Blown film line and process for producing film tubes or film webs

Abstract

Blown film line for the production of film tubes or film webs with - a nozzle head for extruding a film tube - a flattening device and / or squeezing device for flattening and / or squeezing 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 at least one first and at least one 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 an individual layer of the film web can be measured with at least one of the thickness measuring devices, wherein 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.

Description

The invention relates to a blown film plant for producing film hoses or film webs and to a method for producing film hoses or film webs.A plastic film is frequently produced on blown film installations in which a nozzle head is provided which has a nozzle gap which is in particular circular. A plastic melt is extruded or pressed out of this nozzle head, so that a film tube is produced. One of the devices downstream 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 produced from the circular film tube, wherein in particular the two layers are connected to one another along their longitudinal edges. The invention also relates to blown film plants which comprise a stretching unit by means of which the double-layer film web can be stretched in its transport direction. The stretching can influence the properties of the films (strength, permeability to gases, liquids and particles) in a targeted manner. Such a stretching unit is often also referred to as a MDO (machine direction orientation).EP 2 277 681 A1 discloses such a blown film plant. The invention described there has for its object to increase the usable width of the tubular film so that less waste is produced. This is done by producing thin areas in advance at the points at which thickening occurs due to the so-called neck-in within the stretching device, so that the desired film thickness occurs here, which can then be used. Nevertheless, a considerable part of the film must be separated--the so-called edge trimming--which is then usually waste. Apart from the waste costs resulting from this, this is in particular not very environmentally friendly. In addition, the quality of the films produced in this way may be limited due to thickness variations.Further prior art can also be taken from the publications DE 689 26 590 T2 and DE 10 2011 085 735 A1.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 once more in order to further reduce manufacturing costs and the amount of waste.According to the invention, this object is achieved by all the features of claim 1. Possible embodiments of the invention are specified in the dependent claims.According to the present invention, it is provided that downstream a first and at least a second thickness measuring device are provided for measuring the thickness of layers of the film web. It is provided here that the thickness of an individual layer of the film web can be measured with at least one of the thickness measuring devices. Thickness measuring means is understood to mean a device with which a thickness profile of a film web can be produced in a direction which runs at least partially transversely to the transport direction of the film web. Accordingly, a thickness is to be understood as a thickness profile in which a thickness measurement value is assigned in each case to locations in the transverse direction. For example, such a thickness measuring device can comprise a thickness measuring head traversing in the transverse direction of the film web, wherein a thickness measurement can be carried out in each case after a short movement of the measuring head, so that the thickness profile already described can be generated. In cases where the film web is moved further during the measuring process, an oblique thickness profile is produced, which therefore comprises not only a transverse component but also a longitudinal component, which is generally not taken into account, however.According to the invention, not only is a thickness measuring device provided-as disclosed in the prior art-but a second thickness measuring device is provided, wherein at least one of the thickness measuring devices measures 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 portion and a thin portion which are directly on top of one another are not recognized as such. Consequently, it can occur that in applications in which the double-layer film web is separated into two single-layer film webs, defects occur which then ultimately still lead to rejects. This is avoided by the present invention. The measurement of a single layer can be carried out in various ways, which will be 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 having one layer, wherein the separation can be carried out in particular at or near the side edges connecting the double-layer film web. Thus, a longitudinal separating device can 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 open, so that a double-wide, single-layer film web can be produced. Alternatively, two longitudinal separating devices can be provided, with which both side edges can be opened or separated, so that two individual film webs lying one on top of the other can be produced, each with one layer. Here, "severing" can be understood to mean a longitudinal severing directly in the side edge, wherein as a rule no waste arises. By "severing" is meant that a longitudinal severing device is spaced from the relevant side edge so as to produce a strip which belongs to the waste. The separation or detachment of at least one side edge leads to a simple possibility of carrying out a thickness measurement of only one layer.It is particularly advantageous for the thickness of the double-layer film web to be measurable with the first thickness measuring device and for the thickness of an individual layer of the film web to be measurable with the at least one second thickness measuring device. The first thickness measuring device can be arranged close to and downstream of the stretching device. The at least one second thickness measuring device can be arranged at a location at which the two layers already separated by a separation or separation have also been removed from one another. This is preferably effected shortly before the winding of the individual layers of the film web onto a respective roll in a winding device.However, it is also advantageous if the thickness of the layer of the first film web is measurable with the first thickness measuring device and the thickness of the layer of the second film web is measurable with the second measuring