Measuring arrangement with a blow-molding extruder, method for measuring a melt tube or a blow-molding product and method for blow-molding extrusion

Abstract

Measuring arrangement (5) for measuring a melt hose (20), the measuring arrangement (5) comprising: - a melt tube (10) discharged from the blow-molding extruder (1) through the annular gap (8) along an extrusion axis (A) or a blow-molding product (16, 116, 216) extruded along an extrusion axis (A), a measuring device (20) for measuring a wall of the melt tube (10), characterized in that - the measuring device is a THz measuring device (20) with at least one THz sensor (24), wherein the THz sensor (24) has an optics (26) for focusing a THz transmission beam (25) onto a focus (F), - a control device (32) which is designed to receive measurement signals (S1) from the THz measuring device (20) and to determine at least one wall thickness (wd) of a front wall (110) of the melt tube (10), wherein the front wall (110) is located behind the focus (F) in a divergent beam region (25b) of the THz transmission beam (25) or in front of the focus (F) in a convergent beam region (25a), wherein the front wall (110) has an adjustment distance (30) to the focus which lies in a range from above a Rayleigh length (ZR) to a predetermined multiple of the Rayleigh length (ZR), wherein the focal point (F) lies in front of the extrusion axis (A) and / or the axis of symmetry of the melt tube (10) or the blow-molded product (16, 116, 216).

Description

[0001] The invention relates to a measuring arrangement with a blow molding extruder, a method for measuring a melt tube or a blow molding product and a method for blow molding extrusion.

[0002] In blow molding extrusion processes, a starting material, e.g. plastic granulate, is melted and extruded through an annular gap, e.g. vertically, so that a melt tube made of a thin molten plastic material is ejected. The melt tube can be ejected continuously or intermittently. In some cases, a jet of compressed air is also emitted from a nozzle provided within the annular gap, so that the melt tube is blown and conveyed along the extrusion direction or extrusion axis, with the melt tube continuously changing its shape in the jet of compressed air. The exact position of the walls of the melt tube relative to the extrusion axis or axis of symmetry of the blow molding extruder is therefore not fixed.The melt tube is then gripped directly by a tool or fed to the tool via a gripper, so that the blow-molded product is formed in the tool while it is pressurized with compressed air from the inside.

[0003] To create blow molds with suitable properties, precise wall thickness adjustment is required. However, the wall thickness cannot be measured mechanically on the melt tube itself; furthermore, measuring the generally opaque melt tube using optical means, for example, is generally problematic.

[0004] For measuring plastic products such as plastic pipes, THz measuring devices are generally known. These devices utilize a THz sensor that aligns and focuses a THz transmission beam along an optical axis, for example, onto the pipe axis of the measurement object, and detects partial beams reflected at the interfaces of the measurement object. This allows relevant dimensions of the measurement object to be determined, particularly wall thicknesses and inner and outer diameters. For accurate measurement, the THz transmission beam must be aligned perpendicular to the wall of the measurement object so that the reflected partial beams are reflected back to the THz sensor, and the wall thickness is measured perpendicular to the interfaces.

[0005] WO 2017 / 101906 A1 discloses a THz measuring device in which the angle of the THz sensor is adjustable. This allows a pipe as the measurement object to be measured even if its position relative to the THz sensor is not precisely determined by pivoting the THz sensor in a plane perpendicular to the pipe axis and capturing a maximum measurement signal. This allows the wall and other geometric properties of a pipe to be captured even if the position is initially imprecise.

[0006] However, for larger deviations, a simple angle adjustment may not be sufficient. In particular, a melt tube, for example, may be positioned so unfavorably that even a simple swivel of the THz transmission beam fails to capture a relevant measurement signal.

[0007] WO 2011 / 072 650 A1 shows a nozzle for an extruder, which has a housing divided into at least two parts, with a housing base body and an end part, wherein a flowable mass is dispensed through the extruder and subsequently measured for a wall thickness by a thickness measuring sensor.

[0008] DE 10 2019 108 299 B4 describes a THz measuring device for measuring the layer thickness of a wall of a measurement object and / or the distance between interfaces of a measurement object, with a transmitting and receiving unit. In this case, the layer thickness and / or the distance of the wall is determined as the time-of-flight difference between interfaces, with an adjustable optical unit with a deformable reflector provided in the beam path of the transmitting and receiving unit.

