WO2022128941A1

WO2022128941A1 - Thermal transfer printing apparatus with coating module comprising an endless band

Info

Publication number: WO2022128941A1
Application number: PCT/EP2021/085537
Authority: WO
Inventors: Klaus Peter Crone; Alain Saurer; Niklaus Schneeberger; Manuel THOMET; Marc MEINBERGER; Benjamin FERCHAUD
Original assignee: Armor
Priority date: 2020-12-14
Filing date: 2021-12-13
Publication date: 2022-06-23

Abstract

The invention relates to a thermal transfer printing apparatus (200) comprising: an endless ribbon (20) and a coating module (100) to coat the endless ribbon with melted ink. Said coating module comprises an endless band (10) to transport the melted ink on its outer surface; a reservoir assembly (11) designed to hold ink thereon and to feed the outer surface of the endless band (10) with said ink; and a first conveyor system comprising rollers to hold and transport the inner surface of the endless band along a first path in contact with the endless ribbon.

Description

THERMAL TRANSFER PRINTING APPARATUS WITH COATING MODULE COMPRISING AN ENDLESS BAND

FIELD OF INVENTION

The present invention relates to a thermal transfer printing apparatus comprising a coating module to coat a ribbon with ink. The invention also relates to a method to coat a ribbon with ink.

BACKGROUND OF INVENTION

Current solutions involving a thermal transfer printing apparatus use a disposable already coated ribbon. One limitation of these solutions is that the ribbon needs to be replaced periodically as the end of the ribbon has been reached.

To cope with the disposal of such used ribbon along with the remaining un-transferred ink, in an alternative class of thermal transfer printing apparatus, an endless ribbon is continuously coated and exposed to the thermal head while recovering the non-printed ink.

Such solutions offer reusing the remaining portion of the ink that is not used in previous thermal transfer printing cycles and reduce the wastes produced from the disposable ribbon of the printing apparatus.

In an emerging class of thermal transfer printing apparatus, an endless ribbon is transported while a part of the ribbon is exposed to the thermal printhead. It is an objective to use a ribbon capable of withstanding a large number of cycles, for example multiple million cycles through the system. This imposes, among other steps, coating continuously new ink on the ribbon to ensure a homogenous layer of ink on the ribbon while printing.

US2004/135870 teaches such printing apparatus wherein ink is coated with a roller on a gravure endless ribbon.

A first limitation of such a coating system is that the amount of ink coated on the ribbon is hard to control.

A second limitation is that said coating system is not capable to coat a constant thickness on the ribbon at variable speed and with any types of ink.

A third limitation is that the coating process provides damage to the ribbon because of a doctor blade, reducing the time of life of the endless ribbon. The invention aims to provide a method and a coating module avoiding said limitation.

SUMMARY

According to one aspect, the invention relates to a coating module comprising a coater to coat a ribbon with hot melted ink. Said coater comprises an endless band to transport the hot melted ink on its outer surface and a first conveyor system to hold and transport the endless band along a first path. The coating module also comprises a second conveyor system holding and transporting said ribbon. Said endless band and said ribbon are arranged for creating a sliding contact on a coating zone wherein the ink on the endless band is pressed between the endless band and the ribbon to transfer hot melted ink from the endless band to the ribbon.

According to one aspect, the invention relates to a thermal transfer printing apparatus comprising: an endless ribbon; and a coating module to coat the endless ribbon with melted ink comprising: an endless band to transport the melted ink on its outer surface; a reservoir assembly and a first conveyor system comprising rollers to hold and transport the inner surface of the endless band along a first path.

The reservoir assembly is designed to hold ink thereon and to feed the outer surface of the endless band with said ink or is designed to hold ink thereon and to apply said ink on the outer surface of the endless band;

The printing apparatus further comprises a printhead to print a portion of the coated ink on the endless ribbon to a substrate; and a second conveyor system comprising rollers to transport the coated ribbon from the coating module to the printhead and to transport the printed ribbon from the printhead to the coating module to be re-coated and at least one support element holding the endless ribbon.

In one embodiment, the outer face of the endless band transported by the first conveyor system is in contact with a portion of the endless ribbon. Said portion of the endless ribbon is held by the support element along a coating zone. Preferably, said contact ensures a transfer of melted ink from the endless band to the endless ribbon.

In one embodiment, the first conveyor system and the second conveyor system being arranged in such a way that the support element supports indirectly the endless band to cause a contact between the outer face of the endless band and the ribbon to transfer melted ink from the endless band to the ribbon.

In one embodiment, the support element comprises a support roller.

The ink is squeezed between the coater and the ribbon along a length where the endless band slides on the ribbon. The combination of the pressure applied and a relative speed between the ribbon and the endless ribbon cause a shear rate causing the transfer of ink from the endless band to the ribbon. Another advantage is that the present invention allows coating no matter the amount of remaining ink on the ribbon arriving at the coater. Another advantage of squeezing the ink between the endless band and the ribbon is to allow recovering the excess of ink.

In one embodiment, the coating module comprises a speed controller. The speed controller controls a first motor for controlling a first speed of the endless band and/ or controls a second motor for controlling a second speed of the ribbon. The advantage of controlling independently both the speed of the ribbon and the speed of the endless band is to allow controlling the shear rate between the ribbon and the endless band. Such control allows controlling the thickness of the layer of ink coated on the ribbon.

In one embodiment, the speed controller is configured to automatically adjust the first speed to keep the thickness of ink coated on the ribbon constant when the second speed is modified. One advantage is to allow increasing or reducing the speed of printing ensuring a constant ink thickness on the ribbon.

In one embodiment, the speed controller is configured to automatically adjust the first speed to modify the thickness of ink coated on the ribbon keeping the second speed constant.

In one embodiment, the pressure under which the ink is squeezed between the endless band and the ribbon is controlled by a pressure controller. One advantage is to control the thickness of ink coated on the ribbon by adjusting the force applied between the ribbon and the endless band. In one embodiment, the pressure controller comprises a movable element for controlling the pressure under which the ink is squeezed between the ribbon and the endless band. ln one embodiment, the coating zone extends along a segment in a plane sensibly perpendicular to the longitudinal axis of the support roller which is curved.

In one embodiment, the pressure controller is configured to automatically adjust the pressure under which the ink is squeezed between the ribbon and the endless band to modify the thickness of ink coated on the ribbon keeping the second speed constant. In one embodiment the pressure controller is configured to automatically adjust the pressure under which the ink is squeezed between the ribbon and the endless band to modify the second speed constant, keeping the thickness of ink coated on the ribbon constant.

In one embodiment, the pressure controller is configured to control a pressure applied between the endless band and the support element.

In one embodiment, the speed controller comprises at least one pre-stored program wherein the first speed is inferior or superior to the second speed. One advantage is to create a shear rate in the coating zone between the ribbon and the endless band, causing the transfer of ink on the ribbon.

In one embodiment, the second conveyor system is arranged to support the ribbon while the endless band creates a pressure on the ribbon within the coating zone. In one embodiment, the second conveyor system comprises a roller supporting the ribbon and supporting the endless band on the coating zone, the first potion comprises at least a rounded part around said roller. One advantage is to indirectly support the endless band to create a tension on said endless band by rerouting the endless band from its original path designed by the first conveyor system. In one embodiment, the second conveyor system supports the endless band on its outer surface on the coating zone. One advantage is to create a zone where the outer surfaces of both the endless band and the ribbon are in contact, optionally with ink during use.

In one embodiment, the outer surface of the endless band is a smooth surface. One advantage is to transport and hold the ink thanks to the respective surface tension of the endless band and the ink.

