Method for determining sagging of melt in a pipe extruded in an extrusion device

Description

[0001] The invention relates to a method for determining a sagging of melt of a pipe extruded in an extrusion device.

[0002] For example, plastic pipes are extruded in extrusion devices. The pipes emerging from the extrusion device are conveyed at regular intervals along their length. They typically pass through several cooling sections, where a cooling liquid, such as water, is sprayed onto the outer surface of the pipes to cool them. Immediately after exiting the extrusion device, the melt of the extruded pipes is still fluid over large areas, meaning it has not yet solidified. During the cooling process, intensified by the cooling sections, the pipes cool down until they are completely hardened or solidified.

[0003] WO 2016 / 139155 A1 discloses a method for measuring the diameter and / or wall thickness of a pipe using terahertz radiation. This measurement method enables the precise determination of geometric parameters, such as diameter or wall thickness, of pipes extruded in an extrusion device. Particularly when measuring pipes shortly after they emerge from the extrusion device, the determined geometric parameters may deviate from the actual geometric parameters in the fully solidified state of the pipe. In particular, during the solidification of extruded pipes, a downward sagging of the melt due to gravity regularly occurs, which changes the wall thickness ratios between the upper and lower regions of the pipe during cooling. Sagging of the melt cannot be completely prevented.Attempts are being made to anticipate sagging by deliberately adjusting the wall thickness at the exit from the extrusion device. The need for precise control of sagging is a major concern. However, sagging is difficult to measure.

[0004] DE 10 2015 122 205 A1 discloses a method and a terahertz measuring device for measuring a layer thickness and / or a distance. Measurements are performed using different optical axes. The optical axis of the emitted terahertz radiation is adjusted during or between measurements, and one of the multiple measurements is used to determine the layer thickness. Measuring with multiple measuring axes is intended to enable precise measurements, even in the case of pipe position errors in a pipe being measured, with a deviation of the pipe axis from the symmetry axis, such as can occur due to sagging of the pipe, vibrations, or the flexible material of the freshly extruded plastic pipe.

[0005] DE 20 2018 006 144 U1 describes a device for measuring a tubular strand emerging from an extrusion device, in which electromagnetic radiation from a radiation source is directed from the inside to the inside of the tubular strand. To reduce or prevent signal loss when sagging occurs, several radiation transmitters and / or radiation receivers can be arranged side by side.

[0006] EP 2 752 287 A1 discloses a device for measuring industrial products manufactured in extrusion lines using terahertz radiation. This device is intended to provide continuous measurement, which can be used to provide control functions for the extrusion line, for example, based on measured diameter deviations.

[0007] WO 2019 / 086081 A1 discloses a terahertz measuring method and a terahertz measuring device for measuring pipes. The measuring object is measured using a main sensor by emitting a terahertz main beam along a main axis. Furthermore, a measurement is performed along an additional axis, in particular an additional axis adjusted or inclined along the axis of symmetry or conveying direction. Several measuring heads configured as terahertz sensor arrays can be arranged around the pipe to be measured, in particular in a circumferential direction. This is intended to enable wall thicknesses around the pipe to be measured without having to pivot the measuring device. Furthermore, defects are to be detected in this way, with a high probability that reflected radiation can be detected by one of the additional sensors of one of the terahertz sensor arrays.

[0008] J. Hauck et al.: "Terahertz inline wall thickness monitoring system for plastic pipe extrusion" (AIP Conference Proceedings, January 1, 2014, pages 86-89) describes an inline terahertz wall thickness measurement system for plastic pipe extrusion. The wall thickness of a plastic pipe is measured non-contact and with high accuracy using terahertz radiation by evaluating time-of-flight differences at the inner and outer diameters of the pipe. Knowing the refractive index of the pipe material, the geometric layer thickness can be calculated.

[0009] Based on the explained prior art, the object of the invention is to provide a method of the type mentioned at the outset with which the sagging of the melt can be detected more easily and precisely than in the prior art.

[0010] The invention solves the problem by the subject matter of claim 1. Advantageous embodiments can be found in the dependent claims, the description and the figures.

[0011] For a method of the type mentioned at the outset, the invention solves the problem by measuring the wall thickness of the pipe over the circumference of the pipe and creating a wall thickness profile over the circumference of the pipe from the measured wall thicknesses, and by determining a sagging of the melt from the frequency of the created wall thickness profile.

