Film extruder and method for producing plastic film

Abstract

The invention relates to a film extruder, in particular a blown film extruder, for producing a plastic film, comprising: a nozzle, in particular an annular nozzle, for providing a plastic melt; and at least one drawing roller which is arranged downstream of the nozzle at a distance from the nozzle and is used for drawing the plastic melt solidified into the plastic film in the cold solidification zone in the conveying direction, and the drawing roller rotates at a circumferential speed. According to the invention, a speed determination device is provided for determining a local conveying speed of at least one partial surface of the plastic melt and / or the plastic film.

Description

Technical Field

[0001] The invention relates to a film extruder, in particular a blown film extruder, for producing a plastic film according to the preamble of claim 1 and a method for producing a plastic film according to the preamble of claim 7 . Background Art

[0002] To produce plastic film, plastic granules or a mixture of multiple plastic granules are typically melted and liquefied in an extruder under heating and pressure. The plastic melt is fed into a nozzle head in a strip-like shape and distributed over a surface in a distributor. The pressure then forces the distributed plastic melt out of the nozzles of the nozzle head, causing the nozzles to produce the plastic melt. The nozzles can be linear nozzles (flat nozzles) or annular nozzles. The first nozzle is used for so-called flat film extrusion, while the second nozzle is used for blown film extrusion. Accordingly, the melt forms either a flat melt or a melt bubble. Multiple extruders are often used to produce multiple single melts, which are combined into a multi-layer plastic melt in the nozzle head. The single or multi-layer plastic melt is pulled from the nozzle by at least one pulling roller, which is positioned at a distance from the nozzle. On the way to the pulling roller and / or during contact with the pulling roller, the plastic melt cools to such an extent that it forms the plastic film to be produced. The transition region in which the plastic melt solidifies and solidifies into a plastic film is often referred to as the solidification zone or solidification line, since the plastic melt figuratively freezes in this region.

[0003] In a blown film extruder, additional components are arranged in the direction of transport of the plastic melt or plastic film (hereinafter also referred to as "melt" or "film"). These components may include an internal cooling device, which extends from the nozzle head in the direction of transport of the melt or film and is arranged within the nozzle. This internal cooling device can be used to introduce a slight overpressure into the interior of the melt bubble, thereby blowing it up transversely to the direction of transport, i.e., stretching it. The circumferential speed of the pulling roller is usually greater than the melt outflow speed from the annular nozzle, so that the melt can also be stretched or stretched in the direction of transport. In addition, a calibration basket and / or a flattening device can be provided downstream of the annular nozzle. The calibration basket can be used to give the melt bubble or film hose a round shape or maintain this shape. The flattening device can be used to gradually flatten the film hose and convert it into a double-layer film. This allows the film to be pulled by the pulling roller in a simple manner. A pressure roller is often also placed against the pulling roller to prevent the film from slipping on the pulling roller.

[0004] In flat film extrusion plants, the melt, spread across its width, is typically delivered under the influence of gravity to a pulling roll designed as a cooling roll (so-called chill roll), which is located at a distance from the flat nozzle. The circumferential speed of the pulling roll is generally greater than the melt outflow rate from the flat nozzle. On the temperature-controlled chill roll, the melt solidifies into a film, with this area also being referred to as the cold consolidation zone. On the path from the flat nozzle to the pulling roll, the melt's properties can be influenced, for example, by means of electrodes and / or suction elements (so-called vacuum boxes).

[0005] However, it has been proven to be disadvantageous that the flow behavior of the melt that has not yet formed a film is very sensitive to external influences. In this way, the different temperature distributions in the extruder may cause the characteristics of the melt to be uneven. Pollution or other external influences (such as ventilation) in the production workshop where the film extruder is installed may cause the temperature in the melt to behave differently. Such unevenness in turn causes the flow behavior to be different on the melt length between the nozzle and the cold-setting zone and / or in the transverse direction. However, such differences are transferred to the film in the cold-setting zone, so that the film may have different qualities in its length and / or width. However, the discovery of such unevenness in the melt and / or film is sometimes difficult or impossible. Accordingly, inappropriate settings are often made at the film extruder, for example, inappropriate settings are made for the position of the component parts of the machine or for parameter control or regulating loops. Summary of the Invention

[0006] It is therefore an object of the present invention to provide a detection option for inhomogeneities in a melt and / or a film.

