NL2008064C2

NL2008064C2 - Hot-melt ink dosing system.

Info

Publication number: NL2008064C2
Application number: NL2008064A
Authority: NL
Inventors: Jacobus Hendricus Johannes Janssen
Original assignee: Mutracx B V
Priority date: 2012-01-02
Filing date: 2012-01-02
Publication date: 2013-07-03

Description

P30972NL00/MVE Hot-melt ink dosing system

The invention relates to a hot-melt ink dosing system. A hot-melt ink is a material that is jettable from an inkjet system. Because an inherent property of a hot-melt ink is that it is solid at normal room temperatures, it needs to be heated to an elevated temperature to melt, so that it can be jetted towards a substrate with an inkjet system after which the ink can 5 solidify on the substrate to form a desired pattern on the substrate.

Compared with aqueous inks, hot-melt inks have some challenges relating to the supply of the hot-melt ink to print heads of an inkjet system. One of the challenges is to do this in a reliable manner, such that at any time during the printing operation of the inkjet system 10 enough properly prepared hot-melt ink is available for the print heads to jet, i.e. enough hot-melt ink has the right predetermined operating temperature.

A further challenge may be that while doing this, the hot-melt ink may age due to the applied thermal load to get and keep the hot-melt ink at the predetermined operating temperature, 15 which means that the properties of the hot-melt ink change undesirably. Aging is especially a problem when the number of print heads increases, because this typically results in a large size reservoir and thus a large amount of ink which is kept at an elevated temperature for a longer period of time. From the reservoir, the hot-melt ink is then supplied to the respective print heads via corresponding supply lines.

20

Another disadvantage of having a large reservoir is that it takes a relatively long time for the system to heat the corresponding large amount of hot-melt ink during a start-up of the system.

25 It is therefore an object of the invention to provide a hot-melt ink dosing system in which the risk of aging of the hot-melt ink is reduced while ensuring that hot-melt ink at the predetermined operating temperature is available when required.

The object of the invention is achieved by providing a hot-melt ink dosing system according 30 to claim 1.

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The circulation of the hot-melt ink in the closed circuit past the fluid connections has the advantage that the required size of the reservoir is substantially independent of the amount of print heads connected to the hot-melt ink dosing system, while at the same time the predetermined operating temperature of the circulating hot-melt ink can easily be maintained 5 for reliability purposes with respect to prior art hot-melt ink systems in which the hot-melt ink is substantially stationary. It is the length of the fluid line of the closed circuit and the amount of fluid connections that need to be adjusted to the amount and size of print heads. The size of the reservoir can then be designed for the estimated consumption rate of the respective print heads and possibly for the desired replenishing rate to minimize the amount of melted 10 hot-melt ink in the dosing system.

In an embodiment, the reservoir is connectable to a hot-melt ink cartridge containing a predetermined amount of solid hot-melt ink to replenish the closed circuit. The heating system preferably comprises a separate heating element to supply heat to the hot-melt ink 15 cartridge when connected to the reservoir, so that the hot-melt ink can be melted and supplied to the reservoir in the liquid phase. A control system may be provided which controls said heating element in dependency of the amount of hot-melt ink circulating in the closed circuit. The control system may be configured to replenish the closed circuit with melted hot-melt ink when the amount of hot-melt ink in the closed circuit drops below a 20 predetermined minimum value, so that hot-melt ink is only subjected to a thermal load if required by the consumption of the inkjet system. This further reduces the chance of the hot-melt ink changing properties due to aging, because the amount of hot-melt ink in liquid phase in the dosing system is relatively low and thus the average residence time in the hot-melt ink dosing system is low.

25 A relatively small amount of liquid hot-melt ink in the dosing system further has the advantage that the start-up time of the system, in which the hot-melt ink in the closed circuit has to be melted, is reduced.

30 In order to measure the amount of hot-melt ink in the closed circuit, the dosing system may comprise a level sensor to measure the level of hot-melt ink in the reservoir. The output of the level sensor is then supplied to the control system which in turn drives the heating system in dependency thereof.

35 In an embodiment, the level sensor is configured to detect whether the level of hot-melt ink in the reservoir is above or below a predetermined minimum level, wherein the level sensor comprises a tubular measuring chamber having a bottom open end arranged at a height in -3- the reservoir corresponding to the predetermined minimum level, an air volume displacing device connected to the measuring chamber to supply a predetermined volume of air to the measuring chamber, and a pressure sensor to measure an air-pressure difference between an air-pressure in the measuring chamber and an air-pressure in the reservoir above the 5 hot-melt ink.

