Combination stretching device for stretching plastic films

Abstract

The invention relates to a combination drawing unit (3) for drawing plastic films for a film drawing machine (1) with a warm drawing section (5) and a hot drawing section (4), which can be used selectively for drawing a continuous film web (F). The combination drawing unit according to the invention is characterized in that the warm drawing section (5) is arranged upstream of the hot drawing section (4) in the direction of travel of the film web (F). The combination drawing unit (3) can be used in a film drawing machine.

Description

[0001] The invention relates to a combination stretching unit for stretching plastic films for a film stretching machine of the type defined in more detail in the preamble of claim 1. The invention also relates to a film stretching machine with such a combination stretching unit and a method for operating such a combination stretching unit.

[0002] Combination stretching units are known from the prior art and are used in film stretching lines. They serve for the longitudinal stretching of various types of plastic films. They comprise a hot stretching section with, for example, five stretching rollers used for hot stretching, some of which also serve as rollers for feeding and discharging the film web running through the film stretching line. Hot stretching takes place at temperatures of approximately 110°C to 130°C, preferably at approximately 115°C to 125°C. The actual stretching is achieved by adjusting the peripheral speeds of the stretching rollers, which increase in the direction of travel of the film web, over at least one hot stretching gap formed between at least two of the stretching rollers.In a typical setup, this is followed by a conventional longitudinal stretching section in which the film web is stretched at temperatures of 80°C to 100°C, preferably 85°C to 95°C, between two rollers, creating a conventional stretching gap. This stretching gap is larger than the at least one hot stretching gap. In this description, for the sake of clarity, this conventional longitudinal stretching section will be referred to as the hot stretching section, the process as hot stretching, and the stretching gap used as the hot stretching gap, even though this is not a generally accepted technical term.

[0003] This hot-stretching section is often designed as a so-called omega stretching section, in which the two rollers adjacent to the stretching gap are arranged in one plane, while the rollers before and after them in the direction of film web travel are located below this plane. The path of the film web thus roughly follows the shape of the Greek letter omega.

[0004] Such combination stretching machines also use stretching rollers as intermediate rollers or transport rollers to transfer the film web from the hot stretching area, which is arranged first in the direction of travel of the film web, to the warm stretching area following in the direction of travel of the film web.

[0005] The operation of such a combination stretching machine always proceeds in such a way that, depending on the type of plastic film being produced, either the hot stretching section or the warm stretching section is actively operated. The section that is not in use at any given time then only serves as a transport section for the film web, but its rollers still need to be driven, temperature-controlled, and operated. This allows both techniques to be used on a single film stretching machine. Hot stretching is typically used for thinner plastic films with a thickness of up to 15 µm, for example, so-called thermal transfer release films, which are used for transferring barcodes onto packaging and are typically less than 10 µm thick. The warm stretching section, on the other hand, is used for film thicknesses greater than 15 µm, for example, for plastic films with a thickness of 23 µm or more.

[0006] The disadvantage of current combination stretching machines lies in the need to drive and temperature-control many unnecessary rollers, and especially in the size of the structure in the direction of film web travel. This results in additional space requirements at the installation site of the film stretching machine. Furthermore, it makes the machine frame required for the combination stretching machine complex and very long, which can easily lead to increased susceptibility to vibration.

[0007] The object of the present invention is therefore to provide a combination stretching machine that is improved in comparison to these disadvantages, compact and more cost-effective, a film stretching machine with this machine and a method for operating it.

[0008] According to the invention, this problem is solved by a combination stretching machine with the features of claim 1, and in particular in the characterizing part of claim 1. Advantageous embodiments and further developments are described in the dependent claims. A film stretching machine according to claim 15 also solves the problem. Advantageous embodiments of this machine are described in the dependent claims. Finally, a method for operating such a combination stretching machine also solves the problem. Here, too, an advantageous embodiment is described in the dependent claim.

[0009] According to the invention, the heated area is arranged upstream of the hot-stretched area in the direction of travel of the film web. Both areas serve for longitudinal stretching, i.e., stretching the plastic film parallel to the direction of travel of the plastic film, which is also referred to as MDO (Material / Machine Direction Orientation).

[0010] This reversal of the order of the warm-stretching and hot-stretching sections, compared to the prior art, allows for the shared use of various functionalities and rollers. This results in significant savings in the number of rollers, including their drives, temperature control devices, and associated controls. The design can thus be implemented in a very compact manner, which, in addition to reducing the space required within the plant, also saves space in the installation area of ​​a film stretching line equipped with such a combination stretching unit. The more compact design of the combination stretching unit also allows for a more compact machine frame. This makes it less susceptible to vibration and, while maintaining comparable stability, requires a less complex design, thus saving effort in both construction and assembly.The machine frame is also easier to manufacture and requires less material, which also saves costs.

