Low-cost moulded page array printhead from HP

WO 2015/116025 A1
Flexible carrier
Hewlett-Packard Development Company

This review is from a further batch of patent applications describing more details of HP’s moulded printheads. We discussed these in the September/October 2014 issue of Directions. HP introduced their low-cost page array printhead in the OfficeJet Pro X printer in 2013. The printhead used consists of a staggered array of four colour dies.

The new moulded printhead design has some important benefits. First of all it allows the use of much narrower silicon dies. Patent applications related to the first low-cost page array printhead design described the die size as 5 mm wide. The moulded printhead patent applications describe the dies as “slivers” 500 microns wide, so an order of magnitude narrower.

The use of moulded epoxy resin for the body of the printhead significantly reduces the cost of the substrate for the dies – considerably cheaper than others previously considered or used, such as silicon, glass, ceramic and so on. HP is also able to incorporate in-flight drop detection or image scanning into the moulding on the front face. In addition other patent applications describe ink property sensing and ink level detection.

The first of the patent applications we have selected for review describes the manufacturing processes for the moulded printhead, in particular the use of a flexible carrier sheet.

Aug-15aThe first stage of the manufacturing process is to fix a flex circuit 66 with conductors 22 on to a flexible carrier sheet 68 with a thermal release tape 70. The flexible carrier sheet used can be cured epoxy sheet or a high temperature plastic. It is also possible to use semi-rigid and rigid materials such as metal, carbon fibre, composites etc. by adding grooves so that in the final stage of manufacturing the substrate can be peeled away.

The printhead die 12 is then positioned over the gap 72 in the flex circuit. The die is 500 microns wide, 100 microns thick and 26 mm long. Dry etching is used to form the ink flow port 56. This is possible and viable due to the thin structure.

On the front face of the die the ink channels 54 and actuator chamber 50 are formed in the usual way in a spin-on photo-imageable resin, and the nozzle layer formed on top. The die contains not just the heaters for drop actuation, but addressing and drive circuits too. Electrical contacts 24 connect to the flex circuit tracks 22.

Aug-15bAfter the die is in position on the substrate, the body of the printhead14 is formed by transfer moulding with tool 74. The transfer moulding process has been adapted by standard techniques used in the semiconductor industry for device packaging. Preferably no release agent is used in the moulding process, the concern being contamination of the surfaces that will come into contact with the ink.

The stiffness of the moulding can be adjusted, depending on whether the print bar will be used directly or whether it needs to conform to a separate support structure.

Aug-15cThe final step is to strip away the flexible carrier 68 and thermal release tape 70 to leave the finished print bar. Note that with this design there is a single wide ink manifold slot 16 stretching along the full length of the die, with separate ink feeds 56 into each actuator chamber 50. As shown there are probably two rows of nozzles, one each side of the ink feed slot, and therefore using a single colour of ink. This would be a similar configuration to the new HDNA (High Definition Nozzle Architecture) printheads that will be field upgrades for HP’s high-speed web presses. These have 1,200 nozzles/inch in each row, and therefore 2,400 nozzles/ inch for each ink feed slot.

Aug-15dHere a wafer-scale view is shown. There are 5 print bars being made on one wafer, with each print bar having 4 rows of silicon die “slivers”. 10 dies are shown in a staggered arrangement across the width of the print bars, giving a print width of 230 mm. The width of each print bar is around 16mm. With a potential saving on manufacturing costs and printhead size, these printheads are likely to be very competitive with Memjet’s low-cost page wide arrays.

The ability to make such narrow low-cost arrays of nozzles leads to the ability to offer extra rows for redundancy in case any nozzles become defective. However to be able to substitute for non-working nozzles you need to know which ones they are. In the second patent application two schemes for determining these are described, in both cases with all of the functionality within the printhead.

Hollow titanium oxide particles for white inks

WO 2015/047306 A1
White pigment dispersions
Hewlett-Packard Development Company

We have discussed the issues regarding white ink jet inks many times in these pages when we have described the many different approaches employed to get over the unavoidable issue of good white pigments being generally heavy, and the pigment size being necessarily large in order to scatter the light. This leads to settling, and potentially complex ink systems.

