BACKGROUND OF THE INVENTION Printing processes include stencil duplicating (mimeography) and screen-printing. These processes utilize stencils in which ink is allowed to pass through ink permeable areas on to the stencil (image areas) and on to the paper, thus printing an image. Duplicating, usually a rotary process, is used for printing finer details than screen printing which is a flat bed process.

Various stencil making processes have been developed and used over the years, both for mimeography and screen-printing. The process of stencil making involves cutting out image areas in the stencil. This was originally done by hand cutting of the stencil material. Other methods used various mechanical means to cut the image areas in impregnated tissue or in free standing films. Electro-optical methods were also used for stencil cutting by IR absorption or electro-erosion.

Since the 1980's, thermal digital duplicators have been developed. In these machines, the stencil is cut using a thermal head to make holes in a thin polyester layer. This technology gives short preparation time and is suitable for a digital work flow. It is generally used, at present, in all digital duplicators. Present stencil duplicating is limited in its application to simple jobs with very coarse halftone images.

Generally, the master used in digital stencil printing is imaged using thermal technology with consequently limited low-resolution, especially with regard to four colors printing. The quality of thermal stencil printing is generally poorer compared to other commercial printing technologies. The earlier generation of digital stencil machines had a resolution of 300 dpi compared with approximately 1250dpi resolution required for conventional quality printing.

Furthermore, inking systems used in stencil printing provide a high degree of dot gain and an insufficient level of ink drying on the paper. Ink drying is generally only possible on non-coated paper whereas it is common to use coated paper for quality printing.

In addition, digital stencil printing machines generally have poor registration capability between colors used for printing several colors sequentially, further contributing to low print quality. These machines also involve manually changing the drums for each color to be printed.

In addition to the obvious registration problems, operating the machine in four or more color processes involves great complexity on the user side where the attentive presence of the operator is required for each printing job.

Various attempts have been made to solve these problems.

Imaging technology was improved by doubling the resolution capability in the later generation of duplicating machines. However, the resolution is still less than half that considered acceptable for traditional quality printing.

US Patent No. 5513565 to Hasegawa describes a stencil printing device having a supporting device for a plurality of printing drums, one for each color.

The device describes an automatic change of printing drum. However, every time one of the color drums is changed, the stack of printed papers has to be manually brought back to its original starting position. Thus, the printing device also has the previously mentioned disadvantage of requiring manual operation and of having poor paper registration.

U. S. patent No. 5375516 ('516) to Hasegawa, assigned to Riso Kagaku Corporation of Tokyo, describes stencil printing device having a plurality of drums arranged along an incline line. The path for conveying paper between drums is a straight line. The paper is conveyed along a belt where at least four sets of grippers lay on the belt. The paper is conveyed from one set of grippers to the next underneath the printing drums. Each transformation of the paper between two adjacent sets of grippers causes registration misalignment. Furthermore, the plate making system of the'516 patent travels along the four printing drums discharging the imaged master on to each drum. The sequential process of the '516 patent, where discharging misalignments is compounded for each printing drums, reduces the ability of achieving a good and acceptable level of registration.

Similarly, systems having four separate plate making stations, one for each printing drum, also have mis-registration problems.

Thus, current systems, including the'516 patent, which uses the concept of first imaging a master stencil and then placing the stencil on the printing drum

for each color printing drum (that is four times for four color separations), cannot avoid the mis-registration problems.

SUMMARY OF THE INVENTION An object of the present invention is to provide a stencil color printing machine, which overcomes the limitations and disadvantages of prior devices.

The stencil printing machine of the present invention includes a plurality of printing drums, arranged around a common impression drum and utilizes a single laser imaging system.

There is thus provided, in accordance with a preferred embodiment of the present invention, a stencil printing device. The stencil printing device includes a common impression drum and a plurality of printing drums arranged around the common impression drum, each of the plurality of printing drums rotatable around a stationary shaft. The impression drum is engageable with the plurality of printing drums so as to synchronously rotate each of the plurality of printing drums around its shaft, the shaft being stationary.

