Method for determining melt sag of a pipe extruded in an extrusion apparatus

Abstract

The invention relates to a method for determining the melt sag of a pipe extruded in an extrusion device, wherein the wall thickness of the pipe is measured on the circumference of the pipe and a wall thickness profile on the circumference of the pipe is established from the measured wall thickness, and the sag of the melt is determined from the frequency and / or the amplitude of the established wall thickness profile.

Description

Technical Field

[0001] The present invention relates to a method for determining melt sinking of pipe extruded in an extrusion apparatus. Background Technology

[0002] In an extrusion apparatus, plastic pipes, for example, are extruded, where the pipes exiting the extrusion apparatus are generally conveyed along the longitudinal direction of the pipe. Here, the pipes typically pass through multiple cooling sections, in which a coolant (e.g., water) is sprayed onto the outer surface of the pipe to cool it. Directly after exiting the extrusion apparatus, the molten extruded pipe remains flowable over a wide range, i.e., not yet solidified. In the enhanced cooling process through the cooling sections, the pipe is cooled until it is fully hardened or solidified.

[0003] A method for measuring the diameter and / or wall thickness of tubing using terahertz radiation is known from WO2016 / 139155A1. This method allows for the precise determination of geometric parameters, such as diameter or wall thickness, of tubing extruded in an extrusion apparatus. However, when measuring the tubing shortly after it exits the extrusion apparatus, the determined geometric parameters may deviate from the actual geometric parameters when the tubing is fully solidified. Specifically, during the solidification process of the extruded tubing, a downward sinking of the melt due to gravity, also known as sagging, generally occurs, causing a change in the wall thickness ratio between the upper and lower regions of the tubing during cooling. This sinking cannot be completely prevented. Attempts are made to counteract sagging in a desired manner by intentionally adjusting the non-uniform wall thickness at the extrusion apparatus exit. Precise control of sagging is required. However, sagging is technically difficult to detect. Summary of the Invention

[0004] Based on the prior art described herein, the object of the present invention is to provide a method of the type described at the beginning, which, compared with the prior art, enables simpler and more accurate detection of melt sinking.

[0005] The present invention achieves the objective by means of a method for determining the melt sink of a pipe extruded in an extrusion apparatus.

[0006] For the type of method described at the beginning, the present invention achieves the objective by measuring the wall thickness of the pipe around its perimeter, establishing a wall thickness variation curve around the perimeter of the pipe based on the measured wall thickness, and determining the melt sinking based on the frequency and / or amplitude of the established wall thickness variation curve.

[0007] The pipe measured according to the invention can be, for example, a plastic pipe. The pipe is extruded in an extrusion apparatus. In an extrusion apparatus, it is known to melt the extruded material by heating and produce the melt through an extruder nozzle that traces the shape of the object to be extruded. For this purpose, the extrusion nozzle has at least one discharge opening. The extruded pipe is produced from the extrusion apparatus in the longitudinal direction and further conveyed in the longitudinal direction. Particularly during the measurement according to the invention, the pipe is conveyed in the longitudinal direction. It is feasible that, in the wall thickness measurement according to the invention, the pipe is not yet fully solidified, i.e., the pipe still has a flowable portion. The pipe may pass through one or more cooling sections after it exits the extrusion apparatus. In such cooling sections, as illustrated, for example, a coolant (such as water) is sprayed onto the outer surface of the pipe for cooling. Directly after exiting the extrusion apparatus and also through further conveying sections, the pipe is not yet fully solidified and accordingly still has a flowable portion in molten form. As it passes through the cooling sections, the pipe is gradually further solidified until it reaches its final shape. After passing through the first cooling section, the pipe is generally not yet fully solidified and therefore still has a flowable portion.

[0008] As explained, during the solidification process of the tube, a downward sinking of the melt, known as sag, occurs. Also as explained, sag cannot be completely avoided; however, the melt is directly formed into a tube at the head of the extruder nozzle. Sag can be anticipated by intentionally producing the tube at the outlet of the extruder with a greater wall thickness in the upper region than in the lower region. Ideally, the temperature of the melt in the extruder should be set as high as possible, as this corresponds to a high output and corresponding productivity of the extruder. On the other hand, the risk of uncontrolled melt sinking in the lower region of the tube and therefore unacceptable shape deviations in the fully cooled tube increases with the melt temperature.

