CA2873297A1

CA2873297A1 - Plant including a twin-screw extruder for continuous production of rolls of air bubble film

Info

Publication number: CA2873297A1
Application number: CA2873297A
Authority: CA
Inventors: Maurizio Mencarelli
Original assignee: TORNINOVA Srl
Current assignee: Colines Air Bubble Srl
Priority date: 2013-12-09
Filing date: 2014-12-04
Publication date: 2015-06-09
Anticipated expiration: 2034-12-04
Also published as: CA2873297C; ITMI20132048A1

Abstract

It is described the use of at least one twin-screw extruder (10,20,30) in the continuous production of continuous rolls of air bubble film (22), starting from the extrusion of granules of one or more polymers using "cast extrusion".

Description

PLANT INCLUDING A TWIN-SCREW EXTRUDER FOR CONTINUOUS
PRODUCTION OF ROLLS OF AIR BUBBLE FILM
DESCRIPTION
The present invention relates to a plant of continuous production of rolls of air bubble film (or "pluriball" film) using "cast extrusion", starting from the extrusion of polymer granules, and to the respective production process.
The air bubble films are composed of two superimposed films, a thermoformed one for forming the bubble and the other below being the base. Each of said two superimposed films can also consists of several layers of different materials.
More particularly, the present invention relates to such a continuous plant where at least one of the layers forming the air bubble film is obtained by means of a twin-screw extruder.
The production plant of air bubble film is a complex machinery formed by different units: it starts from an extrusion unit which uses granules of plastic material as a starting raw material and produces in continuous packing rolls of air bubble film of various weights, shape, size, length, type.
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 are fed into the channel formed between the screw and the cylinder and begin to melt, the resulting melt polymer is then pressurized while proceeding towards the outlet as the clearance between the screw and the cylinder becomes smaller towards said outlet. The single-screw extruder geometry is then designed so as to allow a high pressurization of the mobilized melt polymer.
The extrusion unit may be a monoextrusion section (when the bubble film is formed by two individual films, one giving the shape of the bubble and one at the base of the underlying layer) or by coextrusion (when the two films forming the bubble film are, each, formed by several layers).

2 The melt plastic material in the high-pressure outlet of each single-screw extruder feeds two flat extrusion dies each comprising at the outlet a thin slit (one for the film forming the base and the other for the film forming the bubble), which impart to the respective melt polymer the shape of flat film.
In the case of multilayer film before the two flat dies a "feedblock"
distributor is arranged that allows to feed with the same melt polymers the two flat extrusion dies.
The two plastic films (optionally multi-layer), still in a fluid state, that have come out from the two flat extrusion dies then come into contact, in the forming station, with a cooled forming cylinder, which provides the thermoforming of the two films giving the typical shape of a pluriball film or air bubble film and its solidification with consequent cooling.
Downstream the abovementioned forming station there may be various other stations for different processing operations to be performed on the air bubble film already obtained: typically there are rolling units which couple other films of different nature (paper, aluminum, polyethylene foam, HDPE films, etc.) to the air bubble film by welding the same to the just formed product; another typical processing consists in the drilling (or pre-cut) which incises the air bubble film without cutting it completely, but allows the final user to easily tear the sheets of product (as in the case of toilet paper or paper towel).
At the end of these units the automatic cutting and roll rewinding stations are placed and finally there are groups dedicated to the unloading and discharge of the obtained rolls.
Currently in the air bubble film market, but more in general 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 to use recycled or of less value materials than the first choice virgin material normally used (polyethylene): however, these materials cannot be processed by the current single-screw extruder.
In fact, in the single-screw extruder the material is fed onto a thermoregulated cylinder pushed by a preset pitch screw that, by gradually decreasing the available space of

