Known to the art and widely used, for example in the building industry, for packaging, and for do-it-yourself are sheets of plastic material, of a substantially uniform and limited thickness with respect to their size, having a shape with alternating ridges and grooves' parallel to each other, so as to have cross section with a sinusoidal shape, or with alternating semi-circles, or with"zigzags" (that is with the grooves having an angular profile) with various configurations. Each groove forms a ridge on the opposite side of the sheet, and each ridge forms a groove on the opposite side of the sheet. The term "corrugated"will be used henceforth to indicate sheets with parallel grooves and ridges with one or more of a variety of possible shapes, that is to say shapes with a sinusoidal or wave-shaped cross-section, shapes with a"zigzag"cross-section, that is with a profile formed by segments at an angle to each other with any configuration, and mixed shapes.

In prior art corrugated sheets, the ridge parts and the groove parts are longitudinally homogeneous, that is to say with the same cross section along their entire length.

Prior art sheets are produced by means of a process which comprises a stage of plasticization of the thermoplastic material with an extruder, a stage of rolling of the web produced, and a subsequent stage of deformation of the web according to shapes having their axes longitudinal to the web, through a series of roller trains.

A previous patent application by the same applicant discloses a vacuum forming process by means of counter-rotating rollers, which allows the production of corrugated webs with corrugations lying transversally to the longitudinal direction of the web.

Although the prior art corrugated sheets have yielded satisfactory results, there is nevertheless a need in the field for corrugated sheets having greater mechanical strength whilst having the same weight.

A further need is to obtain a good thermal insulation with said sheets.

An object of the present invention is therefore to meet the aforementioned needs, in particular by providing corrugated sheets having excellent mechanical strength, easily assemblable with an air gap for good insulation and easy to use for a variety of purposes.

A further object is to allow industrial production of said sheets in an economically advantageous manner.

The stated objects have been achieved with a sheet as stated in claim 1, with an apparatus as stated in claim 12, and with a procedure as stated in claim 13. Combinations of sheets as stated and a web for production of said sheets also form the subject matter of the invention.

In other words, a corrugated sheet according to the invention is shaped with protuberances projecting inside one or more grooves. The protuberances are advantageously spaced apart from one another longitudinally and have a height, measured starting from the bottom of the groove, equal to or less than the height of the groove. Protuberances can be provided only in the grooves opening on one side of the sheet, or on the grooves opening on both sides of the sheet. The protuberances can be hollow or can be filed with stiffening material, for example a plug.

According to a preferred process of the invention, a web of thermoplastic material at a suitable temperature is deformed between counter-rotating rollers with corrugations transverse to the longitudinal extension of the web, the counter-rotating rollers having cooperating protrusions and recesses to form said protuberances in the sheets.

The apparatus comprises at least one cooperating forming roller and"drawing"roller provided with corresponding protrusions and recesses, to form said protuberances in the sheets.

The invention provides plastic sheets having particular strength, able to better withstand being walked upon, and which can be used advantageously in a number of uses, for example for insulation, for roofing and other uses, with better results with respect to prior art materials.

Exemplary embodiments of the invention will now be described with reference to the appended illustrative unrestrictive drawings, in which: Figure 1 is a perspective view of an example of a corrugated sheet according to the invention, with protuberances lower in height than the grooves, Figure 2 is a front view of the sheet of Figure 1, Figure 3 is a perspective view showing two coupled overlapping sheets of the invention, automatically fixed to each other, Figure 4 is a perspective view of a sheet having protuberances of the same height as the grooves and being fixed on joists, Figure 5 is a view of two coupled sheets with an air gap, Figure 6 is a profile view of a plurality of sheets coupled in a honeycomb configuration, Figures 7,7a show the sheets of the invention used as roofing, to support tiles, Figure 8 shows a corrugated sheet with protuberances, Figure 9 shows a sheet with opposite protuberances, Figures 10 and 11 show various types of coupled sheets, Figure 12 is a perspective view of an apparatus for producting sheets according to the invention, Figure 13 is a sectional view taken along a vertical plane through the apparatus of Figure 12.

With reference first to Figure 1, a corrugated sheet according to the invention, of sheet material, is designated as a whole with reference numeral 10 and has a shape comprising a series of alternating ridges and grooves with parallel axes, substantially equal to each other, each ridge forming a groove on the opposite surface of the sheet and each groove forming a ridge on the opposite surface. In particular, the example of Figure 1 is a corrugated sheet which has a configuration with a bottom portion of the grooves (and thus the top surfaces of the ridges) substantially flat and with the side walls inclined with respect thereto. The groove portions open at the bottom in Figure 1 are designated by 12, the groove portions open at the top in the condition in Figure 1 are designated by 14.

According to the invention, the sheet has protuberances 16 extending from the bottom of at least some of the grooves 12. The protuberances 16 are formed in the sheet material and, at least as regards the example in the figure, have a substantially frustoconical shape, with a circular base, and are spaced apart from each other along the length of the groove.

