Method for checking an extruder and the extruder

Abstract

The present invention relates to a method for checking the settings of an extrusion device for producing a tubular strand conveyed along its longitudinal direction, in which the extrusion die of the extrusion device is set to have different exit widths for the extruded material at the upper and lower sides. The method includes the steps of measuring a refractive index across a cross section of at least one wall of the tubular strand using a terahertz measurement device at a first measurement location downstream of at least one first cooling section for the tubular strand, where the tubular strand has not yet completely solidified, further measuring a shape value at at least one measurement location above the tubular strand and at least one measurement location below the tubular strand using the terahertz measurement device at the first measurement location, the shape value comprising a wall thickness and / or an inner diameter and / or an outer diameter of the tubular strand, and verifying a ratio of the measured refractive index and the shape values ​​measured above and below the tubular strand using a predetermined calibration correlation between the refractive index at the first measurement location and a ratio of the exit widths of the extrusion material above and below the extrusion die set in the extrusion apparatus. The present invention also relates to an apparatus for performing the method of the present invention.

Description

[Technical Field]

[0001] The present invention relates to a method for verifying the settings of an extrusion apparatus for manufacturing tubular strands conveyed along its longitudinal direction, wherein the method is set so that the outlet width of the extruded material differs between the upper and lower sides of the extrusion die of the extrusion apparatus. The present invention also relates to an apparatus for performing this method.

[0002] For example, plastic pipes used to supply gases and water to residential and industrial areas, as well as for drainage, are primarily manufactured from materials such as HDPE, PP, and PVC. Typical pipe diameters are up to 3m, and wall thicknesses are up to 250mm. This manufacturing process usually takes place in an extruder, where the plastic pipe material is melted and discharged through a generally annular extrusion die. The extruded pipe is then withdrawn from the extruder and transported longitudinally. The diameter of the withdrawn pipe is then shaped to the desired outer diameter in a downstream calibration device, such as a sleeve-shaped one. During transport, the pipe generally passes through several cooling sections, where it is cooled and the initially fluid molten plastic solidifies sequentially. In the first cooling section, the shaped pipe is prevented from collapsing, for example, by a vacuum. Cooling of the pipe in the cooling sections is often done using a coolant such as water. The cooling water flows around the pipe, rapidly solidifying its outer region. After leaving the first cooling section, the outer surface of the tube is largely solidified, and the external shape of the tube becomes virtually unchanged. However, even after the tube leaves the first cooling section, there is often still a flowable portion of the tube material inside the tube wall. In the further stages of transporting the tubular strand, especially as it passes through further cooling sections, the inside of the tube is also continuously cooled, resulting in solidification of the inner surface as well. The tube is finally cut to the desired length with the help of a flying saw.

[0003] The shaping of the tube during the process of leaving the extruder and completely solidifying is substantially affected by two factors that must be taken into consideration, for example, in order to achieve the goal of making the tube wall thickness as uniform as possible. First, there is the shrinkage of the tube material that occurs during cooling. Second, there is the sagging of portions of the viscous mass that remain fluid during solidification due to the effect of gravity.

[0004] As a countermeasure to these effects that influence the final shape of the tubular strand, it is known to set the extrusion die of the extruder so that the outlet width of the extruded material in the upper region is greater than the outlet width in the lower region. To set the outlet width, it is possible to set the outlet gap of the extrusion die wider in the upper region than in the lower region. Alternatively or additionally, the extrusion die may be heated more strongly in the upper region than in the lower region, resulting in a larger outlet width in the upper region than in the lower region. Both of these measures result in more material being discharged in the upper region of the extrusion die than in the lower region. This intentional asymmetrical discharge of material is intended to compensate for sagging so that the wall thickness of the solidified tube is as constant as possible around its circumference.

[0005] Important geometric values ​​such as pipe wall thickness and diameter cannot be finally measured until the material has completely solidified, that is, after all shrinkage and sagging have occurred at the end of all cooling sections of the pipe. The typical production rate of an extruder for medium-sized pipe cross-sections is approximately 1,000 kg / h. The outlet temperature of the molten material from the extrusion die ranges from approximately 200°C to 240°C, depending on the material. For example, for a pipe with an outer diameter of 330 mm, a wall thickness of 30 mm, and a typical cooling section of 60 m, initial measurements of wall thickness and diameter are often only obtained several hours after the start of production. Only then can geometric deviations from the nominal values ​​be detected, potentially affecting the production parameters of the extruder, but any changes made at this stage can only be re-verified several hours later. After the start of the process, several corrections often required to obtain an optimal process, such as setting the wall thickness constant and to its nominal value over the circumference, can take several days.

[0006] As explained, when the extrusion die is set to compensate for sagging, the exit width is larger in the upper region than in the lower region. Even when sagging occurs, it is often overcompensated for to absolutely prevent the wall thickness from falling below the minimum value, and empirical values ​​are used here. This ultimately leads to the discharge of an unnecessarily large amount of material.

[0007] Therefore, it is desirable to draw conclusions as early as possible regarding the expected shrinkage and sag of the tubes produced by the extruder. For example, measurements of the tube wall thickness and diameter after the first cooling section may not correspond to the desired final values ​​that will exist after the tube has completely cooled, because at this point, only solidified material is present in the outer region of the tube wall, while recrystallized and molten material remains inside. Subsequently, at a measurement position downstream of the first cooling section, the diameter and wall thickness values ​​that will still be subject to shrinkage and sag are recorded. To expedite the start-up process and ensure nominal values ​​are maintained more consistently, predicting the expected shrinkage and sag values ​​as early as possible is economically very important.

[0008] International Patent Publication No. 2022 / 058081 proposes a method for determining the geometric parameters of a strand-like or planar object, by which shrinkage can be predicted. For this purpose, a verification step confirms the correlation between the refractive index of the object and the shrinkage that occurs during its solidification. In the determination step, the refractive index and at least one geometric parameter of the object, which is not yet completely solidified, are determined, particularly downstream of the first cooling section of the object, and the geometric parameters of the object in its fully solidified state are calculated from the determined values, taking into account the correlation confirmed in the verification step. Thus, the shrinkage of the material of the object is predicted during its solidification process, and based on this, the geometric parameters of the object in its fully solidified state, such as the wall thickness, are calculated.

[0009] The method described makes it possible to reliably predict shrinkage, which can be considered one of two major influences that significantly affect the final shape. However, advantageously, measurements are available immediately after the material leaves the extruder, and therefore the final geometric parameters can be calculated early to prevent waste. However, sagging, the second major influence on the final shape of the object, cannot be predicted by known methods for reasons explained in detail below.

