METHOD AND APPARATUS FOR MAKING PLASTIC FILM

Abstract: A method and apparatus for making a plastic film wherein an annular film preform is extruded by an extruder (1). The extruder (1) comprises a nozzle (6) for spreading and flattening the annular film preform into a flat film (5).

Description: 1 METHOD AND APPARATUS FOR MAKING PLASTIC FILM The invention relates to a method for making a plastic film, the method comprising extruding an annular plastic film preform by an extruder. The invention also relates to an apparatus for making a plastic film, the apparatus comprising an extruder for extruding an annular plastic film preform. US patents 3,244,781 and 3,891 ,374 and EP publication 0,182,764 illustrate apparatuses for making a plastic film. These solutions comprise extruding an annular plastic film preform and orientating it thereafter by means of a mandrel or air blast and then flattening it for example by means of rolls into a flat plastic film. However, this method of making a flat piastic film is complicated and difficult, and the apparatus needed is expensive. The object of this invention is to provide a simple and effective method and apparatus for making a flat plastic film. The method of the invention is characterized in that the extruder comprises a nozzle for spreading the annular film preform and flattening it into a flat film. The apparatus of the invention is characterized in that the extruder comprises a nozzle for spreading the annular film preform and flattening it into a flat film. The basic idea of the invention is that an annular film preform is extruded by an extruder, and the extruder comprises a nozzle for spreading and flattening the annular film preform into a flat film. The idea of a preferred embodiment is that the nozzle of the extruder is arranged to widen up to the end portion of the extruder. An advantage of the invention is that flat films can be produced both very simply and effectively. When the nozzle of the extruder is arranged so that it widens up to the end portion of the extruder, the plastic to be extruded is subjected to cross-direction orientation all the time in addition to longitudinal orientation, which makes it considerably easier to orientate the plastic film in the cross-direction at a later processing stage. The invention will be described in greater detail in the following drawings, in which Figure 1 is a schematic cross-sectional side view of an apparatus of the invention, Figure 2 is a partially cross-sectional top view of the apparatus illustrated in Figure 1 , Figure 3 is a cross-sectional view of a detail of the apparatus illustrated in Figure 1 along line A-A, Figure 4 is a cross-sectional view of a detail of the apparatus illustrated in Figure 1 along line B-B, Figure 5 is a cross-sectional view of a detail of the apparatus illustrated in Figure 1 along line C-C, and Figure 6 is a schematic cross-sectional top view of an extruder used in the apparatus of the invention. Figure 1 is a side view of an apparatus according to the invention. The apparatus comprises an extruder 1. The extruder is preferably conical, i.e. it comprises a cone-shaped rotor 2, outside of which there is an outer stator 3 whose surface at least on the rotor 2 side is cone-shaped, and inside the rotor there is an inner stator 4 whose surface at least on the rotor 2 side is cone- shaped. In that case there is an annular feed gap in the shape of a tapering cone inside and outside the rotor. When the rotor 2 rotates, it extrudes material which is between the rotor 2 and the stators 3 and 4 from the extruder 1 in a manner known per se. The basic structure of such an extruder is disclosed e.g. in EP patent 0,422,042. For the sake of clarity the figures do not illustrate e.g. the rotating means of the rotor or the feeding devices for feeding the material to be extruded into the extruder 1. The extruder 1 may comprise more than one rotor 2 and more than two stators 3 and 4. In that case the extruder 1 can be used for extruding multilayer products. The solution with one rotor 2 and two stators 3 and 4 can be used for making two-layer products. The end portion of the inner stator 4 is wide and tapers in the vertical direction so that together with the nozzle 6 it forms a relatively flat and wide gap through which the plastic 5a is extruded. The inner stator 4 may be made of two parts, for example, in which case the end portion of the inner stator is attached to the front portion. After the nozzle 6 there is a calibration piece 7 whose nuts are used for adjusting the height of the gap, which allows to define the thickness of the plastic film 5 to be obtained from the extruder 1. After the extruder 1 the plastic film 5 is cooled by a cooling device 8. The cooling device 8 may comprise e.g. a cooling roll 9, which is arranged in a cooling tank 10 containing a cooling medium, e.g. water. The plastic film 5 is arranged to be pressed against the cooling roll 9. The apparatus according 3 to Figure 1 uses auxiliary rolls 11 for guiding the plastic film 5 at several points. After cooling the plastic film 5 is guided to a machine direction orientation device 12. The machine direction orientation device 12 comprises orientation rolls 13 whose velocities are adjusted so that they can be used for stretching the plastic film 5 and thus for orientation in the machine direction. If desired, the velocity of each orientation roll 