[0009] DE 38 16 273 A1 describes a method for producing hollow bodies from thermoplastics, wherein a tubular preform is formed by exiting a die gap of an extrusion head and expanded in a blow mold. Control is achieved by measuring the position of the preform relative to the blow mold by measuring the weight of a separated lower slug and comparing an actual measured value with a target value.

[0010] The invention is based on the object of creating a measuring arrangement and a method for measuring a melt tube or a blow mold, which enable reliable measurement with relatively little effort.

[0011] This object is achieved by means of a measuring arrangement having the features of claim 1 and the subclaims 2 to 11 as well as a method for measuring a melt tube or a blow-molded product having the features of claim 12 and the subclaims 13 to 16 as well as a method for blow-mold extrusion having the features of claim 17.

[0012] Thus, a targeted defocusing of the THz transmission beam is provided, whereby the focus is set in such a way that the wall of the melt tube or the blow-molded product lies in a divergent or convergent beam area of the emitted THz transmission beam.

[0013] This allows the wall to be detected to be detected with a beam geometry in which a divergent beam is fanned out sufficiently so that a larger wall area of the melt tube or blow mold is illuminated and, furthermore, the radiation detects the wall with a larger angular range of the convergent or divergent beam.

[0014] The distance of the focus from the wall is preferably adjusted as a function of the Rayleigh length. The Rayleigh length depends on the wavelength or center frequency of the THz transmitted beam, and the beam geometry. According to the invention, it is recognized that the Rayleigh length represents a useful reference measure for determining the adjustment distance of the focal point from the wall. The adjustment distance can, in particular, be selected to be greater than the Rayleigh length, e.g., between the Rayleigh length and a predetermined multiple of the Rayleigh length, e.g., 10 times, 5 times, or 2 times the Rayleigh length.

[0015] According to the invention, it is particularly also recognized that with a THz measurement in the frequency range from 0.05 to 50 THz, 20 or 5 THz and suitable optics, distance values are determined using the Rayleigh length, which also represent useful distances in practice. For example, with an upstream lens and typical measuring distances of the THz sensor from the measurement object, Rayleigh lengths of e.g. 10 mm to 100 mm, in particular 40 mm, are determined, i.e. values in a range of a few cm. Thus, with the THz radiation and such defocusing, reliable detection of the measurement object can be achieved in order to reliably detect the front wall in common blow molding extrusion processes and to reliably determine the wall thickness of the front wall, and if necessary also other geometric properties.

[0016] Preferably, the angle of the optical axis of the THz sensor is adjusted by an angle adjustment device, in particular in a plane perpendicular to the extrusion axis and / or in the plane defined by the optical axis and the extrusion axis. This two-dimensional adjustment allows for better accounting for the deformations occurring in the melt tube, particularly when measuring the melt tube, than with a one-dimensional adjustment, as may be required for extruded tubes, since an extruded tube essentially only bends in the plane perpendicular to the extrusion axis.

[0017] The invention can be used in both continuous and discontinuous blow molding extrusion processes.

[0018] According to the invention, a control of the extrusion process is furthermore made possible, in which, depending on the measurement and in particular the determination of the geometric properties of the melt tube, a control of the blow molding extruder is provided, in particular an adjustment of the annular gap or an extruder screw, and / or the nozzle for dispensing the compressed air jet.

[0019] According to the invention, already formed blow molds can also be measured by positioning the blow mold in the geometric area behind the focus, in particular at a measurement distance of one Rayleigh length to a specified multiple of the Rayleigh length. It is recognized that measuring a finished product is generally difficult, since the distance of the blow-molded product from the THz measuring device must be precisely adjusted without the provision of suitable mounts or holders. The defocusing according to the invention allows a sufficiently large illuminated wall area of a blow-molded product to be reliably captured.

[0020] The blow molding extruder preferably takes in free-flowing starting material and melts it via an extrusion screw, and continuously discharges the plastic melt as a melt tube via the annular gap along an extrusion axis.

[0021] The invention is explained in more detail below with reference to some embodiments in the accompanying drawings. They show: Fig. 1 shows an arrangement according to a first embodiment with a blow molding extruder with extruded melt tube and tool; Fig. 2 an arrangement of THz measuring device and various blow molds to be measured; Fig. 3 a representation of the beam geometry of the THz transmission beam a of the front wall to be measured; Fig. 4 a flowchart of a method according to the invention.