[0010] A method for checking the sagging of a tube extruded by an extruder is known from International Patent Publication 2022 / 106180. In this method, the wall thickness of the tube is measured along its circumference, and a wall thickness profile is created from the measured wall thickness along the circumference. The sagging of the molten material is confirmed from the frequency and / or amplitude of the created wall thickness profile. While this method makes it possible to reliably detect sagging, it is not possible to predict sagging.

[0011] Therefore, with respect to the prior art described above, the object of the present invention is to provide a method and apparatus of the type described at the beginning that can reliably check and, if necessary, the exit width of the extruded material set in the extruder, taking into account the sagging of the tubular strand produced in the extruder, particularly in real time, with minimal time delay after the strand exits the extruder, and correct it as needed.

[0012] The present invention achieves this objective by independent claims 1 and 18. Advantageous embodiments are shown in the independent claims, specification, and drawings.

[0013] Regarding the type of method described at the beginning, the present invention achieves this objective by the following steps: Using a terahertz measuring device, the refractive index is measured across a cross-section of at least one wall of the tubular strand at a first measurement position downstream of at least one first cooling section for the tubular strand, while the tubular strand is not yet completely solidified. - Using a terahertz measuring device, further measuring shape values ​​at at least one measurement point on the upper side and at least one measurement point on the lower side of the tubular strand at the first measurement position, wherein the shape values ​​include the wall thickness and / or inner diameter and / or outer diameter of the tubular strand. - The ratio of the measured refractive index to the shape values ​​measured on the upper and lower sides of the tubular strand is used to verify the ratio of the outlet widths of the extruded material on the upper and lower sides of the extrusion die, as set in the extruder, using a predetermined calibration correlation between the refractive index at the first measurement position and the ratio of the outlet widths of the extruded material on the upper and lower sides of the extrusion die.

[0014] The present invention also achieves this objective by providing an apparatus for carrying out the method according to the present invention, comprising a terahertz measuring device and an evaluation device configured to confirm the ratio of set outlet widths of the extruded material on the upper and lower sides of the extrusion die.

[0015] The method and apparatus according to the present invention allow for the prediction of sagging after a first measurement position, i.e., the sagging of the still-fluid, viscous mass portion of the tubular strand as it exits the extruder during solidification due to the effects of gravity. Based on this, the settings of the extruder can be checked and modified as necessary. The tubular strand may be a tube, for example, a plastic tube. As is known, the extruder consists of, for example, a basically annular extrusion die from which the molten plastic material exits first. Also, as is known, after exiting the extruder, the tubular strand passes through a series of cooling sections in a regular manner, in which the strand material is continuously and completely cooled by a coolant such as water, and thus completely solidified. For example, after exiting the first cooling section immediately after leaving the extruder, the outer surface of the tubular strand may already be solidified, and thus the molding of the outer surface of the tubular strand is complete. However, inside the tube wall, the material of the tubular strand still has recrystallized regions and molten regions, and therefore at least a portion is still flowable, and as is known, sagging occurs in addition to shrinkage during the further cooling process of the strand. To finally shape the tubular strand in the first cooling section, the strand material can be pressed against, for example, the cylindrical inner surface of a calibration sleeve in this first cooling section, for example, by applying a vacuum.

[0016] However, in principle, it is also possible to model the expected sag in detail using the Navier-Stokes equations, based on precisely known framework conditions and detailed material properties of the tubular strand, in addition to shrinkage. However, this is not practical for the rapid prediction of sag desired in this case due to the very large numerical load. Therefore, according to the present invention, a reliable prediction of the expected sag, and thus the final shape of the strand, is still intended to be available as soon as possible after it leaves the extruder to prevent waste, so that the manufacturing process can be intervened at an early stage if necessary, for example by adjusting the settings of the extruder. Preferably, the prediction of sag is intended to be made in real time. In either case, the use of the Navier-Stokes equations, which are themselves reliable, is excluded in this application scenario.

[0017] As already explained, unlike shrinkage, sagging cannot be predicted by the method described in International Patent Publication 2022 / 058081. Shrinkage from the extrusion temperature until complete cooling does not depend on the solidification time, especially the cooling rate. Therefore, the shrinkage rate can be easily and reliably predicted by comparing the refractive index measured at the first measurement position with the refractive index at cold. Furthermore, shrinkage is not substantially affected even if parameters of the extruder or cooling parameters are changed, for example. This is a substantially unchangeable process characteristic, but it is relatively easy to predict.

[0018] This differs from sagging, which involves a far more complex process than shrinkage. During sagging, the temperature-dependent flow behavior of the material, its temperature, and the cooling rate play a crucial role. Generally, sagging of the molten material during tube manufacturing is more pronounced the higher the initial temperature of the molten material and the longer the time it takes for the molten material to cool and solidify completely. Varying operating conditions, such as extrusion temperature, extruder capacity, discharge rate, and cooling intensity and duration, affect the degree of sagging. On the other hand, predicting shrinkage using refractive index only requires stopping the system and recording the change in refractive index along with the shrinking wall thickness and diameter over several hours of cooling. This reliable method does not work when predicting sagging, precisely because the sagging of wall thickness is more or less pronounced depending on whether the cooling is performed for a long or short time. Furthermore, unlike shrinkage, sagging can be decisively influenced by changing the parameters of the extruder and the cooling parameters of the tubular strand.

[0019] To solve these problems, according to the present invention, first, a (first) terahertz measuring device is used to measure the average refractive index or the resulting refractive index over a cross section of at least one wall thickness of the tubular strand at a first measurement position downstream of at least one first cooling section of the tubular strand, where the tubular strand is still not completely solidified, i.e., still has flowable recrystallized and / or molten sections; and second, at this first measurement position, shape values ​​of the tubular strand, including wall thickness and / or inner and / or outer diameters, are measured at at least two measurement locations on the upper and lower sides of the tubular strand. For this purpose, the terahertz measuring device consists of a terahertz transmitter and a terahertz detector, which are arranged in substantially the same position and can also be combined with a transceiver in a particularly practical manner. The terahertz radiation used in accordance with the present invention may be, for example, in the frequency range of 1 gigahertz to 6 terahertz. Terahertz radiation is particularly suitable for difficult measurement conditions in the environment of an extruder, such as high temperatures, all kinds of contamination, and steam generation. Therefore, in contrast to laser radiation in the visible frequency band, for example, terahertz radiation is hardly affected by such interference. Terahertz radiation emitted from a transmitter in the direction of terahertz measurement collides with a tubular strand, for example, from above and / or below. In this process, some of the terahertz radiation is reflected off the outer surface of the strand, and some of the terahertz radiation passes through the strand material. As a result, further reflection occurs at the interface of the tubular strand, particularly at the interface between the wall of the strand facing the transmitter and the wall facing away from the transmitter. Also, some of the terahertz radiation exits again from the strand on the opposite side of the transmitter. Here, the terahertz measuring device may include a reflector that reflects the terahertz radiation exiting from the strand side away from the transmitter back into the strand. This radiation, along with the remaining radiation component reflected at the strand interface, is further reflected at the strand interface before reaching a detector, for example, located in the same place as the transmitter, and is detected as a measurement event by this detector.

