EP1225274B1 - Process for treating a web of material and calender - Google Patents

Description

The invention relates to a method for treating a material web, in which the material web is guided through a plurality of soft nips in a roll stack and pressurized there. Furthermore, the invention relates to a calender with a roll stack of a plurality of rollers forming a plurality of soft nips in which a material web is pressurized.

The invention will be described below with reference to a paper web as an example of a material web. However, it is also applicable to other material webs.

Paper webs are guided in the course of their manufacture by calenders and there pressurized and optionally also elevated temperature. On the one hand, this calendering achieves a compaction of the paper web, on the other hand one can achieve certain surface properties, in particular smoothness and gloss.

In calenders, as they are known, for example, from DE-A-195 08 349, center rolls are used, which are covered with a plastic covering. With prolonged use of these calenders, there is an undesirable vibration problem, a so-called barring phenomenon. This barring phenomenon is characterized by strip-shaped markings of the elastic rollers, more precisely, their plastic coverings. These markings are formed when the paper web is being treated in the paper web. As soon as these markings become visible there, the paper web is considered as scrap.

The invention has for its object to reduce the barring phenomenon.

This object is achieved in a method of the type mentioned above in that the material web is pre-smoothed before passing through the nips by being passed without slip through an additional nip formed between two soft rolls.

Here, the following considerations apply: A roll stack, which is formed from several rolls, has a large number of natural frequencies. This does not mean the natural frequencies of the rolls per se, such as bending natural frequencies, but the natural vibration modes that result from the oscillating roll masses on the spring and damper systems of the interposed plastic linings.

A running calender generates exciter forces whose frequencies are composed of multiples of the rolling speeds. These excitation forces can result from inhomogeneities, anisotropies or geometrical errors (roundness) be justified. Also, paper thickness variations of the calender passing through the paper web can excite the roll stack. If one of these excitation frequencies hits a natural frequency, then the vibration system responds with increased oscillations. Due to the large number of possible pathogens and the large number of possible modes of natural vibration, these resonance points can not practically be avoided in terms of design. In general, the vibration system is so strongly damped and the excitation forces are so small that the resulting oscillatory movements are not directly disturbing. Over a more or less extended period of time, however, these oscillatory movements are reflected in the plastic linings of the elastic rollers. As a rule, the nearest integer multiples of the oscillation frequencies are impressed as patterns on the rollers. This results in a feedback of the oscillation. The oscillations then increase exponentially. They express themselves on the one hand in an increased sound level (up to more than 115 dB (A)) and on the other hand in periodic thickness variations of the continuous paper web. In practice, different periods of time are observed in which these feedback phenomena (barrings) form. Most days or weeks pass before this phenomenon has grown so strongly that it disturbs the production process. Under unfavorable conditions, however, it is possible for such a feedback effect to occur after just a few minutes. From this it can be concluded that these must be different imprinting mechanisms. It is believed that the most frequently occurring long-term phenomena in practice are due to periodic wear or periodic compaction or fatigue of the plastic covering.

In order to avoid the above-described feedback effect in the long-term range, the generation mechanism of the periodic surface waviness must be permanently disturbed. It must be avoided that a constant integer pattern with a frequency close to the natural frequency of the system is impressed on the rollers. This can in principle be achieved by a speed variation of the calender. On the one hand, a speed variation leads to a permanent change in the excitation frequencies and, on the other hand, at a constant natural frequency to a different number of impressions in the plastic coverings. However, such an approach is conceivable only with calenders that are not directly integrated into the production process of the paper machine, so-called offline calenders. In these, a permanent speed variation is within certain limits, e.g. ± 5%, around one operating point possible, without disturbing the production process noticeably. However, with calenders integrated in the paper machine's paper flow (on-line machines), short-term speed changes are not tolerable. In this case, the vibration system must be influenced by other measures.

