Film extrusion machine, and method for producing a plastic film

Abstract

The invention relates to a film extrusion machine, in particular a blown film extrusion machine, for producing a plastic film, said machine comprising: a die, in particular an annular die, for providing a plastic melt; and at least one draw-off roller, which is arranged downstream of and at a distance from the die, for drawing off the plastic melt, which has solidified in a frost zone to form a plastic film, in a transport direction, wherein the draw-off roller rotates at a peripheral speed. A speed determination device is provided for determining the local transport speed of at least a partial surface of the plastic melt and / or of the plastic film.

Description

[0001] Film extrusion machine and method for producing a plastic film >

[0002] The invention relates to a film extrusion machine, in particular a blown film extrusion machine, for producing a plastic film according to the preamble of claim 1 and to a method for producing a plastic film according to the preamble of claim 7.

[0003] To produce a plastic film, plastic granules or a mixture of several plastic granules are usually melted and liquefied in an extruder using a heater and the application of pressure. This plastic melt is fed in strand form into a die head and there distributed evenly in a distributor. This evenly distributed plastic melt is then pressed out of the nozzle of the die head due to pressure, so that the nozzle provides the plastic melt. The nozzle can be a linear nozzle (flat nozzle) or an annular nozzle. The former is used for so-called flat film extrusion, the latter for blown film extrusion. Accordingly, the melts form either a flat melt or a melt bubble. Often several extruders are used to produce several individual melts, which are combined in the die head to form a multi-layer plastic melt.This single- or multi-layer plastic melt is drawn out of the nozzle by at least one draw-off roller, which is positioned at a distance from the nozzle. On its way to the draw-off roller and / or during contact with the draw-off roller, the plastic melt cools sufficiently to form the plastic film to be produced. The transition region, where the plastic melt solidifies and hardens into the plastic film, is often referred to as the frost zone or frost line, since the plastic melt, symbolically speaking, freezes in this area.

[0004] In a blown film extrusion machine, further components are arranged in the transport direction of the plastic melt or the plastic film (hereinafter also referred to as “melt” or “film”). These can include an internal cooling device which extends from the die head in the transport direction of the melt or film and is arranged inside the die. With this internal cooling device, a slight overpressure can be introduced into the interior of the melt bubble, causing it to be inflated, i.e. stretched, transversely to the transport direction. The peripheral speed of the take-off rollers is generally greater than the discharge speed of the melt from the ring die, so that the melt can be stretched or is stretched in the transport direction. Furthermore, a calibration basket and / or a flattening device can be provided downstream of the ring die. With a calibration basket, the melt bubble orThe film tube can be formed into a circular shape or held in this shape. With a flattening device, the film tube can be successively flattened and converted into a double-layered film. This allows the film to be easily pulled off the take-off roller. A pressure roller is often attached to the take-off roller to prevent the film from slipping on the take-off roller.

[0005] In a flat film extrusion line, the melt, which has been stretched to its desired width, is fed, usually under the influence of gravity, onto a chill roll (the so-called chill roll). The chill roll is positioned at a distance from the flat die. The peripheral speed of the chill roll is generally greater than the melt's discharge velocity from the flat die. On the temperature-controlled chill roll, the melt solidifies into film; this area is also referred to as the frost zone. On its way from the flat die to the chill roll, its properties can be influenced, for example, by means of electrodes and / or suction elements (so-called vacuum boxes).

[0006] A disadvantage, however, is that the flow behavior of the melt before it has turned into a film is very sensitive to external influences. Different temperature distributions in the extruders can lead to inhomogeneous properties of the melt. Contamination or other external influences such as drafts in the production hall where the film extrusion machine is installed can also lead to different temperature behavior within the melt. Such inhomogeneities, in turn, lead to different flow behavior across the length of the melt between the nozzle and the frost zone and / or in the transverse direction. However, such differences are transferred into the film in the frost zone, so that the film can have different quality across its length and / or width. However, detecting such inhomogeneities in the melt and / or film is sometimes difficult or even impossible.Accordingly, inappropriate settings are often made on film extrusion machines, for example for the positions of machine components, but also for parameter controls or control loops.

[0007] The object of the present invention is therefore to propose a detection possibility for inhomogeneities within the melt and / or the film.

[0008] According to the invention, this object is achieved by all features of claim 1. Possible embodiments of the invention are specified in the dependent claims.

