MEASURING ARRANGEMENT WITH A BLOW-MOULDING EXTRUDER, METHOD FOR MEASURING A MELTSTUBE OR A BLOW-MOULDING PRODUCT AND METHOD FOR BLOW-MOULDING EXTRUSION

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 produce blow-molded products 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 boundary surfaces 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 boundary surfaces.

[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. This is done 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 hose 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 / 072650A1 describes a nozzle for an extruder consisting of a core and a housing divided into at least two parts, comprising a stationary housing base and an end part of the housing. The housing encloses the core such that a flow channel is present between the core and the housing, through which the flowable compound can be discharged. The flow channel is sealed in the region of the separation between the housing base and the adjacent end part by means of an elastically deformable seal, and the end part is mounted on this seal so that it can be tilted in any direction. One or more thickness sensors measure the actual thickness of the compound behind the nozzle.

[0008] DE10 2019108 299 B4 discloses a THz measuring device and a THz measuring method for determining a layer thickness or a distance of a measurement object. In this method, a terahertz transmitter radiates terahertz radiation along an optical axis onto the measurement object, and a terahertz receiver detects reflected terahertz radiation, allowing the wall of the measurement object to be measured. A deformable reflector is provided for modifying the beam cross-section.

[0009] DE 3816273 A1 discloses a method for producing hollow bodies made of thermoplastic material, wherein a tubular preform is formed by exiting a die gap of an extrusion head and expanded in a blow mold. Furthermore, the position of the preform relative to the blow mold is controlled and / or regulated, and finally, the slugs created by the blow mold are separated from the hollow body after leaving the blow mold. Wall thickness programming is performed along the length of the preform using conventional control devices. The wall thickness programming can be performed in the longitudinal or circumferential direction of the preform.

[0010] DE 10 2009 058 361 B3 describes a nozzle with a core and a housing divided into at least two parts, comprising a stationary housing base and an end portion of the housing at which a flowable mass exits the nozzle. In this case, a body with a precisely defined geometry, or even a tubular preform of an extrusion blow molding process, is formed from the flowable mass. Furthermore, methods are mentioned in which, using this nozzle, the wall thickness distribution of a mass discharged from the nozzle is measured continuously or at least at short intervals at at least one point during the ongoing process.

[0011] The invention is based on the object of creating a measuring arrangement and a method for measuring a melt tube or a blow-molded product, which enable reliable measurement with relatively little effort.

[0012] This object is achieved by a measuring arrangement according to claim 1. The subclaims describe preferred developments. Furthermore, a method for producing a blow-molded product according to claim 12 is provided.

[0013] The method according to the invention is carried out in particular with the measuring arrangement according to the invention; the measuring arrangement according to the invention is provided in particular for the implementation of the method according to the invention

[0014] 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.

[0015] 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-molded product is illuminated and, furthermore, the radiation detects the wall with a larger angular range of the convergent or divergent beam.

[0016] According to the invention, the distance of the focus from the wall is 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 helpful reference measure for determining the adjustment distance of the focal point from the wall. According to the invention, the adjustment distance is greater than the Rayleigh length; for example, it can be selected between the Rayleigh length and a predetermined multiple of the Rayleigh length, e.g., 10 times, 5 times, or 2 times the Rayleigh length.

[0017] In the method and the device according to the invention, the THz transmission beam is preferably 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 0.01 or 0.05 to 50 THz, e.g. 0.05 to 5 THz.

[0018] 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 conventional 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, the THz radiation and such defocusing can be used to reliably detect the measurement object 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.

[0019] 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 of the melt tube, particularly when measuring the melt tube, than with a one-dimensional adjustment, as may be required with extruded tubes, since an extruded tube essentially only bends in the plane perpendicular to the extrusion axis.

[0020] The invention can be used in both continuous and discontinuous blow molding extrusion processes.

[0021] 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.

[0022] According to the invention, already formed blow-molded products can also be measured by arranging the blow-molded product in the geometric region behind the focus, at a measuring 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.

[0023] 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.

[0024] The invention is explained in more detail below with reference to some embodiments of the invention and 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 shows an arrangement comprising a THz measuring device and various blow-molded products to be measured; Fig. 3 shows a representation of the beam geometry of the THz transmission beam a of the front wall to be measured; Fig. 4 shows a flow diagram of a method according to the invention.

[0025] According to Figure 1A 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 way, 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.

[0026] The melt tube 10 is subsequently gripped directly by a tool 12, or gripped by a gripper 14 and conveyed to the tool 12. In Figure 1 the tool 12 is arranged below the blow-molding extruder 1 or the extruder head 3; however, the tool 12 can also, in particular, grasp the melt tube 10 directly below the extruder head 3 as soon as it has reached a predetermined length along the extrusion axis A.

[0027] 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 Figure 1 indicated blow molded product 16 is dispensed.

[0028] 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 of which emits 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 , i.e. the XZ plane defined by the extrusion axis A and the THz sensor 24.

[0029] According to Figure 1In particular, a continuous blow molding process can be provided, in which the melt tube 10 is continuously dispensed, possibly 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.

