Method for checking extrusion device and extrusion device

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 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, and also to an apparatus for carrying out this method.

[0002] Plastic pipes, such as those used to supply gas 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 3 m, with wall thicknesses of up to 250 mm. This production typically occurs in an extrusion device, where the plastic pipe material is melted and extruded through a generally annular extrusion die. The extruded pipe is then drawn out of the extrusion device and conveyed longitudinally. The drawn pipe is then shaped to the desired outer diameter in a downstream, e.g., sleeve-shaped, calibration device. During the conveying process, the pipe typically passes through several cooling sections, where the pipe is cooled and the initially still-fluid plastic melt is successively solidified. In the first cooling section, the formed pipe is prevented from collapsing, e.g., by a vacuum. Cooling of the pipe in the cooling sections is often achieved using a cooling liquid, such as water, which flows around the pipe, rapidly solidifying its outer region. After exiting the first cooling section, the outer surface of the tube is generally solidified, and the tube's external shape remains substantially unchanged. However, even after the tube leaves the first cooling section, there is often still a flowable portion of the tube material remaining inside the tube wall. As the tubular strand continues to travel, particularly through additional cooling sections, the interior of the tube also continues to cool, eventually solidifying down to the inner surface of the tube. The tube is finally cut to the desired length with the aid of a flying saw.

[0003] The formation of the tube as it leaves the extrusion device and solidifies completely is substantially influenced by two influences that must be taken into account, for example, to achieve the goal of achieving as uniform a wall thickness as possible: first, shrinkage of the tube material during cooling; and second, sagging of the still-flowing, viscous mass during solidification due to the influence of gravity.

[0004] As a countermeasure to these influences affecting the final shape of the tubular strand, it is known to set the extrusion die of the extrusion device so that the exit width of the extruded material in the upper region is larger than the exit width in the lower region. To set the exit width, the exit gap of the extrusion die can be set wider in the upper region than in the lower region. Alternatively or additionally, the extrusion die can be heated more strongly in the upper region than in the lower region, resulting in a larger exit 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 asymmetric material discharge is intended to compensate for sagging so that the wall thickness of the solidified tube is as constant as possible over the entire circumference.

[0005] Critical geometric parameters such as the wall thickness and diameter of the tube can only be measured after complete solidification, i.e., after all shrinkage and sagging have occurred at the end of the tube's entire cooling section. Typical production rates for extrusion equipment for medium-sized tube cross sections are approximately 1,000 kg / h. The melt exit temperature from the extrusion die ranges from approximately 200 to 240 °C, depending on the material. For example, for a tube 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 a few hours after the start of production. Only then can geometric deviations from the nominal values ​​be identified, which can affect the extrusion equipment's production parameters, but these changes can only be confirmed again a few hours later. After the start of the process, several corrections, often required to obtain an optimal process, can take several days, for example, to make the wall thickness constant over the entire circumference and set it to its nominal value.

[0006] As explained, when an extrusion die is set to compensate for sag, the exit width is made larger in the upper region than in the lower region. Even if sag occurs, the sag is often over-corrected, and empirical values ​​are used, to ensure that the wall thickness does not fall below the minimum value. This ultimately leads to the extrusion of an unnecessarily large amount of material.

[0007] Therefore, it is desirable to draw conclusions as early as possible about the expected shrinkage and sag of the tube produced in the extrusion equipment. For example, measurements of the tube's 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, the outer region of the tube wall contains only solidified material, while the interior contains recrystallized and molten material. Subsequently, measurements downstream of the first cooling section record diameter and wall thickness values ​​that will still be subject to shrinkage and sag. To speed up the ramp-up process and ensure continued adherence to nominal values, it is economically very important to predict expected shrinkage and sag values ​​as early as possible.

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

[0009] The described method allows shrinkage to be reliably predicted and can be considered as one of the two major influences on the final shape, but advantageously, measurements are already available immediately after the material leaves the extrusion device, so that the final geometric parameters can be calculated early to prevent waste. However, sagging, as the second major influence on the final shape of the object, is impossible to predict with known methods, for reasons that will be explained in more detail below.

[0010] A method for determining sagging of a tube extruded in an extrusion device is known from WO 2022 / 106180. Here, the wall thickness of the tube is measured over the circumference of the tube, and a wall thickness profile over the circumference of the tube is generated from the measured wall thickness. The sagging of the molten material is determined from the frequency and / or amplitude of the generated wall thickness profile. While this method allows reliable detection of sagging, it still does not allow prediction of sagging.

[0011] In view of the prior art described, the object of the present invention is therefore to provide a method and an apparatus of the type mentioned at the outset, which makes it possible to reliably check and, if necessary, correct the outlet width of the extruded material set in the extrusion apparatus, taking into account the sagging of the tubular strand produced in the extrusion apparatus, in particular in real time, with a minimum time delay after the strand has left the extrusion apparatus.

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

[0013] For a method of the type mentioned at the beginning, the invention achieves this object by the following steps: 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; measuring further shape values ​​at at least one measurement point above and at least one measurement point below the tubular strand at the first measurement location using the terahertz measurement device, the shape values ​​comprising a wall thickness and / or an inner diameter and / or an outer diameter of the tubular strand; For the ratio of the measured refractive index to the shape values ​​measured on the upper and lower sides of the tubular strand, confirming the ratio of the exit width of the extruded material on the upper and lower sides of the extrusion die set in the extrusion device using a predetermined calibration correlation between the refractive index at the first measurement location and the ratio of the exit width of the extruded material on the upper and lower sides of the extrusion die.

