Thermal inkjet printers typically utilize a printhead that includes an array of orifices (also sometimes called nozzles) through which ink is ejected on to paper or other print media. Ink filled channels feed ink to a firing chamber at each orifice. As a signal is applied individually to addressable thermal elements, resistors for example, ink within a firing chamber is heated, causing the ink to bubble and thus expel ink from the chamber out through the orifice. As ink is expelled, more ink fills the chamber through a channel from the reservoir, allowing for repetition of the ink expulsion sequence. The use of thermal inkjet printing in high throughput commercial applications presents special challenges for maintaining good print quality.
Small droplets released during break-up of the tail of more elongated ink drops ejected by conventional inkjet printheads typically travel more slowly to the print medium than does the main drop (the head of the ejected ink drop). Thus, these trailing, “satellite” droplets land on the print medium away from the main drop, forming extraneous marks along the edges or in the background of the desired images. Such print quality defects often make the images appear fuzzy or smeared. This undesirable characteristic of ejecting elongated ink drops may become more pronounced as printing speed increases and the printhead and print medium move faster and faster with respect to one another.
Clear mode printing, in which substantially all of the ink in the firing chamber is ejected, has been used to eject tail free drops. However, the rate at which ink refills the firing chamber after each ejection in preparation for the next ejection is significantly slower than for printing with elongated ink drops. In “normal”, non-clear mode printing, the collapsing ink bubble tends to drag ink into the firing chamber to help speed refill. In clear mode printing, since the ink bubble is vented completely out through the orifice, there is no collapsing bubble to help draw in refill ink, thus slowing refill. Consequently, conventional clear mode printhead architectures have not proven suitable for inkjet web printing presses and other high speed printing applications.
The structures shown in the figures, which are not to scale, are presented in an illustrative manner to help show pertinent features of the disclosure