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The present invention relates to a mould used for the production of slabs of agglomerated stone material and to a method for the manufacture thereof.

In recent years there has been a growing trend in industry towards the use of techniques for the manufacture of thin and large articles in the form of slabs, in which a mix consisting of a natural stone material or a ceramic material or a lithoid material of predetermined particle size and a binder is deposited in a tray-shaped mould. Within said mould, the mix undergoes a vacuum compaction step by means of application of pressure together with a vibratory movement, and finally it undergoes a hardening step which, together with synthetic resins, for example polyester resins, acting as the binder, requires the use of a catalyst and the application of heat. The tray-shaped moulds used hitherto in the abovementioned techniques have been produced in the form of a sandwich consisting of one or more cloths arranged inside and several rubber layers for coating the outer surface of the moulds, both that of the base of the moulds and that of the cover (see EP-A- 1,027,205 and WO-A-01 42007). The resins used as a binder for the mix emit, in particular during hot-hardening of the abovementioned slabs (involving temperatures in the region of 120-130 ⁰ C), vapours which have an aggressive action on the rubber. In particular the polyester resins emit styrene vapours prone to cause the rapid deterioration of the rubber surface of the moulds. In order to eliminate, or at least limit as far as possible, this aggressive action it is known to deposit a protective sheet on the rubber surface which comes into contact with the mix. The protective sheet consists of a disposable film arranged between the rubber-coated surface and the mix. The material from which the film is made is normally based on PVA (polyvinyl alcohol), which is impermeable to the styrene vapours, resistant to the temperatures reached during hardening of the slabs and facilitates separation of the slabs from the moulds after hardening.

The film is both ready available in rolls, so that it may be directly applied onto the outer surface of the moulds, or may be formed in situ by means of spraying and subsequent drying.

Although undoubtedly effective for the purposes indicated, the use of said film has certain drawbacks, in particular:

- the greater complexity of the method for producing the slabs;

- the consequent need for specific machinery which may be complex and require a notable

financial outlay;

- the contamination of the processing sludge due to dissolving in the process water (which is used during the subsequent slab sizing operations and which clings to the slabs when they are removed from the moulds at the end of the hardening step). It would be desirable — and this forms the main object of the present invention — to improve the structure of the moulds used for the production of slabs of agglomerated stone material so as to not to require the use of said disposable film.

These and other objects may be achieved by means of a structure of the tray-shaped moulds as well as a method for manufacturing the said moulds, having the characteristic features described in the claims below.

In particular, a method according to the invention for manufacturing a mould, of the type and for the use indicated above, comprises the steps of:

- applying onto a layer of rubber forming part of the mould a film suitable for making contact with said mix, said film being manufactured using one or more fluorinated polymers and being applied onto the surface in contact with the rubber using a coating which promotes adhesion with the said rubber during the following vulcanization step;

- carrying out vulcanization of the structure thus obtained so as to ensure permanent bonding between said film and the underlying rubber layer.

It is possible to introduce into the rubber layer a reinforcing layer and/ or obtain a sandwich structure comprising various layers of rubber on various reinforcing layers. Furthermore, advantageously, the rubber is EPM (i.e. a terpolymer obtained from ethylene and propylene monomers) or EPDM (Le. a terpolymer obtained from ethylene and propylene monomers and at least one third monomer consisting of a non-conjugated diene), and said film consists of a layer of PTFE (polytetrafluoroethylene) having a superficial substrate compatible with the rubber on which it is deposited. There is no need to mention that said vulcanization does not influence the physical/ chemical properties of the PTFE.

Preferably, the vulcanization operation is performed at high pressure (about 50 bar) and at a temperature of about 165°C.

In order to clarify more fully the characteristic features and advantages of the invention, the description of a preferred, but not exclusive embodiment will now be provided, with reference to the accompanying drawing in which:

- Fig. 1 shows in schematic form a tray-shaped mould used for the production of slabs and filled with a mix of agglomerated stone material in which the binding agent consists of resins, in particular polyester resins;

- Fig. 2 shows a cross-sectional view of the base of the mould according to Fig. 1, which is able to come into contact with said mix for formation of the slab.

A base 100 and a cover 200 of a tray-shaped mould 10 have a square or rectangular shape. The base 100 consists essentially of a flat surface 12 and four side walls 14 (only two are shown in Fig. 1), while the cover 200 consists solely of a flat surface.

The structure shown in Fig. 2 relates to the flat surface 12 of the base 100, but also the side walls 14 of the base 100 and/or the cover 200 may have the same structure. According to the invention said structure comprises (as well as, in a manner already known, two layers of synthetic rubber, i.e. a lower layer 123 and an upper layer 125, and two reinforcing layers, i.e. a middle layer 124 and an outer layer 122, so as to form a sandwich) a film 130 which is combined with the upper layer of rubber 125 and partly made with one or more fluorinated polymers and partly with material compatible with the rubber of said layer 125. The mix of agglomerated stone material is then spread over said film 130 during manufacture of the slabs until the end of the hardening step and is incorporated in the said structure. In a preferred embodiment of the present invention, said film 130 consists of a layer 131 of

PTFE (polytetrafluoroethylene) lined underneath with a superficial substrate or coating 132 of material compatible with the synthetic rubber from which at least the already mentioned upper layer 125 is made. The superficial substrate or coating 132 acts as a layer promoting adhesion during the following vulcanization step by means of which said structure is made permanent.

The rubber of said layer 125 is preferably EPM (i.e. a terpolymer obtained from ethylene and propylene monomers) or EPDM (Le. a terpolymer obtained from ethylene and propylene monomers and at least one third monomer consisting of a non-conjugated diene) since they are elastomers particularly resistant to high temperatures. It should be pointed out that the accompanying Fig. 2 is not to scale since, in reality, the overall thickness of the film 130 is about 4-5 mm and that of the layer 131 of PTFE is in the region of 0.3 mm.

Since the anti-adhesion and heat-resistance properties of fluorinated polymers, in particular PTFE, are per se well known, it can be understood how the present invention fully achieves the objects indicated in connection with a mould or other rubber-based support used for the production of slabs from a mix of agglomerated stone material.

Other embodiments or variants of the present invention are possible and may be envisaged, without departing from the scope of protection thereof. For example the entire film 130 may also be made using a mixture of one or more fluorinated polymers and synthetic rubber and,

in this case also, incorporated in said structure.