Abstract:
An apparatus fixes ink on a substrate, such as in ink-jet printing. A leveling member is positioned to contact an ink-bearing side of the substrate at a nip. A radiation source is positioned to direct radiation to the ink-bearing side of the substrate at the nip, the radiation suitable for curing the ink on the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Cross-reference is hereby made to the following U.S. Pat. applications, assigned to the assignee hereof: U.S. application Ser. No. 12/256,684, U.S. application Ser. No. 12/256,690 being filed simultaneously herewith; and U.S. application Ser. No. 11/291,284, filed Nov. 30, 2005, now US Patent Application Publication US 2007/0120930 A1. 
     INCORPORATION BY REFERENCE 
     The following documents are incorporated by reference in their entireties for the teachings therein: US Patent Application Publication US 2007/0120930 A1; and US Patent Application Publication US 2008/0122914 A1. 
     TECHNICAL FIELD 
     The present disclosure relates to printing with radiation-curable inks. 
     BACKGROUND 
     US Patent Application Publication US 2008/0122914 A1 discloses compositions for an ultraviolet (UV)-curable ink suitable for use in ink-jet printing. Such inks include one or more co-monomers and a gellant. When exposed to radiation of a predetermined frequency, these co-monomers polymerize and thus bind to any number of types of surfaces. In practical applications, such inks have a viscous property at room temperature, but become more liquid when heated for jetting onto a substrate to form images. 
     US Patent Application Publication US 2007/0120930 A1 discloses a printing apparatus suitable for use with a radiation-curable ink. The apparatus uses a “transfuse” system, wherein ink forming the desired image is first jetted onto an image receptor in the form of a belt, and then transferred from the image receptor onto a print sheet or other substrate. At various locations along the belt path are disposed ultraviolet radiation sources for partially hardening the ink on the belt before transferring to the print sheet. 
     Although the above-described apparatus uses an image receptor to apply ink to a print sheet, it would be desirable to provide a system where such an ink as above described could be applied directly to a print sheet or other substrate. One challenge to such a system is that, in practical applications, such inks tend to have a “mayonnaise” consistency at room temperature, but when heated incidental to jetting, change to a low viscosity liquid. A typical ink-jet printing process heats the ink until it is liquid and then directly fires ink droplets from a piezoelectric print head onto the substrate. Once the ejected ink hits the substrate, it changes phase from the liquid back to its more viscous consistency, thereby reducing its penetration into porous media. Once this ink is exposed to UV radiation, photoinitiators in the ink are bombarded with UV radiation and the incident flux converts the monomers present in the ink into a cross linked polymer matrix resulting in a very hard and durable mark on the paper. 
     However, there is a desire to have the ink leveled prior to having it UV cured. The reason for this is so that gloss is more uniform, missing jets can be masked, and certain applications such as packaging require thin layers of relatively constant thickness. Since these inks have a mayonnaise consistency, they have very little cohesive strength prior to curing. In addition, the inks are typically designed to have good affinity to many materials. This means that conventional methods for flattening a layer of ink tend to fail, because the ink splits and leaves much of the image behind on the device trying to flatten it, such as a traditional fuser roll as familiar in xerography. The present description proposes a way to resolve this issue. 
     SUMMARY 
     According to one aspect, there is provided an apparatus for fixing ink on a substrate. A leveling member is positioned to contact an ink-bearing side of the substrate at a nip. A first radiation source is positioned to direct radiation to the ink-bearing side of the substrate at the nip, the radiation suitable for curing the ink on the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified elevational view of a fixing apparatus, as would be found in a larger printing apparatus, according to a first embodiment. 
         FIG. 2  is a simplified elevational view of a fixing apparatus according to a second embodiment. 
         FIG. 3  is a simplified elevational view of a fixing apparatus according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified elevational view of a fixing apparatus, as would be found in a larger printing apparatus, according to a first embodiment. A sheet or substrate (of any suitable material) S bearing an unfixed ink image I approaches, along a process direction P, a fixing apparatus including a rotatable member, here in the form of an ink-side leveling roller  10 , and a backing member here in the form of a backing roller  20 . In a practical embodiment, the ink image I comprises at this time an uncured, viscous liquid that has not significantly penetrated into the substrate S. At the nip formed between rollers  10  and  20 , the unfixed ink I is mechanically “leveled” by the nip pressure, which effectively causes the various layers of multi-colored inks to assume a consistent total height relative to the surface I of substrate S. 
