AU2001240342A1

AU2001240342A1 - Thermal expansion compensation for modular printhead assembly

Info

Publication number: AU2001240342A1
Application number: AU2001240342A
Authority: AU
Inventors: Kia Silverbrook
Original assignee: Memjet Technology Ltd
Current assignee: Memjet Technology Ltd
Priority date: 2000-03-10
Filing date: 2001-03-09
Publication date: 2001-11-29
Anticipated expiration: 2021-03-09

Description

Title

Thermal Expansion Compensation for Modular Printhead Assembly.

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/AUOO/00579 PCT/AU00/00581 PCT/AU00/00580

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

PCT/AU00/00583 PCT/AU00/00593 PCT/AUOO/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/AUOO/00597

PCT/AU00/00598 PCT/AUOO/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/AUOO/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, are the disclosures of two co-filed PCT applications, PCT/AUO 1/00261 and

PCT/AUO 1/00260 (deriving priority from Australian Provisional Patent Application Nos.

PQ6110 and PQ6111). Further incorporated is the disclosure of two co-pending PCT

applications filed 6 March 2001, application numbers PCT/AUO 1/00238 and

RECTIFIED SHEET (Rule 91) ΪSA/AU PCT/AUOl/00239, which derive their priority from Australian Provisional Patent

Application nos. PQ6059 and 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. The invention will be described with particular reference to silicon printhead chips for digital inkjet printers wherein the nozzles, chambers and actuators of the chip are

formed using MEMS techniques. However, it will be appreciated that this is in no way

restrictive and the invention may also be used in many other applications.

Silicon printhead chips are well suited for use in pagewidth printers having stationary

printheads. These printhead chips extend the width of a page instead of traversing back and

forth across the page, thereby increasing printing speeds. The probability of a production

defect in an eight inch long chip is much higher than a one inch chip. The high defect rate

translates into relatively high production and operating costs.

To reduce the production and operating costs of pagewidth printers, the printhead may be made up of a series of separate printhead modules mounted adjacent one another, each

module having its own printhead chip. 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 a support beam. Once aligned, the printing from

each module precisely abuts the printing from adjacent modules.

RECTIFIED SHEET (Rule 91) ISA/AU 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.

Summary of the Invention

According to a first aspect, the present invention provides a printhead assembly for an

inkjet printer, the printhead assembly including: a composite support member for attachment to the printer, the composite support member being formed of at least two materials and having a unitary mounting element;

a printhead adapted for mounting to the mounting element; wherein,

the materials of the support member are selected and structurally combined such that

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

coefficient of thermal expansion of the printhead.

For the purposes of this specification, "the coefficient of thermal expansion of the

support member" is the effective coefficient of thermal expansion of the mounting element

taking into account any external influences from the rest of the support.

Preferably, the printhead is two or more printhead modules that separately mount to the mounting element, each of the modules having a silicon MEMS chip, wherein the

mounting element is also formed from silicon.

In a particularly preferred form, the support member further includes a metal portion

adapted for attachment to the printer.

Preferably the mounting element is supported by, and adjustably positionable within,

the metal portion. In some embodiments, the printer is a pagewidth printer and the support member is a

beam with an elongate metal shell enclosing a central core formed from silicon.

Conveniently, the beam is adapted to allow limited relative movement between the silicon

core and the metal shell. To achieve this the beam may include an elastomeric layer

interposed between the silicon core and the metal shell. Furthermore, the outer shell may be formed from laminated layers of at least two different metals.

It will be appreciated that through careful design and material selection, the

coefficient of thermal expansion of the mounting section of the support member can be

made to substantially match the coefficient of thermal expansion of the printhead chips.

Without any significant differences between the thermal expansion of the printhead and the

mounting section of the support member, the problems of printhead module misalignment

are avoided. By designing the support member to accommodate some relative movement

between the outer shell and mounting section, the problems of bowing are also avoided.