In one embodiment, the radius of the roller supporting the ribbon and the endless band on the coating zone is ranging from 10 mm to 50 mm. In one embodiment, the sliding contact (or coating zone) extends along a predefined length comprised in the range [1 mm ; 2 cm] or in the range [1 mm ; 5 cm]. In one embodiment, the first conveyor system comprises a squeeze roller arranged to support a portion of the endless band in contact with the ribbon by its inner surface and further wherein the pressure controller is configured to control the squeeze roller to press the endless ribbon and the endless band between the support element and the squeeze roller or to press the ink between the endless ribbon and the endless band. In one embodiment, the squeeze roller comprises an outer layer made of elastomer such as rubber.

In one embodiment, the first conveyor system holds and transports the endless band on its inner surface and the second conveyor system supports the endless band on its outer surface through the coating zone.

In one embodiment, the coater comprises a reservoir intended to hold hot melted ink thereon and further comprises a roller applicator in contact with the ink in the reservoir and in contact with the outer surface of the endless band to apply hot melted ink on the surface of the endless band.

In one embodiment, the width of the roller applicator is smaller than the width of the endless band. One advantage is to limit the passage of ink to the inner surface of the endless band.

In one embodiment, the coater further comprises a guide to center remaining ink on the endless band on a central portion of the surface of the endless band. Said guide is arranged on the path of the endless band from the sliding contact between the endless band and the ribbon to the roller applicator. One advantage is to limit the passage of ink to the inner surface of the endless band by spreading.

In one embodiment, the coater further comprises a blade to adjust the amount of ink carried on the roller applicator. One advantage is to limit the amount of ink transferred to the endless band.

In one embodiment, the ink reservoir is arranged to recover an excess of ink squeezed between the endless band and the ribbon.

In one embodiment, the coating module further comprises a heater to heat a remaining ink on the ribbon before arriving in contact with the endless band of the coater. One advantage is to facilitate the fusion of the ink on the ribbon which has not been printed, increasing the homogeneity of the layer of coating.

According to another aspect, the invention relates to a thermal transfer printing apparatus comprising a coating module according to the first aspect of the invention. The printing apparatus further comprises a printhead, and the ribbon is an endless ribbon; the second conveyor system being designed to hold and transport the ribbon from the coater to the printhead in a cyclic manner.

According to another aspect, the invention relates to a method for coating a ribbon of a thermal transfer printing apparatus according to the invention. Said method comprises:

■ Transporting the endless band along a first path forming a loop;

■ Applying hot melted ink on the outer surface of the endless band;

■ Carrying the ink on the endless band along the first path until the coating zone where the ink is squeezed between the endless band and the ribbon, causing the transfer of the ink from the endless band to the ribbon;

■ Optionally carrying the ink on the ribbon until a printhead to thermally transfer at least a portion of the ink on a substrate; and

■ Optionally carrying a portion of ink which has not been transferred on the substrate to the coating zone.

According to another aspect, the invention relates to a method for printing a substrate using a thermal printing apparatus according to the present invention. The method comprising:

■ Transporting the endless band along a first path forming a loop;

■ Applying hot melted ink on the outer surface of the endless band;

■ Carrying the ink on the endless band along the first path until a coating zone wherein the ribbon and the endless band are in contact and wherein the ink is squeezed between the endless band and the ribbon,

■ Applying a pressure to the ink between the endless band and the ribbon (20) supported by the support element.

■ Carrying the ink on the ribbon until a printhead to thermally transfer at least a portion of the ink on a substrate; and

■ Carrying a portion of ink which has not been transferred to the substrate to the coating zone.

In one embodiment, the method further comprises applying a shear rate on the ink within the coating zone by giving to the endless band a speed different than the speed of the ribbon. In said embodiment, the first and the second conveyor may generate on the ink a force ranging from 0.5 N/mm to 6 N/mm.

In one embodiment, the method further comprises applying a shear rate on the coating zone by giving to the endless band a direction of movement opposite from the direction of movement of the ribbon on the coating zone. In said embodiment, the first and the second conveyor may generate on the endless band a force ranging from 0.01 N/(mm. degree) to 0.4 N/(mm. degree).

In one embodiment, the ink on the endless band after the coating zone is centered in a central portion of the endless band.

In one embodiment, a remaining ink on the ribbon which has not been thermally transferred to the substrate is heated before reaching again the coating zone.

In one embodiment, a part of the ink squeezed between the ribbon and the endless band in excess falls back in a reservoir of ink.

In one embodiment, the ribbon is conveyed by a conveyor belt at least on the coating zone.

In one embodiment, the first speed of the endless band is automatically adjusted to keep the thickness of ink coated on the ribbon constant when the second speed of the ribbon is modified.

In one embodiment, the first speed of the endless band is automatically adjusted to modify the thickness of ink coated on the ribbon keeping the second speed of the ribbon constant.

In one embodiment, the pressure applied to the ribbon by the endless band is automatically adjusted to modify the thickness of ink coated on the ribbon keeping the second speed of the ribbon constant.

In one embodiment, the pressure applied to the inked ribbon by the endless band is automatically adjusted to modify the second speed of the ribbon keeping the thickness of ink coated on the ribbon constant.

BRIEF DESCRIPTION OF FIGURES

Fig. 1 is a perspective view of a portion of a thermal transfer printing apparatus comprising a coater according to one embodiment of the present invention wherein the ribbon is transported by a conveyor belt.

Fig. 2 is a sectional view of a coating module of a thermal transfer printing apparatus according to one embodiment of the present invention. Fig. 3A is a sectional view of a coating module of a thermal transfer printing apparatus according to one embodiment of the present invention wherein the endless band and the ribbon are conveyed along a same direction and at different speed to create a shear rate between them.

Fig. 3B is a sectional view of a coating module of a thermal transfer printing apparatus according to one embodiment of the present invention wherein the endless band and the ribbon are conveyed along opposite directions and at different speeds to create a shear rate between them.

Fig. 4 is a schematic view from above of the coating module of the printing apparatus according to one embodiment of the invention.

Fig. 5 is a perspective view of a printing apparatus according to one embodiment of the invention.

Fig. 6 is a sectional view of a coating module comprising a squeeze roller and wherein ink is provided by a device melting solid ink.

DETAILED DESCRIPTION

In the present application, the term “inner surface” should be understood as the surface of a ribbon or a band forming a loop which is in the interior of said loop or inside of said loop. The term “outer surface” should be understood as the surface of a ribbon or of a band forming a loop which is at the exterior of said loop or outside said loop.

Figure 1 shows an example of a coating module 100 according to one embodiment of the present invention. The coating module 100 comprises a coater 1 and a conveyor system to hold and transport a ribbon 20 to be coated.

A thermal transfer printing apparatus comprising such coating module is illustrated in figure 5. The printing apparatus comprises a conveyor system to hold and transport the ribbon 20 from the coating module 100 to a printhead 201 for printing to a substrate 202.

First conveyor system

The coater 1 comprises an endless band 10. The endless band 10 allows transporting ink on its surface, preferably on its outer surface.

The coater 1 also comprises a first conveyor system. The first conveyor system is arranged to hold and transport the endless band 10 along a first path, especially with respect to the reservoir or with respect to the second conveyor system.

The first conveyor system comprises at least two rollers 13 ,14 to hold and transport the endless band 10. The rollers of the first conveyor system optionally comprises cylinder rollers free in rotation in regard with the frame of printing apparatus around the longitudinal axis of the cylinder. In one embodiment, at least one of the rollers 13, 14, 15, 18 may be joined to the frame with at least one or two degrees of freedom along a plane sensibly perpendicular to the longitudinal axis of the cylinder of said roller.