[0009] According to the present invention, a speed determination device is provided for determining the local transport speed of at least a partial area of ​​the plastic melt and / or the plastic film. The local transport speed of the melt and / or the film, in particular of the surface, is accordingly determined for a partial area. The transport speed of the melt and / or the film generally varies considerably between the nozzle and the frost zone. Directly upon exiting the nozzle, the melt has a low local transport speed, which, however, increases considerably, at least for partial areas, due to the higher peripheral speed of the take-off roll. In blown film systems in particular, the local transport speed increases during film stretching as a result of the internal pressure applied by the internal cooling.As the melt cools, the transport speed approaches the circumferential speed of the take-off roller. The local transport speed is basically a three-dimensional vector whose direction can be oriented differently relative to the transport direction, with one component running parallel to the transport direction. If the local transport speeds are determined for several partial areas, a velocity vector field can be created from this. It is advantageous if the local speeds are determined for partial areas for at least 30%, preferably at least 50%, of the total width or the total circumference of the melt or film after it leaves the nozzle. The local speeds can be absolute local speeds or relative local speeds.To determine relative speeds, the local speeds of a partial area can be recorded over a period of time and an average calculated from these. The deviations from this mean local speed are then the relative local speeds. Instead of or in addition to the temporal averaging, the local speeds of neighboring partial areas can also be used. A computing and control device is provided to calculate these average values ​​and the relative local speeds. The speed determination device preferably comprises detection devices such as sensors or cameras, which in particular enable a spatially resolved determination of the local speeds. These detection devices are naturally connected to the computing and control device via a data line.

[0010] By determining the local velocities, the melt and / or the film can be characterized. It is known that areas of the melt with higher temperatures, in particular, flow more rapidly than other areas. A higher local velocity can therefore be detected in these areas. Such areas can therefore lead to local thin spots in the subsequent film. An inhomogeneous layer composition of the melt can also lead to different local velocities, since the individual layers can exhibit different cooling behavior. Overall, different local velocities represent possible inhomogeneities in the melt and / or the film. The determined local velocities can therefore serve as a basis for improved settings on the film extrusion machine. The advantage here is that the device according to the invention can be used independently of the temperatures in the melt or film.The film also works independently of the (average) thickness of the melt or film.

[0011] The speed-determining device according to the invention, with the configurations just described, can be combined not only with a film extrusion machine, but also, in principle, with machines used in plastics processing in which heated or heated plastics are cooled. One possible combination includes, for example, devices in which a plastic film is heated and then cooled again. This includes a stretching device in which a plastic film is heated to a temperature range between the melting temperature and a temperature up to 20 degrees Celsius below the melting temperature, then stretched in the transport direction, and then cooled again.Such a device comprises a heating device for heating the plastic film, a first stretching roller operated at a peripheral speed, a second stretching roller operated at a peripheral speed greater than the peripheral speed of the first stretching roller, and an optional cooling and / or fixing unit. In this process, the plastic film is stretched longitudinally, whereby the film width decreases and the edges simultaneously thicken (so-called neck-in). The general goal of this process is always to reduce the thickening of the edges, as these thickened areas are of no use in further processing of the film and must be removed. It is therefore expedient to determine local speeds of partial areas of the film using a speed-determining device as described above.A speed determination device according to the invention can therefore be advantageously used in such a stretching device.

[0012] Further devices include those that produce a plastic molded part from plastic material at a temperature above its melting point, for example, by injection molding, blow molding, and the like. Calendering devices can also be combined with a speed-determining device as described above. All of the above and following features can be combined with such devices, even if they are only described in connection with film extrusion machines.

[0013] In an advantageous embodiment of the invention, it is provided that the deviation of the local transport speed of a partial area from the peripheral speed of the take-off roller can be calculated using a computing and control device, wherein if the deviation is below a threshold value, this partial area can be assigned to the plastic film. This means that the local transport speed is determined for one or more partial areas, which in particular is an absolute local transport speed. This transport speed is compared with the peripheral speed of the take-off roller by forming a difference. If the absolute value of this difference is below a threshold value, i.e. within a tolerance range, the compared speeds can be regarded as equal. This partial area is then assigned to the plastic film, i.e. a location at which the plastic film has already been formed.This means that the frost zone has already been passed through. Under constant process conditions in the film extrusion machine, the frost zone is generally stationary. This makes it possible to perform this procedure sequentially or simultaneously for different sections. This allows the precise location of the frost zone to be determined.

