DEVICE FOR THE PRODUCTION OF INLINE STRETCHED TUBULATED FILMS USING THE BLOW MOLDING PROCESS

Description

[0001] The present invention relates to a device for producing inline stretched tubular films by the blow molding process.

[0002] Blown film extrusion lines, which have been in use for a long time, are typically used to produce tubular film using the blow-molding process. Granulated plastics are then plasticized into a viscous mass in extruders under high pressure. This mass, which is at a high temperature due to the pressure, is formed into a ring in a die head and escapes from the die head through a ring die. Immediately after leaving the ring die, the mass already forms a film tube. However, since this film tube has not yet completely cooled down, its diameter can be changed. The diameter is usually increased by injecting compressed air into the interior of the film tube. To ensure that the film tube always has a constant diameter, it is guided at a distance from or directly along film guide elements.This arrangement of the film guide elements is referred to as a calibration basket in the field of blown film extrusion systems.

[0003] After passing through the calibration basket, the film tube can be flattened and laid in a reversing haul-off unit and then stretched monoaxially in the machine direction in a stretching system.

[0004] By stretching thermoplastic films, their properties can be specifically modified. Such properties include transparency or strength. Such stretching, which can be performed in the transverse and / or longitudinal direction of the film web, can be performed inline directly after the extrusion process. The stretching or orienting of thermoplastic films is described, for example, in WO 2006 / 063641 A1 and WO 2011 / 057918 A1.

[0005] Film thickness profile control systems with segmented control zones are used in the production of tubular film. These systems allow the film thickness profiles to be controlled in such a way that thickness deviations are as small as possible across the entire circumference of the tube. Such film thickness profile control systems are disclosed in US 2004 / 0113331 A1, WO 2012 / 080276, and EP1719602A1.

[0006] For example, DE 39 411 85 A1 discloses a method for controlling the film thickness of tubular films from blown film lines with downstream axial or biaxial stretching of the inflated tubular films in an oven, so that a final film is produced which has the smallest possible deviations in thickness.

[0007] During longitudinal stretching in a stretching line, the film is stretched in the machine direction according to the degree of stretching, thereby reducing the film thickness. At the same time, the film constricts in the transverse direction, reducing its width. This constriction results in the stretched film becoming progressively thicker from the center of the film toward the film edges, even though it was previously controlled to a constant thickness during the blow molding process. This increase in thickness is particularly pronounced at the film edge areas. This causes an edge buildup in the film thickness during subsequent winding of the film. The film web is stretched more and more at the edges as the winding diameter increases.

[0008] To produce films with the most uniform thickness profile possible, as is particularly required for printing or lamination, the film edges that do not match the desired thickness profile must be trimmed away. However, this trimming process results in a significant loss of film width. Thus, up to approximately 200 mm of film width is eliminated on each side of the film due to trimming.

[0009] EP 2 277 681 B1 discloses a generic method in which the film thickness profile of the tubular film produced in the blown film line is controlled such that the stretching produces a film with a thickness profile that has the smallest possible deviations from the average film thickness across the entire film width. For this purpose, a tubular film is produced in the blown film process that has two opposing thin spots. When laying the tubular film flat, care is taken to ensure that the thin spots are located in the film edge area and that the stretched film then has a thickness profile with the smallest possible deviations from the average film thickness. The deliberately introduced thin spots therefore compensate for the thickening in the edge area that later occurs during stretching, so that a uniform film thickness extending right into the edge area should result.

[0010] According to EP 2 277 681 B1, segmented control zones are provided in the cooling ring for the targeted introduction of the thin spots. These zones can consist, for example, of heating cartridges. A disadvantage of this type of profile control, however, is that the segmented control zones in the cooling ring often have to be operated at the limit of their possible performance, thus limiting the accuracy and speed of the control process.

