Stretching system and method for reducing uneven temperatures and air currents in a foil stretching system

Description

[0001] The invention relates to a stretching system for stretching a film and a method for reducing uneven temperatures and air flows in a film stretching system.

[0002] Stretching systems are used primarily in the production of plastic films. Typically, the film to be stretched is moved through a furnace by means of a conveyor system in a haul-off direction. In the furnace, the film to be stretched is heated before the stretching process and maintained at a predetermined temperature during the stretching process. The stretched film is then cooled in the furnace.

[0003] Throughout the entire process, it is necessary to maintain a uniform temperature in the transverse direction, i.e., perpendicular to the draw-off direction. Otherwise, processes in the film being stretched will occur at different times in the transverse direction, resulting in film defects and uneven film properties. Uneven temperature control in the transverse direction is caused, for example, by undesirable air currents in the draw-off direction, which cause cold air to flow unevenly into the oven.

[0004] EP 3 398 750 A1 discloses a device for controlling air flow in a furnace for producing stretched films. The device is mounted outside the furnace at the inlet or outlet of the furnace.

[0005] It is therefore an object of the invention to provide a stretching system and a method in which uneven temperatures of the film in its transverse direction are reduced.

[0006] The object is achieved by a stretching system for stretching a film, in particular a transverse, longitudinal and / or simultaneous stretching system, comprising an oven and a compensating device. The oven has a vertical direction, a transverse direction and a draw-off direction. The compensating device has an air conveyor, a first compensating opening and a second compensating opening, wherein the first compensating opening and the second compensating opening open into the oven on opposite sides of a center plane of the oven in the vertical and draw-off directions. The air conveyor is arranged fluidically between the first compensating opening and the second compensating opening, wherein the compensating device is designed to remove a quantity of air from the oven through one of the compensating openings and to feed the removed quantity of air back into the oven through the other of the compensating openings.

[0007] Using the compensating device, it is possible to generate transverse air currents on different sides of the film that counteract the undesired air currents, thus eliminating them. At the same time, the temperature or air balance in the oven remains unchanged, since the amount of air extracted from one side of the center plane is returned to the oven in equal measure on the other side.

[0008] The compensation device may be free of temperature control devices for the conveyed air.

[0009] However, it is also conceivable that a temperature control device, such as a heat exchanger, is included in the compensation device. This can be helpful for specifically counteracting temperature differences in the furnace and / or compensating for heat losses that occur in the compensation device.

[0010] In particular, the balancing device is a closed system.

[0011] For example, the compensation openings do not extend above the center plane of the furnace.

[0012] In order to further increase the uniformity of the air flow, the first compensating opening and the second compensating opening can be of the same design, although this is not a requirement.

[0013] In one aspect, the first compensating opening and the second compensating opening are arranged at substantially the same position in the withdrawal direction so that the compensating device itself does not generate air flows in the longitudinal direction.

[0014] In one embodiment, the first compensating opening and the second compensating opening are arranged in the region of a film inlet into the oven or in the region of a film outlet from the oven, thereby preventing the entry of air that is too cold or too warm from the environment of the stretching system.

[0015] For example, the first equalizing opening and the second equalizing opening are located in a neutral zone of the furnace, a zone before a neutral zone, and / or a zone after a neutral zone. The neutral zone is located between two zones of the furnace with significantly different temperatures, for example, an annealing zone and a cooling zone. This prevents airflow along large temperature gradients, making the stretching system more efficient.

[0016] In one aspect, a film path is defined in the furnace, in particular wherein the first compensating opening and the second compensating opening are arranged outside the film path with respect to the transverse direction and / or are arranged at the level of the film path with respect to the vertical direction. In this way, the film is not affected by the air conveyed by the compensating device.

[0017] In order to prevent the film from being blown directly, the first compensating opening and the second compensating opening can each have two flow openings, one of the two flow openings being arranged above and the other of the two flow openings being arranged below the film track.

[0018] For example, the first compensating opening and the second compensating opening each have a plurality of flow openings in at least one flow region, wherein the flow region extends vertically, in particular from below the film path to above the film path, or wherein the flow region extends transversely above or below the film path. The plurality of flow openings prevents interaction with the film, since the air is drawn from or supplied to the furnace not only at specific points, but over a larger area.

[0019] It is also conceivable that several flow areas are possible.

[0020] To avoid contamination of the treatment room, the air conveyor can be located outside the treatment room.