[0020] As is known, the refractive index can be determined from the detected radiation across the cross-section of one or two opposing walls of the strand at a first measurement position, as will be described in more detail below. The refractive index can be measured on the upper and / or lower sides of the strand. In particular, when radiation passes through two, for example, opposing walls of the strand, it is possible to determine the average refractive index or the resulting refractive index across both irradiated walls. Furthermore, as is known, the distance from the individual interfaces of the strand can be determined, for example, based on the measurement of the transit time, from which it is possible to determine geometric parameters such as the wall thickness of the strand, and / or the inner and / or outer diameter. For example, the upper measurement may be taken at the highest point of the strand, and the lower measurement may be taken at the lowest point of the strand. In particular, the upper and lower wall thicknesses of a tubular strand may be measured at the first measurement position. As explained at the beginning, these wall thicknesses often differ because extruders are generally set up to extrude more material on the upper side than on the lower side of the extrusion die to compensate for sagging. Such asymmetry often still exists in the region of the (first) terahertz measuring device, particularly immediately downstream of the first cooling section.

[0021] A predetermined calibration correlation is used to predict still expected sagging and to verify the extruder according to the present invention. The basis for using the calibration correlation is the refractive index measured at a first measurement position. The calibration correlation assigns the refractive index at the first measurement position to the ratio of the set outlet widths of the extruded material on the upper and lower sides of the extrusion die. The ratio of the outlet widths on the upper and lower sides of the extrusion die, and the ratio of the measured shape values ​​on the upper and lower sides of the strand, can be specified as a simple number ratio, i.e., a percentage. Since the strand, in principle, still consists of recrystallized material and molten material, particularly regions containing still flowable material, the measured refractive index in the region of the first measurement position will differ from its cold value. The refractive index measurement provides information about the flowable material, particularly the recrystallized and / or molten material, that still exists within the tubular strand at the first measurement position. The calibration correlation provides a correspondence between the refractive index measured at a first measurement position and the ratio of the exit widths of the extrusion die required to obtain the desired strand shape in a fully solidified state, based particularly on predetermined geometric parameters of the strand at the first measurement position and predetermined manufacturing parameters of the extruder. This ratio of exit widths obtained according to the calibration correlation of the measured refractive index can be compared to the ratio of the set exit widths of the extruder in order to verify the extruder. As described at the beginning, the exit width can be set, for example, by setting the exit spacing of the extrusion die and / or by operating the heating device of the extrusion die. For example, the basic setting of the exit width can be done by setting the exit spacing of the extrusion die, and any subsequent fine-tuning or other adjustments of the exit width can be done only by controlling the heating device of the extrusion die.

[0022] For example, simply detecting the upper and lower wall thicknesses of a tubular strand at a first measurement position is insufficient for predicting the expected sagging according to the present invention, and thus for confirming the settings of the extruder. This is because the wall thickness ratio measured at the upper / lower first measurement position of the strand does not provide information about the extent to which previous sagging has already occurred or is complete. Only by combining the evaluation of the (resulting) refractive index measured at the first measurement position with a weighting of the wall thickness ratio, preferably measured at the upper / lower first measurement position, can it be estimated whether the sagging will be fully compensated for in the further cooling process so that the strand has a predetermined wall thickness profile in the fully solidified state.

[0023] Due to the simplified method according to the present invention, prediction of sagging still expected downstream of the first measurement position, and the confirmation of the extruder thereby, can be performed quickly and in real time, and the extruder can be rapidly controlled accordingly to set the exit of the strand material from the extruder die, particularly on the upper and lower sides of the extruder die, so that the desired shape of the strand is achieved in a fully solidified state. Often, the desired shape of a tubular strand is one in which the cross section is circular and the wall thickness is constant along the circumference.

[0024] The calibration correlation may be stored in the form of a function or a curve. In a particularly simple method, the calibration correlation may be established empirically. For this purpose, in the related manufacturing process, when setting the outlet width of the extrusion die so that the tubular strand has a predetermined shape, for example, a constant wall thickness over its circumference, after the tubular strand is completely solidified, the measured refractive index value obtained at the first measurement position and the shape values, particularly the measured quotient of the wall thickness, at the upper and lower sides may be stored. In a later process, it is possible to compare the refractive index measured at the first measurement position with this stored refractive index. As is known, the refractive index depends on temperature. If the measured refractive index deviates from the stored refractive index, it can be estimated that there is a change in temperature, particularly in the still fluid part inside the strand at the first measurement position. Therefore, it is further estimated that in order to achieve the desired shape of the solidified strand, correction of the set outlet width of the extrusion die is necessary. Further settings of the outlet width that result in the desired shape of the solidified strand may be appropriately stored for different refractive indices at the first measurement position when creating the calibration correlation empirically.

[0025] In particular, changes in the manufacturing parameters in the upstream region of the first measurement position can have a significant impact on the sagging and thus the shape of the strand in the solidified state. Such changes occur particularly when the manufacturing process is restarted after a production interruption time. This includes, for example, the temperature of the extrusion die rising gently during startup or changes in the output speed of the extrusion device. Another example is, for example, a change in the cooling parameters by changing the composition and / or temperature of the coolant. Such changes can be recognized based not only on the refractive index measured at the first measurement position but also particularly on the shape values measured at the first measurement position. This is especially because a significant proportion of the overall sagging has already occurred by the time it exits the first cooling section. Therefore, according to the present invention, when the manufacturing process is restarted, it is possible to set the optimal process parameters of the extrusion device, particularly at an early stage, based on the calibration correlation.

[0026] Changes in the manufacturing process can be recognized not only from the refractive index but also, in particular, from the shape values recorded at the first measurement position. If the shape values recorded at the first measurement position change, it can be concluded that the expected sag that is taken into account according to the calibration correlation also changes. Thus, according to one embodiment, it is possible to determine the calibration correlation, for example, at the first measurement position, based on the defined shape values of the tubular strand. Then, if a deviation of the shape values measured at the first measurement position from the defined shape values is confirmed, a warning may be output and / or the calibration correlation may be adjusted according to the confirmed deviation of the shape values. For example, the calibration correlation may be adjusted by a coefficient corresponding to the confirmed change. This coefficient can be adjusted based on empirical values and, for example, can be multiplied by a constant coefficient considering the influence of the change on the occurring sag.

