WO2021069474A1

WO2021069474A1 - Blown film installation and method for producing tubular films or film webs

Info

Publication number: WO2021069474A1
Application number: PCT/EP2020/078068
Authority: WO
Inventors: Martin Backmann; Henning Voss
Original assignee: Windmöller & Hölscher Kg
Priority date: 2019-10-09
Filing date: 2020-10-07
Publication date: 2021-04-15
Also published as: DE102019215492A1; EP4041524A1

Abstract

The invention relates to a blown film installation for producing tubular films or film webs, comprising a nozzle head for extruding a tubular film, a lay-flat implement and/or a squeezing device for laying flat and/or squeezing the tubular film in order to produce a double-layer film web from the tubular film, and a stretching device, by which the double-layer film web can be stretched in the longitudinal direction. At least a first and at least a second thickness measuring device for measuring the thickness of layers of the film web are provided downstream of the stretching device, wherein the thickness of a single layer of the film web can be measured by means of at least one of the thickness measuring devices.

Description

Blown film line and process for the production of film tubes or film webs

The invention relates to a blown film system for producing film tubes or film webs and a method for producing film tubes or film webs.

A plastic film is often produced on blown film systems in which a nozzle head is provided which has a nozzle gap which is in particular circular. A plastic melt is pressed or extruded from this nozzle head, so that a film tube is created. One of the devices following in the transport direction of the film tube is a flattening device and / or squeezing device, with which the film tube can be flattened, so that a double-layer film web is created from the circular film tube, the two layers in particular being connected to one another along their longitudinal edges. The invention also relates to such

Blown film systems which comprise a stretching unit with which the double-layer film web can be stretched in its transport direction. By stretching, the properties of the films (strength, permeability for gases, liquids and particles) can be specifically influenced. Such a stretching unit is often referred to as an MDO (Machine Direction Orientation).

EP 2 277 681 A1 discloses such a blown film line. The invention described there has set itself the goal of increasing the usable width of the film tube so that less waste is produced. this happens in that at the points at which a thickening occurs within the stretching device due to the so-called neck-in, thin points are produced in advance, so that the desired film thickness is created here, which can then be used. Nevertheless, a considerable part of the film has to be cut off - the so-called edge trim - which is then usually waste. Apart from the resulting waste costs, this is in particular not very environmentally friendly. In addition, the quality of the films produced in this way can be restricted due to fluctuations in thickness. The object of the present invention is therefore to propose a blown film plant and a method with which the usable width of the film web can be increased again in order to further reduce the cost of filling and the amount of waste. According to the invention, this object is achieved by all of the features of claim 1. Possible configurations of the invention are specified in the dependent claims.

According to the present invention it is provided that a first and at least one second thickness measuring device are provided downstream for measuring the thickness of layers of the film web. It is provided that the thickness of an individual layer of the film web can be measured with at least one of the thickness measuring devices. A thickness measuring device is to be understood as a device with which a thickness profile of a film web can be created in a direction which is at least partially transverse to the transport direction of the film web. Correspondingly, a thickness is to be understood as a thickness profile in which locations in the transverse direction are each assigned a thickness measurement value. For example, such a thickness measuring device can comprise a thickness measuring head traversing in the transverse direction of the film web, a thickness measurement being able to be carried out after a short movement of the measuring head so that the thickness profile already described can be generated. In those cases in which the film web is moved further during the measuring process, an inclined thickness profile is created thus not only includes a transverse component, but also a longitudinal component, which, however, is usually not taken into account.

According to the invention, not only a thickness measuring device is provided - as disclosed in the prior art - but a second thickness measuring device, at least one of the thickness measuring devices measuring a single layer of the film web. In the prior art, only the thickness of the double-layer film web is measured, so that, for example, a thick point and a thin point that lie directly on top of one another are not recognized as such. As a result, it can happen that in applications in which the double-layer film web are separated into two single-layer film webs, defects occur which then ultimately lead to rejects. This is avoided by the present invention. The measurement of a single layer can be carried out in different ways, which are discussed in more detail below.

