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

Abstract

A stretching device for stretching a plastic film in its transport direction, having a first roller and a second roller, wherein the first roller is drivable with a first drive and rotatable at a first rotational speed, the second roller is drivable with a second drive and rotatable at a second rotational speed, wherein the second rotational speed is greater than the first rotational speed, wherein the second roller is arranged downstream of the first roller in the transport path of the plastic film. The device is characterized by means for changing the amount of air between the plastic film and the surface elements of the first roller and / or the second roller on which the film is placed.

Description

TECHNICAL FIELD

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

[0002] Such a stretching device serves to purposefully influence the properties of a plastic film. By such a stretching, inter alia, the orientation of molecules contained in the plastic film (hereinafter also referred to simply as "film") is changed. In order to guide the stretching here, the stretching device comprises a first roller which can be driven with a first drive device and which rotates with a first rotational speed or peripheral speed. Furthermore, the stretching device comprises a second roller which can be driven with a second drive device and which rotates with a second rotational speed or peripheral speed. In the transport path of the plastic film, the second roller is arranged downstream of the first roller.

[0003] The stretching of the plastic film now takes place in that the second rotational speed is greater than the first rotational speed. Rotational speed is to be understood as the rotational speed which the film obtains in connection with the stretching. The rotational speed can therefore also be referred to as the peripheral speed of the first and second rollers.

[0004] However, various problems can also arise in the stretching. Thus, after the stretching, it often happens that monomers which have been outgassed during the manufacture of the plastic film adhere to the plastic film, or that dust which has been pressed into the film by the stretching also adheres to the plastic film. The heat transfer between the film and the rollers is often also unsatisfactory. Furthermore, the properties of the film, in particular the geometric properties such as the thickness profile, can however also be changed along a direction transverse to the transport direction of the film and the planar position of the film can also be changed by the stretching. SUMMARY

[0005] It is therefore the task of the present application to propose a stretching device with which at least some of the problems mentioned can be reduced.

[0006] According to the application, this task is solved by the features of claim 1. Possible design options of the application are specified in the dependent claims.

[0007] In a first aspect of the invention, a stretching device of a class for stretching a plastic film is provided. This stretching device is characterized by means of changing the amount of air between the plastic film and a surface element on which the film is attached, on a first roller and / or a second roller. Therefore, the present invention enables the changing of the amount of air between the plastic film and the surface element on which the film is attached. This includes both a decrease and an increase in the amount of air. The plastic film travels onto the corresponding roller at the impact edge and exits the roller at the discharge edge. The impact edge and the exit edge... Together with its side edges, a total surface element is defined on which the film can lie flat to the maximum extent. However, the actual surface element on which the film is attached may be smaller than the total surface element.

[0008] By varying the air volume, these problems can be specifically avoided. Therefore, increasing the air volume reduces the amount of dust or monomers being forced into the film. Decreasing the air volume significantly improves heat transfer between the roller and the plastic film, as air absorbs only a small amount of heat and is therefore a poor thermal conductor. Thus, reducing the air volume improves overall heat conduction.

[0009] The reduction in air volume can also be used to make the film adhere more strongly to the roller, thus reducing the film's freedom of geometric change. In particular, it can reduce the tendency to change the thickness distribution transversely to the transport direction. The measures according to the invention can also reduce the deterioration of the film's planar position.

[0010] Here, the device for changing the air volume can be arranged only in the edge region of the roller. This avoids the effective width of the membrane traveling on the flattest possible portion of the roller and only the edge region of the membrane being attached. Because these edge regions are often subsequently separated, the negative impact of the device for changing the air volume on the membrane edge region is not negatively affected, while the advantages of the invention are retained.

[0011] In order to be able to reduce the air volume, in particular, in a first inventive refinement it is provided that the surface of the first roller and / or the second roller is provided with recesses. Thereby, in particular, the actual size of all surface elements on which the film is placed is reduced. Furthermore, the area at the edge of these surface elements is recessed with respect to these surface elements and thereby provides volume elements into which air can penetrate. Between the surface elements on which the film is placed and the film itself, now only a small portion of air remains. In this respect, the term "lying flat" should not be equated with a full-area contact of the film and the surface area. It is possible that in some places or even over the entire surface a very small amount of air remains, so that although a full-area contact is achieved macroscopically, it is not full-area microscopically. By the measures described, it is achieved that the film adheres more strongly to the roller, so that the advantages described above are achieved.

[0012] Grooves of an annular or helical design can also be recesses in the sense described in the preceding paragraph. Grooves of a helical design can be single- or multi-start here, wherein multi-start grooves can have the same and / or opposite rotational direction (cross grooves). Such grooves, in particular, are able to accommodate an air volume in the area on which the film is placed and to discharge the air volume again into the surroundings in the area on which the film is not placed. The width and / or depth of such grooves can be less than 2 mm, preferably less than 1 mm and in particular less than 0.5 mm. Here, the transition of the surface element on which the film is placed and the groove can be rounded or provided with a bevel in order to avoid damage to the film.

[0013] In another inventive refinement, the means for changing the air volume comprises through-holes in the surface of the first roller and / or the second roller, wherein the through-holes are connections of the fluid communication between the outer surface and a cavity within the first roller and / or the second roller. With such through-holes it is possible to achieve that at least a portion of the air enclosed between the film and the surface element of the respective roller on which the film is placed can be guided out into the cavity of the roller. Thus, a reduced air volume remains between the film and the surface element, so that the film adheres more strongly to the surface element of the roller surface and exhibits the undesired effects to a reduced extent or not at all.

[0014] In an advantageous design of the application it is provided that the means for changing the air quantity comprise through-holes in the surface of the first and / or second roller, wherein at least a portion of the through-holes can be provided by a porous material, in particular by a micro-porous material. Thereby, a large number of small channels serving as through-holes can be realized. This has the advantage that the openings on the outer surface of the roller are small and that the edges of these openings have no negative effect on the film, for example in the form of indentations. Thereby, the above-mentioned task is solved without having to tolerate new disadvantages. The porous material can here constitute the roller sleeve or at least be a component thereof. The average pore size of the porous material can be between 5 and 100 micrometers, in particular between 10 and 60 micrometers and preferably between 20 and 45 micrometers. In an inventive variant it is provided that the porous material, in particular the micro-porous material, is only an outer coating of the otherwise closed roller surface. Thus, for example, a closed tube, for example made of metal, can be provided, onto which the porous material is applied or onto which the porous material is applied. The cavities provided by the porous material can now accommodate the air between the film and the roller.

