Method and measuring arrangement for determining the position of a frost limit during the production of a tubular film made of thermoplastic material

Abstract

The invention relates to a method for determining the position of a frost limit (11) of a tubular film (2) made of thermoplastic material during the production thereof, comprising the following method steps: detecting a temperature curve (33) of the tubular film (2) along a first measurement section (M1) along the production axis (P);ascertaining a position along the first measurement section (M1), in which position an absolute value of a temperature gradient of the temperature curve (33) in the production direction (R) does not meet a predetermined limit value, and fixing the position as the first preliminary position (h1) of the frost limit (11);detecting a diameter curve (34) of the tubular film (2) along a second measurement section (M2) along the production axis (P);ascertaining a position along the second measurement section (M2), in which position an absolute value of a diameter gradient of the diameter curve (34) in the production direction (R) does not meet a predetermined limit value, and fixing the position as the second preliminary position (h2) of the frost limit (11); and checking a plausibility of the first preliminary position (h1) and the second preliminary position (h2) of the frost limit (11) on the basis of the mutual spacing (Δh) thereof.

Description

[0001] Method and measuring arrangement for determining the position of a freezing point during the production of a tubular film made of thermoplastic material

[0002] Description

[0003] The invention relates to a method for determining the position of a freezing point of a tubular film made of thermoplastic material emerging from a blowing head and drawn off in a production direction along a production axis during its production. The method comprises the steps of recording a temperature profile of the tubular film along a first measuring section along the production axis, determining a position along the first measuring section at which an absolute value of a temperature gradient of the temperature profile in the production direction falls below a predetermined limit, and defining the position as the first provisional position of the freezing point.

[0004] The invention further relates to a measuring device for determining the position of a freezing point of a tubular film made of thermoplastic material emerging from a blowing head and drawn off in a production direction along a production axis during the production of the tubular film. The measuring device comprises a temperature measuring unit configured to record a temperature profile of the tubular film along a first measuring section along the production axis, and a processing unit configured to determine a position along the first measuring section at which an absolute value of a temperature gradient of the temperature profile in the production direction falls below a predetermined limit value, and to establish the position as the first provisional position of the freezing point.

[0005] During the production of a tubular film, plastic melt is extruded from an extrusion tool (blow head) with an annular channel die and drawn in the direction of a production axis. Immediately after exiting the annular channel die, the plasticized, plastically formable and expandable hot tubular film is blown on the outer and often also the inner circumference with cooling air from a cooling ring or an internal cooling device and cooled. The tubular film bubble is guided in the direction of the production axis over a device for guiding the film tube (often also referred to as a guide device, calibration device or calibration basket) and a flattening device. In a draw-off unit, it is squeezed off as a flat tube by conveyor rollers and drawn off. The calibration device has film guide elements distributed over the circumference, which serve to guide the film tube.The film guide elements can, for example, have guide rollers that lie tangentially against the tubular film and guide it. The film guide elements are usually radially adjustable transversely to the production axis of the tubular film and can thus be adapted to tubular films of different diameters. The point at which the tubular film freezes, i.e., the transition of the plastic melt in the tubular film from a plasticized and expandable state to a no longer expandable state, is referred to as the freezing point. To prevent the film guide elements from leaving marks on the tubular film, the calibration device must be positioned downstream of the freezing point; in the case of a tubular film pulled vertically upwards, this position must be above the freezing point.This ensures that the film guide elements of the calibration device touch the tubular film in an area where the plastic melt has already solidified, preventing the film guide elements from leaving marks on the tubular film. Therefore, knowledge of the position of the freezing point is important during tubular film production, and this is often estimated by operating personnel.

[0006] The freezing point does not correspond to a specific temperature (solidification temperature). The solidification point of the melt depends heavily on the formulation of the thermoplastic material and the thickness of the tubular film. The formulation determines the solidification temperature, and the local thickness around the circumference of the tubular film determines the height at which the solidification temperature is reached.

[0007] The freezing points, which vary slightly around the circumference of the tubular film due to local thickness variations, form the so-called freezing point, which ideally appears as a horizontal line with a slightly jagged height gradient. In some transparent formulations, the freezing point is faintly visible as a milky line. In the majority of production runs, however, the freezing point is not visually discernible and can only be estimated based on the bubble shape.

[0008] After passing through the freezing point, no further stretching occurs in the longitudinal or transverse directions. Therefore, the local thickness and diameter of the tubular film remain constant downstream of the freezing point.

[0009] In the tube formation zone, i.e., the area between the exit of the plastic melt from the annular channel die and the freezing point, the thickness of the tubular film or the plastic melt changes continuously. The transition in the thickness of the melt or the plastic melt at the exit of the annular channel die from typically in the range of 2,000 μm to a final thickness of typically between 6 μm and 300 μm occurs through stretching of the tubular film in the transverse and longitudinal directions.

