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

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 specifically influence the properties of a plastic film. Such stretching, in particular, changes the orientation of the molecules contained in the plastic film (hereinafter also referred to as "film"). To effect stretching, the stretching device comprises a first roller, which is drivable by a first drive and rotatable at a first rotational or peripheral speed. Furthermore, the stretching device comprises a second roller, which is drivable by a second drive and rotatable at a second rotational or peripheral speed. In the transport path of the plastic film, the second roller is arranged downstream of the first roller.

[0003] The plastic film is stretched because the second rotation speed is greater than the first rotation speed. In the context of stretching, rotation speed refers to the rotational speed experienced by the film. This can therefore also be referred to as the peripheral speed of the first roller and the second roller.

[0004] However, stretching can also lead to various problems. For example, monomers that outgas during production often adhere to the plastic film after stretching, as do dust particles that are pressed into the film during stretching. Heat transfer between the film and the roller is often unsatisfactory. Furthermore, stretching can also change film properties, particularly geometric properties such as the thickness profile, in a direction transverse to the film's transport direction, as well as the film's flatness.

[0005] DE 20 2020 000 221 U1 shows a generic stretching device in which at least one of the rollers involved can be passed through by air from the outside to the inside.

[0006] However, the effectiveness of this device is impaired because with this roller the air flows from the outside to the inside over the entire roller surface, so that more air flows through than is necessary for the purposes mentioned there.

[0007] The object of the present invention is therefore to propose a stretching device with which at least some of the problems mentioned can be reduced.

[0008] According to the invention, this object is achieved by all features of claim 1. Possible embodiments of the invention are specified in the dependent claims.

[0009] In a first aspect of the invention, a generic stretching device for stretching a plastic film is provided. This stretching device is characterized by means for varying the amount of air between the plastic film and surface elements of the first roller and / or the second roller on which the film rests. This invention thus makes it possible to vary the amount of air between the plastic film and the surface elements on which the film rests. This thus includes reducing the amount of air, but also increasing the amount of air. The plastic film runs onto the respective roller at an impact edge and leaves this roller again at a run-off edge. The impact edge and the release edge, together with the side edges, define the total surface element on which the film can rest at most. The actual surface element on which the film rests can, however, be smaller than the total surface element.

[0010] By changing the air volume, these problems can be avoided. For example, increasing the air volume can reduce the penetration of dust or monomers into the film.

[0011] Reducing the air volume can significantly improve heat transfer between the roller and the plastic film, since air can only absorb a small amount of heat and is therefore a poor heat conductor. Reducing the air volume therefore improves overall heat conduction.

[0012] Reducing the air flow can also help the film adhere more strongly to the roller, thus limiting the film's ability to change geometrically. In particular, the tendency for the thickness profile to change transversely to the transport direction can be reduced. The measure according to the invention can also reduce the deterioration of the film's flatness.

[0013] The stretching device according to the invention further provides that the first and / or second roller comprises at least one cavity, which is circumferentially divided into at least two segments by at least one separating element, wherein at least one segment can be subjected to an air pressure that is lower or higher than the ambient pressure. Each segment can thus be subjected to different air pressures, so that, for example, the amount of air between the plastic film and surface elements of the first roller and / or the second roller on which the film rests can be varied differently in the axial direction of the roller.

[0014] Separating elements divide the inner cavity of one of the aforementioned rollers into segments in the circumferential direction, thus segmenting the cavity. Such separating elements therefore run parallel to the longitudinal direction of the respective roller. In particular, a roller shell can be mounted so as to be rotatable about a roller axis, wherein the separating element or elements can be arranged immovably relative to the axis. This makes it possible, for example, to arrange a segment below the surface elements of the first roller and / or the second roller on which the film rests. Consequently, a negative pressure and / or positive pressure is only generated in the area of ​​these surface elements, while in another segment essentially ambient pressure prevails. In this way, the effectiveness of the means for changing the air volume is increased, since no ambient air is sucked in, but only the air volume in the relevant areas is changed.At least in the area of ​​the release line, the significance of which will be explained further below in connection with a release roller, a segment can be provided that can be subjected to overpressure. This ensures that the film is released from the first and / or second roller along an actual release line that deviates only slightly from the target release line.

[0015] A segment in whose effective area the film is not in contact can be subjected to overpressure, allowing an air flow to be moved from the inside of the roller to the outside. This allows for continuous cleaning of the passage openings, preventing the accumulation of dirt and blockages that occur over time.

[0016] At least three separating elements can also be provided in the circumferential direction, so that at least three segments can be provided. For example, one possible arrangement is one in which a segment subjected to negative pressure is arranged in the run-up area of ​​the film and a segment subjected to positive pressure is arranged in the run-out area. Ambient pressure can prevail in all remaining areas. This means that in the area where the film meets the surface of the roller (run-up area), the entrained air can be sucked out, thereby achieving the advantages of the invention. In the area where the film leaves the surface again (run-out area), however, the film can be released from the surface of the roller using positive pressure, thereby preventing the film from becoming detached at different circumferential angles of the roller across its width.Avoiding this effect leads to improved properties, especially geometric properties, of the film. Applying an overprint can also be used, for example, to clean through-holes in the roll shell.

[0017] The individual segments can be individually controlled with regard to the magnitude of the negative and / or positive pressure. The control can be carried out by a computing and control unit, which compares measurement data from sensors with target data for the control. Such measurement data can be transmitted to the computing and control unit from measuring sensors, which can be used to measure the geometric properties of the film, in particular the film's thickness profile. In this way, a control of the positive and / or negative pressure in each segment can be established.

[0018] In this case, the means for varying the air flow can be arranged only in the edge areas of the roller. This prevents the usable width of the film from running on a part of the roller that is as flat as possible, and only the edge areas of the film adhere. Since these edge areas are often removed later, the negative influences of the means for varying the air flow on the film edge areas do not have a negative impact, while the advantages of the invention are retained.

