In general, inkjet printers include at least one printhead that ejects drops of liquid ink onto a surface of an image receiving member. In an indirect or offset printer, the inkjets eject ink onto the surface of a rotating image receiving member, such as a rotating metal drum or endless belt, before the ink image is transferred to print media. In a direct printer, the inkjets eject ink directly onto print media, which may be in sheet or continuous web form. A phase change inkjet printer employs phase change inks that are solid at ambient temperature, but transition to a liquid phase at an elevated temperature. Once the melted ink is ejected onto the media or image receiving member, depending upon the type of printer, the ink droplets quickly solidify to form an ink image.
Inkjet printers are used to print a wide range of documents using various types and colors of ink. Some printed documents are read by both humans and machines. For example, a check includes printed text that is both human readable and readable by automated check processing equipment. Check processing machines use Magnetic Ink Character Recognition (MICR) to identify printed characters in a check, such as routing and account numbers, quickly and accurately. The magnetic ink readable by MICR machines includes a suspension of magnetic particles, such as iron oxide, which are detectable using a magnetic field. The use of MICR printing is widespread and enables automated processing of checks and other documents even when the printed magnetic ink characters are visually obscured by stamps or other overprinting. Automated check processing machines perform high-speed character recognition using printed magnetic ink characters to identify account and routing numbers. While check processing is one application of magnetic ink printing, magnetic inks can be incorporated in a wide range of printed documents and can be used in conjunction with non-magnetic inks as well.
Solid ink printers receive ink in a solid form, sometimes known as solid ink sticks. The solid ink sticks are typically inserted in an ink loader for the printer and are moved by a feed mechanism and/or gravity toward a heater plate. The heater plate melts the solid ink impinging on the plate into a liquid that is delivered to a melt reservoir. The melt reservoir maintains the ink in a melted state and delivers the ink to a printhead for ejection onto an image receiving surface.
In MICR solid ink, magnetic particles are suspended in a phase change ink. When MICR solid ink is melted and in a liquid state, the metal particles are pulled downwardly by gravity and can collect in the lower regions of melted ink containers and passageways in a printer. The metal particles settling out of the ink can degrade the uniform distribution of magnetic particles in the ink, which can cause the printed ink to be outside the readable limits for MICR readers. Characters printed with the non-uniform ink can be difficult for MICR readers to recognize. Thus, a need exists for devices and methods that help maintain a uniform distribution of magnetic particles in phase change magnetic ink as the ink is used in an inkjet printer.