[0010] This stretching to the final thickness is achieved by expanding the diameter due to the internal pressure in the tubular film formed as a bubble and by pulling it off in the longitudinal direction.

[0011] The stretching takes place with simultaneous cooling of the outside and / or inside of the tubular film in the tube formation zone, as described above, and ends at the freezing point.

[0012] The change in thickness is determined by the continuously changing melt viscosity due to cooling. After the glass transition point, no further changes in thickness or diameter occur. If the temperature profile of the tubular film is continuously recorded along the production axis in the tube formation zone and across the transition to the glass transition point, a significant change in the temperature gradient (temperature decrease per unit distance along the production axis or along the measuring section) is observed in the area of ​​the glass transition point.

[0013] A method of the type mentioned above for determining the position of the freezing point is known from JPH 05-138733 A. In this method, the position at which no further temperature change occurs is determined. This position is assumed to be the position at which the tubular film solidifies. To keep this position constant, a control unit regulates the volume flow of a cooling gas depending on the temperature profile.

[0014] A stable position of the freezing point is essential for the process stability of the plant and at the same time an indicator of a stable process.

[0015] In addition to short-term fluctuations caused by process instabilities, the freezing point can also change its position slowly over a longer production period.

[0016] A glass transition point whose position does not match the production parameters inevitably leads to instabilities in the plastic tube formation zone, which usually manifest as resonant vibrations with varying amplitudes and frequencies. Such resonant vibrations lead to width and thickness variations in the final product in the direction of production, reducing product quality and even leading to rejects.

[0017] Furthermore, the stable position of the freezing point depends on the stability of those production parameters that affect it. For a given film format (final thickness and final width), these influencing parameters include, for example, system performance, thermoplastic material formulation, cooling air flow, cooling air temperature, ambient temperature, and melt temperature. Fluctuations in the aforementioned parameters inevitably lead to fluctuations in the position of the freezing point.

[0018] If an initially stable freezing point position is suddenly subject to such fluctuations, it can be assumed that one or more of the aforementioned parameters are also subject to fluctuations. These fluctuations are not always short-term or dynamic. A change in the ambient temperature in the production hall, for example, leads to a slow drift of the freezing point. Generally speaking, any shift in the freezing point during production is undesirable and leads to disruptions or quality problems. With the advancement of plant automation and the use of increasingly comprehensive production parameter databases for production optimization and waste minimization, the position of the tube formation zone or the freezing point plays a key role. The position of the freezing point must be accurately determined and monitored for largely complete plant automation.

[0019] Such error-free determination also opens up the possibility of implementing control loops that correct the position of the freezing point in the event of deviations. Control loops that act on one or more of the aforementioned process parameters are suitable for this purpose.

[0020] The basic prerequisite for such control loops is the precise knowledge of the position of the freezing limit as well as continuous and error-free monitoring of this position.

[0021] The object of the present invention is therefore to provide a method and a measuring arrangement which enable a reliable determination of the position of the freezing limit.

[0022] The problem is solved by a method for determining the position of a freezing point of a tubular film made of thermoplastic material emerging from a die head and pulled off in a production direction along a production axis during said production. The method comprises the steps of recording a temperature profile of the tubular film along a first measuring section along the production axis, determining a position along the first measuring section at which an absolute value of a temperature gradient of the temperature profile in the production direction falls below a predetermined limit value, and defining the position as the first provisional position of the freezing point. In addition, a diameter profile of the tubular film is recorded along a second measuring section along the production axis.Subsequently, a position along the second measuring section is determined at which the absolute value of a diameter gradient of the diameter profile in the production direction falls below a predetermined limit, and this position is defined as the second provisional position of the freezing point. Finally, the plausibility of the first provisional position and the second provisional position of the freezing point is checked based on their distance from each other.

[0023] To assess the distance between the first provisional position and the second provisional position during the plausibility check, for example, a difference between one of the provisional positions and the other of the two provisional positions can be determined. It is also conceivable to calculate the quotient of the two provisional positions and determine a deviation from the numerical value 1 or a percentage deviation from the numerical value 1. Furthermore, a percentage deviation between one of the provisional positions and the other of the two provisional positions can be determined. In addition, it is possible to determine the distance between one or both provisional positions and an average of the two provisional positions, again in the form of a difference or a percentage deviation.In principle, of course, other mathematical calculation methods known to the expert are also conceivable, which provide a measure of the distance between the two provisional positions.

[0024] According to the invention, the plausibility of the freezing point position is thus checked using two independent parameters: the temperature profile on the one hand and the diameter profile on the other. These two independent parameters can be used, for example, to check whether the determined preliminary freezing point positions can be used for fully automated process control. This provides a redundant system that prevents the use of an inaccurate or incorrect freezing point position for fully automated process control and the resulting waste.