In a first embodiment of the invention, at least one longitudinal separating device is provided with which the double-layer film web can be separated into a first and an optional second film web, each with one layer, the separation being able to be carried out in particular on or near the side edges connecting the double-layer film web. A longitudinal separating device can therefore be provided with which one side edge of the double-layer film web can be opened or separated while the second side edge remains. In this case, the double-layer film web can be folded out so that a double-wide, single-layer

Foil web can be generated. Alternatively, two longitudinal separating devices can be provided, with which both side edges can be opened or separated, so that two individual film webs, one on top of the other, each with one layer can be produced. “Ripping” can be understood to mean a lengthwise cutting directly in the side edge, with generally no waste being produced. “Separating” is understood to mean that a longitudinal separating device takes a distance from the relevant side edge, so that a strip is created which belongs to the waste. The opening or severing of at least one side edge leads a simple way to measure the thickness of just one layer.

It is particularly advantageous with the first thickness measuring device the thickness of the double-layer film web and with the at least one second thickness measuring device the thickness of an individual layer of the film web can be measured. The first thickness measuring device can be arranged near and downstream of the stretching device. The at least one second thickness measuring device can be arranged at a point at which the two layers, which have already been separated by a separation or separation, have also been removed from one another. This is preferably done shortly before the individual layers of the film web are wound onto a winding in a winding device. However, it is also advantageous if the thickness of the layer of the first film web can be measured with the first thickness measuring device and the thickness of the layer of the second film web can be measured with the second measuring device. This means that the layers that have already been separated by a separation or separation are also at a distance from one another. This in turn takes place preferably shortly before the individual layers of the film web are wound up on a winding in each case in a winding device. In a further embodiment of the invention, in particular three thickness measuring devices can be provided downstream of the stretching device, the thickness of the double-layer film web being measured with the first thickness measuring device and the thickness of an individual layer of the film web being measured with the two second thickness measuring devices. With such an arrangement it is possible to react very flexibly to the measurement requirements.

It is also advantageous if the thickness of the layer of the first film web can be measured with the first thickness measuring device and the thickness of the layer of the second film web can be measured with the second measuring device, the thickness measuring devices each having the same distance from the stretching device as seen along the transport paths of the layers. In this case, the measured thickness profiles can be compared directly with one another and thus a regulation of the Improve the film thickness profile so that there is a minimum of waste. In particular, it can also be provided that the traverses of traversing thickness measuring devices are coordinated so that the recording of the thickness profile takes place not only at the same distance in the transport direction of the individual layers of film from the stretching device, but also at the same transverse positions. This makes it possible to clearly assign thick and thin areas to the individual layers and, for example, to recognize deviations in the case of thin areas that should occur in equal proportions in the area of the side edges in both layers. Such a detection is a prerequisite for the fact that deviations can be corrected.

In an advantageous embodiment of the invention, it is provided that the first thickness measuring device and the second thickness measuring device in relation to the transport path of the double-layer film webs and / or the

Transport paths of the first film web and / or the second film web have a different distance from the stretching device, an evaluation device being provided with which a thickness profile can be determined for each layer, with the evaluation device taking into account the distance between the thickness measuring devices. In other words, with the evaluation device, which takes into account the distance between the thickness measuring devices, it is possible to assign the measured thickness profiles to the same point on the double-layer film web. In particular, the evaluation device also takes into account the time that a point on the film web needs to cover the path between the first and the second thickness measuring device. In this way it is made possible for the thickness profiles to be comparable with one another or even for a differential thickness profile to be able to be created. The latter is important in the event that the first thickness measuring device measures a thickness profile of the double-layer film web. In particular, it can also be provided here that in the case of traversing thickness measuring devices the

Traversing are coordinated so that the recording of the thickness profile not only with regard to the distance of the thickness measuring devices in the transport direction of the individual layers of film web to the stretching device, but also the transverse positions of the respective thickness measuring devices can be coordinated.

