Patent Application
20110273513
The present invention relates to micro-fluid ejection devices, such as printers, copiers, graphics plotters, all-in-ones, etc. More particularly, it relates to ejection heads, e.g., inkjet printheads, having wind baffles. The baffles modify airflow beneath an ejection zone where fluid crosses a print gap from the head to a print media.
The art of printing images with micro-fluid technology is relatively well known. Conventionally, a permanent or semi-permanent ejection head has access to a local or remote supply of fluid. The fluid ejects from an ejection zone of the head to a print media in a pattern corresponding to pixels of images being printed. Over time, the heads and fluid drops have become increasingly smaller.
In the course of developing heads with fluid drop sizes smaller than 5 Pico liters, a “tree vein” or “wood grain” print defect has been observed. It consists of dark-toned bands meandering from outboard edges of a printing swath toward a center. The bands are typically present for most of the swath length except for a short portion near the beginning of fluid jetting. The bands have been also observed across any swath width so long as the fluid jetting nozzles are spaced relatively closely together. While reduction of the print gap from the ejection zone of the head to the print media tends to minimize or eliminate the defects, there is a lower practical limit to decreasing the gap. If it becomes too short, inadvertent contact with the media by the head will smear the yet-to-dry fluid.
In the print gap, air velocity varies approximately linearly between the scan speed at the ejection zone (e.g., nozzle plate) of the head and zero at the print medium. Simulation by the inventors has shown that a curtain of fluid drops from a closely spaced array of nozzles in an ejection zone is capable of strongly influencing the print gap airflow. The wakes of the drops effectively constitute a moving barrier that pushes out air as the head scans. The result is a flow field similar to a river flowing around a row of bridge pilings, in which the fluid velocity downstream meanders from side to side in irregularly shifting patterns. It is believed main drops from the head tend to travel to the print media with little deviation due to their large mass. The smaller satellite drops, on the other hand, are believed to slow down and become influenced in direction by the local airflow. The observed wood grain effect is consistent with this hypothesis, i.e., satellite drops are channeled together into concentrated bands by the print gap airflow as modified by the wakes of the main drops.
Simulations further show that the flow field around the head develops in both time and space. This effect occurs in the print gap also: the velocity profile changes with time and varies across the width of the ejection zone even when no fluid ejectors are jetting. The time-dependence of the no-jetting flow field likely contributes to the wood grain print defect by forcing and enhancing local velocity oscillations around the ejectors.
Accordingly, a need exists to minimize or eliminate printing defects, especially when utilizing small volume drops. The need further extends to modifying airflow in the print gap and to do so consistently across as much of the gap as possible. Additional benefits and alternatives are also sought when devising solutions.
The above-mentioned and other problems are minimized by utilizing wind baffles with micro-fluid ejection heads. Broadly, the baffles modify airflow in a print gap between the head and print media as the head scans. The concept introduces surface structures that tend to drag air in the print gap. They are shaped, spaced and located so that the air velocity approximates the scan speed of an ejection zone as it reciprocates over the print media. It is intended to develop airflow conditions as early and as uniformly as possible during the scanning of the head and as consistently as possible over an entirety of the gap. In various designs, multiple wind baffles provide a cascading airflow effect from one wind baffle to the next.
These and other embodiments will be set forth in the description below. Their advantages and features will become readily apparent to skilled artisans. The claims set forth particular limitations.
The accompanying drawings incorporated in and forming a part of the specification, illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings:
In the following detailed description, reference is made to the accompanying drawings where like numerals represent like details. The embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized and that process, electrical, and mechanical changes, etc., may be made without departing from the scope of the invention. The following detailed description, therefore, is not to be taken in a limiting sense and the scope of the invention is defined only by the appended claims and their equivalents. In accordance with the present invention, methods and apparatus describe wind baffles for an ejection head for use in micro-fluid ejection devices.
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Pluralities of baffles 75 extend toward the gap from the surface 73, but not into the gap. They also extend laterally over surface 73 beyond a periphery of the area defined by the ejection zone 70. Certain embodiments even contemplate extensions 79 of the baffles beyond the surface 73. In either, they run generally orthogonal to both the scan 65 and airflow directions 63 and reside fairly symmetrically on opposite sides of the ejection zone. They can number as few as one or as many as eight, or more. The location of a first baffle 75-1 is as close as possible to a leading edge (LE) of scanning movement, with others being evenly spaced and substantially parallel. Their shape can vary, but sloped leading surfaces 77 accommodate inrushing airflow and assist in deflecting paper upon inadvertent contact. Symmetrical shapes also serve to assist in modifying airflow as the head scans in opposite directions during sequential passes over a media. Representative cross sections include, but are not limited to, circles, tubes, triangles, domes, hemispheres, or the like.
During use, air (A) rushes into the paper gap G as the head scans in the printhead movement direction 65. The baffles slow airflow velocity relative to the printhead. In the area beneath the ejection zone 70, airflow speed is caused to approximate the scan speed of the head as it moves past the media. In this manner, fluid drops are more consistently able to reach the media 52 without drifting. Print defects are minimized. In
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The foregoing has been presented for purposes of illustrating the various aspects of the invention. It is not intended to be exhaustive or to limit the claims. Rather, it is chosen to provide the best illustration of the principles of the invention and its practical application to enable one of ordinary skill in the art to utilize the invention, including its various modifications that naturally follow. All such modifications and variations are contemplated within the scope of the invention as determined by the appended claims.
