DEVICE AND METHOD FOR MONOAXIAL LENGTH CHANGE OF FILM WEBS

Description

[0001] The invention relates to a device for the monoaxial length change of film webs in the machine direction according to the preamble of claim 1, a stretching system according to the preamble of claim 6, a blown film system for producing tubular films according to the preamble of claim 8, and a method for the monoaxial length change of film webs in the machine direction. This comprises at least one processing gap defined by two rollers, over which a film web is guided in the machine direction, wherein the first roller in the film transport direction has a first peripheral speed and the second roller in the film transport direction has a second peripheral speed.

[0002] In film production, stretching machines are used to reduce the thickness of the film after production. Stretching is used for both tubular and cast films.

[0003] A stretching line typically consists of heating rollers to prepare the film for stretching, followed by stretching rollers between which the film is stretched. Stretching can be followed by annealing or tempering rollers, where the film undergoes a temperature treatment to influence its properties. Finally, the film is cooled using chill rollers. By stretching the film webs, not only are their geometric dimensions changed, but their properties, such as transparency or strength, can also be specifically influenced.

[0004] Stretching systems are operated offline or inline in blown film lines. Stretching of tubular film can also be performed inline in blown film lines, either directly after the take-off roll pair or after the turner bar system following the take-off roll pair.

[0005] During monoaxial stretching of film webs in the machine direction, also known as longitudinal stretching, a transverse constriction occurs, known as neck-in, with simultaneous thickening of the edges, known as edge build-up. This effect is very unfavorable for further processing of the film. Neck-in is disadvantageous because it complicates the film production process with regard to the required flatness for further processing of the film. The higher the neck-in, the more difficult it becomes to achieve the required film quality in the economical production of monoaxially stretched films. Disadvantages arise such as drooping film edges and other flatness errors when unwinding the produced rolls. This leads to poor winding quality and thus to poorer further processing suitability of the film, for example during printing, lamination or laminating.

[0006] DE 10 2009 046 585 A1 discloses a stretching system for longitudinally stretching film webs and a method therefor. The stretching system essentially consists of a heating device, a stretching unit, and a cooling device. To reduce neck-in and edge buildup during longitudinal stretching of the film web, a pressing device in the form of a support roller is used, which presses the film web against the stretching rollers.

[0007] DE 10 2009 046 593 A1 also discloses a stretching system for longitudinally stretching film webs and a method therefor. To reduce neck-in and edge buildup, a deflection roller is inserted into the stretching gap between the two stretching rollers of the stretching unit. The film necks down less.

[0008] Transverse shrinkage or necking of the monoaxially stretched film is also counteracted by adjusting the temperature of the stretching rolls, as disclosed in DE 10 2011 085 735 A1. Furthermore, necking can be counteracted by adjusting the stretching gap. Reducing the roll gap leads to a reduction in the film's neck-in.

[0009] To improve the flatness of films produced in blown film lines, which are subsequently stretched in stretching lines, thickness control processes, such as those described in DE 10 2009 033 171 A1, are used. This reduces edge buildup during monoaxial stretching in the machine direction. For this purpose, during the production of tubular films, the areas that will later form the edges of the flat film are provided with thin spots, so that after stretching, a final product with the most uniform thickness possible across the film web is created. This process also has a positive effect on reducing transverse shrinkage.

[0010] CN 110 049 856 A describes a device according to the preamble of claim 1.

[0011] The invention is based on the object of creating a solution that enables the production of film webs with improved flatness in devices for monoaxial length change of film webs in the machine direction by reducing transverse necking and thus edge buildup. This increases the quality of the film rolls produced.

[0012] In a device for the monoaxial length change of film webs in the machine direction of the generic type, the object is achieved according to the invention by the characterizing part of the main claim.

[0013] The solution to the problem is achieved according to the invention with a device for the monoaxial length change of film webs in the machine direction mentioned at the outset, in which a vacuum source can be connected to the air-flow roller and the roller can be subjected to vacuum, wherein the air-flow roller is made of an open-porous material in the micrometer range.

[0014] As the film web is transported over the rollers, an air boundary layer is drawn in between the film web and the roller. These air cushions negatively impact the flatness of the film web during stretching in the processing gap.

[0015] By using rollers with air flow from the outside to the inside, the drawn-in air can flow continuously through the air-flow roller into the inside of the roller, and the film lies directly against the rollers across its entire width. The flatness of the film web is improved. These air cushions are particularly effective at high machine speeds.