[0022] According to Fig. 1, a measuring arrangement 5 is provided, which comprises a blow-molding extruder 1, a melt hose 10 discharged from the blow-molding extruder 1, a THz measuring device 20, and preferably a tool 12. The blow-molding extruder 1 comprises, among other things, an extrusion screw 2, a hose head 3, and a variable nozzle 4. A granulate 6 is fed to the extrusion screw 2 in the usual manner, so that the extrusion screw 2 extrudes molten plastic material 7 through an annular gap 8 between the hose head 3 and the variable nozzle 4. Compressed air 9 is continuously blown through the variable nozzle 4 along an extrusion axis A. Thus, the melt tube 10 is blown out as a preform along the extrusion axis A, wherein the melt tube 10 has a wall thickness WD which depends in particular on the setting of the annular gap 8, the temperature of the molten plastic material 7 and the compressed air jet emitted by the variable nozzle 4.Thus, in particular, a distance ID of the melt tube 10 from the extrusion axis A is variable. The melt tube 10 continuously changes its position and its inner diameter or outer diameter, while also being asymmetrical or non-round in the circumferential direction.

[0023] The melt tube 10 is then gripped directly by a tool 12, or by a gripper 14 and transported to the tool 12. In Fig. 1, the tool 12 is arranged below the blow-molding extruder 1 or the extruder head 3; however, the tool 12 can also grip the melt tube 10 directly below the extruder head 3 as soon as it has reached a predetermined length along the extrusion axis A.

[0024] Subsequently, the melt tube 10 is inflated in the usual way in the tool 12 from the inside via the nozzle 4 and formed by the inner shape of the tool 12, so that subsequently the Fig. 1 indicated blow molded product 16 is dispensed.

[0025] A THz measuring device 20 is rigidly connected to the blow molding extruder 1 via a structure 22 and has one or more THz transceivers 24, each emitting a THz transmission beam 25 along an optical axis B into a measuring chamber 23. In this case, the optical axis B runs in a basic position initially perpendicular to the extrusion axis A. The THz sensor 24 is adjustable via an angle adjustment device 28 relative to the structure 22 and thus to the blow molding extruder 1. The angle adjustment device 28 enables in particular an adjustment of an adjustment angle alpha, ie a pivoting in the plane perpendicular to the extrusion axis A, ie the YZ plane, and / or in the image plane of the Fig. 1, ie the XZ plane defined by the extrusion axis A and the THz sensor 24.

[0026] According to Fig. 1, in particular, a continuous blow molding process can be provided, in which the melt tube 10 is continuously dispensed, optionally first grasped and conveyed by the gripper 14, and then picked up by the tool 12 to form the blow-molded products 16. Alternatively, a discontinuous blow molding process or, for example, a continuous process with rotating blow molds can also be used.

[0027] The THz transmission beam 25 is focused on a focus F, which is located at a focal distance FL from the THz sensor 24. For this purpose, the THz sensor 24 typically has a lens 26 and, for example, a transceiver chip. The THz transmission beam 25 forms a beam waist at the focus F, the radius W0 of which Fig. 3. Thus, the THz transmission beam 25 forms a convergent beam region 25a in front of the focus and a divergent beam region 25b behind the focus F.

[0028] In order to reliably detect the continuously moving and fluttering melt tube 10, the focus F is focused in front of the melt tube 10 so that the melt tube 10 is detected by the divergent beam area 25b. According to Fig. 3, the divergent beam region 25b detects an illuminated wall region 31 that is located at a setting distance 30 from the focus F. A distance d110 of the front wall 110 to the THz sensor 24 or the optics 26 thus results from the sum of F and the setting distance 30.

[0029] The Rayleigh length zR is defined such that at a distance of the Rayleigh length ZR from the focus F, the radius W(Z) of the beam cone of the transmitted beam 25 is larger by a factor of √2 (root of two) than the radius W0 of the beam waist at the focus F. The Rayleigh length zR is represented by the following formula for the usual approximation of the transmitted beam 25 as a Gaussian beam or with a Gaussian distribution perpendicular to the optical axis B of the beam:

[0030] According to Fig. 3 a Rayleigh length zR is defined by: zR=n⋅π⋅w02λ0 with zR Rayleigh length n refractive index of the medium, here n = 1 of air, W(Z) radius of the ray cone W0 radius of the beam waist at focus F, λ0 center frequency or vacuum wavelength of the THz transmitted beam 25.