[0019] In order to be able to reduce the amount of air, in a first variant of the invention, the surface of the first roller and / or the second roller is provided with depressions. This, in particular, reduces the actual size of all surface elements on which the film rests. Furthermore, areas at the edges of these surface elements are recessed relative to these surface elements, thus providing volume elements into which the air can penetrate. Only a fraction of the air now remains between the surface elements on which the film rests and the film itself. In this context, it should be explained that the word "rests" does not equate to the film and the surface area being in full contact. A very small amount of air may remain in places or even over the entire surface, so that while macroscopically there is full contact, microscopically it is not full contact.The measure described ensures that the film adheres more strongly to the roller, thus achieving the advantages described above.

[0020] Ring-shaped or spiral-shaped grooves can also be depressions in the sense described in the previous section. Spiral-shaped grooves can be single-start or multi-start, with multi-start grooves being able to rotate in the same and / or opposite directions (cross grooves). In particular, such grooves can absorb a quantity of air in the areas on which the film rests and release it back into the environment in the areas where the film is not resting. The widths and / or depths of such grooves can be less than 2 mm, preferably less than 1 mm and in particular less than 0.5 mm. The transition between the surface elements on which the film rests and the grooves can be rounded or provided with bevels to prevent damage to the film.

[0021] In a further variant of the invention, the means for varying the air quantity comprise through-openings in the surface of the first roller and / or the second roller, wherein the through-openings represent a fluid-communicating connection between the outer surface and a cavity within the first roller and / or the second roller. Such through-openings make it possible for at least a portion of the air trapped between the film and the surface elements of the respective roller on which the film rests to be discharged into the cavity of the roller. Consequently, a reduced amount of air remains between the film and the surface elements, so that the film adheres more strongly to the surface elements of the roller surfaces and the undesirable effects are reduced or eliminated altogether.

[0022] In an advantageous embodiment of the invention, the means for varying the air flow comprise through-openings in the surface of the first roller and / or the second roller, wherein at least some of the through-openings can be provided by a porous material, in particular by a microporous material. This makes it possible to create a large number of small channels that serve as through-openings. This has the advantage that the openings on the outer surface of the roller are small and their edges do not have any negative effects on the film, for example in the form of impressions. The above-described object is thus achieved without having to accept any new disadvantages. The porous material can form the casing of the roller or at least be a component thereof.The average pore size of the porous material can be between 5 and 100, in particular between 10 and 60, and preferably between 20 and 45 micrometers. In one variant of the invention, a porous material, in particular a microporous material, represents only an outer coating of an otherwise closed roller surface. For example, a closed tube, e.g., made of metal, can be provided, to which the porous material is or will be applied. The cavities provided by the porous material can then absorb the air located between the film and the roller.

[0023] Regardless of whether the porous material provides through-holes or merely cavities for air intake, the pore size can be varied in the axial direction of the roller. Particularly at the edges of the roller shell, the volume per surface element provided by the porous material can be larger than in the center, so that the film adheres more strongly to the roller at the edges.

[0024] The outer surface of the roller, which is at least partially formed by porous material, can be provided with a coating to prevent damage to the film, which coating in particular has a lower hardness than the porous material.

[0025] A porous material can be a sintered material, particularly a sintered metal or a sintered plastic. A sintered material is a material produced through a sintering process. In this process, fine-grained materials, which can be ceramic or metallic, or comprise plastic, are heated – often under increased pressure – while the temperatures remain below the melting temperature of the main components, so that the shape of the workpiece is retained. For example, the shape can be sleeve-shaped, creating a sleeve that can later be applied to a base body. The base body and sleeve can then form the roller. Such a sleeve can also be produced using an additive manufacturing process, whereby a three-dimensional body can be created from a powdered raw material through local heating. The graininess of the material results in channels remaining between the grains, which form through openings.The peripheral surface of a roller manufactured in this way can be additionally ground and / or polished to prevent damage to the film. However, to prevent the through-openings from accidentally closing due to abrasion, the surface of the roller can subsequently be treated with an etching process. The through-openings are generally not straight, but this does not impair the ability to divert air into the interior of the roller. A sintered material is usually a very hard material, so that despite the high tensile forces within a stretching device, only slight deflection occurs, which means that the geometric properties of the film are hardly affected. In addition, the production of a sintered material is generally relatively inexpensive.

[0026] In order to reduce the amount of air, according to an alternative or additional embodiment of the invention, the means for changing the amount of air comprise through-openings in at least part of a roll shell of the first and / or second roll, wherein at least part of the through-openings are formed by bores. A roll shell can encompass the roll surface, so that bores consequently extend from the area surrounding the roll into the interior of the roll. It is also possible to design the roll shell in multiple layers. For example, an inner part of a roll shell can be provided which comprises bores. An outer part of the roll shell can comprise a porous, in particular a microporous material. In this way, it is possible to prevent the edges of a bore from becoming visible on the film, which would then render it unusable.

[0027] The holes can be smaller than 2 mm, preferably smaller than 1 mm, and in particular smaller than 0.5 mm. The transitions from the holes to the surface of the part of the roll shell can be rounded or conical. This measure, in turn, prevents damage to the film. A plurality of holes can be evenly distributed over the roll shell, with a certain number of holes being provided per surface element. The distances between any two adjacent holes can, however, be varied. It is particularly advantageous if the number of holes per surface element or the number of through-openings per surface element is varied, in particular in the longitudinal direction of the roll shell, i.e. in the direction of the axis of rotation. In particular at the edges of the roll shell, the number of holes orthe number of through-holes per surface element should be larger than in the middle area so that the film adheres more strongly to the roller at the edges. The edges can each take up one third of the roller shell length so that the middle area also covers one third. However, the middle area can also be larger so that the edges are correspondingly smaller. By varying the number of holes per surface element or the number of through-holes per surface element, the volume flow passing through the wall of the roller can generally be varied. With sintered materials, such a variation can be achieved alternatively or additionally by varying the layer thickness of the sintered material so that there are different resistances for the volume flow.

