Method for measuring the sagging of the melt of a tube extruded by an extrusion device

Abstract

The present invention relates to a method for measuring melt sag of a tube extruded in an extrusion apparatus, wherein the wall thickness of the tube is measured around the circumference of the tube, a wall thickness profile around the circumference of the tube is generated from the measured wall thickness, and melt sag is measured from the frequency and / or amplitude of the generated wall thickness profile. Figure 4 under above under

Description

Technical Field

[0001] The present invention relates to a method for measuring the sagging of a melt of a tube extruded by an extrusion device.

Background Art

[0002] In an extrusion device, for example, plastic tubes are extruded, and the tubes exiting the extrusion device are regularly conveyed along their longitudinal directions. In this process, they typically pass through a plurality of cooling sections where a coolant, such as water, is sprayed onto the outer surface of the tube to cool the tube. Immediately after exiting the extrusion device, the melt of the extruded tube can still flow over a wide area, which means it has not yet solidified. During the cooling process, enhanced by the cooling sections, the tube is cooled until it is completely cured or until each solidifies.

[0003] A method for measuring the diameter and / or wall thickness of a tube by terahertz radiation is known from International Publication No. WO 2016 / 139155. With this measurement method, precise measurement of geometric parameters such as the diameter or wall thickness of a tube extruded within an extrusion device is possible. In particular, when the tube is measured immediately after exiting the extrusion device, the measured geometric parameters may deviate from the actual geometric parameters of the completely solidified state of the tube. In particular, during the solidification of the extruded tube, due to gravity, regular sagging of the melt downward occurs, and as a result, the wall thickness ratio between the upper and lower regions of the tube changes over the course of cooling. It is not possible to completely prevent the sagging of the melt. By intentionally setting a non-uniform wall thickness at the exit from the extrusion device, attempts have been made to cancel out the sagging predictably. In so doing, it is necessary to precisely control the sagging. However, the sagging is difficult to detect by measurement.

Summary of the Invention

Problems to be Solved by the Invention

[0004] Proceeding from the described prior art, the object of the present invention is to provide a method of the type initially mentioned, by means of which the sagging of the melt can be detected more simply and precisely compared to the prior art.

Means for Solving the Problems

[0005] The present invention achieves the object by the subject matter of claim 1. Advantageous embodiments are disclosed in the dependent claims, the description and the drawings.

[0006] In a method of the type initially mentioned, the present invention is characterized in that the wall thickness of the tube is measured over the circumference of the tube, a wall thickness profile over the circumference of the tube is generated from the measured wall thicknesses, and the sagging of the melt is measured from the generated wall thickness profile. From the frequency In this way, the object is achieved.

[0007] The tube measured according to the present invention may be, for example, a plastic tube. This is extruded in an extrusion device. In an extrusion device, as is known, the extruded material is melted by heating and the melt is discharged through an extrusion nozzle that models the shape of the extruded object. For this purpose, the extrusion nozzle has at least one outlet opening. The extruded tube is discharged longitudinally from the extrusion device and further conveyed longitudinally. In particular, the tube is conveyed longitudinally during the measurement according to the present invention. During the measurement of the wall thickness according to the present invention, the tube may not be completely solidified, which means that it still has a fluid component. After leaving the extrusion device, the tube can pass through one or more cooling sections. In such a cooling section, as described, a coolant, such as water, is sprayed onto the outer surface of the tube for cooling. Immediately after leaving the extrusion device and continuing over a further region of the conveying section, the tube is not yet completely solidified and thus still has a fluid component in the form of a melt. After passing through the cooling section, the tube is further continuously solidified until it reaches its final shape. After passing through the first cooling section, the tube typically is not yet completely solidified, which means that it still has a fluid component.

[0008] As described, during the solidification of the tube, a downward sagging of the melt occurs. Also as described, since the melt is directly formed into the tube within the head of the extruder nozzle, it is not possible to completely avoid the sagging. The sagging can be anticipated in that at the exit from the extrusion device, the tube is intentionally discharged with a larger wall thickness in the upper region than in the lower region. In principle, it is desirable to set the temperature of the melt in the extruder as high as possible, because this is synonymous with a high output capacity of the extrusion device and the corresponding productivity. On the other hand, the risk of an unacceptable deviation in the shape of the completely cooled tube due to the uncontrolled sagging of the melt in the lower region of the tube increases with the temperature.

