Doctor blade, inking arrangement and use of doctor blade in flexographic printing

Abstract

A doctor blade (5, 7) for contact with an anilox roller (15) comprises a flat, elongate base element having a thickness of less than about 0.3 mm, which, along a longitudinal region of the doctor blade adapted for contact with said anilox roller, is provided with a coating (43). The coating comprises a metal matrix and at least about 65% by weight of one or more ceramic(s). The coating comprises 0 to 65% by weight of chromium carbide. An inking arrangement comprises an anilox roller and a doctor blade. A doctor blade is used in flexographic printing.

Claims

1. An inking arrangement comprising: an anilox roller; and a doctor blade for contact with the anilox roller, the doctor blade comprising a flat, elongate base element, which, along a longitudinal region of the doctor blade adapted for contact with the anilox roller, is provided with a coating, the coating comprising a metal matrix and at least 65% by weight of at least two ceramics, the at least two ceramics including: chromium carbide in an amount of 10% to 65% by weight of the coating; and tungsten carbide in an amount of 25% to 85% by weight of the coating, wherein the doctor blade further comprises a front face, an adjacent face, and a rounded cross-section along the longitudinal region of the doctor blade adapted for contact with the anilox roller, wherein the rounded cross-section has a diameter comprising a center, and wherein the center of the diameter is located substantially at a bisectrix between the front face and the adjacent face of the doctor blade.

2. The inking arrangement according to claim 1, wherein the coating has a thickness of 15 to 60 μm.

3. The inking arrangement according to claim 1, wherein the coating comprises at least 5% by weight of the metal matrix.

4. The inking arrangement according to claim 1, wherein the metal matrix comprises at least one of nickel, cobalt or chromium.

5. The inking arrangement according to claim 1, wherein the coating comprises 70 to 90% by weight of the at least two ceramics.

6. The inking arrangement according to claim 1, wherein the coating has a thickness of 30 to 40 μm.

7. The inking arrangement according to claim 1, wherein the flat, elongate base element is a steel strip.

8. The inking arrangement according to claim 1, wherein the flat, elongate base element has a thickness of 0.1 to 0.25 mm.

9. The inking arrangement according to claim 1, wherein the rounded cross-section has a diameter of 10 to 50 μm.

10. The inking arrangement according to claim 1, wherein the anilox roller has a surface layer of a ceramic material.

11. The inking arrangement according to claim 10, wherein the ceramic material includes chromium oxide as a main component.

12. The inking arrangement according to claim 1, wherein the flat, elongate base element has a thickness of less than 0.3 mm.

13. The inking arrangement according to claim 1, wherein the coating is provided by high velocity oxygen fuel spraying.

14. A method in flexographic printing, comprising contacting a doctor blade with an anilox roller, the doctor blade comprising a flat, elongate base element, which, along a longitudinal region of the doctor blade, is provided with a coating comprising a metal matrix and at least 65% by weight of at least two ceramics, the at least two ceramics including: chromium carbide in an amount of 10% to 65% by weight of the coating; and tungsten carbide in an amount of 25% to 85% by weight of the coating, wherein the doctor blade further comprises a front face, an adjacent face, and a rounded cross-section along the longitudinal region of the doctor blade adapted for contact with the anilox roller, wherein the rounded cross-section has a diameter comprising a center, and wherein the center of the diameter is located substantially at a bisectrix between the front face and the adjacent face of the doctor blade.

15. The method according to claim 14, wherein the coating has a thickness of 15 to 60 μm.

16. The method according to claim 14, wherein the coating has a thickness of 30 to 40 μm.

17. The method according to claim 14, wherein the coating is provided by high velocity oxygen fuel spraying.

18. The method according to claim 14, wherein a surface of the anilox roller includes a ceramic material having chromium oxide as a main component.

19. The method according to claim 14, further comprising determining that a respective size of all surface defects at the longitudinal region is less than 20 μm.

20. The method according to claim 19, wherein determining the respective size of all surface defects is performed by microscopic imaging.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described in the following with reference to the appended drawings.

(2) FIG. 1 shows diagrammatically in a side view a machine for flexographic printing.

