Method and apparatus for inspecting extrusion device

Abstract

The invention relates to a method for checking the adjustment of an extrusion device which produces a tubular strand conveyed in the longitudinal direction of the extrusion device, the outlet width of the material to be extruded being adjusted differently on the upper and lower sides of the extrusion nozzle of the extrusion device, the method has the following steps: measuring the refractive index on the cross-section of at least one wall of the tubular strip by means of a terahertz measuring device at a first measuring point downstream of at least one first cooling section for the tubular strip, in which first measuring point the tubular strip has not yet completely solidified, and measuring the refractive index on the cross-section of at least one wall of the tubular strip by means of a terahertz measuring device downstream of the at least one first cooling section for the tubular strip, a geometric value is measured at a first measurement point by means of the terahertz measurement device, in addition at at least one measurement point on the upper side of the tubular strip and at least one measurement point on the lower side of the tubular strip, the geometric value having a wall thickness and / or an inner diameter and / or an outer diameter of the tubular strip, for the ratio of the measured refractive index and for the geometric values measured on the upper and lower sides of the tubular strip, the refractive index is measured by means of a previously determined calibration relationship between the refractive index at a first measurement position and the ratio of the outlet width for the extruded material on the upper and lower sides of the extrusion nozzle. The ratio of the outlet width set on the extrusion device for the extruded material on the upper side and the lower side of the extrusion nozzle is checked. The invention further relates to a device for carrying out the method according to the invention.

Description

Technical Field

[0001] The invention relates to a method for checking the setting of an extrusion device which produces a tubular strand conveyed in the longitudinal direction, wherein the outlet width for the extruded material is set differently on the upper side and the lower side of an extrusion nozzle of the extrusion device. The invention also relates to a device for carrying out the method. Background Art

[0002] Plastic pipes, which are used, for example, to supply gas and water to residential and industrial areas, but also for drainage, are mainly made of materials such as HDPE, PP and PVC. Typical pipe diameters are up to 3 m and wall thicknesses up to 250 mm. They are usually manufactured in an extrusion device, in which the plastic raw material is melted and discharged through a generally annular extrusion nozzle. The pipe extruded in this way is pulled out of the extrusion device in the longitudinal direction and thus conveyed. The diameter of the pulled-out pipe is formed to the desired outer diameter in a calibration device, such as a sleeve-shaped device, connected downstream. During the conveying process, the pipe usually passes through a plurality of cooling sections, in which the cooling of the pipe and the continuous consolidation of the plastic melt, which is still initially flowable, are carried out. In the first cooling section, for example, the formed pipe collapses with a vacuum. The cooling of the pipe in the cooling section is often carried out by a cooling liquid, such as water. The cooling water flushes the pipe and quickly consolidates its outer area. That is, after being discharged from the first cooling section, the outer surface of the pipe is usually solidified, so that the outer geometry of the pipe is then basically no longer changed. After exiting the first cooling section, however, the usually still flowable portion of the tube material is located inside the tube wall. During the further transport of the tubular strip, in particular during the passage through further cooling sections, the tube interior also continues to cool and thereby solidify to the inner surface of the tube. The tube is finally cut to the desired length using a flying saw.

[0003] During the complete solidification after exiting the extruder, the shaping of the tube is influenced essentially by two effects, which are to be taken into account for the purpose of achieving a wall thickness of the tube that is, for example, as uniform as possible. On the one hand, there is the shrinkage of the tube material during the cooling process. On the other hand, there is the sagging, i.e. the falling, of the still flowable viscous mass fraction during solidification due to the influence of gravity.

[0004] As a reaction to these effects which influence the final geometry of the tubular strip, it is known to set the extrusion nozzle of the extrusion device with a greater outlet width for the extruded material in the upper region than in the lower region. In order to set the outlet width, the discharge gap of the extrusion nozzle can be set wider in the upper region than in the lower region. Alternatively or additionally, the extrusion nozzle can be heated more strongly in the upper region than in the lower region and thus achieve a greater outlet width in the upper region of the extrusion nozzle than in the region below it. That is, by these two measures, more material is discharged in the upper region of the extrusion nozzle than in the lower region. By this intentionally asymmetrical discharge of the material, the droop should be compensated so that the solidified tube has a wall thickness that remains as constant as possible over its entire circumference.

[0005] The important geometric values ​​of the tube, such as wall thickness and diameter, can be measured only after complete solidification, that is, after all cooling sections of the tube have ended, when shrinkage and sagging have completely ended. The typical output efficiency of the extrusion device is about 1000 kg / h for an average tube cross section. The outlet temperature of the melt from the extrusion nozzle is in the range of about 200°C to 240°C depending on the material. For a tube with an outer diameter of, for example, 330 mm and a wall thickness of 30 mm and a typical cooling section of 60 m for the tube, the first measurement results about wall thickness and diameter often exist only a few hours after the start of production. Only then can the influence on the production parameters of the extrusion device be carried out when determining the geometric deviation of the theoretical value, wherein the changes made can be rechecked again after a few hours. Often after the start of the process, it takes several days for the multiple corrections required for the optimized process to be carried out, so as to adjust the wall thickness evenly around the circumference and toward the nominal value, for example.

[0006] To compensate for the sag, as explained, when setting the extrusion nozzle, the outlet width is set larger in the upper region than in the lower region. In order not to undercut the minimum wall thickness even when sag occurs, an overcompensation of the sag is usually carried out, using empirical values. This ultimately leads to the discharge of more material than is necessary.

[0007] It is therefore desirable to obtain information as early as possible about the expected shrinkage and expected sag of the tube produced in the extrusion device. Measurements of the wall thickness and diameter of the tube after the first cooling section, for example, may not correspond to the desired final values ​​to be found after complete cooling of the tube, since at this point the tube wall only has solidified material in the outer region and still recrystallized portions and melt in the interior. As a result, diameter and wall thickness values ​​that are still subject to shrinkage and sag are detected at the measuring point after the first cooling section. In order to accelerate the start-up process and further continuously guarantee the nominal values, the earliest possible prediction of the expected shrinkage and sag values ​​is of great economic significance.

[0008] WO 2022 / 058081 A1 proposes a method for determining geometric parameters of a strip-shaped or plate-shaped object, using which shrinkage is predicted. To this end, in a determination step, the relationship between the refractive index of the object and the shrinkage that occurs during its consolidation is determined. In the determination step, the refractive index and at least one geometric parameter of the object that has not yet been completely consolidated are determined, especially after the first cooling section of the object, and the geometric parameters of the object in the fully consolidated state are calculated from the determined values ​​taking into account the relationship determined in the determination step. That is, the shrinkage of the material of the object during its consolidation process is predicted and based on this, the geometric parameters, such as the wall thickness of the object in the fully consolidated state, are calculated.

[0009] With the method explained, shrinkage can be reliably predicted and taken into account as one of the two effects that have a decisive influence on the final geometry, wherein the measured values ​​can advantageously already be present shortly after the material has been discharged from the extrusion device and the final geometric parameters can be calculated accordingly at an early stage in order to avoid rejects. For reasons explained in more detail below, sagging as the second decisive effect for the final geometry of the object, in contrast, cannot be predicted with the known methods.