[0030] The THz transmission beam 25 is focused to 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 Figure 3 is shown. 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.

[0031] 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 An illuminated wall area 31 is detected by the divergent beam area 25b, which 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 as the sum of F and the setting distance 30.

[0032] 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 24 is larger by a factor of √2 (root of two) than the radius WO 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 24 as a Gaussian beam or with a Gaussian distribution perpendicular to the optical axis B of the beam: According to Figure 3 a Rayleigh length zR is defined by: z R = n ⋅ π ⋅ w 0 2 λ 0 with zRRayleigh length nRefractive index of the medium, here n = 1 of air, W(Z)Radius of the beam cone WORadius of the beam waist at the focus F, λ0Center frequency or vacuum wavelength of the THz transmitted beam 25.

[0033] 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 the maximum angular fanning is achieved outside the Rayleigh length ZR, e.g. even with a small beam width.

[0034] 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, e.g. with G = 10, ie ten times the value of ZR, preferably G=5 or G=2.

[0035] 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 perpendicularly through the interfaces. The THz transmission beam 24 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 in advance with the measuring chamber 28 empty.

[0036] The angle adjustment device 28 can thus compensate for the current course of the blown melt tube 10 and an accurate measurement of the wall thickness wd can be carried out, 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.

[0037] 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, i.e., 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, i.e., pivot back and forth.

[0038] 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.

[0039] 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.

[0040] Figure 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 prepared and positioned, subsequently in step ST1 the blow molding extruder 1 continuously forms and blows out a melt tube 10, or the blow molded product 16, 116, 216 is positioned, according to step ST2 the THz sensor 24 emits the THz transmission beam 25 along the optical axis B and focuses it as described above. 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 subsequently takes place in order to set the optical axis B perpendicular to the wall 110 of the melt tube 10 or the blow molded product.

[0041] 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 during the adjustment.

[0042] Subsequently, as described above, in the first embodiment, in step ST5, the melt tube 10 is directly gripped by the tool 12 or the intermediate gripper 14 and subsequently deformed in the tool 12 in order to dispense the blow-molded products 16 or 116, 216. List of reference symbols

[0043] 1 Blow molding extruder 2 Extruder screw 3 Extruder head, extruder head 4 Variable nozzle 5 Measuring arrangement 6 Granules 7 Molten plastic material 8 Annular gap 9 Compressed air 10 Melt tube as preform 12 Tool 14 Gripper 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 chamber 24 THz sensor 25 THz transmission 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 tube 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 AExtrusion axis, symmetry axis of the blow molding extruder 1 BOptical axis of the THz sensor 24 FFocus FLFocal length, focal length IDDistance of the melt tube from A WORadius of the beam waist or focal spot d110Distance of the front wall from the optics 26 Th (Theta)Aperture angle of the THz transmission beam 25 after the focus FalphaAdjustment angle of the optical axis BwdWall thickness of the front wall 110 S1Measurement signal S2Angle adjustment signal S3Control signal to extruder ZRRayleigh length

Claims

1. Measuring arrangement (5) for measuring a molten tube (20) or a blow-molding product (16, 116, 216), the measuring arrangement (5) comprising: - a molten tube (10) put out by a blow-molding extruder (1) through a ring gap (8) along an extrusion axis (A) or a blow-molding product (16, 116, 216) extruded along an extrusion axis (A), - a THz measuring device (20) including at least one THz sensor (24), said THz sensor (24) comprising optics (26) for focusing a THz transmission beam (25) onto a focus (F), - a controller means (32), adapted to receive measuring signals (S1) from the THz measuring device (20) and to determine at least one wall thickness (wd) of a front wall (110) of the molten tube (10) or the blow-molding product (16, 116, 216), the front wall (110) lying in a divergent beam area (25b) of the THz transmission beam (25) behind the focus (F) or in a convergent beam area (25a) before the focus (F), where the front wall (110) has a set distance (30) to the focus that lies in an area above a Rayleigh length (ZR) up to a pre-determined multiple of the Rayleigh length (ZR), and the focal point (F) lies before the extrusion axis (A) and / or the axis of symmetry of the molten tube (10) or the blow-molding product (16, 116, 216).

2. Measuring arrangement (5) according to claim 1, characterized in that the controller means (32) is adapted to further compute one or more measured values: a distance (d110) of the font wall (110) from the THz sensor (24), an internal diameter, an external diameter of the molten tube (10) or the blow-molding product (16, 116, 216), a refraction index of the material of the molten tube (10) or of the blow-molding product (16, 116, 216).

3. Measuring arrangement (5) according to claim 1 or 2, characterized in that the set distance lies in an area above the Rayleigh (ZR) length up a tenfold, preferably a fourfold of the Rayleigh length (ZR).

4. Measuring arrangement (5) according to one of the above claims, characterized in that it further comprises an angle adjusting means (28), the THz sensor (24) being adjustable relative to the extrusion axis (A) by means of the angle adjusting means (28), and the optical axis (B) of the THz sensor (24) in its default position being aligned perpendicular to the extrusion axis (A), and said angle adjusting means (28) being designed to adjust the optical axis (B) - in a measuring plane (YZ) perpendicular to the extrusion axis (A); - in a vertical plane (XZ) determined by the optical axis (B) and the extrusion axis (A).