[0012] According to one embodiment, the slumping of the melt can still be determined from the amplitude of the created wall thickness curve.

[0013] The pipe measured according to the invention can, for example, be a plastic pipe. It is extruded in an extrusion device. In extrusion devices, extrusion material is melted by heating, and the melt is extruded through an extruder nozzle that replicates the shape of the object to be extruded. The extruder nozzle has at least one outlet opening for this purpose. The extruded pipe is extruded longitudinally from the extrusion device and further conveyed in the longitudinal direction. In particular, the pipe is conveyed longitudinally during the measurement according to the invention. It is possible that the pipe is not yet fully solidified during the wall thickness measurement according to the invention, i.e., still contains flowable portions. After leaving the extrusion device, the pipe can pass through one or more cooling sections.In such cooling sections, as explained, a cooling liquid, such as water, is sprayed onto the outer surface of the pipe to cool it. Immediately after exiting the extrusion device and continuing along the conveyor line, the pipe is not yet fully solidified and therefore still contains flowable components in the form of a melt. As it passes through the cooling sections, the pipe is successively further solidified until it reaches its final shape. After passing through the first cooling section, the pipe is generally not yet fully solidified and therefore still contains flowable components.

[0014] As explained, as the pipe solidifies, the melt sinks downwards, known as sagging. As also explained, sagging cannot be completely avoided, as the melt is formed into a pipe directly in the head of the extruder nozzle. Sagging can be anticipated by deliberately producing the pipe with a greater wall thickness in the upper area at the exit from the extrusion device than in the lower area. In principle, it is desirable to set the temperature of the melt in the extruder as high as possible, as this is synonymous with high output and corresponding productivity of the extrusion device. On the other hand, as the temperature of the melt increases, the risk of uncontrolled sagging of the melt into the lower area of ​​the pipe and thus of unacceptable shape deviations in the completely cooled pipe increases.

[0015] Adjustment elements, for example plate-shaped adjustment elements, are often provided on the head of the extruder nozzle of such extrusion devices. Such adjustment elements are generally arranged near at least one outlet opening of the extruder nozzle. Using the adjustment elements, the wall thickness at the outlet from the extruder nozzle of the extrusion device can be adjusted at several points around the circumference of the pipe. Common extruder nozzles can, for example, have 10, 16, or 20 such adjustment elements, which are arranged in particular in the region of the inner pipe wall. Each of these adjustment elements can be mechanically adjusted and / or is equipped with a heater to enable variability of the wall thickness in this area. A higher pipe temperature leads to greater flow behavior.It would also be conceivable to supply such adjusting elements with more or less of a lubricant in order to suitably adjust the wall thickness of the pipe over the circumference of the pipe.

[0016] The invention is based on the surprising discovery that the aforementioned adjustment elements lead to a modulation of the wall thickness over the circumference of the tube, which can still be identified metrologically even after the wall has cooled, particularly inside the tube, in particular with a precise wall thickness measurement, for example a terahertz radiation wall thickness measurement, as explained in more detail below. In particular, the wall thickness profile exhibits a metrologically detectable frequency and / or amplitude modulation. The present inventors assume that this modulation is caused by the adjustment 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, with an average wall thickness of approximately 10 mm, the amplitude of the wall thickness modulation is approximately 10 µm, corresponding to approximately 0.1%. With sufficiently precise wall thickness measurement methods, the modulation can still be reliably detected.

[0017] The invention is further based on the finding that the frequency and / or amplitude modulation of the wall thickness profile changes depending on the sagging of the melt. If, for example, a periodic wall thickness profile with the same frequency and a specific amplitude over the circumference of the pipe is present directly at the outlet from an extruder nozzle of the extrusion device, both the frequency and the amplitude of the wall thickness profile can change as the melt sags during the cooling of the pipe. As the melt sags, the wall thickness profile is, so to speak, compressed in the lower region of the pipe, i.e., the frequency of the modulation of the wall thickness profile increases. Furthermore, as the pipe cools, the modulation amplitude of the wall thickness profile also decreases.It is assumed that the reason for this is that the modulation of the initially almost completely present melt due to the solidification of the pipe material, which begins particularly in the cooling sections, initially solidifies with the original phase, while the still liquid melt portion sinks downward due to gravity. This presumably leads to a corresponding reduction in the amplitude of the wall thickness curve.