According to one embodiment, one of the at least two rollers is a drive roller for instance referring to the figure 3A, the roller 13, 14 or 15. The coating module may further comprise a motor to rotate the drive roller 14 and a speed controller COs coupled to said motor generating a rotation of the drive roller 14. Then, the speed of the endless band 10 around its path is related to the speed of rotation of the drive roller 14 and controlled by the speed controller COs.

In one embodiment represented in figure 2, the first conveyor system comprises a third roller 15. The third roller 15 may be a spring-loaded tension roller. The third roller may be attached to a linear slide, groove, rail or slide runner. The spring-loaded tension roller 15 comprises a spring element loaded and arranged to push or to pull the roller along the linear slide in order to maintain a mechanical tension to the endless band 10. In one embodiment, the spring-loaded tension roller is a drive roller.

In one embodiment, the third roller 15 is movable along a slide, preferably along a linear slide. The third roller is controlled by means configured to fix the third roller in a chosen position along said slide. The position of the third roller 15 allows to control the tension applied along the endless band 10. The third roller 15 may be connected to a pressure controller COp controlling the position of the third roller and controlling the tension applied on the endless band 10.

The coater 1 may comprise a reservoir 11 to hold hot melted ink. Preferably, the melting point of the hot melted ink ranges from 50°C to 100°C. The coater 1 comprises means to coat ink on the outer surface of the endless band 10. In one embodiment, a part of the endless band 10 is in contact with ink in the reservoir. In another embodiment, the coater 1 comprises means to transport hot melted ink from the reservoir to the outer surface of the coater 1 .

The coater 1 is arranged to coat a ribbon 20 with hot melted ink.

In one preferred embodiment, the outer surface of the endless band 10, intended to carry the ink on it, is a smooth surface. Said surface is not a textured surface or is not a surface comprising roughness to retain the ink. The ink is carried by the smooth surface of the endless band 10 by capillarity or chemical interactions.

Preferably, the coater 1 is removable from the printing apparatus. This advantageously allows to replace the coater or to remove it to replace the endless band.

The reservoir assembly 11 is intended to store solid, molten ink, liquid ink, or partially melted ink.

In another embodiment illustrated in figure 6, the reservoir assembly 11 is coupled to a device 111 to apply melted ink on the endless band 10. Said device may comprise a house to maintain and melt a piece of solid ink.

In said embodiment, the device 111 comprises means to melt a piece of solid ink and is arranged in such a way that the melted ink is dropped onto the outer surface of the endless band 10 or within the ink molten pool 61 . Said dropping device 111 may comprise means to automatically add new melted ink within the ink molten pool 61 when instructions are received from the ink sensor.

In other embodiments non-illustrated, said device may comprise a slot-die coating device to apply a layer of melted ink on the outer surface of the endless band. Said device could also be chosen among the traditional list of coating device. In some other non-illustrated examples, said device is designed to apply ink to the endless band using any technique available to the skilled person such as but not limited to knife-coating, curtain coating, extrusion coating, transfer coating, flexo coating.

The endless band may comprise a plastic film to transport ink. The plastic film may comprise a polyimide film. The polyimide film advantageously provides high thermal conductivity and robustness.

The endless band may comprise a composite material. Said endless band may be reinforced with additives. The width of the endless band 10 is superior to the width of the endless ribbon 20. In another embodiment, the endless band comprises a fabric. Said fabric may be reinforced with additives. The additives are chosen among additives increasing the thermal conductivity or mechanical robustness of the endless band 10.

Second conveyor system

The thermal transfer printing apparatus further comprises a second conveyor system holding and transporting said ribbon 20. The ribbon 20 may be an endless ribbon 20 forming a loop.

In one embodiment illustrated in figure 1 , the second conveyor system comprises a conveyor belt 23. The conveyor belt 23 is designed and arranged to hold and transport the ribbon 20 along a second path. The conveyor belt 23 fulfills the same function as a continuous track driving the ribbon 20 in one direction of rotation. In one embodiment, the conveyor belt is an endless conveyor belt.

The inner face of the ribbon 20 is held on the outer surface of the conveyor belt 23. In the example of figure 1 , the conveyor belt 23 is supported by three rollers 21 , 25.

The conveyor belt 23 supports a portion of the ribbon 20 during its movement, reducing the tension along the ribbon 20. This supporting function of the conveyor belt 23 aims to offer better distribution of the tensions that apply on the ribbon 20. The conveyor belt 23 advantageously provides more design freedom to orient the ribbon transportation than a roller. The conveyor belt 23 can support the ribbon 20 over a longer length. The tension, that drives the ribbon 20, can be taken by the conveyor 23 instead of the ribbon 20 itself. Therefore, the tension in the conveyor belt 23 can be different, in most of the cases higher, than the tension in the ribbon 20.

The second conveyor system advantageously minimizes the stress on the ribbon 20 which improves the time of life of said ribbon 20, especially on the portion coated with hot melted ink. Furthermore, minimizing stress on the ribbon 20 allows avoiding the creation of a ripple profile on the ribbon 20. Moreover, the use of a conveyor belt 23 reduces the risk of wrinkling and of misalignment of the ribbon 20.

The conveyor belt 23 may comprise a plastic band arranged around at least two rollers 21 , 25. In other embodiments, the conveyor belt 23 may be made in any flexible material such as an elastomer, a thermosetting resin or a thermosetting plastic such as polyimide, a cork band or a sheet or foil of metal, such as stainless steel or titanium. In one preferred embodiment, the conveyor belt 23 comprises a coated metal band. The metal band may be coated with a material ensuring stickiness with the ribbon 20. Said coating comprises preferably plastic material, for example silicone. In such embodiment, the coating ensures the stickiness with the ribbon and softness to avoid deterioration of the ribbon 20. The metal band ensures the stiffness of the conveyor belt.

In one embodiment, the second conveyor system comprises a drive roller. The drive roller is connected to a motor to rotate said drive roller.

The coating module may further comprise a motor to rotate the drive roller of the second conveyor system and a speed controller COs coupled to said motor generating a rotation of the drive roller. Then, the speed of the ribbon 20 along its path is related to the speed of rotation of the drive roller of the second conveyor system and controlled by the speed controller COs.

At least one battery or an electrical alimentation may be implemented in the coating module in order to provide power supply to the motor.

The second conveyor system may comprise a guide for guiding the conveyor belt or the ribbon 20 between two rollers. In one embodiment illustrated in figure 1 , the second conveyor system comprises two guides 22, 24 for guiding the moving conveyor belt 23 along a predefined path, for example a curved path. In particular, the portion of the conveyor belt 23 carrying the ribbon 20 is supported by a partially rounded shape guide 22, 24. The guides 22, 24 are designed and arranged in such a way that the predefined path of the conveyor belt 23 comprises a curved portion. One advantage of the curved guide 22, 24 is to avoid major changes of direction of the ribbon 20, especially between two rollers 25, 21 . The curvature of the guide 22, 24 allows minimizing the probability of folding the ribbon 20 and minimizing variations of orientation along the length of the ribbon 20. This configuration allows homogenizing the distribution of the tension forces within the ribbon 20.

Coating zone

The first conveyor system and the second conveyor system are arranged to create a sliding contact between the endless band 10 and the ribbon 20 along a coating zone A. The contact is made in such a way to allow transferring the hot melted ink from the endless band 10 to the ribbon 20.