[0025] The first provisional position is defined by the absolute value—i.e., a value independent of the mathematical sign—of the temperature gradient in the direction of production falling below a predetermined limit. The first provisional position can be determined using any mathematical method, as long as the condition is met that the absolute value of the temperature gradient of the temperature curve viewed in the direction of production falls below a predetermined limit. For example, it is also possible to check the temperature gradient of the temperature curve opposite to the direction of production to see whether its absolute value exceeds the predetermined limit. The same applies analogously to the definition of the second provisional position.

[0026] In addition, a final position of the freezing limit can be determined based on the first preliminary position and the second preliminary position of the freezing limit.

[0027] When recording the temperature curve and the diameter curve, signal noise occurs in practice, as is common in metrology, due to measurement errors. This can lead to the measured values ​​locally and briefly falling below the corresponding predetermined limit before the freezing point is actually reached. To this end, the specialist will process the signal accordingly, as is common and well-known in metrology, for example, by smoothing the raw data curve before determining the preliminary positions.

[0028] As already explained above, the temperature of the tubular film decreases within the tube formation zone when viewed in the direction of production. In the area of ​​the glass transition point, the temperature decrease slows down significantly, although this depends on the ambient conditions, the formulation of the plastic melt, and similar factors. To determine the initial preliminary position of the glass transition point, the transition from a negative temperature gradient with a high absolute value to a temperature gradient with a low absolute value, possibly close to zero, can be determined when viewed in the direction of production.

[0029] To record the diameter progression, a distance between a diameter measuring unit and the tubular film can be measured. Detecting the distance of a diameter measuring unit from a surface of the tubular film is equivalent to detecting the diameter progression of the tubular film, since the distance to the surface of the tubular film depends on the diameter of the tubular film. The distance of a diameter measuring unit from the surface of the tubular film decreases as the diameter of the tubular film increases. The diameter of the tubular film and the distance of a diameter measuring unit from a surface of the tubular film are therefore considered equivalent within the meaning of the present disclosure.

[0030] When checking plausibility, plausibility can be affirmed if the distance between the first and second provisional freezing limit positions is below a predetermined limit. Thus, the provisional freezing limit positions are only assumed to be plausible if they are sufficiently close to each other and not too far apart.

[0031] Essentially, any position from the first provisional freezing limit position to the second provisional freezing limit position can be determined as the freezing limit position. For example, it is possible to specify one of the two provisional freezing limit positions as the final freezing limit position and use the other provisional freezing limit position only for plausibility check purposes.

[0032] However, it is also conceivable to determine an average of the first and second provisional positions of the freezing point and define it as the final position of the freezing point. This ensures that even if one of the two provisional positions deviates significantly from the actual position of the freezing point, the impact of this deviation on process control is mitigated by additionally taking the second provisional position into account. However, any other mathematical definition is equally conceivable.

[0033] To ensure that the freezing point area is completely covered by at least one of the two measuring sections, the first measuring section and the second measuring section can be arranged in a region where the tubular film transitions from a region of varying diameter to a region of constant diameter. The freezing point is expected to be located in this region.

[0034] In an exemplary method, it can be provided that the temperature profile is detected by means of a non-contact temperature sensor that is moved along the first measuring path, and / or that the diameter profile is detected by means of a non-contact distance sensor that is moved along the second measuring path.

[0035] In principle, it is also conceivable to measure the temperature profile along several first measuring sections distributed around the circumference of the tubular film and / or to measure the diameter profile along several second measuring sections distributed around the circumference of the tubular film. The preliminary positions of the freezing point determined around the circumference can be used to calculate an average. On the other hand, arranging the measuring sections distributed around the circumference also makes it possible to detect an inclination of the freezing point relative to the production axis and to determine the extent of the inclination.

[0036] The invention is further achieved by a measuring device for determining the position of a freezing point of a tubular film made of thermoplastic material emerging from a blowing head and drawn off in a production direction along a production axis during the production of the tubular film. The measuring device comprises a temperature measuring unit configured to record a temperature profile of the tubular film along a first measuring section along the production axis, and a processing unit configured to determine a position along the first measuring section at which an absolute value of a temperature gradient of the temperature profile in the production direction falls below a predetermined limit value, and to establish the position as the first provisional position of the freezing point.The measuring device further comprises a diameter measuring unit for detecting a diameter profile of the tubular film along a second measuring section along the production axis. A processing unit is configured to determine a position along the second measuring section at which an absolute value of a diameter gradient of the diameter profile in the production direction falls below a predetermined limit value and to define this position as the second provisional position of the freezing limit. The measuring device further comprises a processing unit configured to check the plausibility of the first provisional position and the second provisional position of the freezing limit based on their distance from one another. The measuring device can further comprise a processing unit configured to determine a final position of the freezing limit based on the first provisional position and the second provisional position of the freezing limit.

[0037] The processing unit for determining and setting the first provisional position, the processing unit for determining and setting the second provisional position, the processing unit for checking plausibility, and the processing unit for determining the final position can be represented by a single processing unit, for example, a computer or a circuit board with programmable components. Each processing unit can have a memory for storing measurement data during the recording of the temperature and diameter profiles, as well as a processor unit for processing the measurement data during the setting of the first and second provisional positions, during the plausibility check, and / or during the determination of the final position.

