Stretching device and method for stretching a plastic film in its direction of transport

Description

[0001] The invention relates to a stretching device and a method for stretching a plastic film in its transport direction.

[0002] Such a stretching device serves to selectively influence the properties of a plastic film (see, for example, DE1504647A1). In particular, this stretching alters the orientation of the molecules contained in the plastic film (hereinafter also referred to simply as "film"). To effect stretching, the stretching device comprises a first roller, which can be driven by a first drive and rotated at a first peripheral speed. The stretching device also comprises a second roller, which can be driven by a second drive and rotated at a second peripheral speed. In the transport path of the plastic film, the second roller is arranged downstream of the first roller.

[0003] The stretching of the plastic film occurs because the second circumferential speed is greater than the first. Typically, the second circumferential speed is at least twice the first. The ratio of these circumferential speeds is also known as the stretch ratio of the film.

[0004] However, various problems can arise during stretching. The film's properties, particularly geometric characteristics such as its thickness profile perpendicular to the transport direction, and its flatness, can change as a result of the stretching process. In particular, stretched films often exhibit edge thickening while simultaneously becoming narrower than in their original state, a phenomenon known as neck-in. This necessitates trimming the film edges, leading to unwanted waste and high costs.

[0005] The object of the present invention is therefore to propose a stretching device and a method with which at least some of the aforementioned problems can be reduced.

[0006] According to the invention, this problem is solved by all the features of claim 1. Possible embodiments of the invention are specified in the dependent claims.

[0007] According to the present invention, it is provided that at least one first electrode, which can be subjected to a first electrical potential and which is arranged in the angular range of the first roller on which the plastic film rests and / or upstream of this angular range, and the first roller can be subjected to a second electrical potential.

[0008] The first and second rollers are each partially wrapped by the film. The film encounters the roller at a leading edge (which is not necessarily linear), is guided over it through an angular range, and exits the roller at a trailing edge or release edge. The film is guided freely between the trailing edge of the first roller and the leading edge of the second roller, where it is stretched. Specifically, this means that there is no further guiding element, such as another roller, in this intermediate area. This area is also referred to as the stretching gap.

[0009] An electrode according to the invention preferably comprises an electrical conductor, which in particular comprises a metal, wherein the conductor is mounted in a housing which consists at least partially of electrically insulating material. The electrode is connected to parts of a machine frame via the housing. The electrical conductor faces the film, with the space between the electrical conductor and the film being free. The electrical conductor can be in the form of several pins, needles, or even disc-shaped (hereinafter referred to as discs), the ends, tips, or bases of which point towards the film. Preferably, several pins, needles, or discs are arranged in a row or several parallel rows, pointing transversely to the transport direction of the film.The spacing between the pins, needles, or discs is preferably less than 20 mm, more preferably less than 10 mm, and particularly less than 5 mm, in order to generate the most homogeneous electric fields possible on the surface of the film. These pins, needles, or discs are preferably connected to one another by one or more wires. Pins or wires can generate very high electric fields at their ends facing the film. Alternatively, one or more wires may be provided facing the film. The advantage of a wire is that it can generate a homogeneous electric field. A wire is preferably arranged transversely to the transport direction of the film, but can also be bent in a helical or wedge shape. Ribbon-like electrical conductors may also be provided, which are not made of a metal but comprise materials that have a significantly lower conductivity than metal.Additionally or alternatively, conductive ceramics can also be used as electrical conductors in electrodes.

[0010] An electrode also includes an electrical supply line and / or an electrical connection. An electrode can be a charging electrode, which releases or absorbs no or only minimal charge carriers, or a discharging electrode, which intentionally releases charge carriers and can thus directly electrically charge the film.

[0011] Within the scope of the invention, the terms "electrical conductor" or "electrically conductive" are understood to mean an electrical conductivity of at least 1 S / m (Siemens per meter). If the conductivity is lower, the material is referred to as a "non-conductor" or "insulating material" within the scope of the invention.

[0012] An electric generator can be used to impart an electric charge to the electrode, which can be varied. The electric charge can be positive or negative, or its polarity can periodically change by applying alternating current. By imparting an electric charge, the electrode acquires an electrical potential relative to its surroundings.

