EP0116971A1 - Piezoelectrically activated recording head with a duct matrix - Google Patents

Abstract

In order to obtain a mechanically stable piezoelectrically operated write head for ink mosaic writing devices, which is considerably simplified in production, it is provided that the ink channels (13-16) are formed by a channel matrix (10) which consists of a series of piezolelectric strips (20- 27), which are arranged in parallel at a distance from one another, electrically contacted on both sides and covered by a plate (17, 18) on both sides. The channels (13-16) thus formed directly form the writing nozzles. A perforated matrix is therefore not necessary.

Description

The invention relates to a piezoelectrically operated writing head for ink mosaic writing devices with writing nozzles in the form of ink channels receiving writing liquid, from which the writing liquid is ejected dropwise by piezoelectric deformation of a drive element.

So far, two solutions are known for ink mosaic writing devices. In one, the writing head consists of a perforated matrix with a row of nozzles, in front of the inlet opening of which rod-shaped piezoelectric elements are arranged in such a way that the 'piezo elements bend when a voltage is applied and thereby drop-wise write liquid ejects from the corresponding nozzle (DE-PS 25 27 647). In order to obtain a sufficiently high recording quality at all, the individual rod-shaped piezoelectric elements are combined to form a kind of comb and thus connected to one another via a common web. Great demands must be made of the manufacture of the comb and of the perforated die, particularly with regard to the tolerances, in order to ensure that the print head functions properly. For the same reason, the die matrix and comb must be carefully adjusted.

Furthermore, write heads are known which consist of a single casting process made of dielectric plastic manufactured workpiece consist, which contains several channels receiving the writing fluid (DE-PS 25 43 451). These channels are also closed off by a perforated matrix towards the recording medium. Ceramic tubes, which enclose the actual ink channels in a cylindrical shape, serve as the piezoelectric drive elements. In order that the outlet openings can be arranged so close to one another that the desired high recording quality is obtained, the ink channels are directed away from these outlet openings in a radial manner and the piezoceramic tubes are arranged at a distance from the outlet openings. This write head is relatively complex to manufacture and also has a large mass, so that correspondingly large acceleration forces are necessary for the rapid deflection of such a write head.

The object of the present invention is to design the write head mentioned at the outset in a mechanically stable manner and, moreover, to significantly simplify production. In particular for movable writing heads, efforts are also being made to keep the mass to be accelerated as small as possible.

This object is achieved according to the invention in that the ink channels are formed by a channel matrix, which consist of a series of strips of piezoelectric material which are arranged in parallel at a distance from one another, electrically contacted on both sides and covered on both sides by a plate. This should also include the possibility that the strips are formed, for example, by sawing grooves into a solid plate made of piezoelectric material, which is then only on the top be covered with a plate to form the duct matrix. The rest of the solid plate remains the lower plate. In this way, rectangular channels for the ink are created between the strips of piezoelectric material. If, for example, a voltage is now applied to two strips of piezoelectric material, these become narrower and higher, so that the enclosed cross-sectional area of the channel is enlarged. This draws additional writing fluid into this channel. If the voltage at this contact is removed, the strips return to their original shape, as a result of which the channel volume is suddenly reduced. This leads to the discharge of writing fluid.

The dimensions of the strips and the spaces can advantageously be chosen so that the channels formed between the strips form the writing nozzles directly. In contrast to the known recording heads can in this case therefore a separate piercing die dispensed werden.Damit eliminates the otherwise complicated adjustment between perforated die and piezo _- comb or between perforated die and the workpiece with the ink channels.

Due to its "sandwich construction", the channel matrix according to the invention is mechanically robust and nevertheless small and light, so that an extremely high deflection speed of the print head is possible with reasonable forces. Although it might initially appear that the liquid contained in the channels is ejected uniformly in both directions from these, tests have shown that a sufficiently large ejection takes place in the direction of a recording medium arranged in front of the write head. Since the Ink channels on the back are connected directly to a storage chamber for writing liquid, the sudden change in cross-section causes a reflection of the liquid wave running in this direction, so that the majority of the displaced liquid is ejected in the direction of the recording medium.

