KR20220143976A - Printing rollers containing surface coatings with low adhesion - Google Patents

Abstract

A printer for transferring ink onto a base material includes: components to transport the base material along a path through a printer; a printing station having a plurality of printheads provided to transfer solid ink onto the base material transported along the path and arranged along the path; a plurality of metallic backing rollers facing the plurality of printheads and arranged to support the base material passing between the printheads and the backing rollers; and a spreader station accommodating the base material from the printing station and provided to spread the solid ink onto the base material. The spreader station has a spreader roller and a metallic pressing roller facing the spreader roller. The ungreased metallic rollers have a coating having low adhesion to solid ink images but having sufficient lateral friction since the coating does not slip on a paper web or ink.

Description

Printing rollers containing surface coatings with low adhesion

FIELD OF THE INVENTION The present invention relates to printing rollers, and in particular to printing rollers comprising a surface coating having substantially low adhesion.

A printing roller involves discharging ink droplets from an orifice in the print head onto a receiving surface to form an image. The image is composed of a grid-like pattern of potential droplet locations, collectively referred to as pixels. Inkjet printing systems typically use direct printing or offset printing. In a typical direct printing system, the ink is ejected from a jet in the print head onto the final receiving web. In an offset printing system, an image is formed on an intermediate transfer surface and then transferred to a final receiving web. The intermediate transfer surface may take the form of a liquid layer applied to a support surface such as a drum. The print head jets ink onto the intermediate transfer surface to form an ink image. When the ink image is completely deposited, the final receiving web is then brought into contact with the intermediate transfer surface and the ink image is transferred to the final receiving web. U.S. Patent No. 5,389,958, assigned to the applicant of the present invention, is an example of an indirect or offset printing method using a phase change ink. The ink is applied to the intermediate transfer surface from its solid form to its molten molten form. The ink image solidifies on the liquid intermediate transfer surface by cooling to a malleable solid intermediate state as the drum continues to rotate. When imaging is complete, the transfer roller is placed in contact with the drum to form a compression transfer nip between the transfer roller and the intermediate transfer surface/curve surface of the drum. A final receiving web, such as a sheet of media, is then transferred to the transfer nip, and the ink image is transferred to the final receiving web. In this printer, a substantially continuous web or "substrate" (paper, plastic or other printable material) is conveyed through a path by a series of conveying elements such as rollers. The path has a preheater that brings the web to an initial predetermined temperature. Generally speaking, the liquid ink cools rapidly as it strikes the web. Each printhead is associated with a backing member, usually in the form of a bar or roller, disposed substantially opposite the printhead supporting the web on the other side. Each backing member may be heated and controlled in combination with a preheater to cause an adjacent portion of the web to reach a predetermined “ink-receiving” temperature, for example, from about 40°C to about 70°C. The molten solid ink is usually sprayed at a temperature in the range of 100 to 140° C., which is usually significantly higher than the temperature of the receiving web, so in some cases the web temperature can be further adjusted using a blower or fan behind the web at the printing station. Controlled. Past the printing station, the web is transported along a path by a series of tension rollers followed by one or more “middle-heaters”. The mid-heater brings the ink disposed on the web to a temperature suitable for the desired properties as the ink on the web is transported through subsequent “spreader” elements. The spreader element applies pressure and in some cases heat to the web to create a continuous layer by pressure, essentially catching and scrubbing away the isolated ink droplets on the web. A spreader usually has opposing rollers, such as an image side roller and a pressure roller. In one embodiment, the nip pressure between the two rollers is set in the range of about 500 to about 2000 psi lbs/side. Low nip pressures provide less line spread, while high nip pressures can shorten roller life. The spreader may also be equipped with a cleaning/oiling station associated with the image side roller, suitable for cleaning and/or applying a layer of a specific lubricant or other material to the roller surface. Such stations coat the surface of the spreader roller with a lubricant such as amino silicone oil having a viscosity of about 10 to 200 centipoise. Past the spreader, some printers are equipped with a "glosser," whose function is to alter the gloss of an image or to impart a desired surface texture. In certain machines capable of double-sided printing, turn rollers may be provided between the mid-heater and the spreader as well as at the beginning of the print path. In certain printers, 24 backing rollers and two turning rollers are provided. In a typical direct printing press, the pressure rollers are everywhere formed of a relatively soft material having a hardness of about 50D to about 65D and a modulus of elasticity of about 65 MPa to about 115 MPa. In contrast, the opposing image-side roller that contacts the inked side of the web is typically formed of a relatively hard material such as metal. In a particular embodiment, the roller is formed of anodized aluminum. Likewise, the backing roller and the turning roller are formed of the same material, namely anodized aluminum. Each of the anodized aluminum rollers is contacted with a spread and un-spread solid ink image depending on its position in the print path and depending on whether the process is single-sided or double-sided. In any press, it is desirable to minimize the amount of ink offset from the substrate or web onto the rollers. In a printer scheme as described above, if the adhesive force between the ink image and the roller is greater than the cohesive force within the ink image itself, an ink offset onto the aluminum roller will occur. One way to minimize ink offset is to keep the rollers at a relatively low temperature around 30°C. Since the temperature of the ink itself is much higher than this desired temperature, a cooling fan is needed to lower the web and ink temperature at the print station. The web and ink temperature should then be increased to about 60° C. in the spreader for optimal distribution of the ink onto the web. As a result, the process has a narrow operating range that can be energy inefficient.

