METHOD AND DEVICE FOR TESTING AN EXTRUSION DEVICE

Description

[0001] The invention relates to a method for checking the settings of an extrusion device producing a tubular strand conveyed along its longitudinal direction, wherein the outlet widths for extruded material are set differently on an upper side and a lower side of an extrusion nozzle of the extrusion device. The invention also relates to a device for carrying out the method.

[0002] For example, plastic pipes for supplying residential and industrial areas with gas and water, as well as for drainage, are predominantly made from materials such as HDPE, PP, and PVC. Typical pipe diameters are up to 3 m, with wall thicknesses of up to 250 mm. Production usually takes place in extrusion devices in which a plastic raw material is melted and extruded through a usually annular extrusion die. The pipe extruded in this way is drawn lengthwise from the extrusion device and thus conveyed. The diameter of the drawn pipe is shaped to the desired outer diameter in a downstream calibration device, for example, in the form of a sleeve. During the conveying process, the pipe usually passes through several cooling sections in which the pipe is cooled, thus gradually solidifying the initially flowable plastic melt.In a first cooling section, the formed tube is prevented from collapsing, for example by means of a vacuum. The tube is often cooled in the cooling sections using a cooling liquid, such as water. The cooling water flows around the tube and quickly hardens its outer area. After leaving the first cooling section, the outer surface of the tube has usually solidified, meaning that the external geometry of the tube subsequently remains essentially unchanged. However, after leaving the first cooling section, there are usually still flowable portions of the tube material inside the tube wall. As the tubular strand is conveyed further, particularly as it passes through further cooling sections, the interior of the tube gradually cools and thus solidifies, reaching the inner surface of the tube. The tube is finally cut to the desired length using a flying saw.

[0003] The shape of the pipe during complete solidification after exiting the extrusion device is essentially influenced by two effects that must be taken into account to achieve, for example, a pipe wall thickness that is as uniform as possible. First, there is shrinkage of the pipe material during cooling. Second, there is sagging, i.e., the sinking of the still-flowing viscous mass fractions during solidification due to the influence of gravity.

[0004] In response to these effects, which influence the final geometry of the tubular strand, it is known to set the extrusion die of the extrusion device with a larger exit width for extruded material in an upper region than in a lower region. To adjust the exit width, an exit gap of the extrusion die can be set wider in the upper region than in a lower region. Alternatively or additionally, the extrusion die can be heated more strongly in an upper region than in a lower region, thereby achieving a larger exit width in the upper region than in the lower region of the extrusion die. Both measures therefore ensure that more material exits the upper region of the extrusion die than in the lower region.This intentionally asymmetrical application of the material is intended to compensate for sagging, so that the solidified pipe has a wall thickness that is as consistent as possible over its entire circumference.

[0005] Important geometric parameters of the pipe, such as wall thickness and diameter, can only be finally measured after complete solidification, i.e. at the end of all cooling sections of the pipe, when shrinkage and sagging are completely complete. A typical output rate of an extrusion device for medium pipe cross-sections is around 1,000 kg / h. The outlet temperature of the melt from the extrusion nozzle is in the range of 200°C to 240°C, depending on the material. For a pipe with, for example, an outer diameter of 330 mm and a wall thickness of 30 mm and a typical cooling section for the pipe of 60 m, the first measurement results regarding wall thickness and diameter are often only available hours after production has begun. Only then can the production parameters of the extrusion device be influenced if geometric deviations from the target values ​​are detected, although any changes made can only be re-evaluated after several hours.Often, several corrections are required to achieve an optimal process after a process start, which takes several days, for example to adjust the wall thickness uniformly over the circumference and to its nominal value.

[0006] To compensate for sagging, as explained above, the extrusion die's exit width is set larger in the upper section than in the lower section. To ensure a minimum wall thickness is maintained even when sagging occurs, overcompensation for sagging is generally performed, using empirical values. Ultimately, this leads to unnecessary material being extruded.

[0007] It is therefore desirable to obtain information as early as possible about the expected shrinkage and sagging of a pipe produced in an extrusion device. For example, a measurement of the wall thickness and diameter of a pipe after the first cooling section may not correspond to the desired final values ​​that would be found after the pipe has completely cooled because, at this point, the pipe wall only contains solidified material in the outer region and still contains recrystallizing components and melt in the interior. As a result, at a measuring position after the first cooling section, diameter and wall thickness values ​​are recorded that are still subject to shrinkage and sagging. To accelerate the start-up process and ensure continuous compliance with the nominal values, predicting the expected shrinkage and sagging values ​​as early as possible is of great economic importance.

[0008] WO 2022 / 058081 A1 proposes a method for determining geometric parameters of a strand- or plate-shaped object, which predicts shrinkage. For this purpose, a relationship between the refractive index of the object and a shrinkage occurring during its solidification is determined in a determination step. In a determination step, the refractive index and at least one geometric parameter of the not yet fully solidified object are determined, in particular after a first cooling section of the object. From the determined values, a geometric parameter in the fully solidified state of the object is calculated, taking into account the relationship determined in the determination step.This means that the shrinkage of the object's material during its solidification is predicted and, on this basis, a geometric parameter, such as a wall thickness, is calculated in the fully solidified state of the object.

[0009] Using the described method, shrinkage can be reliably predicted and accounted for as one of the two effects that significantly influence the final geometry. The measured values ​​are advantageously available shortly after the material exits the extrusion device, allowing the final geometry parameter to be calculated early enough to avoid scrap. Sagging, however, as the second significant effect on the final geometry of the object, cannot be predicted using the known method for reasons explained in more detail below.

[0010] A method for determining sagging of a pipe extruded in an extrusion device is known from WO 2022 / 106180 A1. The wall thickness of the pipe is measured around the circumference, and a wall thickness profile is created from the measured wall thicknesses. Sagging of the melt is determined from the frequency and / or amplitude of the resulting wall thickness profile. While this method enables reliable detection of sagging, it also cannot predict its occurrence.

[0011] Based on the explained prior art, the invention is therefore based on the object of providing a method and a device of the type mentioned at the outset, with which the exit widths for extruded material set on the extrusion device can be reliably checked and, if necessary, corrected, taking into account the sagging of a tubular strand produced in an extrusion device, in particular in real time and with a small time offset after the strand exits the extrusion device.

[0012] The invention solves the problem by the independent claims 1 and 18. Advantageous embodiments can be found in the dependent claims, the description and the figures.