[0009] An adjusting element, such as a plate-shaped adjusting element, is typically provided at the head of the extruder nozzle of such an extrusion apparatus. This adjusting element is generally arranged near at least one discharge opening of the extruder nozzle. The wall thickness at the outlet of the extruder nozzle can be adjusted at multiple locations around the periphery of the tube using the adjusting element. Commonly used extruder nozzles may have, for example, 10, 16, or 20 such adjusting elements, particularly located in the region of the inner tube wall. Each of these adjusting elements can be mechanically calibrated and / or equipped with a heater to achieve wall thickness variation in this region. Higher tube temperatures result in stronger flow characteristics. It is also conceivable to supply more or less lubricant to these adjusting elements to suitably adjust the tube wall thickness around the periphery.

[0010] This invention is based on the unexpected understanding that the aforementioned regulating element causes modulation of the wall thickness around the periphery of the tube, which can still be identified by measurement techniques, particularly by precise wall thickness measurements, such as terahertz radiation-wall thickness measurements, even after the wall has cooled, especially within the tube itself, as will be explained in more detail below. Specifically, the wall thickness variation curve exhibits frequency modulation and / or amplitude modulation that can be detected by measurement techniques. The inventors started from the fact that this modulation is caused by the regulating element. The wall thickness variation curve can be modulated, in particular, according to a periodic function, such as a cosine or sine function. Although the amplitude of the modulation of the wall thickness is very small. For example, in the case of an average wall thickness of approximately 10 mm, the amplitude of the modulation of the wall thickness is approximately 10 µm, correspondingly approximately 0.1%. However, the modulation can be reliably identified by means of a correspondingly precise wall thickness measurement method.

[0011] This invention is also based on the understanding that the frequency modulation and / or amplitude modulation of the wall thickness variation curve changes according to the sinking of the melt. If, for example, a periodic wall thickness variation curve with the same frequency and a defined amplitude on the periphery of the tube exists directly at the outlet of the extruder nozzle of the extrusion equipment, then during the process of melt sinking that occurs during tube cooling, not only the frequency but also the amplitude of the wall thickness variation curve can change. Due to the sinking of the melt, so-called compression of the wall thickness variation curve occurs in the lower region of the tube, i.e., a larger frequency of modulation of the wall thickness variation curve. Furthermore, a decrease in the modulation amplitude of the wall thickness variation curve also occurs during the tube cooling process. The reason for this is presumably that, due to the solidification of the tube material, particularly from the outside in the cooling section, the modulation of the melt, which is still almost completely present, first solidifies in its original phase, while the portion of the melt still existing in a liquid state sinks downward due to gravity. This may result in a corresponding decrease in the amplitude of the wall thickness variation curve.

[0012] Based on this, the teaching of the present invention is to detect the degree of melt sinking from top to bottom by evaluating the frequency and / or amplitude of the wall thickness variation curve measured on the periphery of the pipe. According to the present invention, this can be achieved in a simpler and more accurate manner compared to the prior art. Of course, it is not mandatory to directly determine or evaluate the frequency of the wall thickness variation curve, for example. For example, parameters determined by the frequency, such as the wavelength or phase or phase shift of the wall thickness variation curve, can also be determined and used for evaluation. The corresponding principle applies to amplitude.

[0013] Therefore, according to the present invention, the wall thickness of the pipe is measured around its perimeter. Here, the wall thickness can be measured continuously or at discrete circumferential intervals around the perimeter of the pipe. Preferably, the wall thickness is measured over the entire perimeter of the pipe, i.e., a 360° angular range. However, it is also conceivable to measure the wall thickness only on a portion of the entire perimeter, particularly the portion characterizing melt sinking, such as the upper or lower quarter, or the upper or lower half of the perimeter, where melt sinking plays a role in the frequency and / or amplitude of the wall thickness variation curve. The measured wall thickness forms a wall thickness variation curve around the perimeter of the pipe, i.e., a curve showing the wall thickness around the perimeter. If the wall thickness is not measured continuously but at discrete intervals around the perimeter, interpolation can be performed between the measurement points to establish a curve showing the wall thickness variation curve. By evaluating the frequency and / or amplitude of the established wall thickness variation curve, melt sinking can be accurately determined technically.

