In traditional inkjet technology, image quality is related to the volume of individual ink droplets. With all else being equal, a smaller drop volume results in higher resolution and better image quality. For example, a drop volume for a 600 dpi×600 dpi resolution inkjet printer is about 16.0 pL, while that for a higher quality 1200 dpi×1200 dpi resolution inkjet printer is only about 4 pL. Sub-picoliter drops are required to obtain printed images at greater than 2400 dpi×2400 dpi resolution.
Printheads capable of producing sub-picoliter drops are challenging to manufacture. More specifically, extremely small orifice holes are needed to achieve such sub-picoliter drops. The dimensional accuracy and uniformity of such orifice holes is beyond the capability of existing micro fabrication technologies. Moreover, it is difficult to operate a printhead with small drop volumes due to problems such as jet straightness. In addition, small orifices tend to become clogged more easily by contaminants. Small orifices also have short latency and are difficult to recover after being idle for a period of time.
Due to finite size of spots made by inkjet droplets on the receiving substrate, a halftoning technique is used to produce various levels of gradation for mid-tone shades. Smaller drop volumes achieve higher image quality by producing a finer level of gradation in the mid-tone shades without introducing objectionable graininess or other noises associated with halftoning. Halftoning also reduces the printing speed due to the required processing time for rendering the halftone image.
Another approach for increasing color image quality uses diluted inks. Because less colorant is present in each diluted ink drop, the effect of smaller drops having higher concentration is achieved. However, certain drawbacks to this approach include a higher cost and more complex printing system, issues related to drying, and media cockle due to excess solvents.
The present invention provides a new and improved apparatus and method which addresses the above-referenced problems.