1. Field of the Invention
The present invention relates to an ink droplet ejection device.
2. Discussion of Related Art
As an ink droplet ejection device, there is known a recording head that is to be incorporated in an inkjet printer. U.S. Pat. No. 6,663,208 (corresponding to JP-2002-160362A) discloses such a recording head including: (a) a cavity unit having (a-1) a plurality of nozzles located in its front portion, (a-2) a plurality of pressure chambers located in its rear portion and held in communication with the respective nozzles, and (a-3) a common ink chamber held in communication with the pressure chambers so as to distribute an ink supplied from an ink source, into the pressure chambers; and (b) a piezoelectric actuator unit fixedly disposed on the rear portion of the cavity unit. The piezoelectric actuator unit includes a plurality of deformable portions serving as actuators. Each of the deformable portions is arranged to be deformable with application of a drive pulse signal (voltage) thereto so as to change a volume of a corresponding one of the pressure chambers and apply an ejection pressure to an ink stored in the corresponding pressure chamber, so that the ink is ejected from the corresponding pressure chamber through one of the nozzles that is held in communication with the corresponding pressure chamber. The ejected ink takes a form of an ink droplet that is received in a recording medium, whereby an ink dot is formed on the recording medium. The recording head is arranged to be reciprocably movable in a main scanning direction (i.e., a width direction of the recording medium) that is perpendicular to a sub-scanning direction (i.e., a feeding direction of the recording medium).
It is common that an inkjet printer is arranged to form various kinds of dots having respective different sizes, so that a recorded area per dot is variable as needed. With combination of the various kinds of dots, an image with a desired gradation can be produced on a medium such as a paper sheet. The various kinds of dots can be categorized into a large-sized dot, a medium-sized dot and a small-sized dot, and also can be categorized into a composite dot that is formed by a plurality of ink droplets and a non-composite dot that is formed by a single ink droplet.
The present inventor conducted an experiment with respect to formations of a large-sized dot, a medium-sized dot and a small-sized dot. In the experiment, a large-sized-dot forming pulse train for forming the large-sized dot was configured such that two ink droplets were successively ejected and then united to each other before or after landing on the medium whereby the dot was formed by the united two ink droplets. A medium-sized-dot forming pulse train for forming the medium-sized dot was configured such that a single ink droplet was ejected whereby the dot was formed by the single ink droplet. A small-sized-dot forming pulse train for forming the small-sized dot was configured such that a single ink droplet was ejected whereby the dot was formed by the single ink droplet. The small-sized dot forming pulse train was different from the medium-sized-dot forming pulse train in that a drive pulse included therein had a pulse width deviated from a maximizing value that maximizes efficiency of the ink ejection, so that the ink droplet ejected by the small-sized dot forming pulse train had a volume smaller than the ink droplet ejected by the medium-sized-dot forming pulse train.
However, due to the above-described construction of the cavity unit in which the ink is distributed from the common chamber into the plurality of pressure chambers, when an ejection pressure is applied to at least one of the pressure chambers, the ejection pressure could be propagated to the other pressure chambers via the common chamber, thereby causing a so-called cross talk between the adjacent pressure chambers and inducing an ink ejection from the other pressure chambers.
The experiment conducted by the present inventor revealed that, when the small-sized or medium-sized dot and the large-sized dot were formed through nozzles adjacent to each other, the ejection velocity of the ink droplet for the small-sized or medium-sized dot was increased or reduced by influence of the cross talk. It was further confirmed that an extra ink in the form of extremely small or minute ink droplets was ejected in addition to the ink droplet forming the small-sized or medium-sized dot. The ejection of the minute ink droplets was caused easily when the ejection velocity of the ink droplet forming the small-sized or medium-sized dot was too large or too small. However, such minute ink droplets were not ejected through the nozzle assigned to successively eject two ink droplets forming the large-sized dot.
The minute ink droplets are not uniform in shape and size, and each of the minute ink droplets has a volume that is still smaller than a volume of each of so-called satellite ink droplets which are described in the above-identified U.S. Pat. No. 6,663,208. While the satellite ink droplets commonly land on the medium, the minute ink droplets are caused to float as ink mists without landing on the medium, due to their small volumes. The floating ink mists could stick inside an image forming apparatus incorporating therein a recording head, thereby causing a risk of malfunction in various operations performed by the image forming apparatus.