Furthermore, in accordance with a preferred embodiment of the present invention, the impression drum includes a first gear ring, attached to the circumference of one edge of the impression drum, and each of the plurality of printing drums includes a second gear ring, attached to the circumference of one edge of each of the plurality of printing drums. The first gear ring is engageable with each of the second gear rings.

Moreover, in accordance with a preferred embodiment of the present invention, the impression drum further includes at least one under blanket fitted thereto.

Additionally, in accordance with a preferred embodiment of the present invention, the nip diameter ratio between the impression drum and each of the plurality of printing drums is equal to an absolute number greater than one.

Still further in accordance with a preferred embodiment of the present invention, each of the plurality of printing drums further includes a plurality of master handling systems, each of the plurality of master handling systems including blank stencil masters. The required length of the blank stencil masters is

dischargable from each of the plurality of master handling systems for wrapping around the corresponding printing drum.

Furthermore, in accordance with a preferred embodiment of the present invention, the blank stencil masters are imaged on the corresponding printing drum.

Moreover, in accordance with a preferred embodiment of the present invention, the impression drum is fully engaged with the plurality of printing drums whereby the impression drum synchronously rotates the plurality of printing drums.

Additionally, in accordance with a preferred embodiment of the present invention, during printing the impression drum is fully engageable with the plurality of printing drums and the under blanket is pressed onto the surface of each of the plurality of printing drums.

Still further, in accordance with a preferred embodiment of the present invention, the impression drum is semi-engaged with the plurality of printing drums allowing the plurality of printing drums to rotate synchronously with the impression drum without the surfaces of the plurality of printing drums touching the under blanket.

Furthermore, in accordance with a preferred embodiment of the present invention, each of the plurality of printing drums includes motion control means for independently rotating each of the plurality of printing drums.

Moreover, in accordance with a preferred embodiment of the present invention, and further including an operational controller for controlling the operation of the device and a laser imaging system coupled to the printing controller, for laser imaging of the stencil masters on press. The operational controller includes a central processing unit (CPU) and a printing controller coupled to the CPU.

Additionally, in accordance with a preferred embodiment of the present invention, the printing controller is coupled to the motion control means.

Still further, in accordance with a preferred embodiment of the present invention, the imaging system is configured to be located within the hollow inner space of the impression drum.

Furthermore, in accordance with a preferred embodiment of the present invention, the laser imaging system includes a scanning system, and an optical system coupled to the scanning system.

Moreover, in accordance with a preferred embodiment of the present invention, the laser imaging system is any one of a group of laser imaging systems including flat field and linear scanning systems.

Additionally, in accordance with a preferred embodiment of the present invention, the imaging system is a single imaging system for exposure onto multiple locations.

Still further, in accordance with a preferred embodiment of the present invention, the optical system includes a plurality of optical elements located in an optical path, the plurality of optical elements including a deflecting element, a high speed polygon spinner having a plurality of facets, and a flat field f-theta lens.

The laser beam is deflected by the polygon spinner through the f-theta lens on to the deflecting element.

Furthermore, in accordance with a preferred embodiment of the present invention, the deflecting element is any one of a group including a rotatable mirror, a diffractive element, and a prism.

Moreover, in accordance with a preferred embodiment of the present invention, the deflecting element is a mirror which is rotatable through a full circle thereby to reflect the laser beam of the imaging system onto the inner circumference of the impression drum. The rotatable mirror is configured to align with the longitudinal axis of the impression drum.

Additionally, in accordance with a preferred embodiment of the present invention, the impression drum includes an elongated slot formed for allowing the laser beam to reach the plurality of printing drums, the axis of the slot being parallel to the longitudinal axis of the impression drum. The optical path to each of the plurality of printing drums is identical.

Still further, in accordance with a preferred embodiment of the present invention, and further including a feeding and delivery system configured to feed a single sheet of substrate at a time to the impression drum.

Furthermore, in accordance with a preferred embodiment of the present invention, the impression drum includes a plurality of grippers attached to the

circumference, the circumferencial distance between each of the plurality of grippers being equal.