[0038] The diameter measuring unit for detecting the diameter profile of the tubular film can be configured to measure a distance between the diameter measuring unit and the tubular film.

[0039] In one embodiment of the measuring arrangement, the temperature measuring unit can comprise a non-contact temperature sensor that is movably driven along the first measuring path. Furthermore, alternatively or additionally, the diameter measuring unit can comprise a non-contact distance sensor that is movably driven along the second measuring path.

[0040] For this purpose, the measuring device can comprise a sensor carrier carriage that is driven to move along the first and second measuring sections and on which the temperature sensor and the distance sensor are mounted. The sensor carrier carriage can be driven, for example, via a spindle, a rack, or a belt drive.

[0041] To protect the measuring system from external influences, it can be provided with a housing in which the sensor carrier slide is movably mounted and which has a slot facing the tubular film, which is aligned with the temperature sensor and / or the distance sensor. This protects all components within the housing and is accessible only through the slot.

[0042] In one embodiment, there are several temperature measuring units arranged around the circumference and / or several diameter measuring units arranged around the circumference.

[0043] According to a specific embodiment, the measuring system comprises a calibration device, for example, a calibration basket, for guiding the tubular film. The calibration device comprises a frame that is height-adjustable relative to the die head along a production axis, a plurality of support arms distributed around the circumference of the frame, whose diameter is synchronously adjustable relative to the production axis, and which have guide elements for guiding the tubular film, and adjustment means for adjusting the support arms. The measuring device is attached to the adjustable support arms, to the adjustment means, or to the frame of the calibration device.

[0044] Since the calibration device is adjusted in height so that it is located behind the tube formation zone in the direction of production, the positioning of the measuring device on the calibration device ensures that the measuring device is always positioned near the freezing point and, depending on the process parameters, is moved into the appropriate position together with the calibration basket. The measuring device should be positioned upstream of the tubular film inlet into the calibration device in the direction of production.

[0045] Preferred embodiments are explained in more detail below with reference to the figures.

[0046] Figure 1 shows a vertical section of a blown film extrusion line with a measuring arrangement according to the invention,

[0047] Figure 2 shows a view of a calibration device of the blown film extrusion line according to Figure 1 in the direction of a production axis, Figure 3 shows a graph of a temperature profile and a diameter profile of the tubular film over the distance to the blowing head,

[0048] Figure 4 shows a graph of a temperature profile and a profile of the distance between a diameter measuring unit and the tubular film over the distance to the blow head,

[0049] Figure 5 is a perspective view of a measuring device and

[0050] Figure 6 is a perspective view of the measuring device according to Figure 5 without

[0051] Housing.

[0052] Figure 1 shows a complete blown film extrusion line 1 for producing a tubular film 2 in a production direction R vertically upwards along a production axis P in vertical section. An extruder 4 for thermoplastic material, on which two feed hoppers 5, 6 can be seen, stands on a foundation 3. The thermoplastic material fed in in granulate form via the feed hoppers 5, 6 is plasticized and homogenized by pressure and additional heating medium in a screw of the extruder 4 and pressed into a die head 7 with a vertical axis that is connected to the extruder 4. The die head 7 has on its upper side 8 an annular channel die 19, shown schematically here, from which the expanding tubular film 2, which is axisymmetric to a production axis P and is made of initially still plasticized, thermoplastic material, emerges.An internal cooling device 9, through which cooling gas is introduced into the tubular film 2, and a gas extraction pipe 10 are attached centrally to the blow head 7. This creates an internal overpressure in the tubular film 2, as a result of which the tubular film 2 initially expands and, at a freezing point 11, transitions into a state in which the plastic melt solidifies and further plastic deformation of the material of the tubular film 2 is no longer possible. As a result of the internal overpressure, which also serves to stabilize it, and as the tubular film 2 is pulled off, the tubular film material initially stretches circumferentially and longitudinally, which are prevented more quickly the more the tubular film material is cooled. After the plastic material of the tubular film 2 has solidified in the area of ​​the freezing point 11, it essentially retains its diameter.The tubular film 2 is further pulled upwards along the production axis P in the pull-off direction and flattened in a flattening device 12 and guided upwards via a pull-off device 13. The flattened tubular film 2 is then wound onto coils (not shown here).