[0013] For a method of the type mentioned above, the invention solves the problem by the following steps: by means of a terahertz measuring device, the refractive index is measured over the cross-section of at least one wall of the tubular strand at a first measuring position downstream of at least one first cooling section for the tubular strand, at which the tubular strand has not yet completely solidified; by means of the terahertz measuring device, geometric values ​​are further measured at the first measuring position at at least one measuring location on the top side and at at least one measuring location on the bottom side of the tubular strand, wherein the geometric values ​​include the wall thickness and / or the inner and / or outer diameter of the tubular strand,For the measured refractive index and for a ratio of the geometric values ​​measured at the top and bottom of the tubular strand, the ratio of the exit widths for extruded material at the top and bottom of the extrusion die set on the extrusion device is checked using a previously determined calibration relationship between the refractive index at the first measuring position and the ratio of the exit widths for extruded material at the top and bottom of the extrusion die.

[0014] The invention also achieves the object by a device for carrying out the method according to the invention, comprising the terahertz measuring device and an evaluation device which is designed to check the ratio of the set outlet widths for extruded material at the top and at the bottom of the extrusion nozzle.

[0015] With the method and device according to the invention, the sagging still to be expected following the first measuring position—that is, the sinking of the still-flowable, viscous mass fractions of the tubular strand emerging from the extrusion device during solidification due to the influence of gravity—can be predicted. On this basis, the settings of the extrusion device can be checked and, if necessary, corrected. The tubular strand can be a pipe, for example, a plastic pipe. The extrusion device has, in a conventional manner, a generally circular extrusion nozzle from which the previously melted plastic material emerges.In a manner also known per se, the tubular strand regularly passes through several cooling sections following its exit from the extrusion device, in which the strand material is successively cooled, for example by a cooling liquid such as water, until it cools down and thus completely solidifies. For example, after exiting the first cooling section immediately following the extrusion device, the outer surface of the tubular strand may already have solidified, so that the shaping of the outer surface of the tubular strand is complete. In the interior of the tube wall, however, the material of the tubular strand still has recrystallizing regions and melt components and is therefore at least partially still flowable, whereby as the strand cools further, sagging is known to occur in addition to shrinkage.For the final shaping of the tubular strand in the first cooling section, it is possible, for example, to press the strand material against the cylindrical inner surface of a calibration sleeve by applying a vacuum.

[0016] In principle, it is possible to model the expected sagging in detail using the Navier-Stokes equations, starting from precisely known framework conditions and material properties of the tubular strand. However, this is numerically so complex that it is impractical for the rapid prediction of sagging that is desired in this case. According to the invention, in order to avoid rejects, a reliable prediction of the expected sagging and thus the final shape of the strand should be available as early as possible after it leaves the extrusion device, so that early intervention in the production process is possible if necessary, for example by adjusting the settings of the extrusion device. The prediction of sagging should preferably be made in real time. Both of these factors preclude the use of the inherently reliable Navier-Stokes equations in this application.

[0017] Unlike shrinkage, sagging, as already mentioned, cannot be predicted in the manner explained in WO 2022 / 058081 A1. Shrinkage from extrusion temperature to complete cooling is independent of the solidification time course, in particular the cooling rate. Therefore, shrinkage can be predicted simply and reliably by comparing the refractive index measured at the first measurement position with the cold value of the refractive index. Furthermore, shrinkage cannot be significantly influenced by changing parameters such as the extrusion device or the cooling parameters. It is essentially an unchanging process property, but one that can be predicted relatively easily.

[0018] This is different with sagging, where the processes are much more complex than with shrinkage. In sagging, the temperature-dependent flow behavior of the material, its temperature, and the rate of cooling play a significant role. In general, it can be stated that sagging of the melt during pipe production will be more pronounced the higher the initial temperature of the melt and the longer the time until cooling and complete solidification of the melt. Varying operating conditions regarding the extrusion temperature, the extruder feed rate, the haul-off speed, and the intensity and duration of cooling influence the degree of sagging.While a plant shutdown is sufficient for shrinkage prediction using the refractive index, during which the change in the refractive index is recorded along with the shrinking wall thickness and diameter over several hours of cooling, this reliable method is not effective for predicting sagging, precisely because the sagging in wall thickness will be significantly more or less pronounced if cooling occurs over a longer or shorter period. Sagging - unlike shrinkage - can also be significantly influenced by changing the parameters of the extrusion device and the cooling parameters for the tubular strand.

[0019] To solve these problems, according to the invention, a (first) terahertz measuring device is used at a first measuring position downstream of at least a first cooling section for the tubular strand, where the strand has not yet completely solidified, i.e., where it still has flowable recrystallizing and / or melt components, to measure, on the one hand, the average or resulting refractive index across the cross section of at least one wall thickness of the tubular strand and, on the other hand, likewise at the first measuring position, at at least two measuring locations on the top and bottom of the tubular strand, geometric values, including the wall thickness and / or the inner and / or outer diameter, of the tubular strand are measured. For this purpose, the terahertz measuring device comprises a terahertz transmitter and a terahertz detector, which are arranged essentially at the same location and, in a particularly practical manner, can also be combined to form a transceiver.The terahertz measurement radiation used according to the invention can, for example, be in a frequency range from 1 gigahertz to 6 terahertz. Terahertz radiation is well suited for difficult measurement conditions in the vicinity of extrusion devices, particularly with the high temperatures, possible contamination, and the presence of steam, etc. Thus, in contrast to laser radiation in the visible frequency range, for example, terahertz radiation is largely insensitive to such interference. The terahertz radiation emitted by the transmitter of the terahertz measurement direction strikes the tubular strand from above and / or below, for example. Part of the terahertz radiation is reflected by the outer surface of the strand, while another part of the terahertz radiation penetrates the strand material.This results in further reflections at the boundaries of the tubular strand, particularly at the boundaries of the wall sections of the strand facing and away from the transmitter. A portion of the terahertz radiation also exits the strand on the side opposite the transmitter. There, the terahertz measuring device can have a reflector that reflects the terahertz radiation exiting on the strand side facing away from the transmitter back to the strand. After further reflections at the boundaries of the strand, it reaches the detector, which may be located at the same location as the transmitter, along with the remaining radiation components reflected at the boundaries.

[0020] In a manner known per se, the refractive index across the cross-section of one or two opposite wall sections of the strand at the first measuring position can be determined from the detected radiation, as will be explained in more detail below. The refractive index can be measured at the top and / or bottom of the strand. In particular, when radiating through two, for example, opposite wall sections of the strand, an average or resulting refractive index can be determined across both radiated wall sections. Furthermore, in a manner known per se, for example based on time-of-flight measurements, the distances to the individual interfaces of the strand can be determined, from which geometric parameters such as wall thicknesses and / or inner and / or outer diameters of the strand can be determined.For example, the measurement at the top can be taken at the highest point of the strand, and the measurement at the bottom can be taken at the lowest point. In particular, the upper wall thickness and the lower wall thickness of the tubular strand can be measured at the first measuring position. These will often differ because, as explained above, the extrusion device is usually set up so that more material exits the top of the extrusion die than the bottom of the extrusion die to compensate for sagging. This asymmetry is often still present in the area of ​​the (first) terahertz measuring device, i.e., particularly immediately downstream of the first cooling section.