(3) FIG. 2 shows in a side view the arrangement contained within the dashed line square of FIG. 1.

(4) FIGS. 3a to 3d are diagrammatic side views of four different embodiments of doctor blades.

(5) FIGS. 4a and 4b are scanning electron microscope (SEM) images of the tip of doctor blades that have been used against an anilox roller.

DETAILED DESCRIPTION

(6) The flexographic printer 1 shown diagrammatically in FIG. 1 in a side view is provided with an inking blade unit 3 with a blade holder 9 carrying two blades 5, 7 to be further described in connection with FIG. 2. Furthermore, the printer 1 has an anilox roller 15 constituted by a steel drum covered with a ceramic sleeve or shell. The inking blade unit 3 is associated with a printing ink container 27, an ink feeding conduit 29 containing an ink feeding pump 31 for the transfer of printing ink from an ink supply 37 to the inking blade unit 3. Furthermore, a return conduit 35 is provided for the return of excessive printing ink to the container 27. The printer 1 is furthermore provided with a printing plate cylinder 21, carrying printing plates 19, and a pressure roller 23. A substrate 25, such as a paper web or a polymer film, for printing travels in the nip between cylinder 21 and roller 23 in the direction indicated in FIG. 1.

(7) In FIG. 2 there is shown by an enlarged side view the arrangement around the inking blade unit 3 as contained within the dashed line square of FIG. 1. The blade holder 9 is provided with two carrier flanges 11, 13, each carrying a blade 5, 7 arranged in butting and trailing positions, respectively, vis-a-vis the anilox roller 15. The anilox roller 15 is comprised of a steel cylinder 15 covered by a ceramic shell or sleeve 17, comprising Cr.sub.2O.sub.3 as a main component. As is seen in FIG. 2, blade 7 has a sealing function, whereas blade 5 has a wiping function removing excess printing ink from the surface of the ceramic sleeve 17. The inking blade unit 3 defines an inking chamber 10 together with the anilox roller 15 with blades 5, 7 in engagement on the surface of the anilox roller 15. Blades 5, 7 are each provided with a coating 43, comprising a metal matrix and at least 65 wt % of a ceramic, facing the surface of the anilox roller 15.

(8) FIG. 3a shows in a side view a steel strip 41 having an edge region 45 coated with a coating 43a comprising a metal matrix and at least 65 wt % of a ceramic. FIG. 3a furthermore illustrates schematically the thickness t and the width w of the base element as referred to herein. FIG. 3a also illustrates schematically the thickness ct and width cw of the coating as referred to herein.

(9) FIG. 3b shows a similar arrangement but with the coating 43b being provided with a rounding 44 at the longitudinal region of the doctor blade adapted for contact with the anilox roller 15.

(10) FIG. 3c shows an embodiment with the steel strip 41 being provided with a bevel 45c on the edge region, the coating 43c having a corresponding triangular configuration. The coating 43c has as well as a rounding 44 at the longitudinal region of the doctor blade adapted for contact with the anilox roller 15.

(11) In FIGS. 3b and 3c, the doctor blade comprises a front face 46, an adjacent face 47, and a rounded cross-section 44 along the longitudinal region of the doctor blade adapted for contact with the anilox roller; the rounded cross-section has a diameter comprising a center and the center of the diameter is located substantially at a bisectrix between the front face and the adjacent face of the doctor blade.

(12) FIG. 3d shows an embodiment of the lamella type, wherein the strip edge region 45d has a recess opposite to the coating 43d. The coating 43d is shown with a square shape, but may alternatively have a rounded shape, at the longitudinal region of the doctor blade adapted for contact with the anilox roller 15.

EXAMPLES

(13) The invention will now be further illustrated by examples disclosing experimental procedures, data and images illustrating the inventive concept. Throughout the examples the symbol wt % is used to denote % by weight. It should, however, be noted that the present invention is in no respect restricted to the conditions and materials disclosed in the examples. Rather, the invention is restricted only as reflected by the scope of the claims.

Example 1. Material Behavior

(14) Pin-on-Disc tribometer tests according to ASTM G 99 were conducted to analyze the abrasion wear and friction behavior of a variety of materials listed in the tables below.