[0027] When establishing the calibration correlation, material parameters of the tubular strand, in particular the thermal conductivity and / or the heat capacity, may be considered. Further, manufacturing parameters, in particular the temperature of the extrusion die and / or the discharge speed of the extrusion device and / or the cooling parameters of the manufactured tubular strand, may be considered. Such parameters also affect the occurring sag.

[0028] According to a further embodiment, the calibration correlation may assign the refractive index at the first measurement position to the nominal ratio of the outlet widths of the extruded material above and below the extrusion die such that the tubular strand has a nominal wall thickness profile, in particular a uniform wall thickness profile, over its circumference after its complete solidification. As already explained, for example, an empirically established calibration correlation may be in the form of a function or a curve. Here, the refractive index at the first measurement position can be plotted as a function of the ratio of the outlet widths of the extruded material above and below (set as the nominal ratio in the extrusion die). In the simplest case, the curve may be a straight line with a negative slope. However, a curve deviating from a straight line is also possible.

[0029] In a further embodiment, if a deviation is observed between the nominal ratio obtained as a result of the exit width of the extrusion die to the refractive index measured at a first measurement position according to the calibration correlation, and the ratio of the set exit width of the extrusion die, the nominal ratio obtained as a result of the refractive index measured according to the calibration correlation may be displayed. Alternatively or additionally, the extruder may be operated to preferably automatically set the nominal ratio obtained as a result of the measured refractive index according to the calibration correlation. This operation may be performed by an evaluation device. Thus, automatic control of the extruder becomes possible. The display of the nominal ratio may be performed on an operator's display.

[0030] As already explained, the expected sag of the tubular strand until it is completely solidified can be predicted based on a comparison of the shape value measured at the first measurement position with the outlet width set in the extruder. It is then possible to display the expected sag and / or the expected value of the measured shape value when the tubular strand is completely solidified, and / or the nominal ratio obtained as a result of the outlet width of the extruded material on the upper and lower sides of the extrusion die according to the calibration correlation can be adjusted based on the predicted sag.

[0031] In a further embodiment, the refractive index and / or shape values ​​can be measured at multiple measurement points along the circumference of the tubular strand at a first measurement position. For this purpose, for example, multiple terahertz transmitters and terahertz detectors may be arranged to be distributed along the circumference of the tubular strand. Preferably, a terahertz measuring device rotating with respect to the tubular strand may include a terahertz transmitter, a terahertz detector, and optionally a reflector, which can be used to generate measurements from the terahertz measuring device distributed along the circumference. In particular, it is possible in principle to completely cover the circumference of the strand in this way. Further advantages are obtained by measuring over a wider area along the circumference, especially on the sides of the strand, compared to measuring only from the top and bottom of the tubular strand. For example, this allows for the recognition of solidification of the side walls of the strand, which may already be further progressing and can prevent or block the downward flow of still flowable portions due to gravity, and as a result, can affect sagging.

[0032] In a further embodiment, the refractive index can be measured by comparing the transit time of the measuring radiation emitted from the terahertz measuring device with the transit time of the measuring radiation with the tubular strand placed in the beam path of the measuring radiation. This approach to measuring the unknown refractive index of an object is described, for example, in European Patent No. 3 265 748 B1. Measurement of the refractive index using a terahertz measuring device can be performed in the manner corresponding to the present invention.

[0033] In a further embodiment, the refractive index can be measured by determining the optical wall thickness of the tubular strand using a terahertz measuring device, further by determining the outer and inner diameters of the tubular strand using a terahertz measuring device, and by determining the refractive index of the tubular strand from a comparison of the determined outer and inner diameters with the determined optical wall thickness. This alternative method for measuring an unknown refractive index is described in German Patent Publication No. 10 2018 128 248A1. This method is also applicable in this case.

[0034] In particular, according to practical embodiments, the shape value of the tubular strand is determined from the measured transit time of the measuring radiation emitted from the terahertz measuring device.

[0035] In a further embodiment, the refractive index and / or wall thickness and / or inner and / or outer diameter of the tubular strand can be measured in an additional terahertz measurement direction at a second measurement position located spaced apart from the first measurement position in the longitudinal direction of the tubular strand, particularly at a second measurement position downstream of the first measurement position where the tubular strand has substantially solidified. Furthermore, at the second measurement position, the wall thickness and / or inner and / or outer diameter of the tubular strand on the upper side and the wall thickness on the lower side of the tubular strand can be measured using an additional terahertz measuring device. Calibration correlations can be confirmed using the measurement results of the additional terahertz measuring device and corrected as necessary.

[0036] The second measurement location may, in principle, be located upstream or downstream of the first measurement location. Preferably, it is located downstream, and particularly preferably, downstream enough to solidify the tubular strand substantially completely. The further terahertz measuring device may, in principle, be provided in the same manner as the (first) terahertz measuring device provided at the first measurement location. The further terahertz measuring device also consists of a (further) terahertz transmitter and a (further) terahertz detector, which can be combined again with a transceiver in a particularly practical manner. A reflector may be located on the far side of the tubular strand opposite the transmitter, and in some cases, the detector may be located in the same place. In a cost-effective manner, the further terahertz measurement direction may be fixed in place, in particular, so as to irradiate the tubular strand with terahertz radiation from vertically above to downward or vertically below to upward. It is also conceivable that the further terahertz measuring device may be a portable measuring device, in particular a so-called handheld measuring device, and therefore not permanently installed at the second measurement location. Next, the refractive index and the above-mentioned geometric parameters may be measured in the manner described above with respect to a terahertz measuring device placed at the first measurement position. At the second measurement position, especially if the tubular strand is substantially completely solidified at this position, the final material and geometric parameters of the strand, including the cold value of the refractive index, can be measured. Further measurements with a terahertz measuring device are substantially more accurate here than, for example, measurements with a mechanical probe. The final parameters measured at the second measurement position are compared with parameters of a calibration correlation, which may be verified based on the actually measured final parameters and adjusted as necessary.

[0037] The embodiments described above reduce the model's dependence on material parameters. In particular, in the field of plastic strands, material parameters are often unknown or rather not well defined. To reduce the dependence on material parameters, the embodiments described above may therefore use additional sensors to measure, for example, the cold values ​​of the refractive index and wall thickness at at least one angular position on the circumference of the strand, or the ratio between at least two wall thicknesses at different angular positions, for example, the ratio between the upper and lower wall thicknesses of the strand in the case of a terahertz measuring device fixedly positioned vertically above the strand. The calibration correlation used according to the present invention can be verified, for example, based on measurements from an additional terahertz measuring device to reduce deviations caused by material parameters that deviate from reality. In particular, as described above, changes in process temperature downstream of the first measurement position that affect sagging can be recognized by an additional terahertz measuring device. An example is a change in the cooling rate of the strand.