The protuberances can be of any desired height, not greater than the depth of the groove and preferably substantially equal to that depth.

Included within the scope of the invention are sheets with protuberances having bases of any shape, for example square, sheets with protuberances extending from both series of grooves 12 and 14, on one side and the other, sheets with any distribution of the protuberances in the grooves, regular or irregular. The sheet in Figure 8 has a shape with sinusoidal corrugations, with protuberances.

Some applications of the sheets according to the present invention are illustrated in Figures 3 to 7.

Figure 3 shows two superimposed sheets 10, 10a, having the respective protuberances 16 and 16a nested into one another, to obtain a good adherence of the sheets and prevent the sheets from sliding on one another.

Figure 4 shows a sheet 10b in which the protuberances, of a height corresponding to the depth of the groove in which they are formed, are provided with inserts 18 of a shape corresponding to the hollow of the protuberance, and of relatively stiff material. This allows the sheet to be fixed on a support 20 such as a board or beam, for example of wood, by means of nails or screws 22. A sheet like 10b has optimal resistance when walked upon, as indicate by the shoe drawn with a dashed line thereon.

Figure 5 shows two sheets according to the invention combined in a facing arrangement to form an air gap therebetween for the purpose of thermal and/or acoustic insulation. In the particular case, a top sheet 10c is disposed with the protuberances facing downward, a bottom sheet 10d is disposed with the protuberances facing upward, and the top of the protuberances of each sheet rests against the top surface of the ridges of the opposite sheet.

Figure 6 shows a plurality of sheets 10, like the one of Figure 1, referenced 10', 10", 10"', the sheets are combined in an arrangement wherein every two adjacent sheets are opposite to each other forming a cross-sectional honeycomb configuration, so as to increase an air chamber comprised therebetween, for better insulation. The combination can be made more stable by applying inserts 24.

The spaces or gaps defined between the sheets arranged in honeycomb configuration can possibly be filled with foam resin.

Figures 7,7a show, in a perspective view and a top view, respectively, two different types of application of sheets according to the invention, on roof slopes, with the purpose of retaining a cladding of tiles. In Figure 7, the corrugated sheet, of type l Ob in Figure 4, with inserts 18 as explained above with reference to Figure 4, is fixed to the beam 20 by means of nails or the like 22, which also retain per se lcnown tongues 26 for retaining the tiles C. Each tile C is retained by the bent end 26'of the respective tongue, in a per se known manner.

In Figure 7a, on the other hand, the task of retaining the tiles is assigned directly to the protuberances 16, spaced apart form each other transversely by such a distance that the edges of the tiles will encounter them in the event of downward sliding.

Figure 9 illustrates a sheet 10e with protuberances 16e on both surfaces.

The uses and combinations of sheets according to the invention can be diverse: for example it is possible to combine sheets with sinusoidal corrugations (Figure 10) instead of sheets with zigzag corrugations, or else sheets of one type and the other (Figure 11), for all those purposes described above by way of example.

The sheets according to the invention can be made in any way accessible to a person skilled in the art, however a procedure and an apparatus particularly suitable for production thereof also form the subject matter of the present invention.

In Figures 12 and 13 an apparatus 50 for continuous production of web to obtain sheets according to the present invention is shown. The apparatus comprises two counter- rotating rollers, in particular a forming roller 52 on the bottom in the figure and a roller cooperating in forming, 54, for the sake of convenience called a"drawing"roller, shown in the top position.

The forming roller 52 has axially extending grooves 55, circumferentially spaced apart by raised parts 56 and has solid end parts 57. Each groove has a shape corresponding to the desired corrugation of the web and thus of the sheet, is connected to a vacuum source (not illustrated), and has axially spaced apart protruding parts or protrusions 58 in at least some of the grooves. The protrusions parts have a shape corresponding to the shape of the protuberances it is desired to have on the sheet.

The drawing roller 54 preferably comprises a series of cylindrical or tubular elements 60 on supporting discs 62 and connected by means of tubes 64 to a cooling system. The tubular elements are circumferentially spaced from each other at a distance correlated to the circumferential distance of the grooves of the roller 52 with which they cooperate.

For a better understanding of the operation of the rollers 52 and 54 reference should be made to Italian patent application No. M12001A 1108 by the same applicant, which describes similar elements for the production of a corrugated web without protuberances.

According to the present invention, the cylindrical or tubular elements 60 are provided with recesses 66 having shapes and positions corresponding to the protrusions 58 of the roller 52.

For production, a continuous web of thermoplastic material at a suitable temperature is passed between counter-rotating rollers 52 and 54 and takes the shape of the surface of the forming roller 52 by operation of vacuum applied and by mechanical action of the drawing roller. Downstream of the apparatus 50 the web is cooled, trimmed along the longitudinal edges, and finally cut into sheets.

It is understood that any variations or modifications to what is described and claimed which are within the reach of a person skilled in the art with normal experience fall within the scope of the present invention.