Circulation apparatus for a liquid mixture of substances in a container

Abstract

The invention relates to an apparatus for circulating a liquid mixture of substances, for example a solution, emulsion or suspension, between a container and a pressure-sensitive component which is supplied from the container, for example for circulating an ink between an ink storage container and a print head of a printing system, comprising a pump, which allows the liquid mixture of substances to circulate in the container through an external conduit circuit, a Venturi tube, the main conduit of which is connected into the external conduit circuit, and also a conduit which connects the output connection of the pressure-sensitive component to the suction connection of the Venturi tube.

Description

[0001] The invention relates to a device for circulating a liquid mixture stored in a container, for example a solution, emulsion or suspension, comprising a circulation pump that circulates the liquid mixture in the container through an external circuit, so that separation within the container is avoided, even if no mechanical stirring device is arranged in it.

[0002] US8833915B2 describes a device according to the preamble of claim 1.

[0003] Various liquids, especially liquid mixtures, including inks for digital printing, are sometimes unstable, and pigments, dissolved substances, etc. can settle if the liquid is not kept in constant motion – not only in a storage container, e.g., an ink tank, but also in a processing unit, e.g., in a printhead. In state-of-the-art technology, for example in laboratories, stirrers are used to prevent the separation of such liquid mixtures.

[0004] Ink tanks also contain such integrated stirrers, which keep the ink constantly moving, as shown in the attached [document / image]. Fig. 2The figure illustrates a typical state of the art: In a printing system 21, ink 25 is supplied to a printhead 24 from an ink reservoir 22 via a line 23. To prevent the ink 25 from separating in the ink reservoir 22, a stirrer 26 is provided, the shaft 27 of which extends through the housing 28 of the ink reservoir 22 to the outside. However, such a stirrer 26 is complex, as the ink reservoir 22 is usually required to be airtight from the atmosphere, while the mechanics or electrical components of the stirrer 26 – i.e., the shaft 27 or cables, etc. – must extend from the ink reservoir 22. Circulation of the ink 25 through the printhead 24 is not provided for in this design.

[0005] Therefore, magnetic stirrers were already incorporated into the JP H05-185600; these are magnetic elements inside an ink tank that react to an externally applied magnetic field, rotate, and thus mix the ink. However, this still requires removing the magnetic element and inserting it into a new ink reservoir every time the reservoir is changed, which is extremely cumbersome.

[0006] DE 600 11 928 T2 describes a different method; here, stirrers within an ink reservoir are completely dispensed with, and instead, ink circulation is provided, wherein the first of two pipes leading into the reservoir is connected to the suction port of a pump, whose pressure port is connected to the second pipe. This keeps the ink constantly in circulation and thus prevents segregation, eliminating the need for a stirrer.

[0007] However, none of these documents addresses the further problem of how to maintain a uniform, constant pressure within a pressure-sensitive component, especially within a printhead, so that the printhead always prints with the same quality.

[0008] Currently available printing systems attempt to achieve this either by installing an actively operated pump device within the inlet line of an inkhead or an actively operated suction device in the outlet line. However, this technique has not proven successful because it is technically almost impossible to make pump or suction devices capable of maintaining a truly constant pressure differential, i.e., one free from pressure fluctuations or waves, over a long period of time.

[0009] On the other hand, attempts have also been made to use two ink reservoirs operating at different internal pressures, so that this pressure difference creates an ink flow. However, this requires a constantly active pressure control system to compensate for the increasing empty volume above the liquid in the reservoir with the higher internal pressure, due to ink consumption, and the correspondingly decreasing empty volume in the other reservoir.

[0010] There is also a variant where two ink reservoirs are located at different heights or have different fill levels, with the pressure difference being caused by these different levels. However, this requires a constantly active fill level control system to compensate for the decreasing liquid volume in the upstream reservoir due to ink consumption and the correspondingly increasing liquid volume in the other reservoir.

[0011] Furthermore, in both of these cases, two ink tanks per printhead or print color are required, which increases the space requirement. Additionally, actively changing the flow rate is difficult with such a system, as fill levels and pressures in the tanks cannot be easily altered without the printhead leaking or drawing in air, etc. Finally, such systems are not easy to retrofit with non-recirculating ink tanks, because a standard non-recirculating ink tank has at most a heater and a full / empty indicator with a refill function; therefore, upgrading the printing system to a recirculating system using two tanks requires replacing everything.

