GB2554445A - A drop on demand printing head and printing method - Google Patents

Abstract

A drop-on-demand printing method comprising performing the following steps in a printing head: discharging a first primary drop 121A of a first liquid from a first nozzle 111A to move along a first path pA with a first speed; discharging a second primary drop 121B of a second liquid from a second nozzle 121B to move along a second path pB with a second, lower speed. The second path pB is inclined with respect to the first path pB at an angle α from 3 to 60 degrees and crosses the first path pA at a connection point. The first drop 121A has a higher kinetic energy than the second drop 121B. The path of flight pC of the combined drop 122 is altered no more than 20 degrees from the axis of the path of flight pA of the first primary drop.

Description

(54) Title of the Invention: A drop on demand printing head and printing method Abstract Title: A drop on demand printing head and method (57) A drop-on-demand printing method comprising performing the following steps in a printing head: discharging a first primary drop 121Aof a first liquid from a first nozzle 111Ato move along a first path pAwith a first speed; discharging a second primary drop 121B of a second liquid from a second nozzle 121B to move along a second path pB with a second, lower speed. The second path pB is inclined with respect to the first path pB at an angle a from 3 to 60 degrees and crosses the first path pA at a connection point. The first drop 121A has a higher kinetic energy than the second drop 121B. The path of flight pC of the combined drop 122 is altered no more than 20 degrees from the axis of the path of flight pA of the first primary drop.

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Fig. 3

A DROP ON DEMAND PRINTING HEAD AND PRINTING METHOD

DESCRIPTION

TECHNICAL FIELD

The present invention relates to drop on demand printing heads and printing methods.

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 (CII) 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 gas 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 a 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 have 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 usage of fast drying solvent, which is well accepted by CIJ technology designed with fast drying solvent in mind, 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 it gets in contact with other objects.

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 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 that it needs certain drop size in order to work well. The other well-known disadvantage of CIJ technology is high usage of 'l·

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

A PCT application WO2016135294A2 discloses a drop-on-demand printing method comprising performing the following steps in a printing head: discharging a first primary drop of a first liquid to move along a first path; discharging a second primary drop of a second liquid to move along a second path; controlling the flight of the first primary drop and the second primary drop to combine the first primary drop with the second primary drop into a combined drop at a connection point within a reaction chamber within the printing head so that a chemical reaction is initiated within a controlled environment of the reaction chamber between the first liquid of the first primary drop and the second liquid of the second primary drop; and controlling the flight of the combined drop through the reaction chamber along a combined drop path such that the combined drop, during movement along the combined drop path starting from the connection point is distanced from the elements of the printing head. In one of the embodiments, the printing head comprises a set of electrodes for altering the path of flight of the second primary drop to a path being in line with the path of flight of the first primary drop before or at the connection point.

There is a need to provide an alternative solution for controlling the flight of the primary drops, with alternative means for controlling the path of flight of the primary drops, and with an aim to improve drop placement accuracy.

SUMMARY

There is disclosed a drop-on-demand printing method comprising performing the following steps in a printing head: discharging a first primary drop of a first liquid from a first nozzle outlet to move along a first path (pA) with a first speed; discharging a second primary drop of a second liquid from a second nozzle outlet to move along a second path (pB) with a second speed, lower than the first speed, wherein the second path (pB) is inclined with respect to the first path (pB) along an axis inclined at an angle (a) from 3 to 60 degrees and crosses the first path (pA) at a connection point; controlling the flight of the first primary drop and the second primary drop to combine the first primary drop with the second primary drop into a combined drop at the connection point so that a chemical reaction is initiated between the first liquid of the first primary drop and the second liquid of the second primary drop; wherein the first primary drop has at the connection point a kinetic energy higher than the second primary drop; and wherein the path of flight (pC) of the combined drop is altered no more than 20 degrees from the axis of the path of flight (pA) of the first primary drop.

The method may comprise discharging the first primary drop of a size larger than the second primary drop.

The method may comprise controlling the timing of discharge of the primary drops.

The method may comprise controlling the relative position of the nozzle outlets.

The connection point can be located within a reaction chamber defined by a cover.

The method may comprise controlling at least one of the following parameters within the reaction chamber: chamber temperature, electric field, ultrasound field, UV light.