Moreover, in accordance with a preferred embodiment of the present invention, the ratio between the diameter of the impression drum and the diameter of each of the plurality of printing drums is equal to the number of the plurality of grippers.

Additionally, in accordance with a preferred embodiment of the present invention, each of the plurality of printing drums includes individual inking units installed within each of the plurality of printing drums. The inking unit includes an ink supply unit and an ink transfer unit.

Still further, in accordance with a preferred embodiment of the present invention, the ink supply unit includes an ink supply connected to the stationary shaft of the printing drum, a pumping unit for pumping the ink supply, and a manifold arrangement connected to the stationary shaft for distribution of the pumped ink.

Furthermore, in accordance with a preferred embodiment of the present invention, the ink transfer unit includes any of a group of transfer units including at least one roller connected to a piston arrangement and a squeegee blade.

Moreover, in accordance with a preferred embodiment of the present invention, the roller includes inner and outer rollers rotatable around their own axes. The outer roller is in contact with the inner circumference of the printing drum and the inner roller is in spaced-apart contact with the outer roller.

There is thus provided, in accordance with a preferred embodiment of the present invention, an optical system for use with an imaging system. The optical system includes a plurality of optical elements located in an optical path. The plurality of optical elements includes a deflecting element, a high speed polygon spinner having a plurality of facets, and a flat field f-theta lens. The laser beam is deflected by the polygon spinner through the f-theta lens on to the deflecting element.

Furthermore, in accordance with a preferred embodiment of the present invention, the deflecting element is any one of a group including a rotatable mirror, a diffractive element and a prism.

Moreover, in accordance with a preferred embodiment of the present invention, the deflecting element is a mirror which is rotatable through a full circle

thereby-to reflect the laser beam of the imaging system onto the inner circumference of an impression drum, the rotatable mirror configured to align with the longitudinal axis of the impression drum.

Additionally, in accordance with a preferred embodiment of the present invention, the imaging system is configured to be located within the hollow inner space of an impression drum.

Still further, in accordance with a preferred embodiment of the present invention, the imaging system further includes a scanning system coupled to the optical system.

Furthermore, in accordance with a preferred embodiment of the present invention, the imaging system is any one of a group of imaging systems including flat field and linear scanning systems.

Moreover, in accordance with a preferred embodiment of the present invention, the imaging system is a single imaging system for exposure onto multiple locations.

Additionally, in accordance with a preferred embodiment of the present invention, the impression drum includes an elongated slot formed for allowing the imaging system to reach a plurality of printing drums arranged around the impression drum, the axis of the slot being parallel to the longitudinal axis of the impression drum, the optical path to each of the plurality of printing drums being identical.

There is further provided, in accordance with a preferred embodiment of the present invention, a method for imaging photo-sensitive masters on press, utilizing a printing device including a printing controller coupled to a laser imaging system, a common impression drum and a plurality of printing drums arranged around the common impression drum. The laser imaging system includes a scanning system and an optical system coupled to the scanning system. The method including the steps of: first rotating the plurality of printing drums and impression drum around their respective axes until the impression drum is aligned with the first drum of the plurality of printing drums, loading a blank photo-sensitive master onto the first drum of the plurality of printing drums,

actuating the optical system to enable the reflected laser beam to strike the blank photo-sensitive master at a pre-determined starting reference point, imaging and curing the blank photo-sensitive master, second rotating the plurality of printing drums and impression drum around their respective axes until the impression drum is aligned with the second drum of the plurality of printing drums, repeating the steps of loading, actuating the optical system, imaging and curing of the photo-sensitive master on the second drum, repeating the steps of: second rotating the plurality of printing drums and impression drum around their respective axes until the impression drum is aligned with the second drum of the plurality of printing drums; and repeating the steps of loading, actuating the optical system, imaging and curing of the photo-sensitive master on the second drum; for all of the remaining printing drums.

Furthermore, in accordance with a preferred embodiment of the present invention, the impression drum includes a first gear ring attached to the circumference of one edge of the impression drum and each of the plurality of printing drums includes a second gear ring, attached to the circumference of one edge of each of the plurality of printing drums. The steps of first and second rotating includes the step of the first gear ring engaging with each of the second gear rings.

