DE69904743T2 - DROPLETS STORAGE APPARATUS - Google Patents

Abstract

Droplet deposition apparatus comprises a fluid chamber comprising an actuator actuable by electrical signals to effect ejection of droplets from the fluid chamber, a drive circuit for supplying the electrical signals; and a conduit for supplying droplet fluid to said fluid chamber, the drive circuit being in substantial thermal contact with the conduit so as to transfer a substantial part of the heat generated in the drive circuit to the droplet fluid.

Description

The present invention relates to a droplet deposition apparatus, such as for inkjet printheads.

The current trend in drop-on-demand inkjet printing towards higher resolution requires an increased density of both the ink ejection nozzles and the associated driver circuitry. However, the increased density of driver circuitry can lead to problems associated with overheating. Similarly, the trend towards ever-increasing printhead widths places correspondingly higher demands on thermal management within printheads. Thermal ("bubble jet") printheads benefit in this respect from having their driver circuitry in close contact with the ink, which has a cooling effect. However, this is offset by the need for special measures to maintain the electrical integrity of the circuitry in the ink environment.

It is an object of at least the preferred embodiments of the present invention to provide a simple way of preventing the driver circuit of a print head from overheating without simultaneously compromising its electrical integrity.

In a first aspect, the present invention provides a droplet deposition apparatus comprising:

a liquid chamber with an actuator or actuating device, which can be actuated by electrical signals to cause the ejection of droplets from the liquid chamber;

a driver circuit device for supplying the electrical signals to the actuator or actuating device; and

Conduit means for conveying droplet liquid to and from the said liquid chamber;

wherein the driver circuit means is in close thermal contact with said conduction means such that a substantial portion of the heat generated in said driver circuit is transferred to said droplet liquid.

Arranging the driver circuitry in such a manner can easily allow the ink in the printhead to act as a sink for the heat generated in the driver circuitry. This can significantly reduce the likelihood of overheating, while avoiding the electrical integrity problems that could arise where the formation of an integrated circuit package containing the circuitry allows it to come into direct contact with the ink.

The apparatus may comprise first conduit means for supplying droplet liquid to said liquid chamber and second conduit means for conducting the droplet liquid from said liquid chamber. If so, the driver circuit means may advantageously be thermally connected to the second conduit means. This may be the most direct route from the printhead for the heat generated in the driver circuit chip and, in the event that the heat generated by the chip changes significantly during operation, any variation in the temperature of the ink in the liquid chamber itself may be minimised. As is known, for example, from WO 97/35167, such a temperature change change or variation can lead to variations in the ejection velocity of the droplets and consequently to deposition errors in the printed image.

When the driver circuit is enclosed in an integrated circuit package of substantially cuboid shape in which at least some of the faces are rectangles, each having a surface area, a face other than the face with the smallest surface area may advantageously be arranged to be substantially parallel to the direction of liquid flow in that part of the conduit which is closest to said face and is in close thermal contact with the liquid. Such an arrangement may ensure significant heat transfer to the droplet liquid. Preferably, the face with the largest surface area is arranged to be parallel to the direction of liquid flow. A circuit architecture which allows such an arrangement may maximise heat transfer from the circuit.

A second aspect of the present invention provides a droplet deposition apparatus comprising:

at least one droplet ejection unit comprising a plurality of liquid chambers, a plurality of actuators, and a plurality of nozzles arranged in a row, said actuators being operable to eject a droplet of liquid from the liquid chamber through a respective nozzle; and

a support member for said at least one droplet ejection unit, said support member having at least one droplet liquid passage communicating with said plurality of liquid chambers and arranged to direct droplet liquid to or from said liquid chambers in a direction substantially parallel to said row of nozzles and to transfer a substantial part of the heat generated during droplet ejection to the droplet liquid being conveyed.

This allows a substantially uniform distribution of heat to be achieved along the length of the support member, which can lead to reduced thermally induced stresses that could otherwise deform the printhead. Such deformation would become more pronounced as the width of the printhead increased, e.g., to that of a page (typically 12.6 inches (32 cm) for the American "Foolscap" standard, and would occur regardless of whether a plurality of narrow ejection units or a single wide ejection unit were used in conjunction with the support member.

