EP0057956B1 - Writing head for an ink jet printer - Google Patents

Description

The invention relates to a write head for inkjet printers with a plurality of nozzle channels, each of which is assigned a drive element for ejecting ink droplets as required and to which the ink is supplied via a common distributor channel and via attenuators assigned via each nozzle channel, the nozzle channels, the distributor channel and the attenuators are arranged in a block forming the print head, in particular made of plastic, and run in such a way that the nozzle channels opening into the distribution channel in the area between the drive elements and outlet nozzles are deflected such that they run parallel to one another in the remaining areas, the distance in the area the outlet nozzles is determined by the required distance of the ink droplets to be applied in a mosaic on the recording medium and in the rear region by the dimensions of the drive elements.

Such a write head is known from DE-OS 2 918 737. There the nozzle channels, the distribution channel and the attenuators are designed as integrated trenches in a plastic block and are covered by a cover plate and thus closed. The individual nozzle channels can only be checked after the complete assembly of the print head and it is difficult to adjust the individual nozzle elements individually, so that either a loss in quality or increased rejects have to be accepted.

A writing head is known from DE-OS 2 659 398, which contains several channels and drive elements, in order to represent characters in the form of an ink drop matrix, in particular as an ink-jet writing mechanism. These print heads essentially consist of two plastic blocks. The tubular nozzle channels with their drive elements are cast into one plastic block. The channels are cast in a fan-shaped and straight line and open at their front ends in a separate nozzle plate, in which the nozzles are then aligned parallel to one another. In addition, each channel is shaped so that it has a parallel cross section in the area of the cylindrical drive elements, which is relatively large. The geometric arrangement of the nozzles in the nozzle plate is designed in such a way that the spacing of the nozzles from one another corresponds to the conditions resulting from the grid-like structure of the characters to be printed. For example, with a character height of 3.2 mm, the nozzle center distance for six nozzles is 0.53 mm. In order to meet this condition, the nozzle channels in the area between the drive elements and the nozzles must be conical. This means more work and materials. The second plastic block contains a trough-shaped recess into which the liquid is introduced. Both plastic blocks are firmly connected to each other at their edges. A further plate is provided between the plastic blocks, in which damping channels (throttles) are arranged, the tube cross section of which is smaller than the cross section of the nozzle channels and which are parallel to one another.

The known compact units place very high demands on production. Despite high accuracy in the manufacture of the individual parts, an offset in the transition zones between the pressure channel and nozzle element and / or between the plate for damping the liquid and the channels cannot be ruled out during their assembly. This fact can significantly hinder the drop emission and even lead to failure of the print head.

The known system is difficult to fill with liquid without trapping gas bubbles in the transition zones. Any misalignment between the pressure channel and the nozzle element promotes the gas bubble inclusion, not only when filling the liquid for the first time, but also during operation, because e.g. B. can shake the liquid back into the interior of the system by shaking and then trapping air bubbles at the critical points. Any offset between the pressure channel and the nozzle element also hinders the pressure line in any case and causes pressure losses.

Furthermore, there is always a gap between the blocks with the nozzle channels and the nozzle plate. This gap also influences the drop ejection and, moreover, is not constant over a long period. It changes, for example, through thermal or chemical influences or through mechanical tensioning. These changes require the system to be operated with new, correspondingly modified control pulses after each change. This means not only additional electronic effort, but also permanent service readiness.

Similarly, in the area of the liquid supply, there is no guarantee that the liquid will be problem-free, i.e. can be supplied without gas inclusions. Rather, by attaching the additional plate with the throttle lines, their exact positioning with respect to one another is also required at this point. This is also a critical zone, as has already been described for the area of the nozzle plate. If the feed bores are even smaller in cross-section than the nozzle channels - advantageous from the spraying point of view - the problems listed when filling the liquid become even greater.

Furthermore, a writing element for inkjet printers is known from US Pat. No. 3,832,579, the tubular nozzle channel of which is made of glass and runs in a straight line from the area of the drive element to the outlet nozzle with the same, relatively large cross-section. Nozzle channel and nozzle consist of one piece. A hose is attached to the end of the nozzle channel facing away from the nozzle, via which the inks are closed drove out of an ink reservoir. This hose has a smaller cross section than the nozzle channel, so that it acts as an attenuator. The electromechanical, preferably piezoelectric drive element is arranged in tubular form around the nozzle channel and, when electrically controlled, exerts a pressure on the liquid in the nozzle channel. This pressure is not only directed towards the outlet nozzle, but also towards the rear in the opposite direction. However, since the liquid should only be ejected towards the front of the nozzle, the damping element is provided on the rear nozzle channel.

