Method for calibrating a THZ measuring device and extrusion and measuring system

Description

[0001] The invention relates to a method for calibrating a THz measuring device in an extrusion line, as well as a measuring and extrusion system.

[0002] THz measuring devices are used, among other things, in extrusion lines to measure extruded profiles. An extruder produces a profile, particularly a tube, which is then measured by a THz measuring device for relevant geometric properties, such as its wall thickness, outer diameter, and inner diameter. Terahertz radiation, which can also be in the radar and microwave range, is emitted by a THz transmitter and partially reflected at the profile's interfaces, allowing the geometric properties to be determined directly or indirectly from the reflected signal's travel time. The refractive index of the profile material is generally also taken into account when determining geometric properties such as wall thickness.

[0003] WO 2017 / 000933 A1 shows a terahertz measuring device in an extrusion line, wherein an extruded tube with an initially unknown refractive index is measured. WO 2016 / 139155 A1 describes a device and a method for measuring the diameter and / or wall thickness of a strand. In an empty measurement, terahertz radiation is first emitted from a transmitter through an empty measuring space to a reflector, and the reflected beam is measured. Subsequently, the measurement is performed with a test object to determine the properties of the strand.

[0004] DE 10 2016 119 728 A1 describes a portable THz measuring device for determining at least one layer thickness of a test object, wherein a shaped aperture with a support contour is provided at the front end area of ​​the THz measuring device, which enables vertical positioning of the THz measuring device on profiles with a predetermined diameter.

[0005] Determining both the geometric properties and the refractive index of a profile is therefore generally difficult. Furthermore, errors or defects in the stationary THz measurement device can lead to deviations in the measurement results.

[0006] US 2015 / 0323452 A1 describes a method for determining the material properties of an object. This involves determining the thickness at a point on the object in a non-invasive manner. Furthermore, it describes performing a terahertz measurement by passing terahertz radiation through the object using a terahertz emitter and measuring the terahertz radiation using a terahertz detector. Furthermore, it describes determining optical properties at the point on the object using the terahertz measurement and the thickness measurement.

[0007] DE 10 2019 119 491 A1 describes a method and a device for measuring a tubular strand emerging from an extrusion device. In a first measuring area, terahertz radiation from at least one first radiation source is directed from the inside onto the inner surface of the tubular strand. In a second measuring area, which is located downstream of the first measuring area in the conveying direction of the strand, terahertz radiation from at least a second radiation source is directed from the outside onto the outer surface of the tubular strand. Changes in the geometric parameters between the measuring points are determined.

[0008] WO 2021 / 259426 A1 discloses a THz measurement method and a THz measurement device for measuring a measurement object, in particular a pipe. A THz sensor arrangement with an open-circuit path between two THz transceivers is provided, a calibration is performed by measuring the propagation time through the open-circuit path, a measurement object is positioned in the measurement area, and two THz measurements are performed from the two measurement positions between which the open-circuit path is formed, whereby a total propagation time through the measurement area with the measurement object is determined.

[0009] From DE 10 2018 128 248 A1 a method for determining the refractive index of a tubular body is known, in which the tubular body is irradiated in a measuring area by a transmitting device with measuring radiation in the gigahertz or terahertz frequency range, wherein the measuring radiation is reflected at boundary surfaces of the tubular body and the reflected measuring radiation is detected by a receiving device, wherein the optical wall thickness of at least one wall section of the tubular body is determined on the basis of the reflected measuring radiation and the refractive index of the body is determined from a comparison of the outer and inner diameter of the tubular body with the determined optical wall thickness.

[0010] DE 20 2020 005 529 U1 describes a device for carrying out a method for determining at least one geometric parameter of a strand- or plate-shaped object that has not yet fully solidified and still contains flowable components. In a determination step, a relationship between the refractive index of the strand- or plate-shaped object and a shrinkage occurring during its complete solidification is determined for the strand- or plate-shaped object. The refractive index and a geometric parameter of the object are then determined, and the geometric parameter in the fully solidified state of the strand- or plate-shaped object is subsequently calculated from these values.

[0011] The invention is based on the object of creating a method and an extrusion and measuring system that enable reliable measurement of profiles.

[0012] This object is achieved by a method and an extrusion and measuring system according to the independent claims. The subclaims describe preferred developments.

[0013] The method according to the invention can be carried out in particular with the extrusion and measuring system according to the invention.