In addition, it can be provided that at least one winding device is provided for winding up at least one film web, with at least one layer of the film web being able to be measured with at least one thickness measuring device when this film web has reached the roll. Consequently, not only the thickness of an individual layer of the film web can be determined, but also a thickness profile of the roll. The thickness profile of the roll or rolls can thus ultimately be able to be taken into account when controlling or regulating the thickness profile in the blown film system.

In order to be able to exploit the advantages of the invention, at least one device is provided for embossing a thickness profile on the film tube and / or the double-layer film web and / or the first and / or the second film web, the device comprising at least one of the following elements:

• Fluid application device, with which a fluid with a temperature and / or with a volume flow on the film tube and / or the

Foil web is conductive

• an infrared radiation source with which a thermal radiation can be given to the film tube and / or the film web

• an electromagnetic radiation source with which electromagnetic radiation can be given to the film tube and / or the film web

• a contact element, in particular a roller, which contacts the film tube and / or the film web and controls the temperature.

In this way, a temperature profile, that is to say locally different temperatures, can be impressed on the film tube in the circumferential direction or on the double-layer film web in the transverse direction. Higher temperatures mean that the material of the film tube or the film web does not yet solidify strongly and consequently continues to flow, so that local thin spots arise. Conversely, lower temperatures lead to solidification, so that thick spots are created here. It is thus possible in different ways to set the thickness profile of the film tube or the film web and to check this with the thickness measuring devices. A fluid application device can be a segmented fluid nozzle, in particular a fluid cooling ring, with which fluid flows can be generated over the width of the film web or over the circumference of the film tube, which can have different temperatures and / or volume flows in segments. A fluid can in particular be in gas, preferably air.

An infrared radiation source can be used to produce local heating at certain points on the circumference of the film tube or in the transverse direction of the film web in order to reduce the thickness of the film tube at these points, in particular to produce thin spots. An electromagnetic radiation source can serve the same purpose, with local heating being able to be caused in particular by absorption of radiation by the material of the film web or the film tube. Another possibility of causing local heating of the material is to provide a contact element which makes contact with the film tube or the film web. Such a contact element can then comprise different, in particular segment-wise, temperatures over the width of the film web and / or over the circumference of the film tube, which are passed on directly to the material of the film web or the film tube. Such a contact element can preferably be a roller or a plurality of rollers.

In a further embodiment of the invention, it is provided that the actual position of a thickness profile impressed on the film tube can be determined and is comparable with the target profile, a device for impressing a thickness profile being controllable with a control device, in particular the thickness profiles of the individual film webs being against one another. In other words, the thickness profile of the film webs is regulated. In particular, a further Thickness measuring device can be provided with which a thickness profile of the film tube can be determined. It is advantageous to compare this thickness profile with the thickness profiles of the individual film layers. Such a comparison is particularly advantageous in blown film systems in which a device for embossing a thickness profile in the transport direction of the

The film tube of the flattening device and / or the squeezing device is arranged upstream and in which a film reversing device is provided, with which the later layers can be shifted against each other during the laying flat and / or the squeezing off. A film reversing device is generally advantageous for producing a good roll, but has the disadvantage that any thick or thin areas that may be embossed also shift in an undesired manner. This effect can be detected with the present invention and compensated for with the aid of the control system just described. In this context it should be mentioned that a stretching device in the transport direction of the

Film tube and / or the film web between the lay-flat device and / or the squeezing device and the film reversing device can be arranged. Here the film material is still warm and can be stretched with less heat. However, the stretching device must also be reversed. An arrangement downstream of the film reversing device is also advantageous, since a stationary system can be designed more simply with regard to the energy supply.

The above-mentioned object is also achieved by a method for setting up film tubes or film webs in which

- a film tube is extruded with a nozzle head,

- with a flattening device and / or squeezing device, the film tube is flattened and / or squeezed off in order to produce a double-layer film web from the film tube, a second thickness measuring device, the thickness of layers of the film web is measured, with at least one of the Thickness measuring devices the thickness of a single layer of the film web is measured.

The same advantages result that have already been described in connection with a blown film line according to the invention.