Supplying the predetermined volume of air to the measuring chamber with the air volume displacing device will result in a pressure difference between the air-pressure in the measuring chamber and the air-pressure in the reservoir above the hot-melt ink if the level of 10 hot-melt ink in the reservoir is above the minimum level, and will not result in a pressure difference between the air-pressure in the measuring chamber and the air-pressure in the reservoir above the hot-melt ink if the level of hot-melt ink in the reservoir is below the minimum level. Hence, regularly supplying the predetermined volume of air to the measuring chamber and measuring the pressure difference provides information about the level of hot-15 melt ink in the reservoir being below or above the predetermined minimum level based on which it can be decided by a control system to replenish the hot-melt ink.

In an embodiment, when the level of hot-melt ink drops below the predetermined minimum level, a predetermined amount of hot-melt ink can be automatically provided to the reservoir 20 from the hot-melt ink cartridge. In that case, it is preferred that the amount of hot-melt ink in the cartridge corresponds to the predetermined amount. However, alternatively, the heating system may be operated to melt the hot-melt ink in the cartridge until the level in the reservoir has risen to a predetermined maximum level. To make this possible, use can be made of a similar level sensor as described above for the minimum level, so that the level 25 sensor is configured to detect whether the level of hot-melt ink in the reservoir is above or below a predetermined maximum level, wherein the level sensor comprises a tubular measuring chamber having an open end arranged at a height in the reservoir corresponding to the predetermined maximum level, an air volume displacing device connected to the measuring chamber to supply a predetermined volume of air to the measuring chamber, and 30 a pressure sensor to measure an air-pressure difference between an air-pressure in the measuring chamber and an air-pressure in the reservoir above the hot-melt ink.

Because the predetermined operating temperature of a hot-melt ink can be above the 100 degrees Celsius and/or the hot-melt ink may in some cases be very aggressive, i.e. have a 35 low pH, the level sensor needs to be able to cope with these conditions. The abovementioned type of sensors are very suitable to be used in these environments due to the use of air-pressure in combination with static components. As a result, the level sensor is -4- reliable due to a lack of moving parts. Further, electrical components, e.g. for the pressure sensor and the driving electronics for the air volume displacing device can be situated at a safe distance from the reservoir and be connected to the measuring chamber and the reservoir by tubing, thereby providing an explosion and spark-free level sensor. The volume 5 of said tubing is preferably small compared to the volume of the measuring chamber.

A further advantage of the level sensor may be that the level sensor is independent of hot-melt ink material and/or temperature.

10 The components which may get into contact with the hot-melt ink, such as the measuring chamber can be made of a suitable material that is inert to the hot-melt ink, e.g. that is capable of withstanding corrosion.

In an embodiment, the reservoir has a surface-area-to-volume ratio of at least 50 [1/m], 15 preferably at least 100 [1/m] and most preferably at least 150 [1/m], This is advantageous as the heating system is usually configured to apply heat to the reservoir via the outer surface of the reservoir, so that the larger the surface-area-to-volume ratio of the reservoir, the faster the volume inside the reservoir is heated through the outer surface. As a large surface-area-to-volume ratio typically results in one of the dimensions getting quite large, the reservoir 20 can be folded to get a U-shaped cross-section, thereby keeping the overall dimensions of the reservoir within predetermined values. Preferably, the reservoir is configured such that inside the reservoir the maximum distance to the nearest wall of the reservoir is at most 10 mm, preferably at most 5 mm.

25 The hot-melt ink cartridges are preferably replaceable units, which are replaced by a full cartridge after being emptied. The reservoir may be configured to be connectable to more than one cartridge at the same time, so that for instance each time the level in the reservoir drops below a predetermined minimum level, a cartridge can be emptied into the reservoir without requiring to manually replace the cartridges immediately. Manual replacement is 30 then only required when the last cartridge has been emptied.

In an embodiment, the hot-melt ink cartridge when connected to the reservoir has a bottom opening in fluid communication with the reservoir, so that melted hot-melt ink will automatically flow towards the reservoir due to gravity forces. Preferably, a spacer is 35 positioned inside the hot-melt ink cartridge at a distance above the opening between solid hot-melt ink and the opening, wherein the spacer has a surface area at least as large as the opening, and wherein the spacer is arranged inside the hot-melt ink cartridge such that -5- melted hot-melt ink has to flow around the spacer towards the opening. As a result, it is prevented that a vacuum is created in the cartridge which would prevent the hot-melt ink from flowing out of the cartridge. Hence, emptying of the cartridges can be ensured, which makes the dosing system more reliable when used in an inkjet system.

