Inkjet print head

Details Inkjet print head Inkjet print head

1999-10-29

2002-08-27

Barlow, John (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06439702

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to inkjet printers and, in particular, it concerns print head configurations for such printers.
Impulse inkjet systems are well known in the art. They generally fall into two categories: continuous systems and drop on demand systems. Continuous inkjet systems operate by continuously ejecting ink droplets at high frequency, some of which are deflected by suitable means prior to reaching the substrate being imprinted, allowing the undeflected drops to form the desired imprinting pattern. Drop on demand systems eject drops selectively as required.
Drop on demand inkjet systems may, in turn, be divided into two general categories on basis of the principle of ejecting the droplets. Most systems in use today are the thermal bubble jet type wherein the ejection of ink droplets is effected by boiling of the ink.
Thermal bubble system, like the one disclosed in Japanese patent application No. 61-59913, includes thermoelectric heating elements. Actuation of a specific element causes the ink in that cavity to boil which causes a sudden rise in pressure, thus ejecting an ink drop through the nozzle. Bubble jet printing systems are advantageous in the ease of their miniaturization. On the other hand, they suffer some disadvantages relative to piezoelectric systems. One such disadvantage is the short useful life of the heating elements due to the high stresses imposed on the resistor protecting layer. In addition, it is relatively difficult to control precisely the volume of the drop and its directionality.
Still another drawback is the low frequency of printing signals which may be applied consistently to the printing head. Still another drawback of the thermal bubble system is that it is limited to special ink formulations which can withstand boiling temperature without mechanical or chemical degradation.
Other drop-on-demand inkjet systems use piezoelectric crystals which deform when a voltage is applied to them, thereby causing the ejection of a drop of ink from an adjoining ink cavity, as will be shown below. Ink is fed to the cavity through a restricted inlet opening, and leaves the cavity through a nozzle. The relative fluid impedance of the restricted inlet opening and the nozzle is such that a suitable amount of ink exits the outlet nozzle during the bending of the diaphragm. Replenishment of the cavity with ink is a result of the capillary action of the ink meniscus in the nozzle and the return motion of the diaphragm. The time taken to replenish depends on the fluid impedance.
In contrast to thermal bubble systems, piezoelectric drivers are not required to operate at elevated temperatures, allowing them to accommodate a much wider selection of inks. Furthermore, the shape, timing and duration of the driving pulses are more easily controlled. Finally, the operational life of the piezoelectric crystal and hence the piezoelectric head is much longer.
Piezoelectric crystal drop-on-demand print heads are well known in the art. Some illustrative examples of such developments include U.S. Pat. Nos. 4,730,197 and 5,087,930. These patents disclose a construction having a series of stainless steel layers. The layers are of various thickness and include various openings and channels. The various layers are stacked and bonded together to form a suitable fluid inlet channel, pressure cavity, fluid outlet channel and orifice plate.
The systems disclosed in the above referenced patents illustrate the use of a fluid inlet channel having a very small aperture, typically 100 microns or less. The use of a very small aperture is dictated by the need to limit the back flow from the ink cavity during ejection of a drop. On the other hand, it is problematic in that the small aperture is susceptible to clogging during the bonding of the layers as well as during normal operation of the print head. Additionally, the techniques used for forming the openings in the orifice plate, which typically include punching, chemical etching or laser drilling, require that the thickness of the orifice plate be equal to, or less than, the orifice diameter, which is itself limited by resolution considerations to about 50 microns. Finally, any air bubbles or other gaseous substance trapped in the flow channels cannot easily be purged, and because bubbles are compressible, their presence in the system can have detrimental effects on the system performance.
Piezoelectric elements are used in inkjet heads in various configurations, each having its implications for the cavity construction. Some examples are: a layered type, as shown schematically in FIG. 53 of U.S. Pat. 5,666,141, in which a rod shaped layered element extends longitudinally as a result of voltage applied to the electrodes, causing a pressure surge in the ink cavity. Another conventional configuration, known as the bimorph-cantilever type, is shown schematically in FIG. 54 of U.S. Pat. No. 5,666,141. In this case, two electrodes are cemented to a piezoelectric element forming a thin leaf. A voltage applied to the electrodes causes the leaf to bend, thereby ejecting a single drop. In more recently developments, the piezoelectric element is typically cemented to a thin plate forming a diaphragm located above the ink cavity.
Two approaches are used to achieve full print coverage of the printed substrate: the conventional construction uses a small printing head containing a limited number of cavities and nozzles (sometimes as low as a single nozzle), each nozzle printing a specific row. To achieve full coverage the printing head is being moved to-and-for while ejecting ink droplets. Each movement of the printing head corresponds to a strip of printed lines, typically one for each nozzle in the head. The printed substrate is also moved forward in steps, the width of the step depending on the number of printing nozzles. This mechanism is commonly used in desk printers and the like. Its main disadvantages are the limited printing speed and the high noise level it produces.
The second approach, to which the present invention primarily relates, is the full array approach. According to this approach, each pixel across one dimension of the substrate is covered by a specific nozzle. Although this approach necessitates a large number of nozzles, it can achieve very high printing speed and silent operation.
In order to provide high nozzle densities over a small area, conventional inkjet print heads are typically formed on silicon or ceramic wafers by use of masking or etching techniques. The use of such wafers renders the structures uneconomical for implementing large two-dimensional arrays of cavities.
As an alternative to the use of a constricted fluid inlet channel aperture with its associated problems mentioned above, it has been suggested that suitable ink flow impedance could be combined with advantageous filtering properties by passing the ink into the cavities through a porous layer. The principles of this approach are described in the parent application of this application, now issued as U.S. Pat. No. 5,940,099 to the present applicant. In order to achieve high quality uniform printing, it is important that the ink supply to the porous layer should uniform with respect to the cavities. However, the parent patent does not address details of how to achieve uniformity of ink supply across the porous layer for large two-dimensional nozzle arrays.
A further issue relating to inkjet print head design is the choice of material for the front face of the printing head. For a range of reasons including mechanical and chemical properties and ease of production, polyimide compositions are frequently preferred. However, it has been found that a polyimide front surface has a tendency to collect small splashes of ink and other residues, leading to inferior printing quality and reduced reliability.
There is therefore a need for an inkjet print head which provides an improved ink supply through a porous layer to a plurality of ink cavities. It would also be highly advantageous to provide an inkjet print head with

Affiliated with

Also associated with

Rating

Inkjet print head does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Inkjet print head, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Inkjet print head will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2946354