EP3061611A1

EP3061611A1 - A printing head

Info

Publication number: EP3061611A1
Application number: EP15202657.1A
Authority: EP
Inventors: Piotr Jeuté; Maciej Zawadzki
Original assignee: Jeute Piotr
Current assignee: JEUTE, PIOTR
Priority date: 2015-02-26
Filing date: 2015-12-24
Publication date: 2016-08-31
Anticipated expiration: 2035-12-24
Also published as: PL411384A1; EP3061611B1; GB2539165A; GB201503296D0; PL226793B1

Abstract

An inkjet printing head comprising a nozzle assembly having a pair of nozzles (111A, 111B; 211A, 211B), each nozzle being connected through a channel (112A, 112B; 212A, 212B) with a separate liquid reservoir (116A, 116B; 216A, 216B) for discharging in a downstream direction a primary drop (121A, 121B; 221A, 221B) of liquid at the nozzle outlet (113A, 113B; 213A, 213B) to combine at a connection point (132; 232) into a combined drop (122; 232). The printing head further comprises a primary enclosure (141; 241) surrounding the nozzle outlets (113A, 113B; 213A, 213B), and having a cross-section narrowing in the downstream direction; and a source of a gas stream configured to flow in the downstream direction inside the primary enclosure (141; 241). The connection point (132; 232) is located within the primary enclosure (141; 241).

Description

TECHNICAL 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.
CIJ 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 are considered as a background for the present invention.
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-IJP) 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. The individual drops are not combined with each other.
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.
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. 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.
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.

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 CIJ 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 CIJ - 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 present invention is related 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 discharging in a downstream direction a primary drop of liquid at the nozzle outlet to combine at a connection point into a combined drop. The printing head further comprises a primary enclosure surrounding the nozzle outlets and having a cross-section narrowing in the downstream direction; and a source of a gas stream configured to flow in the downstream direction inside the primary enclosure. The connection point is located within the primary enclosure.
The primary enclosure may have a first section at its downstream outlet with a diameter larger than the diameter of the combined drop.
The primary enclosure may have a first section at its downstream outlet with a diameter not larger than the diameter of the combined drop.
The nozzles can be configured for discharging the primary drops of liquid at an angle inclined towards the longitudinal axis of the head, preferably at an angle from 5 to 75 degrees, more preferably from 15 to 45 degrees.
Both nozzles can be 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 nozzles can be configured for discharging the primary drops of liquid in parallel to the longitudinal axis of the head.
The primary enclosure may further comprise a third section extending upstream in parallel to the external walls of the nozzles.
The length of the first section at the outlet of the primary enclosure may be not smaller than the diameter of the combined drop.
The printing head may further comprise a set of electrodes at the outlet of the primary enclosure.
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 a set of electrodes at the outlet of the secondary 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.

BRIEF DESCRIPTION OF DRAWINGS

The invention is shown by means of exemplary embodiment on a drawing, in which:

Figs. 1, 2A, 2B, 3 and 4 show schematically a first embodiment of the invention;
Fig. 5 shows schematically a second embodiment of the invention;
Fig. 6 shows schematically a third embodiment of the invention;
Figs. 7A-7C show schematically a fourth embodiment of the invention;
Fig. 8 shows schematically an fifth embodiment of the invention;
Fig. 9 shows schematically a sixth embodiment of the invention;
Fig. 10 shows schematically a seventh embodiment of the invention;
Fig. 11 shows schematically an eighth embodiment of the invention;Figs. 12, 13 and 14 show schematically different devices propelling drop out of the nozzle.

