FR2754471A1 - METHOD AND DEVICE FOR TRANSMITTING LIQUID IN A CONTROLLED WAY, APPLICATION TO PRINTING - Google Patents

Abstract

The invention concerns a method and device for controlled discharging of liquid and its application in printing. It consists in rotating a cylinder (4), impregnating with a liquid at least one external lateral surface (6) of this cylinder and applying, at least in one point (A) of this surface and at predetermined times, a force which co-operates with the centrifugal force resulting from the rotation for causing a discharge of a portion of the liquid (12) in a manner substantially tangential to the surface, so as to splash this portion of liquid directly onto the object.

Description

LIQUID EMISSION METHOD AND DEVICE
CONTROLLED, APPLICATION TO PRINTING
DESCRIPTION
TECHNICAL AREA
The present invention relates to a method and a device for the emission of a liquid in a controlled manner.

 It finds applications in particular in the field of printing.

 The invention is in particular usable for producing ink jet print heads.

PRIOR STATE OF THE ART
In industrial applications of marking objects with an ink jet, a high initial speed is sought for ink drops.

 In fact, unlike applications in the field of office automation, an object to be marked is located at a great distance from the print head, this distance possibly ranging from one to several centimeters.

 A high initial speed is therefore necessary to limit the effect of aerodynamic disturbances on the drops.

 In the method known as the deflected continuous jet method, this speed is of the order of 20 m / s.

 It is obtained by forming an ink jet through a nozzle of small diameter, under a pressure of the order of 3 bars (approximately 3 × 10 5 Pa).

 This jet is fragmented into drops of ink using a piezoelectric crystal.

 Some drops are then electrically charged and deflected in a permanent electric field.

 The uncharged drops are not deflected and are recovered by a gutter before being recycled inside the printer where this process is carried out.

 The individual charge of a drop determines the amplitude of its deflection and therefore its position on the object to be marked in the direction of the electric field.

 It is thus possible to print several positions with a single nozzle.

 The displacement of an object perpendicular to this electric field in synchronization with the ejection of the drops makes it possible to form an image on this object.

 In the method known as the binary continuous jet method, the formation, the charge and the deflection of the ink drops are done in the same way as in the deflected continuous jet method, except that the amplitude of the deflection is identical for each drop.

 A row of regularly spaced nozzles is used, each corresponding to a position to be printed.

 Moving an object perpendicular to this row of nozzles in synchronization with the ejection of the drops makes it possible to form an image on this object.

In all the variants of the deflected or binary continuous jet process, the nozzle is a critical element due to - its reduced dimensions, which make its
difficult realization - of its influence on the efficiency of the fragmentation
jet and - its propensity to become blocked under the effect of
particles contained in ink or evaporation
of some of its constituents.

 In addition, the pressures used and the need to recover and recycle the deflected drops increase the complexity of the material and affect its reliability.

 We can understand the advantage of a process where the initial speed of the drops is obtained without using a nozzle or hydrostatic pressure.

 Such a method is used to disperse liquids into fine droplets, so as to apply paints or agricultural additives uniformly.

 In known devices, making it possible to implement such a method, a rotating cylinder or disc is impregnated with liquid and generates a cloud of small drops over its entire periphery.

 The formation of these drops is explained by the appearance of a standing wave in the liquid located at the periphery of the cylinder.

 Under the action of centrifugal force, the peaks of the waves that form the liquid then detach from the cylinder and are projected outwards.

 The cylinder can also be electrified to charge the drops of liquid, which makes it possible to obtain a better distribution of the drops on the objects to be covered and possibly to control the number or the size of the drops projected.

 Such devices have also been used for printing objects.

 There is then interposed, between the cylinder and an object to be printed, a screen which lets the ink drops pass only at predefined locations.

The following documents will be consulted on this subject.
FR-A-2 483 326 (BERTIN ET CIE.)
FR-A-2 212 782 (IBM).

 In these known devices, a magnetic or electric field is used to deflect the drops of ink when it is desired to print a character.

 It should however be noted that the drops are emitted permanently under the effect of centrifugal force, by the rotating cylinder.