[0021] According to the invention, a previously determined calibration relationship is used to predict the expected sagging and to check the extrusion device. The basis for using the calibration relationship is the refractive index measured at the first measurement position. The calibration relationship assigns the refractive index at the first measurement position to the ratio of the set exit widths for extruded material at the top and bottom of the extrusion die. The ratio of the exit widths at the top and bottom of the extrusion die, as well as the ratio of the measured geometric values ​​at the top and bottom of the strand, can be specified as a simple numerical ratio or, for example, as a percentage.Since the strand in the area of ​​the first measuring position generally still contains areas with recrystallizing material and melt components, in particular still flowable material, the measured refractive index differs from its cold value. The measured value of the refractive index provides information about the flowable components, in particular recrystallizing components and / or melt components, still present inside the tubular strand at the first measuring position. The calibration relationship provides - particularly for defined geometric parameters of the strand at the first measuring position as well as defined production parameters of the extrusion device - a correlation between the refractive index measured at the first measuring position and a ratio of the exit widths of the extrusion nozzle required for a desired strand geometry in the fully solidified state.This exit width ratio, resulting from the calibration relationship for the measured refractive index, can be compared with the ratio of the set exit widths of the extrusion device to check the extrusion device. As explained above, the exit width can be adjusted, for example, by adjusting the exit gap of the extrusion nozzle and / or by controlling the extrusion nozzle's heating devices. It is conceivable, for example, that the basic setting of the exit widths is achieved by adjusting the exit gap of the extrusion nozzle, and subsequent fine-tuning or adjustment of the exit widths is then performed solely by controlling the extrusion nozzle's heating devices.

[0022] Simply recording, for example, the wall thicknesses on the top and bottom of the tubular strand at the first measuring position is not sufficient for the inventive forecast of the sagging still to be expected and thus for checking the settings of the extrusion device. This is because a wall thickness ratio at the top / bottom of the strand determined at the first measuring position does not provide any information about the extent to which previous sagging has already occurred or is even already complete. Only in combination with the evaluation of the (resulting) refractive index measured at the first measuring position and preferably its weighting to the wall thickness ratio at the top / bottom measured at the first measuring position can it be estimated whether this will result in complete compensation of the sagging in the further course of the cooling sections, so that the strand has the specified wall thickness profile in the fully solidified state.

[0023] Due to the simplified approach of the invention, the prediction of the sagging still to be expected downstream of the first measuring position and thus the testing of the extrusion device can be carried out quickly and in real time. This allows, for example, the extrusion device to be controlled quickly to adjust the exit of the strand material from the extrusion die, particularly at the top and bottom of the extrusion die, so that the desired geometry of the strand is achieved in the fully solidified state. The desired geometry of a tubular strand will often be a circular cross-section with a constant wall thickness around the circumference.

[0024] The calibration relationship can be stored in the form of a function or curve. The calibration relationship can be determined empirically in a particularly simple way. For this purpose, for the respective production process, the outlet width of the extrusion nozzle can be adjusted such that the tubular strand has a predetermined geometry around its circumference after complete solidification, for example a uniform wall thickness over its circumference, the measured refractive index resulting at the first measuring position and the measured quotient of the geometric values, in particular the wall thicknesses, on the top and bottom sides can be stored. This stored refractive index can then be compared with the refractive index measured at the first measuring position in a later process. The refractive index is known to be temperature-dependent.If the measured refractive index deviates from the stored refractive index, it can be assumed that there is a change in temperature and thus, in particular, in the still-flowable portions inside the strand at the first measurement position. It can therefore be further assumed that a correction of the set exit widths of the extrusion nozzle is necessary to achieve the desired geometry of the strand in the solidified state. As part of an empirical creation of the calibration relationship, additional exit width settings can be stored for different refractive indices at the first measurement position, which lead to the desired geometry of the strand in the solidified state.

[0025] Changes in production parameters in the area before the first measuring position can have a significant impact on sagging and thus on the geometry of the strand in the solidified state. Such changes occur particularly when starting up a production process after a production shutdown. Examples include a slowly increasing temperature of the extrusion die during start-up or a change in the output of the extrusion device. Another example is a change in cooling parameters, for example due to a change in the composition and / or temperature of the cooling liquid. Such changes can be detected not only from the refractive index measured at the first measuring position but also, in particular, from the geometry values ​​measured at the first measuring position. This is particularly true since a significant portion of the total sagging has already occurred by the time the strand leaves the first cooling section.According to the invention, it is therefore possible to set optimal process parameters, in particular of the extrusion device, at a particularly early stage when restarting a production process based on the calibration context.

[0026] Changes in the production process can be detected not only by the refractive index but also in particular by the geometric values ​​recorded at the first measuring position. If the geometric values ​​recorded at the first measuring position change, this indicates that the expected sagging taken into account according to the calibration relationship has also changed. Accordingly, according to one embodiment, it can be provided that the calibration relationship for defined geometric values ​​of the tubular strand is determined, for example, at the first measuring position. It can then further be provided that if a deviation of the geometric values ​​measured at the first measuring position from the defined geometric values ​​is detected, a warning is issued and / or the calibration relationship is adjusted according to the detected deviation of the geometric values.For example, the calibration relationship can be adjusted by a factor corresponding to the observed change. This factor can be adjusted based on empirical values, for example, by multiplying it by a constant factor that takes into account the influence of the change on the sagging that occurs.

[0027] When determining the calibration relationship, material parameters of the tubular strand, in particular the thermal conductivity and / or the heat capacity, can also be considered. Production parameters, in particular the temperature of the extrusion die and / or the withdrawal speed of the extrusion device and / or the cooling parameters of the produced tubular strand, can also be considered. These parameters also influence the sagging that occurs.

[0028] According to a further embodiment, the calibration relationship can assign the refractive index at the first measuring position to a target ratio of the exit widths for extruded material at the top and bottom of the extrusion die, so that the tubular strand, after it has completely solidified, has a target wall thickness profile, in particular a uniform wall thickness profile, over its circumference. As already mentioned, the calibration relationship, which can be determined empirically, for example, can be in the form of a function or curve. The refractive index at the first measuring position can be plotted as a function of the ratio of the exit widths for extruded material at the top and bottom, which is to be set as the target ratio at the extrusion die. In the simplest case, the curve can be a straight line with a negative gradient. However, it can also be a curve that deviates from a straight line.

