# Patent

## Title
Stable printhead assembly

## Abstract

Disclosed is a stable printhead assembly (1) for carrying a printhead (2) such as modular MEMS printhead. The assembly has an elongated core (5) and for example, a MEMS printhead bonded to the core. The core is contained within an outer laminated shell (4). The shell and core together have an effective coefficient of thermal expansion substantially equal to that of the printhead.

## Background
This is a Continuation Application of U.S. Ser. No. 10/129,503 filed May 6, 2002, now U.S. Pat. No. 6,676,245.


FIELD OF THE INVENTION
The present invention relates to printers, and in particular to digital inkjet printers.
CO-PENDING APPLICATIONS
Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention on 24 May 2000:












PCT/AU00/00578 PCT/AU00/00579 PCT/AU00/00581 PCT/AU00/00580


PCT/AU00/00582 PCT/AU00/00587 PCT/AU00/00588 PCT/AU00/00589


PCT/AU00/00583 PCT/AU00/00593 PCT/AU00/00590 PCT/AU00/00591


PCT/AU00/00592 PCT/AU00/00584 PCT/AU00/00585 PCT/AU00/00586


PCT/AU00/00594 PCT/AU00/00595 PCT/AU00/00596 PCT/AU00/00597


PCT/AU00/00598 PCT/AU00/00516 PCT/AU00/00517 PCT/AU00/00511









Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending application, PCT/AU00/01445 filed by the applicant or assignee of the present invention on 27 Nov. 2000. The disclosures of these co-pending applications are incorporated herein by cross-reference. Also incorporated by cross-reference, is the disclosure of a co-filed PCT application, PCT/AU01/00238 (deriving priority from Australian Provisional Patent Application No. PQ6059).
BACKGROUND OF THE INVENTION
Recently, inkjet printers have been developed which use printheads manufactured by micro-electro mechanical system(s) (MEMS) techniques. Such printheads have arrays of microscopic ink ejector nozzles formed in a silicon chip using MEMS manufacturing techniques.
Printheads of this type are well suited for use in pagewidth printers. Pagewidth printers have stationary printheads that extend the width of the page to increase printing speeds. Pagewidth printheads do not traverse back and forth across the page like conventional inkjet printheads, which allows the paper to be fed past the printhead more quickly.
To reduce production and operating costs, the printheads are made up of separate printhead modules mounted adjacent each other on a support beam in the printer. To ensure that there are no gaps or overlaps in the printing produced by adjacent printhead modules it is necessary to accurately align the modules after they have been mounted to the support beam. Once aligned, the printing from each module precisely abuts the printing from adjacent modules.
Unfortunately, the alignment of the printhead modules at ambient temperature will change when the support beam expands as it heats up during printhead operation. Furthermore, if the printhead modules are accurately aligned when the support beam is at the equilibrium operating temperature, there may be unacceptable misalignments in any printing before the beam has reached the operating temperature. Even if the printhead is not modularized, thereby making the alignment problem irrelevant, the support beam and printhead may bow because of different thermal expansion characteristics. Bowing across the lateral dimension of the support beam does little to affect the operation of the printhead. However, as the length of the beam is its major dimension, longitudinal bowing is more significant and can affect print quality.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a printhead assembly for a digital inkjet printer, the printhead assembly including:
a support member for attachment to the printer;
a printhead adapted for mounting to the support member;
the support member having an outer shell and a core element defining at least one ink reservoir such that the effective coefficient of thermal expansion of the support member is substantially equal to the coefficient of thermal expansion of the printhead.
Preferably, the outer shell is formed from at least two different metals laminated together and the printhead includes a silicon MEMS chip. In a further preferred form, the support member is a beam and the core element is a plastic extrusion defining four separate ink reservoirs. In a particularly preferred form, the metallic outer shell has an odd number of longitudinally extending layers of at least two different metals, wherein layers of the same metal are symmetrically disposed about the central layer.
It will be appreciated that by laminating layers of uniform thickness of the same material on opposite sides of the central layer, and at equal distances therefrom, there is no tendency for the shell to bow because of a dominating effect from any of the layers. However, if desired, bowing can also be eliminated by careful design of the shells cross section and variation of the individual layer thicknesses.
In some embodiments, the printhead is a plurality of printhead modules positioned end to end along the beam.