[0038] The correlation between the refractive index and the crystallization state of the strand material can be used, for example, empirically to determine the droop coefficient:

[0039] SF = k * Δ n however: SF: Sagging coefficient k: coefficient Δ n : The difference in refractive index between the refractive index measured at the first measurement position and the cold refractive index.

[0040] The larger the sag coefficient SF measured in this way, the more pronounced the expected sag will be. The volumetric and mass flow rates of the still flowable viscous material are roughly proportional to the sag coefficient. By accurately knowing the cold value of the refractive index, the sag coefficient, and consequently the calibration correlation, can be determined more precisely.

[0041] Therefore, the model according to the present invention for predicting expected sagging can be further improved. In particular, the sagging coefficient at the relevant measurement location is an indicator that can be used to infer how much the difference in wall thickness emanating from the upper and lower regions of the extrusion die, which is often intentionally set from there, will change in subsequent processes of the extrusion line. Based on this, by setting the annular gap of the extrusion die of the extrusion apparatus and / or the appropriate temperature of the extrusion die, the sagging coefficient can be preset at the measurement location of the terahertz measuring device so that the desired generally uniform wall thickness is obtained over the entire circumference of the strand with the remaining sagging.

[0042] In a further embodiment, the calibration correlation may further assign the refractive index at the first measurement position to the ratio of the upper and lower wall thicknesses of the tubular strand at the first measurement position, and a weighting coefficient is used to convert the ratio of the upper and lower wall thicknesses of the tubular strand at the first measurement position to the nominal ratio of the upper and lower exit widths of the extruded material on the extrusion die, the weighting coefficient taking into account the different degrees of sagging upstream and downstream of the first measurement position. Thus, it is possible to define a weight between the sagging of the strand that has already occurred, which is confirmed by measuring the shape value at the first measurement position, and the sagging that is expected to occur thereafter until it is completely solidified. The sagging that has already occurred is obtained by comparing the shape value measured at the first measurement position with the exit width set on the extrusion die. For example, assuming that this phenomenon continues after the first measurement position, it is still possible to predict the expected sagging 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, albeit to a lesser degree. Conversely, a weighting coefficient can be formed that indicates the degree of sagging upstream of the first measurement position is greater than the degree of sagging downstream of the first measurement position. For example, by forming a first quotient of the vertical gap dimension of the extrusion die in the upper and lower regions and a second quotient of the wall thickness in the upper and lower regions measured at the first measurement position, it is possible to compare the two quotients with each other, for example, by forming a quotient from the first quotient and the second quotient. This quotient may be a weighting coefficient. The weighting coefficient is generally greater than 1 because most of the sagging occurs between the exit of the extruder and the first measurement position, and thereafter the sagging is generally less noticeable. For example, the weighting coefficient may be about 3. Based on this type of weighting coefficient, a generally desired uniform wall thickness of the strand can be obtained circumferentially for a particular strand material and under certain manufacturing conditions. This weighting coefficient, or the weighted quotient formed by the transformation, allows the refractive index at the first measurement position to be assigned in the calibration correlation to either the ratio of the upper and lower wall thicknesses of the strand measured at the first measurement position, or the nominal ratio of the upper and lower exit widths of the extrusion die of the extruder.Therefore, based on the shape value and refractive index measured at the first measurement position, it is possible to directly determine and define the offset set in the extrusion die. Thus, the exit width of the extrusion die can be directly controlled in an open-loop or closed-loop manner based on the measurement value obtained at the first measurement position.

[0043] If a further terahertz measuring device is provided at a second measuring position downstream of the first measuring position, the measurements can be used to fine-tune the model according to the present invention, and by extension, the extruder. This takes advantage of the fact that at the first measuring position downstream of the first cooling section, the molten material has already cooled to the point where, in addition to the cooled outer region, some of the molten material remains inside the strand, particularly inside the tube wall, to recrystallize. Next, it is also possible to form a third quotient of the wall thicknesses of the upper and lower regions measured at the second measuring position of the second terahertz measuring device 27, i.e., in a fully solidified state. This third quotient should ideally be 1, for example, but may then be compared with the first and second quotients. Similarly, it is also possible to form a quotient from the refractive index measured at the first measuring position and the (cold value) refractive index measured at the second measuring position.

[0044] In the apparatus according to the present invention, the measurements of a terahertz measuring device, and, where applicable, an additional terahertz measuring device, are available in the evaluation device. The evaluation device is designed to carry out the evaluation and verification according to the present invention. It is also designed, in particular, to carry out embodiments according to the dependent claims of the method according to the present invention. For this purpose, the evaluation device may include a control device for controlling an extruder, in particular an extrusion die, in the manner described above. Accordingly, the device may also include an additional terahertz measuring device. The device may also include an extruder.

[0045] The present invention will be described in more detail below with reference to the accompanying drawings of exemplary embodiments. [Brief explanation of the drawing]

[0046] [Figure 1] A schematic side view of an apparatus for carrying out the method according to the present invention. [Figure 2] Partial cross-sectional view of the apparatus shown in Figure 1. [Figure 3] A graph showing the temperature dependence of the refractive index. [Figure 4] A graph illustrating the change in refractive index across radial positions within the pipe wall, which has not yet completely solidified. [Figure 5] A graph illustrating the calibration correlation according to the present invention. [Figure 6] A graph showing the radial temperature distribution inside the pipe wall, which has not yet completely solidified. [Figure 7] A graph showing different wall thicknesses measured at the first measurement point along the circumference of a tubular strand.

[0047] Unless otherwise specified, the same reference symbol refers to the same object in the diagram.

[0048] In Figures 1 and 2, a tubular strand 10, in this example a tube 10, specifically a plastic tube 10, is shown, comprising a wall 12, a cavity 14 partitioned by the tube 10, an outer surface 16 with a circular cross-section, and an inner surface 18, also with a circular cross-section, that partitions the cavity 14. In this embodiment, the tube 10 is extruded by an extruder in an extruder 20 and transported along its longitudinal axis from left to right in Figure 1 by a suitable transport device. After exiting the extruder 20, for example, an annular extrusion die, the tube 10 first passes through a first cooling section 22, where the tube 10, having exited the extrusion die, is strongly heated and cooled while it is still not completely solidified, i.e., still consisting of a recrystallized portion and a flowable portion (molten material). The first cooling section 22 may include a calibration device, particularly a calibration sleeve, against which the tube 10 is pressed, for example, by vacuum and atmospheric pressure inside the tube 10. As a result, the outer diameter of the preformed tube 10 by the extrusion die is finally set. In a further step, the tube 10 passes through a first terahertz measuring device 24, where the refractive index and geometric parameters of the tube 10, such as the inner diameter and / or outer diameter and / or wall thickness, are determined in a manner that will be described in more detail below. After the first terahertz measuring device 24, the tube 10 passes through at least one further cooling section 26, where it is cooled further. The dash of the tube 10 indicates that further cooling sections 26 may be provided. After the tube 10 has completely solidified, it is cut into predetermined portions using a length cutting device 28, for example, equipped with a flying saw.