DETAILED DESCRIPTION

First embodiment

The inkjet printing head 100 according to the invention is shown in an overview in Fig. 1. Figs. 2A and 2B show the same longitudinal cross-sectional view, but for clarity of the drawing different elements have been referenced on different figures. Fig. 3 shows a longitudinal cross-sectional view along a section parallel to that in Figs. 2A and 2B. Fig. 4 shows various transverse cross-sectional views.
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. Fig. 1 shows a head with a plurality of nozzle assemblies 110 arranged in parallel to print multi-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 even as much as 156 nozzle assemblies.
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, 116B. At the nozzle outlet 113A, 113B the liquid forms a primary drop 121A, 121B. The primary drops 121A, 121B ejected from the nozzle outlets 113A, 113B move towards a connection point 132, where they combine to form a combined drop 122 and travels towards the surface to be printed.
The primary drops 121A, 121B are guided by streams 171A, 171B and 174A, 174B of gas (such as air or nitrogen, provided from a pressurized gas input 119, having a pressure of preferably 5 bar) inside primary enclosure 141. The shape of the primary enclosure 141 in its upper part helps to direct the stream of gas alongside the nozzles 111A, 111B and guides drops from the outlets 113A, 113B of the nozzles 111A, 111B towards the connection point at the separator tip 132, at which they join to form the combined droplet 122.
The nozzles 112A, 112B have drop generating and propelling devices 161A, 161B for ejecting the droplets, which are only schematically marked in Figs. 2A and 2B, and their schematic types are shown in Figs. 12-14. The drop generating and propelling devices may be for instance of thermal (Fig. 12), piezoelectric (Fig. 13) or valve (Fig. 14) type. In case of the valve the liquid would need to be delivered at some pressure.
The primary enclosure 141 has sections of different shapes. The first section 143, which is located furthest downstream (i.e. towards the direction of flow of the combined drop 122) has preferably a constant, round cross-section of a diameter D1 larger than the diameter dC of the combined drop 122, preferably at least 2 times larger. Therefore, at the outlet of the primary enclosure 141 at the downstream end of the first section 143, which forms a kind of combined drop nozzle, the combined drop 122 is guided by the stream of air 171A, 171B which separates it from the walls of the primary enclosure 141. This improves precision of its movement directly forward, which facilitates precise droplet placement, which in turn improves the print quality. The second section 144 of the primary enclosure 141 is located between the first section 143 and the nozzle outlets 113A, 113B and has a diameter which increases upstream (i.e. opposite the direction of drops flow), such that its upstream diameter encompasses the nozzle outlets 113A, 113B and leaves some space for gas 171A, 171B to flow between the enclosure walls and nozzle outlets 113A, 113B. The same time the cross section of the primary enclosure 141 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. 4). The internal walls of the second section 144 converge downstream, therefore the flowing gas stream 171A, 171B forms an outer gas sleeve that urges the drops 121A, 121B, 122 towards the centre of the enclosure 141.
The primary enclosure 141 may further comprise a third section 145 located upstream the second section, which has internal walls in parallel to the external walls of the nozzles 111A, 111B. As more clearly visible in the cross-section B-B of Fig. 4, the nozzles 111A, 111B are surrounded by the primary enclosure 141 and separated by a blocking element 133 (which is however separate from the nozzles 111A 111B), such that streams of gas 171A, 171B may form between the nozzles 111A, 111B and the primary enclosure 141 and streams of gas 174A, 174B may form between the nozzles 111A, 111B and the blocking element 133.
The stream of gas 171A, 171B that is guided by this section is in parallel to the direction of ejecting of the primary drops 121A, 121B from the nozzle outlets 113A, 113B. Parallel direction of the flowing gas stabilized prior to its contact with primary droplets improves the control over the path of droplets flow starting from the nozzle outlets 113A, 113B, 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 141 needs to be designed in such a way to enhance the appropriate velocity of gas flowing thorough respective sections, i.e. 145, 144, 143. The velocity of the flowing gas should be preferably higher than drop velocity precisely at the nozzle outlets area which is close to the end of section 145, preferably at least not lower than the drop velocity in the area of the section 144 and higher again in the nozzle 143, 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. nozzle 143. Such design would leave some room for gas pressure momentary compensating adjustments while for the short instant the gas flow through the nozzle 143 would slowed down by passing combined drop 122. This momentary pressure increase in the section 144 would preferably add more kinetic energy for the drop 122 on leaving the nozzle 143.
In any case the second section 144 of the enclosure 141 the gas streams 171A, 171B, 174A, 174B are preferably configured to flow with a linear velocity not smaller than the velocity of the primary ink drops 121A, 121B ejected from the nozzle outlets 113A, 113B. The temperature of the gas may be increased to allow better coalescence and mixing of the primary drops 121A, 121B by decreasing the surface tension and viscosity of the ink and the curing agent (polymerization initiator). The geometry of the first section 143 relative to the second section 144 - especially the decrease of cross section surface of section 143 vs. section 144 - is designed such that the gas increases its velocity, preferably from 5 to 10 times, thus increasing the kinetic energy of the coalesced combined drop 122 and stabilizing the flow of the combined drop 122.
