Method of manufacturing of printing apparatus

To allow accurate positioning relative to a printer mechanism, a printhead is provided with a reference surface formed on a reference member. The reference member is attached to the base of the printhead but positioned with reference to a nozzle of an ink ejecting unit mounted on the base member. This obviates the need for the base member to be manufactured to narrow tolerances.

BACKGROUND OF THE INVENTION
 The present invention relates to methods of manufacture of printing
 apparatus, particularly methods of manufacture of droplet deposition
 apparatus such as inkjet printheads.
 Apparatus for deposition of droplets of ink or other fluid are well known.
 As shown, for example, in EP-A-0 278 590 (belonging to the present
 applicant and incorporated herein by reference), they comprise one or more
 ink ejecting chambers from which droplets of ink are ejected, generally
 via a nozzle, towards a substrate on which an image is to be printed.
 To ensure correct positioning of the printed image on the substrate, it is
 necessary to position the ink ejecting chambers and/or their nozzles
 accurately relative to the substrate. This is particularly important where
 several printheads are used to print several overlapping images of
 different colours (normally cyan, magenta, yellow and black) to provide a
 full-colour printed image.
 Typically, a printer mechanism is employed to hold the substrate relative
 to a reference surface on the printhead. The printhead is in turn
 manufactured such that the reference surface lies a fixed distance from
 the ink ejecting chambers and/or their nozzles of the printing unit (where
 a printhead has an array of ink ejecting chambers, the reference surface
 may be positioned relative to a particular one (e.g. the first) of the ink
 ejecting chambers). However, a tight tolerance on this fixed distance is
 necessary if the overall positioning of the printhead relative to the
 substrate is to be to the high accuracy required. Manufacturing the
 printhead to such tight tolerances may be difficult to achieve, however,
 and will depend, inter alia, on the material of the printhead, its form
 and overall dimensions. The present invention seeks to avoid these
 difficulties.
 SUMMARY OF THE INVENTION
 Accordingly, the invention consists a method of locating a reference
 surface on a printing apparatus, the apparatus comprising a printing unit
 having at least one printing element and mounted on a support member; the
 method comprising the steps of: positioning the apparatus such that said
 printing element is located at a first predetermined position; positioning
 a reference member having said reference surface such that said reference
 surface is located at a second predetermined position; said first and
 second positions being in a predetermined spatial relationship; and fixing
 the reference member to said support member, thereby to fix said reference
 surface in a predetermined position relative to said printing element.
 By this method, it is possible to align the printing elements of a printing
 apparatus with the reference surface independently of any intermediate
 printhead structure, thereby avoiding any difficulties that may be
 associated with such an intermediate structure. and any manufacturing
 difficulties associated therewith.
 Where the printing apparatus is droplet deposition apparatus such as an
 inkjet printhead, the term `printing element` covers not only an ink
 ejecting chamber in a printhead but also the respective nozzle, where this
 is present.
 The present invention also consists in a method of manufacturing a printing
 apparatus that includes a printing unit comprising a droplet ejection unit
 having at least one printing element, said at least one printing element
 comprising a nozzle for droplet ejection formed in a nozzle plate, and a
 support member for said droplet ejecting unit; the method comprising the
 steps of: mounting said droplet ejecting unit on said support member;
 thereafter forming at least one nozzle in said nozzle plate of the droplet
 ejecting unit; and thereafter arranging a reference member such that a
 reference surface of said reference member is located at a predetermined
 position relative to said at least one nozzle; and fixing the reference
 member to said support member.
 As before, such a method allows the nozzles of a droplet deposition
 apparatus to be aligned with the reference surface independently of any
 intermediate printhead structure, thereby avoiding any difficulties that
 may be associated with such an intermediate structure. Furthermore, since
 the relative positioning of nozzle and reference surface only takes place
 after the droplet ejecting unit has been mounted on its support member,
 the mounting process need only be carried out to an accuracy appropriate
 to the relative location of the unit and the support member rather than to
 an accuracy appropriate to the relative location of a nozzle of unit and a
 reference surface of the support member.