[0012] The disadvantages of the described prior art result in the problem initiating the invention, namely to further develop a circulation device of the generic type in such a way that a constant, uniform pressure can always be maintained in the pressure-sensitive component - for example in a print head.

[0013] The solution to this problem in a circulation device of this type is achieved by a Venturi tube, the main line of which is connected to the external circuit; and a line which connects an output port of the pressure-sensitive component to the suction port of the Venturi tube.

[0014] With such an arrangement, it is possible to circulate liquid, especially ink, through a pressure-sensitive component, particularly a printhead. In such a printing system, the ink essentially flows past the nozzles and is kept in constant motion to prevent pigment settling, etc. At the same time, nozzle blockages caused by air bubbles can be prevented or rectified because the small bubbles are carried along by the circulation.

[0015] In the current state of the art, there are different printheads; for example, those that require a flow rate of 1-5 ml / min; others require a flow rate of 100-300 ml / min.

[0016] The circulation through the printhead should be as pulseless as possible, meaning a continuous ink flow. Pump or pressure pulses, due to the inertia of the ink, would cause the printhead to drip or draw in air, resulting in improper printing. Therefore, a virtually pulseless pump is required, such as a diaphragm or propeller pump, along with a Venturi tube and suction port. This pump circulates the ink from the ink tank back into the tank, keeping the ink constantly moving and eliminating the need for a stirrer.

[0017] At the point where the ink is pumped from the pump back into the tank, a Venturi tube is connected to the printhead's circulation outlet. Through this Venturi tube, ink is continuously and almost pulselessly drawn through the printhead and collected in a container; preferably the same container that supplies the printhead with ink. Any desired circulation rate can be achieved by adjusting the pump's flow rate and / or by using different Venturi tubes and / or flow restrictors.

[0018] The arrangement according to the invention may make it possible to dispense with an active vacuum or a vacuum control in the ink reservoir, provided that the suction pressure of the Venturi tube is sufficient for the printhead, especially if the ink reservoir is arranged above the printhead.

[0019] The system according to the invention is very small, can be retrofitted at any time, is easy to control and implement, and requires virtually no monitoring.

[0020] It is irrelevant whether the liquid circulates in a small container or is returned to a larger one, etc.

[0021] Of course, the container may also contain a degassing unit, and / or a filter, and / or a heater, etc.

[0022] It has proven advantageous that the external circuit is not closed via the pressure-sensitive component, but is closed directly, thus essentially forming a bypass to the pressure-sensitive component. This completely decouples the mixing process within the storage and feed container from the operation of the pressure-sensitive component – ​​in particular, a printhead.

[0023] For this very reason, the circuit between the storage and / or feed container and the pressure-sensitive component should not be closed via the circulation pump, but should instead represent a bypass to the circulation pump.

[0024] Furthermore, it has proven effective to connect the main line of the Venturi tube to the circulation pump's circuit downstream of the pump. This means that the two circuits—the circulation pump's circuit on the one hand and the pressure-sensitive component's circuit on the other—only merge within the Venturi tube and then flow together back to the tank. Apart from this merging of flows, the two flows are largely decoupled, so that any pressure fluctuations potentially caused by the circulation pump, despite all countermeasures, cannot reach the pressure-sensitive component.

[0025] The pressure-sensitive component is fed directly from the container at its inlet connection; ideally, no pump device is required, but a simple pipe connection suffices; even a pressure gradient - such as could be achieved by raising the container relative to the printhead - is not necessary.

[0026] Rather, the pressure gradient required for the formation of a flow is created by the Venturi tube, which generates a vacuum at its suction port. In other words, the flow within the closed circuit via the pressure-sensitive component is maintained solely by a pressure gradient resulting from the suction power of the Venturi tube.

[0027] By connecting a return line from the pressure-sensitive component to the container via the Venturi tube, the circuit is closed through the pressure-sensitive element, and unused liquid returns to the container where it is immediately available for subsequent use.

[0028] If the closed circuit via the pressure-sensitive component is free of pumps, an optimally uniform pressure gradient results across the pressure-sensitive component, so that it finds optimal operating conditions.

[0029] A check valve can be installed in the suction line between the outlet port of the pressure-sensitive component and the suction port of the Venturi tube, preventing backflow to the outlet port of the pressure-sensitive component. This completely eliminates flow reversal within the pressure-sensitive component, even if the circulation pump should fail.