 The only function of the electric or magnetic field is the deflection of the ink drops.

 As a result, the shape of the characters that can be printed is limited.

 Thus, these known devices have the disadvantage of requiring the renewal of the screen each time it is desired to print a different character, which is incompatible with most current industrial marking applications.

 Another known example of a device where a rotating cylinder is used to generate drops is described in international application WO 96/14212 from the company Jemtex.

 In the latter device, the cylinder rotates inside a tube and has cavities arranged longitudinally to generate periodic pressure waves passing in front of orifices which are formed in the tube and through which ink flows. .

 This ink is channeled to form regularly spaced jets above an object to be printed.

 The pressure variations cause the jets to break into drops and, insofar as the pressure waves are synchronous in the different jets, the instant of separation of the drops is substantially identical for all the jets.

 The drops are then loaded and then deflected by means similar to those used in the continuous jet process (deflected or binary).

 It is remarkable that this device does not have a nozzle and that the cylinder has cavities.

 However, it will be noted that the centrifugal force plays no significant role and that the drops are emitted continuously, regardless of the need to print at a given time.

 Drops not used for printing must therefore be recovered, as in the case of the continuous jet process (deflected or binary).

A similar device is described in international application WO 96/05060 from the company
Komatsu.

 In the latter device, the drops are also emitted continuously but, in this case, under the effect of centrifugal force.

 These drops are electrically charged and then deflected.

 The unwanted drops for printing are recovered.

Other devices are known from the following documents
JP-A-56008289 (NTT)
JP-A-63288757 (FUJI XEROX CY. LTD.).

 These latter devices use electrostatic forces to extract ink drops from a cylinder pierced with nozzles, but the tangential speed of the cylinder plays a negligible role in the initial speed of the drops.

 Therefore, this initial speed of the drops is very low.

 And such devices do not allow objects to be printed at sufficiently large distances, for industrial applications.

STATEMENT OF THE INVENTION
The object of the present invention is to remedy the above drawbacks.

 More generally, the present invention aims to send a liquid in a controlled manner towards an object on which one wishes to deposit this liquid, while allowing this object to be placed at a distance greater than those authorized by the known techniques, mentioned more high.

 Specifically, the present invention relates to a method of emitting liquid to an object, this method being characterized in that one rotates a cylinder, in that one impregnates with a liquid at least the surface lateral lateral of this cylinder, and in that one applies, at least at one point of this external lateral surface and at predetermined times, a force which cooperates with the centrifugal force resulting from the rotation of the cylinder to cause a substantially tangential emission of a portion of liquid to the object at these predetermined times.

 Of course, the liquid is supposed to be sensitive to the force applied.

The present invention also relates to a device for emitting liquid to an object, this device being characterized in that it comprises - a cylinder, - means for rotating this cylinder, - means for impregnation provided to soak up with
liquid at least the outer side surface of this
cylinder, and - means of application, to at least one point of
this outer lateral surface and at times
predetermined, of a force capable of cooperating with the
centrifugal force resulting from the rotation of the cylinder
to cause a substantially tangential emission
a portion of liquid to the object at these times
predetermined.

 In a particular application of the invention, this device is intended for printing the object, said liquid is an ink and said force is able to cooperate with said centrifugal force to cause a substantially tangential emission of a drop of ink towards the object at said predetermined times.

 Such a device is in particular usable for producing ink jet printheads.

 According to a particular embodiment of this device, the external lateral surface of the cylinder comprises cavities capable on the one hand of promoting the accumulation of ink in these cavities to initiate the formation of the drops and on the other hand of controlling the size of these drops.

 The part of the external lateral surface which is between the cavities with respect to the ink has a lower wettability than that of the bottom of these cavities in order to further promote the accumulation of ink in these cavities and to better control the size of the drops.

 The cavities can form circular grooves whose axis is the axis of the cylinder.

 According to a preferred embodiment of the device which is the subject of the invention, said force is an electrostatic force and the ink has a non-zero electrical conductivity.

 In this case, the device can also comprise means for applying another electrostatic force capable of preventing the emission of the ink drop when no printing is desired.