[0014] Thus, a stationary THz measuring device is provided in the extrusion line, which can measure the extruded profile inline, i.e., in the production line. This enables direct and continuous measurement during extrusion and generates a first measurement signal as a stationary THz measurement signal.

[0015] Furthermore, two portable measuring devices are provided for measuring profile pieces that are cut from the extruded profile and subsequently cooled. One of these is a reference measuring device based on a measuring principle alternative to THz measurement and can directly determine the geometric wall thickness. The reference measuring device has a measurement accuracy down to the micrometer range. A mechanical measuring device, an ultrasonic measuring device, and / or a laser measuring device can be provided as a reference measuring device.

[0016] The mechanical measuring device can be designed as a pair of pliers or a clamping device with clamping jaws for contact with the wall surfaces of the profile piece. This allows the portable, mechanical measuring device to be applied to the profile with a defined mechanical preload or predetermined mechanical pressure, enabling reliable, reproducible mechanical measurements, particularly without any significant deformation of the product, and thus generating a second measurement signal as a mobile mechanical measurement signal.

[0017] The ultrasonic measuring device is preferably applied to the profile piece from the outside, ie in particular perpendicular to the outer surface of the profile piece, whereby, for example, a measuring head of the ultrasonic measuring device can be placed on the outer surface of the profile, e.g. with a gel for sound coupling that is applied to the measuring head.

[0018] Furthermore, a portable THz measuring device is provided, which can be applied by the user to the profile piece and – generally similar to the stationary measuring device – measures transit times between the wall surfaces. Thus, a subsequent measurement of the cooled profile piece is preferably performed according to the previous inline measurement of the warm profile, in particular with the same electromagnetic properties such as the frequency or frequency band. Thus, the portable THz measuring device can, in particular, have the same THz transceiver as the stationary inline measuring device; thus, a third measurement signal is generated as a mobile THz measurement signal.

[0019] Thus, profile pieces are preferably cut off after extrusion and preferably cooled first to take the geometric properties of the finally cooled profile pieces into account. Cooling can also take place over a longer period of time. For example, the profile piece can be cut off and further measured the following day.

[0020] The profile piece is then measured twice. The measurement is carried out reproducibly using both the reference measuring device and the portable THz measuring device, in particular at the same measuring point. This ensures the comparability of the measurements. For this purpose, the user can mark the measuring point, so that it can be clearly located with little effort. The measuring point is, in particular, at a defined distance from one end of the profile piece, so that it is not too close to the end and affected by mechanical deformations caused by the cutting process. And, on the other hand, it also enables good accessibility for the reference measuring device, in particular for a mechanical reference measuring device that is applied from the end. The order of the reference measurement and the measurement with the portable THz measuring device is fundamentally arbitrary. For example,Several measurements can also be carried out in the circumferential direction around the profile piece at the defined distance in order to enable an average wall thickness in the defined measuring areas or measuring points.

[0021] By using the reference measurement with the alternative measurement principle, the wall thickness can be reliably determined independently of the refractive index of the THz radiation. This calibrates the subsequently attached portable THz measuring device, allowing the refractive index of the profile piece to be determined.

[0022] The determined refractive index is then used to calibrate the stationary THz measuring device, i.e. the determined refractive index is used for the stationary THz measurements, which makes it possible to determine wall thicknesses from the stationary THz measurement signal.

[0023] Subsequently, the stationary THz measurement can be compared with a measurement by the portable THz measuring device, ie in particular a measurement with the portable THz measuring device can be carried out directly after the stationary measurement on the warm, not yet separated profile, ie unlike the mobile THz measurement on the cooled pipe section.

[0024] This allows calibration of the stationary THz measurement signal of the still warm profile after extrusion to the final wall thickness formed in the cooled profile pieces, which results after shrinkage, so that deviations can be directly detected and corrected inline, e.g., also as a control by suitable control of the extrusion line.

[0025] Furthermore, deviations or errors of the stationary THz measuring device can be determined by comparison.

[0026] Measuring the profile pieces with the portable devices does not delay or interfere with the continuous extrusion of the profile. Furthermore, the user can repeat measurements as needed.

[0027] Such comparative measurements can be carried out repeatedly during production, ensuring high levels of safety and accuracy without delaying production.