3 granule between the cylinder and the screw, significantly increases the pressure, even exponentially, as it approaches the extruder output thus melting the material and allowing the latter to feed the flat die without using any pump.
Said increase in pressure, which is not adjustable or controllable, does not allow the single-screw extruder to process, without incurring the thermomechanical degradation phenomena to which all polymers may be subject, techno-polymers (polyamides, polycarbonates, polystyrene, polyimide and other), polymer alloys (for example, PC/ABS) or blends of polymers with rheological characteristics significantly different one with respect to the other, for example having different Melt Flow Index (MFI) ranging from 0.3 to 3.5: what typically happens by processing together, in single-screw extruders, materials which are not homogeneous in the MFI, rheology, molecular weight distribution (as blends of polymers), is the formation of "gels" or "gel" visible at the output of the flat extrusion dies.
These gels or lumps are agglomerations of molten/semi-molten material that does not have the same viscosity values as the surrounding film and represent breaking points for the film during the production process. The breaking of the film starting from these gels prevents the production of the film itself.
The need to process non-homogeneous polymers from the point of view of MFI, rheology, molecular weight distribution, is due to the fact that more and more often plastic materials that are obviously of lower quality are required to be processed, such as recycled ones, by mixing different products some of which come from plastic recycling, or processing off-specification products coming from the production plants, as these materials have a significantly lower price than that of first choice virgin material.
Also, as mentioned above, since the market is shifting more and more towards the production of air bubble film of reduced weight up to weights of 20-30 g/m2, the raw material are required to have a perfect filmability, i.e. that the melt polymer is suited to be extruded through the double flat die passing through "slots" of a few tenths of a millimeter, without breaking and also having mechanical characteristics allowing them to be subsequently thermoformed.

4 Currently, to produce particularly light films, only an exclusively virgin raw materials with mechanical and rheological well defined characteristics can be used, thus limiting the possible choices (both economical and technical) of raw material supply.
This is due to the fact that the non-adjustable increase in pressure on the polymer, which occurs in the single-screw extruder, induces a degradation of the polymer and especially when this is not a first choice material having homogeneous rheological characteristics.
It is therefore clear that the two requirements are in stark contrast to one another, and the current state of the art does not allow to produce lightweight air bubble films with low-quality materials or techno-polymers.
US2003/0037858 describes the production of a laminate film, different from the bubble wrap or air bubble film, formed by two films coupled so as to have channels and chambers deflated in order to be inflated by the final user in order to limit the bulkiness of the film roll during transport.
Said film is obtained by putting into contact the two films, previously extruded and cooled below their melting temperature, and subsequently by heating only selected portions of one of the films by means of a contact roller and/or of a roller with surface protrusions: in this way the first and the second film are welded to one another only in selected areas thus obtaining inflatable chambers interposed between the first and the second film.
In this process the use of a twin-screw extruder is provided in a blown extrusion process where an annular extrusion die for obtaining the first or second film is used.
The object of the present invention is to overcome, at least in part, the disadvantages of the prior art by providing a method for continuous production, in monoextrusion or coextrusion, of an air bubble film (hereinafter also referred to as pluriball film), even lightweight, starting from techno-polymers or from materials of lower quality derived from recycling with respect to first choice virgin plastics, or off-specification material coming from the production plants.

A further object is to produce said air bubble films using a plant which does not require substantial changes in the lay-out, which is simply managed and obtained.
These and other objects are achieved by the plant according to the invention having

5 the characteristics listed in appended independent claim 1.
Advantageous embodiments of the invention are apparent from the dependent claims.
An object of the present invention concerns the use of at least one twin-screw extruder in continuous production, in monoextrusion or coextrusion, of rolls of air-filled bubble film starting from the extrusion of granules of one or more polymers by way 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 mono-extruding section for forming, by way of "cast extrusion", a bubble film formed by two identical films, each one monolayer; or it can be used as one or many of the extruders of a coextrusion section required for forming, by way of "cast extrusion", an air bubble film formed by two multilayer films.
Another object of the present invention relates to a process for continuous production, in monoextrusion or coextrusion, of rolls of air bubble film, preferably light films with weights of 20-30 g/m2, but may also be up to 500 g/m2, consisting of at least two superimposed films, one forming the base and one forming the bubble, starting from extrusion of granules of one or more polymers, said process comprising at least the steps of monoextruding or coextruding said two planar thin films through two flat extrusion dies fed by one (monoextrusion) or many extruders (coextrusion) of granules wherein at least one extruder is a twin-screw extruder; thermoforming said planar films by coupling said melt films by means of a cylinder, not heated, provided with cavities placed under vacuum, which cooperates with a contact cylinder, so as to obtain a continuous air bubble film.
A further object of the present invention relates to a plant for the continuous production of rolls of air bubble film, such as pluriball film or air bubble film, starting from the extrusion of the polymer granules, comprising in sequence