Alternatively, the head may have no blocking element 133, then the streams of gas 174A, 174B will not be directed in parallel to the axes of the nozzles 111A, 111B. However, due to the directions of streams 171A, 171B, the control over path of movement of the primary droplets 121A, 121B may still be possible.
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%, 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 can be classified as a green technology.
Therefore, the ink drop is combined with the catalyst drop within the head 100, i.e. before combined drop exits the primary enclosure 141. The head construction is such that the nozzle outlets 113A, 113B are separated from each other by the channels for gas streams 174A, 174B, which does not allow the primary drops 121A, 121B to combine at the nozzle outlets 113A, 113B. 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 is no risk of clogging of the primary enclosure 141 at the connection point or downstream the enclosure 141, as the combined drop 122 is already separated from the nozzle outlets 113A, 113B and the streams of gas 171A, 171B, 174A, 174B (which preferably flows continuously) can effectively remove any residuals that would stick to the enclosure walls 141 before solidifying. The enclosure 141 guides the drops 121A, 121B, 122 towards its axis by the gas streams 171A, 171B, therefore the drops 121A, 121B, 122 are guided in a controlled and predictable manner. 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 of the primary drops 121A, 121B, the combined drop 122 would tend to deviate from the axis of the primary enclosure 141, it will be aligned with its axis at the end of the enclosure 141, and therefore exit the enclosure 141 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 141 in a more predictable manner than in the prior art solutions where drops combine in-flight outside the printhead.
Therefore, the primary enclosure 141 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 132.
The nozzles 112A, 112B are preferably symmetrical, i.e. their angles of inclination βA, βB are the same with respect to the axis of the head 100 or of the nozzle arrangement 110. In alternative embodiments, the nozzles 112A, 112B may be asymmetric, i.e. the angles βA, βB may be different, depending on the parameters of liquids supplied from the nozzle outlets 113A, 113B.
The inclination angles βA, βB are preferably from 5 to 75 degrees, and more preferably from 15 to 45 degrees.
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.
The primary enclosure 141 can be replaceable, which allows to assembly the head 110 with a enclosure 141 having parameters corresponding to the type of liquid used for printing. For example, enclosures 141 of different diameters D1 of the first section 143 can be used, depending on the desired size of the combined drop 122. The angles of inclination βA, βB of the nozzles can be adjustable, to adjust the nozzle assembly 110 to parameters of the liquids stored in the reservoirs 116A, 116B.
The first section 143 of the primary enclosure 141 has preferably a length L1 not shorter than the diameter dC of the combined drop 122, and preferably the length L1 equal to a few diameters dC of the combined drop 122, to set its path of movement precisely for precise drop placement control.
The internal surface of the primary enclosure 141, especially at the first section 143 and at the second section 144 has preferably a low friction coefficient to provide low adhesion of the residuals of combination of the primary drops, to keep the device clean and allow the residuals to be blown off by the stream of gas 171A, 171B.
The printing head may further comprise a secondary enclosure 151 which surrounds the primary enclosure 141 and has a shape corresponding to the primary enclosure 141 but a larger cross-sectional width, such that a second stream of gas 172, supplied from the pressurized gas inlet 119, can surround the outlet of the first section 143 of the primary enclosure 141, so that the combined drop 122 exiting the primary enclosure 141 is further guided downstream to facilitate control of its path. The gas stream 172 may further accelerate the drop 122 exiting the primary enclosure 141. The cross-section of the second outlet section 153 of the secondary enclosure 151, which is between the outlet of the primary enclosure and the first outlet section 152 of the secondary enclosure, is preferably decreasing downstream such as to direct the stream of gas 172 towards the central axis. The first outlet section 152 of the secondary enclosure 151 has preferably a round cross-section and a diameter D2 that is preferably larger than the diameter D1 of the outlet of the primary enclosure, such that the combined drop 122 does not touch the internal side all of the secondary enclosure 151 to prevent clogging and is guided by the (now combined) streams of gas 171A, 171B, 172 between the combined drop 122 and the side walls of the secondary enclosure 151. 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 152 is from zero to a multiple of diameters dC of the combined drop 122, 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 122 may be ejected from the printing head, 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 precure before reaching the substrate 190.
In the second outlet section 153 of the secondary enclosure 151 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 122, which remains within the gas stream. Lowering the temperature of the combined drop 122 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 head may further comprise a cover 181 which protects the head components, in particular the nozzle outlets 113A, 113B from the environment, for example prevents them from touching by the user.

