Liquid-jet printing device, comprising at least one chamber for accommodating liquid, a supply system comprising a supply duct for supplying liquid to the chamber, an outlet opening for passing a drop of liquid under pressure out of the chamber to receiving material and a movable element which forms a wall of the chamber and can be moved to a first position wherein the movable element closes off the supply system from the chamber and passes a drop of liquid in the chamber through the outlet opening and can be moved from the first position to a second position wherein the supply system and the chamber for supplying liquid to the chamber are in open communication with each other.

Such a printing device is known from US patent US 4,383,264. With this printing device, the chamber consists of a cavity formed in the plate which contains the outlet opening. The chamber adjacent to the outlet opening is thus very small and it can only accommodate a small amount of liquid. On the side of the chamber facing away from the outlet opening, when the movable element is in the second position, the chamber is in open communication with the supply system for liquid. When moving the movable element into its first position, it presses in the direction of the chamber and in this case presses a drop of liquid from the chamber through the outlet opening while closing off the supply system of the chamber. As, with this known printing device, the liquid supply is closed off very close to the outlet opening, hardly any pressure builds up in the chamber, so that hardly any energy can be stored in the compressed liquid in the chamber near the outlet opening, as is customary with modern piezo drop on-demand inkjet printers. Furthermore, there is a significant risk with the abovementioned known printing device that, when the movable element is moved from its first position to its second position, air will be sucked into the chamber via the outlet opening due to the very short distance between the chamber and the outlet opening. As a result thereof, the flowing out of liquid will stop when the printing device is activated further, because only the air bubble which was sucked in will be compressed.

Another printing device is known from US patent US 3,946,398 and comprises at least one relatively large pressure chamber for accommodating liquid, a supply system comprising a supply duct for supplying liquid to the pressure chamber, an outlet opening for passing a drop of liquid under pressure from the pressure chamber to receiving material and a movable element which forms a closed wall of the pressure chamber and can be moved to a first position in which it pressurizes the liquid present in the pressure chamber for passing a drop of liquid through the outlet opening and can be moved from the first position to a second position in which liquid flows from the supply duct to the pressure chamber. As the movable element forms a closed wall of the pressure chamber which is securely fastened to the other walls of the pressure chamber, with the supply duct directly opening into the - - pressure chamber, it is a drawback that, due to the rigid pressure chamber unit, a lot of force is required to deform the movable element in such a way that a drop of liquid emerges from it as a result of a reduction in volume of the pressure chamber. The reason for this is that the entire, rigidly constructed pressure chamber is subject to deformation. In order in this case to prevent liquid from the pressure chamber from being passed back into the supply duct, the connection between the supply duct and the pressure chamber is of a narrow design. When the volume of the pressure chamber is reduced, this results in a positive pressure wave in the pressure chamber which also propagates in the direction of the supply duct and then returns as a negative pressure wave at the outlet opening from there. This produces a relatively strong negative pressure near the outlet opening, which causes the risk of the formation of undesirable vapour bubbles in the liquid which, in particular with liquids having a low vapour pressure, hinders the further flowing out of drops of liquid.

It is an object of the invention to provide a liquid-jet printing device which does not have these drawbacks.

This object is achieved with a liquid-jet printing device according to the preamble by the fact that the chamber is an elongate pressure chamber for accommodating an amount of liquid which can be pressurized in order to pass a drop of liquid through the outlet opening and that the movable element, in an unbent position, closes off an elongate opening in a wall of the pressure chamber in order to produce said first position.