[0053] Directly above the blow head 7 there is arranged a cooling gas ring 14 with internal outlet nozzles from which cooling gas, usually air, flows out and against the tubular film 2, which is under increased internal pressure, in a ring shape essentially parallel to the surface of the tubular film 2. The tubular film 2 plasticized in this area initially expands in diameter under the aforementioned excess pressure in the interior until it solidifies under the effect of the cooling gas and assumes a constant diameter. Above the freezing point 11, i.e. downstream of the freezing point 11 in the withdrawal direction, there is a calibration device 15, shown only schematically in Figure 1, in the form of a calibration basket with a plurality of guide elements in the form of guide rollers 16 which come into contact with the tubular film 2 and are arranged in a ring around the production axis P and around the circumference of the tubular film 2.To enable adaptation to tubular films 2 of different diameters, the guide rollers 16 are mounted on a frame 17 so as to be adjustable at least approximately radially to the production axis P. For this purpose, the guide rollers 16 are rotatably mounted on support arms (not shown here). The support arms are adjustable, for example, pivotable or linearly movable, via adjustment means not shown here such that the guide rollers 16 are adjustable at least substantially radially to the production axis P.

[0054] A measuring device 18 is attached to the calibration device 15, wherein the measuring device 18 can be attached to one of the support arms, to one of the adjustment means or to the frame 17 of the calibration device 15. The measuring device 18 comprises a temperature measuring unit for recording a temperature profile of the tubular film 2 along a first measuring section M1 along the production axis P, wherein the first measuring section M1 extends beyond the freezing limit 11, i.e. crosses it. In the exemplary embodiment shown, the first measuring section M1 runs parallel to the production axis P. However, the first measuring section M1 can also be arranged at an angle to the production axis P, as long as the temperature profile of the tubular film 2 can be recorded over a sufficient range along the production axis P.

[0055] Furthermore, the measuring device 18 comprises a diameter measuring unit for detecting a diameter profile of the tubular film 2 along a second measuring section M2 along the production axis P, wherein the second measuring section M2 also extends beyond the freezing limit 11, i.e., crosses it. In the illustrated embodiment, the second measuring section M2 also runs parallel to the production axis P. The second measuring section M2 can also be arranged at an angle to the production axis P, as long as the diameter profile of the tubular film 2 can be detected over a sufficient range along the production axis P.

[0056] The detection of the diameter profile of the tubular film 2 is equivalent to the detection of the distance of the measuring device 18 or the diameter measuring unit from a surface of the tubular film 2, since the distance to the surface of the tubular film 2 depends on the diameter of the tubular film 2.

[0057] The first measuring section M1 and the second measuring section M2 can be arranged parallel to one another, whereby this includes that the two measuring sections M1, M2 are identical or are arranged overlapping one another.

[0058] The measuring device 18 is connected to a processing unit 44, for example, a computer, via a data line 43, which can be a wired or wireless data line. However, the processing unit 44 can also be integrated into the measuring device 18, for example, in the form of a circuit board with programmable components.

[0059] Figure 2 shows a view of the calibration device 15 in the direction of the production axis P for guiding the tubular film 2. The movable elements described below are fastened to the frame 17 of the calibration device 15, wherein the frame 17 is arranged so as to be height-adjustable relative to the blow head. The frame 17 forms a central passage through which the tubular film 2 is guided parallel to the production axis P. Six adjustment units 20 are arranged distributed over the circumference. The adjustment units 20 serve to adjust film guide elements 21, 22 in a direction radial to the production axis P. One of the six adjustment units 20 is described below as an example, wherein all adjustment units 20 are constructed identically.

[0060] The adjustment units 20 each have a support arm 23 pivotally mounted on the frame 17. The support arm 23 is pivotable about a pivot axis S, which is arranged parallel to the production axis P.

[0061] Furthermore, the adjustment units 20 each have a carrier 24, which in the illustrated embodiment carries two film guide elements 21, 22 in the form of guide rollers, which are spaced apart from one another and arranged in a V-shaped overlap when viewed in the direction of the production axis P. The carrier 24 is connected to the carrier arm 23 so as to be pivotable about a pivot axis, wherein the pivot axis is arranged parallel to the production axis P.

[0062] Furthermore, the adjustment units 20 each have a coupling rod 25 which is pivotally connected to the carrier 24.

[0063] Finally, the adjustment units 20 each have an adjustment mechanism by means of which the coupling rod 25 is articulated to the frame 17.

[0064] The coupling rod 25 of the adjustment unit 20 is pivotally and slidably connected to the frame 17 via a coupling element 26. The coupling element 26 is pivotally connected to the frame 17, with the coupling rod 25 being slidably coupled to the coupling element 26. For the sake of clarity, the coupling rod 25 and the coupling element 26 are only shown for one of the adjustment units 20.

[0065] In addition, a driver 27 is fastened to the coupling rod 25 and is guided so as to be translationally movable along a guide 28 on the frame 17. In the exemplary embodiment shown, the guide 28 is a groove in a plate 29 which is firmly connected to the frame 17. The guide 28 is curved and adapted such that the carrier 24 is always aligned centrally to the production axis P, regardless of the distance to the production axis P or to the tubular film 2. This ensures precise central alignment of the film guide elements 21, 22 in the form of the two guide rollers relative to the tubular film 2, so that both rollers are always held in contact with the tubular film 2.

