Plastic bubble sheets, generally used in packaging and insulation, are known. These bubble sheets comprise at least two layers or films of plastic, at least one of which is formed into bubbles and welded to the other, at least in the areas around the said bubbles.

In one known process for producing such sheets, two continuous webs of plastic film are preheated to a sufficient temperature to allow them to be deformed and welded ; at least one of the webs is shaped into bubbles by passing it around a roll with a shaped surface connected to a vacuum system; and finally the two webs are joined together on a calender, incorporating ambient air in the airtight bubbles which they form.

Patent application PCT/IT02/00455 discloses a bubble sheet with the bubbles filled with carbon dioxide, and a process for producing it, in an airtight chamber in which a carbon dioxide atmosphere is created.

The prior art also includes apparatuses for the continuous extrusion of a tubular plastic film ; the film is extruded by a circular extrusion head and is then used for tubular articles which are welded or cut into sheets or wound up.

One object of this invention is to produce bubble film more simply and quickly than conventional systems do. Another object is to produce bubble film, especially film in which the bubbles are filled with carbon dioxide or other gas without necessitating the use of an airtight chamber.

These objects have been achieved with a process as stated in its wider definition in Claim 1, a product produced by the process, and an apparatus as defined in Claim 10.

In other words, according to the invention a continuous tube of plastic film is extruded through a circular extrusion head, the tube is adjusted to a temperature that will allow it to be vacuum-deformed and welded, and the tube is passed through a nip between two rolls, of which at least one is a vacuum-forming roll for bubble film. According to the invention it is also possible to supply carbon dioxide or more generally gas or air to fill the

bubbles from a central portion of the extrusion head inside the tube, thus circumventing the need to have an airtight chamber.

The invention achieves the objectives stated above, in particular: it allows rapid production of bubble film, with the bubbles filled with air or other gas, such as carbon dioxide; it reduces investment costs and management costs for the production apparatus; it is easy to use ; and it uses less energy than current processes.

The new apparatus according to the invention can be completely automated with the help of, for example, sensors inside the extruded and inflated tube. Furthermore, existing extrusion apparatuses can be adapted to carry out the method of the invention.

Furthermore, too, the product can be produced with a slight pressure of gas inside the bubbles. This improves its properties for use as packaging, or for other uses, and makes its shape more stable when the material is cooled.

Examples of embodiments of the invention-purely for illustration without implying any limitation-are described below with reference to the accompanying drawings in which: Fig. 1 shows a perspective view of a piece of bubble film which it is wished to produce with the invention ; Fig. 2 shows, on a reduced scale, the central parts of an apparatus for the extrusion forming of a tubutar piastic film ; Figs. 3, 4 and 5 show diagrammatically, in an end view, a side view and a perspective view, respectively, the ringed detai ! marked i) i in Figure 2 ; Fig. 3a shows the shapes A assumed in cross section by the tubular film in the planes indicated at A-A, B-B, C-C in Fig. 4; Fig. 6 is an interrupted section through a variant of the bubble sheet; Fig. 7 is a view similar to Figure 3 but shows a system of rolls for producing the bubble film seen in Fig. 6; Fig. 8 is a section through a bubble film, in another variant; Fig. 9 is a diagrammatic drawing of an apparatus for producing the bubble film of Fig. 8 ; Figs. 10,11 and 12 show an arrangement of an apparatus in vertical section, in a reduced plan view, and with diagrams of the relative positions of forming and/or welding rolls ;

Fig. 13 is a section showing bubble articles capable of being produced by the apparatus seen in Figs. 10 to 12; Figs. 14 and 15 show another arrangement for an apparatus and articles which it can produce; Figs. 16 ; 17 and 18; 19 show other diagrams for production apparatuses; and Figs. 20,21 and 22 show in simplified form a machine for producing the article seen in Fig. 6 and another similar article.

Referring initially to Figure 1, a sheet of bubble material is indicated as a whole by the reference 10 and comprises an essentially flat first layer or film 11, and a second layer or film 12 shaped with circular bubbles 13. The two layers are welded together in the areas around the bubbles, thus closing the bubbles hermetically.