[0029] According to a further embodiment, if a deviation is detected between the target ratio of the extrusion nozzle outlet widths resulting from the refractive index measured at the first measuring position according to the calibration relationship and the ratio of the set extrusion nozzle outlet widths, the target ratio resulting from the measured refractive index according to the calibration relationship can be displayed. Alternatively or additionally, the extrusion device can be controlled, preferably automatically, to set the target ratio resulting from the measured refractive index according to the calibration relationship. The control can be carried out by an evaluation device. This enables automatic control of the extrusion device. The target ratio can be displayed for an operator on an operator display.

[0030] As already explained, based on a comparison of the geometry values ​​measured at the first measuring position with the exit widths set on the extrusion device, the expected sagging of the tubular strand until complete solidification of the tubular strand can be predicted. The predicted sagging and / or expected values ​​of the measured geometry values ​​in the fully solidified state of the tubular strand can then be displayed, and / or the target ratio of the exit widths for extruded material at the top and bottom of the extrusion die resulting from the calibration relationship can be adjusted based on the predicted sagging.

[0031] According to a further embodiment, the refractive index and / or the geometric values ​​can be measured at the first measuring position at a plurality of measuring locations around the circumference of the tubular strand. For this purpose, for example, several terahertz transmitters and terahertz detectors can be arranged distributed around the circumference of the tubular strand. Preferably, a terahertz measuring device rotating around the tubular strand with a terahertz transmitter and terahertz detector and, if appropriate, a reflector can be provided, with which measured values ​​can be generated distributed around the circumference. In particular, complete coverage of the circumference of the strand is conceivable in this way. Compared to measuring only from the top and bottom of the tubular strand, more extensive measurement around the circumference, in particular also on the sides of the strand, offers further advantages.For example, it is possible to detect a possibly advanced solidification of the side walls of the strand, which could influence or prevent the still flowable parts from flowing downwards due to gravity and thus influence sagging.

[0032] According to a further embodiment, the refractive index can be determined by comparing the travel time of the measuring radiation emitted by the terahertz measuring device without a tubular strand arranged in the beam path of the measuring radiation with the travel time of the measuring radiation with the tubular strand arranged in the beam path of the measuring radiation. This procedure for determining an unknown refractive index of an object is described, for example, in EP 3 265 748 B1. The refractive index determination with the terahertz measuring device can be carried out in a corresponding manner in the present case.

[0033] According to a further embodiment, the refractive index can be determined by using the terahertz measuring device to determine the optical wall thickness of the tubular strand, by using the terahertz measuring device to determine an outer diameter and an inner diameter of the tubular strand, and by determining the refractive index of the tubular strand by comparing the determined outer and inner diameters 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 procedure can again be applied in the present case.

[0034] According to a particularly practical embodiment, the geometric values ​​of the tubular strand can be determined from a transit time measurement of measuring radiation emitted by the terahertz measuring device.

[0035] According to a further embodiment, at a second measuring position spaced from the first measuring position in the longitudinal direction of the tubular strand, in particular a second measuring position downstream of the first measuring position at which the tubular strand is substantially completely solidified, the refractive index across the cross-section of at least one wall of the tubular strand and / or the wall thickness and / or the inner and / or outer diameter of the tubular strand can be measured using a further terahertz measuring device. Furthermore, at the second measuring position, the wall thickness and / or the inner and / or outer diameter of the tubular strand at the top side of the tubular strand and the wall thickness at the bottom side of the tubular strand can be measured using the further terahertz measuring device. The calibration relationship can also be checked and, if necessary, corrected based on the measurement results of the further terahertz measuring device.

[0036] The second measuring position can generally be located upstream or downstream of the first measuring position. It is preferably located downstream, and particularly preferably far enough downstream that the tubular strand is essentially completely solidified. The additional terahertz measuring device can generally be equipped like the (first) terahertz measuring device provided at the first measuring position. The additional terahertz measuring device also has a (further) terahertz transmitter and a (further) terahertz detector, which in turn can be combined in a particularly practical manner to form a transceiver. Opposite the transmitter and, if appropriate, the detector arranged at the same location, a reflector can again be arranged on the opposite side of the tubular strand.The additional terahertz measuring device can be arranged cost-effectively at a fixed position, in particular such that it emits terahertz measuring radiation onto the tubular strand from above vertically downwards or from below vertically upwards. It would also be conceivable for the additional terahertz measuring device to be a portable measuring device, in particular a so-called handheld measuring device, which is therefore not permanently arranged at the second measuring position. The measurement of the refractive index and the aforementioned geometric parameters can again be carried out in the manner explained above for the terahertz measuring device arranged in the first measuring position. At the second measuring position, in particular when the tubular strand has essentially completely solidified at this position, the final material and geometric parameters of the strand can be measured, including the cold value of the refractive index.Measurement with an additional terahertz measuring device is significantly more accurate than, for example, a measurement with a mechanical probe. The final parameters measured at the second measurement position can be compared with the parameters of the calibration context, and the calibration context can be verified and, if necessary, adjusted based on the actually measured final parameters.

[0037] The aforementioned embodiment reduces the dependence on material parameters of the model according to the invention. Especially in the area of ​​plastic strands, the material parameters are often not known or defined with sufficient precision. To reduce the dependence on material parameters, an additional sensor can be used in the aforementioned embodiment, for example, to measure the cold value of the refractive index and the wall thickness at at least one angular position around the circumference of the strand or a ratio between at least two wall thicknesses at different angular positions. For example, with a terahertz measuring device fixed vertically above the strand, the ratio between the upper and lower wall thickness of the strand.The calibration relationship used according to the invention can then be verified based on the measured values ​​of the additional terahertz measuring device, for example, to reduce deviations caused by material parameters that deviate from reality. In particular, the additional terahertz measuring device can detect any changes in the process temperature downstream of the first measuring position, which, as explained, influence sagging. An example of this would be a change in the cooling rate of the strand.

[0038] For example, a relationship between the refractive index and the crystallization state of the strand material can be used empirically to determine a sagging factor SF = k * Δ N with: SF: Sagging factor k: factor ΔN : Refractive index difference between the refractive index measured at the first measuring position and the cold value of the refractive index

[0039] The greater the sagging factor determined in this way SF, The greater the sagging, the greater the expected sagging. The volume or mass flow rate of the still-flowing viscous material is roughly proportional to the sagging factor. With precise knowledge of the cold value of the refractive index, the sagging factor and thus the calibration relationship can be determined more precisely.