Some of the more promising white ink jet ink inventions have involved neutral density hollow resin particles – using low refractive index “bubbles” rather than high refractive index dense pigment particles. Others have used a combination of the two. This patent poses the question that if titanium dioxide is the best white pigment, but is too dense, then why not lower its density to give a pigment incorporating the best of both worlds.

The patent suggests two approaches to achieving this, both of which grow the titanium dioxide particles around a template, to give hollow pigment particles with an overall density less than the bulk. The first approach uses a carboxylic acid functional polymer dispersion, such as poly(methyl vinyl ether-alt-maleic acid), in the presence of titanium (IV) oxysulfate. The patent suggests that the carboxylic acid groups of the dispersion strongly coordinate with the Ti4+ groups, resulting in a titanium rich shell. Calcination of this mixture at 550C results in the inventive low density pigment.

The second approach uses an amazing sequence of chemical reactions to form the hollow titanium dioxide particles. A mixture of titanium (IV) oxysulfate is dissolved into water, hydrogen peroxide and urea. The hydrogen peroxide decomposes to water and oxygen, and onto these bubbles adsorbs the titanium oxo species, which are reduced in situ by ammonia (from the hydrolysed urea) to give titanium hydroxide. The resultant sludge is calcined at high temperature to yield the hollow titanium dioxide particles with a claimed 35% reduction in density over standard titanium dioxide.

Inks were made from these lower density pigments and successfully jetted through HP thermal printheads. Pigment settling was claimed to be undetectable during the 45 day experiment.

Metallic effects from a thermal ink jet ink

US 2014/0170384 A1
Neutral gray reflective ink
Hewlett-Packard Development Company

Judging by the number of patents filed, it appears that the ink jet printing of expanded colours and appearances are sought after for home and office decorative printing. Reflective inks, in particular, have been disclosed with a particular emphasis on metal flakes – materials currently used in more traditional metal-effect printing processes. Thermal printheads have significant constraints regarding viscosity and particle size and so large metal flakes are a non-starter and a silver nano-particle approach would be expensive and questionable from a toxicological perspective.

The patent suggests that to look metallic, then only a certain threshold value for specular reflection needs to be achieved. It is suggested that 10% of the incident light must be specularly reflected, but greater than this is always desirable. With this specular reflectivity requirement relaxed, alternative non-metallic materials can be considered, such as magnetite. Magnetite (Fe3O4) can be made up into a stable nano-dispersion, suitable for thermal ink jet, and will give a metallic lustre when printed on glossy paper.

However, magnetite has an inherent yellow-orange hue and so to achieve a neutral silver effect a mixture of blue, cyan or magenta dyes must be added to counteract this. The exemplified ink jet formulations first prepared the magnetite dispersion in water by heavily bead milling a 5.6 wt% mixture of the oxide in water with a polyether alkoxysilane reactive dispersant at 0.5 wt%, relative to the pigment. This was then used (at 36.3 wt%) to make a simple thermal ink jet ink with 14 wt% humectant, 0.1 wt% neutralised styrene acrylic binder, 0.3 wt% surfactants, 5.4 wt% of a cyan and magenta dye mixture and water.

June-14aThis ink was loaded into a HP black cartridge from a Photosmart 8450 printer to print colour test patches. The chart left shows the colour coordinates of the neutral formulation above, compared against the non-colour corrected magnetite dispersion and a comparative silver nano-particle ink. It can be seen that the yellow tint has been fairly well counteracted in comparison to the metal ink samples.

The patent goes on to discuss the importance of the media in achieving a metallic effect and that the pore size of the surface of the media must be smaller than that of the magnetite pigment particles to ensure they remain on the surface as a contiguous layer. Fig 1 below shows this schematically with the dyes (16) absorbing into the media under the ceramic particles (14). There is an obvious issue to such a configuration in that the robustness of the printed image will be compromised if the pigment is left unprotected on the surface, but this is addressed in the patent application US 2014/0170395 A1.

June-14bAlthough it seems that the ultimate specular reflectivity of this approach is not quite that of a metallic silver ink, there do seem to be significant advantages in using such benign and low cost iron oxide materials.