[0015] Whether the porous material provides through-holes or only cavities for accommodating air, it can be provided that the pore size is changed along the axial direction of the roller. The volume provided by the porous material per surface element can in particular be larger on the edge of the roller sleeve than in the middle region, so that the film is attached more strongly to the roller on the edge.

[0016] The outer surface of the roller, which is at least partially constituted by the porous material, can be provided with a coating in order to avoid damage to the film, which coating in particular has a smaller hardness than the porous material.

[0017] The porous material can be a sintered material, in particular a sintered metal or a sintered plastic. A sintered material is a material manufactured by a sintering process. Here, a fine-grained material, which can be ceramic or metallic or include a plastic, is heated, often under increased pressure, wherein the temperature is kept, however, below the melting temperature of the main component, so that the configuration of the workpiece is preserved. The configuration can be, for example, a sleeve shape, which can be applied to a base body after production. The base body and the sleeve can then constitute the roll. The production of such a sleeve can also be carried out in an additive manufacturing method, in which a three-dimensional body can be produced from a powdery raw material by local heating. The granular nature of the material results in channels remaining between the particles, which constitute the through-holes. The peripheral surface of a roll produced in this way can additionally be ground and / or polished in order to avoid damage to the film. However, in order to avoid unintentional closure of the through-holes due to wear, the surface of the roll can subsequently also be treated by an etching method. The through-holes are generally designed to be non-linear, however, without impairing the ability to conduct air out into the interior of the roll. Sintered materials are mostly very hard materials, so that despite high tensile forces within the stretching device only small bending occurs, so that in particular the geometric properties of the film are hardly affected. Furthermore, the production of sintered materials is generally relatively low-cost.

[0018] In order to reduce the amount of air, according to an alternative or additional design of the application, the means for changing the amount of air comprises through-holes at least in a part of the sleeve of the first roll and / or the second roll, wherein at least a part of the through-holes is formed by drilling. The sleeve can here surround the roll surface, so that the drilling then runs from the periphery of the roll into the interior of the roll. The sleeve can also be constructed in multiple layers. For example, an inner part of the sleeve can be provided, which comprises the drilling. An outer part of the sleeve can comprise a porous, in particular micro-porous material. In this way it is possible to avoid the edges of the drilling appearing on the film and thus making it unusable.

[0019] The boreholes can be less than 2 mm, preferably less than 1 mm and in particular less than 0.5 mm. Here, the transition from the borehole to the surface of the part of the roller cover can be rounded or conically shaped. With this measure, damage to the film is also avoided. A plurality of boreholes can be distributed uniformly over the roller cover, wherein a defined number of boreholes can be provided per surface element. However, the spacing between two adjacent boreholes, respectively, can vary. It is particularly advantageous if the number of boreholes per surface element or the number of through-holes per surface element varies, in particular along the longitudinal direction of the roller cover, i.e. along the direction of the axis of rotation. The number of boreholes or through-holes per surface element can in particular be greater at the edges of the roller cover than in the middle region, so that the film is attached more strongly to the roller at the edges. The edges can here each occupy one third of the length of the roller cover, so that the middle region also comprises one third. However, the middle region can also be greater, so that the edges are correspondingly smaller, respectively. By means of this variation in the number of boreholes per surface element or the number of through-holes per surface element, the volume flow through the wall of the roller can thus be changed in general. This variation can alternatively or additionally be achieved for a sintered material by varying the layer thickness of the sintered material, so that there is a different resistance to the volume flow.

[0020] In an advantageous embodiment, it is conceivable for the first roller and / or the second roller to be constructed in a plurality of layers, wherein in particular a layer can be configured displaceable relative to a second layer, wherein the two layers can be detachably connected after the relative displacement. It is thus possible, for example, for an outer layer adapted to the properties of the film to be displaced onto the base body. Furthermore, this embodiment is advantageous in order to have to replace only the outer layer when the roller is contaminated. Such a layer can then be cleaned, while the production of the film can be continued with another outer layer.

[0021] In a further variant of the application, the means for varying the air quantity can comprise at least one cavity, which can be loaded with an air pressure reduced relative to the ambient pressure. It is thus achieved that a part of the air quantity fed to the cavity of one of the rollers can be conducted out of the cavity, for example via a through-hole. The air repeatedly re-introduced by the movement of the film and the roller can thus be continuously conducted out, so that a stationary state in terms of a reduced air quantity between the plastic film and the surface elements of the first roller and / or the second roller on which the film is placed can be adjusted. A rotating sleeve produced via the journal of the respective roller provides a possible solution for loading the cavity. On this rotating sleeve a hose or tube can be latched, which connects the interior space fluidically with a source of negative pressure, in particular a pump.

[0022] It can furthermore be provided that the at least one cavity can be loaded with an increased pressure relative to the ambient pressure. It is thus possible to achieve that dirt is removed from the through-holes by means of a blowing-off process. In addition or alternatively, a suction device can be provided which is arranged in the angle region of the first and / or second roll in which the film lies flat, that is to say between the disengagement line and the feed edge of the film. In this way, dirt can be removed even better from the surface of the roll. Such a suction device can extend at least partially in the direction of the roll axis. Such a suction device can also have a plurality of suction chambers which are separate from one another, the size of which can furthermore be variable, so that the suction device can be loaded with different suction powers. The suction power can be smaller in the region which is directed toward the roll and which does not come into contact with the film than in the remaining region, for example, since less dirt is generally to be feared here. Dirt is often caused by paraffin which is fed to the roll and adheres to the film.