[0028] According to an advantageous embodiment, the profile is marked at the measurement location. This marking can be performed before or after the stationary THz measurement in the stationary THz measurement device. The marking can be done, for example, using a pen. This allows the profile piece to be reliably measured again at the marked location after it has cooled down, thus eliminating the possibility of measurements at different locations and enabling reliable comparison.

[0029] As a profile, in particular a pipe, ie a round pipe, can be extruded and measured, but also a rectangular profile, or a layer, e.g. a film, or a semi-open profile, e.g. a rain gutter.

[0030] According to the invention, an extrusion and measuring system is also provided comprising the extrusion line with the stationary THz measuring device and the two portable measuring devices, i.e. the portable reference measuring device and the portable THz measuring device.

[0031] According to the invention, a method for THz measurement of profiles is preferably further provided, in which the method according to the invention for calibrating the stationary THz measuring device is first carried out and subsequently a profile is extruded and measured with the calibrated stationary THz measuring device. According to a further aspect of the invention, a method for producing and measuring profiles is preferably provided, in which a profile is extruded and subsequently the extruded profile is measured with the method for THz measurement and, depending on the THz measurement with the calibrated stationary THz measuring device, at least one control parameter of the production process, e.g., of an extruder and / or a calibration sleeve, or of a puller for pulling the profile, is changed.

[0032] For the sake of clarity, the measurement signals are hereinafter referred to as the first to third measurement signals, i.e., the stationary first THz measurement signal, the mobile second measurement signal, and the mobile third THz measurement signal. Thus, in this designation, the first and third measurement signals are THz measurement signals, and the second and third measurement signals are mobile measurement signals.

[0033] The invention is explained in more detail below with reference to an embodiment of the invention and the accompanying drawings. They show: Fig. 1 shows an extrusion line with a terahertz measuring system according to an embodiment of the invention; Figs. 2 and 3 show the measurement of a profile piece by the reference measuring device and the portable THz measuring devices; Fig. 4 shows a signal diagram of the stationary THz measuring device; Fig. 5 shows a flowchart of a method according to the invention.

[0034] In an extrusion line 1, according to Figure 1an extruder 2 with calibration sleeve 2a, a stationary THz measuring device 3, a cooling device 5, a discharge device 6 and a separating device 8 are provided.

[0035] Starting material 9, e.g., granules, pellets, powder, or flakes of plastic or rubber, possibly also with additives, is fed to the extruder 2 and output from the calibration sleeve 2a as a continuous profile, here as a tube 4 - extrusion step ST0. The tube 4 is withdrawn from the discharge 6 and cooled in the cooling device 5, so that in particular the feed rate of the starting material 9, the conveying rate in the extruder 2, the discharge speed of the discharge 6, and the settings of the calibration sleeve 2a determine the formation of the tube 4. The tube 4 is subsequently divided in the cutting device 8 into tube pieces 10 of defined length L.

[0036] After extrusion, in step ST1, an inline measurement is performed by the stationary THz measuring device 3, which can have one or more THz sensors 7 arranged around the tube axis or symmetry axis A. The sensors 7 emit THz radiation 11 and detect reflected THz radiation, whereby geometric properties can be measured, in particular the outer diameter D, inner diameter I, and wall thicknesses wd4 of the wall regions of the tube 4. To form a multi-layer tube 4, layer thicknesses or wall thicknesses of several layers can also be determined. In addition to these length dimensions, deformations such as ovality and sagging caused by material flow can also be determined from the measurement data.

[0037] The stationary THz measuring device 3 can, for example, perform direct time-in-flight measurements, possibly also with frequency modulation and / or measurements with pulsed radiation, e.g., in the frequency range from 0.01 to 50 THz, in particular 10 GHz to 30 THz. Thus, the THz radiation 11 can also be in a range that completely or partially coincides with or overlaps radar radiation and / or microwave radiation.

[0038] In stationary THz measurements, the measurement diagram shows the Figure 4A stationary measurement signal S1 with signal peaks at times t1, t2, t3, t4 is determined, corresponding to partial reflections at an outer wall surface 12a and an inner wall surface 12b of the front and rear wall regions. The time differences t2-t1 and t4-t3 thus depend on the geometric wall thickness wd4 and the refractive index n4 of the material of the tube 4. Knowing the refractive index n4 allows the wall thickness wd4 and, accordingly, other geometric dimensions of the tube 4 to be precisely determined—assuming the THz measuring device 3 is intact.

