GB2538220A

GB2538220A - A printing head

Info

Publication number: GB2538220A
Application number: GB1503290.7A
Authority: GB
Inventors: Jeuté Piotr
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-02-26
Filing date: 2015-02-27
Publication date: 2016-11-16
Also published as: PL411383A1; PL226751B1; GB201503290D0

Abstract

An inkjet printing head 100 comprises a nozzle assembly having a pair of nozzles 111A, 111B, each nozzle 111A, 111B being connected through a channel 112A, 112B with a separate liquid reservoir 116A, 116B for forming a primary drop 121A, 121B of liquid at the nozzle outlet 113A, 113B. The head 100 also has a means 131 for guiding the primary drops 121A, 121B within the printing head 100 from the nozzle outlet 113A, 113B to a connection point 132; and means 132 for restricting the freedom of combination of the primary drops 121A, 121B into a combined drop at the connection point 132. The head 100 may further comprise gas-supplying nozzles 119A, 119B for blowing gas towards the separator tip 132. The means for guiding the primary drops (221A, 221B, Fig.4A) may have a form of a primary enclosure (241, Fig.4A) surrounding the nozzle outlets (213A, 213B, Fig.4A) extending downstream and having a first section (243, Fig.4B) for restricting the freedom of combination of primary drops (221A, 221B, Fig.4A) with a diameter (D2, Fig.4B) not smaller than the diameter (D2, Fig.4B) of the combined drop (222, Fig.4A).

Description

A PRINTING HEAD

DESCRIPTION

1ECHNICAL FIELD

The present invention relates to printing heads.

BACKGROUND

Ink jet printing is a type of printing that recreates a digital image by propelling drops of ink onto paper, plastic, or other substrates. There are two main technologies in use: continuous (CIJ) and Drop-on-demand (DOD) inkjet.

In continuous inkjet technology, a high-pressure pump directs the liquid solution of ink and fast drying solvent from a reservoir through a gunbody and a microscopic nozzle, creating a continuous stream of ink drops via the Plateau-Rayleigh instability. A piezoelectric crystal creates an acoustic wave as it vibrates within the gunbody and causes the stream of liquid to break into drops at regular intervals. The ink drops are subjected to an electrostatic field created by a charging electrode as they form; the field varies according to the degree of drop deflection desired. This results in a controlled, variable electrostatic charge on each drop. Charged drops are separated by one or more uncharged "guard drops" to minimize electrostatic repulsion between neighboring drops. The charged drops pass through an electrostatic field and are directed (deflected) by electrostatic deflection plates to print on the receptor material (substrate), or allowed to continue on undeflected to a collection gutter for re-use. The more highly charged drops are deflected to a greater degree. Only a small fraction of the drops is used to print, the majority being recycled. The ink system requires active solvent regulation to counter solvent evaporation during the time of flight (time between nozzle ejection and gutter recycling), and from the venting process whereby air that is drawn into the gutter along with the unused drops is vented from the reservoir. Viscosity is monitored and a solvent (or solvent blend) is added to counteract solvent loss.

Drop-on-demand (DOD) may be divided into low resolution DOD printers using electro valves in order to eject comparatively big drops of inks on printed substrates, or high resolution DOD printers, may eject very small drops of ink by means of using either thermal DOD and piezoelectric DOD method of discharging the drop.

In the thermal inkjet process, the print cartridges contain a series of tiny chambers, each containing a heater. To eject a drop from each chamber, a pulse of current is passed through the heating element causing a rapid vaporization of the ink in the chamber to form a bubble, which causes a large pressure increase, propelling a drop of ink onto the paper. The ink's surface tension, as well as the condensation and thus contraction of the vapor bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir. The inks used are usually water-based and use either pigments or dyes as the colorant. The inks used must have a volatile component to form the vapor bubble, otherwise drop ejection cannot occur.

Piezoelectric DOD use a piezoelectric material in an ink-filled chamber behind each nozzle instead of a heating element. When a voltage is applied, the piezoelectric material changes shape, which generates a pressure pulse in the fluid forcing a drop of ink from the nozzle. A DOD process uses software that directs the heads to apply between zero to eight drops of ink per dot, only where needed.

High resolution printers, alongside the office applications, are also being used in some applications of industrial coding and marking. Thermal Ink Jet more often is used in cartridge based printers mostly for smaller imprints, for example in pharmaceutical industry. Piezoelectric printheads of companies like Spectra or Xaar has been successfully used for high resolution case coding industrial printers.

All DOD printers share one feature in common: the discharged drops of ink have longer drying time compared to CIJ technology when applied on non porous substrate. The reason being fast drying solvent usage, which is well accepted by designed with fast drying solvent in mind CIJ technology, but which usage needs to be limited in DOD technology in general and high resolution DOD in particular. That is because fast drying inks would cause the dry back on the nozzles. In most of known applications the drying time of high resolution DOD printers' imprints on non porous substrates would be at least twice and usually well over three times as long as that of CIJ. This is a disadvantage in certain industrial coding applications, for instance very fast production lines where drying time of few seconds may expose the still wet (not dried) imprint for damage when gets in contact with other object.

Another disadvantage of high resolution DOD technology is limited drop energy, which requires the substrate to be guided very evenly and closely to printing nozzles This also proves to be disadvantageous for some industrial applications. For example when coded surface is not flat, it cannot be all guided very close to nozzles.

CU technology also proves to have inherent limitations. So far CIJ has not been successfully used for high resolution imprints due to the fact it needs certain drop size in order to work well. The other well known disadvantage of CIJ technology is high usage of solvent.

This causes not only high costs of supplies, but also may be hazardous for operators on environment, since most efficient solvents are poisonous, such as the widely used MEK (Methyl Ethyl Ketone).

