FR2534609A1 - SQUEEGEE BLADE FOR CONTROLLED APPLICATION AND SMOOTHING OF A COATING COMPOSITION ON A PAPER STRIP - Google Patents

Abstract

THE PRESENT INVENTION RELATES TO A SQUEEGEE BLADE FOR CONTROLLED APPLICATION AND SMOOTHING OF A COATING AGENT ON A MOVING STRIP OF PAPER. ACCORDING TO THE INVENTION, THE ENTRY ZONE 9 OF THE BLADE 1 INTENDED FOR AN ENGAGEMENT WITH THE STRIP OF PAPER AND AT LEAST PART 4A OF THE BEVEL SURFACE 4 DOWNSTREAM OF THE SAID ZONE IN THE DIRECTION OF THE COURSE 6 OF THE STRIP OF PAPER 7, AS WELL AS PART 5C OF THE ENTRY SIDE 2 OF THE BLADE 1 UPSTREAM OF THE ENTRY ZONE 9 ARE PROVIDED WITH A THIN WEAR-RESISTANT SURFACE COATING 5; THE COATING 5 PREFERABLY HAS A TOTAL THICKNESS OF 0.25 MM OR LESS AND IT CONSISTS OF A MATERIAL OF CERAMICS, METAL OXIDES OR METAL CARBIDES; THE COATING 5 IS PREFERREDLY APPLIED IN THE FORM OF A SERIES OF SUPERIMPOSED THIN COATS, EACH BEING APPLIED BY SPRAYING THE COATING MATERIAL IN THE MELTED STATE. THE INVENTION APPLIES IN PARTICULAR TO THE PAPER INDUSTRY.

Description

1 2534609

  The present invention relates to a squeegee blade for controlled application and smoothing of a coating composition on a paper web continuously.

moving.

  Developments in paper coating technology tend towards coating facilities with higher and higher production rates Currently, there are facilities with paper web speeds of up to 1500 m / min and more, the width of the paper web can sometimes reach 8 meters or more. Such installations require a high accuracy of the squeegee used for such coating, in particular the fact that the amount of the coating composition applied does not usually exceed 25 mm. g / m 2 of the paper surface In this area, it is important that the layer of the coating composition on the strip of

paper is perfectly uniform.

  The result of the coating of such a coating installation is significantly affected by the length of the blade bevel in the direction of travel of the web with respect to the spring force applied to the flexible blade. The applied surface pressure, that is, the pressure per unit area exerted by the bevel on the surface of the paper, known as the surface specific pressure, is of decisive importance for the amount of the coating composition remaining on the web. Therefore, there is a specific relationship between the extent of the bevel and the elastic properties of the blade. To prevent changes in the extent of the bevel surface due to wear during the coating process, such Sleeping blades are therefore now normally pre-sharpened to ensure a corresponding extent and angle of the bevel as precisely as possible to the actual conditions prevailing during the It is also important that the flexibility of the blade is adjusted to the prevailing spring force so that the blade adjusts to any unevenness of the strip and also allows the passage of defects such as sticks or

  fairly thick pellets in the paper strip.

  Conventional squeegee blades provide a good coating result, but they have the disadvantage of being subject to rapid and uneven wear, and therefore need to be replaced while only a small portion of the blade material has been worn. practical reasons, this is due to the fact that the coating blade, which generally cooperates with one side of the coated paper web and a rubber-coated carrier roll

  the other side of the paper strip, is wider than that

  On the other hand, dispersions of clay pigments in water are usually used to coat the paper, which means that the coating composition and the paper web itself have an extremely abrasive effect on the cutting edge of the blade. Furthermore, the parts of the blades placed beyond the paper web, and thus simply in contact with the rubber-coated backing roll during the coating process, are subject to negligible wear. This means that the cutting edge of the blade , initially straight, will wear out and become somewhat concave along the sections where the paper web passes and after a while, the coating through the web will become uneven. Therefore, in practice, the conventional blades need to be replaced at after a few hours of use only This is expensive, not only from the material point of view but also because it requires expensive stops followed by new the periods

start-up.

  Significant work has been done to try to increase the duration of use of conventional coating blades and the properties of the traditional blade have been optimized by an appropriate choice of

  steel composition and by treatments such as annealing.