[0039] Basically, in Fig. 4 a measurement of the rear wall area of ​​the pipe section 10, ie the measurement at times t3, t4, is not necessary, since an all-round measurement of the pipe section 10 can be carried out and thus each wall area can be measured as a front wall area.

[0040] In step ST2, a pipe section 10 is cut from the pipe 4 by the cutting device 8. Subsequently—and also before the cutting—the pipe section 10 cools according to the cooling step ST3, in particular to room temperature, wherein in particular a shrinkage (differential shrinkage) ds, i.e. a relative change in the dimensions, occurs.

[0041] On the severed pipe section 10, one or more measuring points 18 are marked at a distance d18, e.g., 2.5 inches (6.35 cm), from one of the pipe section ends 10a. In step ST4, the following is carried out: Fig. 2, 3a manual measurement using a reference measuring device, which in this embodiment is applied as a mechanical measuring device 14 to one or more measuring points 18. The mechanical measuring device 14 can in particular be designed as a pair of pliers 14 with a micrometer sensor 15, wherein the pliers 14 are mechanically biased (preloaded) by a spring 16, i.e. the pliers jaws 17a and 17b engage the outer wall surface 12a and the inner wall surface 12b at the measuring point 18 with a predetermined force F or predetermined pressure. The user thus applies the mechanical measuring device 14 from the end 10a of the cut pipe section 10 to the measuring point 18 and receives a reference measurement signal S2 with a very precise value of the geometric wall thickness wd10.

[0042] Subsequently, in step ST5, a mobile, manual THz measurement of the pipe section 10 is performed at the same measuring point 18 using a portable THz measuring device 13, which is placed with its support contour 21 on the outer wall surface 12a. The support contour 21 ensures a defined support of the THz measuring device 13 perpendicular to the outer wall surface 12a, i.e., radially to the pipe axis A. In Fig. 2 and 3 the measuring devices 13, 14 are thus placed at the same measuring point 18 on the profile piece 10; only for better illustration, Fig. 2, 3The mechanical measuring device 14 is also shown on the opposite side. The portable THz measuring device 13 has, in particular, the same or similar components as the individual THz sensors 7 of the stationary THz measuring device 3, e.g., the same THz transceiver 20, i.e., it emits THz radiation 111 that corresponds to the THz radiation 11 of the stationary THz measuring device 3, in particular, the same measuring frequency and the same bandwidth. Thus, the portable THz measuring device 13 in turn delivers a measuring signal S3, which basically corresponds to the diagram of the Fig. 4 or at least enables a measurement of the front wall area with the time difference t2-t1.

[0043] ST6, calibration of the second THz measurement signal S3, i.e. determination of the refractive index n10: The determined wall thickness wd10 is used to calibrate the second THz measurement signal S3 of the portable THz measuring device 13. Since the measurements were carried out at the same measuring point 18 at the same temperature, the wall thicknesses wd10 of the two measurements can be equated, so that the refractive index n10 can be determined directly from the second THz measurement signal S3.

[0044] ST7, Calibration of the first THz measurement signal S1: The determined refractive index n10 is used to calibrate or evaluate the first THz measurement signal S1 of the stationary THz measuring device 3. Thus, the first measurement signal S1 or the stationary measuring device 3, which measures the warm material of the pipe 4, can be calibrated to values ​​that correspond to the later, finally formed profile piece 10, which in particular also allows for direct control of the production parameters, in particular of the extruder 2 and the discharge device 6.

[0045] ST8, Comparison step: According to an advantageous embodiment, a supplementary comparative measurement of the stationary THz measuring device 3 with the portable THz measuring device 13 is then carried out on the extruded tube 4, i.e. inline. In particular, it can be assumed that due to identical or similar electronic components, in particular the same THz transceiver 21, the electronic measurements are comparable and, for example, the propagation times t2-t1 of the two THz measurement signals S1, S3 should correspond. This comparative measurement with the portable THz measuring device thus preferably results in a measurement of the inline, still warm tube, unlike the first measurement with the portable THz measuring device with the separated, preferably cooled, tube section.

[0046] Thus, preferably through this supplementary comparative measurement, an accurate determination of the shrinkage ds or a direct calibration of the stationary measurement to the final values ​​of the cooled profile piece is possible; furthermore, measurement errors, in particular defects of the sensors 7 of the stationary THz measuring device 3, can be detected.