[0014] As explained, frequency modulation and / or amplitude modulation of the wall thickness variation curve can be induced, in particular, by an adjustment element for the wall thickness at the outlet of the extrusion apparatus. In this case, a wall thickness variation curve with periodic amplitude modulation may exist, for example, at the outlet of the extrusion nozzle of the extrusion apparatus. This wall thickness variation curve then varies in response to melt sinking, which is measured and evaluated according to the invention. However, the invention can also be applied to extrusion apparatuses with other devices for determining wall thickness, particularly those without such adjustment elements. Variations in other characterizing wall thickness fluctuations present at the outlet of the extrusion apparatus due to melt sinking can then be evaluated. According to the invention, such wall thickness fluctuations can also be introduced intentionally. For example, it is conceivable to form defined circumferential sections with increasing or decreasing wall thickness by appropriately adjusting the wall thickness at the outlet of the extrusion apparatus. For this purpose, at least one corresponding marking element may be provided at the outlet of the extrusion apparatus, which causes an increase or decrease in wall thickness. When the melt sinks, a technically identifiable change in the wall thickness variation curve appears in the region where the wall thickness increases or decreases; for example, the width of the wall segment where the wall thickness may increase or decrease may change. Therefore, an amplitude change appears in the wall thickness variation curve measured and established according to the present invention. The sinking of the melt can then be inferred from this.

[0015] Generally, an extrusion apparatus may have at least one element at its outlet that causes a characterizing property of the wall thickness, which is identifiable or can be identified in a wall thickness variation curve established according to the invention.

[0016] According to one design scheme, melt sinking can be inferred by comparing an established wall thickness variation curve with a reference wall thickness variation curve. Specifically, melt sinking can be inferred by comparing the frequency and / or amplitude of the established wall thickness variation curve with that of the reference wall thickness variation curve. The reference wall thickness variation curve can be determined by measurement or theoretically (particularly computationally). The reference wall thickness variation curve can exist, in particular, for the same peripheral section as the established wall thickness variation curve. If the wall thickness variation curve established according to the invention is established over the entire perimeter of the pipe, then the reference wall thickness variation curve can also exist over the entire perimeter of the pipe. The comparison between the established wall thickness variation curve and the reference wall thickness variation curve particularly simplifies the quantitative determination of melt sinking. The change in the wall thickness variation curve caused by sinking can be determined in a particularly simple manner.

[0017] In a particularly practical manner, the reference wall thickness variation curve can be a periodic reference wall thickness variation curve, such as a sine or cosine form. In any case, when the wall thickness adjustment element is present and uniformly arranged, such a periodic reference wall thickness variation curve should be expected directly at the outlet of the extruder nozzle of the extrusion equipment. This periodic reference wall thickness variation curve is particularly suitable as a raw value for comparison according to the invention to determine sag. In particular, when no such adjustment element is provided, the reference wall thickness variation curve can also be other reference wall thickness variation curves, such as those exhibiting characteristic wall thickness variations that are selectively introduced when necessary.

[0018] The reference wall thickness variation curve can be, correspondingly, the expected or measured reference wall thickness variation curve directly at the outlet of the extrusion equipment, particularly directly at the outlet of the extruder nozzle of the extrusion equipment. As already illustrated, a periodic variation curve, particularly one with a relatively large amplitude, can be expected there. If a subsequently measured wall thickness variation curve deviates from the reference wall thickness variation curve (e.g., deviates from the periodicity of the reference wall thickness variation curve) or has a changed amplitude (e.g., smaller than that of the reference wall thickness variation curve), this is a qualitative and quantitative indicator of melt sinking.

[0019] According to another design scheme, a deviation curve can be established by comparing the established wall thickness variation curve with a reference wall thickness variation curve, particularly on the periphery of the pipe. Therefore, this deviation curve depicts the deviation between the established wall thickness variation curve and the reference wall thickness variation curve, specifically in the form of a curve. The deviation curve can, for example, represent the wall thickness variation and / or phase variation and / or frequency variation and / or amplitude variation between the established wall thickness variation curve and the reference wall thickness variation curve. The deviation curve, or the variation curve corresponding to the deviation curve, can be used in a particularly suitable manner as an input parameter for adjusting the extrusion equipment and / or at least one cooling section located downstream of the extrusion equipment, so as to achieve the desired wall thickness variation curve at the measurement location and / or in the fully cured state of the pipe.