(15) A fixed pin coated by thermal spraying with the respective materials listed in the tables was loaded against rotating discs of cast iron. Cast iron was selected to represent an appropriate counter surface in order to accelerate the wear process to be evaluated. The wear of the pin coating was calculated as the mass loss divided by the sliding distance and the load, and was reported as the pin wear coefficient. The wear of discs was measured as the depth of the wear track, and was reported as the disc wear depth. The pin and disc temperatures were measured. The friction force was calculated as end of test average.

(16) TABLE-US-00001 Material (oxide ceramics, 60 wt % Al.sub.2O.sub.3 97 wt % Al.sub.2O.sub.3 100 wt % comparative examples) 40 wt % ZrO.sub.2 3 wt % TiO.sub.2 Cr.sub.2O.sub.3 Pin wear coefficient 8.64E−10 1.57E−08 1.25E−08 (g m.sup.−1 N.sup.−1) Disc wear depth (μm) 14 243 154 Pin temperature (° C.) 90 90 100 Disc temperature (° C.) 154 154 130 Friction force (N) 80 36 70

(17) TABLE-US-00002 Material 80 wt % CrC 86 wt % WC 73 wt % WC (carbide particles in 17 wt % Ni 10 wt % Co 20 wt % CrC metal matrix) 3 wt % Cr 4 wt % Cr 7 wt % Ni Pin wear coefficient 4.32E−10 1.20E−09 1.67E−09 (g m.sup.−1 N.sup.−1) Disc wear depth (μm) 22 26 20 Pin temperature (° C.) 72 140 100 Disc temperature (° C.) 132 152 150 Friction force (N) 44 94 60

Example 2. Surface Quality of the Blade and Material Compatibility

(18) Doctor blades were manufactured by providing steel strips with coatings comprising CrC in a NiCr matrix by thermal spraying. CrC—Ni—Cr 80/17/3 wt % powders having different CrC particle size (about 5 μm and about 3.5 μm, particle size distribution average, Fisher Sub Sieve Sizer (FSSS) standard) were used as raw materials for the thermal spraying. Doctor blades having CrC—Ni—Cr coatings of different Vickers hardness (1050 Hv and 900 Hv) were obtained. The doctor blades were tested during 138 hours of operation on a full-scale flexographic printer with the following conditions and parameters.

(19) Machine: Windmoeller & Hoelscher—Miraflex CM—8 units

(20) Speed: 300 m/min

(21) Anilox roller (lineation): 300 l/cm

(22) Cell transfer volume: 3.5 cm.sup.3/m.sup.2

(23) Pressure: 1.8 bar

(24) Chambered doctor blade: Yes (negative position)

(25) Work: Process

(26) Ink: Cyan (solvent-based)

(27) Viscosity: 19-20″ DIN cup 4

(28) Substrate: polymer films (BOPP, PET, OPA)

(29) No printing defects were detected. The best result in this configuration was achieved with a CrC carbide size of around 5 microns (particle size distribution average—FSSS standard). It was noted that higher hardness of the coating rendered an increased longevity of the blade.

(30) It is expected that depending on the mechanical stress and physical constraints applied in the printing configuration, other materials could potentially perform better. Such stress and constraints are dependent on many parameters, such as blade contact pressure, counter-face (anilox roller) rotation speed, ink type and amount (lubricant effect). Examples of other CrC based materials could comprise a CrC content of at least 65 wt % and a metal matrix content below 35 wt %.

Example 3. Blade Tip Design

(31) Doctor blades were manufactured by providing steel strips with coatings comprising CrC in a NiCr matrix by thermal spraying. A CrC—Ni—Cr 80/17/3 wt % powder was used as raw material for the thermal spraying. The coatings formed were ground to obtain top and front surfaces meeting at an angle of about 90°, and subsequently polished to obtain a rounded shape of 30 μm diameter at the edge of the doctor blade intended for contact with the anilox roller. The doctor blades were tested on a full-scale flexographic printer with the following conditions and parameters.

