EP0749836B1 - Apparatus to prevent nozzle clogging of an inkjet printhead - Google Patents

Description

The invention relates to an arrangement for keeping the nozzles of an ink print head clean, in particular an ink print head with large numbers of nozzles.
Such ink printheads are intended for use in small, fast printers, which in turn are part of modern machines for franking mail or for printing addresses or for product labeling.
In contrast to the conventional office printer with line-by-line printing, printing is carried out as a single print in one pass through the recording medium. Because of this much larger printing width, approximately one inch - the number of ink nozzles to be arranged one below the other is considerably larger than for ink print heads for office printers. In order to meet modern convenience - clichés with word and picture symbols - for franking machines with good print quality, print resolutions of approximately 200 dpi - dots per inch - are required. That means ink printheads with 200 nozzles.
With these print heads, it should be ensured that safety-relevant print image data, such as value, date and machine number in franking machines, are printed without loss of printing points.
Since quick-drying inks have to be used, there is a great risk that the ink will dry out in the nozzles that are not used for a long time, or that the nozzles will become blocked due to the accumulation of dust and ink residue in the nozzle area.
A large number of different solutions are known for preventing or eliminating nozzle clogging, such as covering and suctioning off the nozzle surface of the ink print head, ejecting ink through all nozzles, wiping the nozzle surface with a wiping lip and supplying cleaning agent to the nozzle surface, see DE 38 10 698 C2 and EP 0 285 155 A1.
All these measures have in common that they only take place during the printing pauses, the spacing of which can be far apart in time for long-term printers such as franking machines. As a result, constipation is likely.

An ink jet printer with several nozzles is known, compare DE 33 11 735 A1, in which detection is carried out using a determination device, when one or more nozzles are for a predetermined Time period is in an unused state, and with the a control device evaluates the result of the determination and for ink is ejected from the applicable nozzles. Different said, with a signal measuring device, the printing pause period is measured and with a flushing device before Print rinsing the nozzles used for printing brought about when the print pause is a predetermined period of time exceeds. To be reasonably certain that the nozzles too printing pauses have to be relatively short must be closed for cleaning purposes interrupt, reducing the printer's uptime and the ink consumption by this cleaning principle is essential is increased.

A cover for the nozzles of a vacuum ink print head working with fast-drying inks is also known, compare EP 0 173 939 A1. The cover is intended on the one hand to prevent the ink from drying out at the nozzle ends during the printing pauses, and on the other hand to ensure that the ink meniscus is neither touched nor pushed back.
For this purpose, the cover has a circumferential sealing element and a membrane, which form a pressure chamber when the cover is placed on the nozzle surface. The inherent tension of the membrane is chosen to be smaller than the surface tension of the ink menisci. In this relatively small pressure chamber, the air quickly becomes saturated with moisture as a result of the ink solvents initially evaporating, so that drying of the nozzles is avoided over shorter printing pauses. However, this arrangement does not offer protection against the drying of nozzles that are not used for a long time during operation.

Another known device, compare DE 38 25 045 A1 and DE 38 25 046 A1, for cleaning the nozzle surface of an ink print head has a wiping element which is movably arranged in front of the nozzle surface and, in addition to the ink nozzles, a nozzle from which cleaning fluid is ejected against a deflector orifice and from it is redirected to the nozzle surface. The wiping element consists of a belt which contains a plurality of cutouts lying next to one another in the belt direction and which is transported past the nozzle surface in one direction. Depending on the position of the belt, the nozzles are released - cutout - or covered. The tape is designed as an endless tape and contained in a tape cassette with a drive. During printing, the tape lies with one of its cutouts in front of the nozzle openings and releases them for ink ejection. When changing from printing to printing pause, cleaning liquid is first discharged dropwise through the cleaning nozzle. The belt is then moved so far in its transport direction that, instead of the cutout, the following belt section comes in front of the nozzle surface and covers it. The edge of the cutout wipes over the nozzle surface and cleans it of any accumulated dirt.
As is evident, this device is only used for rough cleaning of the nozzle surface. A smearing of the nozzle openings when the tape slides past is not excluded, nor is it possible for nozzles to dry out.