device. This means that the layers already separated by a separation or separation are also spaced apart from one another. This is in turn preferably effected shortly before the winding of the individual layers of the film web onto a respective roll 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, wherein the thickness of the double-layer film web can be measured with the first thickness measuring device and the thickness of a single layer of the film web can be 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 furthermore advantageous to measure the thickness of the layer of the first film web with the first thickness measuring device and the thickness of the layer of the second film web with the second measuring device, wherein the thickness measuring devices have 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 directly compared with one another and a regulation of the film thickness profile can thus be carried out in an improved manner, so that a minimum of waste is produced. In particular, it can also be provided if, in the case of traversing thickness measuring devices, the traversings are matched to one another, so that the thickness profile is recorded not only at the same distance in the transport direction of the individual layers of film web to the stretching device, but also at the same time at the same transverse positions. This makes it possible to unambiguously assign thick and thin areas to the individual layers and, for example, to detect deviations in the case of thin areas which should occur in the region of the side edges in both layers in equal proportions. Such a detection is a prerequisite for deviations being correctable.In an advantageous embodiment of the invention, it is provided that the first thickness measuring device and the second thickness measuring device have a different distance from the stretching device with respect 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, wherein an evaluation device is provided with which a thickness profile can be determined for each layer, wherein the distance of the thickness measuring devices can be taken into account with the evaluation device. In other words, the evaluation device, which takes into account the distance between the thickness measurement devices, makes it possible to assign the measured thickness profiles to the same location of the double-layer film web. In particular, the evaluation device also takes into account the time required for a location of the film web to cover the path between the first and the second thickness measurement device. In this way, it is made possible for the thickness profiles to be comparable to one another or even for a differential thickness profile to be produced. The latter is important in the case 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 measurement devices, the traversings are matched to one another, so that the recording of the thickness profile can take place not only with regard to the distance of the thickness measurement devices in the transport direction of the individual layers of film web to the stretching device, but also with a matching of the transverse positions of the respective thickness measurement devices.In addition, it can be provided that at least one winding device is provided for winding at least one film web, wherein at least one layer of the film web can be measured with at least one thickness measuring device when this film web has reached the winding. Consequently, not only the thickness of an individual layer of the film web can be determined, but a thickness profile of the roll. The thickness profile of the roll or of the rolls can thus finally be taken into account in a control or regulation of the thickness profile in the blown film plant.In order to be able to exploit the advantages of the invention, 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 comprises at least one of the following elements:• Fluid application device, with which a fluid can be conducted at a temperature and / or with a volume flow onto the film tube and / or the film web• an infrared radiation source, with which a heat radiation can be emitted to the film tube and / or the film web• an electromagnetic radiation source, with which electromagnetic radiation can be emitted to the film tube and / or the film web• a contact element, in particular a roller, which contacts and tempered the film tube and / or the film web.In this way, a temperature profile, i.e. 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 lead to the material of the film tube or the film web not yet becoming strongly solidified and consequently continuing to flow, so that local thin spots are produced. Conversely, lower temperatures lead to solidification, so that thick spots are produced here. It is thus possible in different ways to set the thickness profile of the film tube or of the film web and to check it 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 fluid flows can have different temperatures and / or volume flows segment by segment. A fluid can be in particular in gas, preferably air.An infrared radiation source can be used to specifically induce local heating at specific 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 in order to produce thin points. An electromagnetic radiation source can serve the same purpose, wherein local heating can be brought about in particular by absorption of radiation through the material of the film web or the film tube. A further possibility for causing local heating of the material is the provision of a contact element which contacts 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 temperatures are applied 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 be comparable to the desired profile, wherein a device for imposing a thickness profile can be controlled by a control device, wherein in particular the thickness profiles of the individual film webs are adjacent to one another. In other words, the thickness profile of the film webs is controlled. In particular, in this context, 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 advantageous in particular in blown film plants in which a device for embossing a thickness profile is arranged upstream of the flattening device and / or the squeezing device in the transport direction of the film tube and in which a film recycling device is provided, with which the later layers can be displaced with respect to one another during flattening and / or squeezing. A film reversing device is generally advantageous for producing a good roll, but has the disadvantage that thick or thin areas which may have been impressed also