[0011] To overcome this problem, WO 2018 / 215945 A1 proposes an additional rotating ring, which additionally tempers the film at the folded edges of the flattened film web in the area of ​​the cooling ring, so that the film tube is already thinned at this point to counteract the subsequent edge thickening. The rotation of the additional ring is synchronized with the rotation of the reversing pull-off device. This approach is very complex and likely to remain inaccurate due to the necessary synchronization.

[0012] EP 0 335 411 A2 and JPS6273922U propose biaxial stretching of a re-inflated, previously flattened film tube. This eliminates the problems of neck-in and edge thickening, eliminating the need to create thin spots beforehand.

[0013] The object of the invention is therefore to increase the accuracy and speed of the known thin-point control system.

[0014] This object is achieved by the features of patent claim 1. Further preferred embodiments can be found in the subclaims.

[0015] The following advantages are achieved by the inventive connection of a second thin-spot control system: Thin spots can be introduced precisely at the desired locations (e.g., fold edges), even during reversal. Inaccurately positioned thin spots from the first thin spot control system are eliminated, resulting in better film flatness during winding. A larger control stroke and higher control speed are available for the introduction of thin spots. In combination with the first thin spot control system, the required heating power of the heating cartridges in the cooling ring can be reduced. The thickness profile of the stretched film has a smaller deviation from the target profile. This reduces edge trimming.

[0016] Further details and advantages of the invention are described with reference to the accompanying drawings. Fig. 1 shows a side view of the blown film line according to the invention, Fig. 2 shows a side view of the downstream stretching line, Fig. 3 shows a side view of the downstream winder and Fig. 4 shows a perspective view of the flattening unit from Fig. 1 .

[0017] Fig. 1 shows a blown film system 1 for producing a film tube. The extruder 2 melts plastic granules and feeds them to the blowing head 4 via line 3. In the blowing head 4, the plastic melt is converted into a cylindrical melt stream, which can then be withdrawn from an annular gap 5 of the cooling ring 8 in the withdrawal direction z as a film tube 6. The not yet solidified film tube 6 is inflated from the inside with a slight overpressure to the diameter of the calibration basket 7.

[0018] In the cooling ring 8 there are segmented control zones of a first thin-point control system which controls the segmented control zones in such a way that during the subsequent stretching a film is produced with a thickness profile with the smallest possible deviations from the average film thickness over the entire film width.

[0019] After the calibration basket 7, the film tube 6 is fed to the flattening unit 9, in which the circular film tube initially forms an elliptical cross-section with increasing eccentricity until it is finally formed into a double-layer film web by the two take-off rollers 10.

[0020] The flattening device 9 is arranged to be rotatable, the axis of rotation being substantially aligned with the hose axis 11, which is in the Figure 1 indicated by a dot-dash line. The rotatability of the flattening device is indicated by arrow 12.

[0021] The blowing head 8 is divided into various circumferential sections. Each circumferential section of the blowing head 8 is capable of subjecting the film tube to an air flow that varies across the circumference of the film tube and / or to a temperature that varies across the circumference of the film tube. The circumferential sections of the film tube that are cooled to a lesser extent form a thin spot 13. Where the cooling effect is greater, a thick spot is formed.

[0022] To ensure that the thin spot always reaches a fixed position on the flattening device, it is also necessary for the thin spot to move along the circumference, which is indicated in the figure by arrow 14. This movement of the thin spot is achieved by changing the parameters of the circumferential section of the die head 8 closest in the direction of arrow 14, in order to create a thin spot adjacent to the circumferential section of the film tube that currently has a thin spot. The current thin spot is retracted by the relevant circumferential section of the die head 8 again exerting a stronger cooling effect on the angle section assigned to it.

[0023] In order to record a thickness profile of the film tube 6, a first measuring unit 18 is provided, which, viewed in the transport direction z, is preferably arranged between the calibration basket 7 and the flattening device 9. The first measuring unit 18 can measure the wall thickness of the film tube at the respective position. To form a profile, the first measuring unit 18 can also be moved around the film tube, which is indicated by the double arrow 19. To move the first measuring unit 18, it is arranged displaceably on a rail 20.