[0016] Transverse constriction caused by stretching the film web with simultaneous edge build-up is also counteracted and reduced by the film being in contact with the roller.

[0017] If this effect is not sufficient, a vacuum source can be connected to the roller, which allows air to flow through from the outside to the inside. The vacuum sucks the film web onto the roller and adheres more strongly to it, reducing neck-in and edge expansion.

[0018] The degree of neck-in and edge build-up depends, among other things, on the adhesion of the film to the rollers that define the processing gap. The film slides more strongly in the outer areas in the transverse direction, i.e., perpendicular to the transport direction of the film web, across the roller surface, thus attempting to compensate for the transverse forces acting on it. This transverse neck-in occurs primarily in the processing gap and on the rollers themselves.

[0019] The goal is to adjust the vacuum on the rollers so that the increased friction improves the adhesion of the film web to the roller and prevents the relative speed in the transverse and longitudinal directions between the film and the roller. This achieves uniform contact between the film and the roller across the entire width of the roller. This creates a defined separation edge of the film web from the roller in the film transport direction at the roller exit, as well as a defined contact edge of the film web against the roller in the film transport direction when it enters the roller.

[0020] The level of negative pressure also depends on the desired length change of the film web, the machine speed and the film properties.

[0021] The force with which the film is sucked onto the roller is determined by the level of the vacuum. This is approximately 100 to 300 mbar; in special cases, it can be selected to be higher or lower. The vacuum is regulated accordingly depending on the properties of the film and the desired length change. According to the invention, at least one of the rollers defining the processing gap of the length-changing device is designed as a roller through which air can flow from the outside to the inside.

[0022] In a preferred embodiment, the second roller after the processing gap is designed as an air-flow roller.

[0023] The drawn-in air flows out through the roller and the film lies on the roller across its entire width.

[0024] If the effect is not sufficient, in a further embodiment of the invention, the air-flow roller is connected to a vacuum source.

[0025] The negative pressure applied to the air-permeable roller draws the film web onto the roller. This increases the friction between the film web and the device's roller, thus reducing transverse shrinkage after the processing nip. The film is evenly drawn onto the roller across its entire width, including the film edges.

[0026] In another embodiment of the invention, the first roller of the device upstream of the processing gap is designed as an air-flow roller. This increases contact between the film web and the roller, preventing the formation of air cushions between them. The film web thus lies evenly on the roller. The film web reaches the processing gap with less transverse constriction. This positively influences the flatness. In another embodiment, this roller can also be connected to a vacuum source. The higher forces enhance the effects.

[0027] In another embodiment, the rollers before and after the processing gap are designed as air-permeable rollers. This improves adhesion of the film web without air pockets between the film web and the rollers, and neck-in and edge buildup on the film are reduced. In a preferred embodiment, the second roller is also connected to a vacuum source. This counteracts transverse necking. In rare cases, both rollers can be connected to a vacuum source.

[0028] In the device according to the invention, the air-flow rollers can work together with or without lay-up rollers. The lay-up rollers serve to additionally fix the film web to the air-flow rollers. They increase the contact between the film and the roller, ensuring that the film rests evenly against the roller.

[0029] Depending on the stretching task, it may also be necessary to apply varying degrees of suction to the film across its width, for example, more strongly at the edges than toward the center. In this case, the air-permeable roller is designed so that varying degrees of negative pressure can be applied across its axial length.

[0030] Depending on the application, a combination of an air-permeable roller and a tempering roller can also be used. This, on the one hand, tempers the film web to ensure the appropriate temperature for the length change, and on the other hand, allows the air drawn between the film web and the roller to flow inward via the air-permeable roller. The direct contact of the film web with the roller also improves the heat transfer from the roller to the film web. If a vacuum is also applied to the rollers to draw the film web more firmly onto the roller, the friction between the film and the roller increases, further reducing neck-in.

[0031] Temperature control can be either heating or cooling. On one side of the combined rollers, the fluid, such as gas, steam, water, or oil, for temperature control is supplied and removed via a rotary union. A vacuum source is connected to the opposite side of the fluid supply and removal via a rotary union, and the air is extracted. If no vacuum is applied, the air escapes through these openings. With a roller that only allows air to flow through, there is no need for fluid supply and removal for temperature control.

[0032] The combined tempering and air-flow roller, like state-of-the-art rollers, also consists of a fluid-tempered double-shell roller, but is additionally equipped with channels for air flow and can be connected to a vacuum source. The channels can also be designed as small holes.