(32) Machine: Fischer & Krecke—Flexpress 16S—8 units

(33) Speed: 250 m/min

(34) Anilox roller (lineation): Harper 420 l/cm and Inoflex 420 l/cm

(35) Cell transfer volume: 3.4 cm.sup.3/m.sup.2

(36) Pressure: 3.4-3.5 bar

(37) Chambered doctor blade: Yes (negative position)

(38) Work: Process

(39) Ink: Cyan (solvent-based Siegwerk NC-402)

(40) Viscosity: 21-22″ DIN cup 4

(41) Substrate: polymer film (LD-PE (white))

(42) The main objective of this test was to investigate the influence of the blade tip design on the doctoring effectiveness and quality in order to optimize the ink dynamics management. A good printing result, at least as good as for a reference lamella type steel blade having a front angle for adaptation to the anilox roller, was achieved with the rounded edge carbide based doctor blades.

(43) It is expected that depending on the fluid dynamics in the application, other similar blade tip designs could potentially perform better. Such hydrodynamic properties are dependent on many parameters, such as blade contact pressure, counter-face (anilox roller) rotation speed, ink type and amount (lubricant effect). Examples of similar blade tip designs involving a rounding could have a diameter in the range of about 10 to 50 μm.

Example 4. Scanning Electron Microscope (SEM) Images

(44) FIGS. 4a and 4b are SEM images of the tip of doctor blades that have been used against an anilox roller. In these figures, the blade top (outside the ink chamber) is denoted by T, the sliding surface (in contact with the anilox roller surface) is denoted by S, and the blade front (inside the ink chamber) is denoted by F.

(45) FIG. 4a is an image of the tip of a doctor blade having an Al.sub.2O.sub.3—ZrO.sub.2 coating comprising 60 wt % Al.sub.2O.sub.3 and 40 wt % ZrO.sub.2. Encircled on the right is a local defect having a size of about 40 μm. Encircled on the left is a smaller defect extending all through the sliding surface, leading to a potential continuous leak of ink. These kinds of defects are common in such ceramic material. Especially defects of the type on the right may be even larger. As a remark, this doctor blade has a narrow sliding surface because it was prematurely removed from a printing unit due to a quality issue.

(46) FIG. 4b is an image of the tip of a doctor blade from Example 2 (CrC—Ni—Cr 80/17/3 wt %, CrC particle size about 5 μm (FSSS)). Encircled is a local defect having a size of no more than about 15 μm. This defect is the largest found in the analyses of worn blades from the full-scale tests of Example 2. The sliding surface is much wider than in FIG. 4a, indicating that this doctor blade has been in operation in the printing unit for a long time without any quality issue.

Example 5. Printing Tests

(47) Doctor blades were manufactured by providing steel strips with coatings comprising CrC and WC in a NiCrCo matrix by thermal spraying. A CrC—WC-metal 45/37/18 wt % powder, the 18 wt % of metal being Ni—Cr—Co 12/3/3 wt %, was used as raw material for the thermal spraying. The CrC particle size was about 5 μm and the WC particle size was around 2.5 μm (particle size distribution average, Fischer Sub Sieve Sizer (FSSS) standard). These doctor blades were compared to the blades of Example 2 in operation on a full-scale flexogaphic printer with the following conditions and parameters.
Machine: Comexi Fi 160-8 units
Speed: 250 m/min
Anilox roller (lineation): Apex (480 l/cm for process ink and 200 l/cm for white ink) and majority Sandon i-Pro (480 l/cm for process ink and 200 l/cm for white ink)
Cell transfer volume: 3.5 cm.sup.3/m.sup.2 for process ink and 10 cm.sup.3/m.sup.2 for white ink
Pressure: 3 bar
Chambered doctor blade: Yes (negative position)
Work: White and process
Ink: solvent-based white and solvent-based magenta
Viscosity: 20″ DIN cup 4
Substrate: transparent foil (PE)
It is known that white ink have a higher abrasiveness than process inks (cyan-magenta-yellow), except black. The potential abrasiveness could be 5 to 10 times higher in the case of white inks.
CrC based coatings brought the highest printing quality. In a more aggressive environment in terms of wear (influenced by the ink type, anilox rotation speed, blade contact pressure, ink amount etc.), CrC—WC based coatings were found to bring an acceptable printing quality and a good productivity.

