ACOUSTIC PRINTHEADS

Abstract: A printhead (11) for an acoustic printer comprises one or more acoustic microlenses (12), each of which brings an acoustic beam (13) to focus approximately at the free surface (15) of a pool (14) of ink for ejecting individual droplets (16) of ink from the pool (14) on demand. As used herein, an "acoustic microlens" is defined as being an acoustic lens (12) having an aperture diameter (A) which is less than an order of magnitude greater than the wavelength of the incident acoustic wave (i. e., the acoustic wave which illuminates the lens).

Description: This invention relates to acoustic printers and, more particularly, to microlenses for such printers. Acoustic printing is a potentially important direct marking technology. It still is in an early stage of development, but the available evidence indicates that it is likely to compare favorably with conventional ink jet systems for printing either on plain paper or on specialized recording media, while providing significant advantages of its own. Drop-on-demand and continuous-stream ink jet printing systems have experienced reliability problems because of their reliance upon nozzles with small ink ejection orifices, which easily clog. Acoustic printing obviates the need for such nozzles, so it not only has greater intrinsic reliability than ordinary ink jet printing systems, but also is compatible with a wider variety of inks, including inks which have relatively high viscosities and inks which contain pigments and other particulate components. Furthermore, it has been found that acoustic printing provides relatively precise positioning of the individual printed picture elements ("pixels"), while permitting the size of those pixels to be adjusted during operation, either by controlling the size of the individual droplets of ink that are ejected or by regulating the number of ink droplets that are used to form the individual pixels of the printed image. When an acoustic beam impinges on a free surface (i. e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which the beam exerts against the surface of the pool may reach a sufficiently high level to eject individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. These principles have been applied to prior ink jet and acoustic printing proposals. For example, K. A. Krause, "Focusing Ink Jet Head," IBM Technical Disclosure Bulletin , Vol 16, No. 4, September 1973, pp 1168-1170 described an ink jet in which an acoustic beam emanating from a concave surface and confined by a conical aperture was used to propel ink droplets out through a small ejection orifice. US-A-4,308,547 showed that the small ejection orifice of the conventional ink jet is unnecessary. To that end, they provided spherical piezoelectric shells as transducers for supplying focused acoustic beams to eject droplets of ink from the free surface of a pool of ink. They also proposed acoustic horns driven by planar transducers to eject droplets of ink from an ink-coated belt. Spherical piezoelectric transducers are suitable for use in low- and moderate-resolution acoustic printers. Such a transducer can be designed so that the acoustic beam it generates comes to an essentially unaberrated focus at or near the free surface of a pool of ink, thereby minimizing the variables that need to be controlled to achieve stable operation. Unfortunately, however, the mechanical strength of known piezoelectric materials imposes a design constraint on the minimum permissible thickness of a shell-like transducer, with the result that the upper end of the useful frequency range for these transducers is somewhere in the vicinity of 25 MHz. In a liquid, such as water, the wavelength of a 25 MHz acoustic beam is approximately 60 µm, so the upper limit on the printing resolution that can be achieved, using an ink having an acoustic speed comparable with that of water, is only about 200 spots per inch. Furthermore, these shells are usually several mm in diameter. EP-A-0,216,589 published 01.04.87 and claiming priority from 16.09.85 discloses on alternative source of focused acoustic beams. It uses at leasts one acoustic wave transducer which is submerged in ink. Each transducer launches a converging cone of acoustic energy at the ink surace to eject droplets on demand. To increase the resolution which can be achieved and to provide a less-cumbersome and lower-cost technique for manufacturing arrays of relatively stable acoustic droplet ejectors, acoustic lenses may be used for focusing the beam. However, acoustic lenses are not limited to use in arrays. Indeed, it has been found that the acoustic lens is extremely well suited to all forms of acoustic printing because its aperture need not be much larger than the wavelength of the acoustic wave in the solid which defines the lens. In accordance with this invention, a printhead for an acoustic printer comprises one or more acoustic