[0010] WO 2022 / 106180 A1 discloses a method for determining the occurrence of droop in a tube extruded in an extrusion device. In this case, the wall thickness of the tube is measured over the circumference of the tube and a wall thickness distribution over the circumference of the tube is constructed from the measured wall thickness. The droop (droop) of the melt is determined from the frequency and / or amplitude of the constructed wall thickness distribution. This method enables reliable detection of droop, which is also not possible to predict with this method. Summary of the invention

[0011] Starting from the explained prior art, the object of the present invention is to provide a method and a device of the type mentioned at the outset, by means of which the outlet width set at the extrusion device for the extruded material can be checked and, if necessary, corrected reliably, in particular in real time and with a small time offset after the strip has emerged from the extrusion device, taking into account the sagging of the tubular strip produced in the extrusion device.

[0012] The invention achieves this object by means of independent claims 1 and 18. Advantageous embodiments can be found in the dependent claims, the description and the drawings.

[0013] The invention achieves the object of a method of the type mentioned at the outset by means of the following steps:

[0014] • measuring the refractive index in a cross section of at least one wall of the tubular strip by means of a terahertz measuring device at a first measuring position downstream of at least one first cooling section for the tubular strip, at which first measuring position the tubular strip has not yet completely solidified,

[0015] • measuring geometrical values ​​by means of the terahertz measuring device at a first measuring position and also at at least one measuring location on the upper side of the tubular strip and at least one measuring location on the lower side of the tubular strip, wherein the geometrical values ​​include the wall thickness and / or the inner diameter and / or the outer diameter of the tubular strip,

[0016] •For the ratio of the measured refractive index and the geometric values ​​measured on the upper and lower sides of the tubular strip, the ratio of the outlet widths set on the extrusion device for the extruded material on the upper and lower sides of the extrusion nozzle is checked with the aid of a previously determined calibration relationship between the refractive index at a first measuring position and the ratio of the outlet widths for the extruded material on the upper and lower sides of the extrusion nozzle.

[0017] The invention further achieves this object by means of a device for carrying out the method according to the invention, which device has a terahertz measuring device and an evaluation device which is designed to check the ratio of the set outlet widths for the extruded material on the upper side and the lower side of the extrusion nozzle.

[0018] By means of the method according to the invention and the device according to the invention, it is possible to predict the sagging, i.e. the falling, of the still flowable viscous mass fraction of the tubular strip discharged from the extrusion device during solidification under the influence of gravity and then expected at the first measuring position, and on this basis the setting of the extrusion device can be checked and corrected if necessary. The tubular strip can be a tube, for example a plastic tube. The extrusion device has, in a manner known per se, for example an annular extrusion nozzle in principle, from which the previously molten plastic material is discharged. In a manner also known per se, the tubular strip follows the discharge from the extrusion device through usually a plurality of cooling sections, in which the extruded material is continuously cooled, for example by a cooling liquid, such as water, until it cools down and thereby completely solidifies. For example, after discharge from the first cooling section directly following the extrusion device, the outer surface of the tubular strip can already solidify, so that the shaping of the outer surface of the tubular strip is completed. Inside the tube wall, the material of the tubular strip, on the other hand, has still recrystallized regions and melt portions and is correspondingly still at least partially flowable, wherein, as is known, sagging occurs in addition to shrinkage during the further cooling of the strip. For the final shaping of the tubular strip in the first cooling section, it is possible to press the strip material onto the, for example, cylindrical inner surface of a calibration sleeve in this first cooling section, for example by applying a vacuum.

[0019] Rather, it is possible in principle to model the expected droop in addition to the shrinkage in detail using the Navier-Stokes equations, starting from the precisely known framework conditions and the material properties of the tubular strip. However, this is so numerically complex that the currently desired rapid prediction of the droop is not feasible. Thus, according to the invention, in order to avoid waste, a reliable prediction of the expected droop of the strip and thus the final shape should be available as early as possible after it has been discharged from the extrusion device, so that the production process can be intervened early, for example by adapting the settings of the extrusion device, if necessary. Preferably, the prediction of the droop should be carried out in real time. Both are against the use of the inherently reliable Navier-Stokes equations in this application case.

[0020] Unlike shrinkage, as already mentioned, sag cannot be predicted in the manner explained in WO 2022 / 058081 A1. The shrinkage from the extrusion temperature until complete cooling is independent of the time course of solidification, in particular the cooling rate. Therefore, the shrinkage can be predicted in a simple and reliable manner by comparing the refractive index measured at the first measuring position and the cold value of the refractive index. Changes in, for example, the parameters of the extrusion device or the cooling parameters also have no significant influence on the shrinkage. It is essentially a process characteristic that is immutable, but can be predicted relatively easily.

[0021] This is different when sagging, where the process is much more complicated than in the case of shrinkage. In sagging, first of all, the temperature-dependent flow characteristics of the material, the material temperature and the cooling rate play an important role. In general, it can be said that the higher the initial temperature of the melt and the more time it takes until the melt cools and completely solidifies, the more obvious the melt sags during the production of the tube. The variable operating conditions of the extrusion temperature, delivery power, pull-out speed and the intensity and duration of the cooling of the extruder affect the degree of sagging. Although it is sufficient to stop the equipment for shrinkage prediction with the help of the refractive index, where the change of the refractive index is recorded together with the shrinkage wall thickness and diameter for several hours of cooling, this reliable method does not work for predicting sagging, precisely because when cooling is carried out at longer or shorter time intervals, the sag of the wall thickness is significantly stronger or weaker. Unlike shrinkage, sagging can also be decisively affected by changes in the parameters of the extrusion device and the cooling parameters for the tubular strip.

[0022] In order to solve the problem, according to the invention, firstly, by means of a (first) terahertz measuring device, at a first measuring position downstream of at least one first cooling section for the tubular strip, on the one hand, an average or synthetic refractive index is measured on the cross section of at least one wall thickness of the tubular strip, and on the other hand, geometrical values ​​of the tubular strip, including the wall thickness and / or the inner diameter and / or the outer diameter, are measured at at least two measuring locations on the upper side and the lower side of the tubular strip at the first measuring position, wherein the tubular strip is not yet completely solidified, i.e., has a recrystallized and / or melt content that is still flowable. For this purpose, the terahertz measuring device has a terahertz transmitter and a terahertz detector, which can be arranged essentially at the same location and can also be combined into a transceiver in a particularly practical manner. The terahertz measuring radiation used according to the invention can be, for example, in the frequency range of 1 GHz to 6 THz. Terahertz radiation is well suited for difficult measuring conditions in the environment of an extruder, in particular with high temperatures, possible contaminants, and the presence of steam and the like. In contrast to laser radiation, for example in the visible frequency range, terahertz radiation is thus largely insensitive to such interference. The terahertz radiation emitted by the transmitter of the terahertz measuring device irradiates the tubular strip, for example, from above and / or from below. Here, a portion of the terahertz radiation is reflected on the surface of the outside of the strip, and another portion of the terahertz radiation is incident on the extruded material. Here, further reflection occurs at the boundary of the tubular strip, in particular at the boundary of the wall section of the strip facing and away from the emitter, respectively. A portion of the terahertz radiation also emerges from the strip again on the side opposite to the emitter. The terahertz measuring device can have a reflector there, which reflects the terahertz radiation emitted on the side of the strip facing away from the emitter back to the strip again, so that the terahertz radiation, after further reflection on the boundary of the strip, arrives together with the remaining radiation portion reflected on the boundary, for example, at a detector arranged at the same location as the emitter and is detected by the detector as a measurement event.