[0018] On this basis, the teaching of the invention is to determine the degree of sagging of the melt from top to bottom by evaluating the frequency and / or amplitude of the wall thickness profile of the pipe measured around the circumference. According to the invention, this is possible in a simpler and more precise manner than in the prior art. Of course, it is not mandatory to directly determine or evaluate, for example, the frequency of the wall thickness profile. It is also possible, for example, to determine a variable determined by the frequency, such as a wavelength or a phase or phase shift of the wall thickness profile, and use it for the evaluation. The same applies to the amplitude.

[0019] According to the invention, the wall thickness of the pipe is thus measured over the circumference of the pipe. The wall thickness can be measured continuously or at discrete circumferential intervals around the circumference of the pipe. It is preferably measured over the entire circumference of the pipe, i.e., an angular range of 360°. However, it is also conceivable for the wall thickness to be measured only over part of the entire circumference, in particular a part characteristic of sagging of the melt, such as an upper or lower quarter, or an upper or lower half of the circumference, where sagging of the melt affects the frequency and / or amplitude of the wall thickness curve. From the measured wall thicknesses, a wall thickness curve is created over the circumference of the pipe, i.e., a curve representing the wall thickness over the circumference.If the wall thickness is not measured continuously but at discrete intervals around the circumference, interpolation can be performed between the measurement points to create a curve representing the wall thickness progression. By evaluating the frequency and / or amplitude of the resulting wall thickness progression, melt sagging can be precisely determined using measurement techniques.

[0020] As explained, a frequency and / or amplitude modulation of the wall thickness profile can be caused in particular by adjustment elements for the wall thickness at the outlet of the extrusion device. In this case, for example, a wall thickness profile with a periodic amplitude modulation can be present at the outlet of an extrusion nozzle of the extrusion device. This wall thickness profile then changes according to a sagging of the melt, which is measured and evaluated according to the invention. However, the invention can also be applied to extrusion devices that have other means for determining the wall thickness, in particular no such adjustment elements. It is then possible to evaluate a change in other characteristic wall thickness fluctuations present at the outlet of the extrusion device caused by a sagging of the melt. Such wall thickness fluctuations can also be deliberately introduced according to the invention.For example, it would be conceivable to form a defined circumferential section with an increased or decreased wall thickness by appropriately adjusting the wall thickness at the outlet of the extrusion device. For this purpose, at least one corresponding marking element can be provided at the outlet of the extrusion device, which causes the increased or decreased wall thickness. If the melt subsides, this then leads to a metrologically identifiable change in the wall thickness profile in the area of ​​the increased or decreased wall thickness; for example, there could be an increase or decrease in the width of the wall section with the increased or decreased wall thickness. This therefore leads to an amplitude change in the wall thickness profile measured and created according to the invention. From this, in turn, it can be concluded that the melt has subsided.

[0021] Generally speaking, the extrusion device may have at least one element at its output that causes a characteristic property of the wall thickness that can be or is identified in the wall thickness profile created according to the invention.

[0022] According to one embodiment, a comparison of the created wall thickness profile with a reference wall thickness profile can be used to infer slumping of the melt. In particular, a comparison of the frequency and / or amplitude of the created wall thickness profile with the frequency and / or amplitude of the reference wall thickness profile can be used to infer slumping of the melt. The reference wall thickness profile can be determined by measurement or theoretically, in particular mathematically. The reference wall thickness profile can in particular be present for the same circumferential section as the created wall thickness profile. If the wall thickness profile created according to the invention is created over the entire circumference of the pipe, the reference wall thickness profile can also be present over the entire circumference of the pipe. The comparison of the created wall thickness profile with a reference wall thickness profile particularly simplifies the quantitative determination of slumping of the melt.The change in the wall thickness caused by subsidence can be determined in a particularly simple manner.