[0009] Setting elements, such as plate-shaped setting elements, are often provided on the head of the extruder nozzle of such an extrusion device. Such setting elements are typically arranged in the vicinity of at least one outlet opening of the extruder nozzle. By means of the setting elements, the wall thickness at the exit from the extruder nozzle of the extrusion device can be set at a plurality of points over the circumference of the tube. A typical extruder nozzle has, for example, 10, 16, or 20 such setting elements, especially arranged in the region of the inner wall of the tube. Each of these setting elements can be adjusted mechanically and / or is equipped with a heater to enable variations in the wall thickness in this region. The higher the temperature of the tube, the more pronounced the flow behavior. It is also conceivable to supply more or less lubricant to such setting elements in order to appropriately set the wall thickness of the tube over its circumference.

[0010] The present invention is based on the surprising recognition that the above-described setting elements bring about a modulation of the wall thickness over the circumference of the tube, which can still be identified, especially after the wall has been cooled, particularly inside the tube, by means of a particularly accurate wall thickness measurement, for example by means of terahertz radiation wall thickness measurement as described in more detail below. In particular, the wall thickness profile has a frequency and / or amplitude modulation that can be detected by the measurement. The inventors presume that this modulation is due to the setting elements. The wall thickness profile can in particular be modulated according to a periodic function such as a cosine or sine function. The amplitude of the modulation of the wall thickness is very small. For example, for an average wall thickness of about 10 mm, the amplitude of the modulation of the wall thickness is about 10 μm and thus about 0.1%. However, the modulation can be reliably detected using an appropriate wall thickness measurement method accordingly.

[0011] The present invention is also based on the recognition that the frequency and / or amplitude modulation of the wall thickness profile changes in response to the melt dripping. For example, if a periodic wall thickness profile with the same frequency and a specific amplitude exists directly at the outlet from the extruder nozzle of an extrusion device over the circumference of the tube, both the frequency and the amplitude of the wall thickness profile can change during the melt dripping that occurs as the tube cools. Due to the melt dripping, a so-called compression of the wall thickness profile occurs in the lower region of the tube, which means that the frequency of the modulation of the wall thickness profile is higher. In addition, the modulation amplitude of the wall thickness profile is also reduced as the tube cools. The reason for this is presumably that the modulation of the melt, which was initially almost completely present, first hardens in the original phase due to the solidification of the tube material starting from the outside, especially in the cooling section, and the still liquid components of the melt drip down as a result of gravity. As a result, a corresponding amplitude reduction of the wall thickness profile probably occurs.

[0012] Based on this, the teaching of the present invention is to detect the degree of melt sag from top to bottom by using the evaluation of the frequency and / or amplitude of the wall thickness profile of the tube measured circumferentially. According to the present invention, this is possible in a simpler and more accurate way compared to the prior art. Of course, it is not essential to directly measure or evaluate, for example, the frequency of the wall thickness profile. For example, it is also possible to measure variables measured by frequency, such as the wavelength or phase of the wall thickness profile, or the phase shift respectively, and use it for evaluation. The same applies to the amplitude.

[0013] Therefore, according to the present invention, the wall thickness of the tube is measured across the circumference of the tube. In this case, the wall thickness can be measured at continuous or discrete circumferential distances across the circumference of the tube. In this case, it is preferably measured over the entire circumference of the tube, i.e., in an angular range of 360°. However, it is also conceivable to measure the wall thickness only over a part of the entire circumference, in particular over the upper or lower quarter, or over the upper or lower half, etc., of the circumference where the melt sag affects the frequency and / or amplitude of the wall thickness profile, i.e., over the characteristic part of the melt sag. From the measured wall thickness, a wall thickness profile across the circumference of the tube is generated, which means a curve showing the wall thickness across the circumference. If the wall thickness is not measured continuously but instead at discrete distances across the circumference, interpolation can be performed between the measurement points in order to generate a curve representing the wall thickness profile. By using the evaluation of the frequency and / or amplitude of the generated wall thickness profile, the melt sag can be accurately measured by measurement.

