Method and apparatus for rewetting a paper web

Abstract

A method for rewetting a paper web (4) is provided, in which the web (4) is guided through a device (2), the device (2) having a processing unit (10) with which the web (4) is processed, the device (2) having a drying unit (12) for drying the web (4) downstream of the processing unit (10), the device (2) having a plurality of humidifying units (18, 20) downstream of the drying unit (12), pre-wetting the web (4) using a first humidifying unit (18) of the plurality of humidifying units, and post-wetting the web (4) using a second humidifying unit (20) of the plurality of humidifying units after a minimum residence time (T), thereby performing multiple-stage rewetting of the web (4) with a wetting agent using the plurality of humidifying units (18, 20), the minimum residence time (T) being selected such that the web (4) in the second humidifying unit (20) has an increased absorption rate of the wetting agent based on the pre-wetting. Furthermore, a device (2) for processing a paper web (4) is provided.

Description

Technical Field

[0001] The present invention relates to a method for rewetting a paper web and an apparatus for processing a paper web.

[0002] Examples of apparatuses for processing a paper web include printing machines that print on such webs and corrugators that produce webs consisting of multiple layers of individual paper layers (specifically corrugated sheet webs).

[0003] In connection with the processing of a paper web, the web is typically dried, for example, after printing in a printing machine. During drying, a significant amount of moisture (i.e., water) is removed from the paper, and this removed moisture is preferably supplied back to the paper later within the framework of rewetting. However, the problem here is that the paper has only a limited absorption capacity for the water applied to the web during rewetting. Therefore, in some cases, it is not possible to completely restore the previously removed moisture. This results in defects such as the occurrence of wrinkles, the Blechdoseneffekt, Roping, etc. Such defects also pose problems when the web is wound up at the end of the apparatus or when the web is directly passed on to a subsequent apparatus in an in-line operation.

[0004] U.S. Patent No. 5,596,930 describes a method for humidifying a moving web of a printed material. In this method, a humidifying agent can be applied to the web in a plurality of steps.

[0005] Against this background, the object of the present invention is to improve the rewetting of a paper web. For this purpose, in particular, a correspondingly improved method and an improved apparatus should be provided.

[0006] According to the present invention, this problem is solved by a method having the features described in claim 1 and an apparatus having the features described in claim 15. Advantageous configurations, developments, and modifications are the subject of the dependent claims. The explanations given in connection with the method apply mutatis mutandis to the apparatus, and vice versa. In the following, insofar as the steps of the method are described, advantageous configurations of the apparatus result from the fact that the apparatus is designed to carry out one or more of those steps.

[0007] The core idea of the present invention is particularly multi-stage rewetting, in which the rewetting of the paper web is divided into a plurality of stages, and in each of those plurality of stages, a wetting agent is applied to the web. Thereby, the effect that the pre-wetted web can absorb moisture much better than the non-pre-wetted web afterwards can be utilized, and as a result, it becomes possible to bring more wetting agent, especially water, and thus moisture, to the paper.

[0008] The method according to the present invention is used for rewetting a paper web and is in particular an operating method for an apparatus in which rewetting is carried out. In the following, without limiting generality, a single-layer paper web, i.e., a paper web in the narrow sense, is assumed, but the present invention is equally applicable to other paper webs, especially multi-layer webs, such as cardboard sheets, or semi-finished products of that kind of web composed of a plurality of single layers of paper.

[0009] The web is guided through the apparatus. For example, the web is unwound by a machine unwind and subsequently processed, and finally either rewound again by a winder of the apparatus or alternatively passed directly to a subsequent apparatus. In the following, without limiting generality, it is assumed that the apparatus is a printing machine, but the explanations given here are generally applicable to any apparatus that processes a paper web and carries out rewetting in relation to that processing. For example, a configuration in which the apparatus is a corrugator for manufacturing cardboard sheets is also particularly advantageous.

[0010] This device generally has a processing unit used to process a web. Further, this device has a drying unit downstream of the processing unit for drying the web (i.e., the processed web). The drying unit has, in particular, one or more hot air dryers and / or infrared dryers for drying. The type of processing and also the type of drying are not initially important; rather, it is more important that, in principle, moisture is removed from the paper of the web during drying. This moisture should be supplied again as completely as possible by rewetting within the framework of this method. For this purpose, the device has a plurality of humidifying units downstream of the drying unit, and a plurality of stages of rewetting with a humidifying agent are performed on the web (i.e., the dried web) using these plurality of humidifying units. Thus, by applying a humidifying agent to the web using the humidifying units, moisture is supplied again to the web after drying. Here, pre-wetting of the web (also referred to as first humidification or "Pre-Wetting") is performed by the first humidifying unit among the plurality of humidifying units, and after a minimum residence time, post-wetting of the web (also referred to as second humidification or "Secondary Wetting") is performed by the second humidifying unit among the plurality of humidifying units, thereby performing a plurality of stages of rewetting of the web with a humidifying agent. The pre-wetting and post-wetting are performed, in particular, on the same side of the web and, in a preferred configuration, even on both sides.

[0011] In particular, the humidifying agent contains water, and in particular, most of it is water (i.e., 95% or more, or rather 99% or more), or rather consists solely of water. The water is preferably, as far as possible, soft water and / or demineralized water. In a particularly advantageous configuration, the humidifying agent is a mixture of water and a wetting additive, and the proportion of the wetting additive is, in particular, at most 1% by volume, particularly preferably at most 0.1% by volume, which is usually sufficient for high-quality wetting additives to achieve the lowest surface tension. The wetting additive is used to lower the surface tension of water (e.g., from 72.7 mN / m (polar component: 51 mN / m, dispersive component: 21.7 mN / m) at 20 °C to 28.5 mN / m (polar component: 5.2 mN / m, dispersive component: 23.3 mN / m)), thus further increasing the absorption rate. Optionally, air or another inert gas (e.g., nitrogen) is further mixed into the humidifying agent.