[0023] In a particularly practical manner, the reference wall thickness profile can be a periodic reference wall thickness profile, for example, a sinusoidal or cosinusoidal reference wall thickness profile. Such a periodic reference wall thickness profile is to be expected immediately at the outlet from an extruder nozzle of the extrusion device, at least if adjustment elements that determine the wall thickness are present and uniformly arranged. Such a periodic reference wall thickness profile is particularly suitable as a starting value for the comparison according to the invention for determining sagging. However, particularly if no such adjustment elements are provided, the reference wall thickness profile can also be a different reference wall thickness profile, for example, a reference wall thickness profile that depicts a possibly deliberately introduced characteristic wall thickness change.

[0024] The reference wall thickness profile can accordingly be an expected or measured reference wall thickness profile directly at the exit of the extrusion device, in particular directly at the exit of an extruder nozzle of the extrusion device. As already explained, a periodic profile, in particular with a relatively large amplitude, can be expected there. If the subsequently measured wall thickness profile deviates from the reference wall thickness profile, for example, a periodicity of the reference wall thickness profile, or exhibits a changed, for example, lower amplitude than the reference wall thickness profile, this is a qualitative and quantitative indicator of melt slumping.

[0025] According to a further embodiment, a deviation curve can be created from the comparison between the created wall thickness curve and the reference wall thickness curve, in particular over the circumference of the pipe. This deviation curve thus represents the deviation between the created wall thickness curve and the reference wall thickness curve, in particular in the form of a curve. The deviation curve can, for example, represent a change in wall thickness and / or a change in phase and / or a change in frequency and / or an amplitude change between the created wall thickness curve and the reference wall thickness curve. The deviation curve orA curve corresponding to the deviation curve can be used in a particularly suitable manner as an input variable for controlling the extrusion device and / or at least one cooling section arranged downstream of the extrusion device in order to achieve a desired wall thickness curve at the measuring point and / or in the fully solidified state of the pipe.

[0026] The wall thickness of the pipe across its circumference can be measured downstream of a first cooling section for the pipe emerging from the extrusion device. In particular, the measurement can be performed after a first cooling section and before a second cooling section. The pipe will then have partially cooled and solidified, particularly on its exterior, but will usually still contain flowable melt components inside.

[0027] It is also possible to use the measured melt slump to predict the expected further melt slump until the pipe fully solidifies. This can be done, for example, by comparing it with a previously determined wall thickness curve for a fully solidified pipe. This allows reliable predictions about the wall thickness geometry in the fully solidified state even when the pipe is not fully solidified.

[0028] According to a further embodiment, a change 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 detected sagging of the melt. The detected sagging thus serves as an important signaling device if unexpected changes occur in the production process, for example, a failure or temperature increase of a cooling fluid for the extrusion device and / or a cooling section arranged downstream of the extrusion device. According to the invention, such unexpected changes in the production process can be detected early, and appropriate countermeasures can be taken.

[0029] According to a further embodiment, at least one control parameter of the extrusion device and / or of at least one cooling section arranged downstream of the extrusion device can be changed based on the determined sagging of the melt. In this way, a stable production process can be established or controlled to such a stable production process with optimal process conditions.

[0030] According to a particularly practical embodiment, at least one control parameter can be changed using the created deviation curve. The deviation curve is particularly well-suited as a controlled variable for automated control.

[0031] According to a further embodiment, the at least one control parameter can be changed using a phase-locked loop. A phase-locked loop, also known as a phase-locked loop (PLL), is a control method in which the phase position or frequency of a variable oscillator is influenced via a closed control loop so that the phase deviation between an external periodic reference signal and the oscillator or a signal derived therefrom is as constant as possible. The reference frequency of the phase-locked loop can, for example, correspond to the number of adjustment elements of an extruder nozzle of the extrusion device. With the aid of a phase detector, a phase deviation between the created wall thickness profile and the reference wall thickness profile is used at the reference frequency to control the frequency of a voltage-controlled oscillator, the control voltage of which represents the frequency change, i.e., in particular, corresponds to the deviation profile.The phase detector can, for example, provide the deviation curve between the measured wall thickness curve and the reference wall thickness curve as an output signal. In this case, such a phase-locked loop can be used particularly well to change the at least one control parameter. Other methods would also be conceivable, such as the use of a bandpass filter with a narrow bandwidth so that only the modulation frequency can pass. The use of a frequency discriminator would also be conceivable. A bandpass filter can, for example, be used in combination with a downstream phase detector, with the phase detector comparing the phases of a measured wall thickness curve filtered by a bandpass filter and a reference wall thickness curve and outputting the phase difference. The phase difference can then be used as the basis for changing the at least one control parameter.Another option would be to use a bandpass filter with a downstream frequency discriminator. The frequency discriminator compares the frequencies of a measured wall thickness profile filtered through a bandpass filter and a reference wall thickness profile and outputs the frequency difference. This can then be used as the basis for changing at least one control parameter.