There is also disclosed a drop-on-demand printing head comprising: a nozzle assembly comprising: a first nozzle connected through a first channel with a first liquid reservoir with a first liquid and having a first drop generating and propelling device for forming on demand a first primary drop of the first liquid and discharging the first primary drop to move along a first path (pA) with a first speed; and a second nozzle connected through a second channel with a second liquid reservoir with a second liquid and having a second drop generating and propelling device for forming on demand a second primary drop of the second liquid and discharging the second primary drop to move along a second path (pB) with a second speed, lower than the first speed, wherein the second path (pB) is inclined with respect to the first path (pB) along an axis inclined at an angle (a) from 3 to 60 degrees and crosses the first path (pA) at a connection point; means for controlling the flight of the first primary drop and the second primary drop to combine the first primary drop with the second primary drop into a combined drop at the connection point so that a chemical reaction is initiated between the first liquid of the first primary drop and the second liquid of the second primary drop; wherein the first primary drop has at the connection point a kinetic energy higher than the second primary drop; and wherein the path of flight (pC) of the combined drop is altered no more than 20 degrees from the axis of the path of flight (pA) of the first primary drop.

The first primary drop may have a size larger than the second primary drop.

The printing head may comprise a controller for controlling the timing of discharge of the primary drops.

The printing head may further comprise means for controlling the relative position of the nozzle outlets.

The connection point can be located within a reaction chamber defined by a cover.

BRIEF DESCRIPTION OF DRAWINGS

The invention is shown by means of exemplary embodiment on a drawing, in which: Fig. 1 shows schematically the overview of the printing head;

Fig. 2 shows schematically an embodiment of the printing head in a cross-section;

Figs. 3, 4, 5 show schematically different devices for propelling a drop out of the nozzle.

DETAILED DESCRIPTION

The details and features of the present invention, its nature and various advantages will become more apparent from the following detailed description of the preferred embodiments of a drop on demand printing head and printing method.

The present invention allows to shorten the time of curing of the ink after its deposition on the surface, by allowing to use fast-curing components which come into chemical reaction in a reaction chamber within the printing head, thereby increasing the efficiency and controllability of the printing process. In other words, the invention provides coalescence in controlled environment.

In the printing head according to the invention, the primary drops can combine into a combined drop wherein a chemical reaction is initiated, without the risk of clogging of the reaction chamber or the outlet of reaction chamber. Preferably, the primary drops combine into the combined drop within the reaction chamber (in the controlled and predictable environment of the printing head, but they may also combine outside the printing head, just before contacting the printed surface. This is achieved by charging the primary drops with opposite charges, so that the primary drops can attract each other and coalesce in flight.

The reaction chamber preferably has at the connection point, wherein the combined drop is formed, a size larger than the size of the expected size of the combined drop, such as to allow good coalescence of the primary drops and prevent the combined drop from touching the walls of the reaction chamber. At the connection point, there is therefore some space available for the primary drops to freely combine.

A chemical reaction is initiated between the component(s) of the first liquid forming the first primary drop and the component(s) of the second liquid forming the second primary drop when the primary drops coalesce to form the combined drop. A variety of substances may be used as components of primary drops. The following examples are to be treated as exemplary only and do not limit the scope of the invention:

a combined drop of polyacrylate may be formed by chemical reaction between the primary drop of a monomer (for example: methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate optionally with addition of colorant) and the second primary drop of an initiator (for example: catalyst such as trimethylolpropane, tris(l-aziridinepropionate) or azaridine, moreover UV light may be used as initiator agent) a combined drop of polyurethane may be formed by chemical reaction between the primary drop of a monomer (for example: methylene diphenyl diisocyanate (MDI), such as 4,4'-methylenediphenyl diisocyanate, or toluene diisocyanate (TDI) or different monomeric diisocyianates either aliphatic or cycloaliphatic) and the second primary drop of an initiator (for example: monohydric alcohol, dihydric alcohol or polyhydric alcohol such as glycerol or glycol; thiols, optionally with addition of colorant) a combined drop of polycarboimide may be formed by reaction between the primary drop of a monomer (for example: carbimides) and the second primary drop of an initiator (for example dicarboxylic acids such as adipic acid, optionally with addition of colorant)

In general, the first liquid may comprise a first polymer-forming system (preferably, one or more compounds such as a monomer, an oligomer (a resin), a polymer etc., or a mixture thereof) and the second liquid may comprise a second polymer-forming system (preferably, one or more compounds such as a monomer, an oligomer (a resin), a polymer, an initiator of a polymerization reaction, one or more crosslinkers ect., or a mixture thereof). The chemical reaction is preferably a polyreaction or copolyreaction, which may involve crosslinking, such as polycondensation, polyaddition, radical polymerization, ionic polymerization or coordination polymerization. In addition, the first liquid and the second liquid may comprise other substances such as solvents, dispersants etc.

