EP0057956A2 - Writing head for an ink jet printer - Google Patents

Abstract

The print head of an inkjet printer contains a plurality of tubular nozzle channels of the same cross section, on each of which cylindrical, assigned drive elements are arranged. Each nozzle channel is formed in one piece with its outlet nozzle and the attenuator arranged at the rear end and opens vertically into the distribution channel, via which the ink liquid is supplied to all nozzle channels. In the area between the drive elements and the nozzles, the nozzle channels are deflected in such a way that they run parallel to one another in the remaining areas, the distances in the area of the nozzles being the required distance of the mosaic-like ink droplets on the recording medium and in the rear area by the dimensions of the Drive elements are determined. The drive elements are activated in such a way that the liquid meniscus in a nozzle is first drawn into the nozzle channel and then pushed out.

Description

The invention relates to a write head for inkjet printers with a plurality of tubular nozzle channels, each of which is assigned a drive element for the drop-shaped ejection of ink droplets as required and to which the ink is supplied via a common distributor channel and separately assigned attenuators, the nozzle channels with their drive elements and the common distributor channel in separate blocks of preferably plastic are arranged, which are interconnected.

Such a write head is known, for example, from DE-OS 26 59 398, which contains a plurality of 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 so 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 speaks. 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 ducts (throttles) are arranged, the pipe cross-section of which is smaller than the cross-section of the nozzle ducts 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 inclusion of glass bubbles, not only when the liquid is filled for the first time, but also during operation because, for example, the liquid can flow back into the interior of the system due to vibrations and then air bubbles are trapped at the critical points when it is rushed forward . Any offset between pressure channel and nozzle element also in any case obstructs the pressure line and causes pressure losses.

Furthermore, there is always a gap between the blocks with the nozzle channels and the nozzle plate. This gap also affects the drop output and is also not constant over a long period of time. 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 point of view of spraying - 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 ink is supplied from 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. Since the ejection of the liquid is only desired 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. However, this diameter 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 object of the invention is to improve on the one hand the filling of the print head with ink liquid, to make the drop emission safer and to provide a simple manufacturing method for such a write head with several channels, and on the other hand to ensure the smallest possible center distances of the outlet nozzles even when the Nozzle channels all over their Length of constant cross-section. In addition, it should be possible to check the print head during manufacture and thereby reduce the number of rejects, without having to do without a compact design.

This object is achieved in that the nozzle channels are integrally formed both with their outlet nozzles and with their damping members, that the nozzle channels open vertically into the distribution channel and that the nozzle channels are deflected in the area between the drive elements and the outlet nozzles in such a way that they are deflected into the the remaining areas run parallel to one another, the distance in the area of the outlet nozzles being determined by the required distance of the mosaic-like ink droplets on the recording medium and in the rear area by the dimensions of the drive elements.

The drive elements themselves are expediently kept freely accessible so that they can be adjusted subsequently and all the 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 art used fabric of the block. Nozzle channels made of glass are particularly advantageous.

A write head designed in this way is also particularly simple to manufacture, the method of which is characterized in accordance with the invention in that each nozzle channel is first brought into the predetermined shape and provided with a drive element, then checked for proper function and drop emission, then in a holder is aligned and poured into a plastic block, that after this process the plastic block is cut 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 relevant drive elements, in the nozzle channel of which there is a drop emission deviating from a predetermined value, is more or less reduced 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 avoids rejects on finished print heads. It is also possible to trim the individual drive elements after the production of the write head and thereby to open all of its nozzle channels Set 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, which is kept relatively small, e.g. in the area of the damping members and the nozzles, e.g. 0.4 mm. In addition, the drive elements are controlled such 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 shows a pre-assembled complete nozzle duct,
• 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 the complete print head with nozzle channels and distribution channel and
• 5 shows the cross section along the section line AB of FIG. 4.

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 print 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. 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 several nozzle channels to be combined to form a write head made of e.g. six nozzles arranged one above the other or from a nozzle matrix, the center distance of the nozzles lying one above the other being 0.53 mm in the case of a character size on the recording medium of 3.2 mm high. 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 reject head requirement can be kept low.