[0032] The at least one control parameter can be, for example, an output power of the extrusion device and / or a melt temperature in the extrusion device and / or a temperature and / or position of adjustment elements of the extrusion device that determine the geometry of the pipe at the outlet of the extrusion device.

[0033] According to a further embodiment, it can be provided that, to measure the wall thickness of the pipe, terahertz radiation is emitted onto the pipe, terahertz radiation reflected by the pipe is detected, and the wall thickness of the pipe is determined from the detected terahertz radiation, in particular the intensity of the detected terahertz radiation. In this embodiment, terahertz radiation is emitted onto the pipe. Some of the terahertz radiation can enter the pipe. It is reflected at (external and optionally internal) boundary surfaces of the pipe and detected by a suitable detector. The frequency of the terahertz radiation can, for example, be in a frequency range from 10 GHz to 3 THz. It can be so-called millimeter waves. A transmitter emitting the terahertz radiation and a detector receiving the reflected terahertz radiation can be arranged essentially at the same location.They can, for example, be integrated into a transceiver. Terahertz radiation can be used to reliably determine geometry parameters, especially in challenging process environments where optical systems such as lasers struggle. Furthermore, this measurement method offers sufficient accuracy to reliably detect the frequency and / or amplitude modulation of the wall thickness profile evaluated according to the invention. A method for determining wall thickness using terahertz radiation is described, for example, in WO 2016 / 139155 A1. Reference is made to this publication accordingly.

[0034] 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 comprise one or more frequency bursts. In particular, a so-called frequency sweep can be used, in which a predetermined frequency range is scanned one or more times. Pulse- or phase-modulated terahertz radiation can be used, for example, using a so-called time-domain reflectometry or frequency-domain reflectometry method. Transmitting several discrete frequencies instead of a frequency spectrum is also conceivable.

[0035] The wall thickness of the tube can be determined from a time-of-flight measurement of the terahertz radiation emitted and reflected by the tube, as described, for example, in WO 2016 / 139155 A1.

[0036] According to a further embodiment, at least one transmitter for emitting the terahertz radiation and at least one detector for detecting the emitted terahertz radiation reflected by the pipe can be rotated around the longitudinal axis of the pipe, preferably along a circular path, during the emission and detection of the terahertz radiation. By rotating or displacing a pair of transmitter and detector, for example a transceiver, wall thickness values ​​can be recorded distributed around the circumference of the pipe. Of course, it would also be conceivable to arrange several pairs of transmitters and receivers distributed around the circumference of the pipe and thus determine several measured values ​​around the circumference.

[0037] An embodiment of the invention is explained in more detail below with reference to the figures. They show schematically: Figure 1 shows a device for carrying out the method according to the invention in a schematic side view, Figure 2 shows a sectioned partial view of the device from Figure 1 , Figure 3 a cross-sectional view of the Figures 1 and 2 shown tube to illustrate a sagging of melt, and Figure 4 three diagrams to illustrate the method according to the invention.

[0038] Unless otherwise stated, the same reference symbols in the figures refer to the same objects.

[0039] In the Figures 1 and 2a pipe 10, in this case a plastic pipe 10, is shown, which has a wall 12, a cavity 14 defined by the pipe 10, an outer surface 16 with a circular cross-section and an inner surface 18 with a circular cross-section, which defines the cavity 14. In the present example, the plastic pipe 10 is extruded by means of an extruder in an extrusion device 20 and conveyed along its longitudinal axis by means of a suitable conveying device, in Figure 1from left to right. After emerging from an extruder nozzle of the extrusion device 20, the pipe 10 first passes through a first cooling section 22, in which the pipe 10, which is highly heated and not yet fully solidified, i.e. still containing flowable components (melt), exits the extrusion device 20 and is cooled. The pipe 10 then passes through a measuring device 24, in which the wall thickness of the pipe 10 is determined over the circumference of the pipe 10 in a manner explained in more detail below. Following the measuring device 24, the pipe 10 passes through further cooling sections 26, in which further cooling takes place. After the pipe 10 has fully solidified, it is cut to predetermined lengths, for example in a cutting device 28.