[0014] As described, the frequency and / or amplitude modulation of the wall thickness profile can be caused, in particular, by setting elements for the wall thickness at the outlet of the extruder. In this case, for example, a wall thickness profile with periodic amplitude modulation can be present at the outlet of the extrusion nozzle of the extruder. This wall thickness profile then changes according to the sagging of the melt which is measured and evaluated according to the invention. However, the invention can also be used for extruders which have different means for establishing the wall thickness, in particular extruders without such setting elements. It is then possible to evaluate the changes in other characteristic wall thickness fluctuations present at the outlet of the extruder which are caused by the sagging of the melt. Such wall thickness fluctuations can also be introduced intentionally according to the invention. For example, by appropriately setting the wall thickness at the outlet of the extruder, it is conceivable to design defined circumferential sections with an increased or decreased wall thickness. For this purpose, at least one corresponding marking element which causes an increased or decreased wall thickness can be provided at the outlet of the extruder. When the melt sags, changes which can be identified by measuring the wall thickness profile in the regions where the wall thickness has increased or decreased can occur, for example an increase or decrease in the width of the wall section where the wall thickness has increased or decreased. Thus, amplitude changes occur in the wall thickness profile which is measured and generated according to the invention. From this, it is then possible to infer the sagging of the melt.

[0015] Generally speaking, the extruder can have at its outlet at least one element which can be identified in the wall thickness profile generated according to the invention or which causes the characteristic properties of the wall thicknesses which are each identified.

[0016] According to one embodiment, the sag of the melt can be inferred from a comparison between the generated wall thickness profile and a reference wall thickness profile. In particular, the sag of the melt can be inferred from a comparison between the frequency and / or amplitude of the generated wall thickness profile and the frequency and / or amplitude of the reference wall thickness profile. The reference wall thickness profile can be measured by measurement or theoretically, particularly mathematically. The reference wall thickness profile can in particular be present in the same circumferential portion as the generated wall thickness profile. When the wall thickness profile generated according to the invention is generated over the entire circumference of the tube, the reference wall thickness profile can also be present over the entire circumference of the tube. The comparison between the generated wall thickness profile and the reference wall thickness profile simplifies in particular the quantitative measurement of the sag of the melt. The change in the wall thickness profile caused by the sag can be confirmed in a particularly simple manner.

[0017] In a particularly practical method, the reference wall thickness profile can be a periodic reference wall thickness profile, for example, a reference wall thickness profile in the shape of a sine or cosine. Such a periodic reference wall thickness profile can be directly predicted at the outlet from the extruder nozzle of the extrusion device if there are at least setting elements for measuring the wall thickness and they are arranged uniformly. Such a periodic reference wall thickness profile is particularly suitable as a starting value for the comparison according to the invention for measuring the sag. However, in particular when such setting elements are not provided, the reference wall thickness profile can also be a different reference wall thickness profile, for example, a reference wall thickness profile that models characteristic wall thickness variations that can be intentionally introduced.

[0018] Accordingly, the reference wall thickness profile can be the reference wall thickness profile that is predicted or measured directly at the exit of the extrusion device, in particular directly at the exit of the extruder nozzle of the extrusion device. As already explained, for example, a periodic profile with a particularly large amplitude can be predicted there. If the subsequently measured wall thickness profile deviates from the reference wall thickness profile, for example from the periodicity of the reference wall thickness profile, or has changed from the reference wall thickness profile, for example has a low amplitude, this is a qualitative and quantitative indicator of the melt dripping.

