ERIKSSON, Tore (Östra Rud, Klässbol, S-671 95, SE)
KARLSSON, Håkan (Vallgatan 38, Säffle, S-661 43, SE)
ERIKSSON, Tore (Östra Rud, Klässbol, S-671 95, SE)
1. A coating device for two-sided coating of paper and/or paperboard, comprising a first coating device (1) arranged to coat a first side of a paper web (3) and a second coating device (2) arranged to coat a second side of said paper web (3), which first coating device (1) comprises a first metering element (4) in the form of a rotating rod and which second coating device (2) comprises a second metering element (5) in the form of a coating blade, characterised in that said second metering element (5) is provided with a soft contact surface (52) against the paper web (3), which contact surface (52) is arranged to have a surface hardness in the range of 50 Shore A - 95 Shore A.
2. A coating device according to claim 1, characterised in that said first metering element (4) is constituted by a hard rotating rod.
3. A coating device according to claim 1, characterised in that said first metering element (4) is constituted by a soft rotating rod.
4. A coating device according to any one of the preceding claims, characterised in that said soft contact surface (52) of said second metering element (5) is arranged to have a surface hardness of 60 Shore A - 95 Shore A.
5. A coating device according to claim 4, characterised in that said metering element (5) is made of a material that has a rebound of at least 50% according to ASTM standard D2632.
6. A coating device according to any one of the preceding claims, characterised in that the angle of incidence α between said second metering element (5) and the paper web (3) is 10-45°, preferably 15-25°.
Coating device for two-sided coating of paper or paperboard
The present invention relates to a coating device for two-sided coating of paper and/or paperboard, comprising a first coating device arranged to coat a first side of a paper web and a second coating device arranged to coat a second side of said paper web, which first coating device comprises a first metering element in the form of a rotating rod and which second coating device comprises a second metering element in the form of a coating blade.
PRIOR ART AND BACKGROUND
A prior art method and device for the coating of a moving paper or paperboard web is known from WO8504824 in which the web is conveyed vertically upwards through closed chambers on each side of the web, which chambers hold a coating slip. The upper delimitation for the chamber on one side consists of a rotating rod that is mounted in bearings in a longitudinal rod cradle. The delimitation for the chamber on the other side consists of a coating blade. The moving web is conveyed through the nip formed between the blade and the rotating rod. The final dosing of coating slip takes place in the nip. The surface of the rotating rod and the contact surface of the coating blade is made of a hard material, i.e. steel or harder.
When the above mentioned coating method is used problems sometimes arise, which is evident from the following explanation:
All paper or paperboard webs have more or less irregularities in the surface structure. These surface structure variations consist of surface "peaks and valleys" or more or less protruding particles of fibres or other contraries.
The above mentioned properties of the material web result in problems in connection with the described process since the two metering elements that have hard surfaces will give a "filling" effect that results in a patchiness of the coating layer, i.e. the surface valleys are filled with coating slip while the surface peaks are uncovered. If the surface has particles that pass through the nip, between the rod and the blade, the blade will be lifted by the particle, which results in a transversally extending line with a larger amount of coating slip on both sides of the particle. If the particle does not pass the nip
but gets stuck in the nip, the result will be a longitudinal line in the coating layer or that the web is torn off.
BRIEF ACCOUNT OF THE INVENTION It is an object of the present invention to eliminate, or at least minimize, the above mentioned problem, which is achieved by a method and a device, respectively, according to the appended claims.
Thanks to the invention a method and device is achieved that substantially eliminates the above mentioned negative aspects. Completely surprisingly it has also been shown that the use of a method according to the invention also leads to a decreased unequal- sidedness which is against the prevailing belief of persons skilled in the art. Thanks to the invention it is accordingly possible to achieve a substantially problem free two-sided coating with good equal-sidedness of the two surfaces.