[0013] Multiple first electrodes can be provided. This means that several first electrodes can be arranged in series along the film's transport direction to enhance the effect on the film described below. All electrodes can have the same electrical potential. However, it is particularly possible to apply different potentials to the first electrodes in order to control the effect on the film more precisely.

[0014] According to the invention, it is further provided that the first roller can also be subjected to an electrical potential. Preferably, the electrical potential of the roller differs from that of the electrode, so that an electrical voltage is generated between the electrode and the roller, which induces an electric field. In particular, however, the first roller is grounded, so that the electrical potential is zero, i.e., at ambient potential. For this purpose, it is advantageous if the roller is electrically conductive.

[0015] All rollers described within the scope of these disclosures can be designed, at least partially, to be electrically conductive, so that the electrical potential arises on their outer surface. Electrically conductive components leading to the environment, and in particular to a generator, can include a rotary bearing of the roller or a sliding contact. Grounding can also be achieved, in particular, by passing currents through a temperature control fluid to maintain the temperature of the roller or the roller interior. All described rollers can also be provided with a coating to prevent damage, especially scratches, to the foil.

[0016] Overall, the aforementioned arrangement results in an electric field acting on the film, which induces an electric charge displacement within the film, which is generally non-conductive. Alternatively or additionally, the film can be charged with electric charges, i.e., electric charge carriers can be added or removed. In this case, the film is (additionally) held against the surface of the first roller by an electric force. This ensures that the film, particularly at its edges, is guided across the full angular range. The constriction or edge thickening that often occurs before the release edge can thus be reduced.If the electrode is positioned near or even upstream of the first roller, and especially at the leading edge of the first roller, the additional electrical force directed towards the roller reduces the amount of air carried along by the film that can penetrate into the area between the film and the roller. This reduced air volume, compared to the prior art, further improves the effect of reduced edge thickening and constriction.

[0017] According to the invention, at least a second electrode, which can be subjected to a third electrical potential and which is arranged in the angular region of the second roller on which the plastic film rests, and the second roller itself, which can be subjected to a fourth electrical potential, are provided. Assuming that the first and second rollers rotate in opposite directions, this means that the reverse side of the film can now be influenced by an electrical potential. The polarity of the third electrical potential is opposite to that of the first electrical potential, thus further intensifying the influence on the film. In particular, this ensures that the film, including its edge regions, experiences a force in the direction of the second roller as quickly as possible. Consequently, the leading edge is as linear as possible.For this purpose, it is advantageous that the second electrode is located directly behind the leading edge in the area or in the transport direction of the film.

[0018] Multiple secondary electrodes can be provided. This means that several electrodes can be arranged in series along the film's transport direction to amplify the described effect on the film. All secondary electrodes can have the same electrical potential. However, it is particularly possible to apply different potentials to the secondary electrodes so that the effect on the film can be controlled more precisely.

[0019] It is further advantageous to provide at least a third electrode, which can be supplied with a fifth electrical potential and which is located in the angular region of the second roller on which the plastic film rests, and downstream of the second electrode, wherein the fifth electrical potential has the opposite polarity to the third potential. However, the potential may have a different magnitude. The advantage of a third electrode is that the charge displacement within the film, caused by the first electrode and—if provided—by the second electrode, can be at least partially reversed. This is therefore referred to as a compensating charge. Consequently, the film is no longer subjected to an electrical force in the direction of the roller in the area of ​​the release edge of the second roller, allowing it to be easily removed.As a rule, follower rollers rotate at comparable peripheral speeds to the second roller, so that only a small tensile force acts on the film. In principle, the inclusion of an electrode to compensate for the film's charge is also conceivable in conjunction with a roller other than the second one.

[0020] In a further advantageous embodiment of the invention, at least a fourth electrode is provided, which can be supplied with a sixth electrical potential, wherein the fourth electrode is arranged downstream of the second roller. This makes it possible to apply a compensating charge to the film from both sides, provided that at least one electrode is provided on each side of the film, so that the charge displacement within the film can be completely compensated. The film can then be processed further without any charge, for example, by winding it up. Measuring devices for characterizing the film can also be operated without being affected by electrical charges. The at least one fourth electrode is preferably supplied with a potential that periodically changes in polarity, thus generating an alternating voltage. This compensates the charge buildup on the film particularly reliably.Several fourth electrodes can be provided on one or both sides of the foil, with the electrical potentials differing, as already described in connection with the first, second, and third electrodes. The fourth electrodes are preferably supplied with a potential such that a voltage between 1 and 10 kV (kilovolts) is generated relative to ground.