The coating zone preferably extends along a predefined length. Said length ensures that the endless band 10 slides on the ribbon 20 during a predefined time to ensure the transfer of the ink from the endless band 10 to the ribbon 20.

The first conveyor system and the second conveyor system may be arranged to create a pressure between the endless band 10 and the ribbon 20. Said pressure may allow squeezing the ink on the endless band 10 between the endless band 10 and the ribbon 20. Both parameters combined: the pressure and/or the difference of speed between the ribbon 20 and the endless band 10 may advantageously allow the coating of ink on the outer surface of the ribbon 20.

As illustrated in figure 2, the second conveyor system may comprise a support roller 21 to hold and transport the ribbon 20. In the illustrated embodiment, the second conveyor system does not comprise a conveyor belt to convey the ribbon 20. However, it will be understood that a conveyor belt may be added as illustrated in figure 1 , figure 4 and figure 5.

In one embodiment, by its fixed position, said roller 21 supports indirectly the endless band 10 on its outer surface. Said arrangement firstly allows creating a length along which the outer surface of the endless band 10 is in contact with the ribbon 20. Secondly, in this arrangement, the endless band 10 applies pressure to the ribbon 20 due to its tension. The coating zone is then an arc of a circle. Preferably, said arc of a circle has a radius sensibly equal to the radius of the roller 21 supporting the endless band 10 and the ribbon 20.

In other words, the outer face of the endless band 10 transported by the first conveyor system is in contact with a portion of the endless ribbon. Said portion of the endless ribbon is held by the support element along a coating zone. Said contact ensures a transfer of melted ink from the endless band 10 to the endless ribbon 20. The contact with the support element 21 provides a support to the endless ribbon 10 to apply pressure to the ink between the endless band and the endless ribbon. Said pressure may be provided by the tension of the endless band or by a squeeze roller as explained in the following of the present description. The support roller 21 of the second conveyor system holds indirectly the endless band 10, in such a way that the tension of the endless band 10 causes it to press itself against the ribbon 20. Said tension may depend on the position of said roller 21 and may also depend on the pressure applied by the spring-loaded tension roller 15.

The amount of ink squeezed between the endless band 10 and the endless ribbon 20 creates a coating zone A.

The coating zone A corresponds to the ink layer between the endless band 10 and the ribbon 20. In one embodiment, the coating zone A is defined by the area wherein the thickness of the ink layer between endless band 10 and the ribbon 20 is sensibly constant. Within the coating zone A, the ink is squeezed against the outer surface of the ribbon 20 supported by the support roller 21 and the endless band 10 containing the ink at the molten state, applying itself to the ribbon’s surface.

The ink is squeezed and shaped as a film through the coating zone A during the conveyance of the endless ribbon.

As illustrated in figure 6, the excess of ink arriving at the coating zone A feeds an ink molten pool 61. The ink molten pool 61 is an amount of excess of melted ink at the junction between the ribbon supported by the support roller 21 and the endless band 10. Indeed, when the melted ink fed to the coating zone A (by both the melted ink 122 provided by the endless band 10 and the residual ink 123 provided by the ribbon) is higher than the quantity of ink (121 , 124) exiting from the coating zone A, the excess of ink accumulates in the ink molten pool 61 .

Creating such ink molten pool 61 advantageously improves the quality of coating. Indeed, it creates a buffer volume of ink, allowing compensating occasional variations of quantity of ink provided to the coating zone A. Furthermore, it advantageously accelerates the melting of the residual ink 123 in the ribbon 20 because said ink is submerged by melted ink.

In said embodiment, a nip is formed within the coating zone A between the endless band and the ribbon and further comprises the ink molten pool 61 which is fluidly connected to the ink within the coating zone A. The nip may also be defined by the area wherein the outer layer 181 is elastically deformed by the contact with the ribbon 20 supported by the support roller 21 . Another advantage is to allow quick acceleration of the speed of the ribbon V2 without lacking melted ink within the nip.

Such ink molten pool 61 is common to all embodiments described in the present description.

In one embodiment, the reservoir assembly 11 further comprises an ink sensor (non-illustrated). The ink sensor is designed to detect when the level of melted ink within the ink molten pool 61 reaches a predefined threshold. Preferably, the reservoir assembly 11 further comprises an ink controller. The ink controller is configured to receive a signal from the ink sensor comprising the data sensed by the ink sensor. The ink controller is configured to control the amount of ink 122 added to the ink molten pool 61 by the squeeze ink roller 13.

In one specific embodiment, the ink controller is configured to automatically feed the endless ribbon with more ink when the sensor detects that the level of melted ink in the ink molten pool 61 reaches a first threshold.

Temperature control

In one embodiment, the temperature of the endless band is controlled by a first heater. Preferably, the coater comprises a heater to heat the endless band. Said heater may be arranged to heat the endless band from its inner surface. The control of the temperature of the endless band ensures a control of temperature of the ink applied on the endless band 10.

Preferably, all the rollers 13,14,15,18 supporting the endless band 10 comprises means to heat said endless band.

In one embodiment, the temperature of the ribbon 20 is controlled on the coating zone by a second heater. Preferably, the second conveyor comprises a second heater to heat the ribbon from its inner surface through the conveyor belt 23. The heater may be arranged in the roller 21 of the second conveyor holding the ribbon 20 and the endless band 10. The heater may also be arranged in a guide 22, 24 holding the conveyor belt 23. The control of the temperature of the ribbon 20 allows to control the temperature of the ink squeezed between the ribbon 20 and the endless band 10.

In one embodiment non-illustrated, the printing apparatus 200 comprises isolating walls. Isolating walls are arranged to isolate the sliding contact on the coating zone. Isolating walls isolate at least partially the coater 1 and the second conveyor system form the printhead 201 of the printing apparatus 200. Isolating walls may isolate at least partially the coater 1 and the second conveyor system form the rest of the printing apparatus. The isolation walls allow improving the control of the temperature of the ink squeezed between the ribbon 20 and the endless band 10.

This control of the temperature advantageously ensures a homogeneous viscosity of the ink squeezed between the endless band 10 and the ribbon 20 increasing the control of the thickness of ink coated on the ribbon.

The heater to control the temperature of this zone is preferably connected to a controller COs, COp. The control of the temperature advantageously allows the control of the viscosity of the ink 12 on the coating zone A and improves the control of the coating.

Control of the thickness of the ink layer on the ribbon

As described above, the motor connected to the drive roller of the first conveyor system and the motor connected to the drive roller of the second conveyor system are controlled by a speed controller COs.

The speed controller COs controls the speed of the ribbon 20 and controls the speed of the endless band 10.

When the speed of the ribbon 20 is different than the speed of the endless band 10, the endless band 10 slides on the ribbon 20 on the coating zone A. The pressure applied by the endless band 10 on the ribbon 20 advantageously ensures a shear rate between the ribbon 20 and the endless band 10 when the endless band 10 slides on the ribbon 20. The shear applied to the ink between the endless band 10 and the ribbon 20 allows the transfer of hot melted ink from the endless band 10 to the ribbon 20.

Indeed, the ink layer squeezed between the ribbon 20 and the endless band 10 is further split after passing through the nip and one portion of said ink is coated on the ribbon 20 exiting the sliding contact on the coating zone A.

Therefore, the ribbon 20 is transported by the second conveyor system to the coating zone A in contact with the endless band 10 wherein ink is transferred from the endless band 10 to the ribbon 20. Then, a layer of ink is coated onto the ribbon 20 exiting from said coating zone. In one embodiment, a pressure controller COp further controls the pressure applied by the endless band 10 to the ink within the coating zone by controlling the tension applied to the endless band 10 or by controlling the position of the third roller 15. In an alternative embodiment, the first conveyor system is mobile relative to the second conveyor system. The position of the first conveyor system is controlled by the pressure controller COp to control the pressure applied to the endless band 10 on the ink on the coating zone.

