Drop-on-demand printhead

0001The present invention relates to drop-on-demand printheads for selectively printing drops of ink in a print line on a web or sheet movable relatively to the printhead.

0002Hitherto drop-on-demand printheads have been applied to form travelling printheads printing the height of one or a few print lines at a time. Certain developments in drop-on-demand printhead design give the prospect of low cost nozzle module assemblies which can be mounted fixed in the printer forming a wide printbar the width of the paper. Recent advances in the printhead reliability make that prospect practical as well as economic.

0003US-A-4,612,554 discloses a drop-on-demand ink drop printhead for selectively printing drops of ink in a printline comprising layers of like print modules of which contiguous modules in adjacent layers are equally laterally offset. In the arrangement described however the modules do not form ink ejectors until they are meshed together when they form a single layer of ink ejectors. The maximum printing resolution thus achievable is limited to the number of ink channels formed when meshing together of the modules is effected. The modules can be meshed either without mutual lateral offsetting or with such offsetting when a laterally extended printhead is to be formed.

0004The present invention is concerned to provide a printhead of modular construction capable of spanning a web or sheet on which printing is to be effected and with which print lines having printing resolution of an integral multiple of two or more times the ejector density in the individual modules are achievable.

0005The present invention consists in a drop-on-demand ink drop printhead for selectively printing drops of ink in a print line on a web or sheet movable relatively to the printhead, comprising layers of like modules of which contiguous modules in respective adjacent layers are equally laterally offset, characterised in that said modules are formed each with a row of parallel directed uniformly spaced ink drop ejectors providing at least one group of ejectors, said groups of ejectors in each of the layers being of equal span laterally and spaced apart by the same amount equal to the product of said span and the reciprocal of one less than the number of layers thereby to provide laterally offset groups in the layers having corresponding segments numbering one less than the number of layers together capable of depositing drops of ink in a particular segment of the print line at a density which is the same for all segments of the print line and is further equal to the product of the drop deposition capability of the groups of ejectors and the number of groups having segments corresponding to said print line segment.

0006Advantageously, ink drop ejectors of the groups are disposed so that drops deposited from segments of ejector groups corresponding with a particular print line segment are interleaved in the print line.

0007In a preferred form, the print modules are formed in stacks of one module per layer and the stacks are disposed in side by side relationship to form the printhead. Suitably, locating means are provided on the modules for assembling the modules into stacks.

0008In the preferred form referred to, advantageously each module normal to the parallel ink ejectors thereof is of rectangular section elongated in the direction of the row of ejectors.

0009The modules of the printheads, according to the invention, are provided with electronic means for actuating the ink ejectors thereof, said means being adapted differentially to delay ejection of ink drops from the ejector groups of the layers to effect printing of ejected drops on the print line. Also, the ink ejectors of the modules are preferably piezo-electrically actuated in shear mode and each includes parallel side walls between said channels thereof one at least of which is actuable to effect drop ejection.

0010A preferred feature of one form of drop-on-demand printhead according to the invention is that make-up ink supply duct means of the modules comprise a riser extending through corresponding modules of the module layers which communicates in each module with the ink ejectors. Suitably, the riser connects with manifold means which in turn connect with the ink ejectors.

0011In a further preferred form of the invention each module is formed with two spaced groups of ink ejectors and with duct means between said groups for supplying make-up ink thereto. Advantageously, the ink supply duct means extend through each module transversely to the module layer and terminate in openings which communicate with openings of modules in adjacent module layers there being provided between said communicating openings, liquid tight sealing means.

0012Preferably air supply duct means of the modules comprise a passage extending through corresponding modules of the module layers transversely to said layers which communicates in each module with a duct which opens at the drop ejection end of the module adjacent the ejector apertures of the ink drop ejectors. Where the modules each have two spaced groups of ejectors the air supply is provided therebetween and comprises a passage section extending through the module transversely to the module layer and a duct which connects with said passage section and opens at the drop ejection end of the module between said groups, the arrangement being such that the passage sections of corresponding modules in the module layers form a continuous air supply passage through the module layers.