[0028] In an advantageous embodiment, it is conceivable for the first and / or second roller to be constructed in multiple layers, wherein in particular one layer can be designed to be displaceable relative to a second layer, wherein both layers can be detachably connected after a relative displacement. This makes it possible, for example, for an outer layer adapted to the properties of the film to be pushed onto a base body. This embodiment is also advantageous in that if the roller becomes dirty, only the outer layer needs to be replaced. Such a layer can then be cleaned, while production of the film can continue with another outer layer.

[0029] In a further variant of the invention, the means for varying the air quantity can comprise at least one cavity which can be pressurised with an air pressure which is lower than the ambient pressure. This ensures that a portion of the air quantity which is supplied to the cavity of one of the rollers can be discharged from the cavity, for example via through openings. In this way, the air which is repeatedly introduced by the movement of the film and the roller can be continuously discharged, so that a stationary state with regard to the reduced air quantity can be set between the plastic film and surface elements of the first roller and / or the second roller on which the film rests. One possibility for pressurising the cavity is provided by a rotary feedthrough via a pin of the respective roller.A hose or a pipe can be connected to this rotary union, which connects the interior in a fluid-communicating manner with a vacuum source, in particular a pump.

[0030] Furthermore, it can be provided that at least one cavity can be subjected to a pressure that is higher than the ambient pressure. This makes it possible to free passage openings of dirt by means of a blow-out process. Additionally or alternatively, a suction device can be provided which is arranged in an angular region of the first and / or second roller in which no film is in contact, i.e. between the detachment line and the inlet edge of the film. In this way, it is even easier to remove dirt from the surface of the roller. Such a suction device can extend at least partially in the direction of the roller axis. Such a suction device can also have several separate suction chambers, the sizes of which can also be variable, so that the suction device can be subjected to different suction powers.For example, in areas near the roller that do not come into contact with the film, the suction power may be lower than in other areas, as these are generally less likely to be contaminated. Contamination often results from paraffin adhering to the film, which is released onto the rollers.

[0031] In order to be able to change the amount of air between the plastic film and surface elements of the first roller and / or the second roller on which the film rests, one embodiment of the invention provides at least one suction device with which air can be sucked out of the area in which the plastic film comes into contact with the first and / or the second roller in the transport direction. In particular, when the film runs onto the roller, the air entrained by the film gets between the surface elements of the roller and the film. This air can then be sucked out in a targeted manner using a suction device, so that significantly less air is introduced into the aforementioned area. The advantages of the invention can thus be achieved.

[0032] Alternatively or additionally, it can be provided to at least partially cover the area of ​​the roller on which the film is not resting with a covering device, so that in this area the ambient pressure is not applied to the roller surface. In particular, if the pressure in the roller body is lower than the ambient pressure and / or if a negative pressure is created by a suction device arranged close to the roller surface, the suction of air from the environment (false air) can be avoided. This leads to greater effectiveness of the invention, since then only the air entrained by the film, but not any air entrained by the roller, needs to be removed. Advantageous arrangement. Consequently, a negative pressure and / or an overpressure is only generated in the area of ​​these surface elements, while in another segment essentially ambient pressure prevails.This increases the effectiveness of the means for varying the air flow, since no ambient air is drawn in; instead, only the air flow in the relevant areas is changed. At least in the area of ​​the release line, the significance of which will be explained further below in connection with a release roller, a segment can be provided that can be subjected to excess pressure. This ensures that the film is released from the first and / or second roller along an actual release line that deviates only slightly from the target release line.

[0033] A segment in whose effective area the film is not in contact can be subjected to overpressure, allowing an air flow to be moved from the inside of the roller to the outside. This allows for continuous cleaning of the passage openings, preventing the accumulation of dirt and blockages that occur over time.

[0034] At least three separating elements can also be provided in the circumferential direction, so that at least three segments can be provided. For example, one possible arrangement is one in which a segment subjected to negative pressure is arranged in the run-up area of ​​the film and a segment subjected to positive pressure is arranged in the run-out area. Ambient pressure can prevail in all remaining areas. This means that in the area where the film meets the surface of the roller (run-up area), the entrained air can be sucked out, thereby achieving the advantages of the invention. In the area where the film leaves the surface again (run-out area), however, the film can be released from the surface of the roller using positive pressure, thereby preventing the film from becoming detached at different circumferential angles of the roller across its width.Avoiding this effect leads to improved properties, particularly the geometric properties of the film. Applying overpressure can also improve the film's performance. For example, in the direction of the roller that does not come into contact with the film, the suction power may be lower than in the other areas, as contamination is generally less likely here. Contamination often results from paraffin adhering to the film, which is released by the rollers.

[0035] In order to be able to change the amount of air between the plastic film and surface elements of the first roller and / or the second roller on which the film rests, one embodiment of the invention provides at least one suction device with which air can be sucked out of the area in which the plastic film comes into contact with the first and / or the second roller in the transport direction. In particular, when the film runs onto the roller, the air entrained by the film gets between the surface elements of the roller and the film. This air can then be sucked out in a targeted manner using a suction device, so that significantly less air is introduced into the aforementioned area. The advantages of the invention can thus be achieved.

[0036] Alternatively or additionally, it can be provided to at least partially cover the area of ​​the roller on which the film is not resting with a covering device, so that in this area the ambient pressure is not applied to the roller surface. In particular, if the pressure in the roller body is lower than the ambient pressure and / or if a negative pressure is created by a suction device arranged close to the roller surface, the suction of air from the environment (false air) can be avoided. This leads to greater effectiveness of the invention, since then only the air entrained by the film, but not any air entrained by the roller, needs to be removed. In this context, it is advantageous if the suction device and the covering device are connected to one another and form a subassembly.