The following documents illustrate various improvements to the ink jet printing technology.

An article ""Double-shot inkjet printing of donor-acceptor-type organic charge-transfer complexes: Wet/nonwet definition and its use for contact engineering" by T. Hasegawa et al (Thin Solid Films 518 (2010) pp. 3988-3991) presents a double-shot inkjet printing (DS-UP) technique, wherein two kinds of picoliter-scale ink drops including soluble component donor (e.g. tetrathiafulvalene, TTF) and acceptor (e.g. tetracyanoquinodimethane, TCNQ) molecules are individually deposited at an identical position on the substrate surfaces to form hardly soluble metal compound films of TTF-TCNQ. The technique utilizes the wet/nonwet surface modification to confine the intermixed drops of individually printed donor and acceptor inks in a predefined area, which results in the picoliter-scale instantaneous complex formation.

A US patent US7429100 presents a method and a device for increasing the number of ink drops in an ink drop jet of a continuously operating inkjet printer, wherein ink drops of at least two separately produced ink drop jets are combined into one ink drop jet, so that the combined ink drop jet fully encloses the separate ink drops of the corresponding separate ink drop jets and therefore has a number of ink drops equal to the sum of the numbers of ink drops in the individual stream. The drops from the individual streams do not collide with each other and are not combined with each other, but remain separate drops in the combined drop jet.

A US patent application US20050174407 presents a method for depositing solid materials, wherein a pair of inkjet printing devices eject ink drops respectively in a direction such that they coincide during flight, forming mixed drops which continue onwards towards a substrate, wherein the mixed drops are formed outside the printing head.

A US patent US8092003 presents systems and methods for digitally printing images onto substrates using digital inks and catalysts which initiate and/or accelerate curing of the inks on the substrates. The ink and catalyst are kept separate from each other while inside the heads of an inkjet printer and combine only after being discharged from the head, i.e. outside the head. This may cause problems in precise control of coalescence of the drops in flight outside the head and corresponding lack of precise control over drop placement on the printed object.

SUMMARY

The problem associated with inkjet printing -especially in its DOD version -is the relatively long time of curing of the ink after its deposition on the surface remains actual.

There is still a need to improve the DOD inkjet printing technology in order to shorten the time of curing of the ink after its deposition on the surface. In addition, it would be advantageous to obtain such result combined with higher drop energy and more precise drop placement in order to code different products of different substrates and shapes.

There is a need to improve the inkjet print technologies in attempt to decrease the drying (or curing) time of the imprint and to increase the energy of the printing drop being discharged from the printer. The present invention combines those two advantages and brings them to the level available so far only from CU printers and unavailable in the area of DOD technology in general (mainly when it comes to drying time) and high resolution DOD technology in particular, where both drying (curing) time and drop energy have been have been very much improved compared to the present state of technology. The present invention addresses also the main disadvantages of CIJ technology leading to min. 10 times reduction of solvent usage and allowing much smaller -compared to those of CU -drops to be discharged with higher velocity, while the resulting imprint could be consolidated on the wide variety of substrates still in a very short time and with very high adhesion.

The invention relates to an inkjet printing head comprising a nozzle assembly having a pair of nozzles, each nozzle being connected through a channel with a separate liquid reservoir for forming a primary drop of liquid at the nozzle outlet. The printing head further comprises means for guiding the primary drops within the printing head from the nozzle outlet to a connection point; and means for restricting the freedom of combination of the primary drops into a combined drop at the connection point.

The means for guiding the primary drops may have a form of separator of a downstream-narrowing cross-section positioned between the nozzle outlets.

The separator may have its side walls adjacent to the nozzle outlets and configured to guide the primary drops along its side walls to combine into a combined drop at the separator tip which forms the means for restricting the freedom of combination of primary drops.

The length of each side wall of the separator can be larger than the diameter of a primary drop exiting the nozzle outlet adjacent to that side wall.

The side walls of the separator can be inclined with respect to the longitudinal axis of the head at an angle from 5 to 75 degrees, preferably from 15 to 45 degrees.

Both side walls of the separator can be inclined with respect to the longitudinal axis of the head at the same angle.

The side walls of the separator can be inclined with respect to the longitudinal axis of the head at different angles.

The side walls of the separator can be inclined with respect to the longitudinal axis of the head at an angle not larger than the angle of inclination of the nozzle channels The side walls of the separator can be inclined with respect to the longitudinal axis of the head at an angle larger than the angle of inclination of the nozzle channels.

The separator can be heated.

The head may further comprise gas-supplying nozzles for blowing gas towards the separator tip.

The means for guiding the primary drops may have a form of a primary enclosure surrounding the nozzle outlets, extending downstream and having a first section for restricting the freedom of combination of primary drops with a diameter not smaller than the diameter of the combined drop and a second section located between the first section and the nozzle outlets and having a width increasing upstream; and further comprising a source of a gas stream to flow downstream inside primary enclosure.

The nozzles can be inclined with respect to the longitudinal axis of the head at an angle from 5 to 75 degrees, preferably from 15 to 45 degrees.

Both nozzles are inclined with respect to the longitudinal axis of the head at the same angle The nozzles can be inclined with respect to the longitudinal axis of the head at different angles.

The primary enclosure may further comprise a third section extending upstream in parallel to the external walls of the nozzles.

The diameter of the first section of the primary enclosure can be equal to the diameter of the combined drop.

The cross-section area of the first section of the primary enclosure may be not smaller than 80% of the cross-section area of the combined drop.

The length of the first section of the primary enclosure may be not smaller than the diameter of the combined drop.