[0024] Furthermore, a control unit 40 is provided, which receives the measured values ​​from the first measuring unit 18 via line 42 and with which the blowing head 8 can be controlled via line 41 in such a way that a desired thickness profile can be generated. The thickness profile and the necessary control parameters can be generated dynamically for the individual segments of the fluid application device, so that the resulting thickness profile moves in phase with the rotation of the flattening device.

[0025] Additionally or alternatively, the control unit 40 can also receive the measured values ​​from a second measuring unit 38 (in Fig. 2 shown) or by a third measuring unit 45 (in Fig. 3 shown).

[0026] The influencing factors of the three measuring units can be taken into account in a weighted manner when modifying the control commands.

[0027] Behind the take-off rollers 10, the double-layer film web passes through a reversing turn-over take-off roller 15, which is responsible for guiding the flattened film tube from the flattening device to the stationary roller 16. The arrow 17 indicates that this film tube is now being guided for further processing.

[0028] Fig. 2 shows a stretching system 30, which is inline with the Figure 1 The film web 24 first passes through a guide roller 31 and then several heating rollers 32. The task of the heating rollers 32 is to bring the already completely or partially cooled film web 24 back to a temperature that is sufficient for a stretching or drawing process.

[0029] After the film web 24 has been brought back to a stretching temperature in the area 28 of the heating rollers 32, it enters the area of ​​the stretching roller 22 and the nip roller 33 and traverses the gap between these two rollers 22, 33. The film web 24 then passes through the stretching gap 21, then reaches the surface of the stretching roller 23 and exits the stretching gap 21. This stretching roller 23 forms a nip with the nip roller 36. Due to the lower peripheral speed of the first pair of rollers 22, 33 compared to the second pair of rollers 23, 36, the film web 24 is elongated, i.e. stretched, in the stretching gap 21. This results in two undesirable effects that make it necessary to sever longitudinal strips from the sides of the film webs. The first effect is a reduction in the film width during stretching (so-called necking). The second effect is a thickening of the edges of the film web.The size of the stretching gap, i.e., the distance between the separation edge of the film web 24 from the roller 22 and the edge where the film web meets the roller 23, can be designed to be variable. This makes it possible to influence the size of the neck-in and / or the thickening of the film web at its edges.

[0030] After passing through the stretching nip, of which there may be several in series, the film web 24 reaches the area 29, which comprises cooling rollers, each designated by the reference numeral 37, in which the film web 24 is cooled again. After leaving this area 29, the film web 24 has reached a somewhat lower temperature so that its surface can survive transport over the guide roller 31 in the transport direction z without further damage. The film web 24 is then conveyed further in the direction of arrow 34 and, at the end of an optional further processing step, is fed to a winding device in which the film web is wound up as a double-layer film web or separately into two individual layers. In principle, it is not excluded that the film web or the individual layers of the film web are given longitudinal cuts and wound up next to one another in several blanks.

[0031] Before the film web reaches the stretching device 30, a cutting or puncturing device 35 can be provided, with which the double-layer film web can be cut or punctured so that air or another gas that might still be present within the double-layer film web can escape. This measure leads to improved quality of the stretching process and to increased accuracy of the thickness profiles of the double-layer film web to be measured. In particular, it can be provided that the film web is cut lengthwise along or near a side edge so that the double-layer film web is only connected via one side edge. Furthermore, it may be desired to cut the double-layer film web at both of its side edges. This is particularly necessary for film webs of certain thicknesses, since the air transport to one side edge of the film web may not be sufficiently fast.

[0032] A second measuring unit 38 is provided downstream of the stretching device 30 in the transport direction, with which a thickness profile of the double-layer film web can be recorded after it has been stretched. However, it should be noted that without further measures, only the total thickness of the film web can be measured, i.e., the sum of the thicknesses of the individual layers. However, it is conceivable, particularly if the double-layer film web has been cut longitudinally, to insert a contrast agent, such as a metal sheet, between the two layers so that each layer can be measured separately with regard to its thickness.