• eine impulsbetätigte Druckzelle,
• eine druckmäßig mit der Druckzelle gekoppelte Tintenzelle, die über einen Tintenkanal an eine Tintenpatrone angeschlossen ist,
• eine Luftzelle, die durch eine Wand mit einer ersten Öffnung von der Tintenzelle getrennt ist und die durch eine Außenöffnung mit der Atmosphäre verbunden ist,
• eine Pumpe, die im Betrieb über eine Luftleitung mit der Luftzelle verbunden ist und die die Luftleitung bei Nichtbetrieb unter Unterdruck setzt,
• eine Abdeckvorrichtung, die vor die Außenöffnung positionierbar ist,
• ein in der Luftleitung enthaltenes Ventil, daß bei nichtdruckendem Betrieb den Druck in der Luftleitung entlastet und zur Vorbereitung des Druckbetriebes den Druck so weit erhöht, daß die in der Luftzelle enthaltene Tinte durch die Außenöffnung hinausgedrückt wird,
• einen in zwei Positionen verstellbaren Auffangbehälter für die aus der Luftzelle gedrückte Tinte,
• einen Tintenstandsensor in der Luftleitung, der die Zufuhr von Tinte abschaltet, wenn er vom Tintenstand erreicht wird.

Obwohl es von Vorteil ist, einen Teil der Reinigungsvorrichtung im Tintendruckkopf zu integrieren, hat diese Lösung noch einige Nachteile. So erfolgt auch hier ein Schutz gegen Eintrocknen und Verstopfen ausschließlich während der Druckpausen. Der konzentrische Aufbau zwecks Führung des Luftstromes ist auf einen Tintendruckkopf mit nur einer Düse abgestimmt; demzufolge auf einen Tintendruckkopf mit vielen Düsen nicht übertragbar. Da die Drucktinte auch zum Feuchthalten in den Druckpausen verwendet und dann jedesmal vor Druckbeginn in einen äußeren Aufnahmebehälter gedrückt wird, ist diese Prozedur zeitaufwendig und außerdem geht jedesmal Tinte verloren; auch der Tintenmeniskus kann unerwünscht weit zurückgedrückt werden.
Der mechanische Aufwand - sowohl ein Abdeckkörper als auch ein Aufnahmebehälter verstellbar außerhalb des Tintenkopfes, dazu eine Pumpe mit Ventil und ein Sensor innerhalb des Tintenkopfes - ist beträchtlich.In a known method, see DE 32 03 014 C2 of this document shows the preamble of claim 1, to prevent the nozzle of an ink printer from becoming blocked, an air cell is placed in front of the nozzle opening, which in turn has an outer opening aligned with the nozzle opening through which the ink drops open fly the record carrier.
In operation, the ink jet is accompanied by an air jet from the air cell which surrounds the ink jet in a ring and is thus accelerated. When printing is paused, the outer opening of the air cell is closed by means of a porous cover body in order to keep the nozzle moist and the moisture in the air cell is increased by injecting ink into the air cell while the air cell is being vented.
To initiate printing, the ink in the air cell is flushed out of it through the outer opening by increased air pressure. For this purpose, the cover body is pivoted away and a receptacle for the ink is brought in front of the outer opening. After the air cell has been emptied, the outside opening is blown free of ink by the inflowing air. The receptacle is removed and the record carrier is brought into position.
The device for performing the method comprises

• a pulse-operated pressure cell,
• an ink cell coupled in terms of pressure to the pressure cell and connected to an ink cartridge via an ink channel,
• an air cell which is separated from the ink cell by a wall with a first opening and which is connected to the atmosphere by an outer opening,
• a pump which is connected to the air cell via an air line during operation and which pressurizes the air line when not in operation,
• a covering device that can be positioned in front of the outer opening,
• a valve contained in the air line that relieves the pressure in the air line during non-printing operation and increases the pressure in preparation for the printing operation to such an extent that the ink contained in the air cell is pushed out through the outer opening,
• a two-position adjustable container for the ink squeezed out of the air cell,
• an ink level sensor in the air line that shuts off the supply of ink when it is reached by the ink level.

Although it is advantageous to integrate part of the cleaning device in the ink print head, this solution still has some disadvantages. Protection against drying out and clogging is only provided during printing breaks. The concentric structure for guiding the air flow is matched to an ink print head with only one nozzle; consequently not transferable to an ink print head with many nozzles. Since the printing ink is also used to keep it moist during the printing pauses and is then pressed into an outer receptacle each time before printing begins, this procedure is time-consuming and, moreover, ink is lost every time; the ink meniscus can also be pushed back undesirably.
The mechanical effort - both a cover body and a receptacle adjustable outside the ink head, plus a pump with valve and a sensor inside the ink head - is considerable.