 In this case also, according to a first particular embodiment, the external lateral surface of the cylinder comprises a first electrically conductive layer and a second layer which covers the first layer and has, with respect to the ink, a lower wettability. to that of this first layer, cavities being formed through this second layer, the first layer forming the bottom of these cavities, the means for applying said electrostatic force comprising at least one electrode intended to be brought to a different electrical potential of the one to which the first layer is worn.

 According to a second particular embodiment, the external lateral surface of the cylinder comprises a first electrically conductive layer, a second electrically insulating layer which covers the first layer and a third electrically insulating layer which covers the second layer and has, opposite ink, a wettability lower than that of this second layer, cavities being formed through this third layer, the second layer forming the bottom of these cavities, the means for applying said electrostatic force comprising at least one electrode intended to be brought to an electrical potential different from that to which the first layer is worn, the device further comprising means capable of electrically charging the drops.

 The device may further comprise an ink tank through which the outer lateral surface of the rotating cylinder passes, and means for removing excess ink from this surface.

 As a variant, the cylinder is hollow and contains ink as well as a porous layer which surrounds the ink contained in the cylinder and through which the ink diffuses towards the outside of the cylinder.

 The hollow cylinder may contain a removable ink cartridge, the porous layer of which constitutes an outer envelope.

 The ink may be hot-melt and the device may further comprise means for heating this hot-melt ink.

 According to a particular embodiment, the means for applying said force are provided for applying this force to a plurality of points arranged along a generatrix of the cylinder, in order to allow the simultaneous tangential emission of a plurality of drops towards the object.

 The means for applying said force may also be capable of varying the time of application and the amplitude of this force so as to vary the quantity of ink contained in each drop emitted and / or the number of drops. emitted and therefore the intensity of the tint obtained on the object.

 In the latter case, it is possible to use 3 or 4 devices in accordance with the invention, the variations in intensity of the tint obtained from each device allowing the printing of three-color or four-color images by superimposition, on said object, of three or four images respectively formed by the devices, each image having one of the basic colors.

BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the appended drawings in which
Figure 1 is a schematic view of a mode of
particular realization of the device object of
the invention, intended for ink projection,
Figure 2 schematically illustrates a cylinder
usable in the invention and provided with cavities
superficial where ink accumulates,
Figure 3 schematically illustrates another
cylinder usable in the invention and provided with
surface cavities forming grooves
circulars admitting the cylinder axis as an axis
common,
Figure 4 is a cross-sectional view
schematic and partial of another cylinder
usable in the invention and provided with cavities
superficial,
Figure 5 is a cross-sectional view
schematic and partial of a variant of
realization of the cylinder of FIG. 4,
Figure 6 is a schematic view of another
particular embodiment of the device
subject of the invention,
Figure 7 schematically illustrates a
device according to the invention,
Figure 8 is a cross-sectional view
schematic and partial of a usable cylinder
in the invention, and
Figure 9 schematically illustrates a
printer with devices conforming to
the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS
In the devices according to the invention, which are shown in these figures, the ink is rotated by a cylinder of suitable diameter, the outer lateral surface of which is impregnated with ink.

 This ink is in equilibrium under the effect of the centrifugal force resulting from the rotation of the cylinder around its axis and from the capillary forces.

 Many parameters are involved in this balance, in particular the surface area of the cylinder, the viscosity of the ink, their relative surface tensions and of course the speed of rotation.

 Drops are emitted from the surface of the cylinder by the application, at predetermined times and at one or more points, of a force cooperating with the centrifugal force.

 One will thus detach one or more drops of ink whose initial speed is substantially equal to the tangential speed of the cylinder.

 We therefore make sure to emit drops only at predetermined places and times, so as to print any desired character or image element.

 Likewise, the size of the drops is controlled by limiting, by various means, the quantity of ink separated from the cylinder.

 As will be seen in more detail below, an electric field is created locally and a drop of ink is detached by electrostatic attraction, the ink used having a non-zero electrical conductivity.

 Conversely, a uniform electric field in the opposite direction can be used to oppose the centrifugal force in places where no ink emission is desired.

 This uniform opposite field can be adjusted to variations in the parameters involved in the balance of the ink surface.

