Patent Application
20130050368
1. Technical Field
The invention relates to inkjet printing. More particularly, the invention relates to ink jet UV pinning for control of gloss.
2. Description of the Background Art
Certain types of printing systems are adapted for printing images on large-scale substrates, such as for museum displays, billboards, sails, bus boards, and banners. Some of these systems use so-called drop on demand ink jet printing. In these systems, a carriage which holds a set of print heads scans across the width of the substrate while the print heads deposit ink as the substrate moves.
Solvent based inks are sometimes used in these systems in which an infrared dryer is used to dry off the solvent after the ink is deposited onto the substrate. Systems using solvent based inks are able to print on flexible substrates such as PVC materials and reinforced vinyl. However, solvent based inks are typically considered to be unusable for printing on rigid substrates such as metals, glass, and plastics. Therefore, to print on rigid, as well as flexible substrates, radiation-curable inks such as UV-curable inks are often preferred. For these systems, the ink is deposited onto the substrate and then cured in a post-printing stage. For instance, after the deposition of the ink, the substrate moves to a curing station. The ink is then cured, for example, by exposing it to UV radiation. In other systems, the UV radiation source for curing is mounted directly on the same carriage that carries the set of print heads.
UV ink jet dot gain is a parameter that is difficult to control. Ink deposited onto a substrate, until it is cured with UV energy, can react by spreading or shrinking depending on the surface tension and surface energy of the ink and substrate. Drop to drop interactions also complicate the control of dot gain and gloss. The time frames of interaction are such that locations of various colors and print heads with respect to the cure lamp result in differential gloss banding, an objectionable printing artifact.
Methods to correct this time-to-lamp problem have been proposed and implemented. For example, Ink Jet Printer with Apparatus for Curing Ink and Method (U.S. Pat. No. 6,145,979) describes a method to prolong, uniformly, the time-to-lamp for an ink jet printer through the use of mirrors or a post cure lamp traveling with the print carriage.
Image Forming Apparatus Having a Plurality of Printing Heads (U.S. Pat. No. 7,152,970) describes a method of positioning UV cure lamps adjacent to each print head color to equalize the time to lamp between print heads and colors. Digital Ink Jet Printing Method and Apparatus and Curing Radiation Application Method (U.S. Pat. No. 7,837,319) describes a method of applying a first and second intensity UV cure energy, each applied at a constant time for all locations on the substrate.
Another method used to mitigate differential gloss banding is to use pinning (aka setting), the application of a low UV energy (the order of 5% of cure energy) to freeze or gel the ink dots on the media as soon as possible after application to the media, where they are later cured by high intensity UV radiation. Examples of this method are disclosed in Systems and Methods for Curing a Fluid (U.S. Pat. No. 6,739,716), which describes two UV cure lamps or reflectors that direct two different power levels onto a substrate as ink jet ink is applied. The result is to freeze each layer of ink that is applied so as to prohibit interaction between the ink layers.
Method of Printing Using Partial Curing by UV Light (U.S. Pat. No. 7,152,969) similarly describes pinning to allow many passes of ink application without drop to drop interaction.
The assignee of the present application, EFI, holds two patents in this area: Apparatus and Method for Setting Radiation Curable Ink (U.S. Pat. No. 6,457,823) and Radiation Treatment for Ink Jet Fluids (U.S. Pat. No. 7,600,867), both of which are aimed at inhibiting ink to ink or ink to substrate interactions.
Methods of controlling ink interactions to minimize gloss banding print artifacts by time-to-lamp or pinning and curing can still result in print artifacts due to other variables. Short times to lamp or pinning result in low dot gain with thick ink build up and loss of color due to small dot size. Gloss banding continues to persist due to bidirectional laydown of droplets, which result in physical reflectance which is directionally viewing dependent.
During the printing process, UV curable ink must be cured within a short time period after it has been deposited on the substrate, otherwise ink with positive dot gain may spread out and flow, or ink with negative dot gain may ball up. UV radiation sources mounted on the carriage are capable of emitting radiation at high enough energies to cure the ink within such time frames. However, a significant amount of power must be supplied to the UV radiation source to enable it to emit these high energies. Typical UV radiation sources are quite inefficient because most of the emitted radiation is unusable. A substantial percentage of the emitted radiation is not used because the source emits radiation with wavelengths over a spectrum which is much wider than the usable spectrum. In addition, to ensure that the required amount of radiation is transmitted to the ink, the carriage must scan across the substrate at moderate speeds, even though the print heads are capable of depositing ink onto the substrate at much higher carriage speeds.