[0019] According to another embodiment, from the comparison between the generated wall thickness profile and the reference wall thickness profile, a deviation profile can be generated, in particular over the circumference of the tube. Thus, this deviation profile models the deviation between the generated wall thickness profile and the reference wall thickness profile, in particular in the form of a curve. The deviation profile can show, for example, a wall thickness change and / or a phase change and / or a frequency change and / or an amplitude change between the generated wall thickness profile and the reference wall thickness profile. The deviation profile, or the profile corresponding to each deviation profile respectively, is used as an input variable, in particular in a suitable way, to adjust the extrusion device and / or at least one cooling section arranged downstream of the extrusion device, in order to achieve the desired wall thickness profile at the measurement position and / or in the fully solidified state of the tube.

[0020] The wall thickness of the tube over its circumference can be measured downstream of the first cooling section of the tube coming from the extrusion device. In particular, the measurement can be carried out behind the first cooling section and in front of the second cooling section. The tube is then cooled and solidified, at least in part, in particular on its outside, but usually still has a melt component that can flow inside.

[0021] It is also possible to predict further sagging of the melt expected before the tube is completely solidified from the measured sagging of the melt. This can be done, for example, by comparison with the previously measured wall thickness profile of the completely solidified tube. Thus, even when the tube is not completely solidified, a reliable description of the wall thickness shape in the completely solidified state can be made.

[0022] According to another embodiment, changes in at least one process parameter of the extrusion device and / or at least one cooling section arranged downstream of the extrusion device can be identified based on the measured sagging of the melt. Thus, when unexpected changes occur in the production process, such as a failure of the extrusion device and / or a cooling section arranged downstream of the extrusion device or an increase in the temperature of the coolant, the measured sagging is an important signal generator. Such unexpected changes in the production process can be detected early according to the present invention and can be cancelled accordingly.

[0023] According to another embodiment, at least one control parameter of the extrusion device and / or at least one cooling section arranged downstream of the extrusion device can be changed based on the measured sagging of the melt. In this way, a stable production process can be set, or such a stable production process having optimal process conditions can be adjusted respectively.

[0024] At least one control parameter can be changed by the generated deviation profile, particularly according to a practical embodiment. The deviation profile is particularly suitable as an adjustment variable for automatic adjustment.

[0025] According to another embodiment, at least one control parameter can be changed by a phase-locked loop. A phase-locked loop (PLL) is a method of adjusting the phase position of a variable oscillator or its respective frequency through a closed control loop such that the phase deviation between an external periodic reference signal and a signal from or derived from an oscillator becomes as constant as possible. The reference frequency of the phase-locked loop can, for example, correspond to the number of setting elements of the extrusion nozzle of an extrusion device. A phase detector is used such that the phase deviation between the generated wall thickness profile and a reference wall thickness profile having a reference frequency is used to adjust the frequency of a voltage-controlled oscillator, the control voltage of which represents a model of the frequency change, which in particular means corresponding to the deviation profile. The phase detector can, for example, provide a deviation profile between the measured wall thickness profile and the reference wall thickness profile as an output signal. Using such a phase-locked loop, in this example, at least one control parameter can be changed in a particularly suitable manner. Of course, other methods are also conceivable, for example, using a band-pass filter having a narrow bandwidth such that only the modulation frequency can pass through. Using a frequency discriminator is also conceivable. The band-pass filter can, for example, be used in combination with a downstream phase detector, which compares the phases of the measured wall thickness profile filtered through the band-pass filter and the reference wall thickness profile and outputs a phase difference. The phase difference can then be used as a basis for a change in at least one control parameter. Using a band-pass filter with a downstream frequency discriminator is also conceivable, the frequency discriminator comparing the frequencies of the measured wall thickness profile filtered through the band-pass filter and the reference wall thickness profile and outputting a frequency difference. This can then be used as a basis for a change in at least one control parameter.

[0026] The at least one control parameter can be, for example, the output capacity of the extrusion device and / or the melt temperature in the extrusion device and / or the temperature and / or position of the setting element of the extrusion device that measures the shape of the tube at the outlet of the extrusion device.