Thermal inkjet head with improved jet straightness

US 2013/0293638 A1
Fluid ejection device having firing chamber with mesa
Hewlett-Packard Development Company, L.P.

There have been several thermal ink jet printhead designs in the past that have attempted to improve jet straightness.  As drops are formed, the bulk of the ink moves rapidly to the substrate.  Some of the ink is contained in the tail, which breaks off.  Often the tail will fragment into fine drops which either slowly follow the main drop to the substrate or fall back on the nozzle plate.  In addition erratic tail break-off can lead to mis-directed drops.

The proposed solution is to redesign the heater chamber floor with a central column or mesa and a circular heater.  The silicon substrate is etched to form a circular pit 230 around a central column 250.  The heater 205 is formed on the outer wall of the pit.  The heater, electrodes 208 and the exposed surface of the substrate are coated with an insulator, which also increases the robustness of the structure.

The column is centrally positioned under the nozzle.  It is thought that the tail then breaks more centrally than otherwise.   In addition, because the heater is not at the centre of the chamber, it is far less likely to be damaged by the collapsing bubble.  Therefore the overcoat layer, which has a protective function as well as being an insulator, can be reduced in thickness and therefore increase the efficiency of the device.

Hewlett-Packard’s nano-colourant dispersion

US 2013/0284050 A1
Colorant dispersion for an ink
Hewlett-Packard Development Company L.P.

Ink jet inks usually contain organic colorants, as these materials when chosen correctly show strong absorption, low density and in the case of pigments are usually available at particle sizes suitable for incorporation into ink jet inks.  Inorganic pigments are used in speciality ink jet inks when the inherent stability to heat and UV is required – tile printing would be one example. However the range of available inorganic pigments is narrow and the colour of such materials tends to be dull.

This patent focuses on a nano-dispersion of Fe3O4 (iron(II,III)oxide) with a particle size of around 30 nm, which at this particle size has a burnt red colour unsuitable for black text printing.  When manufactured at a large particle size, iron(II,III) oxide is used as a black pigment (C.I pigment black 11).

The example takes an ~8 wt% Fe3O4 suspension in water synthesised by a basic precipitation process and adds Silquest A1230 (a reactive alkyleneoxide dispersant, 50 wt% on pigment) and citric acid (10 wt% on pigment).  This mixture was then bead milled to give a dark red dispersion with an average particle size of ~30 nm.  To this dispersion was then added 5 wt% gallic acid (3,4,5-trihydroxybenzoic acid) and it was heated to 60C for one hour, during which the colour changed from dark red to black.  This black dispersion was then made into a basic ink jet ink by dilution with water and 2-pyrolidone as humectant and successfully ink jet printed to give sharp text that was significantly more black than the pale yellow comparative ink produced without the gallic acid heating step.

From the examples it seems that although the heating step significantly improves the colour of the inorganic pigment, the intensity and neutrality is not quite the same as a comparative carbon black ink made to the same formulation.  However, scratch resistance and gloss were significantly better than the carbon black ink.

UV stability is unfortunately not mentioned, as it would be interesting to see if the pigment would revert to its former dark red colour.  If not this could be an interesting approach to the production of small particle size black pigments for extreme ink jet applications.

Optically clear nano-particle ink from Hewlett Packard

US 2013/0222496 A1
Optically clear fluid composition
Hewlett-Packard Development Company, L.P.

Uniform gloss on paper substrates requires a transparent ink to raise the gloss level of the chosen media to the same level of the coloured components of the ink.  If such an ink is to be used then there is a good argument for printing it over the full printed image.  This helps to improve mechanical properties, such as scratch and rub resistance, and potentially introduces other benefits such as UV absorption, therefore increasing the stability of the printed image.

This patent discusses a clear ink jet ink designed to be printed over the full image to equalise gloss, improve mechanical properties and absorb UV, without increasing the overall haze of the printed image.

The novelty of the patent is to use metal oxide nano-particles with a particle size below 140 nm.  This small particle size is necessary for transparency of these materials.  The patent suggests that it is also beneficial if the metal oxide average particle size is similar to that of the colorant pigment particles, in order to stack efficiently and therefore minimise coating roughness and haze.  Such nano-particulate materials also show benefits with respect to stability and printhead performance.