An apparatus for producing the bubble sheet 10 is illustrated in Figures 2 to 5, and indicated as a whole by the reference 20. The apparatus comprises an extruder 22, known per se, with an extrusion head 24 with an annular exit slot for the plasticized material-known per se-and it also comprises a nozzle in the center, that is inside the annular slot, for blowing air at slightly above atmospheric pressure to dilate the extruded tubular material which is still sufficiently hot. This type of extruder is generally used to extrude plastic film in the form of a tube T, whose walls are kept apart by the in-blown air.

At a sufficient distance downstream of the extruder nozzle (in other words up above the nozzle in the example seen in Figure 1) is a so-called funnel, indicated by reference 26, also known per se, which comprises a V-shaped structure 28 which converges with distance from the extrusion head 24, i. e. it converges upwards in the example seen in Figure 1. The structure 28 may take any form, such as (Figs. 3 and 4) having two flat frames 29 carrying two series of idle rollers 30 with transverse axes relative to the axis of the extruded tube T, that is their axes are essentially horizontal in the example seen in Figures 2 to 4. The job of the funnel is to impart gradually (Fig. 3a) to the tube a flattened form to enable it to be passed into a nip defined between two rolls which, in conventional apparatuses, serve to give the tube the flat shape with the two faces by now cool enough not to stick to each other.

According to the invention, two rolls are mounted downstream of the funnel 26 with their axes transverse with respect to the formed tube. They are similar to conventional rolls and work together, but at least one of them is a forming roll 34 and the other may simply be a welding roll 32. Both rolls 32,34 may be of a type known per se. The surface of the forming roll is formed into cavities corresponding in form and dimensions to the form of the bubbles to be produced in the finished sheet, and the cavities are connected to a vacuum apparatus. The welding roll preferably has an essentially smooth cylindrical surface. The number 33 denotes the nip formed between the rolls 32 and 34.

The operation of the apparatus will now be described.

The extrusion head 24 extrudes a continuous tube T of plastic film. Air or other air-like substance (preferably carbon dioxide C02) is blown into the tube through the head 24 at slightly above atmospheric pressure. This helps to keep the tube T inflated with its walls apart. The tube cools as it moves towards the funnel 26. Sometimes the gas is even circulated through the tube, being injected and extracted, to cool the material of the tube T more rapidly.

The tube is then conveyed into the funnel structure 26, which gradually flattens it (see cross sections A, B and C in Fig. 3a), defining two opposite walls which are brought towards each other. The two walls of film, at least one of which is formed into bubbles, are brought together-at a temperature suitable for deformation and welding, depending on the material in question- and are passed through the nip 33 between the rolls 32 and 34, at least one of the walls being deformed, so that the said rolls weld the two walls together between themselves at least in the spaces between the bubbles or at least around the bubbles. The air or other gas held inside the film tube at a slight pressure remains trapped inside the bubbles. The continuous bubble sheet 10 emerges from the pair of rolls 32 and 34 and is sent for cutting or rewinding or other processing.

It should be observed that, although the figures show an apparatus in which the extrusion head is located below, and the pair of forming and welding rolls above, in other versions of the apparatus the extrusion head is located above the rolls, and the tube travels down rather than up.

It should also be observed that if carbon dioxide is used inside the

extruded tube and for filling the bubbles, the carbon dioxide is preferably contained in bottles in liquid form, and its absorption of heat on passing into the gaseous state can be exploited to lower the temperature of the tube T to an appropriate level for deformation and welding. This lowering of the temperature can however also be achieved in other ways accessible to those skilled in the art, as already hinted at above and also, for example, by circulating air around the outside of the tube T.

With the structure described, a bubble sheet F is formed with two films 11 and 12. It is also possible to form bubble sheets with more than two films.