[0021] In a further embodiment of the invention, at least one measuring device is provided for measuring the electrical charge and / or the electrical voltage of the film, in order to measure the charge of the film on one or both sides (front and / or back). This measuring device can be a measuring strip extending transversely across the film with several individual measuring points or with a measuring element extending over the entire transverse direction. Alternatively, a single measuring point or a group of measuring points can be provided, which can be moved across the film by means of a traversing device. Such traversing devices are known to those skilled in the art from measuring devices for measuring film thickness profiles.

[0022] The measurement result can be used, for example, to control or even regulate the compensation charge. Partial or all of the electrodes can also be controlled or regulated with respect to the set potential. While control can be performed by the operator, a computer and control unit is advantageous for regulation. This unit repeatedly compares the measurement results with target values ​​and, in the event of deviations exceeding a tolerance range, adjusts the application of electrical potentials and / or currents to at least some of the electrodes. During the comparison, the determination of the target value, and / or the determination of the tolerance range, the computer and control unit can consider various other parameters, such as the film's stretch ratio, the web speed, or intrinsic properties of the film (material composition, number of layers, total thickness, etc.).Other measured values, such as the measured thickness of the film, can also be incorporated into the control parameters.

[0023] In an advantageous embodiment of the invention, at least some of the aforementioned electrodes can be subjected to a voltage to ground between 5 and 60 kV. At these voltages, the electrodes are supplied with an electric current, preferably between 1 and 50 mA.

[0024] One or, in particular, several high-voltage generators can be provided for each electrode. Multiple generators have a synergistic effect. In conjunction with the control system described above, it can be provided that at least one generator is controlled with respect to its voltage and at least one generator is regulated with respect to its charging current. This results in more stable control.

[0025] A preferred embodiment of the invention includes the fact that at least some of the electrodes are adjustable with respect to their effective width. This means that the effectiveness, i.e., the application of a potential, does not extend over the entire width of the roller, but only over a portion of it. Furthermore, this also makes it possible to adapt the effective width to the width of the film. This enables and makes the application of the potential to the film more effective, especially in the edge regions.

[0026] To realize these advantages, it may be possible to connect needles, pins, or individual wires, each emitting an electric field, individually or in groups, thus enabling the switching on or off of parts of the electrodes. Electrically insulating covers may also be provided, which can be inserted between the aforementioned electrical conductors and the film. These could, for example, be plastic plates that can be attached to the electrode housing. To achieve the aforementioned advantages, insulating cover elements may also be provided, with which sections of the roller or film can be covered. Such cover elements can be arranged directly or indirectly on the machine frame, for example, by being detachably and / or slidably attached.Alternatively or additionally, it may be provided that individual or groups of electrical conductors are attached to support elements, with each pair of support elements being arranged to be displaceable relative to each other in the transverse direction of the film. Instead of a single electrode, two separate electrodes, each smaller than the width of the roller but equal to or greater than half the roller width, may be arranged side by side and slightly offset in the transport direction of the film. These electrodes then form an electrode pair and can be displaced relative to each other in the transverse direction of the film, so that the electrode pair can be adapted to different film widths.

[0027] It is also conceivable that an electrode with a smaller width than the width of the roller is used. Such an electrode can be mounted so that it can be moved axially along the roller and periodically moved back and forth parallel to the roller axis during operation (changing).

[0028] To achieve more effective application of an electrical potential to the edges of the film, the electrode can be positioned only in one of these edges. To accommodate changes in film width, this electrode can, of course, be mounted so that it can be moved transversely to the film's transport direction.

[0029] In an advantageous embodiment of the invention, the distance of at least one of the electrodes to the film is adjustable. If the electrode is located in the area of ​​one of the rollers, this refers to a change in the radial direction of the roller. If the film runs along a plane in the area where the electrode is located, this refers to a change in the direction orthogonal to the plane of the film. Instead of the entire electrode, parts of the electrode can also be radially displaceable. This means that different sections of the electrode can assume different distances to the film in the transverse direction to the transport direction of the film. This allows for better adaptation to different properties of the film in its transverse direction.Changing the distance between the electrode or parts thereof and the foil allows not only the voltage applied to the foil to be varied, but also the distribution of the electric field across the foil. This distance can be adjusted using mechanisms that, for example, allow the electrode to be moved along rails. The distance can be changed manually or automatically, i.e., by a motor. In the case of automatic adjustment, the change in distance can be controlled by the computer and control unit, particularly through a control system.