In general, it is highly preferable that the liquids are selected such that both liquids have a similar and low dynamic viscosity, preferably below 50 mPa*s (cps).

Both liquids shall be selected such as not to form an explosive mixture in the air.

Both liquids shall have an interface surface tension selected to allow the liquids to coalesce in flight and diffuse to form the combined drop, so that a chemical reaction is initiated immediately after coalescence of primary drops. Additives, such as surfactants, may be added to the liquids to lower the interface surface tension.

Particularly good results were obtained for the first liquid being methylene diphenyl diisocyanate (MDI) (which may comprise a pigment) and the second liquid being ethanolamine. A combined drop formed from these liquids coagulated by the way of chemical reaction in about 1 second.

By controlling the environment of the reaction chamber, it is possible to achieve controllable, full coalescence of the primary drops (which occurs only at particular conditions, dependent on the liquids, such as the speed, mass of drops, the surface tension, viscosity, angle of incidence). It is typically not possible to control these parameters at the environment outside the printing head, where the ambient temperature, pressure, humidity, wind speed may vary and have significant impact on the coalescence process (and result in deviation of the paths of flight of the drops, generation of satellite drops (which might clog the interior of the printing head), bouncing off of the primary drops, which may lead to at least loss of quality, if not to full malfunction of the printing process).

By increasing the temperature within the printing head, the surface tension and viscosity of the primary drops can be reduced.

If the coalescence process is under control, the chemical reaction may be initiated evenly within the volume of the combined drop, thereby providing prints of predictable quality. The liquids of the primary drops coalesce by mechanical manner (due to collision between the drops) and mix by diffusion of the components. The speed of diffusion depends on the difference of concentration of components in the individual drops and the temperaturedependent diffusion coefficient. As the temperature is increased, the diffusion coefficient increases, and the speed of diffusion of the components within the combined drop increases. Therefore, increase of temperature leads to combined drops of more even composition and increases the speed of the chemical reaction.

If the combined drop is formed such that it has a temperature higher than the temperature of the surface to be printed, the combined drop, when it hits the printed surface, undergoes rapid cooling, and its viscosity increases, therefore the drop is less prone to move away from the position at which it was deposited. This cooling process should increase the density and viscosity of the combined drop while deposited, however not to the final solidification stage, since the final solidification should result from completed chemical reaction rather than temperature change only. Moreover, as the chemical reaction (i.e. polymerization, curing (crosslinking)) is already initiated in the combined drop, the crosslinking of individual layers of printed matter is improved (which is particularly important for 3D printing).

The presented solution allows to prevent remnants of combined, reacting substance to build up in the proximity of nozzle outlets by means of controlling the path of flight of primary drops after they are discharged from respective nozzle outlets.

The presented drop-on-demand printing head and method can be employed for various applications, including high-quality printing, even on non-porous substrates or surfaces with limited percolation., Very good adhesion of polymers combined with comparatively high drop energy allows for industrial printing and coding with high speeds on a wide variety of products in the last phase of their production process. The control of the gradual solidification, which includes the preliminary density increase allowing the drop to stay where applied, but at the same time allowing the chemical reaction to get completed before the final solidification, makes this technology suitable for advanced 3D printing. The crosslinking between individual layers would allow to avoid anisotropy kind of phenomena in the final 3D printed material, which would be advantageous compared to the great deal of existing 3D ink jet based technology.

An 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 in Fig. 2.