[0033] Ideally, an air-permeable layer is applied, similar to a sintered structure. It is also possible to make the rollers from an open-porous material in the micrometer range. The rollers are made of materials such as steel, stainless steel, or plastic. In a preferred embodiment, the sintered layer is made of stainless steel, for example, which can also be highly polished. In addition, the surface roughness or surface structure of the rollers can influence the friction or adhesion between the film web and the roller.

[0034] If tempering is not necessary, rollers of the same type without a double jacket are used.

[0035] Reducing the transverse shrinkage of the film web results in new physical properties of the film, which lead to improved further processing in printing and lamination processes due to improved flatness. Furthermore, this results in less edge trimming.

[0036] The length-changing device comprises processing gaps, each defined by two rollers. The first roller in the film web transport direction has a first peripheral speed, and the second roller in the film web transport direction has a second peripheral speed, so that the film web undergoes a length change in the processing gap between them.

[0037] If the second roller has a higher peripheral speed than the first roller of the device, a positive change in the film's length results. Depending on the length change ratio, this is referred to as stretching, drawing, or annealing. The transition in the film's length change from annealing to drawing and stretching is fluid.

[0038] Length change ratios of approximately 1:1 to 1:10 can be achieved.

[0039] In the case of annealing, shrinkage of the film web can be permitted in addition to lengthening; in this case, the second roller has a lower peripheral speed than the first roller and a negative change in length results.

[0040] The shrinkage ratio is usually between 0.5:1 and 1:1.

[0041] Both rollers that define the treatment gap can also have the same peripheral speed.

[0042] A stretching system mentioned at the outset is characterized according to the invention in that a component of the stretching system is the device for changing the length of a film web described above and below.

[0043] In one embodiment, the device according to the invention for changing the length of a film web is part of a stretching system.

[0044] In stretching systems, which usually consist of a heating roller, stretching roller, annealing roller and cooling roller arrangement, the stretching roller is replaced by an air-permeable roller after the stretching nip or treatment nip, as this is where the greatest improvement for the stretching process is achieved.

[0045] A blown film line mentioned at the outset is characterized according to the invention in that the device for changing the length of a film web described above and below is arranged inline in the blown film line.

[0046] Such stretching systems can be used inline or offline in a film production plant.

[0047] It is also possible to provide several devices according to the invention, which comprise at least one processing gap delimited by two rollers, in a stretching system.

[0048] However, it is also possible in stretching systems with the device according to the invention to additionally design additional stretching rollers or additional rollers such as the annealing rollers and cooling rollers following the stretching rollers as well as the upstream heating rollers as air-flow rollers in order to further improve the film quality.

[0049] In the production of tubular films, the device according to the invention for changing the length of a film web can be installed inline in a blown film line.

[0050] Blown film lines comprise, in the film transport direction, a plastics dosing device, an extruder, followed by the film blowing head and a cooling system, as well as a calibration basket. This is followed by a flattening device, a take-off device, and a turning device. Finally, the tubular film is wound up in a winder.

[0051] When producing tubular films in blown film lines, the length-changing device according to the invention, with at least one processing gap defined by two rollers, at least one of which is an air-permeable roller, can be arranged directly in the film transport direction above the pair of take-off rollers of the take-off device, before or after the turning bars. Several devices according to the invention, which comprise at least one processing gap defined by two rollers, can also be provided in a blown film line.

[0052] They can also be combined with additional air-permeable rollers. The take-off can be designed as a stationary, rotating, or reversing turntable. It can also be operated without turntable bars.

[0053] The device according to the invention for monoaxial length change can be arranged as a component of a stretching system in a blown film line in the film transport direction above the take-off roller pair before or after the turning bars above the tubular film bubble.

[0054] Alternatively, the device can be installed in a stretching line either inline on the floor next to the blown film line or offline on the floor next to the blown film line. The blown film line can also be designed with or without a reversing haul-off.

[0055] The device for monoaxial length change according to the invention can also be installed inline as part of a stretching system of a so-called MDO system, or in duplicate after a film blowing system. The tubular film is then cut into two film webs at opposite points on both sides, and each film web is further processed in a stretching system.

[0056] To further optimize the quality of film webs, the use of the device according to the invention and the use of thickness control systems can be combined. The combined use of thickness control systems in the production of tubular film, e.g., according to DE 10 2009 033 171 A1, and air-flow rollers in the length-changing device results in a further positive effect: with less neck-in of the film web due to stretching, less energy needs to be introduced into the control system to optimize flatness. Finely controlled control is possible, thus simplifying the process.