[0014] In a further advantageous embodiment of the invention, the film extrusion machine is a blown film extrusion machine with a calibration basket, wherein the calibration basket can be positioned downstream of the frost zone using the computing and control device. In particular, the calibration basket can be positioned a maximum of 2 meters, preferably a maximum of 1 meter, and in particular a maximum of 0.5 meters downstream of the frost zone. A calibration basket serves to ensure that the film, which has become a film tube in the frost zone, no longer changes in diameter and is also approximately circular. For this purpose, a calibration basket comprises film guide elements that are arranged outside the circumference of the film tube and thus circumscribe the film tube.Since contact between the film guide elements and the film tube is desired or – in the case of contactless guide elements – cannot be ruled out, the goal is to always position the calibration basket downstream of the frost zone to prevent damage to the film caused by the film guide elements. With the device according to the invention, the position of the frost zone can be determined even if the calibration basket covers this frost zone, i.e., especially if the calibration basket is not yet properly positioned in the frost zone.

[0015] In an advantageous embodiment of the invention, it is provided that the speed determination device determines the local

[0016] Transport speeds of a plurality of partial surfaces can be determined transversely to the transport direction, whereby deviations of the local transport speeds from an average transport speed can be determined using the computing and control device, and a thickness profile of the plastic film can be derived from the deviations. Particularly in a blown film line, controlling a uniform film thickness of the subsequent film across its width or circumference is particularly important. In known film extrusion machines, the film thickness is measured with thickness measuring systems downstream of the frost zone, and these measured values ​​are used to influence the melt differently transversely to its transport device.However, there is a time lag between the measurement and the influencing of the film, which can be reduced by the film extrusion device according to the invention, since the speed can now also be determined at the melt, i.e., upstream of the frost zone. In this context, it is advantageous if the device comprises a tempering device, in particular a cooling device, which comprises several segments distributed over the circumference of the melt or film, wherein each segment can be used to apply an air stream to the melt or film, which can be adjusted differently with regard to the volume flow and / or temperature. This adjustment can then be made for each segment by the computing and control device.Additionally or alternatively, the invention makes it possible to determine longitudinal tolerances, in particular longitudinal tolerances with regard to the film thickness, i.e. thickness changes, in the transport direction of the film tube.

[0017] It is particularly advantageous that the film extrusion device according to the invention can be used to simultaneously determine deviations from the local transport speeds both in the transport direction and transversely thereto. This means that the local film thickness can be determined not only at one position around the circumference, but also at various positions in the transport direction, i.e. in the longitudinal direction. In this way, it is possible to determine the development of differences in melt or film thickness at least partially in the transport direction. This also makes it possible to determine the quality of the flatness 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 and film thickness can only be determined relative to an average film thickness. A film thickness measuring device can be provided in the film extrusion machine to determine an average film thickness.In this case, the measured values ​​of the film thickness gauge are taken into account by the computing and control device when determining the local deviations.

[0018] In a preferred embodiment of the invention, the speed determination device comprises at least one infrared camera with which infrared images of the plastic melt and / or the plastic film can be recorded, wherein a computing and control device is provided with which an average temperature for a plurality of partial areas of the surface of the plastic melt or the plastic film can be determined from a plurality of infrared images, wherein at least one temperature deviation from the average temperature can be determined, wherein the transport speed of this temperature deviation along the surface of the plastic melt or the plastic film can be determined. A temperature can be determined by means of an infrared camera in such a way that the radiation intensity of a defined infrared wavelength range is determined. The radiation intensity depends, among other things, on the temperature.The computing and control device can average several consecutively recorded temperature values ​​for each partial area, so that an average temperature can be calculated for each partial area. This average temperature can, of course, be calculated dynamically, which means that most recently recorded temperature values ​​are taken into account when averaging is calculated again. However, if a different temperature is determined for a partial area, this represents an inhomogeneity in the film in this partial area. If, after a time interval, this or a similar pattern of temperature deviations is detected in a neighboring or at least a nearby partial area, the local velocity of the melt or the film can be determined.Interferences, particularly interference contours such as an internal cooling device, are easy to detect because the temperature influences they cause are stationary and can therefore be taken into account, for example by being calculated out by the computing and control device. This is particularly advantageous when it comes to determining the absolute temperature. An absolute temperature can be determined for at least a partial area using the computing and control device if a reference temperature is known. For example, a temperature measuring unit can be integrated into the nozzle head, with which the nozzle head temperature can be measured in the area of ​​the nozzle. In this area, this nozzle head temperature is in equilibrium with the melt temperature, so that the melt temperature can be deduced from the temperature measurements.However, it is not necessary to use the temperature values ​​recorded for each partial area and the resulting average values ​​to calculate the local velocities. For example, the described method can be used to determine a temperature distribution of the film bubble, particularly between the nozzle and the calibration device. By determining the deviations from the calculated average value in a specific area, general temperature fluctuations can be determined, for example, to determine changing properties of the film, such as the thickness distribution.

