EP1046505A1 - Inkjet printhead - Google Patents

Abstract

Ink jet printhead comprising:

• at least one ink channel (16) connected to a nozzle (18) and defined between toopposed side walls (20);
• a membrane (22) interconnecting the side walls; and
• actuating means (24) arranged adjacent to the membrane (22) for deflecting thesame,

characterized in that the side walls (20) and the membrane (22) are formed by a one-piecemember (14).

Description

• The invention relates to an ink jet printhead comprising at least one ink channelconnected to a nozzle and defined between two opposed side walls; a membraneinterconnecting the side walls; and actuating means arranged adjacent to themembrane for deflecting the same.
• A conventional printhead of the type, which is disclosed for example in EP-A-0 819 524,comprises a plurality of ink channels which are arranged side by side so that theassociated nozzles form a linear nozzle array. The ink channels and the nozzles areformed by grooves cut into the surface of a substrate which may for example be madeof silicon. The membranes for the various ink channels are formed by a continuous thinsheet which is overlaid on the substrate so that it covers the open top sides of thegrooves. This sheet has to be firmly bonded to the regions of the substrate defining theside walls of the ink channels. The actuators are formed by piezo-electric fingers whichare supported by a common backing plate and have their respective free ends bondedto the top surfaces of the membranes, so that, when a voltage is applied to an individualone of the piezo-electric fingers, the corresponding membrane is flexed into the volumeof the associated ink channel which is filled liquid ink, so that an ink droplet is expelledfrom the nozzle.
• US-A-4 657 631 discloses another type of printhead in which the ink channels areformed by groove-like structures in a metal layer which is formed on the flat surface ofthe substrate. Thus, the bottom wall of each ink channel is formed by a portion of thesubstrate, and rigid side walls and a rigid top wall are formed by the metal layer. Theactuators are disposed inside of each ink channel and are disposed on the surface ofthe substrate, so that they are directly exposed to the ink liquid without a membraneintervening between the actuator and the ink. This type of printhead can bemanufactured by forming a photo-sensitive layer on the surface of the substrate and byexposing and developing this layer, thereby to form a pattern of ridges which have ashape complementary to that of the ink channels, then forming a metal layer on thesurface of the substrate by sputtering and subsequent electronic plating, until the ridgesare buried in the metal layer, and then removing the photo-sensitive material, so that the ink channels are formed in the metal layer.
• It is an object of the invention to provide an ink jet printhead according to the preambleof claim 1, which can be manufactured more easily and with a higher production yield.
• This object is achieved with the features indicated in claim 1.
• According to the invention, the side walls and the membrane are formed by a one-piecemember.
• This construction has the advantage that, on the one hand, the membrane can be madevery thin so that it has a higher flexibility, and, on the other hand, the member formingthis membrane and the side walls of the ink channels can, as a whole, have acomparatively high structural strength. This greatly facilitates the manufacturing processand makes it possible to achieve a high production yield.
• More specific features of the invention are indicated in the dependent claims.
• Preferably, the one-piece member forming the membranes and the side walls of aplurality of ink channels is a metal foil which is obtained in a growth process such aselectroforming. This has the advantage that the membrane can be made very thin andthe thickness thereof can be controlled with high accuracy. Further, since the foil can beformed directly on the surface on the substrate, no assembly process nor a separatebonding step is required for forming a complete channel plate including the substrateand the metal foil which forms the membrane and the side walls of the ink channels.
• By forming the metal foil on the surface of a photoresist which has appropriately beenshaped by means photolithographic techniques, it is possible to obtain a three-dimensionalstructure of the foil which forms not only the membrane and the side wallsof the ink channels but also the nozzles with appropriate sizes and shapes. The cross-sectionof the foil in a plane normal to the axis of the ink channels has a meander-likeshape, with a space formed between the portions of the foil which define the side wallsof two adjacent ink channels. This construction greatly reduces the amount ofmechanical or acoustic coupling between the adjacent ink channels, so that cross-talkamong the various channels of the printhead is reduced significantly. In addition, this cross-sectional shape of the foil has the effect that the thermal expansion of the channelplate is controlled only by the material of the substrate, e.g. silicon, which is particularlyuseful when the printhead is used for hot-melt ink and, accordingly, operates and hightemperatures.
• Since the nozzles are formed directly by the three-dimensionally structured foil, nomechanical finishing of the nozzle front of the printhead is necessary.
• By providing a thin metal layer on the surface of the substrate on which the foil isapplied in a later manufacturing step, it is possible to obtain a design in which the inkchannels and, more important, also the nozzles are completely surrounded by only onetype of material, i.e. metal, so that the directionality of the droplet-formation process willnot be influenced by differences in the adhesiveness of the walls defining the nozzles.