[0031] The setting distance 30 of the focus F from the front boundary surface of the wall 110 of the melt tube 10 is selected as a function of the Rayleigh length ZR, whereby the setting distance 30 is selected to be greater than the Rayleigh length ZR, since outside the Rayleigh length ZR the maximum angular fanning out is achieved with a minimum beam width.

[0032] Advantageously, a range between ZR and an upper value of G*ZR (G multiplied by ZR) is selected as the setting distance 30, ie a value higher than ZR by a factor of G, eg with G = 10, ie ten times the value of ZR, preferably G=5 or G=2.

[0033] The angle adjustment device 28 can be used to adjust the THz sensor 24 such that a maximum measurement signal S1 is detected, resulting in a measurement perpendicular through the wall 110 of the melt tube 10. For this purpose, an adjustment angle alpha is adjusted in two planes, i.e., two adjustment angles, until a maximum measurement signal is determined, at which a measurement is thus performed perpendicular through the interfaces. In this case, the THz transmission beam 25 is partially reflected on the front and rear surfaces of the wall 110, so that the reflection peaks in the reflection beam 27 can be detected and—given a known refractive index n10 of the melt tube 10—the wall thickness wd can thus be determined. If the refractive index n10 is unknown, a calibration measurement can also be performed beforehand with the measuring chamber 23 empty.

[0034] The angle adjustment device 28 can thus compensate for the current course of the blown melt tube 10 and enable an accurate measurement of the wall thickness wd, whereby a large surface area of the melt tube 10 can be detected due to the defocusing and the positioning of the wall 110 in the divergent beam area 25b.

[0035] Thus, the front wall 110 is measured in each case. The rear wall can be correspondingly detected by a THz sensor 24 provided on the other side, ie, the THz measuring device 20 has several static THz sensors 24 arranged around the measuring chamber 23. Furthermore, one or more THz sensors 24 can rotate or reverse completely around the melt hose, ie, pivot back and forth.

[0036] The control of the THz sensors 24 and the evaluation of the measurement signals S1 are carried out via a control device 32. Depending on the measurement, the blow molding extruder 1 can subsequently be controlled, in particular by adjusting the annular gap 8 and / or the extrusion screw 2.

[0037] Fig. Figure 2 shows a further embodiment with a measurement of finished blow-molded products 116, 216 made of a corresponding plastic material. Since the position of the blow-molded product 116, 216 is initially unknown, the THz transmission beam 25 can again be focused to a corresponding setting distance 30 in order to reliably detect the front wall of the blow-molded products 116, 216 in the divergent beam region 25b.

[0038] Fig. 4 thus shows the method according to the invention, with a start in step ST0, in which the measuring arrangement with the THz measuring device 20 is provided and positioned, subsequently in step ST1 the blow-molding extruder 1 continuously forms and blows out a melt tube 10, or the blow-molding product 16, 116, 216 is positioned, according to step ST2, the THz sensor 24 outputs the THz transmission beam 25 along the optical axis B and focuses it as described above.

[0039] According to step ST3, the THz reflection beam 27 is recorded and evaluated by the THz sensor 24, so that in step ST4 the appropriate adjustment of the angle adjustment device 28 is subsequently carried out in order to set the optical axis B perpendicular to the wall 110 of the melt tube 10 or the blow-molded product 16, 116, 216.

[0040] During and after the adjustment, the THz transmission beam 25 is thus again emitted according to step ST2 in order to continuously measure the melt tube 10 or the blow-molded product 16, 116, 216 during the adjustment.

[0041] Subsequently, as described above in the first embodiment, in step ST5, the melt tube 10 is directly grasped by the tool 12 or the gripper 14 arranged therebetween and is subsequently deformed in the tool 12 in order to dispense the blow-molded products 16 or 116, 216. List of reference symbols 1 blow molding extruder 2 extruder screws 3 Hose head, extruder head 4 variable nozzles 5 Measuring arrangement 6 granules 7 molten plastic material 8 Annular gap 9 Compressed air 10 Melt tube as preform 12 tools 14 grippers 16 Blow molded product 20 THz measuring device 22 fixed structure between the blow molding extruder 1 and the angle adjustment device 28 23 Measuring room 24 THz sensor 25 THz transmit beam 25a convergent beam area 25b divergent beam area 26 Optics, lens 27 THz reflection beam 28 Angle adjustment device 30 Adjustment distance between the focus F and the melt hose 10 31 wall area of the front wall 110 illuminated by the THz transmission beam 25 32 Control device 110 front wall of the melt hose 10 116, 216 blow-molded products A Extrusion axis, axis of symmetry of the blow molding extruder 1 B optical axis of the THz sensor 24 F Focus FL focal length ID Distance of the melt hose from A W0 Radius of the beam waist or focal spot d110 Distance of the front wall from the optics 26 Th (Theta) Aperture angle of the THz transmit beam 25 after the focus F alpha adjustment angle of the optical axis B wd wall thickness of the front wall 110 S1 measurement signal S2 angle setting signal S3 control signal to extruder ZR Rayleigh length

