ITMI20132048A1 - PLANT WITH EXTRUDER BIVITE FOR THE CONTINUOUS PRODUCTION OF PLASTIC FILM ROLLS WITH AIR BUBBLES - Google Patents

Description

Description of the invention entitled:

"PLANT WITH TWIN SCREW EXTRUDER FOR THE CONTINUOUS PRODUCTION OF ROLLS OF PLASTIC FILM WITH AIR BUBBLES"

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DESCRIPTION

The present invention refers to a continuous production plant of rolls of plastic film with air bubbles (bubble wrap) continuous by means of "cast extrusion", starting from the extrusion of polymer granules, and to the related process of production.

Air bubble films are composed of two superimposed films, one forming the bubble and the other underlying the base. Each of said two superimposed films can also be composed of several layers of different materials.

More particularly, the present invention relates to such a continuous system where at least one of the layers forming the air bubble film is obtained by means of a twin-screw extruder.

The air bubble plastic film production plant is a complex machinery made up of different units: it starts with an extrusion unit which, using plastic granules as the starting raw material, is able to continuously produce rolls of plastic film with bubbles of packaging air of various weights, formats, lengths and types.

In particular, the plastic granules of each layer forming the air bubble film are dosed in order to feed the respective single screw extruder, the granules advance in the channel formed between the screw and the cylinder starting to melt and the resulting melt is pressurized as it proceeds towards the outlet since the free space between the screw and the cylinder is reduced towards said outlet. The geometry of the single screw extruder is therefore designed in such a way as to allow high pressurization of the handled spindle.

The extrusion unit can be a monoextrusion section (when the bubble film is formed by two single films, the one that gives the shape of the bubble and the base one of the underlying surface) or a co-extrusion section (when the two films that form the bubble film are, each, formed by several layers).

The molten plastic material exiting at high pressure from each single screw extruder feeds two flat extrusion heads which at the exit each have a thin slot (one for the base film and the other for the film forming the bubble), the which impart the shape of a flat film to the respective melt.

In the case of multilayer film, before the two flat heads there is a “feedblock” distributor which allows the two flat extrusion heads to be fed with the same spindles.

The two plastic films (possibly multilayer), still in the fluid state, which have come out of the two flat extrusion heads are then in contact, in the forming station, with a cooled forming cylinder which thermoforms the two films giving them the typical shape of the bubble wrap film and its solidification with consequent cooling.

After the aforementioned forming station there may be various other stations for different processes to be carried out on the bubble wrap film now produced: typical are the lamination units that couple other films of different nature (paper, aluminum, polyethylene foam, HDPE film, etc. .) to the bubble wrap film by welding the same to the newly formed product; another typical process consists in the perforation (or pre-cut) which engraves the bubble wrap without cutting it completely, but allows the end user to easily tear off the product sheets (as in the case of toilet paper or kitchen paper).

At the end of these units there are the automatic cutting and rewinding stations in rolls and finally there are the units assigned to the unloading and ejection of the produced rolls.

Currently in the bubble wrap market, but more generally in the packaging market, there is a need to produce light films (weights of 20-30 grams per square meter) to reduce the weight of packaging and with recycled or less valuable materials than to virgin materials of first choice normally used (polyethylene): however these materials cannot be processed by current single-screw extruders.

In fact, in the single-screw extruder the material advances on a thermoregulated cylinder pushed by a screw with a set pitch which, by gradually decreasing the space available for the granule between cylinder and screw, considerably increases its pressure, even exponentially, as you approach the extruder outlet by melting the material and allowing it to feed the flat head without using any pump.

This increase in pressure, which is not adjustable or controllable, does not allow the single screw extruder, without incurring the phenomena of thermomechanical degradation to which all polymers may be subject, to process technopolymers (polyamides, polycarbonate, polystyrene, polyimide and others), polymer alloys (for example PC / ABS) or blends of polymers with decidedly different rheological characteristics, for example having different Meli Flow Index (MFI) varying from 0.3 to 3.5: what typically happens when working together materials that are not homogeneous in MFI, rheology, distribution of molecular weights (as polymers mixtures) in single-screw extruders is the formation of "gels" or "gels" visible at the exit of the flat extrusion heads.

These gels or lumps are agglomerates of molten / semi-fiy material that do not have the same viscosity values with the surrounding film and represent breaking points for the film during the production process. The breaking of the film triggered by these freezes prevents the production of the film itself.

