Ceramic coated liquid transfer rolls and methods of making them

Liquid transfer rolls such as printing rolls are made by imposing a hard ceramic coating on an incipient liquid transfer roll in an electrolytic bath subjected to a modified shaped wave alternating current, which causes dielectric breakdown and the formation of a hard ceramic coating on the roll. The roll is laser engraved to form liquid carrying reservoirs or cells on the surface of the roll, either before or after the ceramic coating is built on the roll.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to the manufacture of liquid transfer rolls such as
 ceramic coated printing rolls, and particularly to hard engraved or
 embossed surfaces on aluminum and other metal rolls used for transferring
 liquid such as ink in printing.
 2. Description of the Related Art
 It is a well established and accepted practice in the printing industry,
 and in other industries where process materials such as ink, varnish, and
 adhesives are transferred from one surface to another, to apply ceramic
 coatings to liquid carrier rolls fabricated from aluminum, steel and other
 suitable materials. Very hard and wear-resistant ceramic coatings such as
 refractory oxides or metallic carbides may be applied by thermal spray
 technologies. After application of the thermal sprayed coating, the roll
 surface is ground and/or polished to a very smooth finish and then
 engraved with a high power laser to create "inkwells" or cell patterns in
 the coating. These cells carry the printing ink or other liquid process
 materials. Such coatings have virtually revolutionized the printing
 industry over the past two decades. These ceramic coatings enable the
 transfer rolls to withstand the wear generated by the continuous scraping
 of a steel knife (doctor blade) utilized to maintain a uniform film of the
 liquid being transferred on the roll's surface. The ceramic coatings are
 equally advantageous in the transfer of adhesives, varnish and other
 liquids.
 The thermal spray processes rely on the introduction of fine particles of
 the preferred ceramic into a high energy, high temperature gas stream. The
 powder particles are heated to plasticity and propelled onto the surface
 to be coated where they impact and form a mechanical bond with the
 substrate. Additional layers of the coating are applied until the desired
 coating film thickness has been achieved. Precision grinding and polishing
 techniques are then employed to create a smooth, dimensionally stable
 coating. Liquid transfer rolls are engraved at this point with laser
 equipment to form the ink-carrying (or other liquid-carrying) cells. To
 make a cell, the laser partially melts and vaporizes the ceramic coating
 it contacts at a discrete point or line to create an open channel, cell or
 hole, which becomes an inkwell (or container for other liquid). A
 by-product of the laser activity is a semi-molten slag, similar to
 volcanic lava, which forms around the hole and re-solidifies. It appears
 that this phenomenon occurs because the thermal spray coating consists of
 individual powder particles, some of which are not fully plasticized. This
 is the nature of the thermal spray process. The re-solidified ceramic is
 very hard and brittle. When the process roll is placed into service,
 pieces of the brittle ceramic can break off and become embedded in the
 roll surface or in the steel doctor blade. An embedded piece of hard
 ceramic can scratch or cut a groove in the working surface of the roll in
 a very short period of time, destroying the quality of the roll face and
 necessitating its replacement. The embedding problem has become more and
 more frequent as the cells have become smaller with the ever-increasing
 demand for higher print quality.
 A second shortcoming of the thermal sprayed ceramic coating is that a
 roughened surface remains in the cell after the laser engraving operation.
 This rough surface makes cleaning of the roll (removal of ink or other
 material) extremely difficult and time consuming. Valuable time is lost
 and it is not uncommon to damage the cells during the cleaning process.
 A desirable improved process would be one which has little or no ceramic
 re-melt or liquid flow on application of the laser to cut the ink-well
 patterns. Additionally, a desirable process would leave holes or cells
 having sharply defined patterns, having walls which are smooth and of the
 same texture as the surrounding area, to facilitate ink (and other
 material to be transferred) removal and cleanup as well as more precise
 patterns. Easier cleanup increases productivity and minimizes the chances
 of damage to the roll surface. And, a desirable process would be one which
 does not demonstrate a significant shortcoming of the thermal spray
 processes--the sometimes inadequate adherence of the coating to the roll
 substrate. This is an inherent difficulty with the thermal spray processes
 particularly for substrates, such as aluminum, having coefficients of
 thermal expansion considerably different from the ceramic coating.