Additionally, these metal oxide dispersions should be compatible with the printed inks in order to prevent aggregation of these nano-particles which might increase haze.  Generally, such metal oxide dispersions are electrostatically stabilised and can be formulated to be stable at a pH of 8-10, the range for ink series such as HP 38 and HP 70.  Specifically the pH of the metal oxide dispersion should be within 0.5 pH units of the coloured inks.

The specific metal oxide particles used should be transparent to visible light and have a refractive index greater than 1.65.  Zinc and aluminium oxide work well for this technology, whereas silica (with a refractive index of 1.45) does not, as it apparently gives hazy coatings.

Exemplified coating formulations contain around 2 wt% of a polyurethane binder, 2 wt% (dry solids) of the metal oxide particles, 10 wt% of humectants (1,2-hexanediol, 2-pyrolidone or similar), ~0.5 wt% surfactants and biocides, with the remainder water.  The mechanical properties of such coatings were compared to a silver halide print, giving equal or better scratch resistance and very low haze levels.

Hewlett Packard’s microwave-curable media coating

WO 2013/062510 A1
Inkjet recording medium, and method of using the same
Hewlett-Packard Development Company, L.P.

Over the years, much effort has been put into improving the image stability and gloss of photo prints.  Here, ink passes through a porous top coating which is subsequently made to be non-porous, so providing both image protection in the form of wet and dry rub resistance along with good levels of gloss and image distinctness.

The substrate can be any typical medium such as paper, olefin coated paper or polymer film.  It is first coated with an ink receiving coating that will absorb fluid from the ink.  This is a typical porous coating and primarily consists of a porous pigment and a binder.

Protection is given to the image by the particulate-based reactive coating applied above the porous coating.  This is in itself porous to the ink and so the ink will pass through it and be absorbed in the ink receiving coating.  After printing, barrier properties are created by exposure to microwave radiation which will cause cross-linking of some of the particles and coalescence of others.  This is shown in the figure in which ink (26) is shown absorbed in the ink absorbing layer (14), having almost entirely passed through the protective coating (16).

Jun13aJun13b

The coating is applied as a latex in which both reactive and non reactive particles are dispersed.  Suitable reactive polymers include those with epoxy functionality, and those with fatty acid, alkoxy-silane, acetoacetoxy, hydroxyl, amine or carboxyl functional groups.  Self cross-linking polymers can be used or a cross-linking agent can be included.  The uncured particles (0.2 to 10 µm) should have good mechanical stability (Youngs Modulus 600 to 3000 MPa) at a temperature below 110C.

A polar microwave radiation cure promoter may also be included.  This acts to strengthen the dipole relaxation effect in which particles turn or rotate with some lag as the dipole direction changes at high frequency.  Heat is also generated which is helpful to both the curing and coalescence processes.  Suitable agents are calcium acetate monohydrate, calcium propionate and calcium propionate hydrate.  Exposure to microwaves also induces coalescence of the non reactive polymer particles which are generally hydrophobic polymers such as PTFE or hydrocarbon waxes.  It appears that much or all of the coalescence and cure processes will take place at temperatures exceeding 110C.

These media appear to be suitable for most types of aqueous, solvent or latex ink and can be applied using more or less any mode of ink jet.  Microwave exposure can be carried out either in-line or off-line at a frequency of 0.3 to 300 GHz.  Exposure time should be from around 10 seconds to 4 minutes, although the longer times would seem impractical.

Hewlett-Packard’s erasable ink technology

WO 2012/166149 A1
Method of forming an erasable ink
Hewlett-Packard Development Company

Several methods are currently available that may erase printed images from the surface of a medium. For example, inks that include dyes may be erased by subjecting the inks to high intensity UV radiation, causing the dye to break down to a colourless state. Oxidation and bleaching methods can also be used to erase an image with the correct colorants. Some inks can be formulated with thermal dyes that can decolorise when exposed to secondary and tertiary components in the presence of heat, the Toshiba e-blue technology for example.