[0012] Here, "A is located upstream of B", and the equivalent "B is located downstream of A" means that A is located in front of B along the web, i.e., the web first passes through A and then through B. Instead of the terms "upstream / downstream", the term pairs "front / back", "in front / behind" are also used.

[0013] Here, for the sake of simplicity, the handling (treatment, drying, humidifying, etc.) of the "web" is mentioned. Thus, in particular, the web is not processed gradually section by section, i.e., not first completely processed, but rather the whole is first dried and then, for the first time, humidified. Instead, the web is guided continuously (i.e., in-line) through the entire apparatus, and each web part is processed at each location and then passed directly on.

[0014] The minimum residence time is selected such that the web in the second humidifying unit has an increased absorption rate for the humidifying agent based on the pre-humidification (by the first humidifying unit). In a configuration that is appropriate but not essential, this is in particular 3 g / m 2The absorption rate in the range from 0.1 second to 1 second for the amount of the humidifying agent (i.e., thereafter 3 to 30 g / (s×m 2 )) is understood to be so, and for other amounts, it is understood to be the corresponding absorption rate. In other words, by pre-humidification, the humidifying agent is applied to the web and absorbed by the paper during the minimum residence time, so that the web has increased hydrophilicity at the end of the minimum residence time. At this time, the humidifying agent first forms a liquid film on the web during pre-humidification, and this liquid film, based on the drying performed previously, is not immediately completely absorbed by the paper and at least partially remains on the web. In any case, the knowledge that a certain amount of time is required for it to be completely absorbed by the paper is utilized. During that time, the web is so to speak "watered" with the humidifying agent, but since the humidifying agent is not absorbed by the paper, it is meaningless to supply more humidifying agent. The liquid film forms a corresponding barrier and limits the maximum absorption capacity of the paper. This maximum absorption capacity is also limited by the conveyance of the web passing through the apparatus and the available section. Because when the web is further processed or wound up, there should no longer be excess humidifying agent on the web, and in some cases, the excess humidifying agent must be sucked up.

[0015] Accordingly, here the re-humidification is divided into a plurality of stages, i.e., at least two humidifying units, and these humidifying units correspondingly form the first stage and the second stage of the re-humidification. Here, the pre-humidification is not performed with the original target amount of the humidifying agent, but with an amount reduced compared to that target amount of the humidifying agent. The further humidifying agent is supplied for the first time during the post-humidification by the second humidifying unit. However, since the web is pre-humidified here, the humidifying agent of the second humidifying unit penetrates into the paper much faster than when no pre-humidification is performed (i.e., since the absorption rate is increased), so that overall and in the same section, more humidifying agent is brought to the web than when only one-stage re-humidification is performed. This advantage also applies to re-humidification using more stages than two.

[0016] The minimum residence time is used, in particular, to wait until the most suitable point for post-humidification. As already suggested, the humidifying agent first forms a liquid film on the web during pre-humidification, and it is desirable that this liquid film be removed as thoroughly as possible when the web reaches the second humidifying unit. As long as the liquid film is present on the web, the absorption rate of the further applied humidifying agent is substantially zero, and it is only when the liquid film is absorbed by the web that the absorption rate increases. Therefore, the minimum residence time is preferably selected such that when the web reaches the second humidifying unit, the humidifying agent applied by the first humidifying unit is completely absorbed by the web. In particular, in this state and at this point, an increase in the absorption rate occurs. Basically, for example, using a squeegee, it is possible to simply scrape off the liquid film that may occur in some cases. However, in that case, the already applied humidifying agent is correspondingly discharged and remains unused.

[0017] The absorption rate of the humidifying agent into the paper depends greatly on the initial moisture content of the paper (i.e., the initial water content). Dry paper, such as that which exists after passing through a drying device, absorbs water more slowly than pre-humidified paper. The reason is that the adsorbed water molecules increase the hydrophilicity at the capillary walls of the paper. This reduces the contact angle, and thus increases the absorption rate. This relationship is also summarized in the so-called Lucas-Washburn equation (or simply the Washburn equation). According to this equation, with a smaller contact angle, the penetration depth per unit time becomes deeper, and accordingly the absorption rate becomes faster. Equivalent to an increase in the absorption rate are, in particular, an increase in the penetration depth, an increase in hydrophilicity, a reduction in the contact angle, and a reduction in surface tension.

[0018] Particularly problematic for the absorption rate of the web is what is known as the keratinization of the paper. Paper contains paper fibers that absorb moisture and release it again when dry. Keratinization is understood to mean any fiber change (i.e., the actual change in the paper fibers) that occurs during the drying process and the rewetting process. Examples of that kind of fiber change include the irreversible closing of pores (decrease in specific surface area) due to the occurrence of hydrogen bonds between adjacent cell walls or the integration of microfibrils. Keratinization is particularly irreversible. Keratinization usually occurs during drying in the drying unit. In keratinized paper, it becomes very difficult to supply fresh moisture. That is, the absorption rate becomes very slow. The method presented here is not particularly intended to bring as much moisture as possible within a given time in a given section using only a single humidification unit. Rather, it is intended to establish an increase in the absorption rate, especially in a first stage by pre-humidification. Immediately after the absorption rate has been significantly increased, ideally as soon as an appropriate threshold value for the absorption rate has been reached (e.g., from the ranges mentioned above), in a second stage, additional moisture is brought to the paper by post-humidification, but here it is much more efficient than in keratinized paper, so that overall more moisture is brought to the paper in the same time and section. The multi-stage rewetting and, thereby, the greater moisture content of the paper advantageously reduce or completely avoid the keratinization of the paper in the apparatus during drying after rewetting or during drying between the two stages of rewetting.