[0011] A particularly advantageous embodiment of the combination drawing unit according to the invention provides that the warm drawing section and the hot drawing section share the drawing roller located downstream of the drawing gap of the warm drawing section, respectively, in the direction of travel of the film web. For the functionality of warm drawing, this roller is the first roller downstream of the drawing gap that is cooled during warm drawing. Simultaneously, for operation of the combination drawing unit as a hot drawing unit, it can serve as the first heated drawing roller of the hot drawing unit. If required, the warm drawing gap can be used for pre-stretching the plastic film before this first drawing roller of the hot drawing section.

[0012] Another highly advantageous embodiment provides a heating device for heating the film web in the hot-drawing gap. In the combination drawing machine according to the invention, this is formed by two infrared emitters, which can heat both sides, i.e., the top and bottom of the film web, in the drawing gap. By reducing the number of drawing rollers that can be heated as needed, this design is particularly efficient in providing the required temperature in the plastic film for hot-drawing and especially also for hot-drawing.

[0013] According to a particularly advantageous embodiment of the combination drawing unit according to the invention, the hot-drawing section can have two pressure rollers which interact with the first drawing roller located in the direction of travel of the film web in front of the hot-drawing gap. They ensure good adhesion of the plastic film to the first drawing roller and ideally prevent air inclusions between the plastic film and the first drawing roller, which can lead to quality problems, even film tearing.

[0014] Another very advantageous embodiment provides that, in the direction of travel of the film web, a further roller is arranged upstream of the first stretching roller located before the hot stretching gap. The film web is fed to the first stretching roller via this feed roller in such a way that it wraps around it at an angle of 50° to 80°, preferably 60° to 70°, and particularly preferably approximately 65°. Such a feed roller can thus ensure the ideal wrap angle, which guarantees sufficient adhesion of the film to the first stretching roller upstream of the hot stretching gap, so that it can be stretched in the desired manner by the second stretching roller located downstream of the hot stretching gap, which rotates at a higher peripheral speed.

[0015] In a particularly advantageous embodiment, the hot-stretching section of the combination stretching unit according to the invention can comprise, in addition to the shared second stretching roll, a third stretching roll, a fourth stretching roll, and a fifth stretching roll, the fifth stretching roll serving as a transfer roll in a subsequent treatment zone following the direction of travel of the film web. This subsequent treatment zone can be configured in any desired manner. For example, the plastic film can be cooled here, it can be fed to a transverse stretching unit additionally or alternatively, or, if it is already finished after longitudinal stretching, it can be wound up for storage in this treatment zone. Other treatment zones known per se can also be included.

[0016] According to a very advantageous further development thereof, the second stretching roller arranged in the direction of travel of the film web after the hot stretching gap, as well as the stretching rollers following in the direction of travel of the hot stretching area, are each provided with a pressure roller, which ensures good contact of the film web with the stretching roller on the one hand and prevents or minimizes air cushions on the other.

[0017] A further advantageous development of the combination drawing unit may also provide that the hot drawing section has at least two hot drawing gaps, which are formed at least between the first and second drawing rolls of the hot drawing section and between the second and third drawing rolls of the hot drawing section. Additional hot drawing gaps may be provided.

[0018] In hot drawing, unlike warm drawing, the stretching process does not occur in a single step, but can comprise several hot drawing steps after the optional pre-stretching mentioned above. At least one pre-stretching and one main or final stretching step are required, with the stretch factor of the pre-stretching typically being lower than that of the main stretching, although this is not mandatory. Dividing the process into multiple steps can improve the quality and strength of the plastic film. An (optional) pre-stretching, followed by pre-stretching and a final main stretching step offers a very good compromise between the achievable quality of the plastic film and the effort or number of stretching rollers required.

[0019] Each of the hot-drawn gaps is smaller in its dimensions, i.e., its gap width, than the hot-drawn gap. The hot-drawn gap is typically approximately 1.8 to 4.6 times larger than the respective hot-drawn gap.

[0020] A particularly advantageous embodiment can provide that the plastic film or film web wraps around the third and fourth stretching rollers with a wrap angle of more than 120°. The fourth stretching roller, in particular, can have a very large wrap angle, especially one exceeding 150°.