[0049] The design and function of the first terahertz measuring device 24 will be described in more detail with reference to Figure 2. In the illustrated example, the first terahertz measuring device 24 comprises a transceiver 30 that combines a transmitter and a detector for terahertz radiation. The transmitter irradiates the tube 10 with terahertz radiation 32. The terahertz radiation is reflected by different interfaces of the tube 10 and by a reflector 34 positioned opposite the transceiver 30, and returns to the transceiver 30, where it is detected by the detector. The transceiver 30 is further connected to an evaluation device 38 via line 36. The reflected radiation received by the detector generates a corresponding measurement signal, which is transmitted to the evaluation device 38 via line 36. In this way, the evaluation device 38 can determine, for example, the wall thicknesses 40, 42 and the inner and / or outer diameters 44 shown in Figure 2, based, for example, on measured transit times. The evaluation device 38 can also determine the refractive index of the strand material based on the measurement signal received from the detector, for example, as described in International Patent Publication WO2016 / 139155 or German Patent Publication 10 2018 128 248A1.

[0050] The first terahertz measuring device 24 measures the outer diameter 44, wall thickness 40, 42, and refractive index of the tube 10, for example, at a first measurement position shown in Figure 1. At this first measurement position, the tube 10 is not yet completely solidified, meaning it still contains a flowable portion. In this process, the transceiver 30 can rotate in a circular orbit around the tube 10, for example, and determine the geometric parameters and refractive index at various positions on the circumference of the tube 10. The reflector 34 may also rotate around the tube 10. However, the reflector 34 can be omitted.

[0051] A further terahertz measuring device 25 is positioned between at least one further cooling unit 26 and the length cutting device 28. The further terahertz measuring device 25 also includes a transceiver 27 that combines a transmitter and a detector for terahertz radiation. On the opposite side of the tube 10 is a reflector 29 for terahertz radiation. The reflector reflects the terahertz radiation 31 emitted from the transmitter through the tube 10, is reflected at the interface of the tube 10, and then reflected to the detector.

[0052] As will be described in detail below, the additional terahertz measuring device 25 measures the refractive index and at least the wall thickness of the upper and lower sides of the tube 10 at a second measurement position where the tube 10 is substantially solidified. The measurement of the refractive index and the measurement of geometric parameters such as the inner and / or outer diameter and / or wall thickness of the tube 10 may be performed by the additional terahertz measuring device 25 at the second measurement position shown in Figure 1, in this case at a measurement position where the tube 10 is substantially solidified, in the manner described above in relation to the first terahertz measuring device 24. In a particularly simple embodiment, the additional terahertz measuring device 25 may be positioned in a fixed location, or it may simply irradiate the tube 10 vertically from above or below with terahertz radiation, thereby enabling the measurement of the aforementioned geometric parameters, particularly the wall thickness of the upper and lower sides of the tube 10. The additional terahertz measuring device 25 may also be a portable handheld device.

[0053] Figure 3 shows the relationship between refractive index and temperature, or rather, the state of aggregation. First, it can be seen that the correlation between refractive index and temperature, or rather the state of aggregation, is not linear. Second, it can be seen that the refractive index changes particularly significantly during the recrystallization stage, that is, the stage in which the state of aggregation transitions between solid and liquid. This is utilized in the method according to the present invention, in which conclusions can be drawn from the refractive index measured in either case regarding the proportion of material remaining in the recrystallization stage, and, if applicable, the proportion of material remaining in the liquid state. From this, the expected sagging after measurement at the first measurement position can be inferred.

[0054] In FIG. 4, the refractive index is plotted very schematically over the radial position x and the wall thickness. A range from 0 to 1 is plotted on the x-axis. However, a wall thickness of 0 applies to the outer surface, and a value of 1 applies to the inner surface of the wall related to the tube 10. The curve 46 shown as a solid line in FIG. 4 indicates the refractive index. It is shown that the refractive index is constant within the range of the solidification region, that is, in the range of about 0 to 0.4 on the x-axis, and the cold value n of the refractive index cold , for example, corresponds to 1.5 in this embodiment. In the subsequent non-solidification region, when looking radially inward, the refractive index decreases to a minimum value, which is about 1.46 in this embodiment. It should be noted that the actual profile of the refractive index does not necessarily have to be linear, especially in the range of the non-solidification region. The dashed horizontal line in FIG. 4 shows, for example, the refractive index n res measured at the first measurement position across the cross-section of at least one wall of the tube 10, which is, for example, 1.46. By comparing this with the cold value n of the refractive index cold , it can be inferred that a part of the strand material is still in the recrystallized phase and, if applicable, is flowable in the form of a melt.

[0055] In the case of this embodiment, the graphs in FIGS. 3 and 4 were created taking polyethylene, especially HDPE, as an example of the pipe material.

[0056] The refractive index difference between the measured refractive index and the cold value of the refractive index is as follows:

[0057] Δ n =n cold -n res

[0058] For various values of this refractive index difference, it is possible to determine the sag coefficient SF = k * Δ n which is the coefficient k, and its curve profile is very schematically shown as a dashed line with the reference numeral 48 in FIG. 4. In the illustrated example, the sag coefficient SF is the resulting refractive index n resIt behaves substantially complementary to this. The actual profile of the sag coefficient SF may deviate from this linear profile. The actual profile can be empirically established, for example, within the scope of a series of experiments with each fabricated tubular strand.

[0059] As an example, Figure 5 shows a calibration correlation used according to the present invention. This calibration correlation assigns the refractive index at a first measurement position to the ratio of the upper and lower outlet widths of the extrusion die of the extrusion apparatus 20 for the extruded material. In Figure 5, the refractive index at the first measurement position is plotted on the y-axis. The ratio of the upper and lower wall thicknesses of the tube 10, i.e., the offset between the measured upper and lower wall thicknesses measured at the first measurement position, is plotted as a percentage at the top of the X-axis. The related nominal ratio of the upper and lower outlet widths of the extrusion die of the extrusion apparatus 20, i.e., the offset between the upper and lower outlet widths, is plotted as a percentage at the bottom of the X-axis. For a cold refractive index of 1.5, a starting value of 0% indicates that, in each case, the difference between the upper and lower outlet widths of the extrusion die and the wall thickness measured at the upper and lower first measurement positions is zero. This is because, in this purely theoretical case, it is assumed that no sagging occurs. The values ​​greater than 0 at the top of the x-axis in Figure 5 represent the difference (offset) in the wall thickness of pipe 10 required for the desired pipe shape in a fully solidified state, measured at the upper and lower first measurement positions for a low refractive index, in each case, i.e., when there is still a flowable portion in pipe 10. Thus, a value of 5% means that the upper wall thickness is 5% greater than the lower wall thickness, and a value of 10% means that the upper wall thickness is 10% greater than the lower wall thickness.