[0019] As a result, advantageously, far more humidifying agent, in particular water, can be brought to the web, thereby achieving a correspondingly very high water content. Similarly, a very large loss of moisture during drying can advantageously be fully compensated, or rather, overcompensated as required. This reduces or completely prevents defects such as looping, wrinkling, and tin can effects during the conveyance and processing of the web in the apparatus. Particularly important here is that no wrinkles occur in the web, which is ensured by multi-stage rehumidification, especially within the apparatus and at the end of the apparatus. Furthermore, multi-stage rehumidification has a positive effect on web shrinkage in the apparatus, especially by significantly reducing the impact of web shrinkage due to drying and subsequent rehumidification. Also, multi-stage rehumidification can achieve a difference in the moisture profile in the cross-direction of less than 2%, especially a maximum of 1.3%. That is, the difference in moisture at any two points along the width of the web is less than 2% or at most 1.3%. If the difference exceeds 2%, in the normal case, so-called warping occurs during further processing of the web in the corrugator. That is, the cardboard web is distorted. Finally, since the shrinkage of the web is compensated at the appropriate location as required, multi-stage rehumidification enables the high mechanical dynamics of the apparatus, especially with respect to changes in web speed and coating. This can be demonstrated in particular using an appropriate sensor for measuring the width of the web in the apparatus, and is further recognized as an advantage in that an accurate cut is made at the end of the corrugator during further processing in the corrugator.

[0020] The minimum residence time is, in particular, in the range from 0.5 seconds to 30 seconds, particularly preferably in the range from 1 second to 30 seconds, although the upper limit of 30 seconds is basically unnecessary. That is, the minimum residence time is, in particular, at least 0.5 seconds or at least 1 second. Research has shown that such a minimum residence time is sufficient to achieve the above-described effects. The minimum residence time is equal to the minimum section through which the web passes between the first humidifying unit and the second humidifying unit. The conversion between the minimum residence time and the minimum section is performed using the web speed, i.e., the speed at which the web is conveyed through the apparatus. In some cases, the web speed varies according to the job. For the above-mentioned range from 0.5 seconds to 30 seconds, at a suitable web speed from 100 m / min to 450 m / min, a minimum section in the range from 1 m to 225 m is produced nominally, and in any case, a minimum section of at least 2 m is advantageous. In particular, the web path extending throughout the apparatus (i.e., the section through which the web travels) is typically much shorter than 200 m, so the minimum section is significantly shorter than 225 m at most. The minimum section between the first humidifying unit and the second humidifying unit is very preferably in the range from 25 m to 50 m, particularly preferably in the range from 35 m to 40 m. These ranges are advantageously still entirely within the apparatus. Thereby, for the above-mentioned web speed, the minimum residence time is about 4 seconds to 30 seconds, or 5 seconds to 24 seconds.

[0021] In particular, the minimum residence time between the first humidifying stage and the second humidifying stage is selected with respect to the maximum web speed of the apparatus (e.g., 450 m / min). Even at a slower web speed, there are typically no problems. This is because a longer residence time does not result in a significant change in the moisture content of the paper. Accordingly, there is no upper limit for the minimum residence time for the time being. What is preferentially important is that when the web reaches the second humidifying unit, it is still wet and, in any case, not completely dry, so that an increase in the absorption rate occurs for post-humidification. Irrespective of this, the amount of the humidifying agent applied is preferably adjusted depending on the web speed so that the same amount of the humidifying agent is always applied per unit time and per unit area even when the web speed changes.

[0022] The amount of moisture that paper can absorb typically depends on the basis weight of the paper (e.g., from 100 g / m 2 to 300 g / m 2 or more), and also generally depends on the type of paper. The greater the basis weight, i.e., the heavier the paper, the more moisture the paper can absorb. Furthermore, papers with a greater basis weight (especially over 300 g / m 2 ) typically have less drying (when considering moisture in % units), so strong rewetting according to the greater basis weight is usually not required in normal cases. Nevertheless, a greater amount (especially over 3 g / m 2 ) is possible without problems (i.e., especially no wrinkles occur) compared to the case of a smaller basis weight. Therefore, in an advantageous configuration, the amount of wetting agent applied to the web per unit time using the wetting unit is adjusted depending on the basis weight of the paper, and for a greater basis weight, the amount of wetting agent also increases. For the reasons described above, this relationship is not necessarily linear, and in particular, this relationship does not necessarily continue up to a very large basis weight. Similarly, alternatively or additionally, the above amount is adjusted depending on the type of paper. This is based on the consideration that binders in the paper, and in some cases other additives, as well as the porosity and initial moisture of the paper, i.e., the general type of paper, also affect the absorption rate, i.e., the ability of the paper to absorb moisture.

[0023] In particular, by each wetting unit, a maximum of 3 g / m 2 of wetting agent is applied. This amount is optimal, in particular, for the above-described range regarding the normal dimensional design of the apparatus and the minimum residence time, and is also suitable for typical paper types and basis weights. Depending on the basis weight, only one of the wetting units is used, and in particular, an amount of wetting agent from 1 g / m 2 to 3 g / m 2 is applied. As described above, for a greater basis weight, a greater amount is used. Therefore, when using at least two wetting units, a total of 6 g / m 2A humectant can be applied up to that point, and these humectants are also completely absorbed by the paper based on multi-stage rewetting, so a correspondingly high moisture content is achieved. Especially in the case of a very large basis weight (especially over 300 g / m 2 ²), or generally, amounts up to 4, 5, 6 g / m 2 ² or more are preferably applied using only one of the humidifying units within the humidifying unit. The total amount of the humectant becomes correspondingly large.