Advantageously, the passage for the droplet liquid may occupy the majority of the area of the support member when viewed in cross-section. Alternatively or additionally, the passage may have respective areas for the flow of droplet liquid into and out of each liquid chamber. Such flow may assist in the transfer of heat from the liquid chamber (where the main source of heat - the actuator - is located) to the remainder of the support member, whereby temperature differentials may be reduced.

In order to provide effective support for the at least one droplet ejection unit, the cross-section of the support member is preferably larger or wider in the direction of the ink ejection from the nozzles than in the direction of the nozzle row.

In one embodiment, the apparatus comprises a plurality of said droplet ejection units, the support member supporting the droplet ejection units side by side in the direction of the nozzle rows, the support member having at least one droplet liquid passage communicating with at least two of said ejection units and arranged to convey droplet liquid to or from the ejection units in a direction substantially parallel to said nozzle rows and to transfer a substantial portion of the heat generated during droplet ejection to the conveyed droplet liquid.

Heat dissipation can be facilitated by constructing the support member from a material such as aluminum which has high thermal conductivity. Such a material also has advantages in terms of manufacturing and cost. Problems arise, however, when the ejection unit is made from a material with a thermal expansion coefficient that differs significantly from that of the support. This will be the case with an ejection unit having channels formed in a body of piezoelectric material (typically lead zirconium titanate, PZT), which will be described below. As can be readily understood, differential expansion - particularly in the direction of the nozzle row in a "pagewide" device - can lead to deformation and/or breakage of ink seals, actuator components, electrical contacts, etc.

Therefore, it is preferable to provide means for attaching said at least one droplet ejection unit to the support member in order to substantially avoid the transmission of thermal deformation of the support member to said at least one droplet ejection unit.

A third aspect of the present invention provides a droplet deposition apparatus comprising:

a liquid chamber, at least a part of which is made of a first material with a first thermal expansion coefficient, said chamber being associated with actuators which are operable to eject a droplet from the chamber and have an opening for the inlet of droplet liquid into it,

a support member for said liquid chamber and having a passage for supplying droplet liquid to said opening, the support member being at least partially defined by or formed from a second material having a second coefficient of thermal expansion greater than said first coefficient; and

Means for securing the fluid chamber to the support member so as to substantially prevent the transfer of thermal deformation of the support member to said fluid chamber.

Preferably, the fastening means comprises resilient bonding means for bonding the or each liquid chamber to the support member. In an example described below, an adhesive rubber pad is used to bond an extruded aluminum support member to a liquid chamber structure having a channel formed in a body of PZT and closed by a lid member of a material such as molybdenum which is thermally matched to the PZT. The formation of ink supply openings in the lid and ink ejection nozzles in the channeled member can provide a particularly compact design with a small number of components.

Further advantageous embodiments of the invention are set out in the description, the drawings and the dependent claims.

The invention will now be described by way of example with reference to the following drawings in which:

Figure 1 is a front and top perspective view of a first embodiment of the invention;

Fig. 2 is a rear and top perspective view of the print head of Fig. 1;

Fig. 3 is a sectional view of the print head, the section being perpendicular to the direction of the nozzle rows;

Fig. 4 is a perspective view from above and above one end of the print head of Fig. 1;

Fig. 5 is a cross-sectional view taken along a liquid channel of an ink ejection module of the printhead of Fig. 1; and

Fig. 6 is a sectional view of a second embodiment of a droplet deposition apparatus, the section being perpendicular to the direction of extension of the nozzle rows.

Figure 1 shows a first embodiment of a droplet deposition apparatus embodied by a print head 10. The embodiment shown is a "pagewide" device having two rows of nozzles 20, 30 extending (in the direction indicated by arrow 100) across the width of a sheet of paper, allowing ink to be deposited across the entire width of a page in a single pass. The ejection of ink from a nozzle is achieved by the application of an electrical signal to actuators associated with a liquid chamber communicating with the nozzle, as is known, for example, from EP-A-0 277 703, EP-A-0 278 590 and in particular British Application Nos. 9710530, EP 1011977 A, and 9721555, EP 1021302 A. To simplify production and increase yield and performance, the "pagewide" rows of Nozzles are made from a number of modules, one of which is shown at 40. Each module has associated fluid chambers and actuator means and is connected to an associated driver circuit (integrated circuit ("chip") 50) by means of, for example, a flexible circuit 60. Ink supply to and from the printhead is carried out via respective bores (not shown) in the end caps 90.