[0014] The invention also achieves this object by means of an apparatus for carrying out the method according to the invention, comprising a terahertz measurement device and an evaluation device configured to ascertain the ratio of the set exit widths of the extruded material on the upper and lower sides of the extrusion die.

[0015] The method and the device according to the present invention allow for the prediction of the expected sagging after the first measurement point, i.e., the sagging of the still-flowable, viscous mass of the tubular strand exiting the extrusion device during solidification due to the influence of gravity, based on which 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. As is known per se, the extrusion device can, for example, consist of an essentially annular extrusion die from which the molten plastic material emerges. Also known per se, after exiting the extrusion device, the tubular strand passes through several cooling sections in a regular manner, where the strand material is continuously and completely cooled by a cooling liquid, for example, water, and thus completely solidified. For example, after exiting the first cooling section immediately after the extrusion device, the outer surface of the tubular strand can already be solidified, and thus the shaping 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 zones and is therefore at least partially still flowable, and as is known, sagging occurs in addition to shrinkage during further cooling 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 calibrated sleeve of this first cooling section, for example by applying a vacuum.

[0016] In principle, however, it would also be 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 would be highly numerically intensive and therefore not practical for the rapid prediction of sag desired in this case. Therefore, according to the present invention, a reliable prediction of the expected sag, and therefore the final shape of the strand, is intended to be available as soon as possible after exiting the extrusion device in order to prevent waste, thereby making it possible to intervene in the manufacturing process at an early stage, if necessary, by adjusting the settings of the extrusion device. Preferably, the prediction of sag is intended to be performed in real time. Both of these preclude the use of the Navier-Stokes equations, which are themselves reliable, in this application scenario.

[0017] As already explained, unlike shrinkage, sagging cannot be predicted by the method described in WO 2022 / 058081. The shrinkage from the extrusion temperature to complete cooling does not depend on the time required for solidification, in particular the cooling rate. Therefore, by comparing the refractive index measured at the first measurement location with the refractive index at the cold value, the shrinkage rate can be easily and reliably predicted. Furthermore, shrinkage is not substantially affected by changing parameters, for example, of the extrusion equipment or cooling parameters. It is a process characteristic that is virtually unchangeable, but is relatively easy to predict.

[0018] This differs from sagging, which is a much more complex process than shrinkage. During sagging, the temperature-dependent flow behavior of the material, its temperature, and the cooling rate play a key role. Generally, melt sagging during tube production is more pronounced the higher the initial melt temperature and the longer it takes for the melt to cool and completely solidify. Variable operating conditions, such as extrusion temperature, extruder throughput, discharge speed, and cooling intensity and duration, affect the degree of sagging. On the other hand, for predicting shrinkage using refractive index, shutting down the system is sufficient, and the changes in refractive index are recorded along with the shrinking wall thickness and diameter over several hours of cooling. This reliable method does not work for predicting sagging, precisely because the wall thickness sagging is more or less pronounced during long versus short cooling periods. Furthermore, unlike shrinkage, sagging can be decisively influenced by changing the parameters of the extrusion equipment and the cooling parameters of the tubular strand.

[0019] To solve these problems, the present invention first uses a (first) terahertz measuring device to measure the average or resulting refractive index across at least one cross-section of the tubular strand's wall thickness at a first measurement location downstream of at least one first cooling section of the tubular strand, where the tubular strand is not yet completely solidified, i.e., still has flowable recrystallized and / or melted sections. Then, also at this first measurement location, geometrical values ​​of the tubular strand, including the wall thickness and / or the inner and / or outer diameter, are measured at at least two measurement locations above and below the tubular strand. For this purpose, the terahertz measuring device comprises a terahertz transmitter and a terahertz detector, which are approximately co-located and can also be combined in a particularly practical way with a transceiver. The terahertz measuring radiation used in accordance with the present invention may be, for example, in the frequency range from 1 gigahertz to 6 terahertz. Terahertz radiation is particularly well-suited to the challenging measurement conditions of the extrusion device's environment, including high temperatures, any contamination, and steam generation. Therefore, in contrast to, for example, laser radiation in the visible frequency band, terahertz radiation is hardly affected by such interference. Terahertz radiation emitted from a transmitter in the terahertz measurement direction impinges on the tubular strand, for example, from above and / or below. In this process, part of the terahertz radiation is reflected by the outer surface of the strand, and part of the terahertz radiation passes through the strand material. This results in further reflections at the interfaces of the tubular strand, particularly at the interfaces between the walls of the strand facing the transmitter and those facing away from the transmitter. Furthermore, part of the terahertz radiation re-emerges from the strand on the side opposite the transmitter. The terahertz measurement device may include a reflector that reflects the terahertz radiation that leaves the strand away from the transmitter back into the strand. This radiation, together with the remaining radiation components reflected at the interfaces of the strand, is further reflected at the interfaces of the strand before reaching a detector, for example, located at the same location as the transmitter, and is detected as a measurement event by the detector.

[0020] As is known per se, the refractive index can be determined from the detected radiation across a 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, e.g., opposing, walls of the strand, it is possible to determine the average or resulting refractive index across both irradiated walls. Furthermore, as is known per se, the distance from the individual boundaries of the strand can be determined, for example, based on transit time measurements, and from this distance, it is possible to determine the wall thickness and / or geometric parameters of the strand, such as the inner and / or outer diameter. For example, the upper side measurement can be performed at the highest point of the strand, and the lower side measurement can be performed at the lowest point of the strand. In particular, the upper and lower wall thicknesses of a tubular strand can be measured at the first measurement position. As explained at the beginning, these wall thicknesses often differ because extrusion devices are typically configured to allow more material to exit the extrusion die at the upper side than at the lower side to compensate for sagging. Such asymmetries often still exist in the region of the (first) terahertz measuring device, ie especially immediately downstream of the first cooling section.

