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US-12619176-B2 - Image transfer for liquid electro-photographic printing

US12619176B2US 12619176 B2US12619176 B2US 12619176B2US-12619176-B2

Abstract

In one example, an LEP printer includes a belt rotatable in a loop, a series of multiple printing units along the belt each to apply an LEP ink color separation to the belt, a first heater to dry the color separations on the belt to a molten film at a drying temperature, a second heater to heat the molten film on the belt to a transfer temperature higher than the drying temperature, and a pressure roller near the belt to press a printable substrate against the belt at a nip between the pressure roller and the belt.

Inventors

  • Peter Nedelin
  • Mark Sandler

Assignees

  • HEWLETT-PACKARD DEVELOPMENT COMPANY, L.P.

Dates

Publication Date
20260505
Application Date
20211022

Claims (13)

  1. 1 . An LEP printer comprising: a belt rotatable in a loop; a series of multiple LEP printing units along the belt each to apply an LEP ink color separation to the belt; a first heater to dry the color separations on the belt to a molten film at a drying temperature; a second heater to heat the molten film on the belt to a transfer temperature higher than the drying temperature by a beam of light having an exposure time of 2 to 3 milliseconds; and a pressure roller near the belt to press a printable substrate against the belt at a nip between the pressure roller and the belt, wherein the second heater is located before the pressure roller.
  2. 2 . The printer of claim 1 , wherein: the first heater comprises a series of multiple first heaters along the belt, each first heater having a first intensity; and the second heater comprises a laser having a second intensity greater than the first intensity.
  3. 3 . The printer of claim 2 , wherein the second heater is located along the belt such that, when the belt is rotating, the molten film enters the nip within 30 ms after the second heater heats the molten film to the transfer temperature.
  4. 4 . The printer of claim 3 , wherein the second intensity is at least 1.0 W/mm 2 .
  5. 5 . The printer of claim 4 , wherein: the drying temperature is 90° C.-110° C.; and the transfer temperature is 120° C.-160° C.
  6. 6 . The printer of claim 5 , wherein: one or more of the first heaters is located between each pair of adjacent printing units; and one or more of the first heaters is located downstream from all of the printing units.
  7. 7 . An LEP printer, comprising: a belt rotatable in a loop; a series of multiple printing units along the belt each to apply an LEP ink color separation to the belt; a series of multiple first heaters along the belt, each first heater having a first intensity to dry the color separations on the belt to a molten film at a drying temperature; a second heater having a second intensity greater than the first intensity to heat the molten film on the belt to a transfer temperature higher than the drying temperature; a pressure roller near the belt to press a printable substrate against the belt at a nip between the pressure roller and the belt; and a controller programmed to: rotate the belt; and turn off some of the first heaters based on a density of the color separations on the belt.
  8. 8 . The printer of claim 7 , wherein: one or more of the first heaters is located between each pair of adjacent printing units; and one or more of the first heaters is located downstream from all of the printing units.
  9. 9 . The printer of claim 7 , wherein the second heater is located along the belt such that the molten film enters the nip within 30 ms after the second heater heats the molten film to the transfer temperature.
  10. 10 . The printer of claim 7 , wherein the second intensity is at least 1.0 W/mm 2 .
  11. 11 . An LEP printing process, comprising: rotating an intermediate transfer belt in a loop; and during a single rotation of the belt loop: gathering multiple LEP ink color separations together on the rotating belt; drying the color separations on the rotating belt to a molten film at a drying temperature; then heating the molten film on the rotating belt to a transfer temperature greater than the drying temperature by a beam of light having an exposure time of 2 to 3 milliseconds; and within 30 ms after heating the film on the rotating belt to the transfer temperature, transferring the film from the rotating belt to a printable substrate, wherein the heating is performed at a location on the intermediate transfer belt before the transferring.
  12. 12 . The process of claim 11 , wherein: drying the color separations comprises heating the color separations with a first heater having a first intensity; and heating the molten film comprises heating the molten film with a second heater comprising a laser having a second intensity greater than the first intensity.
  13. 13 . The process of claim 12 , wherein: heating the color separations comprises heating the color separations to 90° C.-110° C.; and heating the molten film comprises heating the molten film to 120° C.-160° C.

Description

BACKGROUND 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. DRAWINGS FIG. 1 illustrates one example of an LEP printer in which the heating functions for ink drying and film transfer are divided between two heating systems. FIGS. 2-6 illustrate one example of an LEP print engine such as might be implemented in the printer shown in FIG. 1. FIG. 7 illustrates one example of an LEP printing process in which the film transfer temperature is higher than the ink drying temperature. The same part numbers refer to the same or similar parts throughout the figures. The figures are not necessarily to scale. DESCRIPTION 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 h