If it is now intended to print mosaic-like characters, several such writing elements are to be arranged side by side and parallel to one another. The center distance of the individual outlet nozzles is then determined by the outer diameter of the tubular drive elements. This diameter, however, is considerably larger than the distance between the raster points to be applied to the recording medium, which is required for an easily legible character. The recorded drawing line appears as a row of dots and not as a continuous line.

The invention has for its object to design a write head of the type mentioned with a simple manufacturing possibility so that the function of the individual nozzle channels can be checked and adjusted individually during manufacture and before completion to a writing head, without sacrificing a compact design to have to.

This object is achieved in that all the nozzle channels, including the damping members, are each designed as integrated tubular units and are each provided with a permanently assigned drive element, that the individual nozzle channels are held in a first block in a predetermined assignment to one another in such a way that the feed openings of the Nozzle channels are open to a level of the first block, and that the distribution channel is arranged in a second block such that it forms an open distribution channel to a level corresponding to the level of the first block, and that the level of the first block and the level of the second blocks are joined together.

The drive elements themselves are expediently kept freely accessible, so that they can be adjusted subsequently and all writing nozzles have the same drop emission.

The distribution channel is tubular and has a larger cross section than the cross section of a nozzle channel. The routing of the distribution channel in the separate plastic block is insignificant. It can be guided in a U-shape or in a straight line. It is only essential that the actual feed area for the nozzle channels runs perpendicular to these. The material of the nozzle channels can also be any and depends essentially on the compatibility with the ink liquid and the plastic used for the block. Nozzle channels made of glass are particularly advantageous.

A writing head designed in this way is also particularly simple to manufacture, the method according to the invention of which is characterized in that each nozzle channel is first brought into the predetermined shape and provided with a drive element, then checked for proper function and droplet emission, then in a holder is aligned and poured into a plastic block so that after this process the plastic block is cut flat at its rear end perpendicular to the ends of the nozzle channels and connected to the distributor channel, which is also cast in a plastic block. After assembling the print head and filling the distributor channel with liquid, the drop emission of the individual nozzle channels is measured and coordinated with one another by trimming the drive elements. This trimming can take place in that one of the electrodes of the drive elements in question, in whose nozzle channel there is a drop emission that deviates from a predetermined value, is reduced more or less until the predetermined value is reached.

The write head according to the invention has the advantage that the individual nozzle channels with their drive elements can be checked individually for their functionality before being poured into the plastic block during its manufacture. This practically eliminates rejects on finished print heads. It is also possible to trim the individual drive elements after the production of the write head and thus to set all of its nozzle channels to the same properties with the same drive pulses for all channels. The control electronics for the write head can thus be designed in a simple manner.

According to a further embodiment of the invention, the nozzle channels have the same cross-section over their entire length, except in the area of the damping members and the nozzles, which is kept relatively small, e.g. B. 0.4 mm. In addition, the drive elements are controlled in such a way that the liquid meniscus present in the nozzle is first drawn into the front part of the nozzle channel by the applied voltage pulse and the voltage is switched off as soon as the liquid meniscus has reached its maximum in the ejection direction. The fact that the liquid is drawn in first and then ejected results in a higher initial velocity of the ejected droplet. In addition, only a low control voltage is required to emit the liquid. This is a negative control in contrast to the known arrangements in which there is a positive control, i.e. the liquid meniscus is immediately moved in the direction of the discharge.

The invention is described in more detail using an exemplary embodiment shown in the drawing.

• Fig. 1 ein vormontierter kompletter Düsenkanal,
• Fig. 2 den Zusammenbau mehrerer Düsenkanäle nach Fig. 1 in einem ersten Modul,
• Fig. 3 den Verteilerkanal in einem zweiten Modul,
• Fig. 4 den kompletten Schreibkopf mit Düsenkanälen und Verteilerkanal und
• Fig. 5 den Querschnitt entlang der Schnittlinie AB der Fig. 4.

Show it:

• 1 is a pre-assembled complete nozzle channel,
• 2 shows the assembly of several nozzle channels according to FIG. 1 in a first module,
• 3 the distribution channel in a second module,
• Fig. 4 shows the complete write head with nozzle channels and distribution channel and
• 5 shows the cross section along the section line AB of FIG. 4th

The write head for an inkjet printer, in which the individual ink droplets are ejected from the nozzles and then hit a recording medium in free flight, i.e. are not deflected in an electromagnetic field, consists of a tubular nozzle channel 1 with a drive element 2 surrounding it. The drive element is preferably glued to the nozzle channel. Each nozzle channel 1 is deflected or bent between the drive element 2 and the outlet nozzle 3 such that it runs parallel to the adjacent nozzle channels of the write head outside this area, the center distance of the nozzles in the front area being due to the required distance of the mosaic-like ink droplets on the Record carriers, e.g. B. 0.53 mm, and in the rear area is determined by the dimensions of the drive elements 2. At the rear end of the nozzle channel 1, a constriction is provided which serves as an integrated damping 4 for the liquid vibrations.