[0030] Furthermore, it is possible to install a reducing device in the suction line between the outlet port of the pressure-sensitive component and the suction port of the Venturi tube to limit the flow rate from the outlet port of the pressure-sensitive component. This allows the circulation rate to be individually adjusted to the requirements of the pressure-sensitive component.

[0031] Preferably, such a reducing device is designed as an adjustable reducing valve so that the circulation speed can be readjusted at any time by the pressure-sensitive component.

[0032] It is within the scope of the invention that the inlet and outlet pipes open onto the storage or feed container in the area of ​​its housing base, so that circulation can be maintained as long as there is still enough liquid in the container to prevent the outlet pipe opening from running dry.

[0033] Since no stirrer is required in the reservoir, the reservoir can be kept free of moving parts inside. Therefore, the ink reservoir can also be designed as a replaceable, reusable or disposable container and can be quickly replaced at any time, so that the printing process only needs to be interrupted briefly.

[0034] Furthermore, the invention recommends using a continuously operating pump as the circulation pump, for example a diaphragm pump or a propeller pump. The fewer pressure fluctuations such a pump generates, the less the operation of the pressure-sensitive assembly is affected.

[0035] Finally, it is in accordance with the teaching of the invention that the container is an ink reservoir, and / or that the pressure-sensitive component is an ink printhead. The invention can be used to particular advantage in printing systems because a high degree of consistency in the pressure gradient across the ink printhead is usually required there to achieve optimal printing results.

[0036] Further features, details, advantages and effects based on the invention will become apparent from the following description of a preferred embodiment of the invention and from the drawing. The drawing shows: Fig. 1 shows a schematic piping plan of an inkjet printing system according to the invention; and Fig. 2 shows an inkjet printing system according to the prior art.

[0037] In the context of a printing system 1 according to the invention, an ink reservoir 2 is provided which supplies a printhead 4 with ink 5 via a line 3.

[0038] However, the ink reservoir 2 does not contain an agitator. Instead, separation of the ink 5 is prevented by keeping it constantly in motion via an external circuit 6. This is achieved by a circulation pump 7, which is connected to the ink reservoir 2 via its suction line 8 and its pressure line 9, 10, so that the ink 5 circulates through the external circuit 6.

[0039] In the pressure line 9, 10 of the circulation pump 7, i.e. downstream of it, a Venturi tube 11 is connected in such a way that it is flowed through by the ink 5 circulating in the circuit 6.

[0040] The Venturi tube 11 consists of a smooth-walled tube section whose inner lumen narrows at a point in its cross-section, for example to 90% of the normal inner tube cross-section or less, preferably to 80% of the normal inner tube cross-section or less, more preferably to 60% of the normal inner tube cross-section or less, in particular to 40% of the normal inner tube cross-section or less, or even to 30% of the normal inner tube cross-section or less. Even greater constrictions are conceivable, for example to 25% of the normal inner tube cross-section or less, more preferably to 20% of the normal inner tube cross-section or less, more preferably to 15% of the normal inner tube cross-section or less, in particular to 10% of the normal inner tube cross-section or less.

[0041] The narrowing of the cross-section of the Venturi tube 11 can be achieved, for example, by two opposing, conical tube sections 12, 13 that taper from their peripheral ends to their mutually facing ends and are connected at their point of smallest diameter. A third, lateral connection 14 is provided at this point, at which a vacuum is present during operation of the Venturi nozzle 11, and which is therefore to be referred to as the suction connection 14.

[0042] Preferably, the Venturi tube 11 has no further openings besides the peripheral openings of its two tube sections 12, 13 and the suction port 14, and is therefore independent of the surrounding atmospheric pressure in the interior.

[0043] The Venturi tube 11 should be made of a solid material, such as metal or a solid, structurally stable plastic, so that it does not deform under the influence of a pressure difference between the outside and inside.

[0044] Furthermore, the jacket of the Venturi tube 11 should be both liquid-tight to prevent ink from leaking out and gas-tight so that neither air nor any other gas diffuses into the interior in the event of an internal negative pressure.

[0045] Another special feature of the invention is that the central, tapered section of the Venturi tube 11 should preferably be at the same level as the two tube sections 12, 13. This can be achieved, for example, by aligning the Venturi tube 11 horizontally as shown in Fig. 1 reproduced.

