EP2823100A1

EP2823100A1 - Electrolytically puls-plated doctor blade with a multiple layer coating

Info

Publication number: EP2823100A1
Application number: EP13757582.5A
Authority: EP
Inventors: Debbie ÅGREN; Jan-Åke HAGLUND; Erik MÅNSSON; Wolfgang Hansal; Gabriela SANDULACHE; Martina Halmdienst
Original assignee: Swedev AB
Current assignee: Swedev AB
Priority date: 2012-03-08
Filing date: 2013-03-08
Publication date: 2015-01-14
Also published as: EP2823100A4; WO2013133762A1

Abstract

The invention relates to a doctor blade (1), having a multiple layer coating. The coating comprises at least one electrolytically puls-plated abrasion resistant nickel composite layer and at least one outer electrolytically puls-plated low-friction nickel and/or nickel composite layer. It also relates to the use of a coated steel strip for a doctor blade and a method for the manufacture of said doctor blade from a steel strip. The blade according to the invention exhibits both an even and a smooth surface with a lubricating effect, a good abrasion resistance and the blade may be tailored to needs of the individual printer.

Description

ELECTROLYTICALLY PULS-PLATED DOCTOR BLADE WITH A MULTIPLE

LAYER COATING

TECHNICAL FIELD

The present invention relates to doctor blades provided with a pulse plated coating. It also relates to a coated steel strip for a doctor blade, the strip has a width of 5-80 mm, a thickness of 0.05-1 mm and a length of up to 500 m, wherein the coated steel strip is provided with a multiple layer coating comprising at least one electrolytically pulse- plated abrasion resistant nickel composite layer and at least one outer electrolytically pulse-plated low-friction nickel or nickel composite layer. The present invention also relates to the method for the manufacture of said coated steel strip.

BACKGROUND OF THE INVENTION

Doctor blades are used in the printing industry, in order to scrape printing ink from a rotating roll. In this connection, problems with wear of the roll and of the doctor blade, arise. The problem of wearing of a blade of doctor type has been addressed in a number of patent applications, e.g. WO 2011088583, WO 2010040236, WO 2006112522 and WO 2003064157, by the provision of a blade that has an abrasion resistant coating.

However, some problems are not solved in the mentioned prior art. For example in so called flexographic printing, the doctor blade butts against a ceramic screen roll which is very expensive and which moreover gives rise to a quite considerable wear of the doctor blade when the roll is new. If the blade coating is hard and brittle, as some nickel-phosphorus layers containing abrasion resistant particles can be, small pieces can chip out and be trapped between the blade and the roll, causing scratches ("scoring lines") in the expensive screen roll. Another effect of the chipping is problems with print quality such as lines in the print ("streaks") due to incomplete ink-scraping by chipped blade edges.

Another problem which is not solved in the mentioned prior art is uneven wear of the blade. In e.g. so called rotogravure printing there is, after initial wearing, formed an abutment surface on the doctor blade which is to abut closely against the print roll during the entire number of copies printed, so that colour pigment does not pass and before the doctor blade is exchanged. However, usually only about 10-20% of the wear section of the doctor blade is used at the pattern surface of the printing roll, before a change is made. This is due to uneven wear, in which a lubrication with the used ink takes place at the pattern surface, while the doctor blade is worn much faster outside the pattern surface and at the ends of the printing roll, perhaps all the way down to the part of the doctor blade which is outside the actual wear section. Due to this intense wear at the ends of the doctor blade, ink leaks onto the pattern surface and it is moreover not rare that fissures form in the surface layer of the doctor blade due to effect of forces, whereby the printing must be stopped for exchanging of the doctor blade. Accordingly, this has to be done despite the fact that the doctor blade has not been more than 10-20% worn at the pattern surface. Attempts to solve this problem have been made, there having been presented a doctor blade which exhibits a larger material thickness at the ends, i.e. in the parts which are intended to be positioned outside the pattern surface. In this case, the doctor blade has been ground with a conventional lamella grinding in the wear section but not in the end parts. This grinding is however very complicated to perform and moreover leads to that the doctor blade only can be manufactured at final lengths and not in longer pieces for cutting in connection with its use.

Another problem, which is not solved in the mentioned prior art is insufficient ink- scraping at high printing speeds. When the blade has a hard surface it takes a longer time to form an abutment surface on the doctor blade, which is to abut closely against the print roll during the entire number of copies printed. At high printing speeds the doctor blades with stiffer, harder and less conforming surfaces can tend to "ride" over a film of ink ("ink-planing") so that colour pigment passes and discolouring

("toning/hazing/fogging") occurs.

Another problem that may arise is an unacceptably long idling time ("start up time") with a new doctor blade, due to the new blade containing irregularities in the initial unused blade surface. Defects as small as 10 μιη or less can cause problems with streaks or hazing, depending on the material to be printed. When the blade has a hard surface it takes a longer time to wear through surface irregularities and to form a smooth abutment surface on the doctor blade, which is to abut closely against the print roll. The development of higher-resolution printing applications places higher demands on the surface finish of doctor blades, and as printers experience a growing number of shorter printing jobs, they demand a quicker start up for each job to minimize downtime and waste. Another problem that exists is the combined need for minimized downtime (i.e. long blade life) as well as a short start up time. This places different requirements on the same blade over the service period of the blade, i.e. a surface quickly adapting to the printing cylinder at the beginning of printing and yet a wear resistance allowing the blade to be used for a long period of time to minimize downtime for changing blades.