[0060] The relevant required difference (offset) between the upper and lower outlet widths of the extrusion die, shown in the bottom row of the x-axis in Figure 5, is obtained, as previously described, by multiplying the offset at the first measurement position by a weighting coefficient that takes into account the different sags upstream and downstream of the first measurement position. As previously described, the weighting coefficient takes into account the extent to which the sag between the extruder 20 and the first measurement position is greater than the sag that still occurs after the first measurement position. In the example in Figure 5, we assumed a weighting coefficient of 3. This weighting coefficient can be determined empirically by the method described. The calibration correlation can be established empirically, for example, by the method described above. Furthermore, it can be determined for the defined shape value of the tube 10 at the first measurement position by the method described above. Thus, the calibration correlation represents the nominal ratio of the upper and lower outlet widths of the extrusion die of the extruder 20 to the refractive index of the tube 10 measured at the first measurement position, and this nominal ratio is the predetermined shape of the tube 10 in a fully solidified state, in particular the uniform wall thickness of the tube 10 along its circumference. As can be seen from Figure 5, the refractive index decreases as the ratio of the upper and lower exit widths of the extrusion die increases.

[0061] Figure 6 shows the radial temperature profile within the wall of the tube 10 from inside to outside, occurring at the first measurement position of the first terahertz measuring device 24, for the purpose of illustrating two different refractive indices measured at the first measurement position. Here, temperature (in °C) is plotted over, for example, radial position (in millimeters). The solid curve corresponds to a larger measured refractive index than the dashed curve. Therefore, the temperature profile of the dashed curve is higher than the temperature profile of the solid curve. The temperature of the outer surface of the tube is significantly cooled and solidified by the coolant applied to the outside in the first cooling section 22. In contrast, the temperature rises strongly radially inward because, even though the outer wall is cooled, these areas are not cooled to the point of solidification. Here, there are still flowable portions. Above approximately 120°C, the material becomes fluid, and sagging occurs.

[0062] Figure 7 shows the wall thickness measured at a first measurement position on the circumference of pipe 10 for two different cases as an example, with the wall thickness of pipe 10 (in millimeters) plotted over angular positions (in degrees). 180° is the upper side of the pipe, and 0° and 360° are the lower side. The dashed lines correspond to the temperature distribution shown as dashed lines in Figure 6, for example, and the solid lines correspond to the temperature distribution shown as solid lines in Figure 6, for example. The curves shown in Figure 7 are empirically established, and in all cases, the wall thickness is uniform over the circumference of pipe 10. It can be seen that a larger offset between the outlet width at the upper side (180°) of pipe 10 and the outlet width at the lower side (0°) of pipe 10 is necessary to compensate for sagging, more so in the case of high temperature distributions than in the case of low temperature distributions. For example, if the desired pipe shape in a fully solidified state is shown by a solid line, an upper / lower wall thickness offset of approximately 6.4% (33 mm to 31 mm) is required. From the calibration correlation in Figure 5, this 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 upper / lower wall thickness ratio measured at the first measurement position must be 1.064, or 6.4% in terms of offset percentage. Converting using the weighting coefficient as described above, the ratio of the exit widths of the extruded material in the extrusion die must be 1.192, or 19.2% in terms of offset percentage. Therefore, this ratio becomes the nominal ratio for which the extrusion die should be set. Then, accordingly, different values ​​corresponding to different refractive indices in Figure 5 are obtained for the dashed line in Figure 7. As shown in Figure 7, by performing multiple measurements, the calibration correlation in Figure 5 can be empirically formed, making it possible, for example, to interpolate between individual measured values.

[0063] To carry out the method according to the present invention using the apparatus according to the present invention shown in the figure, as described above, the refractive index across the cross-section of at least one wall of the pipe 10 and the shape values ​​of the pipe 10, particularly the wall thickness, and / or inner and / or outer diameter, are measured at a first measurement position immediately downstream of the first cooling section 22 by the first terahertz measuring device 24 at a plurality of measurement points distributed in the circumferential direction of the pipe 10. Based on the measurement of the shape values, it is preferable to confirm whether the shape values ​​present at the first measurement position correspond to the shape values ​​for which a calibration correlation has been created. If not, a warning is issued, or, for example, the calibration correlation can be changed by a coefficient corresponding to the deviation of the confirmed shape values. Furthermore, with respect to the refractive index and shape values ​​measured at the first measurement position, the nominal ratio of the upper and lower outlet widths of the extrusion die of the extrusion device 20 obtained according to the calibration correlation is determined by the calibration correlation shown in Figure 5, and this nominal ratio is compared with the actually set ratio of the upper and lower outlet widths of the extrusion die of the extrusion device 20. If these ratios differ from each other, a warning is issued and / or the extruder 20 may be operated so that the ratio of the upper and lower exit widths of the extruder die conforms to the nominal ratio obtained according to the calibration correlation. Such operation of the extruder 20 may be performed automatically. The measurements from the terahertz measuring device 24 are available in the evaluation device 38. The calibration correlation can also be stored in the evaluation device 38. The evaluation device 38 can perform the evaluations described and, if applicable, the operation of the extruder 20.

[0064] Furthermore, at a second measurement location located downstream of the first measurement location, where the tube 10 is substantially completely solidified, the refractive index and / or wall thickness and / or inner diameter and / or outer diameter of the tube 10 are measured by an additional terahertz measuring device 27. For example, the refractive index at the second measurement location may, in particular, correspond to the cold value of the refractive index, and the wall thickness at least on the upper and lower sides of the tube 10 may also be measured by the additional terahertz measuring device 27. The measurement results from the additional measuring device 27 are available to the evaluation device 38. Based on the measurements from the additional terahertz measuring device 27, which correspond to the final parameters of the tube 10 because the tube 10 is already substantially completely solidified, the calibration correlation, used as well as the cold value of the refractive index and the predicted sag, may be verified using the actual parameters of the tube 10 and modified as necessary. This is done by the evaluation device 38. If necessary, the calibration correlation may be adjusted based on this.