6 an extrusion unit of the "cast extrusion" type (monoextrusion or coextrusion) comprising at least one extruder, preferably two extruders, optional respective dosing pumps, and two flat extrusion dies so as to obtain a film forming the base and a film forming the bubble, a thermoforming station, with a thermoregulated forming cylinder provided with cavities placed under vacuum, and cooperating with a contact cylinder to thermoform the two aforesaid films in the form of air bubble films, one or more optional rolling units for coupling the bubble film with one or more layers of different nature (paper, aluminum, foam polyethylene, HDPE film, etc.), and/or an optional pre-cut (or drilling) unit, a series of automatic cutting stations, roll rewinding of the bubble film, possibly coupled, and unloading of the obtained rolls, where at least one of the extruders upstream of the two extrusion dies is a twin-screw extruder adapted to have at the outlet a pressure allowing to fill parts of the plant immediately downstream of said extruder, preferably allowing to feed in continuous said two extrusion dies, more preferably allowing to fill a respective gear pump, if present, located downstream of said twin-screw extruder.
Said twin-screw extruder preferably has an end segment L for pumping the melt polymer wherein the profile of the two screws in said portion is adapted to generate at the extruder outlet a pressure allowing to feed and fill parts of the plant immediately downstream of said extruder, which can be said two extrusion dies, or a respective gear pump, if present, located downstream of said twin-screw extruder and upstream of said extrusion dies.
More specifically, said twin-screw extruder advantageously has a very short pumping segment L much reduced and a space between cylinder and screws in said segment L
which gradually decreases approaching the outlet of the extruder.
This design expedient is thus for guaranteeing a much higher pressure at the outlet of the extruder with respect to the conventional twin-screw extruders which are in fact used for other purposes such as production of polymer granules and not to feed flat extrusion dies.
In the segment L therefore metering of the polymer takes place so as to effectively fill the two extrusion dies without degrading too much the melt polymer: this segment L

7 is, as much as possible, placed near the outlet of the extruder (end of the cylinder) to limit and reduce the possibility of giving rise to phenomena of degradation.
Generally in the conventional twin-screw extruders the output pressure of the melt polymer is sufficiently low, and for this reason they are not used in the production of flat films which require a high pressure in order to be extruded through the thin opening of the flat extrusion die: in fact, the twin-screw extruders are used for mixing two or more polymers, for the manufacturing of compounds, master-batch, extrusion/manufacturing of foams (PE, PP, XPS foam) and some types of injection molding (molding), i.e. when there is no need of a high pressure at the outlet of the extruder, but rather where it is necessary to minimize the pressure as much as possible to avoid problems of product degradation.
The twin-screw extruder then allows to mix very different materials and to obtain at the output an almost homogeneous material from the rheological point of view, i.e. the same viscosity at the same temperature. Moreover, thanks to the presence of two screws that penetrate each other, the volumetric flow of the twin-screw extruder, is more constant as compared with respect to a single-screw one.
The known twin-screw extruders thus allow to obtain a better mixing of the batch as compared with what is obtainable by the conventional single-screw extruders and a perfect rheological homogeneity of the output product as the geometry of the twin-screw extruder allows to mix therein the plastic materials several times as compared to the same segment of the single-screw.
This greater mixing in the melting phase of the material allows the twin-screw extruder to process mixtures of different materials and of limited quality outputting as a result a melt polymer almost homogeneous from the rheological point of view with no presence of gels or imperfections i.e. preventing the formation of lumps of plastic material with different rheological characteristics that may give rise breakage of the film and consequent stop of the production process (in continuous).
However, the use of a twin-screw extruder has never been provided in the production lines of air bubble film because of its operation outlet pressure, which in general is