According to further aspects of the invention: the first metering element is in the form of a hard rotating rod, whereby it has been possible to achieve particularly good results in connection with a device according to the invention. - the coating blade is made of a material with a rebound of at least 50% according to ASTM standard D2632, whereby particularly good results have been achieved in tests. the angle of incidence α between said second metering element and the paper web is 10-45°, preferably 15-25°, whereby it has been possible to achieve particularly good results in tests.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention will be described in greater detail with reference to the attached drawing figures, of which: Fig. 1 shows the principles of a device according to a first embodiment of the invention, as seen from the side, Fig. 2 shows a magnification of a part of Fig. 1, and Fig. 3 shows a modified embodiment according to the invention.
Fig. 1 shows a side view over a coating arrangement according to the invention, consisting of a first coating device 1 in the figure arranged to the right of an upwards
moving paper web 3, and a second coating device 2 in the figure arranged to the left of the paper web 3. The first coating device 1 is arranged to apply a first coating layer 9 on one side of the paper web 3, and the second coating device 2 is arranged to apply a second coating layer 8 on the other side of the paper web 3.
The paper web 3 moves upwards in the figure. Accordingly, the paper web 3 enters from below, as seen in the figure, without any coating slip. Fountain applicators 6, 7 on both sides of the paper web 3 apply a coating layer 8, 9 on the respective sides of the paper web. Conventional protective/recycling plates 10, 11 are arranged in connection with the fountain applicators 6, 7. Thereafter, there is a short distance (preferably in the range of 200 mm - 2000 mm, corresponding to a retention time of 0.01 sec - 1.0 sec, partly depending on the quality of the paper web) before the paper web 3 with the onto it sprayed coating layers 8, 9 is led to a first wiper 4 arranged on one side of the paper web and a second wiper 5 arranged on the other side of the paper web 3.
The first final metering element 4 is constituted by a hard rotating rod that is rotary mounted in a holder body 15. The holder body 15 is in its turn fixed in a support structure 14 preferably provided with an adjustment mechanism that among other things can be arranged to control the pressure at which the rotating rod 4 is held in the holder body 15.
On the other side of the paper web 3, in relation to the rotating rod 4, a second final metering element 5 is arranged, comprising a blade 50 with a soft covering 51 with a resilient contact surface 52. The body material is preferably chosen such that the rebound value is at least 50% according to ASTM standard D2632, more preferably in the range of 60-90%. In a manner that is conventional per se, the blade is held in a holder device 12 that is provided with a groove 13 for positioning of the blade 50. The holder device 12 is arranged to be adjustable, such that the linear load at which the blade 50 bears against the paper web 3 and via the latter bears against the rotating rod 4, can be varied. With the purpose of achieving the surprisingly good results that have been accomplished by aid of the invention, the line load should be in the range of 0.25 N/lmm - 3.0 N/lmm, preferably 0.5 N/lmm - 2.0 N/lmm. Moreover, the velocity of the web 3 should preferably be in the range of 0.8 m/sec - 33 m/sec, preferably 1.6 m/sec - 25 m/sec. In addition, the hardness of the bearing surface of the soft blade 50 should be in the range of 50 Shore A - 95 Shore A, preferably 60 Shore A - 90 Shore A in order to achieve said surprising results.
Fig. 3 shows a modified embodiment of a device according to the invention. One modification consists of using another form of a soft metering element 5 than the one shown in Figs. 1 and 2. Instead of using a per se known conventional blade, a new type of metering element is used that is described in detail in WO 2007/061378, which is hereby incorporated herein by reference. Briefly, it is clear from the figures that instead of a conventional blade 50 with a soft body 51 , a rotary unit 53 arranged at the blade 50 is used, at which unit there is arranged at least one soft blade device 54 that has the soft contact surface 52.
According to the performed tests it has surprisingly been shown that, instead of an increased unequal-sidedness, the unequal-sidedness will decrease when a soft blade is used against a rotating rod. This is true for the coating of paper as well as paperboard with basis weights of up to about 450 g/m 2 .
The invention is not limited to what has been described above but may be varied within the scope of the appended claims. It is realised among other things that a hard rotating rod as well as a soft rotating rod can be used in connection with the invention, the choice of rod being affected by the existing circumstances. It is also realised that a variety of materials and material combinations can be used in order to achieve the desired material properties with the purpose of fulfilling the required function of the soft part of the metering element 5.