DESCRIPTION OF PREFERRED EMBODIMENTS
 FIG. 1 depicts an inkjet printhead 5 manufactured according to the present
 invention and comprising an ink ejection unit or units 10 mounted at one
 end of a base member 15. Base member 15 may be made of a thermally
 conductive material such as aluminium so as to carry away heat generated
 both in the ink ejection units and in printhead driving circuitry mounted
 on circuit board 20. Driving signals are conveyed from one end of the
 circuit board to the ink ejection units, for example by wire bonds 25,
 whilst print data and power arrive at the other end of the circuit board
 via connector 30.
 As shown, four manifolds 35 supply ink of four different colours (generally
 cyan, magenta, yellow and black) to four neighbouring ink ejection units,
 although these manifolds could equally well supply the same colour ink to
 all ink ejection units or be replaced by a single ink manifold. As
 explained hereafter, registration between the channels of the different
 ink ejection units is achieved e.g. by forming all four units in a single
 base member. Manifolds 35 are clamped in sealing contact with the ink
 ejection units 10 by means of a bar (not shown) that sits in recesses 36
 and which in turn is secured--e.g. by means of bolts--to chassis 15. These
 features are known in the art, e.g. from WO97/04963 belonging to the
 applicant and incorporated herein by reference, and consequently do not
 require discussion in any further detail. Ink ejection takes place from a
 line of nozzles 40 formed in a nozzle plate 45, with each nozzle
 communicating with a respective ink-ejecting chamber of the ink ejecting
 unit 10.
 Base 15 is formed on its lower surface with a groove 50 in which a rod 55
 is located so as to protrude from one side of the base as illustrated in
 FIG. 1. The end surface 60 of rod 55 serves as a reference surface/datum
 face, registering with another datum on a printhead support structure (not
 shown) so as to ensure the correct positioning in the nozzle array (X)
 direction of the printhead within that support structure. This in turn
 requires that the reference surface to lie perpendicular to the nozzle
 array direction and, where the reference member is prismatic having a
 prism axis, that this axis lies parallel to the nozzle array direction.
 In particular, rod 55 allows the ink ejecting nozzles to be correctly
 located within the printhead support structure which in turn ensures the
 correct positioning in the X-direction of the ink droplets ejected from
 the nozzles on the substrate to be printed (obviously, any variation from
 printhead to printhead in the positioning of the printed image on the
 substrate is undesirable).
 This is achieved according to the method illustrated schematically in FIG.
 2: the printhead 5 together with the rod 55 located--but not secured--in
 groove 50 is placed in a jig (not shown) and the first nozzle 65 in the
 row of nozzles 40 aligned with a first jig reference plane 70. Next, the
 rod 55 is moved along the groove so as to align end face 60 with a second
 jig reference plane 75, spaced from the first jig reference plane by a
 fixed distance A. The rod is subsequently immovably secured in the groove
 and the printhead removed from the jig. When the printhead is subsequently
 mounted in a printhead support structure using this datum, the user can be
 certain that the first nozzle 65--and hence the whole nozzle array 40--of
 the printhead will be positioned a fixed distance relative to the support
 structure.
 It will be appreciated that in practice, the first jig reference plane 70
 will generally be defined by the cross-hairs of a microscope whilst the
 second jig reference plane will be defined by an abutment for the end of
 the rod 55. Although alignment with the nozzle 65 at the end of the nozzle
 row 40 is shown in the example of FIG. 2, alignment may be sought with
 nozzles located elsewhere in the row 40. Furthermore, some other feature
 having a position related to that of the nozzle--e.g. the ink channel
 located behind the nozzle and visible from the front of the printhead
 through the translucent material of the nozzle plate--may be used in the
 alignment process in place of the nozzle.