These two patents present an erasable ink that is erased chemically with a second ink. A second patent (WO 2012/166160 A1) uses an additional electrolytic stage to accelerate the bleaching process. The main novelty over similar chemical whitening patents is the “relatively human friendly” nature of the materials used.

Thus the patent suggests that colorants containing ionic complexes that can change between coloured and non-coloured states on oxidation and reduction are ideal for this technology. A particularly favoured colorant mentioned in these patents is iron(II)ascorbate which has a dark purple colour in solution and when dry, with a  useful UV-vis absorption range from 350-700nm. This can give a workable “black” ink when formulated at around 3wt%.

In particular, such an ink can be relatively easily oxidised back to the colourless iron(III)ascorbate under mild conditions, lending itself to an easy decolorisation step. This ease of decolorisation does require the presence of a reducing agent  (<1% sodium bisulfite) in solution to prevent atmospheric oxygen decolorising the ink. The colour of the iron(II)ascorbate is also pH sensitive and so a buffer such as 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO) should be included in the final ink formulation at about 0.2wt%. Other components included in the ink are the usual humectants, in this case 5wt% glycerol and 10wt% 1,2-propanediol and surfactants, 0.25% surfynol 465. This ink was printed onto HP recycled paper using the black ink cartridge in an HP Photosmart 8450 printer.

An erasure fluid was formulated with 3wt% hydrogen peroxide, 20wt% 1,2 propane diol and 5wt% glycerol and this was also printed using an HP Photosmart printer. When printed over the previous iron(II)ascorbate, the colour was bleached over a period of a few hours. The page was then reprinted with the “black” ink to give an image with similar density to the initial print.

Other fluid combinations gave slightly improved bleaching times which is where the second patent (WO 2012/166160 A1) comes in. This patent proposes to use a significantly less aggressive decolorising liquid coated onto the iron(II)ascorbate printed media, which is then contacted this liquid against a series of electrodes. When a voltage of up to 10V was applied across these electrodes the image was be decolorised in a matter of seconds.

These electrodes were positioned across the paper, but the main body of the patent discusses the use of a pair of spiral wrapped electrode wires contacting the wetted media surface, see fig below. In comparison to the the hydrogen peroxide bleaching agent, this approach works with simple surfactant solutions. The electrolysis of the water content of the decolorising solution generates hydrogen peroxide in situ, and judging by the rapid bleaching times.

Dec12

Although this approach to a re-usable media solution is interesting, there is the problem of a build up of the erasing solution in the paper after multiple cycles lending an oily feel. This is mentioned in the text, but not really fully addressed.

Careful manufacturing process for improved latex ink

US 2012/0232215 A1 
Single batch latex ink compositions and methods 
Hewlett Packard Development Company, L.P.

The addition of polymers to a pigmented ink jet ink will generally improve printed image robustness at the expense of a higher ink viscosity which, depending on the printhead technology used, may thus preclude the use of such materials. As frequently discussed here, polymer encapsulated pigments potentially offer the best of both worlds without any compromise in the ink’s viscosity.

However, even with the best encapsulated pigments, as with everything, there is still some compromise and that comes in the form of a more involved manufacturing process, and it is this point that the patent hopes to address, with a relatively simple approach.

The example used (4.6 wt%) of the monomers styrene, hexamethylacrylate and acrylic acid at a ratio of 40:55:5, and added 0.1 wt% of azobisisobutyronitrile (AIBN) and 2.3 wt% of a pigment powder (BASF D7086 cyan). This paste was then added to water (92.3 wt%) in which had been dissolved sodium dodecyl sulphate (0.7 wt%). This was then mixed at high shear in a microfluidiser at 10,000 psi for 15 minutes to produce the dispersed pigment emulsion.

This emulsion was then charged into a reaction vessel and degassed under argon before heating to 80C for two hours. After this time a portion of 0.1 wt% of potassium persulfate, dissolved in a small amount of water, was added over the course of three hours, with a further 3 hours to ensure full reaction of the monomers. The resultant dispersion was allowed to cool, neutralised with potassium hydroxide and filtered.

