Plant comprising a twin-screw extruder for the continuous production of rolls of plastic stretch film

Abstract

A twin-screw extruder is configured to produce rolls of plastic stretch film on a continuous basis, starting from the extrusion of granules of one or more polymers an using cast extrusion.

Claims

1. A plant (100) for continuous production of rolls of plastic stretch film (50) starting from granules of one or more polymeric plastic materials, said plant comprising: an extrusion unit (80) that is a cast extrusion unit, in monoextrusion or in coextrusion, and that comprises a flat extrusion die (6) and one or more extruders (2,3) positioned upstream of said flat extrusion die (6), to form a melt of said one or more polymeric plastic materials; a solidification unit of said melt comprising at least one chill-roll (7) with a continuous surface, positioned immediately downstream of said flat extrusion die (6) so as to solidify said melt, by cooling, to obtain said plastic stretch film (50); and one or more winding units of the plastic stretch film into rolls, adapted to discharge said rolls after said rolls are produced, wherein at least one of the extruders (2,3) of said one or more polymeric plastic materials is a twin-screw extruder (2), and wherein said twin-screw extruder (2) supplies a respective amount of the melt of the one or more polymeric plastic materials with a pressure, at an outlet mouth, so as to fill parts of said plant positioned immediately downstream of said twin-screw extruder (2).

2. The plant according to claim 1, wherein said twin-screw extruder (2) is provided with a final metering section (L) of the respective amount of the melt, and wherein a free volume between a cylinder and a profile of screws of said twin-screw extruder (2) that occupy said section (L) decreases towards the outlet mouth of said twin-screw extruder (2) so as to generate the pressure at the outlet mouth of said twin-screw extruder (2) which is such as to fill said parts of the plant positioned immediately downstream of said twin-screw extruder (2).

3. The plant according to claim 1, wherein, when the plastic stretch film (50) is multilayer, a feed-block distributing device is provided upstream of said flat extrusion die (6), adapted to divide melts of said one or more polymeric plastic materials leaving said extruders (2,3) into a same number of streams as a layers forming the stretch film (50).

4. The plant according to claim 1, wherein the extrusion unit (80) comprises at least one gravimetric dosing system (1) configured to feed granules of the one or more polymeric plastic materials to the one or more extruders (2,3).

5. The plant according to claim 1, further comprising a measuring unit of a thickness of the plastic stretch film (50) after solidification, positioned downstream of a solidification station.

6. The plant according claim 1, further comprising an oscillation unit, positioned immediately upstream of said one or more winding units of the plastic stretch film (50) into rolls.

7. A process for continuous production of rolls of plastic stretch film (50) starting from extrusion of granules of one or more polymeric plastic materials, said process comprising: extruding by cast extrusion said stretch film (50) through a flat extrusion die (6) fed by one or more extruders (2,3) of said granules, wherein at least one of the extruders (2,3) is a twin-screw extruder (2) which supplies the one or more polymeric plastic materials in molten state, with a pressure, at an outlet mouth, which is such as to fill parts of equipment positioned immediately downstream of said twin-screw extruder (2); cooling and solidifying said molten plastic film with at least one chill-roll (7) so as to obtain a solidified plastic stretch film (50); optionally, controlling a thickness of the solidified stretch film (50) to keep said thickness constant with respect to a width of the stretch film (50); and optionally, treating the stretch film (50) by oscillation, in order to improve an aesthetical appearance of the rolls of the plastic stretch film (50).

8. (canceled)

9. The plant according to claim 1, wherein the pressure is such to continuously feed one or both of said flat extrusion die (6) or a respective gear pump (5), if present, positioned downstream of said twin-screw extruder (2) and upstream of said extrusion die (6).

Description

[0073] Further characteristics of the invention will appear more evident from the following detailed description, referring to a purely illustrative and therefore non-limiting constructive form of an embodiment illustrated in the enclosed drawings, in which:

[0074] FIG. 1 is a side view of the complete plant for the production of a stretch film according to the invention;

[0075] FIG. 2 is a plan view of the extrusion section alone of the plant illustrated in FIG. 1;

[0076] FIG. 3 is an enlarged view of the cooling station inside the rectangle indicated with A in FIG. 1;

[0077] FIGS. 4a-4d are side views of the three sections a), b) c) respectively, that form each screw of the twin-screw extruder, including an overall view (4d) of the screw according to the invention.

[0078] In the following description, every reference to stretch film should be considered as referring to any plastic stretch film.

[0079] FIG. 1 schematically illustrates the complete plant for the production of a plastic stretch film starting from granules, indicated as a whole with the reference number 100.

[0080] Said plant 100 comprises, in sequence, an extrusion unit, a cooling and solidification station of the molten film, a control unit of the thickness, an oscillation unit, a winding unit into rolls including cutting means and weighing means, a discharge and expulsion unit of the rolls produced.

[0081] The extrusion unit, indicated as a whole with reference number 80 (FIG. 2), is positioned upstream with respect to the other units, and in this embodiment, it is positioned at a greater height with respect to the other sections of the plant 100.

[0082] Said extrusion unit 80 comprises one or more gravimetric dosing systems 1 for feeding, with granules of polymeric plastic materials, two respective extruders 2 and 3, both positioned downstream of said dosing systems 1.

[0083] In particular, the granules are sucked from a storage and then suitably dosed for feeding said extruders 2, 3 so as to have a prefixed weight ratio between the different polymeric materials.

[0084] At least two of the two extruders 2, 3 must be a twin-screw extruder 2.