[0021] According to the present invention, a predetermined calibration correlation is used to predict the expected sag and to validate the extrusion device. The basis for the calibration correlation is the refractive index measured at the first measurement location. The calibration correlation assigns the refractive index at the first measurement location to the ratio of the set exit widths of the extrusion material at the upper and lower sides of the extrusion die. The ratio of the upper and lower exit widths of the extrusion die and the ratio of the measured shape values ​​at the upper and lower sides of the strand can be specified as a simple numerical ratio, i.e., for example, a percentage. Since the strand is essentially still composed of recrystallized material and molten zones, especially regions containing still-flowable material, the measured refractive index in the region of the first measurement location differs from its cold value. The measured refractive index provides information about the flow zones, especially recrystallized zones and / or molten zones, still present in the tubular strand at the first measurement location. The calibration correlation provides a correspondence between the refractive index measured at the first measurement location and the ratio of the extrusion die exit widths required to obtain a desired strand shape in a fully solidified state, based on predetermined geometric parameters of the strand at the first measurement location and predetermined manufacturing parameters of the extrusion device. This ratio of the exit widths obtained according to the calibration correlation of the measured refractive indexes can be compared to the set ratio of the extrusion device exit widths to verify the extrusion device. As explained above, the exit width can be set, for example, by setting the exit spacing of the extrusion die and / or by operating the extrusion die heating device. For example, it is conceivable that the basic setting of the exit width is performed by setting the exit spacing of the extrusion die, and subsequent fine-tuning or other adjustments of the exit width are performed only by controlling the extrusion die heating device.

[0022] For example, simply detecting the wall thicknesses of the upper and lower sides of the tubular strand at the first measurement locations is not sufficient for the prediction of the expected sagging according to the present invention and therefore for validating the settings of the extrusion device, since the wall thickness ratio measured at the first measurement locations at the top / bottom of the strand does not provide information about the extent to which previous sagging has already occurred or is already complete.Only in combination with the evaluation of the (resulting) refractive index measured at the first measurement locations and preferably with a weighting of the wall thickness ratio measured at the first measurement locations at the top / bottom can it be estimated whether the sagging will be fully compensated in the further course of the cooling section so that the strand will have a predetermined wall thickness profile in the fully solidified state.

[0023] Due to the simplified approach according to the invention, the prediction of the sag still to be expected downstream of the first measurement location, and thus the validation of the extrusion device, can be performed quickly and in real time, allowing the extrusion device to be rapidly controlled accordingly, for example to set the exit of the strand material from the extrusion die, particularly above and below the extrusion die, so that the desired shape of the strand is achieved in the fully solidified state. In many cases, the desired shape of the tubular strand is circular in cross section and has a constant wall thickness around its circumference.

[0024] The calibration correlation may be stored in the form of a function or curve. In a particularly simple way, the calibration correlation may be established empirically. For this purpose, when the exit width of the extrusion die is set in the relevant manufacturing process so that the tubular strand has a predetermined shape around its circumference after fully solidifying, e.g., a constant wall thickness around its circumference, the refractive index measurement value obtained at the first measurement position and the measured shape values, particularly the wall thickness measurement quotient, at the upper and lower sides may be stored. Later in the process, the refractive index measured at the first measurement position can be compared 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 assumed that there is a change in temperature, particularly in the still-fluid portion of the strand interior at the first measurement position. Therefore, it can be further assumed that a correction of the set exit width of the extrusion die is necessary to achieve the desired shape of the solidified strand. Further settings of the exit 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 empirically creating the calibration correlation.

[0025] In particular, changes in production parameters in the region upstream of the first measurement location can significantly affect the strand sagging and thus its shape in the solidified state. Such changes occur especially when the production process is restarted after a production interruption. This includes, for example, a gradual increase in the temperature of the extrusion die during start-up or a change in the output speed of the extrusion device. Another example is a change in cooling parameters, for example, by changing the composition and / or temperature of the coolant. Such changes can be recognized not only based on the refractive index measured at the first measurement location, but also on the shape values ​​measured at the first measurement location. This is especially true because a significant proportion of the overall sagging has already occurred by the time the strand leaves the first cooling section. Therefore, according to the present invention, optimal process parameters, especially for the extrusion device, can be set particularly early when the production process is restarted based on the calibration correlation.

[0026] Changes in the manufacturing process can be recognized not only from the refractive index but also from the shape value recorded at the first measurement location. If the shape value recorded at the first measurement location changes, it can be concluded that the expected sag considered according to the calibration correlation has also changed. Thus, according to one embodiment, a calibration correlation can be determined based on a defined shape value of the tubular strand, for example at the first measurement location. If a deviation of the shape value measured at the first measurement location from the defined shape value is then identified, a warning can be output and / or the calibration correlation can be adjusted according to the identified deviation of the shape value. For example, the calibration correlation can be adjusted by a coefficient corresponding to the identified change. This coefficient can be adjusted based on empirical values, for example, multiplied by a constant coefficient that takes into account the impact of the change on the resulting sag.