Accordingly, there is a need for a roller structure that reduces the risk of ink offset onto the roller under conditions that optimize the printing process and the energy efficiency of the process. In addition, there is a need for a low-adhesive coating with poor affinity for solid ink images or with low adhesion.

According to one aspect, a printing apparatus has a plurality of rollers in contact with the ink image on a substrate, the surfaces of which have a coating with poor adhesion to the solid ink image. A press or printer for transferring ink onto a substrate comprises a plurality of components for transporting a substrate along a path through the printer, a plurality of components disposed along the path and configured to transfer solid ink onto a substrate transported along the path. a print station having a printhead, and a plurality of metallic backing rollers facing the plurality of printheads and disposed to support a substrate passing between the backing roller and the printheads. Each backing roller has a coating that has poor adhesion to solid ink. In certain embodiments, the coating is also oleophobic, preferably superoleophobic. In certain embodiments, the coating has a hexadecane sliding angle of less than 30 degrees. In another embodiment, the coating has a sliding angle of less than 30 degrees with the solid ink. The coating may have a thickness of 10 to 100 microns. The printer may further include a spreader station configured to receive the substrate from the print station and to spread the solid ink onto the substrate. The spreader station has a spreader roller and a metal pressure roller opposing the spreader roller. The pressure roller may have a low adhesion coating, an oleophobic coating or a super-oleophobic coating. In a particular embodiment, the transport elements of the printer are configured to transport a substrate for duplex printing. The elements have at least one metallic turning roller, which may have a low adhesion coating, an oleophobic coating or a super-oleophobic coating.

SUMMARY OF THE INVENTION According to the present invention, a roller structure is provided that reduces the risk of ink offset onto the roller under conditions optimizing the printing process and the energy efficiency of the process, and a low tack coating with poor affinity or low adhesion to solid ink images. is also provided.