[0037] In a further development, it is provided that the suction device and / or the covering device is divided into sections transversely to the transport direction of the plastic film. In this case, it is possible for a suction device extending across the entire width of the plastic film to be divided into segments. However, individual, independent sections can also be provided. This case is particularly advantageous when it is a matter of suctioning out air in the area of ​​the edges of the plastic film in order to, for example, minimize the influence of stretching on the geometric properties of the film in its edge region. If such independent sections are provided, it is advantageous if they can be positioned in the transverse direction of the film, i.e. parallel to the axis of the respective roller, by means of a positioning device in order to be able to adapt the sections of the suction device to different film widths.

[0038] The individual sections can be individually controlled with regard to their suction power. The control can be carried out by a computing and control unit, which compares measurement data from sensors with target data for the control. Such measurement data can be transmitted to the computing and control unit from measuring sensors, which can be used to measure the geometric properties of the film, in particular the film's thickness profile. In this way, a closed-loop control of the air extraction can be established.

[0039] An advantageous embodiment of the invention provides that the means for changing the amount of air between the plastic film and surface elements of the first roller and / or the second roller on which the film rests, comprise at least one overpressure device with which the surface of the plastic film facing away from the roller can be subjected to pressurized fluid, in particular air. In this way, the back of the film, i.e. the side facing away from the roller, is subjected to a force applied by the pressurized air. This allows the film to be pressed against the roller with greater force, so that a smaller proportion of the air can reach the area between the film and the roller. Like the suction device described above, the overpressure device can be divided into sections, which can also be positionally adjustable in the direction of the roller axis.Control and / or regulation of the overpressure and / or volume flow can also be provided. This can also be done individually, particularly in sections.

[0040] If, in particular, the means for varying the air flow, designed as an overpressure device, is arranged in the edge regions of the roller, it can be provided that the fluid with which the edge regions of the roller, on which in particular the edge regions of the film rest, is subjected to a cold fluid. "Cold fluid" refers to a fluid that has a temperature below 50 degrees Celsius, preferably below 30 degrees Celsius, and preferably below 15 degrees Celsius. It is advantageous that the edge regions of the film have a higher stiffness than the usable area of ​​the film before running onto the respective roller, so that the edge regions no longer migrate inward as strongly. In other words, neck-in is reduced.

[0041] To reduce neck-in, a fixing device can be provided. This can be located, for example, in the area between the first and second rollers. In this case, the fixing device can consist of two disc-like pairs of rollers, each with an edge of the film running through its nip. In this way, the forces caused by longitudinal stretching and acting on the film edges are compensated for or at least reduced. Fixing devices can also be arranged on or at the first and / or second roller. For example, these can be surface elements that provide greater adhesion to the film than the other areas of the roller surface. A fixing device can be moved in the transverse direction of the film, i.e. in the axial direction of the first and / or second roller.

[0042] In order to increase the adhesion of the film to the first and / or second roller, it can be provided that the means for changing the amount of air between the plastic film and surface elements of the first roller and / or the second roller on which the film rests, comprise at least one electrode with which the plastic film can be electrostatically charged upstream or at the line of contact between the plastic film and the roller. Due to the electrostatic charge of the film, an electrostatic force of attraction or repulsion is generated between the film and the roller. If an attractive force is present, i.e. a force which, as seen from the film, is directed towards the roller, a small portion of the air entrained by the film enters the space between the film and the roller, so that the film adheres better to the roller.At least two consecutive electrodes can be provided in the axial direction of the respective roller, the electrical voltages of which can be adjusted independently of one another. The electrical voltages can be controlled by means of a control device. In this way, it is possible to change and, in particular, optimize the line of impact of the film on the roller. Ideally, this line of impact is a straight line. This design variant can help to improve the flatness of the film. In addition, it can be achieved that the edges of the film adhere to the roller with a different voltage and thus with a different induced charge, just like the middle areas of the film, despite the greater thickness.

[0043] In a further embodiment of the invention, an electrode can be combined with a pressure relief device, such as that described above. This makes it possible to electrostatically charge the fluid flowing from the pressure relief device toward the film, and this electrostatic charge can be applied to the film. This further improves the adhesion of the film to the roller.

[0044] In connection with an electrode, it can be advantageous if a discharge device is provided behind the first and / or second roller in the transport direction of the film. This in turn can reduce or even completely eliminate the electrostatic charge on the film, so that further processing, such as winding, is not negatively affected. In particular, it is advantageous if the discharge device is arranged at the desired release point of the film from the first and / or second roller or upstream of it in the transport direction of the film, so that the film is discharged no later than in the area of ​​the release line. This ensures that areas of the film do not remain stuck to the roller for different lengths of time, which could lead to flatness errors. A discharge device can be designed, for example, as a grounded roller and / or a brush roller.

[0045] A first and / or second roller suitable for the embodiment of the invention can comprise at least partially a metal such as aluminum and / or at least partially a plastic, in particular a fiber-reinforced plastic. The aforementioned materials can be used, for example, for a base body of a first and / or a second roller.

[0046] Furthermore, the first and / or second roller can have a specially machined surface. As already described above, the surface can comprise a sintered material. Alternatively or additionally, the surface can be rubberized, at least partially coated with polytetrafluoroethylene (PTFE), known as Teflon, and / or at least partially chrome-plated in order to better adapt the surface properties to the properties of the film. In particular, scratches can be avoided on sensitive films. In the case where sintered material has been used for the first and / or second roller, a PTFE can already be embedded before the sintering process to prevent the through-holes from becoming blocked.

[0047] In order to improve the flatness of a film in particular, at least one detachment roller can be provided which can be adjusted to the first and / or the second roller. Ideally, a film should leave the first or the second roller at a desired detachment line which runs parallel to the longitudinal axis of the roller. The desired detachment line lies on a surface which forms a common tangential surface of the first and / or second roller and the following roller. In reality, this detachment line is not a straight line, which can lead to defects in the film. A detachment roller is positioned such that it forms a nip with the first and / or second roller in the desired detachment line or behind it in the transport direction of the film. This ensures that the actual detachment line adapts to the desired detachment line. In particular, the detachment roller can be displaceable in the circumferential direction of the first and / or second roller.