DISCLOSURE OF THE INVENTION The general object of the present invention is to provide a doctor blade having an overall better performance than known doctor blades.

A particular object of the invention is to provide a doctor blade having improved running in characteristics, in particular a reduced running in time and a reduced risk for streaks or other defects during start up.

Another object is to further improve the ink scraping properties of the doctor blade, in particular at high printing speeds so as to avoid undesirable discoloration. A further object is to provide a doctor blade, which reduces the printing downtime not only by a shorter running in time but also by a longer life time.

The foregoing objects, as well as additional advantages are achieved to a significant measure by the provision of doctor blade as set out in the claims.

The doctor blade has a property profile fulfilling the increasing demands in the printing industry. In particular, the blade according to the invention exhibits an even and smooth surface and may have a lubricating effect. The blade according to the invention exhibits as well a good abrasion resistance of the inner coated layers.

The blade according to the invention can combine these two features of good inner abrasion resistance and smooth lubricating surface to exhibit a good wear resistance without causing increased wear on a rotating roll, which the blade bears against. The blade may be tailored to needs of the individual printer. Yet another objective of the present invention is to present a method for continuous electrolytically pulse-plated nickel and/or nickel-alloy coating of such a blade, in at least two, preferably three layers. These and other objectives are accomplished by the doctor blade according to the invention and by the method according to the invention, as these are presented in the claims.

The invention is defined in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail with reference to the drawings, of which:

Fig. 1 is showing, in cross-section, a doctor blade according to the invention, which butts against a roll,

Fig. 2 is showing a block diagram, in side view, over the pulse-plating process according to the invention,

Fig. 3 is showing, in perspective, an example of how the masking of the doctor blade can be accomplished during the coating process

DETAILED DESCRIPTION OF THE INVENTION According to one aspect of the invention, the blade exhibits a coating which is thicker on the underneath side than on the top side, at least at a wear section of the blade, i.e. a front part of the blade where the steel core exhibits a thickness of about 30-300 μιη. At the wear section, the coating may exhibit a total thickness of 8-25 μη on the underneath side, preferably 10-20 μηι and even more preferred 13-18 μηι, while the coating on the top side typically exhibits a total thickness of 3-15 μηι, preferably 3-10 μιη, at the wear section. This design of the coating aims at that the forces which the blade is exposed to should be absorbed in the most favourable way. In this connection, it is the case that the blade is exposed to the largest forces on its underneath side, due to the underneath side being the first to meet the roll at its rotation, with certain abutment forces, whereby accordingly the need of a thick coating is largest on the underneath side of the blade. According to another aspect of the invention, the blade exhibits a section of the coating on its top side, in the following denoted a reinforcement section, which exhibits a largest thicloiess which is larger than the thicloiess on the top side of the wear section of the blade and preferably also larger than the thickness of the coating on the underneath side of the wear section of the blade, as seen in the normal against the surface of the blade. The reinforcement section normally exhibits a largest thickness of 10-40 μηι, preferably 15-35 μηι, as seen in the normal against the surface of the blade. This reinforcement section is arranged at the transition section between the wear section of the blade and the rear part of the blade, on the top side of the blade, with the puipose of absorbing stresses in the surface layer of the blade when the blade has been worn all the way down to or in the vicinity of this transition section, normally first at the parts of the blade that are positioned outside the pattern surface, i.e. the ends of the blade. Thanks to the reinforcement section, the wear is stopped and the stresses are diverted into the doctor blade. Hereby, fissure forming is prevented at the transition section between the wear section and the rear part of the blade. Hereby, the life term of the blade may be considerably prolonged, since the wear section may be used to considerably more than the conventional 10-20% before it has to be exchanged due to wear and thereby following fissure formation in the ends of the blade. The different thicknesses of the coatings, including the reinforcement section, are achieved in a continuous process for electrolytic nickel and/or nickel-alloy coating in two or more steps, by use of a total or partial masking of the different parts of the blade. Other process parameters too, such as current density, duty cycle, frequency, pulse polarity, positioning of the strip in relation to the electrodes, i.e. the distance between the same, and the like, may be used in order to control the formation of the coatings in different positions of the blade. The process and the masking according to the invention are described in greater detail in connection with the drawings description below.