It is desirable, therefore, to set or pre-cure the ink rather than fully cure it as the ink is deposited on the substrate so that the ink does not spread or ball up, even though it is still in a quasi-fluid state, i.e. the ink is not completely hardened. Such an arrangement requires less power, and, therefore, facilitates using smaller UV radiation sources. In addition, a lower energy output requirement would allow the carriage to operate at a higher speed. Hence, images can be printed at a higher rate, resulting in a higher throughput.
Embodiments of the invention implement an apparatus and method for setting radiation curable ink deposited on a substrate. Specifically, in one aspect of the invention, an ink jet printing system includes a UV energy source which emits UV radiation to polymerize or pin a fluid that is deposited onto a substrate by one or more ink jet print heads. The fluid can be an ink that is UV curable, or the fluid can be any other type of polymerizable fluid that does not necessarily contain a dye or pigment.
An embodiment of the invention uses controlled pinning energy to adjust the amount of ink interaction between drops, substrate, and ink layers, resulting in virtual elimination of gloss banding and control of the finished gloss level from a gloss level of approximately 85 to a gloss level of approximately 5. This is a significant feature in UV ink jet printing, i.e. to be able to control gloss within the printing system.
The invention thus provides a significant improvement in the technology of setting (aka pinning) and curing UV ink. That is, by controlling the pinning energy, the amount of drop to drop interaction can be controlled in a way that allows the finished gloss or matt content of the final image to be controlled. An added benefit of this gloss control is that a well known artifact of gloss banding or differential gloss banding is significantly reduced.
The invention provides a significant improvement in the technology of setting (aka pinning) and curing UV ink. That is, by controlling the pinning energy, the amount of drop to drop interaction can be controlled in a way that allows the finished gloss or matt content of the final image to be controlled. An added benefit of this gloss control is that a well known artifact of gloss banding or differential gloss banding is significantly reduced.
Mills, et al., Radiation treatment for ink jet fluids, U.S. Pat. No. 7,600,867 (Oct. 13, 2009) (incorporated herein in its entirety by this reference thereto) discloses an apparatus and method for setting radiation curable ink deposited on a substrate. Specifically, in one aspect thereof, an ink jet printing system includes a UV energy source which emits pulsed UV radiation to polymerize a fluid that is deposited onto a substrate by one or more ink jet print heads. In some cases, the radiation emitted by the energy source is adjustable. The energy source emits low energy UV radiation to set the fluid, as well as a higher energy UV radiation to cure the fluid. In certain cases, the fluid is first set and subsequently cured. Thus, it is known to use different levels of energy to set the fluid and to cure the fluid via a common radiation source, but not to control pinning to influence the finished gloss or matte content of a final image.
In contrast thereto, embodiments of the invention herein manage ink jet drop interactions (gloss) by the control of pinning energy. Previously, pinning was used to prevent ink jet drop interactions with application of a low UV energy. A presently preferred embodiment of the invention allows control of UV ink drop interactions by adjusting the amount of pinning energy applied. References herein to pinning or setting are to freezing or gelling the ink to prevent interaction.
A typical printer includes the following components (not shown) a base, a transport belt which moves the substrate through the printing system, and a rail system attached to the base. A carriage 24 is coupled to the rail system. The carriage holds a series of inkjet print heads and one or more radiation sources, such as UV radiation sources, and is attached to a belt which wraps around a pair of pulleys positioned on either end of the rail system. A carriage motor is coupled to one of the pulleys and rotates the pulley during the printing process. As such, when the carriage motor causes the pulley to rotate, the carriage moves linearly back and forth along the rail system.
In
The print heads and the UV radiation sources are mounted to the carriage. The UV radiation sources are attached to and positioned on either side of a carriage frame. A series of drop on demand inkjet print heads 23 is also mounted on the carriage frame and positioned between the UV radiation sources. In an embodiment, the series of inkjet print heads includes a set of black (K) print heads, a set of yellow (Y) print heads, a set of magenta (M) print heads, and a set of cyan (C) print heads. Each set of print heads is positioned on either side of an axis that is substantially orthogonal to an axis along which the carriage traverses. In an embodiment, the print heads are arranged so that during the printing process the black print heads first deposit black ink, then the yellow print heads deposit yellow colored ink, followed by the deposition of magenta ink from the magenta print heads, and finally the cyan print heads deposit cyan colored ink. These colors alone and in combination are used to create a desired image on a substrate. Thus, the image is made of regions having no ink or one to four layers of ink. For example, a green region of the image is produced by depositing two layers of ink, namely, yellow and cyan. And an intense black region of the image results from dispensing all four colors, cyan, magenta, yellow, and black. As such, this intense black region is made of four layers of ink.
Although certain regions of the image are made with multiple layers of ink, and all four sets of the print heads may simultaneously deposit ink onto the substrate, only one layer of ink is deposited at a given time on the portion of the substrate that is positioned beneath a respective set of print heads as the carriage scans across the substrate. As the ink is applied to the substrate, embodiments of the invention apply selected amounts of energy at selected times for selected intervals to the pin lamps to pin the ink to prevent gloss. The cure lamps are used to effect ink cure and the use of both the pin lamps and cure lamps may be coordinated to optimize print quality.
