Printing apparatus

1. An apparatus for maintaining an inkjet printer printhead in a working state, the printer defining an array of orifices from which in use jets of ink are projected, the apparatus comprising:

means for applying a pressure pulse to ink within the printhead sufficient to discharge ink from each orifice when not in use to project ink, the pressure pulse having a relatively rapidly rising leading edge and a relatively slowly falling trailing edge.

2. An apparatus according to claim 1 , wherein the pulse has a duration from initiation of the leading edge to initiation of the trailing edge of less than 1 second.

3. An apparatus according to claim 2 , wherein the said duration is less than 300 ms.

4. An apparatus according to claim 3 , wherein the said duration is less than 100 ms.

5. An apparatus according to claim 4 , wherein the said duration is from 10 ms to 50 ms.

6. An apparatus according to claim 1 , comprising means for applying the pressure pulse after a predetermined number of printing operations have been performed by projecting ink from the orifices.

7. An apparatus according to claim 6 , wherein the predetermined number is 1.

8. An apparatus according to claim 1 , wherein the pressure applying means limits the maximum pressure applied to the ink to 300,000 Pa (3 bar).

9. An apparatus according to claim 8 , wherein the maximum pressure is limited to 100,000 Pa (1 bar).

10. An apparatus according to claim 9 , wherein the maximum pressure is limited to a pressure within the range of 20000 to 80000 Pa (0.2 to 0.8 bar).

11. An apparatus for maintaining an inkjet printer printhead in a working state, the printer defining an array of orifices from which in use jets of ink are projected, the apparatus comprising:

means for applying a pressure pulse to ink within the printhead sufficient to discharge ink from each orifice when not in use to project ink, the pressure pulse having a relatively rapidly rising leading edge and a relatively slowly falling trailing edge, wherein the pressure applying means comprises a valve which is switchable between a first condition in which a source of compressed air is connected to an ink supply arrangement, and a second condition in which the ink supply arrangement is connected to an exhaust conduit which communicates with the atmosphere via an airflow restrictor.

12. An apparatus according to claim 11 , in which the airflow restrictor is manually adjustable to enable control of the slope of the trailing edge of the pressure pulse.

13. An apparatus for maintaining an inkjet printer printhead in a working state, the printer defining an array of orifices from which in use jets of ink are projected, the apparatus comprising:

a valve for applying a pressure pulse to ink within the printhead sufficient to discharge ink from each orifice when not in use to project ink, the pressure pulse having a relatively rapidly rising leading edge and a relatively slowly falling trailing edge, wherein the valve is switchable between a first condition in which a source of compressed air is connected to an ink supply arrangement, and a second condition in which the ink supply arrangement is connected to an exhaust conduit which communicates with an atmosphere via an airflow restrictor.

14. An method for maintaining an inkjet printer printhead in a working state, the printer defining an array of orifices from which in use jets of ink are projected, the method comprising:

applying a pressure pulse to ink within the printhead sufficient to discharge ink from each orifice when not in use to project ink, wherein the pressure pulse having a relatively rapidly rising leading edge and a relatively slowly falling trailing edge.