[0024] Preferably, using a plurality of humidifying units, at least 5 g / m 2 ² of the humectant is applied, or the web is rewetted to at least 4% (i.e., here, also in other cases, by weight) of residual moisture. Preferably, using a plurality of humidifying units, the coated paper web is rewetted to at least 4% to 5% of residual moisture, and / or the uncoated paper web is rewetted to at least 5% to 7% of residual moisture. Preferably, in order to achieve the target water content (equivalent to the target residual moisture) by rewetting, and also at the end of the device, the target amount of the humectant is set, and this target amount of the humectant is distributed to a plurality of humidifying units, whereby the total target amount of the humectant is applied to the web in multiple stages. Residual moisture represents the amount of moisture in the web relative to the weight of the web. Depending on the basis weight, an amount of humectant from 5 g / m 2 ² to 3 g / m 2 ² is preferred, and as described above, for a larger basis weight, a larger amount is used. Alternatively or additionally, especially regardless of the basis weight, a residual moisture in the range of 5% to 6% is preferred. Usually, the drying device dries the web to a residual moisture in the range of 2.5% to 3.5% (uncoated paper), and correspondingly, by rewetting, a residual moisture of 1.5% to 3.5% is supplemented, which is typically not achieved by only one-stage rewetting. Different from uncoated paper, coated paper usually stores less moisture based on the layer thickness of the paper and is typically dried from 5% initial moisture to 1% to 1.5%, so in order to obtain 4% to 5% moisture, a residual moisture of 2.5% to 4% is supplemented by rewetting.

[0025] Preferably, each humidifying unit is formed as a spray bar. This type of spray bar is characterized by very low requirements for structural space and high economy. Furthermore, since the spray bar preferably does not operate in the circulation mode, only fresh humidifying agent is always applied to the web. Basically, other configurations are also conceivable, for example, a humidifying device equipped with rollers or spray disks, but in particular the latter is not as economical as the spray bar and also requires more structural space. In particular, the requirement for structural space is an important aspect in the multi-stage rehumidification as described here. Because this type of multi-stage rehumidification requires humidifying units to be installed at at least two different locations within the device. However, the device is usually designed to be as small as possible and a plurality of components are combined in a narrow space, so multi-stage rehumidification by a plurality of large humidifying units (for example, spray disks) cannot be easily realized. In particular, in the existing device design, retrofitting is not possible unless the device is redesigned from the ground up. In contrast, the spray bar is particularly small and can be retrofitted at various locations in the existing device without having to abandon the previous device design.

[0026] The use of a spray bar typically has the further advantage that, since the spray bar has a plurality of spray nozzles arranged along the width of the web, non-uniform moisture profiles can be easily compensated for, i.e., made uniform, by selective control of the individual spray nozzles, for example by on / off or adjustment of the amount of humidifying agent applied.

[0027] Regarding the arrangement of a plurality of humidifying units within the device, there are a plurality of suitable configurations, and several particularly preferred configurations will be described below. These configurations can basically be combined with each other.

[0028] This device has at least one device section, which includes the processing unit and the drying unit described above. However, depending on the configuration, the device has another device section where the web is guided successively, i.e., the web is directly passed to the subsequent device section at the end of the device section, thereby enabling multiple processes and / or general handling of the web. The last device section is, among other things, a winder where the web is finally wound into a roll, or a handover section where the web is directly passed to the subsequent device without being wound.

[0029] In a suitable configuration, the humidifying unit is arranged downstream of the cooling unit of the device, and the cooling unit is arranged downstream of the drying unit. The cooling unit is used to cool the web again at the subsequent stage of the drying unit. Here, the humidifying unit is not integrated with this cooling unit and is arranged separately at another location, particularly downstream of the cooling unit.

[0030] In another suitable configuration, between the first humidifying unit and the second humidifying unit, the web is not dried, especially the web is only conveyed. Therefore, between the first humidifying unit and the second humidifying unit, no other processing unit or drying unit is arranged, and only the absorption of the humidifying agent by the paper is carried out. Therefore, as long as the device has multiple device sections, the first humidifying unit and the second humidifying unit are arranged between two device sections, or at the start or end of a device section, or a combination thereof. The first humidifying unit, the second humidifying unit, and the smallest section between them are collectively also referred to as a two-stage humidifying section.

[0031] In another suitable configuration, the apparatus has a plurality of apparatus sections, each having a processing unit web for processing the web and a subsequent drying unit for drying the web, and at least one of the plurality of humidifying units is arranged downstream of each drying unit, so that each time drying is performed, and in particular before subsequent drying is performed, or in front of the end of the apparatus, the web is humidified at least once. Unlike the two-stage humidification section, in this case, the plurality of humidifying units are distributed throughout the apparatus and are not arranged in series. This is based on the consideration that it is typically harmless as long as the desired residual moisture is achieved as a whole at the end of the apparatus when drying is performed between two humidifying units. The humidification between two drying operations advantageously prevents the aforementioned keratinization of the paper, so that even if the section between the two humidifying units is longer than the minimum section, and even if a drying unit is located between them as required, the humidifying unit located further downstream has an increased absorption rate caused by the humidifying unit located further upstream.

[0032] In a preferred configuration, the apparatus is a printing machine, in particular a digital printing machine. Additionally or alternatively, the processing unit is suitably a printing unit for printing the web using ink, especially water-based ink and / or inkjet ink. In the case of a digital printing machine, the printing unit typically has a plurality of printing bars, especially for different colors (e.g., CMYK). In a printing machine, the drying unit is a printed matter drying unit for drying the ink. The multi-stage rehumidification described here is very suitable for application in a printing machine. This is because in the normal case here, strong drying of the web is performed to dry the printed matter brought about by the ink during printing. As a secondary effect, moisture is also removed from the paper, and the removed moisture is supplied again by rehumidification.

[0033] In a suitable configuration, the apparatus has a primer application device and a primer drying unit upstream of the printing unit, and a varnish application device and a varnish drying unit downstream of the printing unit. The web is processed by the primer application device such that a primer, which is used in particular as a base for subsequent printing, is applied. The applied primer is dried by the primer drying unit. Similarly, the web is processed by the varnish application device such that a varnish, which is used as a finish for the previously performed printing, is applied. The applied varnish is dried by the varnish drying unit. The varnish application device is analog. That is, the varnish is applied, for example, via a roller or a trough, or sprayed. Alternatively, the varnish application device is digital. That is, the varnish is printed as in the case of a digital printing unit having one or more printing bars. The primer application device and the varnish application device are separate processing units of the apparatus.