shift in an undesirable manner. This effect can be detected with the present invention and compensated by means of the control just described. In this connection, it should be mentioned that a stretching device can be arranged in the transport direction of the film tube and / or the film web between the flattening device and / or the squeezing device and the film recycling device. Here, the film material is still warm and can be stretched with a lower heat input. However, the stretching device must also be reversed. An arrangement downstream of the film recycling device is also advantageous, since a stationary installation can be designed more simply with regard to the energy supply.The above-mentioned object is also achieved by a method for producing film hoses or film webs, in whicha tubular film is extruded using a die head,a flattening device and / or a squeezing device is used to flatten and / or squeeze the film tube in order to produce a double-layer film web from the film tube,stretching the double-layer film web in the longitudinal direction with a stretching device,wherein additionally 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, wherein the thickness of an individual layer of the film web is measured with at least one of the thickness measuring devices.The same advantages result which have already been described in connection with a blown film plant according to the invention.The invention described is particularly advantageous in combination with films containing raw materials having a density of more than 0.93 g / cm 3. Such films have a high rigidity, so that an adjustment according to the invention of the thickness profile leads to a particularly high quality of the films. Preferred raw materials here are polyethylenes, in particular HDPE (high-density polyethylene)Further advantages, features and details of the invention will become apparent 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 subsequent description can be essential to the invention individually or in any desired combinations of mentioned features. Within the scope of the entire disclosure, features and details which are described in connection with the method according to the invention naturally also apply in connection with the blown film plant according to the invention and vice versa, so that with regard to the disclosure reference is or can always be made reciprocally to the individual aspects of the invention. The individual figures show: FIG. 1 is a side view of a device for producing a tubular film FIG. 2 shows a representation of a stretching device FIG. 3 shows a representation of a winding deviceFIG. 1 shows a device 1 for producing a film tube, which first comprises at least one extruder 2, with which plastic present in granular form, for example, can be plasticised. The plastic melt thus produced is fed via a line 3 to a nozzle head 4, where this melt is transferred into a cylindrical melt stream, so that this melt stream can be drawn out of an annular gap 5, which is not visible in this figure, in the draw-off direction z. Now there is a film tube 6 which has not yet been consolidated. This is inflated from the inside by a slight overpressure, so that it has a larger diameter inside the optional calibration device 7. The tubular film is consolidated by a fluid application device 8, which is often also referred to as a cooling ring because of its ring-like configuration enclosing the tubular film. Because of the cooling effect, a temperature control device is often also referred to.After passing the calibration device 7, the film tube 6 reaches the operative region of a flattening device 9, in which the circular film tube is initially transferred into an ellipsoidal cross section with an increasing eccentricity, until it finally forms, in the influence region of the squeezing device, two film webs lying one on top of the other, which are connected to one another at their side edges. In other words, a double-layer film web 24 is now present.The flattening device 9 is rotatably arranged, wherein the axis of rotation is substantially aligned with the tube axis 11, which is indicated in FIG. 1 by a dot-dash line. The rotation of the flattening device is indicated by the arrow 12.The fluid application device 8 is divided into different circumferential sections. Each circumferential section of the fluid application device is capable of applying a volume flow (amount of fluid per unit time) varying over the circumference of the film tube to the film tube and / or a volume flow having a temperature varying over the circumference of the film tube. Air is preferably provided as the fluid. The circumferential section of the film tube assigned to the relevant circumferential section of the fluid application device can thus be individually temperature-controlled, in particular cooled to a lesser extent or even heated. The circumferential sections of the film tube, which "flow" strongly due to the lower cooling action of the fluid application device, form a thin point 13. In the case of a greater cooling effect, on the other hand, a flow-out is reduced, so that thick spots are formed here. Thick spots and thin spots have a greater or lesser thickness compared to the average thickness of the film tube.In order for the thin point to always reach a fixed position of the flattening 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 "migration" of the thin area is realized by the fact that the circumferential section of the fluid application device closest in the direction of the arrow 14 is changed in its parameters in order to now produce a thin area adjacent to the circumferential section of the film tube which has just one thin area. The current thin section is retracted by the relevant circumferential section of the fluid application device now acting on the angle section assigned to it again in a more cooling manner.