FIELD OF THE INVENTION This invention is generally in the field of printing techniques and relates to an inkjet digital printing apparatus. More specifically, the invention relates to ink systems for use in inkjet"drop on demand"digital printers.

BACKGROUND THE INVENTION "Drop on demand"digital printers, and more specifically, those used for <BR> <BR> wide-format printing (e. g. , of about 0.7 meter length of the printing line) and very wide format printing (up to 5 meter length of the printing line), typically comprise a central (main) ink reservoir located near the supporting base of a printer, and a printing head system located remotely from the main ink reservoir. The printing head system includes an array of secondary ink reservoirs, typically one or two reservoirs for each printing color, thus totally-four or eight such secondary reservoirs. Each of the secondary ink reservoirs is housed in a printing head assembly, that includes a nozzle and additional elements, e. g. , for drop actuation, vacuum maintenance and temperature control.

Digital printing techniques typically require maintaining a desired viscosity <BR> <BR> of ink in a secondary ink reservoir (s), e. g. , about 10-20cps. The desired ink viscosity can be maintained by providing desired temperature conditions within the reservoirs. Usually, desired temperature conditions are obtained by utilizing separate heating means within each reservoir controlled by a local sensor. The heating means of each reservoir can also be integrated with the separate actuation means needed for releasing the ink drop (for example, in thermal or piezoelectric actuation mechanisms).

US Patent No. 5,622, 897 discloses a drop-on-demand inkjet printhead having a thermoelectric temperature control apparatus incorporated therewith for the selective heating or cooling thereof. The inkjet printhead includes a channel array constructed of a thermally conductive material, a spaced series of internal ink- carrying channels which extend rearwardly from a front side surface and a piezoelectric actuator acoustically coupled to each of the ink-carrying channels for selectively imparting pressure pulses thereto. N-type and P-type thermoelectric carriers are mounted to a top side surface of the channel array and then serially connected to a temperature controller. By applying electrical power to the N-type and P-type thermoelectric carriers, heat will be transferred between the channel array and a cover plate mounted to the top side surfaces of the N-type and P-type thermoelectric carriers. In this manner, the channel array may be selectively heated or cooled.

US Patent No. 5,920, 331 discloses a method and apparatus for producing a thermal pulse for a drop-on-demand printer actuator. A varying voltage pulse is applied to a resistance heater forming part of the actuator, which generates time varying power in the resistance heater. The power varies with respect to time in a manner comprising: (1) a pre-heating stage, which raises the temperature of the actuator, but is of insufficient total energy to actuate the printing actuator; (2) a stage of increased power which rapidly raises the temperature of the actuator to the required temperature for operation; (3) a stage of decreased power, which is less than the power in stage (b) but is sufficient to maintain the temperature of the temperature at the required temperature for operation; (4) a stage of low or zero power, during which the temperature of the temperature rapidly falls below the required temperature for operation. Accurate control over the temperature history at critical points in a device (such as the nozzle tip in a thermal, drop-on-demand print head) can be achieved by determining the required power function by applying an initial power function to a dynamic finite element simulation of the required structure, and iteratively refining the power function.

"Drop-on-demand"digital printing techniques typically also require maintaining a desired vacuum condition within the secondary ink reservoir in order to prevent the undesirable ink from dropping through the nozzles. In the conventional printers of the kind specified, the ink reservoir is typically equipped with a sensor operable to control the vacuum level in the reservoir and initiate the vacuum level correction when required.

The following patent publications exemplify the known vacuum control systems aimed at supplying under vacuum a required ink volume from an external main reservoir to a secondary reservoir, which is a part of a print head system: U. S.

4.942, 937 ; U. S. 5,920, 331 ; and WO 00/23281.

SUMMARY OF THE INVENTION There is a need in the art to improve the operation and simplify the construction of a digital"drop-on-demand"printer, by eliminating the need for using a plurality of separately operable temperature controllers, as well as a plurality of separately operable vacuum sensors, especially in modern color printers utilizing four and more printing colors and consequently a corresponding number of ink reservoirs.