[0040] This allows the inventive model for predicting the expected sagging to be further improved. In particular, the sagging factor at the respective measuring position represents an indicator from which it can be deduced to what extent the regularly deliberately adjusted difference in the wall thickness emerging from the extrusion die in the upper and lower regions will change further along the extrusion line. On this basis, the sagging factor can be determined by adjusting the annular gap of the extrusion die and / or by appropriately setting the temperature of the extrusion die of the extrusion device. SF at the measuring location of the terahertz measuring device so that the remaining sagging results in the desired, usually uniform, wall thickness of the strand over the entire circumference.

[0041] According to a further embodiment, it can be provided that the calibration relationship further assigns the refractive index at the first measuring position to a ratio of the wall thicknesses at the top and bottom of the tubular strand at the first measuring position, wherein a weighting factor is applied to convert the ratio of the wall thicknesses at the top and bottom of the tubular strand at the first measuring position to the target ratio of the exit widths for extruded material at the top and bottom of the extrusion nozzle, which takes into account a different degree of sagging upstream of the first measuring position and downstream of the first measuring position. Thus, a weighting can be defined between the sagging of the strand that has already occurred, as determined by measuring the geometric values ​​at the first measuring position, and the sagging that is still to be expected until complete solidification.The sagging that has already occurred is determined by comparing the geometry values ​​measured at the first measurement position with the exit widths set at the extrusion nozzle. For example, if it is assumed that this will continue after the first measurement position, but to a lesser extent, the expected sagging 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 measurement position. Conversely, a weighting factor can be calculated that indicates the extent by which the sagging before the first measurement position is greater than the sagging after the first measurement position.For example, by forming a first quotient of the vertical gap dimensions of the extrusion die in the upper and lower regions and a second quotient of the wall thicknesses measured at the first measuring position in the upper and lower regions, two quotients can be compared with each other, for example a quotient can be formed from the first and second quotient. This can then form the weighting factor. Since the majority of the sagging occurs between the exit from the extrusion device and the first measuring position, and thereafter only minor sagging generally occurs, the weighting factor will generally be greater than 1. For example, the weighting factor could be approximately 3. For a specific strand material and constant production conditions, the usually desired uniform wall thickness of the strand across the circumference can be obtained on the basis of such a weighting factor. Using this weighting factor orIn the calibration context, the refractive index at the first measuring position can be assigned to the ratio of the wall thicknesses of the strand at its top and bottom measured at the first measuring position or to the target ratio of the exit widths at the top and bottom of the extrusion die of the extrusion device using a weighting quotient formed by inversion. This makes it possible to directly determine and specify the offset to be set at the extrusion die based on the geometry values ​​measured at the first measuring position and the measured refractive index. The exit widths of the extrusion die can thus be controlled or regulated directly based on the measured values ​​obtained at the first measuring position.

[0042] If another terahertz measuring device is provided at a second measuring position downstream of the first measuring position, its measured values ​​can be used for fine adjustment of the model according to the invention and thus of the extrusion device. This takes advantage of the fact that at the first measuring position, after a first cooling section, the melt has already cooled to such an extent that, in addition to a cooled outer zone, a portion of recrystallizing melt is still present inside the strand, in particular within the pipe wall. It is then also possible to calculate a third quotient of the wall thicknesses in the upper and lower regions measured at the second measuring position of the second terahertz measuring device 27, i.e., in the fully solidified state. This third quotient, which ideally should be 1, for example, can then be compared with the first and second quotients.Similarly, a quotient could be formed from the refractive index measured at the first measuring position and the (cold value of) the refractive index measured at the second measuring position.

[0043] In the device according to the invention, the measured values ​​of the terahertz measuring device and, if applicable, the additional terahertz measuring device are applied to the evaluation device. The evaluation device is designed to carry out the evaluation and testing according to the invention. It is also designed, in particular, to carry out the embodiments according to the dependent claims of the method according to the invention. For this purpose, the evaluation device can comprise a control device, in particular for controlling the extrusion device, in particular the extrusion nozzle, in the manner explained above. The device can accordingly also comprise the additional terahertz measuring device. The device can also comprise the extrusion device.

[0044] An embodiment of the invention is explained in more detail below with reference to the figures. They show schematically: Figure 1 shows a device for carrying out the method according to the invention in a schematic side view, Figure 2 shows a sectioned partial view of the device from Fig. 1 , Figure 3 is a diagram illustrating the temperature dependence of the refractive index, Figure 4 is a diagram illustrating the variability of the refractive index over the radial position within the wall of a tube that has not yet fully solidified, Figure 5 is a diagram illustrating the calibration relationship according to the invention, Figure 6 is a diagram illustrating the temperature distribution in the radial direction within the wall of a tube that has not yet fully solidified, and Figure 7 is a diagram illustrating different wall thicknesses measured at the first measuring position over the circumference of a tubular strand.

[0045] Unless otherwise stated, the same reference symbols in the figures refer to the same objects.

[0046] In the Figures 1 and 2 a tubular strand 10, in this case a pipe 10, in particular a plastic pipe 10, is shown, which has a wall 12, a cavity 14 defined by the pipe 10, an outer surface 16 with a circular cross-section and an inner surface 18 with a circular cross-section, which defines the cavity 14. In the present example, the pipe 10 is extruded by means of an extruder in an extrusion device 20 and conveyed along its longitudinal axis by means of a suitable conveying device, in Figure 1from left to right. After emerging from the, for example, annular extrusion die of the extrusion device 20, the tube 10 first passes through a first cooling section 22, in which the tube 10, which emerges from the extrusion die at a high temperature and not yet fully solidified, i.e., still containing recrystallizing and flowable components (melt), is cooled. The first cooling section 22 can comprise a calibration device, in particular a calibration sleeve, against which the tube 10 is pressed, for example, by a vacuum and the atmospheric pressure inside the tube 10. This definitively determines the outer diameter of the tube 10 preformed by the extrusion die. Subsequently, 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 and / or outer diameter and / or wall thickness, are determined in a manner explained in more detail below.Following the first terahertz measurement direction 24, the tube 10 passes through at least one additional cooling section 26, in which further cooling takes place. The dashed line of the tube 10 illustrates that additional cooling sections 26 may be provided. After the tube 10 has completely solidified, it is cut into predetermined lengths in a cutting device 28, for example, a flying saw.

