Print head for an ink jet printer

Abstract

A print head for an ink jet printer includes at least one ink supply channel and at least one nozzle having a nozzle channel and an inflow opening, and ink can be pressed from the ink supply channel through the inflow opening into the nozzle channel and ejected therefrom, wherein the nozzle is arranged in a stationary manner on a side wall of the ink supply channel and in the ink supply channel a ram is provided, which can be moved back and forth between a reversal point that has a minimal distance from the inflow opening of the nozzle and a reversal point that has a maximal distance from the inflow opening of the nozzle, wherein a first limiting device limits the movement of a ram end face to a movement between the reversal points; and a second external device is provided for applying a negative pressure relative to the ambient air pressure to the ink in the ink supply channel.

Claims

1. A print head for an ink jet printer, the print head comprising at least one ink supply channel and at least one nozzle with a nozzle channel and inflow opening, ink being pressed through the inflow opening from the ink supply channel into the nozzle channel and ejected from the latter, the nozzle being arranged in a stationary manner on a side wall of the ink supply channel and a ram is assigned to the at least one nozzle with a ram end face in the ink supply channel which ram end face is spaced apart opposite the inflow opening, the print head comprising a first moving device for moving the ram end face in the ink supply channel between a first reversal point that has a minimum distance from the inflow opening of the nozzle and a second reversal point that has a maximum distance from the inflow opening of the nozzle, wherein the first moving device delimits the movement of the ram end face to a movement between the first and second reversal points and a pressurizer is provided for pressurizing the ink in the ink supply channel with a negative pressure relative to the environmental air pressure, wherein the inflow opening of the nozzle is arranged in a section of the side wall opposite the ram, which side wall is delimited by a cylindrical delimiting face forming with the movement of the ram in the ink, wherein a section of the ram opposite an end face of the ram is connected securely with a movable ram rod pressurized by a restoring force acting in the direction of the nozzle, wherein the ram rod is guided movably at least in part in a hollow shaft parallel to a longitudinal middle straight line of the hollow shaft, wherein a radially peripheral seal is provided between the guide rod and the hollow shaft.

2. The print head as claimed in claim 1, wherein at the first reversal point at a minimum distance the distance between a ram end face and the inflow opening is greater than zero.

3. The print head as claimed in claim 1, wherein a distance of a ram outer edge to a side wall of the ink supply channel in a direction vertical to the nozzle axis is greater than zero, preferably greater than 1 mm and more preferably greater than 3 mm.

4. The print head as claimed in claim 1, wherein there are a plurality of nozzles and a ram is assigned to each nozzle.

5. The print head as claimed in claim 1, wherein at least in the case of the at least one nozzle no side wall is configured in one piece with the nozzle and an end face of the at least one nozzle surrounding the inflow opening is formed flush with an inner surface of a side wall of the ink supply channel that is in contact with the ink.

6. The print head as claimed in claim 1, wherein a longitudinal middle axis of the at least one nozzle extends normally to the surface of the ink supply channel.

7. The print head as claimed in claim 1, wherein the inflow opening of the nozzle is conical, in the form of a funnel that tapers in the direction of an outlet opening.

8. The print head as claimed in claim 7, wherein the outlet opening of the nozzle is designed to be cylindrical.

9. The print head as claimed in claim 1, wherein the nozzle has a length which is a multiple but at least twice the maximum diameter of the nozzle.

10. The print head as claimed in claim 1, wherein the nozzle is of ceramic, hard metal or surface-treated steel and/or the end face of the ram is made at least in part of ceramic, hard metal or surface-treated steel.

11. The print head as claimed in claim 1, wherein the ram has an external diameter of preferably between 3.0 to 5.0 mm.

12. The print head as claimed in claim 1, wherein a nozzle has an internal diameter of preferably between 200 to 350 ?m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention and further advantages are explained in more detail with reference to several non-restrictive exemplary embodiments which are represented in the drawings.

(2) In the latter in a much simplified schematic representation:

(3) FIG. 1 shows a partial cross section of a print head;

(4) FIG. 2 shows a portion of the print head of FIG. 1 in more detail;

(5) FIG. 3 shows a pump chamber formed between an end face of a ram and a nozzle;

(6) FIG. 4 shows the principle of functioning of an imaginary pump chamber and

(7) FIG. 5 shows a theoretically calculated print distribution under the end face of a ram.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(8) First of all, it should be noted that in the variously described exemplary embodiments the same parts have been given the same reference numerals and the same component names, whereby the disclosures contained throughout the entire description can be applied to the same parts with the same reference numerals and same component names. Also details relating to position used in the description, such as e.g. top, bottom, side etc. relate to the currently described and represented figure and in case of a change in position should be adjusted to the new position.