[0048] In a further embodiment of the invention, the first and / or second rollers have a shape other than cylindrical. For example, the roller may have a conical, concave, or convex shape. Such a roller configuration can counteract various defects, such as flatness errors, depending partly on the properties of the film.

[0049] In principle, it is advantageous to provide a temperature control device with which the film regions resting on the first and / or second roller or located in the section between the first and second rollers can be changed in terms of their temperature. An increase or decrease in the temperature of the film may be desired. In particular, a temperature control device can be provided with which the aforementioned film regions can be brought to different temperatures in the transverse direction, i.e., transversely to their transport direction. For example, it can be provided to cool the edge regions of the film more intensively than the other regions in order to reduce neck-in.

[0050] Such a temperature control device can be integrated in particular in the first and / or second roller, so that the efficiency of this temperature control device is high. In this case, the film itself is temperature-controlled by the heat conduction of the roller. For example, the roller body can be traversed by at least one pipe through which a temperature-controlled fluid can be conducted. Such a pipe can extend in the axial direction of the roller to ensure rapid fluid distribution. This pipe can be designed, for example, as a bore in the roller shell. However, the pipe can also have a different course, such as a helical shape, in order to generate greater heat transfer. In this case, the pipe can be formed by a pipe or a hose which, in the case of a hollow roller shell, is arranged within the roller shell.If the roll shell is multi-layered, the pipe can also be arranged between two layers. Multiple pipes are also conceivable to apply different temperatures to different zones of the roll. The temperature-controlled fluid is preferably fed into the roll through one or more rotary unions on the end face.

[0051] Particularly when designing the first and / or second roller in combination with sintered materials, it is conceivable to design the roller shell in at least two layers. An inner layer can comprise larger pores than the outer layer, allowing the fluid to flow through these pores. According to the invention, the outer layer can reduce the amount of air between the film and the roller by dissipating air.

[0052] In a further variant of the invention, the first and / or second roller can comprise at least one heating wire embedded in a surface layer of the roller. This embedding can be produced, in particular, in combination with a design of this surface layer made of a sintered material.

[0053] In an advantageous embodiment of the invention, a tempering device can be configured as an induction device. In particular, a device for providing a magnetic field, which constitutes part of the induction device, can be arranged inside the roller to increase the efficiency of this tempering device. However, to ensure a simpler design, an arrangement outside the roller is also conceivable.

[0054] Alternative or additional temperature control devices based on other physical principles are also conceivable. For example, the temperature of the film can be changed by a device for generating infrared radiation. This is preferably arranged outside one of the rollers. A temperature-controlled gas, which can be directed directly onto the film from at least one nozzle, can also be provided for temperature control. Air is particularly suitable as the gas. Devices for generating infrared radiation and / or a nozzle can also be arranged inside the roller.

[0055] In a further advantageous embodiment of the invention, the means for varying the air quantity comprise at least one line arranged within the roller, in particular within the roller shell. Furthermore, in this embodiment, connecting lines are provided which are in fluid communication with the lines and open with their other ends into the roller surface. Such connecting lines can in turn be bores or channels which arise, for example, in a sintered material. Thus, the air located between the film and the roller surface can be discharged through the connecting lines and the lines. The lines can open, for example, into one of the end faces of the respective roller. In particular, the lines can be axial lines and / or designed as axial bores.In a multi-layered construction of the shell of the first and / or second roller, an inner layer that is impermeable to air can also be provided. Using spacers, an annular space can be provided between the inner layer and the outer layer, in which the air can be collected. The annular space can thus also be understood as a conduit in the sense described above. An annular space is particularly advantageous when the outer layer is formed from a sintered material.

[0056] In an embodiment with axial lines, these can end at an end face of the roller or at the end face of an attachment, such as a journal. A component that is stationary relative to the roller and encloses a circular segment-shaped cavity can be attached to this end face. The ends of the lines can then sweep over this cavity as the roller rotates. If a negative pressure or positive pressure is now applied to this cavity, each line that enters the effective range of the cavity as a result of the rotation of the roller is also subjected to a negative pressure or positive pressure. This pressure is released again when the respective line is again outside the effective range.A cavity subjected to negative pressure is preferably arranged such that those lines that are in fluid communication with connecting lines whose ends open into the surface areas on which the film rests can be subjected to negative pressure. This prevents excessive ambient air from being sucked in. A cavity subjected to positive pressure is preferably arranged such that those lines that are in fluid communication with connecting lines whose ends open into the surface areas on which the film does not rest and / or at the detachment line of the film can be subjected to positive pressure.

[0057] Accordingly, several components of the type described can be provided, for example one which can be subjected to an overpressure and one which can be subjected to a negative pressure.

[0058] The above-mentioned object is additionally achieved by a method for stretching a plastic film in its transport direction, comprising a first roller which is driven by a first drive and rotates at a first rotational speed, and a second roller which is driven by a second drive and rotates at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film, the existing air quantity being changed by means for changing the air quantity between the plastic film and surface elements of the first roller and / or the second roller on which the film rests, and the first and / or the second roller comprising at least one cavity which is divided into at least two segments in the circumferential direction by at least one separating element,wherein at least one segment is subjected to an air pressure that is reduced or increased compared to the ambient pressure.,

[0059] With this method according to the invention, the same advantages can be achieved as have already been described in connection with a device according to the invention.

