PLANT FOR THE PRODUCTION OF PLASTIC FILMS TO BE THEN SUBJECTED TO A STRETCHING PROCESS AND RELATED METHOD

Abstract

A plant produces plastic films subjected to stretching. The plant includes a flat extrusion head made of polyethylene-based plastic material, first and second rollers forming a rolling press, and a third cooling/stabilizing roller. Molten plastic material/melt exiting the flat head passes through the first and second rollers before being directed onto the third cooling/stabilizing roller. The extrusion head has a width of 1,000-5,000 mm, the first and second rollers each have a diameter of 200-800 mm. The third roller has a diameter of 400-1,000 mm. The first and second rollers are made of ferrous material, with a conductive heat transfer coefficient of at least 15 W/(mK). The ferrous material is a construction steel with a chrome-plated and mirror-polished surface, having a roughness (Ra)<1 m, copper coatings, or coatings having thermal-conductivity and surface-roughness values corresponding to the rollers. The first and/or second roller has a deformable and non-stick coating.

Claims

1. A plant for production of plastic film subjectable to a stretching process, said plant comprising in succession: a flat extrusion head of polyethylene-based plastic material, at least a first roller and a second roller, arranged to form a rolling press, and at least a third cooling/stabilizing roller, wherein molten plastic material or melt exiting from the flat head is passed through said at least first roller and second roller facing each other and collaborating with each other before being directed onto said third cooling/stabilizing roller, wherein said flat extrusion head has a width dimension ranging from at least 1,000 mm up to 5,000 mm, said first roller of said rolling press has a diameter ranging from at least 200 mm to 800 mm, said second roller of said rolling press has a diameter ranging from at least 200 mm up to 800 mm, said at least a third cooling/stabilizing roller has a diameter ranging from at least 400 mm up to 1,000 mm, wherein said at least first roller and second roller are made of ferrous material, with a conductive heat transfer coefficient of at least 15 W/(mK), said ferrous material being a construction steel with a chrome-plated and mirror-polished surface, having a roughness (Ra)<1 m, copper coatings, or coatings having a conductive heat transfer coefficient of at least 15 W/(mK), and surface-roughness of (Ra)<1 m, wherein, at least one of said first roller and second roller comprises a coating of deformable and non-stick material.

2. The plant according to claim 1, comprising a pressure roller arranged downstream of said at least first roller and second roller, before said at least one third cooling/stabilizing roller, in contact with said cooling/stabilizing roller.

3. The plant according to claim 1, comprising one or two counter-pressure rollers is provided positioned for cooling, that are directly operating on said first roller.

4. The plant according to claim 1, wherein said one of said first roller and second roller comprises a coating of deformable and non-stick material comprising a coating of a silicon rubber.

5. The plant according to claim 1, wherein said one of said first roller and second roller comprises a coating of deformable and non-stick material in the form of a coating of composite materials, containing minerals that raise an operating temperature without compromising characteristics of hardness and high elastic recovery.

6. The plant according to claim 1, wherein said one of said first roller and second roller comprises a coating of deformable and non-stick material in the form of an external Teflon sock, with a thickness ranging from 0.5 mm to 5 mm.

7. The plant according to claim 1, wherein said one of said first roller and second roller comprises a coating of deformable and non-stick material having hardness values that range from 50 Sh up to 80 Sh.

8. A method for production of plastic film subjectable to a stretching process, said method being implemented in a plant that comprises in succession: a flat extrusion head of polyethylene-based plastic material, at least a first roller and a second roller, arranged to form a rolling press, and at least a third cooling/stabilizing roller, wherein said at least first roller and second roller are facing each other and collaborate with each other, wherein said flat extrusion head has a width dimension ranging from at least 1,000 mm up to 5,000 mm, said first roller of said rolling press has a diameter ranging from at least 200 mm to 800 mm, said second roller of said rolling press has a diameter ranging from at least 200 mm up to 800 mm, said at least third cooling/stabilizing roller has a diameter ranging from at least 400 mm up to 1,000 mm, wherein said at least first roller and second roller are made of ferrous material, with a conductive heat transfer coefficient of at least 15 W/(mK), said ferrous material being a construction steel with a chrome-plated and mirror-polished surface, having a roughness (Ra)<1 m, copper coatings, or coatings having a conductive heat transfer coefficient of at least 15 W/(mK), and surface-roughness values of (Ra)<1 m, wherein, at least one of said first roller and second roller comprises a coating of deformable and non-stick material; said method comprising: extruding at least 95% polyethylene-based plastic material from the flat extrusion head to form molten plastic material or melt exiting from the flat head passing said molten plastic material or melt exiting from the flat head into said at least first roller and second roller facing each other and collaborating with each other before being directed onto the at least a third cooling/stabilizing roller.

9. The method for the production of plastic film to be subsequently subjected to a stretching process according to claim 8, wherein at least 95% polyethylene is used.