(9) The Figures are described in general in the following.

(10) According to FIG. 1 and FIG. 2 a print head 1 for an ink jet printer according to the invention comprises at least one ink supply channel 2 and at least one nozzle 3 for ejecting the ink from the ink supply channel 2.

(11) In the print head 1 a plurality of ink channels can be provided arranged parallel to one another and extended in length, in which, as shown in FIG. 1, nozzles 3 and movable rams 6 are arranged at regular intervals. The ink channels, nozzles 3 and ram 6 are configured or arranged in this case like the ink channel 2, nozzle 3 and ram 6 described in the following. The ink channel 2 is used for supplying ink to the nozzle 3.

(12) The ink can flow continually through the ink channel 2 in order to avoid sedimentation of the ink. The drop in pressure in the ink channel 2 is preferably very small, which can be achieved by having a cross section of the ink channel that is as large as possible.

(13) The nozzle 3 is arranged in a stationary manner on a side wall 4 of the ink supply channel 2 and in the ink supply channel 2 a ram 6 is provided which moves back and forth between two reversal points opposite the inflow opening of the nozzle, reversal point (U1) and reversal point (U2). The nozzle 3 can be arranged replaceably on the side wall 4, for example the nozzle 3 can be screwed into the side wall 4 of the ink supply channel 2. In order resist abrasive pigments in the ink, the nozzle 3 can be made of ceramic, hard metal or glass etc.

(14) An end face 7 of the nozzle 3 comprising the inflow opening 5 can be formed to be flush with an inner surface 8 of the side wall 4 of the ink supply channel 2 in contact with the ink. By means of the flush arrangement of the nozzles 3 on their inner end faces 7 with the inner wall of the ink channel 2 the ink flow is disrupted as little as possible and sedimentation is avoided. A longitudinal middle axis a of the nozzle 3 can in this case run normally to the surface 8 of the ink supply channel 2.

(15) A section of the ram 6 opposite an end face 9 of the ram 6 can be connected securely to a movable ram rod 10 pressurized with a restoring force acting in the direction of the nozzle 3. The ram 6 can be withdrawn from the nozzle by means of an actuator. An electronic actuator can be provided as the actuator for operating the ram, which is for example in the form of a solenoid armature 11 connected to the ram rod 10, which works with a coil 12 which can be wound around a core 13. The ram 6 can be drawn upwards by the armature 11 of the magnet. In this case a spring denoted in FIG. 1 by the reference number 14 is tensioned, which in a currentless state of the actuator pushes the ram 6 back down. When the ram 6 approaches the nozzle 3 or when the solenoid armature 11 approaches the core 13, strong damping occurs caused by the radial film flow and prevents the hard slamming of the ram 6 or the solenoid armature 11 on the ink or ink film between the ram and the nozzle or the inner wall of the ink supply channel and thus increases the lifetime of the ram 6 and the actuator. Instead of the magnet other kinds of drive are also possible for the ram, for example in the form of a compressed air or piezo-actuator.

(16) The ram rod 10 can be guided movably at least in part in a hollow shaft 15 parallel to a longitudinal middle straight line of the guide shaft or hollow shaft 15, wherein a radially peripheral seal 16 can be provided between the guide rod 10 and the hollow shaft 15.

(17) As shown in FIGS. 1 and 2 the guide shaft 15 can protrude from the delimiting wall 17 of the ink channel 2 opposite the nozzle 3 into the ink channel 2. By sealing the transition between the guide shaft 15 and the ram 6 it is possible to prevent the ink penetrating into the guide shaft 15. The guide shaft 15 can be configured to be slim, in order to create as little flow resistance as possible in the ink channel. It is particularly advantageous in this case if the guide shaft or hollow shaft 15 has smooth and/or rounded surface. The guide shaft or hollow shaft 15 can have a circular, elliptical or similar cross section for example.

(18) A ceramic part or part made from a different material than that of the ram can be inserted into the ram end face 9, in order to increase the lifetime of the end face 9 in the presence of abrasive pigments.

(19) As shown in FIG. 3, the inflow opening 5 of the nozzle 3 can be configured to be conical, in the form of funnel that tapers in the direction of an outlet opening 18. The outlet opening 18 of the nozzle 3 can be configured to be cylindrical. Furthermore, the nozzle 3 can have a length L which is multiple, but at least twice the maximum diameter d of the nozzle 3.

(20) Preferably, the inflow opening 5 of the nozzle 3 is arranged in an area of the side wall 4 opposite the ram 6, which lies inside a pump chamber 19 and delimits the latter in one direction.