[0023] According to an advantageous design of the stretching device according to the application, the first roll and / or the second roll comprises at least one cavity which is divided into at least two sections along the axial direction and / or along the peripheral direction by at least one dividing element, wherein at least one section can be loaded with an air pressure which is reduced or increased relative to the ambient pressure. Each section can thus be loaded with a different air pressure, so that the air volume between the plastic film and the surface element of the first roll and / or of the second roll on which the film is placed can be varied differently, for example, along the axial direction of the roll. In particular, two dividing elements are advantageous, since the edge region and the intermediate region can then be loaded with different negative pressures, for example, as viewed along the axial direction. In particular, it is provided that a higher negative pressure is applied to the film in the edge region of the film than in the use region of the film, so that damage to the film in the use region is avoided. The dividing elements can be arranged immovably relative to the roll jacket. It can be advantageous here that the dividing elements are arranged displaceably relative to the roll axis.

[0024] The dividing elements alternatively or additionally can divide the interior cavity of one of the mentioned rollers into sections, i.e. sectioned, along the circumferential direction. Such dividing elements then run parallel to the longitudinal direction of the respective roller. In particular the roller sleeve can be rotatably supported around a roller axis, wherein one or more dividing elements can be arranged immovable relative to this axis. Thus, for example, a section can be arranged below the surface elements of the first and / or second roller on which the film is placed. Then only in the area of these surface elements a negative pressure and / or an overpressure is generated, while in another section essentially the ambient pressure prevails. In this way the effectiveness of the means for changing the air quantity is improved, since ambient air is not sucked in, but only the air quantity at the relevant area is changed. At least in the area of the disengagement line, which is explained below in connection with the disengagement roller, a section can be provided which can be loaded with an overpressure. As a result, the film is disengaged from the first and / or second roller along an actual disengagement line which only slightly deviates from the nominal disengagement line.

[0025] Sections in whose area of action the film is not lying flat can be loaded with an overpressure, wherein an air flow can move from the interior of the roller outward. Thus, a continuous cleaning of the through-holes can be achieved, so that an increasing soiling and clogging of the through-holes over time can be avoided.

[0026] At least three dividing elements can also be provided along the circumferential direction, so that at least three sections can be provided. For example, an arrangement option arises in which sections loaded with a negative pressure are arranged in the area of the film's run-on (Auflaufbereich) and sections loaded with an overpressure are arranged in the run-off area (Auslaufbereich). In all remaining areas the ambient pressure can prevail. Thus, in the area in which the film runs onto the roller surface (run-on area), the air brought in can be sucked away, whereby the advantages according to the invention are achieved. In contrast, in the area in which the film leaves the surface again (run-off area), the film can be detached from the surface of the roller with an overpressure, whereby it is avoided that the film detaches at different circumferential angles of the roller, viewed over the width of the film. Avoiding this effect contributes to an improved behavior, in particular a geometric behavior, of the film. The application of an overpressure can also be used, for example, for cleaning the through-holes in the roller sleeve.

[0027] Individual sections can be individually manipulated in terms of the size of the negative pressure and / or the overpressure. The manipulation can be carried out by a computing control unit which compares measurement data of sensors with target data for the manipulation. These measurement data can be transmitted to the computing control unit by measurement sensors with which a measurement of the geometric behavior of the film, in particular a thickness profile of the film, can be carried out. In this way, an adjustment of the overpressure and / or the negative pressure can be built into each section.

[0028] In order to be able to change the amount of air between the plastic film and the surface elements of the first and / or second roller on which the film is placed, in one embodiment of the application at least one suction device is provided with which air can be sucked away from the region in which the plastic film comes into contact with the first and / or second roller in the direction of transport. In particular when the film runs onto the roller, air is dragged along by the film which reaches between the surface elements of the roller and the film. By means of the suction device it is now possible to suck this air in a targeted manner, so that significantly less air is introduced into the mentioned region. This makes it possible to achieve the advantages according to the application.

[0029] Alternatively or additionally it can be provided that the regions of the roller which are not lying flat with the film are at least partially covered with a cover device, so that in this region the ambient pressure does not meet the roller surface. This can avoid the intake of air from the environment (ingress of air) in particular when a lower pressure than the ambient pressure is applied in the roller body and / or when a negative pressure is induced by a suction device arranged near the roller surface. This promotes greater efficiency of the application, because then only the air dragged along by the film has to be conducted away, without the air possibly dragged along by the roller having to be conducted away. It is advantageous in this respect if the suction device and the cover device are connected to one another and form a unit.

[0030] In one refinement it is provided that the suction device and / or the cover device is divided into sections transversely to the direction of transport of the plastic film. It is possible here for a suction device which extends over the entire width of the plastic film to be divided into sections. However, it is also possible to provide individual sections which are independent of one another. This is particularly advantageous if air in the edge region of the plastic film is to be sucked away, for example in order to minimize the influence of the stretching on the geometric properties of the film in its edge region. If such independent sections are provided, it is advantageous if these sections can be positioned along the transverse direction of the film, that is to say parallel to the axis of the respective roller, by means of positioning devices in order to adapt the sections of the suction device to different film widths.

[0031] The individual sections can be individually controlled in terms of their suction power. The control can be carried out by a computing control unit which compares measurement data of sensors with target data for the control. Such measurement data can be transmitted to the computing control unit by measurement sensors with which measurements of the geometric properties of the film, in particular the thickness profile of the film, can be carried out. In this way, an adjustment of the air suction can be created.

[0032] An advantageous design variant of the application provides that the device for changing the amount of air between the plastic film and the surface elements of the first and / or second roll on which the film is placed comprises at least one overpressure mechanism with which fluid, in particular air, under overpressure can be applied to the surface of the plastic film facing away from the roll. In this way, the back of the film, that is to say the side facing away from the roll, is loaded with a force exerted by the air under overpressure. As a result, the film can be pressed onto the roll with greater force, so that a smaller proportion of air can be reached in the region between the film and the roll. As with the previously described suction mechanism, the overpressure mechanism can be divided into segments which can additionally be displaced in the direction of the roll axis. Control and / or regulation of the overpressure and / or volume flow can also be provided. This can also be done segment-wise and individually, in particular.