[0023] In a manner known per se, as explained in more detail below, the refractive index on the cross section of one or a second opposing wall section of the strip can be determined from the detected radiation at a first measuring position. The refractive index can be measured on the upper side and / or the lower side of the strip. In particular, in the case of two, for example, opposing wall sections of the transmitted strip, the average or combined refractive index through the two transmitted wall sections can be determined. In addition, the distance to each boundary surface of the strip can be determined in a manner known per se, for example by means of a propagation time measurement, from which geometric parameters of the strip such as the wall thickness and / or the inner diameter and / or the outer diameter can be determined. For example, the measurement on the upper side can be carried out at the highest point of the strip and the measurement on the lower side can be carried out at the lowest point of the strip. In particular, the upper wall thickness and the lower wall thickness of the tubular strip can be measured at the first measuring position. The upper wall thickness and the lower wall thickness are usually different because - as explained at the beginning - the extrusion device is usually set up so that, in order to compensate for the sag, more material is discharged on the upper side of the extrusion nozzle than on the lower side of the extrusion nozzle. In the region of the (first) terahertz measuring device, ie in particular directly downstream of the first cooling section, this asymmetry usually still exists.

[0024] According to the invention, for the prediction of the expected droop and the inspection of the extrusion device, a previously determined calibration relationship is used. The basis for using the calibration relationship is the refractive index measured at the first measuring position. The calibration relationship assigns the refractive index at the first measuring position to the ratio of the set outlet width of the extruded material on the upper and lower sides of the extrusion nozzle. Like the ratio of the measured geometric values ​​on the upper and lower sides of the strip, the ratio of the outlet width on the upper and lower sides of the extrusion nozzle can be given as a simple quantitative ratio or, for example, a percentage. Since the strip usually also has a region containing recrystallized material and melt content, especially thus still flowable material in the region of the first measuring position, the measured refractive index is different from its cold value. The measured value of the refractive index provides information about the flowable content, especially the recrystallized content and / or the melt content, still present inside the tubular strip at the first measuring position. The calibration relationship - especially for the defined geometric parameters of the strip at the first measuring position and the defined production parameters of the extrusion device - provides an assignment relationship between the refractive index measured at the first measuring position and the ratio of the outlet width of the extrusion nozzle required for the desired strip geometry in the fully solidified state. This ratio of the outlet width obtained according to the calibration relationship to the measured refractive index can be compared with the ratio of the set outlet width of the extrusion device for checking the extrusion device. As explained at the beginning, the outlet width can be set, for example, by setting the outlet gap of the extrusion nozzle and / or by controlling the heating device of the extrusion nozzle. For example, it is conceivable to carry out a basic setting of the outlet width by setting the outlet gap of the extrusion nozzle and then only to carry out a subsequent fine adjustment or adjustment of the outlet width by controlling the heating device of the extrusion nozzle.

[0025] Merely detecting the wall thickness at the first measuring position, for example on the top and bottom of a tubular strip, is not sufficient for the prediction according to the invention about the expected sag and thereby the checking of the setting of the extrusion device. This is because the wall thickness ratio above / below the strip determined at the first measuring position does not provide information about the extent to which the previous sag has already occurred or even ended. Only by combining the (synthetic) refractive index measured at the first measuring position and preferably evaluating it weighted to the top / bottom wall thickness ratio measured at the first measuring position can it be assessed whether the sag is completely compensated in the further course of the cooling section, so that the strip has a predetermined wall thickness distribution in the fully solidified state.

[0026] Based on the simplified measures according to the invention, the prediction of the sag still to be expected downstream of the first measuring point and the checking of the extrusion device can be carried out quickly and in real time, so that, for example, the extrusion device can be controlled correspondingly quickly in order to adjust the discharge of the strip material from the extrusion nozzle, especially on the upper and lower sides of the extrusion nozzle, so that the desired geometry of the strip in the completely solidified state is achieved. The desired geometry of a tubular strip is often a circular geometry in cross section with a wall thickness that is constant over the circumference.

[0027] The calibration relationship can be stored in the form of a function or a curve. The calibration relationship can be determined empirically in a particularly simple manner. For this purpose, the corresponding production process can be such a situation in the setting of the outlet width of the extrusion nozzle, that is, so that the tubular strip has a predetermined geometric structure on its circumference after it is completely solidified, for example, a uniform wall thickness on its circumference, and the measured refractive index and the geometric values ​​on the upper side and the lower side, especially the quotient of the measurement of the wall thickness, obtained at the first measuring position, are stored. Then, the refractive index measured at the first measuring position can be compared with the stored refractive index in a subsequent process. The refractive index is related to temperature as known. If the measured refractive index deviates from the stored refractive index, it can be based on the following situation, that is, the change of the temperature and thereby the still flowable portion in the strip is present at the first measuring position. It can be further based on the following situation, that is, the correction of the outlet width of the setting of the extrusion nozzle is required to achieve the desired geometric structure of the strip in the solidified state. Within the scope of the empirical formation of the calibration relationship, other settings of the exit width can be stored correspondingly for different refractive indices at the first measuring position, which settings lead to the desired geometry of the strip in the frozen state.

[0028] In particular, changes in production parameters in the area before the first measuring position can have a significant effect on the sag and thereby the geometry of the strip in the solidified state. Such changes occur in particular when starting the production process after a production pause. By way of example, the temperature of the extrusion nozzle, which increases slowly during the start-up process, or a change in the output efficiency of the extrusion device is indicated. Another example is a change in cooling parameters, for example by a change in the composition and / or temperature of the coolant. In addition to the refractive index measured at the first measuring position, such changes can be identified, in particular, also by means of geometric values ​​measured at the first measuring position. This is especially because most of the sag that occurs overall has already occurred by the time of discharge from the first cooling section. That is, according to the invention, it is possible to adjust the optimized process parameters, in particular the extrusion device, particularly early when the production process is restarted with the aid of calibration relationships.

[0029] In addition to the refractive index, changes in the production process can also be identified, in particular, by means of the geometric values ​​detected at the first measuring position. If the geometric values ​​detected at the first measuring position change, this can be inferred that the expected sag considered according to the calibration relationship has also changed. Correspondingly, it can be provided according to a design that, for example, a calibration relationship for a defined geometric structure value for a tubular strip is determined at the first measuring position. It can then be further provided that, in the case of a determined deviation of the geometric values ​​measured at the first measuring position from the defined geometric values, a warning is output and / or the calibration relationship is adapted to the determined deviation of the geometric values. For example, the calibration relationship can be adapted by a factor corresponding to a determined change. The factor can be adapted with the aid of an empirical value, for example by multiplying it with a constant factor that takes into account the influence of the change on the sag that occurs.