[0027] When establishing the calibration correlation, material parameters of the tubular strand may be taken into account, in particular the thermal conductivity and / or heat capacity. Furthermore, manufacturing parameters may be taken into account, in particular the temperature of the extrusion die and / or the discharge speed of the extrusion device and / or the cooling parameters of the tubular strand being produced. Such parameters also influence the sagging that occurs.

[0028] According to a further embodiment, the calibration correlation may assign the refractive index at the first measurement location to a nominal ratio of the extrusion material exit widths at the upper and lower sides of the extrusion die, such that the tubular strand has a nominal wall thickness profile, in particular a uniform wall thickness profile, around its circumference after its full solidification. As already explained, the calibration correlation, established, for example empirically, may be in the form of a function or curve. Here, the refractive index at the first measurement location can be plotted as a function of the ratio of the extrusion material exit widths at the upper and lower sides (set at the extrusion die as a nominal ratio). In the simplest case, the curve may be a straight line with a negative slope. However, it may also be a curve that deviates from a straight line.

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

[0030] As previously described, the expected sag of the tubular strand until it is fully solidified can be predicted based on a comparison of the shape value measured at the first measurement location with the exit width set on the extrusion device. The expected value of the predicted sag and / or the measured shape value at the fully solidified state of the tubular strand can then be displayed and / or the resulting nominal ratio of the exit width of the extruded material above and below the extrusion die according to a calibration correlation can be adjusted based on the predicted sag.

[0031] According to a further embodiment, the refractive index and / or shape value can be measured at multiple measurement points around the circumference of the tubular strand at the first measurement position. For this purpose, for example, multiple terahertz transmitters and terahertz detectors can be arranged distributed around the circumference of the tubular strand. Preferably, a terahertz measurement device that rotates relative to the tubular strand can include a terahertz transmitter, a terahertz detector, and optionally a reflector, with which measurements of the terahertz measurement device can be generated distributed around the circumference. In particular, such complete coverage of the circumference of the strand is possible in principle. Further advantages can be obtained by measuring more extensively around the circumference, especially on the side surfaces 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 have progressed further and may prevent or prevent still-flowable parts from flowing downwards due to gravity, thereby affecting sagging.

[0032] According to a further embodiment, the refractive index can be measured by comparing the transit time of the measurement radiation when the tubular strand is not placed in the beam path of the measurement radiation emitted from the terahertz measurement device with the transit time of the measurement radiation when the tubular strand is placed in the beam path of the measurement radiation. This approach to measuring the unknown refractive index of an object is described, for example, in EP 3 265 748 B1. The measurement of the refractive index using a terahertz measurement device can be performed in a manner corresponding to the present invention.

[0033] According to a further embodiment, the refractive index can be measured by determining the optical wall thickness of the tubular strand using a terahertz measurement device, and further by determining the outer and inner diameters of the tubular strand using a terahertz measurement device, and 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 DE 10 2018 128 248 A1. This approach is also applicable in the present case.

[0034] According to a particularly practical embodiment, the shape values ​​of the tubular strand are determined from transit time measurements of the measuring radiation emitted by the terahertz measuring device.

[0035] According to a further embodiment, at a second measurement position spaced apart from the first measurement position in the longitudinal direction of the tubular strand, in particular downstream of the first measurement position where the tubular strand has substantially completely solidified, the refractive index across a 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 in a further terahertz measurement direction. Furthermore, at the second measurement position, the wall thickness and / or the inner and / or outer diameter of the tubular strand at the upper side of the tubular strand and the wall thickness at the lower side of the tubular strand can be measured using a further terahertz measurement device. The calibration correlation can be verified and corrected, if necessary, using the measurement results of the further terahertz measurement device.

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

[0037] The aforementioned embodiment reduces the dependence of the model according to the present invention on material parameters. Especially in the field of plastic strands, material parameters are often unknown or not precisely defined enough. To reduce the dependence on material parameters, the aforementioned embodiment can therefore use additional sensors to measure, for example, the cold values ​​of the refractive index and wall thickness at at least one angular position around the strand, or the ratio between at least two wall thicknesses at different angular positions, e.g., the ratio between the wall thicknesses at the top and bottom of the strand in the case of a terahertz measurement 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 measurement device to reduce deviations caused by unrealistic material parameters. In particular, as explained above, changes in the process temperature downstream of the first measurement location, which affect sagging, can be detected by the additional terahertz measurement device. Examples include changes 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 to determine, for example empirically, the sag coefficient:

[0039] SF=k*Δ n however: SF: Sag factor k: coefficient Δ n : The difference between the refractive index measured at the first measurement position and the cold value of the refractive index

[0040] The higher the sag factor SF measured in this way, the more significant the expected sag. The volumetric and mass flow rates of viscous materials that are still flowable are roughly proportional to the sag factor. Accurate knowledge of the cold value of the refractive index allows for a more accurate determination of the sag factor and therefore the calibration correlation.

[0041] Therefore, the model according to the invention for predicting the expected sag can be further improved. In particular, the sag coefficient at the relevant measurement location is an indicator of how the difference in wall thickness between the upper and lower regions of the extrusion die, which is often intentionally set, will subsequently change in the subsequent steps of the extrusion line. On this basis, by setting the annular gap of the extrusion die and / or the appropriate temperature of the extrusion die of the extrusion device, it is possible to pre-set the sag coefficient at the measurement location of the terahertz measurement device in such a way that the desired, approximately uniform wall thickness is obtained with the remaining sag around the entire circumference of the strand.