Compared to the printing device known from US 3,946,398, this results, when a pressure is exerted in the pressure chamber, in the pressure chamber being closed off from the supply system and, when the pressure in the pressure chamber is released, an open and relatively large connection being produced between the supply system and the pressure chamber and thus no large negative pressure being created in the liquid as a result of the formation of a negative pressure wave in the liquid, thus preventing the formation of vapour bubbles in the liquid. Furthermore, compared to the printing device known from US 4,383,264, closing off the elongate opening in the pressure chamber by means of the likewise elongate movable element achieves the effect that the movement of the movable element from its second position to its first position along its entire length contributes to the build-up of pressure in the liquid in the pressure chamber which is required to expel the drop of liquid. According to Figures 1 , 3 and 5 of the printing device known from US 4,383,264, an elongate movable element only cooperates with a liquid chamber in the centre of the movable element, as a result of which the movable element also displaces liquid in the supply duct at its ends during its movement into its first position, which requires unnecessary additional energy and a damping of the displacement of the movable element, which does not benefit the efficiency of the printing device.

In a first embodiment of a printing device according to the invention, the - - movable element is an elongate flexible strip which, in its unbent position, closes off the elongate opening in the wall of the pressure chamber in order to produce the

aforementioned first position and, in a bent position, is at least partly clear of the elongate opening in the wall of the pressure chamber in order to produce the aforementioned second position. As a result thereof, a relatively large open connection is produced between the supply system and the pressure chamber in the bent position of the flexible strip in order to further restrict the formation of undesirable vapour bubbles in the liquid, thus rendering the printing device very suitable for depositing liquids with a low vapour pressure.

In an attractive form of the first embodiment of a printing device according to the invention which is formed by a first block in which a number of parallel grooves are provided, each of which forms a pressure chamber, an outlet opening being present in a wall of each groove in the form of a nozzle, each groove can be covered by a strip which is loosely arranged thereon, the ends of which strip which protrude beyond the groove are provided with a recess which fits around a projection which is provided on the first block in order to keep the strip in position, a cavity is formed in the first block outside the grooves and forms a part of the supply system in which the strips are situated, and a supply line which is connected to the cavity opens in the upper surface of the first block and a film/foil connected to all the strips forms a sealed connection with the first block around the grooves and the cavity.

In a second embodiment of a printing device according to the invention, the printing device is formed by a block which is provided with at least one elongate groove which forms the pressure chamber, a nozzle-shaped outlet opening being present in a wall of the groove, which groove can be covered by a film/foil which is at least partly loosely arranged thereon and a hollowing is present in the block outside the groove and forms a part of the supply system, which hollowing is closed off from the pressure chamber by the film/foil in its first unbent position and is in open communication with the pressure chamber in the second bent position of the film/foil. This ensures that the energy which is required to bend the element (the film/foil) which covers the pressure chamber is transmitted directly and not via a flexible film/foil onto a metal strip which covers the pressure chamber, as is the case in the first embodiment. As a result thereof, relatively less energy is required to squirt a drop of liquid onto receiving material. This is caused by the fact that the bending stiffness of the flexible film/foil is much lower than the bending stiffness of a piezoelectric element and of a metal strip. In an attractive form of the second embodiment, the hollowing is produced in the form of ducts which are arranged on either side of the elongate pressure chamber and separated therefrom by its narrow ridges.

Other characteristic features and advantages of the invention will become clear from the following description of an embodiment according to the invention. - -

The invention will be explained below with reference to the attached drawings, in which:

Fig. 1 shows a first embodiment of a liquid-jet printing device according to the invention in perspective;

Fig. 2 shows a view in longitudinal section along the line A-A from Fig. 1 ;

Fig. 2a shows the longitudinal section from Fig. 2 in a position in which liquid is supplied to the pressure chamber;

Fig. 3 shows a cross section along line B-B from Fig. 1 ;

Fig. 3a shows the cross section from Fig. 3 in a position in which liquid is supplied to the pressure chamber;

Fig. 4 shows a cut-away top view of the device illustrated in Fig. 1 ;

Fig. 5 shows a second embodiment of a liquid-jet printing device in longitudinal section; and

Fig. 6 shows a longitudinal section along line C-C from Fig. 5.