• Preferred embodiments of the invention will now be described in conjunction with theaccompanying drawings, in which:
• Fig. 1 is a perspective view of essential parts of an ink jet printhead according to theinvention;
• Fig. 2 is a perspective bottom view of the printhead with parts of a substrate beingremoved for illustration purposes;
• Fig. 3 is a cross-sectional view of a member defining the ink channels of a printheadaccording to a modified example; and
• Fig. 4-9 illustrate a sequence of steps for manufacturing the printhead according to theinvention.
• As is shown in figure 1, an ink jet printhead comprises achannel plate 10 which has asubstrate 12 made of silicon, and a three-dimensionally structuredmetal foil 14 formedon the flat top surface of thesubstrate 12. Thefoil 14 defines a plurality ofparallel inkchannels 16 which are arranged side-by-side and each converge to anozzle 18, so thatthe nozzles form a linear array along the front side of thesubstrate 12.Eachink channel 16 has a pair ofopposed side walls 20 which are interconnected by amembrane 22, so that themembrane 22 forms a top wall of the ink channel opposite tothesubstrate 12. Theside walls 20 andmembranes 22 of all theink channels 16 areintegrally formed by a one-piece member, i.e. by themetal foil 14.
• The printed further comprises a plurality of piezo-electric actuators 24 only one of whichis shown in Fig. 1. Theactuators 24 are disposed on each of themembranes 22, sothat, when they are electrically energized, they perform expansion stroke, and themembrane 22 flexes into the interior of the ink channel. Thus, when the ink channel isfilled with liquid ink, an ink droplet will be expelled from thenozzle 18.
• Thefoil 14 also forms afront wall 26 in which thenozzles 18 are defined. Each nozzle isconnected to the associatedink channel 16 by afunnel portion 28 which is also anintegrated part of thefoil 14. Further, thefoil 14 forms crater-like reinforcementmembers 30 which are partly cut by thefront wall 26 and are provided between thenozzles 18 for improving the structural strength of thefront wall 26.
• As can be seen more clearly in Figs. 2 and 3, thefoil 14 has a meander-like cross-sectionalshape in a plane normal to the longitudinal direction of theink channels 16,with V-shapedconcave spaces 32 formed between theside walls 20 of each pair ofadjacent ink channels. Thus, when anindividual actuator 24 is energized and anacoustic pressure wave is generated in the ink liquid contained in the associatedinkchannel 16, thespace 32 will prevent this pressure wave from being transmitted to theneighboring ink channels, so that cross-talk among the various channels is avoided. Inaddition, due to its meander-like cross-section, thefoil 14 can behave like an expansionbellow and can adapt itself to thermal expansions of thesubstrate 12 without causing adistortion of thechannel plate 10 as a whole. On the other hand, when exposed tovertical pressure, thefoil 14 has a comparatively high strength and is surprisinglyrobust, even when the thickness of the foil is only in the order of several µm.
• In a typical embodiment, the thickness of the foil in the portion forming themembranes22 is between 3 and 30 µm, preferably 15 µm, with an accuracy of ±1 µm. This assuresa high flexibility of themembranes 22 so that the mechanical energy of theactuators 24is readily transmitted to the ink liquid. The high level of accuracy in the membranethickness assures a uniform performance of all the ink channels.
• Fig. 3 illustrates a modified embodiment in which an outwardly projecting ridge orbump34 is formed along the longitudinal center line of eachmembrane 22 which allows for ahigh amount of deflexion of the membrane with little strain on the foil material, even when the width of theactuator 24 is comparatively large.
• A process of manufacturing thechannel plate 10 will now be described in conjunctionwith Figs. 4 - 9.
• At first, as is shown in Fig. 4, athin layer 36 of metal, e.g. of nickel or nickel alloy isformed on the flat top surface of thesubstrate 12. Thelayer 36 covers the whole surfaceof the substrate with the exception of aportion 38 directly adjacent to the nozzle side ofsubstrate and with the exception of ahole 40 near the rear end of each ink channel.Then, a three-dimensionally structuredphotoresist 42 is applied on the substrate and onthelayer 36 by means of photolithographic techniques. The shape of thephotoresist 42is complementary to the relief of the bottom side of thefoil 14 shown in Fig. 2.
• In the next step, thefoil 14 is formed on the surface of thelayer 36 and on the surface ofthephotoresist 42 by means of a nickel-electroforming process. The result is shown inFig. 6. Then, as is shown in Fig. 7, anink feed channel 44 is formed in thesubstrate 12by etching, laser drilling, powder blasting or the like.
• In order to form thenozzle face 46 of the channel plate Fig. 8, a dicing cut is performedin thesubstrate 12, thephotoresist 42, and thefoil 14. The plane of this dicing cut isslightly offset from thefront wall 26 of thefoil 14, so that a smallhorizontal flange 48 isformed along the upper edge of thefront wall 26.
• Finally, thephotoresist 42 is removed, so that theink channel 16 and thenozzle 18 areformed, as is shown in Fig. 9. The ink channel communicates with theink feed channel44 through thehole 40 formed in thelayer 36. As is also shown in Fig. 9, the front edgeof thelayer 36 is located in the same longitudinal position as the upper and outer edgeof the wall of thenozzle 18, so that the mouth of thenozzle 18 lies in a plane normal toplane of thesubstrate 12 and is slightly recessed from the main part of thefront wall 26.As a result, the droplets will be expelled in a direction strictly aligned with thelongitudinal direction of the ink channel, and no finishing needs to be applied to thenozzle face.