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 121 A, 121B ejected from a pair of nozzles 111 A, 11 IB. The embodiment can be enhanced by using more than two nozzles. 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, 11 IB of the pair of nozzles in the nozzle assembly 110 has a channel 112 A, I12B for conducting liquid from a reservoir 116 A, 116B. At the nozzle outlet 113A, 113B the liquid is formed into primary drops 121 A, 121B and ejected as a result of operation of drop generating and propelling devices 161 A, 16IB shown in a more detailed manner on Figs. 3, 4, 5. The drop generating and propelling devices may be for instance of thermal (Fig. 3), piezoelectric (Fig. 4) or valve (Fig. 5) type. In case of the valve the liquid would need to be delivered at some pressure. One nozzle 111A is arranged preferably in parallel to the main axis Aa of the printing head - for that reason, it will be called shortly a “parallel axis nozzle”. The other nozzle 11 IB is arranged at an angle a to the first nozzle 111A - for that reason, it will be called shortly an “inclined axis nozzle”. Therefore, the first nozzle 111A is configured to eject the first primary drop 121A to move along a first path and the second nozzle 11 IB is configured to eject the second primary drop 121B to move along a second path. The nozzle outlets 113 A, 113B are distanced from each other by a distance equal to at least the size of the larger of the primary drops generated at the outlets 113 A, 113B, so that the primary drops 121A, 121B do not touch each other when they are still at the nozzle outlets 113A, 113B. This prevents forming of a combined drop at the nozzle outlets 113 A, 113B and subsequent clogging the outlets 113A, 113B with a solidified ink. Preferably, the angle a is a narrow angle, preferably from 3 to 60 degrees, and more preferably from 5 to 25 degrees (which aids in alignment the two drops before coalescence). In such a case, the outlet 113 A of the parallel axis nozzle 111A is distanced from the outlet of the printing head by a distance larger by “x” than the outlet 113B of the inclined axis nozzle 11 IB. The path of flight of the second drop 12IB crosses with the path of flight of the first drop 121A at a connection point 132.

The liquid produced by combination of drops from the two reservoirs 116 A, 116B is a product of a chemical reaction of a first liquid supplied from a first reservoir 116A and a second liquid supplied from the second reservoir 116B (preferably a reactive ink composed of an ink base and a catalyst for initiating curing of the ink base). The ink base may be composed of polymerizable monomers or polymer resins with rheology modifiers and colorant. The catalyst (which may be also called a curing agent) may be a cross-linking reagent in the case of polymer resins or polymerization catalyst in the case of polymerizable resins. The nature of the ink base and the curing agent is such that immediately after mixing at the connection point 132 a chemical reaction starts to occur leading to solidification of the mixture on the printed material surface, 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 50 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.

The liquids supplied by the two reservoirs 116A, 116B can be various substances, selected such that immediately after mixing a chemical reaction leading to transformation of the first and second liquid to a reaction product starts to occur. Thus chemical reaction transforming the first and second liquid into a reaction product is initiated within the reaction chamber within the printing head. Therefore, a chemical reaction is initiated before the combined drop leaves the printing head enclosure and reaches the printed material surface.

Typically, the ink drop will be larger than the catalyst drop.

The control of the path of flight of the primary drops 121A, 121B is controlled by setting at least one of:

- a particular speed of the primary drops (to provide adequate kinetic energy for the drops) ejected from the nozzle outlets;

- the size of the primary drops;

- the position of the nozzle outlets.

The parameters of the primary drops are selected such that the kinetic energy of the drop ejected from the parallel axis nozzle is higher, preferably much higher (for example, at least 2 times, or at least 4 times, or at least 8 times, or at least 10 times, or at least 20 times, or at least 50 times, or at least 100 times) than the kinetic energy of the drop ejected from the inclined axis nozzle, at the connection point. Therefore, when the primary drops collide at the connection point, the combined drop travels along a path pC that is aligned substantially by the path pA of the primary drop. Preferably, the path pC of the combined drop 122 is altered no more than 20 degrees, preferably no more than 10 degrees, preferably no more than 5 degrees, from the axis of the path of flight pA of the first primary drop 121 A.

Preferably, the drops have different sizes, wherein the larger drop 121A is ejected from the parallel axis nozzle 111A, and the smaller drop 121B is ejected from the inclined axis nozzle 11 IB. For example, the larger drop 121A may be at least 2 times, or at least 4 times, or at least 8 times, or at least 10 times larger than the smaller drop 121B.