According to another aspect of the invention, the coatings, are at least on the underneath side of the blade at its wear section and a short distance beyond the transition section between the wear section and the rear part of the blade, formed of two or more layers having different compositions. At least two layers, preferably three or four layers, of different compositions and particle size distributions are formed by the continuous process for electrolytically pulse-plated nickel and/or nickel-alloy coating in several steps (several cells), at least one of these layers comprising particles that increase the abrasion resistance of the coating (abrasion resistant particles) arranged either homogeneously or in a gradient (smaller to larger or larger to smaller) within the coating. Such particles may e.g. be constituted by metal oxides, carbides or nitrides, e.g. Zr0₂, A1₂0₃, Si0₂, SiO, Ti0₂, ZnO, SiC, TiC, VC, WC, SiN and/or cubic BN. Most preferred is use of SiC and/or cubic BN. Besides giving an increased hardness, such a layer counteracts the formation of burrs, and can be tailored to suit the particular printing conditions (e.g. ink pigment size).

It is preferred that at least one other of these layers also comprises particles that increase the lubricating effect of the coating, preferably hexagonal BN. An alternative second layer or a third, outermost layer is preferably constituted by an electrolytically pulse- plated nickel coating essentially without a content of abrasion resistant or lubricating particles, whereby the outemiost layer instead can be constituted by an electrolytically pulse-plated nickel coating which is free from additives apart from the additives that conventionally are used in connection with the application of such coatings or an electrolytically pulse-plated nickel coating which comprises additives of Teflon/PFTE type. By the concept "of Teflon/PTFE type" it is hereby meant additives such that the surface of the doctor blade exhibits properties obstructing the adhesion of ingredients in the ink which is used by the end user together with the doctor blade.

Also on the top side of the blade, including the reinforcement section, the coating may be constituted by two, three or more layers according to the above, optionally of the same type and in the same order as on the underneath side. Suitably, but not necessarily, the greater part of the thickness of the coating at the reinforcement section may be constituted by a layer with abrasion resistant particles, the other layers exhibiting in the main the same thickness at the reinforcement section as at the wear section, on the top side of the blade. It is however also conceivable to use only one coating layer on the top side of the blade, which in that case suitably consists of a layer comprising abrasion resistant particles. As an alternative, there is made use of more than one layer both on the top side and on the underneath side, the number of layers however being greater on the underneath side than on the top side.

According to yet another aspect of the invention, the blade, in the rear part of its top and underneath side, only exhibits one coating layer, which is preferably constituted by an electrolytically pulse-plated nickel coating essentially without a content of particles or an electrolytically pulse-plated nickel coating comprising additives of the type

Teflon/PTFE, MoS₂ or WS₂. However, it is of course also conceivable that the layer instead comprises other particles according to the above. Here, the coating layer suitably has a thickness of about 1-10 μιη, preferably 1-6 μηι. Alternatively, the rear part may exhibit two or more layers according to the above, the outermost layer being constituted by an electrolytically pulse-plated nickel coating essentially without a content of particles or an electrolytically pulse-plated nickel coating comprising additives of the type Teflon/PTFE.

According to yet another aspect of the invention, the outermost coating layer of the blade, preferably without any additives or only having additives of the type

Teflon/PTFE, may be the same over the entire blade, whereby this outermost layer suitably is applied in a final electrolytic cell without masking.

The particle density of the particles used in the layers, depend to a certain degree on the particle size of the pigment which is to be used in the printing, when the blade is a doctor blade. The less the size of the pigment particles, the less the size of the abrasion resistant particles, and the greater the particle density in the layers. Typically, the lubricating particles, e.g. hexagonal BN should be smaller than 4 μηι, the abrasion resistant particles, e.g. SiC, should be smaller than 5 μηι and the additives of the type Teflon/PTE should be smaller than 5 μιη. The thinner the layer, the smaller the particles. Typical content of particles in the respective layers are 0-20 weight-%, preferably 2-15 weight-% and even more preferred 3-7 weight-%. In the case of two or more layers with particles, the presence and distribution of particles can be individually set for each layer. A such construction can for example consist of the innermost layer containing particles with particle size distribution D90%=5 μιη, D50%>=1,5 μηι, D10%=0,5 μιη with the larger particles existing closest to the steel base, an intermediate layer containing particles with particle size distribution D90%o=l μηι, D50%=0,2 μιη, D 10%=50 nm with the larger particles existing closest to the inner layer, and an outermost layer consisting of fine-grained nickel without embedded particles.

When an outermost coating layer comprising additives of Teflon/PTFE or similar is used, the coating process is finished with a heat treatment step, e.g. at about 200-600°C, typically about 400°C , for a few minutes, typically 30 minutes at the most. In this heat treatment, superficial particles of PTFE will flow out into a thin, mainly even, surface layer of the outermost coating layer. According to the invention, this heat treatment may be combined with, i.e. performed at the same time as, a heat treatment step which is required to achieve an increased hardness in the layers when the electrolyte bath is of nickel alloy type, preferably at 100-500°C, and even more preferably at 100-300°C for at least 30 minutes. Typically there is achieved a hardness of about 600 - 800 HV, in a coating layer comprising SiC according to the invention, when heat treatment is not used. When heat treatment is used, in connection with nickel alloy baths or Ni baths including metal salts, including SiC, the hardness of this layer may be up to 800 HV, preferably up to 900 HV and even more preferred up to 1000 HV for a phosphorus content of 2 - 20% in the coating. The hardness of a coating layer comprising hexagonal BN is typically about 620 -700 HV, and always lower than the layer comprising abrasion resistant particles, however higher than the hardness of the steel in the core of the blade. The hardness of a coating layer comprising pure nickel is typically about 300-500 HV. When used as an outer layer this softer nickel can adapt quickly to the harder printing cylinder surface and decrease the running in time of the blade. All HV values are given for a load of 100g.