[0047] Thus, an extrusion and measuring system 22 is created, which comprises the extrusion line 1 with the extruder 2, the stationary THz measuring device 3, preferably the take-off device 6 for taking off the extruded profile 4, the separating device 8, the reference measuring device 14, the portable THz measuring device 13, and the control and evaluation device 25. List of reference symbols

[0048] 1Extrusion line 2Extruder 2aCalibration sleeve 3Stationary (inline) THz measuring device 4Profile, in particular pipe 5Cooling device 6Taper 7Sensor of the stationary THz measuring device 3 8Separation device 9Starting material 10Profile pieces, in particular pipe pieces 11THz radiation of the stationary THz measuring device 3 12aOuter wall surface 12bInner wall surface 13Portable THz measuring device 14Reference measuring device, in particular pliers 15Micrometer sensor 16Spring for mechanical preload of the pliers jaws 17a, 17bJaws 18Measuring point 19End of the pipe section 10 20THz transceiver in the stationary THz measuring device 3 and the portable THz measuring device 13 21Support contour 22Extrusion and measuring system for production and measurement of profile pieces 10 25Control and evaluation device 111THz radiation of the portable THz measuring device 13 DOuter diameter IInner diameter d18Distance wd10Wall thickness of the profile piece 10 n4Refractive index of the profile 4 n10Refractive index of the profile piece 10 S1first measurement signal, stationary THz measurement signal S2second measurement signal, mobile reference measurement signal S3third measurement signal, mobile THz measurement signal wd4 geometric wall thickness of profile 4 wd10 geometric wall thickness of profile section 10

Claims

1. Method for calibrating a stationary THz measuring device (3) which measures geometric properties (wd4, D, I) of a profile (4) during an extrusion of the profile (4) by means of one or more THz sensors (7), comprising at least the following steps: - after extrusion of the profile (4) (Step ST0) step of stationary inline THz measurement by outputting first THz radiation (11) from one or more THz sensors (7) of the stationary THz measuring device (3) onto the profile (4), detecting the THz radiation reflected at boundary surfaces (12a, 21b) of the profile (4) and outputting a first measurement signal (S1) of the stationary THz measuring device (3) (Step ST1 - stationary inline THz measurement), - reference measurement of a wall thickness (wd10) at a measuring point (18) of a profile piece (10) by means of a reference measuring device (14) on the basis of a reference measuring method alternative to THz measurement, with generation of a second measuring signal (S2) (Step ST4 - reference measuring), - measurement of the profile piece (10) at the measuring point (18) by means of a portable THz measuring device (13) with generation of a mobile third measuring signal (S3) (Step ST5 - mobile THz measuring), - calibration of the third measurement signal (S3) on the basis of the second measurement signal (S2), with determination of a refractive index (n10) of the profile piece (10) (Step ST6 - calibration of the third measurement signal), - calibration of the first measurement signal (S1) of the stationary THz measurement device (3) on the basis of the determined refractive index (n10) of the profile piece (10) (Step ST7 - stationary calibration).

2. Method according to claim 1, characterised in that after the calibration of the first measuring signal (S1) (Step ST7), a comparison step (ST8) is subsequently carried out, in which the extruded profile (4) is measured by the stationary THz measuring device (3) and subsequently still before the separation (8) for comparison with the portable THz measuring device (13), and the first measuring signal (S1) of the stationary THz measuring device (3) is compared with the third measuring signal (S3) of the portable THz measuring device (13), and depending on the comparison, the calibration is evaluated as successful (St8 - comparison step).

3. Method according to claim 2, characterised in that in the step of comparison (ST8) it is determined whether the determined values of the wall thicknesses (wd4) lie within an accuracy range, e.g. by comparing a difference of the values with a limit value, and if the accuracy range is reached, e.g. if a difference of the values lies below a limit value, the calibration is evaluated as successful.

4. Method according to claim 2 or 3, characterised in that the comparison measurement is performed by the portable THz measuring device (13) (Step ST8) directly after the stationary THz measurement (Step ST1) and at the same measuring point (18) of the profile (4), for better comparability of the first measuring signal (S1 and third measuring signal (S3) (ST8 - comparison step).

5. Method according to one of the above claims, characterised in that the profile piece (10) is produced by separating it from the extruded profile (4) by means of a separating device (8) after the extrusion (Step St0) and the step of stationary in-line measurement (Step ST2).