[0057] The length-changing device can also be used in a stretching or annealing system for treating cast films.

[0058] In a method according to the invention for the monoaxial length change of film webs mentioned above, the film web is guided over at least one processing gap delimited by two rollers. The first roller in the film transport direction has a first peripheral speed and the second roller in the film transport direction has a second peripheral speed. At least one of these rollers is an air-flow roller, which ensures that the film web guided over it rests against it in order to counteract transverse constriction and edge build-up of the film web and to prevent the formation of air cushions between the rollers and the film web. A vacuum source can be connected to the air-flow roller, and the roller can be subjected to vacuum. The air-flow roller is made of an open-porous material in the micrometer range.

[0059] A combination of an air-flow and a temperature-controlled roller also increases heat transfer between the film and the roller, as the insulating air layer between them can escape through the air-flow roller. This results in a stable stretching process and, as a result, a more homogeneous film without flatness defects, as the film lies more evenly across the roller's width.

[0060] The effect is enhanced by applying negative pressure. This allows the film's detachment edge from the roller at the roller's exit in the direction of film transport, and the film's contact edge to the roller at the roller's inlet in the direction of film transport, to be better secured by suction, resulting in a smaller relative speed difference in the transverse and longitudinal directions across the width of the film and the roller.

[0061] To further optimise the film web quality, the process is combined with film thickness control as described in DE 10 2009 033 171 A1. Here, a tubular film is subjected to film thickness control during its production in a blown film line. The thickness profile of the tubular film is controlled in such a way that the tubular film has thin spots at those points which, in a downstream inline stretching line, represent the edge areas of the flattened tubular film. So that after stretching the film in a stretching line, a film is created with the smallest possible deviations from the average film thickness across the film width. If, during stretching in the stretching line, the film web is also guided in the length-changing device over an air-flow roller with a vacuum source, which sucks in the film and thus prevents transverse constriction and edge build-up, films of even higher quality are produced.By applying a vacuum source to the air-flow roller, the quality is further improved. The flattened tubular film can be a blocked film, single-sided or double-sided, a double-flattened tubular film, or a single-layer film processed in two stretching lines.

[0062] The use of a permeable roller, with or without connection to a vacuum source, as a roller, especially as a stretching roller, increases the adhesion of the film web to the roller, thus leading to less transverse necking and a lower edge buildup of the film web, and thus improved film flatness. This reduces the previously necessary edge waste. Both of these factors increase the cost-effectiveness of stretched film production.

[0063] Further details, features and advantages of the subject matter of the invention emerge from the subclaims and from the following description of the associated drawings, in which a preferred embodiment of the invention is shown by way of example.

[0064] In the drawings shows: Figure 1 a stretching system with the device according to the invention for monoaxial length change of a film web in the machine direction and Figure 2 the device for monoaxial length change of a film web in the machine direction

[0065] The Figure 1 shows the device according to the invention for the monoaxial length change of a film web in the machine direction in a stretching system, also called MDO system, as one possible application of the device according to the invention.

[0066] The film web (1) is fed into the stretching system from above. First, it is guided over a heating section (II) with heating rollers (3). The film web (1) is then fed to the inventive device for changing the length (IV), here the stretching device, with the rollers (5a, 5b), here the stretching rollers and an intermediate processing gap, here the stretching gap, and is stretched. The stretching rollers (5a, 5b) can have a smaller diameter than the heating rollers. The film web (1) is then fed to an annealing section (VII) with annealing rollers (6) for temperature control, followed by a cooling section (IX) with cooling rollers (9). Finally, the film (1) leaves the MDO system and can be wound up in the winder. The film web is held on the various rollers by lay-on rollers (4).

[0067] The Figure 2shows the length-changing device (IV) of the stretching system in detail. The film web (1) is guided over a first stretching roller (5a) to the second stretching roller (5b). The stretching gap is located between the stretching rollers (5a) and (5b). In this stretching system, the second stretching roller (5b) has a higher peripheral speed than the first stretching roller (5a), resulting in a positive length change of the film web. The lay-on rollers (4) serve to additionally fix the film web (1) on the stretching rollers (5a) and (5b).

[0068] The film web (1) then leaves the device (IV) according to the invention. The stretching roller (5b) is designed according to the invention as an air-flow roller with the option of connecting to a vacuum source, not shown here. The stretching roller (5a) and the stretching roller (5b) can also be designed as tempering rollers. In this case, the stretching roller (5b) is a combination of an air-flow roller and a tempering roller.