First Itemized List of Embodiments

(48) 1. A doctor blade for contact with an anilox roller, the doctor blade comprising a flat, elongate base element having a thickness of less than about 0.3 mm, which, along a longitudinal region of the doctor blade adapted for contact with said anilox roller, is provided with a coating, wherein the coating comprises a metal matrix and at least about 65% by weight of one or more ceramic(s) and wherein the coating comprises 0 to 65% by weight of chromium carbide.
2. A doctor blade according to item 1, the one or more ceramic(s) being one or more carbide ceramic(s), nitride ceramic(s) and/or oxide ceramic(s), the carbide ceramic(s) preferably being one or more metal carbide(s), more preferably one or more of chromium carbide, tungsten carbide and silicon carbide, most preferably one or both of chromium carbide and tungsten carbide.
3. A doctor blade according to item 1, the one or more ceramic(s) being one or more carbide ceramic(s) among which chromium carbide is present, preferably one or more metal carbide(s) among which chromium carbide is present, more preferably chromium carbide and none, one or both of tungsten carbide and silicon carbide.
4. A doctor blade according to any one of the preceding items, wherein the coating comprises 0 to 60% by weight of chromium carbide, preferably 0 to 30% by weigh of chromium carbide, more preferably wherein the coating is essentially free from chromium carbide.
5. A doctor blade according to any one of the preceding items, wherein chromium carbide is present in the coating in an amount of up to 65% by weight, preferably from 0.1 to 65% by weight, more preferably from 10 to 60% by weight, most preferably from 10 to 30 or from 30 to 60% by weight.
6. A doctor blade according to any one of the preceding items, wherein the coating comprises 0 to 85% by weight, preferably 0 to 65% by weight, more preferably 0 to 55% by weight, of tungsten carbide.
7. A doctor blade according to any one of the preceding items, wherein tungsten carbide is present in the coating in an amount of up to 90% by weight, preferably from 0.1 to 90% by weight, more preferably from 25 to 85% by weight, most preferably from 25 to 55 or from 55 to 85% by weight.
8. A doctor blade according to any one of the preceding items, wherein chromium carbide and tungsten carbide are present in the coating in an amount of 10 to 60% by weight of CrC and 25 to 85% by weight of WC, more preferably 10 to 30% by weight of CrC and 55 to 85% by weight of WC, or 30 to 60% by weight of CrC and 25 to 55% by weight of WC.
9. A doctor blade according to any one of the preceding items, wherein silicon carbide is present in the coating in an amount of up to 90% by weight, preferably from 0.1 to 90% by weight, more preferably from 25 to 85% by weight, most preferably from 25 to 55 or from 55 to 85% by weight.
10. A doctor blade according to any one of the preceding items, wherein the coating comprises at least about 5% by weight, preferably at least about 10% by weight, of the metal matrix.
11. A doctor blade according to any one of the preceding items, wherein the metal matrix comprises nickel, cobalt or chromium, or a combination thereof, preferably nickel and chromium.
12. A doctor blade according to any one of the preceding items, wherein the coating comprises about 70 to 90% by weight, preferably about 75 to 85% by weight, of the one or more ceramic(s).
13. A doctor blade according to any one of the preceding items, wherein the coating has a thickness of about 15 to 60 μm, preferably of about 30 to 40 μm.
14. A doctor blade according to any one of the preceding items, wherein the base element is a steel strip.
15. A doctor blade according to any one of the preceding items, wherein the base element has a thickness of about 0.1 to 0.25 mm, preferably of about 0.15 to 0.25 mm.
16. A doctor blade according to any one of the preceding items, having a rounded cross-section along the longitudinal region of the doctor blade adapted for contact with said anilox roller.
17. A doctor blade according to item 16, wherein the rounded cross-section has a diameter of about 10 to 50 μm, preferably of about 20 to 40 μm, more preferably of about 25 to 35 μm.
18. A doctor blade according to any one of the preceding items, wherein said anilox roller has a surface layer of a ceramic material, such as a ceramic coating, shell or sleeve, the ceramic material preferably comprising Cr.sub.2O.sub.3 as a main component.
19. An inking arrangement comprising an anilox roller and a doctor blade for contact with the anilox roller, the doctor blade comprising a flat, elongate base element, which, along a longitudinal region of the doctor blade adapted for contact with said anilox roller, is provided with a coating, wherein the coating comprises a metal matrix and at least about 65% by weight of one or more ceramic(s) and wherein the coating comprises 0 to 65% by weight of chromium carbide.
20. An inking arrangement according to item 19, further defined as in any one of items 2 to 14 or items 16 to 18.
21. An inking arrangement according to item 19 or 20, wherein the base element has a thickness of less than about 0.3 mm, preferably about 0.1 to 0.25 mm, more preferably about 0.15 to 0.25 mm.
22. Use of a doctor blade comprising a flat, elongate base element, which, along a longitudinal region of the doctor blade, is provided with a coating, wherein the coating comprises a metal matrix and at least about 65% by weight of one or more ceramic(s) and wherein the coating comprises 0 to 65% by weight of chromium carbide, in flexographic printing, preferably for contact with an anilox roller.
23. Use according to item 22, further defined as in any one of items 1 to 21.