[0021] It is also conceivable to provide a filter in the compensating device, particularly in a pipe of the compensating device. This can improve the purity of the air in the furnace, thereby also increasing the quality of the produced film.

[0022] The air conveyor can be a radial fan to make the balancing device efficient.

[0023] For example, the first equalizing opening is fluidically connected to the air conveyor by a first pipe and the second equalizing opening is fluidically connected to the air conveyor by a second pipe in order to obtain defined flow paths.

[0024] The pipes are particularly insulated to prevent heat losses through the compensating device, which would lead to temperature differences in the furnace.

[0025] In one embodiment, the compensating device comprises a first flap box fluidically connected to the pressure side of the air conveyor, and a second flap box fluidically connected to the suction side of the air conveyor. The first pipeline and the second pipeline each branch into two branches, with one branch fluidically connected to the first flap box and the other branch fluidically connected to the second flap box. In this way, the air flow can be controlled.

[0026] The folding boxes, for example, are 2-way distributors.

[0027] For example, in a first position of the flap boxes, the first pipe is fluidically connected to the suction side of the air conveyor, and the second pipe is fluidically connected to the pressure side of the air conveyor. In a second position of the flap boxes, the first pipe is fluidically connected to the pressure side of the air conveyor, and the second pipe is fluidically connected to the suction side of the air conveyor. This allows the air flow within the balancing device to be reversed.

[0028] In the case of automatic control of the compensating device, the stretching system can have one or more sensors for the direction of an air flow and / or one or more pressure sensors.

[0029] In one embodiment, the stretching system has a transport system designed to transport the film through the oven in the withdrawal direction, in particular along the film travel path, thereby achieving a high level of process reliability.

[0030] Such transport systems are known, for example from WO2014 / 094803.

[0031] The compensation openings can be provided outside the transport system in the transverse direction and / or next to the transport system.

[0032] The problem is further solved by a method for reducing uneven temperatures and air flows in a stretching system, comprising the following steps: Measuring the size of the air flow in the withdrawal direction and / or the pressure on one side of the center plane of the oven of the stretching system, and removing, by means of a compensating device, a quantity of air from the oven through a first compensating opening on one side of the center plane of the oven and supplying the removed quantity of air on the other side of the center plane to the oven through a second compensating opening depending on the result of the measurement.

[0033] The measurement of the air flow size can be done automatically using sensors or manually, e.g. using a manual flow meter.

[0034] The features and advantages discussed for the stretching system apply equally to the process and vice versa.

[0035] Further features and advantages of the invention will become apparent from the following description and the accompanying drawings, to which reference is made. In the drawings: Figure 1 extremely schematic view of a stretching system according to the invention in plan view, Figure 2 a schematic section through the stretching system according to Figure 1 along line II-II, Figure 3 a perspective view of a part of a compensating device of the stretching system according to Figure 1 , Figure 4 a schematic, shortened section through the stretching system according to Figure 1 , the Figures 5, 6 and 7 a second, third and fourth embodiment of a stretching system according to the invention in a shortened section, Figure 8 a fifth embodiment of a stretching system according to the invention in a shortened section, and Figure 9 an enlarged sectional view of the Figure 8 along line AA.

[0036] In Figure 1 A stretching system 10 according to the invention is shown very schematically.

[0037] In the first embodiment shown, the stretching system is a transverse stretching system, also called a TDO ("Transverse Direction Orienter"). It is also conceivable that the stretching system could be a longitudinal or simultaneous stretching system.

[0038] The stretching system 10 has a furnace 12, a transport system 14 and a compensating device 16.

[0039] The furnace 12 has a withdrawal direction R that corresponds to the direction of movement of the film to be stretched. The transverse direction Q of the furnace 12 runs transversely to the withdrawal direction R and horizontally, and the vertical direction H runs vertically.

[0040] The oven 12 has different zones along the withdrawal direction R for treating the film to be stretched.

[0041] In the first zone 22, also called the preheating zone, the film is heated. In the subsequent second zone 24 ("stretching zone"), the film is stretched in the transverse direction Q, so that it has a greater width at the end of the second zone 24 than at the beginning.

[0042] After stretching, the film then passes through the third zone 26 (called "heat treatment zone", "further heating zone" and / or "annealing zone"), where the film can be relaxed at high temperatures.

[0043] The film then passes through a fourth zone 28 and a fifth zone 30 ("cooling zone"), where the film is cooled in the fifth zone 30.

[0044] The fourth zone 28 is called the neutral zone and serves to separate the third zone 26 and the fifth zone 30. The neutral zone is, for example, an empty room without ventilation.