 Further, it would be desirable to have a process wherein the laser-produced
 cell patterns are imparted to the roll surface prior to application of the
 ceramic coating rather than after, since application of the laser after
 coating incorporates all the above possible defects and shortcomings, and
 results in vertical surfaces in the cells which are different in
 composition from the horizontal surfaces. Coating after engraving is not
 feasible with sprayed ceramics.
 The reader will be interested in the disclosure of U.S. Pat. No. 5,616,229
 to Samsonov and Hiterer, which proposes the formation of ceramic coatings
 of up to 300 microns within about 90 minutes through the use of an
 alternating current of at least 700 volts having a shaped wave (not the
 conventional sinusoidal form) which rises from zero to its maximum height
 and falls to below 40% of its maximum height within less than a quarter of
 its full alternating cycle, thereby causing dielectric breakdown, the
 alternating current being imposed on an electrolytic bath in which the
 metal subject is an electrode, the bath comprising initially an alkali
 metal hydroxide and in a later step including an oxyacid salt of an alkali
 metal, such as sodium tetrasilicate. While the '229 patent speaks of
 forming coatings on aluminum surfaces, the authors do not treat the
 possible use of such a coating process for the manufacture of aluminum
 process rolls, where the coating step is integrated with a laser system to
 engrave or emboss a three-dimensional pattern for holding liquid.
 Laser engraving of hard-coated rolls is described in U.S. Pat. Nos.
 4,794,680, 5,089,683, 5,093,180, and 5,143,578, which are incorporated
 herein, in their entirety, by reference.
 SUMMARY OF THE INVENTION
 My invention combines the use of laser embossing and/or engraving systems
 with the formation of hard ceramic layers on aluminum-based printing and
 other process rolls. The engraving or embossing step may be used either
 before or after the coating is formed on the aluminum roll. In either
 case, the oxide ceramic coatings utilized according to this invention on
 the surface of process rolls exhibit surface hardnesses of at least 1000
 Kn.sub.100 and preferably 1300 Kn.sub.100 or more, and a density greater
 than 90% of theoretical, preferably greater than 97%, and a surface
 roughness after finishing of less than 8 micro-inches Ra, preferably less
 than about 4 micro-inches. In addition to the process described in the
 above-mentioned Samsonov U.S. Pat. No. 5,616,229, such hardnesses and
 densities may be achieved by the methods described by Hradcovsky in U.S.
 Pat. Nos.4,659,440 and 5,069,763, Hanagata U.S Pat. No. 5,147,515, and
 Erokhine et al. U.S. Pat. No. 5,720,866, all of which are incorporated
 herein in their entireties by reference. I may use any method of forming a
 hard surface on an aluminum roll; metals other than aluminum are also
 useful in my invention; particularly rolls fabricated from titanium,
 magnesium, beryllium, hafnium, zirconium, and alloys of these with or
 without aluminum, having coatings of the hardnesses and densities
 described above.
 Whether the engraving step is performed before coating or after, in a
 preferred method I use a modified shaped-wave electrolytic process to form
 a hard coating on the process roll. The process may use the teachings of
 U.S. Pat. No. 5,616,229 and accordingly that patent is hereby incorporated
 by reference, in its entirety, into this disclosure. However, the '229
 patent uses two distinct electrolytic baths for the substrates discussed,
 and I have found it is not necessary to do so for liquid transfer rolls,
 particularly of aluminum.