[0066] The coupling element 26, the driver 27 and the guide 28 together form adjustment means via which the coupling rod 25 is connected to the frame 17 and the support arm 23 is adjusted.

[0067] The support arm 23 is fixedly connected to a pivot plate 29, which is pivotably attached to the frame 17 about the pivot axis S, such that the support arm 23 is pivotally arranged on the frame 17 via the pivot plate 29. A drive 30 is also attached to the frame 17. The drive 30 is in the form of a solenoid, with which an actuator 31 in the form of a piston rod can be linearly driven. The actuator 31 is pivotally connected to the pivot plate 29 of one of the adjustment units 20. Furthermore, a housing of the drive 30 is pivotally connected to the frame 17. The drive 30 is thus supported against the frame 17, and the support arm 23 can be pivoted by adjusting the actuator 31.If the actuator 31 is moved from a retracted position to an extended position, the support arm 23, which is connected to the drive 30 via the pivot plate 29, is pivoted inwards so that the film guide elements 21, 22 enclose a smaller diameter and can thus guide a tubular film 2 with a smaller diameter.

[0068] The calibration device 15 has a synchronization mechanism to synchronize the movement of all adjustment units 20. The synchronization mechanism has push rods 32, each of which couples the pivot plates 29 to one another over the circumference of adjacent adjustment units 20. For this purpose, the push rods 32 are each pivotally connected to the two pivot plates 29 of adjacent adjustment units 20. Thus, the pivoting movement of the adjustment unit 20 connected to the drive 30 is transmitted to the remaining adjustment units 20, so that all adjustment units 20 are moved synchronously. The measuring device 18 is shown in three different positions. In a first position, the measuring device 18 is arranged radially outside the frame 17 and fixedly connected to the frame 17. In a second position, the measuring device 18 is arranged flush with the frame 17, viewed in the production axis P, and is fastened to it.In a third position, the measuring device 18 is located on a support 24 of one of the adjustment units 20. In all three positions, it is ensured that the measuring device 18, together with the entire calibration device 15, is height-adjustable relative to the blow head, so that the measuring device 18 can always be arranged in the area of ​​the freezing limit. In the third position, in which the measuring device 18 is connected to a support 24 of one of the adjustment units 20, the measuring device 18 is also adjusted relative to the production axis P when the film guide elements 21, 22 are radially adjusted. This ensures that the measuring device 18 always has a constant radial distance from the tubular film 2, whereby the measuring accuracy can be increased.

[0069] The three positions shown can alternatively be used as mounting positions for the measuring device 18. However, it is also conceivable that these positions be used in different combinations for the arrangement of the measuring device 18. Furthermore, several measuring devices 18 can be arranged distributed around the circumference, for example, to determine an inclination of the freezing limit relative to the production axis P.

[0070] Figure 3 shows a graph of a temperature curve 33 and a diameter curve 34 of the tubular film. The distance to the blowing head (height position) is plotted from left to right along the abscissa axis (X-axis). The temperature and the diameter of the tubular film are plotted from bottom to top along the ordinate axis (Y-axis). Instead of the diameter curve 34, the distance 45 between the diameter measuring unit and the tubular film can also be plotted, as shown in Figure 4. These two values ​​correlate with each other and are to be considered equivalent within the meaning of the present disclosure.

[0071] It can be seen that with increasing distance from the die head, the temperature 33 initially decreases rapidly and continuously, and from a certain height position, decreases only slightly or remains approximately constant. The diameter 34 of the tubular film increases rapidly and continuously toward higher height positions, with the distance 45 between the diameter measuring unit and the tubular film decreasing inversely proportionally. From a certain height position, the values ​​change only slightly or both values ​​remain approximately constant.

[0072] In this case, at an altitude position h1, the absolute value of the temperature gradient of the temperature curve 33 falls below a predetermined limit. The temperature gradient corresponds to the slope of the temperature curve 33 and, for simplicity, is used as an absolute value without a sign. If a predetermined limit is undershot, it can therefore be assumed that the temperature 33 decreases sufficiently slowly to assume the position of the freezing point. This altitude position is determined as the first provisional position h1 of the freezing point.

[0073] In the present case, at a height position h2, the absolute value of the diameter gradient of the diameter curve 34 or the absolute value of the distance gradient of the distance curve 45 between the diameter unit and the tubular film falls below a predetermined limit value. The diameter gradient and the distance gradient correspond to the slope of the diameter curve 34 or the distance curve 45 between the diameter unit and the tubular film and can each be used as absolute values ​​without a sign for simplification. If a predetermined limit value is undershot, it can therefore be assumed that the diameter 34 is increasing sufficiently slowly or the distance 45 between the diameter unit and the tubular film is decreasing sufficiently slowly to be able to assume the position of the freezing limit. This height position is determined as the second provisional position h2 of the freezing limit.