As used herein, the term “printer” encompasses any device that performs a printing function, such as a digital copier or printer. Although this disclosure addresses phase change inkjet applications, other printing techniques through which a substrate carrying an ink image passes while in contact with a pressure element or guide element are contemplated. Although a roll or roller is described herein as a pressing element or a guiding element, other configurations are contemplated wherein the surface is in contact with the ink image on the substrate. The pressing element or guiding element has a backing roller, an image side roller and a turning roller. These rollers are believed to be non-oiled rollers within the printer. One measure of the risk of ink offset for a particular ink material and a particular pressing element relates to the adhesion between the ink material and the surface. The sliding angle is the angle of inclination at which the droplet begins to slide when the resting surface is tilted. The sliding angle can be used to measure the adhesion between the droplet and the surface. The smaller the sliding angle, the lower the adhesion. When the adhesion of the droplet to the surface is high, the droplet will not slide up to a 90 degree inclination angle. The inevitable result of the sliding angle is the contact angle, which is the angle at which the liquid/gas interface meets the solid surface. For a completely wet surface, the contact angle is 0°, meaning that the liquid is completely spread across the surface. Conversely, a never-wetted surface has a contact angle of 180°, meaning that the liquid is in the form of spherical droplets resting on the surface. With respect to the aluminum surface of the aforementioned pressing element, the contact angle of water is between 50° and 68°, the contact angle of hexadecane is between ˜4° and 6°, and the contact angle of the phase-change ink (measured at ˜105° C.) is 1.6° to 4.2°. Droplets or molten ink do not slide but do flow when tilted, indicating the ink's stickiness to the aluminum drum surface. For these aluminum elements, the low ink contact angle and tack indicate that the aluminum surface is insufficient to avoid the ink offset problem described above. The risk of ink offset requires strict temperature control throughout the ink application process and prior to the spreader station to increase ink agglomeration. Therefore, as mentioned above, the printed image should be maintained at a temperature of about 30 DEG C to minimize ink offset. In one embodiment suitable for organic solid inks, a low tack coating is applied to the pressure element or guide element of the printer. In particular, this coating is applied to the printer's non-lubricated rollers, including backing rollers and redirection rollers. In some embodiments, the coating may be applied to the pressure rollers of the spreader station. The coating exhibits low adhesion to the solid ink image, but sufficient lateral friction to not slip against the ink or paper web. As described herein, the low tack coating significantly reduces the risk of ink offset even at high operating temperatures. Thus, the entire printing process can be done at the high temperatures required in the spread station. In the system described above, the mid-heater raises the temperature of the web to 60° C. so that the ink can be spread by the spreader drum and pressure roller. Thus, with the low tack coating described herein, the web can be maintained at this 60° C. throughout its entire path through the print station. Moreover, the temperature during the entire process does not need to be tightly controlled as in conventional systems. In some systems, the mid-heater and air circulation elements can be eliminated, which reduces the overall energy requirement in the printer. This increased flexibility in temperature control may also enable the use of unheated backing rollers, where the web temperature is achieved with only a preheater. Thus, in certain embodiments, the web may be preheated to a temperature of 100° C. such that the temperature of the web decreases along the printing path to reach a predetermined 60° C. temperature at the spreader station. The low tack coatings disclosed herein exhibit abrasive properties suitable for use in printers to prevent excessive wear of the printer's rotating rollers. Suitable coatings can be made by cross-linking the diisocyanate with the hydroxyl-functionalized polyester in the presence of a polysiloxane additive and optimally a fluorolink cross-linking agent. In one embodiment, these components were formed in a polyurethane coating solution and applied to the surface of an aluminum drum. Suitable techniques for applying the coating include spraying, flowing and dipping. This transparent film can be obtained after curing the coating in a heating oven. Specific examples of the coatings disclosed herein include hydroxyl-terminated polyacrylates with Desmophen A870 BA from Bayer Material Science as component 1, and Desmodur N-3300A from Bayer Material Science as component 2 in n-butyl acetate. It can be made by mixing. A polysiloxane additive sold under the trade name Silclean™ 3700, a hydroxyl functional silicone modified polyacrylate from BYK, is added in various amounts, usually in an amount of 2 to 10% by weight relative to the main polymer. After coating and drying at 135° C. for 30 to 60 minutes, the low adhesion coating disclosed herein can be obtained. Optionally, 0.01% to 5% of a fluoro cross-linker known as Fluorolink, especially Fluorolink-D from Solvay Solexis, can be added to the coating solution to increase the contact angle of the final coating. The table below summarizes the data. The data in the table above for PTFE TEFLON, a well-known low surface energy material, is provided for comparison. PTFE TEFLON has a fairly high contact angle, but a fairly large sliding angle, indicating that it is not a suitable coating for aluminum drums for use in solid-ink presses. In fact, the contact angle and sliding angle for the solid ink under the same conditions are ˜63° and 90° for the PTFE TEFLON layer, indicating that the solid ink will stick to its surface. In certain instances, the contact angle and sliding angle (at 105° C.) of the solid ink on some of the films were found to be 50°-80° and 10°-25°, respectively, which indicates that the solid ink is low for these coatings. It indicates that it has adhesion. The data thus show that the described coatings are oleophobic. In some embodiments, it is superoleophobic. The low tack coatings described herein may be applied to the pressing element using any suitable technique including spraying, dipping, flow coating, or draw down coating. In certain embodiments, the coating is applied to a thickness of 10 to 100 microns. In some embodiments, the coating may be superoleophobic. It is contemplated that the superoleophobic coating may require more specialized techniques for application to the rollers described above. The claims, originally filed and amended, cover variations, substitutions and modifications of the embodiments and spirits described herein, including those not currently foreseen or unknown, and which may arise, for example, from others. , improvements, equivalents, and substantial equivalents.

Claims (1)

A printer for transferring ink onto a substrate includes components for transporting a substrate along a path through the printer, disposed along the path and configured to transfer solid ink onto the substrate as the substrate is transported along the path. a print station having a plurality of printheads configured and a plurality of metallic backing rollers facing the plurality of printheads and disposed to support the substrate passing between the backing rollers and the printheads; and each of the rollers comprises a surface coating having low adhesion to the solid ink.