BRIEF DESCRIPTION OF THE DRAWING.
A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing in which:
FIG. 1 is a schematic cross section of a printhead assembly according to the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the figure, the printhead assembly 1 includes a printhead 2 mounted to a support member 3. The support member 3 has an outer shell 4 and a core element 5 defining four separate ink reservoirs 6, 7, 8 and 9. The outer shell 4 is a hot rolled trilayer laminate of two different metals. The first metal layer 10 is sandwiched between layers of the second metal 11. The metals forming the trilayer shell are selected such that the effective coefficient of thermal expansion of the shell as a whole is substantially equal to that of silicon even though the coefficients of the core and the individual metals may significantly differ from that of silicon. Provided that the core or one of the metals has a coefficient of thermal expansion greater than that of silicon, and another has a coefficient less than that of silicon, the effective coefficient can be made to match that of silicon by using different layer thicknesses in the laminate.
Typically, the outer layers 11 are made of invar which has a coefficient of thermal expansion of 1.3×10−6 m/° C. The coefficient of thermal expansion of silicon is about 2.5×10−6 m/° C. and therefore the central layer must have a coefficient greater than this to give the support beam an overall effective coefficient substantially the same as silicon.
The printhead 2 includes a micro moulding 12 that is bonded to the core element 5. A silicon printhead chip 13 constructed using MEMS techniques provides the ink nozzles, chambers and actuators.
As the effective coefficient of thermal expansion of the support beam is substantially equal to that of the silicon printhead chip, the distortions in the printhead assembly will be minimized as it heats up to operational temperature. Accordingly, if the assembly includes a plurality of aligned printhead modules, the alignment between modules will not change significantly. Furthermore, as the laminated structure of the outer shell is symmetrical in the sense that different metals are symmetrically disposed around a central layer, there is no tendency of the shell to bow because of greater expansion or contraction of any one metal in the laminar structure. Of course, a non-symmetrical laminar structure could also be prevented from bowing by careful design of the lateral cross section of the shell.
The invention has been described herein by way of example only. Skilled workers in this field will readily recognise that the invention may be embodied in many other forms.

## Claims

1. A printhead assembly for carrying a printhead, comprising:
an elongated core; 
a MEMS printhead bonded to the core; 
the core being contained within an outer laminated shell, the shell and core together having an effective coefficient of thermal expansion substantially equal to that of the printhead.

2. A printhead assembly according to claim 1, wherein:
the outer shell is formed from different materials laminated together, the laminate having inner and outer layers which are of the same metal.

3. A printhead assembly according to claim 2, wherein:
the laminated shell comprises inner and outer layers of invar.

4. A printhead assembly according to claim 1, wherein:
the printhead is fabricated from silicon.

5. A printhead assembly according to claim 1, wherein:
the outer shell has an odd number of longitudinally extending layers, being at least three in number, layers being arranged symmetrically about a central layer.

6. A printhead assembly according to claim 5, wherein:
the laminated shell comprises two or more different materials, each having a different coefficient of thermal expansion.

7. A printhead assembly according to claim 1, wherein:
a coefficient of thermal expansion of the core and a coefficient of thermal expansion of the shell are different.

8. A printhead assembly according to claim 1, wherein:
the extrusion comprises adjacent reservoirs which collectively lead to an area adapted to receive a printhead which is carried by the core.

9. A printhead assembly according to claim 1, wherein:
the core is an extruded and elongated body having a plurality of interior reservoirs, the reservoirs each having an ink exit opening, the openings converging into an area adapted to receive the printhead.

10. A printhead assembly according to claim 1, wherein:
the body is a plastic extrusion.

11. A printhead assembly according to claim 10, wherein:
the body is internally subdivided by extruded membranes to define the reservoirs.

12. A printhead assembly according to claim 11, wherein:
the reservoirs are four in number.

13. A printhead assembly according to claim 1, wherein:
the core is adapted to be encased by the shell, the body and shell.

14. A printhead assembly according to claim 13, wherein:
the body includes a portion which protrudes beyond the shell, this portion receiving the printhead.

15. A printhead assembly according to claim 1, wherein:
the core has a coefficient of expansion which is less than the coefficient of expansion of silicon, and the shell has a coefficient of expansion which is greater than the coefficient of expansion of silicon.

16. A printhead assembly according to claim 1, wherein:
the shell comprises at least two materials bonded to one another and having coefficients of expansion which are different than the coefficient of expansion of silicon, one material having a coefficient of expansion which is greater than the coefficient of expansion of silicon and another material having a coefficient of expansion which is less than the coefficient of expansion of silicon.

17. A printhead assembly according to claim 1, wherein:
the laminated shell comprises hot rolled layers of metal.

18. A printhead assembly according to claim 1, further comprising:
a modular printhead bonded to the core, the printhead comprising a plurality of modules disposed along the core.

19. A printhead assembly according to claim 18, wherein:
each module is fabricated from silicon.

20. A printhead assembly according to claim 19, wherein:
each module further comprises a MEMS structure.