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

Abstract

A method for determining sagging of melt of a tube extruded in an extrusion device includes structuring a measuring device to rotate about the tube. The measuring device is configured to measure a wall thickness of the tube over a circumference of the tube and determine a wall-thickness profile over the circumference of the tube from the measured wall thicknesses. The wall-thickness profile includes a frequency and an amplitude. The method further includes determining a sagging of the melt from at least one of (i) the frequency and (ii) the amplitude of the wall-thickness profile.

Claims

1. A method for determining sagging of melt of a tube extruded in an extrusion process, comprising: measuring a wall thickness of the tube over a circumference of the tube; determining a wall-thickness profile over the circumference of the tube from the measured wall thicknesses, wherein the wall-thickness profile includes a frequency; comparing the wall-thickness profile with a reference wall-thickness profile; determining the sagging of the melt from a change in the frequency between the reference wall-thickness profile and the determined wall-thickness profile; and applying a change to at least one control parameter of the extrusion process based on the determined sagging of the melt to control the sagging of the melt.

2. The method according to claim 1, wherein the reference wall-thickness profile comprises a periodic reference wall-thickness profile.

3. The method according to claim 1, wherein the reference wall-thickness profile comprises a measured reference wall-thickness profile directly at an outlet of an extrusion device used in the extrusion process.

4. The method according to claim 1, further comprising generating a deviation profile from the comparison between the wall-thickness profile and the reference wall-thickness profile.

5. The method according to claim 4, further comprising: measuring the wall thickness of the tube over the circumference of the tube at a position downstream of a first cooling section; and cooling the tube emerging from an extrusion device via the first cooling section.

6. The method according to claim 5, further comprising identifying the at least one control parameter of at least one of (i) the extrusion device and (ii) the first cooling section based on the determined sagging of the melt.

7. The method according to claim 6, further comprising changing the at least one control parameter based on the deviation profile.

8. The method according to claim 6, further comprising changing the at least one control parameter by means of a phase-locked loop.

9. The method according to claim 6, wherein the at least one control parameter is at least one of: (i) an output capacity of the extrusion device; (ii) a melt temperature in the extrusion device; (iii) a temperature; and (iv) a position of setting elements of the extrusion device determining a geometry of the tube at an outlet of the extrusion device.

10. The method according to claim 1, further comprising predicting further sagging of the melt before the tube completely solidifies from the determined sagging of the melt.

11. The method according to claim 1, wherein the measuring the wall thickness of the tube over the circumference of the tube comprises: emitting terahertz radiation towards the tube and over the circumference of the tube; detecting the terahertz radiation reflected by the tube; generating measuring signals corresponding to the detected terahertz radiation; and determining the wall thickness over the circumference of the tube based on a propagation time of the measuring signals.

12. The method according to claim 11, wherein the terahertz radiation comprises modulated continuous wave terahertz radiation.

13. The method according to claim 11, wherein: at least one transmitter is configured to emit the terahertz radiation; and at least one detector is configured to detect the terahertz radiation emitted and reflected by the tube, wherein the at least one transmitter is configured to be rotated about a longitudinal axis of the tube during the emission and detection of the terahertz radiation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An exemplary embodiment of the invention is explained below in greater detail with reference to figures.

(2) FIG. 1 is a schematic side view of an embodiment of a device for carrying out the method according to the invention.

(3) FIG. 2 is a partial sectional view of the embodiment of FIG. 1.

(4) FIG. 3 is a cross-sectional view of an embodiment of a tube from FIGS. 1 and 2 illustrating sagging of melt.

(5) FIG. 4 shows three diagrams to illustrate the method according to the invention.

(6) The same reference numbers refer to the same objects in the figures unless indicated otherwise.

DETAILED DESCRIPTION OF THE INVENTION

(7) A tube 10, in the present case a plastic tube 10, is depicted in FIGS. 1 and 2, which has a wall 12, a hollow space 14 delimited by the tube 10, an outer surface 16 which is circular in cross-section and an inner surface 18 which is likewise circular in cross-section, which delimits the hollow space 14. The plastic tube 10 is, in the present example, extruded with the aid of an extruder in an extrusion device 20 and conveyed along its longitudinal axis by means of a suitable conveying apparatus, from left to right in FIG. 1. After exiting from an extruder nozzle of the extrusion device 20, the tube 10 initially passes through a first cooling section 22, in which the tube 10, which exits the extrusion device 20 heated to a great extent and not yet completely solidified, that is to say still having flowable components (melt), is cooled down. Further along in the procedure, the tube 10 passes through a measuring device 24, in which the wall thickness of the tube 10 is determined over the circumference of the tube 10 in a manner explained in more detail below. Following the measuring device 24, the tube 10 passes through further cooling sections 26, in which further cooling occurs. After the tube 10 has completely solidified, it is cut to predefined lengths, for example in a cutting-to-length apparatus 28.