The described invention is particularly advantageous in combination with films containing raw materials with a density of more than 0.93 g / cm ³ . Such films have great rigidity, so that an adjustment of the thickness profile according to the invention leads to a particularly high quality of the films. Preferred raw materials here are polyethylenes, in particular FIDPE (High-Density Polyethylene)

Further advantages, features and details of the invention emerge from the following description, in which various exemplary embodiments are explained in detail with reference to the figures. The features mentioned in the claims and in the preceding and / or following description can be essential to the invention individually or in any combination of the features mentioned. In the context of the entire disclosure, features and details that are described in connection with the method according to the invention naturally also apply in connection with the blown film line according to the invention and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference is or will always be made to each other can. The individual figures show:

Fig. 1 side view of a device for Fierstellung a

Foil tube

Fig. 2 representation of a stretching device

3 shows a winding device

FIG. 1 shows a device 1 for setting a film tube, which initially comprises at least one extruder 2 with which plastic, for example in granulate form, can be plasticized. Via a line 3 the plastic melt produced in this way is fed to a nozzle head 4, where this melt is converted into a cylindrical melt flow, so that this melt flow can be withdrawn from an annular gap 5 (not visible in this figure) in the withdrawal direction z. Now there is a film tube 6 that has not yet solidified. This is inflated from the inside by a slight overpressure, so that it has a larger diameter within the optional calibration device 7. The film tube is solidified by a fluid application device 8, which is also often referred to as a cooling ring because of its ring-like configuration surrounding the film tube. Because of the cooling effect, one often speaks of a temperature control device.

After passing the calibration device 7, the film tube 6 reaches the effective area of a lay-flat device 9, in which the circular film tube is first converted into an elliptical cross-section with increasing eccentricity, until it finally has two film webs lying on top of one another at their side edges in the area of influence of the squeezing device are interconnected, forms. In other words, there is now a double-layer film web 24.

The flattening device 9 is arranged to be rotatable, the axis of rotation essentially being aligned with the hose axis 11, which is indicated in FIG. 1 by a dash-dotted line. The rotatability of the flattening device is indicated by arrow 12.

The fluid application device 8 is divided into different circumferential sections. Each circumferential section of the fluid application device is capable of the film tube with one over the circumference of the

To act on the film tube different volume flow (amount of fluid per unit of time) and / or with a volume flow with a temperature varying over the circumference of the film tube. Air is preferably provided as the fluid. This allows the person concerned

The circumferential section of the film tube assigned to the circumferential section of the fluid application device is individually temperature-controlled, in particular less cooled or even heated. The circumferential sections of the film tube, which due to the lower cooling effect of the fluid application device, “flow away” to a great extent, form a thin point 13. With a greater cooling effect, on the other hand, flowing is reduced, so that thick spots are formed here. Thick places and thin places have a greater or lesser thickness compared to the average thickness of the film tube.

