AU2001240343A1 - Thermal expansion compensation for modular printhead assembly - Google Patents

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

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

PCT/AUOO/00583 PCT/AUOO/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/AUOO/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/AU01/00261 and

PCT/AUO 1/00259 (deriving priority from Australian Provisional Patent Application No.

PQ6110 and PQ6158). Further incorporated are the disclosures of two co-pending PCT

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

RECTIFIED SHEET (Rule 91)

ISA/AU PCT/AUO 1/00239, 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 temperature it maintains during

operation.

Summary of the Invention Accordingly, the present invention provides a system for aligning two or more printhead modules mounted to a support member in a printer, the system including: positioning the printhead modules on the support member such that they align when

the support member is at its operating temperature but not necessarily at other temperatures.

Preferably, the support member is a beam and the printhead modules include MEMS

manufactured chips having at least one fiducial on each;

wherein,

the fiducials are used to misalign the printhead modules by a distance calculated from:

i) the difference between the coefficient of thermal expansion of the beam and

the printhead chips; ii) the spacing of the printhead chips along the beam; and,

iii) the difference between the production temperature and the operating

temperature.

Conveniently, the beam may have a core of silicon and an outer metal shell. In a

further preferred embodiment, 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 metal shell. In other forms, the

outer shell may be formed from laminated layers of at least two different metals. It will be appreciated that this system requires the coefficient of thermal expansion of

the printhead chips to be greater than or equal to the coefficient of thermal expansion of the

beam, otherwise the "gaps" left between the printhead modules as compensation at ambient

temperature will not close as the beam reaches the operating temperature.

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:

Figure 1 shows a schematic cross section of a printhead assembly according to the

present invention.

Detailed Description of Preferred Embodiments

Referring to the figure the printhead assembly 1 has a plurality of printhead modules 2

mounted to a support member 3 in a printer (not shown). The printhead module includes a

silicon printhead chip 4 in which the nozzles, chambers, and actuators are manufactured

using MEMS techniques. Each printhead chip 4 has at least 1 fiducial (not shown) for

aligning the printheads. Fiducials are reference markings placed on silicon chips and the

like so that they may be accurately positioned using a microscope.

According to one embodiment of the invention, the printheads are aligned while the

printer is operational and the assembly is at the printing temperature. If it is not possible to

view the fiducial marks while the printer is operating, an alternative system of alignment is

to misalign the printhead modules on the support beam 3 such that when the printhead

assembly heats up to the operating temperature, the printheads move into alignment. This is

easily achieved by adjusting the microscope by the set amount of misalignment required or

simply misaligning the printhead modules by the required amount. The required amount is calculated using the difference between the coefficients of

thermal expansion of the printhead modules and the support beam, the length of each

individual printhead module and the difference between ambient temperature and the

operating temperature. The printer is designed to operate with acceptable module

alignment within a temperature range that will encompass the vast majority of environments in which it expected to work. A typical temperature range may be 0°C to 40°C. During

operation, the operating temperature of the printhead rise a fixed amount above the ambient temperature in which the printer is operating at the time. Say this increase is 50°C, the

temperature range in which the alignment of the modules must be within the acceptable

limits is 50°C to 90°C. Therefore, when misaligning the modules during production of the

printhead, the production temperature should be carefully maintained at 20°C to ensure that

the alignment is within acceptable limits for the entire range of predetermined ambient

temperatures (i.e. 0°C to 40°C).

To minimize the difference in coefficient of thermal expansion between the printhead

modules and the support beam 3, the support beam has a silicon core 5 mounted within a

metal channel 6. The metal channel 6 provides a strong cost effective structure for

mounting within a printer while the silicon core provides the mounting points for the

printhead modules and also helps to reduce the coefficient of thermal expansion of the

support beam 3 as a whole. To further isolate the silicon core from the high coefficient of

thermal expansion in the metal channel 6 an elastomeric layer 7 is positioned between the

core 5 and the channel 6. The elastomeric layer 7 allows limited movement between the

metal channel 6 and the silicon core 5. The invention has been described with reference to specific embodiments. The

ordinary worker in this field will readily recognise that the invention may be embodied in

many other forms.

Claims

CLAIMS :-

1. A method for aligning two or more printhead modules mounted to a support member

in a printer, the method including:

positioning the printhead modules on the support member such that they align when the support member is at its operating temperature but not necessarily at other temperatures.

2. A system for aligning a plurality of printhead modules mounted on a support member

in a printer wherein the support member is a beam and the printhead modules include

MEMS manufactured chips having at least one fiducial on each;

wherein,

the fiducials are used to misalign the printhead modules at ambient temperature by a

distance calculated from:

i) the difference in coefficient thermal expansion between the beam and the

printhead chips; ii) the spacing of the printhead chips along the beam; and,

iii) the difference between the production temperature and the operating

temperature.

3. A system for aligning a plurality of printhead modules mounted to a support member

and a printer according to claim 2 wherein the beam has a core of silicon and an outer metal

shell.

4. A system for aligning a plurality of printhead modules mounted to a support member

in a printer according to claim 3 wherein the beam is adapted to allow limited relative

movement between the silicon core and the metal shell.

5. A system for aligning a plurality of printhead modules mounted to a support member

in a printer according to claim 4 wherein the beam has an elastomeric layer between the silicon core and metal shell to permit the limited relative movement.

6. A system for aligning a plurality of printhead modules mounted to a support member

in a printer according to claim 5 wherein the outer shell is formed from laminated layers of

at least two different metals.