A bubble sheet 50 as illustrated in Figure 6 can be produced, according to the invention, by a system of rolls 60 as illustrated in Figure 7, mounted downstream of the extrusion head. The sheet 50 comprises three layers of plastic film 51,52, 54 welded together to form bubbles 53 between layers 51 and 52, and spaces 55 enclosed between the bubbles, between layers 52 and 54. The bubbles 53 may be filled with air at atmospheric pressure or at a slightly higher pressure, or with gas, such as carbon dioxide, at a slight pressure. The spaces 55 may be filled with air at atmospheric pressure or with gas, such as carbon dioxide, at a slight pressure.

The system of rolls 60 (Fig. 7) comprises a forming roll 62 and a welding roll 64 similar to the rolls 32 and 34, respectively, described in relation to the first embodiment ; while a roll 65 and a roll 66, both welding and both known per se, are provided in series. In the example shown in Figure 7 the tube T is shown being fed from above into the nip 63 between the forming and welding rolls, but it could of course be fed from underneath as in the apparatus shown in Fig. 2. The intermediate bubble sheet emerging from the rolls 62,64 is fed into the nip 67 between the welding rolls 65,66, together with a web N of smooth plastic film, preferably at a temperature sufficient for welding, which will form the film 54. Coming out from the rolls 65,66 is the bubble sheet 50 seen in Fig. 6 in continuous form. To obtain the spaces 55 filled with compressed air or with carbon dioxide, the nip 67 between the rolls 65 and 66 is located in an airtight chamber 68 in which air or CO2 or other gas is maintained at pressure.

A bubble sheet 70 as illustrated in Figure 8 can be produced, according to the invention, by a system of rolls 80 (81 to 86) as illustrated in Figure 9,

mounted downstream of extrusion heads E1 and E2. The film comprises four layers of plastic film 71,72, 74,75 welded to each other to form bubbles 73 between layers 71 and 72, and bubbles 76 between layers 73 and 74, in addition to spaces 77 enclosed between the bubbles and situated between layers 72 and 75. The bubbles 73 and/or 76 can be filled with air at atmospheric pressure or at a slightly higher pressure, or with gas, such as carbon dioxide, at a slight pressure. The spaces 77 may themselves be filled with air at atmospheric pressure or with gas, such as carbon dioxide, at a slight pressure.

The system 80 shown in Fig. 9 comprises a first extruder E1 and a second extruder E2, the respective extrusion heads of which produce a first tubular film T1 and a second tubular film T2, respectively. The tubular film T1 is flattened, shaped into bubbles and welded as stated above in respect of the first embodiment, between the rolls 81 and 82, generating a bubble sheet F1, which comprises the layers 71,72. The second tubular film T2 is flattened, shaped into bubbles and welded as stated above in respect of the first embodiment, between the rolls 83 and 84, to form a bubble sheet F2 comprising the layers 74, 75. The bubble sheets F1, F2 arejoined together via the bubble parts 73, 75, head to head, and welded by the rolls 85, 86 to give the sheet 70. As with the embodiments described earlier, the bubbles 73 <BR> <BR> <BR> <BR> and/or 76 may be filled with compressed air, or with C02 or other gas by injecting cornpressd gas through the center of the extrusion head of the extruder E1, and likewise of the extruder E2, respectively. The spacs 77 may be filled with compressed air or gas-such as carbon dioxide-by enclosing at least rolls 85 and 86 in an airtight chamber 88 containing an atmosphere of the gas under the desired conditions.

Figs. 10 to 13 show a version with an extruder 101, which may be single, double or triple, as in Fig. 11 where there are three extruders 101A, 101B, 101C. The extrusion head 103 outputs the tubular film Tb (with one or two or three layers) which travels to the pair of rolls 105 and 107, one of which may be a forming roll and the other simply a welding roll, or both may be forming rolls, with versions as indicated in Fig. 12; the pair 105A, 107A is designed to form a tube of morphology A as seen in Fig. 14; the pair 105B is designed to form a tube of morphology B as seen in Fig. 14; and the pair

105C, 107C is designed to form a tube of morphology C, as seen in Fig. 14.