[0030] In determining the calibration relationship, material parameters of the tubular strip, in particular thermal conductivity and / or heat capacity, can also be taken into account. Production parameters, in particular the temperature of the extrusion nozzle and / or the withdrawal speed of the extrusion device and / or cooling parameters of the produced tubular strip can also be taken into account. Such parameters also have an influence on the droop that occurs.

[0031] According to another design, the calibration relationship can be configured to the refractive index at the first measuring position to the theoretical ratio of the outlet width of the extruded material on the upper side and the lower side of the extrusion nozzle, so that the tubular strip has a theoretical wall thickness distribution, especially a uniform wall thickness distribution, on its circumference after it is completely solidified. As already mentioned, the calibration relationship determined empirically, for example, can exist in the form of a function or a curve. The refractive index at the first measuring position can be plotted here depending on the ratio of the outlet width of the material to be extruded on the upper side and the lower side to be set to the theoretical ratio on the extrusion nozzle. In the simplest case, the curve can be a straight line with a negative slope. But the curve can also be a curve that deviates from a straight line.

[0032] According to another design, when determining the deviation between the theoretical ratio of the outlet width of the extrusion nozzle for the refractive index measured at the first measuring position according to the calibration relationship and the ratio of the adjusted outlet width of the extrusion nozzle, the theoretical ratio for the measured refractive index according to the calibration relationship can be displayed. Alternatively or additionally, the extrusion device can be controlled, preferably automatically, to adjust the theoretical ratio for the measured refractive index according to the calibration relationship. The control can be implemented by an evaluation device. Automatic control of the extrusion device is possible. The display of the theoretical ratio can be performed on an operator display for an operator.

[0033] As already explained, the expected sag of the tubular strip until the complete solidification of the tubular strip can be predicted based on a comparison of the geometric values ​​measured at the first measuring position and the outlet width set at the extrusion device. The predicted sag in the state of complete solidification of the tubular strip and / or the expected values ​​of the measured geometric values ​​and / or the theoretical ratio obtained according to the calibration relationship for the outlet width of the extruded material on the upper side and the lower side of the extrusion nozzle can then be displayed again based on the predicted sag.

[0034] According to another design, the refractive index and / or the geometric value can be measured at a plurality of measuring locations on the circumference of the tubular strip at the first measuring position. For this purpose, for example, a plurality of terahertz emitters and terahertz detectors can be arranged distributedly on the circumference of the tubular strip. The terahertz measuring device preferably rotating around the tubular strip can be provided with terahertz emitters and terahertz detectors and, if necessary, reflectors, with which measured values ​​can be generated distributedly on the circumference. In particular, in principle, it is conceivable to completely cover the circumference of the strip in this way. Compared to the measurement only from the upper and lower sides of the tubular strip, extensive measurement on the circumference, especially also on the side of the strip, provides other advantages. For example, the solidification of the side wall of the strip that may have continued to progress can be identified in this way, which may affect or prevent the downward outflow of the portion that can still flow in relation to gravity and may thereby affect sagging.

[0035] According to another embodiment, the refractive index is determined by comparing the propagation time of the measuring radiation emitted by the terahertz measuring device without the tubular strip arranged in the beam path of the measuring radiation with the propagation time of the measuring radiation with the tubular strip arranged in the beam path of the measuring radiation. This method of operation for determining the unknown refractive index of an object is described, for example, in EP 3 265 748 B1. The determination of the refractive index using a terahertz measuring device can be performed in a corresponding manner.

[0036] According to another design, the refractive index can be determined by determining the optical wall thickness of the tubular strip using the terahertz measuring device, further by determining the outer diameter and inner diameter of the tubular strip using the terahertz measuring device, and by determining the refractive index of the tubular strip by comparing the determined outer diameter and inner diameter with the determined optical wall thickness. This alternative method for determining an unknown refractive index is described in DE 10 2018 128 248 A1. This operating method can be used here again.

[0037] According to a particularly practical embodiment, the geometrical value of the tubular strip can be determined from a propagation time measurement of the measuring radiation emitted by the terahertz measuring device.

[0038] According to another embodiment, at a second measuring position spaced apart from the first measuring position in the longitudinal direction of the tubular strip, in particular at a second measuring position downstream of the first measuring position, a refractive index on a cross section of at least one wall of the tubular strip and / or a wall thickness and / or an inner diameter and / or an outer diameter of the tubular strip can be measured using a further terahertz measuring device, at which second measuring position the tubular strip is substantially completely solidified. In addition, at the second measuring position, the wall thickness and / or the inner diameter and / or the outer diameter of the tubular strip on the upper side of the tubular strip and the wall thickness on the lower side of the tubular strip can be measured using the further terahertz measuring device. The calibration relationship can also be checked and, if necessary, corrected using the measurement results of the further terahertz measuring device.

[0039] The second measuring position can be located upstream or downstream of the first measuring position in principle. Preferably, the second measuring position is located downstream, and in particular, it is preferably located so far downstream that the tubular strip is substantially completely solidified. The other terahertz measuring device can be equipped in principle like the (first) terahertz measuring device arranged at the first measuring position. The other terahertz measuring device also has a (other) terahertz transmitter and a (other) terahertz detector that can be combined into a transceiver in a particularly practical manner. Opposite to the transmitter and the detector arranged at the same location as necessary, a reflector can be arranged on the side away from the tubular strip again. The other terahertz measuring device can be arranged in a fixed position in a low-cost manner, in particular, so that it emits terahertz measurement radiation from the top vertically downward or from the bottom vertically upward onto the tubular strip. It is also conceivable that the other terahertz measuring device is a portable measuring device, in particular a so-called handheld measuring device, that is, it is not permanently arranged at the second measuring position. The refractive index and the geometrical parameters can be measured again in the manner explained above for the terahertz measuring device arranged in the first measuring position. At the second measuring position, the final material and geometrical parameters of the strip, including the cold value of the refractive index, can be measured, in particular when the tubular strip is substantially completely solidified at this second measuring position. The measurement with the additional terahertz measuring device is significantly more accurate than, for example, a measurement with a mechanical detector. The final parameters measured at the second measuring position can be compared with the parameters of the calibration relation and the calibration relation can be checked and, if necessary, adapted based on the final parameters actually measured.

[0040] The above-mentioned design is used to reduce the dependency of the material parameters of the model according to the invention. It is precisely in the area of ​​the plastic strip that the material parameters are often not known or defined accurately enough. In other words, in order to reduce the dependency of the material parameters, additional sensors can be used in the above-mentioned design to measure, for example, the cold value of the refractive index and the wall thickness at at least one angular position on the circumference of the strip or the ratio between at least two wall thicknesses at different angular positions, for example, the ratio between the wall thickness above and below the strip in a terahertz measuring device fixedly positioned vertically above the strip. The calibration relationship used according to the invention can then be verified based on the measured values ​​of the additional terahertz measuring device in order to, for example, reduce the deviation caused by the material parameters that deviate from the actual. In particular, possible changes in the process temperature downstream of the first measuring position can be detected by the additional terahertz measuring device, which, as explained, have an effect on the droop. The changed cooling rate of the strip is indicated by way of example.