[0074] The plausibility of these two measured values ​​can be checked based on the two preliminary positions h1, h2 of the freezing limit. For example, the distance Ah between the two preliminary positions h1, h2 can be calculated. As long as the distance Ah does not exceed a predetermined limit, the plausibility can be confirmed. In this case, for example, one of the two preliminary positions h1, h2 can be determined as the final position of the freezing limit, or any value in between, for example the average of the two preliminary positions h1, h2.

[0075] Figures 5 and 6 show different perspective views of the measuring device 18 and are described together below.

[0076] The measuring device 18 has a non-contact temperature sensor 35 and a non-contact distance sensor 36, which are mounted on a sensor carrier slide 37. In the illustrated embodiment, the sensor carrier slide is driven via a spindle 38 so as to be movable along a longitudinal axis L. Alternatively, a drive via a rack and pinion or a belt drive, as well as other drive concepts for the linear adjustment of the sensor carrier slide 37, is also conceivable. The temperature sensor 35 and the distance sensor 36 are arranged one behind the other parallel to the longitudinal axis L, so that the first measuring section of the temperature sensor 35 and the second measuring section of the distance sensor 36 partially overlap and are offset from one another in the direction of the longitudinal axis L by the axial distance between the temperature sensor 35 and the distance sensor 36.

[0077] The temperature sensor 35 and the distance sensor 36 are connected to electrical components 40 via electrical cables routed in a cable guide 39. The entire assembly is housed in a housing 41 and protected from the outside. The housing 41 has a slot 42 that runs parallel to the longitudinal axis L in the housing 41 and is aligned with the temperature sensor 35 and the distance sensor 36, allowing them to determine the temperature and distance of the tubular film from within the housing 41.

[0078] List of reference symbols

[0079] 1 blown film extrusion line

[0080] 2 tubular film

[0081] 3 Foundation

[0082] 4 extruders

[0083] 5 feed hoppers

[0084] 6 feed hoppers

[0085] 7 Blow head

[0086] 8 Top

[0087] 9 Internal cooling device

[0088] 10 Gas extraction pipe

[0089] 11 Freezing limit

[0090] 12 Flattening device

[0091] 13 Puller

[0092] 14 Cooling gas ring

[0093] 15 Calibration device

[0094] 16 guide rollers

[0095] 17 frames

[0096] 18 Measuring device

[0097] 19 Ring channel nozzle

[0098] 20 adjustment unit

[0099] 21 Film guide element

[0100] 22 Film guide element

[0101] 23 Support arm

[0102] 24 carriers

[0103] 25 coupling rod

[0104] 26 coupling element

[0105] 27 Driver 28 Guide

[0106] 29 Swivel plate

[0107] 30 drive

[0108] 31 Actuator

[0109] 32 push rod

[0110] 33 Temperature curve

[0111] 34 Diameter progression

[0112] 35 Temperature sensor

[0113] 36 Distance sensor

[0114] 37 sensor carrier slides

[0115] 38 spindle

[0116] 39 Cable routing

[0117] 40 electronic components

[0118] 41 housings

[0119] 42 slot

[0120] 43 Data line

[0121] 44 processing unit

[0122] 45 Course of the distance

[0123] Ah distance h1 first provisional position h2 second provisional position

[0124] L Longitudinal axis

[0125] M1 first measuring section

[0126] M2 second measuring section

[0127] P Production axis

[0128] R Production direction

[0129] S swivel axis

Claims

Method and measuring arrangement for determining the position of a freezing point during the production of a tubular film made of thermoplastic material Claims 1 . Method for determining the position of a freezing limit (11) of a tubular film (2) made of thermoplastic material emerging from a blow head (7) and drawn off in a production direction (R) along a production axis (P) during its production, comprising the following method steps: detecting a temperature profile (33) of the tubular film (2) along a first measuring section (M1) along the production axis (P), and Determining a position along the first measuring section (M1) at which an absolute value of a temperature gradient of the temperature profile (33) in the production direction (R) falls below a predetermined limit value and defining the position as the first provisional position (h1) of the freezing limit (11), characterized by the method steps: Detecting a diameter profile (34) of the tubular film (2) along a second measuring section (M2) along the production axis (P), Determining a position along the second measuring section (M2) at which an absolute value of a diameter gradient of the diameter profile (34) in the production direction (R) falls below a predetermined limit value and defining the position as a second provisional position (h2) of the freezing limit (11), and Checking a plausibility of the first provisional position (h1 ) and the second provisional position (h2) of the freezing limit (11 ) based on their distance (Ah) from one another.