[0007] This object is achieved according to the invention by all the features of claim 1. Possible embodiments of the invention are specified in the dependent claims.

[0008] According to the invention, a speed determination device is provided for determining the local conveying speed of at least one partial surface of the plastic melt and / or the plastic film.

[0009] Accordingly, a local conveying velocity is determined for a sub-surface from the melt and / or film, particularly from the surface. The conveying velocity of the melt and / or film typically varies significantly between the nozzle and the cold consolidation zone. Immediately upon exiting the nozzle, the melt has a lower local conveying velocity, but this velocity increases significantly for at least a sub-surface due to the higher circumferential speed of the pulling rolls. In particular, in blown film plants, the local conveying velocity increases during film stretching due to the internal pressure exerted by internal cooling. As the melt cools, the conveying velocity approaches the circumferential speed of the pulling rolls in magnitude. In principle, the local conveying velocity is a three-dimensional vector whose direction can be oriented differently relative to the conveying direction, with one component running parallel to the conveying direction. Therefore, if the local conveying velocity is determined for multiple sub-surfaces, a velocity vector field can be generated. Advantageously, the local velocity is determined for a sub-surface over at least 30%, preferably at least 50%, of the total width or circumference of the melt or film after exiting the nozzle. The local velocity can be an absolute or relative local velocity. To determine the relative velocity, the velocities of the partial surfaces can be recorded over a period of time and averaged. The deviation from the average local velocity is then the relative local velocity. Instead of or in addition to the temporal averaging, the local velocities of adjacent partial surfaces can also be considered. A computing and control device is provided to calculate the average value and the relative local velocity. The velocity determination device preferably includes a detection device (such as a sensor or a camera), which in particular enables spatially resolved determination of the local velocity. The detection device is, of course, connected to the computing and control device via a data link.

[0010] The melt and / or film can be characterized by determining the local velocity. It is known that areas of the melt, in particular, with a higher temperature melt more significantly than other areas. Therefore, higher local velocities can be detected in these areas. Therefore, such areas may lead to locally thin areas in the subsequent film. The uneven layer composition of the melt may also lead to different local velocities, because the individual layers may have different cooling behaviors. In general, different local velocities represent possible inhomogeneities in the melt and / or film. Therefore, the measured local velocities can be used as a basis for improved settings at the film extruder. Advantageously, the device according to the invention works independently of the plastic material processed in the melt or film. It is also independent of the (average) thickness of the melt or film.

[0011] The speed determination device according to the present invention, having the design described above, can be combined not only with film extruders but also, in principle, with machines in plastics processing in which heated or heated plastics are cooled. Possible combinations include, for example, devices that heat the plastic film and then cool it again. This includes a stretching device in which the plastic film is heated to a temperature range between the melt temperature and a temperature up to 20 degrees Celsius below the melt temperature, then stretched in a conveyor device and then cooled again. Such a device includes a heating device for heating the plastic film, a first stretching roller operating at a circumferential speed, a second stretching roller operating at a greater circumferential speed than the first stretching roller, and an optional cooling and / or fixing unit. In this process, the plastic film is stretched longitudinally, with the film width decreasing and the edges thickening (so-called necking). The general goal of this process is always to minimize edge thickening, as this thickening is undesirable and must be eliminated during further processing of the film. Therefore, using the speed determination device described above to determine the local speed of a subregion of the film is target-oriented. Therefore, the speed determination device according to the invention can be used advantageously in such a stretching device.

[0012] Another device is one that produces plastic moldings from plastic material at a temperature above its melting point (e.g., by injection molding, blow molding, etc.). A calendering device can also be combined with the speed determination device described above. All features described above and below can also be combined with such a device, even if they have been described only in conjunction with a film extruder.