[0040] Based on Figure 2The structure and function of the measuring device 24 will be explained in more detail. In the example shown, 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 onto the pipe 10. The terahertz radiation is reflected at various interfaces of the pipe 10 and at a reflector 34 arranged opposite the transceiver 30 and returns to the transceiver 30, where it is detected by the detector. The transceiver 30 is further connected via a line 36 to an evaluation device 38. The reflected radiation received by the detector generates corresponding measurement signals, which are forwarded via line 36 to the evaluation device 38. In this way, the evaluation device 38 can Figure 2 Determine the wall thicknesses 40, 42 shown, for example based on transit time measurements.

[0041] The measuring device 24 is rotated around the longitudinal axis of the pipe during the measurement of, for example, the wall thickness 40, whereby the wall thickness is measured continuously or at discrete intervals over the entire circumference of the pipe 10 and a wall thickness profile is created therefrom over the circumference of the pipe.

[0042] In Figure 3The pipe 10 is shown in a cross-section, wherein the rays 46 drawn at regular angular intervals illustrate interfaces between adjacent plate-shaped adjustment elements of the extruder nozzle of the extrusion device 20. The adjustment elements are formed in the wall geometry of the extruded pipe until it cools, particularly in the area of ​​the inner pipe walls. Without the occurrence of sagging of the melt, these adjustment elements would have to be formed on the inner wall 18 of the pipe 10 according to their original spacing according to the rays 46. In fact, the sagging of the melt during cooling leads to a displacement of the areas formed by the adjustment elements, starting from the angular position designated by φ 0, corresponding to the top side of the pipe 10, particularly initially to an expansion, as in Figure 3characterized by the rays 46'at the angular positions φ 1 , φ 2 and φ 3 and the areas 48, and subsequently a compression to the bottom of the tube 10.

[0043] In Figure 4 This effect is shown for a wall thickness curve over the circumference of the pipe, in particular from 0° to +180° and 0° to -180°, where 0° is the top of the pipe. Figure 4 In the two upper diagrams, the wall thickness is plotted against the circumferential angle. Figure 4 The top diagram shows a reference wall thickness curve 50, in the example shown a cosine-shaped curve with constant frequency and amplitude. The reference wall thickness curve is the curve that can be expected immediately at the exit from the extruder nozzle of the extrusion device 20. For illustration, Figure 4The adjustment elements 51 of the extruder nozzle and the interfaces 52 formed between them are shown. The frequency of the reference wall thickness profile 50 corresponds to the frequency of the adjustment elements 51 or interfaces 52 of the extruder nozzle, which are evenly distributed over the circumference.

[0044] The middle diagram in Figure 4 shows schematically a Figure 1 shown measuring position of the measuring device 24 over the circumference of the pipe 10 measured wall thickness profile 54. It can be seen on the one hand that the amplitude of the measured wall thickness profile 54 is smaller than the amplitude of the reference wall thickness profile 50. It can also be seen that with increasing circumferential angle starting from the uppermost position of the pipe at 0° downwards according to the Figure 3shown displacement of the beams 46' results in a deviation of the frequency from the reference wall thickness profile 50, in particular initially a reduction of the frequency up to the angular position φ 3 and subsequently an increase of the frequency up to the bottom of the pipe at 180°.

[0045] In the bottom diagram in Figure 4a deviation curve 56 is shown, created from a comparison between the measured wall thickness curve 54 and the reference wall thickness curve 50. The deviation curve shows a phase shift φ of the measured wall thickness curve 54 compared to the reference wall thickness curve 50. The deviation curve 54 can form the output of a phase detector, on the basis of which at least one control parameter of the extrusion device and / or the first cooling section 22 or also the further cooling sections 26 is changed in order to generate a desired wall thickness curve, for example a periodic wall thickness curve, at the measuring location of the measuring device 24 or in the completely cooled state of the pipe 10.