[0060] In a further aspect of the invention, a blown film line is provided with at least one extruder for producing a plastic melt, a nozzle head for producing a film tube from the plastic melt, a flattening device for converting the film tube into a double-layer plastic film, a driven take-off device for pulling off and further transporting the double-layer plastic film, and a winding device for winding at least one layer of the double-layer plastic film, wherein a stretching device is provided which is designed according to one of claims 1 to 12 and / or according to the above description. Such a blown film line can optionally comprise a calibration device which is arranged upstream of the flattening device in the transport direction.Furthermore, a reversing device can be provided which is arranged downstream of the take-off device and with which errors, in particular deviations from the average film thickness, can be offset across the film width. A stretching device according to the invention can then be arranged downstream of the take-off device, for example between the take-off device and an optional reversing device. An arrangement of the stretching device between a reversing device and the winding device is also conceivable. The plastic film can be fed to the stretching device as a flattened film tube or as a double-layer plastic film cut on one or both sides. A plastic film cut on one side can have been unfolded beforehand and fed to the stretching device as a single-layer plastic film, which is in particular twice as wide.A plastic film cut on both sides can be separated into its individual layers, with each layer being fed to a separate stretching device. A winding device can comprise a winding station for winding an uncut, one-sidedly cut, or two-sidedly cut double-layer plastic film. A single-layer—i.e., unfolded—plastic film cut on one side can also be wound. However, two winding stations can also be provided, each for winding one layer of the previously double-layer plastic film.

[0061] 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 may be essential to the invention individually or in any combination of mentioned features. 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 with regard to the disclosure, reference is always made to the individual aspects of the invention. The individual figures show: Fig. 1Schematic view of a stretching device according to the invention Fig. 2A stretching roller of an embodiment of the invention Fig. 3A stretching roller of a further embodiment of the invention Fig. 4A stretching roller of a further embodiment of the invention Fig. 5An embodiment of the invention with a suction device Fig. 6An embodiment of the invention with an electrode Fig. 7A blown film plant according to the invention with a stretching device Fig. 8A further blown film plant according to the invention with a stretching device Fig. 9An embodiment of the invention with a nozzle Fig. 10An embodiment of a device according to the invention with a covering device. Fig. 11A variant of a covering device. Fig. 12An embodiment of a device according to the invention, Fig. 13An embodiment of a device according to the invention.

[0062] The Figure 1shows a schematic representation of a stretching device according to the invention. The means for changing the amount of air between the plastic film and surface elements of the first roller and / or the second roller on which the film rests are not visible in this figure and are explained in more detail with reference to the following figures. The web-shaped plastic film 101 runs into 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. The task of a preheating roller is to bring the film to a predefined temperature. For this purpose, a preheating roller is usually temperature-controlled, with a temperature-controlled fluid often being introduced into the preheating roller.

[0063] 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, a dedicated electric motor, which drives the roller 110 at a first rotational speed.

[0064] The first roller 110 is preferably associated with a first application roller 111, which, together with the first roller 110, provides an inlet gap for the film. Preferably, the inlet gap or the path of the film 101 is configured such that the film 101 runs tangentially to the rollers 110, 111 in the inlet gap. The roller gap already serves to minimize the air between the stretching roller 110 and the film.

[0065] The first roller 110 is also assigned a second application roller 112, which forms an outlet gap with the roller 110. The second application roller 112 can be adjustable in the circumferential direction of the first roller 110. The second application 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 roller 110.

[0066] 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 a separate electric motor, which drives the roller 120 to rotate at a second rotational speed. The second rotational speed is greater than the first rotational speed, with the second stretching roller then having a greater peripheral speed than the first stretching roller. This results in the film 101 being stretched between the outlet gap of the first roller 110 and the inlet edge of the second roller 120 in the ratio of the peripheral speeds in its transport direction. The distance between the outlet gap and the inlet edge is often also referred to as the stretching gap.

[0067] It is possible for the first roller 110 and the second roller 120 to be movable relative to each other. This allows the stretching gap to be influenced. A change in the stretching gap can influence the properties of the film.

[0068] The second roller 120 is preferably associated with a third application roller 121, which, together with the first roller 120, provides a second inlet gap for the film. Preferably, the inlet gap or the path of the film 101 is configured such that the film 101 runs tangentially to the rollers 120, 121 in the inlet gap. The roller gap itself serves to minimize the air between the stretching roller 120 and the film.

[0069] In the case that the stretching gap is small, the third application roller 121 would collide with the roller 110, so that in this case the third application roller would have to be pivoted away.

[0070] The second roller 120 is optionally also assigned a fourth application roller 122, which forms an outlet gap with the roller 120. The second application roller 122 can also be adjustable in the circumferential direction of the first roller 120. The second application roller 122 serves to ensure that the film leaves the first roller 120 along a line that runs parallel to the axial direction of the roller 120.

[0071] In principle, one or more contact rollers can be dispensed with in a stretching device according to the invention. Nevertheless, the term "inlet gap" or "outlet gap" can be used. This refers to the line along which the film contacts the stretching roller or separates from the stretching roller.

[0072] Further stretching rollers may be provided, in particular each having one or two contact rollers, wherein two stretching rollers arranged directly one after the other are driven in such a way that the stretching roller arranged downstream has a higher peripheral speed than the preceding stretching roller.

[0073] 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 is solidified.

[0074] Based on the Figure 2An embodiment of the stretching device according to the invention is explained below. A perspective view of a stretching roller is shown. In the example shown, it is the stretching roller 120. This comprises a roller shell 140 and end faces, of which the end face 141 is visible. The stretching roller has, in particular, a shaft, an axle or axle stub. Of the aforementioned components, only the part 142 protruding from the end face 141 is visible. The roller shell 140 now comprises depressions 143, which in the present example are shown as a circumferential groove. Further examples of depressions have already been given earlier in the description of the invention. The characteristic of depressions is that they only protrude into the roller shell, but do not extend through the roller shell 140 into the interior of the roller defined by the roller shell and the end faces.

[0075] The Figure 3 resembles the Figure 2 However, here the roller 120 includes through-openings 150. The precise design of the through-openings has already been described above. The through-openings extend into the interior of the roller and thus form a fluid-communicating connection between the interior and the surroundings of the roller 120. Instead of through-openings, openings can also protrude from the surface, which only comprise cavities open to the outside, for example, the open areas of porous material.