The main point exemplified in the text is that the addition of the thermal initiator over time is critical, as without this approach the particle size is generally significantly below 250 nm and unsuitable for ink jet, presumably due to viscosity issues. Thus by starting polymerisation with a relatively low number of initiator species, then the number of polymer particles nucleated will be relatively low and these particles will then grow during the subsequent polymerisation and further nucleation will be minimal. In comparison, if all the initiator is added at the start of the polymerisation, then there will be significantly higher polymer particle nucleation resulting in more, smaller particles in the final ink.

Emulsion-based thermal ink jet inks

WO 2012/105950 A1
Emulsion-based thermal inkjet inks
Hewlett-Packard Development Company

Thermal ink jet technology requires an ink to explosively boil when the relevant firing resistor is activated, and thus print a droplet.  This generally requires inks that use carrier liquids, such as water, that boil at relatively low temperatures.  There are a huge range of applications that require the jetting of fluids that do not boil, either because their boiling points are too high at atmospheric pressure, or they do not have a clearly defined liquid/gas phase transition.

So although piezoelectric printheads are more expensive to produce, they are able to jet a significantly larger range of ink jet ink chemistries than the cheaper, but more limited, thermal printheads.  Some progress has been made in which UV curable formulations have been jetted through thermal printheads, but these have used water soluble UV monomers as they still require water (or some similar organic solvent) to form the bubble.

Using water as a solvent for such aqueous soluble UV curable materials has its own problems, one being that as the concentration of monomers is increased and the proportion of water decreased, the first order phase transition to the gas phase can be smeared out and so the bubble production is impacted.  Ideally more typical hydrophobic monomers would be used in thermal printheads, but as these show very limited water solubility they have seen limited success.

This patent presents an approach to allowing a wide range of high boiling point, hydrophobic solvents, to be used in thermal printheads without resorting to low boiling co-solvents as the drive fluid.  The inventive step is to formulate an ink that is mainly a hydrophobic, high boiling liquid with water dispersed in this continuous phase as a stable water-in-oil emulsion.  It is suggested that other low boiling solvents could be used in this way, but water is the main target.

Aug12

The figure left demonstrates the theory behind this approach.  A surfactant 12 is added to the high boiling point solvent at an amount above its critical micelle concentration (CMC).  Then, in the absence of water, the surfactant will form micellar structures 14 in which the surfactant polar groups 12a are situated on the inside of a spherical structure.  Small additions of water will hydrate the core 14a of the micelles to give the structure 18, but at this point the water is effectively dissolved in the surfactant and so will not possess bulk water properties.  As the water content is increased, there is a gradual transition to a vesicular structure 20, where a water core 20a is surrounded by a surfactant shell.  In this state the water present possesses its bulk properties and if the vesicle size is small enough it can form a stable dispersion in the continuous oil phase.  Ideally this stable state is a micro-emulsion.

The patent suggests that it has been experimentally shown that 5% water in such a micro-emulsion is enough to jet a hydrocarbon oil-based ink or commercially available UV ink.  In one example 5 wt% of an anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and 7.7 wt% water were added to Isopar L and mixed to give a micro-emulsion of water-in-oil.  A magenta ink was then formulated by the addition of 2 wt% pigment and this was successfully jetted through a thermal printer, giving a drop weight of 10 pl and a drop speed of 7.5 m/s with a 25V/1.7µs drive pulse.

A similar example took an HP Scitex FB221 light magenta ink and added 8 wt% AOT, 8 wt% water, 8 wt% Isopar L and 15 wt% lauric acid and mixed to give a micro-emulsion.  The lauric acid was included as a viscosity modifier to bring the viscosity below 4 cps.  This formulation was again jetted through a thermal printhead giving a drop weight of around 10 pl and a drop velocity of 7.5 m/s using a drive pulse of 24 V at 3 µs and a precursor pulse of 0.5 µs.

Although these examples seem somewhat preliminary, they seem to prove an interesting point that complex inks, viscosity allowing, could be jetted through thermal printheads.  It will be interesting to see if more specific and complete patents will be published using this chemistry technology to allow the printing of other complex and functional inks through thermal printheads.