0013The invention will now be described by way of example by reference to the accompanying somewhat diagrammatic drawings, in which:- <ul id="ul0001" list-style="none"><li>FIGURE 1 shows a module part of an array drop-on-demand printhead of the type installed in co-pending European Patent Application No. 0 278 590 published 17th August 1988.</li><li>FIGURE 2(a), 2(b) and 2(c) each show a printbar assembly in section in which the modules are grouped in stacks having respectively three, four or five layers of modules;</li><li>FIGURE 3 shows a printbar assembly in isometric projection of the type in which stacks are grouped having three layers of modules;</li><li>FIGURE 4 shows an isometric projection view of a single module particularly illustrating feed-through ducts for the supply of ink and air flow to and from housekeeping manifolds;</li><li>FIGURE 5 shows a section view of a stack comprising four layers of laterally overlapping modules of the type illustrated in Figure 4;</li><li>FIGURE 6 shows an exploded isometric view of the module, nozzle plate and housekeeping manifold;</li><li>FIGURE 7 shows an enlarged view (with increased vertical scale) of a section of the housekeeping manifold parallel to the nozzle plate, the portion of the figure to the left of the chain dotted line being taken on the line C-C of the portion thereof to the right of the chain dotted line; and</li><li>FIGURE 8 shows a further enlarged view of a section of the housekeeping manifold normal to the nozzle plate in the plane of the air flow shields.</li></ul>

0014Figure 1 shows a module 10 of a piezo-electric shear mode actuated drop-on-demand printhead of the type illustrated in our co-pending European Patent Application No. 0 278 590 published 17th August 1988 and No. 0 277 703 published 10th August 1988, the contents of which are incorporated herein by reference.

0015Printhead modules of the invention referred to are employed to describe the present invention, but the invention is not thereby limited. However piezo-electrically driven ink drop ejectors prior to that invention were limited to a channel spacing of 1 to 2 channels per mm. The modules illustrated are able to be produced at higher densities, for example, 4, 5¹/3 and 8 channels per mm. These can be conveniently assembled into a wide printbar having 16 ink channels and printing 16 independently deposited drops per mm into a print line by stacking 5, 4 or 3 layers of laterally overlapping modules which combine 4, 3 or 2 rows of nozzles respectively to generate interleaved segments of the print line at the full design density.

0016The method of the invention can be readily adapted to form a variety of print line densities both above and below 16 per mm, and is best suited to combining small numbers of modules (3-6) into stacks and to grouping multiple lines of stacks to form multi-colour printbars. It is also readily applied to types of printhead other than those which are piezo-electrically actuated, including thermal and air assisted types.

0017Figure 1 shows a module 10 of a printhead 1 energised via a drive chip 12 and drive tracks 14. Each drive track 14 is connected to a corresponding ink channel 16 supplied via a manifold with make up ink from supply 15. The ink channels 16 are terminated with corresponding nozzles 18. These are illustrated for clarity formed in a nozzle plate 17 of the module shown separate from a body part thereof. The ink channels 16 and the corresponding nozzles 18 form a continuous row 19 of independently actuable ink drop ejectors occupying a substantial part of the width of the module 10 at a linear density of N drops per unit length.

0018The modules 10 are conveniently incorporated into a printbar having drop densities of 2N, 3N or 4N (rN) etc. drops per unit length by combining the modules in separate stacks having 3, 4 or 5, (r + 1) etc. layers of overlapping modules in a stack respectively, as illustrated in the parts of Figure 2. Thus Figure 2(a) illustrates a printhead 1 made up of separable stacks 20a, 20b, 20c of laterally overlapping like modules having three laterally offset layers, 22, 24, 26 and providing a print density of 2N where N is the density of ink channels in one module. The horizontal line drawn in each module represents a line of nozzles located so that the nozzles from different layers interleave one another when projected onto the print line. One segment of the print line is made up from drops printed from the right hand side of the top layer modules 22a-d of the corresponding stack 20a-d and the left hand side of the middle layer modules 24a-d. A second segment is made up from drops printed from the right hand side of the middle layer modules 24a-d of the stack and the left hand side of the bottom layer modules 26a-d. The third segment is made up of the right hand side of the bottom layer modules 26a-d of one stack and the left hand side of the top layer modules of the adjacent stack 20b-e. The necessary print delay associated with operation of modules in each layer needed to effect collinear deposition of the drops from the different layers of modules is readily accomplished by data storage in the chip or data distribution system.

0019Figure 2(b) shows a corresponding arrangement of stacks 30a-d having four layers of laterally overlapping like modules 32, 34, 36 and 38 in each layer and providing a print density of 3N. Similarly Figure 2(c) shows corresponding stacks 40a-c having five layers of like modules per stack and achieving a print density of 4N. In each case the extra layer provides an interval between the overlapping modules in each layer to butt the adjacent modules at the same time providing for the supply of ink to the ink channels and air or solvent flow to the housekeeping manifolds as hereinafter described.

0020It will be apparent from the foregoing description that the spacing between the groups of ejectors in each layer is the product of the span of the group and the reciprocal of one less than the number of layers.