8 lower than that required to effectively fill the subsequent parts of the plant, for example, the flat extrusion dies and/or any gear dosing pumps upstream of said dies.
In particular, in the implementation described herein (which is not 'imitative of the invention) the twin-screw extruder preferably has, downstream from its outlet, a filter-changer to filter the material leaving the extruder in order to ensure total purity and quality of the final product, subsequently connected to a dosing pump that feeds the two flat extrusion dies. Similar implementations may have a different sequence of the different elements, or the lack of some of them.
In particular, to obtain the film through the slits of the flat dies it is necessary to supply the melt polymer at high-pressure, while avoiding an undesired phenomenon of pulsating flow which might cause periodic changes in the film thickness in the machine direction.
Therefore, the twin-screw extruder used in the present plant is preferably modified in an appropriate way with respect to the known twin-screw extruders in order to assure the flow of the melt plastic material necessary for the mono-extrusion or co-extrusion plant, and the final pressure required, using the fewer number of screw diameters (length of the extruder) so as to not degrade the plastic material or to limit its degradation.
From previous art it is known that the degradation phenomenon of the polymers is related to mechanical and thermal stress. The mechanical stress is in turn associated 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 by the pressure on the polymer.
The twin-screw extruder used herein is suitably modified in order to control, in very specific points of the screw/cylinder system, the pressure on the polymer as well as the shear-rate which influences the degradation phenomenon by way of the stress on the polymer.
For "shear-rate" it is meant the speed gradient, i.e. the rate of change in speed at which a layer of fluid passes to an adjacent layer (strain-rate).

9 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 plasticization capacity of mixtures with various viscosities, ensuring that the desired pressure at the rated flow rate needed to feed the respective dosing pump feeding the flat extrusion dies are met.
In the formation of the air bubble film by means of "cast extrusion" according to the present invention there is the need to have considerable pressure at the outlet of the extruder as downstream of the outlet the melt polymer will have to pass first, by means of suitable thermoregulated pipes ("necks"), also through the optional steel filter ("filter-changer"), and then will have to enter and properly fill also the optional gear pump, which in the present plant has a dosing pump function.
This pump has a minimum operating pressure, for example in the range of 20 to bar, below which it is impossible to process, as the pump itself would not fill properly in order to feed at the proper pressure the flat extrusion dies. When the air bubble film (pluriball film) consists of two films, each one multi-layer, the need is felt to exactly control the relative percentages of the individual extruders with respect to the total flow rate.
Since this metering of the flow rate is actually obtained by controlling the revolutions of the dosing pump, it is clear how important it is for the process that the pump is properly filled.
Therefore, the twin-screw extruder used herein advantageously has a profile of the two screws adapted to generate a pressure at the outlet of the extruder such as to continuously feed said two extrusion dies, preferably such as to appropriately fill the gear pump downstream of the extruder itself but not so highly and uncontrolled as to degrade the polymer with excessive overpressures and/or mechanical and thermal stresses.
In particular, in the twin-screw extruder used herein, depending on the polymer to be processed, the segment L, in which an increase of pressure takes place, is determined in the design phase: this is done simply by varying the geometry of the screws and their positioning along the shaft, then selecting the start point of the pumping step (pressurization) of the polymer, without having to change 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 initiating degradation phenomena.
A11 this entails the possibility of increased plasticization of polymer blends with various viscosities without initiating degradation phenomena as the step of mixing and

10 plasticization occurs at a lower average pressure than in a single-screw extruder.
To summarize, it can be stated that advantageously the twin-screw extruder used herein preferably has a final portion suitably modified which acts as the final pumping portion of a single-screw extruder.
Thus dimensioned the twin-screw extruder can be inserted inside a production plant of air bubble film overcoming the current state of the art limits.
A further consideration must be made about the purely economic aspects of the invention, specifically, the high cost of the twin-screw extruders compared to single-screw extruder is balanced by their greater energy efficiency and by their greater output in terms of kilograms of production with respect to the screw diameters.
In particular to ensure a certain flow rate (in kg/h) the single-screw extruders should be of a size (screw diameter) greater than the twin-screw extruders, this allows (keeping fixed the hourly flow rate of the machine) 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 resulting cost savings.
To sumarize it can be said that the cost per kilo of finished product turns out to be lower by using a machine provided with a twin-screw extruder, this leads to deferre the higher initial cost of the twin-screw extruder suitably modified.