[0041] For example, the relationship between the refractive index and the crystalline state of the strip material can be used empirically to determine the droop factor

[0042] SF = k * Δ N

[0043] in

[0044] SF is the droop factor

[0045] k is a factor

[0046] Δ N is the refractive index difference between the refractive index measured at the first measurement position and the cold value of the refractive index

[0047] The larger the droop factor SF determined in this way, the greater the droop is to be expected. The volume or mass flow rate of the still flowable viscous material is approximately proportional to the droop factor. With an accurate knowledge of the cold value of the refractive index, the droop factor can be determined more accurately and the relationship can be calibrated therewith.

[0048] The model according to the invention for predicting the expected sag can thereby be further improved. In particular, the sag factor at the corresponding measuring point constitutes an indicator from which it can be deduced to what extent the usually intentionally set difference in the wall thickness emerging from the extrusion nozzle in the upper region and the lower region changes in the further course of the extrusion line. Based on this, the sag factor SF pre-set in this way at the measuring point of the terahertz measuring device can be set by setting the annular gap of the extrusion nozzle of the extrusion device and / or a suitable temperature setting of the extrusion nozzle, so that the desired usually uniform wall thickness of the strip is produced over the entire circumference with the remaining sag.

[0049] According to another design, it can be provided that the calibration relationship further assigns the refractive index at the first measuring position to the ratio of the wall thickness on the upper side and the lower side of the tubular strip at the first measuring position, wherein a weighting factor is used for the conversion between the ratio of the wall thickness on the upper side and the lower side of the tubular strip at the first measuring position and the theoretical ratio of the outlet width of the extruded material on the upper side and the lower side of the extrusion nozzle, which weighting factor takes into account the different sagging degrees upstream of the first measuring position and downstream of the first measuring position. That is, a weighting can be defined between the sagging of the strip that has already occurred, which is determined by the measurement of the geometric values ​​at the first measuring position, and the sagging that is expected to occur until complete solidification. The comparison of the geometric values ​​measured at the first measuring position with the outlet width set on the extrusion nozzle results in the sagging that has already occurred. If, for example, the following situation is taken into account, that is, the sagging continues after the first measuring position, but in a smaller range, then the sagging that is expected can be predicted by multiplying the sagging that has already occurred by a number less than 1, taking into account the refractive index measured at the first measuring position. On the contrary, a weighting factor can be formed, which indicates to what extent the sag before the first measuring position is greater than the sag after the first measuring position. For example, by forming a first quotient of the vertical gap size of the extrusion nozzle in the upper and lower regions and a second quotient of the wall thickness measured at the first measuring position in the upper and lower regions, the two quotients can be compared with each other, for example a quotient is formed from the first and second quotients. This quotient can then form a weighting factor. Since the majority of the sag occurs between the outlet from the extrusion device and the first measuring position, and only a small sag usually occurs thereafter, the weighting factor is usually greater than 1. By way of example, the weighting factor may be approximately 3. For a certain extrusion material and constant production conditions, a generally desired uniform wall thickness of the strip can be obtained on the circumference based on such a weighting factor. By means of this weighting factor or the weighted quotient formed by the inverse, the refractive index at the first measuring position can be optionally assigned in the calibration relationship to the ratio of the wall thickness of the strip on its upper side and its lower side measured at the first measuring position or to the theoretical ratio of the outlet width on the upper side and the lower side of the extrusion nozzle of the extrusion device. It is thereby possible to directly find and predetermine the offset to be set on the extrusion nozzle by means of the geometric values ​​measured at the first measuring position and the measured refractive index. The outlet width of the extrusion nozzle can thus be controlled or set directly based on the measured values ​​taken at the first measuring position.

[0050] As long as an additional terahertz measuring device is set at the second measuring position downstream of the first measuring position, the measured values ​​of the additional terahertz measuring device can be used for the fine adjustment of the model according to the present invention and the extrusion device therewith. Here, it is used that at the first measuring position, after the first cooling section, the melt has been cooled so much that, in addition to the cold outer zone, there is also a portion of the recrystallized melt inside the strip, especially inside the tube wall. At this time, it is also possible to form a third quotient of the wall thickness measured at the second measuring position of the second terahertz measuring device 27, that is, in the completely solidified state in the upper and lower regions. This third quotient, which should be 1 in an ideal case, can then be compared with the first and second quotients. In a similar manner, it is also possible to form a quotient from the refractive index measured at the first measuring position and the refractive index (cold value) measured at the second measuring position.

[0051] In the device according to the invention, the measured values ​​of the terahertz measuring device and, if applicable, the further terahertz measuring device are applied to the evaluation device. The evaluation device is designed to carry out the evaluation and checking according to the invention. The evaluation device is also designed in particular to carry out the embodiments according to the dependent claims of the method according to the invention. The evaluation device can have a control device for this purpose, which is used in particular to control the extrusion device, in particular the extrusion nozzle, in the manner explained above. The device can also have a further terahertz measuring device accordingly. The device can also have an extrusion device. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Next, an embodiment of the present invention will be further explained with the aid of the accompanying drawings. Schematically shown:

[0053] Figure 1 A schematic side view of an apparatus for carrying out the method according to the invention;

[0054] Figure 2 Figure 1 A partial cross-sectional view of the equipment;

[0055] Figure 3 Graphs to illustrate the temperature dependence of the refractive index;

[0056] Figure 4 A graph illustrating the variability of the refractive index through radial positions within the wall of a tube that has not yet fully solidified;

[0057] Figure 5 a diagram for illustrating calibration relationships according to the present invention;

[0058] Figure 6 a diagram for illustrating the temperature distribution in the radial direction within the wall of a tube which has not yet completely solidified; and

[0059] Figure 7 Graph for illustrating different wall thicknesses measured at first measuring points over the circumference of a tubular strip. DETAILED DESCRIPTION

[0060] Unless otherwise indicated, the same reference numerals in the figures denote the same objects.

[0061] exist Figure 1 and 2 1 shows a tubular strip 10, here a tube 10, in particular a plastic tube 10, which has a wall 12, a cavity 14 defined by the tube 10, an outer surface 16 which is circular in cross section and an inner surface 18 which is also circular in cross section and defines the cavity 14. In this example, the tube 10 is extruded by means of an extruder in an extrusion device 20 and is conveyed by means of a suitable conveying device along the longitudinal axis of the tube, in the Figure 1 , the tube 10 is conveyed from left to right in the extrusion nozzle 20. After being discharged from the extrusion nozzle of the extrusion device 20, for example, an annular extrusion nozzle, the tube 10 first passes through a first cooling section 22, in which the tube 10 is cooled, the tube is strongly heated and is not yet completely solidified, that is, it is discharged from the extrusion nozzle with a recrystallized and flowable portion (melt). The first cooling section 22 can have a calibration device, in particular a calibration sleeve, to which the tube 10 is extruded, for example, by vacuum and normal pressure inside the tube 10. The outer diameter of the tube 10 preformed by the extrusion nozzle is finally determined. In the further course, the tube 10 passes through a first terahertz measuring device 24, in which the refractive index and geometric parameters of the tube 10, such as the inner diameter and / or the outer diameter and / or the wall thickness, are determined in a manner further explained below. Following the first terahertz measuring device 24, the tube 10 passes through at least one other cooling section 26, in which further cooling is performed. The dotted line of the tube 10 illustrates that other cooling sections 26 can be provided. After the tube 10 has been completely solidified, it is cut into predetermined sections in a cutting device 28 , for example with a flying saw.

