Anisotropic thermal conductivity on a heated platen

Details Anisotropic thermal conductivity on a heated platen Anisotropic thermal conductivity on a heated platen

2000-12-08

2002-06-18

Gordon, Raquel Yvette (Department: 2853)

Incremental printing of symbolic information

Ink jet

Medium and processing means

Reexamination Certificate

active

06406140

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to hard copy apparatus, more specifically to conductive heating of print media, and particularly to the heating of print media that is advancing through the printing zone of an ink-jet printer.
2. Description of the Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, facsimile machines, and multifunctional peripheral (“MFP”) hard copy apparatus employ ink-jet technology for producing hard copy. The basics of this technology are disclosed in various articles in the
Hewlett
-
Packard Journal
, for example, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992, and Vol. 45, No. 1 (February 1994) editions. Ink-Jet devices are also describe by W. L. Lloyd and H. T. Taub in
Output Hardcopy Devices
, chapter 13 (Ed. R.C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
In order to simplify the description of the present invention the term “paper” is used as synonymous with all types of print media; the term “ink” is used as synonymous with all compositions of colorant; the term “pen” is used as synonymous with all types of ink-jet writing instruments. While the present invention is described for convenience in terms of application to ink-jet printing, it is to be recognized by those skilled in the art that many of the concepts are applicable to any hard copy apparatus using a wet colorant for creating a print. No limitations on the scope of the invention are intended nor should any be implied.
One important factor affecting the print quality of ink-jet printers is drying time. Print media movement must be controlled to ensure that the liquid ink dries properly once printed. Bleed of one color into another can occur when two wet droplets come into contact. Any touching of the printed surface before the ink is dry can result in smearing. An additional concern is paper cockle, which is an uncontrolled, localized warping of the paper that occurs as liquid ink saturates the fibers.
Active heating devices can be and are used to heat a printing sheet in order to speed the drying time. However, heat must be applied carefully to avoid the introduction of other problems. For example, the paper can be scorched. Furthermore, if heat is not applied correctly, the resultant uneven drying time of a color area of an image can produce undesirable variations in the hue characteristics, known as hue shift. Another problem attributable to improperly applying heat is a noticeable warping of the sheet. Normally, paper carries at least some moisture content. For example, a sealed ream of standard office paper has about 4.5-percent moisture content. High ambient humidity can increase the moisture content as the paper sheets lie in an intray. As heat is applied to part of the paper, uneven drying and shrinkage occurs. Uneven shrinkage causes the paper to warp. Some print media, such as polyester-based transparencies, will carry insignificant amounts of water and, therefore, will not buckle a s a result of uneven shrinkage. Such media, however, may warp or even burn if all or portions of it are overheated. Thus, uniform, controlled heating of the media is important for high print quality, irrespective of the type of print media.
A METHOD OF MULTIPLE ZONE HEATING OF INKJET MEDIA USING A SCREEN PLATEN is taught in U.S. Pat. No. 5,668,584 by Broder et al., i s sued Sep. 16, 1997 (assigned to the common assignee herein and incorporated by reference). Pre-printing, print zone, and post-printing heating is shown using an open screen type platen. Other specific methods and apparatus for CONDUCTIVE HEATING OF PRINT MEDIA is described by common inventor Wotton et al., in U.S. patent application Ser. No. 09/1412842, filed Oct. 5, 1999 (“Wotton et al.” hereinafter), co-pending herewith, assigned to the common assignee herein, and incorporated herein by reference in it s entirety, particularly discussing vacuum holddown type platen technology.
If heat is to be applied to the print sheet, it is useful to have it in the print zone. Heating in the print zone rapidly drives off a substantial portion of the liquid component of the ink so that cockle is unable to form, or at least is minimized. However, when one attempts to heat the media in the print zone, it is important to ensure that the applied heat is not directed into the printhead. If an ink-jet printhead overheats, drop trajectory and other characteristics of the printhead can change, again negatively affecting print quality. Moreover, the heat should not be applied in a manner, such as by convection, that itself may directly alter droplet trajectory.
Another prior art solution is shown by Vincent et al. in U.S. Pat. No. 5,510,822, issued Apr. 23, 1996 for an INK-JET PRINTER WITH HEATED PRINT ZONE (assigned to the common assignee herein and incorporated by reference).
A close study of the thermodynamics of a print zone heater has shown that the problem is more complex than previously thought. Along the x-axis, some of the thermal loads that can cause a temperature imbalance include paper type and size, ink composition and presence or absence (i.e., dotted and not-dotted pixels) in regions of the printing sheet, and airflow such as occurs when using a vacuum-type platen as in Wotton et al. It has been found that airflow near the edge of the printing sheet creates the largest thermal load. This load has been found to create temperatures drops near the edge of as much as 30-degrees Centigrade. Edge-to-edge printing, known as full bleed further exacerbates the problem. At the same time, as a variety of different sized media is usually used in a printer, the edge of the sheet from page to page may be indeterminate. Thus, the load position from airflow near the edge of the paper is a variable factor. Therefore, there is a need for method and apparatus to provide substantially infinitely adjustable power densities along the x-axis in order to ensure a uniform temperature profile.
One solution is to have a very fine heater resolution. However, such is expensive in and of itself and also requires extensive control subsystems.
It has been found that anisotropic thermal conductivity on a heated platen, i.e., having different levels of thermal conductivity in the x-axis, y-axis, and z-axis, provides significant advantages and advancement in the state of the art.
GLOSSARY
As used herein, the term “high thermal conductivity” shall mean: greater than approximately one-hundred (100) W/m.K (Watts/meter Kelvin). An example of a material having a relatively high thermal conductivity is aluminum.
As used herein, the term “low thermal conductivity” shall mean: less than approximately ten (10) W/m.K. An example of a material having relatively low thermal conductivity is plastic.
As used herein, for operating temperatures between about 40° C. and 150° C., the term “high thermal resistance” shall mean:
in the y-axis,
surface temperature change (&Dgr;T/L
Y
) ≧1.0° C./mm for surface
temperature changes of up to 90° C. between two points; and
in the z-axis,
Power
watts z-axis
≦0.15 Power
watts total
.
As used herein, the term “low thermal resistance” shall mean: length÷thermal conductivity>fifteen percent heat flow (or power flow in Watts).
As used herein, the term “high thermal mass” shall mean: a mass having a response time of greater than 60 seconds to change temperature by 100° C.; or, as a calculable m·C
p
(mass-specific heat), a specific implementation contemplated by the inventors being m·C
p
≧1200 J/K (Joules/Kelvin). A preferred implementation (an example of a component in the current context having a relatively high thermal mass would be the entire platen
42
) should have a response time measured in minutes rather than seconds.
As used herein, the term “low thermal mass” shall mean: a mass having a response time of less than 30 seconds to change temperature by 100° C.; as a ca

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