11 Furthermore, the greater capacity to homogenize plastic materials with different rheological properties, together with a greater energy efficiency compared to the single-screw extruder, allows to expand the choice of raw materials used for the production of particularly light plastic films.
In particular, thanks to the use of the twin-screw extruder, lower quality materials can be prioritized, and therefore cheaper, without affecting the proper functioning of the entire plant 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 dies is not binding for the purposes of the present invention and depends mainly on the number of different non-raw materials or techno-polymers to be extruded and not necessarily on the number of layers of the air bubble film.
Further characteristics of the invention will become clear from the detailed description that follows, referring to a purely exemplary, and therefore non-limiting, embodiment illustrated in the appended drawings, wherein:
Figure 1 is a schematic view of an extrusion unit according to the invention for producing a five layer (5 + 5) air bubble film;
Figure 2 is a side view of the complete production plant of air bubble film according to the invention;
Figure 3 is a side view of the extrusion and lamination section of the production plant of air bubble film shown in fig. 2;
Figure 3a 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 production plant of air bubble film shown in fig. 2;
Figures 5a-5d are exploded side views of the three sections a), h), c) that compose each screw of the twin-screw extruder and as a whole of each screw according to the invention;
Figures 6-7 are plan views of the section of the plant respectively illustrated in Figure 3 and 4.

12 In figure 1 is illustrated in a schematic way the co-extrusion section of a multilayer (5 + 5) bubble film according to the present invention, in particular by means of "Cast Extrusion".
The extrusion unit comprises a gravimetric dosing system 15 (shown also in Figure 2) for feeding 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, i.e. 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, which is fed with the polyethylene granules A, is a twin-screw extruder, although it is to be understood that this is not binding for the purposes of the present invention and therefore twin-screw extruders for the polymers B and/or C can be used, in alternative or in addition to that used for the polymer A.
In the case where the twin-screw extruder 10 is dedicated to the above polymer A, it has an end segment L (corresponding to the pumping portion c of Fig. 5) of processing length in the range of 8 to 15 times the diameter of the screw.
Within each extruder 10,20,30 (single-screw or twin-screw), the polymer granules are mixed, pressed and then melt, coming out in the form of melt polymer from the outlet of the extruder to feed a respective filter-changer 25 and subsequently the respective dosing gear pump 16 (figs. 3 and 3a), which conveys the respective melt polymer to a feed-block distributor device 200 (shown schematically only in Figure 1), in itself already known in the technology of film co-extrusion: it has the purpose of dividing the three currents A, B, C of melt polymer in as many currents as the layers in the final bubble film 22 (fig.1) then placing the different currents of melt polymer in a preset sequence of layers feeding the two flat extrusion dies 40, for example ABCBA
to form