Preferably, the drops have different speeds, wherein the primary drop 121A is ejected from the parallel axis nozzle 111A with a higher speed than the primary drop 121B ejected from the inclined axis nozzle 11 IB. For example, the primary drop 121A may be ejected with a speed at least 2 times, or at least 4 times, or at least 8 times, or at least 10 times higher than the primary drop 121B. The speed of ejection of the second primary drop 121B can be set to a minimum speed allowable by the particular nozzle, for example 2 m/s. The speed of ejection of the first primary drop 121A can be set to a maximum speed allowable by the particular nozzle, for example 6 m/s or even higher.

For example, if the first primary drop 121A is four times larger than the second primary drop 12IB and is ejected with a speed 3 times higher, it will have about 36 times higher kinetic energy. Thus the path of flight pC of the combined drop towards the printed surface would not be substantially altered from the path of flight pA of the first primary drop.

Thanks to this feature slight changes in the way the first primary drop and the second primary drop would collide with each other at the connection point would not substantially change the path of flight of the combined drop, which would remain consistently repeatable, providing the high accuracy drop placement of the printed surface.

The position of the nozzle outlets may be regulated in order to fine-tune the position of the connection point, so that the drops collide in a manner such that the path of flight of the combined drop is most closely aligned to the path of flight of the parallel axis primary drop 121A. For example, this can be achieved by a guide 141 along which one of the nozzles 121B may slide, wherein the guide 141 is movable by an actuator 142 (or the nozzle is movable).

The primary drops are preferably combined within the head 100, i.e. before the drops leave the outlet 185 of the head..

The process of generation of primary drops 121A, 121B is controlled by a controller of the drop generating and propelling devices 161A, 161B (not shown in the drawing for clarity), which generates trigger signals and controls the time of ejection of the drops. The primary drops are therefore generated on demand, in contrast to CIJ technology where a continuous stream of drops is generated at nozzle outlets. Each of the generated primary drops is then directed to the surface to be printed, in contrast to CIJ technology where only a portion of the drops is output and the other drops are fed back to a gutter.

In yet another embodiment, more than two primary drops may be generated, i.e. the combined drop 122 may be formed by coalescence (simultaneous or sequential) of more than two drops, e.g. three drops ejected from three nozzles, of which at least two have their axes inclined with respect to the desired axis of flow Ac of the combined drop 122.

The axis of flow Ac of the combined drop 122 is preferably the main axis of the printing head, but it can be another axis as well. The printing head may comprise additional means for improving drop placement control.

Furthermore, the printing head may comprise means for speeding up the curing of the combined drop 122 before it leaves the printing head, e.g. a UV light source (not shown in the drawing) for affecting a UV-sensitive curing agent in the combined drop 122.

The liquids in the reservoirs 116A, 116B may be preheated or the nozzle outlets can be heated by heaters installed at the nozzle outlets, such that the ejected primary drops have an increased temperature. The increased temperature of working fluids (i.e. ink and catalyst) may 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 having a temperature lower than the temperature of the combined drop. The temperature of the ejected primary drops should therefore be higher than the temperature of the surface to be printed, wherein the temperature difference should be adjusted to particular working fluid properties. The rapid cooling of the coalesced drop after placement on the printing surface (having a temperature lower than the ink) increases the viscosity of the drop preventing drop flow due to gravitation.

The printing head further comprises a cover 181 which protects the head components, in particular the nozzle outlets 113A, 113B and the area around the connection point 132, from the environment, for example prevents them from touching by the user or the printed substrate. The cover 181 forms the reaction chamber. Because the connection point 132 is within the reaction chamber, the process of combining primary drops can be precisely and predictably controlled, as the process occurs in an environment separated from the surrounding of the printing head. The environment within the printing head is controllable and the environment conditions (such as the air flow paths, pressure, temperature) are known and therefore the coalescence process can occur in a predictable manner.

Moreover, the cover 181 may comprise heating elements (not shown in the drawing) for heating the volume within the cover 181, i.e. the volume surrounding of the nozzle outlets

113A, 113B and liquid reservoirs 116A, 166B to a predetermined temperature elevated in respect to the ambient temperature, for example from 40°C to 80°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 may be positioned within the cover 181 to sense the temperature. The higher temperature within the printing head facilitates better mixing of coalesced drop by means of diffusion. Additionally, the increased temperature increases the speed of chemical reaction starting at the moment of mixing. Ink reacting on the surface of printed material allows for better adhesion of the printed image.