In another embodiment illustrated in figure 6, the pressure controller COp is designed to control the pressure applied by a squeeze roller to the ink within the coating zone.

The pressure controller COp may control the position of the third roller along a linear slide or may control the rigidity of a spring element of the spring-loaded tension roller. The pressure controller may also control the position of the first conveyor system. The control of the tension of the endless band 10 or the pressure applied by the endless band to the ink within the coating zone advantageously improves the control of the coating.

The speed controller COs may control or automatically adjust the speed of the endless band when the speed of the ribbon is modified to keep a constant ratio between the speed of the ribbon 20 and the speed of the endless band 10.

Advantageously, the control of the speed of the endless band 10 allows controlling the thickness of the layer coated on the ribbon 20 while the speed of ribbon 20 is constant. More advantageously, the control of the speed of both the ribbon 20 and the endless band 10 allows modifying the speed of the ribbon 20 with a constant layer of ink coated on said ribbon 20.

The coating module advantageously controls the thickness of ink at a given viscosity coated 124 on the ribbon 20 and the speed of the ribbon 20 independently of each other.

Indeed, the viscosity of the layer of ink is directly linked to the ink temperature and the shear rate applied to the hot melted ink between the outer surface of the endless band 10 and the ribbon 20 in the coating zone A. Said shear rate directly depends on the ratio between the speed of the ribbon 20 and the speed of the endless band 10. Said shear rate also directly depends on the pressure applied in the ink squeezed between the ribbon 20 and the endless band 10. The thickness of the layer of ink coated 124 on the ribbon 20 and the speed of the ribbon 20 may be advantageously modified independently.

Forward mode and Reverse mode

In a first embodiment called “forward mode” and illustrated in figure 3A, the ribbon 20 is conveyed in the same direction as the endless band 10 on the coating zone A. The shear rate is created by giving to the ribbon 20 a speed higher or lower than the endless band 10 speed. Preferably, the ratio of the speed of the ribbon (V2) over the speed of the endless band (V1 ) is inferior to 0.9 or superior to 1 .1 . This ratio advantageously allows the creation of a shear rate enabling the transfer of ink to the ribbon 20.

In “forward mode”, when the endless band moves faster than the ribbon, the speed variation between the endless band 10 speed V1 and the ribbon 20 speed V2 can be comprised between 1.05 and 2.5 and preferably between 1.1 and 2. The speed variation, when the endless band moves slower than the ribbon, may also range between 0.2 and 0.9, preferably between 0.4 and 0.8. Said ratio may be comprised between 1.05 and 2.5 or between 0.2 and 0.9, allowing advantageously creation of a shear rate due to the differential speed. Preferably, said ratio is comprised between 1.1 and 2 or between 0.4 and 0.8, reducing the force to be applied and improving the lifetime of the coating module.

Said speed variation may be calculated by the formula (V1 -V2) / V2.

The pressure between the endless band 10 and the ribbon 20 can vary between 0.5 N/mm and 6 N/mm, and preferably between 1 N/mm and 4 N/mm when the endless band 10 moves faster than the ribbon 20. The pressure between the endless band 10 and the ribbon 20 can range between 0.1 N/mm and 8 N/mm, preferably between 0.5 and 6 N/mm.

A second embodiment called “reverse mode” is illustrated in figure 3B. On the coating zone A, the vector of the speed of the ribbon 20 (V2) is in a opposite direction than the vector of the speed of the endless band 10 (V1 ). The shear rate is created by the shift between the ribbon 20 and the endless band 10. The “reverse mode” advantageously allows the coating of the ribbon with sensibly lower pressure than on the “forward mode”.

For example, the pressure to be applied on the reverse mode for the same speed of the ribbon to coat a layer of ink of the same thickness on the ribbon should be reduced by approximately 20 times. In this mode, the pressure between the endless band 10 and the ribbon 20 can vary between 0.01 N/(mm. degree) and 0.4 N/(mm. degree), and preferably between 0.05 N/(mm. degree) and 0.3 N/(mm. degree).

The pressure is obtained by the force applied (in Newton) divided by the width of the ribbon 20 (in millimeters) and multiplied by the angle a of the ribbon in the coating zone A (in degree).

In “reverse mode”, the variation of the speed proportion between the endless band 10 speed V1 and the ribbon 20 speed V2 can be between 0.5 and 0.3 and preferably between 0.55 and 0.8, said speed proportion being calculated as the absolute value of V2/ V1 .

Controller

The printing apparatus comprises a speed controller COs to operate the various components of the printing apparatus, including the drive roller of the first conveyor system and the drive roller of the second conveyor system.

For example, the speed controller COs and/or the pressure controller COp includes a hardware processor and software to control the printing apparatus 200, including controlling the speed of the ribbon 20, controlling the speed of the endless band 10 and controlling the position of the third roller and/or of the first conveyor system. The delivery of data to be printed by the printhead is received from a memory. The data flow is preferably sequenced by a calculator in function of the printing speed. The data can be changed during the printing process while the ribbon 20 and substrate 202 continue to move at the same speed.

In one example, the controllers may receive commands from a control interface such as a button ON/OFF or printing mode instructions or settings.

Preferably the speed of rotation of the ribbon 20 is driven by the speed controller COs by controlling the speed of rotation of the drive roller of the second conveyor system. The speed of rotation of the endless band 10 may also be driven by the controller by controlling the speed of rotation of the drive roller of the first conveyor system. The speed controller COs can be connected to the motor to rotate the drive roller of the second conveyor system and/ or connected to the motor to rotate the drive roller of the first conveyor system.

In one embodiment, the speed controller COs comprises at least one pre-stored program. Said pre-stored program may comprise instructions to control the speed of the ribbon 20 and the speed of the endless band 10 in such a way that the speed of the endless band 10 is inferior to the speed of the ribbon 20. Said pre-stored program may comprise instructions to maintain the proportion of the first speed and the second speed constant.

The pre-stored program may comprise instructions, when the speed of the ribbon 20 is modified, to automatically adjust the speed of the endless band 10 to keep the thickness of the layer of ink coated 124 on the ribbon 20 constant. The printing apparatus according to said embodiment advantageously allows to automatically keep a thickness of ink coated 124 on the ribbon 20 constant when the speed of the ribbon 20 is modified, e.g. when the speed of printing is increased.

The pre-stored program may comprise instructions to automatically adjust the speed of the endless band 10 when the speed of the ribbon 20 is constant to modify the thickness of the layer of ink coated 124 on the ribbon 20. The printing apparatus according to said embodiment advantageously allows modifying the thickness of ink on the ribbon 20 with the same speed as the ribbon 20. Said pre-stored program may comprise instructions to control the speed of the ribbon 20 and the speed of the endless band 10 in such a way that the speed of the endless band 10 is equal or sensibly equal to the speed of the ribbon 20.

The controllers can include or can be coupled with one or more sensors to assist in carrying out its functions. Moreover, the controllers can be divided into various subcomponents which can operate in cooperation with each other or separately control each component of the apparatus 1 .

The controllers can also be coupled to a printhead of the printing apparatus or to the reservoir 11 to control the temperature of the ink and/or the feeding of the reservoir 2.