With an arrangement according to JP-A-63236654 this document also shows the preamble of claim 1 to keep a Ink printhead with a plurality of nozzles is in front of the nozzle surface an anteroom provided by a front wall of the housing the ink printhead is complete. Are in the front wall there are openings opposite the nozzles that are larger than that Diameter of a nozzle opening. In this way, the expelled Ink droplets pass freely through the front wall. In the Anteroom is enriched with moisture via an external connection Air is blown in to accelerate the ink droplets. Whether the airflow actually works in the desired direction is questionable.

The situation is similar for a solution according to GB-A-2280149, this document also shows the preamble of claim 1. The purpose of the invention is to increase the reliability of Ink printheads of the type described at the beginning with little Expenditure.

The invention has for its object to find a solution with of all the nozzles of an ink print head both when printing and are protected against dehydration even during printing breaks. About that In addition, the nozzles should largely prevent contamination from paper dust be preserved and cleaning when deposits of ink remain be possible on the nozzle surface.

According to the invention, this object is achieved according to the main claim. Further advantageous features of the invention can be found in the subclaims.
The proposed solution offers a number of advantages.

The microclimate of solvent vapors during printing and mostly also protects during printing breaks all nozzles before drying. The vestibule has only a small volume, because the distance between the nozzle surface and the recording medium in the Rule must not exceed 2 mm; consequently, no essential one occurs Solvent consumption and therefore no additional environmental pollution on. The solvent is usually colorless, so that Shadow formation due to slightly escaping solvent vapors be prevented on the record carrier. If in the solvent enough volatile components are contained, then the ink printhead temperature is already sufficient for evaporation. Otherwise, the solvent becomes additional when entering the anteroom warmed up. Due to the evaporation will be in the anteroom always a slightly higher pressure compared to the outside atmosphere to adjust. This overpressure prevents dust from entering the anteroom and so that it reaches the nozzle surface. If there is ink residue over time have accumulated in the area of the nozzle openings, these will eliminated by rinsing with the solvent. For this purpose the vestibule closed, rinsed and the detergent sucked off and filtered. Appropriately, as usual, the vestibule closed during longer pauses in printing.

The solution according to the invention is suitable both for ink printheads with piezo actuators and for ink printheads using bubble jet technology. In both cases it is possible to use parts of the ink printhead to implement the solution.
The vestibule is formed by a corresponding arrangement of the ink print head in its housing, which is required anyway, in that the nozzle surface is set back somewhat in relation to the front wall of the housing. If the ink print head has rows of nozzles, adapted openings are provided in the front wall, the width b of which is greater than the diameter d of a nozzle opening. The openings are expediently designed as elongated slots. But it can also be holes. Slots would be easier to make and clean. The vestibule is connected to a container for solvents for the ink via at least one nozzle and associated channel.
An area at the edge of the anteroom is trough-shaped. The solvent for the formation of the solvent vapors is admitted into this area via at least one nozzle. This nozzle is designed in the same way as the ink nozzles and also has the same drive - pressure chamber with actuator. In this way, the solvent can be supplied in the desired fine dosage. To support this effect, this part of the antechamber is designed as a capillary space. The nozzle and supply is advantageously part of the ink print head. However, it is also possible to integrate both into the housing.
The other area of the vestibule is intended to receive the solvent vapors and covers the entire area in front of the ink nozzles.
The trough-shaped area is connected to the rest of the antechamber via slots.
A sealing cap is adjustably mounted adjacent to the housing so that it can be non-positively arranged in front of the front wall. The sequence of movements is designed so that a wiper lip coupled to the sealing cap is wiped over the openings / slots. Dust deposits are wiped off on the front wall. A suction plate is interchangeably arranged in the sealing cap, with which the openings are tightly sealed. The suction plate can be made of foam or felt, so that liquid residues are sucked up from the openings during flushing operations in the anteroom and dripping is avoided. The housing is usually closed with the sealing cap during longer pauses in printing.
In order to be able to carry out cleaning or rinsing processes on the ink print head quickly, at least one nozzle and its associated channel to the container with the solvent are larger than the other nozzles and their channels.
A valve is arranged in the channel to prevent unwanted solvent leakage. A pump, which is inserted in the solvent channel, is actuated for the rinsing process. The detergent flows back into the solvent container through an exchangeable filter. In order to ensure a good distribution of the solvent vapors in the anteroom and an effective flushing, the distance between the inside, front wall and nozzle surface is so large that a capillary effect is excluded.
A heating device is arranged to support the evaporation in the trough-shaped area when the operating temperature of the ink print head is not sufficient to generate sufficient solvent vapors.
It is also possible to arrange the heater around the nozzle. This heating device is particularly advantageous if a bubble jet ink print head is used, the ink of which is mixed together in a known manner on a water basis.