5

In an embodiment, the spacer is a plate with ridges, which ridges automatically provide the desired distance between plate and bottom of the hot-melt ink cartridge.

In an embodiment, the spacer is a plate with sideways extending protrusions to provide a 10 desired distance between plate and sidewall of the hot-melt ink cartridge.

In an embodiment, the hot-melt ink cartridge is connectable to a connection element of the reservoir, wherein the connection element comprises a siphon to provide a gas separation between air inside the reservoir and air outside the reservoir. Even in case no cartridge is 15 connected to the reservoir, fumes or gases due to the relatively high temperature inside the reservoir are not able to escape from the reservoir through the connection element thereby preventing a hazardous situation for other components or people working near the dosing system.

20 In an embodiment, the dosing valve is operated by air-pressure, which provides the same advantages as the level sensor as due to the use of air-pressure, the actuation of the dosing valves is explosion and spark-free.

The invention also relates to a method for dosing hot-melt ink to multiple print heads of an 25 inkjet system, said method comprising the following steps: - heating a portion of hot-melt ink to a predetermined operating temperature to allow the hot-melt ink to flow; - circulating the heated hot-melt ink in a closed circuit; - tap heated hot-melt ink from the closed circuit to a print head if required.

30

In an embodiment, the method further comprises the step of replenishing the hot-melt ink in case the amount of hot-melt ink in the closed circuit drops below a predetermined minimum value. Preferably, the replenishing stops when the amount of hot-melt ink in the closed circuit reaches a predetermined maximum value.

35

The invention also relates to a level sensor to detect whether the level of hot-melt ink in the container is above or below a predetermined level, wherein the level sensor comprises a -6- tubular measuring chamber having an open end arrangable at a height in the container corresponding to the predetermined level, an air volume displacing device connected to the measuring chamber to supply a predetermined volume of air to the measuring chamber, and a pressure sensor to measure an air-pressure difference between an air-pressure in the 5 measuring chamber and an air-pressure in the container above the hot-melt ink.

The invention further relates to a hot-melt ink cartridge for an inkjet system, comprising an opening, wherein a spacer is positioned inside the hot-melt ink cartridge at a distance from the opening between solid hot-melt ink and the opening, wherein the spacer has a surface 10 area at least as large as the opening, and wherein the spacer is arranged inside the hot-melt ink cartridge such that melted hot-melt ink has to flow around the spacer towards the opening to leave the hot-melt ink cartridge.

The invention further relates to an inkjet system, in particular a drop-on-demand inkjet 15 system comprising a hot-melt ink dosing system according to the invention.

Different aspects of the invention may be combined with each other when conceivable.

The invention will now be described in more detail in a non-limiting way and with reference 20 to the accompanying drawings in which like parts are indicated with like reference numerals and in which

Fig. 1 depicts schematically an inkjet system according to the invention

Fig. 2 depicts schematically a hot-melt ink dosing system according to the invention

Fig. 3 depicts schematically a reservoir of the dosing system of fig. 2 and 25 Fig. 4 depicts schematically a hot-melt ink cartridge according to the invention

Fig. 1 depicts an inkjet system IS according to an embodiment of the invention for depositing ink fluid in a desired pattern on a substrate S by jetting liquid droplets DR of the ink fluid in a jetting direction JD towards the substrate S. The inkjet system is preferably a drop-on-30 demand inkjet system in which a droplet is only jetted when required. This is in contrast to continuous inkjet systems in which droplets are continuously jetted at a predetermined frequency and wherein droplets required to form the pattern are directed towards the substrate and the remaining droplets are captured and thus prevented from reaching the substrate.

35

The inkjet system of Fig. 1 is an industrial inkjet system, for instance an inkjet system used to deposit resist material as a mask layer on a printed circuit board (PCB) as an alternative -7- to the more traditional process of providing a mask layer using lithography. Because the mask layer can be deposited directly by the inkjet system, the amount of process steps can be reduced dramatically and thus the time for PCB manufacturing. However, such an application requires a high droplet placement accuracy and a high reliability (substantially 5 every droplet counts).

