The invention relates to an ink jet recording device including a recording head compπsing a substantially linear array of nozzles for the emission of droplets of ink, the array of nozzles extending in a first direction, the device further comprising transportation means for transporting a record carrier in such a manner that a surface of the record carrier faces the nozzles and moves in a second direction transverse to the first direction.

A device of this kind is known from US Re 28 219. The known device is a continuous ink jet system in which from each nozzle a continuous stream of ink droplets is emitted. The known device comprises a plurality of rows of parallel, equidistant nozzles. For constructional reasons, the distance between adjacent nozzles in the row is comparatively large. Consequently, gaps are formed between dots that are formed on the record carrier by droplets of ink emitted by adjacent nozzles. To enable the device to print a continuous line that extends over the whole width of the record carrier, the rows are laterally staggered. If droplets are emitted from a first row of nozzles with a suitable delay relative to the emission of droplets from a second row which is arranged upstream of the first row in the direction of transport of the record carrier, the droplets from the first and second rows will be deposited on a common line on the record carrier. If a sufficient number of rows is provided and if the delays between the emission of droplets from the different rows is controlled very precisely, it is possible to deposit a continuous line of droplets on the record carrier. It is clear that the device will be large due to the high number of rows of nozzles. Moreover, an expensive control mechanism having a high precision is required.

It is an object of the invention to provide an ink jet recording device that can print continuous monochrome lines on the record carrier with only a single row of nozzles. To achieve this object, the device in accordance with the invention is characterized in that the recording device is a droplet on demand recording device, each nozzle

communicating with a pressure chamber associated with actuator means for emitting an ink droplet only when a predetermined signal is applied to the actuator means, the nozzles being formed by a plurality of first through-holes in a nozzle plate that forms a wall of the pressure chambers, a deflection plate being provided on a face of the nozzle plate that is directed away from the pressure chambers, the deflection plate comprising a number of second through-holes equal to the number of first through-holes, each second through-hole having a diameter that exceeds the diameter of the corresponding first through-hole, the deflection plate being movable relative to the nozzle plate in a direction parallel to the first direction, the device further comprising driving means for imparting to the deflection plate an oscillating motion in said direction parallel to the first direction, the oscillating motion being centred around a rest position in which the coπesponding first and second through-holes are substantially aligned, the device further comprising a control unit conceived to activate the recording head in such a manner that the emission of droplets of ink from each nozzle is correlated with the momentary position of the deflection plate relative to the nozzle plate. In the device according to the invention, the ink is ejected through a channel that is composed of the first and second through-holes. When the deflection plate is in its rest position, this channel is substantially circularly symmetric. When the deflection plate is in any other position, the channel is asymmetric. During the passage of the ink through the channel, the ink wets the surface of the channel. Due to the surface tension of the meniscus, which is asymmetrical compared to the nozzle, a force in a direction parallel to the first direction is exerted on the ink droplet being emitted. This causes the ink droplet to follow a trajectory that deviates from the axis of the nozzle. Consequently, the ink droplet is deflected in a direction parallel to the first direction and it lands on the record carrier in a position that is not exactly opposite the nozzle. The angle of deflection depends on the momentary position of the deflection plate and during a period of the oscillating motion it is possible to deposit ink droplets on a plurality of positions between the positions opposite the nozzles. The only element of the recording head that undergoes the oscillating motion is the deflection plate. This plate can be constructed to be very light in weight and, consequently, little energy is required to move it. A prefeπed embodiment of the device in accordance with the invention is characterized in that the oscillating motion has an amplitude at least equal to one half of the difference between the diameters of the second and first through-holes. It has been found that in this embodiment it is easily possible for droplets of ink from a single nozzle to be deposited over a width that substantially coπesponds to the centre to centre distance (pitch)

of the nozzles.

A further prefeπed embodiment is characterized in that position detection means are provided for locating the momentary position of the deflection plate relative to the nozzle plate. The momentary position of the deflection plate could be deduced from the time that has elapsed after the deflection plate has passed a predetermined position (e.g. its rest position or one of the extreme positions of the oscillating motion), but the use of the position detection means enables a more accurate determination of the momentary position of the deflection plate.

A very simple embodiment of the device in accordance with the invention is characterized in that the driving means comprise a permanent magnet fixed to the deflection plate and a coil of electrically conductive wire fixed to the nozzle plate. These and other aspects of the invention will be apparent from the embodiments described hereinafter.