5. Measuring arrangement (5) according to claim 4, characterized in that the controller means (32) is adapted to control the angle adjusting means (28) for adjusting the THz sensor (24), the controller means (32) being designed to control the angle adjusting means (28) depending on the measuring signal (S1) in such a way that a reflected measuring signal (S1) of the boundary surfaces of the front wall (110) is at a maximum, so as to attain perpendicular irradiation of the THz transmission beam (25) onto the front wall (110).

6. Measuring arrangement (5) according to one of the above claims, characterized in that the THz sensor (24) is designed to emit the THz radiation in a frequency band of 0.01 to 50 THz, in particular, 0.05 to 5 THz, in particular, fully electronically, as a frequency modulated radar wave, or pulsed beam or by means of direct time-of-flight measurement.

7. Measuring arrangement (5) according to one of the above claims, characterized in that the optics (26) is designed to focus the THz transmission beam (25) punctiform or elliptical onto the focal point (F).

8. Measuring arrangement (5) according to one of the above claims, characterized in that the controller means (32) puts out a quality signal depending on the evaluation of the measuring signal (S1) and the determination of the wall thickness (wd), by comparing the determined wall thickness (wd) with pre-determined wall thickness reference values.

9. Measuring arrangement (5) according to one of the above claims, characterized in that it further comprises - a blow-molding extruder (1) with an extrusion head (2) and an optional pressurized air nozzle (4), between which a ring gap (8) is formed for extruding the molten tube (10), the blow-molding extruder (1) being designed to put out the molten tube along the extrusion axis (A) through the ring gap by means of the jet of compressed air from the pressurized air nozzle.

10. Measuring arrangement (5) according to claim 9, characterized in that the controller means (32) is designed to control the blow-molding extruder (1) for regulation, depending on the determined wall thickness (wd) of the molten tube (10) and / or a determined wall thickness (wd) of the finished blow-molding product (16, 116,216), in particular, by adjusting a ring gap (8) of the blow-molding extruder (1).

11. Measuring arrangement (5) according to claim 9 or 10, characterized in that it comprises a tool (12) for blow-molding the molten tube (10), the controller means (32) being designed and adapted to control the tool (12) and / or a grab (14) in such a manner that the molten tube (10) is grabbed in consecutive shaping processes always directly by the tool (12) or by means of a grab (14) and transformed into a blow-molding product (516, 116, 216).

12. Method for measuring a molten tube (10) for an extrusion blow-molding process or a blow-molding product (516, 116, 216), including at least the following steps: providing a THz measuring device (20) including a THz sensor (24) which comprises an optical axis (B), (ST0) extruding a molten tube (10) through a blow-molding extruder (1) along an extrusion axis (A) (ST1), or positioning a blow-molding product (16, 116, 216) determined by an extrusion axis (10), (ST1) emitting a THz transmission beam (25) from the THz sensor (24) along its optical axis (B) in such a manner that the front wall (110) of the molten tube (10) or the blow-molding product (16, 116, 216) lies in a divergent beam area (25b) of the THz transmission beam (25) behind the focus (F) or in a convergent beam area (25a) before the focus (F), (ST2) receiving a reflected beam (27) reflected from the molten tube (10) or the blow-molding product (16, 116, 216) and evaluating a measuring signal received in the reflected beam (27), adjusting an adjustment angle (alpha, 90°-alpha) between the optical axis (B) of the THz sensor (24) on the one hand and the extrusion axis (A) or an orthogonal to the extrusion axis (A) on the other, depending on the measuring signal (S1), in such a way that the measuring signal in the reflected beam (27) is brought to a maximum, so as to attain perpendicular irradiation of the optical axis (B) onto the front wall (110) of the molten tube (B) or the blow-molding product (16, 116, 216), and determining the wall thickness (wd) of the front wall (110), (ST4) where the focus (F) is or is being set such that a set distance (30) of the focus (F) before or behind the front wall (110) lies in an area above a Rayleigh length (FZ) and below a pre-determined multiple of the Rayleigh length (ZR).

13. Method according to claim 12, characterized in that the angular adjustment happens 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 the claims 12 or 13, characterized in that the multiple of the Rayleigh length is the 10-fold or fivefold of the Rayleigh length.

15. Method according to one of the claims 12 through 14, characterized in that, depending on an evaluation of the measuring signal (S1), a quality indication or quality signal is put out for evaluating the molten tube (10) or the blow-molding product (16, 116, 216), in particular, for determining an error of the wall thickness (wd).

16. Method for blow-mold extruding blow-molding products (16, 116, 216), where a molten tube (10) is put out by the blow-molding extruder (1) using a method according to one of the claims 12 to 15 (ST1) and measured by the THz measuring device (20) with adjustment of the adjustment angle (90°-alpha), (ST2, ST3, ST4), and subsequently, the molten tube (10) is grabbed by a tool (12) directly or via a grab (14), where the molten tube is inflated from within and its shape is determined from the outside by the tool (12), (ST5), as a continuous or dis-continuous process.