 Once detached, the drops are driven at an initial speed substantially equal to the tangential speed of the cylinder towards the object to be printed, which moves perpendicular to the axis of the cylinder, in synchronization with the emission of the drops.

 By juxtaposing, along a generator of the cylinder, electrodes making it possible to locally vary the electric field, it is possible to simultaneously emit several drops of ink to form a portion of the image to be printed.

 The size of the drops is mainly determined by a spatial factor and by a temporal factor.

 The spatial factor notably involves the geometry of the external lateral surface of the cylinder.

 This surface can be provided with cavities to facilitate the accumulation of ink in certain places, in order to initiate the formation of the drops and to limit their volume.

 We can also play on local differences in wettability of this surface.

 The temporal factor involves the form and duration of application of the electric extraction field.

 This second factor conditions the kinetics of a drop formation until it separates from the cylinder and the first moments of its path in the air.

 By playing on this second factor, it is possible to emit more or less large drops and / or to emit more or less drops, which makes it possible to obtain more or less strong shades, for example for printing four-color images.

 In addition, deliberate delays can be introduced in the control of the electrodes to compensate for possible geometric defects of the device.

 One of the advantages of the invention is that the entire height of the cylinder (counted parallel to the axis thereof) can be used to generate drops.

 The printing width is only limited by the technical possibilities of producing the cylinder and the electrodes.

 It is thus possible to reach printing widths of the order of a few tens of centimeters or even more.

 The resolution of the device depends on the minimum space necessary on the one hand to form the drops on the surface of the cylinder and on the other hand to juxtapose the electrodes.

 It is therefore understandable that the invention has several intrinsic advantages: simplicity of production, writing over long distances, variation in size or number of drops, large printing width.

 The dynamic and electrical phenomena used make it possible to envisage the use of a wide range of inks, in particular the use of hot-melt inks.

 Figure 1 is a schematic cross-sectional view of a device according to the invention.

 In the example of FIG. 1, this device is intended to project drops of ink onto a support to be printed 2.

 The device of FIG. 1 comprises a cylinder 4 whose external lateral surface 6 is impregnated with ink by means not shown.

 This cylinder is driven in a rotational movement along the arrow F, around the axis X of this cylinder 4, by means not shown.

The print medium 2, which is for example a strip of paper, moves in translation, by means not shown, along the arrow
F1, in a plane parallel to the X axis of cylinder 4.

 The device of FIG. 1 also comprises means intended to apply an electrostatic force at a point A of the surface 6 impregnated with ink.

 These means for applying the electrostatic force comprise at least one electrode 8 which is in a plane containing the axis X of the cylinder and parallel to the plane of the support to be printed 2, as seen in FIG. 1.

 The means for applying the electrostatic force also comprise means for controlling each electrode 8 referenced 10 which are capable of bringing this electrode 8 to an appropriate electrical potential which cooperates with the centrifugal force resulting from the rotation of the cylinder to extract a drop of ink from surface 6 when the ink arrives at point A located opposite electrode 8.

 A drop of ink 12 is then projected towards the support 2 along a trajectory T substantially tangent to the surface 6 of the cylinder whose tangential speed (equal to the product of the angular speed of the cylinder by the radius thereof) is sufficiently large for this purpose.

 The tangential speed of the cylinder is determined so that the radial speed initially communicated to the drop 12 is much lower than this tangential speed.

 It is thus possible to print the desired patterns 14 on the support 2 by appropriately controlling each electrode 8.

 FIG. 2 is a schematic top view of the cylinder 4 of FIG. 1 showing that this cylinder comprises surface cavities 16 intended to promote the formation of the drops of ink which it is desired to project onto the support 2.

 We will return in more detail to these cavities 16 in the description of FIG. 4.

 Figure 3 schematically illustrates a top view of another cylinder 18 according to the invention, provided with surface cavities 20 further intended to promote the formation of ink drops.

 In the case of FIG. 3, these cavities 20 are circular surface grooves which all have the same axis, this axis being the axis X1 of the cylinder 8.

 One of the advantages of the cylinder of FIG. 3 is that the unused ink can be continuously refreshed during its passage through a reservoir (not shown in FIG. 3).