2. Method according to claim 1, characterized in that that a final position of the freezing limit (11) is determined based on the first provisional position (h1) and the second provisional position (h2) of the freezing limit (11). Method according to claim 1 or 2, characterized in that, to detect the diameter profile, a distance between a diameter measuring unit and the tubular film (2) is measured. Method according to one of claims 1 to 3, characterized in that, in the method step of checking plausibility, the plausibility is affirmed if the distance (Ah) between the first provisional position (h1) and the second provisional position (h2) of the freezing limit (11) is below a predetermined limit value.Method according to one of claims 1 to 4, characterized in that the first provisional position (h1), the second provisional position (h2) or a position between the first provisional position (h1) and the second provisional position (h2) is determined as the final position of the freezing limit (11). Method according to one of claims 1 to 5, characterized in that an average value of the first provisional position (h1) and the second provisional position (h2) of the freezing limit (11) is set as the final position of the freezing limit (11). Method according to one of claims 1 to 6, characterized in that the first measuring section (M1) and the second measuring section (M2) are arranged in a region in which the tubular film (2) is separated from an area. with a changing diameter into a region with a constant diameter. Method according to one of claims 1 to 7, characterized in that the temperature profile (33) is detected by means of a non-contact temperature sensor (35) which is moved along the first measuring section (M1), and / or that the diameter profile (34) is detected by means of a non-contact distance sensor (36) which is moved along the second measuring section (M2). Method according to one of claims 1 to 8, characterized in that the temperature profile (33) is measured along a plurality of first measuring sections (M1) arranged distributed over the circumference of the tubular film (2) and / or that the diameter profile (34) is measured along a plurality of second measuring sections (M2) arranged distributed over the circumference of the tubular film (2).Measuring arrangement for determining the position of a freezing limit (11) of a tubular film (2) made of thermoplastic material, which emerges from a blowing head (7) and is pulled off in a production direction (R) along a production axis (P), during the production thereof, with a measuring device (18) which comprises the following: a temperature measuring unit which is set up to record a temperature profile (33) of the tubular film (2) along a first measuring section (M1) along the production axis (P), and a processing unit (44) which is set up to determine a position along the first measuring section (M1) at which an absolute value of a temperature gradient of the temperature profile (33) in the production direction (R) falls below a predetermined limit value, and to establish the position as a first provisional position (h1) of the freezing limit (11). characterized by a diameter measuring unit which is used to record a diameter profile (34) of the tubular film (2) along a second measuring section (M2) along the production axis (P), a processing unit (44) which is set up to determine a position along the second measuring section (M2) at which an absolute value of a diameter gradient of the diameter profile (34) in the production direction (R) falls below a predetermined limit value, and to define the position as a second provisional position (h2) of the freezing limit (11), and a processing unit (44) which is set up to check a plausibility of the first provisional position (h1) and the second provisional position (h2) of the freezing limit (11) based on their distance (Ah) from one another.Measuring arrangement according to claim 10, characterized in that the measuring arrangement comprises a processing unit (44) configured to determine a final position of the freezing boundary (11) based on the first preliminary position (h1) and the second preliminary position (h2) of the freezing boundary (11). Measuring arrangement according to claim 10 or 11, characterized in that the diameter measuring unit is configured to detect the diameter profile. (34) of the tubular film (2) is designed to measure a distance between the diameter measuring unit and the tubular film (2). Measuring arrangement according to one of claims 10 to 12, characterized in that the temperature measuring unit comprises a contactless temperature sensor (35) which is movably driven along the first measuring section (M1), and / or that the diameter measuring unit comprises a contactless distance sensor (36) which is driven to move along the second measuring section (M2). Measuring arrangement according to claim 13, characterized in that the measuring device (18) comprises a sensor carrier carriage (37) which is driven to be movable along the first measuring section (M1) and the second measuring section (M2) and on which the temperature sensor (35) and the distance sensor (36) are mounted. Measuring arrangement according to claim 14, characterized in that the sensor carrier carriage (37) is driven via a spindle (38), a rack, or a belt drive. Measuring arrangement according to claim 14 or 15, characterized in that the measuring device (18) has a housing (41) in which the sensor carrier carriage (37) is arranged to be movable and which has a slot (42) facing the tubular film (2) and aligned with the temperature sensor (35) and / or the distance sensor (36).Measuring arrangement according to one of claims 10 to 16, characterized in that there are several temperature measuring units distributed over the circumference of the tubular film (2) and / or that there are several diameter measuring units distributed over the circumference of the tubular film (2). Measuring arrangement according to one of claims 10 to 17, characterized in that the measuring arrangement has a calibration device (15) for guiding the tubular film (2), wherein the calibration device (15) comprises the following: a frame (17) which is designed to be height-adjustable along the production axis (P) relative to the blow head (7). a plurality of support arms (23) distributed over the circumference of the frame (17) and synchronously adjustable in diameter relative to the production axis (P) with film guide elements (21, 22) for guiding the tubular film (2), and Adjustment means (26, 27, 28) for adjusting the support arms (23), and that the measuring device (18) is attached to the adjustable support arms (23), to the adjustment means (26, 27, 28), or to the frame (17) of the calibration device (15). Measuring arrangement according to claim 18, characterized in that the measuring device (18) is arranged in the production direction (R) upstream of an inlet of the tubular film (2) into the calibration device (15).

Images