[0076] The interior of the roller 120 can optionally be subjected to a vacuum. For this purpose, the axle, shaft, or axle stub can be provided with fluid lines. In the case of a shaft, fluid lines comprise a rotary union. The fluid line of the axle, shaft, or axle stub is connected to another fluid line, such as a hose 151, which leads to a vacuum source.

[0077] The Figure 4 shows further possible features of a roller 120 having through-openings. Here, the interior, which is now visible due to the missing representation of the roller shell, can be divided into several individual chambers by partition walls, in this example two partition walls 160, 161. This makes it possible to apply different air pressures to the various individual chambers. In the present case with two individual chambers, one of them can, for example, be subjected to a negative pressure, while no positive or negative pressure can be applied to the second individual chamber, so that the ambient pressure prevails here.

[0078] From the Figure 5An embodiment can be seen in which a suction device 165, which extends transversely with respect to the transport direction of the film 101, is arranged upstream of the stretching roller, here in particular the stretching roller 120. This suction device comprises a closed box 166 with one or more suction openings 167. The box can in particular be subjected to a negative pressure, so that air from the inlet gap passes through the suction openings in the direction of arrow L into the suction device and is discharged.

[0079] The Figure 6shows a further embodiment of a stretching device according to the invention, in which an electrode 170 is arranged upstream of the inlet gap of a stretching roller, in the embodiment shown, the stretching roller 120. This electrode can be subjected to an electrical potential, which differs in particular from the electrical potential of the film 101. This electrically charges the film, which leads to the film being additionally held on the stretching roller by an electrical force.

[0080] The Figure 7shows a device 1 for producing a film tube, namely a blown film system, which initially comprises at least one extruder 2, with which plastic, for example in granulate form, can be plasticized. Via a line 3, the plastic melt thus produced is fed to an extrusion tool 4, which can also be referred to as a nozzle 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 withdrawal direction z. A film tube 6 is now present which has 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 tempering device 8, which is often referred to as a cooling ring because of its ring-like design enclosing the film tube.

[0081] After passing the calibration device, the film tube 6 enters the effective area of ​​a flattening device 9, in which the circular film tube is converted into an elliptical cross-section with an increasing eccentricity until it finally forms a double-layered plastic film in the area of ​​influence of the take-off rollers 10, which are connected to each other at their sides.

[0082] The flattening device is arranged to rotate, the axis of rotation being essentially 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.

[0083] The Figure 7further 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.

[0084] Downstream of the reversing device 15, a stretching device 100 according to the invention is arranged, which has already been described in connection with the Figures 1 to 6 and the further description. The information contained in the Figure 7 The stretching device shown corresponds to the one shown in the Figure 1 It should also be noted that a cutting device can be positioned upstream of the stretching device 100, with which one or two edges of the fold can be cut open or cut off. Additionally, a separating device can be provided with which the double-layer plastic film can be divided into one or more single-layer films.

[0085] The arrow 17 indicates that this film tube, after passing through the stretching device 100, is guided to further processing, which is not specified in more detail here.

[0086] The Figure 8 shows a further embodiment of a blown film line according to the invention, in which the stretching device 100 is now arranged between the take-off rolls 10 and the reversing device 15. It should be noted that the rolls within the stretching device are now arranged in such a sequence that the plastic film can be transported from bottom to top.

[0087] The Figure 9shows a further embodiment of a stretching device according to the invention, in which at least one nozzle 180 is arranged upstream of the inlet gap of a stretching roller, in the embodiment shown, the stretching roller 120. Through this nozzle, the film on the side facing away from the roller can be subjected to a pressurized fluid 181, in particular compressed air. This ensures that the film is pressed against the roller 120 with a force, so that less air is drawn into the area between the film and the roller 120 by the movement of the film.

[0088] Numerous possible features of the invention are specified in the description and claims. Even if the free combination of various features is not always mentioned, this is to be considered as part of the disclosure, provided no contradictions arise.

[0089] The Figure 10shows a further embodiment of a device according to the invention, in which a covering device 185 is shown for one of the stretching rollers. Such a covering device can also be provided on each additional stretching roller. With this covering device 185, the angular region of the stretching roller not wrapped by the film can be covered, so that a negative pressure applied to areas of the roller surface and / or its interior is not disturbed and is maintained as far as possible. The covering device can interact with the stretching roller without contact. The covering device can, for example, be a bent sheet metal. The covering device can extend in the axial direction almost over the entire length of the roller. Seals can be provided on the lateral edges of the covering device, which additionally seal the area between the roller and the covering device in order to further improve the reduction of false air.The covering device can be composed of several cover elements that can be moved relative to each other. This allows for optimal adjustment of the cover element, even for different film widths and different film wrap angles.

[0090] The Figure 11 shows a variant of a covering device, wherein the covering device comprises at least one circulating belt 186, which is also guided by the stretching roller. At least two deflection rollers 187 are provided for guiding and / or driving the circulating belt. The deflection rollers can be displaceable in the circumferential direction and / or in the radial direction of the stretching roller.

[0091] In order to avoid the Figures 10 and 11 The described and avoidable effect of false air intake can be minimized according to the embodiment of the Figure 12It may be provided that the fourth application roller 122 is dimensioned and / or positioned in such a way that the area not wrapped by the film is as small as possible. Since this unwrapped area depends on a multitude of parameters, no specific information can be given regarding the position and dimensions of the roller 122. It is possible for a person skilled in the art to determine such information.