[0046] The peripheral ends of the conical pipe sections 12, 13 are connected to the pressure line 9, 10 of the circulation pump 7 and form the main line of the Venturi nozzle or the Venturi tube 11. The suction port 14 of the Venturi tube is connected via a suction line 15 to the outlet-side port 16 of the printhead 4, so that the negative pressure compared to the normal operating pressure at the inlet-side port 17 of the printhead 4 causes a pressure difference, which results in a flow.

[0047] In order to adjust this pressure differential precisely to the value required by the printhead 4 in question, a pressure-reducing device can be installed in the suction line 15. This can be an adjustable pressure-reducing valve 18.

[0048] Furthermore, the invention provides that a check valve 19 is installed in the suction line 15 to prevent backflow of ink into the suction line 15, even if the circulation pump 7 is switched off or fails.

[0049] Since the pressure difference p Δ above the printhead 4 is equal to the height of the liquid level 20 of the ink 5 in the ink reservoir and / or feed container 2, this container 2 can be operated without refilling as long as the lines 3, 8 do not run dry.

[0050] For the Venturi tube, 11 the Bernoulli equation applies, where the index 1 refers to the parameters in the inlet connection or pipe section 12, and the index 2 to the parameters in the suction connection 14: p 1 − p 2 + ρ * g * h 1 − h 2 = 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 , where p is the pressure at the respective location, h is the height at the respective location, A is the clear flow cross-section at the respective location, v is the flow velocity at the respective location, p is the density of the ink and g is the acceleration due to gravity, or: p 2 − p 1 = ρ * g * h 1 − h 2 − 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 ; p 2 = p 1 + ρ * g * h 1 − h 2 − 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 .

[0051] If the height difference (h 1 - h 2 ) is negligibly small - that is, in particular, if the Venturi tube 11 is horizontally oriented or if its dimensions are small - the formula simplifies to: p 2 = p 1 − 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 .

[0052] In the following, it will be assumed that the density ρ of the ink is approximately 1 kg / dm 3<.

[0053] Furthermore, it can be assumed that the pressure p1; when the circulation pump 7 is running, is greater than the weight pressure pt at the bottom of the ink reservoir 2 and also greater than the surrounding atmospheric pressure pa. In particular, with the pressure increase pz resulting from the power of the circulation pump 7: p 1 = p t + ρ * g * h t − h 1 + p z ,

[0054] The weight pressure pd at the inlet-side port 17 of the printhead 4 depends on the height hd of the inlet-side port 17 of the printhead 4 in relation to the height ht of the outlet at the bottom of the ink reservoir 2: p d = p t + ρ * g * h t − h d .

[0055] However, a vacuum p 2 should be present at the suction port 14, such that a vacuum p Δ < 0 prevails at the outflow-side port 16 of the printhead 4 compared to the weight pressure pd at the inlet-side port 17 of the printhead 4: p 2 = p d + p Δ = p t + ρ * g * h t − h d + p Δ , and reduced by more than the difference ρ * g * (h 2 - hd ) : p 2 + ρ * g * h 2 − h d = p d + p Δ = p t + ρ * g * h t − h d + p Δ , dh: p 2 + ρ * g * h 2 − h d = p t + ρ * g * h t − h d + p Δ , p 2 = p t + ρ * g * h t − h d − ρ * g * h 2 − h d + p Δ , p 2 = p t + ρ * g * h t − h 2 + p Δ . p 2 = p 1 − 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 = p t + ρ * g * h t − h 2 + p Δ . p 2 = p t + ρ * g * h t − h 1 + p z − 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 = = p t + ρ * g * h t − h 2 + p Δ .

[0056] If h 1 ≈ h 2, then: p 2 ≈ p t + ρ * g * h t − h 2 + p z − 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 = = p t + ρ * g * h t − h 2 + p Δ . p z − 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 = p Δ . 1 2 * ρ * v 2 2 * 1 − A 2 / A 1 2 = p z − p Δ . v 2 2 * 1 − A 2 / A 1 2 = 2 * p z + p Δ / ρ . v 1 2 * A 1 / A 2 2 * 1 − A 2 / A 1 2 = p z + p Δ * 2 / ρ . v 1 2 * A 1 / A 2 2 − 1 = p z + p Δ * 2 / ρ . A 1 / A 2 2 − 1 = p z + p Δ * 2 / ρ * v 1 2 .