The need to process non-homogeneous polymers from the point of view of MFI, rheology, distribution of molecular weights, arises from the fact that more and more often we try to work plastic materials that are obviously of lower quality, for example from recycling, to mix different products, some of which come from plastic recycling, or to process off-specification products from production plants, since these materials have a significantly lower price than that of first-choice virgin material.

Furthermore, as mentioned above, since the market is moving more and more towards the production of bubble wrap films with reduced weight reaching weights of 20-30 g / m <2>, it is necessary that the raw material has a perfect filmability, that is that the melt is able to be extruded through the double flat head passing through "slots" of the order of a few tenths of a millimeter, without breaking and also having mechanical characteristics such as to be subsequently thermoformed.

Currently, to produce particularly light films, it is possible to use only and only virgin raw materials with well-defined mechanical and rheological characteristics, limiting the possible choices (both of an economic and technical nature) of the procurement of raw materials. This is due to the fact that the non-adjustable increase in pressure on the polymer that occurs in the single screw extruder induces a degradation of the polymer and especially when this is not a first choice material with homogeneous rheological characteristics.

It therefore appears evident that the two needs are in stark contrast to each other, and the current state of the art does not allow the production of light air bubble films with low-quality materials or technopolymers.

The purpose of the present invention is to overcome, at least in part, the disadvantages of the known art by providing a method for producing in continuous, in monoextrusion or in coextrusion, air bubble films (hereinafter also referred to as bubble wrap films), even light ones. , starting from technopolymers or lower quality materials compared to first choice virgin plastics such as plastics coming from recycling, or out of specification coming from production plants.

A further object is to produce such bubble wrap films using a plant which does not require substantial changes in the layout, which is simple to manage and manufacture.

These and other purposes are achieved by the system in accordance with the invention having the characteristics listed in the attached independent claim 1.

Advantageous embodiments of the invention appear from the dependent claims.

An object of the present invention relates to the use of at least one twin-screw extruder in the continuous production, in monoextrusion or in coextrusion, of rolls of plastic film with air bubbles starting from the extrusion of granules of one or more polymers by means of "cast extrusion ".

Without departing from the scope of the present invention, said twin-screw extruder can be used as the only extruder of a monoextrusion section to form a bubble film formed by two identical films, each shown; or it can be used as one or more of the extruders of a coextrusion section necessary to form a bubble film formed by two multilayer films.

Another object of the present invention relates to a process for the continuous production, in monoextrusion or coextrusion, of rolls of plastic film with air bubbles, preferably light films with weights of 20-30 g / m, but which can also reach up to 500 g / m <2>, consisting of at least two superimposed films, one base and one forming the bubble, starting from the extrusion of granules of one or more polymers, said process comprising at least the monoextrusion or coextrusion phases of said two thin planar films through two flat extrusion heads fed by one (monoextrusion) or more extruders (coextrusion) of granules in which at least one extruder is a twin-screw extruder;

forming of said planar films by coupling said films so as to obtain a continuous air bubble film.

A further object of the present invention relates to a continuous production plant of rolls of plastic film with air bubbles (Pluriball film or Bubble wrap), starting from the extrusion of polymer granules, comprising in sequence

a "cast extrusion" type extrusion unit (monoextrusion or coextrusion) comprising at least one extruder, preferably two extruders, optional respective dosing pumps, and two flat extrusion heads so as to obtain a base film and a film forming the bubble,

a forming station, with a thermoregulated forming cylinder to thermoform the two aforementioned films in the form of an air bubble film,

one or more optional lamination units for laminating the bubble film with one or more layers of different nature (paper, aluminum, polyethylene foam, HDPE film, etc.), and / or

an optional pre-cutting (or perforation) unit,

a series of automatic cutting stations, rewinding in rolls of the bubble film, possibly coupled, and unloading of the produced rolls,

where at least one of the extruders upstream of the two extrusion heads is a twin-screw extruder able to have a pressure at the outlet such as to fill parts of the plant immediately downstream of said extruder, preferably such as to feed continuously said two extrusion heads, more preferably such as to fill a respective gear pump, if present, located downstream of said twin-screw extruder.

Said twin-screw extruder preferably has a final section L for pumping the molten polymer in which the profile of the two screws in said section is able to generate at the outlet of the extruder a pressure such as to feed and fill parts of the plant immediately downstream. of said extruder, which can be called two extrusion heads, or a respective gear pump, if present, located downstream of said twin-screw extruder and upstream of said extrusion heads.