In the following, the invention is exemplified by a doctor blade 1 (Fig. 1), which is intended to be used to scrape off printing ink from a rotating roll 2, which roll normally is a so called anilox roll or engraving roll. During operation, the doctor blade 1 is exposed to forces indicated by arrows.

The doctor blade 1 exhibits a steel core, with about 0.35-1.2% C, which has been hardened to a hardness of about 550-750 HV and has been lamella ground. By the concept of lamella grinding it is meant that the blade exhibits a rear, thicker part 3, normally 0.15-0.6 mm thick, for clamping in a holder (not shown) for the blade, and a front, thinner part 4, normally about 50 μηι thick, which constitutes a wear section. At the transition between the rear part 3 and the wear section 4, the blade exhibits a sharp edge 5 on its top side, and thereafter a soft, gradual transition 6 down towards the wear section 4. On the underneath side, the blade 1 is entirely flat, except at the tip 7, which may be softly chamfered. The blade 1 may exhibit a total extension (width) of 8-120 mm in the shown cross-section. Normally, the edge 5 is situated less than 10 mm from the tip 7 of the blade.

On its underneath side, the blade 1 exhibits a coating 8, which is formed from at least two different layers 8a, 8b, 8c and which exhibits a total thickness of 10-20 μηι. This underneath coating 8 may extend over the entire or essentially the entire underneath side of the blade, or only over the wear section 4 and a short distance past the transition section 5,6. A coating 8 is arranged on the top side of the blade, which coating is formed from at least one layer 9a, 9b and which exhibits a total thickness of 3-15 μηι, up to about 70% of the extension of the wear section, as seen from the tip of the blade. After these about 70% of the extension of the wear section, there is formed a

reinforcement section 10, which has preferably been formed by the same type of layer as the coating 9, but in greater thicknesses, according to the above. The rear part 3 also exhibits at least one coating layer 11.

The top coating 9a and underneath coatings 8a, 8b, 8c may exhibit numerous variations in particle size distribution, due to that the particle size distribution can be controlled by the pulsing parameters in each coating step. A particle content of 2 - 300 g/1 in the bath may typically be used.

In Fig. 1 , there is shown a rounded edge tip of the blade, which is a result of the original shaved edge profile remaining after grinding of the lamella in the steel strip substrate. In cross-section radius edge deviation (RED) of the coating surface may be defined as the length of a protrusion or intrusion expressed as a % of the blade edge radius, e.g. a protrusion of ΙΟμιη may be expressed as a RED of 6.7% of a 0.15 mm tip edge radius.

A conceivable embodiment of a doctor blade of steel, having a multiple layer coating comprising at least one electrolytically pulse-plated abrasion resistant nickel composite layer comprising a nickel-based matrix and up to 30 weight % in total of other alloying elements, in particular anyone of P, Co, Sn, Cu, Fe, W, Mn, Mo and abrasion resistant particles and optionally lubricating particles and/or additives, and at least one outer electrolytically pulse-plated low-friction nickel or nickel composite layer, wherein at least one outer low-friction nickel layer comprises nickel and up to 30 weight % in total of other alloying elements, in particular anyone of P, Co, Sn, Cu, Fe, W, Mn, Mo and optionally lubricating particles and/or additives, said outer layer fulfils at least one of the following conditions regarding surface roughness (Ra) and radius edge deviations (RED): Ra < 0.10 μιη, preferably 0.01-0.05 μιη and RED < 7%, preferably < 5%, wherein the RED is defined as the length of a protrusion or intrusion from a cross- section radius edge of the blade expressed as a % of the blade edge radius.

In Fig. 2, there is shown a block diagram in side view intended to illustrate the process for the electrolytically pulse-plated nickel or nickel alloy coating according to the invention. After suitable degreasing and activation stages, the doctor blade 1 is brought to pass as a continuous strip through at least two, in the shown embodiment three electrolytic cells and rinses 21, 22, 23 with contact polarisation of the blade 1 via cathodic electrode rollers 25. Between one and four pulse rectifiers may be connected to each cell, e.g. 27a,27b,27c,27d to cell 21, 28a,28b,28c,28d to cell 22 and