The printing head may further comprise a secondary enclosure surrounding the primary enclosure and connected to the source of a gas stream and comprising a first section extending downstream from the outlet of the first section of the primary enclosure and having a diameter decreasing downstream to a diameter larger than the diameter of the combined drop.

The printing head may further comprise deflecting electrodes at the outlet of the secondary enclosure.

The printing head may further comprise charging electrodes at the outlet of the primary enclosure.

The printing head may comprise a plurality of nozzle assembles arranged in parallel. The nozzle outlets can be heated.

The printing head may further comprise a cover enclosing the nozzle outlets and the connection point.

The cover may comprise heating elements for heating the volume within the cover.

BRIEF DESCRIPTION OF DRAWINGS

The invention is shown by means of exemplary embodiment on a drawing, in which: Figs. 1, 2A and 2B show schematically the first embodiment of the invention; Figs. 3, 4A, 4B, 5 and 6 show schematically the second embodiment of the invention; Fig. 7 shows schematically the third embodiment of the invention; Fig. 8 shows schematically the fourth embodiment of the invention; Figs. 9, 10, 11 show schematically different devices for propelling a drop out of the nozzle.

DETAILED DESCRIPTION

First embodiment A first embodiment of the inkjet printing head 100 according to the invention is shown in an overview in Fig. 1 and in a detailed cross-sectional view on Figs. 2A and 2B, which show the same cross-sectional view, but for clarity of the drawing different elements have been referenced on different figures.

The inkjet printing head 100 may comprise one or more nozzle assemblies 110, each configured to produce a combined drop 122 formed of two primary drops 121A, 121B ejected from a pair of nozzles 111A, 111B separated by a separator 131. Fig. 1 shows a head with 8 nozzle assemblies 110 arranged in parallel to print 8-dot rows 191 on a substrate 190, it is worth noting that the printing head in alternative embodiments may comprise only a single nozzle assembly 110 or more or less than 8 nozzle assemblies, even as much as 256 nozzle assemblies or more for higher-resolution print.

Each nozzle 111A, 111B of the pair of nozzles in the nozzle assembly 110 has a channel 112A, 112B for conducting liquid from a reservoir 116A, 1168. At the nozzle outlet 113A, 113B the liquid is formed into primary drops 121A, 121B as a result of operation propelling devices shown on Figs. 9, 10, 11. The nozzle outlets 113A, 113B are adjacent to a separator 131 having a downstream-narrowing cross-section (preferably in a shape of a longitudinal wedge or a cone) that separates the nozzle outlets 113A, 113B and thus prevents the undesirable contact between primary drops 121A and 121B prior their full discharge from their respective nozzle outlets 113A and 113B. The primary drops 121A, 121B ejected from the nozzle outlets 113A, 113B move along the separator 131 towards its tip 132, where they combine to form a combined drop 122, which separates from the separator tip 132 and travels towards the surface to be printed.

The liquids supplied from the two reservoirs 116A, 116B are preferably an ink and a catalyst for initiating curing of the ink. This allows initiation of curing of the ink in the combined drop before it reaches the surface to be printed, so that the ink may adhere more easily to the printed surface and/or cure more quickly at the printed surface.

For example, the ink may comprise acrylic acid ester (from 50 to 80 parts by weight), acrylic acid (from 5 to 15 parts by weight), pigment (from 3 to 40 parts by weight), surfactant (from 0 to 5 parts by weight), glycerin (from 0 to 5 parts by weight), viscosity modifier (from 0 to 5 parts by weight). The catalyst may comprise azaridine based curing agent (from 30 to 50 parts by weight), pigment (from 3 to 40 parts by weight), surfactant (from 0 to 5 parts by weight), glycerin (from 0 to 5 parts by weight), viscosity modifier (from 0 to 5 parts by weight), solvent (from 0 to 30 parts by weight). The liquids may have a viscosity from 1 to 30 mPas and surface tension from 20 -50 mN/m. Other inks and catalysts known from the prior art can be used as well. Preferably, the solvent amounts to a maximum of 10%, preferably a maximum of 5% by weight of the combined drop. This allows to significantly decrease the content of the solvent in the printing process, which makes the technology according to the invention more environmentally-friendly than the current CIJ technologies, where the content of solvents usually exceeds 50% of the total mass of the drop during printing process. For this reason, the present invention is considered to be a green technology.

In the first embodiment, the ink drop is combined with the catalyst drop within the head 100, i.e. when the drops are in contact with the components of the head 100, in particular at the separator tip 132. However, the head construction is such that the nozzle outlets 113A, 113B are separated from each other by the separator 131 and therefore the ink and the catalyst will not mix directly at the nozzle outlets 113A, 113B, which prevents the nozzle outlets 113A, 113B from clogging. Once the drops are combined to a combined drop 122, there risk of clogging of the separator tip 132 is minimized, as the separator tip 132 has a small surface and the kinetic energy of the moving combined drop 122 is high enough to detach the combined drop 122 from the separator tip 132. The separator 131 guides the drops 121A, 121B along its surface, therefore the drops 121A, 121B are guided in a controlled and predictable manner until they meet each other. It enables much better control over the coalescence process of two primary drops as well as the control over the direction the combined drop will follow after its discharge from the separator tip 132. It is therefore easy to control drop placement of the combined drop 122 on the surface to be printed. Even if, due to differences in size or density or kinetic energy of the primary drops 121A, 121B, the combined drop 122 would not exit the head perpendicularly (as shown in Figs. 2A and 2B) but at an inclined angle, that angle would be relatively constant and predictable for all drops, therefore it could be taken into account during the printing process. Even relatively large-size drops -like the those used for instance in low resolution valve based ink jet printers -can be combined due to the use of the separator 131 in a more predictable manner than in the prior art solutions where drops combine in-flight outside the printhead.