Second embodiment

In a second embodiment, shown schematically in Fig. 5, one or both of the liquids stored in liquid reservoirs 116A, 116B 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 combination of primary drops 121A, 121B to a combined drop 122. As shown in Fig. 5, the outlet of the primary enclosure 141 may contain a set of electrodes 162, which generate electrical field that forces the charged combined drop 122 to be aligned with the longitudinal axis of the head. Moreover, the outlet of the secondary enclosure 151 may contain a set of electrodes 163, which generate electrical field that forces the charged combined drop 122 to be aligned with the longitudinal axis of the head. Both or only one of the electrodes set 164, 165 may be used. Preferably, the sets 164, 165 each comprise at least 3 electrodes, or preferably 4 electrodes, which are distributed evenly along the circumference of a circle, such as to force the drop 122 towards the central axis. Therefore, the sets of electrodes 164, 165 aid in drop placement. The other elements are equivalent to the first embodiment.

Third embodiment

In a third embodiment, shown schematically in Fig. 6, only the primary enclosure 141 is present, without the secondary enclosure 151. The primary enclosure 141 has a longer first section 143 as compared to the first embodiment, which facilitates control over drop placement and may allow to increase the energy of the outlet combined droplet. The other elements are equivalent to the first embodiment.

Fourth embodiment

A fourth embodiment, shown schematically in Fig. 7A and 7B, 7C (which are schematic cross-sections along the line A-A of Fig. 7A), differs from the first embodiment of Fig. 2A by the following. The nozzles 111A, 111B have the end sections of their channels 112A, 112B arranged substantially perpendicularly to the main axis of the printing head) and the nozzle outlets 113A, 113B are configured to eject the primary drops 121A, 121B such that they move along respectively a first path and a second path which are initially directed in parallel to the main axis X of the printing head.
Such arrangement of the end sections of the nozzle channels 112A, 112B further allows to position relatively large (for example, piezoelectric) drop generating and propelling devices 161A, 161B, as shown in Fig. 7B.
Fig. 7C shows a variant with a possibility to implement more than two (e.g. six) nozzles 112A-112F, each having its own drop generating and propelling device 161A-161F, each connected to an individual liquid reservoir, in order to allow generation of a combined drop from more than two primary drops. It shall be noted that in such case not all combined drops have to be combined from six drops, it is possible that for a particular combined drop only some of the nozzles 112A-112F provide primary drops, e.g. two, three, four or five nozzles, depending on the desired properties of the combined drop.
After being ejected, the primary drops 121A, 121B are guided by the streams of gas 171A, 171B within the primary enclosure 141, such that the first path and the second path are changed to cross each other at the connection point 132, which is located preferably at the downstream section 143 of the primary enclosure 141, which has preferably a constant, round cross-section of a diameter not larger than, and preferably substantially equal, to the desired diameter of the combined drop 122.