[0020] The wall thickness of the pipe at its periphery can be measured downstream of the first cooling section of the pipe used from the extrusion equipment. In particular, the measurement can be performed after the first cooling section and before the second cooling section. Thus, although the pipe is partially, especially on its outer side, cooled and solidified, the pipe generally has a still flowable melt portion inside.

[0021] Furthermore, the determined melt sink can be used to predict further melt sink until the pipe is fully cured. This can be done, for example, by comparing it with a previously determined wall thickness variation curve for a fully cured pipe. Thus, reliable conclusions about the wall thickness geometry in the fully cured state can be given even in the incompletely cured state of the pipe.

[0022] According to another design, changes in at least one process parameter of the extrusion equipment and / or at least one cooling section located downstream of the extrusion equipment can be identified based on the determined melt sink. Therefore, the determined sink is an important indicator if unexpected changes occur during production, such as coolant failure or temperature rise in the extrusion equipment and / or the cooling section located downstream of the extrusion equipment. According to the invention, such unexpected changes in the production process can be identified early, and appropriate responses can be made.

[0023] According to another design scheme, at least one control parameter of the extrusion equipment and / or at least one cooling section located downstream of the extrusion equipment can be changed based on the determined melt sink. In this way, a stable production process can be regulated, or optimized process conditions can be adjusted to achieve such a stable production process.

[0024] The at least one control parameter can be changed using an established deviation variation curve according to a particularly practical design scheme. This deviation variation curve is particularly well-suited as a control variable for automated regulation.

[0025] According to another design, the at least one control parameter can also be changed using a phase-locked loop (PLL). A PLL is a regulation method in which the phase or frequency of a variable oscillator is influenced by a closed regulation loop, such that the phase deviation between an external periodic reference signal and the oscillator or a signal derived therefrom is kept as constant as possible. The reference frequency of the PLL can, for example, correspond to the number of regulating elements at the extruder orifice of an extrusion device. The frequency of a voltage-controlled oscillator (VCO) is adjusted using the phase deviation between an established wall thickness variation curve and a reference wall thickness variation curve, with the reference frequency employed by a phase detector. The control voltage of the VCO maps the frequency change, i.e., specifically the deviation variation curve. The phase detector can provide, for example, the deviation variation curve between the measured wall thickness variation curve and the reference wall thickness variation curve as an output signal. Using such a PLL, the at least one control parameter can be changed in a particularly suitable manner under current conditions. Of course, other methods can also be considered, such as using a bandpass filter with a narrow bandwidth, allowing only the modulation frequency to pass through. A frequency discriminator can also be considered. A bandpass filter can be used, for example, in combination with a downstream phase detector, wherein the phase detector compares the phase of the measured wall thickness variation curve filtered by the bandpass filter with the phase of a reference wall thickness variation curve and outputs a phase difference. This phase difference can then be used as a basis for changing the at least one control parameter. Alternatively, a bandpass filter with a downstream frequency discriminator can be used, wherein the frequency discriminator compares the frequency of the measured wall thickness variation curve filtered by the bandpass filter with the frequency of a reference wall thickness variation curve and outputs a frequency difference. This, in turn, can be used as a basis for changing the at least one control parameter.

[0026] The at least one control parameter may be, for example, the extrusion power of the extrusion equipment and / or the melt temperature in the extrusion equipment and / or the temperature and / or position of the regulating element of the extrusion equipment, which determines the geometry of the tube at the outlet of the extrusion equipment.