[0019] It is advantageous if the infrared camera comprises a two-dimensional sensor so that the radiation originating from different sub-areas of the surface of the melt or the film can be detected at the same time. With such a sensor, more than one detection (so-called "images"), preferably more than five detections per second, can generally be realized. An "image" can be understood as a two-dimensional matrix of radiation intensity values. With the latter detection frequency, it is also conceivable for several detections to be added together. Although this reduces the effective frequency, it improves the signal-to-noise ratio. In order to improve the signal-to-noise ratio, several cameras can be provided so that either the images of the same sub-areas are correlated between the cameras or several sensor areas within a camera can record the radiation from a single sub-area of ​​the melt or film.of the film. Additional cameras can also be provided to detect larger areas of the melt or film. In the case of a blown film line, it is conceivable to distribute at least two cameras, preferably at least three cameras, and especially at least four cameras, particularly evenly around the circumference. Instead of one or more infrared cameras, cameras with other types of sensors can also be provided, for example, color cameras or cameras with vibration sensors.

[0020] It is further advantageous if the device comprises a data storage device in which at least one speed profile or at least one speed vector field of a film production process can be stored. For a current film production process, for example, the current speed profile can be compared with an older speed profile using the control and computing device, and advantageous adjustments to the process and / or setting parameters for the current film production of the film extrusion machine can be derived. A stored speed profile can also be transmitted to another film extrusion machine via a data connection. There, the values ​​can be scaled to different machine parameters, so that a stored speed profile is also available on this additional film extrusion machine.

[0021] The above-mentioned object is also achieved by a method for producing a plastic film using a film extrusion machine, in particular a blown film extrusion machine, wherein a plastic melt is provided using a nozzle of the film extrusion machine, in particular an annular nozzle, and wherein the plastic melt, which has solidified into a plastic film in a frost zone, is drawn off in a transport direction using at least one draw-off roller arranged downstream of and at a distance from the nozzle, wherein the draw-off roller rotates at a peripheral speed.

[0022] The method according to the invention is characterized in that the local transport speed of at least a partial surface of the plastic melt and / or the plastic film is determined using a speed-determining device. This method achieves the same advantages that have already been described in connection with the film extrusion device according to the invention. Furthermore, this method is not limited to a film extrusion machine in combination with the production of a plastic film, but can also be combined with other devices, as described above.

[0023] 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 description may be essential to the invention individually or in any combination of mentioned features. Within the scope of the entire disclosure, features and details described in connection with the method according to the invention naturally also apply in connection with the film extrusion machine according to the invention, and vice versa, so that with regard to the disclosure, reference is always made to the individual aspects of the invention. The individual figures show:

[0024] Fig. 1 Side view of a blown film line according to the invention

[0025] Fig. 2 is an enlarged detail from Figure 1

[0026] Figure 1 shows a film extrusion machine, namely a blown film system 1, which comprises at least one extruder 2, with which plastic, for example in granulate form, can be plasticized. The plastic melt thus produced is fed via a line 3 to a nozzle head 4, from which this melt can be converted into a film bubble 6 by being forced out of a non-visible annular nozzle 5. The melt emerging from the nozzle can now be drawn off. A melt bubble 6, which has not yet solidified, is now present. This is inflated from the inside in a tube formation zone by a slight overpressure so that it has a larger diameter within the calibration basket 7. For this purpose, an internal cooling device 13 is provided, which is located inside the annular nozzle 5 and extends partially in the transport direction. This internal cooling device 13 can be supplied with air through the extrusion tool.

[0027] The film bubble solidifies through cooling, with some of the film bubble's heat being dissipated into the environment. Cooling air is supplied to the film bubble, particularly by a temperature control device 8, which is often referred to as a cooling ring due to its ring-like design enclosing the film tube. The cooling air flow can be adjusted to varying degrees in terms of its volume flow and / or temperature at different angular ranges, thus affecting the film tube differently in different circumferential sections.

[0028] The melt solidifies into a film in frost zone 18.

[0029] A calibration basket 7 is preferably positioned downstream of the frost zone 18, the mode of operation of which has already been described above.