In order to prevent coupling of the mechanical deformations of an activated channel to an adjacent one, it is provided in a further development of the invention that only every second channel is provided for writing and the channels in between are filled with an elastic material or with air. Virtually no mechanical coupling is obtained through the air. Penetration of writing fluid into these intermediate channels can be prevented by the fact that they do not go all the way to the end facing the storage chamber.

If the strips of piezoelectric material are rigidly connected to a carrier plate, it is particularly advantageous if this carrier plate is kept so thin that the longitudinal expansion of the piezoelectric strips does not cause the carrier plate and thus the entire channel matrix to bend. Particularly favorable mechanical properties are obtained if the strips are additionally reinforced on the upper side by a layer - in particular made of metal - which offers approximately the same resistance to the longitudinal expansion as the lower carrier plate.

If the carrier plate is made directly of metal, it can be used as a common electrode for all strips of piezoelectric material.

The channel matrix is particularly simple to manufacture if a bilaminar material made of carrier material and piezoelectric material is used and the strip structure is produced, for example, by sawing in the piezoelectric material. After the corresponding contact of the strips on the top, only a cover plate is applied as a finish. Every second channel can in turn be filled with an elastic material or air.

Another particularly advantageous production of the print head is based on a laminate of piezoelectric material provided on both sides with a metal layer, from which longitudinal channels are worked out alternately from one side and from the other. The depth of the channels extends approximately over a metal layer and the piezoelectric material. After appropriate contacting, the channel plate produced in this way is closed off at least on one side with a continuous plate. This construction also has the advantage that the channel plate can be intentionally broken in the longitudinal direction by different layers, so that the tensile stress that occurs when a voltage is applied can no longer spread. Nevertheless, the mechanical stability of the entire arrangement is completely preserved.

Further advantages result from the subclaims.

• Fig.1 eine schematische Ansicht einer Tintenmosaikeinrichtung,
• Fig.2 in einer perspektivischen Darstellung den Schreibkopf,
• Fig.3 einen Querschnitt durch eine erste mögliliche Kanalmatrize,
• Fig.4 einen Ausschnitt der Kanalmatrize nach Fig.3 in zwei unterschiedlichen Zuständen,
• Fig.5 wiederum in einem Querschnitt eine weitere Ausführungsform der Kanalmatrize und
• Fig.6 ein Querschnitt durch eine doppelte Kanalmatrize,
• Fig.7 einen Längsschnitt durch einen Kanal und
• Fig.8 einen Schreibkopf mit vier Kanalmatrizen.

Exemplary embodiments of the invention are explained and described in more detail below with reference to 8 figures. The same parts are provided with the same reference numerals. It shows

• 1 shows a schematic view of an ink mosaic device,
• 2 shows a perspective view of the write head,
• 3 shows a cross section through a first possible channel matrix,
• 4 shows a section of the channel matrix according to FIG. 3 in two different states,
• 5 again in a cross section a further embodiment of the channel matrix and
• 6 shows a cross section through a double channel matrix,
• 7 shows a longitudinal section through a channel and
• 8 shows a write head with four channel matrices.

The outer basic structure of an ink mosaic writing device can be seen from FIG. The recording medium 3, for example normal registration paper, is pulled past the front face 6 of the housing 7 in the direction of arrow 4 via transport rollers 1 and 2. In the housing 7, the connecting line 8 is guided, which is provided at its free end with a plug 9 for connection to a corresponding control device which receives the control signals for recording the desired courses, characters or Supplies images. The housing 7 contains the actual write head, of which a possible embodiment is shown in perspective in FIG.

The writing head consists of a channel matrix 10 and a reservoir 11 for writing liquid 12. In the channel matrix 10, ink channels 13 to 16 are indicated by dashed lines, which are formed by two plates 17 and 18 and the strips 20 to 27 between them made of piezoelectric material. For the sake of clarity, the electrical contacting of the piezoelectric strips has been omitted here. For example, the plate 17 can be made of metal and form a common electrode for all common strips 20 to 27 made of piezoelectric material. The other side of the strips must then be contacted in pairs. If the plate 18 is also made of conductive material, insulation must be provided between it and the contacts.