Moreover, in accordance with a preferred embodiment of the present invention, each of the plurality of printing drums includes a motion control unit for independently rotating each of the plurality of printing drums.

Additionally, in accordance with a preferred embodiment of the present invention, the optical system includes a plurality of optical elements including a deflecting element, a high speed polygon spinner having a plurality of facets and a flat field f-theta lens. The imaging step includes the step of deflecting a laser beam by the polygon spinner through the f-theta lens on to the deflecting element.

Still further, in accordance with a preferred embodiment of the present invention, the deflecting element is a mirror which is rotatable through a full circle.

The step of deflecting includes configuring the rotatable mirror to align with the longitudinal axis of the impression drum and reflecting the laser beam of the imaging system onto the inner circumference of the impression drum.

There is still further provided, in accordance with a preferred embodiment of the present invention, an imaging system for sequentially imaging at least two radiation sensitive elements, positioned at different locations. The imaging system includes a laser system, an optical system, a control system, and at least two carriers disposed vis-a-vis the laser beam. The optical system includes a deflection element for diverting the laser beam of the laser towards the radiation sensitive elements. The control system is coupled to the optical system and the laser system for scanning the laser beam to image the radiation sensitive elements. The carriers are configured to support the radiation sensitive elements.

The diverting of the laser beam and the imaging of radiation sensitive elements are performed sequentially.

Furthermore, in accordance with a preferred embodiment of the present invention, the radiation sensitive elements are disposed in a circular pattern around the laser system.

Moreover, in accordance with a preferred embodiment of the present invention, the radiation sensitive elements are disposed in a linear pattern vis-a-vis the laser system.

Additionally, in accordance with a preferred embodiment of the present invention, the carriers are coupled to a hollow impression drum used in a printing process.

Still further, in accordance with a preferred embodiment of the present invention, the laser and the imaging system are disposed within the hollow impression drum, and the drum includes at least one elongated imaging slot formed parallel to the drum axis.

Furthermore, in accordance with a preferred embodiment of the present invention, the optical system further includes a high speed polygon spinner having a plurality of facets and a flat field f-theta lens. The laser beam scans the radiation sensitive element in a first direction by the polygon spinner through the f-theta lens on to the deflecting element and the radiation sensitive elements, and the deflecting element is sequentially rotated in a second direction.

Moreover, in accordance with a preferred embodiment of the present invention, the deflecting element is any one of a group including a rotatable mirror, a diffractive element and a prism.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which: Fig. 1 is a schematic cross-section of a digitally operated stencil printing machine constructed and operative according to a preferred embodiment of the present invention; Figs. 2A and 2B are schematic isometric view of the digitally operated stencil printing machine of Fig. 1, illustrating the optical system; Figs. 2C and 2D are schematic illustrations of examples showing the arrangement of exposed surfaces in-line with the deflecting element alongside ; Fig. 3 is a further schematic isometric view of the digitally operated stencil printing machine of Fig. 1; Fig. 4 is a schematic block diagram illustration of the printing machine control system ; Figs. 5A and 5B are flow chart illustrations of the imaging process ; Fig. 6 is a schematic isometric view of the interior of the impression drum used in the digitally operated stencil printing machine of Fig. 1; Figs. 7 and 8 are schematic side views of the interior of the printing drum showing the inking system; and Fig. 9 is a flow chart illustration of the printing process.

DETAILED DESCRIPTION OF THE PRESENT INVENTION Reference is now made to Figs 1-3. Fig. 1 is a schematic cross-section of a compact digitally operated stencil printing machine, generally designated 10, constructed and operative according to a preferred embodiment of the present invention. Figs. 2 and 3 are isometric views of stencil printing machine 10.

Stencil printing machine 10 comprises a plurality of printing drums, generally designated 12, arranged around a common impression drum 14 and a laser imaging system (Fig. 1). As will be described hereinbelow, this arrangement of impression drum 14 and printing drums 12 allows for laser imaging of the stencil masters on press.

The exemplary embodiment illustrated comprises five printing drums which allows for the printing of four color separations (CMYK), a single color on each drum plus a fifth drum which may be used for spot color or lacquering, for example.