An arrow on
Exemplary Parameters
The following discussion and accompanying tables and figures provide exemplary parameters for use in practicing one or more embodiments of the invention. These parameters are not intended to limit the scope of the invention.
In an exemplary embodiment of the invention, the lamps and dosages listed below in Table 1 can be used for pinning. Circuitry for operation of such lamps is known, for example, from Mills, et al., Radiation treatment for ink jet fluids, U.S. Pat. No. 7,600,867 (Oct. 13, 2009) (incorporated herein in its entirety by this reference thereto).
For example, in some print jobs it may be desirable to allow some dot spread between the time the print head lays down an ink and the final cure. The energy profile is determined by such factors as, for example, the variables that control ink spread, the surface tension of the particular ink that is used, the mechanism that is used to vary the energy delivered to the pinning lamps, the color or the type of image, etc. Ink volume in any one location is one of the variables that controls ink spread. The ink itself could be different concentrations of photo initiator, which would change the rate of cure. Another variable is UV intensity. The wavelength of the UV is also a variable. Further, different formulations of inks have different characteristics. Factors that affect the ink include, for example, the ink formulation, the color, and the combinations of different inks with one another.
The presently preferred embodiment of the invention employs an initial adjustment that sets the energy supplied to the pinning lamps (and light output by the pinning lamps) to any one or more of the optimal wavelength, intensity, duration for any particular ink.
Another embodiment of the invention provides an adjustment associated with the printer that allows one to vary the amount of gloss. This adjustment can be a software controlled adjustment, such as would be made by user interaction with a computer or printer based GUI, or it may be a hardware adjustment, such as a control knob on the printer. The adjustment takes into account all factors that affect gloss, as discussed above, for a particular ink and medium. The control need not be infinitely variable, but can have preset selections, such as high gloss, medium gloss, low gloss, and matte finish. As set, the desired gloss (or lack therefor) is produced in accordance with the energy delivered to the pinning lamps. For example, if it is desired to print a job for a department store, then it may be desirable to have one sort of gloss and, if the print job is for another type of application, then it may be desirable for it to be very glossy or very matte.
For purposes of the discussion herein, various tables are provided below that provide a value for gloss. In these tables a value of 100 on the gloss level is perfectly smooth, almost like glass, and very highly reflective; and zero is substantially flat and reflects very little, if at all. A presently preferred embodiment controls energy to the pinning lamps to provide a flat image that looks fairly uniform. This alleviates some print artifacts. In this embodiment, the gloss levels are typically 10 to 15. Those skilled in the art will appreciate that gloss level is determined as desired for any particular application. This is a key advantage of the invention: one can control energy supplied to the pinning lamps to control gloss.
In some embodiments, it is possible to vary the intensity of the pinning lamps depending on the image content on a per page or per area basis. Thus, different pages or different areas of an image to be printed can have different levels of gloss. For example, text may be low gloss and an image may have a higher level of gloss.
In some embodiments, a change of ink type may require resetting of the variables that adjust the level of gloss. Typically, a printer is designed around a particular ink type. In an embodiment, the printer is sold with a specific ink type. In some embodiments, a replacement ink or different type of ink includes either new printer software, instructions for making new settings on the printer, or new pinning lamps that match the ink. In some embodiments, the user can patch printer driver software with the new optimization levels. This approach allows a variety of ink types to be used on a printer, as long as the lamps are capable of pinning and curing the ink at the right wavelengths and energy levels.
In some embodiments, the foregoing techniques increase the printer throughput because, while the state of the art freezes ink dots immediately when they are applied to the media, the invention allows one to control the rate at which the ink is cured, allowing the media to be moved more quickly during the curing process, thus producing a better image quality at the higher speeds. Further, because the approach herein controls the ink drop interaction, it is possible to print with less interlacing and get a better result. It is difficult to do this with the Hg arc lamps without repositioning them. It is possible to get more control of the individual LEDs, and in some embodiments it is possible to reposition the lamps to change how the pinning is applied to the print.
Pinning Energy vs. Gloss
Table 2 below shows pin energy vs. gloss. From Table 2 and
Pinning Vs. Ink and Wavelength
Table 8 below shows gloss vs. pin lamp type and ink type.
Table 9 (below) shows the intensity of the pinning lamp at various levels of energy and the level of gloss for each level of energy relative to a specific test print. In this case of Table 9, the pinning lamp is an LED. As can be seen from Table 9, a high energy pin produces a certain amount or lack of gloss, as measured with a gloss meter.
Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the Claims included below.