The first embodiment of a liquid-jet printing device which is illustrated in Figure 1 in perspective is formed by a rectangular bottom block 1 and a rectangular top block 2 of identical dimensions. Blocks 1 and 2 can be fastened to one another by means which are not shown. On its upper side, the bottom block 1 is provided with a shallow cavity 3 of rectangular dimensions. In the bottom of the cavity 3, a number of parallel ducts 4 are formed which extend in the longitudinal direction of the cavity 3 in order to form pressure chambers for the liquid to be printed. In the centre of each duct 4, an outlet opening 5 is formed, as can clearly be seen in Figure 2. In the bottom block 1 , small tubes 6 are provided, each of which connects an outlet opening 5 to the underside of the bottom block 1 in order to transport a drop of liquid from one of the pressure chambers formed by the ducts 4 to a receiving material guided along the underside of bottom block 1 , as will be explained in detail below.

On the bottom block 1 , a thin flexible film/foil 8 is arranged, for example a 20 μιτι thick Kapton film, or a film/foil of stainless steel or glass, which covers virtually the entire bottom block. On the underside of this film/foil 8, a number of elongate thin metal strips 9 are provided, each of which covers one of the ducts 4. To this end, the size of each strip 9 is greater than the size of a duct 4 and, in the rest position, the edge zones of each strip 9 are in contact with the bottom of the cavity 3 around a duct 4 in order thus to make each duct 4 into a pressure chamber. The thickness of the strips 9 corresponds to the depth of the cavity 3, so that, in the rest position, the film/foil 8 extends in a flat position from the edges of the bottom block 1. Near the edges of the bottom block 1 , a circular groove 10 is formed in the upper surface. In this groove 10, a rubber sealing ring 11 is provided which, in the fitted position of the bottom block 1 and the top block 2, forms a liquid-tight sealing for the space - - formed, on one side, by the cavity 3 and ducts 4 and, on the other side, by the film/foil 8. Furthermore, a supply line 12 is provided in the bottom block 1 and opens, near the upper surface of the bottom block 1 , in an area which is situated inside the liquid-tight space formed by sealing ring 1 1.

Opposite each strip 9, a piezoelectric element 14, for example of the bimorphous type, is attached to the film/foil 8. When the piezoelectric element 14 is excited, for example by applying a suitable electrical voltage by some means which are not shown, the element bends in its longitudinal direction and consequently the strip 9 which is connected thereto also bends, so that it becomes unattached from the edges of the associated duct 4 along a significant part of its length in order to allow the passage of liquid from the cavity 3 to the respective duct 4. It is important that the bending stiffness of the strip 9 is smaller than the bending stiffness of the piezoelectric element 14, for example 0.2.10 ¹¹ N/m ² for the strip and 0.7.10 ¹¹ N/m ² for the piezoelectric element.

At each of its ends, each strip 9 has a recess 15 which fits around a projection 16 which is formed in the cavity 3.

Flexible bands 17, each provided in a cavity on the underside of the top block 2, press elastically on the ends of strips 9 in order to keep these ends in position during deformation of the strip.

By detaching the top block 2 from the bottom block 1 , the film/foil 8 which is loosely arranged on the bottom block can readily be removed in order to replace it, together with the strip 9 which is attached thereto and the piezoelectric element 14, for example in case of damage.

In a suitable embodiment, the length of each duct is 5 to 10 mm and 5 parallel ducts per mm can be provided.

The operation of the first embodiment of the liquid-jet printing device illustrated in Figures 1 to 4 will now be explained with reference to these figures.

The supply duct which is formed by the cavity 3 is filled with liquid via the supply line 12. By applying a suitable electrical voltage to each piezoelectric element 14, the latter is made to bend, which will also cause the strip 9 which is connected thereto to bend, as can clearly be seen, in particular, in Fig. 2a. Liquid now flows from the cavity 3 into each duct 4 which forms a pressure chamber via the gaps which have formed between the edges of the duct 4 and the bent strip 9, as can clearly be seen, in particular, in Fig. 3a. The ends of the strip 9 are essentially kept in position by the projections 16 and by the flexible bands.