[0027] According to another design scheme, to measure the wall thickness of a pipe, terahertz radiation is emitted onto the pipe, the terahertz radiation reflected by the pipe is detected, and the wall thickness of the pipe is determined by the intensity of the detected terahertz radiation. In this design, terahertz radiation is emitted onto the pipe. The terahertz radiation can partially penetrate into the pipe. The terahertz radiation is reflected at the (external and, if necessary, internal) boundary surfaces of the pipe and detected by a suitable detector. The frequency of the terahertz radiation can, for example, be in the range of 10 GHz to 3 THz. The terahertz radiation can be so-called millimeter waves. The transmitter emitting the terahertz radiation and the detector receiving the reflected terahertz radiation can be positioned substantially in the same location. The transmitter and detector can, for example, be integrated into a transceiver. Terahertz radiation can be used to reliably determine geometric parameters, especially in challenging process environments where optical systems, such as lasers, are difficult to use. Furthermore, this measurement method provides sufficient accuracy to reliably detect the frequency modulation and / or amplitude modulation of the wall thickness variation curve as evaluated according to the present invention. The wall thickness is determined using terahertz radiation, for example, as described in WO2016 / 139155A1. Refer to that document accordingly.

[0028] Terahertz radiation can be modulated continuous-wave terahertz radiation, particularly frequency-modulated continuous-wave terahertz radiation. Terahertz radiation can also be pulse-modulated or phase-modulated. Frequency modulation can include one or more frequency bursts. In particular, so-called frequency sweeps can be performed, in which a predetermined frequency range is traversed once or multiple times. Pulse- or phase-modulated terahertz radiation can be transmitted, for example, using so-called time-domain reflection (VRF) or frequency-domain reflection (VRF). It is also conceivable to transmit multiple discrete frequencies instead of a spectrum.

[0029] The wall thickness of the tube can be determined by measuring the propagation time of the emitted terahertz radiation reflected by the tube, as described, for example, in WO2016 / 139155A1.

[0030] According to another design, at least one transmitter for emitting terahertz radiation and at least one detector for detecting the emitted terahertz radiation reflected by the tube rotate about the longitudinal axis of the tube, preferably along a circular trajectory, during the emission and detection of the terahertz radiation. By rotating or moving the pair of transmitter and detector (e.g., transceiver), values ​​for the wall thickness distributed around the perimeter of the tube can be detected. Of course, it is also conceivable to arrange multiple pairs of transmitters and receivers distributed around the perimeter of the tube, and to determine multiple measurements around the perimeter in this way. Attached Figure Description

[0031] Embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. The drawings schematically illustrate:

[0032] Figure 1 A schematic side view shows an apparatus for implementing the method according to the invention;

[0033] Figure 2 Show Figure 1 A sectional view of the equipment in the image;

[0034] Figure 3 Show Figure 1 and 2 The cross-sectional view of the pipe shown is used to illustrate the sinking of the melt; and

[0035] Figure 4 Three diagrams are shown to illustrate the method according to the present invention. Detailed Implementation

[0036] Unless otherwise stated, the same reference numerals in the various figures denote the same objects.

[0037] exist Figure 1 and Figure 2 The image shows a pipe 10 (here, a plastic pipe 10), which has a wall 12, a cavity 14 defined by the pipe 10, an outer surface 16 that is circular in cross-section, and an inner surface 18 that is also circular in cross-section, the inner surface defining the cavity 14. In this example, the plastic pipe 10 is extruded in an extrusion apparatus 20 by means of an extruder and conveyed along its longitudinal axis by means of a suitable conveying device. Figure 1The tube 10 is conveyed from left to right. After exiting the extruder nozzle of the extrusion unit 20, the tube 10 first passes through a first cooling section 22, where it is cooled. The tube 10, which has been intensely heated and is not yet fully solidified (i.e., still has a flowable portion, i.e., melt), exits the extrusion unit 20. In a further process, the tube 10 passes through a measuring device 24, where the wall thickness of the tube 10 is determined around its perimeter in a manner described in more detail below. After the measuring device 24, the tube 10 passes through another cooling section 26 for further cooling. After the tube 10 is fully solidified, it is cut to predetermined lengths, for example, in a length-cutting device 28.

[0038] With the help of the appendix Figure 2 The structure and function of the measuring device 24 are described in more detail. In the example shown, the measuring device 24 includes a transceiver 30, in which a transmitter and a detector for terahertz radiation are combined. The transmitter emits terahertz radiation 32 onto 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, returning to the transceiver 30, where it is detected by the detector. The transceiver 30 is also connected to an evaluation device 38 via line 36. The reflected radiation received by the detector generates a corresponding measurement signal, which is further transmitted to the evaluation device 38 via line 36. The evaluation device 38 can determine the terahertz radiation in this way. Figure 2 The wall thicknesses of 40 and 42, as indicated by the bid, are determined, for example, by means of propagation time measurement.