[0021] In addition to the typically used stretching rollers with hard-chrome-plated surfaces, ceramic surfaces, or surfaces made from a combination of chromium and PTFE, according to a highly advantageous embodiment of the combination stretching unit according to the invention, at least some of the stretching rollers can have a surface made of a laser-structured hard metal material. This material can be, in particular, tungsten, preferably in the form of tungsten carbide. Such rollers have proven to be particularly efficient for the combination stretching unit constructed according to the invention, as they enable high functionality in the relevant temperature ranges and ensure very good quality of the produced film.

[0022] According to a particularly advantageous embodiment, laser structuring allows for the creation of microchannels open towards the surface. These microchannels, when used as intended, are open to an area of ​​the stretching rollers free of the film web, such as an edge or end face of the coating. The microchannels preferably do not run linearly, but rather in a more or less random pattern across the surface. They are designed such that each channel is open to the environment at least on one side, i.e., at one of its ends. This end can terminate at an edge of the hard metal coating or at the edge of the stretching roller where no plastic film is present during normal use.This ensures that any air that might become trapped between the film web and the surface of the stretching roller can escape through the microchannels. The microchannels themselves are so small that while sufficient airflow is guaranteed, the surface of the stretching roller exhibits so few macroscopic structures that any impairment of the surface of the manufactured plastic films is impossible. In practice, such surface coatings appear completely smooth to the naked eye and to the touch; the microstructures are only visible under a microscope.

[0023] Existing combination drawing units are relatively complex with regard to maintenance, as adjustment of the ordinary (longitudinal) drawing gap, i.e., the hot drawing gap as defined in this description, is often necessary, not to adjust it during operation, but to allow access to the underlying components during maintenance. According to a highly advantageous embodiment, the compact combination drawing unit according to the invention can be designed so that the pressure rollers on one side, particularly those arranged above the film web, are housed in a common pressure roller frame. This frame is mounted in or connected to the machine frame in such a way that it is either movable or can be easily lifted off, for example, using a crane. The entire assembly of pressure rollers can thus be removed for maintenance purposes without any change in their relative positions to one another.This eliminates the need to adjust the hot drawing gap, making the design simpler compared to state-of-the-art combination drawing machines.

[0024] A film stretching line can now be equipped with such a combination stretching unit. The film stretching line itself will include further processing zones, for example, the production of the plastic film via a casting roller, a subsequent heating zone, the combination stretching unit, and other processing zones as already described above.

[0025] Within such a film stretching system, a highly advantageous further development allows the last roller of the heating zone to be used for the targeted feeding of the film web to the first stretching roller. This last preheating roller in the heating zone thus eliminates the need for another roller within the hot stretching section of the combination stretching unit, making it even more compact and requiring even fewer rollers specifically for the unit.

[0026] According to a highly advantageous further development, at least one of the preheating rollers in the heating zone can have a surface made of a laser-structured hard metal material. In particular, like the stretching rollers described above, they can be designed with a laser-structured tungsten carbide surface featuring microchannels that, in normal use, are open to a point on the heating roller not covered by the film web, thus efficiently preventing air inclusions.

[0027] According to the inventive method, the combination drawing machine can now be operated in such a way that, during operation of the hot drawing section (i.e., hot drawing as defined here), the drawing rollers arranged adjacent to the hot drawing gap are operated at a circumferential speed that increases in the direction of travel of the film web. The first drawing roller thus has a lower circumferential speed, while the second drawing roller, located after the hot drawing gap, has a higher speed. This stretches the film over the hot drawing gap in the desired manner. The subsequent drawing rollers of the hot drawing section then rotate at approximately the same speed as the last drawing roller of the hot drawing section, whereby slightly adjusted speeds can be set here due to shrinkage occurring during cooling, a desired post-stretching, or the like.During hot stretching, the plastic film in the stretching gap ideally has a temperature of 80°C to 100°C, preferably 85°C to 95°C. The film web, typically emerging from the heating zone, runs onto the first stretching roller, which is heated, is stretched, and then cooled in the area of ​​the second stretching roller to fix the stretched state. The other stretching rollers in the hot stretching area can also be temperature-controlled as needed.

