# Patent

## Title
Printhead assembly incorporating heat aligning printhead modules

## Abstract

The present invention relates to a printhead assembly for a printer. The assembly includes a support member. A plurality of printhead modules is mounted to the support member. Each printhead module includes an integrated circuit manufactured using micro-electromechanical systems (MEMS) techniques. The printhead modules are configured to move into alignment with each other upon heating of the printhead assembly.

## Background
CROSS REFERENCE TO RELATED APPLICATION
The present application is a continuation of U.S. application Ser. No. 11/330,057 filed on Jan. 12, 2006, now issued U.S. Pat. No. 7,404,620, which is a continuation of U.S. application Ser. No. 10/882,764 filed on Jul. 2, 2004, now issued U.S. Pat. No. 7,040,736, which is a continuation of U.S. application Ser. No. 10/129,437 filed on May 6, 2002, now issued as U.S. Pat. No. 6,793,323, which is a 371 of PCT/AU01/00260 filed on Mar. 9, 2001, all of which are herein incorporated by reference.


FIELD OF THE INVENTION
The present invention relates to printers, and in particular to 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 are the disclosures of two co-filed PCT applications, PCT/AU01/00261 and PCT/AU01/00259 (deriving priority from Australian Provisional Patent Application No. PQ6110 and PQ6158). Further incorporated are the disclosures of two co-pending PCT applications filed 6 Mar. 2001, application numbers PCT/AU01/00238 and PCT/AU01/00239, 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.
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 printhead assembly for an inkjet printer, the assembly comprising:
an elongate support member for mounting in an inkjet printer body; and,
two or more printhead modules detachably mounted in series along the support member, the support member having a structural component for strengthening the member, and a mounting component for mounting the printhead modules; wherein,
the mounting component is connected to the structural component to allow relative thermal expansion.
By allowing the structural component of the support to thermally expand relative to the mounting component, the structural components can be strong, inexpensive metal despite the co-efficient of thermal expansion (CTE). Only the mounting component need approximate the CTE of the printhead chips to maintain a suitable alignment of the modules.
Optionally, each of the printhead modules has an array of inkjet nozzles formed on a monolithic substrate and the coefficient of thermal expansion of the monolithic substrates is similar to that of the mounting component. The mounting component may be connected to the structural component via an elastomeric element that allows relative thermal expansion and contraction.
Optionally the structural component is a beam with a channel section and the mounting component is a core house within the channel section. The channel may be an aluminium extrusion and the core may have ink supply channels for the printhead modules.
Optionally the monolithic substrates and the core are predominantly formed from silicon.
Optionally the printhead assembly is a pagewidth printhead.
According to an aspect related to the present invention, there is provided a system for aligning two or more printhead modules mounted to a support member in a printer, the support member having a production temperature when the printhead modules are mounted to the support member, and an operating temperature reached during normal operation of the printer, the system including:
positioning the printhead modules on the support member to misalign with each other by an amount determined by the coefficient of thermal expansion of the support member, the printhead module spacing on the support member and the difference between the production temperature and the operating temperature, such that they align when the support member is at the operating temperature.
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:
FIG. 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

1. A printhead assembly for a printer, the assembly comprising:
a support member; and
a plurality of printhead modules mounted on the support member, each printhead module including an integrated circuit manufactured using micro-electromechanical Systems (MEMS) techniques;
wherein the printhead modules and support members are configured to move into alignment with each other upon heating of the printhead assembly,
the support beam is a support beam having a silicon core mounted within a metal channel, and
each printhead module is mounted to the silicon core with a neck portion.

2. A printhead assembly as claimed in claim 1, wherein the printhead modules and support member are configured to move into alignment with each other when the printhead assembly is at operating temperature.

3. A printhead assembly as claimed in claim 2, wherein said operating temperature is about 50° C. above ambient temperature.

4. A printhead assembly as claimed in claim 1, wherein each printhead module further includes at least one fiducial for positioning the printhead modules along the support member.