[0039] The measuring device 24 rotates around the longitudinal axis of the pipe during the measurement of, for example, wall thickness 40, wherein the wall thickness is measured continuously or at discrete intervals over the entire periphery of the pipe 10 and thereby establishes a wall thickness variation curve over the periphery of the pipe.

[0040] exist Figure 3 The tube 10 is shown in a cross-sectional view, where the boundary surfaces between adjacent plate-shaped regulating elements at the extruder orifice of the extrusion apparatus 20 are shown by rays 46 drawn at regular angular intervals. The regulating elements trace the wall geometry in the extruded tube, particularly in the region of the inner tube wall, until the tube cools. Without melt sinking, these regulating elements must trace the inner wall 18 of the tube 10 according to the initial spacing indicated by the rays 46. In practice, due to melt sinking (descending) during the cooling process, the area traced by the regulating elements is displaced from the angular position marked φ0 (corresponding to the upper side of the tube 10), particularly initially stretched, as shown in… Figure 3As indicated by rays 46' and region 48 at angular positions φ1, φ2 and φ3, and then compressed until the underside of the tube 10.

[0041] This effect is Figure 4 The diagram shows the wall thickness variation curves around the perimeter of the pipe, specifically from 0° to +180° and from 0° to -180°, where 0° represents the top side of the pipe. Figure 4 In the middle, the wall thickness at the circumferential corner is plotted in two upper diagrams. Figure 4 In the topmost graph, a reference wall thickness variation curve 50 is shown. In the example shown, this curve is a cosine-shaped variation curve with a constant frequency and amplitude. The reference wall thickness variation curve is the variation curve expected directly at the outlet of the extruder nozzle of the extrusion unit 20. For clarity, in... Figure 4 The adjustment elements 51 of the extruder orifice and the boundary surfaces 52 formed between the adjustment elements are marked. The frequency of the reference wall thickness variation curve 50 corresponds to the frequency of the adjustment elements 51, or boundary surfaces 52, which are uniformly distributed around the periphery of the extruder orifice.

[0042] Figure 4 The diagram in the middle of the image schematically shows the measurement device 24. Figure 1 The wall thickness variation curve 54, measured at the measurement location shown, is located on the periphery of the pipe 10. It can be seen that the amplitude of the measured wall thickness variation curve 54 is smaller than the amplitude of the reference wall thickness variation curve 50. Furthermore, it can be seen that as the circumferential angle increases downwards from the uppermost position at 0° on the pipe, the wall thickness variation curve 54... Figure 3 The movement of ray 46' shown in the figure corresponds to the frequency deviation from the reference wall thickness variation curve 50. In particular, the frequency first decreases until the angle position φ3, and then the frequency increases until the lower side of the pipe at 180°.

[0043] exist Figure 4 The bottommost chart shows a deviation curve 56 established by comparing the measured wall thickness variation curve 54 with a reference wall thickness variation curve 50. The deviation curve shows the phase shift φ of the measured wall thickness variation curve 54 relative to the reference wall thickness variation curve 50. The deviation curve 54 can form the output of a phase detector, based on which at least one control parameter of the extrusion equipment and / or the first cooling section 22 or another cooling section 26 is modified to produce a desired wall thickness variation curve, such as a periodic wall thickness variation curve, at the measurement position of the measuring device 24 or when the tube 10 is fully cooled.

[0044] For example, it is also possible to predict further melt sinking until the pipe 10 is fully solidified by means of a wall thickness variation curve 54 established at the measurement position of the measuring device 24 (at which the pipe 10 generally still has a fusible portion), for example by comparing it with a wall thickness variation curve established in the corresponding fully solidified state of the pipe 10.

