Ink jet printers have print heads that operate a plurality of ejection jets from which liquid ink is expelled. The ink may be stored in reservoirs located within cartridges installed in the printer, or the ink may be provided in a solid form and then melted to generate liquid ink for printing. In these solid ink printers, the solid ink may be in either pellets, ink sticks, granules or any other shape. The solid ink pellets or ink sticks are typically placed in an “ink loader” that is adjacent to a feed chute or channel. A feed mechanism moves the solid ink sticks from the ink loader into the feed channel and then urges the ink sticks through the feed channel to a heater assembly where the ink is melted. In some solid ink printers, gravity pulls solid ink sticks through the feed channel to the heater assembly. Typically, a heater plate (“melt plate”) in the heater assembly melts the solid ink impinging on it into a liquid that is delivered to a print head for jetting onto a recording medium.
A typical inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media or it may be a rotating intermediate imaging member, such as a print drum or belt. In the print head, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, or voltage level, of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a print head controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” or “ink drop positions.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
Ejections of ink drops from different inkjet ejectors in the same printhead are not always uniform. Slight variations in the drop ejection angles of the inkjet ejectors and different lengths of flight time for ink drops result in ink drops not landing at their intended locations. The different lengths of flight times for inkjet ejectors may arise from changing velocities for the ink drops as they are expelled from inkjet ejectors. For example, some inkjet ejector may eject an ink drop after some period of inactivity with a different velocity than an ink drop expelled after a series of ejections. Ink drops fired at different velocities from one or more rows of inkjet ejectors across the face of the printhead are likely to land at different positions in the process direction. This phenomenon may be visually detected as a ragged edge in an image. “Process direction” refers to the direction in which the image receiving member is moving as it passes the printhead and “cross-process direction” refers to the direction across the width of the image receiving member. Efforts to identify image data patterns that reduce ragged edges in images that arise from differences in ink drop velocities are worthwhile.