[0033] The second measuring unit 38 can also be arranged to be movable along a rail extending at least partially transversely to the transport direction.

[0034] Fig. 3shows a film cutting device 50, which is shown here in connection with two winding stations 60, 61, but can also be provided independently. The film web 34 can already have undergone a pre-treatment before entering the film cutting device. In particular, an initial edge trim can already have taken place in order to cut off some of the thickened areas at the edge, which leads to improved quality of an optional pre-treatment. Within the film cutting device, further cutting devices 51, in particular in the form of cutting knives, are provided, each of which carries out the final edge trimming on an edge of the still double-layered film web. For this purpose, the double-layered film web is guided over rollers 52 and 53, which primarily ensure the web tension required for the edge trimming. At the latest after the edge trimming, the double-layered film web is separated into two individual layers, which, however, still lie directly on top of one another.

[0035] The actual separation of the layers occurs through rollers 54, 55, which form a nip. After passing through the nip, the first layer is fed to the first winding station 60, where it passes over several additional rollers and is wound onto the roll 62.

[0036] Downstream of the separating device, a third measuring unit 45 is provided, the structure and operation of which are preferably similar to the second measuring unit 38. The measurement results (in raw form or as an evaluated thickness profile) are fed to the evaluation and control device 40 via a data line 44, for example, by wire and / or wirelessly.

[0037] The second layer can be fed to winding station 61, whose structure and function are identical to the first winding station. A second thickness gauge can also be provided for measuring the second layer. Regarding the structure and function, reference is made to the description in the previous paragraph.

[0038] The thickness measurement profiles recorded downstream of the separating device can be continuously added together by the evaluation and / or control device to create a roll total profile, i.e., the sum of the thickness profiles of the individual layers in a roll. In a blown film line with a reversing device but without a stretching device, deviations in film thickness from the average film thickness, i.e., thick and / or thin spots, are distributed along the axial direction of the roll, resulting in an overall uniform circumference of the roll. However, if a stretching device is provided, this can result in additional thick or thin spots that cannot be compensated for by reversing.By forming a roll sum profile as described above, the formation of, for example, piston rings (local thickenings) on the winding can be detected at an early stage and taken into account when setting the control parameters for the fluid application device.

[0039] Fig. 4 shows a perspective view of the flat-laying unit from Fig. 1 . Identical parts are identified by the same reference numerals as in Fig. 1 so that in this respect the description according to Fig. 1 The direction designations in the following description are based on the x, y, and z coordinates of the 105 coordinate system.

[0040] At the lower edge of the two triangular side areas of the flattening unit 9 are two heat sources 101 and 103, each positioned opposite the other. The two heat sources 101 and 103 are fixed to the crossbeams 102 and 104, respectively, with the crossbeams 102 and 104 moving synchronously with the reversing movement of the flattening unit 9. This synchronous movement ensures that the profiling is applied precisely at the desired location (e.g., in the area of ​​the folded edge of the double-layer film web formed by the take-off rollers 10). In addition, the heat sources 101 and 103 are precisely adjustable in their positions in the x-direction and y-direction to further increase the accuracy of the profiling.

[0041] The two heat sources 101 and 103 are controlled by a second thin-spot control system, whereby the second thin-spot control system can be connected to the first thin-spot control system. Profiling is achieved by heat input from heat sources 101 and 103, for example, by convection or IR radiation. The heat input locally heats the film to a temperature sufficient to enable plastic deformation due to the internal bubble pressure. A thin spot is thus created in the heated and stretched area. The extent of the thin spot is measured by the downstream measuring units 38 and / or 45 and controlled by varying the heating power.