It will be appreciated by persons skilled in the art that the present invention is not limited to five printing drums, but may include any number, the maximum number of drums being determined by the overall dimensions and the compactness of the digitally operated stencil printing machine.

The impression drum 14, which is rotatable around its central axial line by a motor 16 (Fig 3), is engageable with printing drums 12 so as to synchronously rotate each of the printing drums 12 around its stationary shaft 24.

A shaft 20 extends through impression drum 14 and is supported at one end thereof by support elements 22. Motor 16 is suitable attached to the supported end of shaft 20 (best seen in Fig 3).

Impression drum 14 comprises a gear ring 18 (best seen in Fig. 2A) attached to the circumference of one drum edge. A second gear ring, generally designated 28 (individually referenced by the printing drum suffix (a, b, etc), is attached to the circumference of each of the printing drums 12. An under-blanket 30 is fitted to the impression drum 14 (best seen in Fig 1). The under-blanket 30, which is composed of a flexible material similar to that commonly used in offset printing, supports the paper 85 (or substrate) for receiving the image.

Five printing drums 12, (individually referenced by a suffix (a, b, c, d and e), that is 12a, 12b, 12c, 12d and 12e) illustrated in the preferred embodiment of

Figs 1-3, are rotatable around their central axis by unit of a stationary shaft 24.

Axle 24 extends through each printing drum 12, which is supported at each end thereof by suitable support elements 22.

In the preferred embodiment machine 10, the nip diameter ratio between the drum 14 and the printing drums 12 are precisely equal to four (4) to ensure good registration. The ratio between gear ring 18 and gear ring 28 is similar (that is 4). However, it will be appreciated that any other absolute ratio may be used.

Stencil printing machine 10 further comprises a plurality of master handling systems, generally designated 25 and a plurality of ultra-violet (UV) light systems, generally designated 27, one for each of the printing drums 12. Each master handling system 25 discharges a blank stencil master 29 (to be imaged) around its corresponding printing drum 12. Stencil master 29 may be fabricated as described in US Application No. 09/177,074, assigned to the Assignees of the present application, or any other suitable fabrication system. Master handling system 25 can discharge the required length of stencil master 29 and wrap it around printing drum 12. The stencil master 29 is generally made from several layers of different substances.

Each master handling system 25 has the following major operation capabilities : a)-Can contain and store a quantity of none imaged stencil masters 29. b)-Discharge a portion of the none-imaged stencil master 29 onto the printing drum 12. c)-Peel a layer of the stencil master. d)-Collect the imaged stencil off the printing drums 12, contain and store a quantity of used imaged stencil master portions and used layers.

There are three operating modes of the stencil printing machine 10 for engagement of the printing drums 12 and impression drum 14. Firstly, impression drum 14 can be fully engaged with printing drums 12 so that the rotation of impression drum 14 synchronously rotates printing drums 12. In this mode, under-blanket 30 is pressed onto the surface of printing drums 12 during printing.

Secondly, in a semi-engaged state, where the teeth of a gear ring 18 and gear rings 28 are semi-engaged allowing printing drums 12 to rotate synchronously with impression drum 14 without the surfaces of the printing drums 12 touching the under-blanket 30.

Thirdly, the two gear rings 18,28 are totally disengaged, allowing the printing drums 12 to rotate independently, each according to its motion control unit 34.

Reference is now briefly made to Fig. 4, which is a schematic block diagram illustration of the operational controller, generally designated 32 of the digitally operated stencil printing machine 10. Each of the printing drums 12 and master handling systems 25 are independently controllable, through its individual motion control unit 34a to 34e. The motion control unit 34 of printing drum 12 and master handling system 25 comprise any suitable system for rotational movement, such as a motor 36 coupled to an reading sensor 35, such as an encoder, for example, and a local electronic control board 40, for example. Each local electronic control board 40 is coupled to a central electronic printing controller 42.

Operational controller 32 comprises a central processing unit (CPU) 44 coupled to an imaging system. Imaging system is also coupled to printing controller 42. Data to be imaged and printed is input via a suitable computer terminal 48 coupled to CPU 44.