 Figure 4 is a schematic and partial cross-sectional view of the cylinder 4 of Figure 1 in the vicinity of the ink drop emission area.

 As seen above, this zone is located opposite an electrode 8.

 In the example of FIG. 4, the cylinder comprises a first layer 22 and a second layer 24 which covers the layer 22.

 Layer 22 is electrically conductive.

 The layer 24, which covers the layer 22, can be electrically conductive but it is preferably electrically insulating.

 In addition, the wettability of the layer 22 with respect to the ink is greater than the wettability of the layer 24.

 This layer 24 is perforated to define the cavities 16 which are mentioned in the description of FIG. 2 and the bottom of which is then constituted by the layer 22.

 It can be seen that portions of ink 26 are thus kept in contact with the layer 22 in the cavities 16 provided in the layer 24.

 The conductive layer 22 is maintained at an electrical potential VO by suitable means, not shown.

 Each electrode 8 is, in turn, brought to an electrical potential VE by means of the control means 10 when it is desired to project a drop of ink with this electrode 8.

 The potential VE is chosen different from VO.

 Arriving under the electrode 8, a drop 26 is extracted from its cavity by the electric field resulting from the potential difference VE - VO which cooperates with the centrifugal force of the cylinder.

 The drop is then projected substantially tangentially to the surface of the cylinder whose tangential speed is determined for this purpose.

 We see in Figure 4 the drop 28 thus projected (towards the support not shown in Figure 4).

 FIG. 5 schematically illustrates an alternative embodiment of the cylinder shown in FIG. 4.

 In the case of FIG. 5, the cylinder does not have two surface layers as in the case of FIG. 4 but three surface layers respectively referenced 30, 32 and 34.

 Layer 30 is the innermost of the three and this layer 30 is electrically conductive.

 Layer 32 covers this layer 30 and it is made of an electrically insulating material.

 The layer 34 itself covers this layer 32 and it is made of an electrically insulating material whose wettability with respect to the ink is lower than that of the layer 32.

 In the case of FIG. 5, the cavities 16 are formed through this layer 34 and the bottom of these cavities is also constituted by the underlying layer 32.

 The layer 30 is brought to the potential VO by suitable means, not shown.

 The ink drops 26 are still formed in the cavities 16.

 Electric charges are deposited in these ink drops 26 by suitable means not shown in FIG. 5.

 For this purpose, one can for example use a scraper which will be discussed later.

 In this case, an electrically conductive scraper is used.

 This scraper is then brought to a potential of the same polarity as VO.

 The drops 26 thus charged are still extracted when they pass under the electrode 8 brought to the potential VE different from VO and ejected substantially tangentially in the direction of the support (not shown in FIG. 5).

 The choice of the form and the duration of application of the electric field generated by the electrode 8 makes it possible to control the separation of the drops and to control the size and / or the number thereof.

 FIG. 6 is a schematic view of another device according to the invention comprising the cylinder 4 of FIG. 2.

 We also see in Figure 6 the support 2 that we want to print with the device and which moves along the arrow F1 while the cylinder 4 rotates about its axis X along the arrow F.

 In the case of this FIG. 6, the means for applying the electrostatic force comprise a row of electrodes 8 arranged opposite the cylinder 4 parallel to the axis X thereof.

 The device of FIG. 6 also comprises means 10 for controlling this row of electrodes 8 making it possible to subject these electrodes, independently of one another, to electric potentials allowing, in cooperation with the centrifugal force of the rotating cylinder, the 'emission of a set of drops following trajectories T1 substantially tangential to the surface of the cylinder 4 and parallel to each other.

 It is specified that the electrodes 8 are electrically isolated from each other by dielectric elements 40.

 Figure 7 is a schematic cross-sectional view of a device according to the invention.

 This device of FIG. 7 also comprises a cylinder of the kind of that of FIG. 1 referenced 4.

 The device in FIG. 7 also includes a motor 42 which rotates a first pinion 44.

 A second pinion 46, driven in rotation by the pinion 44 by means of a belt 48, itself drives the cylinder 4 in rotation about the axis X thereof.