10. The plant according to claim 1 wherein said one of said first roller and said second roller comprises a coating of deformable and non-stick material in the form of a coating of composite materials, containing silicon or other minerals that raise an operating temperature without compromising characteristics of hardness and elastic recovery.

Description

[0047] The structural and functional features of the present invention and the advantages thereof over the prior art will become even more apparent from a discussion of the following description, also referring to the accompanying diagrammatic drawings, which show an embodiment of the invention itself. In the drawings:

[0048] FIG. 1 diagrammatically shows a first generalized embodiment of a plant according to the invention for the production of plastic films to be then subjected to a stretching process which is also capable of implementing the method of the invention;

[0049] FIGS. 2 and 3 show summary diagrams of second embodiments of a plant according to the invention;

[0050] FIGS. 4 and 5 show summary diagrams of third embodiments of a plant according to the invention;

[0051] FIGS. 6 and 7 show summary diagrams of fourth embodiments of a plant according to the invention.

[0052] In the following description, in order to illustrate the figures, the same reference numerals are used to indicate constructional elements with the same function. Moreover, for clarity of illustration, some reference numerals cannot have been repeated in all figures.

[0053] Indications such as vertical and horizontal, upper and lower (in the absence of other indications) must be read with reference to the assembly (or operating) conditions and referring to the normal terminology in use in current jargon, where vertical indicates a direction substantially parallel to that of the gravity force vector g and horizontal indicates a direction perpendicular thereto.

[0054] With reference to the figures, which are illustrative and non-limiting, various embodiments of a plant for the production of plastic films to be then subjected to a stretching process according to the invention are shown.

[0055] The concept underlying the suggested plant is that the presence of a small or large amount of air between the melt and the casting roller is the real limitation inherent in cast technology, i.e., it will never be possible to have sufficient control over the melt cooling process if such a random condition is relied upon.

[0056] It is worth emphasizing that such a limitation arises if the film thus produced must be then subjected to a stretching operation, where any morphological differences in the product are emphasized, effectively making the final film unusable.

[0057] Needless to say, for the production of films that do not need to be subjected to subsequent stretching operations, on the other hand, the quality ensured by the current casting process is absolutely adequate for the market needs.

[0058] The solution suggested by the present invention is as simple as it is effective; indeed, it aims at performing a sort of rolling of molten plastic material (melt), i.e., implementing the passage through a nip created between two facing and cooperating rollers, so as to avoid air entrapment between the molten plastic material or melt and the rollers themselves.

[0059] It is thus possible to effectively manage the heat transfer between the elements involved with given values not affected by an amount of air that would otherwise be very difficult to measure. Obviously, the dimensional and geometric features of such rollers must be well defined so as to ensure the possibility of adequately handling a molten plastic material that must be maintained in a non-solid state for as long as possible.

[0060] In particular, with reference to FIG. 1, in the first embodiment of the plant, a series of elements, which are indicated below, is provided and arranged in sequence.

[0061] Firstly, there is a flat extrusion head 10, the width dimension of which can vary preferably but not exclusively between about 1000 mm up to about 5000 mm.

[0062] A first rolling press roller 11 then follows, the diameter of which can vary preferably but not exclusively from about 200 mm up to about 800 mm.

[0063] The first rolling press roller 11 cooperates with a second rolling press roller 12, the diameter of which can vary preferably but not exclusively from about 200 mm up to about 800 mm. Such an arrangement creates a passage through a nip obtained between the two facing and cooperating rollers, which advantageously allows avoiding the entrapment of air between the molten plastic material or melt exiting the flat extrusion head 10 and the rolling press rollers 11 and 12 themselves which will form a film 15.

[0064] In the embodiment shown in FIG. 1, the presence of a cooling/stabilizing roller 13 in the sequence is noticed, the diameter of which can vary preferably but not exclusively from about 400 mm up to about 1000 mm, and on which the film formed between the two rolling press rollers 11 and 12 is wound. It is worth noting that a pressure roller 14 is also present in this sequence, which cooperates in the ingress and arrangement of the film 15 exiting from the two rollers 11 and 12 onto the third cooling/stabilizing roller 13.

[0065] According to the invention, special attention must be paid to the construction and surface finish, especially of the rolling press rollers 11 and 12, which are the ones that give the film its optical and flatness qualities.

[0066] Since the maximum heat transfer must be ensured in order to accurately manage the cooling process, the rolling press roller 12 should preferably (but not exclusively) be made of ferrous material, which has an indicative conductive heat transfer coefficient of at least 15 W/(mK).

[0067] Typical examples can be structural steels with chrome-plated and mirror-polished surfaces, or with intrinsically highly low roughness (Ra<1 m), copper fillers, or others.

[0068] Even more important and delicate is the configuration of the rolling press roller 11, which should fulfill multiple purposes, such as: [0069] ensuring the correct temperature control of the film face in contact therewith, [0070] ensuring an even contact meniscus between the two rollers, which is fundamental to having an even film thickness, [0071] having non-stick properties to avoid the melt from sticking to the surface thereof, [0072] having surface finish features adapted to not score or damage the film, i.e., ensuring proper surface flatness.