Reference is now particularly made to Figs. 2A and 2B. Fig. 2B is a detailed view of the various elements and the optical path of the imaging system.

Imaging system is configured to be located within the hollow inner space of the impression drum 14. The impression drum 14 rotates around the stationary imaging system, which is attached to a solid optical bench 50. The emplacement of the imaging system in the hollow inner space of the impression drum 14 is a space-saving feature which together with the circular configuration of the printing drums 12 allows for a compact digital printing machine 10.

Imaging system is a laser based system, that can obtain the higher resolutions (up to 2500 dpi) associated with conventional quality printing.

The laser imaging system together with the optical system are features of the invention, details of a preferred embodiment of which are described hereinbelow. The main features of the imaging/optical system of the invention can be summarized, as follows : a. The imaging system is a single imaging system, which is used for exposure onto multiple locations ;

b. The imaging system and the exposed elements do not need to be moved using complex mechanical arrangements; c. Optical components are used to deflect the exposing beam onto the different exposure locations ; d. In order to change the exposure location, the orientation and/or location in space of at least one of the optical components ("dynamic component"or"deflecting element") is changed; and e. Optical components may be placed in the optical path before or after the"dynamic component".

Details of a preferred embodiment of the invention are described hereinbelow with respect to Figs. 2A and 2B, but it will be appreciated by persons knowledgeable in the art that numerous variations may be made with respect to the imaging/optical system. For example, the optical. system may be linear scanning or other suitable type and is not restricted to the flat field type.

Furthermore, the dynamic component can be a mirror, a diffractive element, a prism or other suitable deflecting element. The exposed elements can be cylinders or flat surfaces or any other surface.

In addition, the exposed surfaces can be arranged in any of numerous ways around the deflecting element, for example without the necessity of being in a perfect circle. The exposed surfaces can also be arranged in-line with the deflecting element alongside, for example, as shown in the examples of Figs 2C and 2D.

Laser imaging system comprises a flat field scanning system (well known in the art) having, for example, a fiber laser source 47 connected to the fiber end 54, coupled to an optical system. The optical system comprises various optical components, including a focussing lens 58, beam expansion lenses 59 and 61, a piezo mirror 60, folding mirror 62 and an acousto-optic modulator 64 for altering the beam. The beam expansion lenses 59 and 61 are utilized to locate and correct the beam if necessary.

Laser imaging system further comprises a high speed polygon spinner 66 having a plurality of facets 68, a flat field f-theta lens 70 and a rotatable mirror 72.

The optical bench 50 comprises a"L"-shaped angle bar 75 suitably attached to one of the support elements 22, one leg of which extends inwardly to

the hollow space of impression drum 14. The optical path of the laser beam is shown by thick black line 74 (Figs. 1,2A, 2B).

Polygon spinner 66 is pivotally supported about its mid point, referenced 76. In a preferred embodiment, polygon spinner 66 has ten facets 68, but as will be appreciated, the polygon spinner 66 is not limited to a specific number of facets.

Rotatable mirror 72, which is pivotally supported at each end by vertical members 78 suitably connected to optical bench 50, can rotate through a full circle and reflect the laser beam 74 onto the inner circumference of impression drum 14.

The axis 73 of rotatable mirror 72 is configured to align with the axis of impression drum 14. The rotatable mirror 72 is linked to motion control unit 34.

As best seen in Figs. 2B and 3, impression drum 14 has an elongated slot 80 formed therein, the axis of slot 80 being parallel to the longitudinal axis of impression drum 14. Slot 80 enables the laser beam 74 to reach the printing drums 12 arranged around the impression drum 14. The optical path to each of the printing drums 12 is identical. Since the laser moves sideways to scan each printing drum, only one optical system located within the impression drum 14 need be used.

It is a feature of the invention that imaging of the master on press can be obtained by using a single stationary laser system 46 together with a relatively small and easily controlled moving element, rotatable mirror 72. Thus, by arranging each of the plurality of printing drums 12 in a circle around a single impression drum 14, the master 29, wrapped around each printing drum, may be imaged. This arrangement avoids the use of multiple imaging systems or a complex mechanical selection arrangement for imaging.