Therefore, the separator 131 functions as a guide for the primary drops 121A, 121B within the printing head 100 from the nozzle outlet 113A, 113B to a connection point, i.e. the separator tip 132. The separator tip 132 restricts the freedom of combination of primary drops 121A, 121B into a combined drop 122, i.e. the combined drop may form only under the separator tip 132, which impacts its further path of travel -downwards, towards the opening in the cover 181. In other words, in the presented inkjet head, the drops 121A, 121B of at least two components, which before the combination have properties of stable liquids, are guided to a connection point wherein they are still kept in contact with the components of the head, i.e. with the separator 131 down to its tip 132. Therefore, during combination and coalescence of the primary drops 121A,121B, they are in contact with the head components.

The nozzles 112A, 112B have drop generating and propelling devices 161A, 161B for ejecting the drops, which are only schematically marked in Figs. 2A and 2B, and their schematic types are shown in Figs. 9 -11. The drop generating and propelling devices may be for instance of thermal (Fig. 9), piezoelectric (Fig. 10) or valve (Fig. 11) type. In case of the valve the liquid would need to be delivered at some pressure.

The separator 131 as shown in Figs. 2A and 2B is symmetrical, i.e. the inclination angles aA, aB of its side walls 114A, 1148 are the same with respect to the axis of the head 100 or of the nozzle arrangement 110. In alternative embodiments, the separator may be asymmetric, i.e. the angles aA, aB may be different, depending on the parameters of liquids supplied from the nozzle outlets 113A, 113B.

The inclination angles aA, aB are preferably from 5 to 75 degrees, and more preferably from 15 to 45 degrees.

Preferably, the inclination angles f3A, PB of the nozzle channels 112A, 112B are not smaller (as shown in Fig. 2B) than the inclination angles aA, aB of the corresponding separator walls 114A, 114B, so that the ejected primary drops 121A, 121B are forced into contact with the separator walls 114A, 114B.

The separator 131 can be replaceable, which allows to assembly the head 110 with a separator 131 having parameters corresponding to the type of liquid used for printing.

The separator 131 preferably has a length LA, LB of its side wall 114A, 114B, measured from the nozzle outlet 113A, 113B to the separator tip 132, not shorter than the diameter dA, dB of the primary drop 121A, 121B exiting the nozzle outlet 113A, 113B at that side wall 114A, 114B. This prevents the primary drops 121A, 121B from merging before they exit the nozzle outlets 113A, 1138.

The surface of the separator 131 has preferably a low friction coefficient to provide low adhesion of the drops 121A, 121B, 122, such as not to limit their movement and not introduce spin rotation of the primary drops 121A, 1218. In order to decrease adhesion between the separator and the drops 121A, 121B, 122, the separator and/or the nozzle outlets 113A, 113B may be heated to a temperature higher than the temperature of the environment. The liquids in the reservoirs 116A, 116B may be also preheated. Increased temperature of working fluids (i.e. ink and catalyst) may also lead to improved coalescence process of primary drops and preferably increase adhesion and decrease the curing time of the combined drop 122 when applied on the substrate.

As shown in Fig. 1, the separator 131 may be common for a plurality of nozzle assemblies 110. In alternative embodiments, each nozzle assembly 110 may have its own separator 131 and/or cover 181 or a sub-group of nozzle assemblies 110 may have its own common separator 131 and/or cover 181 The printing head may further comprise a cover 181 which protects the head components, in particular the separator tip 132 and the nozzle outlets 113A, 113B, from the environment, for example prevents them from touching by the user or the printed substrate.

Moreover, the cover 181 may comprise heating elements 182 for heating the volume within the cover 181, i.e. the volume surrounding of the nozzle outlets 113A, 113B and the separator 131 to a predetermined temperature, for example from 40°C to 60°C (other temperatures are possible as well, depending on the parameters of the drops), such as to provide stable conditions for combining of the drops. A temperature sensor 183 may be positioned within the cover 181 to sense the temperature.

Moreover, the printing head 110 may comprise gas-supplying nozzles 119A, 119B for blowing gas (such as air or nitrogen), preferably heated, towards the separator tip 132, in order to decrease the curing time, increase the dynamics of movement of the drops and to blow away any residuals that could be formed at the nozzles outlets 113A, 113B separator tip 132.

Second embodiment A second embodiment of the inkjet printing head 200 according to the invention is shown in an overview in Fig. 3. Figs. 4A and 4B show the same longitudinal cross-sectional view, but for clarity of the drawing different elements have been referenced on different figures. Fig. 5 shows a longitudinal cross-sectional view along a section parallel to that in Figs. 4A and 4B. Fig. 6 shows various transverse cross-sectional views.

The inkjet printing head 200 may comprise one or more nozzle assemblies 210, each configured to produce a combined drop 222 formed of two primary drops 221A, 221B ejected from a pair of nozzles 211A, 211B. Fig. 3 shows a head with a plurality of nozzle assemblies 210 arranged in parallel to print multi-dot rows 291 on a substrate 290, it is worth noting that the printing head may comprise only a single nozzle assembly 210 or even as much as 256 nozzle assemblies or more.

Each nozzle 211A, 211B of the pair of nozzles in the nozzle assembly 210 has a channel 212A, 212B for conducting liquid from a reservoir 216A, 216B. At the nozzle outlet 213A, 213B the liquid forms a primary drop 221A, 221B. At the nozzle outlet 213A, 213B the liquid is formed into primary drops 221A, 221B as a result of operation propelling devices shown on Figs. 9, 10, 11. The nozzle outlets 213A, 213B are adjacent to a conical-shaped separator 231 that separates the nozzle outlets 213A, 213B. The primary drops ejected from the nozzle outlets 213A, 213B move along the separator 231 towards its tip 232, where they combine to form a combined drop 222, which separates from the separator tip 232 and travels towards the surface to be printed.

