DROP-ON-DEMAND INK-JET PRINTING APPARATUS

Abstract: A drop-on-demand ink-jet printing apparatus comprises, an ink chamber connected to an ink supply means and filled with ink supplied by the ink supply means, the ink chamber including a nozzle for projecting an ink droplet and an elastic surface for changing the volume of the ink chamber by its deflection, a plurality of pressure vibration modes having a plurality of antinodes being generated within the ink chamber, and a piezoelectric transducer attached to the el­ astic surface at a position corresponding to one of the antino­ des of one of the pressure vibration modes, whereby the pie­ zoelectric transducer excites one of the pressure vibration modes in the ink chamber and projects the ink droplet from the nozzle.

Description: The present invention relates to a drop-on-demand ink-jet printing apparatus, and more particularly, to an ink-jet printing head in which a droplet of printing fluid is ejected from a nozzle by volume displacement. The ink-jet printing apparatus is well known in the art, which prints a desired pattern on a recording medium such as paper by depositing discrete droplets of printing fluid (ink) on the recording medium. Such an ink-jet printing apparatus is disclosed in US patent 4,189,734 issued to Kyser et al. Kyser et al teaches a structure of a printing head which includes a base plate and a deflection plate bonded to the base plate to form a chamber. The chamber is filled with the ink and provided with a nozzle at one end. A piezoelectric transducer is bonded to the deflection plate and connected to a driver. Upon application of voltage across the piezoelectric transducer, the transducer contracts to cause the plate to deflect inward into the chamber. Thus, the volume of the chamber is reduced to eject a droplet of the printing fluid from the orifice of the nozzle. In the conventional apparatus, the piezoelectric transducer is fixed on the deflection plate at a position unrelated to the pressure vibration modes of the ink in the chamber. Accordingly, the piezoelectric transducer generates the pressure vibration wave combining a plurality of the pressure vibration modes. In order to print Chinese characters, half-tone images and the like with the high printing resolution the ink-jet printing apparatus is required to generate fine droplet of the ink, ie, to reduce the volume of the droplet. In general, the droplet volume Q is represented by the sectional area A of the nozzle, the droplet velocity v(t) at the orifice of the nozzle, the time t₁ when the pressure of the piezoelectric transducer is applied to the ink in the chamber, and the time t₂ when the droplet of ink is separated from the orifice of the nozzle, as follows: The droplet velocity v(t) is proportional to the voltage applied to the piezoelectric transucer. The period of time from t₁ to t₂ is determined by the configuration of the chamber and the disposition of the piezoelectric transducer with respect to the chamber. According to formula (1), the droplet volume is reduced if the sectional area A of the nozzle can be decreased. However, it is difficult to manufacture the fine nozzle, and the fine nozzle is apt to be choked up with the ink. The other manner to reduce the droplet volume Q is to decrease the droplet velocity v(t), ie, to decrease the voltage to the piezoelectric transducer. However, the low speed droplet is difficult to control to project to an accurate position due to the deflection of its locus. Accordingly, the fine droplet is difficult to obtain in the conventional ink-jet printing head. Therefore an object of the present invention is to provide a drop-on-demand ink-jet printing apparatus capable of generating fine droplets of the ink without reducing the sectional area of the nozzle and the droplet velocity. In US-A-4,417,255, there is described an ink-jet printer in which third harmonic vibration is imparted to ink inside a nozzle in addition to the fundamental harmonic vibration. The present invention is defined in claim 1. One embodiment of the invention will be described below, by way of example, with reference to the accompanying drawings. Brief description of the drawings: FIGS. 1(a) and 1(b) are plan views of an ink-jet printing apparatus which is known to the applicants and FIG. 1(c) is a sectional view taken along the line A-A' of Fig. 1(a); FIGS. 2(a) and 2(b) illustrate a positional relationship between natural pressure vibration modes and a chamber shown in FIG. 1(b); FIGS. 3(a) and 3(b) illustrate a fixed position of a piezoelectric transducer shown in Fig. 1(a); FIGS. 4(a) and 4(b) are graphs showing an ink velocity as a function of time; FIGS. 5(a) and 5(b) are plan views of an ink-jet printing apparatus according to the present