The process of imaging portions of a blank stencil master 29 around printing drums 12 is now described with respect to Fig. 5, which is a flow chart illustration of the imaging process.

The stencil printing machine 10 is placed in its semi-engaged state (step 202), that is the teeth of a gear ring 18 and gear rings 28 are semi-engaged allowing impression drum 14 to rotate synchronously with printing drums 12. In this state of semi-engagement, the surface of the printing drum 12 is not in

contact with the surface of the impression drum 14. Thus, ink is not transferred to under-blanket 30.

In the event that paper 85 is not fed to the printing drum 12, the semi-engaged mode is automatically activated so that ink does not go on to the drum.

The impression drum 14 is rotated until the slot 80 on its circumference is placed underneath the first color printing drum 12a (step 204). The first color printing drum 12a is then disengaged completely from the impression drum 14 (step 206). The old (imaged) stencil master (if applicable) is unloaded (step 207).

While independently rotating around its axis, first color printing drum 12a is loaded with a blank stencil master 29 (step 208) by the master handling system 25a. At the end of one full rotation, the blank stencil master 29 is smoothly wrapped around the printing drum 12a. This method contrasts with existing thermal printing systems, where the master is first imaged and then placed on the printing drum.

The rotatable mirror 72 is then angled (step 210) so that laser beam 74a (from f-theta lens 70) is deflected by rotatable mirror 72 (beam 74b) through slot 80 of impression drum 14 to strike the blank stencil master 29 wrapped around the first color printing drum 12a. The motion control unit 34a, which controls both the printing drum 12a and the angle of rotatable mirror 72, is actuated to adjust the position of printing drum and rotatable mirror 72 (step 212) to enable the reflected laser beam 74b to strike the blank stencil master 29 at an exact pre-determined starting point (or"home position").

The"home position"point is a permanent reference point on the encoder 38 of each printing drum 12. The"home position"points for each printing drum 12 are calibrated to a registration accuracy of a few microns. This relatively high level of accuracy can be achieved due to the fine resolution of the reading sensor 38. The calibration is preferably carried out when all the gears, that is gears 28 of the printing drums 12 and gear 18 of the impression drum 14, are fully engaged.

As is well known in the art, the gears 28 of each printing drum 12 are assembled in a manner that no interference of the teeth occurs at the"home position"engagement point of the common impression drum 14.

The high speed polygon spinner 66 and the first color printing drum 12a are then actuated (step 214) to rotate synchronously with the modulator 64.

Modulator 64 modulates the laser beam 74 and creates, for example, an ablation imaging process on the surface of blank stencil master 29. Modulation and optical correction are controlled by signals received from the imaging data 46.

At the end of the imaging process, the printing drum 12a is rotated and the UV light from the corresponding UV light system 27 adjacent to the printing drum 12a cures the imaged stencil master 29 (step 216), as described, for example, in US Application No. 09/177,074. The UV light system can be rotated from its normal position facing impression drum 14 to operate on printing drum 12.

At the end of the curing process, the polyester layer of the old imaged stencil layer is unloaded (step 217), The first color printing drum 12a and the common impression drum 14 are semi-engaged (step 218) (through gears 28 and 18, respectively) at the pre-determined starting point ("home position"). The action of re-engagement, which is undertaken by the motion control system 34, described hereinabove, ensures that registration errors are avoided.

All printing drums 12 and impression drum 14 are then rotated around their respective axes until the slot 80 on the circumference of impression drum 14 is directly underneath the second color printing drum 12b (step 220).

Steps 206-220 are then repeated (step 222), that is, the second color printing drum 12b is then disengaged from the impression drum 14 (repeating step 206) and the second blank stencil master 29 is loaded on to second color printing drum 12b (repeating step 208), by the specific master handling system 25b, in a similar manner to that described hereinabove for the first color drum 12a.