 My method comprises engraving, preferably by laser, a three-dimensional
 pattern on an incipient liquid transfer roll, and forming a hard coating
 on the incipient liquid transfer roll by immersing it first in an
 electrolytic bath comprising (deionized) water, an alkali metal salt or
 hydroxide (preferably potassium hydroxide) as an electrolytic agent, at a
 concentration of 0.5-7 grams per liter, and, as a passivating agent, a
 colloidal suspension of sodium silicate in the form Na.sub.2 O.xSiO.sub.2
 (x=&gt;2.55 by weight) at a concentration of 2.0-9.5 grams per liter while
 conducting through the bath a modified shaped-wave alternating electric
 current from a source of at least 250-800 volts through the surface of the
 incipient liquid process roll. The modified shaped-wave electric current
 rises from zero to its maximum height and falls to below 40% of its
 maximum height (amplitude) within less than a quarter of a full
 alternating cycle, thereby causing dielectric breakdown and the formation
 of a compact ceramic film on the roll surface.
 In another version of my invention, a hard wear resistant ceramic coating
 is formed on an incipient process roll having a blank metal surface by
 immersing it in an selectrolytic bath comprising (deionized) water, an
 electrolytic agent comprising an alkali metal salt or hydroxide
 (preferably potassium hydroxide) at a concentration of 0.5-7 grams per
 liter, and, as a passivating agent, a colloidal suspension of sodium
 silicate in the form Na.sub.2 O.xSiO.sub.2 (x=&gt;2.55 by weight) at a
 concentration of 2.0-9.5 grams per liter while conducting through the bath
 a modified shaped-wave alternating electric current from a source of at
 least 250-800 volts through the surface of the incipient printing
 (process) roll. The modified shaped-wave electric current rises from zero
 to its maximum height and falls to below 40% of its maximum height within
 less than a quarter of a full alternating cycle, thereby causing
 dielectric breakdown and the formation of a compact ceramic film on the
 roll surface. I then remove the roll from the bath, grind or polish the
 formed ceramic coating to a suitably smooth finish, and engrave or emboss
 its surface, preferably by laser, to impose a three-dimensional pattern on
 the ceramic coating surface of the roll.
 Terms
 Throughout this specification, I use the term "liquid transfer roll" to
 mean a roll designed and manufactured to carry liquid on its surface and
 transfer it to another surface. As described above, liquid transfer rolls
 have small depressions or cells placed on their surfaces, or channels,
 which serve as miniature reservoirs for liquid. The cells may have various
 shapes and are typically made by laser beams. See U.S. Pat. Nos. 5,093,180
 and 5,221,562 for descriptions of liquid transfer rolls made by laser
 engraving a hard-surface roll. A common type of liquid transfer roll is a
 printing roll. In some passages in the present specification, I may refer
 to a liquid transfer roll as simply a process roll. The liquid process
 roll may be in the form of a tube, and the term "liquid process roll" is
 used herein to include a tube or a sleeve having a cylindrical surface for
 placement over a base or holder of any suitable geometry or material.
 Surface roughness measurements described herein in Ra units reflect the
 average surface roughness measured in micro-inches according to ANSI
 Method B46.1.

DETAILED DESCRIPTION OF THE INVENTION
 The electrolytic treatment of the incipient liquid transfer roll will
 generally take about 30 to about 240 minutes to form a ceramic coating of
 25 to 300 microns (0.001 to 0.012 inch) thick. A preferred thickness for
 the coating is 100 to 200 microns (0.004 to 0.012 inch) thick. Where the
 roll is aluminum, during the process cycle the substrate temperature is
 preferably maintained at less than 60.degree. C. (140.degree. F.). The
 incipient blank liquid transfer roll will preferably have an aluminum
 surface, but it may be made of aluminum, magnesium, titanium, zirconium,
 beryllium, hafnium or alloys thereof.
 The coating process can utilize a single electrolytic bath comprising water
 and a solution of an alkali metal hydroxide concentrated at 0.5 to 2 grams
 per liter, a second bath containing water, a solution of alkali metal
 hydroxide (0.5 to 2 grams per liter) and a low concentration (1 to 2 grams
 per liter) of sodium tetrasilicate, and a third bath containing water, an
 alkali metal hydroxide concentrated at 0.5 to 5 grams per liter and a 1 to
 5 grams per liter concentration of sodium tetreasilicate.