invention, and FIG. 5(c) is a sectional view taken along the line B-B' of Fig. 5(a); FIGS. 6(a) and 6(b) illustrate fixed positions of piezoelectric transducer shown in Fig. 5(a); FIG. 7 is a block diagram of a drive means for the ink-jet printing apparatus of the present invention; FIGS. 8(a) to 8(f) are timing charts of the drive means shown in Fig. 7; and FIGS. 9(a) to 9(e) illustrate transmission of vibration in a chamber shown in Fig. 5(b). Referring to Figs. 1(a), 1(b) and 1(c), a known ink-jet printing head 10 comprises a base plate 8 on which concaves are formed. An elastic plate 7 is fixed on the base plate 8 to form an ink reservoir 5 and an ink chamber 9. The ink chamber 9 includes a nozzle portion 1, an ink path portion 2, a pressure applied portion (main chamber) 3 and an ink supply path portion 4. The ink reservoir 5 stores ink supplied by an ink source 11 and supplies it to the ink chamber 9 via the ink supply path 4. A piezoelectric transducer 6 is fixedly secured on the elastic plate 7 at a portion above the main chamber 3. The piezoelectric transducer 6 is connected to a drive circuit 12 which supplies drive pulse thereto and generates an ink droplet D. In the Fig. 1 arrangement, the axial lengths l₁, l₂, l₃, and l₄ of the nozzle 1, the ink path portion 2, the main chamber 3 and the ink supply path portion 4 are 0.8 mm, 9mm, 11mm and 4.5 mm, respectively. The widths w₁,w₃, w₄ and w ' / 4 of the nozzle 1, the main chamber 3, the narrow portion and the wide portion of the ink supply path portion 4 are 70 µm, 1.6 mm, 70 µm and 1.6 mm, respectively. The depths d₁,d₃,d₄ and d'₄ of the nozzle 1, the main chamber 3, the narrow and wide portion of the ink supply path portion 4 are 40 µm, 50 µm, 40µm and 50 µm, respectively. The thicknesses S₆, S₇ and S₈ of the piezoelectrical transducer 6, the elastic plate 7 and the base plate 8 are 0.2 mm, 0.1 mm and 1.5 mm, respectively. The elastic plate 7 and the base plate 8 are made of stainless steel. Figs. 2(a) and 2(b) show the positional relationship between the chamber 9 shown in Fig. 1 and natural pressure vibration modes generated in the chamber 9. As shown in Fig. 2(b), the amplitude of pressure vibration on both edges of the chamber 9 are always 0. That is, the vibration does not occur at the nozzle 1 and the ink supply path 4. Between the both edges, 1st to 5th order mode for the pressure vibration harmonics are generated. For instance, the 2nd order mode for the pressure vibration has twice frequency the 1st order mode, and has two antinodes and one node. The natural periods t₁ to t₅ for the 1st to 5th order modes are measured at 87.8 µ sec, 22.3 µ sec, 12.8 in sec, 9.1 µ sec and 6.9 µ sec. Referring to Figs. 3(a) and 3(b), the piezoelectric transducer 6 is provided to excite the 3rd order mode for the pressure vibration harmonics in the first embodiment of the present invention. The transducer 6 is fixed on the elastic plate 7 at the position corresponding to second antinode AN₂ of the 3rd order mode, ie, the length L₁ of the transducer 6 is equal to the length between first and second modes N₁ and N₂ apart from the nozzle end by 8.6 mm and 17.4 mm, respectively. The velocity of the ink at the nozzle 1 is illustrated in Fig. 4(b). Since the piezoelectric transducer 6 excites the 3rd order mode, the ink ejecting time represented from t₁ to t₂ is shortened in comparison with Fig. 4(a) which illustrates in the case of the conventional head. That is, the area is smaller than the area a , with the result that the droplet volume is reduced. Figs. 5(a) to S(c) show an embodiment of the present invention. As shown in Fig. 5(b), the configuration of a chamber 9 is the same as the known arrangement of Fig.1. Under the chamber 9, there is provided with another ink supply path 14 which connects the ink reservoir 5 to the nozzle 1 via an ink supply hole 13 and a small ink reservoir 12 as described in US patent 4 549 191. The ink supply path 14 is formed on an ink supply plate 15 which is bonded to the base plate 8. The ink reservoir 5 is supplied with ink by the ink source 11 via a tube 18. First and second piezoelectric transducer 16 and 17 are fixed on the elastic plate 7 at the positions described below. Referring to Figs. 6(a) and 6(b), the first and second piezoelectric transducer 16 and 17 are provided to excite the 5th order mode for the pressure vibration harmonics. The first transducer 16 is fixed on the elastic plate 7 at the position corresponding to third antinode AN'₃ of the 5th order mode, and the second transducer 17 is fixed at the position corresponding to fourth antinode AN'₄ The lenth L'₁ of the first transducer 16 and the length L'₂ of the second transducer 17 are substantially equal to the length between nodes N'₂ and N'₃ and, between