microlenses, each of which brings an acoustic beam to focus approximately at the free surface of a pool of ink for ejecting individual droplets of ink from the pool on demand. As used herein, an "acoustic microlens" is defined as being an acoustic lens having an aperture diameter which is less than an order of magnitude greater than the wavelength of the incident acoustic wave (i. e., the acoustic wave which illuminates the lens). Still other features and advantages of this invention will become apparent when the following detailed description is read in conjunction with the attached drawings, in which: Fig. 1 a sectional view of an acoustic printhead comprising an acoustic microlens which is constructed in accordance with the present invention, and Figs. 2A and 2B are sectional views of printheads having acoustic microlenses in combination with certain optional features and in alternative system configurations. In Fig. 1 there is a acoustic printhead 11 (shown only in relevant part) comprising an acoustic microlens 12 which is illuminated during operation by an ultrasonic acoustic wave, such that the lens 12 launches a converging acoustic beam 13 into a pool of ink 14. The focal length of the lens 12 is selected so that the beam 13 comes to focus on or near the free surface 15 of the pool 14, thereby enabling individual droplets 16 of ink to be ejected from the pool 14 on demand, as more fully described below. As illustrated, a microlenses 12 is defined by a small part-spherical depression or indentation which is formed in the upper surface of a solid substrate 21. A piezoelectric transducer 22 is deposited on, or otherwise intimately mechanically coupled to, the opposite old lower surface of the substrate 21, and a rf drive voltage (supplied by means not shown) is applied to the transducer 22 during operation to excite it into oscillation. The oscillation of the transducer 22 generates an ultrasonic acoustic wave 23 which propagates through the substrate 21 to illuminate the microlens 12. To carry out this invention, the substrate 21 is composed of a material having an acoustic velocity which is much higher than the acoustic velocity of the ink 14, Typically, the ink 14 has an acoustic velocity of about 1km/sec. - 2km/sec., so the substrate 21 consists of a material, such as silicon, silicon nitride, silicon carbide, alumina, sapphire, fused quartz, and certain glasses, having an acoustic velocity which exceeds that of the ink 14 sufficiently to reduce the aberrations of the acoustic beam 13 to an acceptably low level, if not effectively eliminate them. For example, the substrate 21 may be composed of a material having an acoustic velocity which is about 2.5 times that of the ink 14, if small aberrations of the acoustic beam 13 are tolerable. If, on the other hand, it is necessary or desirable to reduce the aberrations of the acoustic beam 13 to a negligibly-low level, the substrate 21 is fabricated from a material having an acoustic velocity which is at least four times that of the ink 14. In accordance with the present invention, it has been found that the microlens 12 provides sufficient convergence of the acoustic beam 13 to eject or propel individual droplets 16 of ink from the pool 14 on demand, even though its aperture diameter, A, is less than an order of magnitude (i. e., ten times) greater than the wavelength of the acoustic wave 23 which is illuminating it. The focal length of the lens 12 typically is approximately equal to its aperture diameter, A, such that the lens 12 has an f-number. That, in turn, means that the waist diameter of the acoustic beam 13 at focus is approximately equal to the wavelength, λ i , of the beam 13 in the ink 14. Experiments have shown that the microlens 12 retains its ability to bring the acoustic beam 13 to an essentially diffraction-limited focus, even if its aperture diameter, A, is only about 1.5 times the wavelength, λ s , of the acoustic wave 23 in the substrate 21. While the minimum permissible aperture diameter to wavelength ratio has not been ascertained as yet, the performance of the small aperture microlenses which have been tested to date is surprisingly consistent and stable. As a general rule, the transducer 22 has a relatively narrow band resonant response characteristic, so the radiation pressure of the acoustic beam 13 may controlled as required for drop-on-demand printing, not only by modulating the amplitude or duration of the rf drive voltage applied to the transducer 22, but also by modulating its frequency. The threshold pressure required to eject individual droplets 16 of ink from the pool 14 is a function of the particular ink that is employed and can be determined empirically to establish an appropriate reference level for the droplet ejection control process. The relatively small aperture diameter, A, of the microlens 12 permits arrays of such lenses to be fabricated for various forms of parallel