 However, a preferred method of coating is to utilize a single bath wherein
 the electrolyte solution comprises deionized water, potassium hydroxide
 concentrated at 0.5 -7 grams per liter and a colloidal suspension of
 sodium silicate in the form Na.sub.2 O.xSiO.sub.2 (x=&gt;2.55 by weight). The
 roll to be coated comprises one electrode and the container for the
 electrolyte comprises the other electrode. A modified shaped-wave charge
 of at least 250 volts is passed through the surface of the incipient roll
 causing dielectric breakdown and formation of a compact ceramic film on
 the surface of the roll. The ceramic thus formed comprises aluminum and
 silicon oxides the composition (oxide content and proportions of Al and
 Si) of which may vary somewhat as influenced by the substrate metal and
 the conditions of formation.
 Voltages greater than 800 are unnecessary to the formation of the ceramic
 and voltages in excess of 800 are not recommended because they will
 overheat the electrolytic solution. Voltages less than 250 are not
 recommended because uniform breakdown of the electrolyte will not occur
 and film growth rates will not be efficient or uniform. Amperages and
 cycles are more or less conventional--100 amperes per square foot of
 treated surface is adequate and 50-70 cycles per second is satisfactory.
 The electrolytic fluid is an aqueous solution comprising 2 to 60 or more
 grams per liter, preferably 2 to 15 grams per liter, of a passivating
 agent comprising a soluble silicate, polyphosphate, chromate, molybdate,
 vanadate, tungstate or aluminate salt, the preferred passivating agent
 being sodium silicate (Na.sub.2 SiO.sub.3) in the form of a colloidal
 suspension, and, as an electrolytic agent, 0.5 to 3 grams per liter of a
 strong acid, strong alkali or strong acid or alkaline salt; suitable
 electrolytic agents are H.sub.2 SO.sub.4, KOH, NaOH, NaF, Na.sub.2
 SO.sub.4, H.sub.3 PO.sub.4, and NaPO.sub.4, the preferred electrolytic
 agent being KOH. Any known or commercially used passivating agent may be
 used, such as Na.sub.2 SiO.sub.3, K.sub.2 SiO.sub.3, Na.sub.6 P.sub.6
 O.sub.18, Na.sub.2 Cr.sub.2 O.sub.2, Na.sub.2 Cr.sub.2 O.sub.7, Na.sub.2
 Mo.sub.2 O.sub.7, K.sub.2 Cr.sub.2 O.sub.7, Na.sub.2 V.sub.2 O.sub.7,
 K.sub.2 V.sub.2 O.sub.7, Na.sub.2 WO.sub.4, K.sub.2 WO.sub.4 and
 KAlO.sub.2.
 A laser engraving process is preferred for placing the desired cell pattern
 on the coated surface of the roll. Process rolls so engraved or embossed
 are utilized to distribute, among other materials, printing inks, hot
 melts, cold seals, and overvarnishes. Additionally, continuous designs
 including woodgrain patterns, wall coverings and specialty applications
 are achievable. Engraving patterns are achievable to improve laydown of
 waterborne inks and coatings as well as solvents. Honeycomb configurations
 used in flexographic printing and coating applications are readily
 produced in the ceramic coating of this invention as are hexagonal,
 diamond, square or channel cell shapes. Typically the depth of the
 "inkwells" or cells will be on the order of 50-75 microns (0.002 to 0.003
 inch); thus, where the coating is already on the roll, the cells will not
 be made deeper than the thickness of the coating. When the coating is
 placed on the roll after the laser engraving, the engraving dimensions
 need not be calculated to allow for a thick coating on top of the engraved
 surface in three dimensions, because, as indicated above, the fully formed
 coating will in most cases not protrude beyond the original
 dimensions--that is, the coating will not add to the diameter of the roll.