The primary drops 221A, 221B are guided along the surface of the separator 231 by streams 271A, 271B of gas (such as air or nitrogen, provided from a pressurized gas input 219, having a pressure of preferably 5 bar) inside a primary enclosure 241. The shape of the primary enclosure 241 in its upper part helps to direct the stream of gas alongside the nozzles 211A, 211B and guides drops from the outlets 213A, 2138 of the nozzles 211A, 211B towards the connection point at the separator tip 232, at which they join to form the combined drop 222.

The nozzles 212A, 212B have drop generating and propelling devices 261A, 261B for ejecting the drops, which are only schematically marked in Figs. 4A and 4B, and their schematic types are shown in Figs. 9 -11. The drop generating and propelling devices may be for instance of thermal (Fig. 9), piezoelectric (Fig. 10) or valve (Fig. 11) type. In case of the valve the liquid would need to be delivered at some pressure.

The primary enclosure 241 has sections of different shapes. The first section 243, which is located furthest downstream (i.e. towards the direction of flow of the combined drop 222) has preferably a constant, round cross-section of a diameter Dl substantially equal to the desired diameter dC of the combined drop 222. Alternatively, the cross-section of the first section 243, is preferably not smaller than at least 80% of the cross-section of the combined drop 222. Therefore, at the outlet of the primary enclosure 241 at the downstream end of the first section 243, there is formed a kind of combined drop nozzle, through which the drop is pushed thanks to its kinetic energy enhanced by moving gas. This improves precision of its movement directly forward, which facilitates precise drop placement, which in turn improves the print quality. The second section 244 (of primary enclosure 241) is located between the first section 243 and the nozzle outlets 213A, 213B and has a diameter which increases upstream (i.e. opposite the direction of drops flow), such that its upstream diameter encompasses the nozzle outlets 213A, 213B and leaves some space for gas 271A, 271B to flow between the enclosure walls and nozzle outlets 213A, 213B. The same time the cross section of the primary enclosure 241 changes upstream from round to elliptical one, since the width of the cross section increases more with length upstream, than its depth (cf cross section E-E on Fig. 6). The internal walls of the second section 244 converge downstream, therefore the flowing gas stream 271A, 271B forms an outer gas sleeve that urges the drops 221A, 221B, 222 towards the centre of the enclosure 241.

The primary enclosure 241 may further comprise a third section 245 located upstream the second section, which has internal walls in parallel to the external walls of the nozzles 211A, 211B. As more clearly visible in the cross-section B-B (shown for the left part only) of Fig. 6, the nozzle 211A is surrounded by the primary enclosure 241 and separated from the nozzle 211 B by the blocking element 233, such that the stream of gas 271 A flows only at the outer periphery of the nozzles 211A, 211B but not between the nozzles 211A, 211B wherein it is blocked by the blocking element 233, which then forms the separator 231 The stream of gas 271 A, 271 B that is guided by this section is in parallel to the direction of ejecting of the primary drops 221A, 221B from the nozzle outlets 213A, 213B. Parallel direction of the flowing gas stabilized prior to its contact with primary drops improves the control over the path of drops flow starting from the nozzle outlets 213A, 213B, since from the very moment of discharge, their flow is supported in terms of energy and direction by the flowing gas. It is worth noticing, that the shape of the primary enclosure 241 is preferably designed in such a way to enhance the appropriate velocity of gas flowing thorough respective sections, i.e. 245, 244, 243. The velocity of the flowing gas is preferably higher than drop velocity precisely at the nozzle outlets area, which is close to the end of section 245, preferably at least not lower than the drop velocity in the area of the section 244 and higher again in the nozzle 243, where the flow will be forced to be of higher velocity again due to the smaller cross section surface of the outflow channel, i.e. the nozzle 243. Such design would leave some room for gas pressure momentary compensating adjustments while for the short instant the gas flow through the nozzle 243 would slowed down by passing combined drop 222. This momentary pressure increase in the section 244 would preferably add more kinetic energy for the drop 222 on leaving the nozzle 243.

In any case in the second section 244 of the primary enclosure 241 the gas stream 271A, 271B is preferably configured to flow with a linear velocity not smaller than the velocity of the primary ink drops 221A, 221B ejected from the nozzle outlets 213A, 213B. The temperature of the gas may be increased to allow better coalescence and mixing of the primary drops 221 A, 221 B by decreasing the surface tension and viscosity of the ink and the curing agent (polymerization initiator). The geometry of the first section 243 relative to the second section 244 -especially the decrease of cross section surface of section 243 vs. section 244 -is designed such that the gas increases its velocity, preferably from 5 to 20 times, thus increasing the kinetic energy of the coalesced combined drop 222 and stabilizing the flow of the combined drop 222.

The liquids supplied from the two reservoirs 216A, 216B are preferably an ink and a catalyst for initiating curing of the ink, as described with reference to the first embodiment.