[0092] In order to avoid the Figures 10 and 11 To minimize the effect of false air intake described and to be avoided, as described in the Figure 13As shown, a first guide roller 190 and a second guide roller 191 can also be provided. Each of the guide rollers can touch the film. The first guide roller is arranged upstream of the respective stretching roller, the second guide roller downstream of the stretching roller. The first guide roller can be identical to the second application roller 112. A sealing element 192 is arranged between the two guide rollers 190 and 191, which sealing element preferably contacts the circumferential surfaces of the guide rollers. The resulting cavity consisting of the stretching roller, the film, the guide rollers and the sealing element is closed in particular by seals arranged on the end face of the cavity, but not shown. List of reference symbols 100 Stretching device 101 plastic film 102 Preheat roller 110 First roller 111 First application roller 112 Second feed roller 120 Second stretching roller 121 Third application roller 122 Second feed roller 130 Cooling roller 140 roller shell 141 frontal surface 142 Part protruding from the front surface 141 143 Deepenings 150 passage opening 151 Hose 160 partition 161 partition 165 Suction device 166 Closed box 167 suction opening 170 electrode 180 nozzle 181 Fluid 185 Covering device 186 Circumferential band 187 pulleys 190 First guide roller 191 Second guide roller 192 Sealing element 1 Device for producing a film tube 2 Extruder 3 Line 4 Extrusion tool 5 Invisible annular gap 6 Film tube not yet solidified 7 Optional calibration device 8 Tempering device 9 Flattening device 10 Take-off rollers 11 Hose axis 12 Arrow to illustrate the rotatability of the flat-laying device 13 14 15 Reversing device 16 Stationary roller 17 Arrow indicating further processing T Transport direction Z Pull-off direction

Claims

1. Stretching device (100) for stretching a plastic film (101) in its transport direction, with a first roller (110) that can be driven by a first drive and can be rotated at a first rotational speed, and with a second roller (120) that can be driven by a second drive and can be rotated at a second rotational speed, wherein the second rotational speed is greater than the first rotational speed, wherein the second roller (120) is arranged downstream of the first roller (110) in the transport path of the plastic film (101), wherein means for varying the amount of air between the plastic film (101) and surface elements of the first roller (110) and / or the second roller (120) on which the plastic film (101) rests are provided, characterized in that the first roller (110) and / or the second roller (120) comprise at least one cavity which is divided into at least two segments in the circumferential direction by at least one separating element (160, 161), wherein at least one segment can be subjected to an air pressure that is reduced or increased compared to the ambient pressure.

2. Stretching device (100) according to claim 1, characterized in that the means for varying the amount of air comprise depressions (143) in the surface of the first roller (110) and / or the second roller (120), wherein air can be received in the depressions.

3. Stretching device (100) according to any one of the preceding claims, characterized in that the means for varying the amount of air comprise through-openings (150) in the surface of the first roller (110) and / or the second roller (120), wherein the through-openings (150) provide a fluid-communicating connection between the outer surface (140) and a cavity within the first roller (110) and / or the second roller (120).

4. Stretching device (100) according to any one of the preceding claims, characterized in that the means for varying the amount of air comprise through-openings (150) in the surface (140) of the first roller (110) and / or the second roller (120), wherein at least part of the through-openings (150) can be provided by a porous material, in particular by a microporous material.

5. Stretching device (100) according to any one of the preceding claims, characterized in that the means for varying the amount of air comprise through-openings (150) in the surface of the first roller (110) and / or the second roller (120), wherein at least part of the through-openings (150) can be provided by a porous material, in particular by a microporous material, wherein the porous material is a sintered material, in particular, a sintered metal.

6. Stretching device (100) according to any one of the preceding claims, characterized in that the means for varying the amount of air comprise through-openings (150) in at least part of a roll shell (140) of the first roller (110) and / or of the second roller (120), wherein at least one part of the through-openings are formed by bores.

7. Stretching device (100) according to any one of the preceding claims, characterized in that the first roller (110) and / or the second roller (120) comprise at least one cavity which can be subjected to an air pressure that is lower than the ambient pressure.

8. Stretching device (100) according to any one of the preceding claims, characterized in that the means for varying the amount of air between the plastic film and surface elements of the first roller (110) and / or the second roller (120) on which the plastic film (101) rests comprise at least one suction apparatus (165) with which air can be sucked out of the region in which the plastic film (101) comes into contact with the first roller (110) and / or the second roller (120) in the transport direction.

9. Stretching device (100) according to the preceding claim, characterized in that the suction apparatus (165) is divided into sections transversely to the transport direction of the plastic film (101).

10. Stretching device (100) according to any one of the preceding claims, characterized in that the means for varying the amount of air between the plastic film (101) and surface elements of the first roller (110) and / or the second roller (120) on which the plastic film (101) rests comprise at least one overpressure apparatus (180) with which the surface of the plastic film (101) facing away from the roller (110, 120) can be acted upon with air under overpressure.

11. Stretching device (100) according to the preceding claim, characterized in that the means for varying the amount of air between the plastic film (101) and surface elements of the first roller (110) and / or the second roller (120) on which the plastic film (101) rests comprise at least one electrode with which the plastic film (101) can be electrostatically charged upstream or at the line of contact of the plastic film (101) with the roller (110, 120).

12. Method for stretching a plastic film (101) in its transport direction with a first roller (110) which is driven by a first drive and rotated at a first rotational speed, and with a second roller (120) which is driven by a second drive and rotated at a second rotational speed, wherein the second rotational speed is greater than the first rotational speed, wherein the second roller (120) is arranged downstream of the first roller (110) in the transport path of the plastic film (101), wherein the amount of air present between the plastic film (101) and surface elements of the first roller (110) and / or the second roller (120) on which the plastic film (101) rests is varied by means for varying the amount of air, characterized in that the first roller (110) and / or the second roller (120) comprise at least one cavity which is divided into at least two segments in the circumferential direction by at least one separating element (160, 161), wherein at least one segment can be subjected to an air pressure which is reduced or increased compared to the ambient pressure.

13. Blow film installation (1) with at least one extruder (2) for producing a plastic melt, a die head (4) for producing a film tube (6), a flattening apparatus (9) for converting the film tube (6) into a double-layer plastic film, a driven extraction device (10) for drawing off and further transporting the double-layer plastic film, and a winding apparatus for winding up at least one layer of the double-layer plastic film, characterized by at least one stretching device (100) according to any one of claims 1 to 11.