[0065] As described, the extruder 20 is operable by an evaluation device 38, which may be equipped with a corresponding control device for this purpose, based on the diagram relating to the evaluation and calibration correlation of the measured values ​​according to the present invention. In particular, the ratio of the outlet widths of the extruded material on the upper and lower sides of the extrusion die of the extruder 20 can be set in the manner described by the evaluation device 38 so that, in the solidified state of the pipe 10, a desired wall shape of the pipe 10, in particular, a wall thickness that is as uniform as possible over the circumference, can be obtained. For this purpose, the control device of the evaluation device 38 can appropriately set, for example, the spacing width of the annular outlets of the extruder die on the upper and lower sides of the extruder die. The control device of the evaluation device 38 can also appropriately influence the heating element of the extruder die. [Explanation of symbols]

[0066] 10 tubes 12 walls 14 Cavity 16 Exterior 18. Inner self 20 Extruder 22, 26 Cooling section 24 Terahertz measuring device 25 Further Terahertz Measurement Devices 27 Transceivers 28 Length cutting device 29 Reflector 30 transceivers 31 Terahertz radiation 32 Terahertz radiation 34 Reflectors 36 lines 38 Evaluation device 40, 42 wall thickness 44 diameter 46 Refractive Index Curve 48. Sagging coefficient curve

Claims

1. A method for verifying the settings of an extruder (20) that produces tubular strands (10) that are transported along their longitudinal direction, wherein the extruder is set so that the outlet width of the extruded material is different on the upper and lower sides of the extrusion die, and the method includes the following steps: - Using a terahertz measuring device (24), measure the refractive index across the cross-section of at least one wall of the tubular strand (10) at a first measurement position downstream of at least one first cooling section (22) for the tubular strand (10), while the tubular strand (10) has not yet completely solidified; - Further measuring the shape values ​​at the first measurement position using the terahertz measuring device (24), wherein the shape values ​​include the wall thickness (40, 42) and / or inner diameter and / or outer diameter (44) of the tubular strand (10); - The ratio of the measured refractive index to the shape values ​​measured on the upper and lower sides of the tubular strand (10) is determined by using a predetermined calibration correlation between the refractive index at the first measurement position and the ratio of the outlet widths of the extruded material on the upper and lower sides of the extrusion die, and confirming the ratio of the outlet widths of the extruded material on the upper and lower sides of the extrusion die set in the extrusion device (20).

2. The method according to claim 1, characterized in that the calibration correlation is determined based on the defined shape value of the tubular strand (10).

3. The method according to claim 2, characterized in that if a deviation of the shape value measured at the first measurement position from a defined shape value is detected, a warning is issued and / or the calibration correlation is adjusted according to the detected deviation of the shape value.

4. The method according to claim 1, characterized in that when establishing the calibration correlation, material parameters of the tubular strand (10), particularly thermal conductivity and / or heat capacity, and / or manufacturing parameters, particularly the temperature of the extrusion die and / or the discharge speed of the extrusion apparatus (20), and / or the cooling parameters of the tubular strand (10) being manufactured are taken into consideration.

5. The method according to claim 1, characterized in that the calibration correlation assigns the refractive index at the first measurement position to the nominal ratio of the outlet widths of the extruded material above and below the extrusion die, such that the tubular strand (10) has a nominal wall thickness profile, particularly a uniform wall thickness profile, over its circumference after complete solidification.

6. The method according to claim 5, characterized in that, if a deviation is found between the nominal ratio obtained as a result of the exit width of the extrusion die to the refractive index measured at the first measurement position according to the calibration correlation and the ratio of the set exit width of the extrusion die, the nominal ratio obtained as a result of the refractive index measured according to the calibration correlation is displayed, and / or the extrusion device (20) is operated to preferably automatically set the nominal ratio obtained as a result of the measured refractive index according to the calibration correlation.

7. The method according to claim 5, characterized in that the calibration correlation further assigns the refractive index at the first measurement position to the ratio of the wall thicknesses of the upper and lower sides of the tubular strand (10) at the first measurement position, and a weighting coefficient is used to convert the ratio of the wall thicknesses of the upper and lower sides of the tubular strand (10) at the first measurement position to the nominal ratio of the outlet widths of the extruded material at the upper and lower sides of the extrusion die, wherein the weighting coefficient takes into account the different degrees of sagging upstream and downstream of the first measurement position.

8. The method according to claim 1, characterized in that the expected sagging of the tubular strand (10) until the tubular strand (10) is completely solidified is predicted based on a comparison between the shape value measured at the first measurement position and the outlet width set in the extruder (20).

9. The method according to claim 5, characterized in that the expected sag and / or the expected value of the measured shape value of the tubular strand (10) in a fully solidified state is displayed, and / or the nominal ratio obtained as a result of the outlet width of the extruded material on the upper and lower sides of the extrusion die according to the calibration correlation is adjusted based on the expected sag.

10. The method according to claim 1, characterized in that the refractive index is measured at the first measurement position on the upper and / or lower side of the tubular strand (10).

11. The method according to claim 1, characterized in that the refractive index and / or the shape value are measured at a plurality of measurement points along the circumference of the tubular strand (10).

12. The method according to claim 1, characterized in that the refractive index is measured by comparing the transit time of the measuring radiation (32) when the tubular strand (10) is not placed in the beam path of the measuring radiation (32) emitted from the terahertz measuring device (24) with the transit time of the measuring radiation (32) when the tubular strand (10) is placed in the beam path of the measuring radiation (32).

13. The method according to claim 1, characterized in that the refractive index is measured by measuring the optical wall thickness (40, 42) of the tubular strand (10) using the terahertz measuring device (24), further measuring the outer diameter (44) and inner diameter of the tubular strand (10) using the terahertz measuring device (24), and measuring the refractive index of the tubular strand (10) by comparing the measured outer diameter and inner diameter (44) with the measured optical wall thickness (40, 42).

14. The method according to claim 1, characterized in that the shape value of the tubular strand (10) is measured from the transit time measurement of measuring radiation (32) emitted from the terahertz measuring device (24).

15. The method according to claim 1, characterized in that the refractive index across a cross section of at least one wall of the tubular strand (10), and / or the wall thickness (40, 42), and / or the inner diameter and / or outer diameter (44) of the tubular strand (10) are measured by a further terahertz measuring device (25) at a second measuring position located spaced apart from the first measuring position in the longitudinal direction of the tubular strand (10), particularly at a second measuring position downstream of the first measuring position where the tubular strand (10) has been substantially solidified, the terahertz measuring device (25).

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

17. The method according to claim 15, characterized in that the calibration correlation is confirmed using the measurement results of the further terahertz measuring device (25) and corrected as necessary.

18. An apparatus for carrying out the method according to one of claims 1 to 17, comprising: a terahertz measuring device (24); and an evaluation device (38) configured to confirm the ratio of set outlet widths of the extruded material on the upper and lower sides of the extrusion die.

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