Droplet deposition apparatus

Incremental printing of symbolic information – Ink jet – Ejector mechanism

Reexamination Certificate

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Details

C347S068000

Reexamination Certificate

active

06286943

ABSTRACT:

The present invention relates to droplet deposition apparatus, in particular inkjet printheads, having a bottom sheet comprising piezo-electric material and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls and by a bottom surface extending between said side walls, at least the side walls comprising said piezo-electric material. The present invention also includes methods of manufacture of such apparatus.
Such printheads are known in the art—e.g. from EP-A-0 277 703, EP-A-0 278 590 and EP-A-0 364 136—and employ piezoelectric material actuated in its shear mode to vary the volume of an ink chamber and thereby eject an ink drop. EP-A-0 341 929 describes a method for driving such actuators and makes clear that the actuators each have a capacitive load. As will be generally understood, such a capacitive load makes current and power demands which affect both size and cost of the driving circuitry. Furthermore, it has been recognised by the present inventors that current flowing in the circuitry and electrodes of the printhead itself may also lead to heat generation which in turn will affect the viscosity of the ink: as is the case in many types of inkjet apparatus, variation in ink viscosity will result in a variation in the velocity of the ejected ink droplet which will in turn manifest itself as a droplet-placement error in the printed image.
It has been proposed to decrease the capacitive load of a printhead by restricting the use of piezoelectric material in the printhead to those areas where piezoelectric activity is actually required—i.e. in the channel walls of the “active” (closed) portions of the channels—and using material having a lower dielectric coefficient for the remainder of the printhead. This may be achieved in practice by “letting in” an insert of piezo-electric material into a lower dielectric base, forming channels that pass through both the insert and the base and depositing electrodes along the length of each channel. Only that part of each channel wall which comprises piezoelectric material will distort in response to an electric field applied via the electrodes, with the remainder of each wall and any associated connection area only presenting a low capacitive load. The “letting in” of the piezoelectric insert is a complex process however and therefore renders this technique expensive.
The present invention has as an objective printheads that have a lower capacitive load than known constructions and yet are not complex to produce. Methods of manufacture of such printheads are also comprised in the present invention.
Accordingly, the present invention consists in droplet deposition apparatus comprising
a bottom sheet comprising piezo-electric material and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls and by a bottom surface extending between said side walls;
at least the side walls comprising said piezo-electric material and electrodes for applying an electric field to said piezo-electric material, thereby to effect transverse displacement of said side wall;
a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof;
the channels being supplied with droplet liquid fluid and communicating with a nozzle for ejection of droplets therefrom; each channel having one portion open on a side lying parallel to the channel axis for communication with droplet fluid supply means and another portion which is closed on all sides lying parallel to the channel axis;
characterised in that
the respective side walls comprising said piezo-electric material in said one portion of each channel are disabled such that transverse displacement of said side walls in said one portion of each channel does not take place.
It has been recognised by the present inventors that not only is transverse displacement of the walls in ‘open’ sections of the channel unnecessary but that by disabling the walls in such ‘open’ sections, a reduction in the overall capacitative load can be achieved in printheads manufactured according to the substantially conventional technique of forming channels in a base comprising a uniform layer of piezo-electric material. The complex manufacturing methods described above can therefore be avoided.
According to a first preferred aspect, the invention consists in droplet deposition apparatus comprising a bottom sheet having a layer of piezo-electric material poled in a direction normal to said layer and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls and by a bottom surface extending between said side walls, at least the side walls comprising said piezo-electric material;
a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof;
the channels being supplied with droplet liquid fluid and communicating with a nozzle for ejection of droplets therefrom;
each channel having one portion open on a side lying parallel to the channel axis for communication with droplet fluid supply means and another portion which is closed on all sides lying parallel to the channel axis,;
electrodes provided on opposite sides of said side walls, thereby to form shear mode actuators for effecting droplet expulsion from the channels associated with said actuators, each electrode extending substantially the length of the channel;
characterised in that,
in said one portion of said channel a layer of material having a dielectric constant lower than that of the piezoelectric material is interposed between said piezoelectric material and at least one of said electrodes provided on opposite sides of said side walls.
Such apparatus achieves a lower capacitance than conventional designs without the complexity associated with “let-in” piezoelectric inserts as described above. In said one portion of the channel which is open to ink supply means, the walls do not need to be displaceable. Consequently the electrodes in such a portion can be separated from the piezoelectric material of the channel walls by a layer of material having a lower dielectric constant than the piezoelectric material thereby disabling that part of the piezo electric material. The resulting capacitive load between the electrodes on opposite sides of the channel wall in this one portion is lower than that obtained with piezoelectric material alone (as is the case in said another “closed” portion of the channel) and thereby contributes to a reduction in the total capacitive load of the printhead.
According to a particular embodiment, said-one and another portions are each defined at least in part by side walls having co-planar top surfaces and being of substantially constant height, whereby the height of the respective side walls of said one and another portions is substantially equal. Such an embodiment is particularly amenable to manufacture, there being no variation in channel machining depth required in the forward part of the channels at least. In such an embodiment, said another portion of the channel may extend at a constant depth as far as the nozzle. Independently of this, said one and another portions may be contiguous.
Preferably the electrodes provided on opposite sides of the side walls are located at the top of each channel side wall, remote from said bottom surface. Such an arrangement is particularly amenable to deposition using the -known “angled plating” principle. In particular, the electrode may advantageously extend over approximately half the depth of the respective channel wall in said another portion whilst in said one portion, the electrode preferably extends over 10% or less of the height of the respective channel wa

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