To provide a high accuracy inkjet system, the inkjet system IS comprises a force frame FF which supports a metrology frame MF from the ground GR. Between the force frame FF and the metrology frame MF a vibration isolation system VIS is provided to support the metrology 10 frame MF from the force frame FF while isolating the metrology frame MF from vibrations in the force frame FF. As a result, a relatively steady and quiet printing environment can be created on the metrology frame MF which is advantageous for accuracy.

The inkjet system further comprises a print head assembly with one or more print heads PH 15 which are held by a print head holder H, and a substrate holder SH to hold the substrate S. The print heads PH each comprise one or more, typically dozens of, nozzles from which droplets DR can be ejected towards the substrate S. The nozzles are preferably arranged in an array, i.e. in one or more rows. The print heads together define a printing plane perpendicular to the jetting direction JD, said printing plane indicating where the substrate 20 has to be positioned in order to receive jetted droplets from the print heads.

The substrate holder SH is moveable relative to the print heads PH in a printing direction PD parallel to the Y-direction and thus parallel to the printing plane in order to let a substrate S pass below the print head assembly. In this application a distinction is made between 25 passing the print head assembly while moving from left to right in Fig. 1, i.e. moving the substrate holder in the positive Y-direction, and passing the print head assembly while moving from right to left, i.e. moving the substrate holder in the negative Y-direction. The right to left movement will be referred to as a forward swath and the left to right movement will be referred to as a backward swath.

30

In order to be able to cover the entire top surface TS of the substrate S, many configurations are possible. In a first configuration, the printing plane in the X-direction is at least as large as the largest possible dimension in X-direction of a substrate S that can be held by the substrate holder SH. In that case, a single swath of the substrate holder SH may suffice to 35 cover the entire top surface with droplets. In a second configuration, the printing plane in X-direction is smaller than the largest possible dimension in X-direction of a substrate S that can be held by the substrate holder SH. In that case, multiple parallel swaths are necessary -8- to cover the entire top surface TS of the substrate S. To allow multiple parallel swaths, the print head assembly and/or the substrate holder SH is moveable in the X-direction perpendicular to the printing direction PD.

5 In case of the printing plane in the X-direction being at least as large as the largest possible dimension in X-direction of a substrate S that can be held by the substrate holder SH, multiple swaths may still be necessary in order to obtain the required printing resolution, because the nozzles in the print heads PH may be arranged at a larger distance than the corresponding pitch from each other, e.g. to prevent or reduce cross talk between adjacent 10 nozzles. The substrate is then passing the print head assembly multiple times, wherein each time the substrate has been moved in X-direction corresponding to the resolution in order to print the entire pattern.

In this embodiment, the print head assembly has a printing plane with a dimension in X-15 direction at least as large as the largest possible dimension in X-direction of a substrate the substrate holder SH can handle. As a result, the print head assembly can be mounted stationary with respect to the metrology frame MF.

In the embodiment of Fig. 1, the substrate holder SH is supported by a substrate positioning 20 stage PS, which substrate positioning stage PS in turn is supported by the metrology frame MF. The substrate positioning stage PS is supported by the metrology frame such that it is moveable in the printing direction PD, thereby allowing to position the substrate holder SH and thus the substrate S in the Y-direction. Positioning of the substrate positioning stage PS is done using a stage positioning device SD, which is able to apply forces F between the 25 substrate positioning stage PS and the force frame FF. As a result, the forces F do not introduce disturbances to the metrology frame MF, but are transmitted to the ground via the force frame FF, which results in a higher obtainable accuracy of the inkjet system.

Between the substrate positioning stage PS and the substrate holder SH, a holder 30 positioning device HD is provided in order to position the substrate holder SH in one or more degrees of freedom, preferably at least in the printing direction PD, relative to the substrate positioning stage PS. Using this configuration, the stage positioning device SD can be used for coarse positioning the substrate holder SH in the printing direction while the holder positioning device HD can be used for fine positioning of the substrate holder in the printing 35 direction relative to the print head assembly. If required, the holder positioning device HD may also be used for fine positioning of the substrate holder in other directions as well, e.g. the X-direction and/or the Z-direction, and may even fine position the substrate holder in -9- rotational directions such as Rx, Ry and Rz as well. Preferably, the holder positioning device HD is able to position the substrate holder relative to the substrate positioning stage in six degrees of freedom.