13 can be adjusted separately. The machine direction orientation device 12 may also comprise heating means 14, such as radiation heaters, for heating the plastic film 5 in a manner known per se. The orientation rolls 13 can also be used for heating the plastic film by supplying a heating medium, such as heated oil, to the orientation rolls 13 so that the orientation rolls 13 become warm. If desired, the temperature of each orientation roll 13 can be adjusted separately. The orientation rolls are arranged in a discharge chamber 15. Pressurized gas, preferably air, is fed into the discharge chamber 15 along a gas supply pipe 16. Instead of air, nitrogen or another gas or gas mixture, for instance, may be used as the gas to be fed. The gas to be fed may also be selected according to the desired electric properties. For example, in respect of the preformlectric strength of the product it would be advisable to use sulphurhexafluoride SF 6 and in respect of chargeability e.g. argon. The pressure of the gas to be fed is relatively small compared to the typical foaming methods, being preferably about 10 bars, but it may vary between 3 and 20 bars, for instance. Suitable material, such as calcium carbonate particles, is mixed into the plastic 5a of the plastic film 5, and due to the influence of the particles the joint surfaces of the plastic molecules and the mixed material are torn during orientation, and thus cavitation bubbles are formed. When orientation is performed by arranging pressurized gas to act on the plastic film 5, the gas in question diffuses in the cavitation bubbles and causes overpressure in the bubbles. In the discharge chamber 15 the pressurized gas can act on both sides of the plastic film 5, and thus gas bubbles are formed evenly in the plastic film 5. The discharge chamber 15 is sealed at the entry and exit of the plastic film 5 in a manner known per se. After the machine direction orientation device 12 the plastic film 5 is supplied to a cross-direction orientation device 17. In the cross-direction orientation device 17 the plastic film 5 is stretched in the cross-direction, i.e. orientation is performed in the direction substantially perpendicular to the direction of the orientation performed in the machine direction device 12. Due 4 to the overpressure of the gas in the bubbles and cross-direction stretching the bubbles can grow sideways and also to some extent in the vertical direction in the cross-direction orientation device 17. In that case the foaming degree of the film is for example about 70 to 90%. The foaming degree can be adjusted simply by adjusting the pressure of the gas to be fed into the discharge chamber 15. The cross-direction orientation device 17 comprises two orientation wheels 18, and an orientation band 19 is arranged against both of the wheels. The orientation band 19 is an endless band which is guided by means of band guide rolls 20. The edges of the plastic film 5 are arranged between the orientation wheel 18 and the orientation band 19. The orientation band 19 presses the edges of the plastic film 5 firmly and evenly between the orientation wheel 18 and the orientation band 19 substantially along the whole travel the cross-direction orientation device 17, in which case the film is not subjected to varying pressure stress or tensile strain, and thus the plastic film stretches sideways without tearing. In Figure 1 the plastic film 5, orientation wheel 18 and orientation band 19 are illustrated at a distance from one another for the sake of clarity, but in reality these parts are pressed firmly against one another. The orientation wheels 18 and the orientation bands 19 are arranged so that in the direction of the plastic film they are further away from one another at the end than at the beginning, as is illustrated in Figure 2, and thus the cross-direction orientation device 17 stretches and simultaneously orientates the plastic film 5 in the cross-direction. The deviation of the angle between the orientation wheels 18 and the orientation bands 19 from the machine direction can be adjusted according to the desired degree of cross-direction stretching. One or more band guide rolls 20 can be arranged to be rotated by the rotating means. Since the bands 19 are firmly pressed against the orientation wheels 18, the orientation wheels 18 do not necessarily need rotating means but may rotate freely. For the sake of clarity the enclosed figures do not illustrate rotating means or other actuators of the apparatus. A curved support plate 21 , which has substantially the same shape as the circumference of the orientation wheels 18, ' is arranged between the orientation wheels 18 to support the plastic film 5. The cross-direction orientation device 17 can be placed in a casing 26 of its own. If desired, the casing 26 can be provided with heaters known per se, such as radiation heaters, to heat the plastic film 5. After the cross-direction orientation device 17 the plastic film 5 is led to a relaxation unit 22. In the relaxation unit 22 the plastic film 5 is relaxed, and thus the plastic film shrinks a bit in a manner known per se. Finally, the plastic film 5 is wound on a reel 23. Figure 2 is a cross-sectional top view of the apparatus of the invention at the extruder 1. For the sake of clarity Figure 2 does not illustrate the plastic film 5 or the support structures of the apparatus onto which the