DROPLET DEPOSITION APPARATUS
The present invention relates to droplet deposition apparatus, and in particular to channeled ink jet print heads using piezoelectric actuation.
Channeled ink jet components are well known and a wide variety of configurations of channel and actuator have previously been proposed which utilize piezoelectric materials to provide actuation. Specifications for such components include lifecycle, frequency of operation, and operating voltage to name but a few. In an attempt to provide improved parameters, manufacturing complexity and cost are typically increased.
It is an aim of the present invention to provide a droplet deposition apparatus having improved performance with reduced cost and complexity.
According to a first aspect of the invention, there is provided droplet deposition apparatus comprising at least one ejection chamber in the form of a channel having a pair of channel walls, said walls being deformable in an actuation direction perpendicular to the plane of the channel walls; a roof formed of piezoelectric material and being deformable under the application of an electric field so as to expand or contract in the actuation direction; said roof being attached to said walls so as to close the top of the channel and such that said expansion or contraction of the roof causes said at least one wall to deflect into the channel, thereby to causing a change in volume of the channel.
The piezoelectric roof does not act directly to change the volume of the channel substantially, but rather uses the deformable channel walls to do so.
The channel walls can therefore act as a mechanical amplifier, and relatively small displacements of the piezoelectric element can provide a sufficient volume displacement of the channel to provide desired droplet deposition characteristics. In order to optimise this amplification effect it is preferable for the aspect ratio of the height of said channel to the width of said channel to be
greater than 2, and preferably greater than 5. The aspect ratio is important since it must be optimized to provide an appropriate channel pitch, nozzle cross sectional area, and wall compliance amongst other factors.
It is desirable for the piezoelectric material to be high-activity material, preferably having an activity greater than 1000 pm/V (pico-meters per volt or equivalents pC/N), and more preferably greater than 1500 or even 2000 pm/V. Advantageously the piezoelectric is formed of a single crystal ceramic. The term 'single crystal ceramic' is used to define a class of ceramics having no or very few grain boundaries, as opposed to more common polycrystalline ceramics. Examples of such ceramics include PMN-PT-28 manufactured by Morgan EiectroCeramics Ltd (www.morqanelectroceramics.com) or PZT-4.5% PT manufactured by TRS Ceramics lnc (www.trsceramics.com) So called 'textured ceramics' are intended to be encompassed by this term.
Use of high activity piezoelectric allows a similar displacement to be achieved, with a roof having a shorter dimension in the actuation direction, than with a conventional piezoelectric material. This in turn allows a higher channel density when arranging channels side by side, and densities of 180 dpi for example are possible.
In a second aspect the invention provides droplet deposition apparatus comprising a body having formed therein an array of channels arranged side by side in an array direction, each channel having channel walls deformable in an actuation direction parallel with said array direction, each channel having a roof formed of piezoelectric material attached to said body; and a plurality of electrodes adapted to apply an electric field to selected roofs of said channels to cause said selected roofs to expand or contract in the actuation direction, and thereby deform selected channel walls into said channel in said actuation direction.
In a third aspect the invention provides a method of forming a multichannel droplet deposition apparatus comprising providing a body having formed therein a plurality of channels arranged side by side in an array direction, attaching a sheet of piezoelectric material to said body so as to close the tops of said channels, separating the piezolelectic material between adjacent channels, each separated portion being associated with a respective channel, wherein actuation of said separated portions causes deflection in the array direction of the walls of said respective channels.
Preferably said body is formed of silicon, and the channels can advantageously be formed by etching.
The present invention will now be described by way of example with reference to the accompanying drawings in which
Figure 1 illustrates a print head according to an embodiment of the invention Figure 2 illustrates a print head according to an alternative embodiment of the invention.
Figure 1 shows two parallel channels 10 formed on a base 12. Each channel is supplied with fluid, typically ink, at one or both of its ends (not shown) and ejected via a nozzle 28 formed in the base at a position mid-way along the channel 10.