So that the thin point always arrives at a fixed position of the lay-flat device, it is also necessary for the thin point to move along the circumference, which is indicated in the figure by the arrow 14. This “wandering” of the thin point is achieved in that the parameters of the circumferential section of the fluid application device closest in the direction of arrow 14 are changed in order to now generate a thin point adjacent to the circumferential section of the film tube which currently has a thin point. The current thin point is moved back in that the relevant circumferential section of the fluid application device now again has a stronger cooling effect on the angular section assigned to it. In order to be able to record a thickness profile of the film tube 6, a thickness measuring device 18 can be provided which, viewed in the transport direction z, is preferably arranged between the calibration device 7 and the flattening device 9. The thickness measuring device 18 comprises, for example, a measuring head which can determine the thickness of the wall of the film tube at its current position. To form the profile, the measuring head can be designed to be movable around the film tube in order to be able to repeat the measurement at different positions, which is represented by the double arrow 19. The distance between two positions at which a thickness measurement can be carried out can be variably adjustable . To move the measuring head, it can be arranged displaceably on a rail 20, the rail 20 engaging in a ring around the film tube. Furthermore, an evaluation and / or control device 40 is provided with which the fluid application device can be controlled so that a desired thickness profile can be generated. This thickness profile or the control parameters required for this can be generated dynamically for the individual segments of the fluid application device, so that the resulting thickness profile moves in phase and in particular with an offset with the rotation of the flattening device. At the turning points of the flattening device, the offset is preferably 0. The data line 41 is available for the transmission of control commands. As already described, the thickness measuring device 18 can measure a thickness profile. Measured values (in raw form or already as a thickness profile) are fed to the evaluation and / or control device 40 via the data line 42. The evaluation and / or control device 40 can now evaluate the measured thickness profile and, in particular, modify the control parameters in such a way that the measured thickness profile matches the desired thickness profile. A control loop is thus made available. According to the invention, it is additionally provided that the evaluation and / or control device 40 also take into account the thickness profiles that have been recorded with the thickness measuring device 38 and with at least one of the thickness measuring devices 45. The influencing factors of these individual thickness measuring devices can be weighted and taken into account when modifying the control commands. In particular, it is conceivable that the thickness profiles measured by the thickness measuring device 45 are primarily taken into account for influencing the offset. It can be advantageous that the control parameters that are necessary for setting the target profile are stored in the event of an inflow to a turning point and are mirrored again or taken into account in the event of an expiration from a turning point. For example, if the offset decreases at the point of inflection, the values can be taken into account in such a way that the increase in the offset is set with the same values. This prevents the sluggishness of the control loop from leading to undesirable thick or thin areas. It must be taken into account that in the description of the figures, too, the expression “fluid application device” is a synonym for all possibilities of influencing the thickness profile of the film web. It can thus other or further devices for impressing a thickness profile on the film tube and / or the double-layer film web and / or the first and / or the second film web can be provided.

FIG. 1 also shows a reversing device 15, which has the task of guiding the flattened film tube from the flattening device to the stationary roller 16 without damage occurring. The arrow 17 indicates that this film tube is now being guided for further processing, which is explained in more detail in the following figures. "Reversing" means that various elements, in particular the individual deflection rollers and turning bars, move back and forth between two end points. This movement is preferably a rotary or pivoting movement. The end points can therefore also be referred to as turning points.

FIG. 2 now shows, by way of example, a stretching device 30 which is connected inline to the device 1 shown in FIG. In the context of the present invention, however, a stretching device is not limited to the embodiment described below. Inline here means that the double-layer flat web 24 is supplied from the device 1 directly from the production process, that is to say without transporting the film web in a manageable form. The film web 24 runs into the stretching device 30 along the web transport direction z. Here it is first guided from the guide roller 31 to the sizing rollers 32, each of which is designated by the reference numeral 32. The task of the sizing rollers 32 is to bring the film web 24, which has already been completely or partially cooled, back to a temperature which is sufficient for a stretching or stretching process. Stretching units usually carry out stretching processes, ie the film has already cooled down and has to be brought back to the stretching temperature. Stretching processes, such as those used in blown film extrusion, are also conceivable (especially if the stretching device follows a film extrusion line in-line). In this case, the film web has not yet cooled down completely.

In particular if a stretching device is arranged directly downstream of a blown film line, i. H. if the stretching takes place "inline", it should be possible to speak of a stretching device in the case of a stretching device. However, this is more of a definition than a technical question.

After the film web 24 has been brought back to a stretching temperature in the area 28 of the heating rollers 32, it runs into the area of the stretching roller 22 and the nip roller 33 and crosses the gap between these two rollers 22, 33 Draw nip 21 in order to then reach the surface of the draw roller 23 and leave the draw nip 21. This stretching roller 23 forms a roller gap with the nip roller 36. Due to a lower peripheral speed of the first pair of rollers 22, 33 compared to the second pair of rollers 23, 36, the film web 24 is elongated, that is to say stretched, in the stretching gap 21. Two effects occur here which are not desired and which make it necessary to separate longitudinal strips on the sides of the film webs. The first effect is a reduction in the film width during stretching (the so-called neck in). The second effect is a thickening of the edges of the film web. Provision can be made for the size of the stretching gap, that is to say the distance between the detachment edge of the film web 24 from the roller 22 to the edge of the film web to hit the roller 23, to be variable. In this way it is possible to influence the size of the neck-in and / or the thickening of the film web at its edges.