[0085] Even if this is not binding for the purposes of the present invention, two extruders are provided in the plant 100 illustrated in FIG. 1, to allow a two-layer stretch film to be obtained, for example B and C, one of which is generally and preferably an adhesive layer capable of sticking to itself and around the objects on which it is wound.

[0086] The thickness of said stretch film is therefore composed, microscopically, of two thin layers B and C. For this type of embodiment, one of the two extruders 2,3 is fed with a percentage of materials having a high adhesion capacity such as low and very low density linear polyethylenes (VLLDPE-ULLDPE) and/or various kinds of elastomers, whereas the other extruder is fed with granules of a different polymeric material.

[0087] The twin-screw extruder 2 has an end section L (corresponding to the pumping section c of FIG. 5) having a length within the range of 815 times the screw diameter (D).

[0088] It should be noted, however, that in the single-screw extruders currently used in production plants of stretch film with an L/D ratio typically ranging from 28 to 32, where the geometry is designed so as to allow a high pressurization of the melt moved, the pressure at the outlet and above all the stress of the material are much greater.

[0089] The granules of polymer are mixed, pressed and then melted inside the twin-screw extruder 2, exiting in the form of a melt from the outlet mouth of the twin-screw extruder 2 for feeding a screen changer 4, and subsequently a dosing gear pump 5 (FIG. 2) which sends the respective melt to the flat extrusion die 6, possibly passing first, in the case of multilayer film, through a feed-block distributing device already known per s in the film coextrusion technology, whose function is to divide the streams of melt leaving the extruders into the same number of streams as the layers of multilayer stretch film, so as to distribute the various streams of melt in a predetermined sequence of layers being fed to the flat extrusion die.

[0090] After the two-layer melt has left the respective flat die 6, it passes to the typical solidification section of stretch film: in this step, the melt passes to a cooled stretch film so as to be solid, indicated with reference number 50 in FIG. 3.

[0091] The cooling and solidification unit of the melt indicated in FIG. 1 inside the box A, comprises (FIG. 3) a first chill-roll or cooling cylinder 7 positioned immediately downstream of the flat extrusion die 6, and it can comprise a second chill-roll or cooling cylinder 8 for cooling the other side of the film that has not come into contact with the first chill-roll 7.

[0092] Said chill-rolls 7,8 have a continuous smooth surface, without bumps, recesses or the like, in order to guarantee surface uniformity of the solidified film. Furthermore, each of said chill-rolls 7,8 is thermoregulated by means of water cooling.

[0093] Each of said chill-rolls 7,8 cooperates with one or more idle rolls in order to ensure the complete contact of the film with the respective chill-roll 7,8 in order to avoid non-uniformity of the surface and/or damage to the film.

[0094] In correspondence with the contact point of the melt with the first chill-roll 7, a suction chamber is provided, which suitably generates a depression between the smooth surface of the first chill-roll 7 and the molten film, while the chill-roll 7 is rotating, preventing the formation of air bubbles that can jeopardize the correct contact between the molten plastic and the roll, thus ensuring the immediate solidification of the film as soon as the latter, in the molten state, comes into contact with the first chill-roll 7.

[0095] The chill-roll 7 that rotates at a high rate, also acts as a stretching element of the molten film leaving the mouth of the flat die, the lesser the final thickness required, the more the film is stretched.

[0096] Downstream of the above cooling and solidification section, the following units are preferably present, each already known per s in the state of the art: [0097] a measuring station of the thickness 11, known in itself in the state of the art, which sends signals to the thermal plugs 9 positioned on the flat extrusion die for regulating the thickness of the stretch film; [0098] a sliding or oscillation group 12 whose function is to level the surface of the solidified stretch film if there are sections of surface outside the thickness (cordons) that could cause undulations in the roll resulting from the winding of the film.

[0099] As indicated above, the regulation of the thickness of the stretch film is effected with methods already known per s in the art, for example by means of a closed ring feedback control: at the moment X, the thickness of the solidified film is read and on the basis of this reading a correction is effected on the thickness of the film that is leaving the extrusion die.

[0100] In addition, the following units, already known in the art, are present as essential units downstream of the cooling section: [0101] a winding unit 13 comprising cutting and weighing means for producing rolls having various weights, formats, dimensions, lengths, types; [0102] a roll discharge unit 14.

[0103] The plant is also equipped with an electric panel and control board, already known per s in the art in plants for the production of stretch film, in addition to the normal plant utilities (water pumping, management of side trimmings).

[0104] With reference to the twin-screw extruder used in the present plant, this preferably has a screw configuration as illustrated in FIGS. 4a)-4d), in particular when it is fed with PE granules.

[0105] Each screw is advantageously composed of three distinct sections a), b), c).

[0106] The screws are composed of elements, the elements with threadings are indicated herein with reference numbers 20, 30, 40, whereas the mixing elements are indicated with reference numbers 60 and 70.

[0107] The first section a) is dedicated to transporting the material, the second section b) to mixing and the third section c) is destined for compressing and pumping the material and corresponds to the section indicated above with the initial L.

[0108] The diameter of the screws and cylinder remain constant along said section L: the thickening of the screw pitch in said section L ensures that the free volume between the cylinder and the screws decreases gradually towards the outlet, causing an increase in pressure.

[0109] In a preferred embodiment, the pumping length is equal to about 13 diameters, i.e. a much shorter length than that of a single-screw extruder.

[0110] The present invention is not limited to the particular embodiments previously described and illustrated in the enclosed drawings, but numerous modifications of detail can be applied, available to skilled persons in the field, without being excluded from the scope of the invention itself, as defined in the enclosed claims.