPDM type. The thickness of each rubber layer is in the order of a few millimetres, starting from a minimum thickness of 1 mm. In turn, the glassfibre based structure can be either in form of a mat or of a netting of the open-mesh type, as already mentioned, so as to allow the rubber of the two layers between which the netting is arranged to pass through and therefore to mix up, since the adhesion of glassfibre to the rubber is very poor, if not close to zero. For this reason, when a netting of the open-mesh type is used, the size of the meshes is at least 4 x 4 mm^ even larger sizes obviously also being possible, in accordance with the desired effect. As regards the desired low coefficient of friction of at least one surface of the composite material, this characteristic feature is obtained by impregnating the outer surface of the bottom wall of the tray-shaped mould with a cotton gauze able to withstand up to 2000C. Since adhesion of rubber to cotton also is very poor, the fluid rubber must pass through the meshes of the gauze, thus allowing a mechanical bonding. In order to form the composite material according to the present invention, the various constituent layers are arranged on top of each other staffing from the layer of cotton gauze: a first layer of rubber is arranged thereon, followed by the glassfibre- based structure in form either of a mat or a netting, and by a second layer of rubber. If so required, on top of this layer of rubber a new glassfibre-based structure and then a layer of rubber are arranged, and so on, until a stack is formed consisting of the desired number of rubber layers spaced with glassfibre-based structures (either mats or open-mesh nettings). The stack thus formed is then introduced into a conventional vulcanization press where a pressure in the order of 30-60 kg/ cm2 for a few tens of minutes and a heating temperature of the pressing plates (as required by the vulcanization of the rubber forming the aforementioned layers) are applied, the said temperature being of about 170°C in the case of an EPM or EPDM rubber. The resultant tray-shaped mould shows the following advantageous features: - the inner surface is formed by a vulcanized rubber of the desired chemical and physical properties; - the outer surface, particularly at the bottom wall of the mould, is formed by the layer of cotton gauze which is of a low coefficient of friction and bonded to the overlying rubber; - the composite material is entirely formed by elements resistant to high temperatures; - . the mould is dimensionally stable, since the glassfibre-based structures (mats or nettings) do not undergo any dimensional modification at high temperatures; - the glassfibre is well bonded to the rubber without a need for the glassfibre to be preliminarily treated with, additives adapted to improve its adhesion, owing to the possibility given to the rubber to flow through the the glassfibre-based structures and to mix up.. The invention will be better understood and appreciated on the basis of the following description of a preferred though not unique embodiment with a reference to the attached drawing where : - figure 1 shows a typical tray-shaped mould used for the manufacture of slabs of conglomerate stone material; - figure 2 shows a cross-section of the bottom wall of a tray-shaped mould made in accordance with, the present invention. The mould 10 of figure 1 has a rectangular or square shape with a bottom wall 12 and the peripheral edges of the sidewall 14 turned outwards. It can be easily appreciated from figure 2 that the composite material used for the mould of the invention consists of the following components: - three layers of synthetic rubber, identified by the reference numbers Ia, Ib and Ic; - two layers of glassfibre-based structures (either mats or nettings), identified by the reference numbers 2a and 2b; - one layer of cotton gauze, identified by the reference number 3. It has been found that a tray-shaped mould made with the composite material of the present invention is no longer subject to shrinkage and may be reused up to the occurrence of mechanical breakages or superficial deteriorations of the rubber in contact with the material deposited into the mould, with obvious advantages from an industrial point of view. Depending on the type of application it is possible to select the rubber of a suitable nature and the number of layers and/ or the characteristics of the glassfibre- based structures in order to obtain a given level of rigidity or flexibility. Other modifications and/or variants of the present invention which are conceptually equivalent are possible and may be envisaged, without departing from the scope of the appended claims. For example, in place of layers of cotton gauze, it is possible to use layers of other materials having a similar capability.