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 in each case can be variably adjustable. For the movement of the measuring head, the latter can be arranged displaceably on a rail 20, the rail 20 engaging annularly 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 necessary for this can be generated dynamically for the individual segments of the fluid application device, so that the thickness profile formed moves along in phase and in particular with an offset with the rotation of the flattening device. In the inflection points of the flattening device, the offset is preferably 0. The data line 41 is available for transmitting control commands. The thickness measurement device 18 can measure a thickness profile, as already described. Measured values (in raw form or already as thickness profile) are supplied 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 such that the measured thickness profile corresponds to the desired thickness profile. A control loop is thus made available therewith. According to the invention, it is additionally provided that evaluation and / or control device 40 also takes 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 taken into account in a weighted manner when modifying the control commands. In particular, it is conceivable that the thickness profiles measured by the thickness measuring device 45 are predominantly taken into account for influencing the offset. It can be advantageous that the control parameters that are necessary for setting the setpoint profile are stored when an inlet to an inflection point is made and are applied again or taken into account when an outlet is reflected from an inflection point. Thus, the values can be taken into account when the offset in the inflection point decreases in such a way that the increase in the offset is adjusted with the same values. This prevents the inertia of the control loop from leading to undesired thick or thin spots. It should be taken into account that, even in the description of the figures, the expression "fluid application device" is synonymous with all possibilities for influencing the thickness profile of the film web. Thus, other or further devices 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 can also 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 guided for further processing, which is explained in more detail in the following figures. "Reversing" means here that different elements, in particular the individual deflection rollers and turning stands, move back and forth between two end points. This movement is preferably rotational or pivoting movements. The end points can therefore also be referred to as inflection points.FIG. 2 now shows, by way of example, a stretching device 30 which adjoins the apparatus 1 shown in FIG. 1 in an inline manner. However, in the present invention, a stretching device is not limited to the embodiment described below. Inline means that the double-layer flat web 24 is supplied from the device 1 directly from the production process, i.e. without a transport of the film web in a form that can be handled.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 heat rollers 32, each of which is denoted by reference numeral 32. The function of the heating rollers 32 is to bring the already fully or partially cooled film web 24 back to a temperature which is sufficient for a stretching or stretching process. Stretching processes are generally carried out by stretching units, i.e. the film has already cooled and has to be brought back to stretching temperature. Stretching processes, as are used, for example, in blown film extrusion, are likewise conceivable (in particular if the stretching device follows in-line in a film extrusion plant). In this case, the film web has not yet been fully cooled.In particular, if a stretching device is arranged directly downstream of a blown film plant, i.e. if the stretching takes place "in-line", it would be possible to refer to a stretching unit in a stretching device. However, this is a definition rather than a technical matter.After the film web 24 has been brought back to a stretching temperature in the region 28 of the heating rollers 32, it enters the region of the stretching roller 22 and nip roller 33 and passes through the nip between these two rollers 22, 33. The film web 24 then passes through the stretching nip 21 in order then to reach the surface of the stretching roller 23 and to leave the stretching nip 21. This stretching roller 23 forms a nip with the nip roller 36. Due to a lower circumferential speed of the first roller pair 22, 33 compared to the second roller pair 23, 36, the film web 24 is elongated, i.e. stretched, in the stretching nip 21. There are two effects which are not desired and 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. It can be provided that the size of the stretching nip, i.e. the distance between the detachment edge of the film web 24 from the roller 22 as far as the impact edge of the film web on the roller 23, can be made variable. In this way, it is possible to influence the size of the neck-in and / or the thickening of the film web at their edges.After passing through the stretching nip, of which there may also be a plurality of rolls in series, the film web 24 reaches the region 29 which comprises cooling rolls, each designated by the reference numeral 37, in which the film web 24 is cooled again. After leaving this region 29, the film web 24 has once again reached a somewhat lower temperature, so that its surface can withstand the transport via the guide roller 31 in the transport direction z without further damage. The film web 24 is subsequently conveyed further in the direction of the 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 into 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 a plurality of blanks.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 in or punctured, so that air or another gas, which could still be located 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 of the double-layer film web to be measured. In particular, it can be provided that the film web is cut longitudinally along or near a side edge, so that the double-layer film web is only still connected via one side edge. Further, it may be desired to cut the double-layer film web at both side edges thereof. 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.Behind the stretching device 30 in the transport direction, a thickness measuring device 38 is provided, with which a thickness profile of the double-layer film web can be recorded after its stretching. However, it should be taken into account that, without further measures, only the total thickness of the film web can be measured here, that is to say the sum of the thicknesses of the individual layers. However, it is conceivable, in particular if 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 in each case one layer can be measured separately with respect to its thickness.The thickness measuring device 38 can in turn be a measuring head which is arranged on a rail which extends movably along an at least partially transverse to the transport direction. The measuring head is in turn capable of performing a thickness measurement 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 must be stopped for measuring the thickness. Instead, it can basically be provided that the measuring head carries