[0028] In the hot-stretch section, the hot-stretch gap serves for optional pre-stretching of the plastic film. The second stretch roll, i.e., the one in the direction of film web travel after the hot-stretch gap, forms the first stretch roll of the hot-stretch section. The third and fourth stretch rolls follow as the second and third stretch rolls of the hot-stretch section. They rotate at increasingly higher peripheral speeds to stretch the plastic film over at least two hot-stretch gaps. A first hot-stretch gap preferably forms between the first and second stretch rolls of the hot-stretch section for pre-stretching the plastic film. The second hot-stretch gap, the main stretch gap for the primary stretching of the plastic film, then forms between the second and third stretch rolls of the hot-stretch section.The first two stretching rollers in the hot stretching section are heated, while the third stretching roller, i.e., the fourth stretching roller of the combination stretching unit, is cooled to ensure that the stretched state of the plastic film is fixed. The temperatures during hot stretching are approximately 110°C to 130°C, preferably approximately 115°C to 125°C.

[0029] The inventive method can provide that in the one of the two stretching areas which is not currently being actively used, whose stretching rollers are therefore only driven and heated for transport purposes, the pressure rollers are lifted off in order to reduce friction and energy consumption and to minimize the risk of damage to the surface of the plastic film.

[0030] Further advantageous embodiments of the combination stretching machine, the film stretching system and the method for operating the combination stretching machine can also be seen from the exemplary embodiment, which is described in more detail below with reference to the figures.

[0031] This shows: Fig. 1. A section of a film stretching system with a combination stretching unit in a design according to the state of the art; and Fig. 2 the section of the foil stretching system according to Fig. 1 with a possible embodiment of a combination stretching system according to the invention.

[0032] In the presentation of the Fig. Figure 1 schematically indicates a section of a film stretching machine 1 for a plastic film, relevant to the description of the innovation presented here, according to the prior art. A film web F, shown with a dashed line, passes through the film stretching machine 1 in the illustration of the Fig. 1 from left to right. Outside the depicted section of the film stretching machine 1 on the far left, i.e., in the direction of travel of the film web F before the section shown here, the plastic film is produced in the machine using cooled casting rollers. This is followed by a heating zone 2 with several heated preheating rollers. These are numbered according to their arrangement and each is designated with the abbreviation PH (for Pre-Heat). In the illustration of the Fig. Figure 1 shows only the last three of a total of n preheating rollers PHn-2, PHn-1, and PHn in heating zone 2. The reference symbol PH without a number denotes one or more arbitrary preheating rollers PH; its use with a number denotes the preheating roller with that exact name.

[0033] After passing over the last preheating roller PHn, the film web F enters a combination drawing unit 3, which comprises two separate sections. In the prior art, Fig. In the structure shown in Figure 1, following the heating zone 2 in the direction of travel of the film web F, there is first a hot-stretching area designated 4 and outlined with a dotted line. This is followed by a conventional longitudinal stretching area 5, outlined with a dashed line, in which the plastic film is stretched over a stretching gap R. In the hot-stretching area 4, the longitudinal stretching of the plastic film typically takes place at temperatures on the order of 110°C to 130°C, preferably at approximately 115°C to 125°C. To more easily distinguish the conventional longitudinal stretching area 5, in which the temperature of the plastic film during stretching is typically 80°C to 100°C, preferably 85°C to 95°C, from this hot-stretching area 4, it is referred to here – as already explained above – as the warm-stretching area 5, even though this is not a generally accepted technical term.

[0034] The hot stretching section 5 and the hot stretching section 4 in the combination stretching unit 3 are used for different applications, particularly for different types of plastic films. They are never operated simultaneously, but always alternately, so that while one type of film is being produced, for example, the hot stretching section 5 is in use. Typically, the film thicknesses are then greater than 15 µm; a common film thickness here is, for example, 23 µm.

[0035] The hot stretch area 5 begins in the representation of the Fig. 1 with a first stretching roller S15, where S stands for stretch. The reference symbol S without a number denotes one or more stretching rollers S; its use with a number denotes the stretching roller with that exact name. The stretching roller S15 could also be understood as a transfer roller between the hot stretching section 4 and the warm stretching section 5. This is followed by the stretching roller S16, for the targeted feeding of the film web F to the subsequent stretching roller S17, which is arranged in the direction of travel of the film web F immediately before the stretching gap or warm stretching gap R. During operation of the warm stretching section 5, the film web F is pressed onto the first stretching roller S17, located adjacent to the warm stretching gap R, by means of two pressure rollers or hold-down rollers N15 and N16. The reference symbol N without a number denotes one or more pressure rollers N; its use with a number denotes the pressure roller with that exact name.The pressure rollers N serve two purposes: firstly, to increase the friction between the plastic film and the stretching roller S to enable stretching, and secondly, and more importantly, to prevent air pockets between the plastic film and the stretching roller S. Such air pockets can lead to a reduction in local heat input into the plastic film and, secondly, to inhomogeneity in the friction between the stretching roller S and the plastic film. This can result in uneven stretching due to both of these independent factors, negatively impacting quality and significantly increasing the risk of film web tearing F.