5 Position information about the substrate holder SH relative to the metrology frame MF is measured by a measurement system MS. The measurement system is at least configured to measure a position quantity, i.e. actual position, velocity or acceleration, of the substrate holder in the printing direction PD. In an embodiment, the measurement system measures position information about the substrate holder in six degrees of freedom, depending on the 10 level of control that is applied/required.

The output of the measurement system MS is provided to control electronics CE. The control electronics are here depicted as a black box that controls all processes in the inkjet system IS. As an example, the output of the measurement system MS can be used by the control 15 electronics to drive the stage positioning device SD and the holder positioning device HD (as shown in dashed lines) in order to position the substrate holder accurately relative to the print head assembly. The control electronics may further send driving signals to the print heads PH (see dashed lines) in order to print a desired pattern on the substrate while the substrate S passes the print heads PH.

20

The inkjet system IS further comprises a droplet detection device DD which measures the position of placed droplets on the substrate, e.g. by emitting light towards the substrate and detecting the reflected light. The obtained information is also send to the control electronics, which may comprise a calibration unit in order to adjust the position of the print heads 25 relative to each other based on the droplet position information obtained by the droplet detection device. The droplet detection device DD may further be used to calibrate the timing for firing the nozzles.

The inkjet system further comprises an ink dosing system for dosing ink to the multiple print 30 heads of the inkjet system. In the embodiment of Fig. 1, the ink used is hot-melt ink, which has the property that it is a solid material at room temperature and thus needs to be heated to an elevated temperature to become liquid and therefore jettable. The ink dosing system is thus a hot-melt ink dosing system and is not shown in fig. 1 for simplicity reasons, but schematically in Fig. 2 instead. To show the connections to the multiple print heads, some 35 print heads PH are shown in fig. 2 as well.

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Fig. 2 depicts schematically a hot-melt ink dosing system 1 for dosing a hot-melt ink to multiple print heads PH according to an embodiment of the invention. The hot-melt ink dosing system 1 of Fig. 2 is suitable to be used in the inkjet system according to Fig. 1.

5 The dosing system 1 comprises a closed circuit including a fluid line 3, and arranged in the fluid line a reservoir 5 and a pump 7. The reservoir 5 is configured to hold hot-melt ink, and the pump 7 is configured to circulate hot-melt ink in the closed circuit in a direction indicated by arrow 9.

10 Hot-melt ink has the property that it needs to be heated to an elevated temperature in order to be able to flow through the closed circuit. The dosing system therefore comprises a heating system configured to heat the hot-melt ink to a predetermined operating temperature allowing the hot-melt ink to flow in the closed circuit. In this embodiment, the heating system comprises a set of heating elements 11 which are able to supply heat to the fluid line and 15 the reservoir. The heating elements 11 are shown as individual blocks for simplicity reasons, but may in practice be multiple heating elements distributed along the closed circuit. A heating element may for instance also be integrated into the pump 7.

The dosing system further comprises a fluid connection 13 per print head PH, which fluid 20 connection is in fluid communication with the fluid line 3 of the closed circuit to drain hot-melt ink from the closed circuit to the print head PH. To control the amount of hot-melt ink to the print head PH, a dosing valve 15 is provided in each fluid connection.

Due to the fact that the hot-melt ink is circulated in the closed circuit, as many print heads 25 can be connected to the closed circuit without significantly influencing the reliability of the dosing system. The size of the dosing system can be adapted to the consumption rate of the set of print heads connected to the dosing system and the corresponding replenishing rate that is desired.

30 For instance, the volume of hot-melt ink in the dosing system 1 for about sixty print heads PH may be in the order of 2 liters. This has the advantage that the residence time in the closed circuit is limited and the chance of the hot-melt ink changing properties due to aging is reduced. The small volume also has the advantage that the start-up time, i.e. the time it takes to heat the volume of hot-melt ink in the closed circuit to the predetermined operating 35 temperature is reduced with respect to prior art dosing systems. Further, the circulation of the hot-melt ink has the advantage that the predetermined operating temperature of the hot-melt ink can be easily maintained, as a disturbance, e.g. a thermal disturbance, at a specific -11 - location in the closed circuit is in principle averaged over the entire closed circuit by the circulation and can easily be compensated at another location in the closed circuit. This is also advantageous from reliability point of view.

5 To measure the amount of hot-melt ink in the closed circuit, the reservoir comprises a level sensor 17. It may be desirable that the amount of hot-melt ink in the closed circuit is above a predetermined minimum level so that it can be ensured that enough hot-melt ink is available for the print heads. The level sensor is therefore preferably configured to detect whether the level of hot-melt ink is below or above a predetermined minimum level.