29a,29b,29c,29d to cell 23, depending on the coating construction to be manufactured. It is preferred that the cells are adequately wide so that two or more blades can be coated at the same time during continuous operation. Anodic electrodes 26 are arranged in the cells 21, 22, 23. There is a continuous flow of electrolyte through the cells allowing electrical contact between the anodes and cathodic strip. Due to carrying between the cells, the formed coating layers may be brought to contain a small amount of particles other than the ones specified as "nominal" for each layer. This is true also for layers stated to be without particles. However, this deviation from the nominal composition is so small that it will not affect the concept of the invention to any considerable degree. Each cell 21, 22, 23 contains a nickel electrolyte bath of the type well known in the art i.e. normally comprising NiS0₄, NiCl₂, H₃B0_3; and optionally Ni-sulfamate, NiBr_2, CoCl₂, SnCl₂, H₃P0₃, H₃P0₄, FeS0₄, CuS0₄, Na₂W0₄, NH_3> and/or saccharin, and at least in one of the cells additives in the form of abrasion resistant particles and/or lubricating particles and/or additives of the PTFE/Teflon type. Normally, the electrolytic cells operate at a temperature of about 40-60°C and for direct current plating a current density of up to 20 A/dm². In pulse-plating, the current density may rise to about 200 A/dm during both anodic and cathodic pulse peaks, preferably 0-100 A/dm and even more preferably 3-50 A/dm². Anodic and cathodic pulse peaks may be alternated in a large number of combinations between the cells, resulting in a wide range of variations in plating processing. The larger number of rectifiers may be used to create gradients in particle size distribution within each coating layer. The order between the cells and the masking in the same, according to below, may be varied and naturally depends on the desired end product.

Pulse plating techniques present a larger number of available parameters than direct current plating, such as pulse waveform, peak current density, off-time, frequency and duty cycle which can be used to optimize the features of the plated coating. This larger number of parameters can give a wide variation in results, depending on how well the parameters are adjusted to the electrolytic conditions. Well-controlled pulse plating techniques can produce finer-grained deposits with higher density, higher hardness, lower porosity, lower hydrogen content, lower incorporation of impurities, higher coiTosion resistance, improved control of deposition thickness and improved surface finish. Pulse plating parameters can be optimized to produce nickel and nickel-alloy composite coatings with a higher incorporation and more uniform distribution of particles in the metal matrix, a smaller Ni crystallite size, an orientation of crystallite- growth along the plane of the coating rather than columnar, giving coatings with enhanced mechanical properties such as higher hardness due to smaller crystallite size and dispersion-hardening, improved abrasion resistance and finer surface finish. It is suggested that the maximum particle concentration is achieved when the thickness deposited per cycle approaches the diameter of the particles to be embedded. The favourable coating properties achieved by pulse plating can decrease or eliminate the need for surfactant additives in the plating process.

In pulse plating, there are basically two groups of pulse waveforms: unipolar, e.g. all cathodic pulses, and bipolar, a combination of cathodic and anodic pulses. There are many variants of each of these groups. In some pulse sequences, superimposed pulses, which are unipolar pulses with a base current, are used to deposit different materials at different potentials, with e.g. a base current density that is up to about 60% of the peak current density, a peak pulse length between 0.5 ms and 100 times the base pulse length which is determined by the frequency.. During a cathodic pulse, the deposit can grow by charge transfer, adsorption, nucleation, diffusion and growth of crystals. High cathodic peak current density will promote the formation of many nuclei in the deposition. The deposit tends to grow faster at geometrical extremities such as corners of the substrate, where the current density is higher. During anodic pulse time, the deposit can dissolve and passivate by oxidation, and dissolves faster at geometrical extremities and at higher anodic pulse current density. Bipolar pulse sequences can balance the adsorption and desorption of atoms giving the deposit a more even thickness, avoiding the "dog bone" effect at edges, decreasing the content of impurities, increasing the compactness of each plated layer and thereby avoiding pores reaching down to the substrate. This can be advantageous for increasing both the hardness and surface finish of the inner and outer deposited layers to give a surface roughness and shape that shortens the running-in time of a doctor blade, and an inner hardness that increases the lifetime of the doctor blade.

In some pulse sequences, off-times in which no current is applied, are used to affect the deposition layer structure. During off-time, hydrogen bubbles formed at high current densities may escape from the cathode surface, however if the off-time is too short they may adhere, causing cracking problems. The off-time can also be used at high current densities for the replenishment of nickel ions near the cathode surface, reducing the risk of a rough surface finish. An off-time that is too long can lead to grain growth and a breakdown of the deposited layer, also risking a rough surface finish. Therefore a correct off-time interval preferably shorter than 2 seconds is required for optimal deposition layer properties. The duty cycle in pulse plating is defined as the time with current on divided by the total on- and off-time, expressed as %. In the invention, duty cycles are preferably 10 - 90%, and even more preferably 20 - 80%. The pulse frequency can affect the surface roughness of the plated coating. Lower frequencies can result in rougher surfaces due more surface diffusion and larger grain sizes. At higher frequencies, that is shorter pulses, a lower concentration gradient of the plating metal species over the diffusion layer will decrease the diffusion movement and grain growth, resulting in a finer-grained smoother surface. In the invention, frequencies are preferably 1 - 1000 Hz, and even more preferably 10 - 400 Hz.

The pulse-plating of composite layers comprising nickel and nickel-alloy together with dispersed particles presents additional requirements in the optimization of pulse parameters. For example, lower duty cycles favour a higher incorporation of particles, while in bipolar pulsing, a too high anodic peak current density will discourage the incorporation of particles. An optimal combination of parameters will give both a high particle incorporation and small grain size, favouring a superior wear resistance.