Similarly, with two systems 111 and 113 similar to that shown in Fig. 13, combined as shown in Fig. 14, four-film tubes can be formed as indicated in Fig. 15. The extruders of the two systems 111 and 113 form two tubes Te and Tf which travel to the respective pairs of forming and/or welding rolls 115,117 and 119,121. There are also an auxiliary roll 123 next to the roll 117, and an auxiliary roll 125 next to the roll 119. The two auxiliary rolls 123 and 125 are also next to each other to weld together the tubes coming from the pairs of rolls 115, 117 and 119,121 of the two systems 111 and 112. The rolls 123 and/or 125 and/or the rolls 117 and 119 may be suitably heated to give a degree of plasticity to the two tubes arriving from the two systems 111 and 113, so that these two tubes can be welded as indicated in one or other of combinations D, E, F, G, H illustrated in Fig. 15, depending on the relative positions of the bubbles formed by the pairs of rolls 115,117 and 119,121.

Fig. 16 shows an alternative form to that seen in Fig. 14, with two extrusion heads 211 and 213 and a single shaft with two opposed screws and a single motor.

Figs. 17 and 18 show another alternative, with a single extruder 400 feeding both extrusion heads 400A and 400B.

Fig. 19 shows a system 500 with extruder 501 and extruder head 503 at the top, for forming a bubble Tg which extends downwards and through the pair of forming and/or welding rolls 505, 507, which may be operating in a similar way to that illustrated in either of the previous examples.

When producing a bubble film 50 as illustrated in Fig. 6 or other similar films, with marginal welds 57, a welding arrangement as indicated in Figs. 20 and 21 is envisaged. The tube 51,52, 53 of Fig. 6 formed by the forming cylinders 662,664 (similar to 62,64 in Fig. 7), in which the two films 51,52 are cut along the edges, is passed around a roll 666 next to a roll 668 having annular projections 668A to make the edge welds 57. These projections and/or the roll 666 will be heated to weld the two films 51 and 52 together. If the tube seen in Fig. 6 is completed by the additional film 54, this film 54 is fed in by a roll 670, passes between the rolls 666 and 668 and is welded at 57 together with the films 51 and 52 by the annular projections 668A. The product shown in Fig. 6 and produced in this way may also be given another

film 700 fed in by a roll 672 and welded, again at the edges, by the annular projections 668A, to produce the product shown in Fig. 22.

Some of the figures illustrating the tubes extruded by the annular extrusion heads and closed by the forming and/or welding rolls show the presence of a rod 800 rising from the surface of the extrusion head inside the annular extrusion nozzle forming the bubble, where there are also holes for supplying the gas (air or C02 or the like) into the bubble and for optional circulation of gas to heat or cool the material of the developing bubbles. This rod 800 can be adjusted for length and it supports at its end a unit 802 which may contain various instruments, specifically temperature and pressure sensors and the like. By this means it is possible to ensure-with the aid of a data processing unit and suitable programs-that the apparatus operates correctly and produces the materials correctly in relation to possible sources of perturbation, and carry out the appropriate corrections, such as correcting the pressure, the extrusion rate, the temperatures of the materials and of the heated members, etc.

Plastics suitable for the process are all the plastics that can be extruded with bubble apparatuses and that are biodegradable, coextrudates, metallocenes.

In a bubble film as defined above in which a flat sheet film is present, the material of this flat film may be a material such as paper, board, aluminum film or the like, made heat-weldable or may be a resin - such as polyester, polypropylene or other resins-having very different properties from those of the bubble-shaped film.

Alternatives to the embodiments of the invention illustrated and described are possible. It is intended that alternatives accessible to those skilled in the art with ordinary experience be understood to come within the scope of the present invention. For example, a product of the type seen in Fig. 22 may have sheets 51 and 52 welded to form the bubbles, and sheets 54 and 700 welded only along the longitudinal edges such as 57; the two films 54 and 700 will be welded to the films 51 and 52 in the process of forming a product.