3 shows a cross section through the channel matrix corresponding to FIG. 2. A nickel foil is used as the carrier plate 17, for example. The piezoelectric material 20 to 27 is applied in strips. These strips are provided in pairs with electrical contacts 30 to 33. The end is formed by the end plate 18, which in this case should consist of a non-conductive material. Furthermore, this representation indicates that every second channel 34 to 36 formed is filled with an elastic material, for example silicone rubber. Assuming, for example, that the width of the piezoelectric strips is 50 µm and the distance between the adjacent strips is the same, one obtains a distance of the writing nozzles (cannula 13 to 16) of 200 µm. This means that five writing nozzles per mm are provided, with which very good recording quality can already be achieved. The thickness of the strips can be of the same order of magnitude. In order to obtain a sufficiently large ink output without excessive voltage amplitudes, the length of the channel matrix can be approximately 10 mm. The thickness of the carrier plate 17 and the cover plate 18 is approximately 20 μm.

4 shows an enlarged section of the representation according to FIG. 3 in two different states. The state in which a voltage is applied to the contacts to the two strips 20 and 21 made of piezoelectric material is represented by solid lines, so that these strips become narrower and higher. The state in which the strips of piezoelectric material have returned to their original shape is shown in dashed lines. As can be seen from this illustration, when a voltage is applied to the piezoelectric strips, the cross-sectional area of the channel is enlarged and in this way additional writing liquid is sucked into the channel. At the. Short-circuiting the voltage returns the strips to their original shape and thereby reduces the enclosed channel volume in such a way that the writing fluid to be displaced is ejected as drops at the front of the channel matrix.

5 again shows in cross section a further embodiment of a channel die 40. This essentially consists of a laminate made of piezoelectric material 41, which is provided on both sides with a metal layer 42 or 43. Alternating from the top and Channels 44 to 47 are introduced into the bottom of this laminate, for example by sawing. In the present exemplary embodiment, the channels each extend over a metal layer and the layer made of piezoelectric material. It should be pointed out at this point that these channels do not necessarily have to pass through the entire layer of piezoelectric material. Every second channel can in turn contain air and be sealed off from the storage chamber.

Despite sawing the channels, a mechanically connected laminate is still obtained. In this arrangement, since the strips of piezoelectric material are equally provided with a metal layer on both sides, that is to say are reinforced, there are symmetrical relationships when a voltage is applied to two strips, so that the deformation of the piezoelectric element cannot cause the channel matrix 40 to bend . As shown, the end is again made up of a plate 48 and 49 or else only a plate 49. It should be noted at this point that this reinforcement can also be used with the channel matrix 10 according to the exemplary embodiment discussed above. An advantage of this arrangement is that the electrical contacts can remain dry, i.e. do not come into contact with the writing fluid.

By preventing the strips of piezoelectric material from expanding in the longitudinal direction by means of reinforcement on the top of these strips, the strips become practically narrower than usual when a voltage is applied. In this way, an approximately 30 percent stronger effect can be achieved with the same achieve the applied voltage. In other words, the same effect can be achieved with a reduced voltage and thus overall with a reduced power to be applied.

Another advantage of the channel matrix according to the exemplary embodiment according to FIG. 5 is that the tensile stress in the longitudinal direction caused by the deformation of the strips of piezoelectric material can be suppressed by intentionally breaking the piezoelectric material several times in the longitudinal direction. The fine transverse cracks that occur do not change the mechanical stability of the entire channel matrix, but the tensile stresses can no longer spread in the longitudinal direction.