[0036] In the direction of travel of the film web F, following the hot stretching gap R, the second stretching roller S18 rotates at a faster peripheral speed than the first stretching roller S17 in order to achieve the desired stretching of the plastic film over the hot stretching gap R. Above the hot stretching gap R, whose width in the direction of travel of the film web F is typically approximately 300 to 450 mm, a heating device 6, in particular an infrared radiator, is also arranged in this configuration. A pressure roller N17 is assigned to the second stretching roller S18 following the hot stretching gap R. The stretching rollers S15 to S17 are typically heated during operation of the hot stretching section 5. The stretching roller S18 is cooled to fix the stretched state of the plastic film before it passes over the last stretching roller S19. The film web F is deflected downwards from the left over the stretching roller S16 and then runs upwards after the stretching roller S16 to the stretching roller S17.In the hot stretching gap R, the film web F runs essentially horizontally before being deflected downwards by the stretching roller S18 and then diagonally upwards again by the stretching roller S19. This path of the film web F thus roughly corresponds to the shape of the Greek letter Omega. The hot stretching section 5 in the setup shown here is therefore also referred to as the Omega stretching section.

[0037] In the area of ​​the last stretching roller S19, its speed can optionally be adjusted for post-stretching of the film web F. The film web F is then transferred to a further treatment zone 7, not shown here. During operation of the warm stretching area 5, the stretching rollers S10 to S14 of the hot stretching area 4 only function as transport rollers. However, they must be driven, temperature-controlled, and operated for this purpose.

[0038] Depending on the requirements of the film stretching system 1, treatment zone 7 can involve, for example, a post-treatment such as cooling, winding the film web F for further transport, or a so-called transverse stretching, in which the film is stretched again transversely to the film direction in addition to the longitudinal stretching along the film direction. This is also referred to as TDO (Transversal Direction Orientation) stretching.

[0039] The scenario changes when switching to a thinner film web F, which is to be hot-stretched in film stretching unit 1. In this case, the setup described above, with stretching rollers S15 to S19, only serves as a transport unit. The film web is fed directly to the hot-stretching section 4 via the last preheating roller PHn of heating zone 2. The film web F then passes over a first stretching roller S10 with a pressure roller N10 and is optionally pre-stretched between stretching roller S10 and the subsequent stretching roller S11. The actual hot-stretching then takes place between stretching rollers S11, S12, and S13, with the speed of these stretching rollers increasing accordingly as the plastic web F progresses. Thus, the speed of stretching roller S12 is higher than that of stretching roller S11, and that of stretching roller S13 is higher than that of stretching roller S12. Stretching rollers S10 to S12 are heated during this process.The actual hot stretching gap can be divided into a first hot stretching gap HR1 for pre-stretching and a second hot stretching gap HR2 for the main stretching. These hot stretching gaps are located in the direction of travel of the film web F upstream of the stretching roller S12 and the stretching roller S13, with the stretching roller S13 being cooled to fix the stretched state of the film web F. Subsequently, the film web is transferred via the stretching roller S14 to the hot stretching section 5, which in this case only runs alongside the hot stretching section as a transport unit. Here, too, corresponding pressure rollers N11 to N14 are provided, which are used during the operation of the hot stretching section 4.

[0040] The described structure can now be represented as shown in Fig. 2. Optimize with regard to the required construction volume, the number of required rollers and pressure rollers including their drive and control. The in Fig. The setup shown in section 2 enables exactly the same functionality as within the framework of the Fig. 1 described, but with far fewer components that need to be driven, temperature controlled and controlled, and a significantly reduced construction volume.

[0041] The main difference compared to the one in Fig. The prior art described in Figure 1 consists in reversing the order of the warm stretching section 5 and the hot stretching section 4 in the direction of travel of the film web F. This makes it possible to use different rollers twice, thus saving components and material.