Each channel is bounded by walls 14 which are deflectable as shown by arrow 16 in a direction perpendicular to the axis of the channel 10 and parallel to the array direction A of the channels 10. Deflection of the wall generates an acoustic wave in the fluid in the channel which results in droplet ejection as known e.g. from EP0277703 and EP0278590.
Deflection of a wall is achieved by means of an actuator 18 which displaces the
top of the wall as indicated by arrow 20. In the embodiment shown, actuator 18 is formed of a piezoelectric sheet 22 attached to the top of the channel walls, and forming the roof of each channel 10 thereby sealing the channels. Electrodes 38, 40 are provided to apply an electric field to selected channel roofs so as to cause contraction or expansion in the plane of the sheet parallel to the array direction A.
One possible poling scheme is shown, with the piezoelectric material being poled in the thickness direction of sheet 22 as indicated by arrow 36. An electric field applied perpendicular to the plane of the sheet 22 then causes contraction in direction A in the 31 (indirect) mode. However, alternative schemes are possible, for example poling and an applied field in the plane of the sheet causing actuation in direction A in the 33 (direct) mode. Other forms of piezo material may use other poling schemes will be apparent to one skilled in the art.
Advantageously, a single functional portion of the sheet, as shown at 18, is attached at both of its ends to respective ends of the two walls bounding a particular channel. In this way, actuation of a single portion of the sheet results in both walls of channel being deflected as shown by arrows 20 and 20'. It is this movement of the walls - even though they are themselves inactive - which provides the volume change in the channel necessary for droplet ejection.
Cut-outs 24, partial or preferably through the full thickness of the piezoelectric sheet 22 allow only that portion of the sheet bounding a particular channel to be deflected. Cut outs may be formed in situ or may be preformed before attaching the piezoelectric sheet to the channel structure. Alternatively a combination of process steps may be used to isolate the channel roofs. For example cut outs could be preformed in the piezoelectric at the appropriate pitch, and the sheet attached to the channels, with the cut out side of the sheet facing the tops of the channel walls. The back surface of the sheet could then
be removed (for example by grinding) to separate the roofs.
In this way, the roof portions are effectively pre-formed but held together with a backbone which is removed after attachment to the channel structure. This same principle couid be achieved by metallizing a sheet of piezoelectric material to form a metal backbone, which could later be etched away to release the channel roofs.
Similarly the walls of adjacent channels are de-coupled, or separated, in this case by a cut out 26, ensuring that actuation of one channel for droplet ejection does not result in a droplet being ejected from its neighbour Additionally, the separate walls substantially isolate the acoustic waves in neighbouring channels which can be advantageous to the control of droplet ejection.
Figure 2 shows an alternative embodiment of the present invention. The basic channel structure is similar to that of Figure 1, with channels 202 having deflectable side walls 204 formed on a base 206, each channel having a roof 208 formed of piezoelectric material, forming the channel actuator, and capable of deflecting the side walls into the channel on actuation to cause a volume change in the channel.
In the embodiment of Figure 2 however, the nozzles 210 are formed not in the base, but in the roof of the channel, through the piezoelectric material. It can also be seen that in the embodiment of Figure 2 the roofs of adjacent channels are completely separated. In this embodiment it is possible for the sheet forming the roofs of the channels to be separated at the same time as separating the walls of adjacent channels, in a single sawing operation for example.
The present invention has been described by way of example, and it should be understood that alternative embodiments are included within the scope of the
claims. For example, although channel wall structures having a U shaped profile are shown, other wall constructions are possible which still allow deflection perpendicular to the plane of the walls. Manufacturing techniques such as sawing and etching have been described, however other techniques such as moulding are envisaged. The piezoelectric material forming the roof of the channels may be a single simple portion of material, however it could equally be a composite or multi-layer material providing equivalent overall desired actuation characteristics.