According to yet another aspect of the invention, specific selection of pulse plating parameters is used to improve the surface finish and coating shape regularity, which gives the doctor blade a shorter running in time. Surface roughness measurements at or below Ra 0,10 μιη and/or radius edge deviations < 7% are accomplished, and together these significantly shorten the running in time of the blade. According to yet another aspect of the invention, these specific pulse plating parameters are combined with an outer nickel layer without particles to give a very smooth lubricating outer layer of the doctor blade for shortened running in time of the blade and immediately improved printing quality. In Fig. 3, there is shown an example of how the strip 1, which is constituted by the doctor blade, continuously runs in the cells 21, 22, 23 according to Fig. 2. In each of these cells, or at least in one or some of them, there is arranged one or more masking devices, whereof the shown masking devices 31, 32 constitute one example of how it can look in one of the cells. The masking devices are fixed in the electrolyte bath in a direction which corresponds to the running direction a of the strip, but are somewhat displaceable in the cross direction. In the shown embodiment, the masking devices are arranged so that a front part of the wear section 4 of the blade 1 is partly masked by the masking device 31. The masking device 31 is arranged to extend about the tip of the blade 1, at a distance approx. 0.5 - 5 mm from the blade, and exhibits through holes 33 so that a minor part of the flowing electrolyte liquid is allowed to flow over the tip of the blade, despite the masking, in order there to form a thin coating. The masking device also gives a lower current density at the masked sections, which may however be somewhat increased by aid of the holes 33. A masking device 32 is also arranged to mask the top side of the doctor blade, at its rear part 3. The transition section 6 and the underneath side of the doctor blade are however not masked in the shown embodiment, leading to that thicker coatings 8, 10 (Fig.1) can be formed there. It is to be understood that the shape of the through holes 33 may be varied, they may be circular or oblong e.g., rectangular or oval e.g.

By use of masking devices of different types in the different cells 21, 22 and 23, there is obtained a possibility to form different coating layers in combination with each other, having different thickness and different compositions in different positions of the blade. Accordingly, one may e.g. mask the entire rear part 3 of the blade, i.e. both its top side and its underneath side, in a first step (in a first cell), and only coat the front 10 millimetres of the blade by a first coating layer 8a, 9a (Fig.1) of nickel comprising abrasion resistant particles. At the same time, one may by aid of masking, current density, frequency, duty cycle, the distance between the strip and the electrodes and other process parameters, control the physical forming of the coating layers and the particle size distribution within each layer according to the above. Thereafter, a covering layer without abrasion resistant particles but including lubricating particles may be applied on top of the particles in the first layer, in a second step (in a second cell 2) with essentially the same masking as in step 1. Finally, the front part of the blade may be masked entirely and its rear part 3 may instead be coated, e.g. by a pure Ni layer, in a third step (in a third cell 23).

EXAMPLES

In the following, there is exemplified in Table 1 a number of different conceivable variants of electrolytically coated blade according to the invention. By front part is meant the wear section and reinforcement section, the front part of the underneath side extending all the way to and including the reinforcement section which is arranged on the top side. By "Ni" or "NiX" is meant a nickel coating or nickel alloy coating which has been created by aid of electrolytic nickel coating according to the description above The coating layers used have been numbered so that layer 1 is the layer closest to the blade. By the designations is meant:

A NiP comprising 4-5%P and abrasion resistant particles with even distribution

B NiP comprising 4-5% P and abrasion resistant particles with gradient distribution C NiP comprising 6-8%P and abrasion resistant particles with even distribution

D NiP comprising 6-8%P and abrasion resistant particles with gradient distribution

E NiP comprising 9-12% P and abrasion resistant particles with even distribution

F NiP comprising 9-12% P and abrasion resistant particles with gradient distribution

G NiSn comprising abrasion resistant particles with even distribution

H NiSn comprising abrasion resistant particles with gradient distribution

I Ni comprising abrasion resistant particles with even distribution

J Ni comprising abrasion resistant particles with gradient distribution L Ni comprising lubricating particles

T Ni comprising additives of the type Teflon/PTFE

AL Ni comprising both abrasion resistant and lubricating particles

W Ni without any additives

Table 1

Another example particularly favourable to short running-in time comprises an innennost layer containing electrolytically pulse-plated nickel with 3-7 wt-% SiC particles with size distribution and intermediate layer containing electrolytically pulse-plated nickel with 1-3 wt-% SiC particles with size distribution ϋ90%=1μιχι, D50%=0,2 μιη, D10%=50nm and an outer layer containing electrolytically pulse-plated nickel with no SiC particles. Table 2 exemplifies a number of different conceivable variants of pulse plating parameters according to the invention.

Table 2

The examples are mainly intended to illustrate the great number of variants that can be achieved according to the invention. The skilled man will also realise that a number of other combinations can be made.