In the second embodiment, the ink drop is combined with the catalyst drop within the head 200, i.e. before combined drop 222 exits the primary enclosure 241. The head construction is such that the nozzle outlets 213A, 213B are separated from each other by the separator 231, which does not allow the primary drops 221A, 221B to combine at the nozzle outlets 213A, 213B. Therefore, the ink and the catalyst will not mix directly at the nozzle outlets 213A, 213B, which prevents the nozzle outlets 213A, 213B from clogging. Once the drops are combined to a combined drop 222, there is no risk of clogging of the primary enclosure 241 at the connection point or downstream the enclosure 241, as the combined drop 222 is already separated from the nozzle outlets 213A, 213B and the stream of gas 271A, 271B (which preferably flows continuously) can effectively remove any residuals that would stick to the separator 231 or enclosure walls 241 before solidifying. The enclosure 241 guides the drops 221A, 221B, 222 towards its axis, therefore the drops 221A, 221B, 222 are guided in a controlled and predictable manner. It is therefore easy to control drop placement of the combined drop 222 on the surface to be printed. Even if, due to differences in size or density of the primary drops 221A, 221B, the combined drop 222 would tend to deviate from the axis of the primary enclosure 241, it will be aligned with its axis at the end of the enclosure 241, and therefore exit the enclosure 241 along its axis. Therefore, even relatively large-size drops and primary drops having different sizes can be combined due to the use of the primary enclosure 241 in a more predictable manner than in the prior art solutions where drops combine in-flight outside the printhead.

Therefore, the separator 231 and primary enclosure 241 function as a guide for the primary drops 221 A, 221B within the printing head 200 from the nozzle outlet 213A, 213B to a connection point 232. The separator 231 and the first section 243 of the primary enclosure restricts the freedom of combination of primary drops 221A, 221B into a combined drop 222, i.e. the combined drop 222 forms to a shape and dimensions defined by the inlet of the first section 243, and the separator 231 and the first section 243 impact the further path of travel of the combined drop 222 -downwards, towards the outlet of the first section 243. In other words, in the presented inkjet head, the drops 221A, 221B of at least two components, which before the combination have properties of stable liquids, are guided to a connection point 232 wherein they are still kept in contact with the components of the head, i.e. with the side walls of the first section 243 of the primary enclosure 241. Therefore, during combination and coalescence of the primary drops 221A, 221B, they are in contact with the head components.

The separator 231 may have the same properties as the separator 131 described for the first embodiment.

The inclination angles OA, ps of the nozzle channels 212A, 212B as shown in Figs. 4A and 4B are the same as the inclination angles ctA, WEI of the side walls of the separator 231, so that the primary drops 221A, 221B are ejected from the nozzles in parallel to the separator walls. In alternate embodiments, they may be larger than the corresponding inclination angles aA, aB of the separator walls, so that the ejected primary drops 221A, 221B are forced into contact with the separator walls.

However, an alternate embodiment is possible, wherein the inclination angles f3A, 13B of the nozzle channels 212A, 212B are smaller than the inclination angles aA, aB of the side walls of the separator 231, which may cause the ejected primary drops to separate from the side walls of the separator 231 and combine further downstream, i.e. below the tip of the separator. In such a case the separator 231 functions as a guide for the primary drops 221A, 221B only partially and its main function is to separate the nozzle outlets 213A, 213B so as to prevent them from clogging. In that case, it is mostly the stream of gas 271A, 271B formed by the inside walls of the preliminary enclosure 241 that acts this way (i.e. via moving gas) as means for guiding the primary drops 221A, 2218 within the printing head 200 from the nozzle outlet 213A, 213B to a connection point. The freedom of combination of primary drops 221A, 221B into the combined drop 222 at the connection point is then also restricted by the force of the stream of gas 271A, 271B formed by the internal walls of the primary enclosure 241.

The nozzles 212A, 212B shown in Fig. 3 are symmetrical, i.e. their angles of inclination 13A, pB are the same with respect to the axis of the head 200 or of the nozzle arrangement 210. in alternative embodiments, the nozzles 212A, 212B may be asymmetric, i.e. the angles 13A, j3B may be different, depending on the parameters of liquids supplied from the nozzle outlets 213A, 213B.

The inclination angles 13A, 03 are preferably from 5 to 75 degrees, and more preferably from 15 to 45 degrees.

The primary enclosure 241 can be replaceable, which allows to assembly the head 210 with an enclosure 241 having parameters corresponding to the type of liquid used for printing. For example, enclosures 241 of different diameters D1 of the first section 243 can be used, depending on the actual features and size, as well as desired exit velocity of the combined drop 222. The angles of inclination 13A, 13B of the nozzles 211A, 211B can be adjustable, to adjust the nozzle assembly 210 to parameters of the liquids stored in the reservoirs 216A, 216B.

The first section 243 of the primary enclosure 241 has preferably a length LI not shorter than the diameter dC of the combined drop 222, and preferably the length LI equal to a few diameters dC of the combined drop 222, to set its path of movement precisely for precise drop placement control.

The internal surface of the primary enclosure 241, especially at the first section 243 and at the second section 244 has preferably a low friction coefficient and low adhesion in order to prevent the drops 221 A, 221B, 222 or residuals of their combination from adhering to the surface, helping to keep the device clean and allow the eventual residuals to be blown off by the stream of gas 271A, 271B. The stream of gas preferably prevents also any particles from the outside environment to contaminate the inside of the primary enclosure 243.