[0042] According to a further embodiment, the calibration correlation may further assign the refractive index at the first measurement location to the ratio of the upper and lower wall thicknesses of the tubular strand at the first measurement location, and a weighting factor is used to convert the ratio of the upper and lower wall thicknesses of the tubular strand at the first measurement location to the nominal ratio of the upper and lower exit widths of the extrusion die for the extruded material. This weighting factor takes into account the different degrees of sagging upstream and downstream of the first measurement location. Therefore, it is possible to define a weighting factor between the strand sagging already occurring, as determined by measuring the shape value at the first measurement location, and the expected sagging until complete solidification. The already occurring sagging is obtained by comparing the shape value measured at the first measurement location with the exit width set at the extrusion die. For example, if this phenomenon continues after the first measurement location, it is still possible to predict the expected sagging, albeit to a lesser extent, by multiplying the already occurring sagging by a number less than one, taking into account the refractive index measured at the first measurement location. Conversely, a weighting factor can be formed that indicates a greater degree of sag upstream of the first measurement location than downstream of the first measurement location. 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 in the upper and lower regions measured at the first measurement location, the two quotients can be compared with each other, e.g., a quotient can be formed from the first quotient and the second quotient. This quotient can be the weighting factor. Because the majority of the sag occurs between the time of exiting the extrusion device and the time of the first measurement location and is generally less significant thereafter, the weighting factor is generally greater than 1. As an example, the weighting factor can be approximately 3. Based on this type of weighting factor, a generally desired uniform wall thickness of the strand can be obtained circumferentially for a particular strand material and given manufacturing conditions. By means of this weighting factor, or weighting quotient formed by the transformation, the refractive index at the first measurement location can be assigned in a calibration correlation to either the ratio of the upper and lower wall thicknesses of the strand measured at the first measurement location, or the nominal ratio of the upper and lower exit widths of the extrusion die of the extrusion device.Therefore, the offset to be set in the extrusion die can be directly determined and defined based on the shape value measured at the first measurement location and the measured refractive index, and the exit width of the extrusion die can be directly controlled in an open-loop or closed-loop manner based on the measurements obtained at the first measurement location.

[0043] If an additional terahertz measuring device is provided at a second measuring position downstream of the first measuring position, its measurements can be used to fine-tune the model according to the present invention, and thus the extrusion device. This takes advantage of the fact that at the first measuring position downstream of the first cooling section, the melt has already cooled to the point where, in addition to the cooled outer region, some of the melt remains inside the strand, particularly the tube wall, to recrystallize. It is then 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 the fully solidified state. This third quotient, which should ideally be 1, can 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) refractive index measured at the second measuring position.

[0044] In the device according to the invention, the measurements of the terahertz measuring device and, if applicable, the further terahertz measuring device are available in an evaluation device. The evaluation device is designed to carry out the evaluation and verification 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 may in particular comprise a control device for controlling the extrusion device, in particular the extrusion die, in the aforementioned manner. Accordingly, this device may also comprise a further terahertz measuring device. This device may also comprise an extrusion device.

[0045] The invention will be explained in more detail below on the basis of the accompanying drawings of exemplary embodiments. [Brief explanation of the drawings]

[0046] [Figure 1] 1 is a schematic side view of an apparatus for carrying out the method according to the invention; [Figure 2] FIG. 2 is a partial cross-sectional view of the device of FIG. 1. [Figure 3] 1 is a graph showing the temperature dependence of the refractive index. [Figure 4] 10 is a graph illustrating the variation in refractive index over radial position within a tube wall that has not yet fully solidified. [Figure 5] 10 is a graph illustrating a calibration correlation according to the present invention. [Figure 6] Graph showing the radial temperature distribution within a tube wall that has not yet completely solidified. [Figure 7] 2 is a graph showing different wall thicknesses measured at a first measurement location around the circumference of a tubular strand.

[0047] Unless otherwise specified, the same reference symbols refer to the same objects in the figures.

[0048] 1 and 2 show a tubular strand 10, in this case a tube 10, in particular a plastic tube 10, comprising a wall 12, a cavity 14 defined by the tube 10, an outer surface 16 of circular cross section, and an inner surface 18 also of circular cross section defining the cavity 14. In this example, the tube 10 is extruded by an extruder in an extrusion apparatus 20 and transported along its longitudinal axis, from left to right in FIG. 1, by a suitable transport device. After leaving the extrusion apparatus 20, e.g., an annular extrusion die, the tube 10 first passes through a first cooling section 22, where the tube 10 is intensely heated upon leaving the extrusion die and cooled while still not completely solidified, i.e., still consisting of a recrystallized portion and a flowable portion (melt). The first cooling section 22 may comprise a calibration device, in particular a calibration sleeve, against which the tube 10 is pressed, e.g., by vacuum and atmospheric pressure within the tube 10. As a result, the outer diameter of the preformed tube 10 is finally set by the extrusion die. In a further step, the tube 10 passes through a first terahertz measurement device 24, where 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 described in more detail below. After the first terahertz measurement direction 24, the tube 10 passes through at least one further cooling section 26, where it is further cooled. The dashes in the tube 10 indicate that further cooling sections 26 may be provided. After the tube 10 has completely solidified, it is cut into predetermined sections by a length cutting device 28, which may comprise, for example, a flying saw.