The invention is not limited to the described embodiments but may be varied within the scope of the claims. Especially, it is realised that the skilled man, without any inventive work, can compose other combinations of coating layers and how these are to be manufactured in the process according to the invention, by use of in series arranged electrolytic cells with pulse-current rectifiers having masking adapted to the desired product. INDUSTRIAL APPLICABILITY

The doctor blade of the present invention is particular useful in demanding printing applications requiring short start up times in combination with excellent printing quality and a high durability of the doctor blade.

Claims

Doctor blade (1) of steel, having a multiple layer coating, characterised in that said multiple layer coating comprises

a) at least one electrolytically pulse-plated abrasion resistant nickel composite layer comprising

a 1 ) nickel-based matrix

al .1) the nickel-based matrix comprises up to 30 weight% in total of other

elements, in particular

anyone of P, Co, Sn, Cu, Fe, W, Mn, Mo

a2) abrasion resistant particles, and optionally

a3) lubricating particles and/or additives

b) at least one outer electrolytically pulse-plated low- friction nickel and/or nickel

composite layer,

bl) wherein the outer nickel layer comprises

bl.l) nickel

bl .2) up to 30 weight % in total of other alloying elements, in particular

anyone of P, Co, Sn, Cu, Fe, W, Mn, Mo

and, optionally

bl .3) lubricating particles and/or additives

b2) and wherein said outer layer fulfils at least one of the following

conditions regarding surface roughness (Ra) and radius edge deviations

(RED):

b2.1) Ra < 0.10 μηι preferably 0.01 - 0.05 μιη and b2.2) RED < 7% preferably < 5%

wherein the RED is defined as the length of a protrusion or intrusion

from a cross-section radius edge of the blade expressed as a

% of the

blade edge radius. Doctor blade (1) according to claim 1, characterised in that said nickel composite layer comprises a first coating layer (8a; 8b; 8c; 9a; 9b), which is arranged at least on an underneath side of a front part (4) of the blade (1).

Doctor blade according to claims 1 or 2, characterised in that the blade (1) also exhibits a second coating layer (8a;8b;8c;9a;9b), at least on the underneath side of the front part (4), which second coating layer comprises an electrolytically pulse-plated layer as defined in claim 1 feature a and/or feature b, comprising lubricating particles and/or additives of Teflon/PTFE, MoS₂ or WS₂type, or is in the main free from abrasion resistant or lubricating particles and additives of Teflon/PTFE, MoS₂ or WS₂ type.

Doctor blade according to any of the preceding claims, characterised in that the blade (1) of steel exhibits a front part (4), which is thinner than a rear part (3), said front part (4) constituting a wear section while said rear part (3) constitutes an attachment part of the blade (1).

Doctor blade according to any of the preceding claims, characterised in that the blade (1) exhibits at least one electrolytic nickel layer as defined in claim 1 feature a and/or feature b on a top side (9a, 9b) of a front part of the blade (4,5) and at least two electrolytic nickel layers as defined in claim 1 feature a and/or feature b on an underneath side (8a, 8b, 8c) of the front part of the blade, the number of electrolytic nickel layers as defined in claim 1 feature a and/or feature b being greater on the underneath side (8a, 8b, 8c) of the blade than on its top side (9a, 9b).

Doctor blade according to any of the preceding claims, characterised in that two or more of the nickel layers as defined in claim 1 feature a and/or feature b, preferably all nickel layers as defined in claim 1 feature a and/or feature b are electrolytically pulse-plated nickel layers as defined in claim 1 feature a and/or feature b, said at least one electrolytic nickel layer as defined in claim 1 feature a and/or feature b on the top side (9a,9b) of the front part (4) comprises an electrolytic nickel layer comprising abrasion resistant particles.

Doctor blade according to any of the preceding claims, characterised in that said abrasion resistant particles exist in an amount of 0-20 wt-%, preferably 2-15 wt-% and even more preferred 3-7 wt-% in the first and/or second coating layer, that they exhibit a particle size less than 5 μη , and that they are constituted by one or more metal oxides, metal carbides or metal nitrides, preferably chosen from the group that consists of Zr0₂, A1₂0₃, Si0₂, SiO, Ti0₂, ZnO, SiC, TiC, VC, WC, SiN and cubic BN.

8. Doctor blade according to any of the preceding claims 3-7, characterised in that said at least one electrolytic nickel layer (9a, 9b) as defined in claim 1 feature a and/or feature b on the top side of the front part (4) also comprises an electrolytic nickel layer comprising lubricating particles and/or additives of Teflon/PTFE, MoS₂ or WS₂ type, or which is in the main free from abrasion resistant or lubricating particles and additives of Teflon/PTFE, MoS₂ or WS₂ type.

9. Doctor blade according to any of the preceding claims 3 or 8, characterised in that said lubricating particles and/or said additives of Teflon/PTFE type exist in an amount of 0-20 wt-%, preferably 2-15 wt-% and even more preferred 3-7 wt- % in the second and/or third coating layer, that they exhibit a particle size less than 5 μηι, and that they are constituted by hexagonal BN and/or Teflon PTFE, MoS₂ or WS₂ type.