According to the broad aspect of the present invention, there is provided a print head system for use in a digital printer apparatus, the print head system comprising a predetermined number of print head assemblies each comprising at least one secondary ink reservoir associated with a corresponding main reservoir at a main reservoir station, the print head system being characterized in at least one of the following: (i) the secondary ink reservoirs are associated with an external vacuum system that supplies required vacuum level to the secondary ink reservoirs, said vacuum system being configured for self-controlling a vacuum level therein thereby allowing to maintain the required vacuum level in the secondary ink reservoirs;

(ii) the secondary ink reservoir is at least partially made of a heat conducting material, all the secondary ink reservoirs being surrounded by a common heat conductor connected to a central heating system for receiving heat from the central heating system and thus maintaining certain temperature conditions within all the secondary ink reservoirs; (iii) the main reservoir station is configured for removably receiving at least one additional main ink reservoir and operable to selectively connect said at least one additional main ink reservoir to at least one of the secondary ink reservoirs.

The central vacuum system may be of the homeostatic hydraulic type, or may comprise an air-operated pneumatic venturi pump. The heat conductor may comprise a heat conducting support structure for mounting all the printing head assemblies thereon.

The heat conductor may be configured to enable fluid circulation therethrough, the required temperature of the fluid being maintained by the heat supplied from the central heating system. The central heating system is a closed fluid system including a fluid circulator reservoir associated with the heat conductor ; a central heater station including a heat body and a temperature control sensor for heating fluids; and a pump and tubing means for supplying heated fluids to said heat conductor.

The system of the present invention can advantageously be used in a digital printing apparatus for implementing drop on demand ink output from the secondary ink reservoir.

According to another broad aspect of the present invention, there is provided a method for use in digital printing utilizing a predetermined number of print head assemblies each including at least one secondary ink reservoir associated with a corresponding main reservoir at a main reservoir station, the method being characterized in at least one of the following: (a) supplying vacuum to each of said secondary ink reservoirs with certain required vacuum level from a central vacuum system that is

configured for self-controlling a vacuum level therein thereby allowing to maintain the required vacuum level in all the secondary ink reservoirs; (b) surrounding all the secondary ink reservoirs by a common heat conductor associated with a central heating system that supplies heat to and maintains a certain required temperature of said heat conductor; (c) selectively enabling printing of at least one additional color, in addition to those defined by the ink in said predetermined number of secondary ink reservoirs, by removably mounting at least one additional main ink reservoir to said main ink reservoir station and operating the station to selectively connect said at least one additional main ink reservoir to at least one of said secondary ink reservoirs.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of an ink system according to the invention for use in a digital printer; Fig. 2 is a schematic illustration of a printing head assembly according to the invention; Fig. 3 is a schematic illustration of an ink system according to one embodiment of the invention to be used in a digital printing apparatus; Fig. 4 is a schematic illustration of another embodiment of an ink system according to the invention; Fig. 5 is a schematic illustration of an ink system according to the invention that utilizes a separate ink reservoir station for"spot color"printing.

DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1, there is illustrated an ink system 10 according to the present invention suitable to be used in a digital printer apparatus. The digital printer apparatus is of the kind utilizing the so-called"drop-on-demand"printing technology. The ink system 10 thus comprises the following main components: an array of main ink reservoirs 12 (only one such reservoir being shown in the figure), for example including two main ink reservoirs for each printing color (e. g. cyan, magenta, yellow, and black); an array of secondary ink reservoirs 14 (only one such reservoir being shown in the figure) each associated with the corresponding one of the main reservoirs 12 and associated with a printing head assembly 16.

The ink from the secondary ink reservoir 14 is controllably dropped through nozzles 18. It should be noted, although not specifically shown, that the system also includes an appropriate supply means for supplying ink from the main reservoir 12 to the respective secondary reservoir 14. This does not form part of the present invention, and may utilize any known suitable technique, for example vacuum supply. The ink in the secondary ink reservoirs 14 is kept under certain temperature and vacuum conditions in order to maintain a desired level of the ink viscosity and prevent the undesirable ink dropping through the nozzles 18.