The invention is based on the assumption that a paper web entering the calender is never completely free of variations in basis weight or thickness before treatment. In addition, a paper web is still very rough. If one analyzes the fluctuations on their frequencies, then one usually detects a broadband noise, in which all frequencies are contained. Thus, an excitation of one or more natural frequencies of the roller system through the paper web is inevitable. The calender reacts at these natural frequencies with an increased amplitude, which initially, as already described above, is not disturbing. Over time, however, the increased amplitudes are reflected in the plastic pads, and thus there is feedback in the vibration system. Although such feedback can not be completely avoided, the additional step of pre-smoothing the web of material allows for the amount of time needed to make the markings visible. If a barringfreier period is reached, which is greater than the surface wear-related life of the soft rolls in the calender, then this task is solved. If one Vorglättet the web prior to their entry into the stack of rolls, then one first reaches a reduced excitation amplitude of vibrations, which is caused by the roughness of the material web. Due to the leveling caused by pre-smoothing, it is even possible that some exciter frequencies disappear almost completely. Although the Barringerscheinung is not eliminated. However, the formation of the barring takes much longer. It has been found that the operating time necessary for barrings to become visible is greater than the service life of the soft pad provided with the resilient pad, the service life being due to surface wear.

The use of an additional nip formed between two soft rolls is a particularly simple means for pre-smoothing the web. Although such a nip is generally considered to be practically ineffective for the treatment of a material web. But it is enough to give the paper web a "provisional" or provisional smoothing, which leads to the formation of a barring phenomenon takes much longer than before. If you pass the web without slippage through the additional nip, then there is no sliding area in the contact zone between the web and the two soft rolls that form the nip. The entire contact zone between the rollers and the material web, which is relatively wide due to the elastic surface covering of the soft rolls and the associated flattening of the rolls in the nip, forms an adhesive area. Since wear can only occur in connection with friction or relative movement, such as in the soft nips of a calender, in which a hard runs against a soft roll, the periodic wear is excluded by a superimposed oscillatory motion. The rollers level the rough web without wearing it out. In the end, with a pre-smoothed material web, fewer exciter frequencies and lower excitation amplitudes are produced, which are introduced into the calender.

Preferably, one drives the two rolls forming the additional nip. This makes it possible to reduce the wear of these two rolls. The smaller the wear, the longer the two rolls forming the additional nip can be used. In addition, however, the risk is kept small that one makes an additional barringverursachende impression in the web in these two rolls.

Preferably, the web is treated the same on both sides. So you use on both sides of the web like rolls, ie rolls with the same diameter and the same surface properties.

By such a symmetrical treatment of the web, the risk is minimized that the additional nip forming rollers are damaged by the web excitations similar to the calender rolls.

Preferably, one arranges the additional nip outside a plane in which the nips of the roll stack are arranged. This minimizes the risk of vibrations being transferred from the calender itself to the rollers of the additional nip.

The object is also achieved by a calender of the type mentioned above, namely, that an additional nip is provided outside of the roll stack, which is formed by two soft driven rollers with the same surface properties, which have a peripheral speed, the speed of the continuous web equivalent.

As discussed above in connection with the method, the additional nip serves to pre-smooth the web of material. By Vorglätten is achieved that less exciter frequencies and reduced exciter amplitudes are entered into the actual roll stack. The material web is thus guided without slippage through the nip, so that the rollers that form this additional nip, can not wear out.

It is preferred that a trajectory of the additional nip is provided in a nip which is bounded by an end roll. This nip forms the first nip of the roll stack, which passes through the material web. It is therefore necessary to smooth the material web before it is treated in one of the nips of the roll stack.

This ensures that in each nip an already pre-smoothed material web is present and thus vibration risks are kept small.

Preferably, the two rolls forming the additional nip are arranged laterally offset in the web running direction relative to the roll stack. This has the advantage that vibrations in the calender in the roll level can not propagate to the rolls, which form those additional nip. Although complete decoupling between the roller stack and the two additional rollers will not be achieved. However, if already the vibration direction can no longer hit the location of the additional nip, sufficient damping is ensured.

Preferably, the two rollers forming the additional nip have the same diameter and the same surface properties. This ensures that the web is treated evenly or symmetrically from its two sides. Due to the pre-smoothing in the additional nip so no additional disturbances one-sided type are entered into the web.

It is particularly preferred that both rollers are driven with the same drive power. This again gives symmetrical conditions in the nip.

The invention will be described below with reference to a preferred embodiment in conjunction with the drawings. Herein, the only FIG. Shows a calender.