Second Itemized List of Embodiments

(49) 1. A doctor blade for contact with an anilox roller, the doctor blade comprising a flat, elongate base element, which, along a longitudinal region of the doctor blade adapted for contact with said anilox roller, is provided with a coating, the coating comprising a metal matrix and at least about 65% by weight of a ceramic, such as a carbide ceramic, a nitride ceramic or an oxide ceramic.
2. A doctor blade according to item 1, wherein the ceramic is a metal carbide, preferably chromium carbide.
3. A doctor blade according to item 1 or 2, wherein the coating comprises at least about 5% by weight, preferably at least about 10% by weight, of the metal matrix.
4. A doctor blade according to any one of the preceding items, wherein the metal matrix comprises nickel, cobalt or chromium, or a combination thereof, preferably nickel and chromium.
5. A doctor blade according to any one of the preceding items, wherein the coating comprises about 70 to 90% by weight, preferably about 75 to 85% by weight, of the ceramic.
6. A doctor blade according to any one of the preceding items, wherein the coating has a thickness of about 15 to 60 μm, preferably of about 30 to 40 μm.
7. A doctor blade according to any one of the preceding items, wherein the base element is a steel strip.
8. A doctor blade according to any one of the preceding items, wherein the base element has a thickness of less than about 0.3 mm, preferably of about 0.1 to 0.25 mm, more preferably of about 0.15 to 0.25 mm.
9. A doctor blade according to any one of the preceding items, having a rounded cross-section along the longitudinal region of the doctor blade adapted for contact with said anilox roller.
10. A doctor blade according to item 9, wherein the rounded cross-section has a diameter of about 10 to 50 μm, preferably of about 20 to 40 μm, more preferably of about 25 to 35 μm.
11. A doctor blade according to any one of the preceding items, wherein said anilox roller has a surface layer of a ceramic material, such as a ceramic coating, shell or sleeve, the ceramic material preferably comprising Cr.sub.2O.sub.3 as a main component.
12. An inking arrangement comprising an anilox roller and a doctor blade for contact with the anilox roller, the doctor blade comprising a flat, elongate base element, which, along a longitudinal region of the doctor blade adapted for contact with said anilox roller, is provided with a coating, the coating comprising a metal matrix and at least about 65% by weight of a ceramic, such as a carbide ceramic, a nitride ceramic or an oxide ceramic.
13. An inking arrangement according to item 12, further defined as in any one of items 2 to 11.
14. Use of a doctor blade comprising a flat, elongate base element, which, along a longitudinal region of the doctor blade, is provided with a coating comprising a metal matrix and at least about 65% by weight of a ceramic, such as a carbide ceramic, a nitride ceramic or an oxide ceramic, in flexographic printing, preferably for contact with an anilox roller.
15. Use according to item 14, further defined as in any one of items 1 to 13.