[0073] All these features could be met using a rolling press roller 11 with similar features to those of the rolling press roller 12 (i.e., characterized by a construction of ferrous material and a chrome-plated, mirror-polished surface, for example) if the thickness of the film to be produced were significant (preferably but not exclusively over 500 m) so that it could be rightly considered as a slab.

[0074] Regretfully, however, the reference thicknesses of films suitable for the applications mentioned above (such as BOPET film replacement) are about 20-30 m after stretching; while considering a stretching ratio of up to about 6:1, which for the products of interest herein is practically the upper limit, it means producing a pre-MDO film of about 180-200 m at most.

[0075] Indeed, as mentioned earlier, a thickness considered as acceptable by the market for a MOPE film that could advantageously replace (i.e., with similar mechanical features such as not compromising the final result of the complete package) the current BOPET film should have a thickness of at least 20-30 m; considering that the maximum stretch ratio to which, according to current technological and chemical knowledge, polyethylene-based films can be subjected is not more than 6:1, it results that the maximum reference thickness for the primary film should be about 306=180 microns, possibly even less since the general trend is to minimize the thickness of such a type of film.

[0076] For this kind of thickness, the concept of rolling between two rigid rollers becomes difficult to apply, due to the dimensional tolerances of the rollers dangerously becoming of the same order of magnitude as the film to be produced. Indeed, with the rolling between two rigid rollers, the contact meniscus is substantially reduced to a line, therefore the ensured thickness uniformity is provided by the presence of the so-called buildup of molten material above such a joining line.

[0077] Clearly, the thinner the film to be produced, the more delicate the balance of such a situation, and the unevenness of the film that must be then subjected to a stretching process is unfortunately amplified by the process itself, resulting in a nonconformity of the final product.

[0078] A solution suggested by this invention is to use a rolling press roller 11 with a coating of deformable, non-stick material, which can thus greatly increase the contact area (or meniscus) between the two rolling press rollers and thus compensate for any unevenness in the buildup.

[0079] This is all because the present invention aims at applying such a concept in the production of a film that must then be subjected to a stretching process (preferably but not exclusively MD). Incidentally, the stretching process can take place either immediately downstream of the rolling press unit (typical in-line configuration) or even on a different production plant at a later time (typical off-line configuration).

[0080] The coating of such a rolling press roller 11 can thus be of various kinds, such as: [0081] a coating of silicone rubber, suitable for working at high temperatures and especially having a remarkable elastic recovery even under hot working conditions, [0082] a coating of composite materials, containing silicon or other minerals, for example, which raise the operating temperature thereof without compromising the low hardness and high elastic recovery properties thereof, [0083] an outer Teflon sleeve, with a thickness typically but not exclusively varying between 0.5 mm and 5 mm, which ensures thermal resistance and non-stick properties suitable for the process.

[0084] Typical hardness values for a solution like that suggested can range from 50 Sh up to 80 Sh, although there is still the possibility of using significantly lower or higher hardnesses as well (said range is simply shown as the most suitable for the application). The mechanical characterization of the production plant can obviously take different embodiments from those shown in FIG. 1, since it is possible to provide embodiments that in some ways follow similar arrangements to those of current rolling presses.

[0085] For example, FIGS. 2 and 3 briefly show second embodiments of a plant according to the invention.

[0086] FIG. 2 shows a rolling press arrangement comprising three rolling press rollers 11, 12, 113, the last rolling press roller 113 also having a cooling/stabilizing function. The three rollers 11, 12, 113 are arranged along a common vertical axis, one on top of the other.

[0087] FIG. 3 shows a rolling press arrangement also comprising three rolling press rollers 11, 12, 113, the last rolling press roller 113 also having a cooling/stabilizing function with horizontal flat extrusion head 10. The three rollers 11, 12, 113 are arranged according to a common horizontal axis, placed side by side with a vertical flat extrusion head 10.

[0088] FIG. 4 shows a rolling arrangement similar in all respects to that in FIG. 2, in which one or two cooling counter-pressure rollers 16, 16 directly operating on the first roller 11 are further located.

[0089] FIG. 5 shows a rolling arrangement similar in all respects to that in FIG. 3, in which one or two cooling counter-pressure rollers 16, 16 directly operating on the first roller 11 are also further located.

[0090] The cooling counter-pressure rollers 16, 16 can also have a construction typically (but not exclusively) of a ferrous nature and with chrome-plated and polished surfaces that ensure proper heat removal from the roller 11 by means of direct contact therewith.

[0091] FIGS. 6 and 7 compared with FIGS. 4 and 5 involve the placement of an additional roller 17 to make a four-roller rolling press assembly 11, 12, 113, and 17 which develops the advantages of the present invention.

[0092] Likewise, it is possible to use the pressure roller 14 shown in FIG. 1 or not, depending on the final configuration of the production line.

[0093] The object mentioned in the preamble of the description is thus achieved.

[0094] The scope of protection of the present invention is defined by the appended claims.