powder to spray the ceramic coating; thermal spraying of the duplex (bond coating and ... Keywords anilox rolls, microstructure ofCr203 coatings, plasma.
JTTEE5 5:317-334 9 International
Technology of Thermally Sprayed Anilox Rolls: State of Art, Problems, and Perspectives L. Pawlowski
This paper deals with the surfacing technology of ceramic anilox rolls. The rolls are used in the printing industry to transport the precisely determined quantity of ink in the flexographic printing machines. The technology of roll surfacing is discussed by taking the following aspects into account: preparation of the powder to spray the ceramic coating; thermal spraying of the duplex (bond coating and ceramic top coating); postspray finishing by grinding and polishing; and laser engraving. The powder used as the top coating of the aniloxes is chromium oxide. This powder might be prepared by such techniques as agglomeration, fusing, and crushing, etc. The preparation technique influences coating properties, such as microstructure (tested with SEM, OM, XRD, and XPS), open porosity, microhardness, and modulus of elasticity. Comparison of these properties enables optimum powder preparation techniques to be found. APS technique is used to coat the anilox rolls. Optimization of the plasma spraying parameters is discussed. Aniloxes are submitted to the grinding and polishing of the ceramic coating before laser engraving occurs. The final roughness of the finished coating is discussed in view of an optimum absorption of the laser light energy at engraving. Possible ways of reducing the spraying time are discussed, and future research toward improving the anilox roll quality is proposed. Nomenclature
A AD APS AR AS C CTE FC FS HEPS HVOF HV IPS LD MIP NA NC OM q Rz SC SEM SPS VPS XPS XRD
powder production technique by agglomeration (spray drying) powder production technique by agglomeration and subsequent densification atmospheric plasma spraying anilox rolls arc spraying powder production technique enabling crystalline particles to be obtained coefficient of thermal expansion powder production technique by fusing and crushing flame spraying high energy plasma spraying high-velocity oxygen fuel Vickers microhardness inert plasma spraying line density mercury intrusion porosimeter powder of NiAI powder of NiCr optical microscope powder feed rate coefficient describing roughness powder production technique by sintering and crushing scanning electron microscope shrouded plasma spraying vacuum plasma spraying x-ray photoelectron spectroscopy x-ray diffraction
L. Pawlowski, Universit6 d'Artois, Facult6 des Sciences Appliqu6es, Laboratoire Laser-Plasma-Mat6riaux, Technoparc Futura, F-62400 B6thune, France.
Journal of Thermal Spray Technology
Keywords aniloxrolls, microstructureofCr203 coatings,plasma sprayingof ceramics,propertiesof Cr203 coatings
1. Introduction AN1LOX rolls (AR) used in flexographic machines started to be coated with ceramic plasma sprayed coatings about 10 years ago. Apart from thermal spraying, the AR surfacing technology includes mechanical grinding and polishing and laser engraving. The greatest difficulty results from the diversity of technological processes coming from the distant fields of the surfacing, mechanical, and optical engineering. The present review is based on professional records of the author, available catalogs of the workshops producing AR, and discussions with the experts working in the field.
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2
Fig. 1 Partof a flexographic printing machine: 1, anilox roll; 2 doctor blade; 3, ink container; 4, printing roll
Volume 5(3) September 1996---317
The anilox rolls are used in printing machines working in the flexographic system (see Fig. 1). The roll (1), being in contact with a doctor blade (2), transports the precisely defined quantity of ink from the ink container (3) onto the printing roll (4).The used inks are water or solvent based and have different viscosities (depending on the solid content in the ink varying from 25 to 50 vol%).The quantity of the transported ink is determined by depth and density of the cell engraved in the roll surface. Excessive ink is removed by the doctor blade or the doctor roll. A typical roll is shown in Fig. 2. The prior conventional technology of AR includes galvanic deposition of copper or nickel bond coating, followed by mechanical engraving with diamond indenters, and finally, the galvanic deposition of a wear-resistant chromium top coating. Modern rolls are ceramic with plasma sprayed chromium oxide top coating deposited onto a metallic bond coat. The top coating is ground and polished, engraved with a laser, and polished prior to service. The quality of the ceramic top coating is decisive in the AR quality in service. The properties of this coating are specified in Table 1 (Ref 1).
The specified properties could be fulfilled by a plasma sprayed alumina coating. However, alumina becomes translucent after laser engraving, and it is impossible to make the microscopic measurements of the depth of the engraved cells. Another possible coating, for example alumina titania alloy, interacts with some printing inks (Ref 2). Finally, the chromium oxide coating is used and discussed in this paper.
2. Materials 2.1 Roll New rolls are presently made of stainless steel. The older, refurbished ones are made of lower grade steels that corrode in contact with water-based inks. The concentricity of the roll must be at least 0.02 mm, and the roll must be well balanced. The weight of the roll ranges from a few hundred to several thousand kilograms.
2.2 Powders (Wires)for Bond Coatings
Table I A preliminary specification for the ceramic top coating of anilox rolls Coating property
No.
Property
specification
Remarks
1
Open and interconnected porosity
Low (