[0045] List of reference numerals

[0046] 10 pipes

[0047] 12 walls

[0048] 14 cavity

[0049] 16 outer surfaces

[0050] 18 Inner Wall

[0051] 20 Extrusion Equipment

[0052] Cooling sections 22 and 26

[0053] 24 Measuring Equipment

[0054] 28 fixed-length cutting device

[0055] 30 transceivers

[0056] 32 terahertz radiation

[0057] 34 reflectors

[0058] Route 36

[0059] 38 evaluation devices

[0060] 40, 42 wall thickness

[0061] 46, 46' rays

[0062] 48 areas

[0063] 50 Reference wall thickness variation curve

[0064] 51 Adjustment Element

[0065] 52 boundary surfaces

[0066] 54 Wall thickness variation curve

[0067] 56 Deviation Change Curve

Claims

1. A method for determining melt sinking of a tube (10) extruded in an extrusion apparatus (20), characterized in that, The wall thickness (40) of the pipe (10) is measured around the periphery of the pipe (10), and a wall thickness variation curve (54) is established around the periphery of the pipe (10) based on the measured wall thickness (40). The wall thickness variation curve is modulated according to a periodic function. The frequency modulation of the wall thickness variation curve is caused by at least one adjustment element, and the sinking of the melt is inferred from the comparison of the frequency of the established wall thickness variation curve with the frequency of a reference wall thickness variation curve (50), which is a periodic reference wall thickness variation curve.

2. The method according to claim 1, characterized in that, The reference wall thickness variation curve (50) is the reference wall thickness variation curve (50) expected or measured directly at the outlet of the extrusion equipment (20).

3. The method according to claim 1 or 2, characterized in that, The deviation curve (56) is established by comparing the established wall thickness variation curve (54) with the reference wall thickness variation curve (50).

4. The method according to claim 1 or 2, characterized in that, The wall thickness (40) of the tube (10) is measured on the periphery of the tube (10) downstream of the first cooling section (22) for the tube (10) from the extrusion equipment (20).

5. The method according to claim 1 or 2, characterized in that, The determined melt sink is used to predict the further melt sink expected until the pipe (10) is fully cured.

6. The method according to claim 1 or 2, characterized in that, The variation of at least one process parameter of the extrusion equipment (20) and / or at least one cooling section located downstream of the extrusion equipment (20) is identified based on the determined melt sink.

7. The method according to claim 1 or 2, characterized in that, Based on the determined melt sinking, change at least one control parameter of the extrusion equipment (20) and / or at least one cooling section located downstream of the extrusion equipment (20).

8. The method according to claim 7, characterized in that, The deviation curve (56) is established by comparing the established wall thickness variation curve (54) with the reference wall thickness variation curve (50).

9. The method according to claim 8, characterized in that, The at least one control parameter is changed by means of the established deviation change curve (56).

10. The method according to claim 7, characterized in that, The at least one control parameter is changed by means of a phase-locked loop.

11. The method according to claim 7, characterized in that, The at least one control parameter is the ejection power of the extrusion device (20) and / or the melt temperature in the extrusion device (20) and / or the temperature and / or position of the regulating element (51) of the extrusion device (20), which determines the geometry of the tube (10) at the outlet of the extrusion device (20).

12. The method according to claim 1 or 2, characterized in that, In order to measure the wall thickness (40) of the tube (10), terahertz radiation (32) is emitted onto the tube (10) at the periphery of the tube (10), the terahertz radiation (32) reflected by the tube (10) is detected, and the wall thickness (40) at the periphery of the tube (10) is determined by the detected terahertz radiation (32).

13. The method according to claim 12, characterized in that, The terahertz radiation (32) is modulated continuous wave terahertz radiation, and / or the terahertz radiation (32) is pulse-modulated terahertz radiation or phase-modulated terahertz radiation.

14. The method according to claim 13, characterized in that, The terahertz radiation (32) is frequency-modulated continuous wave terahertz radiation.

15. The method according to claim 12, characterized in that, The wall thickness (40) of the tube (10) is determined by measuring the propagation time of the terahertz radiation (32) emitted and reflected by the tube (10).

16. The method according to claim 12, characterized in that, At least one transmitter for emitting terahertz radiation (32) and at least one detector for detecting the emitted terahertz radiation (32) reflected by the tube (10) rotate around the longitudinal axis of the tube (10) during the emission and detection of terahertz radiation.

17. The method according to claim 16, characterized in that, The at least one transmitter and the at least one detector rotate along a circular trajectory during the emission and detection of terahertz radiation.

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