13 a film forming the base 21' and a film forming the bubble 21, both of five layers in the present embodiment.
In Figures 2-3 no single-screw extruders have been shown for sake of simplicity of illustration.
After each coextruded sheet 21, 21' multilayer ABCBA is contemporarily delivered by the respective flat die 40, the process moves to the typical thermoforming section of air bubble films known in the art: in said section the air bubbles thermoforming of the present air bubble film 22 takes place.
The thermoforming section or unit consists of a forming unit 50 (figg.2-3) that comprises a forming cylinder 300, arranged immediately downstream of the two flat extrusion dies 40, and at least one contact cylinder 301, with no heating, which cooperates with said forming cylinder 300 to facilitate the adhesion of the two films 21, 21' by increasing the contact pressure between said two films 21, 21'.
Said forming cylinder 300, which is not heated nor has heating action, is thermoregulated (for example by water cooling) to maintain constant the temperature of the molten films with which it comes into contact so as to ensure a thermoforming and an adhesion without problems.
The forming cylinder 300 is also provided with recesses or grooves connected to a vacuum source: when the first molten film outgoing from the first extrusion die is cast directly onto the cylinder 300, the areas of film that covers the recesses arc pulled inside said recesses obtaining thermoformed bubbles.
The remaining hot areas of said first film still "flat" are instead free to melt with the second molten film outgoing from the second flat extrusion die that is cast on the rear part of the first thermoformed film, instantly adhering to said first thermoformed film, thanks the contact pressure exerted by the contact cylinder 301 on the second film when said contact cylinder 301 cooperates with the forming cylinder 300.
The points of contact between said films 21, 21' then compenctrate one into the other as said films, not yet solidified, are in molten form: in this way the heat of the two

14 melt polymers, of the thermoformed film 21 forming the bubble and of the underlying film forming the base 21', can be used eliminating the need for subsequent heating at different points when welding the two films 21, 21 to each other in the presence of bubbles is required.
Therefore said forming cylinder 300 provides the thermoforming of the air bubbles of the air bubble film deriving from the two multilayer films 21 and 21'.
Downstream of the forming station 50 various other stations for different machining operations to be performed on the already obtained air bubble films 22 may be present:
typically the rolling unit 60 with heated cylinder (fig. 3) which couple to the air bubble film 22 at least another film 31 (fig. 3) of different nature (paper, aluminum, polyethylene foam, HDPE film, etc.), coming from at least one specific unwinding section 7 of a third layer 31, by welding said additional film 31 to the newly formed air bubble product 22.
In particular, in relation to the preferred embodiment illustrated in fig. 2, different film (paper, aluminum, polyethylene foam, HDPE film, etc.), possibly already coupled together, are unwound alternately by the unwinders 7 and 8 (fig. 4, unwinder for PE
foam) and then laminated to the air bubble film obtained in the rolling assembly 60 to form the third layer 31 over the film forming the bubble 21.
It is also possible to envisage the production of a bubble film having a fourth layer 32 (fig. 3) arranged below the film forming the base 21': in this ease the fourth layer is fed by an unwinder 17 placed upstream of the thermoforming assembly 50; said unwinder 17 feeds a film 32 (paper, aluminum, polyethylene foam, HDPE film, etc.), making it unwind and arrive simultaneously to the melt polymer directly inside the forming cylinder 300 in the forming station 50. As a result, the film obtained will present also the lamination of an additional film beneath the film forming the base 21'.
Another typical process consists in perforation (or pre-cutting) which incises the air bubble film without cutting it completely, allowing the final user to easily tear the sheets of product (as in the case of toilet paper or paper towel).

As illustrated in Figure 4, at the end of these units the automatic cutting station 9, the station 100 for rewinding in rolls and the unloading and ejection units 11 of the obtained rolls can be found.
5 The automatic cutting station 9, in itself already known in the art, may also provide a trimming collection system 14 (fig. 7).
The plant is also provided with an electrical panel 13 (fig. 6) and with a control panel, already known per se in the art of the air bubble film production plant, in addition to 10 provide the normal utilities of a plant (water pumping 19, diathermic oil control unit 18 for the heating rollers - fig.6).
With reference to the twin-screw extruder used in the present plant, it is preferable that it has a screw configuration as shown in fig. 5, in particular when it is fed with PE

15 granules.
In particular, three separate portions a), b), c), each having at least two different thread pitches form each screw.
The threads are denoted here with reference numbers 1, 2, 3, while the mixing elements are denoted with the reference numbers 4 and 5.
The first portion a) is dedicated to the transport of the batch, the second portion b) is intended to mix it while the third portion c) is dedicated to the compression of the material and corresponds to the portion indicated above with the initials L.
In a preferred embodiment, the length of the pumping is equal to about 13 diameters, i.e. a length much lower than that of a single-screw extruder.
It should be noted that normally in the twin-screw extruders each section has only one type of pitch and/or thread, which differs from one section to another: it is only in single-screw extruders that the last outlet portion may have two different thread pitches.