 Befitting its reference function, rod 55 is preferably of a material having
 a low coefficient of expansion, for example quartz or a ceramic such as
 alumina. In the example shown, the rod is of 2 mm diameter and protrudes
 approximately 1 mm from the side of the body 15. Groove 50 is ideally
 located as near as possible to the plane of the nozzle plate so as to
 minimize errors due to expansion of the printhead base 15.
 FIG. 3 is an enlarged section through the front end of the printhead of
 FIG. 1 taken normal to the nozzle array direction X. At 100 is indicated
 the adhesive used to bond rod 55 into groove 50. Advantageously, this
 adhesive is chosen to be radiation (e.g. UV) curable and the material of
 the rod itself is chosen to be radiation transmitting (e.g. quartz) such
 that when in the jig and with the rod and printhead correctly positioned,
 one end of the rod can be exposed to UV light which is then transmitted
 along the rod and to the adhesive, which promptly cures, fixing the rod in
 position. The adhesive can, of course, fill the entire depth of groove 50.
 As an alternative, a thermally-deformable material such as a thermoplastic
 material or a so-called "Woods Metal" can be used: the latter are low
 melting point (typically 60.degree. C.) metals that can be kept liquid by
 means of a modest heat source until the rod is correctly positioned.
 Removal of the heat source then allows the metal to solidify, fixing the
 rod in place. The cooling of a thermoplastic material would have a similar
 fixing effect. However, methods--including conventional room-temperature
 curing adhesives--that avoid possible errors due to thermal expansion of
 the printhead (particular the aluminium base 15) are to be preferred.
 The base of a second printhead, mounted in a so-called "back-to-back"
 relationship to the first printhead 5, is shown hatched at 110.
 Preferably, this second printhead also has its own datum rod, allowing the
 nozzles of both printheads to be accurately positioned relative to one
 another. In particular, the location of the datum rod in the second
 printhead can be chosen such that when assembled together, the nozzles of
 the second printhead are interleaved with those of the first printhead so
 as to give double printing resolution.
 It will be appreciated that the above arrangement is given only by way of
 example and is not intended to restrict the scope of the present
 invention. The movable datum element/reference member need not have rod
 shaped form, nor does it have to sit beneath or within the printhead base.
 However, in order to reduce errors due to expansion of the base, the
 reference member is preferably secured to the base 15 over the same length
 as is the ink ejecting unit 10, ideally to that region of the base lying
 closest to the nozzle array 40. However, design considerations may dictate
 a smaller rod, perhaps restricted to a location adjacent the edge of the
 printhead. Further, it should be noted that whilst the reference/datum
 surface of the examples lies outside the spatial envelope of the printing
 unit and support member of the printhead, this need not be the case and
 that arrangements whereby the reference surface is located e.g. within the
 support member can be envisaged.
 Various methods of securing the datum element, including conventional
 room-temperature curing adhesives and UV-initiated adhesives, may be
 employed. The technique can of course also be used to locate the printhead
 in other directions, particularly to ensure a constant nozzle plate to
 substrate distance (direction Z in FIG. 1) and may be used at more than
 one location on a printhead.
 FIG. 4 is a schematic front view (in the X direction) of several printheads
 5 arranged in a butted, side-by-side relationship. Each printhead has an
 array of nozzles 40 and reference rods 80a, 80b on the left and right-hand
 side of each printhead respectively. Rod 80a is aligned in accordance with
 the present invention so as to be a predetermined distance from the
 furthest-left nozzle in array 40 whilst rod 80b is similarly aligned to be
 a predetermined distance from the furthest-right nozzle in array 40. It
 will be appreciated that such an arrangement allows the separation N of
 the adjacent nozzles of neighbouring printheads to be accurately
 controlled. Rods 80a, 80b are preferably joined together directly by a
 rigid bond 90 (e.g. epoxy adhesive) rather than via the printhead base so
 as to avoid errors due to the greater thermal expansion of the base
 material.