[0033] If the device for changing the amount of air designed as an overpressure mechanism is arranged in particular in the edge region of the roll, it can be provided that the fluid applied to the edge region of the roll on which in particular the edge region of the film is placed is a cold fluid. By "cold fluid" is meant a fluid having a temperature of less than 50 degrees Celsius, preferably less than 30 degrees Celsius, preferably less than 15 degrees Celsius. It is advantageous here that the edge region of the film has a higher stiffness than the use region of the film before it travels onto the relevant roll, so that the edge region no longer moves inwards so strongly. In other words, the neck-in is thereby reduced.

[0034] In order to reduce the neck-in, a fixing mechanism can be provided. This fixing mechanism can be provided, for example, in the region between the first roll and the second roll. In this case, the fixing mechanism can consist of two disc-shaped roll pairs through the roll gap of which the edge of the film travels respectively. In this way, the forces caused by the longitudinal stretching and acting on the film edge are compensated or at least reduced. However, the fixing mechanism can also be arranged at or on the first roll and / or the second roll. This can be, for example, a surface element which provides the film with greater adhesion than the rest of the roll surface. The fixing mechanism can be displaced in the transverse direction of the film, that is to say in the axial direction of the first roll and / or the second roll.

[0035] In order to improve the adhesion of the film on the first and / or second roller, it can be provided that the means for changing the amount of air between the plastic film and the surface element of the first and / or second roller on which the film is applied comprises at least one electrode with which the plastic film can be electrostatically charged upstream or at the contact line of the plastic film with the roller. Due to the electrostatic charging of the film, an electrostatic attractive or repulsive force arises between the film and the roller. If an attractive force, i.e. a force directed from the film towards the roller, is present, a smaller portion of the air dragged along with the film reaches the intermediate space between the film and the roller, so that the film adheres better to the roller. At least two electrodes following one another can be provided, which are adjustable independently of one another in terms of voltage, viewed in the axial direction of the respective roller. The voltage can be controlled by means of a control device. In this way, the line of impact of the film on the roller can be changed and, in particular, optimized. In the ideal case, this line of impact is a straight line. This embodiment variant can contribute to improving the planar position of the film. Furthermore, it can be achieved that the edges of the film adhere to the roller with another voltage and, thus, with another induced charge, as well as the middle region of the film, despite the greater thickness.

[0036] In another design variant of the application, an electrode can be combined with, for example, the overpressure mechanism described above. Thus, the fluid flowing from the overpressure mechanism in the direction of the film can be electrostatically charged, wherein this electrostatic charge can be applied to the film. The adhesion of the film on the roller can again be improved thereby.

[0037] It can be advantageous in connection with the electrode to provide a discharge mechanism downstream of the first roller and / or downstream of the second roller in the transport direction of the film. Thereby, the electrostatic charging of the film can again be reduced or even completely eliminated, so that the further processing, like, for example, the winding, is not negatively affected. It is particularly advantageous if the discharge mechanism is arranged in the intended point of detachment of the film from the first and / or second roller or is arranged before it in the transport direction of the film, in order to achieve the discharge of the film at the latest in the region of the detachment line. Thereby, it is ensured that regions of the film do not remain attached to the roller for different lengths of time, which can lead to planar position errors. The discharge mechanism can be designed, for example, as a grounded roller and / or a brush roller.

[0038] The first and / or second roller suitable for the design variants of the application can comprise at least partially a metal, like, for example, aluminum, and / or at least partially a plastic, in particular a fiber-reinforced plastic. The materials mentioned at the outset can be used, for example, for the base body of the first and / or second roller.

[0039] Furthermore, the first roller and / or the second roller can comprise a particularly processed surface. It has already been described above that the surface can comprise a sintered material. It is conceivable that, alternatively or additionally, the surface comprises a rubberized layer, is at least partially coated with polytetrafluoroethylene (PTFE), known as Teflon, and / or is at least partially chrome-plated, in order to be able to better adapt the surface properties to the properties of the film. Scratches can be avoided, in particular for sensitive films. In the case where a sintered material has been used as a material for the first roller and / or the second roller, PTFE can have been embedded before the sintering process, in order to avoid clogging of the through-holes.

[0040] In order to improve, in particular, the so-called flat position of the film, at least one disengagement roller can be provided, which can be pressed onto the first roller and / or the second roller. The film should ideally leave the first roller or the second roller at a nominal disengagement line, which runs parallel to the longitudinal axis of the roller. Here, the nominal disengagement line lies on a plane, which forms a common tangential plane of the first roller and / or the second roller and the subsequent roller. In reality, this disengagement line is not a straight line, which can lead to defects in the film. Here, the disengagement roller is positioned such that it forms a roller gap with the first roller and / or the second roller in or behind the nominal disengagement line in the transport direction of the film. As a result, the actual disengagement line is matched to the nominal disengagement line. The disengagement roller can in particular be displaced along the peripheral direction of the first roller and / or the second roller.

[0041] In another embodiment of the application, it is provided that the first roller and / or the second roller have a shape which differs from the cylindrical shape. Thus, for example, it can be provided that the roller has a conical, concave or convex shape. With such a design of the roller, different defects, for example flat position errors, can be prevented in part depending on the properties of the film.

[0042] It is in principle advantageous to provide a temperature control device with which a film region, which is lying on the first roller and / or the second roller or is in a section between the first roller and the second roller, can be changed in terms of its temperature. It can be desirable for the temperature of the film to be increased, but it can also be desirable for the temperature of the film to be reduced. In particular, a temperature control device can be provided with which the mentioned film region can be brought to different temperatures along the transverse direction, i.e. transverse to its transport direction. Thus, it can be provided that the edge region of the film is cooled more intensively than the remaining region, in order to reduce the shrinkage.

[0043] Such a temperature control device can be integrated in particular in the first roller and / or in the second roller, so that the efficiency of the temperature control device is high. In this case, the film itself is temperature-controlled by the heat conduction of the roller. Thus, the roller body can be penetrated for example by at least one tube through which the temperature-controlled fluid can be conducted. Such a tube can here extend along the axial direction of the roller in order to guarantee a rapid fluid distribution. The tube can be implemented for example as a bore in a roller sleeve. However, the course of the tube can also have other courses, for example a helical course, in order to be able to generate a greater heat transfer. In this case, the tube can be formed by a tube or a hose which is arranged inside a hollow roller sleeve. If the roller sleeve is constructed in several layers, the tube can also be arranged between two layers. A plurality of tubes can also be envisaged in order to be able to load different zones of the roller with different temperatures. The temperature-controlled fluid is preferably fed into the roller by means of a rotating sleeve of one or more end faces.