0021Replaceable stacks of like laterally offset modules combined in laterally overlapping stacks of modules of this arrangement provide a number of advantages. One advantage of overlapping modules is that the ink modules can be conveniently butted in each layer leaving a region between the ink channels of adjoining modules containing no ink channels. The nozzles for supplying the corresponding region in the print line are made up from the other layers of modules. Since the outermost channels in each are located inwardly from the sides of the module, the modules have a robust construction. The next benefit is that by forming a print bar out of a number of replaceable stacks, field servicing of a wide printbar is more readily accomplished than by replacing the entire printbar. Modules in each stack may also optionally be replaced.

0022Another benefit is that a simple alignment procedure can be used for assembling the modules together into stacks using physical guides (such as dowels or pre-cut grooves and location bars) or optical means (using a vernier system of readily observed optical fringes). The same alignment procedure can be used progressively to locate nozzles relative to the modules during nozzle manufacture, to assemble modules into a stack and to assemble the stacks into the printbar so that the nozzles and nozzle plates are automatically aligned by appropriately designed jigging in manufacture relative to a fixed datum in the printbar. In this way all the nozzles in the stack are correctly interleaved in alignment with the printbar.

0023A particular advantage of having nozzles interleaved from different layers of the stack is that even if failure of a whole module occurs, the print line shows only a change in the print shade and the drawing or written page is substantially readable.

0024Another design advantage is that whereas modules and stacks are individually replaceable, housekeeping manifold supplies, electronic power and data are organised on a printbar basis.

0025A further advantage is that the same design of the ink channels 16 having the same density N and chip drive voltage can be incorporated into printbars having a multiple density of 2N, 3N and 4N etc., providing for a range of print quality from the same modular parts.

0026Figure 3 shows an isometric perspective view of a three layer stack, in which the relative locations of the overlapping modules 10, stacks 20 and printbar 2 can be visualised. Segments of the print line 3 are each made up of nozzles interleaved from two modules in any section. To better illustrate this the print line is shown below the module layers. It is of course in practice to be found on the web or sheet which moves across the face of the printhead.

0027The modules assembled in printbars in Figure 2 at first appear to be unconstrained in the number of nozzles per module and hence module size. Obviously once the resolution of nozzles N/mm in each module and the number of rows of nozzles r which are interleaved to form any particular section of the print line is decided, then if the number of layers of modules in a stack is (r + 1), the print line density is constrained to the integral multiple rN dots/mm.

0028In practice however the number of ink channels energised by one chip is usually a binary number, for example 32 (5 bit) 64 (6 bit) or 128 (7 bit) etc: in addition one module may carry more than one chip. Thus the length of the continuous row of nozzles in one module is limited to only certain values such as <maths id="math0001"><math display="inline"><mrow><msub><mrow><mtext>L = 32</mtext></mrow><mrow><mtext>/N</mtext></mrow></msub><msub><mrow><mtext>mm, 64</mtext></mrow><mrow><mtext>/N</mtext></mrow></msub><msub><mrow><mtext>mm, 128</mtext></mrow><mrow><mtext>/N</mtext></mrow></msub><mtext>mm etc.</mtext></mrow></math><img file="EP0339926B1_D0001.tif" /></maths> and the pitch of the stacks are also limited to values<maths id="math0002"><img file="EP0339926B1_D0002.tif" /></maths> Hence there is a limited set of stack pitches for 16 dots/mm print density given by the table. <tables id="tabl0001" num="0001"><img file="EP0339926B1_D0003.tif" /></tables>

0029It will be obvious that certain other cases can also be constructed. For example the number of layers of modules in a stack can be trivially modified to have (r + 2) or 2(r + 1) layers: alternatively stacks can (as will later be illustrated) be doubled in width to incorporate two rows of nozzles in each laterally overlapping module part, with the advantage that feed-throughs can be delivered centrally rather than at the edge of the modules. These alternative cases do not alter the basic principles involved of combining laterally overlapping modules into the stacks.

0030Thus the pitch interval of the stacks is found to be constrained once other choices are made to a limited number of preferred values from which printbars can be assembled.

0031A particular feature of the stack construction is that the supplies of ink, the housekeeping manifold fluids and electronic power and data are organised on a printbar basis but are distributed through each stack individually. Accordingly the modules in each stack are designed to feed the supplies from one module to another vertically through the stack.

0032The feed-throughs vertically through the stack connecting the modules are illustrated in Figures 4 and 5. Figure 5 shows the printbar 2 on which is mounted a stack 30 having modules 32, 34, 36, 38 each made with two rows of nozzles 19 which communicate with ejector channels contained in the spaces 116. The modules are placed in four overlapping layers as previously illustrated in Figure 2(b).