 Although the present invention is not limited to any particular kind of
 printing--particularly inkjet--apparatus, the arrangement described by way
 of example above and shown in FIG. 5 with the nozzle plate removed
 incorporates an ink ejecting unit that utilizes shear mode wall actuators.
 FIG. 6 shows sectional detail of these ink ejecting units 10 and the line
 of ink-ejecting chambers 105. These are of the kind disclosed in the
 aforementioned WO97/04963 or in EP-A-0 364 136 (also belonging to the
 applicant and incorporated herein by reference) and comprise ink-ejecting
 channels 105 having a longitudinal axis D and defined by actuator side
 walls 200 of poled piezoelectric material such as lead zirconium titanate
 (PZT). By means of electrodes 210 arranged in or on the walls, an electric
 field is applied to the piezoelectric material and normal to the direction
 P of polarization thereof so as to cause the walls to deflect by shear
 mode into the ink channel (as indicated by broken lines in FIG. 6) thereby
 to eject a droplet from a respective nozzle. For ease of manufacture, the
 entire ink ejecting unit comprising channel walls 200, base 205 and cover
 215 may be made of piezoelectric material (the material of the cover need
 not be poled). Furthermore, several channel groups for ejecting several
 different colours of ink may be formed in a single base 205--registration
 between channels of different channel groups is thereby guaranteed.
 The nozzle plate is arranged at one end of the channels 105 (in the plane
 of the paper in FIG. 3a) and is in sealing contact with the end of the ink
 ejecting unit, namely channel walls 200, base 205 and cover 210.
 FIG. 7 shows an example of a nozzle plate/printhead body adhesive bond 220
 prior to nozzle formation, the axis of the ink channel 105 being indicated
 by arrow D. The rear of the nozzle plate is scalloped as described in
 WO95/11131 (belonging to the present applicant and incorporated herein by
 reference) and has grooves 225 formed above and below the channels to
 accommodate excess glue that might otherwise seep into and obstruct the
 channels themselves. Further grooves 230 may also be formed at the
 junction of the nozzle plate with the top and bottom surfaces of cover 215
 and base 205 respectively. Excess adhesive collecting in these channels
 forms fillets 235 which further strengthen the nozzle plate/ink ejecting
 unit bond.
 FIG. 8 is a sectional view showing the nozzle plate support 110 which
 surrounds the ink ejecting unit 10 and comprises first and second members
 300,305. The reference member (rod 55) of the present invention has been
 omitted for clarity.
 First member 300 has a front face 320 to which the nozzle plate 45 is
 bonded (for example using the adhesive bonding techniques outlined in the
 aforementioned WO95/11131). It has been found that excess adhesive may
 collect as a meniscus along the line of intersection between the inner
 surface of the aperture 115 with the nozzle plate 45. To avoid
 interference between this meniscus and the front of the ink ejecting unit
 10, aperture 115 may be made wider as indicated by dashed lines 340, with
 the aperture 350 in the second member 305 remaining a tight clearance fit
 on the ink ejecting unit 10 so as to aid location of the printhead within
 the second member.
 The nozzle plate 45 also extends both above and below the ink ejecting unit
 10 so as to provide a large peripheral region (reference number 50 in FIG.
 1) against which the cap of a conventional printhead maintenance device
 can seal. To this end, the front face 320 of the first member is made flat
 to within 10 .mu.m, this value having been found by the present inventors
 as being necessary to ensure good sealing with a cap. Materials suitable
 for the first member include ceramics, which are easily machined--for
 example by lapping--to the required flatness.