Fifth embodiment

The fifth embodiment, shown schematically in Fig. 8, differs from the first embodiment by the following. At least one of the nozzles, in that example the first nozzle 111A, is connected to a mixing chamber 117, wherein liquid is mixed from a plurality of reservoirs 116A1, 116A2, from which the liquid is dosed by valves 117.1, 117.2. For example, the separate reservoirs 116A1, 116A2 may store inks of different colors, in order to supply from the first nozzle 111A a primary drop of ink having a desired color.

Sixth embodiment

The sixth embodiment, shown schematically in Fig. 9, differs from the fourth embodiment of Fig. 7A-7C by the following. The nozzles are arranged in a plurality of levels. The first level of nozzles 111A.1, 111.B. (connected to liquid reservoirs 116A.1, 116B.1) is arranged such that they produce first level primary drops 121A.1, 121B.1 within the primary enclosure 141, which are guided by the streams of gas to combine into a first level combined drop 122.1. The second level of nozzles 111A.2, 111B.2 (connected to liquid reservoirs 116A.2, 116B.2) is arranged such that they produce second level primary drops 121A.2, 121B.2 within the secondary enclosure 151, which are guided by the streams of gas to combine into a second level combined drop 122.2. The second level combined drop 122.1 may be formed of only the second level primary drops 121A.2, 121B.2 (which allows to increase the drop generation frequency or variety of drop types that can be generated) or may be formed of the second level primary drops 121A.2, 121B.2 combined with the first level combined drop 122.1 (which allows to increase the variety of drop types from more than two components that can be generated).

Seventh embodiment

The seventh embodiment is shown schematically in a longitudinal cross-section on Fig. 10. It has most of its features in common with the first embodiment, with the following differences.
The outlets 113A, 113B of the nozzles 111A, 111B are separated by 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 to 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 respectively a first path and a second path 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. Therefore, the separator 131 functions as means for controlling the flight of the first primary drop 121A and the second primary drop 121B to allow the first primary drop 121A to combine with the second primary drop 121B at the connection point 132 into the combined drop 122.
The separator 131 functions as a guide for the primary drops 121A, 121B within the primary enclosure 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 separator 131 preferably has a length of its side wall 114A, 114B, respectively, measured from the nozzle outlet 113A, 113B to the separator tip 132, not shorter than the diameter 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, 113B.
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, 121B. Moreover, the side walls of the separator 131 are inclined such as to have a high wetting angle between the side walls and the primary drops, such as to decrease adhesion. 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.
Moreover, at the first section 143 of the primary enclosure 141 and at the first section 152 of the secondary enclosure 151, there are charging electrodes 162, 163 which apply electrostatic charge to the combined drop 122.
Moreover, downstream, behind at the first outlet section 152 of the secondary enclosure 151 there are deflecting electrodes 164A, 164B which deflect the direction of the flow of the charged drops 122 in a controllable direction. Thereby, the drop 122 placement can be effectively controlled. In order to allow change of the outlet path of the drops 122 from the inside of the head 100, the output opening 181O of the cover 181 has an appropriate width so that the deflected drop 122 does not come into contact with the cover 181.
The charging electrodes 162, 163 and the deflecting electrodes 164A, 164B can be designed in a manner known in the art from CIJ technology and therefore do not require further clarification on details.