  In theory, it may be possible to increase the service life of such a bedding blade by using a material for the board, which in itself is more wear-resistant than spring steel. Conventional materials apparently suitable such as hard metals and cermets are not always sufficiently flexible In reality, such materials are often extremely brittle and will therefore easily break down due to the stresses that normally occur from time to time during of

  the use of a squeegee blade.

  In the technical fields other than coating, attempts have already been made to solve wear problems by attaching pieces or strips of a more wear-resistant material to the support material used. Hard chrome plating or a plating of another metal has also been suggested as a means of giving to support materials which inherently are soft a better wear surface. Intensive experiments carried out to solve the problem of wear in Similarly, similar solutions have proved unsuccessful with the thin coating blades used in papermaking. These known solutions have proved difficult to use technically with the thin material used for the production of paper. blade and it has also been found that the desirable properties of the thin base material of the blade, such as flexibility, needed for good coating results, were significantly reduced by the measures adopted to improve the wear resistance It is of course essential that a coating blade having a better wear resistance in comparison with conventional blades not only has a longer duration of use, but moreover gives a perfectly satisfactory result of

sleeping, if we must adopt it.

  The present invention relates to an improved scraper blade for the specified purpose, capable of producing a satisfactory coating performance and also having an extended duration of use in comparison with

conventional blades.

  According to this invention, there is provided a scraper blade for controlled application and smoothing of a coating composition on a moving paper web, said blade consisting of a flexible strip of steel where an active area of the blade is provided a thin surface coating with respect to the thickness of the steel strip, having a

  superior wear resistance than the steel blade.

  In a preferred embodiment of the invention, the wear-resistant coating is staggered on the blade by sequentially applying a number of thin layers of said heat-resistant material to one another over the other. wear, the material being put into

  spot by melt-spraying.

  Embodiments of the Invention Hereinafter are described by way of example with reference to the accompanying drawings in which: Figures 1 to 8 are schematic side views illustrating respective embodiments

different from the invention.

  The squeegee blades shown in the drawings are for use in coating moving paper webs In such a coating process, the paper web is generally forced to partially surround a rubber-clad roll of support, rotating in the direction By means of a rotating application roll, partially immersed in a pit of a coating composition, an excess of the coating composition is applied to one side of the paper web and a film blade. squeegee, arranged in principle in a blade holder of a conventional type, is pressed against the surface of the strip coated in this way, in order to smooth the layer of applied coating composition The contact angle of the blade against the strip , and the pressure at which the blade bears against the band are preferably adjustable, and a suitable adjustment of the contact pressure and the contact angle of the blade against the pappier strip makes it possible to determine, with great precision, the quantity of the coating composition remaining on the strip. The squeegee blades shown in the drawings are used in the same way as conventional squeegee blades in a

such a method.

  Only the free active end of the respective squeegee blade is shown in each of Figures 1 to 8. In each of the embodiments to which these figures relate, the squeegee blade comprises a strip of a steel backing material having parallel and opposite major faces 2 and 3 Each of the blades shown has a working surface 4 by which, in use, it engages the paper web The extent of the surface 4 is indicated in the drawings by a broken line 8 and comprises also part of an entrance area 9 indicated by a thick and unbroken line, which is the area o, in use, one side of a paper strip 7, moving

  in the direction of arrow 6, comes into contact with-

  The other side of the paper web generally engages a support roll, not shown, which rotates in the direction of the arrow 6, although in some arrangements the web is not necessarily supported by such a roll on the entire web. its area of engagement with the squeegee blade The portion of the flat surface 2 facing the paper web 7 has a thin coating 5 of a more wear-resistant material than the material 1 of the blade The composition and the structure of the coating 5 will be described in more detail below. The working surface of each squeegee blade is thus designed to cooperate, in use, with the paper web through a layer of paper coating, not shown for simplicity, between the strip and said work surface The coating 5 is thus applied to the flat surface 2 of the blade 1 which first meets the paper web and faces

  The flat surface 2 will also be called,

  After, the entry side of the blade The coating 5 of the blade is confined to a coating zone, whose width, along the inlet side 2, is preferably at most 20 mm.

  Figure 1, the coating area is designated 5 a.

  In the embodiments of Figures 1, 2 and 4 to 6, the working surface 4 is formed by a bevelled surface extending from a free side face of the blade to a parallel portion. at the face 2, of the surface of the coating 5 on the face 2 and which is slightly inclined, to

  an obtuse angle with respect to the face 2.