acoustic printing. Even more generally, however, it facilitates the design of compact printheads for acoustic printing over a broad range of resolutions, including resolutions that are substantially higher than those which can be achieved using known alternative printhead technologies, such as the spherical piezoelectric transducer, for supplying a sharply-focused acoustic beam. For example, microlens-based printheads have been operated at 50 MHz. for 10 spots per mm ( 250 s.p.i.) printing, which is typical of the resolution that is provided by commercially-available, higher-quality, non-acoustic printers. Referring to Figs. 2A and 2B, the basic components of the printhead 51 are essentially the same as those of the printhead 11 (Fig. 1), so like reference numerals have been used to identify like parts. However, as illustrated in Figs. 2A and 2B, a λ z /4 thick layer 52 of impedance-matching material (where λ z = the wavelength of the acoustic beam 13 in the impedance-matching material) may be coated on the outer concave surface of the microlens 12 to suppress unwanted reflections. Furthermore, an overcoating 53, which has an acoustic impedance and an acoustic speed intermediate those of the ink 14 and the substrate 22, may be deposited on the lens-bearing upper surface of the substrate 22 to planarize the printhead 51. The overcoating 53 fills the lens 12 and has a generally planar outer surface. Microlens-based printheads also are compatible with various system configurations, For example, as shown in Fig. 1, such a printhead 11 may be immersed in the pool of ink 14. Alternatively, as shown in Figs. 2A and 2B, the ink 14 may be carried on a transport 55, such as a thin film of 'Mylar' (trademark), and the printhead 51 may be acoustically coupled to the ink 14, either by causing the transport 55 to bear against the printhead 51 (Fig. 2A) or by maintaining a thin layer of liquid 56 (Fig. 2B) between the printhead 51 and the transport 55. The present invention provides an acoustic microlens which may be utilized to fabricate reliable printheads for acoustic printing over a broad range of resolutions, including resolutions which are sufficient for high-quality printing. While part-spherical microlenses are provided for printing generally-circular pixels, it will be appreciated that the geometry of the microlens may be modified to print non-circular pixels, such as elliptical pixels or elongated stripe-like pixels.

Claims: An acoustic printhead (11) for ejecting individual droplets of ink on demand from a free surface of an ink supply, the ink having a known acoustic speed; comprising a solid substrate (21) composed of a material having an acoustic speed which is substantially higher than the acoustic speed of the ink, the substrate being oriented with its uppermost horizontal surface having at least one concave indentation (12) formed therein to define an acoustic microlens having a predetermined aperture diameter and a predetermined focal length; a piezoelectric transducer (22) mechanically coupled to another surface of the substrate for generating an acoustic wave in the substrate for 'illuminating' the microlens, such that the microlens launches a converging acoustic beam upwardly, with the focal length of the microlens being selected to cause the beam to come to a focus at a prescribed distance from the uppermost surface; the acoustic wave having a wavelength in the substrate such that the aperture diameter of the microlens is less than an order of magnitude greater than the wavelength. The printhead of Claim 1, wherein the concave indentation is coated with a quarter-wavelength thick layer (52) of impedance-matching material to form an anti-reflective surface coating on the microlens. The printhead of Claim 1 or 2, wherein the uppermost surface of the substrate is overcoated with a layer of material having an acoustic impedance and an acoustic speed intermediate those of the ink and the substrate, and wherein the overcoat fills the indentation to provide a generally-planar output surface for the printhead. The printhead of any preceding claim, wherein the concave indentation is essentially part-spherical to define a microlens for printing generally-circular pixels. The printhead of any preceding claim, further including a thin film transport for carrying the ink, the transport bearing against the printhead surface to couple the microlens acoustically to the ink. The printhead of Claim 5, further including means for introducing a layer of liquid between the printhead and the ink transport. A printhead as claimed in any preceding claim, adapted to be immersed in ink so that its uppermost surface is spaced below the free surface of the ink by a specified distance.

Inventors: ELROD, SCOTT ALAN, KHURI-YAKUB, BUTRUS T., QUATE, CALVIN F., VANZANDT, THOMAS ROY
Applicants: XEROX CORPORATION
Application year: 1987
Main classification: B41J2
IPC codes: B41J2