[0027] According to another embodiment, in order to measure the wall thickness of a tube, terahertz radiation is emitted towards the tube, the terahertz radiation reflected from the tube is detected, and the wall thickness of the tube can be measured from the detected terahertz radiation, in particular the intensity of the detected terahertz radiation. In this embodiment, terahertz radiation is emitted towards the tube. The terahertz radiation can partially enter the tube. This is reflected at the (outer and optionally inner) interface of the tube and detected by a suitable detector. The frequency of the terahertz radiation can be, for example, within the frequency range of 10 GHz to 3 THz. This can be so-called millimeter waves. The transmitter that emits terahertz radiation and the detector that receives the reflected terahertz radiation can be arranged at substantially the same position. They can be integrated, for example, into a transceiver. The geometric parameters can be measured in a reliable way using terahertz radiation, especially in a difficult process environment where an optical system such as a laser experiences difficulties. Furthermore, this measurement method provides sufficient accuracy to reliably detect the frequency and / or amplitude modulation of the wall thickness profile evaluated according to the present invention. The measurement of the wall thickness by terahertz radiation is described, for example, in WO 2016 / 139155. Therefore, this document is incorporated by reference.

[0028] The terahertz radiation can be modulated continuous wave terahertz radiation, in particular frequency modulated continuous wave terahertz radiation. The terahertz radiation can also be pulse-modulated terahertz radiation or phase-modulated terahertz radiation. The frequency modulation can include frequency bursts or a plurality of frequency bursts. In particular, so-called frequency sweeps can occur, where a given frequency range is traversed once or multiple times. So-called time domain reflectometry or frequency domain reflectometry can be developed, for example, as pulse-modulated terahertz radiation or phase-modulated terahertz radiation. Instead of a single frequency spectrum, it is also conceivable that a plurality of discrete frequencies are transmitted.

[0029] The wall thickness of the tube can be measured from the propagation time measurement values of the terahertz radiation emitted and reflected by the tube, for example, as described in International Publication No. 2016 / 139155.

[0030] According to a further embodiment, at least one transmitter for emitting terahertz radiation and at least one detector for detecting the terahertz radiation emitted and reflected by the tube can be rotated about the longitudinal axis of the tube, preferably along a circular path, during the emission and detection of the terahertz radiation. By rotating a pair consisting of a transmitter and a detector, for example, a transceiver, or shifting each, it is possible to detect the values of the wall thickness distributed over the circumference of the tube. Of course, it is also conceivable to distribute and arrange a plurality of pairs of transmitters and receivers over the circumference of the tube, and in this way, measure a plurality of measurement values over the circumference.

[0031] Exemplary embodiments of the present invention will be described in more detail below with reference to the drawings.

Brief Description of the Drawings

[0032]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0033] The same reference numerals refer to the same objects in the drawings unless otherwise indicated.

[0034] The tube 10, which in this example is a plastic tube 10, has a wall 12, a hollow space 14 defined by the tube 10, an outer surface 16 with a circular cross-section, and an inner surface 18 with a circular cross-section that also defines the hollow space 14, and is shown in FIGS. 1 and 2. The plastic tube 10 is, in this example, extruded with the help of an extruder within an extrusion device 20 and is conveyed along its longitudinal axis from left to right in FIG. 1 by a suitable conveying device. After emerging from the extruder nozzle of the extrusion device 20, the tube 10 first passes through a first cooling section 22, where the tube 10 is cooled after emerging from the extrusion device 20, which is considerably heated and not yet completely solidified, i.e., still has a fluid component (melt). Continuing along this procedure, the tube 10 passes through a measuring device 24, where the wall thickness of the tube 10 is measured over the circumference of the tube 10 in a manner that will be explained in more detail below. Following the measuring device 24, the tube 10 passes through a further cooling section 26 where further cooling takes place. After the tube 10 has completely solidified, it is cut to a predetermined length, for example by a length cutting device 28.

