Patent Application
20240419105
Liquid electro-photographic (LEP) printing uses a special kind of ink to form images on paper and other printable substrates. LEP ink contains tiny pigments encapsulated in a polymer resin, forming particles that are dispersed in a carrier liquid. The polymer particles are sometimes referred to as toner particles and, accordingly, LEP ink is sometimes called liquid toner. In an LEP printing process, an electrostatic pattern of the desired printed image is formed on a photoconductor for each color of the image. Each color is developed by applying a thin layer of LEP ink to the photoconductor. Charged polymer particles in the ink adhere to the electrostatic pattern on the photoconductor to form the desired pattern of liquid ink for that color. Each color pattern is commonly referred to as a “separation.” Each liquid ink color separation is transferred from a photoconductor to an intermediate transfer member, heated to dry the ink and melt the polymer particles, and pressed on to the cooler substrate as a molten film and “frozen” in place at a nip between the intermediate transfer member and a pressure roller.
In some LEP printing processes, each color separation is transferred individually from the intermediate transfer member to the substrate sequentially one after another to form the printed image. In other LEP printing processes, the color separations are gathered together on the intermediate transfer member sequentially one after another and then transferred as a group from the intermediate transfer member to the substrate to form the printed image.
The same part numbers refer to the same or similar parts throughout the figures. The figures are not necessarily to scale.
In some LEP printers, the intermediate transfer member is a belt that rotates in an endless loop past a series of printing units. Each printing unit applies a liquid ink color separation to the surface of the rotating belt to form a liquid ink image on the belt. The belt is heated to dry the liquid ink image to a molten film. The molten film is transferred from the belt to the print substrate at a nip between the belt and a pressure roller. Infrared lamps are commonly used to heat the intermediate transfer belt to dry the ink and to keep the molten film hot to the point of transfer. The best temperature for drying the ink is often less than the best temperature for transferring the molten film to the print substrate. In addition, the temperature of the belt may be lower to prevent over-drying the molten film, for example on longer belts used for printing with six colors of ink. If the temperature of the molten film is too low at the point of transfer, as is often the case, a primer is applied to the substrate to improve adhesion, thus increasing the cost of the substrate and shrinking the universe of usable substrates.
A new technique has been developed to add flexibility to LEP transfer belt processes by dividing the heating functions for ink drying and film transfer between two heating systems. In an example, the LEP printing process includes rotating an intermediate transfer belt in a loop, gathering multiple LEP ink color separations together on the rotating belt, drying the color separations to a molten film, and then, just before transferring the molten film to the print substrate, heating the molten film to a transfer temperature much higher than the drying temperature. For example, a series of IR lamps along the rotating belt heat the liquid ink color separations to 90° C.-110° C. to dry the ink to a molten film. The drying time and/or heating intensity may be varied depending on the density of the color separations that make up the ink image, for example by turning on or off some of the lamps. Once the ink dries to a molten film, any remaining dryer lamps along the belt are turned off. Then, just before the point of transfer, a laser, LED array, or other suitable high intensity focused heater rapidly heats the molten film to a tacky transfer temperature of 120° C. (or more).
A separate, higher intensity heating system near the point of transfer may be optimized for transfer heating without compromising ink drying, for example with higher, more effective transfer temperatures. With a higher intensity heating system near the point of transfer, the drying heating system no longer has to maintain the molten film at an acceptable, but lower and less effective, transfer temperature. A separate, lower intensity heating system for drying the ink may be optimized for drying without compromising transfer heating, for example with lower temperatures, shorter drying times, and/or to prevent over-drying the molten film.
These and other examples shown in the figures and described herein illustrate but do not limit the scope of the patent, which is defined in the Claims following this Description.
As used in this document “and/or” means one or more of the connected things; a “computer readable medium” means any non-transitory tangible medium that can embody, contain, store, or maintain programming for use by a computer processor and may include, for example, circuits, integrated circuits, ASICs, hard drives, random access memory (RAM), and read-only memory (ROM); and “LEP ink” means a liquid that includes polymer particles in a carrier liquid suitable for electro-photographic printing.
Print engine 12 in
Drying heating system 28 may include multiple IR lamps commonly used in LEP belt printers, or other suitable lower intensity heaters, positioned along belt 24 to maintain uniform heating long enough for the liquid ink to dry to a molten film. The use of multiple heaters for drying also allows controller 14 executing control instructions 20 to vary the drying time by turning heaters on and off, for example to dry more and less dense color separations. Transfer heating system 30 includes a laser or other suitable higher intensity heater(s) that heat the molten film very rapidly to the desired transfer temperature just before the film is transferred to the print substrate. The laser(s) may be turned on and off in bursts or their intensity adjusted to achieve the desired transfer temperatures for more or less dense films. As noted above, printer controller 14 may include distinct control elements for individual printer systems including, for example, a system controller for each heating system 28, 30. “Lower” and “higher” in this context refer to the relative intensity of the heaters in the two heating systems, not to an absolute range or threshold.
Referring to
IR lamps commonly used in LEP belt printers or other suitable lower intensity heaters may be used for each drying heaters 46, 48. In the example shown in
An intermediate transfer belt 24 usually includes a replaceable “blanket” covering a core. The comparatively soft, compliant blanket that carries the ink color separations is heated to the drying temperature, 90° C.-110° C. for example, by heaters 46, 48. The drying temperature is maintained along belt 24 for the desired drying time. The drying time and/or temperature may be varied depending on the density of the color separations that make up the ink image, for example by turning on or off some of the heaters 48 (or some of the lamps 58 in multi-lamp heaters 48). Once the ink dries to the desired molten film, heating of the film may be stopped, for example by turning off some of the heaters 48 along the upper run of belt 24. Because print engine 12 includes a separate, downstream heating system 30 to heat the molten film to the desired transfer temperature, above 120° C. for example, drying heating system 28 does not need to keep the molten ink hot all the way to the point of transfer. Accordingly, more or fewer heaters 48 may be used to optimize drying without compromising the transfer temperature.
In one example, transfer heater 50 is implemented as an array of lasers spanning the width of belt 24 as shown in
Belt blankets currently used in LEP belt printers absorb light across a wide band of wavelengths and, thus, may be used with a VCSEL type heater 50. In this example, belt 24 was exposed to beams 54 for 2-3 ms with the post-heating time varied between 20 ms-30 ms (the time between exposure to beams 54 and contact with print substrate 52). Other suitable configurations for transfer heater 50 are possible. For one example, other types of lasers or even non-laser, narrowly focused heat sources may be used for heater 50. The power of each laser and/or the size of the array may be varied to achieve the desired heating characteristics. Also, the wavelength of light beams 54 and the absorption characteristics of the blanket on belt 24 may be tuned to one another to help improve both the effectiveness and the efficiency of heater 50 while maintaining the desired drying characteristics of drying heating system 28.
While the characteristics of heater 50 may vary depending on the particular printing application, it is expected that a heater 50 delivering a heat energy greater than 3.2 mJ/mm2 will be adequate to achieve the desired transfer temperature in the range of 120° C.-160° C., raising the temperature of the molten film 30° C. or more in less than 3 ms. In one example, a molten film 44 in
As noted above, these and other examples shown in the figures and described herein illustrate but do not limit the scope of the patent, which is defined in the following Claims.
“A” and “an” in the Claims means one or more. For example, a heater means one or more heaters and subsequent reference to the heater means the one or more heaters.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/056246 | 10/22/2021 | WO |