  In the following description of these modes of

  realization, the work surface is also called the

bevel 4.

  In the embodiment of Figure 8, the working surface 4 is formed of a slightly concave surface which extends obliquely with respect to the faces 2 and 3, from the surface of the coating 5 on the face 2 to the coating 5 on the face 3, the working surface 4 in this embodiment, thus forming the face

side of the blade.

  In the arrangements of Figures 1, 2 and 8, the surface 4 intersects the support material and thus, in these embodiments, the total work surface comprises both a portion 4 formed by the coating 5 and a drift portion In FIG. 1, the bevelled portion 4 is thus in the same plane as the remainder of the bevelled surface 4 and comprises the inlet zone 9 and the end 5b of the most close to the free edge of the blank 5b refers to the flat surface of the coating 5 of the blade parallel to the face 2,

upstream of the entrance area 9.

  Figure 2 shows a variant of the blade

  1, where the coating 5 upstream of the inlet zone 9 and / or in the inlet zone 9 is advantageously rounded. This rounding can be obtained, for example,

  by-post proper grinding of the coating.

  In the embodiment of the invention shown in Figures 3 to 7, the entire work surface of the blade is formed by the wear-resistant coating. Figure 3 shows a dragging blade o the surface

7 2534609

  working is formed by the surface, substantially parallel

  on the face 2, the coating 5 on the face 2.

  FIG. 4 shows a variant of the blade shown in FIG. 3, where the inlet zone 9 and the adjacent remainder of the surface 4 are formed of a bevelled surface, but do not cut the support material and ri: It is formed by angularly grinding a section of the coating. The transition from the inlet-side coating 5c to the entry zone 9 is preferably rounded in the

  embodiment of Figures 1, 2, 4 to 6 and 8.

  In the embodiments of Figures 5, 6 and the wear-resistant coating on the inlet side 2 of the blade has been deposited on a bevelled surface 10 previously formed on the steel strip 1 and extending from the face 2

to the free edge of the strip 1.

  In the embodiment of Figure 6, the surface 5c of the coating 5 upstream of the transition to the inlet zone 9, has received a suitable convex shape per po;

grinding of the coating.

  In the embodiment of FIG. 7, the working surface 4, formed completely by the coating 5, has received a roughly rounded shape by means of a step-by-step grinding, to form a surface part, upstream of the zone 9, parallel to the face 2, a slightly inclined surfai part comprising the inlet zone 9, and a more inclined surface portion downstream of the inlet zone 9 Thus, in the context of the In the invention, the bevelled surface 4 can be post-ground to give a suitable surface

for any specific purpose.

  In the embodiment of Figure 8, the working surface 4 comprises an inlet portion 4a, an outlet portion 4c and a central portion 4b which is between said inlet portion 4a and said portion The inlet portion 4a and the outlet portion 4c are formed of the wear resistant material, while the central portion 4b comprises the unsupported support material of 1 blade. The blade of Figure 8 is preferably manufactured by applying a wear-resistant coating to the flat and opposite surfaces of a strip of the support material and then the strip is longitudinally cut along the coated regions and the cut edge is ground or

  sharpened to form the oblique working surface 4.

  In each of the embodiments described, the blade support material advantageously comprises a steel strip having a thickness of 0.10 0.70 mm and a hardness of at least 22 mm. the steel strip is usually made of surface-hardened carbon steel of the spring steel type, for example having the following composition

C 1.02%

  If 0.20% Mn 0.40% Pmax 0.03% Smax 0.025% The material used for the blade is thus manufactured with the above-mentioned cold-rolled and hardened steel strip and is generally available from our days, to

  standard dimensions such as 0.254, 0.305 or 0.381 mm.

  The coating 5 has a maximum thickness of 0.25 mm and a minimum total thickness of 0.02 mm and is formed

  several thin layers that are applied to each other

  above the others in succession, by spraying the melt-resistant material, each thin layer, in this case, having a thickness of 0.002 0.030 mm, by

example.

  In some cases, particularly if the coating area is not too wide, it is possible to obtain an extremely high wear resistance by applying a somewhat thicker coating, having a total thickness

about 0.35 mm.