[0034] Preferably, between the primer drying unit and the printing unit, two of the plurality of humidifying units are arranged. Alternatively, only one of the plurality of humidifying units is arranged. Alternatively or additionally, appropriately, between the printed matter drying unit and the varnish coating device, another printing unit of the plurality of printing units is arranged. Alternatively, rather, two of the plurality of humidifying units are arranged. Alternatively or additionally, especially on the downstream side of the varnish drying unit, two other humidifying units of the plurality of humidifying units are arranged. Alternatively, only one of the plurality of humidifying units is arranged. In one particularly preferred configuration, the apparatus has five humidifying units, for which two of the plurality of humidifying units are arranged between the primer drying unit and the printing unit, another one of the humidifying units is arranged between the printed matter drying unit and the varnish coating device, and on the downstream side of the varnish drying unit, two other humidifying units of the plurality of humidifying units are arranged. Which of these plurality of humidifying units is the first humidifying unit or the second humidifying unit described above is basically arbitrary and is preferably adjusted according to the specific operation mode and especially depending on the paper type and basis weight. Basically, the two humidifying units between the primer drying unit and the printing unit are the first humidifying unit and the second humidifying unit for pre-humidification and post-humidification as described above, and / or it is advantageous if the two humidifying units on the downstream side of the varnish drying unit are the first humidifying unit and the second humidifying unit for pre-humidification and post-humidification as described above. In this case, even before the web is processed in each subsequent processing unit, or before the web is wound up at the end of the apparatus or delivered to a subsequent apparatus, at the corresponding locations, multi-stage re-humidification, especially a two-stage humidification section, is realized.

[0035] The web is not wound up at the end of the device. As long as it is passed to a subsequent device, such as a collator, using a handover section, preferably, along the handover section, yet another humidifying unit, particularly a sixth humidifying unit, is arranged. The handover section and the other humidifying unit are still particularly part of the device.

[0036] The operation of each humidifying unit is not necessarily linked to the operation of the processing unit located upstream thereof. However, in a suitable configuration, humidification is performed regardless of whether the processing is carried out in advance. This is advantageous when the drying unit upstream of the humidifying unit is active or when the drying unit still functions during a job change even though the associated processing unit is inactive. Because in this case, the web is further dried and rehumidification continues to be correspondingly advantageous. For example, instead of a primer application device being required and becoming active for each job, the primer application device is inactive as needed for much of the time. However, in these cases, the primer drying unit is still active, drying the web, and before or while the web enters the printing unit, this web is subsequently preferably rehumidified by one or more subsequent humidifying units.

[0037] In a suitable configuration, the apparatus has an analog varnish application device downstream of the printing unit and additionally has a digital varnish application device downstream of the analog varnish application device. Optionally, the apparatus has a primer application device upstream of the printing unit and also has a primer drying unit as described above. The above description applies to analog and digital application devices. In particular, the apparatus has a respective varnish drying unit downstream of each varnish application device, and this varnish drying unit is used to dry each applied varnish. Among other things, one of the plurality of humidifying units is arranged between the printing unit and the analog varnish application unit (and in this case, downstream of the printed matter drying unit). Another one of the plurality of humidifying units is arranged between the analog varnish application device and the digital varnish application unit (and in this case, downstream of the varnish drying unit for the analog varnish application device). Yet another one of the plurality of humidifying units is arranged downstream of the digital varnish application device (more precisely, downstream of the varnish drying unit for the digital varnish application device). Thus, the apparatus has a total of three humidifying units. Which of these plurality of humidifying units is the above-mentioned first humidifying unit or the second humidifying unit is basically arbitrary and is preferably adjusted according to the specific operating mode and especially depending on the paper type and basis weight. Also, it is advantageous to use all three humidifying units together. Alternatively, one of the above-mentioned three humidifying units is omitted, whereby the apparatus has only two humidifying units, which is already advantageous in itself.

[0038] Basically, in as many locations as possible, one humidifying unit is installed at each location, and in this way, excessive shrinkage of the web within the apparatus is prevented, and the dimensional stability of the web is maintained over as long a section as possible within the apparatus, which thereby benefits one or more processes accordingly.

[0039] The device according to the present invention is used for processing a paper web. The device has a processing unit for processing the web and a drying unit for drying the web, which is arranged downstream of the processing unit. Further, the device has a plurality of humidifying units arranged downstream of the drying unit, and the plurality of humidifying units perform pre-humidification of the web using a first humidifying unit among the plurality of humidifying units, and after a minimum residence time, post-humidification of the web is performed using a second humidifying unit among the plurality of humidifying units, whereby multi-stage re-humidification of the web with a humidifying agent is carried out. Here, the minimum residence time is selected such that the web in the second humidifying unit has an increased absorption rate of the humidifying agent based on the pre-humidification. The above description applies equally to the device.

[0040] Hereinafter, embodiments of the present invention will be described based on the drawings. Each figure schematically shows the following.

Brief Description of the Drawings

[0041]

Figure 1

Figure 2

Figure 3

Figure 4

[0042] In FIGS. 1, 3 and 4, modified examples of the device 2 for processing the paper web 4 are shown respectively. In particular, in FIGS. 3 and 4, without loss of generality, a digital printing press as the device 2 for printing on a single-layer paper web 4 is assumed. However, the description in this context applies equally to other devices 2, such as collators, and other webs 4, such as multi-layer paper webs 4.

[0043] Web 4 is guided through device 2, for example, unwound by an unwinder (not explicitly shown in device 2), and finally, either rewound by winder 6 of device 2 or, alternatively, directly passed to a subsequent device (not shown) via delivery section 8.