[0046] It is also possible, for example, to predict, on the basis of the wall thickness profile 54 created at the measuring location of the measuring device 24, at which the pipe 10 regularly still has meltable portions, a further sagging of the melt to be expected until the pipe 10 has completely solidified, for example by comparison with a wall thickness profile created in the completely solidified state of a corresponding pipe 10. List of reference symbols

[0047] 10Pipe 12Wall 14Cavity 16Outer surface 18Inner wall 20Extrusion device 22, 26Cooling section 24Measuring device 28Cutting device 30Transceiver 32Terahertz radiation 34Reflector 36Line 38Evaluation device 40, 42Wall thickness 46, 46'Beams 48Areas 50Reference wall thickness profile 51Adjustment elements 52Interfaces 54Wall thickness profile 56Deviation profile

Claims

1. A method for determining sagging of molten mass of a tube (10) extruded in 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 curve (54) over the circumference of the tube (10) is created from the measured wall thicknesses (40), characterized in that sagging of the molten mass is determined from the frequency of the created wall thickness curve (54).

2. The method according to claim 1, characterized in that the sagging of the molten mass is further determined from the amplitude of the created wall thickness curve (54).

3. The method according to one of claim 1 or 2, characterized in that sagging of the molten mass is concluded from a comparison of the created wall thickness curve (54) with a reference wall thickness curve (50).

4. The method according to one of the preceding claims, characterized in that the reference wall thickness curve (50) is a periodic reference wall thickness curve.

5. The method according to one of claims 3 or 4, characterized in that the reference wall thickness curve (50) is an expected or measured reference wall thickness curve (50) directly at the outlet of the extrusion device (20).

6. The method according to one of claims 3 to 5, characterized in that a deviation curve (56) is created from the comparison between the created wall thickness curve (54) and the reference wall thickness curve (50).

7. The method according to one of the preceding claims, characterized in that the wall thickness (40) of the tube (10) over the circumference of the tube (10) is measured downstream of a first cooling section (22) for the tube (10) coming out of the extrusion device (20).

8. The method according to one of the preceding claims, characterized in that further sagging of the molten mass to be expected until complete solidification of the tube (10) is predicted from the determined sagging of the molten mass.

9. The method according to one of the preceding claims, characterized in that a change in at least one process parameter of the extrusion device (20) and / or of at least one cooling section (22, 26) arranged downstream of the extrusion device (20) is identified on the basis of the determined sagging of the molten mass.

10. The method according to one of the preceding claims, characterized in that a control parameter of the extrusion device (20) and / or of at least one cooling section (22, 26) arranged downstream of the extrusion device (20) is changed on the basis of the determined sagging of the molten mass.

11. The method according to claims 6 and 10, characterized in that the at least one control parameter is changed by means of the created deviation curve (56).

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

13. The method according to one of claims 10 to 12, characterized in that the at least one control parameter is an output capacity of the extrusion device (20) and / or a melting temperature in the extrusion device (20) and / or a temperature and / or position of adjustment elements (51) of the extrusion device (20) that define the geometry of the tube (10) at the outlet of the extrusion device (20).

14. The method according to one of the preceding claims, characterized in that, in order to measure the wall thickness (40) of the tube (10), terahertz radiation (32) is emitted onto the tube (10) over the circumference of the tube (10), terahertz radiation (32) reflected by the tube (10) is detected, and the wall thickness (40) over the circumference of the tube (10) is ascertained from the detected terahertz radiation (32), wherein the terahertz radiation (32) is preferably modulated continuous wave terahertz radiation, in particular frequency-modulated continuous wave terahertz radiation and / or in that the terahertz radiation (32) is pulse-modulated terahertz radiation or phase-modulated terahertz radiation.

15. The method according to claim 14, characterized in that the wall thickness (40) of the tube (10) is ascertained from a transit time measurement of the emitted terahertz radiation (32) reflected by the tube (10).

16. The method according to one of claims 14 or 15, characterized in that at least one transmitter for emitting the terahertz radiation (32) and at least one detector for detecting the emitted terahertz radiation (32) reflected by the tube (10) is rotated about the longitudinal axis of the tube (10), preferably along a circular path, during emission and detection of the terahertz radiation.