[0044] In particular in the design variant of the first roller and / or of the second roller in combination with a sintered material, it can be envisaged that the roller sleeve is designed in at least two layers. Here, the inner layer can comprise coarser holes than the outer layer, through which the fluid can be conducted. Here, the outer layer can reduce the amount of air between the film and the roller according to the invention by the conduction of air.

[0045] In another variant of the invention, the first roller and / or the second roller can comprise at least one heating wire which is embedded in the surface layer of the roller. This embedding can be produced in particular in combination with the design variant of this surface layer by a sintered material.

[0046] In an advantageous embodiment of the invention, the temperature control device can be designed as an induction mechanism. The mechanism which forms part of the induction mechanism for providing a magnetic field can be arranged in particular inside the roller in order to increase the efficiency of the temperature control device. However, in order to guarantee a simpler construction, it can also be envisaged to arrange it outside the roller.

[0047] Alternative or additional temperature control devices based on other physical principles can also be envisaged. Thus, for example, the temperature of the film can be changed by means for generating infrared radiation. This means is preferably arranged outside one of the rollers. It is also possible to provide a temperature-controlled gas which can be directed directly onto the film from at least one nozzle for temperature control. Air is provided in particular as the gas. The means for generating infrared radiation and / or the nozzle can also be arranged inside the roller.

[0048] In another advantageous embodiment of the application, provision is made for the device for changing the amount of air to comprise at least one duct which is arranged within the roll, in particular within the roll cover. Furthermore, in this embodiment provision is made for a connecting duct which is in fluid-communicating connection with the duct and which merges at its further end into the roll surface. Such a connecting duct can in turn be a drilled hole or a channel, for example, in the sintered material. Thus, air which is present between the film and the roll surface can be discharged through the connecting duct and the duct. The duct can merge into one of the end faces of the respective roll, for example. The duct can in particular be an axial duct and / or be designed as an axial drilled hole. For the multi-layer structure of the cover of the first roll and / or the second roll, provision can also be made for a gas-impermeable inner layer. In the case of the use of a spacer pad, an annular space can be provided between the inner layer and the outer layer, into which annular space air can be collected. The annular space can thus also be understood as a duct in the above-described sense. The annular space is particularly advantageous if the outer layer is composed of sintered material.

[0049] In an embodiment with axial ducts, these ducts can terminate on the end face of the roll or on the end face of the mounting, like for example on a journal. On this end face a component can be placed which is fixed relative to the roll and which comprises a circular-arc-segment-shaped cavity. Now, when the roll is rotated, the end of the duct can sweep through this cavity. If this cavity is now connected to an underpressure or an overpressure, each duct is also connected to an underpressure or an overpressure, the duct reaching the area of action of the cavity by the rotation of the roll. When the respective duct is again outside the area of action, this loading is correspondingly cancelled. The cavity which is loaded with underpressure is preferably arranged in such a way that the ducts which are in fluid-communicating connection with the connecting ducts whose ends merge into the surface area on which the film is placed can be loaded with underpressure. Thus, the intake of too much ambient air is avoided. The cavity which is loaded with overpressure is preferably arranged in such a way that the ducts which are in fluid-communicating connection with the connecting ducts whose ends merge into the surface area on which the film is not placed and / or at the line of detachment of the film can be loaded with overpressure.

[0050] A plurality of components of the described type, for example components which can be loaded with overpressure and components which can be loaded with underpressure, can be provided accordingly.

[0051] The above mentioned tasks are additionally solved by a method for stretching a plastic film in its transport direction, which makes use of a first roller and a second roller, wherein the first roller is driven with a first drive means and rotates with a first rotational speed, the second roller is driven with a second drive means and rotates with a second rotational speed, wherein the second rotational speed is greater than the first rotational speed, wherein the second roller is arranged downstream of the first roller in the transport path of the plastic film, wherein the amount of air present is changed with means for changing the amount of air between the plastic film and the surface elements of the first and / or second roller on which the film is placed.

[0052] With this method according to the invention the same advantages can be achieved as already described in connection with the device according to the invention.

[0053] In another aspect of the invention a film blowing apparatus is provided, which has at least one extruder for producing a plastic melt, a nozzle head for manufacturing a film tube from the plastic melt, a laying device for converting the film tube into a double-layered plastic film, a driven extraction device for extracting and continuing to transport the double-layered plastic film, and a winding device for winding at least one layer of the double-layered plastic film, wherein a stretching device according to any one of claims 1 to 12 and / or designed according to the above description is provided. Such a film blowing apparatus can optionally comprise a calibration device, which is arranged before the laying device in the transport direction. Furthermore, a reversing device can be provided, which is arranged after the extraction device and with which defects, in particular deviations from the average film thickness, can be removed laterally over the film width. The stretching device according to the invention can now be arranged downstream of the extraction device, for example between the extraction device and the optional reversing device. It is also conceivable to arrange the stretching device between the reversing device and the winding device. The plastic film can be delivered to the stretching device as a laid film tube or as a single- or double-sided cut double-layered plastic film. A single-sided cut plastic film can have been previously turned over and delivered to the stretching device as a single-layered plastic film, which is in particular double-wide. A double-sided cut plastic film can be divided into its individual layers, wherein these layers can be delivered to individual stretching devices, respectively. The winding device can comprise winding stations for winding the uncut, single- or double-sided cut double-layered plastic film. A single-sided cut, single-layered, i.e. turned over, plastic film can also be wound. But it is also possible to provide two winding stations for winding the layers of the previously double-layered plastic film, respectively. BRIEF DESCRIPTION OF DRAWINGS