By then selectively applying another suitable signal to a piezoelectric element 14, the strip 9 is moved to its unbent position, illustrated in Fig. 2 and Fig. 3, and pushed entirely against the respective duct 4 in order to briefly place the liquid under pressure in the pressure chamber which has thus been formed, as a result of which a drop of liquid is emitted from - - the pressure chamber via the outlet opening 5 which acts as a nozzle and applied to the receiving material which is moving past. During the subsequent excitation and deformation of the piezoelectric element 14 to its bent shape, as illustrated in Fig. 2a and in Fig. 3a, the strip 9 which is connected thereto will become detached from the duct 4 forming the pressure chamber along a significant part of its length and liquid can flow to the pressure chamber from the cavity 3 from all sides.

Deforming a strip 9 which is loosely arranged on a duct 4 which forms the pressure chamber does not result in a deformation of the duct itself. However, by deforming the film/foil 8 which connects a number of strips 3, the strips situated adjacent to the excited strip 3 will bend slightly concomitantly, but this does not have any effect on the emission of liquid from other outlet openings. However, this so-called cross-talk does present a real risk with known liquid-jet printing devices in which the piezoelectric elements which ensure deformation of the pressure chambers each form a fixed wall of this pressure chamber.

The integration density (the number of ducts and or nozzles per mm) of a liquid-jet printing device according to the invention may be high due to the fact that, even if the width of the duct is small, no excessive force is required to cause a strip, which causes the change in volume of the pressure chamber, to bend in its longitudinal direction. This is in contrast with known liquid-jet printing devices, for example from US patent 7,481 ,519 B2, which use closed pressure chambers and in which a wall of the pressure chamber which is fixedly connected to the other walls can only bend in its width direction. In case of a significant integration density, a very large amount of energy is required to achieve the desired bending and also results in a significant amount of cross-talk. Cross-talk is the phenomenon where bending of a pressure chamber also causes an adjacent pressure chamber to bend slightly, which may affect the outflow behaviour thereof.

A variant of the first embodiment of a liquid-jet printing device according to the invention (not shown) consists of an assembly of a bottom block and a top block, wherein a single duct is provided in the bottom block and a film/foil with a single piezoelectric element and a single deformable strip which can cover the duct is clamped between the bottom block and the top block. The bottom block and the top block are each for example 2 mm high and are both 20 mm long and 4.5 mm wide. This so-called 'single device' is outstandingly suitable for DNA research, in which 20 or more drops to be investigated are deposited on a receiving substrate. Many of these liquids contain 5% or more alcohol.

Alcohol has a very low vapour pressure and is consequently highly susceptible to the formation of air bubbles during the periodically occurring negative pressure.

The second embodiment of a liquid-jet printing device according to the invention, which is illustrated in Figures 5 and 6, is also of the above-described 'single device' type, but nevertheless deviates significantly therefrom, inter alia by a different configuration of the - - piezoelectric element and said film/foil.

The 'single device' liquid-jet printing device illustrated in Figures 5 and 6 comprises a block 15 in which an elongate hollowing 16 having rectangular dimensions is formed at the upper side. This hollowing 16 serves as a pressure chamber, as will be explained below.

A liquid supply line 17 is provided in the block 15 and also opens in the upper surface of the block 15. As can clearly be seen in Fig. 6, ducts 18 and 19 are formed on either side of the hollowing 16 forming the pressure chamber, the depth of which essentially corresponds to the depth of the hollowing 16. At one end, the ducts 18 and 19 are connected to each other by a connecting duct 20 which also opens in the upper surface of the block 15 and which is in open communication with the liquid supply line 17. When liquid which is to be printed is supplied through line 17 to the ducts 18 and 19 via duct 20, these ducts are constantly filled with liquid, as is illustrated in Fig. 6. Between, respectively, the ducts 18 and 19 and the hollowing 16 which forms the pressure chamber, narrow ridges 21 are formed which have a width of, for example, 0.25 mm.