10. Doctor blade according to any of the preceding claims, characterised in that the blade (1), on a rear part (3) thereof, exhibits at least one electrolytic nickel layer (11) as defined in claim 1 feature a and/or feature b, preferably not more than one such layer, which exhibits a thickness of 1-10 μιη, preferably 1-6 μιη.

11. Doctor blade according to any one of the preceding claims, characterised in that it, as an outermost coating layer (8c,9b,l 1) exhibits a uniform electrolytic nickel layer covering essentially the entire blade (1).

12. Doctor blade according to any of the preceding claims, characterised in that a total coating on an underneath side (8a, 8b, 8c) of a front part (4) of the blade (1) exhibits a greater thickness than a total coating on a top side (9a, 9b) of the front part of the blade (1), the total thickness of the coating on the underneath side (8a, 8b, 8c) being 8-25 μηι, preferably 10-20 μπι and even more preferred 13-18 μιη, while the total thickness of the coating on the top side (9a, 9b) is 13-15 μηι, preferably 3-10 μιη.

13. Doctor blade according to any of the preceding claims, characterised in that the blade (1) comprises a reinforcement section (10), composed of at least one coating layer (9a, 9b) on a top side of the blade, at a transition section (5,6) between a front part (4) of the blade, which front part constitutes a wear section, and a rear part (3) of the blade, which reinforcement section (10) exhibits a largest thickness which is greater than a thickness of a total coating on a top side (9a, 9b) of the front part (4) of the blade (1) and preferably also greater than a thickness of a total coating on an underneath side (8a, 8b, 8c) of the front part (4) of the blade (1), as seen in the normal against the surface of the blade, the largest thickness of the reinforcement section preferably being 10-40 μηι and even more preferred 13-35 μηι.

14. A method of producing a coated steel strip for a doctor blade, in particular for a doctor blade as defined in any of the preceding claims 1-13, wherein the coated steel strip having a width of 5-80 mm, a thickness of 0.05-1 mm and preferably a length of up to 500 m, is provided with at least one electrolytically pulse-plated nickel layer comprising abrasion resistant particles as defined in claim 1 feature a, and an outer electrolytically pulse-plated nickel layer as defined in claim 1 feature b, the method comprises the following steps: i) providing a steel strip having a width of 5-80 mm, a thickness of 0.05-1 mm on a reel; ii) subjecting said steel strip to continuous electrolytic plating in one or more electrolytic cells (21;22;23) holding an electrolyte liquid comprising at least one nickel cell optionally with other metal salts and additives, at least one of these cells also comprising abrasion resistant particles; iii) subjecting the steel strip to pulse-plating in at least one cell comprising the abrasion resistant particles, thereby providing an electrolytically pulse- plated nickel layer comprising the abrasion resistant particles as defined in claim 1 feature a and/or feature b, and optionally 2-20 % P, on the steel strip; iv) cutting the strip to the desired length.

15. A method according to claim 14 further comprising one or more of the following steps: v) masking completely or partially one or more sections of the blade (1) in at least one of said cells (21 ;22;23), for a flow of electrolytic liquid and or current density by the use of one or more masking devices (31 , 32); vi) providing a first coating layer (8a;8b;8c;9a;9b), which comprises an electrolytically pulse-plated nickel layer comprising abrasion resistant particles as defined in claim 1 feature a and/or feature b, at least on an underneath side of a front part (4) of the blade (1); vii) arranging one or more masking devices (31 ,32), in at least one of the cells, as seen in a running direction of the blade (1); viii) performing the pulse-plating in said cells (21 ;22;23) with pulsing

sequences with duty cycle 10-90%, preferably 20 - 80% , frequency 1 - 1000 Hz, preferably 10 - 400 Hz and peak current density 0-100 A/dm², preferably 3-50 A/dm², unipolar and/or bipolar waveforms including superimposed pulses with base current density up to 60% of peak current density and length 0.5 ms - 100 times the base pulse length and including off-time 1 ms - 2 seconds. ix) performing contact polarisation of the blade (1) said cells (21;22;23) via cathodic electrode rollers (25) and anodic electrodes (26) arranged in the cell(s); x) polishing the coated steel strip, preferably in several steps by the use of several polishing discs; xi) heat treating the strip, after having been coated by the nickel and/or

nickel-alloy coating, preferably at 100-500°C, and even more preferably 100-300°C for at least 30 minutes. xii) providing two or more layers, preferably all layers by means of pulse- plating.

16. A method of producing a coated steel strip for a doctor blade, wherein the blade is defined in any of the preceding claims 1-13, and wherein the strip is produced according to any of claims 14 and 15.

17. A method according to any one of claims 14-16, characterised in that the buildup of the nickel coating as defined in claim 1 feature a and/or feature b formed on the strip is controlled by said masking and preferably also by controlling of the current density in the cell and/or by controlling of a distance between the strip and the electrodes (26) arranged in the cell(s).

18. A steel strip coating having at least one electrolytically pulse-plated nickel layer comprising abrasion resistant particles as defined in claim 1 feature a, wherein the steel strip coating is preferably obtained by the method of claims 14-17.