Figure 1 is a schematic view of an embodiment of an ink jet recording device in accordance with the invention,

Figure 2 is a cross-section of an embodiment of an ink jet recording head for the device shown in Figure 1 , Figure 3 is a section according to the line Ill-Ill of a part of the recording head shown in Figure 2,

Figure 4 is a schematic representation of various stages during emission of a droplet of ink from a nozzle,

Figure 5 is a detailed view of a part of a recording head of the device shown in Figure 1 illustrating the operation of the deflection plate, and

Figure 6 shows some further details of the recording head of the device shown in Figure 1.

Figure 1 is a schematic representation of an ink jet recording device showing at least those parts that are necessary to provide sufficient background for understanding the invention. The device comprises a frame 1 on which a recording head 3 is mounted by means of mounting means 5. The recording head 3 comprises an array of nozzles (not visible in Figure 1) that extends in a first direction 7. A deflection plate 8 is

slidably mounted on the recording head 3 so that it is movable relative to the nozzles in a direction parallel to the first direction 7. A driving means 9 is attached to the frame 1 for transporting the deflection plate 8 in the first direction 7. The driving means 9 may, for example, be a linear motor or a piezoelectric actuator. The driving means 9 imparts to the deflection plate 8 an oscillating motion in the first direction 7, the frequency of this motion being, for example, between 50 Hz and 150 Hz.

Various types of recording heads can be used in a recording device according to the invention. Examples of ink jet recording heads comprising a linear array of nozzles are described e.g. in US-A-4 599 628, EP-A-0 516 188 and the copending patent application No. ... (PHN 15.079). A cross-section of a recording head described in the latter publication is shown by way of example in Figure 2. This recording head comprises a piezoelectric actuator element 11 in the form of an actuator plate that consists of a plurality of layers of a ceramic piezoelectric material alternated with electrode layers. As shown schematically in Figure 2, the first, third, fifth, etc. electrode layers 13 are connected to a first terminal 15 and the second, fourth, sixth, etc. electrode layers 17 are connected to a second terminal 19. The odd-numbered electrode layers 13 are interrupted in a first zone 21 so as to isolate them from the second terminal 19 and the even-numbered electrode layers 17 are interrupted in a second zone 23 so as to isolate them from the first terminal 15. Between the first and second zones 21, 23 an active part of the actuator plate 1 1 is situated. The terminals 15, 19 receive voltage pulses from a control unit as will be discussed later. When a voltage is applied between the terminals 15 and 19, the dimension of at least the active part of the actuator plate 11 in the vertical direction in Figure 2 is varied. In other words, the actuator plate 11 changes its thickness upon application of a voltage. The direction in which the dimension of an actuator plate is changed upon application of a voltage is called its active direction. The active part of the actuator plate 11 is provided with a recess 25 that forms a pressure chamber.

A nozzle plate 27 has a first face 29 that is connected to the first face 31 of the actuator plate 11 so as to form a first wall of the pressure chamber 25. The nozzle plate 27 comprises a plurality of first through-holes 33 that form the nozzles, one of which is visible in Figure 2.

A base plate 35 has a first face 37 that is connected to the second face 39 of the actuator plate 11 so as to form a second wall of the pressure chamber 25. An ink reservoir 41 communicates with the pressure chamber 25 via an ink supply channel 43. The ink reservoir 41 and the ink supply channel 43 are formed as recesses in the second face 39

of the actuator plate 11. As can be seen more clearly in Figure 3, the ink reservoir 41 is a relatively wide duct interconnecting the ink supply channels 43. In order to prevent a pressure wave in one of the pressure chambers 25 from causing a rise of pressure in one or more of the other pressure chambers, each ink supply channel 43 comprises a restricted portion 43a that serves as a choke. The first face 37 of the base plate 35 covers the ink reservoir 41 and the ink supply channel 43. The base plate 35 comprises one or more filling channels 45 formed as through-holes (shown in dotted lines in Figure 2) to connect the ink reservoir 41 to an ink supply system comprising an ink container (not shown in Figures 2 and 3). The construction of the recording head 3 from the actuator plate 11, the nozzle plate 27 and the base plate 35 is very simple. The parts are aligned by means of an alignment pin 47 that extends through alignment holes 49 provided in all three components of the recording head.