[0030] After passing through the calibration basket 7, the film bubble 6 enters the effective area of ​​a flattening device 9, in which the circular film tube is converted into an elliptical cross-section with an increasing eccentricity, until it finally forms a double-layered plastic film in the area of ​​influence of the take-off device comprising in particular two take-off rollers 10, which are connected to one another at their sides.

[0031] The flattening device is arranged so that it can rotate, with the axis of rotation essentially aligned with the hose or symmetry axis 11, which is indicated by a dot-dash line in Figure 1. The rotatability of the flattening device is indicated by arrow 12.

[0032] Figure 1 also shows a reversing device 15, which is designed to guide the flattened film tube from the flattening device to the stationary roller 16 without causing damage. Arrow 17 indicates that this film tube, after passing through the reversing device 15, is guided to further processing, which is not specified in detail here.

[0033] Viewed in the transport direction z, at least one detection device 20 is arranged between the annular nozzle 5 and the calibration basket 7 as a component of a speed determination device, with which at least partial surface areas of the surface of the film bubble 6 can be detected. The detection device 20 is arranged outside the film bubble 6, but directed towards it. The detection device 20 can be attached directly or indirectly to any component of the blown film line 1. However, it is also conceivable to install the detection device 20 independently of the blown film line 1 on a separate frame, for example a tripod, within the production facility.

[0034] The detection measurement values ​​of the detection device 20 are fed to the computing and control unit 22 via a data line 21, with which the data processing described above can be carried out.

[0035] The processing and control unit can issue control commands to various components of the film extrusion system via control lines 23. The arrows on the control lines 23 indicate, by way of example, that the cooling ring is controllable. Here, the volume flow and / or the temperature of the cooling ring can be adjusted segment by segment. Furthermore, the calibration basket 7 can be moved in or against the transport direction z via the control commands.

[0036] Figure 2 shows an enlarged section of Figure 1. The peripheral surface of the melt or film bubble is virtually divided into partial areas 25. For each of these partial areas 25, the sensor can determine a measured value, for example a temperature value, for determining the local velocity of the melt or film.

Claims

Patent claims Film extrusion machine, in particular blown film extrusion machine, for producing a plastic film with a nozzle, in particular an annular nozzle, for providing a plastic melt, at least one take-off roller arranged downstream and at a distance from the nozzle for taking off the plastic melt solidified in a frost zone to form a plastic film in a transport direction, wherein the take-off roller rotates at a peripheral speed, characterized in that a speed determining device is provided for determining the local transport speed of at least one partial area of ​​the plastic melt and / or the plastic film.Film extrusion machine according to claim 1, characterized in that a computing and control device is provided with which the deviation between the transport speed and the peripheral speed of the take-off roller can be calculated, wherein, if the deviation is below a threshold value, this partial area can be assigned to the plastic film. Film extrusion machine according to one of the preceding claims. characterized in that the speed-determining device can be used to determine the local transport speed of a plurality of partial areas, wherein the deviation can be determined for each partial area and at least some of the partial areas can be assigned to the plastic film or the plastic melt, wherein a spatial position of the frost zones can be determined from the assignment. Film extrusion machine according to one of the preceding claims, characterized in that the film extrusion machine is a blown film extrusion machine with a calibration basket, wherein the calibration basket can be positioned downstream of the position of the frost zone using the computing and control device.Film extrusion machine according to one of the preceding claims, characterized in that the speed determining device can be used to determine the local transport speeds of a plurality of partial surfaces transversely to the transport direction, wherein deviations of the local transport speeds from an average transport speed can be determined with the computing and control device, wherein a thickness profile of the plastic film can be derived from the deviations.Film extrusion machine according to one of the preceding claims, characterized in that the speed determination device comprises at least one infrared camera with which infrared images of the plastic melt and / or the plastic film can be recorded, wherein a computing and control device is provided with which an average temperature for a plurality of partial areas of the surface of the plastic melt or the plastic film can be determined from a plurality of infrared images, wherein at least one temperature deviation from the average temperature can be determined, wherein the transport speed of this temperature deviation along the surface of the plastic melt or the plastic film can be determined. Method for producing a plastic film using a film extrusion machine, in particular a blown film extrusion machine, wherein a plastic melt is provided using a nozzle of the film extrusion machine, in particular an annular nozzle, with at least one take-off roller arranged downstream and at a distance from the nozzle, which plastic melt solidified into a plastic film in a frost zone is drawn off in a transport direction, wherein the take-off roller rotates at a peripheral speed, characterized in that the local transport speed of at least one partial surface of the plastic melt and / or the plastic film is determined using a speed determining device.

Images