[0044] Device 2 generally has a processing unit 10 used to process web 4. Further, device 2 has a drying unit 12 downstream of processing unit 10 for drying web 4. Drying unit 12 has one or more hot air dryers 14 and / or infrared dryers 16 for drying. During drying, in principle, moisture is removed from the paper of web 4, but this moisture should be resupplied as completely as possible by rehumidification. For this purpose, device 2 has a plurality of humidifying units 18, 20 downstream of drying unit 12, and multiple-stage rehumidification of web 4 with a humidifying agent is performed using these plurality of humidifying units 18, 20. Here, multiple-stage rehumidification with a humidifying agent is performed by preliminarily humidifying web 4 with the first humidifying unit 18 and then post-humidifying it with the second humidifying unit 20 after a minimum residence time T.

[0045] The humidifying agent here contains sufficient water and is further a mixture of water and a wetting additive for reducing the surface tension of water. Optionally, air or the like is further mixed into the humidifying agent.

[0046] The minimum residence time T is selected such that the web 4 in the second humidifying unit 20 has an increased absorption rate of the humidifying agent based on pre-humidification. At this time, the humidifying agent first forms a liquid film on the web 4 during pre-humidification. Based on the previous drying, this liquid film is not immediately completely absorbed by the paper and at least partially remains on the web. In any case, the knowledge that a certain amount of time is required for the paper to completely absorb it is utilized. During that time, the web 4 is so to speak "watered" with the humidifying agent, but since the humidifying agent is not absorbed by the paper, it is meaningless to supply more humidifying agent. This is shown in FIG. 2, where the absorption of the humidifying agent by the web 4 at different times T1, T2, T3 is shown. It is shown that the humidifying agent applied by the pre-humidification B1 is different from the humidifying agent applied by the post-humidification B2. Also shown is the absorption capacity A of the paper during post-humidification at different times T1, T2, T3 (this absorption capacity is a measure related to the absorption rate). At time T1 immediately after pre-humidification, the liquid film forms a corresponding barrier and limits the maximum absorption capacity of the paper. The paper fibers 22 form a pore structure into which the humidifying agent derived from the pre-humidification B1 is "watered", thereby making post-humidification difficult. The enlarged part of FIG. 2 at time T1 shows in detail the movement of water molecules through the pore structure by capillary action and the subsequent diffusion into the paper fibers 22. At time T2, the humidifying agent derived from the pre-humidification has been completely absorbed by the paper, the pore structure has been pre-humidified, and the liquid film has disappeared, so the contact angle becomes particularly 0, the absorption rate is faster than at time T1, and also faster than the absorption rate before pre-humidification. At time T3 as well, the absorption rate remains fast. Due to the expansion of the paper fibers 22, the pore structure has become slightly narrower, and the absorption capacity does not decrease slightly again.

[0047] Correspondingly, here the rewetting is divided into a plurality of stages, that is, at least divided into two humidifying units 18, 20, and these humidifying units 18, 20 correspondingly form the first stage and the second stage of the rewetting. Here, the preliminary humidification is not carried out with the original target amount of the humidifying agent, but with an amount reduced compared to that target amount of the humidifying agent. Further humidifying agent is supplied only during post-humidification. Since the web 4 is preliminarily humidified here, the humidifying agent of the second humidifying unit 20 is brought to the paper much faster than if no preliminary humidification were carried out. Thus, overall and in the same section, more humidifying agent is brought to the web 4 than if only one-stage rewetting were carried out.

[0048] The minimum residence time T is used to wait until the most suitable time for post-humidification. As long as the liquid film exists on the web 4, the absorption rate of the further applied humidifying agent is substantially zero, and it is only when the liquid film is absorbed by the web 4 that the absorption rate increases. Accordingly, the minimum residence time T is selected such that when the web 4 reaches the second humidifying unit 20, the humidifying agent applied by the first humidifying unit 18 has been completely absorbed by the web 4 (that is, the times T2 and T3 in FIG. 2). Here, in particular, an increase in the absorption rate occurs.

[0049] In the present invention, first, in the first stage, an increase in the absorption rate occurs due to preliminary humidification. As soon as the absorption rate is significantly increased, ideally as soon as an appropriate threshold value regarding the absorption rate is reached, in the second stage, further moisture is brought to the paper by post-humidification, so that overall, in the same time and section, more moisture is brought to the paper.

[0050] The minimum residence time T is, for example, in the range from 0.5 second to 30 seconds. The minimum residence time T is equal to the minimum section through which the web 4 passes between the first humidifying unit 18 and the second humidifying unit 20. The conversion between the minimum residence time T and the minimum section is carried out using the web speed, that is, the speed at which the web 4 is conveyed through the apparatus 2.

[0051] The amount of moisture that paper can absorb typically depends on the basis weight of the paper (e.g., in the range from 100 g / m 2 to 300 g / m 2 ), and also generally depends on the type of paper. The greater the basis weight, i.e., the heavier the paper, the more moisture the paper can absorb. Thus, for example, the amount of humidifying agent applied to the web 4 per unit time using the humidifying units 18, 20 is adjusted depending on the basis weight of the paper, and for a greater basis weight, the amount of humidifying agent also increases. Similarly, alternatively or additionally, the above amounts are adjusted depending on the type of paper.

[0052] In the embodiments illustrated herein, each of the humidifying units 18, 20 applies a maximum of 3 g / m 2 of humidifying agent. Depending on the basis weight, a single one of the plurality of humidifying units 18, 20 applies from 1 g / m 2 to 3 g / m 2 or more of the humidifying agent. Further, here by way of example, together the plurality of humidifying units 18, 20 apply at least 5 g / m 2 of humidifying agent, or the web 4 is rehumidified to at least 4% to at least 7% (i.e., by weight) of residual moisture, depending in particular on whether the paper is coated or not.

[0053] In the embodiments of FIGS. 3 and 4, the humidifying units 18, 20 are each formed as spray bars. Basically, other configurations, for example, humidifying tools with rollers or spray disks are also conceivable.

[0054] Regarding the arrangement of the plurality of humidifying units 18, 20 within the apparatus 2, there are a plurality of suitable configurations, two of which are shown in FIGS. 3 and 4. The individual aspects of those configurations can basically be arbitrarily combined with each other.