 This device also comprises a frame 50 in which there is a cylindrical space where the cylinder 4 is housed.

 The bottom of this space constitutes an ink reservoir 52 through which the rotating cylinder 4 passes, which thus becomes charged with ink on the surface.

 A scraper 54 is also provided above this ink tank so as to remove the excess ink on the surface of the cylinder 4.

 In the case where it is desired to charge the ink drops, an electrically conductive scraper 54 is used, brought to a potential of the same polarity as VO by appropriate means 56.

 Also seen in Figure 7 one of the electrodes 8 of the row of electrodes mentioned above which is parallel to the axis X and extends substantially over the entire height of the cylinder (counted parallel to the axis X) .

 As seen in Figure 7, between the scraper 54 which also extends substantially along the entire height of the cylinder parallel to the axis X and this row of electrodes 8 is an electrode 60 forming a portion of cylinder axis X spaced from the cylinder 4 and facing it and electrically isolated from the row of electrodes 8 and the scraper 54 by dielectric elements 62.

 This electrode 60 is brought to a potential VR by appropriate means 64.

 The potential VR of the electrode 60 is chosen so as to generate an electric field which opposes the centrifugal force.

 this electrode 60 can be used as well in the case where

 This cylinder 4 of FIG. 8 is hollow and provided, on one of its sides, with a removable cover 66 allowing the introduction into this cylinder 4 of an ink cartridge 68 also removable so that it can be replaced.

 The outer envelope of this ink cartridge 68 is constituted by a porous layer 70 allowing the diffusion of the ink 71 towards the outside of the cylinder.

 As seen in Figure 8, the side wall of the cylinder 4 is pierced with a multitude of holes 72 allowing the passage of ink.

 The embodiment of FIG. 8 is particularly advantageous when the object to be printed moves horizontally, the axis of the cylinder 4 then being vertical.

 In this case, when the cylinder is rotated around the X axis, the surface of the ink contained in the cartridge takes substantially the form of a portion of paraboloid of revolution around the X axis. .

 In the invention, a hot-melt ink can be used.

 Returning to FIG. 7, another ink tank 74 can be provided, the bottom of which communicates by a pipe 76 with the bottom of the tank 52.

 This pipe makes it possible to bring the ink into the reservoir 52 from the reservoir 74.

 When the ink is hot-melt, heating means 78 are provided for the ink contained in the reservoirs 74 and 52 and in the pipeline 76.

 FIG. 9 schematically and partially illustrates a printer using devices according to the invention, in which the application time and the amplitude of the electrostatic force generated by each electrode 8 are varied, so as to vary either the quantity of ink contained in each drop emitted or the number of drops emitted or both and therefore the intensity of the shade obtained on the support to be printed 2 from each device.

 Three devices 80, 82 and 84 are used which are arranged one after the other and which allow the printing of three-color images on the support 2 which passes in front of these devices, by the superimposition, on this support 2, of three images respectively formed by these devices 80, 82 and 84.

 These images have the basic colors yellow, cyan and magenta respectively (the inks of the devices being chosen for this purpose).

 One can also add an additional device 86 capable of forming images of black colors.

 This allows the printing of four-color images by the superimposition, on the support 2, of four images respectively formed by the devices 80, 82, 84 and 86, these images having the basic colors yellow, cyan, magenta and black respectively.

 The present invention is not limited to the use of a force of the electrostatic type.

 It is also possible to use, in place of the electrodes 8 associated with the control means 10, means capable of generating a force of magnetic type to extract the drops of ink, this ink then being sensitive to the magnetic force.

 In addition, the invention can be implemented with liquids other than inks.