[0047] Based on Figure 2The structure and function of the first terahertz measuring device 24 will be explained in more detail. In the example shown, the first terahertz measuring device 24 comprises a transceiver 30, in which a transmitter and a detector for terahertz radiation are combined. The transmitter emits terahertz radiation 32 onto the pipe 10. The terahertz radiation is reflected at various interfaces of the pipe 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 further connected via a line 36 to an evaluation device 38. The reflected radiation received by the detector generates corresponding measurement signals, which are forwarded via line 36 to the evaluation device 38. In this way, the evaluation device 38 can, for example, Figure 2The wall thicknesses 40, 42 shown in the drawing, as well as the inner and / or outer diameter 44, can be determined, for example, based on time-of-flight measurements. The evaluation device 38 can also determine the refractive index of the strand material based on the measurement signals received from the detector, as described, for example, in WO 2016 / 139155 A1 or DE 10 2018 128 248 A1.

[0048] The first terahertz measuring device 24 is used to measure, for example, the outer diameter 44 and the wall thicknesses 40, 42 of the tube 10 as well as the refractive index at the Figure 1The measurement position shown is determined at which the tube 10 is not yet completely solidified, i.e., still contains flowable components. The transceiver 30 can, for example, rotate along a circular path around the tube 10 and thus determine the geometric parameters and the refractive index at various locations around the circumference of the tube 10. The reflector 34 can also rotate around the tube 10. However, it is also possible to dispense with the reflector 34.

[0049] Between the at least one additional cooling section 26 and the cutting device 28 is another terahertz measuring device 25. The additional terahertz measuring device 25 also includes a transceiver 27, which combines a transmitter and a detector for terahertz radiation. A reflector 29 for terahertz radiation is arranged on an opposite side of the tube 10. This reflects the terahertz radiation 31 emitted by the transmitter after passing through the tube 10 and reflecting off the interfaces of the tube 10 back to the detector.

[0050] As will be explained in more detail below, the further terahertz measuring device 25 measures the refractive index and at least the wall thickness of the tube 10 at the top and bottom at a second measuring position, where the tube 10 is essentially completely solidified. The measurement of the refractive index and the geometric parameters, such as the inner and / or outer diameter and / or the wall thickness of the tube 10, can be carried out by the further terahertz measuring device 25 at the Figure 1The measurement can be carried out in the second measuring position shown in the manner explained above for the first terahertz measuring device 24, in this case at a measuring position at which the tube 10 is essentially completely solidified. The additional terahertz measuring device 25 can be arranged in a particularly simple manner in a fixed position and simply emit terahertz radiation onto the tube 10 vertically from above or below, so that the aforementioned geometric parameters, in particular the wall thicknesses on the top and bottom of the tube 10, are measured. The additional terahertz measuring device 25 can also be a portable handheld device.

[0051] Figure 3shows the dependence of the refractive index on temperature or state of matter. On the one hand, it is evident that the relationship between the refractive index and temperature or state of matter is non-linear. On the other hand, it is evident that the refractive index changes particularly strongly in the recrystallization phase, i.e., at the transition between the solid and liquid states. This is utilized in the method according to the invention by using the measured refractive index to determine the remaining proportion of material in the recrystallization phase and, if applicable, in the liquid state, and from this, inferring any sagging to be expected following the measurement at the first measuring position.

[0052] In Figure 4The refractive index is plotted very schematically against the radial position x over the wall thickness. On the x-axis, a range from 0 to 1 is plotted, with the wall thickness 0 corresponding to the outer surface and the value 1 to the inner surface of the considered wall of the tube 10. The Figure 4 Curve 46, shown as a solid line, shows the refractive index. It is clearly constant in the solidified zone, i.e., in the range from approximately 0 to 0.4 on the x-axis, and corresponds to the cold value of the refractive index. n cold , In the present case, for example, 1.5. In the non-solidified zone, which follows radially inwards, the refractive index decreases to a minimum value of approximately 1.46. It should be noted that the actual course of the refractive index, particularly in the area of ​​the non-solidified zone, does not necessarily have to be linear. The horizontal dashed line in Figure 4For example, a resulting refractive index measured at the first measuring position over the cross section of at least one wall of the tube 10 n res of, for example, 1.46. From a comparison with the cold value of the refractive index n cold It can therefore be deduced that a portion of the strand material is still in the recrystallization phase and may still be flowable in the form of melt.

[0053] The diagrams of the Figures 3 and 4 were created using the example of polyethylene as pipe material, in particular HDPE.

[0054] The refractive index difference of the measured refractive index to the cold value of the refractive index is accordingly: Δn = n kalt − n res

[0055] For different values ​​of this refractive index difference, a sagging factor can be SF = k * Δn, with a factor k, determine the curve in Figure 4very schematically shown as a dashed line at the reference numeral 48. In the example shown, the sagging factor SF essentially complementary to the resulting refractive index n res . The actual sagging factor (SF) may deviate from this linear curve. The actual curve can be determined empirically, for example, in a series of tests for the respective tubular strand produced.

[0056] Figure 5 shows an example of a calibration relationship used according to the invention. This relationship assigns the refractive index at the first measurement position to a ratio of the exit widths at the top and bottom of the extrusion nozzle of the extrusion device 20 for extruded material. Figure 5The refractive index at the first measuring position is plotted on the y-axis. On the x-axis, the ratio of the wall thicknesses of the tube 10 on its top and bottom, measured at the first measuring position, i.e. the offset between the measured wall thicknesses on the top and bottom, is plotted in percent in the upper row. In the lower row on the x-axis, the corresponding target ratio of the outlet widths on the top and bottom of the extrusion nozzle of the extrusion device 20, i.e. the offset between the outlet widths on the top and bottom, is plotted in percent. The starting value of 0% indicates a required difference of zero between the outlet widths on the top and bottom of the extrusion nozzle and the wall thicknesses measured on the top and bottom at the first measuring position, in relation to the cold value of the refractive index of 1.5. This is because in this purely theoretical case, no sagging is to be expected.The values ​​of the upper row of the x-axis in , which are increased compared to 0. Figure 5 At lower refractive indices, i.e., when the tube 10 still contains flowable components, they indicate the difference (offset) required for the desired tube geometry in the fully solidified state between the wall thicknesses of the tube 10 measured at the first measurement position on the top and bottom. A value of 5% therefore means that the upper wall thickness is 5% greater than the lower wall thickness. Accordingly, a value of 10% means that the upper wall thickness is 10% greater than the lower wall thickness.