With the exception of the outlet nozzle 3 and the attenuator 4, the cross section of the nozzle channel is the same over its entire length and is e.g. 0.4 mm. This relatively small cross section allows the combination of several nozzle channels to form a write head made of e.g. B. six superposed nozzles or from a nozzle matrix, with a character size on the record carrier of 3.2 mm height, the center distance of the superimposed nozzles is 0.53 mm. The constant cross-section enables better control of the liquid discharge through the drive elements.

The drive element 2 acts as an electromechanical transducer and is designed in particular as a piezoelectric transducer. It consists of an inner electrode 5 lying directly on the outer surface of the nozzle channel 1, an outer electrode 6 and a piezoelectric element 7 arranged in between. The inner electrode 5 is on one side around the end face of the piezoelectric element 7 up to the top thereof drawn and separated from the outer electrode 6 by an electrically non-conductive gap. This provides a simple connection of the connecting wires 8 and 9. This element shown in FIG. 1 is a functional drop generator and, if necessary, can be checked for its functionality before being assembled with other such drop generators. As a result, the number of rejects on print heads can be kept low.

A module is now produced from several such nozzle channels 1, in which e.g. six nozzle channels 1, which are bent in a precisely defined manner, including their drive elements 2, outlet nozzles 3 and damping members 4, are immovably combined in a block 10. 2 shows that this can be done, for example, with a casting resin. The drive elements 2 themselves are not cast in, in order to keep free access to them, not to hinder their movements by casting resin or other fixings, and also to isolate them mechanically from one another. However, the drive elements can also be cast in the plastic block 10. Then, however, the change described later and the advantages mentioned must be dispensed with. The combination into a module therefore expediently only takes place on the nozzle channels themselves. This can be done in such a way that the outlet nozzles 3 are at a certain distance from one another, which is determined by the character matrix to be generated, and the side of the channels opposite the outlet nozzles 3 in the area of the damping parts 4 are also combined, in such a way that the nozzle channels 1 in this area 11 are cast in a wider zone than would be required for the finished module.

The individual nozzle channels are poured in a mold. The holder in the area of the outlet nozzles 3 can be carried out in a known manner by means of a holding comb 12 or a perforated plate which ensures the required nozzle spacing. This bracket 12 is cast together with the individual nozzle channels 1. On the other hand, the nozzle channels 1 are placed in a divisible holder of the casting mold, not shown, in such a way that the drive elements 2 are located in the recess 14 formed by the holder and the nozzle channels protrude on both sides of this recess 14. The feedthroughs in the mold for the nozzle channels 10 must be tight so that no casting resin reaches the drive elements 2. The nozzle channels 1 now held in the areas of the outlet nozzles 3 and the drive elements 2 are placed in a mold, not shown, which determines the shape of the module, care being taken to ensure that no casting resin can penetrate into the interior of the nozzle channels 1. This can be done in a known manner by flexible sealing materials, e.g. Silicone rubber, reach.

Pouring the mold prepared in this way is a known process. This creates a raw module as shown in FIG. 2. This is cut off in a simple operation in the area 11 to the desired length along the line 13, so that a flat plane perpendicular to the nozzle channels 1 is created.

In a departure from the example described, the casting mold can also be designed such that it already ends at the section line 13. However, since shrinkage tolerances cannot be ruled out, there is a risk that the edge will not be flat and the ends of the nozzle channels will protrude somewhat from this edge surface. As described later, this area must be sealed against the distribution channel.

The tubular distribution channel is produced in a separate operation (FIG. 3). For this purpose, a U-shaped channel 16 is formed in a plastic block 15, for. B. by casting a correspondingly shaped wire, which is then etched out, or in another suitable form. On the side of the web, the mold, not shown, is also kept somewhat larger. In the area of this web, the plastic block 15 is then brought to the desired shape along the cutting line 20. This section line 20 is to be laid so that the web of the distribution channel is open to the outside. The distribution channel is at least larger in cross section than the cross section of the individual nozzle channels 1 in the area of the web and runs perpendicular to these.

Both module parts according to Figures 2 and 3 are coupled together with their cut sides. As a result, the distribution channel represents an integral liquid supply to the nozzle channels because there is no special assignment of bores to specific channels. The complete write head is shown in Fig. 4. The U-shaped channel 16 consists of the web 17 as the actual distribution channel and of the legs 18 and 19. The feed line 21 for the writing fluid is connected to the leg 18. The leg 19 contains the venting device 22. This module is already a simple writing head with which characters can be constructed in a mosaic-like manner.

The print head allows liquid to be filled in easily without trapping gas bubbles. This is achieved in that there are no disruptive edges in the course of the liquid paths.