After passing through the stretching gap, of which there can also be several in a row, the film web 24 reaches the region 29, which comprises cooling rollers each designated by the reference numeral 37, in which the film web 24 is cooled again. After leaving this area 29, the film web 24 has again reached a somewhat lower temperature, so that its surface can withstand the transport over the guide roller 31 in the transport direction z without further damage. The film web 24 is then in Further conveyed in the direction of arrow 34 and at the end of an optional further processing is fed to a winding device in which the film web is wound up as a double-layer film web or separately in two individual layers. In principle, it is not excluded that the film web or the individual layers of the film web receive longitudinal cuts and are wound up next to one another in several panels.

Before the film web reaches the stretching device 30, a cutting or puncturing device 35 can be provided with which the double-layer film web can be cut or punctured so that air or another gas that could still be within the double-layer film web could escape. This measure leads to an improved quality of the stretching process and to an increased accuracy of the thickness profiles to be measured of the double-layer film web. In particular, it can be provided that the film web along or near one

Side edge is cut lengthways so that the double-layer film web is only connected via one side edge. It may also be desirable to cut the two-ply film web at both of its side edges. This is necessary in particular from certain thicknesses of the film web, since the air transport to a side edge of the film web may not take place sufficiently quickly.

In the transport direction behind that of the stretching device 30, a thickness measuring device 38 is provided with which a thickness profile of the double-layer film web can be recorded after it has been stretched. However, it must be taken into account that, without further measures, only the total thickness of the film web can be measured here, i.e. the sum of the thicknesses of the individual layers. However, it is conceivable, especially when the double-layer film web has received a longitudinal section, to introduce a contrast medium, such as a metal sheet, between the two layers so that each layer can be measured separately with regard to its thickness.

The thickness measuring device 38 can in turn be a measuring head that can be moved along an at least partially transverse to Transport direction extending rail is arranged. The measuring head is in turn able to measure the thickness of the film web 24 at its current position. The measuring head can then be moved to a further position at which a further measurement can be carried out. However, it is not necessary that the measuring head has to be stopped to measure the thickness. Rather, it can basically be provided that the measuring head carries out measurements at adjustable time intervals, but the speed of movement is variable. For example, in connection with the present invention it can be fundamentally desirable for the measuring head to be moved more slowly at the edges of the film web in order to increase the density of the measurements here, which increases the accuracy of the thickness profile at the edges. In addition, it can in principle be conceivable that the

Thickness measuring device 38 is designed and set up to also measure beyond the edge of the film web in order to be able to make statements about the current width of the film web.

Finally, FIG. 3 shows a film separating device 50, which is shown here in connection with two winding points 60, 61, but can also be provided independently of this. The film web 34 can already have been subjected to a pretreatment before entering the film separating device. In particular, a first edge trim can already have taken place in order to cut off part of the thickened areas at the edge, which leads to an improved quality of an optional pretreatment. Further cutting devices 51, in particular in the form of cutting knives, are provided within the film separating device, each of which carries out the final edge trimming on one edge of the still double-layered film web. For this purpose, the double-layer film web is guided over rollers 52 and 53, which primarily ensure the web tension necessary for the edge trimming. At the latest after the edge has been trimmed, the double-layer film web is separated into two individual layers, which, however, are still directly on top of one another.

The actual separation of the layers takes place by the rollers 54, 55, which form a roller gap. After passing through the nip, the The first layer is fed to the first winding point 60, where it runs over various additional rollers and is wound onto the winding 62.

A second thickness measuring device 45 is provided downstream of the separating device, the structure and mode of operation of which is preferably similar to that of the thickness measuring device 38. The measurement results (in raw form or as an evaluated thickness profile) are fed to the evaluation and control device 40 via a data line 44, for example by wire and / or wirelessly.

