Printhead chip with nozzle-clearing structures

1. A printhead chip which comprises

a substrate that defines ink supply conduits;

drive circuitry positioned on the substrate;

nozzle chamber assemblies positioned on the substrate, each nozzle chamber assembly defining a nozzle chamber in fluid communication with a respective ink supply conduit and having an upper portion that defines an ink ejection port in fluid communication with the nozzle chamber and a lower portion that extends from the substrate, the upper portion being reciprocally displaceable with respect to the lower portion so that ink is ejected from the ink ejection port; and

actuators connected to the drive circuitry and to respective said upper portions to displace said upper portions to eject ink, wherein

nozzle-clearing structures are mounted on the substrate in respective nozzle chambers, the nozzle-clearing structures being dimensioned to project through respective ink ejection ports at some stage during displacement of respective upper portions to clear any blockages in the ink ejection ports.

2. A printhead chip as claimed in claim 1 , in which the nozzle-clearing structures comprise bridge members that traverse respective supply conduits and rods mounted on respective bridge members so that, when the upper portions are displaced towards the substrate, the rods project through respective ink ejection ports.

3. A printhead chip as claimed in claim 1 , in which each actuator is elongate, with one end connected to the substrate to receive electrical signals from the drive circuitry and an opposite end connected to a respective upper portion, and is laminated, with one layer forming a heating circuit and another layer forming a constraining cantilever so that when the heating circuit is activated and undergoes thermal expansion, the actuator experiences differential thermal expansion resulting in the actuator bending and thus displacing the respective upper portion.

4. A printhead chip as claimed in claim 3 , in which each actuator has at least one longitudinal portion with a substantially U-shaped cross section for structural integrity.

5. A printhead chip as claimed in claim 1 , in which each upper portion comprises a roof portion and downwardly depending sidewalls, and each lower portion comprises upwardly extending sidewalls that bound the respective ink supply conduit, the sidewalls overlapping telescopically when the upper portion is displaced towards the substrate.

6. A printhead chip as claimed in claim 5 , in which the ink ejection ports are defined by respective nozzle rims that extend from the roof portions.

7. A printhead chip as claimed in claim 6 , in which the nozzle rims have an asymmetrical plan profile to compensate for non-perpendicular movement of the roof portions with respect to the substrate.

8. A printhead chip as claimed in claim 5 , in which the relative dimensions of the nozzle chambers and the respective ink supply conduits are such that a viscous drag is set up between the ink, the sidewalls and walls of the ink supply conduits while the roof portions are displaced towards the substrate to facilitate the ejection of ink.