[0054] Further advantages, features and details of the present application are known from the following description, in which different embodiments are explained in detail with reference to the drawings. Herein, features mentioned in the claims and in the specification are respectively important for the present application, either individually or in any combination of the mentioned features. Throughout the disclosure it is apparent that features and details described in connection with the method according to the present application are also applicable in connection with the stretching device according to the present application and vice versa accordingly, such that with respect to the disclosure related to the individual aspects of the present application always or can be referred to each other. The figures show:

[0055] Figure 1 a schematic view of a stretching device according to the present application,

[0056] Figure 2 a stretching roller showing an embodiment of the present application,

[0057] Figure 3 a stretching roller showing another embodiment of the present application,

[0058] Figure 4 a stretching roller showing another embodiment of the present application,

[0059] Figure 5 an embodiment of the present application with a suction mechanism,

[0060] Figure 6 an embodiment of the present application with an electrode,

[0061] Figure 7 a film blowing apparatus according to the present application with a stretching device,

[0062] Figure 8 another film blowing apparatus according to the present application with a stretching device,

[0063] Figure 9 an embodiment of the present application with a nozzle,

[0064] Figure 10 an embodiment of the device according to the present application with a covering device,

[0065] Figure 11 a variant of the covering device,

[0066] Figure 12 an embodiment of the device according to the present application,

[0067] Figure 13 an embodiment of the device according to the present application. DETAILED DESCRIPTION

[0068] Figure 1A schematic view of a stretching device according to the application is shown. The means for changing the amount of air between the plastic film and the surface elements of the first and / or second roll on which the film is applied are not visible in this figure and are explained in detail by means of the following figures. The plastic film 101 in web form enters the stretching device 100 along a transport direction T. The plastic film first travels onto a preheating roll or, successively, onto a plurality of preheating rolls, only one of which is shown, 102. The task of the preheating rolls is to bring the film to a predefined temperature. To this end, the preheating rolls are generally temperature-regulated, wherein a temperature-regulated fluid is often introduced into the preheating rolls.

[0069] After leaving the preheating roll(s) 102, the film web 101 reaches a first roll 110, which can also generally be referred to as a first stretching roll 110. This stretching roll is connected with a drive, not shown, for example an electric motor of its own, which rotates the roll 110 at a first rotational speed.

[0070] The first roll 110 is preferably equipped with a first counter roll 111, which together with the first roll 110 provides a feed gap for the film. The feed gap or travel path of the film 101 is preferably so set up that the film 101 travels tangentially to the rolls 110, 111 in the feed gap. The roll gap already helps to minimize the air between the stretching roll 110 and the film.

[0071] In addition, the first roll 110 is also equipped with a second counter roll 112, which forms a discharge gap with the roll 110. The second counter roll 112 can be adjusted in the peripheral direction of the first roll 110. The second counter roll 112 serves to exit the film from the first roll 110 along a line that runs parallel to the axial direction of the roll 110.

[0072] Viewed in the transport direction T of the film 101, a second roll 120, which can be referred to as a second stretching roll 120, is arranged downstream. This stretching roll 120 is also connected with a further drive, not shown, for example an electric motor of its own, which rotates the roll 120 at a second rotational speed. The second rotational speed is greater than the first rotational speed here, wherein the second stretching roll then has a greater peripheral speed than the first stretching roll. This results in the film 101 being stretched between the discharge gap of the first roll 110 and the feed edge of the second roll 120 in the ratio of the peripheral speeds along its transport direction. The distance between the discharge gap and the feed edge is also generally referred to as the stretching gap.

[0073] It is possible for the first roll 110 and the second roll 120 to be movable relative to one another. By this it is possible to influence the stretching gap. A change in the stretching gap can influence the properties of the film.

[0074] Preferably, the second roller 120 is equipped with a third abutment roller 121, which, together with the first roller 120, provides a second feed gap for the membrane. Preferably, the feed gap or travel path of the membrane 101 is configured such that the membrane 101 travels tangentially with the rollers 120, 121 within the feed gap. This roller gap already helps to minimize air between the stretching roller 120 and the membrane.

[0075] In the case of a small tension gap, the third abutment roller 121 will collide with the roller 110, so in this case the third abutment roller must be deflected.

[0076] Furthermore, the second roller 120 may optionally be equipped with a fourth abutment roller 122, which forms a discharge gap with the roller 120. The second abutment roller 122 can also be adjusted along the periphery of the first roller 120. The second abutment roller 122 is used to cause the film to leave the first roller 120 along a line parallel to the axial direction of the roller 120.

[0077] In principle, one or more abutment rollers may be omitted in the stretching apparatus according to the invention. However, the feed gap or discharge gap can still be referred to. This feed gap or discharge gap is thus understood as the line along which the film abuts against the stretching roller or at which the film detaches from the stretching roller.

[0078] Additional stretching rollers can also be provided, especially stretching rollers with one or two abutment rollers, wherein two stretching rollers arranged directly in succession are driven such that the stretching roller arranged downstream has a higher peripheral speed than the stretching roller in front.

[0079] A cooling roller 130 is arranged downstream of the stretching rollers 110 and 120. This cooling roller can be used to cool the film 101 again, so that the new molecular orientation within the film caused by the stretching can be solidified.

[0080] The following uses Figure 2To explain one embodiment of the stretching device according to the invention, a perspective view of a stretching roller is shown. In the example shown, it is the stretching roller 120. The stretching roller includes a roller sleeve 140 and an end face, of which end face 141 is visible. The stretching roller particularly has a well, an axle, or an axle stummel. Only a portion 142 extending from the end face 141 is visible among the previously mentioned structural elements. The roller sleeve 140 now includes a recess 143, which is shown in this example as a surrounding groove. Other examples of recesses have been described above in the description of the invention. The characteristic of these recesses is that they only extend into the roller sleeve, but do not extend through the roller sleeve 140 into the internal space of the roller defined by the roller sleeve and the end face.

[0081] Figure 3 and Figure 2 Similarly, however, the roller 120 described herein includes a through-hole 150. The detailed design of the through-hole has been described above. The through-hole extends into the interior space of the roller and thus forms a fluid communication connection between the interior space of the roller 120 and the surrounding environment. Instead of a through-hole, an opening may be provided on the surface, which may simply consist of outwardly open cavities, such as open areas of a porous material.