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.

The environment in the reaction chamber may be controlled by controlling at least one of the following parameters: chamber temperature (e.g. by means of a heater within the reaction chamber), velocity of the streams of gas (e.g. by controlling the pressure of gas delivered), gas components (e.g. by controlling the composition of gas delivered from various sources), electric field (e.g. by controlling the electrodes), ultrasound field (e.g. by providing additional ultrasound generators within the reaction chamber, not shown in the drawings), UV light (e.g. by providing additional UV light generators within the reaction chamber, not shown in the drawings), etc.

A skilled person will realize that the features of the embodiments described above can be further mixed with features known from other DOD printing heads. For example there can be 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 print head as described above.

The primary drops may be ejected at different angles than that shown in Fig. 2, for example both paths of flight pA, pB may be non-parallel with respect to the main axis of the printing head.

Claims (11)

1. A drop-on-demand printing method comprising performing the following steps in a printing head:

5 - discharging a first primary drop of a first liquid from a first nozzle outlet to move along a first path (pA) with a first speed;

discharging a second primary drop of a second liquid from a second nozzle outlet to move along a second path (pB) with a second speed, lower than the first speed, wherein the second path (pB) is inclined with respect to the first path (pB) along an axis inclined at an

10 angle (a) from 3 to 60 degrees and crosses the first path (pA) at a connection point;

controlling the flight of the first primary drop and the second primary drop to combine the first primary drop with the second primary drop into a combined drop at the connection point so that a chemical reaction is initiated between the first liquid of the first primary drop and the second liquid of the second primary drop;

15 - wherein the first primary drop has at the connection point a kinetic energy higher than the second primary drop; and

- wherein the path of flight (pC) of the combined drop is altered no more than 20 degrees from the axis of the path of flight (pA) of the first primary drop.

20

2. The method according to any of previous claims, comprising discharging the first primary drop of a size larger than the second primary drop.

3. The method according to any of previous claims, comprising controlling the timing of discharge of the primary drops.

4. The method according to any of previous claims, comprising controlling the relative position of the nozzle outlets.

5. The method according to any of previous claim, wherein the connection point is

30 located within a reaction chamber defined by a cover.

6. The method according to claim 6, further comprising controlling at least one of the following parameters within the reaction chamber: chamber temperature, electric field, ultrasound field, UV light.

7. A drop-on-demand printing head comprising: a nozzle assembly comprising:

a first nozzle connected through a first channel with a first liquid reservoir with a first

5 liquid and having a first drop generating and propelling device for forming on demand a first primary drop of the first liquid and discharging the first primary drop to move along a first path (pA) with a first speed; and a second nozzle connected through a second channel with a second liquid reservoir with a second liquid and having a second drop generating and propelling device for

10 forming on demand a second primary drop of the second liquid and discharging the second primary drop to move along a second path (pB) with a second speed, lower than the first speed, wherein the second path (pB) is inclined with respect to the first path (pB) along an axis inclined at an angle (a) from 3 to 60 degrees and crosses the first path (pA) at a connection point;

15 - means for controlling the flight of the first primary drop and the second primary drop to combine the first primary drop with the second primary drop into a combined drop at the connection point so that a chemical reaction is initiated between the first liquid of the first primary drop and the second liquid of the second primary drop;

- wherein the first primary drop has at the connection point a kinetic energy higher than the

20 second primary drop; and

- wherein the path of flight (pC) of the combined drop is altered no more than 20 degrees from the axis of the path of flight (pA) of the first primary drop.

8. The printing head according to claim 8, wherein the first primary drop has a size larger

25 than the second primary drop.

9. The printing head according to any of claims 8-

10, further comprising a controller for controlling the timing of discharge of the primary drops.

30 10. The printing head according to any of claims 8-11, further comprising means for controlling the relative position of the nozzle outlets.

11. The printing head according to any of claims 8-12, wherein the connection point is located within a reaction chamber defined by a cover.

Intellectual

Property

Office

Application No: Claims searched:

GB1616473.3

1-11