[0042] The construction of the combination yardarm 3 according to Fig. 2 uses the last preheating roller PHn of heating zone 2 as an additional roller of the hot stretching area 5. The preheating roller PHn thus replaces the one in Fig. 1. Roller designated S16 ensures the targeted feeding of the film web F to the first stretching roller S1 such that the film web F receives a wrap of approximately 65° on the stretching roller S1. During operation of the hot stretching section 5, the film web F is held down on the first stretching roller S1, which is heated, by two pressure rollers N1 and N2. Additionally, heating takes place in the hot stretching gap R via a heating device 6, which is configured according to Fig. 2 is formed in the form of two infrared emitters on the two sides of the film web F. Via a comparably large hot stretch gap R, as in Fig. 1. During operation of the hot stretching section 5, the plastic film is stretched. The second stretching roller S2, located downstream of the hot stretching gap R in the direction of travel of the film web F, is cooled during this operation to ensure the stretched state of the plastic film. It also interacts with a pressure roller N3. Considering the preheating roller PHn and the stretching roller S3, the path of the film web F in the hot stretching section 5 again corresponds to the Greek letter Omega. Therefore, it can still be referred to as an Omega stretching section.

[0043] During operation of the hot stretching section 5, the other stretching rollers S3, S4, and S5 are now used only as transport rollers and optionally for temperature control, as described above. Their pressure rollers N4, N5, and N6 may be lifted during operation of the hot stretching section 5. The next treatment zone 7 can then be connected in the manner described above.

[0044] For the operation of the hot stretching section 4, the hot stretching gap R between the first stretching roll S1 and the second stretching roll S2 is used for optional pre-stretching, analogous to the hot stretching gap R between rolls S10 and S11 in the prior art. The hot stretching gap R has a dimension of approximately 380 mm, as mentioned above. The actual hot stretching then takes place between the second stretching roll S2, the third stretching roll S3, and the fourth stretching roll S4, whereby a first hot stretching gap HR1 forms between the second and third stretching rolls S2 and S3, and a second hot stretching gap HR2 forms between the third and fourth stretching rolls S3 and S4. These two hot stretching gaps HR1 and HR2 are smaller than the hot stretching gap R. They are, for example, in the range of 80 to 250 mm. The first hot stretching gap HR1 is typically the smaller one and serves to pre-stretch the plastic film. The second hot-drawing slot HR1 then serves for the main drawing or final drawing.In this process, stretching rollers S2 and S3 are heated, while stretching roller S4 is cooled to fix the stretched state of the plastic film. The subsequent stretching roller S5 is preferably also cooled. During operation of the hot stretching section 4, stretching rollers S2, S3, S4, and S5 are therefore in operation, and their associated pressure rollers N3 to N6 are engaged. Pressure rollers N1 and N2 in the section of the warm stretching area 5 can instead be disengaged, even in the case of optional pre-stretching, since the adhesion of the plastic film to the first stretching roller S1 alone is sufficient to achieve pre-stretching. If necessary, however, pressure rollers N1 and / or N2 can also be used.

[0045] In addition to using the last preheating roller PHn, as a further roller of the combination stretching unit 3, for the desired method of feeding the film web F into the hot stretching section 5, the stretching roller S2 with its pressure roller N3 is used twice, as can also be seen from the corresponding outlines of the hot stretching section 4 and the hot stretching section 5. Together with its pressure roller N3, it is used as stretching roller S2 after the stretching gap R during operation of the hot stretching section 5 and is cooled in this case, while during operation of the hot stretching section 4, it is also used together with its pressure roller N3 and is heated in this case.

[0046] All in all, this design enables the realization of a combination drawing unit 3 with the five drawing rollers S1 to S5 and the six printing rollers N1 to N6, as is the case in the Fig. 2 can be seen. The same functionality is achieved in the prior art only by using twice the number of ten stretching rollers S10 to S19 and the also larger number of eight pressure rollers N10 to N17. The design according to Fig. 2 can dispense with five of the stretching rollers S and the two of the printing rollers N, each including their drives, temperature control, control system, and space requirements. This allows the machine frame for the combination stretching unit 3 to be designed according to Fig. 2. They can be made much smaller and more compact. This makes vibrations much less problematic, so the machine frame can not only be smaller, but also typically simpler in design, more cost-effective and lighter.

[0047] The described sequence of the warm stretching section 5 and the hot stretching section 4 in the combination stretching unit 3 can be implemented particularly efficiently if suitable heating and stretching rollers PH, S are used, which prevent excessive adhesion of the film web F to the surfaces of the rollers as well as critical air inclusions between the surface of the rollers and the film web F. In addition to these properties, the friction between the surface of the stretching rollers S and the film web F should also be designed to be sufficient to reliably ensure stretching, for example, across the stretching gap R during operation of the warm stretching section 5.Previously frequently used coatings based on chromium and polytetrafluoroethylene (PTFE) are, in the inventor's experience, unsuitable here because they reduce the friction between the surface of the stretching roller S and the film web F to such an extent that, at least in the area of ​​the first stretching roller S1 and the second stretching roller S2, the functionality suffers during operation of the hot stretching area 5, or the surface quality of the plastic film is impaired by excessively high contact forces of the pressure rollers N1, N2 and N3.