 Following is an exemplary procedure for the manufacture of a liquid
 transfer roll by first placing the ceramic coating on a roll and then
 engraving it with a laser.
 Procedure 1
 Fabrication of the starting process roll is accomplished in accordance with
 conventional machining and/or grinding practices to at or near the final
 dimensions specified. The substrate material is preferably aluminum, but
 may be titanium, magnesium, hafnium, zirconium, beryllium or alloys
 thereof. In some cases it may be feasible to fabricate a sleeve from any
 of these metals to be fitted to a process roll originally fabricated from
 other materials such as steel or other base material.
 The fabricated roll or sleeve is cleaned of surface contaminants by any
 suitable method.
 The fabricated roll or sleeve is attached to a fixture or mechanism such
 that it may be immersed in an aqueous electrolyte bath containing an
 electrolytic agent and a passivating agent. The mechanism positions the
 process roll in the electrolyte and is connected to an alternating current
 voltage source. The bath container is connected to the voltage source such
 that a modified shaped-wave electric current of at least 250 volts is
 conducted through the surface of the process roll, causing dielectric
 breakdown and the formation of a compact oxide ceramic film on the roll
 surface. The process roll or sleeve remains in the electrolyte, connected
 to the voltage source, for a predetermined time period, usually from about
 30 minutes to about 240 minutes, sufficient to allow formation of an oxide
 ceramic film of from 25 microns (0.001 inch) to 300 microns (0.012 inch)
 thick. Formation of the oxide ceramic film does not substantially increase
 the dimension of the process roll.
 Upon completion of the coating formation cycle, the process roll or sleeve
 is removed from the electrolyte container, fixturing mechanisms are
 removed and the process roll or sleeve is subjected to grinding or
 polishing techniques well known to those skilled in the art of finishing
 ceramic coatings and/or materials such that a suitably smooth finish of
 about 12 micro-inches Ra, preferably less than 8 micro-inches Ra, is
 achieved. The surface finish may be enhanced to less than 4 micro-inches
 Ra utilizing microfinishing (also known as superfinishing) techniques such
 as continuously moving a film backed diamond tape over the surface of the
 roll, that might be rotated continuously, until the value is achieved.
 Having achieved a surface roughness value of about 12 micro-inches Ra or
 less on the oxide ceramic surface, the process roll or sleeve may be laser
 engraved according to practices will known to those skilled in the
 techniques of laser engraving oxide ceramic coatings (U.S. Pat. Nos.
 4,794,680, 5,089,683, and 5,143,578) or those disclosed in U.S. Pat. Nos.
 5,093,180 and 5,221,5621, also incorporated herein by reference.
 Following laser engraving of the oxide ceramic film on the process roll or
 sleeve, it may be desirable to again employ the microfinishing steps noted
 above to achieve a surface roughness value on the land areas between the
 inkwells or cells of 2 to 6 microinches Ra.
 Following is an exemplary procedure for the manufacture of a liquid
 transfer roll by first engraving a blank aluminum surface roll with a
 laser, and then forming a hard ceramic coating on it.
 Procedure 2
 Fabrication of the starting process roll is accomplished in accordance with
 conventional machining and/or grinding practices to at or near the final
 dimensions specified. The substrate material is preferably aluminum, but
 may be titanium, magnesium, hafnium, zirconium, beryllium or alloys
 thereof. In some cases it may be feasible to fabricate a sleeve from any
 of these metals to be fitted to a process roll or other support originally
 fabricated from other materials such as steel or other base material.
 Following fabrication and the final finishing operations, the uncoated roll
 or sleeve is laser engraved as described elsewhere herein to develop
 inkwells or cell patterns as desired in the final operating condition of
 the roll or sleeve. It is not necessary to substantially change the
 inkwell or cell dimensions to allow for the subsequent formation of the
 oxide ceramic film, nor is it necessary to employ laser engraving
 techniques that are any different than one skilled in the practice might
 utilize when laser engraving the desired cell configuration into a
 substrate material that does not have a surface film or coating applied.