According to one aspect of the present invention, the print head assembly is configured so as to eliminate the need for a separate temperature sensor and controller in each secondary reservoir, but rather utilizes the heat supply to all the secondary reservoirs from an external common heat source. This is implemented by making at least a part of the secondary ink reservoir 14 from a heat conducting material such as aluminum or copper, and attaching the reservoir 14 to a heat conductor 20, which is associated with a central heating system 22. The desired temperature conditions at the secondary ink reservoir 14 are thus maintained by maintaining the temperature conditions in the central heating system 22.

According to another aspect of the present invention, the print head assembly 16 is configured so as to eliminate the need for separate vacuum sensors

and controllers, but rather utilizes a desired vacuum supply from an external vacuum source, which in turn is configured for self-controlling the vacuum level therein. To this end, the secondary ink reservoir 14 is associated with a central vacuum system 24 that supplies vacuum to the ink reservoir 14 with the desired vacuum level. The desired vacuum level at the secondary ink reservoir 14 is thus maintained by maintaining the vacuum conditions at the central vacuum system 24.

Referring to Fig. 2, there is more specifically illustrated a print head assembly 30 according to the invention, which in the present example utilizes both of the above aspects. The print head assembly 30 comprises inter alia a secondary ink reservoir 32 mounted in an insert adapter 34 made of a heat conducting material to be heated by fluid circulation through a heat conductor (20 in Fig. 1). The secondary ink reservoir 32 is associated with the nozzle array 36, and a vacuum chamber (not shown) located in the upper part of the ink reservoir 32. A connector 38 connects the inside of the vacuum chamber to a central vacuum control system (24 in Fig. 1). A connector 40 connects the inside of the ink reservoir 32 to a main ink reservoir (12 in Fig. 1). Electronic circuits 42 (printed circuit boards) are appropriately provided for operating the nozzle array. In this specific example, the printing head assembly is a two-part assembly, both head-parts receiving ink from the same secondary ink reservoir. Alternatively, each of the printing heads can be associated with its own secondary ink reservoir.

It should be noted that the insert adapter 34 can be integral with the printing head assembly 30, and the secondary ink reservoir 32 or at least a corresponding part thereof can be made of a heat conductive material such as aluminum or copper.

It should also be noted that the printing head assemblies (16 in Fig. 1 or 30 in Fig. 2) are arranged in a linear array, adjacent to one another and affixed to a common supporting structure. The present invention takes advantage of the fact that all the printing head assemblies in the multi-assembly printing head system are characterized by having substantially the same temperature-viscosity relation. The technique of the present invention provides for a common heating mechanism for concurrently identically heating all the secondary ink reservoirs. To this end, the

supporting structure is at least partially made of a heat conducting material and is configured as a fluid reservoir (20 in Fig. 1) associated with the central heating system (22 in Fig. 1).

Reference is made to Fig. 3, illustrating an ink heating system 60 according to the invention for maintaining required temperature conditions of ink in the secondary ink reservoir (s) of a digital printer apparatus. A heat conductor 62 is composed of an adapter frame 64 for supporting a corresponding printing head assembly 34, such that the adapter frame surrounds the respective secondary reservoir 32, and all the adapters 64 are mounted on a common frame 66. The heating system 60 utilizes an additional frame located close to the frame 66 and surrounding the frame 66 all along its perimeter, or the frame 66 itself, provided that frame 66 is configured like a tube reservoir for enabling fluids (e. g. water or oil) circulation therein. When using the heating system of the present invention, the adapter 64 is made of a heat conducting material, such as aluminum, copper, etc.

Thus, the heating system 60 comprises a fluid-circulator reservoir associated with the support frame 66, a central heater station 68 that includes a heating tank 69 (with a heating body and temperature control sensor), and a pump means for supplying heated water into the frame circulator 66. It should be noted that the fluids are circulated in and out of the heat conductor 66 (support frame 66) in a substantially continuous manner, through the central heater station 68, thus supplying the heat conductor 66 with a substantially steady level of temperature that falls within the range of the desired operational temperature levels. It should be understood that the invention is not limited by the type of a central heating system station and technique, and any other heating systems can be used without deviating from the scope of the invention.