  More particularly, each thin layer is applied by a thermal spraying technique, where the molten coating material is sprayed, against the surface to be coated. In this case, plasma spraying or flame are suitable methods. Plasma, preferable in many cases, a gas is so intensely heated by an arc that it reaches the plasma state A plasma state, the gas is sprayed through a tubing into a jet, and the material to be used for the coating is fed to the jet of plasma in powder form by a carrier gas The powder thus melts immediately and it is thrown by the jet, in the molten state, on the surface to be coated To prevent the extremely thin steel strip which is coated is not damaged by heat, the coating, very thin in itself, is formed in layers, and is left or forced to cool after

each step.

  Even with the wear-resistant coating, the preferred embodiments of the scraper blade provided according to the invention have substantially the same flexibility as the original, uncoated blade of steel, and a result

  perfectly satisfactory sleeping is thus guaranteed.

  The staggered accumulation of the very thin surface coating as proposed, ensures good flexibility and

  a minimum fragility of the coating layer itself.

  To obtain a coating having the finest quality with respect to the uniformity of the coating of the paper web, the coating of the front blade and / or in the entry zone and possibly also on the subsequent bevel surface is formed by a bevelled or convexly rounded surface This means that the coating of the blade in these regions can receive a curved shape

  without sharp or sharp edges, this being advantageously ob-

  held by an equivalent post-grinding of the coating in stages.

  With regard to the structure of the wear-resistant surface coating, before the first thin layer is applied to the support material, the support material must be subjected, in the surface area to be coated, to a treatment. appropriate surface preparation such as a light carborundum spray, preferably

  immediately before application of the first thin layer.

  In some cases, it may be appropriate to apply a layer of a binder, such as a nickel alloy or the like, between the pretreated surface of the support material and the first layer. The various thin layers are then applied in succession so as to not to affect the flexibility and the original surface regularity of the band

steel.

  Finally, the finished surface coating can, if desired, be grinded to produce a finish

less than 3.0 g Ra.

  The wear-resistant material used in the coating according to the invention may comprise cermets, one or more metal oxides or one or more metal carbides or combinations thereof. Of course, the wear-resistant coating material may be selected. the most suitable in any particular case, taking into account, for example, the quality of the desired paper coating in individual cases Although some coating materials, such as chromium oxide, offer good wear resistance It has been noted that such a coating may, after a certain period of use, produce a slight deterioration of the coating. However, this deterioration is negligible in most cases.

commercialized coatings.

  Surprisingly, the coatings of the blade consisting mainly of alumina have been found to be particularly suitable for coating a high quality paper web which in some cases has to be achieved particularly well. the alumina slide (A 1203) with a small amount of another metal oxide, such as

  titanium (Ti O 2), in respective preferred proportions of 97 and 3%.

  The following experiments with squeegee blades according to the invention confirm the desired improvement.

  compared to conventional blades.

  Experiment 1 A blade according to the invention, having a wear-resistant surface coating formed of alumina and titanium oxide was used to coat a wood-free printing paper The speed of the paper web was 500 m / min and the coating composition used was a dispersion in water of 20% kaolin and 80% carbonate

of calcium.

  The blade could be used for 30 hours with

a good sleeping result.

  A conventional coating blade, without wear-resistant coating, used under equivalent conditions, had to be changed after 4 hours

use.

  EXPERIMENT 2 A blade according to the invention, with a wear-resistant coating formed of alumina, was used to coat a wood-free paper, a coating composition based on a dispersion of a pigment in water The speed of the paper tape was 400 m / min The blade gave a perfectly satisfactory sleeping result on

a period of 60 hours.

  A control experiment using a conventional non-wear-resistant coated blade has shown that this conventional blade needs to be replaced at the end

  a usage time of 8 hours.

  EXPERIMENT 3 The experiment 2 above was repeated with a blade according to the invention having a chromium oxide coating. From the point of view of wear, this blade could be used during

  considerably more than 60 hours.

  However, after a few hours of use, the result of sleeping was slightly worse, but

  probably quite acceptable in most cases.

  As previously mentioned, the width of the wear-resistant coating layer should not exceed 20 mm. In practice, the coating layer should be as narrow as possible to reduce the material consumption and eliminate the risk of cracks in the layer during the displacement of the support material Therefore, in practice, a width should be preferred

  wear-resistant coating of less than 10 mm.