A calender 1 has a roll stack of several, in the present case eight, rolls 3-10, which are arranged in a stiffening 11. Here, the rollers 3, 5, 6, 8, 10 are formed as "soft rolls", i. they have an elastic surface covering, preferably made of a plastic. The remaining rollers 4, 7, 9 are formed as so-called "hard" rollers, i. the surface of these rollers 4, 7, 9 consists of a largely unyielding metal. In a manner known per se, the intermediate or middle rollers 4-9 are mounted on levers 12. When the lower end roll 10, which is supported by a piston-cylinder drive 13, is lowered, then the rolls can move away from each other.

The rollers 3-10 form therebetween nips 14-20, of which the nips 14, 15 and 17-20 are formed as soft nips, while the nip 16 is formed as a change nip. The soft nips are limited by a soft roller and a hard roller, while the alternating nip is limited by two soft rollers.

Through the nips 14-20 a web of material 21, in this case a paper web, guided, which is unwound from a roll 22. However, it is also possible for the material web 21 to come directly from a production machine, for example a paper machine.

The roll stack 2 is arranged at an angle of about 45 ° to the vertical.

The roll stack 2 may also have less or more than the illustrated eight rolls. However, it should have at least four rollers, so that at least three nips can be formed.

Before the first nip 14, i. the nip, which is delimited by an end roll 3, an additional nip 23 is provided, which is formed by two soft rolls 24, 25. The two soft rolls 24, 25 have the same structure, i. They have the same diameter and the same surface and are otherwise structurally identical. The two rollers 24, 25 each have a drive 26, 27, so are driven. Here, the drive power is distributed symmetrically. The drive is effected so that the rollers 24, 25 have a circumferential speed which corresponds to the speed of the material web 21. In this case, the contact between the web 21 and the two rollers 24, 25 in the nip 23 is completely slip-free, i. the contact zone has no sliding area. The entire contact zone forms a detention area. Since wear can only arise in connection with friction or relative movement, such as in a calender, where a hard runs against a soft roll, the periodic wear is excluded by a superimposed oscillatory motion.

In the additional nip 23, the web 21 is flattened by the two soft rolls 24, 25, i. a rough material web is pre-smoothed without the two rollers 24, 25 itself wear. As a result, fewer exciter frequencies and lower exciter amplitudes are introduced into the calender by means of a pre-smoothed material web 21.

As can be seen from the drawing, the two rollers 24, 25 are arranged outside a press plane of the roll stack 2, that is, the axes of the soft rolls 24, 25, which form the additional nip 23 lie not in the same plane as the axes of the rolls of the roll stack. This is based on the consideration that the vibrations are particularly critical, which propagate in the direction of the press plane, ie the plane in which the axes of the rollers 3-10 of the roll stack 2 are arranged. If one then takes out the two rollers 24, 25 of the additional nip 23 from this direction of propagation, one has already achieved a principal decoupling of the vibrations in the roll stack 2.

Claims (9)

1. Process for treating a web of material (21), in which the web of material (21) is led through a plurality of soft nips (14, 15, 17-20) in a roll stack (2) and has pressure applied to it there, characterized in that, before passing through the nips (14, 15, 17-20), the web of material is pre-calendered by being led without slippage through an additional nip (23), which is formed between two soft rolls (24, 25).
2. Process according to Claim 1, characterized in that the two rolls (24, 25) forming the additional nip (23) are driven.
3. Process according to Claim 1 or 2, characterized in that the web of material (21) is treated equally on both sides.
4. Process according to one of Claims 1 to 3, characterized in that the additional nip (23) is arranged outside a plane in which the nips of the roll stack (2) are arranged.
5. Calender having a roll stack (2) comprising a plurality of rolls which form a plurality of soft nips (14, 15, 17-20) in which pressure can be applied to a web of material (21), characterized in that, outside the roll stack (2), an additional nip (23) is provided, which is formed by two soft driven rolls (24, 25) having the same surface characteristics, which have a circumferential speed which corresponds to the speed of the web of material (21) passing through.
6. Calender according to Claim 5, characterized in that a web run is provided from the additional nip (23) into a nip (14) which is bounded by an end roll (3).
7. Calender according to Claim 5 or 6, characterized in that the two rolls (24, 25) forming the additional nip (23) are arranged to be offset laterally with respect to the roll stack (2) in the web running direction.
8. Calender according to one of Claims 5 to 7, characterized in that the two rolls (24, 25) forming the additional nip (23) have the same diameter.
9. Calender according to Claim 8, characterized in that the two rolls (24, 25) are driven with the same drive power.

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