out measurements at adjustable time intervals, but that the movement speed is variable. Thus, for example, in connection with the present invention, it may 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 be conceivable in principle for the thickness measuring device 38 to be designed and configured to also measure beyond the edge of the film web in order to also be able to make statements about the current width of the film web.FIG. 3 finally shows a film separating device 50, which is shown here in connection with two winding points 60, 61, but can also be provided independently thereof. The film web 34 may already have been subjected to a pretreatment before entering the film separating device. In particular, a first edge trimming may already have been carried out in order to already cut off part of the thickened portions at the edge, which leads to an improved quality of an optional pretreatment. Within the film separating device, further cutting devices 51, in particular in the form of separating knives, are provided, which each perform the final edge trimming on an edge of the still double-layer film web. For this purpose, the double-layer film web is guided over rollers 52 and 53, which ensure above all the web tension required for edge trimming. At the latest after the edge trimming, the double-layer film web is separated into two individual layers, which however still lie directly on top of one another.The actual separation of the layers is effected by the rollers 54, 55, which form a roller nip. After passing through the roll nip, the first layer is fed to the first winding location 60, where it runs over various further rolls and is wound onto the roll 62.Downstream of the separating device, a second thickness measuring device 45 is provided, the structure and mode of operation of which are preferably the same as those of the thickness measuring device 38. Via a data line 44, the measurement results (in raw form or as an evaluated thickness profile), for example by wire and / or wirelessly, are fed to the evaluation and control device 40.The second layer can be fed to the winding location 61, the structure and function of which is identical to the first winding location. A second thickness measuring device can also be provided for measuring the second layer. In this case, reference is made to the description in the preceding paragraph with respect to structure and mode of operation.The thickness measurement profiles that can be recorded downstream of the separating device can be continuously added up by the evaluation and / or control device in order to also 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 plant with a reversing device, but without a stretching device, deviations of the film thicknesses from the average film thicknesses, 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 produced. If, however, a stretching device is provided, this can result in further thick or thin spots which can no longer be compensated for by reversing. By forming a roll sum profile as described above, the occurrence of piston rings (local thickenings) on the roll can be detected early and taken into account in the setting of the control parameters for the fluid application device.

Claims

Blown film plant for producing film hoses or film webs, having - a die head for extruding a film hose - a flattening device and / or squeezing device for flattening and / or squeezing the film hose in order to produce a double-layer film web from the film hose - 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 are provided for measuring the thickness of layers of the film web, wherein the thickness of an individual layer of the film web can be measured with at least one of the thickness measuring devices, wherein 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.Blown film plant according to claim 1da, characterised 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 with in each case one layer, wherein the separation can be carried out in particular at or near the side edges connecting the double-layer film web.Blown film plant 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 with respect 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, wherein an evaluation device is provided with which a thickness profile can be determined for each layer, wherein the distance of the thickness measuring devices can be taken into account with the evaluation device.Blown film plant according to one of the preceding claims, characterized in that at least one winding device is provided for winding up at least one film web, wherein at least one layer of the film web can be measured with at least one thickness measuring device when this film web has reached the winding.Blown film plant 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 comprises at least one of the following elements: • fluid application device, with which a fluid can be conducted at a temperature and / or with a volume flow onto the film tube and / or the film web • an infrared radiation source, with which a heat radiation can be emitted to the film tube and / or the film web • an electromagnetic radiation source, with which an electromagnetic radiation can be emitted to the film tube and / or the film web • a contact element, in particular a roller, which contacts and thermally controls the film tube and / or the film web.Blown film plant according to one of the preceding claims, characterized in that an actual position of a thickness profile impressed on the film tube can be determined and is comparable to a desired profile, wherein a device for imposing a thickness profile can be controlled, wherein in particular the thickness profiles of the individual film webs are adjacent to one another.Method for producing film hoses or film webs, in which - a film hose is extruded with a die head - the film hose is flattened and / or squeezed with a flattening device and / or squeezing device, in order to produce a double-layer film web from the film hose - the double-layer film web is stretched in the longitudinal direction with a stretching device, characterized in that the thickness of layers of the film web is measured downstream of the stretching device with at least one first and at least one second thickness measuring device, wherein the thickness of an individual layer of the film web is measured with at least one of the thickness measuring devices, wherein the thickness of the double-layer film web is measured with the first thickness measuring device and the thickness of an individual layer of the film web is measured with the at least one second thickness measuring device.

Images