  The blades described with reference to the drawings have various advantages Applying a relatively thin and durable surface coating to only a small portion of the coating blade allows the blade to retain the necessary flexibility as well as other properties of the coating. support material The extreme thinness of the surface coating reduces the brittleness and the risk of cracks in the wear-resistant coating This, in combination with the fact that such a blade has the same flexibility as a traditional blade, makes it possible to use such blades directly in conventional slide holders on existing paper coating machines. The preferred method of applying the wear-resistant coating is that described in co-pending French patent application claiming the priority of the Swedish patent application No. 82-05806-6,

to which we must refer.

Claims (18)

  1 squeegee blade for controlled application and one   smoothing of a coating composition on a strip of   (7) in displacement, characterized in that it comprises a flexible steel strip in which an active zone of the blade (1) is provided with a surface coating (5), thin with respect to the thickness of the steel strip, having a   superior wear resistance to the steel band.   2 squeegee blade according to claim 1, characterized   in that the flexible steel strip has a thickness of 0.7 mm or less.   3 squeegee blade according to any one of the   1 or 2, characterized in that the steel strip has generally parallel and opposite major surfaces (2, 3), the blade (1) comprising a bevelled surface (4, 10) inclined at an obtuse angle with respect to at one of said major surfaces (2) of the steel strip, and inclined sharply with respect to the other of said major surfaces (3), said bevelled surface (4) forming the active zone of the blade ( 1), said active area being disposed in the region of the portion of said beveled surface (4) upstream near its junction with said first major surface (2) and comprising an inlet area (4 a, 9), an area upstream and adjacent to the entry zone (4a, 9) and a downstream zone and   adjacent to the entrance area (4a, 9).   4 squeegee blade according to any one of the   cations, characterized in that the coating (5)   aforementioned comprises a ceramic material.   Squeegee blade according to any of the claims   cations 1 to 3, characterized in that the coating (5)   cited comprises at least one metal oxide. 2531 to 609   6 Raclette blade according to any one of the claims   1 to 3, characterized in that the coating (5)   cited comprises at least one metal carbide.   Squeegee blade according to one of Claims 1 to 3, characterized in that the coating (5)   wear-resistant consists substantially of alumina.   8 Squeegee blade according to any one of the   1 to 3, characterized in that the wear-resistant coating (5) is composed of alumina containing a small amount of   quantity of another metal oxide.   9 squeegee blade according to claim 8, characterized   in that the other metal oxide is titanium oxide.   Squeegee blade according to claim 9, characterized   in that the wear-resistant coating (5) contains 97% of A 1203 and 3% of Ti O 2.   11 squeegee blade according to claim 1, characterized   in that the surface formed by the coating (5) comprises a face forming the inlet zone (9), a relatively inclined face upstream of said inlet zone (9) and a face   relatively inclined downstream of said inlet zone (9).   Blade according to one of the preceding claims   characterized in that the total thickness of the coating   wear resistant (5) does not exceed 0.35 mm.   Blade according to one of the preceding claims   characterized in that the total thickness of the coating   wear resistant (5) does not exceed 0,25 mm.   14 Squeegee blade according to any one of the   characterized in that the wear-resistant coating (5) comprises a plurality of layers of said wear-resistant material applied one above the other in succession by spraying the material in the molten state.   Squeegee blade according to claim 3, characterized   in that the bevelled surface (4) of the strip is covered by   completely kills said wear-resistant coating (5).   16 Squeegee blade according to claim 3, characterized   in that the bevelled surface (4) further comprises a   exit zone (4 c) in the area of the junction of the sur-   bevel face (4) with the other of the major surfaces (3) of the blade (1), and an intermediate central zone (4b), said inlet zone (4a) and said exit zone (4c). ) being formed by respective coatings (5) resistant to   wear while the central area (4b) is uncoated.   Squeegee blade according to claim 3, characterized   in that the surface formed by the coating (5), resists   Both wear and tear is bevelled or rounded upstream of the area   input (9) or in the input area (9).   18 Squeegee blade according to claim 3, characterized   characterized in that the steel strip has a major surface (2), an E side face and a bevelled surface (4, 10) extending between said major surface (2) and said side face, and in that that said bevelled surface (4, 10) of the strip is fully coated with said wear resistant material, and the coating (5) extending on said bevelled surface (4,) of the strip forms said surface bevel (4, 10) of the raclette blade.   19 Squeegee blade according to any one of the   the active zone E is formed on one side of the blade (1) close to its edge, and in that the width of the zone, on the said side of the blade   (1), where the coating is deposited, is at most 20 mm.