Fig. 2 is a rear perspective view of the printhead of Fig. 1 and with the end caps 90 removed to reveal the printhead support structure 200 which includes ink flow passages 210, 220, 230 extending across the width of the printhead. Ink enters the printhead and ink supply passage 220 via a bore in one of the end caps 90 (which are omitted in the views of Figs. 2 and 3), as shown at 215 in Fig. 2. As it flows along the passage, it is drawn into the respective ink chambers, as shown in Fig. 3, which is a sectional view of the printhead, the section being perpendicular to the direction of extension of the nozzle array. From the passage 220, the ink flows into first and second parallel rows of ink chambers (indicated at 300 and 310, respectively) via an opening 320 formed in the assembly 200 (shown in cross-hatching). After flowing through the first and second rows of ink chambers, the ink exits via openings 330 and 340 to join with the ink stream along respective first and second ink outlet passages 210, 230, as indicated at 235. These join at a common ink outlet (not shown) formed in the end cap located at the opposite end of the printhead from that in which the inlet bore is formed.

Each row of chambers 300 and 310 has associated therewith respective driver circuits 360, 370. The driver circuits are mounted in close thermal contact with that part of the structure 200 which serves as a conduit and which defines the ink flow passages so as to allow a substantial portion of the heat generated by the circuits during their operation to be transferred to the ink via the conduit structure. To this end, the structure 200 of the embodiment of Figs. 1 to 3 is made of a material with good thermal conductivity properties. Among such materials, aluminum is particularly preferred because it can be easily and inexpensively formed by extrusion. The circuits 360, 370 are then positioned on the outer surface of the assembly 200 so that they are in thermal contact with the assembly, with thermally conductive pads or adhesives optionally used to reduce the resistance to heat transfer between the circuit and the assembly.

In the embodiment shown, the cuboid-shaped driver circuit wafers 360, 370 are arranged such that a largest (rectangular or square) surface of each wafer is substantially parallel to the direction (indicated at 235) of fluid flow in the respective portions of the conduits 210, 230 closest to those surfaces. This helps to maximize heat transfer between the circuit and the ink, which is also facilitated by minimizing the thickness of the structure that separates the ink channel and the circuit, as well as by making the structure from a material that has good thermal conductivity.

Reference is now made to Fig. 4 which is a perspective view from above and above one end of the printhead with all but one of the modules 40 removed to reveal the outer and inner individual ities of the assembly 200 more clearly. The assembly includes recesses 500 to accommodate the driver circuits 370 and lips 510, 520 to further hold circuit boards 530 loaded with those components not suitable for incorporation into the driver circuits 370. The formation of the rear lip 520 on a separate component 540 as shown in Fig. 4 allows these boards to be clamped in place by the action of fasteners (e.g., screws) inserted through holes 240 shown in Fig. 2 and engaging a rail (not shown) disposed in the channel 550. Preferably, the rail is made of a strong material, such as steel, which is capable of accommodating screw threads and reinforcing the aluminum structure 200, particularly with respect to forces generated when the printhead is installed and connected.

In the present embodiment, another circuit board is also formed with pins (Fig. 3, 420) for supplying power and data to the printhead and with pins 560 for supplying power and data, which are suitably processed, to the driver circuits 370 via flexible connectors 570. Such connection techniques are well known in the art and will therefore not be discussed in further detail.

As explained above, heat generated in the driver circuits is transferred to the ink as it is dissipated throughout the assembly 200 as a result of the aforementioned ink flow paths. Heat generated in the ink chambers by the associated actuators is also dissipated in this manner. As a result, temperature differences occurring within the assembly 200 are small and do not give rise to significant internal forces and/or deformation.