 The fabricated and engraved roll or sleeve is attached to a fixture or
 mechanism such that it may be immersed in an aqueous electrolyte bath
 containing an electrolytic agent and a passivating agent. Said mechanism
 positions the process roll or sleeve in the electrolyte and is connected
 to an alternating current voltage source. The bath container is connected
 to the voltage source such that a modified shaped-wave electric current of
 at least 250 volts is conducted through the surface of the process roll,
 causing dielectric breakdown and the formation of a compact oxide ceramic
 film on the roll surface. The process roll or sleeve remains in the
 electrolyte, connected to the voltage source, for a predetermined time
 period, usually from about 30 minutes to about 240 minutes, sufficient to
 allow formation of an oxide ceramic film of from 25 microns (0.001 inch)
 to 300 microns(0.012 inch) thick. Formation of the oxide ceramic film does
 not substantially increase the dimension of the process roll. A preferred
 oxide ceramic film thickness for a process roll or sleeve that has been
 laser engraved prior to the film application is about 50 to 75 microns
 (0.002 to 0.003 inch) thick.
 Upon achieving the desired oxide ceramic film thickness, the process roll
 or sleeve is removed from the electrolyte container, fixturing mechanisms
 removed and the process roll or sleeve is microfinished according to the
 techniques and steps described herein to achieve a surface roughness of
 less than 12 micro-inches Ra, preferably less than 8, and most preferably
 about 4 micro-inches Ra or less. The preferred surface has a density of at
 least 95% of theoretical; see U.S. Pat. No. 5,093,180.
 I prefer to perform process steps in addition to those recited above. In
 particular, I believe it is advantageous to perform the following steps to
 make a liquid transfer roll of high quality:
 1. The blank roll, having a metal surface such as described above, as
 received from the manufacturer or after having been manufactured in house,
 is cleaned to remove whatever contaminants may be present on the surface;
 2. The surface is converted to a ceramic by the procedure described above.
 During the electrolytic process, some of the aluminum or other metal on
 the surface is converted to an oxide such as A1.sub.2 O.sub.3 ; some
 SiO.sub.2 is incorporated into the surface of the roll. Temperatures at
 the surface during dielectric breakdown reach over 1000.degree. F. and as
 high as 3000.degree. F., and the silica is substantially welded to the
 aluminum oxide and the substrate. Each spark or waveform creates a spark
 and/or dielectric breakdown causing such high temperatures. The ceramic
 will tend to have a crystalline structure which will vary with the
 particulars of the conditions and materials used. I call the coating an
 "oxide ceramic" coating. It is believed the oxygen which combines with the
 aluminum or other substrate material is contributed by the electrolytic
 agent from strong acids, strong alkalis or strong salts such as H.sub.2
 SO.sub.4, KOH, NaOH, NaF, Na.sub.2 SO.sub.4, H.sub.3 PO.sub.4, and
 NaPO.sub.4, the preferred electrolytic agent being KOH.
 3. The roll is then sealed to protect against the possibility that there
 may be cracks or other defects in the ceramic surface which will be
 vulnerable to corrosion from the liquids to be transferred. I prefer to
 seal with an epoxy or similar sealant such as LOCTITE 12-90 which is
 commercially available from Loctite Corporation, Newington Conn.
 4. The cylindrical shape of the roll is then checked and assured.
 5. The oxide ceramic surface is then ground smooth, to a surface roughness
 of about 12 micro inches Ra or finer. Microfinishing (also known as
 superfinishing) techniques may subsequently be employed to achieve a
 surface finish of about 4 or less micro-inches Ra.
 6. The roll is then laser engraved. Instead of melting as in the prior art,
 the coating will substantially evaporate, leaving clean, sharp sides on
 the cells.