Device for covering jet nozzle of printing head

The invention relates to a device for covering the nozzles of an ink writing head with deformable material.

Such a device is known from DE-AS 2702663. The print head with its nozzles is displaceable on guide rails along a recording medium for the purpose of writing. The guide rails are longer than the writing width, so that the print head is in its rest position at times of non-writing at the beginning or end of the writing area. To avoid drying of the nozzles in this rest position, the nozzle plates of the printhead are covered by a pad of a material having visco-elastic properties. Since the printhead is normally moved linearly, the capping device is pivotally mounted outside the writing area on the housing of the inkjet printer. As soon as the write head has assumed its rest position, the cover device is moved in the direction of the nozzles, so that they are covered.

This known covering device has the advantage that the per se fast-drying ink from the nozzles penetrates into the cavities of the viscous material and thus achieves a dense covering of the nozzles. While such surface material is normally reversible in such material, permanent deformation still occurs to some extent upon prolonged use. A long-term useful coverage of the nozzles is thus not achieved, especially after the swiveling of the capping of the nozzles, the remaining ink on the viscous elastic pad ink also dries very quickly and generates a tension on the surface of the pad, which counteracts a reversible deformation. This process can be alleviated by pressing the capping device against the nozzles under a relatively high pressure, which contact pressure must be steadily increased the longer the capping device is in use. Otherwise, the viscous elastic pad must be replaced constantly.

From U.S. Patent No. 4,228,442 there is also known an ink jet recording head in which a solvent-immersed absorbent material is positioned in front of the mouth of the nozzle. However, the mouth of the nozzle is not covered, and so a fuming of the nozzle or a drying of the ink is thus delayed, but there is no assurance that, for example, a drying of the ink even at longer periods in which is not written is always completely prevented.

The invention has for its object to provide a capping for the nozzles of an ink jet print head, in which contamination of the nozzles and the cover material are avoided by drying ink or a drying moisturizing agent as well as a transition of the humectant in the nozzle channel. In addition, the device should be as simple as possible mechanically.

This object is achieved by a container filled with a humectant whose side facing the nozzles is covered, at least in the area of ​​the nozzles, by a semipermeable membrane which can be placed against the mouths of the nozzles. In this case, the humectant may be miscible with the ink in the nozzles or be soluble or insoluble therein. It is expedient to use a moisture-evaporating liquid as the humectant, for example polyethylene glycol, ethylene glycol, diethylene glycol or glycerol.

The filling of the container with the humectant can be done by repeated filling in the container or by constant shrinkage from a reservoir. It is particularly advantageous if the membrane is clamped loosely over the side of the container facing the nozzles and the moisturizing agent in the container is pressed under pressure against the membrane, so that it bulges against the nozzles of the print head. It is advantageous to use cellulose hydrate as the material for the membrane.

The invention will be explained below with reference to two exemplary embodiments illustrated in the drawings. Fig. 1 shows schematically in section a device for covering the nozzles of an ink writing head, wherein the membrane is stretched slack over the side of the container which faces the nozzles of the print head, and Fig.2 shows in section a modification of the device according to Fig.l, in which the membrane is taut on the relevant side of the container.

Nr.375879

In ink printers, it is important that the ink emerging from the nozzles dry as soon as the ink appears on the recording medium. This prevents penetration of the ink into the material of the recording medium. Penetration into the record carrier would result in poor font quality. This short drying time is opposed by a desired longer drying time in the nozzles. This longer drying time is desirable in order to avoid clogging of the nozzles with longer non-writing times. However, as good writing quality is more important, devices are used which cover the nozzles in the non-writing times and thus try to protect them against drying out.

The illustrated apparatus for capping the nozzles of an ink writing head consists of a container -1-, which in the present examples has a cylindrical shape. This container -1- has on the side facing the print head -5-, a conical end portion -13-, which is open at least in the region of the nozzle -6-. The opposite side of the container -1- is closed by a removable bottom -16-.