6. Method according to claim 5, characterised in that the profile piece (10) is actively or passively cooled after separation (Step ST2), in particular to a final temperature (Step ST3 - cooling off).

7. Method according to one of the above claims, characterised in that one or more measurement points (18) are marked before or after the stationary THz measurement (Step ST1), for unambiguous location of the one or more measurement points (18).

8. Method according to claim 7, characterised in that several measuring points (18) are marked and measured at a defined distance (d18) from the end of the profile piece (10) and around the profile piece (10) (Step ST4), in particular, for a defined accessibility of the reference measuring device (14) and / or to avoid the detection of deformations caused by the separation process (Step St2).

9. Method according to one of the above claims, characterised in that the stationary THz measuring device (3) and the portable THz measuring device (13) perform THz measurements with the same and / or overlapping frequencies or frequency range, in particular, with the same transceiver (2) (Step ST1, Step ST5).

10. Method according to one of the above claims, characterised in that the stationary THz measuring device (3) and / or the portable THz measuring device (13) output THz radiation (11) in the frequency range from 0.01 to 50 THz, in particular 0.02 to 30 THz, preferably 0.05 to 10 THz, in particular as time-of-flight measurement and / or frequency modulation and / or pulsed radiation (Step ST1, Step ST5).

11. Method according to one of the above claims, characterised in that a pipe (4) is extruded as a profile and subsequently separated pipe segments (10) are measured (Step ST4, Step ST5), wherein one or more of the following geometric properties of the pipe (4) and / or the pipe pieces are determined: a wall thickness (wd), an outer diameter (A), an inner diameter (I), deformations, in particular an ovality and / or sagging due to hot material.

12. Method according to one of the above claims, characterised in that during the reference measurement (Step ST4) and / or the measurement by means of the portable THz measuring device (13) (Step ST5) the reference measuring device (14) and / or the portable THz measuring device (13) is placed manually against an outer wall surface (12a) of the profile (4) with a contact surface, e.g., a defined contact contour (21).

13. Method according to one of the above claims, characterised in that in the reference measurement step (Step St4) one or more of the following measuring devices are used as reference measuring device (14): - a mechanical measuring device (14) for mechanical measurement of the profile (4), - an ultrasonic measuring device for ultrasonic measurement of the profile (4), and - a laser for laser measurement of the profile (4).

14. Method according to claim 13, characterised in that as the mechanical measuring device (14), a pair of pliers is inserted into the profile piece (10) from one profile piece end (10a) and clamping jaws (14a, 14b) are applied to the inner surface (12b) and the outer surface (12a) of the profile (4), in particular, with a defined mechanical bias (F) (Step ST4).

15. Method according to claim 13, characterised in that the ultrasonic measuring device is applied to the profile piece (10) from the outside, e.g. with a gel applied to a measuring head of the ultrasonic measuring device (Step ST4).

16. Extrusion and measuring system (22) for producing and measuring profile pieces (10), comprising: an extrusion line (1) comprising an extruder (2) adapted to extrude profiles (4), a stationary THz measuring device (3) arranged downstream of the extruder (2) for measuring an extruded profile (4), the THz measuring device (3) comprising one or more THz sensors (7) aligned with an axis of symmetry (A) of the extrusion line (1), for measuring geometric properties of the extruded profile (4), a haul-off for hauling off the extruded profile (4), a separating device (8) for separating profile pieces (10) from the profile (4), a reference measuring device (14) which is set up to measure a reference wall thickness (wd10) of a separated profile piece (10) using a measuring method which is alternative to THz measuring, a portable THz measuring device (13) with a support contour (21), for placing on a separated profile piece (10) and for measuring it, the portable THz measuring device (13) being programmable and being designed to record a value of a determined wall thickness (wd10) for calibration purposes, and a control and evaluation device (25) set up and designed to record via an interface a determined refractive index (n10) which is determined by means of the reference wall thickness (wd10) and the wall thickness determined by the portable THz measuring device (13), and to determine geometric properties of the profile piece (10) from at least the first measurement signal (S1) of the stationary THz measuring device (3) on the basis of the determined refractive index (n10).

17. Extrusion and measuring system (22) according to claim 16, characterised in that the reference measuring device (14) is realized as one or more of the following measuring devices: - a mechanical measuring device (14) for mechanical measurement of the profile (4), - an ultrasonic measuring device for ultrasonic measurement of the profile (4), - a laser for laser measurement of the profile (4).