Reference is made to Fig. 4, illustrating an ink system 70 according of the invention for maintaining a required vacuum level in the secondary ink reservoir (s) 32 of a digital printer apparatus. A central vacuum control system 72 is associated with the secondary ink reservoirs 32 of all the printing head assemblies 30 in the printing head system 74. This eliminates the need for any vacuum sensor inside the

secondary reservoirs 32. As shown, the central vacuum control system 72 is a homeostatic hydraulic system having a cell 74 with an inner wall 76 defining a vacuum chamber 78 at one side of the wall, and above an oil containing region 80.

An inlet means 82 serves for feeding oil into the region 80 and allows air to flow into a chamber 84 at the other side of the wall 76. The dimensions of the cell 74 and those of the oil containing region, as well as the length of the wall's region inside the oil, are defined by the required vacuum condition in the cell 74.

Generally, any known suitable homeostatic hydraulic system can be used. Thus, according to the invention, such a common (central) self-controlled vacuum control system is used with all the secondary reservoirs of a digital printer apparatus to replace the conventional multi-sensor system. For example, for the printing head assemblies of the printer apparatus commercially available from Spectra UK that operate with the vacuum condition of 10-150mmH20, the vacuum control system of the present invention provides for substantially stable vacuum conditions of about 70mm20.

It should be understood that the invention is not limited by the type and technique of the central vacuum system used for providing the desired vacuum condition. By another embodiment of the present invention, a central air-operated, pneumatic venturi vacuum system can be used, for example by utilizing venturi <BR> <BR> vacuum pumps commercially available from Korea Pneumatic System Co. , South Korea. Such vacuum pumps are widely used in various industrial areas and are known to be safe, energy-efficient, dependable, and cover a wide vacuum range, including those needed for digital printers.

Referring to Fig. 5, there is schematically illustrated the main functional elements of the entire ink supply system 86 of the present invention for use in a multi-color digital printer apparatus. In the present example, the printer apparatus operates with eight colors: four main colors (i. e. yellow, magenta, cyan or black, noted as Y, M, C, K) and four"light"colors (i. e. light yellow, light magenta, light cyan or light black, noted as LY, LM, LC, LK), enabling together to obtain a full color reproduction. Accordingly, the printing head system (carriage 26) comprises

sixteen printing head assemblies, each having its secondary ink reservoir 32. The ink supply system 86 thus comprises a main ink reservoir array 88 having eight main reservoirs, each containing a corresponding one of eight colors and connected to the corresponding pair of the secondary reservoirs 32.

It is a common problem in the digital printing technique to achieve a single color printing, the so-called"spot printing", especially important for white color printing on a transparent information carrier, as well as for orange and green print.

This problem is associated with the specific gravity (density) of white color ink (e. g. Titanium dioxide), which is higher than that of the other colors and thus requires specific ink processing (e. g. additional circulation) prior to being supplied to the secondary ink reservoir. In conventional printers, single color printing is achieved by the combination of the other colors. The present invention solves this problem by providing in the ink supply system 86 a separate reservoir station 90 that can be intended for a specific color. The station 90 includes a reservoir 92, which is removably attachable to the print head system 26, and is either directly connectable to the respective secondary reservoirs 96 or connectable through a circulator 94 in cases of a specific density color, such as white. The circulator 94 can be integrated with the secondary reservoirs 96 (i. e. to form a part of the respective printing head assembly 32 intended for printing this specific color). The construction may be such that the circulator 94 is integrated with the removable ink reservoir 92 or is a separate unit.

Although in the present example of Fig. 5 the station 90 is shown as being associated with an addition printing head assembly (i. e. , additional secondary reservoir), it should be understood that the reservoir 92 can be connectable to either one of the eight-color secondary reservoirs. It should also be noted that the removable ink reservoir 92 can be located within the main reservoir array 88.

The present invention has been described with a certain degree of particularity, but those versed in the art will readily appreciate that various alterations and modifications may be carried out without departing from the scope of the following claims.