More specifically, said twin-screw extruder advantageously has a very reduced pumping section L and a space between the cylinder and screws in said section L which gradually decreases as the extruder outlet approaches.

This design expedient is such as to guarantee a much higher pressure at the outlet of the extruder mouth than conventional twin-screw extruders which are in fact used for other purposes such as the production of polymer granules and not to feed flat extrusion heads. .

In the section L the polymer is pumped (metering) such as to effectively fill the two extrusion heads (die) without excessively degrading the molten polymer: this section L is placed as close as possible to the extruder outlet. (end of the cylinder) to limit and reduce the possibility of triggering degradative phenomena.

Generally in conventional twin-screw extruders the melt outlet pressure is quite low, and for this reason they are not used in the production of flat films which instead require a high pressure to be extruded through the thin opening of the flat extrusion head. : in fact, twin-screw extruders are used for the mixing of two or more polymers, for the manufacture of compounds, for master-batches, for the extrusion / manufacture of foams (PE, PP, XPS foam) and some types of injection presses (molding), i.e. when a high pressure is not needed at the outlet of the extruder but rather where it is necessary to minimize it as much as possible due to product degradation problems.

The twin-screw extruder therefore allows you to mix very different materials and to have an almost homogeneous material at the outlet from the rheological point of view, i.e. same viscosity, at the same temperature. Moreover, thanks to the presence of two screws that interpenetrate each other, the volumetric flow rate in the twin screw extruder is more constant than a single screw.

The known twin-screw extruders therefore allow to obtain a greater mixing of material with respect to that obtained by traditional single-screw extruders and a perfect rheological homogeneity of the output product since the geometry of the twin-screw extruder allows to mix the plastic materials inside it more times than at the same section of single screw.

This greater mixing in the melting phase of the material allows the twin-screw extruder to process mixtures of different and limited quality materials, releasing a melt that is almost homogeneous from the rheological point of view without the presence of gels or imperfections or preventing the formation of lumps of plastic material with different rheologies that could trigger breakages of the film with consequent stoppage of the production process (continuously).

However, the insertion of a twin-screw extruder has never been carried out in the bubble wrap production lines since it usually has a pressure at the outlet of the mouth which is generally lower than that necessary to effectively fill the subsequent parts. of the plant, for example the flat extrusion heads and / or any gear metering pumps upstream of said heads.

In particular in the execution described here (which is not limiting of the invention) the twin-screw extruder will preferably have a filter changer downstream of its mouth to filter the material leaving the extruder in order to guarantee the total purity and quality of the final product, subsequently connected. to a dosing pump that feeds the two flat extrusion heads. Assimilable executions may present a different sequence of the different elements, or the lack of some of them.

In particular, to obtain film through the slots of the flat heads, the melt must be fed at high pressure, avoiding an undesired phenomenon of pulsating flow which could cause periodic changes in the thickness of the film in the direction of the machine.

Therefore, the twin-screw extruder used in the present plant is preferably modified in a suitable way with respect to the known twin-screw extruders so as to always ensure the flow rate of the molten plastic material necessary for monoextrusion or co-extrusion and the final pressure necessary, using the lowest number of screw diameters (length of the extruder) so as not to degrade the plastic material or to limit its degradation.

From the literature it is known that the degradation phenomenon of polymers is linked to mechanical and thermal stress. The mechanical stress is then linked to the geometric characteristics of the screw / cylinder system and to the viscosity of the polymer; the latter is strongly influenced by the temperature and pressure on the polymer.

The twin-screw extruder used here is suitably modified so as to be able to control, in specific points of the screw / cylinder system, the pressure on the polymer as well as the shear-rate which influences the degradation phenomenon through stress on the polymer.

By “shear-rate” we mean the velocity gradient, ie the rate of change of the velocity at which a layer of fluid passes to an adjacent layer (deformation rate).

In a twin-screw extruder, given that the geometric configuration is flexible and the plasticization of the polymer within the screw / cylinder system is controllable, it can be asserted that the degradation phenomenon of the polymer is under control while ensuring in the meantime the maximum capacity of plasticization of mixtures with various viscosities, ensuring the desired pressure at the design flow rate necessary to feed the respective dosing pump feeding the flat extrusion heads.