The printing head may further comprise a secondary enclosure 251 which surrounds the primary enclosure 241 and has a shape corresponding to the primary enclosure 241 but a larger cross-sectional width, such that a second stream of gas 272, supplied from the pressurized gas inlet 219, can surround the outlet of the first section 243 of the primary enclosure 241, so that the combined drop 222 exiting the primary enclosure 241 is further guided downstream to facilitate control of its path. The gas stream 272 may further increase its velocity in the area of second outlet section 253 due to its shape and thus further accelerate the drop 222 exiting the primary enclosure 241. The surface of the cross section of the gas stream 272 decreases downwards which would cause the stream of gas 272 to reach the velocity not lower, but preferably higher than that of the combined drop 222 in the moment of leaving the section 243 of primary enclosure 241. In order to further increase the drop velocity the cross-section of the second outlet section 253 of the secondary enclosure 251, which is between the outlet of the primary enclosure and the first outlet section 252 of the secondary enclosure, is preferably decreasing downstream such as to direct the stream of gas 272 towards the central axis. The first outlet section 252 of the secondary enclosure 251 has preferably a round cross-section and a diameter D2 that is preferably larger (preferably, at least 2 times larger) than the diameter Dl of the outlet (243) of the primary enclosure, such that the combined drop 222 does not touch the internal side all of the secondary enclosure 251 to prevent clogging and is guided by the (now combined) streams of gas 271A, 271B, 272 between the combined drop 222 and the side walls of the secondary enclosure 251. Preferably, the diameter D2 is at least 2 times greater than the diameter dC of the combined drop. Preferably, the length L2 of the first outlet section 252 is from zero to a multiple of diameters dC of the combined drop 222, such as 10, 100 or even 1000 times the diameter dC, in order to guide the drop in a controllable manner and provide it with desired kinetic energy. This may significantly increase the distance at which the combined drop 222 may be ejected from the printing head and still maintain the precise drop placement on the printed surface, which allows to print objects of variable surface. Moreover, this may allow to eject drops at an angle to the vector of gravity, while keeping satisfactory drop placement control.

Moreover, relatively high length L2 may allow the combined drop to pre-cure before reaching the substrate 290.

In the second outlet section 253 of the secondary enclosure 251 the gas increases its velocity thus decreasing its pressure and consequently lowering its temperature. This may cause the increase of velocity and the decrease of the temperature of the combined drop 222, which remains within the gas stream. Lowering the temperature of the combined drop 222 may increase its viscosity and adhesion, which is desirable in the moment of reaching the substrate by the drop helping the drop to remain in the target point and preventing it from flowing sidewise.

The second embodiment may further comprise a cover 281, having configuration and functionality as described for the cover 181 of the first embodiment, including the heating elements and temperature sensor (not shown for clarity of drawing).

Third embodiment The third embodiment of the head 300 is shown schematically in a longitudinal cross-section on Fig. 7. It has most of the features in common with the second embodiment, with the following differences.

At the first section 343 of the primary enclosure 341, there are charging electrodes 362A, 362B which apply electrostatic charge to the combined drop 322. Moreover, at the first outlet section 352 of the secondary enclosure 351 there are deflecting electrodes 363A, 363B which deflect the direction of the flow of the charged drops 322 in a controllable direction. Thereby, the drop 322 placement can be effectively controlled. In order to allow change of the outlet path of the drops 322 from the inside of the head 300, the output opening 3810 of the cover 381 has an appropriate width so that the deflected drop 322 does not come into contact with the cover 381.

The charging electrodes 362A, 362B and the deflecting electrodes 363A, 363B can be designed in a manner known in the art from CIJ technology and therefore do not require further clarification on details.

The other elements, having references starting with 3 (3xx) correspond to the elements of the second embodiment having references starting with 2 (2xx).

Fourth embodiment The fourth embodiment of the head 400 is shown schematically in a longitudinal cross-section on Fig. 8. It has most of the features in common with the second embodiment, with the following differences.

The fourth embodiment lacks the blocking element 233 between the nozzles and the separator 231 of the second embodiment. Therefore, the gas stream from the gas supply 419 is free to flow within the primary enclosure 441 also along its longitudinal axis between the nozzles 411A, 411B, forming a central gas stream 473.

Therefore, the primary drops 421A, 421B, after being ejected from the nozzle outlets 413A, 413B are guided downstream and towards the central axis of the primary enclosure 441 by the streams 471A, 471 B and 473 of gas. The converging shape of the second section of the primary enclosure 441, as in the second embodiment, causes the primary drops to be pushed towards the centre by the converting gas streams 471A, 471B to combine at a connection point 432 to form the combined drop 422.

In the fourth embodiment, the ink drop is combined with the catalyst drop within the head 400, i.e. before combined drop exits the primary enclosure 441. The head construction is such that the nozzle outlets 413A, 413B are separated from each other by some free space, preferably having cross-sectional dimensions larger than the cross-sections of the primary drops 421A, 421B formed at the nozzle outlets 413A, 413B, through which gas stream 473 flows such as to prevent the drops from combination while still in contact with the nozzle outlets 413A, 413B, which prevents the nozzle outlets from clogging. Once the drops are combined to a combined drop 422, there is no risk of clogging of the primary enclosure 441, as the combined drop 422 is already separated from the nozzle outlets 413A, 4138 and the streams of gas 471A, 471B, 473 can effectively remove any residuals that would stick to the enclosure walls 441 before solidifying. The enclosure 441 guides via the streams of gas the drops 421 A, 421B, 422 towards its axis, therefore the drops 421 A, 421 B, 422 are guided in a controlled and predictable manner. It is therefore easy to control drop placement of the combined drop 422 on the surface to be printed.

Therefore, primary enclosure 441 functions as a guide for the primary drops 421A, 421B within the printing head 400 from the nozzle outlet 413A, 413B to the connection point 432. The first section 443 of the primary enclosure restricts the freedom of combination of primary drops 421A, 421B into a combined drop 422, i.e. the combined drop 422 forms to a shape and dimensions defined by the inlet of the first section of the primary enclosure 441, and the first section 443 impacts the further path of travel of the combined drop 422 downwards, towards the outlet of the first section 443. In other words, in the presented inkjet head, the drops 421A, 421B of at least two components, which before the combination have properties of stable liquids, are guided to a connection point 432 and after combination they are in contact with the components of the head, i.e. with the side walls of the first section 443 of the primary enclosure 441. Therefore, the combined drop 422 is in contact with the head components before it exits the head.