Brief Description of the Drawing

A preferred embodiment of the present invention will now be described by way of

example only, with reference to the accompanying drawing, in which:

Figure 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 has a printhead module 2, is fixed to

a support beam 3 adapted for mounting in a digital printer (not shown). The printhead

module 2 has a silicon printhead chip 4. The chip has an array of ink nozzles, chambers and

actuators formed using MEMS techniques. To ensure that any misalignment of the printing produced by adjacent printhead

modules 2 does not exceed a predetermined maximum, the coefficient of thermal expansion

of the support beam 3 should closely match the coefficient of thermal expansion of silicon.

The maximum and minimum allowable coefficients of thermal expansion for the support

beam 3 can be calculated using: the maximum permissible misalignment between adjacent printheads; and,

the difference between ambient temperature (or more particularly the temperature at

which the modules 2 are mounted and aligned on the support beam 3) and the equilibrium operating temperature and the length of the printhead chips using the following formula:

where:

ΔX_max is the maximum acceptable misalignment between printhead modules;

ΔT is the difference between the temperature when the modules were mounted and

aligned on the support beam and the equilibrium operating temperature of the printer;

L is the length of the printhead chip.

M_CTE is the coefficient of thermal expansion of the support beam; and

P_CTE is the coefficient of thermal expansion of the printhead chip.

It will be appreciated that for:

ΔX_max =1 x 10 °m

ΔT = 40°C

L = 20 mm

then:

IM_CT_E - P_CTEI < 1.25 x 10^"6 m/°C and if a silicon printhead is used

P_CΓE = 2.6 x 10^"6m/°C

then the maximum and minimum values for the coefficient of thermal expansion of the

support beam are:

Mere = 2.6 ± 1.25 x 10 ^Dm °C.

This provides a parameter that can be used to select appropriate materials and structural configurations for the support beam 3. In one preferred form, the beam 3 includes

a silicon core element 5 bonded to a metallic outer shell 6 by an intermediate layer 7. The

modules 2 mount to the core element 5 which helps to reduce the effective coefficient of

thermal expansion of the support beam 3 such that it falls within the acceptable range.

An elastomeric layer 7 may be interposed between the outer shell 6 and the core

element 5 such that the influence of the outer shell on the coefficient of thermal expansion

of the silicon core element is reduced.

Alternatively, the silicon core element 5 may be mounted for limited sliding within

the outer shell 6 in order to negate or reduce any influence from the generally high

coefficients of thermal expansion of metals.

The present invention has been described herein with reference to specific examples.

Skilled workers in this field would readily recognise that the invention may be embodied in

many other forms.

Claims

CLAIMS :-

1. A printhead assembly for an inkjet printer, the printhead assembly including:

a composite support member for attachment to the printer, the composite support

member being formed of at least two materials and having a unitary mounting element; a printhead adapted for mounting to the mounting element; wherein,

the materials of the support member are selected and structurally combined such that the coefficient of thermal expansion of the support member is substantially equal to the

coefficient of thermal expansion of the printhead.

2. A printhead assembly according to claim 1 wherein the printhead is two or more

printhead modules that separately mount to the mounting element, each of the modules

having a silicon MEMS chip, wherein the mounting element is also formed from silicon.

3. A printhead assembly according to claim 2 wherein the support member further includes a metal portion adapted for attachment to the printer.

4. A printhead assembly according to claim 3 wherein the mounting element is

supported by, and adjustably positionable within, the metal portion.

5. A printhead assembly according to any one of claims 1 to 4 wherein the printer is a

pagewidth printer and the support member is a beam with an elongate metal shell enclosing

a central core formed from silicon.

6. A printhead assembly according to claim 5 wherein the beam is adapted to allow

limited relative movement between the silicon core and the metal shell.

7. A printhead assembly according to claim 2 wherein the beam has an elastomeric layer

interposed between the silicon core and the metal shell.

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

shell is formed from laminated layers of at least two different metals.