In one embodiment, the pressure controller COp controls the position of the third roller 15 to control the pressure between the endless band 10 and the ribbon 20. In other words, the pressure controller COp may control the third roller to adjust the pressure with which the ink is squeezed between the ribbon 20 and the endless band 10. Therefore, the pressure controller COp is advantageously able to control the thickness of ink coated 124 on the ribbon 20 by modifying the position of the third roller or by modifying the pressure applied to the ink squeezed on the coating zone.

In one embodiment, the pressure controller COp comprises at least one pre-stored program. Said pre-stored program may comprise instructions to automatically adjust the pressure applied by the endless band to the ribbon 20 (or to the ink within the coating zone) when the speed of the endless ribbon V2 is increased or decreased in such a way that the thickness of the layer of coated ink is kept constant.

In one embodiment, a controller controls the temperature of the ink in the reservoir. Said controller may be connected to a heater arranged to heat the ink 12 in the reservoir 11. The temperature being linked to the dynamic viscosity of the ink, said embodiment advantageously allows controlling the thickness of the layer of ink coated 124 on the ribbon 20 by adjusting the temperature of the ink.

In one embodiment, a unique controller includes the pressure controller, the speed controller and preferably the controller controlling the temperature.

Detailed descriptions of figures

As illustrated in figure 2, the printing apparatus may comprise a reservoir intended to stock hot melted ink 12. The coater 1 may also comprise a roller applicator 16.

The roller applicator 16 is a device to transport molten ink from the reservoir 11 to the endless band 10. The roller applicator 16 may also be called “ink pick-up roller”. Said roller applicator may reside at least partially within the reservoir 11 and is adjacent to or in contact with the outer surface of the endless band 10. The roller applicator, by rotation, transports the molten ink from the reservoir 11 to the outer surface of the endless band 10. In one embodiment, the coater 1 may comprise a blade doctor 30. The blade doctor 30 is in contact with the material of the roller applicator to control the amount of ink transported by said roller applicator. The blade doctor 30 may be arranged in such a way that the exceed of ink on the roller applicator falls back in the reservoir 11 . As illustrated in figure 3A, the printing apparatus may comprise a heater 22 to heat the ink 123 on the ribbon 20 before arriving in the coating zone A in contact with the endless band 10. The heater advantageously melts again the ink on the ribbon 20 before arriving on the coating zone A. In one embodiment, the heater is arranged to heat the ink through the ribbon 20.

As illustrated in figure 4, the width of the roller applicator is inferior to the width of the endless band 10 and of the ribbon 20. One advantage is to avoid passage of ink on the inner surface of the endless band 10 and the passage of ink on the inner surface of the ribbon 20.

In another embodiment non-illustrated, the coater 1 comprises a pump intended to be fed by hot melted ink and arranged to provide a continuous flow of ink in contact to the endless band 10.

The coater 1 may also comprise a guide 17 to center ink 121 on the endless band 10 on a central portion. The guide is arranged along the path of the endless band 10 from the coating zone A and the roller application 16. The guide 17 center the ink on the endless band 10 before being re-fed with new ink. The guide 17 advantageously avoid the ink on the endless band 10 to reach the lateral edge of the endless band 10 and to reach the inner surface of the endless band 10.

In another embodiment illustrated in figure 6, the coater 1 comprises a squeeze roller 18. The squeeze roller is designed and arranged to support and transport the endless band 10 by its inner surface. The squeeze roller 18 is arranged to support the portion of the endless band 10 in contact with the endless ribbon 20. In said embodiment, the support roller 21 is arranged to support the portion of the ribbon 20 in contact with the endless band 10.

One advantage of such squeeze roller 18 is to increase the angle at which the endless band 10 enters (5) and exits (P) the ink meniscus in the coating zone. Indeed, the thickness of the coated layer of ink 124 in the ribbon depends on these angles.

For this purpose, the radius of the squeeze roller and its position in regard with the support roller 21 and the other rollers 14, 13 of the first conveyor system are configured to present an enter angle 5 superior to 20° or 45° or/and an exit angle p superior to 20° or 45°.

The enter and exit angles are defined by the angle between the endless band and an axis T which is the tangent of the endless band in the center of the coating zone. In another embodiment, the axis T is an axis perpendicular to the axis passing through the longitudinal axis of rotation of both the squeeze roller 18 and the support roller 21 .

In one embodiment, the squeeze ink roller 18 is controlled by the pressure controller COp.

In one example, the squeeze roller 18 is joined to the frame of the printing apparatus with at least one degree of freedom in translation, preferably along an axis perpendicular to its longitudinal axis.

The pressure controller COp is configured to control the movement of the squeeze roller 18 along said axis to apply a pressure W to the ink within the coating zone by pressing the squeeze roller toward the support roller 21 . In one embodiment, the pressure controller comprises communication means to receive instructions, for example from an interface device. The pressure applied by the pressure controller may depend on the data within the instructions. For examples, instructions may comprise a value of thickness of the coated layer of ink onto the ribbon.

Preferably, the squeeze roller 18 comprises an outer layer 181 made of an elastic material. Said outer layer 181 may consist in an elastomeric layer or a rubber layer.

The outer layer 181 is preferably elastically deformable. When the squeeze roller 18 is compressed in contact with the ribbon 20 against the support element 21 holding the ribbon, the outer layer 181 is elastically deformed.

One advantage is to increase the contact surface between the endless band 10 and the ribbon 20. As explained later, the homogeneity of the transfer is improved by increasing the time during which the ink is subjected to the shear in the contact zone A.

Another advantage is to ensure a level of shear rate to the ink between the ribbon 20 and the endless band 10 (i.e. , within the nip). The shear applied between the squeeze roller 18 and the ribbon 20 allows the transfer of hot melted ink from the squeeze roller 18 to the ribbon 20.

The outer layer 181 may comprise or may be made of elastomers. The outer layer is preferably made of or comprises a rubber (natural rubber or synthetic rubber) such as EPDM rubber (for ethylene propylene diene monomer rubber). In one embodiment, the outer layer is made of or comprises HNBR (for hydrogenated nitrile butadiene rubber). One advantage of HNBR is that they have higher thermal and chemical inertia or stability than other synthetic rubbers, which improves the time life of the outer layer.

Preferably, the thickness of the outer layer 181 is comprised between 0.5 and 20 mm, preferably between 1 mm and 4 mm. The squeeze roller 18 preferably comprises a rigid core. The outer layer 181 is arranged on the rigid core or radially on the outer side of the rigid core. The rigid core advantageously provides a rigid support to the outer layer 181 , increasing the squeezing of the ink in the coating zone A. In one embodiment, the circumferential surface of the rigid core is in contact with the outer layer 131. Preferably, the circumferential surface of the rigid core comprises a coating. Said coating of the rigid core may have a wettability sensibly equal to the wettability of the material of the outer layer 181. Preferably, the rigid core is made of a material comprising a metal or a metal alloy such as aluminum.

The hardness of the outer layer 181 preferably ranges from 30 to 90 shore A. Preferably, the hardness of the outer surface of the support roller 21 is superior to the hardness of the outer layer 181 of the squeeze roller 18. The hardness of the rigid frame of the squeeze roller 18 is superior to the hardness of the outer layer 181 of the squeeze roller 18.

In one alternative embodiment, the squeeze roller does not comprise an elastic outer layer 181 but only comprises the rigid core. In said embodiment, the endless band comprises a layer of elastic material. Said layer of the endless band may be similar to the outer layer described for the squeeze ink roller.

The nip may also be defined by the area wherein the outer layer 181 is elastically deformed by the contact with the ribbon 20 supported by the support roller 21 . In the embodiment illustrated in figure 6, the nip may also be defined by the area wherein the thickness of the ink layer between endless band 10 and the ribbon 20 is sensibly constant.