Claims

[1] Measuring arrangement (5) for measuring a melt hose (20), the measuring arrangement (5) comprising: - a melt tube (10) discharged from the blow-molding extruder (1) through the annular gap (8) along an extrusion axis (A) or a blow-molding product (16, 116, 216) extruded along an extrusion axis (A), a measuring device (20) for measuring a wall of the melt tube (10), characterized by , that - the measuring device is a THz measuring device (20) with at least one THz sensor (24), wherein the THz sensor (24) has an optics (26) for focusing a THz transmission beam (25) onto a focus (F), - a control device (32) which is designed to receive measurement signals (S1) from the THz measuring device (20) and to determine at least one wall thickness (wd) of a front wall (110) of the melt tube (10), wherein the front wall (110) is located behind the focus (F) in a divergent beam region (25b) of the THz transmission beam (25) or in front of the focus (F) in a convergent beam region (25a), wherein the front wall (110) has an adjustment distance (30) to the focus which lies in a range from above a Rayleigh length (ZR) to a predetermined multiple of the Rayleigh length (ZR), wherein the focal point (F) lies in front of the extrusion axis (A) and / or the axis of symmetry of the melt tube (10) or the blow-molded product (16, 116, 216). [2] Measuring arrangement (5) according to claim 1, characterized by that the control device (32) is arranged to further calculate one or more measured variables: a distance (d110) of the front wall (110) from the THz sensor (24), an inner diameter, an outer diameter of the melt tube (10) a refractive index of the material of the melt tube (10). [3] Measuring arrangement (5) according to claim 1 or 2, characterized by that the adjustment distance (30) lies in a range from above one Rayleigh length (ZR) up to ten times, preferably up to four times, the Rayleigh length (ZR). [4] Measuring arrangement (5) according to one of the preceding claims, characterized by , that it further comprises an angle adjustment device (28), the THz sensor (24) is adjustable relative to the extrusion axis (A) by the angle adjustment device (28), and the control device (32) is configured to control the angle adjustment device (28) for adjusting the THz sensor (24), wherein the optical axis (B) of the THz sensor (24) is aligned perpendicular to the extrusion axis (A) in its basic position, and the angle adjustment device (28) is configured to adjust the optical axis (B) - in a measuring plane (YZ) perpendicular to the extrusion axis (A) and / or - in a vertical plane (XZ) defined by the optical axis (B) and the extrusion axis (A). [5] Measuring arrangement (5) according to claim 4, characterized by that the control device (32) is designed to control the angle adjustment device (28) as a function of the measurement signal (S1) in such a way that a reflected measurement signal (S1) of the boundary surfaces of the front wall (110) is maximum, in order to form a perpendicular incidence of the THz transmission beam (25) onto the front wall (110). [6] Measuring arrangement (5) according to one of the preceding claims, characterized by that the THz sensor (24) is designed to output the THz radiation in a frequency range of 0.01 to 20 THz, in particular 0.05 to 5 THz, in particular fully electronically, as a frequency-modulated radar wave, or pulsed beam or by direct time-of-flight measurement. [7] Measuring arrangement (5) according to one of the preceding claims, characterized bythat the optics (26) are designed to focus the THz transmission beam (25) point-like or elliptically onto the focal point (F). [8] Measuring arrangement (5) according to one of the preceding claims, characterized by that the control device (32) outputs a quality signal as a function of the evaluation of the measuring signal (S1) and the determination of the wall thickness (wd), in particular by comparing the determined wall thickness (wd) with predetermined wall thickness comparison values. [9] Measuring arrangement (5) according to one of the preceding claims, characterized by that it continues to have - a blow-molding extruder (1) with an extruder head (2) and an optional compressed air nozzle (4), between which an annular gap (8) is formed for extruding the melt tube (10), wherein the blow-molding extruder (1) is designed to discharge the melt tube along the extrusion axis (A) through the annular gap by means of the compressed air jet of the compressed