 Preferably, the material of the first member has a thermal expansion
 coefficient (T.sub.EC) that is substantially matched to that of the
 material of the printhead body: were this not the case, differences in the
 amount of thermal expansion between the ink ejecting unit 10 and first
 member 300 would lead to stresses in that (unsupported) part 325 of the
 nozzle plate 45 lying between the two members. Where, as in the present
 example, the printhead body is made of PZT (T.sub.CE =3.times.10.sup.-6),
 suitable materials may include alumina, PZT itself and borosilicate
 glasses having T.sub.CE values lying within 3% of that of PZT.
 Although such materials are by themselves brittle and easily broken, the
 assembly of first member 300 attached--for example by means of an adhesive
 layer 330--to a second support member 305 of a tougher material has proved
 robust. Again, this tougher material preferably has a T.sub.CE
 substantially matched to that of the first member. Aluminium, in
 particular, meets this criterion and furthermore is easily manufactured to
 the required dimensions (as shown in FIG. 2, the height H1 of the ink
 ejecting unit 10 is typically 2 mm, the height and width H2,W of the
 nozzle plate support typically 10 mm and 100 mm respectively.
 Expressed in broad terms, the droplet ejection apparatus described above
 comprises at least one chamber formed in a body and communicating with
 droplet liquid supply means and with a respective nozzle formed in a
 separate nozzle plate; electrically actuable means for imparting pressure
 pulses to droplet liquid in the chamber to effect ejection of droplets
 from the nozzle; wherein the outlet of each respective nozzle is formed in
 a first surface of the nozzle plate having a first area, the nozzle plate
 and body being in sealing contact with one another over a second area
 smaller than the first area; and wherein the apparatus further comprises
 support means for supporting the periphery of the nozzle plate and
 comprising a first member having a surface that is flat to within 10 .mu.m
 and to which the nozzle plate is attached, and a second member for
 supporting said first member.
 Such a construction allows the nozzle plate to be supported by a material
 (preferably a ceramic such as alumina) that can easily be machined to the
 flatness required, whilst ensuring the robustness of the construction by
 supporting this material by a second member made of a tougher material
 such as aluminium. Robustness is required to withstand the forces to which
 a printhead might be exposed during its lifetime, in particular those
 generated during engagement/disengagement of a sealing cap from the nozzle
 plate.
 One preferred method of assembly is as follows: nozzle plate support 110 is
 assembled from the first and second members 300, 305; nozzle plate 45 is
 attached to the nozzle plate support; adhesive is applied to the end face
 of ink ejection unit 10; nozzle plate support 110 is slid over the end of
 ink ejection unit 10 and the nozzle plate 45 is bonded to the end face of
 the ink ejection unit 10; support 110 is attached at its rear face 315 to
 the base 15 and, optionally, to the manifold 35 by compliant bonds 310.
 Preferably, the compliant bonds hold the nozzle plate pressed against
 front of the nozzle plate, causing the nozzle plate to bow out slightly.
 FIG. 9 illustrates an alternative embodiment of the nozzle plate support of
 FIG. 8 in which aperture 350 is increased in height. This allows a
 temperature sensor 360 to be mounted at the front of the printhead and
 allows the chassis 15 to extend nearly to the front of the printhead,
 thereby facilitating the conduction of heat away from this area. FIG. 9
 also shows a circuit board comprised of primary and secondary boards 20A,
 20B electrically connected to one another and to the printhead 10 by means
 e.g. of wire bonds 370. Secondary circuit board 20B may be formed with
 conductive tracks spaced at an especially narrow pitch suited to
 connection to an integrated circuit 380 and/or the electrodes 210 of
 individual printhead channels 105 but inappropriate for the remaining,
 larger-scale components of the drive circuit. These can be mounted on
 primary circuit board 20A which, because it is formed with conductive
 tracks at a larger pitch, is less expensive to manufacture. Such a
 two-part arrangement helps minimize the cost of the printhead as a whole.