10

The level sensor 17 of Fig. 2 comprises a tubular measuring chamber 19 having an open end 21 at the bottom of the tubular measuring chamber 19 which can be closed off by the hot-melt ink if the level of the hot-melt ink in the reservoir is above the height at which the open end 21 is situated. The measuring chamber 19 is connected to an air-volume 15 displacing device 23 which is configured to supply a predetermined volume of air into the measuring chamber 19. In this embodiment, the air-volume displacing device 23 is a moveable piston 25 in a cylinder 27. By moving the piston 25 up and down in the cylinder 27, air is moved in and out of the measuring chamber 19.

20 The level sensor further comprises a pressure sensor 29 to measure a pressure difference between the air-pressure in the measuring chamber 19 and an air-pressure in the reservoir above the hot-melt ink. In this embodiment, the pressure sensor 29 is connected to the reservoir via a tubular member 31 extending in the reservoir, said tubular member 31 being in fluid communication with the inside of the reservoir via an open end 33.

25

In the reservoir, three possible levels of the hot-melt ink are shown, namely a low level 35, a high level 37 and an intermediate level 39. When the level of the hot-melt ink in the reservoir is for instance at the low level 35, the measuring chamber 19 is in fluid communication with the tubular member 31, so that supplying air to the measuring chamber 19 using the air-30 volume displacing device 23 will not result in a pressure difference between the air-pressure in the measuring chamber 19 and the air-pressure in the tubular member 31. When the level rises to above the open end 21 of the measuring chamber 19, e.g. to the intermediate level 39 or the high level 37, the measuring chamber 19 is closed off by the hot-melt ink and is no longer in fluid communication with the tubular member 31. If air is now introduced into the 35 measuring chamber 19, the air-pressure inside the measuring chamber 19 will increase relative to the air-pressure inside the tubular member 31. Hence, an air-pressure difference in favor of the air-pressure in the measuring chamber 19 indicates whether the level of the - 12 - hot-melt ink in the reservoir is below or above the level corresponding to the height at which the open end 21 of the measuring chamber 19 is situated.

By moving the piston 25 up and down at regular intervals, the level inside the reservoir can 5 also be measured at regular intervals. Other alternatives for the air-volume displacing devices are also envisaged.

The level sensor of Fig. 2 further comprises a tubular measuring chamber 41 having an open end 43 at the bottom of the tubular measuring chamber 41 which can be closed off by 10 the hot-melt ink if the level of the hot-melt ink in the reservoir is above the height at which the open end 43 is situated. The measuring chamber 41 is connected to an air-volume displacing device 45 which is configured to supply a predetermined volume of air into the measuring chamber 41. In this embodiment, like the air-volume displacing device 23, the air-volume displacing device 45 is a moveable piston 47 in a cylinder 49. By moving the piston 15 47 up and down in the cylinder 49, air is moved in and out of the measuring chamber 41.

The level sensor further comprises a pressure sensor 51 to measure a pressure difference between the air-pressure in the measuring chamber 41 and the air-pressure in the reservoir above the hot-melt ink, in this embodiment implemented via a connection between the 20 pressure sensor 51 and the tubular member 31. When the level of hot-melt ink inside the reservoir is below the open end 43, e.g. at the intermediate level 39 or the low level 35, supplying air to the measuring chamber 41 using the air-volume displacing device 45 will not result in a pressure difference between the air-pressure in the measuring chamber 41 and the air-pressure in the tubular member 31. When the level rises to above the open end 43 of 25 the measuring chamber 41, e.g. to the high level 37, the measuring chamber 41 is closed off by the hot-melt ink and is no longer in fluid communication with the tubular member 31. If air is now introduced into the measuring chamber 41, the air-pressure inside the measuring chamber 41 will increase relative to the air-pressure inside the tubular member 31. Hence, an air-pressure difference in favor of the air-pressure in the measuring chamber 41 indicates 30 whether the level of the hot-melt ink in the reservoir is below or above the level corresponding to the height at which the open end 43 of the measuring chamber 41 is situated.

By moving the piston 47 up and down at regular intervals, the level inside the reservoir can 35 also be measured at regular intervals. Other alternatives for the air-volume displacing devices are also envisaged.