Fig. 1

Eine perspektivische Ansicht eines Tintendruckkopfes mit Gehäuse; im Frontbereich teilweise geschnitten,

Fig. 2

eine perspektivische Ansicht eines Tintendruckkopfes mit Gehäuse und aufgesetzter Dichtkappe; stufenweise geschnitten,

Fig. 3

einen Längsschnitt durch einen Tintendruckkopf entlang einer Düsenreihe mit gleichen Düsen,

Fig. 4

einen Längsschnitt durch einen Tintendruckkopf mit aufgesetzter Dichtkappe entlang einer Düsenreihe mit unterschiedlichen großen Düsen.

The invention is explained in more detail below using the exemplary embodiment.
Show it:

Fig. 1

A perspective view of an ink printhead with a housing; partially cut in the front area,

Fig. 2

a perspective view of an ink printhead with housing and attached sealing cap; gradually cut,

Fig. 3

a longitudinal section through an ink print head along a row of nozzles with the same nozzles,

Fig. 4

a longitudinal section through an ink print head with a sealing cap attached along a row of nozzles with different sized nozzles.

The figures are for ease of illustration and understanding drawn schematically.

1, an ink print head 2 is mounted in a housing 1. The ink print head 2 has four rows of nozzles 22 with nozzles 221 for ink 31 and nozzles 26, 261 for solvent 41, see also FIG. 3. A heating device 123 is used to generate solvent vapors 411. Slot-shaped openings 111 are formed in the front wall 11 of the housing 1. The slots 111 are aligned with the nozzle rows 22 in the ink flight direction. The width b of a slot 111 is so much larger than the diameter d of a nozzle 221, 26 that the ink drops can pass through the slot 111 unhindered.
The diameter e of an enlarged nozzle 261 for solvent 41 is dimensioned such that the vestibule 12 is filled with solvent 41 and rinsed through sufficiently quickly. The vestibule 12 is formed by the volume between the front wall 11 and the nozzle surface 21.

2, the ink 31 is fed from a container 3 via a common ink supply channel 251 to an individual ink supply channel 25 and from there into an ink pressure chamber 24. The ink pressure chamber 24 is driven by actuators, not shown. In the case of a piezo ink print head, these are piezo converters or actuators. In the case of a bubble jet print head, these are heating resistors in the form of emitter resistors in transistor driver circuits. The ink 31 passes from the ink pressure chamber 24 through a nozzle channel 23 to the nozzle or nozzle opening 221 and from there onto a recording medium.
The solvent 41 is passed from a container 4 via a solvent supply channel 29 to a solvent pressure chamber 28 and from there via a nozzle channel 27 to a nozzle 26 for the purpose of generating solvent vapors 411, see also FIG. 3.
The solvent pressure chamber 28 is driven in the same way as the ink pressure chamber 24. In principle, flushing is also possible in this way, with a correspondingly greater actuation energy and longer filling time for the vestibule 12, but not effectively.

For this purpose, there is a further connection from the container 4 through an enlarged solvent channel 291 via a pump 5 to an enlarged nozzle opening 261 in the antechamber 12. The antechamber 12 is covered during the flushing process by means of a sealing cap 8 placed on the front wall 11, see also FIG 4.
The sealing cap 8 is non-positively seated on the front surface of the front wall 11 with a circumferential wiper lip 81. A suction plate 82 is interchangeably fastened in the sealing cap 8, in which escaping solvent 41 is bound. The suction plate 82 can consist of a fleece or sponge. The backflow of the solvent 41 used for rinsing takes place via the nozzles 262 into the solvent channel 292, through an exchangeable filter 6 and through a valve 7 into the container 4, cf. FIG. 4.
The ink residues contained in the solvent 41 are bound in the filter 6. The valve 7 and the pump 5 prevent solvent 41 from getting into the antechamber 12 during this printing operation.