[0030] Furthermore, it is advantageous if the housing, and thus the electrical conductors of the electrode, can be adjusted in their angular position relative to the vertical on the film. In other words, when looking into the plane of the film, the electrodes can be tilted to the right or left. This allows the shape of the electric field on the film surface to be changed. Particularly with very small stretching gaps, this allows the electric field to be more effectively directed into the gap. Changing the angular position of the electrode can also be done manually or automatically, especially as part of a control system.

[0031] Furthermore, it is advantageous if the electrode, when positioned in the area of ​​one of the rollers, is pivotably mounted in the circumferential direction of the roller. With variable roller positions, particularly of the first and second rollers, the position of the leading edge and / or the releasing edge can change. This measure makes it possible to bring the electrode closer to the leading edge or the releasing edge.

[0032] It is advantageous if the electrode can be positioned for cleaning and / or starting up in a way that allows it to be at least 50 cm away from the roller surface or the film transport path between two rollers. This facilitates cleaning the roller and / or feeding the film into the stretching device during the start-up phase.

[0033] Additionally, a drive unit can be provided to move the electrode into the cleaning and / or approach position without manual intervention. The other position changes of the electrode described above can also be performed without manual intervention by at least one drive unit. A position change can be triggered by the computer and control unit, for example, when a track break is detected by a track break sensor.

[0034] In the stretching device according to the invention, a cleaning unit for cleaning the circumferential surface of at least one roller can be provided. Irregularities can be detected by regularly measuring the current to or within at least one electrode and / or to or within at least one roller. Thus, a change in the current determined over time can indicate contamination of one of the rollers or one of the electrodes. This indication can be displayed to the operator on a display device, enabling the operator to initiate or carry out cleaning.

[0035] Finally, the electrode can be rotated around the orthogonal side of the foil for fine positioning.

[0036] In a stretching device according to the invention, machine frame elements can be provided in which feed rollers can be stored and removed. Additionally, in this case, it can be provided that at least some of the electrodes are replaceable. With this measure, at least one electrode can be replaced by a feed roller. The functionality of feed rollers is disclosed in patent specification EP 2 498 976 B1, which is hereby deemed to be incorporated in this application. The advantage is that the optimal means for reducing neck-in and constriction within a stretching device can be used for different film types as well. A feed roller can be provided, in particular, at the leading edge of one or both rollers, preferably the first roller, in order to minimize the air ingress between the film and the roller as described above.

[0037] In a further advantageous embodiment of the invention, the first and / or the second roller comprises openings in its surface which can be subjected to a vacuum. This allows the force acting on the film towards the roller surface to be increased even further, so that the positive effects of the invention can be even more pronounced, particularly depending on the properties of the film. If such a roller has a coating as described above, it is additionally provided that this coating is permeable to air.

[0038] A stretching device according to the invention is particularly advantageous when combined with a blown film line, but also with a cast film line. With these lines, it is possible to influence the edge thickening and / or neck-in, and thus this effect within the stretching device, already during film production from polymer melts by appropriately controlling the film profile. The properties of the electrodes and / or the rollers and their electrical operating parameters can be taken into account in the control of the film profile. Conversely, machine and film production parameters within the blown film or flat film machine can be used to adjust the electrical parameters within the stretching device, especially if these electrical parameters are controlled by the computer and control unit.

[0039] The stretching device according to the invention can also advantageously process a film that is supplied wound onto a roll and unwound in an unwinding device. The film parameters for each roll can be stored, for example in a roll log, and supplied to the computer and control unit of the stretching device. This allows electrical machine and / or operating parameters to be set for each position of the film along its longitudinal direction in order to best fulfill the aforementioned task.

[0040] Films that can be processed in the stretching device can be, for example, so-called "breathable films" (i.e., films with perforations for the controlled passage of gases), but also barrier films. Films can also be, in particular, blocked or unblocked polyethylene films.