[0082] Optionally, negative pressure can be applied to the internal space of the roller 120. For this purpose, the rotating shaft, shaft, or short shaft can be equipped with fluid conduits. In the case of the shaft, the fluid conduit includes a rotating sleeve. The fluid conduit of the rotating shaft, shaft, or short shaft is connected to another fluid conduit, such as a hose 151, leading to a negative pressure source.

[0083] Figure 4 Other possible features of the roller 120 with through holes are shown. Here, the internal space, now visible due to the lack of a roller sleeve illustration, is divided into multiple individual spaces by separating walls, in this example two separating walls 180, 161. Therefore, different air pressures can be applied to different individual spaces. In the current case with two individual spaces, it is possible to apply negative pressure to one of the individual spaces, for example, while no overpressure or negative pressure can be applied to the second individual space, thus ambient pressure exists here.

[0084] from Figure 5 One embodiment can be seen in which a suction mechanism 165 is arranged upstream of the stretching roller, particularly the stretching roller 120, and extends laterally relative to the transport direction of the film 101. This suction mechanism includes a closed box 166 having one or more suction ports 167. This box can be particularly subjected to a negative pressure, causing air to enter the suction mechanism from the feed gap along the direction of arrow L through the suction ports and be discharged.

[0085] Figure 6 Another embodiment of a stretching device according to the application is shown, in which an electrode 170 is arranged before the feed gap of the stretching roller, in the shown embodiment the stretching roller 120. This electrode can be loaded with an electric potential, which is in particular different from the potential of the film 101. Thereby a charging of the film is achieved, which leads to an additional holding of the film on the stretching roller with electrical power.

[0086] Figure 7 An apparatus 1 for producing a film tube, i.e. a blown film apparatus, is shown, which initially comprises at least one extruder 2 with which plastic, which is present in the form of granules, can be plasticized, for example. The plastic melt thus produced is fed via a line 3 to an extrusion tool 4, which can also be referred to as a nozzle head, from which this melt is converted into a film tube 6, so that this melt flow can be drawn off from a circular gap 5, which is not visible in this drawing, in the direction of extraction z. There is now a film tube 6 which has not yet solidified. This film tube is inflated from the inside by a slight overpressure, so that it has a larger diameter within an optional calibration device 7. The solidification of the film tube takes place, inter alia, by means of a tempering device 8, which is often also referred to as a cooling ring due to its annular configuration, which surrounds the film tube.

[0087] After passing through the calibration device, the film tube 6 reaches the area of action of a flattening device 9, in which the circular film tube is converted into an elliptical cross section with increasing eccentricity until it finally forms a double-layered plastic film in the area of action of a draw-off roller 10, which is connected to one another on their sides.

[0088] The flattening device is arranged rotatably, wherein the axis of rotation is essentially aligned with the tube axis 11, which is indicated in Figure 1 by a dot-dash line. The rotatability of the flattening device is indicated by an arrow 12.

[0089] Figure 7 A reversing device 15 is also shown, the task of which is to guide the flattened film tube from the flattening device to a stationary roller 16 without damage occurring.

[0090] Now, downstream of the reversing device 15, a stretching device 100 according to the application is arranged, which has been explained in connection with Figures 1 to 6 and in-depth description. The stretching device shown in Figure 7 corresponds to the one shown in Figure 1The stretching device is shown in the middle. It is also noted that a cutting mechanism can be positioned upstream of the stretching device 100, with which one or both edges of the pleat can be cut open (aufschneiden) or cut out (abschneiden). In addition, a separating device can be provided, with which the double-layered plastic film can be separated into one or more single-layered films.

[0091] The film hose, after passing through the stretching device 100, is guided to the further processing, which is not shown in detail here, by means of the arrow 17.

[0092] Figure 8 Another embodiment of the film blowing apparatus according to the application is shown, in which the stretching device 100 is now arranged between the draw-off roll 10 and the reversing device 15. It is noted here that the rolls within the stretching device are now arranged in such a sequence that the plastic film can be transported from below to above.

[0093] Figure 9 Another embodiment of the stretching device according to the application is shown, in which at least one nozzle 180 is arranged before the feed gap of the stretching roll, in the shown embodiment the stretching roll 120. By means of this nozzle, the film can be loaded on the side facing away from the roll with a fluid 181 under pressure, in particular compressed air. Thereby it is achieved that the film is pressed with a force towards the roll 120, so that less air is sucked into the area between the film and the roll 120 by the movement of the film.

[0094] A large number of possible features of the application are described in the description and the claims. Even if the combinability of the different features is not always mentioned, the combinability can be considered as being disclosed as long as no contradictions arise.

[0095] Figure 10 Another design of the device according to the application is shown, in which a cover device 185 is shown in one of the stretching rolls. Such a cover device can also be provided on each of the further stretching rolls. With this cover device 185, the angle area of the stretching roll, which is not wound with film, can be covered, so that the roll surface area and / or the negative pressure loaded inside it is disturbed as little as possible and is kept as much as possible. The cover device can cooperate with the stretching roll contactlessly. The cover device can be a curved sheet, for example. The cover device can extend along the axial direction almost over the entire length of the roll. On the edges of the sides of the cover device, seals can be provided, which additionally seal the area between the roll and the cover device in order to promote the reduction of the penetrating air again. The cover device can be composed of a plurality of cover elements, which can be displaced relative to one another. Thus, the cover elements can be optimally adapted even in the case of different film widths and different wrapping angles of the film.

[0096] Figure 11 A variation of the covering device is shown, wherein the covering device includes at least one circumferential belt 186, which is also guided by the stretch roller. At least two deflector rollers 187 are provided for guiding and / or driving the circumferential belt. The deflector rollers can be moved along the circumferential direction and / or radial direction of the stretch roller.

[0097] To minimize the combination Figure 10 and Figure 11 The described and to be avoided inhalation effect of infiltrated air can be followed as follows Figure 12 The embodiments are configured such that the fourth abutment roller 122 is sized and / or positioned such that the area unwound by the film is minimized. Since this unwound area depends on several parameters, the position and size of the roller 122 cannot be specifically described. Such descriptions are readily available to those skilled in the art.