However, the overall heating of the printhead during operation may result in differential expansions of the structure 200 and the body in which the liquid chambers 300, 310 are formed if the two parts are made of materials having significantly different coefficients of thermal expansion CTE. This is the case in the present embodiment which has liquid chambers formed in a body of piezoelectric material in accordance with the above-mentioned British Application No. 9721555, EP 1021302 A.

As shown in Fig. 5, which is a cross-sectional view taken along a fluid channel of a module 40, channels 11 are formed in a bottom member 860 of piezoelectric material such that piezoelectric channel walls are formed between them. These walls are subsequently coated with electrodes to form actuators for the channel walls, as is known for example from the aforementioned EP-0-0 277 703, a break in the electrodes at 810 allowing the channel walls in each half of the channel to be operated independently by means of electrical signals applied via electrical inputs (flexible circuits 60).

Each channel half is closed along a length 600, 610 by respective portions 820, 830 of a lid member 620 which is also formed with openings 630, 640, 650 which allow ink to be supplied to and from each channel half for cleaning and heat dissipation purposes as is well known. As is also well known, the lid member 620 is preferably made of a material which is thermally matched to the piezoelectric material of the channeled member. Ink ejection from each channel half is via openings 840, 850 which connect the channel to the opposite surface of the piezoelectric base member in which the channel is formed. Nozzles 870, 880 for ejecting ink are provided. subsequently formed in a nozzle plate 890 which is attached to the piezoelectric component.

To avoid deformation of the print head that might otherwise occur as a result of different thermal expansion properties of the piezoelectric material of the fluid chambers and the aluminum of the structure 200, tie rods can be inserted into bore 580 in the structure and tightened to hold the structure 200 in compression. Although any material having a CTE value less than that of the structure - steel in the case of an aluminum structure - is suitable for the tie rods, it will be recognized that low values of CTE are preferred.

In addition, the cover member 620 may be secured to the assembly 200 by means of a resilient bond - adhesive coated rubber is shown at 430 in Figure 3 - allowing any relative expansion that occurs despite the presence of the tie rods (and which may be on the order of 0.3 mm over a typical 12.6" (32 cm) length of a printhead) to occur at this less critical interface and not create stresses and distortions in the printhead module 40 itself. As shown in Figure 4, the cover 620 may be inserted into a well or wall 590 formed in the assembly 200 and may additionally extend to either side of the printhead to provide mounting surfaces for the printhead. Molybdenum, which has high strength and thermal conductivity, in addition to being thermally matched to PZT, making it a particularly suitable material for the lid.

Fig. 6 shows a cross-sectional view of a second embodiment of a droplet deposition apparatus, wherein the cross-section is perpendicular to the direction of the extension of the nozzle rows.

Similar to the first embodiment shown in Figure 3, the printhead support structure 900 includes ink flow passages 910, 920 that extend across the width of the printhead. Ink enters the printhead and the ink supply passage 920 as indicated at 915 in Figure 6. As it flows along the passage, it is drawn into the respective ink chambers 925 via an opening 930 formed in the structure 900. After flowing through the ink chambers, the ink exits through openings 940 and 950 to join the ink flow along the ink outlet passage 910 as indicated at 935.

A flat alumina substrate 960 is attached to the structure 900 via an alumina interlayer 970. The interlayer 970 is preferably bonded to the structure 900 using thermally conductive adhesive of approximately 100 microns thickness, with the substrate 960 in turn bonded to the interlayer 970 using a thermally conductive adhesive.

Driver circuit chips 980 are mounted on a low density flexible circuit board 985. To facilitate manufacturing of the printhead and reduce costs, the portions of the circuit boards carrying the chips 980 are mounted directly on the surface of the alumina substrate 960. To avoid overheating of the driver circuit, other heat generating components of the driver circuit, such as resistors 990, are mounted in substantial thermal communication with that portion of the structure 900 which serves as a conduit, thus allowing a substantial amount of the heat generated by these components 990 during their operation to be transferred to the ink via the conduit structure.

In addition to the alumina substrate and the interlayer, an alumina plate 995 is mounted on the underside of the structure 900 to limit expansion of the aluminum structure 900 at that position, thereby substantially preventing bending of the structure due to thermal expansion.