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The measuring chamber 19 can thus be used to indicate a low level inside the reservoir and the measuring chamber 41 can be used to indicate a high level inside the reservoir, so that a control system is able to keep the level of hot-melt ink in the reservoir substantially between these two levels so that it is ensured that enough hot-melt ink is available to the print heads 5 PH while the amount of hot-melt ink is kept below a predetermined maximum value to avoid hot-melt ink unnecessarily being subjected to a thermal load, thereby reducing the chance of aging.

Connectable to the reservoir is a hot-melt ink cartridge 53. The hot-melt ink cartridge 53 is in 10 Fig. 2 shown in a connected state, but can be disconnected when necessary so that an empty hot-melt ink cartridge can be replaced by a filled hot-melt ink cartridge.

The hot-melt ink cartridge comprises solid hot-melt ink 54 in a filled state. The heating system of the dosing system comprises a heating element 55, which is able, when the 15 cartridge is connected to the reservoir, to apply heat to the cartridge to melt the hot-melt ink which allows the hot-melt ink to flow into the reservoir to replenish the hot-melt ink in the closed circuit.

In this embodiment, the hot-melt ink cartridge allows to melt hot-melt ink only when required 20 by the dosing system. Hence, when the level in the reservoir drops below the predetermined minimum level set by measuring chamber 19, the heating element 55 may be operated to melt hot-melt ink in the cartridge until the level has increased sufficiently. Replenishing the reservoir may be done until the level set by measuring chamber 41 is reached, but the size of cartridge may also be such that an intermediate level is obtained when emptying the 25 cartridge, such that the maximum level may only be present for safety reasons or can be omitted.

The reservoir in the embodiment of Fig. 2 comprises a siphon 57 to which the cartridge is connectable. When the cartridge is removed from the reservoir, the siphon ensures that 30 there is a gas separation between the inside of the reservoir and the environment, which provides a safe working situation. The heating system may further comprise a heating element 59 in order to melt solidified hot-melt in the siphon when necessary.

The cartridge, when connected to the reservoir comprises a bottom opening 58 through 35 which melted ink is able to flow to the siphon 57. Due to the fact that hot-melt ink is present inside the ink cartridge in the solid and liquid phase there is a high chance of vacuum formation inside the cartridge preventing ink to flow out of the cartridge if no measures are - 14- taken. In the embodiment of Fig. 2 this is prevented by providing a spacer 56 above the opening 58, wherein the spacer has a surface area at least as large as the opening to cover the opening entirely, and wherein the spacer is arranged inside the cartridge between the solid hot-melt 54 and the opening such that melted ink has to flow around the spacer to get 5 to the opening.

Fig. 3 depicts a cross-sectional view of a reservoir 5 according to an embodiment of the invention. The cross-section of the reservoir has a U-shape and therefore provides a large surface-area-to-volume ratio to the reservoir 5. As a result, the maximum distance inside the 10 reservoir to the nearest wall of the reservoir is limited, so that when heat is applied through the wall to the interior of the reservoir, the hot-melt ink is heated up relatively quickly. The advantage of the U-shape is that the overall dimensions of the reservoir are within certain limits.

15 Fig. 4 depicts a cross sectional view of a hot-melt ink cartridge 53 according to an embodiment of the invention. The cartridge is a container having at least an opening 58 which is connectable to a reservoir. The orientation of the cartridge is such that the opening faces downwards, so that ink is able to flow out of the cartridge due to gravity forces. When the cartridge is not connected to the reservoir, the opening may be closed by a removable 20 closing member.

Inside the cartridge a spacer 56 is provided at a distance from the opening between the opening and the solid hot-melt ink. As a result, melted ink has to flow around the spacer towards the opening 58 as indicated by arrows AR. In order to correctly position the spacer 25 inside the cartridge, the spacer may be provided with protrusions 56A which extend sideways from the spacer towards a side wall of the cartridge. Ink is then able to flow between the protrusions towards the opening. To keep the spacer at a distance from the opening, the spacer may comprise extensions 58B, which may be formed by ridges. The extensions 58B and the protrusions 58A may also be used as heat conductors, so that heat 30 applied to the lower portion of the cartridge to melt the hot-melt ink is also conducted to the spacer via the extensions 58B and protrusions 58A.