0033The ink supply system which feeds make up inks vertically through each stack to replenish ink ejected from the print modules is shown in Figure 5 in the upper two modules 32 and 34, which are sectioned on AA in Figure 4 in the rear of each module. The modules are constructed as shown for modules 32 and 34 with ink feed manifolds 102 and 104 which are cut laterally across each module in opposite directions and are shown by the cross-hatching filled with ink. These manifolds connect with the ink channels 116 in Figure 4 (16 in Figure 1), so that suction is created in the manifolds when drops are ejected by actuation of the ink channels.

0034The modules are cut away with apertures 105 and 107 on their upper and lower faces. These are offset so that corresponding apertures are in alignment when the modules are assembled as an overlapping stack and are sealed by means of an 0-ring 109 (or similar means) inserted round the periphery of the apertures. The apertures 105, 107 are also connected by a riser 108. A cover 110 is employed to seal the riser at the top of the stack. The feed-through vertically through the stack formed by the apertures 105, 107, the risers 108 and the manifold branches 102, 104 etc. are made as large as practical to minimise the viscous resistance of the replenishment ink flow. The air flows which are fed to and from the housekeeping manifold are ducted through feed-throughs in each stack as illustrated in Figure 5 by the lower two modules 36 and 38. These are sectioned on BB in Figure 4 at the forward end of each module. The flow supplied to or from one portion of the housekeeping manifold is delivered through the bore 114 and the flow supplied to or from the other portion of the housekeeping manifold is delivered via bore 112. The bores 112 and 114 both exit the front face of the modules 32--38 and penetrate a substantial distance back through the modules between the space occupied by the ink channels 116. The bore 112 is connected to apertures on the upper and lower faces of each module of which aperture 115 is seen in Figure 4 whilst aperture 117 is shown in Figure 5. The apertures 115 and 117 are assembled in an overlapping stack. The apertures are sealed by means of 0-rings. The bore 114 is similarly connected to apertures 115′ on the upper faces of the modules immediately behind and separate from the former apertures 115. Apertures (not shown) offset with respect to apertures 115′ are provided on the lower faces of the modules so that the modules can be similarly assembled and sealed. The stack assembly formed in this way enables a flow of ducted air to be delivered to or ducted from the modules in each stack by pressure and suction on the corresponding ducts in the printbar.

0035The description above shows that both ink and ducted air flows can be fed from the printbar to modules stacked in laterally overlapping form of assembly for the continuous operation of the modules. If the modules provided a single group of ejectors rather than two groups, the ink supply duct would extend through the stacks rearwardly of the ink channels 116 where it would be connected to those channels, for example, by way of a manifold.

0036The supply of ducted air to housekeeping manifolds, which are illustrated in Figures 6, 7 and 8, is employed to enhance the operating reliability of the drop-on-demand printhead 1 compared with prior art printheads in which the nozzle plate faces the print paper, without the benefit of environmental control.

0037The general construction of the housekeeping manifolds applied to modules 10 will first be described. Figure 6 shows an exploded view of the module 10 with two groups of closely spaced ink channels 16 placed on each side of the module in the majority of its width. Ducts for supplying air flows to or from the housekeeping manifold are labelled 112 and 114. Separated from the module is a nozzle plate 17 having two continuous rows 19 of ink ejector nozzles which selectively eject drops through the nozzles 18. The nozzle plates are made with apertures opposite the ducts 112 and 114. Displaced again from the nozzle plate 17 is the housekeeping manifold 50. This is shown sectioned parallel to the nozzle plate to reveal the internal structure, there being simply added a cover 51 to the material illustrated. The housekeeping manifold also has a trench 53 cut right through in the location opposite each row of nozzles 18 so that ejected drops (see Figure 8a) are shot through the trench 53.

0038The module assembly is made by bonding these parts together as illustrated in Figure 7 and 8. The nozzle plate 17 is first bonded to the module 10, and the housekeeping manifold is next bonded to the nozzle plate. Air ducted from the bore 114 of the duct feed-throughs consequently enters the lower section of the housekeeping manifold, where it spreads with uniform velocity by reason of the tapered section and exhausts through the row of apertures 55 in the trench wall into the trench. Suction from the printbar through bore 112 similarly exhausts air from the other side of the trench 53: alternatively the air flow from bore 112 can be reversed and ducted out through the row of apertures 55 which join the trench 53 to the manifold to combine with and augment the flow already exhausting into the trench from the lower manifold.

0039The application of the air flows provided by the housekeeping depend on the phase of operation of the printhead 1, and also on the detailed specifications of the routines required to maintain reliable operation of the printhead. This enables two longstanding reliability problems of drop-on-demand operation to be substantially eliminated.