In the formation of the bubble wrap film by "cast extrusion" in accordance with the present invention, there is a need to have a considerable pressure at the extruder outlet since after the mouth the melt will first have to pass through suitable thermoregulated pipes (" colli ") also through the possible steel filter (" filter changer "), and then the gear pump, if any, must enter and fill it appropriately, which in the system in question has the function of dosing pump.

This pump has a minimum operating pressure, for example in the range of 20 ÷ 80 bar, below which it is impossible to work, as the pump itself would not fill properly in order to feed the flat extrusion heads at the correct pressure. When the bubble wrap film is made up of two films, each multilayer, it is necessary to precisely control the relative percentages of the individual extruders with respect to the overall flow rate.

Since this metering of the flow rates is obtained precisely by controlling the revolutions of the metering pump, it is clear how important it is for the process to have the pump suitably full.

Therefore, the twin-screw extruder used here advantageously has a profile of the two screws capable of generating at the outlet of the extruder a pressure such as to continuously feed said two extrusion heads, preferably such as to suitably fill the gear pump downstream of the extruder itself but not so high and uncontrolled as to degrade the polymer with excessive overpressures and / or mechanical and thermal stresses.

In particular, in the twin-screw extruder used here, section L is established in the design phase, depending on the polymer to be processed, in which there is an increase in pressure: this is done simply by varying the geometry of the screws and their positioning along the shaft, thus choosing the starting point of the pumping phase (pressurization) of the polymer, without having to modify the length or the cylinder itself.

Obviously, this pumping start point will be as close as possible to the end of the cylinder to limit and reduce the possibility of triggering degradation phenomena.

All this with the possibility of greater plasticization of polymeric mixtures with various viscosities without triggering degradation phenomena as the mixing and plasticizing phase takes place at lower average pressures than in a single screw.

In summary, it can be stated that advantageously the twin screw extruder used here preferably has a suitably modified final section which acts as the final pumping section of a single screw extruder.

Once dimensioned in this way, the twin-screw extruder can be inserted inside an air bubble film production plant, going beyond the current limits of the state of the art.

A further consideration should be made regarding the purely economic aspects of the invention, specifically, the high cost of twin-screw extruders compared to single-screw extruders is balanced by their greater energy efficiency and their greater yield in terms of kilos of production on screw diameters.

In particular, to guarantee a certain flow rate (in kg / h), the single-screw extruders must be of larger dimensions (screw diameter) than the twin-screw extruders, this allows (keeping the hourly flow rate of the machine fixed) to reduce the size of the twin-screw extruder reducing at the same time the size of the motor that moves it. All this leads to energy savings with consequent economic savings.

Ultimately it can be said that the cost per kilo of finished product is lower using a machine equipped with a twin-screw extruder, this leads to amortization of the higher initial cost of the suitably modified twin-screw extruder.

Furthermore, the greater ability to homogenize plastic materials with different rheological properties, accompanied by greater energy efficiency compared to single screw, allows to expand the choice of raw materials that can be used for the production of particularly light plastic films.

In particular, thanks to the use of the twin-screw extruder, it will be possible to favor poorer materials, and therefore cheaper, without compromising the correct functioning of all regrets and the properties of the final product.

It should be noted that in the present plant the number of twin-screw extruders provided upstream of the two flat extrusion heads is not binding for the purposes of the present invention and mainly depends on the number of different non-precious materials or technopolymers to be extruded and not necessarily on the number of layers of bubble wrap film.

Further features of the invention will become clearer from the detailed description that follows, referring to a purely exemplary, and therefore non-limiting, embodiment illustrated in the attached drawings, in which:

Figure 1 is a schematic view of an extrusion unit according to the invention to produce a five-layer bubble wrap film (5 + 5);

Figure 2 is a side view of the complete air bubble film production plant according to the invention;

figure 3 is a side view of the extrusion and lamination section of the air bubble film production plant illustrated in fig. 2;

figure 3 a is an enlarged view of the detail enclosed in the circle indicated by A in fig. 3;

figure 4 is a side view of the cutting and rewinding section of the air bubble film production plant illustrated in fig. 2;

Figures 5a-5d are exploded side views of the three sections a), b), c) that make up each screw of the twin-screw extruder and of each screw as a whole according to the invention;

Figures 6-7 are a plan view of the plant section illustrated in Figure 3 and 4 respectively.

Figure 1 schematically illustrates the co-extrusion section of a multilayer bubble film (5 + 5) according to the present invention, in particular by means of “Cast Extrusion”.