[0041] The above-mentioned problem is additionally solved by a method for stretching a plastic film in its transport direction, wherein the plastic film is guided over a first roller (SD), the roller being driven by a first drive and rotating at a first peripheral speed, and wherein the plastic film is guided over a second roller (FD), the roller being driven by a second drive and rotating at a second peripheral speed, the second peripheral speed being greater than the first peripheral speed, wherein in the transport path of the plastic film the second roller is arranged downstream of the first roller, so that the plastic film is stretched in the free area between the first and the second roller.

[0042] The method according to the invention is characterized in that the plastic film is subjected to an electric field by means of at least one first electrode in the angular region of the first roller on which the plastic film rests and / or upstream of this angular region, wherein the electrode is subjected to a first electric potential, and that the first roller is subjected to a second electric potential. The method according to the invention achieves the same advantages as those already explained in connection with the stretching device according to the invention.

[0043] Further advantages, features, and details of the invention will become apparent from the following description, in which various exemplary embodiments are explained in detail with reference to the figures. The features mentioned in the claims and in the description can each be essential to the invention individually or in any combination thereof. Within the scope of the entire disclosure, features and details described in connection with the method according to the invention naturally also apply in connection with the stretching device according to the invention, and vice versa, so that the disclosure always refers, or can refer, to the individual aspects of the invention. The individual figures show: Fig. 1 Schematic view of a track system according to the invention. Fig. 2 A close-up of the Figure 1Fig. 3 A blown film line with a drawing unit according to the invention. Fig. 4 Another blown film line according to the invention with a drawing unit.

[0044] The Figure 1 Figure 1 shows a schematic representation of a stretching device 100 according to the invention. The web-shaped plastic film 101 enters the stretching device 100 in the transport direction T. The plastic film first runs onto one or successively onto several preheating rollers, of which only one preheating roller 102 is shown. A preheating roller has the function of bringing the film to a predefined temperature. For this purpose, a preheating roller is usually temperature-controlled, often by introducing a temperature-controlled fluid into the preheating roller.

[0045] After leaving the preheating roller(s) 102, the film web 101 reaches a first roller 110, which can also generally be referred to as the first stretching roller 110. This stretching roller is connected to a drive (not shown), for example, its own electric motor, which rotates the roller 110 at a first rotational speed in the direction of arrow R. This roller can also be referred to as an SD roller (SD stands for "slow drive"). The film approaches the first roller 110 at the leading edge 140. Ideally, this leading edge 140 runs vertically as a line into the plane of the drawing. In practice, the leading edge often deviates from a straight line.

[0046] A first feed roller 111 is preferably associated with the first roller 110, which together with the first roller 110 provides an entry gap for the film. Preferably, the entry gap or the path of the film 101 is arranged such that the film 101 runs tangentially to the rollers 110 and 111 in the entry gap. In other words, the entry gap coincides with the lead-in edge. The roller gap already serves to minimize the air space between the stretching roller 110 and the film 101. The roller gap also serves to make the lead-in edge straighter.

[0047] The first roller 110 is optionally assigned a second feed roller 112, which forms a run-out gap with the first roller 110. The second feed roller 112 can be adjustable in the circumferential direction of the first roller 110. The second feed roller 112 serves to ensure that the film leaves the first roller 110 along a line that runs parallel to the axial direction of the first roller 110. However, for space reasons, the actual release edge 141 is often located behind the second feed roller 112. The film runs off the first roller 110 at the lead-in edge 140. Ideally, this release edge 141 runs perpendicularly as a line into the drawing plane. In practice, however, the release edge also frequently deviates from a straight line. These deviations, in particular, lead to the effects of edge thickening and neck-in described at the beginning.

[0048] The area between the leading edge 140 and the release edge 141 is the angular area of ​​the roller 110 on which the film rests, i.e. the wrap angle of the first roller 110.

[0049] Viewed in the transport direction T of the film 101, a second roller 120 is arranged downstream, which can be referred to as the second stretching roller 120. This stretching roller 120 is also connected to a further drive (not shown), for example, its own electric motor, which drives the roller 120 at a second peripheral speed in the direction of arrow R'. The second stretching roller has a higher peripheral speed than the first stretching roller, which is why it can also be referred to as an FD roller (FD stands for "fast drive"). This results in the film 101 being stretched in its transport direction between the release edge 141 of the first roller 110 and the lead-in edge 150 of the second roller 120 in proportion to the peripheral speeds. The distance between the release gap and the entry edge is often also referred to as the stretching gap.