In a further exemplary embodiment in FIG. 6 it is again shown in a schematic cross-sectional representation that two channel matrices 50 and 60 can be arranged tightly packed in order to increase the resolution, for example. For the sake of simplicity of illustration, only two channel matrices 50 and 60 have been assumed. This should not preclude the fact that a plurality of such single-row channel matrices can be combined to form a block if necessary. As can also be seen in FIG. 6, the middle plate 51 is simultaneously used as a cover plate for the upper and lower row of channels. If one thinks of this plate 51 again from a conductive material, this results in a common electrode for both the upper and the lower strips made of piezoelectric material. In the example according to FIG. 6, it is further assumed that the ink channels 52, 53 and 61 in the two rows hen are staggered. This has the advantage that an increased resolution can be obtained in a simple manner. It only requires that the two rows be driven at different times in accordance with the relative speed between the writing head and the writing paper. In addition, there is also the possibility of supplying different rows of ink channels with differently colored writing liquids, so that even multicolored recording is possible.

7 shows an ink channel 70 in longitudinal section. In addition to the two cover plates 71 and 72, a part of the piezoelectric material 73 can be seen in the right end region, which reduces the height of the outlet opening. As a result, the ink channel 70 has a larger volume and, as a result, a larger ink output without the size of the drops changing. Since only the height of the ink channels has been changed, the mutual distance can still correspond to the required resolution.

FIG. 8 shows a writing head 80. Similar to that shown in FIG. 2, only that four channel matrices 81-84 are provided densely packed and also four separate storage chambers 85-88 for writing liquid of different colors. If you choose colors, red, blue, yellow and black, you can use it to create flawless color recordings - controlled e.g. from a still image monitor.

Claims (13)

1. Piezoelectrically operated writing head for ink mosaic writing devices with writing nozzles in the form of ink channels receiving writing liquid, from which the writing liquid is ejected dropwise by piezoelectric deformation of a drive element, characterized in that the ink channels (13-16; 44.45; 52.53.61) are formed by a channel matrix (10; 40; 50, 60), which consists of a series of strips (20-27) made of piezoelectric material, which are arranged in parallel at a distance from each other, electrically contacted on both sides, and on both sides by a plate (17th , 18; 48.49) are covered. 2. Print head according to claim. 1, characterized in that the thickness of the strips (20-27) and their mutual spacing are selected such that the channels (13-16; 44.45; 52.53.61) formed between the strips (20-27) form the writing nozzles directly. 3. Writing head according to claim 1 or 2, characterized in that only every second channel (13-16; 44,45) is used as a writing nozzle. 4. Writing head according to one of claims 1 to 3, characterized in that the plate (47) on one side of the piezoelectric strips (20-27) consists of metal and serves as a common electrode for all strips (20-27). 5. Print head according to claim 4, characterized in that at least this plate (17) is so thin that the deformation of the piezoelectric strips (20-27) does not cause any bending. 5. Print head according to one of claims 1 to 5, characterized in that the piezoelectric strips are additionally provided on the top with a metal layer. 7. Print head according to one of claims _1-6 , characterized in that the height of _{the ink channels (70)} decreases in _{the loading area of the outlet opening} . 8. The method for producing a write head according to one of claims 1 to 7, characterized in that a carrier plate (17) is provided on a surface at least in strips with a conductive layer, that piezoelectric material is applied in strips thereof, that the piezoelectric strips ( 20-27) are electrically contacted in pairs and that a cover plate (18) is applied in isolation. 9. A method for producing a write head according to claim 8, characterized in that a bilaminar plate made of carrier material and piezoelectric material is provided, in which the stripe structure is produced by regionally working out piezoelectric material. 10. The method according to claim 8 or 9, characterized in that metal is provided as the carrier material. 11. A method of manufacturing a printhead one of claims 1 to 7, characterized in that a laminate of piezoelectric material (41) provided on both sides with a metal layer (42, 43) is formed, that alternately extending from one side and from the other over approximately two layers Strips are worked out from the laminate, that the remaining strips of piezoelectric material are electrically contacted in pairs, and that the plate thus obtained is closed at least on one side with a continuous plate (48, 49). 12. A method of manufacturing for manufacturing a write head according to claim 11, characterized in that the piezoelectric material is broken at least once in the longitudinal direction of the strips to be formed.

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