[0057] Accordingly, the cross-sectional ratio (A 1 / A 2 ) between the inlet 12 of the Venturi tube 11 and its suction port 14 can be determined according to the above formula from the pump pressure pz of the circulation pump 7, the desired pressure difference p Δ across the print head 4 and the flow velocity v 1 in the inlet 12, taking into account the density ρ of the ink.

[0058] Various modifications of the arrangement according to the invention are possible. In particular, different valves and / or cross-sectional reductions can each be inserted at a different point in the relevant circuit. Furthermore, according to this principle, several printheads of a printing system can be connected to a common Venturi tube, or each printhead can be connected to its own Venturi tube. Reference symbol list 1 Printing system 26 Stirrer 2 ink reservoir 27 Wave 3 Line 28 Housing 4 printhead 5 ink 6 circulatory system 7 circulation pump 8 Suction line 9 Pressure line 10 Pressure line 11 Venturi tube 12 inlet-side pipe section 13 downstream pipe section 14 Suction connection 15 Suction line 16 downstream connection 17 Inlet-side connection 18 Reducing valve 19 non-return valve 20 liquid level 21 Printing system 22 ink reservoir 23 Line 24 printhead 25 ink

Claims

1. An apparatus for circulating a liquid mixture of substances (5) that is stocked in a container (2), for example a solution, emulsion or suspension, comprising a) a circulation pump (7), which allows the liquid mixture of substances (5) to circulate in the container (2) through an external conduit circuit (6), so that a separation is prevented within the container (2), even if a mechanical stirring device is not arranged in said container, b) a Venturi tube (11), the main conduit of which (12, 13) is connected into the external conduit circuit (6), as well as c) a line (15) which connects an output connection (16) of the pressure-sensitive component (4) to the suction connection (14) of the Venturi tube (11), characterized in that the circuit closed via the pressure-sensitive component (4) is free of pumps.

2. The apparatus according to claim 1, characterized in that the external conduit circuit (6) is not closed via the pressure-sensitive component (4), rather is closed directly, i.e., constitutes virtually a bypass to the pressure-sensitive component (4).

3. The apparatus according to claim 1 or 2, characterized in that the conduit circuit between the container (2) and the pressure-sensitive component (4) is not closed via the circulation pump (7), but constitutes a bypass to the circulation pump (7).

4. The apparatus according to one of claims 1 to 3, characterized in that the main conduit (12, 13) of the Venturi tube (11) is connected downstream of the circulation pump (7) into the external conduit circuit (6), in particular into the pressure conduit (9, 10) of the circulation pump (7).

5. The apparatus according to one of the preceding claims, characterized in that the pressure-sensitive component (4) is supplied at the input connection (17) thereof from the container (2).

6. The apparatus according to one of the preceding claims, characterized in that a return line from the pressure-sensitive component (4) to the container (2) is switched via the Venturi tube (11).

7. The apparatus according to one of the preceding claims, characterized in that the flow within the circuit closed via the pressure-sensitive component (4) is maintained only via a pressure gradient, which can be attributed to the suction power of the Venturi tube (11).

8. The apparatus according to one of the preceding claims, characterized in that a non-return valve (19) is provided in the suction line (15) between the output connection (16) of the pressure-sensitive component (4) and the suction connection (14) of the Venturi tube (11), which non-return valve prevents a reverse flow to the output connection (16) of the pressure-sensitive component (4).

9. The apparatus according to one of the preceding claims, characterized in that a reducing device is provided in the suction line (15) between the output connection (16) of the pressure-sensitive component (4) and the suction connection (14) of the Venturi tube (11), in order to limit the flow quantity from the output connection (16) of the pressure-sensitive component (4).

10. The apparatus according to claim 9, characterized in that the reducing device is embodied as an adjustable reducing valve (18).

11. The apparatus according to one of the preceding claims, characterized in that the inlet and / or outlet line(s) (3, 8, 10) discharge(s) at the storage container (2) in the region of the housing base thereof.

12. The apparatus according to one of the preceding claims, characterized in that a stirrer is not arranged in the storage container (2), rather that the storage container (2) is free of movable elements in the interior.

13. The apparatus according to one of the preceding claims, characterized in that the circulation pump (7) is a continuously conveying pump, for example a membrane pump or propeller pump.

14. The apparatus according to one of the preceding claims, characterized in that the container (2) is an ink storage container.

15. The apparatus according to one of the preceding claims, characterized in that the pressure-sensitive component (4) is an ink print head.

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