Eighth embodiment

The eighth embodiment of the head 200 is shown in an overview in Fig. 11. The seventh embodiment 200 is adapted particularly for use with large-size drop generating and propelling devices.
The primary drops 221A, 221B are ejected from the nozzle outlets 213A, 213B of nozzles 211A, 211B which preferably have at least the end sections of their channels 212A, 212B arranged substantially perpendicularly to the main axis X of the printing head. The nozzle channels 212A, 212B may accommodate large-size (e.g. piezoelectric) drop generating and propelling devices 261A, 261B. The primary drops 221A, 221B are formed of a first liquid and second liquid from the reservoirs 216A, 216B.
The primary drops 221A, 2211B are ejected to move along respectively the first and second path, which are initially arranged substantially in parallel to the main axis X. The primary drops 221A, 221B are then guided within a primary enclosure 241 by streams of gas 271A, 271B which may be generated within the primary enclosure 241 from appropriate gas source, e.g. a gas supplying nozzle. The primary enclosure 241 has a downstream-narrowing cross section. The outlet section 243 of the primary enclosure 241 has preferably a constant, round cross-section of a diameter substantially equal to the desired diameter of the combined drop 222, and may be further configured such as described with respect to the section 143 of the first embodiment as shown in Figs. 2A-2B.
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 inkjet 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

An inkjet printing head comprising a nozzle assembly having a pair of nozzles (111A, 111B; 211A, 211B), each nozzle being connected through a channel (112A, 112B; 212A, 212B) with a separate liquid reservoir (116A, 116B; 216A, 216B) for discharging in a downstream direction a primary drop (121A, 121B; 221A, 221B) of liquid at the nozzle outlet (113A, 113B; 213A, 213B) to combine at a connection point (132; 232) into a combined drop (122; 232), characterized in that it further comprises:
- a primary enclosure (141; 241) surrounding the nozzle outlets (113A, 113B; 213A, 213B), and having a cross-section narrowing in the downstream direction; and

- a source of a gas stream configured to flow in the downstream direction inside the primary enclosure (141; 241);

- wherein the connection point (132; 232) is located within the primary enclosure (141; 241).
The printing head according to claim 1, wherein the primary enclosure (141; 241) has a first section at its downstream outlet with a diameter larger than the diameter of the combined drop (122; 222).
The printing head according to claim 1, wherein the primary enclosure (141; 241) has a first section at its downstream outlet with a diameter not larger than the diameter of the combined drop (122).
The printing head according to any of previous claims, wherein the nozzles (111A, 111B; 211A, 211B) are configured for discharging the primary drops (121A, 121B; 221A, 221B) of liquid at an angle inclined towards the longitudinal axis of the head, preferably at an angle from 5 to 75 degrees, more preferably from 15 to 45 degrees.
The printing head according to claim 4, wherein both nozzles (111A, 111B; 211A, 211B) are inclined with respect to the longitudinal axis of the head at the same angle.
The printing head according to claim 4, wherein the nozzles (111A, 111B; 211A, 211B) are inclined with respect to the longitudinal axis of the head at different angles.
The printing head according to any of claims 1-3, wherein the nozzles (111A, 111B; 211A, 211B) are configured for discharging the primary drops (121A, 121B; 221A, 221B) of liquid in parallel to the longitudinal axis of the head.
The printing head according to any of previous claims, wherein the primary enclosure (141) further comprises a third section (145) extending upstream in parallel to the external walls of the nozzles.
The printing head according to any of previous claims, wherein the length of the first section (141) at the outlet of the of the primary enclosure (141) is not smaller than the diameter of the combined drop (122).
The printing head according to any of previous claims, further comprising a set of electrodes (162) at the outlet of the primary enclosure.
The printing head according to any of previous claims, further comprising a secondary enclosure (151) surrounding the primary enclosure (141) and connected to the source of a gas stream (191) 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 according to claim 11, further comprising a set of electrodes (163) at the outlet of the secondary enclosure.
The printing head according to any of previous claims, comprising a plurality of nozzle assembles (110) arranged in parallel.
The printing head according to any of previous claims, wherein the nozzle outlets (113A, 113B; 213A, 213B) are heated.
The printing head according to any of previous claims, further comprising a cover (181) enclosing the nozzle outlets and the connection point.