Claims (19)

1. A droplet deposition apparatus comprising: a liquid chamber (300, 310) having an actuator operable by electrical signals to cause the ejection of droplets from the liquid chamber; a driver circuit device (360, 370) for supplying the electrical signals to the actuator or actuating device; and Conduit means (200) for conveying droplet liquid to and from said liquid chamber, the driver circuit means (360, 370) being in close thermal contact with said conduit means such that a substantial portion of the heat generated in said driver circuit is transferred to said droplet liquid. 2. Apparatus according to claim 1, comprising first conduit means for supplying droplet liquid to said liquid chamber and second conduit means for conducting the droplet liquid from said liquid chamber. 3. Apparatus according to claim 2, wherein said driver circuit means is thermally connected to the second conduction means. 4. Apparatus according to any preceding claim, wherein the driver circuit is enclosed in the housing of an integrated circuit of substantially cuboid shape in which at least some of the faces are rectangles, each having a surface area, one face other than the face with the smallest surface area being arranged to be substantially parallel to the direction of liquid flow in that part of the conduit closest to said face and in close or substantial thermal contact with the liquid. 5. Apparatus according to claim 4, wherein the surface having the largest surface area is arranged so as to be parallel to the direction of liquid flow. 6. Apparatus according to any preceding claim, comprising a support member for said liquid chamber, said support member comprising said conduit means. 7. Apparatus according to claim 6, comprising at least one droplet ejection unit mounted on said support member and having a plurality of said liquid chambers and a plurality of nozzles arranged in a row, said actuator means being operable to eject a droplet of liquid from a liquid chamber through a respective nozzle. 8. Apparatus according to claim 7, wherein the conduit means comprises droplet liquid passages communicating with said plurality of liquid chambers and arranged to convey droplet liquid to and from said liquid chambers in a direction substantially parallel to said row of nozzles and to transfer a substantial portion of the heat generated during droplet ejection to the conveyed droplet liquid. 9. Apparatus according to claim 8, comprising a plurality of said droplet ejection units, the support member supporting the droplet ejection units side by side in the direction of the nozzle rows, the liquid passages communicating with at least two of said ejection units and arranged to convey droplet liquid to or from the ejection units in a direction which is substantially parallel to said nozzle rows and to transfer a substantial portion of the heat generated during droplet ejection to the droplet liquid being conveyed. 10. Apparatus according to claim 8 or 9, wherein the passages for the droplet liquid occupy a major part of the cross-sectional area of the support member. 11. Apparatus according to any one of claims 8 to 10, wherein the droplet liquid passages have respective regions for directing droplet liquid into and out of each liquid chamber. 12. An apparatus according to any one of claims 7 to 11, wherein the cross section of the supporting member is wider in the direction of the nozzle row than in the direction of ink ejection from the nozzle row. 13. Apparatus according to any one of claims 7 to 12, wherein the support member comprises a material with a higher thermal conductivity than said at least one droplet ejection unit. 14. Apparatus according to claim 13, comprising means for attaching said at least one droplet ejection unit to the support member to substantially prevent the transmission of thermal deformation of the support member to said at least one droplet ejection unit. 15. Apparatus according to claim 6, wherein at least a portion of said liquid chamber is made of a first material having a first coefficient of thermal expansion and said support member is at least partially made of a second material having a second coefficient of thermal expansion greater than said first coefficient, said apparatus comprising means for attaching the liquid chamber to the support member to substantially prevent the transmission of thermal deformation of the support member to said liquid chamber. 16. Apparatus according to claim 14 or 15, wherein the fastening means comprises a resilient connecting or bonding means for connecting or bonding the liquid chamber to the support member. 17. Apparatus according to any one of claims 14 to 16, wherein the or each liquid chamber comprises a channel formed in a body of piezoelectric material and closed by a cover or lid member, the cover or lid member being... The cover part is essentially or largely thermally adapted to the piezoelectric material. 18. An apparatus according to claim 17, wherein ink supply ports are formed in said cover. 19. Apparatus according to claim 17 or 18, wherein at least one ink ejection nozzle is formed in said body of piezoelectric material.