The extrusion unit comprises a gravimetric dosing system 15 (also indicated in fig. 2) to feed the granules of the respective plastic material A, B, C (fig. 1) to the respective extruder 10, 20, 30, (also in fig. 6), all arranged downstream of said metering system 15.

Generally the plastic material A is a polyethylene (PE), the polymer B is a tie-layer, that is an adhesive polymer (glue), the polymer C is generally a barrier polymer such as nylon (PA6).

Each of said materials A, B, C is extruded by an extruder and at least one of said extruders is a twin-screw extruder: preferably the extruder 10 fed with the polyethylene granules A is preferably a twin-screw extruder, even if it is to be understood that this is not is binding for the purposes of the present invention and therefore twin-screw extruders can be used for polymers B and / or C, alternatively or in addition to that used for polymer A.

In the event that the twin-screw extruder 10 is intended for the aforementioned polymer A, it has a terminal section L (corresponding to the pumping section c of fig.

5) of process length in the range of 8 ÷ 15 times the screw diameter.

Inside each extruder 10,20,30 (single screw or twin screw), the polymer granules are mixed, pressed and then melted, coming out in the form of melt from the outlet of the extruder to feed a respective filter changer 25 and subsequently the respective metering gear pump 16 (figs. 3 and 3 a) which conveys the respective melt to a feed-block distributor device 200 (shown schematically only in fig. 1), per se already known in film coextrusion technology : it has the purpose of dividing the three melt streams A, B, C into as many streams as there are layers in the final bubble film 22 (Fig. 1), then arranging the different melt streams in a predetermined sequence of feeding layers to the two flat extrusion heads 40, for example ABCBA to form a base film 21 'and a film forming the bubble 21, both having five layers in the present embodiment.

Figures 2-3 do not show any single screw extruders for simplicity of illustration.

After the exit from the respective flat head 40 of each coextruded sheet 21, 2Γ multilayer ABCBA, one passes to the forming section comprising a forming unit 50 (fig. 2-3), where a thermoregulated (cooled) forming cylinder 300 provides for the thermoforming of the two multilayer films 21 and 2Γ giving them the typical shape of the bubble wrap film 22, and at the same time allowing solidification with consequent cooling.

After the forming station 50, various other stations can be present for different processes to be carried out on the now produced bubble wrap film 22: typical are the lamination units 60 with heated cylinder (fig. 3) which couple at least one other film 31 to the bubble wrap film 22 (fig. 3) of different nature (paper, aluminum, polyethylene foam, HDPE film, etc.), coming from at least a specific unwinding section 7 of a third layer 31, welding said additional film 31 to the bubble wrap product 22 as soon as format.

In particular, in relation to the preferred embodiment illustrated in fig. 2, different films (paper, aluminum, polyethylene foam, HDPE film, etc.), possibly already coupled together, are unwound alternately by unwinders 7 and 8 (fig. 4, unwinder for PE foam) and then laminated to the air bubble film being produced in the lamination assembly 60 to form the third 31 layer above the bubble forming film 21.

It is also possible to provide for the production of a bubble film having a fourth layer 32 (fig. 3) placed below the base film 2Γ: in this case the fourth layer is fed by an unwinder 17 placed before the thermoforming unit 50: this unwinder 17 feeds a film 32 (paper, aluminum, polyethylene foam, HDPE film, etc.) making it unwind and at the same time reaching the melt directly into the forming cylinder 300 in the forming station 50. As a result, the film produced will also present the lamination of a further film below the base film 2Γ.

Another typical process consists in the perforation (or pre-cut) that engraves the bubble wrap without cutting it completely, allowing the end user to easily tear off the product sheets (as in the case of toilet paper or kitchen paper).

As shown in Figure 4, at the end of these units there are the automatic cutting stations 9, rewinding in rolls 100 and unloading and ejection units 11 for the rolls produced.

The automatic cutting station 9, per se already known in the art, can also provide a trimming recovery system 14 (Fig. 7).

The plant is also equipped with an electrical panel 13 (fig. 6) and a control panel, already known per se in the art in bubble wrap production plants, as well as providing the normal utilities of a plant (pumping water 19, diathermic oil control unit for heating rollers 18 - fig. 6).

With reference to the twin-screw extruder used in this system, it is preferable that it has a screw configuration as shown in Fig. 5, in particular when it is fed with PE granules.

In particular, each screw is formed by three distinct portions a), b), c), each having at least two different thread pitches.