[0050] It is possible that the first roller 110 and the second roller 120 are movable relative to each other. This allows the stretching gap to be influenced. A change in the stretching gap can affect the properties of the film.

[0051] The second roller 120 is optionally assigned a third feed roller 122, which forms a run-out gap with the roller 120. The third feed roller 122 can also be adjustable in the circumferential direction of the second roller 120. The third feed roller 122 serves to ensure that the film leaves the first roller 120 along the release edge 151, which runs parallel to the axial direction of the roller 120. However, for space reasons, the actual release edge 151 is often located behind the third feed roller 122. Ideally, this release edge 151 runs perpendicularly as a line into the drawing plane. In practice, however, the release edge 151 often deviates from a straight line.

[0052] The area between the leading edge 150 and the release edge 151 is the angular area of ​​the roller 120 on which the film rests, i.e. the wrapping angle of the first roller 120.

[0053] In principle, one or more feed rollers can be omitted in a stretching device according to the invention. Nevertheless, the term "entry gap" or "exit gap" can still be used.

[0054] Further stretching rollers, in particular with one or two feed rollers each, may be provided, wherein two stretching rollers arranged immediately one after the other are driven in such a way that the stretching roller arranged downstream has a higher circumferential speed than the preceding stretching roller.

[0055] Downstream of the stretching rollers 110, 120 a cooling roller 130 is arranged, with which the film 101 can be cooled again, so that the new molecular orientation within the film resulting from the stretching process is solidified.

[0056] To increase the holding force of the film on the first roller, a first electrode 160 can be provided in the region of the wrap angle of the first roller 110 or upstream thereof, which adds an electric charge to the side of the film facing it and / or induces a charge displacement within the film. In the Figure 1 This is symbolized by a hyphen in an ellipse. Any number of further electrodes 161 can be arranged downstream of a first electrode. In the Figure 1 Two further electrodes are shown. Each of the first electrodes can be arranged in the transport direction T of the film in front of or behind the optional feed roller 112.

[0057] Likewise, a second electrode 165 can be provided in the area of ​​the wrapping angle of the first roller 120, which adds an electric charge to the side of the film facing it and / or induces a charge displacement within the film. In the Figure 1 This is symbolized by a "-" sign in an ellipse, which means that the polarity of the charge or charge displacement corresponds to that caused by electrode 160 or 161, respectively. Any number of further electrodes 166 can be arranged downstream of a first electrode. In the Figure 1 Two further electrodes are shown. The second electrode, 165 or 166, also applies the same polarity to the film as electrodes 160 or 161. However, now the side of the film that has contacted the roller 110 is affected. It is advantageous if electrode 165 acts on the film as close as possible to the leading edge 150.

[0058] Upstream of the detachment edge 151 of the second electrode, another electrode 170 is directed towards the foil, which also imparts charges to the foil or causes a charge displacement within the foil. However, the polarity of the potential of electrode 170 is opposite to that of the previous electrodes 160, 161, 165, and 166, which is indicated by a "+" sign. This reduces or, as far as possible, completely compensates for the charging of the foil or the charge displacement within the electrode.

[0059] Alternatively or additionally, an electrode 171 is provided, which is preferably arranged in the area of ​​the release edge 151. Its mode of operation corresponds to that of the electrode 170.

[0060] After the film leaves the second roller 120 at the release edge 151, it may still be electrically charged. To dissipate this charge and enable the film to be transported and / or processed in a state that is as electrically neutral as possible, at least one further electrode 180, 180' is provided. Preferably, two further electrodes are provided opposite each other, between which the film is transported. In contrast to the electrodes described above, it is preferably provided that the electrode 180, 180' is supplied with an alternating voltage. Further electrodes 181 may be provided if the charge of the film has not yet been compensated.

[0061] To determine the charge of the film, at least one charge measuring device 182 is provided, preferably arranged downstream of the detachment point 151. This device can be positioned upstream or downstream of the electrode 180, 180', and in the case of multiple charge measuring devices, at both positions. Measuring devices 182 can be provided on both sides of the film to determine the charge on both of its surfaces. The measuring device 182 can be used not only to determine whether charge compensation has been completed, but can also be configured to determine the local charge of the film across its width on a single surface.