 In a further embodiment, the nozzle plate 45 may be bonded to the front
 face 320 of the first member 300 prior to attaching the nozzle plate to
 the ink ejecting unit 10. This first step is carried out at a temperature
 that is significantly (approximately 40.degree. C.) higher than the
 temperature which the nozzle plate will reach during printhead operation
 (typically 50.degree. C.) such that, once the nozzle plate has bonded to
 the first member (generally a heat curing epoxy such as Epotek or Hi-Sol
 is used) and the assembly has been allowed to cool, the nozzle plate is
 held taut over the aperture (115, FIG. 2) in the first member.
 This effect does, of course, rely on the T.sub.CE of the nozzle plate
 material being greater than that of nozzle plate support such that the
 nozzle plate will shrink more on cooling than will the surrounding support
 and thereby be stretched like a drum skin over the support. T.sub.CE
 values for the nozzle plate materials of polyimide, polycarbonate,
 polyester, polyetheretherketone and the like mentioned above lie in the
 range 20-50.times.10.sup.-6 /.degree. C.
 The stretched, tensioned nozzle plate 45 can subsequently be placed in
 sealing contact with the ink-ejecting channels 105 and the surrounding
 printhead body, preferably using an adhesive that cures at the nozzle
 plate operating temperature and/or has low shrinkage on curing and/or is
 elastic so as to avoid further distortion of the nozzle plate. Note that
 this step can generally be carried out at ambient working temperature--no
 problems have been encountered due to differentials between this and the
 nozzle plate operating temperature.
 Expressed in broad terms, the droplet ejection apparatus described in the
 further embodiment above comprises an array of chambers formed in a body
 and having a respective array of outlets communicating with a respective
 array of nozzles formed in a separate nozzle plate; each chamber further
 communicating with droplet liquid supply means; the apparatus further
 comprising electrically actuable means for imparting pulses to droplet
 liquid in the chambers to effect ejection of droplets from respective
 nozzles; wherein that portion of the nozzle plate in which said array of
 nozzles is formed remains in substantially uniform tension in the nozzle
 array direction when the apparatus is at its operating temperature. Such a
 construction results in a nozzle plate that is held taut (like a drum
 skin) over the array of chamber outlets and which is consequently
 uniformly flat over the length of the array. This in turn increases the
 likelihood of the nozzles formed in the nozzle plate being of a uniform
 quality.
 It will be appreciated that some allowance for thermal expansion will have
 to be made in the choice of material for the first and second members
 300,305 and/or in the nature of the bond 330 between them.
 Indeed, where the flatness of the periphery of the nozzle plate is not an
 issue--for example in a printhead where no capping is required--it may be
 desirable to make the nozzle plate support of a single material as shown
 in FIG. 10. As a material having a T.sub.CE that is less than that of the
 material of the nozzle plate 45 and yet matched to the PZT of the ink
 ejecting unit 10, INVAR (an iron/nickel alloy) has proved suitable.
 Alternatively, if the strength of the support is not critical, alumina may
 also be used, either as a single element or as the first and second
 members of a sandwich nozzle plate support construction as shown in FIGS.
 8 and 9.
 Releasable (e.g. hot melt) adhesives may be used to attached such a support
 to the ink ejection unit so as to allow the support/nozzle plate assembly
 to be replaced should the step of nozzle manufacture prove unsuccessful.
 Whilst the present invention has been described with reference to
 piezoelectric inkjet printheads, it should be understood that this is by
 way of example only. The invention is equally applicable to other kinds of
 inkjet printhead--including thermal--and other kinds of printer having
 printing elements--including thermal transfer and wire dot printing
 elements.
 The text of the abstract filed herewith is repeated here as part of the
 specification:
 To allow accurate positioning relative to a printer mechanism, a printhead
 5 is provided with a reference surface 60 formed on a reference member 55.
 Member 55 is attached to the base 15 of the printhead but positioned with
 reference to a nozzle 40 of an ink ejecting unit 10 mounted on the base
 member. This obviates the need for the base member to be manufactured to
 narrow tolerances.