One advantage of the pressure controller COp is to apply a stress to the ink between the endless band and the endless ribbon within the coating zone. Said stress advantageously allows reaching conditions leading to a change of the behavior of the melted ink, decreasing the viscosity of the ink within the coating zone, improving the control and the quality of the coating. Decreasing the viscosity of the ink along a distance within the coating zone advantageously allows coating uniform layer of coated ink with a thickness inferior to 10pm.

Another advantage of this embodiment is to perform a coating with a reduced friction between the endless band and the endless ribbon. Indeed, the pressure of the squeeze roller allows to reduce or avoid the differential of speed between the endless band and the endless ribbon. The time of life of the endless band 10 and of the ribbon 20 is advantageously improved. In said embodiment, the speed of the ribbon is sensibly equal or equal to the speed of the endless band.

Another advantage is to perform a coated layer thinner than without such outer layer 181 .

Preferably, the ratio of the speed of the ribbon (V2) over the speed of the endless band (V1 ) ranges from 0.95 to 1 .05.

In said embodiment wherein the first conveyor system comprises a squeeze roller, the support roller 21 may be replaced by a support element fixed in rotation and in translation with the frame. The support element may comprise a support plate comprising a surface in contact with the ribbon. Said surface may be convex to ensure a contact with the ribbon. The ribbon, driven by a drive roller, is sliding along said surface.

In one embodiment, the circumferential surface of the squeeze roller and/or of the support roller is a smooth surface. One advantage is that the stress profile applied to the ink within the coating zone A in a plane perpendicular to the longitudinal axis of rotation of the squeeze roller presents a shape of bell curve or a parabolic curve at least in a center portion of the coating zone A. Therefore, it ensures that the stress applied remains above a threshold corresponding to a sufficient stress.

The expression “smooth surface” must be understood as at least one of the following definitions.

Preferably, the smooth surface is not a textured surface.

The expression “smooth surface” could be understood as a flat surface.

The expression “smooth surface” could be understood as a surface which does not comprise asperities able to retain the ink. The expression “smooth surface” could be understood as a surface which does not comprise a rastered or textured surface structure.

In one embodiment, a “smooth surface” must be understood as a surface roughness Ra inferior to 2 micrometers wherein Ra is defined by the arithmetic average of the deviations from the mean line. Preferably, the roughness Ra of the smooth surface is inferior to 0,5 micrometers.

In one embodiment, a “smooth surface” must be understood as a surface comprising a peak to valley roughness inferior to 8 pm, preferably inferior to 2pm, which gives better results. The peak to valley roughness is given by the vertical height of the surface between the highest peak and the lowest valley on the measured surface.

The smooth surface advantageously allows the ink to be pressed between the ribbon and the squeeze ink roller outer surface, improving the shear stress to which the ink is submitted to.

Printhead

The printing apparatus 200 comprises a printhead 201. In one preferred embodiment, the printhead 201 is a thermal transfer printhead.

In a first mode, the printhead is in contact with the inner face of the ribbon 20 to enable the thermal transfer of the ink located in the outer face of the ribbon 20. During this printing process, the outer face of the ribbon 20 is in contact (preferably in pressurized contact) with a substrate 202 to transfer the part of ink intended for printing the substrate.

In a second mode, the printhead 201 is not in contact with the ribbon 20. This mode may be engaged when the printing apparatus is switched off or during two successive printing sequences. The alternance of the first and second mode may be configured depending on the printing mode.

At least one print roller 203 can be used to transport a substrate 202 proximate to the ribbon 20. The thermal transfer printhead 201 is preferably in the vicinity of the substrate 202 and is used to transfer hot melt ink 124 from the ribbon 20 to the substrate 202. The arrangement between the printhead 201 , the ribbon 20 and the substrate 202 may be ensured by mechanical components which are precisely set according to a desired printing precision. Some guides and position control components may be implemented in order to ensure a predefined arrangement between at least the printhead 201 and the ribbon 20.

The print rollers 203 ensure a sufficient pressure on the substrate 202 in order to maintain the substrate 202 in contact with the ribbon 20 when printing process is engaged. In this configuration, said ribbon 20 is maintained in a moving sandwich layer between the substrate 202 and the printhead 201 during the printing process. The movement of the substrate 202 is in the same direction as the displacement direction of the ribbon 20 in the vicinity of the printhead. This movement in the vicinity of the printhead is preferably a rectilinear movement.

Ribbon

The ribbon 20 of the printing apparatus 200 allows the transport of the ink from the coating module 100 to the printhead 201 on its outer face. The ribbon 20 preferably forms a loop. In such configuration, the residual ink 123, not used during the printing process, is conveyed from the printhead to the coating zone A in contact with the endless band 10. In consequence, the same ribbon 20 is used continuously for conveying ink for printing and for conveying residual ink after the printing to the coating zone to be re-coated. The printing process is implemented such as to form a continuous looping process where residual ink is retrieved automatically. This configuration allows retrieving ink which has not been printed. This ink may be advantageously reused on a next turn of the ribbon 20.

One advantage is to provide and autonomous printing apparatus where at least a part, preferably 100% or substantially 100% of the ink is used, i.e. without ink loss.

The ribbon 20 can be made of various materials. The ribbon 20 is preferably made of a material with high temperature resistance properties, such as a temperature resistance up to 300°C, and high chemical resistance properties, for example a chemical resistant to alcohol, ink or solvents, etc. Preferably, the ribbon 20 is made of polyimide. The polyimide allows the ribbon to be used at temperatures up to the range [340°-380°] of temperatures without undergoing deformation. The ribbon 20 may also be made of metal or metal alloy such as titanium alloy. The ribbon 20 is preferably made of a material that has a heat transfer rate greater than 0.120 Watts/(meter. Kelvin).

The thickness and the composition of the ribbon material is designed to create heat transfer through the ribbon 20 allowing the printing.

Preferably, the thickness of the ribbon 20 is inferior to 50 pm or to 20 pm. Said thickness advantageously allows low heat transfer resistance between its inner face and outer face, improving the quality of printing. The thickness of the ribbon 20 may be comprised between substantially 0.5 pm and 50 pm, most preferably between 0.5 pm and 20 pm. In one example, the thickness of the ribbon 20 is chosen in the range [3-25 pm] or [5-10 pm].

In one embodiment, the length of the endless ribbon is inferior to 150 cm. In another embodiment, the length of the endless ribbon is superior to 150 cm or superior to 250 cm.

The ribbon 20 is designed to retain molten ink on its outer surface. The outer face of the ribbon 20 is designed to retain ink on its surface.

For this purpose, the ribbon is a non-porous ribbon. The ribbon or its outer face is preferably hermetic to fluids. Even when the ribbon 20 is pressed between the squeeze ink roller 13 and the support roller 21 , the composition of the ribbon 20 enables the ink to penetrate in the volume of the ribbon 20. In one other embodiment, the width of the ribbon is higher than the width of both the support roller and the squeeze ink roller. Therefore, during the coating, the inner face of the ribbon does not contain ink thereon, which would be harmful to the printhead.

In one embodiment, the ribbon does not comprise textiles or fabric.

Functional description

The operation of a printing apparatus according to one embodiment of the invention is described here above.

On a first step, the endless band 10 is transported by the first conveyor system along a first path. The endless band 10 and the first path form a loop.

On a second step, hot melted ink is coated on the outer surface of the endless band 10. Hot melted ink may be coated on the endless band 10 by a roller applicator. Preferably, hot melted ink is continuously coated on the outer surface of the endless band 10 while said endless band 10 is transported along its first path.