[0057] The value defined by the lower row of the x-axis in Figure 5The corresponding required difference (offset) of the outlet widths at the top and bottom of the extrusion nozzle is calculated by multiplying the offset at the first measurement position by a weighting factor that takes into account the different sagging upstream and downstream of the first measurement position, as explained above. As explained above, the weighting factor takes into account the degree to which the sagging between the extrusion device 20 and the first measurement position is higher than the sagging still occurring after the first measurement position. Figure 5In the example shown, a weighting factor of 3 was assumed. This weighting factor can be determined empirically, as explained above. The calibration relationship can, for example, be determined empirically, as explained above. It can also be determined for defined geometric values ​​of the tube 10 at the first measuring position, as explained above. The calibration relationship thus specifies, for the refractive index of the tube 10 measured at the first measuring position, the desired ratio of the outlet widths at the top and bottom of the extrusion nozzle of the extrusion device 20, which leads to a predetermined geometry of the tube 10 in the fully solidified state, in particular to a uniform wall thickness over the circumference of the tube 10. As in Figure 5 As can be seen, the refractive index decreases with increasing ratio between the exit widths at the top and bottom of the extrusion nozzle.

[0058] Figure 6shows, for illustration, the radial temperature profile within a wall of the tube 10 from the inside to the outside for two different refractive indices measured at the first measuring position, as it results at the first measuring position of the first terahertz measuring device 24. The temperature is plotted in °C against the radial position, for example in mm. The solid curve corresponds to a higher measured refractive index than the dashed curve. Accordingly, the temperature profile indicated by the dashed curve is higher than the temperature profile indicated by the solid line. The temperature on the outside of the tube has cooled significantly and solidified due to the cooling liquid applied to the outside in the first cooling section 22. In contrast, the temperature rises significantly in the radially inward direction, since the cooling of the outer wall has not yet cooled these areas to the point of solidification.Accordingly, there are still flowable components present. Above approximately 120°C, the material is flowable, and sagging occurs.

[0059] Figure 7 shows, for example, two different cases, measured wall thicknesses at the first measuring position around the circumference of the pipe 10, where the wall thickness of the pipe 10 in mm is plotted against the angular position in °. 180° is the top of the pipe and 0° or 360° the bottom. The dashed line corresponds, for example, to the Figure 6 The dashed temperature distribution and the solid line corresponds, for example, to the temperature distribution shown in Figure 6 The temperature distribution shown in solid lines. Figure 7The curves shown were determined empirically and each result in a uniform wall thickness across the circumference of the tube 10. It can be seen that to compensate for the sagging for the higher temperature distribution, a greater deviation (offset) of the outlet width at the top of the tube 10 (180°) from the bottom of the tube 10 (0°) is required than in the case of the lower temperature distribution. For example, in the case of the solid line, for the desired tube geometry in the fully solidified state, an offset of the top / bottom wall thicknesses of approximately 6.4% (33 mm to 31 mm) is required. From the calibration context according to Figure 5This corresponds to a refractive index of approximately 1.46 at the first measurement position. Therefore, if a refractive index of 1.46 is measured at the first measurement position, the wall thickness ratio measured at the first measurement position must be 1.064, or 6.4% as a percentage of the offset. By converting using the weighting factor, as explained above, this means that the exit width ratio for extruded material at the extrusion die must be 1.192, or 19.2% as a percentage of the offset. This ratio therefore forms the target ratio to which the extrusion die must be adjusted. For the dashed line in Figure 7 corresponding, different values ​​result, which in turn correspond to a different refractive index in Figure 5 By performing a number of measurements as described in Figure 7 As shown, the calibration relationship can be Figure 5be constructed empirically, whereby, for example, interpolation can be carried out between individual measured values.

[0060] To carry out the method according to the invention with the device according to the invention shown in the figures, the refractive index over the cross section of at least one wall of the tube 10 and geometric values, in particular the wall thickness and / or the inner and / or outer diameter of the tube 10, are measured by means of the first terahertz measuring device 24 at the first measuring position immediately downstream of the first cooling section 22 at a plurality of measuring locations distributed over the circumference of the tube 10, as explained above. Based on 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 the calibration relationship was created. If this is not the case, a warning can be issued or the calibration relationship can be changed, for example, by a factor corresponding to the determined deviation of the geometric values. Furthermore, by means of the Figure 5The target ratio of the outlet widths at the top and bottom of the extrusion nozzle of the extrusion device 20 resulting from the calibration relationship shown is determined by relating the refractive index measured at the first measuring position and the geometry values ​​measured at the first measuring position and is compared with the actually set ratio of the outlet widths at the top and bottom of the extrusion nozzle of the extrusion device 20. If these ratios deviate from one another, a warning can be issued and / or the extrusion device 20 can be controlled such that the ratio of the outlet widths at the top and bottom of the extrusion nozzle is adjusted to the target ratio resulting from the calibration relationship. This control of the extrusion device 20 can take place automatically. The measured values ​​of the terahertz measuring device 24 are applied to the evaluation device 38.The calibration context can also be stored in the evaluation device 38. The evaluation device 38 can perform the described evaluation and, if necessary, control the extrusion device 20.

[0061] By means of the further terahertz measuring device 27, the refractive index and / or the wall thickness and / or the inner and / or outer diameter of the tube 10 are also measured at a second measuring position located downstream of the first measuring position, at which the tube 10 is essentially completely solidified. For example, the further terahertz measuring device 27 can measure the refractive index at the second measuring position, which can in particular correspond to the cold value of the refractive index, as well as the wall thickness of the tube 10 at least on its top side and on its bottom side. The measurement results of the further terahertz measuring device 27 are also available to the evaluation device 38.Based on the measured values ​​of the additional terahertz measuring device 27, which correspond to the final parameters of the tube 10 due to the already essentially complete solidification of the tube 10, the calibration relationship used, as well as the cold value of the refractive index and the predicted sagging, can be checked and, if necessary, corrected based on the actual parameters of the tube 10. This is done by the evaluation device 38. If necessary, the calibration relationship can be adjusted on this basis.

[0062] As explained, the extrusion device 20 can be controlled on the basis of the inventive evaluation of the measured values ​​and by using the calibration context by the evaluation device 38, which can comprise a corresponding control device for this purpose. In particular, the ratio of the outlet widths for extruded material at the top and bottom of the extrusion nozzle of the extrusion device 20 can be adjusted by the evaluation device 38, as explained, such that the desired wall geometry of the pipe 10, in particular a wall thickness that is as uniform as possible over the circumference, is obtained in the solidified state of the pipe 10. For this purpose, the control device of the evaluation device 38 can, for example, adjust the gap width of the annular outlet gap of the extrusion nozzle at the top and bottom of the extrusion nozzle accordingly.It is also possible for the control device of the evaluation device 38 to influence heating elements of the extrusion nozzle in a suitable manner. List of reference symbols