From several such nozzle channels 1, a module is now produced, in which, for example, six nozzle channels 1 bent in a precisely defined manner, including their drive elements 2, outlet nozzles 3 and damping members 4, are combined in a block 10. The exemplary embodiment according to FIG. 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. It is therefore expedient to only summarize a module the nozzle channels themselves. This can be done in such a way that the outlet nozzles 3 at a certain distance from one another, which is determined by the character matrix that may be generated, and the side of the channels opposite the outlet nozzles 3 in the region of the damping parts 4 are also combined, and in such a way that the nozzle channels 1 are cast in this area 11 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 region 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, not shown, of the casting mold 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 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 done in a simple Work in the area 11 cut 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 distributor channel is produced in a separate operation (Fig. 3). For this purpose, a U-shaped channel .16 is formed in a plastic block 15, e.g. 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 here is also kept somewhat larger. The plastic block 15 is then brought into the desired shape along the cutting line 20 in the region of this web. This section line 20 is to be laid so that the web of the distribution channel is open to the outside. At least in the area of the web, the distribution channel is larger in cross section than the cross section of the individual nozzle channels 1 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 print head is in

Fig.4 shown. 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 liquid 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 made as a plate-like element put. If several such modules are stacked and the outlet nozzles 3 of the one module are shifted in a suitable manner, multiple writing 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.Print head for inkjet printers with a plurality of tubular nozzle channels, each of which is assigned a drive element for dropping ink droplets as required and to which the ink is supplied via a common distributor channel and separately assigned attenuators, the nozzle channels with their drive elements and the common distributor channel in separate blocks Made of preferably plastic, which are connected to each other, characterized in that the nozzle channels (1) are integrally formed both with their outlet nozzles (3) and with their damping members (4), that the nozzle channels (1) are perpendicular to the distribution channel (17) open and that the nozzle channels (1) in the area between the drive elements (2) and the outlet nozzles (3) are deflected such that they run parallel to each other in the remaining areas, the distance in the area of the outlet nozzles (3) the required distance of the mosaic Carried ink droplets on the record carrier and in the rear area are determined by the dimensions of the drive elements (2). 2. Print head according to claim 1, characterized in that
that the drive elements (2) of the nozzle channels (1) are freely accessible in a recess (14) of the associated block (10). 3. Print head according to claims 1 and 2, characterized in that
that the drive elements (2) are tubular and are glued to the nozzle channels (1). 4. printhead according to claims 1 to 3, characterized in
that the drive elements (2) are arranged parallel to each other. 5. printhead according to claims 1 to 4, characterized in
that the inner electrode (5) of the drive element (2) is pulled up to the outside thereof. 6. printhead according to claims 1 to 5, characterized in
that the cross section of a nozzle channel (1) is the same over its entire length except in the region of the damping member (4) and the outlet nozzle (3). 7. Print head according to claims 1 to 6, characterized in that
that the distribution channel (17) is tubular and is larger in cross section than the cross section of a nozzle channel (1). 8. printhead according to claims 1 to 7, characterized in
that the distribution channel (17) is U-shaped, one leg (18) being the supply channel for the liquid, the web (17) being the actual supply channel and the other leg (19) being the venting channel. 9. Manufacturing method for a printhead According to claims 1 to 8, characterized in that each nozzle channel (1) is first brought into the predetermined shape and provided with a drive element (2), then checked for proper function and droplet emission, then aligned in a holder (12) and is poured into a plastic block (10) so that after this process the plastic block (10) is cut flat at its rear end perpendicular to the ends of the nozzle channels (1) and connected to the distributor channel (17) which is also cast in a plastic block (15). 10. The method according to claim 9, characterized in
that after the assembly of the print head and filling of 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. 11. The method according to claim 10, characterized in
that the nozzle channels are tuned in that one of the electrodes of the drive elements in question. at the nozzle channel of which there is a drop emission that deviates from a predetermined value, is more or less reduced until the predetermined value is reached. 12. A method for controlling the drive elements of a write head according to claims 1 to 8, characterized in that
that the liquid meniscus located in the associated nozzle is initially drawn slightly into the nozzle channel by the applied control voltage and only then is expelled. 13. Multiple write head, characterized in that two or more write heads according to claims 1 to 8 are arranged next to each other and are firmly connected.

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