[0035] The structure and function of the measuring device 24 are explained in more detail with reference to FIG. 2. In the illustrated example, the measuring device 24 comprises a transceiver 30 in which a transmitter and a detector for terahertz radiation are combined. The transmitter emits terahertz radiation 32 towards the tube 10. The terahertz radiation is reflected by different interfaces of the tube 10 and a reflector 34 arranged on the opposite side of the transceiver 30 and returns to the transceiver 30, where it is detected by the detector. Further, the transceiver 30 is connected to an evaluation device 38 via a line 36. The reflected radiation received by the detector generates a corresponding measurement signal that is transferred via the line 36 to the evaluation device 38. In this way, the evaluation device 38 can measure, for example, the wall thicknesses 40, 42 depicted in FIG. 2 using propagation time measurements.

[0036] In this case, the measuring device 24 rotates about the longitudinal axis of the tube, for example during the measurement of the wall thickness 40, and the wall thickness is measured continuously or at discrete distances over the entire circumference of the tube 10, from which a wall thickness profile over the circumference of the tube is generated.

[0037] The tube 10 is shown in cross-section in FIG. 3, and the interface between adjacent plate-shaped setting elements of the extruder nozzle of the extrusion device 20 is indicated by a light ray 46 drawn at a constant angular distance. The setting elements are modeled in the wall shape of the extruded tube, especially before being cooled in the region of the inner wall of the tube. Without any drooping of the melt occurring, these setting elements are reflected onto the inner wall 18 of the tube 10 corresponding to their original distance by the light ray 46. In reality, due to the drooping of the melt during cooling, a shift of the region modeled by the setting elements occurs, which starts from the angular position specified by φ0, and accordingly, as shown in FIG. 3 by the light ray 46' at the angular positions φ1, φ2, and φ3 as well as in the region 48, especially the upper side of the tube 10 first stretches and subsequently compression towards the lower side of the tube 10 occurs.

[0038] This effect is shown in FIG. 4 with respect to the wall thickness profile over the circumference of the tube, especially for 0° to +180° and 0° to -180°, where 0° is the upper side of the tube. In FIG. 4, in the two upper diagrams, the wall thickness is plotted in both cases over the circumferential angle. In the uppermost diagram of FIG. 4, a reference wall thickness profile 50 is shown, which is a cosine-shaped profile with a certain frequency and amplitude in the illustrated example. The reference wall thickness profile is the profile directly predicted at the outlet from the extruder nozzle of the extrusion device 20. For the sake of explanation, the setting elements 51 of the extruder nozzle and the interfaces 52 formed between them are shown in FIG. 4. The frequency of the reference wall thickness profile 50 corresponds respectively to the frequency of the setting elements 51 or the interfaces 52 of the extruder nozzle that are uniformly distributed over the circumference.

[0039] The middle figure of FIG. 4 schematically shows the wall thickness profile 54 measured over the circumference of the tube 10 at the measurement position of the measuring device 24 shown in FIG. 1. On the other hand, it can be seen that the amplitude of the measured wall thickness profile 54 is smaller than the amplitude of the reference wall thickness profile 50. As the circumferential angle increases, corresponding to the shift of the light ray 46' shown in FIG. 3, starting from the uppermost position of the tube below 0°, a frequency deviation from the reference wall thickness profile 50 occurs, and it can be seen that in particular the frequency decreases up to the angular position φ3 first, and then increases up to 180° on the lower side of the tube.

[0040] The lowermost figure of FIG. 4 shows the deviation profile 56 generated from the comparison between the measured wall thickness profile 54 and the reference wall thickness profile 50. The deviation profile shows the phase shift φ of the measured wall thickness profile 54 compared to the reference wall thickness profile 50. The deviation profile 54 can form the output of the phase detector, based on which at least one control parameter of the extruding device and / or the first cooling section 22 or the additional cooling section 26 is changed at the measurement position of the measuring device 24 or in the fully cooled state of the tube 10 to generate a desired wall thickness profile, for example a periodic wall thickness profile.

[0041] For example, by comparing with, for example, the wall thickness profile generated in the fully solidified state of the corresponding tube 10, it is also possible to predict a further sagging of the melt expected before the tube 10 is fully solidified using the wall thickness profile 54 generated at the measurement position of the measuring device 24 where the tube 10 still regularly has a fluid component.