Claims (10)

1. Ink jet printhead comprising:

   at least one ink channel (16) connected to a nozzle (18) and defined between toopposed side walls (20);

   a membrane (22) interconnecting the side walls; and

   actuating means (24) arranged adjacent to the membrane (22) for deflecting thesame,

   characterized in that the side walls (20) and the membrane (22) are formed by a one-piecemember (14).
2. Ink jet printhead according to claim 1, wherein the one-piece member (14) is made ofmetal.
3. Ink jet printhead according to claim 1 or 2, wherein is said one-piece member (14) isa member that has been obtained by a growth process.
4. Ink jet printhead according to claims 2 and 3, wherein is said one-piece member is ametal foil (14) that has been obtained by electroforming.
5. Ink jet printhead according to claim 3 or 4, wherein a side of the ink channel (16)opposite to the membrane (22) is closed by a substrate (12) on which the one-piecemember (14) is grown.
6. Ink jet printhead according to any of the preceding claims, wherein the nozzle (18)and a funnel portion (28) connecting the ink channel (16) to the nozzle (18) are alsodefined by said one-piece member (14).
7. Ink jet printhead according to claims 5 and 6, wherein a portion of the substrate (12)delimiting the ink channel (16), the funnel portion (28) and the nozzle (18) is covered bya layer (36) made of the same material as the one-piece member (14).
8. Ink jet printhead according to any of the preceding claims, comprising a plurality of inkchannels (16) arranged side-by-side, so that the associated nozzles (18) from a lineararray, wherein said one-piece member is a continuous metal foil (14) forming the side walls (20) and the membranes (22) of the plurality of ink channels and having ameander-shaped cross-section in a plane normal to the ink channels.
9. Ink jet printhead according to claim 8, wherein the metal foil (14) forms a front wall(26) in which the linear array of nozzles (18) is defined and wherein the mouths of theindividual nozzles (18) are recessed relative to the front wall (26).
10. Ink jet printhead according to any of the preceding claims, wherein the actuatingmeans (24) engage an outwardly projecting bump (34) formed in a central portion of themembrane (22).

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