AU2001237147A1

AU2001237147A1 - Thermal expansion compensation for modular printhead assemblies

Info

Publication number: AU2001237147A1
Application number: AU2001237147A
Authority: AU
Inventors: Kia Silverbrook
Original assignee: Memjet Technology Ltd
Current assignee: Memjet Technology Ltd
Priority date: 2000-03-06
Filing date: 2001-03-06
Publication date: 2001-11-29
Anticipated expiration: 2021-03-06

Description

Title

Thermal Expansion Compensation for Modular Printhead Assemblies.

Field of the Invention

The present invention relates to modular printheads for digital printers and in

particular to pagewidth 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/AUOO/00581 PCT/AU00/00580

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

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

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

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

PCT/AUOO/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 November 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/AUO 1/00239 (deriving

priority from Australian Provisional Patent Application No. PQ6058). Background of the Invention.

Recently, inkjet printers have been developed which use printheads manufactured by

micro electro mechanical systems (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 printers are able to print more quickly than conventional printers

because the printhead does not traverse back and forth across the page.

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, 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 to the operating temperature of the

printer. Furthermore, if the printhead modules are accurately aligned when the support beam is at the equilibrium operating temperature of the printer, then unacceptable

misalignments in the printing may occur before the beam reaches the operating temperature.

Even if the printhead is not modularized thereby making the alignment problem irrelevant,

the support beam and printhead may bow and distort the printing because of the different

thermal expansion characteristics. Summary of the Invention.

Accordingly, the present invention provides a printhead assembly for a printer, the

printhead assembly including:

an elongate support member for attachment to the printer;

a printhead adapted to mount the support member, the printhead having and array of ink ejector nozzles formed in a substrate material; wherein,

the support member is formed from a plurality of different materials having different coefficients of thermal expansion and configured such that the effective coefficient of

thermal expansion of the support member is substantially equal to the coefficient of thermal

expansion of the substrate material.

In some embodiments, the support member is a laminar beam with any odd number of

longitudinally extending layers of at least two different materials wherein layers of the same

material are symmetrically disposed about the central layer. In a particularly preferred

form, the laminar beam has three longitudinally extending layers where the two outer layers

are a first material and the central layer is a second material.

In other embodiments, the printhead is made up of a plurality of printhead modules

adapted to mount to the support member at respective mounting points spaced along the

support member; and

the support member is a composite beam made up of segments of at least two

different materials arranged end to end, wherein,

between any two of the mounting points of the printhead modules there is at least part

of at least two of the segments such that the effective coefficient of thermal expansion of

the support member between the points is substantially equal to the coefficient of thermal

expansion of the substrate material. Preferably, the substrate material is silicon and the arrays of ink ejector nozzles are

formed using MEMS techniques.

In some preferred forms, one of the materials is invar, and at least one of the other

materials has a coefficient of thermal expansion greater than that of silicon.

It will be appreciated that the use of a composite support member made from at least

two different materials having different coefficients of thermal expansion provide an effective coefficient of thermal expansion that is substantially the same as silicon.

Forming the composite beam by bonding different segments of material end to end

will prevent bowing as long as the segment combinations repeat in accordance with the

module mounting 'pitch' or spacing. Each combination of different materials extending

between the mounting points of the printhead modules must have generally the same

effective coefficient of thermal expansion as silicon. Simply ensuring that the effective

coefficient of thermal expansion of the whole beam is about the same as silicon will not

ensure that the modules remain aligned as the coefficient between any two adjacent

mounting points may be higher or lower than silicon, thus causing misalignment.

Brief Description of the Drawing.

A prefeπed embodiment of the invention will now be described, by way of example

only, with reference to the accompanying drawing in which:

Figure 1 is a schematic longitudinal cross section of a first embodiment of a printhead

assembly according to the present invention; and,

Figure 2 is a schematic longitudinal cross section of a second embodiment of a

printhead assembly according to the present invention. Detailed Description of the Preferred Embodiment.

Referring to Figure 1, the printhead assembly has a support beam 1 supporting a

plurality of printhead modules 2 each having a silicon MEMS printhead chip. The support

beam 1 is a hot rolled three-layer laminate consisting of two different materials. The outer

layers 3 and 4 are formed from invar which typically has a coefficient of thermal expansion

of about 1.3 x 10^"6 metres per degree Celsius. The coefficient of thermal expansion of

silicon is about 2.5 x 10^"6 metres per degree Celsius and therefore the central layer 5 must

have a coefficient of thermal expansion greater than this in order to give the support beam as a whole a coefficient of thermal expansion substantially equal to that of silicon.

It will be appreciated that the effective coefficient of thermal expansion of the support

beam will depend on the coefficient of thermal expansion of both metals, the Young's

Modulus of both metals and the thickness of each layer. In order to prevent the beam from

bowing, the outer layers 3 and 4 should be the same thickness.

Referring to Figure 2, the printhead assembly shown as an elongate support beam 1

supporting the printhead modules 2. Each printhead module has a silicon MEMS printhead

chip.

The support beam 1 is formed from two different materials 3 and 4 bonded together

end to end. Again, one of the materials has a coefficient of thermal expansion less than that

of silicon and the other material has one greater than that of silicon. The length of each

segment is selected such that the printhead spacing, or printhead pitch A, has an effective

coefficient of thermal expansion substantially equal to that of silicon.

It will be appreciated that the present invention has been described herein by way of

example only. Skilled workers in this field would recognize many other embodiments and

variations which do not depart from the scope of the invention.

Claims

C LAI MS:

1. A printhead assembly for a printer, the printhead assembly including:

an elongate support member for attachment to the printer;

a printhead adapted to mount to the support member, the printhead having an array of

i ink ejector nozzles formed in a substrate material; wherein,

the support member is formed from at least two different materials having different coefficients of thermal expansion and configured such that the effective coefficient of

expansion of the substrate material.

2. A printhead assembly according to claim 1, wherein the support member is a laminar

beam with any odd number of longitudinally extending layers of at least two different

materials wherein layers of the same material are symmetrically disposed about the central

layer.

3. A printhead assembly according to claim 2, wherein the odd number is three.

4. A printhead assembly according to claim 1, wherein the printhead is made up of a

plurality of printhead modules adapted to mount to the support member at respective

mounting points spaced along the support member; and

the support member is a composite beam made up of segments of at least two

different materials arranged end to end, wherein, between any two of the mounting points of the printhead modules there is at least part

of at least two of the segments such that the effective coefficient of thermal expansion of the support member between the points is substantially equal to the coefficient of thermal

expansion of the substrate material.

5. A printhead assembly according to any one of the preceding claims wherein the

substrate material is silicon and the arrays of ink ejector nozzles are formed using MEMS

techniques.

6. A printhead assembly according to any one of the preceding claims wherein one of

the materials is invar, and at least one of the other materials has a coefficient of thermal expansion greater than that of silicon.