[0045] The transport system 14 comprises, in a manner known per se, two transport rails 32 which are arranged mirror-symmetrically with respect to a central plane M of the stretching system 10 or the furnace 12 and which extend at least partially in the furnace 12.

[0046] In an inlet zone 34 and an outlet zone 36, in which the film to be stretched is fed into or removed from the stretching system 10, the transport rails 32 run outside the oven 12.

[0047] The film is gripped in a manner known per se by clips (not shown) of the transport system 14, which are guided along the transport rails 32, and transported in the withdrawal direction R through the furnace 12. The film runs in a film track F ( Fig. 2 ) which is defined in the furnace 12 by the transport system 14. The film path F intersects the center plane M.

[0048] Figure 2 shows a section through the stretching system 10 along the line II-II.

[0049] In the furnace 12, the transport rails 32 are shown, with the film path F being indicated by a dash-dotted line and the film by a dashed line.

[0050] In addition, several nozzle boxes 38 of the oven 12 are shown, which convey hot air toward the film web F, as indicated by the arrows. The film is heated or maintained at a predetermined temperature by means of the hot air.

[0051] In the area of ​​the film track F or the transport rails 32, at least two sensors 40 are optionally provided, one on each side of the center plane M. The sensors 40 are designed to measure the direction of an air flow and / or the air pressure prevailing at the sensor 40. Instead of the sensors 40, the direction of the air flow can also be determined manually, e.g., using a manual flow meter positioned at the measuring point.

[0052] The compensating device 16 has an air conveyor 42, a first folding box 44, a second folding box 46, a first pipe 48, a second pipe 50, a first compensating opening 52 and a second compensating opening 54.

[0053] The air conveyor 42, the flap boxes 44, 46, and parts of the piping 48, 50 are provided outside the furnace 12. For example, they are mounted on top of a casing of the furnace 12.

[0054] The pipes 48, 50 extend into the furnace 12, and the equalizing openings 52, 54 are finally provided within the furnace 12.

[0055] The compensating openings 52, 54 are arranged on different sides of the center plane M in the furnace 12. In particular, the compensating openings 52, 54 are located outside the film path F and outside the transport system 14 with respect to the transverse direction Q. In other words, the compensating openings 52, 54 are provided next to the transport system 14.

[0056] In the withdrawal direction R, the compensating openings 52, 54 are arranged at substantially the same position. This means, for example, that the offset of the compensating openings 52, 54 from one another in the withdrawal direction R is less than 2 m, in particular less than 1 m.

[0057] For example, the compensating openings 52, 54 are arranged within the same zone 22, 24, 26, 28, 30 or between the zones.

[0058] In the embodiment shown, the compensating openings 52, 54 are provided in the area of ​​a film inlet, ie in the area within the furnace 12 which directly adjoins the inlet zone 34. In the example according to Figure 1 the compensation openings 52, 54 are located in the first zone 22.

[0059] It is also conceivable for the compensating openings 52, 54 to be provided in the area of ​​a film outlet, i.e., within the furnace adjacent to the outlet zone 36. Likewise, the compensating openings 52, 54 can be arranged in the fourth zone 28, i.e., the neutral zone. It is also conceivable for the compensating openings 52, 54 to be arranged in the third zone 26 ("heat treatment zone") or in the fifth zone 30 ("cooling zone"). Accordingly, the sensors 40, if present, are to be offset in the withdrawal direction R.

[0060] Several compensating devices 16 can also be provided in different zones.

[0061] As in Figure 2 As can be seen, the first equalizing opening 52 is fluidically connected to the air conveyor 42 by means of the first pipe 48. Similarly, the second equalizing opening 54 is fluidically connected to the air conveyor 42 by means of the second pipe 50, so that the air conveyor 42 is fluidically arranged between the two equalizing openings 52, 54 in order to convey air from one of the equalizing openings 52, 54 to the other of the equalizing openings 54, 52.

[0062] The compensation device 16 is a closed system, so that the amount of air that is taken from the furnace 12 at one of the compensation openings 52, 54 is supplied to the furnace 12 at the other compensation opening 54, 52.

[0063] In the illustrated embodiment, the compensating device 16 is designed without any temperature control devices, such as heating or cooling. However, the pipes 48, 50 are insulated outside the furnace 12 to prevent temperature fluctuations between the compensating openings 52, 54.

[0064] In an alternative embodiment, a temperature control device 55, for example a heat exchanger, can be arranged in one or both of the pipes 48, 50.