[0013] In an advantageous embodiment of the present invention, the calculation and control device calculates the deviation between the local conveying speed of a sub-surface and the circumferential speed of the pulling roller. If the deviation falls below a threshold, the sub-surface can be associated with the plastic film. This means that a local conveying speed is determined for one or more sub-surfaces, wherein the local conveying speed is particularly the absolute local conveying speed. The conveying speed is compared with the circumferential speed of the pulling roller by determining the difference. If the magnitude of the difference falls below a threshold, i.e., is within a tolerance range, the compared speeds are considered identical. These sub-surfaces are then associated with the plastic film, i.e., with the location where the plastic film has already been formed. This means that the cold consolidation zone has been passed. Under constant process conditions in a film extruder, the cold consolidation zone is generally stationary. Therefore, it is possible to carry out this procedure for different sub-surfaces sequentially or simultaneously. In this way, the exact location of the cold consolidation zone can be determined.

[0014] In another advantageous embodiment of the present invention, the film extruder is a blown film extruder with a calibration basket, wherein the calibration basket can be positioned downstream of the cold-cure zone using a computing and control device. In particular, the calibration basket can be positioned up to 2 meters, preferably up to 1 meter, and in particular up to 0.5 meters downstream of the cold-cure zone. The calibration basket ensures that the film, which is transformed into a film hose in the cold-cure zone, no longer changes in diameter and furthermore makes it approximately circular. To this end, the calibration basket includes film guide elements that are arranged externally around the circumference of the film hose and thus enclose the film hose. Since contact between the film guide elements and the film hose is intentional or (in the case of non-contact guide elements) cannot be avoided, the goal is to always position the calibration basket downstream of the cold-cure zone to prevent the film from being damaged by the film guide elements. If the calibration basket covers the cold-cure zone, i.e., in particular if the calibration basket is not properly positioned in the cold-cure zone, the position of the cold-cure zone can also be determined using the device according to the present invention.

[0015] In an advantageous embodiment of the present invention, a speed determination device is used to determine the local conveying speed of multiple partial surfaces transversely to the conveying direction. Deviations of the local conveying speed from the average conveying speed can be determined using a computing and control device, and the thickness profile of the plastic film can be derived from these deviations. In particular, in blown film plants, maintaining a uniform film thickness across the width or circumference of the subsequent film is particularly important. In known film extruders, the film thickness is measured downstream of the cold-consolidation zone using a thickness measurement system, and the measured values are used to influence the melt differently transversely to the conveying system. However, there is a time lag between the measurement and the influence on the film. The film extrusion device according to the present invention reduces this lag because the speed determination can now also be performed at the melt, i.e., upstream of the cold-consolidation zone. In this regard, it is advantageous if the device includes a temperature control device, in particular a cooling device, comprising multiple segments distributed over the circumference of the melt or film. Each segment can be used to apply an air flow to the melt or film, which can be set differently in terms of volume flow and / or temperature. This setting can then be made by the calculation and control unit for each section. Additionally or alternatively, the invention makes it possible to determine longitudinal tolerances in the conveying direction of the film hose, in particular longitudinal tolerances related to the film thickness, ie thickness variations.

[0016] It is particularly advantageous that, with the film extrusion device according to the invention, deviations from the local conveying speed can be determined simultaneously in the conveying direction and transversely to the conveying direction. Thus, the local film thickness can be determined not only at one location on the circumference, but also at different locations in the conveying direction, i.e. in the longitudinal direction. In this way, it is also possible to determine, at least partially, the differential development in the melt or film thickness in the conveying direction. It is thus also possible to determine the flatness quality of the film with the device according to the invention. In particular, a control loop can also be provided to improve the flatness of the film. However, the melt or film thickness can only be determined relative to an average film thickness. In order to determine the average film thickness, a film thickness measuring device can be provided in the film extruder. In this case, the measured values of the film thickness measuring device are taken into account by the computing and control device when determining the local deviations.

