Inkjet printer and method of controlling same

1. An inkjet printer, comprising:

an ink reservoir for holding a supply of liquid ink;

a printhead including at least one nozzle for discharging a continuous stream of ink drops towards a substrate for printing thereon, a gutter for intercepting the ink drops not to be printed, an inlet for receiving ink from said ink reservoir, and an outlet for returning to said ink reservoir the ink not printed on said substrate;

a pressure-regulated tank having an ink inlet connected to said ink reservoir for receiving ink therefrom, and an ink outlet connected to said printhead inlet for supplying pressurized ink thereto;

a bypass line between said tank outlet and said ink reservoir for directing the ink from said tank to said ink reservoir while bypassing said printhead;

and a bypass control valve in said bypass line for controlling the flow rate of ink via said bypass line from the tank outlet to said ink reservoir enabling said flow rate to be preset according to a desired value.

2. The printer according to claim 1 , wherein the printer further comprises a purgeing valve connected between said tank outlet and said bypass line and adapted to be opened in order to enable said bypass control valve to be preset or pre-calibrated while said bypass line directs ink from said tank to said ink reservoir to bypass said printhead.

3. The printer according to claim 2 , wherein said tank includes a high-level sensor and a low-level sensor defining a known volume between said sensors, such that said bypass control valve may used to preset said flow rate by measuring the elapsed time during which the ink in the tank drains from said high-level to said low-level.

4. The printer according to claim 1 , wherein the printer further comprises a pump between said ink reservoir and said tank inlet for pressurizing the ink supplied to said tank to a desired pressure, said pump being controllable to enable presetting or pre-calibrating the pump during the filling of said tank.

5. The printer according to claim 4 , wherein said tank includes a high-level sensor and a low-level sensor defining a known volume between said sensors, such that said pump may be preset or pre-calibrated by measuring the elapsed time during which the tank is filled with ink from said low-level to said high-level.

6. The printer according to claim 1 , wherein said pressure-regulated tank includes a compressed air inlet for pressurizing the tank by an air cushion over the liquid ink therein, a pressure sensor for sensing the pressure in said tank, and a controller for controlling the inletted compressed air in response to said sensed pressure.

7. The printer according to claim 1 , wherein said printhead includes a plurality of nozzles each capable of discharging a continuous stream of ink drops towards the substrate for printing thereon, and a plurality of gutters each for intercepting the ink drops not to be printed by its respective nozzle and for returning same via said printhead outlet to said ink reservoir.

8. The printer according to claim 1 , wherein said bypass control valve is a motorized needle valve.

9. The printer according to claim 1 ,

wherein said apparatus further comprises: a pump between said ink reservoir and said tank inlet for pressurizing the ink supplied to said tank to a preset pressure; and a purgeing valve connected between said tank outlet and said bypass line;

and wherein said tank includes a high-level sensor and a low-level sensor defining a known volume between said sensors, such that said pump may be used to preset or pre-calibrate the pump flow-rate by measuring the elapsed time during which the pump is operated in order to fill the tank with ink from said low-level to said high-level, and said bypass control valve may be preset or pre-calibrated by measuring the elapsed time during which the ink in the tank drains from said high-level to said low-level while said purgeing valve is open.

10. The printer according to claim 9 , wherein said pressure-regulated tank includes a compressed air inlet for pressurizing the tank by an air cushion over the liquid ink therein, a pressure sensor for sensing the pressure in said tank, and a controller for controlling the inletted compressed air in response to said sensed pressure.

11. A method of controlling an inkjet printer, comprising:

an ink reservoir for holding a supply of liquid ink;

a printhead including at least one nozzle for discharging a continuous stream of ink drops towards a substrate for printing thereon during a printing operation, a gutter for intercepting the ink drops not to be printed, an inlet for receiving ink from said ink reservoir, and an outlet for returning to said ink reservoir the ink not printed on said substrate;

and a pressure-regulated tank having an ink inlet connected to said ink reservoir for receiving ink therefrom, an outlet connected to said printhead inlet for supplying pressurized ink thereto, and a sensor for sensing a predetermined level of ink in said tank;

said method comprising:

determining a nominal flow rate of the pump for pumping the liquid ink through said printhead;

operating said pump to fill the tank to or above said predetermined level during a non-printing operation;

and controlling said pump during a printing operation to produce a flow rate slightly-below said nominal flow rate when the level of the ink in said tank is at or above said predetermined level, and a flow rate slightly-above said nominal flow rate when the level of the ink in said tank is below said predetermined level.

12. The method according to claim 11 , wherein said inkjet printer is comprises an ink reservoir for holding a supply of liquid ink;

a printhead including at least one nozzle for discharging a continuous stream of ink drops towards a substrate for printing thereon, a gutter for intercepting the ink drops not to be printed, an inlet for receiving ink from said ink reservoir, and an outlet for returning to said ink reservoir the ink not printed on said substrate;

a pressure-regulated tank having an ink inlet connected to said ink reservoir for receiving ink therefrom, and an ink outlet connected to said printhead inlet for supplying pressurized ink thereto;

a bypass line between said tank outlet and said ink reservoir for directing the ink from said tank to said ink reservoir while bypassing said printhead;

and a bypass control valve in said bypass line for controlling the flow rate of ink via said bypass line from the tank outlet to said ink reservoir enabling said flow rate to be preset according to a desired value.

13. The method according to claim 11 , wherein said slightly-below flow rate is 0.85-0.95 of the nominal flow rate.

14. The method according to claim 11 , wherein said slightly-below flow rate is about 0.9 of the nominal flow rate.

15. The method according to claim 11 , wherein said slightly-above flow rate is 1.05-1.15 of the nominal flow rate.

16. The method according to claim 11 , where said slightly-above flow rate is about 1.10 of the nominal flow rate.

17. The method according to claim 11 , wherein said inkjet printer further comprises a bypass line between said tank outlet and said ink reservoir for directing the ink from said tank to said ink reservoir while bypassing said printhead; and a bypass control valve in said bypass line;

and wherein said method further comprises presetting or pre-calibrating said bypass control valve during a draining operation of the tank.

18. The method according to claim 16 , wherein said tank includes a high-level sensor and a low-level sensor defining a known volume, such that said nominal flow rate of the pump may be measured by measuring the elapsed time during the filling of the tank from said low-level to said high-level, and said bypass control valve may be preset or pre-calibrated by measuring the elapsed time the draining of said tank from said high-level to said low-level.

19. The method according to claim 11 , wherein said printhead includes a plurality of nozzles each capable of discharging a continuous stream of ink drops towards the substrate for printing thereon, and a plurality of gutters each for intercepting the ink drops not to be printed by its respective nozzle and for returning same via said printhead ink outlet to said ink reservoir.

20. The method according to claim 11 , wherein said pressure-regulated tank includes a compressed air inlet for pressurizing the tank by an air cushion over the liquid ink therein, a pressure sensor for sensing the pressure in said tank, and a controller for controlling the inletted compressed air in response to said sensed pressure.