[0065] It is also conceivable that one or more filters 57 are provided in one or both of the pipes 48, 50, which filter the air flowing through the compensation device 16.

[0066] Figure 3 shows the section of the compensating device 16 around the air conveyor 42 in a perspective and enlarged view.

[0067] In the embodiment shown, the air conveyor 42 is a radial fan and thus has a pressure side and a suction side.

[0068] The first flap box 44 is directly connected, i.e., fluidically connected, to the pressure side of the air conveyor 42. Similarly, the second flap box 46 is fluidically connected to the suction side of the air conveyor 42.

[0069] The valve boxes 44, 46 are, for example, 2-way distributors and are both fluidically connected to both the first pipeline 48 and the second pipeline 50.

[0070] For this purpose, the pipes 48, 50 each branch into two branches 56, 58, wherein the first branch 56, ie the first branch 56 of the first pipe 48 and the first branch 56 of the second pipe 50, is fluidically connected to the first flap box 44. Similarly, the second branches 58 of the pipes 48, 50 are each fluidically connected to the second flap box 46.

[0071] The flap boxes 44, 46 can assume two positions, wherein in the first position the flap boxes 44, 46 are switched such that the first flap box 44 fluidically connects the second pipe 50 to the pressure side of the air conveyor 42 and the second flap box 46 fluidically connects the first pipe 48 to the suction side of the air conveyor.

[0072] In this first position, air is extracted from the furnace 12 at the first equalizing opening 52 and fed back into the furnace 12 at the second equalizing opening 54. In this case, the first equalizing opening 52 is an air outlet, and the second equalizing opening 54 is an air inlet for the furnace 12.

[0073] In the second position of the folding boxes 44, 46, the first folding box 44 fluidically connects the first pipe 48 to the pressure side of the air conveyor 42 and the second folding box 46 connects the second pipe 50 to the suction side of the air conveyor 42. In this position, the second compensation opening 54 forms the air outlet and the first compensation opening 52 forms the air inlet for the furnace 12. The flow direction of the air is thus reversed in the second position compared to the first position of the folding boxes 44, 46.

[0074] Figure 4illustrates again the position of the compensation openings 52, 54 relative to the foil track F shown in dash-dotted lines in schematic form.

[0075] In the first embodiment, the compensation openings 52, 54 are located in the transverse direction next to the transport system 14 and the film track F. In the vertical direction H, the compensation openings 52, 54 are located at the level of the film track F.

[0076] In particular, the compensation openings do not extend beyond the median plane M.

[0077] In addition, the first compensation opening 52 and the second compensation opening 54 are of the same design.

[0078] During operation of conventional stretching systems 10, undesirable air currents in the withdrawal direction R are formed, which are opposite on the different sides of the center plane M, as indicated by the arrows marked L in Figure 1is indicated. In order to reduce these undesirable air currents L, the following method according to the invention is carried out.

[0079] First, the magnitude of the air flow L in the discharge direction R is measured. This can be done using sensors 40. Instead of sensors 40, the direction and magnitude of the air flow can also be determined manually, e.g., using a thread positioned at the measuring point.

[0080] It is also conceivable that the pressure, for example the pressure difference, is determined on both sides of the median plane M.

[0081] In Figure 1 illustrated case, before the commissioning of the balancing device 16 on the Figure 1 upper side of the median plane M (left side with respect to Figure 2 ) an air flow L into the furnace 12 and on the other side of the central plane M ( Fig. 1 lower side; Fig. 2right side) an air flow out of the furnace 12.

[0082] After measuring the air flows L, which can also be done manually, the compensating device 16 is now put into operation. For this purpose, the position of the flap boxes 44, 46 is first selected so that air is supplied to the furnace 12 through the compensating device 16 at that one of the compensating openings 52, 54 that is located on the side of the center plane M through which the undesired air flow L enters the furnace 12. This compensating opening then serves as an air inlet for the furnace 12.

[0083] In the embodiment shown, this is the first compensation opening 52, so that the folding boxes 44, 46 are thus brought into the second position.

[0084] The air conveyor 42 is then put into operation, whereby air is removed from the furnace 12 through the second equalizing opening 54 and is fed back into the furnace 12 in the same quantity at the first equalizing opening 52.

[0085] In this way, a counterflow to the air flow L is created, which weakens the unwanted air flow L.

[0086] The speed of the air conveyor 42 is increased until the undesired air flow L is minimized or completely stopped. The speed can be controlled automatically or adjusted manually.