[0017] In a preferred embodiment of the present invention, the speed determination device includes at least one infrared camera, which can be used to capture infrared images of the plastic melt and / or plastic film. A computing and control device is provided, which can determine the average temperature of multiple sub-surfaces of the surface of the plastic melt or plastic film from the multiple infrared images, wherein at least one temperature deviation from the average temperature can be determined, and wherein the propagation speed of this temperature deviation along the surface of the plastic melt or plastic film can be determined. The temperature can be determined by means of the infrared camera by measuring the radiation intensity in a specified infrared wavelength range. In particular, the radiation intensity is temperature-dependent. The computing and control device can average multiple successively acquired temperature values for each sub-surface, thereby determining an average temperature for each sub-surface. This average temperature can, of course, be determined dynamically, meaning that the most recently acquired temperature value is taken into account when determining the average again. However, if a deviating temperature is now determined for a sub-surface, this temperature represents an inhomogeneity in the film in that sub-region. If, after a certain time interval, this or a similar temperature deviation pattern is detected in adjacent or at least nearby subregions, the local velocity of the melt or film can be determined from this. Interference influences, especially interference contours (such as internal cooling devices), can be easily detected because the temperature influences caused by them are static and can therefore be taken into account by, for example, calculating the temperature influence using a computing and control device. This is particularly advantageous when determining absolute temperatures. When a reference temperature is known, the absolute temperature can be determined for at least one subregion using a computing and control device. For example, a temperature measuring unit can be integrated into the nozzle head, which can be used to measure the nozzle head temperature in the nozzle area. In this area, the nozzle head temperature is in equilibrium with the melt temperature, so the melt temperature can be inferred from the temperature measurement values. However, it is not necessary to obtain a temperature value for each subsurface and to determine the average value thereof for calculating the local velocity. In this way, the described method can be used, for example, to determine the temperature distribution of a film bubble, especially between the nozzle and the calibration device. By determining the deviation from the determined mean value in a subregion, fluctuations in the temperature can generally be determined in order to be able to determine, for example, varying properties of the film, such as the thickness distribution.

[0018] Advantageously, the infrared camera includes a two-dimensional sensor, allowing it to detect radiation from different subareas of the melt or film surface at the same time. Furthermore, such a sensor typically allows for more than one detection (a so-called "image") per second, preferably more than five detections per second. Here, an "image" is understood to be a two-dimensional matrix of radiation intensity values. With this latter detection frequency, it is also conceivable to add multiple detections. While this reduces the effective frequency, it improves the signal-to-noise ratio. To improve the signal-to-noise ratio, multiple cameras can also be provided, either correlating images of the same subarea between cameras or having multiple sensor areas within a single camera detect radiation from a single subarea of the melt or film. Additional cameras can also be provided to detect larger areas of the melt or film. Thus, in a blown film system, it is conceivable to have at least two cameras, preferably at least three cameras, and in particular at least four cameras, distributed evenly around the circumference. Instead of one or more infrared cameras, cameras with other sensor types, such as color cameras or cameras with vibration sensors, can also be provided.

[0019] Furthermore, the device advantageously includes a data memory in which at least one speed profile or at least one speed vector field of a film product can be stored. For example, for a current film production run, the control and computing device can compare the current speed profile with previous speed profiles and derive an advantageous method and / or parameter setting adaptation for the current film production run of the film extruder. The stored speed profile can also be transmitted to other film extruders via a data link. There, these values can be scaled to deviating machine parameters, so that the stored speed profile is also available at the other film extruders.

[0020] Furthermore, the above-mentioned object is achieved by a method for producing a plastic film using a film extruder, in particular a blown film extruder, wherein a plastic melt is provided by means of a nozzle, in particular an annular nozzle, of the film extruder, and wherein the plastic melt solidified into a plastic film in a cold solidification zone is pulled in a conveying direction by means of at least one pulling roller arranged downstream of the nozzle and at a distance therefrom, wherein the pulling roller rotates at a circumferential speed.