The second layer can be fed to the winding point 61, the structure and function of which is identical to the first winding point. A second thickness measuring device can also be provided for measuring the second position. Reference is made here to the description in the previous paragraph with regard to structure and mode of operation.

The thickness measurement profiles that are recorded downstream of the separating device can be continuously added up by the evaluation and / or control device in order to be able to record a roll sum profile, i.e. the addition of the thickness profiles of the individual layers in a roll. In a blown film line with a reversing device, but without a stretching device, deviations in the film thickness from the mean film thickness, i.e. thick and / or thin areas, are distributed in the axial direction of the roll, so that overall a uniform circumference of the roll is created. However, if a stretching device is provided, this can result in further thick or thin areas that can no longer be compensated for with a reversal. By forming a roll sum profile as described above, the occurrence of piston rings (local thickening) on the roll, for example, can be recognized early and taken into account when setting the control parameters for the fluid application device.

Claims

1. Blown film line for the production of film tubes or film webs with

- a nozzle head for the extrusion of a film tube

a flattening device and / or squeezing device for laying flat and / or squeezing off the film tube in order to produce a double-layer film web from the film tube

- a stretching device with which the double-layer film web can be stretched in the longitudinal direction, characterized in that downstream of the stretching device at least one first and at least one second thickness measuring device for measuring the thickness of layers of the film web are provided, with at least one of the thickness measuring devices the thickness a single layer of the film web can be measured.

2. Blown film line according to claim 1, characterized in that at least one longitudinal separating device is provided with which the double-layer film web can be separated into a first and a second film web, each with one layer, the separation in particular on or near the side edges connecting the double-layer film web is feasible.

3. Blown film system according to one of the preceding claims, characterized in that the thickness of the double-layer film web can be measured with the first thickness measuring device and the thickness of an individual layer of the film web can be measured with the at least one second thickness measuring device.

4. Blown film system according to one of the preceding claims, characterized in that the thickness of the layer of the first film web can be measured with the first thickness measuring device and the thickness of the layer of the second film web can be measured with the second thickness measuring device.

5. Blown film line according to one of the preceding claims, characterized in that the thickness of the layer of the first film web can be measured with the first thickness measuring device and the thickness of the layer of the second film web can be measured with the second measuring device, the thickness measuring devices seen along the transport paths of the layers each have the same distance from the stretching device.

6. Blown film system according to one of the preceding claims, characterized in that the first thickness measuring device and the second thickness measuring device have a different distance from the stretching device in relation to the transport path of the double-layer film webs and / or the transport paths of the first film web and / or the second film web, An evaluation device is provided with which a thickness profile can be determined for each layer, the distance between the thickness measuring devices being able to be taken into account with the evaluation device.

7. Blown film plant according to one of the preceding claims, characterized in that at least one winding device for winding at least one film web is provided, with at least one thickness measuring device at least one layer of the film web can be measured when this film web has the winding it is enough.

8. Blown film line according to one of the preceding claims, characterized in that at least one device for embossing a thickness profile on the film tube and / or the double-layer film web and / or the first and / or the second film web is provided, wherein the device is at least one of the following elements:

• Fluid application device with which a fluid at a temperature and / or with a volume flow can be directed onto the film tube and / or the film web

• an electromagnetic radiation source, with which an electromagnetic radiation cal the film tube and / or the film web can be given

9. Blown film line according to one of the preceding claims, characterized in that the actual position of a thickness profile impressed on the film tube can be determined and is comparable with the target profile, a device for impressing a thickness profile being controllable, in particular the thickness profiles of the individual film webs being against one another.

10. Method for the Fierstellung of film tubes or film webs, in which

- A film tube is extruded with a nozzle head

- With a flattening device and / or squeezing device, the film tube is flattened and / or squeezed off in order to produce a double-layer film web from the film tube

- With a stretching device, the double-layer film web in the longitudinal direction is stretched, characterized in that downstream of the stretching device the thickness of layers of the film web is measured with at least one first and at least one second thickness measuring device, the thickness of an individual layer of the film web being measured with at least one of the thickness measuring devices.