[0062] With the help of Figure 2The design and function of the first terahertz measuring device 24 should be explained in more detail. In the example shown, the first terahertz measuring device 24 has a transceiver 30 in which an emitter and a detector for terahertz radiation are combined. The emitter emits terahertz radiation 32 onto the tube 10. The terahertz radiation is reflected at different interfaces of the tube 10 and at a reflector 34 arranged opposite the transceiver 30 and returns to the transceiver 30, where it is detected by the detector. The transceiver 30 is also connected to an evaluation device 38 via a line 36. The reflected radiation received by the detector generates a corresponding measurement signal, which is transmitted to the evaluation device 38 via the line 36. In this way, the evaluation device 38 can determine, for example, by means of a propagation time measurement, the reflected radiation received by the detector. Figure 2 The wall thicknesses 40, 42 and the inner and / or outer diameter 44 are plotted in FIG. The evaluation device 38 can also determine the refractive index of the strip material based on the measurement signals received by the detector, as is described, for example, in WO 2016 / 139155 A1 or DE 10 2018 128 248 A1.

[0063] Using the first terahertz measurement device 24 Figure 1 4, 5 and 6. In the first measuring position shown in FIG. 4, for example, the outer diameter 44 and the wall thicknesses 40, 42 and the refractive index of the tube 10 are determined, at which the tube 10 is not yet completely solidified, i.e., has a still flowable portion. In this case, the transceiver 30 can be rotated around the tube 10, for example, along a circular path and thus determine the geometrical parameters and the refractive index at different locations on the circumference of the tube 10. The reflector 34 can also be rotated around the tube 10. However, it is also possible to dispense with the reflector 34.

[0064] The further terahertz measuring device 25 is located between the at least one further cooling section 26 and the shut-off device 28. The further terahertz measuring device 25 also has a transceiver 27 in which an emitter and a detector for terahertz radiation are combined. A reflector 29 for terahertz radiation is again arranged on the opposite side of the tube 10. The reflector reflects the terahertz radiation 31 emitted by the emitter back to the detector after passing through the tube 10 and being reflected on the interface of the tube 10.

[0065] As explained further below, the further terahertz measuring device 25 measures the refractive index and the wall thickness of the tube 10 at least on the upper side and the lower side at a second measuring position, at which the tube 10 is substantially completely solidified. Figure 1, in this case at a measuring position where the tube 10 is substantially completely frozen, the refractive index and geometrical parameters of the tube 10, such as the inner diameter and / or the outer diameter and / or the wall thickness, are measured in the manner explained above for the first terahertz measuring device 24. The further terahertz measuring device 25 can be arranged in a particularly simple manner in a fixed position and only emits terahertz radiation vertically from above or below onto the tube 10, thereby measuring the geometrical parameters, in particular the wall thickness on the upper side and the lower side of the tube 10. The further terahertz measuring device 25 can also be a portable handheld instrument.

[0066] Figure 3 The dependence of the refractive index on the temperature or the state of aggregation is shown. On the one hand, it can be seen that the relationship between the refractive index and the temperature or the state of aggregation is nonlinear. On the other hand, it can be seen that the refractive index changes particularly strongly in the recrystallization phase, that is, at the transition between the solid and liquid states of aggregation. This is utilized in the method according to the invention by inferring the remaining proportion of material in the recrystallization phase and, if appropriate, in the liquid state from the respectively measured refractive index and thereby deriving the sag that is still to be expected in relation to the measurement at the first measuring position.

[0067] exist Figure 4 The refractive index is plotted very schematically in relation to the radial position on the wall thickness. A range of 0 to 1 is plotted on the x-axis, wherein a wall thickness of 0 is correspondingly present on the outer surface of the observed wall of the tube 10 and a value of 1 is present on the inner surface of the observed wall of the tube. Figure 4 The curve 46 shown as a solid line in FIG. 4 shows the refractive index. It can be seen that the refractive index is constant in the region of the solidified zone, ie in the range of approximately 0 to 0.4 on the x-axis, and corresponds to the cold value n of the refractive index. kalt , here for example 1.5. Looking radially inwards at the adjacent unsolidified zone, the refractive index drops to a minimum value, here about 1.46. It should be noted that the actual distribution of the refractive index, especially in the region of the unsolidified zone, is not actually necessarily linear. Figure 4 On the horizontal dashed line in FIG. 1 , there is a resultant refractive index n of, for example, 1.46, measured, for example, at a first measuring position through a cross section of at least one wall of the tube 10 . res Therefore, the cold value of the refractive index n kalt A comparison of φ and φ allows the conclusion that a portion of the strip material is still in the recrystallization phase and is possibly still flowable in the form of a melt.

[0068] Current examples of polyethylene, especially HDPE, as pipe material Figure 3 and 4 of charts.

[0069] The refractive index difference between the measured refractive index and the cold value of the refractive index is correspondingly:

[0070]

[0071] For different values ​​of this refractive index difference, the droop factor SF=k*Δn can be determined using the factor k, which curves in Figure 4 The droop factor SF is shown very schematically as a dashed line in the reference numeral 48. In the example shown, the droop factor SF is substantially proportional to the resultant refractive index n res The actual distribution of the droop factor SF may deviate from this linear distribution. For example, the actual distribution may be determined empirically within the scope of a test series for respectively produced tubular strips.

[0072] Figure 5 The calibration relationship used according to the invention is shown by way of example. This calibration relationship assigns the refractive index at a first measuring position to the ratio of the outlet widths on the upper side and the lower side of the extrusion nozzle of the extrusion device 20 for extruding the material. Figure 5 In the y-axis, the refractive index at the first measuring position is recorded. On the x-axis, in the upper sequence, the ratio of the wall thickness of the tube 10 on its upper and lower sides measured at the first measuring position, i.e. the offset between the wall thickness measured on the upper and lower sides, is recorded as a percentage. In the lower sequence on the x-axis, the theoretical ratio of the outlet width on the upper and lower sides of the extrusion nozzle of the extrusion device 20, i.e. the offset between the outlet width on the upper and lower sides, is recorded as a percentage. The initial value 0% gives the required difference from zero for the outlet width on the upper and lower sides of the extrusion nozzle and the wall thickness measured on the upper and lower sides at the first measuring position for the cold value of 1.5 for the refractive index. This is because no droop is to be expected in this purely theoretical case. Figure 5 The values ​​of the upper sequence of the x-axis in , which increase relative to 0, respectively indicate the required difference (shift) of the wall thickness measured on the top and bottom of the tube 10 at the first measuring position for the desired tube geometry in the completely solidified state for a lower refractive index, i.e. when the tube 10 has a still flowable portion. Thus, a value of 5% means that the upper wall thickness is 5% greater than the lower wall thickness. A value of 10% correspondingly means that the upper wall thickness is 10% greater than the lower wall thickness.