[0048] For this reason, instead of the rollers usually coated with chromium, PTFE, and / or ceramic, preheating rollers PH and / or stretching rollers S can now be used, which have a surface made of a laser-structured hard metal material. Preferably, this coating of the surface of the corresponding preheating rollers PH and / or stretching rollers S contains or consists of tungsten carbide. Microchannels are introduced into the surface via laser structuring, and these channels are designed so that, during the intended use of the corresponding preheating rollers PH and / or stretching rollers S, they always extend to an end face of the coating, an edge region, or an area not covered by the film. The microchannels have a very small cross-section.This cross-section is sufficient, however, to reliably prevent air pockets between the surface of the preheating rollers PH and / or stretching rollers S and the film web F during operation. Instead, the air can flow out into the areas not covered by the film and / or at the end faces.

[0049] As mentioned above, such air inclusions can lead to inhomogeneities during heating and / or friction, negatively impacting the quality of the plastic film and significantly increasing the risk of film web tearing. The laser-structured microchannels reduce this and, due to their very small dimensions, do not impair the surface of the plastic film. Under a microscope, the channels are not linear; instead, they exhibit a structure similar to that found, for example, in the feeding galleries of bark beetles under the bark of tree trunks. The use of such rollers with a laser-structured tungsten carbide surface is particularly suitable for the last preheating rollers PH in the heating zone 2 along the direction of film web F, such as the three preheating rollers PHn-2, PHn-1, and PHn shown here, as well as the stretching rollers S1 to S4.The S5 stretching roller can be provided with a conventional (hard) chrome coating.

[0050] To ensure accessibility of the assembly of the combination pulley system 3 for maintenance purposes, the previous in Fig. In the setup shown in Figure 1, for example, the width of the stretching gap R can be adjusted so that areas requiring maintenance become accessible. This is extremely complex in its design. In the compact combination stretching unit 3, as shown in the illustration of the Fig.As can be seen in Figure 2, for example, the printing rollers N1, N2, N3 and N5 lying on top of the film web F can be arranged on a common printing roller frame, which either can be moved as a whole relative to a machine frame of the combination drawing unit 3 for maintenance purposes, or which in a particularly simple design is connected to the machine frame in such a way that it can be lifted off as a whole, for example by a crane, in order to make the areas of the combination drawing unit 3 underneath easily accessible. Reference symbol list 1 foil stretching system 2 heating zones 3 combination stretcher 4 Hot stretch area 5 Hot stretch area (ordinary longitudinal stretch area) 6 Heating system 7 treatment zones F foil web HR1, HR2 Hot gap N printing rollers (also N1, N2, ..., N17) PH preheating rollers (also PHn, PHn-1, PHn-2) R (Warm) stretch gap S stretching roller (also S1, S2, ..., S19)