[0098] To minimize the combination Figure 10 and 11 The described and to be avoided inhalation effects of infiltrated air, such as in Figure 13 As shown, a first guide roller 190 and a second guide roller 191 can also be provided. Each of the guide rollers can contact the film. The first guide roller is arranged upstream of the associated stretching roller, and the second guide roller is arranged downstream of the stretching roller. The first guide roller can be the same as the second abutment roller 112. A sealing element 192 is arranged between the two guide rollers 190 and 191, which preferably contacts the peripheral surface of the guide roller. The cavity thus created by the stretching roller, the film, the guide roller, and the sealing element is closed, in particular, by a seal, but not shown, arranged at the end of the cavity.

[0099] List of reference numerals

[0100] 100 tensioning device

[0101] 101 plastic film

[0102] 102 preheating roller

[0103] 110 First Roller

[0104] 111 First Abutting Roller

[0105] 112 Second Abutting Roller

[0106] 120 Second stretching roller

[0107] 121 Third Abutment Roller

[0108] 122 Second Abutting Roller

[0109] 130 cooling rollers

[0110] 140 roller sleeve

[0111] 141 end face

[0112] 142 portion extending from the end face 141

[0113] 143 recess

[0114] 150 through hole

[0115] 151 hose

[0116] 160 separation wall

[0117] 161 separation wall

[0118] 165 suction means

[0119] 166 closed box

[0120] 167 suction opening

[0121] 170 electrode

[0122] 180 nozzle

[0123] 181 fluid

[0124] 185 covering means

[0125] 186 surrounding band

[0126] 187 deflection roller

[0127] 190 first guide roller

[0128] 191 second guide roller

[0129] 192 sealing element

[0130] 1 device for manufacturing a film hose

[0131] 2 extruder

[0132] 3 line

[0133] 4 extrusion tool

[0134] 5 invisible annular gap

[0135] 8 not yet cured film hose

[0136] 7 optional calibration means

[0137] 8 temperature regulating means

[0138] 9 laying means

[0139] 10 withdrawal roller

[0140] 11 hose axis

[0141] 12 arrow for illustrating the rotatability of the laying device

[0142] 15 reversing device

[0143] 18 stationary roller

[0144] 17 arrow for illustrating the continuation of the process

[0145] T transport direction

[0146] Z extraction direction

Claims

1. Stretching device for stretching a plastic film in its transport direction, said stretching device having a first roller and a second roller, wherein said first roller is drivable with a first drive device and rotatable at a first rotational speed, said second roller is drivable with a second drive device and rotatable at a second rotational speed, wherein, the second rotational speed is greater than the first rotational speed, wherein the second roller is arranged downstream of the first roller in the transport path of the plastic film, characterized in that the means for changing the amount of air comprise at least one recess in the surface of the first roller and / or the second roller, wherein air can be contained in the recess.

2. The stretching device according to claim 1, characterized in that the means for changing the amount of air comprise at least one recess in the surface of the first roller and / or the second roller, wherein air can be contained in the recess.

3. The stretching device according to any of the preceding claims, characterized in that the means for changing the amount of air comprise at least one recess in the surface of the first roller and / or the second roller, wherein air can be contained in the recess.

4. The stretching device according to any of the preceding claims, characterized in that the means for changing the amount of air comprise at least one recess in the surface of the first roller and / or the second roller, wherein air can be contained in the recess.

5. The stretching device according to claim 4, characterized in that the means for changing the amount of air comprise at least one recess in the surface of the first roller and / or the second roller, wherein air can be contained in the recess.

6. The stretching device according to claim 5, characterized in that the porous material is a sintered material.

7. The stretching device according to claim 6, characterized in that the porous material is a sintered metal.

8. The stretching device according to any of the preceding claims, characterized in that the means for changing the amount of air comprise at least one recess in the surface of the first roller and / or the second roller, wherein air can be contained in the recess.

9. The stretching device according to any of the preceding claims, characterized in that the first roller and / or the second roller comprise at least one cavity which can be loaded with an air pressure which is reduced relative to the ambient pressure.

10. The stretching device according to any of the preceding claims, characterized in that the cavity of the first roller and / or the second roller is also divided into at least two sections in the axial direction by at least one dividing element, wherein at least one section can be loaded with an air pressure which is reduced or increased relative to the ambient pressure.

11. The stretching device according to any of the preceding claims, characterized in that ​ The device for changing the amount of air between the plastic film and the surface elements of the first and / or second roller on which the film is placed comprises at least one suction device with which air can be sucked from the region in which the plastic film comes into contact with the first and / or second roller in the transport direction.

12. Stretching device according to the preceding claim, characterized in that the suction device is divided into sections transversely to the transport direction of the plastic film.

13. Stretching device according to any of the preceding claims, characterized in that the device for changing the amount of air between the plastic film and the surface elements of the first and / or second roller on which the film is placed comprises at least one overpressure device with which air at overpressure can be loaded onto the surface of the plastic film facing away from the roller.

14. Stretching device according to the preceding claim, characterized in that the device for changing the amount of air between the plastic film and the surface elements of the first and / or second roller on which the film is placed comprises at least one electrode with which the plastic film can be electrostatically charged upstream of or at the contact line with the roller.

15. Method for stretching a plastic film in its transport direction, said method making use of a first roller and a second roller, wherein said first roller is driven with a first driving means and rotates with a first rotational speed, said second roller is driven with a second driving means and rotates with a second rotational speed, wherein, the second rotational speed is greater than the first rotational speed, wherein the second roller is arranged downstream of the first roller in the transport path of the plastic film, characterized in that the amount of air present between the plastic film and the surface elements of the first and / or second roller on which the film is placed is changed using a device for changing the amount of air, wherein the first and / or second roller comprises at least one cavity which is divided into at least two sections in the peripheral direction by at least one dividing element, wherein at least one section can be loaded with an air pressure which is reduced or increased relative to the ambient pressure.

16. Blown film plant having at least one extruder for producing a plastic melt, a nozzle head for producing a film tube, a laying device for converting the film tube into a double-layer plastic film, a driven take-off device for taking 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, characterized in that at least one stretching device according to any of claims 1 to 14.

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