[0073] As explained above, the outlet width on the upper and lower sides of the extrusion nozzle is generated by multiplying the offset at the first measuring position with a weighting factor that takes into account the different sags upstream and downstream of the first measuring position. Figure 5 The following sequence of the x-axis in gives the corresponding required difference (offset). As explained above, the weighting factor takes into account the extent to which the sag between the extrusion device 20 and the first measuring position is greater than the sag that still occurs after the first measuring position. Figure 5In the example shown in , a weighting factor of 3 is assumed. This weighting factor can be determined empirically, as explained. As explained above, for example, the calibration relationship can be determined empirically. As explained above, the calibration relationship can also be determined for defined geometric values ​​of the tube 10 at the first measuring position. The calibration relationship thus gives a theoretical ratio of the outlet widths on the upper and lower sides of the extrusion nozzle of the extrusion device 20 for the refractive index of the tube 10 measured at the first measuring position, which theoretical ratio leads to a predetermined geometry of the tube 10 in the fully solidified state, in particular a uniform wall thickness over the circumference of the tube 10. As shown in Figure 5 As can be seen in FIG. 1 , the refractive index decreases with increasing ratio between the outlet widths on the upper side and the lower side of the extrusion nozzle.

[0074] Figure 6 In order to illustrate two different refractive indices measured at the first measuring position, a radial temperature distribution is shown in the wall of the tube 10 from the inside to the outside as generated at the first measuring position of the first terahertz measuring device 24. Here, the temperature in ° C is recorded with respect to the radial position in mm, for example. The solid curve corresponds to a measured refractive index that is greater than the dashed curve. Correspondingly, the temperature distribution shown by the dashed curve is higher than the temperature distribution shown by the solid line. The temperature on the outside of the tube is strongly cooled and solidified by the coolant applied to the outside in the first cooling section 22. In contrast, the temperature rises strongly inward in the radial direction because the cooling of the outer wall has not yet cooled these areas until solidification. There is a corresponding portion that can still flow here. Above about 120°C, the material can flow and sags accordingly.

[0075] Figure 7 By way of example, the wall thickness measured at a first measuring position on the circumference of the tube 10 is shown for two different cases, wherein the wall thickness of the tube 10 in mm is recorded with respect to the angular position in degrees. 180° is the top side of the tube and 0° or 360° is the bottom side. The dashed line corresponds, for example, to the angular position in degrees. Figure 6 The dashed line shows the temperature distribution and the solid line corresponds to e.g. Figure 6 The solid line shows the temperature distribution. Figure 7 The curves shown in are empirically determined and each result in a uniform wall thickness over the circumference of the tube 10. It can be seen that, in order to compensate for the sag, a greater deviation (shift) of the outlet width on the upper side (180°) of the tube 10 and on the lower side (0°) of the tube 10 is required for a higher temperature distribution than for a lower temperature distribution. For example, in the case of the solid line, a shift of approximately 6.4% (33 mm to 31 mm) of the upper / lower wall thickness is required for the desired tube geometry in the fully solidified state. Figure 5This corresponds to a refractive index of approximately 1.46 at the first measuring position. That is, if a refractive index of 1.46 is measured at the first measuring position, the ratio of the upper / lower wall thickness measured at the first measuring position must be 1.064 or a percentage shift of 6.4%. As explained above, by conversion with weighting factors, this means that the ratio for the outlet width of the extruded material at the extrusion nozzle must be 1.192 or a percentage shift of 19.2%. That is, this ratio forms the theoretical ratio to which the extrusion nozzle is adjusted. For Figure 7 The dashed line in the figure gives the corresponding other values, which again correspond to the Figure 5 Another refractive index in. As in Figure 7 As shown in , by performing multiple measurements, it is possible to empirically construct Figure 5 A calibration relationship is thus defined, wherein, for example, interpolation can be performed between the individual measured values.

[0076] As explained above, in order to implement the method according to the invention using the device according to the invention shown in the figures, the refractive index on the cross section of at least one wall of the tube 10 and geometric values ​​of the tube 10, in particular the wall thickness and / or the inner diameter and / or the outer diameter, are measured at a first measuring position directly downstream of the first cooling section 22 at a plurality of measuring locations distributed around the circumference of the tube 10 by means of a first terahertz measuring device 24. By means of the measurement of the geometric values, it is preferably checked whether the geometric values ​​present at the first measuring position correspond to the geometric values ​​for which a calibration relationship is established. If this is not the case, a warning can be output or the calibration relationship can be changed, for example, by a factor corresponding to a determined deviation of the geometric values. In addition, by means of the refractive index measured at the first measuring position and the geometric values ​​measured at the first measuring position, a calibration relationship can be established. Figure 5 The calibration relationship shown in , determines the theoretical ratio of the outlet width on the upper side and the lower side of the extrusion nozzle of the extrusion device 20 obtained according to the calibration relationship and compares the theoretical ratio with the actually set ratio of the outlet width on the upper side and the lower side of the extrusion nozzle of the extrusion device 20. If these ratios deviate from each other, a warning can be output and / or the extrusion device 20 can be controlled in such a way that the ratio of the outlet width on the upper side and the lower side of the extrusion nozzle is adapted to the theoretical ratio obtained according to the calibration relationship. This control of the extrusion device 20 can be performed automatically. The measured values ​​of the terahertz measuring device 24 are present on the evaluation device 38. The calibration relationship can also be stored in the evaluation device 38. The evaluation device 38 can implement the explained evaluation and, if necessary, control of the extrusion device 20.

[0077] By means of the further terahertz measuring device 27, the refractive index and / or the wall thickness and / or the inner diameter and / or the outer diameter of the tube 10 are also measured at a second measuring position arranged downstream of the first measuring position, at which the tube 10 is substantially completely solidified. For example, the refractive index, which can correspond in particular to the cold value of the refractive index, and the wall thickness of the tube 10 at least on its upper and lower sides can be measured by means of the further terahertz measuring device 27 at the second measuring position. The measurement results of the further terahertz measuring device 27 are also available on the evaluation device 38. By means of the measured values ​​of the further terahertz measuring device 27, which correspond to the final parameters of the tube 10 based on the already substantially complete solidification of the tube 10, the calibration relationships used and the cold value of the refractive index and the predicted sag can be checked and, if necessary, corrected by means of the actual parameters of the tube 10. This is done by means of the evaluation device 38. If necessary, the calibration relationships can be adapted based on this.

[0078] As explained, the extrusion device 20 can be controlled based on the evaluation according to the invention of the measured values ​​and taking into account the calibration relationship by the evaluation device 38, which can have a corresponding control device for this purpose. In particular, the ratio of the outlet widths for the extruded material on the upper side and the lower side of the extrusion nozzle of the extrusion device 20 can be set by the evaluation device 38 as explained so that in the solidified state of the tube 10, the desired wall geometry of the tube 10, in particular a wall thickness that is as uniform as possible over the circumference, is produced. For this purpose, the gap width of the annular outlet gap of the extrusion nozzle on the upper side and the lower side of the extrusion nozzle can be set accordingly by the control device of the evaluation device 38, for example. It is also possible to influence the heating element of the extrusion nozzle in a suitable manner by the control device of the evaluation device 38.