[0087] For example, the air conveyor 42 conveys up to 4000 m 3< of air per hour, in particular m 3< / h.

[0088] Thus, the compensation device 16 minimizes or completely prevents undesired air flows L, which would lead to temperature differences of the film in the transverse direction Q.

[0089] Temperature differences and air currents, which can reduce the quality of the produced film, are therefore reliably avoided, thereby improving the quality of the produced film.

[0090] The Figures 5, 6 , 7 and 8 correspond in the representation of the Figure 4 They show further embodiments of a stretching system 10 according to the invention, which essentially corresponds to the first embodiment. Therefore, only the differences will be discussed below, and identical and functionally equivalent parts are provided with the same reference numerals.

[0091] In the second embodiment according to Figure 5 the first compensating opening 52 and the second compensating opening 54 each have two flow openings 60.

[0092] The flow openings 60 of the first compensating opening 52 are thus fluidically connected to the first pipeline, and the flow openings 60 of the second compensating opening 54 are fluidly connected to the second pipeline 50.

[0093] In the transverse direction Q, the flow openings 60 are arranged next to, ie outside, the film track F, as in the first embodiment.

[0094] In the vertical direction H, however, the flow openings 60 are arranged above and below the film path F, respectively. More precisely, one of the flow openings 60 of the first compensating opening 52 and one of the flow openings 60 of the second compensating opening 54 are arranged above the film path F, and the other of the flow openings 60 of the first compensating opening 52 and the other of the flow openings 60 of the second compensating opening 54 are arranged below the film path F. In this way, the film is not blown directly with air from the compensating device.

[0095] Between the two flow areas 60 of each compensating opening, a throttle valve 61 (in Figure 5 indicated by dashed lines). The amount of air extracted or supplied above and below the film track F can be adjusted by means of the throttle valves 61.

[0096] In Figure 6 a third embodiment of the stretching system 10 is shown, wherein the first compensating opening 52 and the second compensating opening 54 each have a plurality of flow openings 60 which are provided within a flow region 62 in a pipe.

[0097] The flow openings 60 can be designed as slots, round holes, holes with an elliptical contour, holes with a polygonal contour, or a combination thereof. The size of the holes can also vary.

[0098] In the third embodiment, the flow regions 62 extend in the vertical direction H, namely from above the film track F to below the film track F, so that flow openings 60 are located both above, below and at the same height as the film track F.

[0099] In the transverse direction Q, the flow areas 62 are arranged next to the film track F.

[0100] In the fourth embodiment according to Figure 7 are also flow areas 62 as in the third embodiment according to Figure 6 provided.

[0101] In contrast to the third embodiment, several flow areas 62 are provided per compensation opening 52, 54 and the flow areas 62 extend in the fourth embodiment according to Figure 7 in the transverse direction Q.

[0102] More precisely, two flow regions 62 are provided for each compensation opening 52, 54, with one flow region 62 of each compensation opening 52, 54 extending above the film track F and the other below the film track F.

[0103] In the transverse direction Q, the flow regions 62 extend towards the center plane M without crossing it.

[0104] For example, all flow regions 62 extend in the transverse direction Q along the transport rails 32 and partly along the film path F. The flow regions 62 can thus partly protrude beyond the transport system 32 and beyond the film in the transverse direction Q.

[0105] The flow openings 60 are open in and / or against the discharge direction R.

[0106] In the Figures 8 and 9A fifth embodiment of the stretching system 10 according to the invention is shown. The fifth embodiment essentially corresponds to the fourth embodiment according to Figure 7 .

[0107] In contrast to the fourth embodiment, the flow openings 60 are open in or against the vertical direction H.

[0108] How good in Figure 9 As can be seen, the flow openings 60 are aligned towards the film track F.

[0109] The flow openings 60 of the flow regions 62, which extend below the film path F, are formed on the top side of the corresponding tube. Similarly, the flow openings 60 of the flow regions 62, which extend above the film path F, are formed on the bottom side of the corresponding tube.

[0110] The air flows emerging from the flow openings 60 thus run in or against the vertical direction H and essentially perpendicular to the film track F.

[0111] The flow regions 62 of the third, fourth and fifth embodiments further reduce interactions with the film.