(8) The structure and the function of the measuring device 24 are to be explained in greater detail with reference to FIG. 2. In the depicted example, the measuring device 24 comprises a transceiver 30, in which a transmitter and a detector for terahertz radiation are combined. The transmitter emits terahertz radiation 32 toward the tube 10. The terahertz radiation is reflected at different boundary surfaces of the tube 10 and at a reflector 34 arranged opposite the transceiver 30 and travels back to the transceiver 30 where it is detected by the detector. The transceiver 30 is, furthermore, connected to an evaluating apparatus 38 via a line 36. The reflected radiation received by the detector generates corresponding measuring signals which are forwarded to the evaluating apparatus 38 via the line 36. In this way, the evaluating apparatus 38 can determine the wall thicknesses 40, 42 drawn in in FIG. 2 for example, using propagation time measurements.

(9) In this case, the measuring device 24 is rotated about the longitudinal axis of the tube during the measurement, for example, of the wall thickness 40, wherein the wall thickness is measured continuously or at discrete distances over the complete circumference of the tube 10 and a wall-thickness profile over the circumference of the tube is produced from this.

(10) The tube 10 is depicted in a cross-section in FIG. 3, wherein boundary surfaces between adjacent plate-shaped setting elements of the extruder nozzle of the extrusion device 20 are illustrated by the rays 46 drawn in at regular angular distances. The setting elements are modeled in the wall geometry in the extruded tube before it cools down, in particular in the region of the inner tube walls. Without sagging of the melt occurring, these setting elements would be reflected on the inner wall 18 of the tube 10 corresponding to their original distances according to the rays 46. Actually, due to the sagging of the melt while it cools down, a shift of the regions modeled by the setting elements occurs, starting from the angular position designated with 0, accordingly the upper side of the tube 10 in particular initially stretches, as marked in FIG. 3 by the rays 46 at the angular positions 1, 2 and 3 and the regions 48, and subsequently a compressing to the underside of the tube 10 occurs.

(11) This effect is depicted in FIG. 4 for a wall-thickness profile over the circumference of the tube, in particular from 0 to +180 and 0 to 180, wherein 0 is the upper side of the tube. In FIG. 4, in the two top diagrams, the wall thickness is plotted in each case over the circumferential angle. In the uppermost diagram in FIG. 4, a reference wall-thickness profile 50 is shown, in the depicted example a cosine-shaped profile with constant frequency and amplitude. The reference wall-thickness profile is the profile expected directly at the exit from the extruder nozzle of the extrusion device 20. For illustration, the setting elements 51 of the extruder nozzle and the boundary surfaces 52 formed between them are drawn in in FIG. 4. The frequency of the reference wall-thickness profile 50 corresponds to the frequency of the setting elements 51 or, respectively, boundary surfaces 52 of the extruder nozzle distributed uniformly over the circumference.

(12) The middle diagram in FIG. 4 schematically shows a wall-thickness profile 54 measured over the circumference of the tube 10 at the measuring position of the measuring device 24 shown in FIG. 1. On the one hand, it can be seen that the amplitude of the measured wall-thickness profile 54 is smaller than the amplitude of the reference wall-thickness profile 50. It can also be seen that, as the circumferential angle increases, starting from the uppermost position of the tube at 0 downward, corresponding to the shift of the rays 46 shown in FIG. 3, a deviation of the frequency from the reference wall-thickness profile 50 occurs, in particular initially a decrease in the frequency until the angular position .sub.3 and then an increase in the frequency until the underside of the tube at 180.

(13) In the lowermost diagram in FIG. 4, a deviation profile 56 produced from a comparison between the measured wall-thickness profile 54 and the reference wall-thickness profile 50 is depicted. The deviation profile shows a phase shift of the measured wall-thickness profile 54 compared to the reference wall-thickness profile 50. The deviation profile 56 can form the output of a phase detector, on the basis of which at least one control parameter of the extrusion device and or the first cooling section 22 or also the additional cooling sections 26 is changed in order to generate a desired wall-thickness profile, for example, a periodic wall-thickness profile, at the measurement location of the measuring device 24 or, respectively, in the completely cooled-down state of the tube 10.

(14) It is also possible, for example, to predict, for example by comparing with a wall-thickness profile produced in the completely solidified state of a corresponding tube 10, further sagging of the melt to be expected before the tube 10 completely solidifies using the wall-thickness profile 54 produced at the measurement location of the measuring device 24, at which the tube 10 regularly still has flowable components.

LIST OF REFERENCE SIGNS

(15) 10 Tube 12 Wall 14 Hollow space 16 Outer surface 18 Inner wall 20 Extrusion device 22, 26 Cooling section 24 Measuring device 28 Cutting-to-length apparatus 30 Transceiver 32 Terahertz radiation 34 Reflector 36 Line 38 Evaluation apparatus 40,42 Wall thickness 46,46 Rays 48 Regions 50 Reference wall-thickness profile 51 Setting elements 52 Boundary surfaces 54 Wall-thickness profile 56 Deviation profile