[0063] 10 Pipe 12 Wall 14 Cavity 16 Outer surface 18 Inner surface 20 Extrusion device 22, 26 Cooling section 24 Terahertz measuring device 25 Additional terahertz measuring device 27 Transceiver 28 Cutting device 29 Reflector 30 Transceiver 31 Terahertz radiation 32 Terahertz radiation 34 Reflector 36 Line 38 Evaluation device 40, 42 Wall thickness 44 Diameter 46 Refractive index curve 48 Sagging factor curve

Claims

1. A method for checking the setting of an extrusion device (20) which produces a tubular strand (10) conveyed along the longitudinal direction thereof, wherein the exit widths for extruded material on a top side and on a bottom side of an extrusion die of the extrusion device are set differently, wherein the method comprises the following steps: • by means of a terahertz measuring apparatus (24), the refractive index is measured over the cross-section of at least one wall of the tubular strand (10) at a first measuring position downstream of at least one first cooling section (22) for the tubular strand (10), at which the tubular strand (10) has not yet completely solidified, • by means of the terahertz measuring apparatus (24), geometry values are further measured at the first measuring position at at least one measuring location on the top side and at at least one measuring location on the bottom side of the tubular strand (10), wherein the geometry values comprise the wall thickness (40, 42) and / or the internal and / or external diameter (44) of the tubular strand (10), • for the measured refractive index and for a ratio of the geometry values measured on the top side and on the bottom side of the tubular strand (10), the ratio of the exit widths set on the extrusion device (20) for extruded material on the top side and on the bottom side of the extrusion die is checked using a previously determined calibration correlation between the refractive index at the first measuring position and the ratio of the exit widths for extruded material on the top side and on the bottom side of the extrusion die.

2. The method according to Claim 1, characterized in that the calibration correlation is determined for defined geometry values of the tubular strand (10).

3. The method according to Claim 2, characterized in that, if a deviation of the geometry values measured at the first measuring position from the defined geometry values is ascertained, a warning is output and / or the calibration correlation is adjusted in accordance with the ascertained deviation of the geometry values.

4. The method according to one of the preceding claims, characterized in that, when establishing the calibration correlation, material parameters of the tubular strand (10), in particular the thermal conductivity and / or the thermal capacity, and / or production parameters, in particular the temperature of the extrusion die and / or the haul-off speed of the extrusion device (20) and / or cooling parameters of the tubular strand (10) produced, are taken into account.

5. The method according to one of the preceding claims, characterized in that the calibration correlation assigns the refractive index at the first measuring position to a nominal ratio of the exit widths for extruded material on the top side and on the bottom side of the extrusion die, so that the tubular strand (10) has a nominal wall thickness profile, in particular a uniform wall thickness profile, over its circumference following its complete solidification.

6. The method according to Claim 5, characterized in that, if a deviation is established between the resulting nominal ratio of the exit widths of the extrusion die for the refractive index measured at the first measuring position according to the calibration correlation and the ratio of the set exit widths of the extrusion die, the resulting nominal ratio for the measured refractive index according to the calibration correlation is displayed, and / or in that the extrusion device (20) is triggered in order to set the resulting nominal ratio for the measured refractive index according to the calibration correlation, preferably automatically.

7. The method according to one of Claims 5 or 6, characterized in that the calibration correlation further assigns the refractive index at the first measuring position to a ratio of the wall thicknesses on the top side and on the bottom side of the tubular strand (10) at the first measuring position, wherein a weighting factor is utilized to convert between the ratio of the wall thicknesses on the top side and on the bottom side of the tubular strand (10) at the first measuring position and the nominal ratio of the exit widths for extruded material on the top side and on the bottom side of the extrusion die, which weighting factor takes into account a different degree of sagging upstream of the first measuring position and downstream of the first measuring position.

8. The method according to one of the preceding claims, characterized in that, on the basis of a comparison of the geometry values measured at the first measuring position with the exit widths set on the extrusion device (20), an expected sagging of the tubular strand (10) up to the complete solidification of the tubular strand (10) is predicted.

9. The method according to Claim 8 and one of Claims 5 to 7, characterized in that the predicted sagging and / or expected values of the measured geometry values in the completely solidified state of the tubular strand (10) is / are displayed, and / or in that the resulting nominal ratio of the exit widths for extruded material on the top side and on the bottom side of the extrusion die according to the calibration correlation is adjusted on the basis of the predicted sagging.

10. The method according to one of the preceding claims, characterized in that the refractive index is measured at the first measuring position on the top side and / or the bottom side of the tubular strand (10).

11. The method according to one of the preceding claims, characterized in that the refractive index and / or the geometry values is / are measured at a plurality of measuring locations over the circumference of the tubular strand (10).

12. The method according to one of Claims 1 to 11, characterized in that the refractive index is determined from a comparison of the transit time of measuring radiation (32) emitted by the terahertz measuring apparatus (24) without a tubular strand (10) arranged in the beam path of the measuring radiation (32) with the transit time of the measuring radiation (32) with a tubular strand (10) arranged in the beam path of the measuring radiation (32).

13. The method according to one of Claims 1 to 11, characterized in that the refractive index is determined by determining the optical wall thickness (40, 42) of the tubular strand (10) with the terahertz measuring apparatus (24), further by determining an external diameter (44) and an internal diameter of the tubular strand (10) with the terahertz measuring apparatus (24), and by determining the refractive index of the tubular strand (10) from a comparison of the determined external and internal diameter (44) with the determined optical wall thickness (40, 42).

14. The method according to one of the preceding claims, characterized in that the geometry values of the tubular strand (10) are determined from a transit time measurement of measuring radiation (32) emitted by the terahertz measuring apparatus (24).

15. The method according to one of the preceding claims, characterized in that, at a second measuring position at a distance from the first measuring position in the longitudinal direction of the tubular strand (10), in particular a second measuring position downstream of the first measuring position at which the tubular strand (10) has substantially completely solidified, the refractive index over the cross-section of at least one wall of the tubular strand (10) and / or the wall thickness (40, 42) and / or the internal and / or external diameter (44) of the tubular strand (10) is / are measured with a further terahertz measuring apparatus (25).

16. The method according to Claim 15, characterized in that at the second measuring position, the wall thickness (40, 42) and / or the internal and / or external diameter (44) of the tubular strand (10) on the top side of the tubular strand (10) and on the bottom side of the tubular strand (10) is / are measured with the further terahertz measuring apparatus (25).

17. The method according to one of Claims 15 or 16, characterized in that the calibration correlation is checked and, if necessary, corrected using the measuring results of the further terahertz measuring apparatus (25).

18. A device for carrying out the method according to one of the preceding claims, comprising the terahertz measuring apparatus (24) and an evaluation apparatus (38) which is configured to check the ratio of the set exit widths for extruded material on the top side and on the bottom side of the extrusion die.