[0021] The method according to the invention is distinguished in that a local conveying speed of at least one partial surface of the plastic melt and / or the plastic film is determined by means of a speed determination device.

[0022] The same advantages as those described above in connection with the film extrusion device according to the invention are achieved with this method. The method is not limited to being combined with the production of plastic film on a film extruder, but can also be combined with other devices as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Further advantages, features, and details of the present invention are apparent from the following description, in which various exemplary embodiments are explained in detail with reference to the accompanying drawings. The features mentioned in the claims and the description may be essential to the present invention individually or in any combination thereof. Within the scope of the entire disclosure, the features and details described in conjunction with the method according to the invention also apply to the film extruder according to the invention, and vice versa, so that the disclosures of the various aspects of the present invention are always mutually referenced or can be mutually referenced. Among them:

[0024] Figure 1 Shows a side view of a blown film installation according to the invention

[0025] Figure 2 Shown Figure 1 A partial enlarged view of DETAILED DESCRIPTION

[0026] Figure 1 The film extruder, or blown film plant 1, is shown. It comprises at least one extruder 2, with which, for example, plastics in granular form can be plasticized. The resulting plastic melt is supplied via line 3 to a nozzle head 4, from which it is converted into a film bubble 6 by being extruded through an annular nozzle 5 (not visible). The melt exiting the nozzle can now be pulled. A melt bubble 6, which has not yet solidified, is now present. In the tube forming area, the melt bubble is blown up from the inside by a slight overpressure, giving it a larger diameter within the calibration basket 7. For this purpose, an internal cooling device 13 is provided, which is located within the annular nozzle 5 and extends partially in the conveying direction. Air can be supplied to this internal cooling device 13 through the extrusion tool.

[0027] The film bubble solidifies by cooling, wherein some of the heat of the film bubble is dissipated to the environment. Cooling air is directed toward the film bubble, in particular, by a temperature control device 8 (often also referred to as a cooling ring due to its annular design surrounding the film tube). It can be provided that the cooling air flow can be set differently in terms of its volume flow and / or its temperature within different angular ranges, thereby influencing the film tube differently in different circumferential sections.

[0028] The melt solidifies into a film in the cold consolidation zone 18 .

[0029] Preferably, the calibration basket 7 is positioned downstream of the cold consolidation zone 18 , the functioning of which has already been described above.

[0030] After passing the calibration basket 7, the film bubble 6 reaches the effective area of the flattening device 9, in which the round film tube is transformed into an elliptical cross-section with increasing eccentricity until the film tube finally forms a double-layer plastic film in the influence area of the pulling device, which in particular includes two pulling rollers 10, which is connected to each other at its sides.

[0031] The flattening device is rotatably arranged, wherein the axis of rotation is substantially aligned with the Figure 1 The hose or symmetry axis 11 shown by a dashed line is aligned. The rotatability of the flattening device is indicated by an arrow 12.

[0032] also, Figure 1 A reversing device 15 is shown, the task of which is to guide the flattened film hose from the flattening device to a stationary roller 16 without causing damage.

[0033] Arrow 17 indicates that the film hose is guided after passing through the reversing device 15 for further processing, which is not explained in detail here.

[0034] As viewed in the conveying direction z, at least one detection device 20 is arranged between the annular nozzle 5 and the calibration basket 7 as part of the speed determination device. This detection device can at least partially detect the surface area of the film bubble 6. The detection device 20 is arranged outside the film bubble 6 but is directed toward it. The detection device 20 can be attached directly or indirectly to any component of the blown film system 1. However, it is also conceivable for the detection device 20 to be installed independently of the blown film system 1 on its own stand (e.g., a tripod) within the production plant.

[0035] The detected measured values of the detection device 20 are supplied via a data line 21 to a calculation and control unit 22 , with which the above-described data processing can be carried out.