Explanation of Reference Numerals

[0042] 10 Tube 12 Wall 14 Hollow Space 16 Outer Surface 18 Inner Wall 20 Extruding Device 22, 26 Cooling Section 24 Measuring Device 28 Length Cutting Device 30 Transceiver 32 Terahertz radiation 34 Reflector 36 Line 38 Evaluation device 40, 42 Wall thickness 46, 46’ Light ray 48 Region 50 Reference wall thickness profile 51 Setting element 52 Interface 54 Wall thickness profile 56 Deviation profile

Claims

1. A method for measuring the sagging of the melt of a tube (10) extruded within an extrusion device (20), wherein the wall thickness (40) of the tube (10) is measured over the circumference of the tube (10), and a wall thickness profile (54) over the circumference of the tube (10) is generated from the measured wall thickness (40), and the sagging of the melt is measured from the frequency of the generated wall thickness profile (54).

2. The method according to claim 1, characterized in that the sagging of the melt is inferred from a comparison between the generated wall thickness profile (54) and a reference wall thickness profile (50).

3. The method according to claim 2, characterized in that the reference wall thickness profile (50) is a periodic reference wall thickness profile.

4. The method according to claim 2 or 3, characterized in that the reference wall thickness profile (50) is a reference wall thickness profile (50) predicted or measured directly at the outlet of the extrusion device (20).

5. The method according to any one of claims 2 to 4, characterized in that a deviation profile (56) is generated from a comparison between the generated wall thickness profile (54) and the reference wall thickness profile (50).

6. The method according to any one of claims 1 to 5, characterized in that the wall thickness (40) of the tube (10) is measured over the circumference of the tube (10) downstream of a first cooling section (22) of the tube (10) emerging from the extrusion device (20).

7. The method according to any one of claims 1 to 6, characterized in that a further sagging of the melt expected before the tube (10) is completely solidified is predicted from the measured sagging of the melt.

8. The method according to any one of claims 1 to 7, characterized in that a change in at least one process parameter of the extrusion device (20) and / or at least one cooling section (22, 26) arranged downstream of the extrusion device (20) is identified based on the measured sagging of the melt.

9. The method according to any one of claims 1 to 8, characterized in that at least one control parameter of the extrusion device (20) and / or at least one cooling unit (22, 26) arranged downstream of the extrusion device (20) is changed based on the measured sagging of the melt.

10. The method according to claim 9, characterized in that the at least one control parameter is changed by a deviation profile (56) generated from a comparison of the generated wall thickness profile (54) and a reference wall thickness profile (50).

11. The method according to any one of claims 9 or 10, characterized in that the at least one control parameter is changed by a phase-locked loop.

12. The method according to any one of claims 9 to 11, characterized in that the at least one control parameter is the output capacity of the extrusion device (20) and / or the melt temperature in the extrusion device (20) and / or the temperature and / or position of a setting element (51) of the extrusion device (20) that measures the shape of the tube (10) at the outlet of the extrusion device (20).

13. For measuring the wall thickness (40) of the tube (10), terahertz radiation (32) is emitted towards the tube (10) across the circumference of the tube (10), the terahertz radiation (32) reflected by the tube (10) is detected, and the wall thickness (40) across the circumference of the tube (10) is measured from the detected terahertz radiation (32). The method according to any one of claims 1 to 12, characterized in that it is as described.

14. The method according to claim 13, characterized in that the terahertz radiation (32) is modulated continuous wave terahertz radiation, in particular frequency-modulated continuous wave terahertz radiation, and / or the terahertz radiation (32) is pulse-modulated terahertz radiation or phase-modulated terahertz radiation.

15. The method according to claim 13 or 14, characterized in that the wall thickness (40) of the tube (10) is measured from the propagation time measurement of the terahertz radiation (32) emitted and reflected by the tube (10).

16. At least one transmitter for emitting the terahertz radiation (32) and at least one detector for detecting the terahertz radiation (32) emitted and reflected by the tube (10) rotate about the longitudinal axis of the tube (10) during the emission and detection of the terahertz radiation, according to any one of claims 13 to 15.

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