Figure 2 also shows the deflection plate 8 that is provided on the second face 30 of the nozzle plate 27, which is directed away from the pressure chambers 25. The deflection plate 8 comprises a number of second through-holes 48 equal to the number of first through-holes 33. Each second through-hole 48 has a diameter that exceeds the diameter of the coπesponding first through-hole 33. As has been explained above, the deflection plate 8 can perform an oscillating motion. This oscillating motion is centred around a rest position in which the corresponding first and second through-holes 33, 48 are substantially aligned. After the recording head 3 has been completed, the ink reservoir 41 , the ink supply channels 43 and the pressure chambers 25 are filled with a suitable ink. When a voltage of a predetermined polarity is applied between the terminals 15 and 19, the thickness of the actuator plate 11 increases so that the volume of the pressure chamber 25 grows. The surface tension then causes ink to flow from the ink reservoir 41 through the ink supply channel 43 to the pressure chamber 25. When the voltage between the terminals 15 and 19 is reduced to zero or when its polarity is suddenly reversed, the pressure chamber suddenly contracts so that a droplet of ink is expelled through the first through-hole 33 and subsequently travels through the second through-hole 48. The very small cross-section of the restricted portion 43a of the ink supply channel prevents the flow of ink from the pressure chamber 25 back to the ink reservoir 41 as a result of the contraction of the pressure chamber. This serves to reduce cross-talk between the nozzles 33 of a recording head 3 via the ink reservoir 41. Cross-talk via the actuator plate 1 1 is reduced by the provision of slits 51 between the pressure chambers 25, said slits extending through the active part of the actuator plate beyond the first zones 21. Due to these slits the actuator plate 11 is split into a

plurality of fingers 53, each finger comprising one of the pressure chambers 25. The electrode layers 13, 17 in adjacent fingers 53 are electrically isolated by the slit 51 between these fmgers and the fingers themselves are mechanically substantially isolated by the slits. As shown clearly in Figure 3, the nozzles 33 extend in a linear array 55. The length of the array 55 can be chosen freely. Usually it is prefeπed to choose this length equal to the width of a sheet of paper. For paper having the size A4 this amounts to an array length of about 210 mm. If the centre to centre distance of adjacent nozzles 33 (pitch) is chosen to be 1 mm, the recording head would then comprise 210 nozzles. Of course, Figure 3 shows only a small part of the recording head 3, which part comprises only four nozzles 33.

The ink jet recording device shown in Figure 1 further comprises transportation means including a pair of pinch rollers 57 driven by a motor 59. The pinch rollers 57 cooperate to transport a record carrier 61, for example a sheet of paper, in a second direction indicated by an arrow 63. As can be seen in Figure 1 , the second direction 63 is transverse to the first direction 7. The record canier 61 is transported in such a manner that a surface of the record carrier (the upper surface in Figure 1) faces the nozzles 33 of the recording head 3. If no record carrier 61 is present, the nozzles 33 face a waiting and servicing station 65 that serves to keep the recording head in a proper condition when the device is not in use. An ink container 67 is fixed to the frame 1 for supplying ink to the ink reservoir 41 via a pipe system that comprises a flexible tube 69. The device further comprises a control unit 71 that is conceived to activate the recording head 3 by applying voltage pulses as discussed above in such a manner that the emission of droplets of ink from each nozzle 33 is correlated with the momentary position of the deflection plate 8 relative to the nozzle plate 27 during the oscillating motion of the deflection plate. The control unit 71 may comprise a suitably programmed microprocessor (not shown).

In order to provide some background information that is useful in understanding the effect of the deflection plate 8, Figure 4 illustrates the successive stages of the ink droplet ejection. Figure 4 shows a nozzle of a nozzle plate without a deflection plate. The starting point is a dry nozzle, Figure 4A. When the associated pressure chamber 25 (Figure 2) starts to contract, the still concave meniscus of the ink is made convex, the nozzle orifice being filled with liquid until a given value of the curvature of the meniscus is reached, Figure 4B. The diameter of the parabolic curvature is determined by the diameter of the nozzle. From a given curvature, which depends on the structure of the internal limiting nozzle wall and also on the boundary surface tension of the nozzle plate material a lateral