[0069] Depending on the stretching task, individual or all of the lay-up rollers (4) can be omitted.

[0070] Stretching systems can also have more than two stretching rollers (5) and thus have several devices (IV).

[0071] The stretching system can have 2, 3 or more heating, stretching, annealing or cooling rollers. List of reference symbols

[0072] 1Film web 3Heating rollers 4Lay-on rollers 5Rollers / stretching rollers 5aFirst roller / stretching roller 5bSecond stretching roller / roller 6Annealing rollers 7Cooling rollers IIHeating section IVDevice for length change / stretching device VIIAnnealing section IXCooling section

Claims

1. Apparatus (IV) for monoaxially changing the length of film webs (1) in machine direction, comprising at least one processing gap defined by two rollers (5a, 5b) over which a film web (1) is guided in machine direction, wherein the first roller (5a) in film transport direction has a first circumferential speed and the second roller (5b) in film transport direction has a second circumferential speed, wherein at least one of the rollers (5a, 5b) is a roller (5a, 5b) through which air can flow from the outside to the inside, characterised in that a negative pressure source can be connected to the roller (5a, 5b) through which air can flow and the roller (5a, 5b) can be subjected to negative pressure, wherein the roller (5a, 5b) through which air can flow being made of an open-porous material in the micrometre range.

2. Device (IV) according to claim 1, characterised in that the roller (5a, 5b) through which air can flow can be subjected to varying degrees of negative pressure over its length.

3. Device (IV) according to claim 1 or 2, characterised in that at least one of the rollers (5a, 5b) through which air can flow can be combined with temperature control.

4. Device (IV) according to claim 1, 2 or 3, characterised in that the first roller (5a) in film transport direction has a lower circumferential speed than the second roller (5b) in film transport direction and the length change of the film web (1) is a positive length change.

5. Device (IV) according to claim 1, 2 or 3, characterised in that the first roller (5a) in film transport direction has a greater circumferential speed than the second roller (5b) in film transport direction of the film web (1) and the length change of the film web is a negative length change.

6. Stretching system, characterised in that a component of the stretching system is the device (IV) according to any one of claims 1 to 5.

7. Stretching system according to claim 6, characterised in that further rollers of the stretching system are designed as rollers through which air can flow, with and without an adjacent vacuum source.

8. Blown film line for the production of tubular films, characterised in that the device (IV) according to any one of claims 1 to 5 is arranged inline in the blown film line.

9. A blown film line according to claim 8, characterised in that the device (IV) according to any one of claims 1 to 5 is arranged in film transport direction after the pair of take-off rollers and before or after turning bars of the blown film line.

10. A blown film line according to claim 8 or 9, characterised in that the blown film line comprises a plurality of devices (IV) according to any one of claims 1 to 5.

11. Blown film line according to any one of claims 8 to 10, characterised in that the device (IV) according to any one of claims 1 to 5 is arranged in a stretching line layer arranged inline in the blown film line, in particular according to any one of claims 6 or 7 in the blown film line12. Method for monoaxially changing the length of film webs (1) in machine direction, in which a film web (1) is guided through at least one processing gap delimited by two rollers (5a, 5b), wherein the first roller (5a) in film transport direction has a first circumferential speed and the second roller (5b) in film transport direction has a second circumferential speed, wherein the film web (1) is undergoing a length change in the processing gap and at least one of the rollers (5a, 5b) is designed as a roller (5a, 5b) through which air flows from the outside to the inside, the air which accumulates between the roller (5a, 5b) and the film web (1) flowing off through the roller (5a, 5b) and the film web (1) resting against the roller (5a, 5b), characterised in that a negative pressure source is connected to the roller (5a, 5b) through which air flows and the roller (5a, 5b) is subjected to negative pressure, and wherein the roller (5a, 5b) through which air flows is made of an open-porous material in the micrometre range.

13. Method according to claim 12, characterised in that the film is subjected to a film thickness control during its production in a blown film line, so that the film thickness profile of the tubular film produced in the blown film line is controlled in such a way that the predetermined target thickness profile has deviations from a uniform film thickness, which serve to compensate for the change in film thickness over the film width which occurs during the subsequent monoaxial stretching in machine direction, so that the stretching produces a film web (1) with a transverse thickness profile with the smallest possible deviations from the average film thickness over the entire film width.