3 and 4, the vestibule 12 consists of a trough-shaped area 122 located in the edge area and an area 121 in front of the ink nozzles 221. The areas 121 and 122 are connected to one another via capillary slots 124, from which the solvent vapors are made from area 122 to area 121. The area 122 is designed as a circumferential capillary space. The heating device 123 is also located in this.
The containers 3 for ink 31 and containers 4 for solvents 4, which are shown schematically in the ink print head 2, are kept relatively small and are connected to correspondingly large outer containers in a manner not shown by suitable connections

Reference symbols used

1

Housing for ink printhead

11

Front wall of the housing

111

slot-shaped opening (s) in the front wall 11, slots

12th

Anteroom

121

Area of vestibule 12 for solvent vapors 411

122

trough-shaped area of the vestibule 12 for solvent 41, Capillary space

123

Heater

124

Slot from area 122 to area 121

2nd

Ink printhead

21

Nozzle area of the ink print head 2

22

Row of nozzles in the ink printhead 2

221

Nozzles or nozzle openings for ink 31

23

Nozzle channels to the ink pressure chambers 24

24th

Ink pressure chamber

25th

Ink supply channel

251

common ink supply channel

26

Nozzles or nozzle openings for solvents 41

261

enlarged nozzle or nozzle opening for solvent 41, inflowing

262

enlarged nozzle or nozzle opening for solvent 41, draining away

27

Nozzle channels to the solvent pressure chamber 28

28

Solvent pressure chamber

29

Solvent supply channel

291

enlarged solvent supply channel, flowing in

292

enlarged solvent channel, draining

3rd

Ink containers

31

ink

4th

Containers for solvents

41

Ink solvent

411

Solvent vapors

5

pump

6

filter

7

Valve in channel 262

8th

Sealing cap

81

Wiper lip

82

Suction plate

b

Width of the slots 111

d

Diameter of the nozzles 221, 26

e

Diameter of the enlarged nozzles 261, 262

Claims (7)

1. An arrangement for keeping clean the nozzles of an ink print head, in particular of an ink print head with a large number of nozzles having, in front of its nozzle area, an anteroom that is closed by a front wall of the ink print head housing in which openings are provided in front of the nozzles that are bigger than the diameter of a nozzle opening,
   characterized by the following features:

   the anteroom (12) is connected by at least one nozzle (26, 261, 262) and the associated channels (27, 29, 291, 292) with a receptacle (4) for solvent (41) for the ink (31), at least one nozzle (261, 262) and the channel (291, 292) assigned to it being bigger than the other nozzles (221, 26) and their respective channels (23,27),
   said nozzle (26, 261, 262) being closable by means of a valve (7),
   that a pump (5) being arranged in at least one channel (27, 291, 292), that a replaceable filter (6) being arranged in at least one channel (292);

   the openings (111) in the front wall (11) have the form of oblong slots, one slot being provided for each line of nozzles (22) of the ink print head (2) composed of ink print modules;

   the anteroom (12) has a trough-shaped area (122) for the solvent (41) and an area (121) for the solvent vapours (411), the trough-shaped area (122) being formed as a capillary space with capillary slits (124) to the area (121) for the solvent vapours and being provided with a heating device and the area (121) for the solvent vapours being dimensioned in such manner that a capillary effect is excluded;

   a sealing cap (8) for the front wall (11) with the openings (111) is adjustably provided adjacent to the housing (1).
2. An arrangement according to Claim 1, characterized in that the nozzle (26, 261, 261) and the associated channel (27, 291, 292) for the solvent (41) are integrated in the ink print head (2).
3. An arrangement according to Claim 1, characterized in that the drive for at least one nozzle (26) is formed analogously to the drive for the ink nozzles (221).
4. An arrangement according to Claim 1, characterized in that the nozzle (26, 261, 261) and the associated channel (27, 291, 292) for the solvent (41) are formed into the housing (1).
5. An arrangement according to Claim 1, characterized in that a heating device (123) is arranged in the area of nozzle (262).
6. An arrangement according to Claim 1, characterized in that the sealing cap (8) is provided with a wiping lip (81).
7. An arrangement according to Claim 1, characterized in that a replaceable suction plate (82) is provided in the sealing cap (8).

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