On a third step, hot melted ink 122 is carried by the endless band 10 until the coating zone A wherein the outer surface of the endless band 10 is in contact with the ribbon 20. Along said coating zone A, the tension of the endless band 10 squeezes the ink between the ribbon 20 and the endless band 10.

In a first embodiment illustrated in figure 3A, the ribbon 20 and the endless band 10 are transported along a same direction on the coating zone but are transported at different speeds to create a shear rate. An ink thickness is squeezed between the ribbon 20 and the endless band 10. This thickness depends on the (dynamic) viscosity of the ink, the proportion of the speed variation between the endless band 10 speed over the ribbon 20 speed, the radius of the roller 21 and the endless band 10 tension.

The shear rate and the endless band 10 tension cause the ink layer, at the end of the coating zone A, to split between the ribbon 20 and the endless band 10: a first part of the ink 121 remains on the endless band 10 and a second part of the ink has been coated on the ribbon 20, forming a layer of ink 124 on the outer surface of the ribbon 20.

In a second embodiment illustrated in figure 3B, the ribbon 20 and the endless band 10 are transported in opposite directions to create a shear rate. The shear rate and the endless band 10 tension cause the ink, at the end of the coating zone A, to split between the ribbon 20 and the endless band 10: a first part of the ink 121 remains on the endless band 10 and a second part of the ink has been coated on the ribbon 20, forming a layer of ink 124 on the outer surface of the ribbon 20.

On a fourth step, the ink 121 remaining on the endless band 10 after the coating zone A is carried along the rest of the loop of the first path to be re-inked according to the second step. As illustrated on figure 4, before being re-inked, the ink 121 remaining on the endless band 10 may be centered in a central portion of the endless band 10 by at least one guide 17.

The guide 17 allows centering ink along the longitudinal axis of the endless band 10 to advantageously avoid the spread of the ink and to avoid ink to pass on the inner surface of the endless band 10. Regarding the ribbon 20 exiting the coating zone A and coated with ink 124, the ribbon 20 is conveyed by the second conveyor system to the printhead for printing.

During printing, a portion of the ink 124 is thermally transferred to the substrate and the remaining ink remains on the ribbon 20.

The remaining ink is then transported by the ribbon 20 to the coater 1 , following the second path of the ribbon 20. In one embodiment, before arriving to the coater 1 , the remaining ink 123 on the ribbon 20 is heated, preferably above its melting point. The heating of the remaining ink 123 advantageously melts the ink to provide molten ink to the coating zone A.

One advantage of the present invention is to handle the thickness of the layer of ink coated on the ribbon, independently of the quantity of remaining ink 132 on the ribbon arriving on the coating zone A.

In one embodiment, the excess of ink 125 in the coating zone A falls back in the reservoir 11 during coating. One advantage is to reuse all ink which has not been printed on the substrate and to reduce the loss of ink.

The present invention advantageously allows controlling the thickness of the layer of ink 124 coated on the ribbon 20 by adjusting the ratio of the speeds of respectively the ribbon and the endless band and/or by adjusting the pressure between the endless band 10 and the ribbon 20 on the coating zone A. In one embodiment, said thickness can also be controlled by the relative direction of respectively the endless band and the ribbon.

Claims

CLAIMS Thermal transfer printing apparatus (200) comprising:

■ an endless ribbon (20);

■ a coating module (100) to coat the endless ribbon (20) with melted ink comprising:

■ an endless band (10) to transport the melted ink (12) on its outer surface;

■ a reservoir assembly (11 ) designed to hold ink thereon and to apply said ink on the outer surface of the endless band;

■ a first conveyor system comprising at least two rollers (13, 14, 15, 18) to hold and transport the inner surface of the endless band (10) along a first path,

■ a printhead (201 ) to print a portion of the coated ink on the endless ribbon (20) to a substrate (202); and

■ a second conveyor system comprising at least two rollers (21 , 25, 30) to transport the coated ribbon from the coating module (100) to the printhead (201 ) and to transport the printed ribbon from the printhead (201 ) to the coating module (100) to be recoated and at least one support element (21 ) holding the endless ribbon (20) wherein the outer face of the endless band (10) transported by the first conveyor system (13, 14, 15, 18) is in contact with a portion of the endless ribbon (20); said portion of the endless ribbon being held by the support element (21 ) along a coating zone (A). Thermal transfer printing apparatus according to claim 1 , wherein the support element (21 ) comprises a support roller. Thermal transfer printing apparatus according to claim 1 or claim 2, wherein the first conveyor system comprises a first drive roller (14) driving the transport of the endless band (10) and the coating module (100) further comprises a first motor to rotate said first drive roller (14) and comprises a speed controller (COs) configured to control a first speed of rotation of said first drive roller.

4. Thermal transfer printing apparatus according to claim 3, wherein the second conveyor system comprises a second drive roller (21 ) driving the transport of the endless ribbon (20) and a second motor to rotate said second drive roller; the speed controller (COs) being configured to control a second speed of rotation of the second drive roller.

5. Thermal transfer printing apparatus according to claim 3 and claim 4, wherein the speed controller (COs) is configured to automatically adjust the first speed to keep the thickness of ink coated (124) on the ribbon constant when the second speed is modified.

6. Thermal transfer printing apparatus according to claim 3 and claim 4, wherein the speed controller (COs) is configured to automatically adjust the first speed to modify the thickness of ink coated (124) on the ribbon keeping the second speed constant.

7. Thermal transfer printing apparatus according to anyone of claims 1 to 6, further a pressure controller (COp) configured to control a pressure applied between the endless band (10) and the support element (21 ).

8. Thermal transfer printing apparatus according to anyone of claims 1 to 6, wherein the first conveyor system is mobile relative to the second conveyor system and wherein the movement of the first conveyor is controlled by the pressure controller (COp).

9. Thermal transfer printing apparatus according to claim 7 or claim 8, wherein the pressure controller (COp) is configured to automatically adjust the pressure applied between the endless band (10) and the support element (21 ) to keep the thickness of ink coated (124) on the ribbon constant when the second speed of rotation is modified.

10. Thermal transfer printing apparatus according to any of claims 7 to 9, wherein the first conveyor system comprises a squeeze roller (18) arranged to support a portion of the endless band in contact with the ribbon (20) by its inner surface and further wherein the pressure controller (COp) is configured to control the squeeze roller (18) to press the endless ribbon and the endless band between the support element (21 ) and the squeeze roller (18). Thermal transfer printing apparatus according to claim 10, wherein the squeeze roller (18) comprises an outer layer made of elastomer such as rubber. Thermal transfer printing apparatus according to claim 2 to 9, wherein the coating zone extends along a segment in a plane sensibly perpendicular to the longitudinal axis of the support roller (21 ) which is curved. Method for printing a substrate using the thermal transfer printing apparatus according to any of claims 1 to 12, the method comprising:

■ Transporting the endless band (10) along a first path forming a loop;

■ Applying hot melted ink (122) on the outer surface of the endless band (10);

■ Carrying the ink (122) on the endless band (10) along the first path until a coating zone (A) wherein the ribbon (20) and the endless band (10) are in contact and wherein the ink (122) is squeezed between the endless band (10) and the ribbon (20),

■ Applying a pressure to the ink between the endless band and the ribbon (20) supported by the support element (21 ).

■ Carrying the ink (124) on the ribbon (20) until a printhead (201 ) to thermally transfer at least a portion of the ink on a substrate (202); and

■ Carrying a portion of ink (123) which has not been transferred on the substrate (202) to the coating zone (A).