[0055] Generally, the apparatus 2 has at least one apparatus section 24, which includes the processing unit 10 and the drying unit 12 already described. However, depending on the configuration, and particularly in FIGS. 3 and 4, the apparatus 2 optionally has a further separate apparatus section 24 through which the web 4 is successively guided. Here, the winder 6 and the delivery section 8 are each the last apparatus section 24. Particularly in FIG. 4, a configuration is shown in which both the winder 6 and the delivery section 8 are present, and, optionally, are selected from the winder 6 and the delivery section 8 (alternatively or additionally, an inline operation is also realized here).

[0056] In FIGS. 3 and 4, the humidifying units 18, 20 are each arranged downstream of the respective cooling unit 26 of the apparatus 2, and the cooling unit 26 is arranged downstream of the respective drying unit 12. The cooling unit 26 is used to cool the web 4 again after the drying unit 12. The humidifying units 18, 20 are not integrated here with this cooling unit 26 but are instead arranged separately at another location.

[0057] Furthermore, particularly in FIG. 3, it can be seen that between some of the plurality of humidifying units 18, 20, the web 4 is not dried, nor is any processing performed, but only conveyance. Accordingly, no further processing unit 10 or drying unit 12 is arranged between the first humidifying unit 18 and the second humidifying unit 20. In FIG. 3, the first humidifying unit 18 and the second humidifying unit 20 are arranged at the end of the apparatus section 24, but they could also be arranged at other positions, and such arrangements would also be suitable. The first humidifying unit 18, the second humidifying unit 20, and the minimum section therebetween are together also referred to as a two-stage humidifying section, and the apparatus 2 in FIG. 3 accordingly has two such two-stage humidifying sections of that kind, while the apparatus 2 in FIG. 4 does not have a two-stage humidifying section.

[0058] In FIGS. 3 and 4, each apparatus 2 has a plurality of apparatus sections 24 each having a processing unit 10 and a subsequent drying unit 12, and at least one of the plurality of humidifying units 18, 20 is arranged downstream of each drying unit 12, so that the web 4 is humidified at least once each time drying is performed, before subsequent drying is performed, or before the end of the apparatus 2. Different from the two-stage humidifying section, the humidifying units 18, 20 are dispersed over the apparatus 2 and are not arranged continuously, which is the case particularly in FIG. 4, but basically also applies to FIG. 3. This is because also in FIG. 3, a plurality of humidifying units 18, 20 are arranged between the respective drying units 12 and at the end of the facility 2.

[0059] In the configurations of FIGS. 3 and 4 shown by way of example, the apparatus 2 is a digital printing machine, and one of the processing units 10 is a printing unit 28 for printing the web 4 with ink. The corresponding drying unit 12 is a printed matter drying unit 30 for drying the ink. In FIG. 3, the printed matter drying unit 30 has, by way of example, five infrared dryers 16 and four hot air dryers 18. In FIG. 4, the printed matter drying unit 30 is not shown in detail.

[0060] In FIG. 3, the apparatus 2 has a primer application device 32 and a primer drying unit 34 upstream of the printing unit 28, and a varnish application device 36 and a varnish drying unit 38 downstream of the printing unit 28. The primer application device 32 and the varnish application device 36 are another processing unit 10 of the apparatus 2. The primer is applied by the primer application device 32, and this primer is used as a base for subsequent printing and dried by the primer drying unit 34. Similarly, the varnish is applied by the varnish application device 36, and this varnish is used as a finish for the previously performed printing and dried by the varnish drying unit 38. The varnish application device 36 is analog in FIG. 3. That is, the varnish is applied, for example, via a roller or a liquid bath, or sprayed. Instead, a digital varnish application device 40 is also suitable. That is, the varnish is printed as in the case of a digital printing unit provided with one or more printing bars.

[0061] The apparatus 2 according to FIG. 3 has five humidifying units 18, 20. Between the primer drying unit 34 and the printing unit 28, two of the plurality of humidifying units 18, 20 are arranged. Between the printed matter drying unit 30 and the varnish coating apparatus 36, another one of the plurality of humidifying units 18, 20 is arranged. Also, on the downstream side of the varnish drying unit 38, two other humidifying units 18, 20 of the plurality of humidifying units 18, 20 are arranged. In FIG. 3, an exemplary association of which of the plurality of humidifying units 18, 20 is the first humidifying unit 18 and which is the second humidifying unit 20 is also shown. However, this association can basically be changed. In FIG. 3, the two humidifying units 18, 20 between the primer drying unit 32 and the printing unit 28 are, as described above, the first humidifying unit 18 and the second humidifying unit 20 for preliminary humidification and post-humidification. Also, the two humidifying units 18, 20 on the downstream side of the varnish drying unit 38 are, as described above, the first humidifying unit 18 and the second humidifying unit 20 for preliminary humidification and post-humidification. Thus, even before the web 4 is processed in each subsequent processing unit 10, or even before the web 4 is wound up at the end portion of the apparatus 2 or delivered to a subsequent apparatus, multi-stage re-humidification, particularly a two-stage humidification section, is realized at corresponding locations. As long as the web 4 is not wound up at the end portion of the apparatus 2 and is passed to a subsequent apparatus, for example, a collator, using the delivery section 8, optionally, another sixth humidifying unit 18, 20 is arranged along the delivery section 8 (not shown here).

[0062] The apparatus 2 according to FIG. 4 has, on the downstream side of the printing unit 28, an analog sizing applicator 36, and further has, on the downstream side of the sizing applicator 36, a digital sizing applicator 40. One of the plurality of humidifying units 18, 20 is arranged between the printing unit 28 and the analog sizing applicator 36, and another one of the plurality of humidifying units 18, 20 is arranged between the analog sizing applicator 36 and the digital sizing applicator 40. Yet another one of the plurality of humidifying units 18, 20 is arranged on the downstream side of the digital sizing applicator 40. Accordingly, the apparatus 2 has a total of three humidifying units 18, 20, but alternatively, if one of these three humidifying units 18, 20 is omitted, the apparatus 2 has only two humidifying units 18, 20.