(21) The pump effect provided and required for each ink jet-print head is illustrated by an imaginary pump chamber 19 and can be formed by a cylindrical area which is delimited by the diameter d1 of the ram end face 9 and the distance a1 ram-end face to the inner nozzle end face or to the end face of the inflow opening 5. Said space need not necessarily be cylindrical however.

(22) The pump chamber 19 is the space in which a change in volume occurs. The pump chamber 19 always has two openings 20 and 21, as shown in FIG. 4 which illustrates the principle of action of a pump chamber, one for the inflow and one for the outflow. During the refill cycle the ram 6 moves upwards and ink flows through the inflow 20 and outflow opening 21 into the space of the pump chamber 19. During the ejection cycle the ram 6 moves downwards and ink flows through the inflow 20 and outflow opening 21 out of the space of the pump chamber 19.

(23) In order that that the print head 1 can eject a droplet more ink needs to flow out through the outflow opening 21 of the pump chamber than flows back through the inflow opening of the pump chamber 19 into the ink channel 2. For suctioning ink into the pump chamber 19 the opposite is true. FIG. 4 illustrates this graphically, the direction of movement of the ram 6 and the flow directions of ink when suctioning in or ejecting ink from the pump chamber 19 being indicated by arrows in FIG. 4. The different arrow thicknesses show the different amounts of inflowing or outflowing ink.

(24) When the ram 6 moves ink is displaced or suctioned in from the pump chamber 19. The ram movement causes at the same time a change in volume and pressure in the pump chamber 19. In this case, as shown in FIG. 5, a pressure gradient is formed along the ram radius r.

(25) Where said pressure gradient is at a maximum or minimum, according to FIG. 3 a cylindrical delimiting face 22 is formed with the so-called stagnation radius rs. Outside said delimiting face 22 the ink flows into or out of the ink channel, inside said delimiting face 22 the ink flows out of the nozzle or into nozzle, depending on the movement direction of the ram 6. The stagnation radius rs is dependent on the movement direction and speed of the ram 6, on the distance a1 of the ram 6 to the nozzle and on the pressure at radius r and radius ri. As a result volumes of ink flowing in both directions are formed during the back and forth movement of the ram 6.

(26) The inflow opening 5 of the nozzle 3 can be arranged directly at the outflow opening 21 of the pump chamber 19. In this embodiment the inflow opening 5 of the nozzle 3 is arranged in a section of the side wall 4 opposite the ram 6, which during the movement of the ram 6 is delimited by the delimiting face 22 forming by the associated pressure gradient in the ink.

(27) FIG. 5 shows a theoretically calculated radius-dependent pressure gradient under the ram 6. The stagnation radius rs is located where the pressure has the maximum value.

(28) In commercially available ink jet print heads the length L in flow direction is usually such that the ink volume in the nozzle corresponds approximately to the droplet volume. As the nozzle diameter in a print head for structural printers has to be greater than in commercially available ink jet print heads in order to achieve the required droplet volume, the capillary pressure is substantially smaller and during the refill cycle more ink is suctioned back through the nozzle 3 into the pump chamber 19. For this reason the length of the nozzle L can be substantially increased so that the nozzle cannot be completely emptied during the refill cycle and air cannot enter into the pump chamber 19.

(29) The length L of the nozzle consists of the lengths for the cylindrical part l2 and for the conical part l1. Lastly, as a point of formality it should be noted that for a better understanding of the structure of device according to the invention the latter and its components have not been represented true to scale in part and/or have been enlarged and/or reduced in size.

(30) Finally, as a point of formality, it should be noted that for a better understanding of the structure of the device according to the invention the latter and its components have not been represented true to scale in part and/or have been enlarged and/or reduced in size.

(31) In a preferred embodiment of the present invention a plurality of ink supply channels 2 in a print head are aligned parallel to one another according to length, in which a plurality of nozzles 3 with nozzle channel and inflow opening 5 are preferably arranged at equal distances from one another on the ink supply channel wall. First means 23, such as at least one actuator or at least one actuator and at least one spring are provided for the movement of a ram end face 9 or the ram 6, which delimit the ram end face 9 to a movement between the reversal points (U1, U2). A second external means, such as a vacuum pump, which is arranged e.g. in an ink intermediate tank that is connected by at least one ink supply line to the print head and which pump is arranged in the airspace above a fluid level, is provided for pressurizing the ink in the ink supply channel 2 with negative pressure relative to the environmental air pressure. A third external means, such as a circulating pump is provided e.g. in an ink intermediate tank, which pumps the ink preferably continuously through the at least one ink supply line and ink supply channels of the print head.