The other elements, having references starting with 4 (4xx) correspond to the elements of the second embodiment having references starting with 2 (2xx).

Further embodiments A skilled person will realize that the features of the embodiments described above can be further mixed between the embodiments. For example there can be well more than two nozzles directing more than two primary drops in order to form one combined drop by means of using the same principles of discharging, guiding, forming, also by means of controlled coalescence, and accelerating drops within the printhead as described above.

Moreover, one or both of the liquids stored in liquid reservoirs may be pre-charged with a predetermined electrostatic charge, such that one or both of the primary drops exiting the nozzle outlets are charged, which may facilitate drops combination to a combined drop.

It shall be noted that the drawings are schematic and not in scale and are used only to illustrate the embodiments for better understanding of the principles of operation.

The present invention is particularly applicable for high resolution DOD inkjet printers. However, the present invention can be also applied to low resolution DOD based on valves allowing to discharge drops of pressurized ink.

Moreover, the present invention uniquely combines the features and advantages of two well known ink jet technologies by means of delivering the working drop ink in the way DOD printers work -including high resolution ones -but being able to deflect and control its flight path in the way CIJ printers work, with the drying or curing time of the imprint also closer to CIJ standards. Such invention improves technical possibilities to apply high quality durable digital imprints on vast variety of substrates and products. This feature will prove to be especially advantageous in majority of industrial marking and coding applications.

Claims

CLAIMS1. An inkjet printing head comprising a nozzle assembly having a pair of nozzles, each nozzle being connected through a channel with a separate liquid reservoir for forming a primary drop of liquid at the nozzle outlet, characterized in that it further comprises: means for guiding the primary drops within the printing head from the nozzle outlet to a connection point; and means for restricting the freedom of combination of the primary drops into a combined drop at the connection point.
2. The printing head according to claim 1, wherein the means for guiding the primary drops have a form of separator of a downstream-narrowing cross-section positioned between the nozzle outlets.
3. The printing head according to claim 2, wherein the separator has its side walls adjacent to the nozzle outlets and configured to guide the primary drops along its side walls to combine into a combined drop at the separator tip which forms the means for restricting the freedom of combination of primary drops.
4. The printing head according to any of claims 2-3, wherein the length of each side wall of the separator is larger than the diameter of a primary drop exiting the nozzle outlet adjacent to that side wall.
5. The printing head according to any of claims 2-4, wherein the side walls of the separator are inclined with respect to the longitudinal axis of the head at an angle from 5 to 75 degrees, preferably from 15 to 45 degrees.
6. The printing head according to claim 5, wherein both side walls of the separator are inclined with respect to the longitudinal axis of the head at the same angle.
7. The printing head according to any of claims 5, wherein the side walls of the separator are inclined with respect to the longitudinal axis of the head at different angles.
8. The printing head according to any of claims 2-7, wherein the side walls of the separator are inclined with respect to the longitudinal axis of the head at an angle not larger than the angle of inclination of the nozzle channels.
9. The printing head according to any of claims 2-7, wherein the side walls of the separator are inclined with respect to the longitudinal axis of the head at an angle larger than the angle of inclination of the nozzle channels.
10. The printing head according to any of claims 2-9, wherein the separator s heated.
11. The printing head according to any claims 2-10, further comprising gas-supplying nozzles for blowing gas towards the separator tip.
12. The printing head according to any of previous claims, wherein the means for guiding the primary drops have a form of a primary enclosure surrounding the nozzle outlets, extending downstream and having a first section for restricting the freedom of combination of primary drops with a diameter not smaller than the diameter of the combined drop and a second section located between the first section and the nozzle outlets and having a width increasing upstream; and further comprising a source of a gas stream to flow downstream inside primary enclosure.
13. The printing head according to claim 12, wherein the nozzles are inclined with respect to the longitudinal axis of the head at an angle from 5 to 75 degrees, preferably from 15 to 45 degrees.
14. The printing head according to any of claims 12-13, wherein both nozzles are nclined with respect to the longitudinal axis of the head at the same angle.
15. The printing head according to any of claims 12-13, wherein the nozzles are inclined with respect to the longitudinal axis of the head at different angles.
16. The printing head according to any of claims 12-15, wherein the primary enclosure further comprises a third section extending upstream in parallel to the external walls of the nozzles.
17. The printing head according to any of claims 12-16, wherein the diameter of the first section of the primary enclosure is equal to the diameter of the combined drop.
18. The printing head according to any of claims 12-16, wherein the cross-section area of the first section of the primary enclosure is not smaller than 80% of the cross-section area of the combined drop.
19. The printing head according to any of claims 12-18, wherein the length of the first section of the primary enclosure is not smaller than the diameter of the combined drop.
20. The printing head according to any of claims 12-19, further comprising a secondary enclosure surrounding the primary enclosure and connected to the source of a gas stream and comprising a first section extending downstream from the outlet of the first section of the primary enclosure and having a diameter decreasing downstream to a diameter larger than the diameter of the combined drop.
21. The printing head according to any of previous claims 12-20, further comprising deflecting electrodes at the outlet of the secondary enclosure.
22. The printing head according to claim 21, further comprising charging electrodes at the outlet of the primary enclosure.
23. The printing head according to any of previous claims, comprising a plurality of nozzle assembles arranged in parallel.
24. The printing head according to any of previous claims, wherein the nozzle outlets are heated.
25. The printing head according to any of previous claims, further comprising a cover enclosing the nozzle outlets and the connection point.
26. The printing head according to claim 25, wherein the cover comprises heating elements for heating the volume within the cover.