Claims

1. Stretching unit (10) for stretching a film, in particular a transverse direction orienter, a machine direction orienter and / or simultaneous stretching unit, comprising an oven (12) and a compensation device (16), wherein the oven (12) comprises a vertical direction (H), a transverse direction (Q) and a drawing direction (R), characterized in that the compensation device (16) comprises an air conveyor (42), a first compensation opening (52) and a second compensation opening (54), wherein the first compensation opening (52) and the second compensation opening (54) open into the oven (12) on opposing sides of a central plane (M) of the oven (12) in the vertical and drawing directions (H, R), in that the air conveyor (42) is located in terms of flow between the first compensation opening (52) and the second compensation opening (54), in that the compensation device (16) is configured to remove a volume of air from the oven (12) through one of the compensation openings (52, 54) and to feed the removed volume of air into the oven (12) through the other one of the compensation openings (54, 52).

2. Stretching unit according to claim 1, characterized in that the first compensation opening (52) and the second compensation opening (54) are designed identically.

3. Stretching unit according to claim 1 or 2, characterized in that the first compensation opening (52) and the second compensation opening (54) are located substantially at the same position in the drawing direction (R).

4. Stretching unit according to any of the preceding claims, characterized in that the first compensation opening (52) and the second compensation opening (54) are located in the region of a film infeed into the oven (12) or in the region of a film outfeed out of the oven (12).

5. Stretching unit according to any of the preceding claims, characterized in that the first compensation opening (52) and the second compensation opening (54) are located in the region of a neutral zone (28) of the oven (12), a zone (26) of the oven (12) before a neutral zone (28) and / or a zone (30) of the oven (12) after a neutral zone (28).

6. Stretching unit according to any of the preceding claims, characterized in that a film track (F) is defined in the oven (12), in particular wherein the first compensation opening (52) and the second compensation opening (54) are located in respect to the transverse direction (Q) outside the film track (F) and / or are located in respect to the vertical direction (H) at the height of the film track (F).

7. Stretching unit according to claim 6, characterized in that the first compensation opening (52) and the second compensation opening (54) each comprise two flow openings (60), wherein in each case one of the two flow openings (60) is located above and the other one of the two flow openings (60) is located below the film track (F).

8. Stretching unit according to claim 6, characterized in that the first compensation opening (52) and the second compensation opening (54) each comprise a plurality of flow openings (60) in at least one flow region (62), wherein the flow region (62) extends in the vertical direction (H), in particular, from below the film track (F) to above the film track (F), or wherein the flow region (62) extends in the transverse direction (Q) above or below the film track (F).

9. Stretching unit according to any one of the preceding claims, characterized in that the air conveyor (42) is located outside the treatment space (12) and / or that the air conveyor (42) is a radial fan.

10. Stretching unit according to any of the preceding claims, characterized in that the first compensation opening (52) is flow-connected to the air conveyor (42) via a first pipe (48) and the second compensation opening (54) is flow-connected to the air conveyor (42) via a second pipe (50).

11. Stretching unit according to claim 10, characterized in that the compensation device (16) comprises a first hinged box (44) that is flow-connected to the outlet side of the air conveyor (42) and a second hinged box (46) that is flow-connected to the intake side of the air conveyor (42), wherein the first pipe (48) and the second pipe (50) each branch into two branches (56, 58) and in each case one branch (56) is flow-connected to the first hinged box (44) and the corresponding other branch (58) to the second hinged box (46).

12. Stretching unit according to claim 11, characterized in that in a first position of the hinged boxes (44, 46), the first pipe (48) is flow-connected to the intake side of the air conveyor (42) and the second pipe (50) is flow-connected to the outlet side of the air conveyor (42), and in a second position of the hinged boxes (44, 46), the first pipe (48) is flow-connected to the outlet side of the air conveyor (42) and the second pipe (50) is flow-connected to the intake side of the air conveyor (42).

13. Stretching unit according to any of the preceding claims, characterized in that stretching unit (10) comprises a sensor (40) for the direction of an air flow (L) and / or a pressure sensor.

14. Stretching unit according to any of the preceding claims, characterized in that the stretching unit comprises a transport system (14) that is configured to transport a film through the oven (12) in the drawing direction (R), in particular along the film track (F).

15. Method for reducing non-uniform temperatures and air flows in a stretching unit (10), comprising the following steps: - measurement of the magnitude of the air flow (L) in the drawing direction (R) and / or the pressure on a side of the central plane (M) of the oven (12) of the stretching unit (10), and - characterized by removal of a volume of air from the oven (12) through a first compensation opening (52) on a side of the central plane (M) of the oven (12) by means of a compensation device (16) and feeding the removed volume of air into the oven (12) on the other side of the central plane (M) through a second compensation opening (54) depending on the results of the measurement.