rolls, reels and plates of the apparatus are attached, for instance. Figure 3 is a cross-sectional view of a detail of the extruder 1 along line A-A of Figure 1. Here both the outer stator and the inner stator 4 are round in cross-section. Thus the plastic material 5a is also in an annular feeding channel, i.e. at this stage the extruder forms an annular plastic film preform directly from the material without any intermediate steps. Figure 4 is a cross-sectional view of a detail of the extruder 1 along line B-B of Figure 1. Here we see the wide tip of the inner stator 4 and the shape of the nozzle 6 which extrude the plastic 5a into the wide and flat gap, and thus a flat plastic film 5 is formed from the annular plastic film preform. If the first plastic material 5a fed outside the rotor 2 differs from the second plastic material 5a' fed inside the rotor 2, a two-layer annular plastic film preform is obtained first and thereafter a multilayer plastic film 5. Figure 5 is a cross-sectional view of a detail of the cross-direction orientation device 17 along line C-C of Figure 1. It is seen in Figure 5 how the orientation wheel and the orientation band are pushed against each other and press the plastic film 5 between each other. The surface of the support plate 21 against the plastic film 5 may be heated e.g. by providing it with heating resistors, and thus the plastic film 5 slides along the sliding surface in question very easily. Furthermore, propellant, such as air, can be blown from the support plate 21 through the gaps 21a, in which case the propellant flowing through the gaps 21 a provides a sliding bearing between the support plate 21 and the plastic film 5. The gas in question may be heated, if desired, and thus the sliding surface of the support plate 21 and the plastic film 5 are heated with the propellant flowing through the gaps 21 a. Figure 6 illustrates an extruder 1 used in the apparatus according to the invention. The nozzle 6 of the extruder 1 widens up to the end portion of the extruder, i.e. up to the point where the plastic film 5 exits from the extruder 1. In the nozzle 6 of the extruder 1 the plastic 5a is thus subjected to cross- direction orientation all the time in addition to longitudinal orientation, which makes it considerably easier to orientate the plastic film in the cross-direction direction at a later processing stage. The plastic film 5 can be used for several purposes in a manner known per se. At least one surface of the plastic film 5 can be provided with an electrically conductive coating, for instance, in which case the solution can be used e.g. as a microphone or loudspeaker in several acoustic applications, including sound attenuation. The plastic film 5 may also be permanently electrically charged. The drawings and the related description are only intended to illustrate the inventive concept. The details of the invention may vary within the scope of the claims.

Claims: CLAIMS 1. A method for making a plastic film, the method comprising extruding an annular plastic film preform by an extruder (1), characterized in that the extruder (1) comprises a nozzle (6) for spreading and flattening the annular film preform into a flat film (5). 2. A method according to claim ^ characterized in that the film (5) is spread up to the end portion of the extruder (1) by the nozzle (6). 3. A method according to claim 1 or 2, characterized in that the thickness of the plastic film (5) is adjusted by adjusting the height of the gap in the calibration piece (7) of the extruder (1 ). 4. A method according to any one of the preceding claims, characterized in that at least a two-layer annular plastic film preform is extruded by the extruder (1 ), and the preform is flattened into a multilayer film (5). 5. A method according to any one of the preceding claims, characterized in that the extruder (1) comprises at least one rotor (2) and at least one stator (3, 4), and there is an annular feed gap in the shape of a tapering cone between the rotor (2) and the stator (3, 4). 6. An apparatus for making a plastic film, the apparatus comprising an extruder (1 ) for extruding an annular film preform, characterized in that the extruder (1) comprises a nozzle (6) for spreading and flattening the annular film preform into a flat film (5). 7. An apparatus according to claim 6, characterized in that the nozzle (6) is arranged to widen up to the end portion of the extruder (1). 8. An apparatus according to claim 7, characterized in that the apparatus comprises a calibration piece (7) arranged in the extruder (1 ) for adjusting the thickness of the film (5) by adjusting the height of the gap in the calibration piece (7). 9. An apparatus according to any one of claims 6 to 8, characterized in that the extruder (1 ) is arranged to extrude at least a two-layer annular film preform from which a multilayer film (5) is made. 10. An apparatus according to any one of claims 6 to 9, characterized in that the extruder (1 ) comprises at least one rotor (2) and at least one stator (3, 4), and at least their surfaces against each other are 8 cone-shaped so that an annular feed gap in the shape of a tapering cone is formed between them.

Inventors: KIRJAVAINEN KARI, KIRJAVAINEN, KARI
Applicants: KIRJAVAINEN KARI, NEXTROM HOLDING SA, KIRJAVAINEN, KARI, NEXTROM HOLDING S.A.
Application year: 1999
Main classification: B29C47
IPC codes: B29B11, B29C47, B32B37