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

Claims

1. Plant of continuous production of rolls of air-filled bubble film (22) weighing up to 500 g/m2, more preferably of light film weighing 20-30 g/m2, said film (22) comprising at least two superimposed films (21, 21'), one forming the base and the other forming the bubble, said plant consisting essentially of the following units, in sequence, an extrusion unit of the "cast extrusion" type, for monoextrusion or coextrusion, comprising two flat extrusion dies (40) and at least one extruder (10,20,30) located upstream of said flat extrusion dies (40) so as to form said film forming the base (21') and said film forming the bubble (21), a thermoforming station, comprising a non-heating forming cylinder (300), which is connected to a vacuum source and is provided with recesses adapted to thermoform said bubbles of said air bubble film, said forming cylinder (300) being immediately downstream of said two flat extrusion dies so as to couple said two films (21, 21') in molten form to obtain said air-filled bubble film (22), a series of stations of automatic cutting, rewinding of air bubble film in rolls and unloading of said obtained rolls, characterized in that at least one of the extruders (10,20,30) is a twin-screw extruder, said twin-screw extruder (10,20,30) imparting pressure to the melt polymer, at the outlet, allowing to fill parts of said plant placed immediately downstream of said twin-screw extruder, preferably allowing to continuously feed said two extrusion dies (40) and/or a respective gear pump (16), if present, located downstream of said twin-screw extruder and upstream of said extrusion dies (40).

2. The plant according to claim 1, wherein said twin-screw extruder (10,20,30) is provided with an end portion (L) for pumping the melt polymer wherein the free space between the cylinder and the screw profile of said twin-screw extruder occupying said portion (L) decreases approaching the outlet of said twin-screw extruder (10,20,30) so as to generate at the outlet of said twin-screw extruder (10,20,30) pressure allowing to fill said successive parts of the plant immediately downstream of said twin-screw extruder.

3. The plant according to any one of the preceding claims wherein the number of twin-screw extruders depends on the number of different polymers to be extruded.

4. The plant according to any one of the preceding claims wherein each flat die (40) is adapted to extrude each of the two films (21,21') forming the air bubble film (22) in the form of multilayer film.

5. The plant according to Claim 4 wherein each multilayer film (21, 21') is formed by five layers in the sequence A/B/C/B/A where A is a polyethylene, B is an adhesive polymer "tie-layer", C is a barrier polymer such as nylon (PA6).

6. The plant according to any one of the preceding claims wherein a feed-block distributor (20) is provided upstream of said two flat extrusion dies (40), adapted to divide the melt polymers outgoing from one or more of said extruders (10,20,30) in as many currents as the number of the layers in the final air bubble film (22).

7. The plant according to any one of the preceding claims, wherein the extrusion unit further comprises a gravimetric dosing system (15) for feeding the granules of one or more polymeric plastic materials (A,B,C) to the respective extruders (10,20,30).

8. A process for the continuous production of rolls of air bubble film (22) formed by at least two superimposed films (21,21'), one forming the base and the other 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 planar thin film (21, 21') through two flat extrusion dies (40) fed by one or more extruders (10,20,30) of granules wherein at least one of the extruders (10,20, 30) is a twin-screw extruder imparting pressure to the melt polymer, at the outlet, allowing to fill parts of the plant placed immediately downstream of said twin-screw extruder, thermoforming said planar film (21, 21') by coupling said molten film (21, 21), at a non-heating forming cylinder (300), which is connected to a vacuum source and is provided with recesses adapted to thermoform said bubbles of said air bubble film, so as to obtain a continuous air-filled bubble film (22).

9. A use of at least one twin-screw extruder (10,20,30) in the continuous production of rolls of continuous air-filled bubble film (22), starting from the extrusion of granules of one or more polymers using "cast extrusion".