The open side of the container -1- is covered by a semi-permeable membrane -4-, which is clamped between the conical end portion -13- and the inside of a coupling ring -11-. This is achieved by screwing after placing the membrane -4- over the cone -13- the coupling ring -11- to the container -1- with the interposition of a sealing ring -12-. The membrane -4- consists of a material which must be permeable to the moisturizer in the container -2-. On the other hand, it can be permeable or impermeable to the ink as needed.

In the example of FIG. 1, the membrane -4 is laid slack over the cone 13. In addition, in the container -1- a piston -8- arranged, which is pressed by a spring -7- in the direction of the open side of the container -1-. As a result, the humectant -2- filled in the container -1- is pressed against the diaphragm -4-, which consequently bulges outwardly against the mouths of the nozzles -6- and bears against them. The humectant -2- is filled by removing the bottom -16- with the piston -8-, bringing the bottom and the piston back into the drawn position after filling with the humectant. In modification of the drawn example, instead of the spring -7- a device can be used, with which it is possible to push the piston -8- as needed back and forth. This has the advantage that when the piston is withdrawn, the curvature of the membrane -4- is also canceled, so that the nozzles -6- become free. Such an arrangement can be used when the container -1- is stationary and not swinging away. If the printhead -5- be moved away to write, so in this case the piston -8- retract. At the time in which the print head -5- again assumes its rest position, then the piston -8- is advanced again, so that due to the curvature of the membrane -4- this applies again against the nozzle -6-.

Another advantage of the domed membrane is that even write heads can be sealed with non-planar ends, such as curved nozzle plates, ring nozzles, conical nozzles, etc.

In the embodiment of Figure 2, the membrane -4- taut over the cone -13- stretched. In this case, to cover the mouths of the nozzles -6-, the container -1- is to be pivoted against the nozzles -6-. In order not to have to regularly refill the humectant -2-, a nozzle -15- can be provided, via which the container -1- is connected to a reservoir, not shown, for the humectant -2-. Of course, in the embodiment of Fig.l such a nozzle -15- are provided. In this case, however, since the humectant -2- is under pressure in the container -1- to provide a check valve which prevents backflow of the humectant in the reservoir.

The container -1- is externally surrounded by an additional container -10-, in which an absorbent material -9- is contained. The additional container -10- has a drain -14-. This additional container serves to -3- collected on the membrane -4- ink and absorb impurities and discharged via the drain -14-. Also, ink that is emitted to clean the nozzles and hits the membrane, as well as through the membrane -4- exiting humectant -2- flow into the container -10- from.

Nr.375879

- 4 The humectant -2- may be soluble in the ink, miscible with it or insoluble. It can easily evaporate or be volatile. However, it is preferable to use a low-volatile and ink-miscible humectant. If a pressurized humectant -2- used, the membrane permeability and the pressure generated by the piston -8- must be adjusted so that only small amounts of humectant penetrate through the membrane -4-, otherwise the supply of humectant is quickly exhausted , For this reason, an arrangement with feeding the humectant from a reservoir is advantageous.

It is also possible to form the entire covering device as a cartridge, which is replaced after consumption of the humectant by a new filled cartridge. In this case, the holding device of the cover device is to be designed accordingly. It could for example be a clamping device for the cartridge.

The described device for covering the nozzles of an ink writing head has the advantage that the humectant is not contaminated by the ink and that the position of the nozzles in the region of the cover can be arbitrary.

The covering device described has proven excellent in tests. The nozzles -6- were operated with an ink -3- consisting of dye, humectant and about 90% water. The container was filled with diethylene glycol as a humectant, and the membrane was made of cellulose hydrate 22 pm thick. Immediately after completion of the writing process, the membrane was brought into contact with the nozzles. After 18 days, the container -1- was removed. The nozzles -6- sprayed immediately and wrote after about 300 pulses perfectly and deep black. After about 30 minutes of operation, the nozzles were switched off and left uncovered. After 15 minutes, the nozzles were completely dried and no longer usable. The membrane -4-, which is always slightly damp outside, therefore prevents the nozzles from drying even after days.