[0049] The design and function of the first terahertz measuring device 24 will be explained in more detail with reference to FIG. 2. In the illustrated example, the first terahertz measuring device 24 comprises a transceiver 30, which is a combined transmitter and 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 arranged opposite the transceiver 30, and returns to the transceiver 30, where it is detected by a detector. The transceiver 30 is further connected via a line 36 to an evaluation device 38. The reflected radiation received by the detector generates a corresponding measurement signal, which is transferred via the line 36 to the evaluation device 38. In this way, the evaluation device 38 can determine, for example, the wall thicknesses 40 and 42 depicted in FIG. 2 or the inner and / or outer diameter 44, for example, based on the transit time measurements. The evaluation device 38 can also determine the refractive index of the strand material based on the measurement signal received from the detector, as described, for example, in International Patent Publication WO 2016 / 139155 or German Patent Publication 10 2018 128 248 A1.

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

[0051] A further terahertz measurement device 25 is arranged between the at least one further cooling section 26 and the length cutting device 28. The further terahertz measurement device 25 also comprises a transceiver 27 which is a combined emitter and detector for terahertz radiation. On the opposite side of the tube 10 is arranged a reflector 29 for terahertz radiation which reflects the terahertz radiation 31 emitted by the emitter to the detector after passing through the tube 10 and being reflected by the boundary surface of the tube 10.

[0052] As described in more detail below, the further terahertz measurement 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 has substantially completely solidified. Measurements of the refractive index and geometric parameters, such as the inner and / or outer diameter and / or wall thickness of the tube 10, may be performed by the further terahertz measurement device 25 at the second measurement position shown in FIG. 1 , in this case where the tube 10 has substantially completely solidified, in the manner described above in connection with the first terahertz measurement device 24. In a particularly simple embodiment, the further terahertz measurement device 25 may be located at a fixed position and may simply irradiate the tube 10 with terahertz radiation vertically from above or below, thereby measuring the aforementioned geometric parameters, in particular the upper and lower wall thicknesses of the tube 10. The further terahertz measurement device 25 may also be a portable handheld device.

[0053] Figure 3 shows the dependence of the refractive index on temperature, or rather on the state of aggregation. First, it can be seen that the correlation between the refractive index and the temperature, or rather on the state of aggregation, is not linear. Second, it can be seen that the refractive index changes particularly strongly during the recrystallization phase, i.e., the phase in which the state of aggregation transitions between solid and liquid. This is utilized in the method according to the invention, in that the refractive index measured in each case allows for conclusions to be drawn about the remaining proportion of material in the recrystallization phase and, if applicable, the remaining proportion of material in the liquid state, and from this the sagging to be expected after the measurement at the first measurement position.

[0054] In Figure 4, the refractive index is plotted very diagrammatically over radial position x and wall thickness. The x-axis is plotted with a range from 0 to 1, where 0 applies to the outer surface and 1 applies to the inner surface of the wall of the tube 10. The refractive index is shown as a solid curve 46 in Figure 4. The refractive index is shown to be constant within the solidified region, i.e., from about 0 to 0.4 on the x-axis, and the cold value of the refractive index n cold , which corresponds to, for example, 1.5 in this example. In the subsequent unsolidified region, looking radially inward, the refractive index drops to a minimum, which in this example is about 1.46. It should be noted that the actual profile of the refractive index need not actually be linear, especially in the range of the unsolidified region. The dashed horizontal line in Figure 4 represents, for example, the resulting refractive index n measured at a first measurement location across a cross section of at least one wall of the tube 10. res is, for example, 1.46. This is the cold value of the refractive index n cold By comparing the strand thickness with the melt thickness, it can be inferred that part of the strand material is still in the recrystallization phase and, if applicable, is flowable in the melt form.

[0055] In this example, the graphs of FIGS. 3 and 4 were created using polyethylene, particularly HDPE, as the pipe material.

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

[0057] Δ n =n cold -n res

[0058] For various values ​​of this refractive index difference, the sag factor SF=k*Δ n can be determined, the curve profile of which is shown very diagrammatically as a dashed line at 48 in Figure 4. In the example shown, the sag factor SF is determined by the resulting refractive index n resThe actual profile of the sag factor SF may deviate from this linear profile. The actual profile can be established empirically, for example, within a series of experiments on each manufactured tubular strand.

[0059] By way of example, FIG. 5 illustrates a calibration correlation used in accordance with the present invention. This calibration correlation assigns the refractive index at a first measurement location to the ratio of the upper and lower exit widths of the extrusion die of the extrusion device 20 for the extrusion material. In FIG. 5, the refractive index at the first measurement location 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 values ​​of the upper and lower wall thicknesses at the first measurement location, is plotted in percent on the top line of the x-axis. The associated nominal ratio of the upper and lower exit widths of the extrusion die of the extrusion device 20, i.e., the offset between the upper and lower exit widths, is plotted in percent on the bottom line of the x-axis. For a cold value of 1.5 for the refractive index, a starting value of 0% indicates that the required difference between the upper and lower exit widths of the extrusion die and the wall thicknesses measured at the upper and lower first measurement locations is zero in each case, since in this purely theoretical case, no sagging is expected to occur. The values ​​greater than zero at the top of the x-axis in Figure 5 indicate the difference (offset) in wall thickness of tube 10 required for the desired tube shape in the fully solidified state, measured at the first measurement locations above and below for a low refractive index, i.e., when there is still a flowable portion of tube 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 exit widths of the extrusion die, shown at the bottom of the x-axis in FIG. 5, is obtained by multiplying the offset at the first measurement location by a weighting factor that accounts for the different sags upstream and downstream of the first measurement location, as previously described. As previously described, the weighting factor considers the degree to which the sag between the extrusion device 20 and the first measurement location is higher than the sag that still occurs after the first measurement location. In the example of FIG. 5, a weighting factor of 3 was assumed. This weighting factor can be empirically determined using the methods described above. A calibration correlation can be established empirically, for example, using the methods previously described. Furthermore, it can be determined for a defined shape value of the tube 10 at the first measurement location using the methods previously described. The calibration correlation thus indicates the nominal ratio of the upper and lower exit widths of the extrusion die of the extrusion device 20 to the refractive index of the tube 10 measured at the first measurement location, which corresponds to a predetermined shape of the tube 10 in a fully solidified state, in particular, a uniform wall thickness of the tube 10 around its circumference. As can be seen from FIG. 5, the refractive index decreases as the ratio of the upper and lower exit widths of the extrusion die increases.