(32) In a preferred embodiment of the printing method the ink in the ink supply channel 2 is pressurized by negative pressure in the region of more than zero to preferably 5 mbar relative to the environmental air pressure with a nozzle internal diameter at the outflow opening of the nozzle of 300 ?m. For example, other negative pressures can be used however, wherein the ink pressure may not be less than the capillary pressure to ensure that no air is suctioned through the nozzle into the ink supply channel. To clarify: negative pressure in the region of more than zero to preferably 5 mbar relative to the environmental air pressure means a pressure which in the region of more than zero to 5 mbar is lower than the environmental air pressure.

(33) The ink output pumped through the ink supply channel per time unit X is in a preferred embodiment greater by a specific factor adapted to the system than the sum of the amount of ink that can be ejected at the maximum through all nozzles during the printing operation, whereby the rule is that the ink pressure has to be adapted in combination with the capillary pressure so that no air is suctioned through the nozzle channel into the ink supply channel and no ink exits unintentionally out of the nozzle channel.

(34) The ram 6 is not arranged in a sealing manner in the ink supply channel 2 on an ink channel wall, whereby a distance of the ram outer edge to a side wall of the ink supply channel in a direction vertical to the nozzle axis is preferably greater than 1 mm and more preferably greater than 3 mm.

(35) In a preferred embodiment of the print head device the ram 6 has an external diameter of preferably between 3.0 to 5.0 mm. The nozzle 3 in this case has an internal diameter of preferably between 200 to 350 ?m.

(36) In a preferred embodiment of the printing method the change in direction at reversal point (U1) has a frequency of preferably up to 1.1 kHz and more preferably up to 1.0 kHz. If the ram external diameter is selected to be smaller it is possible to print at much higher frequencies.

(37) In the printing method the ram/nozzle distance at reversal point (U1) is maintained to be greater than zero and preferably up to 100 ?m and the ram/nozzle distance at reversal point (U2) is greater than 250 ?m and preferably up to 400 ?m.

(38) The method can be used for printing inks of varying viscosity, wherein-preferably the ink has a viscosity ? of ?=50-100 mPa.Math.sec and/or the ink for printing comprises pigments with a pigment size of up to 10 ?m, wherein the ink has a pigment size of preferably up to 5 ?m. Inks with a low or higher viscosity than said viscosity ? of ?=50-100 mPa.Math.sec and/or with larger pigment sizes than 10 ?m can also be used in principle, as long as an optimal printing stability can be ensured.

(39) If inks are used in the printing process with pigments, which have a particle size g, of course at least a ram/nozzle distance should be selected at a reversal point that is at least greater than the corresponding particle size g in order to avoid collisions of the ram with pigments that may be present on the nozzle surface.

(40) If the ram end face 9 moves from a starting point towards the inflow opening there is a change in volume and pressure in an area of the inflow opening of the nozzle and a specific amount of ink and thus droplet size is ejected from the nozzle. A main droplet is ejected from the nozzle, but depending on the printing method together with the main droplets more or less undesirable small droplets, so-called satellites, are ejected with the main droplets.

(41) In other embodiments of the present invention a plurality of print heads can be mounted in a staggered or any other arrangement and thereby can be arranged such that in the printing process at least one nozzle of a print head overlaps with at least one nozzle of another print head in at least one direction and/or that the nozzles of a row of nozzles of a print head are displaced relative to the nozzles of a row of nozzles of another print head by a specific nozzle distance.

(42) If the nozzles are displaced relative to one another greater droplet densities and thereby higher image resolutions can be achieved.

(43) In a further embodiment of the present invention, in which the print heads are moved in a secondary scanning direction Y and main printing direction X in the printing process, the rows of nozzles of the print heads can be adjusted parallel to one another and obliquely at an angle relative to a secondary scanning direction Y, so that the nozzle distance between the individual nozzles of a print head in a main printing direction X has a nozzle spacing Y, so that printing is possible in the main scanning direction X with a higher resolution than the native resolution of a print head.

(44) The present invention is not intended to be restricted to the embodiments of the print head device given by way of example and in the drawings.

LIST OF REFERENCE NUMERALS

(45) 1 print head 2 ink supply channel 3 nozzle 4 side wall 5 inflow opening 6 ram 7 end face of nozzle 8 surface 9 ram end face or end side 10 ram rod 11 solenoid armature 12 coil 13 core 14 spring 15 hollow shaft 16 seal 17 delimiting wall 18 outlet opening of the nozzle 19 pump chamber 20 inflow opening 21 outflow opening 22 delimiting face