[0042] The described second thin-spot control system is operated during the stretching process in combination with the first thin-spot control system (which introduces the thin spots via segmented control zones in the cooling ring). This is because with the first thin-spot control system, the segmented control zones are often at the limit of their possible performance, thus limiting the accuracy and speed of the control process. Profiling can therefore operate overall at a higher control speed and with a larger control stroke than would be possible with control using the first thin-spot control system alone. It also ensures that the introduced thin spots are positioned precisely in the area of ​​the folded edge. By using the second thin-spot control system, the heating cartridges in the cooling ring can operate at lower power and thus have a longer control stroke to regulate the film profile. This increases profile quality and control speed. List of reference symbols 1 Blown film line 2 Extruder 3 Line 4 Blow head 5 Annular gap 6 film tube 7 Calibration basket 8 cooling ring 9 Flat-laying unit 10 Take-off roller 11 Hose axis 12 Arrow indicating the rotatability of the flat-laying device 13 Thin spot 14 Arrow to indicate the movement of the thin spot 15 Reversible trigger 16 Stationary roller 17 Arrow indicating the film tube for further processing 18 measuring unit 19 Double arrow to indicate the movement of the measuring unit 18 20 rail 21 Stretch gap 22 First pair of rollers 23 Stretching roller 24 film web 25 26 27 28 29 Area that includes cooling rollers 37 30 Route device 31 guide roller 32 33 Nip roller 34 film web 35 Puncturing device 36 Second pair of rollers 37 38 Second measuring unit 39 40 Evaluation and control device 41 42 43 44 data line 45 Third measuring unit 50 Film separator 51 Cutting devices 52 roller 53 roller 54 roller 55 roller 60 First changing station 61 Changing station 62 Wrap 100 101 heat source 102 traverse 103 heat source 104 traverse 105 Coordinate system

Claims

1. Device for producing inline-oriented tubular films (6, 24) in a blowing process, with a blown-film system (1) consisting of an extruder (2), a blow head (4), a cooling ring (8), a calibration cage (7), a lay-flat unit (9) and a reversing turn-around take-off means (15), wherein plastics materials in granulated form can be plasticized in the extruder (2) to form a viscous mass, wherein the mass can be formed into a ring shape in the blow head (4) and escapes through an annular nozzle (5) of the blow head (4), wherein the mass forms a film tube (6), wherein the film tube (6) with an enlarged diameter can be guided into the calibration cage (7), wherein the film tube (6) after the calibration cage can be fed to the lay-flat unit (9), in which the circular film tube (6) forms an elliptical cross-section with an increasing eccentricity which can be formed by two take-off rollers (10) into a double-layered film web (24), with an orientation system (30) for monoaxial drawing of the laid-flat film (24) in the machine direction, with a first thin-points regulation system (18, 40) which controls segmented regulation zones in the cooling ring (8) in order to mark thin points (13) such that, during drawing, a film (24) is produced which has a thickness profile having the lowest possible deviations from the average film thickness over the entire film width, with a second thin-points regulation system, characterized in that the second thin-points regulation system controls two opposite heat sources (101, 103) between the calibration cage (7) and the lay-flat unit (9), wherein the second thin-points regulation system can be connected to the first thin-points regulation system.

2. Device according to claim 1, characterized in that the two opposite heat sources (101, 103) are located at the lower edge of the two triangular side areas of the lay-flat unit (9).

3. Device according to any one of the preceding claims, characterized in that the two heat sources (101, 103) are fixed on traverses (102, 104), wherein the traverses (102, 104) move synchronously with a reversing movement of the lay-flat unit (9).

4. Device according to any one of the preceding claims, characterized in that the heat sources (101, 103) can be adjusted along and across the traverses (102, 104).

5. Device according to any one of the preceding claims, characterized in that the second thin-points regulation system can be connected when the segmented regulation zones of the first thin-points regulation system are at their power limit.

6. Device according to any one of the preceding claims, characterized in that the second thin-points regulation system can be connected in order to regulate the thin points exactly in the area of the two folding edges of the lay-flat unit (9).

7. Device according to any one of the preceding claims, characterized in that a measuring unit (38, 45) is provided downstream of the orientation unit (30), as viewed in the transport direction, for detecting the thickness profile of the film (34) produced.