Explanation of Signs

[0063] 2 Apparatus 4 Web 6 Take-up machine 8 Handover section 10 Processing unit 12 Drying unit 14 Hot air dryer 16 Infrared dryer 18 (First) humidifying unit 20 (Second) humidifying unit 22 Paper fiber 24 Apparatus section 26 Cooling unit 28 Printing unit (processing unit) 30 Printed matter drying unit (drying unit) 32 Primer coating device (processing unit) 34 Primer drying unit (drying unit) 36 Sizing applicator (analog processing unit) 38 Sizing drying unit (drying unit) 40 Sizing applicator (digital processing unit) A Absorbing capacity Humidifying agent applied by preheating B1, B1 Humidifying agent applied by post-humidifying B2, B2 T Minimum residence time T1, T2, T3 Time

Claims

Claim 1 In a method for rewetting a paper web (4), - the web (4) is guided through a device (2), - the device (2) has a processing unit (10), and the web (4) is processed using the processing unit (10), - the device (2) has a drying unit (12) for drying the web (4) downstream of the processing unit (10), - the device (2) has a plurality of humidifying units (18, 20) downstream of the drying unit (12), and pre-wetting of the web (4) is performed using a first humidifying unit (18) among the plurality of humidifying units, and after a minimum residence time (T), post-wetting of the web (4) is performed using a second humidifying unit (20) among the plurality of humidifying units, whereby multi-stage rewetting of the web (4) with a humidifying agent is performed using the plurality of humidifying units (18, 20), - the minimum residence time (T) is selected such that the web (4) in the second humidifying unit (20) has an increased absorption rate of the humidifying agent based on the pre-wetting, A method, characterized in that. Claim 2 The method according to claim 1, characterized in that the minimum residence time (T) is selected such that the humidifying agent applied by the first humidifying unit (18) is completely absorbed by the web (4) when the web (4) reaches the second humidifying unit (20). Claim 3 The method according to claim 1 or 2, characterized in that the minimum residence time (T) ranges from 0.5 seconds to 30 seconds. Claim 4 The amount of the humidifying agent applied to the web (4) per unit time using the plurality of humidifying units (18, 20) is adjusted depending on the basis weight of the paper, The method according to any one of claims 1 to 3, characterized in that the amount is also increased for a larger basis weight. Claim 5 By each humidifying unit (18, 20), a maximum of 3 g / m ２ of a humidifying agent is applied, characterized in that the method according to any one of claims 1 to 4. Claim 6 Together, by means of the plurality of humidifying units (18, 20), at least 5 g / m ２ of a humidifying agent is applied, or the web (4) is rehumidified to at least 4% residual moisture, characterized in that the method according to any one of claims 1 to 5. Claim 7 The method according to any one of claims 1 to 6, characterized in that the humidifying agent is a mixture of water and a wetting additive. Claim 8 The method according to any one of claims 1 to 7, characterized in that the plurality of humidifying units (18, 20) are each formed as a spray bar. Claim 9 The method according to any one of claims 1 to 8, characterized in that the plurality of humidifying units (18, 20) are arranged downstream of a cooling unit (26) which is arranged downstream of the drying unit (12).

10. The method according to any one of claims 1 to 9, characterized in that the web (4) is not dried between the first humidifying unit (18) and the second humidifying unit (20).

11. The apparatus (2) has a plurality of apparatus sections (24), each comprising a processing unit (10) and a drying unit (12) following the processing unit (10). The method according to any one of claims 1 to 10, characterized in that at least one of the plurality of humidifying units (18, 20) is arranged downstream of each drying unit (12), whereby the web (4) is humidified each time drying is carried out.

12. The method according to any one of claims 1 to 11, characterized in that the apparatus (2) is a printing machine in which the processing unit (10) is a printing unit (28) for printing the web (4) with ink, and the drying unit (12) is a print drying unit (30) for drying the ink.

13. The apparatus (2) has a primer application device (32) and a primer drying unit (34) upstream of the printing unit (28), and a varnish application device (36, 40) and a varnish drying unit (38) downstream of the printing unit (28). Two of the plurality of humidifying units (18, 20) are arranged between the primer drying unit (34) and the printing unit (28). Another one of the plurality of humidifying units (18, 20) is arranged between the print drying unit (30) and the varnish application device (36, 40). The method according to claim 12, characterized in that two other of the plurality of humidifying units (18, 20) are arranged downstream of the varnish drying unit (38).

14. The apparatus (2) has an analog varnish application device (36) downstream of the printing unit (28), and a digital varnish application device (40) downstream of the analog varnish application device (36). One of the plurality of humidifying units (18, 20) is arranged between the printing unit (28) and the analog sizing device (36). Another one of the plurality of humidifying units (18, 20) is arranged between the analog sizing device (36) and the digital sizing device (40). Another one of the plurality of humidifying units (18, 20) is arranged downstream of the digital sizing device (40). The method according to claim 12, characterized in that.

15. In an apparatus (2) for processing a paper web (4), - a processing unit (10) for processing the web (4); - a drying unit (12) for drying the web (4), arranged downstream of the processing unit (10); - a plurality of humidifying units (18, 20) arranged downstream of the drying unit (12), the plurality of humidifying units (18, 20) being designed to perform multi-stage rehumidification of the web (4) with a humectant by pre-humidifying the web (4) using a first humidifying unit (18) of the plurality of humidifying units and performing post-humidification of the web (4) using a second humidifying unit (20) of the plurality of humidifying units after a minimum residence time (T). - The minimum residence time (T) is selected such that the web (4) in the second humidifying unit (20) has an increased absorption rate of the humectant based on the pre-humidification. An apparatus (2), characterized by this.

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