In a modification of the example described, the ink supply can also take place in such a way that the distribution channel 17 is led straight up and down out of the block 15.

According to the manufacturing method described, the drive elements 2 are freely accessible on the finished write head. If at the same time one possibility is provided on each drive element 2 to trim it, the entire write head, i.e. set all of its nozzle channels 1 to the same properties with the same control impulses for all channels. When using piezoelectric transducers, this can be done in a simple manner by partially burning away or etching away one of the electrodes 5 or 6. The efficiency of the individual drive elements can thus be set so that all nozzle channels 1 react in the same way to the same control pulse.

The module can be manufactured as a plate-like element. If several such modules are stacked and the outlet nozzles 3 of one module are shifted in a suitable manner, multiple print heads can be produced with many drop generators working simultaneously, which at the same time allow many matrix patterns determined by the arrangement of the outlet nozzles.

Claims (13)

1. A writing head for ink jet printers, comprising a plurality of nozzle ducts, each of which has associated with it a drive member for the drop- letwise ejection of ink droplets at appropriate instants, the ink being applied thereto via a common distribution duct (17) and damping members (4) associated with each nozzle duct (1), the nozzle ducts (1), the distribution duct (17) and the damping members (4) being accomodated in a block (15) which constitutes the writing head and which is notably made of a plastics material in such a manner that the nozzle ducts (1) which open into the distribution ducts (17) are bent in the zone between the drive members (2) and the ejection nozzles (3) in such a manner that they extend parallel to one another in the other zones, the distance at the area of the ejection nozzles (3) being determined by the required distance between the ink droplets which are to be mosaic- wise deposited on the record carrier and at the distance the rear area by the dimensions of the drive members (2), characterized in that all nozzle ducts (1), including the damping members (4), are constructed as respective, integrated, tubular units with which a respective drive member (2) is permanently associated, the individual nozzle ducts (1) being retained in a first block (10) in a predetermined arrangement in such a manner that the inlet openings of the nozzle ducts (1) open towards a plane (13) of the first block, the distribution duct (17) being arranged in a second block (15) in such a manner that it forms a distribution duct (17) which opens towards a plane (20) which corresponds to the plane (13) of the first block, the plane (13) of the first block (10) and the plane (20) of the second block (15) being positioned so that they coincide.

2. A writing head as claimed in Claim 1, characterized in that the drive members (2) of the nozzle ducts (1) are arranged to be freely accessible in a recess (14) in the associated block (10).

3. A writing head as claimed in the Claims 1 and 2, characterized in that the drive members (2) have a tubular construction and are glued onto the nozzle ducts (1).

4. A writing head as claimed in the Claims 1 to 3, characterized in that the drive members (2) are arranged to extend parallel to one another.

5. A writing head as claimed in the Claims 1 to 4, characterized in that the inner electrode (5) of the drive member (2) extends as far as the outer side thereof.

6. A writing head as claimed in the Claims 1 to 5, characterized in that the cross-section of a nozzle duct (1) is the same over its entire length . with the exception of the area of the damping member (4) and the ejection nozzle (3).

7. A writing head as claimed in the Claims 1 to 6, characterized in that the distribution duct (17) is constructed to be tubular, its cross-section being larger than the cross-section of a nozzle duct (1).

8. A writing head as claimed in the Claims 1 to 7, characterized in that the distribution duct (17) is constructed to be U-shaped, one leg (18) serving as a supply duct for the fluid, the connection piece (17) serving as the actual supply duct, whilst the other leg (19) serves as a venting duct.

9. A method of manufacturing a writing head as claimed in the Claims 1 to 8, characterized in that each nozzle duct (1) first obtains the predetermined shape and is provided with a drive member (2), after which it is tested for suitable operation and droplet emission, followed by alignment in a mount (12) and encapsulation in a plastics block (10), the plastics block (10) subsequently being cut at its rear to be perpendicular to the ends of the nozzle ducts (1) and being connected to the distribution ducts (17) which is also encapsulated in a plastics block (15).

10. A method as claimed in Claim 9, characterized in that after the assembly of the writing head and the filling of the distribution duct with fluid, the droplet emission of the individual nozzle ducts is measured and matched by trimming the drive members.

11. A method as claimed in Claim 10, characterized in that the matching of the nozzle ducts is performed by more or less reducing one of the electrodes of the relevant drive members whose nozzle duct exhibits a droplet emission which deviates from a predetermined value until the predetermined value is obtained.

12. A method of controlling the drive members of a writing head as claimed in the Claims 1 to 8, characterized in that under the influence of the control voltage applied, the fluid meniscus in the associated nozzle is slightly drawn into the nozzle duct before being ejected.

13. A multiple writing head, characterized in that two or more writing heads as claimed in the Claims 1 to 8 are adjacently arranged and rigidly interconnected.

Images