[0062] The Figure 2 is a cropped enlargement of the Figure 1and illustrates the possible positional changes of any electrode using electrode 165 as an example. First, the electrode can be moved in a direction perpendicular to the plane defined by the film, or, if the film rests on a roller, perpendicular to the tangential plane, as indicated by the double arrow 200. For this purpose, a first adjustment mechanism, not shown in the figure, can be provided in the stretching direction. The perpendicular to the tangential plane 201 is shown with a dotted line.

[0063] To position the electrode at specific points on the film, it is designed to be possible to move the electrode in the film's transport direction. If the film runs on a roller, as is the case in the Figure 2As shown, the electrode can be repositioned in the direction of the circumference of the roller 120. This is indicated by arrow 202. The pivot axis preferably coincides with the axis of rotation of the roller 120. A corresponding pivoting mechanism is provided, but not shown.

[0064] To allow for fine adjustment, the electrode can be rotated about the perpendicular to the tangential plane 201, as illustrated by the double arrow 203.

[0065] Additionally, the electrode can be rotated about an axis of rotation 204 in the direction of the double arrow 205, whereby this axis of rotation 204 runs parallel to the plane of the foil, but through the electrode.

[0066] Furthermore, the electrode can be pivoted about a second pivot axis (not shown) to allow it to be completely removed from the roller. This is helpful to create space when, for example, the roller needs to be cleaned or a new film needs to be fed into the stretching device. The second pivot axis is located parallel to the axis of rotation, but outside the roller and preferably outside the electrode. This results in the pivot direction 206. However, the second pivot axis can be located differently, resulting in a different pivot direction.

[0067] The Figure 3Figure 1 shows a device 1 for producing a film tube, namely a blown film line, which initially comprises at least one extruder 2 with which, for example, plastic in granular form can be plasticized. The plastic melt thus produced is fed via a line 3 to an extrusion die 4, which can also be called a die head, from which this melt is transferred into a film tube 6, so that this melt stream can be drawn out of an annular gap 5 (not visible in this figure) in the draw-off direction z. The resulting film tube 6 is not yet solidified. This is inflated from the inside by a slight overpressure, so that it has a larger diameter within the optional calibration device 7.The film tube is solidified in particular by a temperature control device 8, which is often also referred to as a cooling ring because of its ring-like design surrounding the film tube.

[0068] After passing through the calibration device, the film tube 6 enters the working area of ​​a flattening device 9, in which the circular film tube is transformed into an elliptical cross-section with increasing eccentricity until it finally forms a double-layered plastic film, connected at its sides, in the influence area of ​​the take-off rollers 10.

[0069] The flattening device is rotatably arranged, with the axis of rotation essentially coinciding with the hose axis 11, which is located in the Figure 1 The alignment is indicated by a dashed line. The rotatability of the flattening device is indicated by arrow 12.

[0070] The Figure 3further shows a reversing device 15, which has the task of guiding the flattened film tube from the flattening device to the stationary roller 16 without causing damage.

[0071] Downstream of the reversing device 15, a stretching device 100 according to the invention is now arranged, which is already in connection with the Figures 1 to 2 and has been explained in the preceding description. The one in the Figure 3 The stretching device shown may correspond to the one described in the Figure 1The diagram shows, although not all electrodes and measuring devices are depicted. It should also be noted that a cutting device may be positioned upstream of the stretching unit 100, with which one or two edges of the fold can be cut open or closed. Additionally, a separating device may be provided with which the double-layered plastic film can be divided into one or more single-layered films.

[0072] Arrow 17 indicates that this foil tube, after passing through the stretching device 100, is guided to further processing, which is not specified in more detail here.