[0036] The computing and control unit can output control commands to various components of the film extrusion system via control lines 23. The arrows on control lines 23 exemplify that the cooling ring is controllable. The volume flow and / or temperature of the cooling ring can be adjusted in sections. Furthermore, the calibration basket 7 can be moved in or against the conveying direction z via control commands.

[0037] Figure 2 Shown Figure 1 The circumferential surface of the melt bubble or film bubble is virtually divided into partial surfaces 25. For each of these partial surfaces 25, a measured value, such as a temperature measured value, can be determined by a sensor to determine the local velocity of the melt or film.

[0038] Reference Signs List

[0039] 1 Blown film facility

[0040] 2 Extruder

[0041] 3 lines

[0042] 4 nozzle tips

[0043] 5 Ring nozzle

[0044] 6Film bubble / melt bubble

[0045] 7 Calibration basket

[0046] 8. Temperature control device

[0047] 9 Flattening device

[0048] 10 pulling rollers

[0049] 11 Hose and axis of symmetry

[0050] 12 Arrows

[0051] 13 Internal cooling equipment 14

[0053] 15 Reversal Device

[0054] 16 fixed position rollers

[0055] 17 Arrow

[0056] 18 Cold solidification zone 19

[0058] 20 Testing Equipment

[0059] 21 Data Line

[0060] 22 Computing and control unit

[0061] 23 Control lines twenty four

[0063] 25 partial noodles

[0064] z conveying direction

Claims

1. A film extruder for producing a plastic film, in particular a blown film extruder, comprising a nozzle, in particular an annular nozzle, for providing a plastic melt, At least one pulling roller is arranged downstream of the nozzle and at a distance from the nozzle, and is used to pull the plastic melt solidified into a plastic film in the cold solidification zone in the conveying direction, wherein The pulling roller rotates at a circumferential speed, It is characterized by: A speed determination device is provided for determining a local conveying speed of at least one partial surface of the plastic melt and / or the plastic film.

2. The film extruder according to claim 1, It is characterized by: A calculation and control device is provided, with which a deviation of the transport speed from the circumferential speed of the pulling roller can be calculated, wherein if the deviation falls below a threshold value, the partial surface can be associated with the plastic film.

3. A film extruder according to any one of the preceding claims, It is characterized by: The speed determination device can be used to determine the local conveying speed of multiple partial surfaces, wherein the deviation can be determined for each partial surface, and at least some of the partial surfaces can be associated with the plastic film or plastic melt, wherein the spatial position of the cold consolidation zone can be determined from the association.

4. A film extruder according to any one of the preceding claims, It is characterized by: The film extruder is a blown film extruder having a calibration basket, wherein the calibration basket can be positioned downstream of the coldset zone position using the computing and control device.

5. A film extruder according to any one of the preceding claims, It is characterized by: The speed determination device can be used to determine local conveying speeds of a plurality of partial surfaces transversely to the conveying direction, wherein deviations of the local conveying speeds from an average conveying speed can be determined using the calculation and control device, wherein the thickness profile of the plastic film can be derived from the deviations.

6. A film extruder according to any one of the preceding claims, It is characterized by: The speed determination device includes at least one infrared camera, with which infrared images of the plastic melt and / or plastic film can be acquired, wherein a computing and control device is provided, with which the average temperature of multiple partial surfaces of the surface of the plastic melt or plastic film can be determined from multiple infrared images, wherein at least one temperature deviation from the average temperature can be determined, wherein the transmission speed of the temperature deviation can be determined along the surface of the plastic melt or plastic film.

7. A method for producing a plastic film using a film extruder, in particular a blown film extruder, wherein: Providing a plastic melt by means of a nozzle, in particular an annular nozzle, of the film extruder, The plastic melt solidified into a plastic film in the cold solidification zone is pulled in the conveying direction by at least one pulling roller arranged downstream of the nozzle and at a distance from the nozzle, wherein the pulling roller rotates at a circumferential speed, It is characterized by: The local conveying speed of at least one partial surface of the plastic melt and / or the plastic film is determined by means of a speed determination device.

Images