     Simultaneous with the mechanical pressure applied at the nip, radiant energy is applied to the ink I, the radiant energy including suitable wavelengths, typically UV, for chemical curing of the ink I on substrate S as any small area of substrate S passes through the nip. For this purpose there is disposed within leveling roller  10  a radiation source  30 , which may include for this embodiment one or more UV lamps or a UV-emitting LED array, directing radiation to the ink I in the nip as the substrate S moves therethrough. The power of source  30  or multiple sources is such that the ink I is fully cured by the time it leaves the nip for a given process speed. 
     In such an embodiment, the walls of leveling roller  10  are effectively transmissive of the curing radiation, so the radiation can efficiently reach the ink I in the nip. According to possible embodiments, leveling roller  10  is comprised of a quartz core with a shrink fit release layer surface. The outer layer of leveling roller  10  is a low surface energy material that also passes UV radiation such as clear PTFE, but other alternatives, such as fluorocarbons, are available. The backing roller  20  is typically formed of silicone over metal. 
     Also shown in  FIG. 1  are IR lamps  40 , or equivalents, for pre-heating a substrate S as needed given a particular material set (ink and substrate). A temperature sensor  50  of known type can measure the surface temperature of leveling roller  10  just upstream of the nip, the recorded temperature being useful for a control system. 
     The curing of ink I is simultaneous with the mechanical pressure formed at the nip so that sufficient cross linking of monomer chains in the ink is initiated while still under a leveling condition such that polymerization is substantially complete by the time the image I leaves the nip formed by rollers  10  and  20 . The process of polymerization results in a solid durable material that experiences some shrinkage. The shrinkage and hardness combined with the low surface energy layer on roller  10  lead to a condition whereby the image tends to self strip from the roller  10 . 
       FIG. 2  is a simplified elevational view of a fixing apparatus, as would be found in a larger printing apparatus, according to a second embodiment. Like reference numbers from  FIG. 1  indicate analogous elements in  FIG. 2 . The  FIG. 2  embodiment differs from  FIG. 1  in that, in lieu of the backing roller, there is provided a rotatable backing belt  22 , which forms a nip along a significant wrap angle around the leveling roller  10 . The belt  22  can be entrained around any number of inner rollers  24  to provide a necessary nip pressure against leveling roller  10 . The backing belt  22  provides a significantly longer dwell time for ink under mechanical pressure to be cured by radiation source  30 . One basic composition of backing belt  22  includes polyimide with a silicone overcoat. 
       FIG. 3  is a simplified elevational view of a fixing apparatus, as would be found in a larger printing apparatus, according to a third embodiment. Like reference numbers from  FIG. 1  or  FIG. 2  indicate analogous elements in  FIG. 3 . In this embodiment, in lieu of a leveling roller, there is provided a leveling belt  12 , entrained on any number of inner rollers  14 , forming a nip against backing belt  22 . An adjustable pressure roller  16  disposed within leveling belt  22  can urge a portion of the belt, along a point in the nip, against backing belt  22 , which can be supported with a pressure pad  26 , as shown. 
     The leveling belt  12  includes multiple layers. An inner layer provides a durable surface that serves as support and a drive surface. One suitable material is a clear (to UV) polyimide. The outer layer of leveling belt  12  includes a low surface energy material that also passes UV radiation; one suitable material is clear PTFE, but other alternatives, such as fluorocarbons, are possible. The adhesive between the layers must also be effectively transmissive of UV. 
     The nip pressure is held constant through the length of the nip by the slightly curved pressure pad  26  inside the backing belt  22  that applies force normal to the backing belt  22 , thereby pushing it into the leveling belt  12 , and causing substrates S passing therethrough to be bent outward with respect to the uncured ink I thereon. The outward bending aids in the self-stripping of the ink. 
     Further as can be seen in  FIG. 3 , IR lamps  40  as described above are disposed within leveling belt  12  at an early part of the nip along the process direction P. These lamps, or equivalents, are used to bring the ink I and substrate S to a predetermined temperature prior to curing, as needed. Following the adjustable pressure roller  16 , the UV sources  30  cure the ink I onto substrate S. 
     Although the two radiation sources in the illustrated embodiment provide first IR for heating and then UV for curing, different applications may require different arrangements of radiation sources. For example, if a plurality of inks is placed on substrate S, such as for different primary colors or other attributes such as magnetic properties, it may be desired to cure one ink (having one particular curing wavelength) before the other (having another particular curing wavelength). The radiation sources can be arranged to effect this ordered curing. Alternatively, multiple radiation sources may differ in other aspects, such as amplitude, to obtain desired print properties, such as gloss, given a particular material set. 
     The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.