[0061] FIG. 6 shows the radial temperature profile within the wall of the tube 10 from inside to outside at a first measurement location of the first terahertz measurement device 24, for illustrative purposes, for two different refractive indices measured at the first measurement location. Here, temperature (in °C) is plotted across radial position (in millimeters), for example. The solid curve corresponds to a higher measured refractive index than the dashed curve. Therefore, the temperature profile of the dashed curve is higher than that of the solid curve. The temperature of the outer surface of the tube is significantly cooled and solidified by the cooling liquid applied to the outside by the first cooling section 22. In contrast, the temperature increases strongly radially inward because cooling the outer wall does not cool these regions enough to solidify them. Thus, flowable portions still exist. Above approximately 120 °C, the material becomes flowable and sagging occurs.

[0062] FIG. 7 shows the wall thickness measured at a first measurement location around the circumference of the tube 10 for two different cases, plotting the wall thickness (in millimeters) of the tube 10 over the angular position (in degrees). 180° is the top of the tube, and 0° and 360° are the bottom. The dashed line corresponds to the temperature distribution shown, for example, in FIG. 6, and the solid line corresponds to the temperature distribution shown, for example, in FIG. 6. The curves shown in FIG. 7 were established empirically, and in both cases the wall thickness is uniform around the circumference of the tube 10. It can be seen that a larger offset of the exit width at the top (180°) of the tube 10 from the exit width at the bottom (0°) of the tube 10 is required for high temperature distributions than for low temperature distributions to compensate for sagging. For example, for the desired tube shape at full solidification shown by the solid line, an upper / lower wall thickness offset of approximately 6.4% (33 mm to 31 mm) is required. From the calibration correlation according to FIG. 5, this corresponds to a refractive index of approximately 1.46 at the first measurement location. Therefore, if a refractive index of 1.46 is measured at the first measurement location, the upper / lower wall thickness ratio measured at the first measurement location should be 1.064, or a 6.4% offset percentage. Converting using the weighting factors in the manner described above, the ratio of the extrusion die's exit widths to the extrusion material should be 1.192, or a 19.2% offset percentage. This ratio is therefore the nominal ratio to which the extrusion die should be set. Correspondingly, different values ​​corresponding to different refractive indices in FIG. 5 are then obtained for the dashed line in FIG. 7. As shown in FIG. 7, the calibration correlation according to FIG. 5 can be empirically formed by performing multiple measurements, for example, by interpolating between individual measurements.

[0063] To implement the method according to the present invention using the apparatus according to the present invention shown in the figures, as described above, the refractive index across at least one wall cross-section of the tube 10 and the geometrical values ​​of the tube 10, in particular the wall thickness and / or the inner and / or outer diameter, are measured at a first measurement location immediately downstream of the first cooling section 22 by the first terahertz measuring device 24 at multiple measurement points distributed around the circumference of the tube 10. Based on the measured geometrical values, it is preferably determined whether the geometrical values ​​present at the first measurement location correspond to the geometrical values ​​for which the calibration correlation was created. If not, a warning is issued or, for example, the calibration correlation can be modified by a factor corresponding to the deviation of the determined geometrical values. Furthermore, for the refractive index measured at the first measurement location and the geometrical values ​​measured at the first measurement location, the calibration correlation shown in FIG. 5 determines the nominal ratio of the upper and lower exit widths of the extrusion die of the extrusion device 20 obtained according to the calibration correlation, and this nominal ratio is compared with the actually set ratio of the upper and lower exit widths of the extrusion die of the extrusion device 20. If these ratios differ from each other, a warning may be output and / or the extrusion apparatus 20 may be operated so that the ratio of the upper and lower exit widths of the extrusion die matches the nominal ratio obtained according to the calibration correlation. Such operation of the extrusion apparatus 20 may be performed automatically. The measurements of the terahertz measurement device 24 are available to the evaluation device 38. The calibration correlation may also be stored in the evaluation device 38. The evaluation device 38 may perform the evaluations described above and, if applicable, operation of the extrusion apparatus 20.

[0064] Furthermore, at a second measurement location, arranged downstream of the first measurement location and where the tube 10 has substantially completely solidified, the refractive index and / or wall thickness and / or inner and / or outer diameter of the tube 10 are measured by the further terahertz measurement device 27. For example, the refractive index at the second measurement location may correspond, in particular, 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 be measured by the further terahertz measurement device 27. The measurement results of the further measurement device 27 are available to the evaluation device 38. Based on the measurements of the further terahertz measurement device 27, which correspond to the final parameters of the tube 10 because the tube 10 has already substantially completely solidified, the calibration correlations used, as well as the cold value of the refractive index and the predicted sag, may be verified and, if necessary, corrected using the actual parameters of the tube 10. This is done by the evaluation device 38. If necessary, the calibration correlations may be adjusted accordingly.