Claims

A hot-melt ink dispensing system for dispensing hot-melt ink to a plurality of printheads of an inkjet system, comprising: 5. a closed circuit with a liquid line, a reservoir, a pump and a heating system, the reservoir being in the liquid line is arranged and arranged to hold hot-melt ink, wherein the pump is placed in the liquid line and is arranged to circulate hot-melt ink in the circuit, and wherein the heating system is arranged to hold the hot-melt ink in the circuit. heat the closed circuit to a predetermined operating temperature through which the hot melt ink can flow through the circuit; a fluid connection per printhead, wherein the fluid connection is connected to the fluid line of the closed circuit, each fluid connection having a dosing valve for dispensing the amount of hot melt ink supplied to the respective print head.

A hot melt ink dispensing system according to claim 1, wherein the reservoir can be connected to a hot melt ink cartridge containing an amount of hot melt ink for replenishing the closed circuit with hot melt ink.

3. A hot melt ink dispensing system according to claim 2, wherein the heating system comprises a heater for supplying heat to the hot melt ink cartridge in case the reservoir is connected to the hot melt ink cartridge.

A hot melt ink dispensing system according to claim 1, comprising a level sensor for detecting the hot melt ink level in the reservoir.

A hot melt ink dispensing system according to claim 4, wherein the level sensor is adapted to detect whether the hot melt ink level in the reservoir is below or above a certain minimum level, the level sensor having a tubular measuring chamber with an open end placed at a height in the reservoir corresponding to the minimum level, an air volume displacement device connected to the measuring chamber for supplying a predetermined amount of air to the measuring chamber, and a pressure sensor for measuring an air pressure difference between an air pressure in the measuring chamber and an air pressure in the reservoir above the hot melt ink.

A hot-melt ink dispensing system according to claim 4, wherein the level sensor is adapted to detect whether the hot-melt ink level in the reservoir is below or above a certain maximum level, the level sensor having a tubular measuring chamber with an open end placed at a height in the reservoir corresponding to the maximum level, an air volume displacement device connected to the measuring chamber for supplying a predetermined amount of air to the measuring chamber, and a pressure sensor for measuring an air pressure difference between a air pressure in the measuring chamber and an air pressure in the reservoir above the hot melt ink.

A hot melt ink dispensing system according to claim 1, wherein the reservoir has a ratio between outer surface and content of at least 50 (1 / m), preferably at least 100 (1 / m) and more preferably at least 150 ( 1 m).

A hot melt ink dispensing system according to claim 7, wherein the reservoir has a U-shaped cross section.

A hot melt ink dispensing system according to claim 7, wherein the reservoir is arranged such that in the reservoir the maximum distance to the closest wall of the reservoir is at most 10 mm, preferably at most 5 mm.

10. A hot melt ink dispensing system according to claim 2, comprising at least one hot melt ink cartridge.

A hot melt ink dispensing system according to claim 10, wherein, when the hot melt ink cartridge is connected to the reservoir, the hot melt ink cartridge comprises an opening in the bottom of the hot melt ink cartridge such that molten hot ink melt ink is able to flow into the reservoir by gravity, with a spacer disposed in the reservoir at a distance from the opening between the solid hot melt ink and the opening, the spacer having a surface that is at least just is as large as the opening, and wherein the spacer is placed in the reservoir such that melted hot melt ink must flow around the spacer to the opening.

A hot-melt ink dispensing system according to claim 11, wherein the spacer is a plate with ridges, which ridges automatically provide the desired distance between plate and bottom of the hot-melt ink cartridge.

13. A hot melt ink dispensing system according to claim 11, wherein the spacer is a plate with laterally extending protrusions for providing a desired distance between plate and side wall of the hot melt ink cartridge.

A hot-melt ink dispensing system according to claim 2, wherein the hot-melt ink cartridge can be connected to a connecting element of the reservoir, and wherein the connecting element comprises a siphon for providing a gas separation between air outside the reservoir and air in the reservoir.

A hot melt ink dispensing system according to claim 1, wherein the dispensing valve is operated with air pressure.

A method for dispensing hot-melt ink to a plurality of printheads of an ink-jet system, comprising the following steps: - heating a portion of the hot-melt ink to a predetermined operating temperature such that the hot-melt ink can flow; - circulating the heated hot melt ink in a closed circuit; - draining heated hot melt ink from the closed circuit to a printhead when necessary.

A method according to claim 16, further comprising the step of replenishing hot melt ink in the event that the amount of hot melt ink in the closed circuit drops below a predetermined minimum value.

18. A method according to claim 17, wherein the replenishment stops when the amount of hot melt ink in the closed circuit reaches a predetermined maximum value.