Ink jet printers operate by ejecting ink onto a print substrate, such as paper, in controlled patterns of dots. By selectively regulating the pattern of ink droplets, such ink jet printers can be used to produce a wide variety of printed images, including text, graphics, and the like. Moreover, ink jet printers are capable of recording permanent images on a wide variety of substrates, including both light reflective and light transmissive substrates.
Ink jet printers utilize a variety of inks, including thermal phase change inks. In general, phase change inks are solid at ambient temperatures and liquid at the elevated operating temperatures of an ink jet printing device. Liquid phase ink droplets are ejected from the printing device at an elevated operating temperature and, when the ink droplets contact the surface of a substrate, they rapidly solidify.
Early references to phase change inks for ink jet printing involved monochrome inks jetted by electrostatic printing devices. Thus, for example, U.S. Pat. No. 3,653,932 discloses a low melting point (30.degree. C. to 50.degree. C.) ink having a base comprising diesters of sebacic acid. In a similar process, U.S. Pat. No. 3,715,219 describes low melting point (30.degree. C. to 60.degree. C.) inks including a paraffin alcohol-based ink. One disadvantage of printing with low melting point phase change inks is that they frequently exhibit offset problems. Specifically, when substrates printed with these inks are stacked and stored for subsequent use, the ink adheres to adjacent surfaces, particularly if the printed substrates are exposed to high ambient temperatures.
Phase change inks are well known in the art. U.S. Pat. Nos. 4,390,369 and 4,484,948 describe methods for producing monochrome phase change inks that employ a natural wax ink base, such as Japan wax, candelilla wax, and carnauba wax, which are subsequently printed from a drop-on-demand ink jet device at a temperature ranging between 65.degree. C. and 75.degree. C. U.S. Pat. No. 4,659,383 discloses a monochrome ink composition having an ink base including a C20-24 acid or alcohol, a ketone, and an acrylic resin plasticizer. These monochrome ink compositions are not durable and, when printed, may become smudged upon routine handling and folding.
Japanese Patent Application No. 128,053/78 discloses the use of aliphatic and aromatic amides that are solid at room temperature, such as acetamide, as printing inks. U.S. Pat. No. 4,684,956 is directed to monochrome phase change inks utilizing synthetic microcrystalline wax (hydrocarbon wax) and microcrystalline polyethylene wax. This molten composition can be applied to a variety of porous and non-porous substrates using drop-on-demand ink jet application techniques.
European Patent Application Nos. 0 187 352 and 0 206 286 disclose phase change ink jet printing in color. The ink bases for these systems include fatty acids, a thermoplastic polyethylene and a phase change material in the first application; and the alcohol portion of a thermosetting resin pair, a mixture of organic solvents (o- and p-toluene sulfonamide) and a dye in the second application.
The development of phase change inks that are substantially transparent, i.e., inks that transmit substantially all of the light that impinges on them, has improved the quality of images printed on light transmissive substrates. Phase change ink compositions disclosed in U.S. Pat. No. 4,889,761 are exemplary and may be used for a variety of applications.
Ink jet printers typically utilize a support surface to support the substrate during printing. A print head having multiple ink orifices ejects ink droplets as it is reciprocated in close proximity to the surface of the print substrate. The print substrate is generally indexed at predetermined intervals to position different areas of the substrate for printing.
Precise placement of ink droplets is required to provide high quality printed images. For ink jet printing devices with reciprocating print heads to provide precise ink drop placement, the distance between the print substrate and the print head must be maintained to a very close tolerance. A multi-orifice print head must also have a tight parallelism tolerance between the jet orifice plane and the printed substrate plane.
Thermal phase change ink print quality is furthermore affected by the rate of ink droplet solidification on the print substrate. Rapid solidification of the ink droplets reduces migration of ink along the print substrate and fusing of adjacent ink droplets, thereby providing high quality images on a wide variety of print substrates. The substrate support is an important factor influencing the rate of ink droplet solidification for any given printing speed, ink jet array, and print substrate. Substrate support surfaces that cannot effectively dissipate thermal energy during printing operations are therefore undesirable because they tend to reduce the rate of ink drop solidification as they are heated during printing operations.
Substrate support surfaces that cannot effectively dissipate thermal energy during printing operations may also cause non-uniform expansion and wrinkling of print substrates during printing operations. Substrate expansion and wrinkling may be caused by thermal expansion or changes in the moisture content of the print substrate, or a combination of both factors. Many types of print substrates, including a variety of papers, are prone to expansion and wrinkling at high temperatures. This may result in substrate feed problems, as well as lower print quality resulting from variations in the distance between the print substrate and the print head.
It would therefore be desirable to provide a substrate support surface having thermal properties that are conducive to printing of thermal phase change inks. The substrate support should provide a thermal backing for the substrate that optimizes print quality and the ability to control the ink applied to the substrate. Moreover, the support surface desirably accommodates a clamp assembly that holds the leading edge of the print substrate during printing and releases the printed substrate after the printing operation has been completed.