Claims

[1] Combination stretching unit (3) for stretching plastic films for a film stretching system (1) with a warm stretching area (5) and with a hot stretching area (4), which can optionally be used for stretching a continuous film web (F), characterized by , that the warm stretching area (5) is arranged in the direction of travel of the film web (F) in front of the hot stretching area (4). [2] Combination yarding system (3) according to claim 1, characterized by , that the warm stretching area (5) and the hot stretching area (4) jointly use the stretching roller (S2) arranged in the direction of travel of the film web (F) after a warm stretching gap (R). [3] Combination drawing device (3) according to claim 2, characterized by , that a heating device (6) is provided for heating the film web (F) in the hot stretch gap (R), which has infrared emitters on both sides of the film web (F). [4] Combination pulley (3) according to claim 2 or 3, characterized by, that the hot stretching area (5) has two pressure rollers (N1, N2) which cooperate with the first stretching roller (S1) located in the direction of travel of the film web (F) in front of the hot stretching gap (R). [5] Combination pulley (3) according to any one of claims 2 to 4, characterized by , that in the direction of travel of the film web (F) in front of the first stretching roll (S1) which is located in the direction of travel of the film web (F) in front of the hot stretching gap (R) a further roll (PHn) is arranged, over which the film web (F) is fed to the first stretching roll (S1) in such a way that the film web (F) wraps around the first stretching roll (S1) at an angle of 50° to 80°, preferably 60° to 70°, particularly preferably about 65°. [6] Combination pulley (3) according to any one of claims 2 to 5, characterized by, that the hot stretching area (4) includes, in addition to the second stretching roller (S2) used jointly with the hot stretching area (5), a third stretching roller (S3), a fourth stretching roller (S4) and a fifth stretching roller (S5), wherein the fifth stretching roller (S5) serves as a transfer roller (S5) into a further treatment zone (7) adjoining in the direction of travel of the film web (F). [7] Combination drawing device (3) according to claim 6, characterized by , that the second stretching roller (S2) arranged in the direction of travel of the film web (F) after the hot stretching gap (R) and the stretching rollers (S3, S4, S5) of the hot stretching area (4) following in the direction of travel of the film web (F) each cooperate with a pressure roller (N3, N4, N5, N6). [8] Combination pulley (3) according to claim 6 or 7, characterized by that the second stretching roller (S2) and the third stretching roller (S3) are arranged in one plane and rotate in opposite directions. [9] Combination pulley (3) according to one of claims 6, 7 or 8, characterized by, that the hot stretching area (4) has at least two hot stretching gaps (HR1, HR2) which are formed at least between the first and second stretching rolls (S2, S3) of the hot stretching area (4) and between the second and third stretching rolls (S3, S4) of the hot stretching area (4). [10] Combination pulley (3) according to any one of claims 6 to 9, characterized by , that the film web (F) wraps around the third and fourth stretching rollers (S3, S4) with a wrap angle of more than 120° each. [11] Combination pulley (3) according to one of the preceding claims, characterized by that at least one of the stretching rollers (S) has a surface made of a laser-structured hard metal material. [12] Combination drawing device (3) according to claim 11, characterized by that the hard metal material contains tungsten, especially in the form of tungsten carbide. [13] Combination pulley (3) according to claim 11 or 12, characterized by, that the laser-structured surface has microchannels open towards the surface, which in the intended use of the stretching rollers (S) are also open towards an area of ​​the stretching rollers (S) free from the film web (F). [14] Combination yarding system (3) according to one of the preceding claims, characterized by , that the printing rollers (N1, N2, N3, N5) arranged on one of the sides of the film web (F) are accommodated in a common printing roller frame which is movable or liftable relative to a machine frame of the combination stretching unit (3) for maintenance purposes. [15] Film stretching system (1) with a combination stretching unit (3) according to one of claims 1 to 14. [16] Film stretching machine (1) according to claim 15 with a combination stretching unit (3) according to one of claims 5 to 14 characterized bya heating zone (2) in the direction of travel of the film web (F) in front of the combination stretching unit (3), which includes several preheating rollers (PH), wherein the last preheating roller (PHn) in the direction of travel of the film web (F) is the further roller of the combination stretching unit (3). [17] Film stretching machine (1) according to claim 16, characterized by , that at least one of the preheating rollers (PH) of the heating zone (2) has a surface made of a laser-structured hard metal material. [18] Method for operating a combination pulley system (3) according to any one of claims 1 to 14, characterized by , that During operation of the hot stretching section (5), the stretching rollers (S1, S2) arranged adjacent to the hot stretching gap (R) are operated at a circumferential speed that increases in the direction of travel of the film web (F), while the other stretching rollers (S3, S4, S5) of the hot stretching section (4) rotate at a circumferential speed adapted to the circumferential speed of the last stretching roller (S2) of the hot stretching section (5), wherein a temperature of the plastic film in the area of ​​the hot stretching gap (R) of approximately 80°C to 100°C, preferably 85°C to 95°C, is set, wherein during operation of the hot stretching section (5) the first stretching roller (S1) is heated and the second stretching roller (S2) is cooled, and that During operation of the hot stretching section (4), the stretching rollers (S2, S3, S4) arranged in the direction of travel of the film web (F) after the hot stretching gap (R) rotate at an increasing peripheral speed, so that at least two hot stretching gaps (HR1, HR2) are formed, while the first stretching roller (S1) of the hot stretching section (5) rotates at approximately the same peripheral speed as the second stretching roller S2 after the hot stretching gap (R), wherein a temperature of the plastic film in the area of ​​the first two stretching rollers (S2, S3) of the hot stretching section (4) is set to approximately 110°C to 130°C, preferably approximately 115°C to 125°C, and wherein cooling takes place in the direction of travel of the film web (F) from the fourth stretching roller (S4). [19] Method according to claim 18, characterized by , that the printing rollers (N) in the area used only for transporting the film web (F) are lifted off from the stretching rollers (S) to which they are assigned.

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