The threads have been indicated here with the reference numbers 1, 2, 3, while the mixing elements have been indicated with the reference numbers 4 and 5.

The first section a) is the one dedicated to the transport of the material, the second section b) is dedicated to mixing while the third section c) is the one intended for the compression of the material and corresponds to the section indicated above with the initials L.

In a preferred embodiment, the pumping length is equal to about 13 diameters, ie a length much shorter than that of a single screw extruder.

It should be noted that normally in twin-screw extruders each section has only one type of pitch and / or thread that differs from one section to another: it is only in single-screw extruders that the last outlet section can have two different thread pitches.

The present invention is not limited to the particular embodiments previously described and illustrated in the annexed drawings, but numerous detailed modifications can be made to it, within the reach of the person skilled in the art, without thereby departing from the scope of the invention itself, such as defined in the appended claims.

Claims (10)

CLAIMS 1. Plant for the continuous production of rolls of plastic film with air bubbles (22) with weights up to 500 g / m <2>, more preferably a light film with weights of 20-30 g / m <2>, said film (22) comprising at least two superimposed films (21, 2 Γ), one at the base and the other forming the bubble, said system comprising, in sequence, an extrusion unit, in monoextrusion or coextrusion, preferably of the "cast extrusion" type, comprising at least one extruder (10,20,30) upstream of two flat extrusion heads (40) for planar films so as to form said film base (2Γ) and said bubble-forming film (21), a forming and cooling station, with a thermoregulated forming cylinder (300) to couple said two films (21, 2Γ) so as to obtain air bubble films (22), a series of automatic cutting stations, rewinding the bubble film into rolls and unloading said product rolls, characterized by the fact that at least one of the extruders (10,20,30) is a twin screw extruder, called twin screw extruder (10,20,30) providing the molten polymer with such a pressure, at the outlet, to fill the parts of said plant placed immediately downstream of said twin-screw extruder, preferably such as to continuously feed said two extrusion heads (40) and / or a respective gear pump (16), if present, located downstream of said twin-screw extruder and upstream of said heads extrusion (40). 2. Plant according to claim 1 in which said twin-screw extruder (10,20,30) is equipped with a final section (L) for pumping the molten polymer in which the profile of the screws of said twin-screw extruder occupying said section (L) it is able to generate at the outlet of said twin-screw extruder (10,20,30) a pressure such as to fill said successive parts of the system placed immediately downstream of said twin-screw extruder. 3. Plant according to claim 2 in which the free space between the cylinder and the screws housed in said section (L) decreases approaching the outlet of said twin-screw extruder (10,20,30). 4. Plant according to any one of the preceding claims in which the number of twin-screw extruders depends on the number of different polymers to be extruded. 5. Plant according to any of the preceding claims in which each of the two films (21,21 ') of the air bubble film (22) is a multilayer film. 6. Plant according to claim 5 wherein each multilayer film (21, 2Γ) is formed by five layers in sequence A / B / C / B / A where A is a polyethylene, B is a "tie-layer" adhesive polymer, C is a barrier polymer such as nylon (PA6). 7. Plant according to any one of the preceding claims, in which a feed-block distributor (20) is provided upstream of said two flat extrusion heads (40), able to divide the melts coming out of one or more of said extruders (10, 20,30) in as many streams as there are layers in the final air bubble film (22). 8. Plant according to any one of the preceding claims, in which the extrusion unit further comprises a gravimetric metering system (15) for feeding the granules of one or more polymeric plastic materials (A, B, C) to the respective extruders (10, 20.30). 9. Process for the continuous production of rolls of plastic film with air bubbles (22) composed of at least two superimposed films (2 1.21 '), one base and one forming the bubble, starting from the extrusion of granules of one or more polymers, said process comprising at least the steps of extrusion by cast extrusion of said two thin planar films (21, 21 ') through two flat extrusion heads (40) fed by one or more extruders (10,20,30) of granules in which at least one of the extruders (10,20 , 30) is a twin-screw extruder which imparts such pressure to the molten polymer at the outlet that it fills the parts of the plant immediately downstream of said twin-screw extruder, forming of said planar films (21, 2Γ) by coupling said films (21, 21 ') so as to obtain a continuous air bubble film (22). 10. Use of at least one twin-screw extruder (10,20,30) in the continuous production of plastic film rolls with continuous air bubbles (22) starting from the extrusion of granules of one or more polymers by "cast extrusion" .