The upper surface of the block 15 is covered by a thin flexible film/foil 22, for example a 20 μπι thick Kapton film or a glass film/foil. This film/foil 22, which is electrically insulating, covers the entire block 15 and, in the rest position, is in contact with the ridges 21 and thus closes off the hollowing 16 so as to form the pressure chamber and also closes off the ducts 18, 19 and 20 on the upper side. At its edges, the film/foil 22 is bonded to the block 15, but is arranged loosely on the ridges 21. Two platelets 25 and 26 are attached to each other on the film/foil 22, for example by means of a suitable layer of adhesive between platelet 25 and the film/foil 22 and between the platelets 25 and 26. Both platelets may be piezoelectric elements which, upon excitation of one of these elements, are caused to bend in their longitudinal direction and, upon excitation of the other element, straighten out again. It is also possible for only platelet 26 to be piezoelectric and for this to cause both platelets to bend upon excitation and to straighten both platelets out again when the excitation ceases. At their ends, the platelets 25 and 26 rest hard on the block 15. An outlet opening 28 is formed at the end of the hollowing 16 opposite the liquid supply line 17. This outlet opening may also be formed in the bottom of the hollowing 16.

The operation of the 'single device' illustrated in Figures 5 and 6 is as follows. Upon excitation of the piezoelectric element 26, for example by means (not shown) applying a suitable electrical voltage, the element bends in its longitudinal direction and at the same time concomitantly lifts up the part of the film/foil 22 which is connected thereto. As a result thereof, the film/foil 22 becomes detached from the ridges 21 , so that liquid can readily flow from the ducts 18 and 19 along the entire length of the ridges 21 into the hollowing 16. When the excitation of the piezoelectric element 26 ceases (or when platelet 25, if it is also a piezoelectric element, is excited), the platelets 25 and 26 straighten out again and pressure is exerted on the liquid which has flowed into the pressure chamber 16, as a result of which a drop of liquid emerges from the outlet opening 28 in order to be deposited on a receiving material which is moving past.

Since in the embodiment of a liquid-jet printing device according to the invention illustrated in Figures 5 and 6, the energy of the piezoelectric element 26 does not have to be transmitted to a metal or ceramic strip situated underneath via a film/foil as is the case with the embodiment according to Figures 1 to 4, relatively less energy is required to squirt a drop of liquid onto the receiving material. The reason for this is that the bending stiffness of the flexible film/foil is much lower than the bending stiffness of a piezoelectric element and of a metal strip.

It will be clear that the 'single device' according to Figures 5 and 6 can also be configured as an 'array device' according to Figures 1 to 4. In that case, a duct 18 or 19 is situated between two neighbouring pressure chambers 16 which is common to neighbouring pressure chambers. In the case of a 'single device', the function of the film/foil can also be fulfilled by an adhesive layer on the piezoelectric element which is situated on the block 15 and is attached thereto at the edge. In this case, the piezoelectric element has to be relatively large.

A particular characteristic feature of both above-described embodiments of liquid-jet printing devices operating according to the invention is the omnidirectional supply of liquid when the pressure chamber is replenished. As a result thereof, the negative pressure which occurs during replenishing is distributed over a large area, as a result of which the negative pressure is much smaller and no vapour bubbles form in the pressure chamber. In addition, the supply of liquid to the pressure chamber is variable, that is to say that the supply by the omnidirectional supply is initially relatively large, but becomes increasingly smaller as the pressure chamber closes and stops entirely when the pressure chamber is closed. This is in contrast to known liquid-jet printing devices where liquid can partly flow back into the liquid supply line opening in the pressure chamber when pressure builds up in the pressure chamber, as a result of which the pressure build-up is less efficient.