[0073] The Figure 4Figure 1 shows a further embodiment of a blown film system according to the invention, in which the stretching device 100 is now arranged between the take-off rollers 10 and the reversing device 15. It should be noted that the rollers within the stretching device are now arranged in such a sequence that the plastic film can be transported from bottom to top. Reference symbol list 100 Stretching device 101 plastic film 102 preheating roller 110 First roller 111 First roller 112 Second feed roller 120 Second stretching roller 122 Second feed roller 130 Cooling roller 140 Lead-in edge 141 Detachment edge 150 Lead-in edge 151 Detachment edge 160 First electrode 161 Additional electrodes 165 Second electrode 166 Additional electrodes 170 electrode 171 electrode 180 electrode 180' 181 electrode 182 Charge meter 200 Double arrow 201 tangent plane 202 Arrow 203 Double arrow 204 axis of rotation 205 Double arrow 206 direction of rotation 1 Device for producing a foil tube 2 Extruder 3 Line 4 Extrusion tool 5 Invisible annular gap 6 Unsolidified foil tube 7 Optional calibration device 8 Temperature control device 9 Flattening device 10 Take-off rollers 11 Hose axle 12 Arrow to illustrate the rotatability of the flattening device 13 14 15 Reversing device 16 Stationary roller 17 Arrow indicating further processing RR' Arrow T Direction of transport Z Direction of withdrawal

Claims

1. Stretching device (1) for stretching a plastics material film (101) in the transport direction (T) thereof, having a first roller (SD) (110), via which the plastics material film (101) can be guided and which can be driven with a first drive and rotated at a first circumferential speed, and having a second roller (FD) (120), via which the plastics material film (101) can be guided and which can be driven with a second drive and rotated at a second circumferential speed, wherein the second circumferential speed is greater than the first circumferential speed, wherein in the transport path of the plastics material film (101) the second roller (120) is arranged downstream of the first roller (110) so that the plastics material film can be stretched in the free region between the first and the second roller, wherein at least a first electrode (160) which can be acted on with a first electrical potential and which is arranged in the angular range of the first roller (110) on which the plastics material film (101) rests and / or upstream of this angular range, and in that the first roller (110) can be acted on with a second electrical potential, characterized in that at least a second electrode (165) which can be acted on with a third electrical potential and which is arranged in the angular range of the second roller (120) on which the plastics material film (101) rests, and in that the second roller (120) can be acted on with a fourth electrical potential, wherein the polarity of the third electrical potential is opposed to the first electrical potential, wherein the first roller and the second roller rotate in opposing directions so that the rear side of the film (101) can be acted on with the third electrical potential.

2. Stretching device (100) according to claim 1, characterized in that at least a third electrode is provided, which can be acted on with a fifth electrical potential and which is arranged in the angular range of the second roller (120) on which the plastics material film (101) rests and downstream of the second electrode (165), wherein the fifth electrical potential has a polarity which is opposed in comparison with the third potential.

3. Stretching device (100) according to claim 1 or 2, characterized in that at least a fourth electrode which can be acted on with a sixth electrical potential is provided, wherein the fourth electrode is arranged downstream of the second roller (120).

4. Stretching device (100) according to any one of the preceding claims, characterized in that the spacing of at least one of the electrodes relative to the film (101) can be changed.

5. Stretching device (100) according to any one of the preceding claims, characterized in that the width of at least one of the electrodes can be varied.

6. Stretching device (100) according to any one of the preceding claims, characterized in that the angular position of at least one of the electrodes which are arranged in an angular range of one of the rollers on which the plastics material film rests can be changed.

7. Method for stretching a plastics material film (101) in the transport direction (T) thereof, wherein the plastics material film (101) is guided via a first roller (SD) (110), wherein the roller (110) is driven with a first drive and rotates at a first circumferential speed, and wherein the plastics material film (101) is guided via a second roller (FD) (120), wherein the roller (110) is driven with a second drive and rotates at a second circumferential speed, wherein the second circumferential speed is greater than the first circumferential speed, wherein in the transport path of the plastics material film (101) the second roller (120) is arranged downstream of the first roller (110) so that the plastics material film (101) is stretched in the free region between the first roller and the second roller, wherein the plastics material film (101) in the angular range of the first roller (110) on which the plastics material film (101) rests and / or upstream of this angular range is acted on by means of at least a first electrode (160) with an electrical field, wherein the electrode (160) is acted on with a first electrical potential, and the first roller (110) is acted on with a second electrical potential, characterized in that at least a second electrode (165) which is arranged in the angular range of the second roller (120) on which the plastics material film (101) rests, is acted on with a third electrical potential and in that the second roller (120) is acted on with a fourth electrical potential, wherein the polarity of the third electrical potential is opposed to the first electrical potential, wherein the first roller and the second roller rotate in opposing directions so that the rear side of the film (101) is acted on with the third electrical potential.