[0065] As explained, the extrusion device 20 can be operated by the evaluation device 38, which may be equipped with a corresponding control device for this purpose, based on the inventive evaluation of the measured values ​​and diagrams for the calibration correlation. In particular, the ratio of the outlet widths of the extrusion material at the upper and lower extrusion dies of the extrusion device 20 can be set in the described manner by the evaluation device 38, so that in the solidified state of the tube 10, the desired wall shape of the tube 10 is obtained, in particular a wall thickness that is as uniform as possible over the circumference. For this purpose, the control device of the evaluation device 38 can, for example, appropriately set the spacing width of the annular outlet gaps of the extrusion dies at the upper and lower ends of the extrusion die. The control device of the evaluation device 38 can also appropriately influence the heating elements of the extrusion die. [Explanation of symbols]

[0066] 10 tubes 12 Wall 14 Cavity 16 Exterior 18 Inner 20 Extrusion equipment 22, 26 Cooling section 24 Terahertz measurement equipment 25 Further terahertz measurement equipment 27 Transceiver 28 Length cutting device 29 Reflector 30 Transceiver 31 Terahertz radiation 32 Terahertz radiation 34 Reflector 36 lines 38 Evaluation equipment 40, 42 wall thickness 44 diameter 46 Refractive Index Curve 48 Sag Coefficient Curve

Claims

1. 1. A method for checking the settings of an extrusion device (20) for producing a tubular strand (10) conveyed along its longitudinal direction, the extrusion device being set so that the upper and lower extrusion die outlet widths of the extrusion material are different, the method comprising the steps of: measuring the refractive index across a cross section of at least one wall of the tubular strand (10) using a terahertz measurement device (24) at a first measurement location downstream of at least one first cooling section (22) for the tubular strand (10), where the tubular strand (10) has not yet completely solidified; - measuring further 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 (10) at the first measurement position using the terahertz measurement device (24), the shape values ​​comprising the wall thickness (40, 42) and / or the inner diameter and / or the outer diameter (44) of the tubular strand (10); - For the ratio of the measured refractive index and the shape values ​​measured on the upper and lower sides of the tubular strand (10), confirming the ratio of the exit widths of the extrusion material on the upper and lower sides of the extrusion die set in the extrusion device (20) using a pre-determined calibration correlation between the refractive index at the first measurement location and the ratio of the exit widths of the extrusion material on the upper and lower sides of the extrusion die.

2. 2. The method of claim 1, wherein the calibration correlation is determined based on defined shape values ​​of the tubular strand (10).

3. 3. The method of claim 2, wherein if a deviation of the shape value measured at the first measurement location from a defined shape value is identified, a warning is output and / or the calibration correlation is adjusted according to the identified deviation of the shape value.

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

5. 2. The method of claim 1, wherein the calibration correlation assigns the refractive index at the first measurement location to a nominal ratio of the exit widths of the extruded material on the upper and lower sides of the extrusion die, such that the tubular strand (10) has a nominal wall thickness profile, in particular a uniform wall thickness profile, around its circumference after full solidification.

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

7. 6. The method of claim 5, wherein the calibration correlation further assigns the refractive index at the first measurement location to a wall thickness ratio of the upper and lower sides of the tubular strand at the first measurement location, and a weighting factor is used to convert the wall thickness ratio of the upper and lower sides of the tubular strand at the first measurement location to a nominal ratio of the exit widths of the extruded material at the upper and lower sides of the extrusion die, the weighting factor taking into account different degrees of sag upstream of the first measurement location and downstream of the first measurement location.

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

9. 6. The method according to claim 5, characterized in that a predicted sag and / or an expectation value of the measured shape value in the fully solidified state of the tubular strand (10) is displayed and / or the resulting nominal ratio of the exit widths of the extruded material at the upper and lower sides of the extrusion die according to the calibration correlation is adjusted based on the predicted sag.

10. 2. The method of claim 1, wherein the refractive index is measured at the first measurement location on the upper and / or lower side of the tubular strand (10).

11. 2. 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 around the circumference of the tubular strand (10).

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

13. 2. The method of claim 1, wherein the refractive index is measured by measuring an optical wall thickness (40, 42) of the tubular strand (10) using the terahertz measurement device (24), and further by measuring an outer diameter (44) and an inner diameter (44) of the tubular strand (10) using the terahertz measurement device (24), and determining the refractive index of the tubular strand (10) from a comparison of the measured outer and inner diameters (44) with the measured optical wall thickness (40, 42).

14. 2. The method of claim 1, wherein the shape values ​​of the tubular strand (10) are determined from transit time measurements of measurement radiation (32) emitted by the terahertz measurement device (24).

15. 2. 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 and / or the wall thickness and / or the inner and / or outer diameter of the tubular strand are measured with a further terahertz measuring device at a second measuring position spaced apart from the first measuring position in the longitudinal direction of the tubular strand, in particular at a second measuring position downstream of the first measuring position at which the tubular strand has substantially completely solidified.

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

17. 16. The method of claim 15, wherein the calibration correlation is verified and, if necessary, corrected using measurements of the further terahertz measuring device (25).

18. 18. An apparatus for carrying out the method according to one of claims 1 to 17, comprising the terahertz measurement device (24) and an evaluation device (38) configured to ascertain the ratio of set exit widths of the extrusion material on the upper and lower sides of the extrusion die.

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