external wetting of the exterior outer surface (sideways pointing arrows in Figure 4C) occurs in addition to the desired ejection direction (aπow pointing upward from the nozzle). As a result, a ring of nozzle plate material suπounding the nozzle is wetted by the ink. The size of the wetting ring depends on the boundary surface tension, the flow rate of the ink and the shape of the pulse generated by the control unit 71. If the wetting power of the near nozzle brim is approximately equal in all radial directions, the ink drops will be ejected in the axial direction as shown in Figure 4D. This is because the forces acting on the ink droplet due to the adhesion of the ink to the nozzle plate surface are substantially equal in all radial directions. If for some reason the wetting power of the near nozzle brim is asymmetrically distributed, a part of the brim area may still be wetted after the meniscus has become concave again, other parts being dry. This is shown in Figure 4E where a part to the right of the nozzle is wetted, the other parts being dry. The pulse of the control unit then results unavoidably in a deflection of the ejected ink droplet (Figure 4F) as the lateral forces then acting on this droplet are different in different directions. These forces are the greater according as ink stays behind on a larger section of the nozzle brim. This phenomenon has for a long time been considered as undesirable because it could introduce an uncertainty in the direction in which ink droplets are ejected. In the ink jet recording device in accordance with the invention, the described phenomenon is used to bring about a controlled deflection of ink droplets in order to enable the deposition at positions on the record carrier 61 that are not directly opposite a nozzle 33.

Figure 5 shows a cross-section of a part of the nozzle plate 27 and the deflection plate 8 with a first through-hole 33 having a relatively small diameter d and a second trough-hole 48 having a relatively large diameter D. Figure 5A shows the situation in which the axis 75 of the first through-hole 33 is aligned with the axis 73 of the second through-hole 48. The surface area of the nozzle plate 27 that is available for wetting by the ink is then symmetrical. Consequently, no net lateral forces are exerted on an ink droplet ejected from the nozzle and the droplet is ejected in a direction 77 that substantially coincides with the axis 73 of the first through-hole 33. Figure 5B shows the situation in which the axis 75 of the second through-hole 48 is not aligned with the axis 73 of the first through-hole 33. The nozzle plate surface that is available for wetting by the ink is now asymmetrical. It can easily be seen in Figure 5B that the area available to the left of the nozzle is considerably larger than the area available to the right of the nozzle. Consequently, the adhesion of the ink to the surface on the left side will exceed the adhesion on the right side and a net lateral force will be exerted on the droplet of ink that is ejected. As a result, the droplet will be

ejected in a direction 79 that encloses an angle a with the axis 73 of the first through-hole 33. The value of α depends on the difference between the two diameters D and d and on the distance over which the deflection plate 8 has been moved in the direction 7 relative to its rest position shown in Figure 5A in which the axes 73 and 75 coincide. Preferably, the amplitude of the oscillating motion of the deflection plate 8 is at least equal to ' z(D-d). In that case at least the position shown in Figure 5B is reached during the oscillating motion.

From the foregoing it is clear that the momentary position of the deflection plate 8 at the moment when a droplet of ink is ejected determines the deflection angle α and, consequently, the position on the record carrier 61 where the droplet will land. In order to determine the moment when a droplet of ink must be ejected from one of the nozzles, the control unit 71 can deduce the position of the deflection plate 8 from the phase of the oscillating motion of the recording head as imparted by the driving means 9. A more accurate measurement of the position of the deflection plate 8 can be performed if position detection means are provided for locating its momentary position relative to the nozzle plate 3.

Figure 6 schematically shows a view of a part of an embodiment of the recording head 3 in which such position detection means are provided. The position detection means comprise a first portion 81 attached to the frame 1 and a second portion 83 attached to the recording deflection plate 8. The first portion 81 is electrically connected to the control unit 71. There are many different types of position detection devices that are suitable to be used in the ink jet recording device of Figure 1, such as optical, capacitive or magnetic devices. A few examples can be found e.g. in US-A-5 136 192 and US-A-5 279 044. The position detection device needs to sense the position of the deflection plate 8 only within a relatively small distance range, approximately equal to the difference in diameters D-d. The first driving means 9 (see Figure 1) may be implemented very simply by providing a system similar to a well-known loudspeaker system as shown schematically in Figure 5. This system may comprise a permanent magnet 85 attached to the deflection plate 8 and a coil 87 of electrically conductive wire which is fixed to the nozzle plate 27. The coil 87 may, for example be attached to the recording head 3 or the frame 1. The coil 87 is electrically connected to the control unit 71. When it is energized, a magnetic field is generated that cooperates with the field of the permanent magnet 85 to exert a force in the first direction 7 on the deflection plate 8. Such a driving system is very compact. Another example of a feasible driving means is a driving means comprising a piezoelectric actuator, e.g. a stack of piezoelectric elements or a CMA.