air nozzle. [10] Measuring arrangement (5) according to claim 9, characterized by that the control device (32) is designed to control the blow molding extruder (1) for regulation depending on the determined wall thickness (wd) of the melt tube (10) and / or a determined wall thickness (wd) of the finished blow molded product (16), in particular by adjusting an annular gap (8) of the blow molding extruder (1). [11] Measuring arrangement (5) according to claim 9 or 10, characterized by in that it has a tool (12) for blow-molding the melt tube (10), wherein the control device (32) is designed and configured to control the tool (12) and / or a gripper (14) in such a way that the melt tube (10) is grasped in successive molding processes directly by the tool (12) or by means of a gripper (14) and is formed into a blow-molded product (16). [12] Method for measuring a melt tube (10) of an extrusion blow molding process or a blow molded product (16, 116, 216), comprising at least the following steps: Positioning or providing a THz measuring device (20) with a THz sensor (24) having an optical axis (B), (ST0) Extruding a melt tube (10) through a blow-molding extruder (1) along an extrusion axis (A) or positioning a blow-molding product (16, 116, 216) defined by an extrusion axis (A), (ST1) Emitting a THz transmission beam (25) from the THz sensor (24) along its optical axis (B) such that the front wall (110) of the melt tube (10) or the blow-molded product (16, 116, 216) lies in a divergent beam region (25b) of the THz transmission beam (25) behind the focus (F) or in a convergent beam region (25a) in front of the focus (F), (ST2) Receiving a reflection beam (27) reflected by the melt tube (10) or the blow-molded product (16, 116, 216) and evaluating a measurement signal in the reflection beam (27), (ST3) Adjusting an adjustment angle (alpha, 90°-alpha) between, on the one hand, the optical axis (B) of the THz sensor (24) and, on the other hand, the extrusion axis (A) or an orthogonal to Extrusion axis (A), depending on the measuring signal (S1) such that the measuring signal in the reflection beam (27) is maximum, for setting a perpendicular incidence of the optical axis (B) on the front wall (110) of the melt tube (10),and determining the wall thickness (wd) of the front wall (110) (ST4), wherein the focus (F) is or is adjusted such that an adjustment distance (30) of the focus (F) in front of or behind the front wall (110) lies in a range above a Rayleigh length (ZR) and below a predetermined multiple of the Rayleigh length (ZR), wherein the focal point (F) lies in front of the extrusion axis (A) and / or the axis of symmetry of the melt tube (10) or the blow-molded product (16, 116, 216). [13] Method according to claim 12, characterized by that the angular adjustment takes place in a plane (yz) perpendicular to the extrusion axis (A) and / or in a plane (xz) defined by the THz sensor (24) and the extrusion axis (A). [14] Method according to one of claims 12 to 13, characterized bythat the setting distance (30) of the focus (F) in front of or behind the front wall (110) is in a range above a Rayleigh length (ZR) and below 10 times or five times the Rayleigh length (ZR). [15] Method according to one of claims 12 to 14, characterized by that, depending on an evaluation of the measuring signal (S1), a quality display or quality signal is output for evaluating the melt hose (10), in particular for determining an error in the wall thickness (wd). [16] Method according to one of claims 12 to 15, characterized by that the THz transmission beam (25) is output as a continuous or temporally interrupted transmission beam, in particular as a frequency-modulated radar wave and / or pulsed transmission beam and / or direct time-of-flight measurement, in particular in the frequency range from 0.05 to 50 THz, e.g. 0.05 to 5 THz. [17] Method for blow-molding extrusion of blow-molded products (16, 116, 216), wherein, with a method according to one of claims 12 to 16, a melt tube (10) is output from the blow-molding extruder (1) (ST1) and is measured by the THz measuring device (20) by adjusting the setting angle (90°-alpha), (ST2, ST3, ST4), and the melt tube (10) is subsequently gripped by a tool (12) directly or via a gripper (14), wherein the melt tube (10) is inflated from the inside and its shape is fixed from the outside by the tool (12), (ST5), as a continuous or discontinuous process.

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