Claims (17)

 1. Method for emitting liquid to an object (2), this method being characterized in that a cylinder (4, 18) is rotated, in that at least the lateral surface is impregnated with a liquid outside (6) of this cylinder, and in that one applies, at least at a point (A) of this external lateral surface and at predetermined times, a force which cooperates with the centrifugal force resulting from the rotation of the cylinder to causing a substantially tangential emission of a portion of liquid (12, 28) towards the object at these predetermined times.  2. Device for emitting liquid to an object (2), this device being characterized in that it comprises - a cylinder (4, 18), - means (42, 44, 46, 48) for rotating of this  cylinder, - impregnation means (52) provided for impregnating  of a liquid at least the external lateral surface  of this cylinder, and - means (8, 10) for applying, to at least one point  (A) of this outer lateral surface and at  predetermined moments, of a force capable of cooperating  with the centrifugal force resulting from the rotation of the  cylinder to cause emission substantially  tangential of a portion of liquid (12, 28) towards  the object at these predetermined times.  3. Device according to claim 2, intended for printing the object (2), wherein said liquid is an ink and said force is able to cooperate with said centrifugal force to cause a substantially tangential emission of a drop of ink ( 12, 28) towards the object at said predetermined times.  4. Device according to claim 3, in which the external lateral surface of the cylinder comprises cavities (16, 20) able on the one hand to promote the accumulation of ink in these cavities to initiate the formation of drops and on the other hand to control the size of these drops.  5. Device according to claim 4, in which the part of the external lateral surface which is between the cavities with respect to the ink has a lower wettability than that of the bottom of these cavities in order to further promote accumulation. ink in these cavities and better control the size of the drops.  6. Device according to any one of claims 4 and 5, wherein the cavities form circular grooves (20) whose axis is the axis (X1) of the cylinder.  7. Device according to any one of claims 3 to 6, wherein said force is an electrostatic force and the ink has a non-zero electrical conductivity.  8. Device according to claim 7, further comprising means (60, 64) for applying another electrostatic force capable of preventing the emission of the ink drop when no printing is desired.  9. Device according to any one of claims 7 and 8, wherein the outer lateral surface of the cylinder comprises a first electrically conductive layer (22) and a second layer (24) which covers the first layer and has, vis-à-vis screw ink, wettability lower than that of this first layer, cavities (16) being formed through this second layer, the first layer forming the bottom of these cavities, the means for applying said electrostatic force comprising at at least one electrode (8) intended to be brought to an electrical potential different from that to which the first layer is worn.  10. Device according to any one of claims 7 and 8, wherein the outer lateral surface of the cylinder comprises a first electrically conductive layer (30), a second electrically insulating layer (32) which covers the first layer (34) and a third electrically insulating layer which covers the second layer and has, with respect to the ink, a lower wettability than that of this second layer, cavities (16) being formed through this third layer, the second layer forming the bottom of these cavities, the means for applying said electrostatic force comprising at least one electrode (8) intended to be brought to an electrical potential different from that to which the first layer is carried, the device further comprising means (54, 56) suitable for electrically charging drops.  11. Device according to any one of claims 3 to 10, further comprising an ink tank (52), through which the outer lateral surface of the rotating cylinder passes, and means (54) for eliminating the surplus of ink from this surface.  12. Device according to any one of claims 3 to 10, in which the cylinder (4) is hollow and contains ink as well as a porous layer (70) which surrounds the ink (71) contained in the cylinder and through which the ink diffuses towards the outside of the cylinder.  13. Device according to claim 12, in which the hollow cylinder (4) contains a removable ink cartridge (68) whose porous layer (70) constitutes an outer envelope.  14. Device according to any one of claims 3 to 13, in which the ink is hot-melt, the device further comprising means (78) for heating this hot-melt ink.  15. Device according to any one of claims 3 to 14, in which the means (8, 10) for applying said force are provided for applying this force to a plurality of points arranged along a generatrix of the cylinder, in order to allow the simultaneous tangential emission of a plurality of drops towards the object (2).  16. Device according to any one of claims 3 to 15, in which the means (8, 10) for applying said force are further capable of varying the application time and the amplitude of this force so as to make vary the quantity of ink contained in each drop emitted and / or the number of drops emitted and therefore the intensity of the tint obtained on the object (2).  17. Printer comprising a number N of devices according to claim 16, N being equal to 3 or 4, the variations in intensity of the shade obtained from each device (80, 82, 84, 86) allowing the printing of 'three-color or four-color images by the superimposition, on said object (2), of three or four images respectively formed by the devices, each image having one of the basic colors.