[0079] Reference numerals list

[0080] 10 tubes

[0081] 12 walls

[0082] 14 Cavity

[0083] 16 External Surface

[0084] 18 Internal Surface

[0085] 20 Extrusion device

[0086] 22, 26 Cooling section

[0087] 24 Terahertz measurement device

[0088] 25 Additional terahertz measurement devices

[0089] 27 Transceiver

[0090] 28 Cutting device

[0091] 29 Reflector

[0092] 30 transceiver

[0093] 31 Terahertz radiation

[0094] 32 terahertz radiation

[0095] 34 reflectors

[0096] 36 wires

[0097] 38 Evaluation device

[0098] 40, 42 wall thickness

[0099] 44 diameter

[0100] 46 Refractive Index Curve

[0101] 48 Droop Factor Curve

Claims

1. A method for checking the setting of an extrusion device (20) which produces a tubular strip (10) conveyed in its longitudinal direction, in, The outlet widths for the extruded material are set differently on the upper side and the lower side of the extrusion nozzle of the extrusion device, wherein the method comprises the following steps: • measuring the refractive index in a cross section of at least one wall of the tubular strip (10) by means of a terahertz measuring device (24) at a first measuring position downstream of at least one first cooling section (22) for the tubular strip (10), at which first measuring position the tubular strip (10) has not yet completely solidified, • measuring geometrical values ​​at a first measuring position and also at at least one measuring location on the upper side of the tubular strip (10) and at least one measuring location on the lower side of the tubular strip by means of the terahertz measuring device (24), wherein the geometrical values ​​include a wall thickness (40, 42) and / or an inner diameter and / or an outer diameter (44) of the tubular strip (10), • For the ratio of the measured refractive index and the geometric values ​​measured on the upper and lower sides of the tubular strip (10), the ratio of the outlet widths set on the extrusion device (20) for the extruded material on the upper and lower sides of the extrusion nozzle is checked with the aid of a previously determined calibration relationship between the refractive index at a first measuring position and the ratio of the outlet widths for the extruded material on the upper and lower sides of the extrusion nozzle.

2. The method according to claim 1, It is characterized in that The calibration relationship is determined for defined geometric values ​​of the tubular strip (10).

3. The method according to claim 2, It is characterized in that In the event of a determined deviation of the geometric value measured at the first measuring position from the defined geometric value, a warning is output and / or the calibration relationship is adapted in accordance with the determined deviation of the geometric value.

4. The method according to any one of the preceding claims, It is characterized in that When determining the calibration relationship, material parameters of the tubular strip (10), in particular thermal conductivity and / or heat capacity, and / or production parameters, in particular the temperature of the extrusion nozzle and / or the withdrawal speed of the extrusion device (20) and / or cooling parameters of the produced tubular strip (10) are taken into account.

5. The method according to any one of the preceding claims, It is characterized in that The calibration relationship assigns the refractive index at the first measuring point to a theoretical ratio for the outlet width of the extruded material at the upper side and the lower side of the extrusion nozzle, so that the tubular strip (10) has a theoretical wall thickness distribution, in particular a uniform wall thickness distribution, over its circumference after its complete solidification.

6. The method according to claim 5, It is characterized in that When it is determined that there is a deviation between a theoretical ratio of the outlet width of the extrusion nozzle to the refractive index measured at the first measuring position according to the calibration relationship and a ratio of the adjusted outlet width of the extrusion nozzle, the theoretical ratio of the measured refractive index according to the calibration relationship is displayed, and / or the extrusion device (20) is controlled, preferably automatically controlled, to adjust the theoretical ratio of the measured refractive index according to the calibration relationship.

7. The method according to claim 5 or 6, It is characterized in that The calibration relationship further assigns the refractive index at the first measuring position to the ratio of the wall thicknesses on the upper side and the lower side of the tubular strip (10) at the first measuring position, wherein a weighting factor is used for the conversion between the ratio of the wall thicknesses on the upper side and the lower side of the tubular strip (10) at the first measuring position and a theoretical ratio for the outlet width of the extruded material on the upper side and the lower side of the extrusion nozzle, the weighting factor taking into account the different degrees of sag upstream of the first measuring position and downstream of the first measuring position.

8. The method according to any one of the preceding claims, It is characterized in that Based on a comparison of the geometric values ​​measured at the first measuring point with the outlet width set at the extrusion device (20), an expected sag of the tubular strip (10) until complete solidification of the tubular strip (10) is predicted.

9. The method according to claim 8 and any one of claims 5 to 7, It is characterized in that The predicted sag and / or expected values ​​of measured geometrical values ​​in the fully solidified state of the tubular strip (10) are displayed, and / or a theoretical ratio obtained according to the calibration relationship is adapted for the outlet width of the extruded material on the upper side and the lower side of the extrusion nozzle based on the predicted sag.

10. The method according to any one of the preceding claims, It is characterized in that At a first measuring point, the refractive index is measured on the upper side and / or the lower side of the tubular strip (10).

11. The method according to any one of the preceding claims, It is characterized in that The refractive index and / or the geometrical value are measured at a plurality of measuring locations around the circumference of the tubular strip (10).

12. The method according to any one of claims 1 to 11, It is characterized in that The refractive index is determined by comparing a propagation time of a measuring radiation (32) emitted by the terahertz measuring device (24) without a tubular strip (10) arranged in the beam path of the measuring radiation (32) with a propagation time of the measuring radiation (32) with the tubular strip (10) arranged in the beam path of the measuring radiation (32).

13. The method according to any one of claims 1 to 11, It is characterized in that The refractive index is determined by determining an optical wall thickness (40, 42) of the tubular strip (10) using the terahertz measuring device (24), further by determining an outer diameter (44) and an inner diameter of the tubular strip (10) using the terahertz measuring device (24), and by determining the refractive index of the tubular strip (10) by comparing the determined outer diameter and inner diameter (44) with the determined optical wall thickness (40, 42).

14. The method according to any one of the preceding claims, It is characterized in that Geometric values ​​of the tubular strip (10) are determined by measuring the propagation time of the measuring radiation (32) emitted by the terahertz measuring device (24).

15. The method according to any one of the preceding claims, It is characterized in that At a second measuring position spaced apart from the first measuring position in the longitudinal direction of the tubular strip (10), in particular downstream of the first measuring position, a refractive index in a cross section of at least one wall of the tubular strip (10) and / or a wall thickness (40, 42) and / or an inner diameter and / or an outer diameter (44) of the tubular strip (10) are measured using a further terahertz measuring device (25), wherein the tubular strip (10) is substantially completely solidified.

16. The method according to claim 15, It is characterized in that At a second measuring position, the wall thickness (40, 42) and / or the inner diameter and / or the outer diameter (44) of the tubular strip (10) is measured at the upper side of the tubular strip (10) and at the lower side of the tubular strip (10) using the further terahertz measuring device (25).

17. The method according to claim 15 or 16, It is characterized in that The calibration relationship is checked and, if necessary, corrected using the measurement results of the further terahertz measuring device (25).

18. A device for carrying out the method according to claim 1, comprising a terahertz measuring device (24) and an evaluation device (38) which is designed to check the ratio of the set outlet widths for the extruded material on the upper side and the lower side of the extrusion nozzle.

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