nodes N'₃ and N'₄ respectively. The distances from the front end of the nozzle 1 to the nodes N'₂, N'₃ and N'₄ are 9.9 mm, 14.8 mm and 19.7 mm. The first and second transducer 16 and 17 are connected to drive circuits 37 and 38, respectively, as shown in Fig. 7. Print timing pulse generators 33 and 34 send a drive signal to the drive circuits 37 and 38 via AND gates 35 and 36, respectively. The AND gates 35 and 36 are opened by a print data signal 30. The pulse generator 33 is supplied with a print timing signal 31 via a delay circuit 32 and the pulse generator 34 is directly supplied with the same. Referring to Figures 8(a) to 8(f), the print timing signal 31 is generated after the print data signal 30 (Fig. 8(b) ) turns to "1" as shown in Fig. 8(a). The pulse generator 34 generates a first print pulse d having a pulse width in response to the print timing signal 31 as shown in Fig. 8(d). The drive circuit 38 receives the print pulse d via the AND gate 36 and generates a drive pulse f for actuating the transducer 17 as shown in FIg. 8(f). Thus, the print timing signal 31 is delayed by the time period and enables the pulse generator 33 to generate a second print pulse c . The drive circuit 37 generates a drive pulse e for actuating a transducer 16 as shown in Fig. 8(e). Accordingly, the second transducer 17 is actuated at first, and then the first transducer 16 is actuated with a time delay of Figs. 9(a) to 9(e) illustrate the transmission of the vibration in the ink chamber 9 caused by the drive pulses e and f . When the second transducer 17 is actuated at the time t is a positive pressure is generated at the position corresponding to the antinode AN'₄ (Fig. 6(b)) as shown in Fig. 9(a). Next, when the positive pressure is transmitted to the antinodes AN'₃ and AN'₅, ie, when the time t is ¾t₅, the first transducer 16 is actuated to enhance the vibration as shown in Fig. 9(b). The pressure vibration wave thus generated is gradually transmitted to the antinode AN'₂ (Fig. 9(c)) and the antinode AN'₁ (Fig. 9(d)). Thus, the 5th order mode for the pressure vibration shown in Fig. 6(b) is formed at the time 9 / 4t₅. The droplet of the ink ejects when the pressure on the antinode AN'₁, ie, the pressure on the nozzle 1 is minimised. That is, the droplet is generated at t = 9 / 4t₅ (Fig. 9(d). It is noted that the 5th order mode natural period t₅ is shorter than the 3rd order mode natural period t₃. since the ink ejecting time period t₁ to t₂ is substantially equal to the period the droplet volume is further reduced in comparison with the Fig. 1 arrangement. In the Fig. 5 embodiment the sectional area A has a rectangular configuration and its size of 40 µm x 70 µm. The diameter of the droplet is 40 µm when the droplet velocity is 4 m/s. As described above, in the embodiment shown in Fig. 5 the piezoelectric transducer is provided at the position corresponding to the antinode of the n-th order mode for the pressure vibration harmonics of the ink chamber. Accordingly, the piezoelectric transducer excites only the n-th order mode and the pressure vibration wave generated in the ink chamber has a high frequency so as to shorten the ink ejecting time period. As a result, fine droplets of the ink can be generated without decreasing the droplet velocity. Further, the satellites (excess minute droplets) are not generated since the component of the pressure vibration wave include only the n-th order mode harmonics.

Claims: A drop-on-demand ink printing apparatus comprising, an ink chamber (9) connected to an ink supply means whereby it may be filled with ink supplied from the ink supply means, the ink chamber (9) including a nozzle (1) for projecting an ink droplet and an elastic surface (7) for changing the volume of the ink chamber (9) by its deflection, a plurality of pressure vibration modes generating a plurality of antinodes within the ink chamber (9), a plurality of piezoelectric transducers (16)(17) each being fixed to the elastic surface (7) at a position corresponding to a respective antinode of a pressure vibration mode, and driving means (37)(38) for actuating the piezoelectric transducers (16)(17), the driving means (37)(38) being arranged to actuate first one of the transducers (16)(17) and then the other of the transducers (16)(17) characterised in that the period of time between the actuation of the first one and the other of the transducers is one half of one period of a pressure vibration mode. A drop-on-demand ink-jet printing apparatus as claimed in claim 1, characterised in that the pressure vibration mode is of either the 3rd or the 5th order.

Inventors: FUKUCHI, HIROMICHI, UEMATSU, RYOSUKE, USHIODA, TOYOJI
Applicants: NEC CORPORATION
Application year: 1986
Main classification: B41J2
IPC codes: B41J2