0040These are: <ul id="ul0002" list-style="none"><li>(1) Ingress of atmospheric dust.</li><li>(2) Evaporation of solvent from the ink menisci at the nozzle plate.</li></ul>

0041The collection of dust on the nozzle plate is tolerated on travelling head drop-on-demand printers. The dust can be removed by high speed drop ejection or wiping. Such a routine is not acceptable on a wide bed drop-on-demand printer, where long term trouble free operation must be assured over the range of duty cycles experienced in the field.

0042Dust is inherently part of the environment of a printer; it is carried in by electrostatic fields, convection currents and with paper movement and often originates from the paper. Operation of some jets causes dust to be pumped by convection into neighbouring jets. It is therefore evident that the provision of filtered dust free air past the printhead nozzles is essential for reliable operation.

0043Filtered air flow to protect the nozzles from dust is conveniently provided by the housekeeping manifold 50. This is conveniently made practical by supplying the ducted air flow into the trench 53 in front of the nozzles as illustrated in Figure 8(a).

0044It will be evident that the housekeeping manifold 50 need not be confined to the module construction but can also be applied to a nozzle plate the full width of the printhead; or to a travelling printhead.

0045In operation the housekeeping air flow is needed during periods of operation of the printhead (Figure 8(a)) but need not be employed when the printhead is dormant or waiting to be used, which is the status of a printer during the majority of its use. The trench 53 may therefore be covered by a sliding cover 57 (Figure 8(b)) during dormant periods.

0046During operation periods the ducted air flow supplied to the housekeeping manifold causes scavenging air to flow in the trench and to remove solvent vapour evaporated from the ink meniscus. There are a number of strategies for preventing solvent evaporation or limiting the deleterious effects of solvent evaporation from the ink meniscus, provided by the housekeeping manifold.

0047First (and particularly with water based ink) the ducted air can be modified to contain a proportion of solvent vapour (i.e. by controlled humidity). In many cases the partial pressure of the ink at operating temperature is low so that the solvent humidity necessary to avoid encrustation or formation of a film over the ink meniscus is low: but even high vapour pressure solvents (such as ethanol) can be held in a print ready status this way.

0048Second the ducted air means that the conditions obtaining and therefore the degree of evaporation that has occurred at every nozzle is known. It is usually found that an ink will tolerate a known period such as 100 to 1000 seconds before ink drying becomes serious. Most inks have low vapour pressure additives that reduce the rate of evaporation of the low boiling point constituents. It is possible in that case to eject drops periodically from all under or unutilised nozzles, so that they are replenished with new ink as evaporation occurs, before the nozzle plug becomes too viscous, and inhibits printing.

0049A further strategy is to make the printhead dormant for short periods (e.g. 15 seconds) at intervals, to circulate air with a higher solvent mass ratio so that any menisci which have a reduced solvent partial pressure (i.e. are dry) are restored. This is found to occur rapidly (e.g. in less than 15 seconds) and print ready status is restored. It may be preferred to close the sliding cover 52 over the trench 55 during this operation. However when there is no printing taking place, the tendency of ejected drops to set up flows which draw dust in is minimised. Thus solvent circulation can occur without closing the sliding cover with very little solvent loss. It will therefore be seen that the housekeeping manifold provides substantial opportunities to reduce and substantially eliminate the principal causes of drop-on-demand printhead unreliability and therefore to assure the levels of availability demanded of a wide array printhead.

0050The housekeeping manifold further enables the printhead to be kept at a print ready status during dormant periods. This is obtained by closing the trench 53 with the sliding cover (or by another means) at the beginning of a dormant period and at the same time briefly circulating solvent rich air. It is sufficient to repeat this intermittently (i.e. every 1/2hr. to 1hr., depending on the temperature and other conditions) to maintain the menisci in a print ready status.

0051When the dormant period is very long, or the printer is disconnected from the power supply, however, the housekeeping manifold can be used to supply liquid solvent in the region of the printhead. In that case the ducted air flows may be used in a different sequence at start up to remove the solvent from the housekeeping supply ducts and to reestablish a print ready status.

0052Electrical connection of the modules in a stack typically involves the connection of <ul id="ul0003" list-style="dash"><li>Data lines -- 1</li><li>Clock lines -- 2</li><li>Voltage lines -- 2</li><li>Earth lines -- 1</li></ul>

0053The connection is simplified by the realisation that every chip can be connected either in series or in parallel. One series of 8 parallel tracks can therefore be connected layer by layer through the stack to every chip. Electrical connection of a stack does not present serious problems even if double the number of parallel lines is required.