At the end of one full rotation, the blank stencil master 29 is smoothly wrapped around the second printing drum 12b (repeat of step 208). Steps 210 and 212 are repeated for second printing drum 12b, that is the rotatable mirror 72 and the second color printing drum 12b are positioned so that the laser beam 74b is directed to the"home position"of second drum 12b. The"home position"of the second printing drum 12b is positioned with great registration accuracy relative to the first color printing drum 12a"home position".

The imaging and curing for the second printing drum 12b is similar to that of the first printing drum 12a, described hereinabove (repeating steps 214-216).

The second color printing drum 12b is then semi-engaged with the common impression drum 14 at the"home position"of the second printing drum 12b (repeating step 218).

The imaging of data of remaining color separations for the printing drums 12c, 12d are carried out in a similar manner (step 224). Thus, at the end of the imaging process, all color separations, for example, Cyan, Magenta, yellow, Black (CMYK) have been imaged in a fully electronic controlled process The stencil master 29 remains clamped on the printing drums 12 unchanged from the start of the imaging process until the end of the printing job.

Referring back to Fig. 1, stencil printing machine 10 further comprises a feeding system, generally designated 82 and a delivery system, generally designated 84 located above the feeding system 82. Both the feeding system 82 and the delivery system 84 are situated on the same side of the printing machine 10. The feeding system 82 comprises a tray 86 and is configured to feed a single sheet of substrate, such as paper 85 at a time from the tray 86 to the impression drum 14.

Stencil printing machine 10 also comprises in it's a preferred exemplary embodiment, four sets of grippers 88 suitably attached and equally spaced around the circumference of impression drum 14. It will be appreciated that any number of sets can be used. In order to assure synchronous motion, the number of sets of grippers 88 should be the same as the nip diameter ratio number between the drum 14 and the printing drums 12. The printing phase of the stencil machine 10 will now be described with reference to Figs. 6,7 and 8.

Fig. 6 is a schematic cut-away isometric view and Figs. 7 and 8 are schematic side views of the interior of printing drums 12, showing the inking system, generally designated 90. Each printing drum 12 has its own individual inking system 90 installed within the hollow printing drum 12.

Each inking system 90 comprises an ink supply system and an ink transfer system. The ink supply system 90 comprises a manifold 92 and ink supply 102. The ink transfer system comprises any of several alternative systems, comprising components such as inner and outer rollers, 94 and 96 respectively and a piston arrangement 98 (Figs 6,7), or a squeegee blade 100 (Fig. 8), The ink supply 102 is connected to the stationary shaft 24 of the printing drum 12. Pump arrangement 107 pumps ink from the ink supply 102 to the

stationary shaft 24. An opening (not visible) is formed approximately in the center of the stationary shaft 24. The manifold 92, which is any suitable manifold arrangement, comprises an inlet 104 connected to the opening in stationary shaft 24 and a plurality of outlets 106 connected to the inlet 104, so that ink (referenced 105) may flow unimpeded from ink supply 102 to outlets 106.

During the printing operation, the ink 105 is transferred from the manifold 92 to the master wrapped around the printing cylinder 12 in one of the several alternative ways, depending on the type of master, ink viscosity and printing drum structure : a. By capillary forces acting on the ink- The tissue of the stencil master tends to'pull'the ink through the fine fibrous structure of the drum; b. By a set of rollers circulating around the inner circumference of the printing drum 12 (Fig. 7); and c. By the action of a squeegee blade 100 receiving ink from the manifold 92 (Fig. 8).

The walls of the printing drums 12 are relatively thin and perforated and are covered with a few layers of fine mesh steel. Thus, the ink 105 can pass through the perforated thin wall of the printing drum 12 to the imaged blank stencil master 29 on to the paper 85.

Fig. 7 illustrates the action of the set of rollers. Inner and outer rollers, 94 and 96 respectively, are pivotable, supported by piston arrangement 98, so as to be able to rotate about their own axes. Piston arrangement 98 is suitably attached to stationary shaft 24. Outer roller 96 is in contact with the inner circumference of the printing drum 12 and inner roller 94 is in spaced-apart contact with outer roller 96. The space between the inner and outer rollers, 94 and 96 is pre-determined to define the correct ink metering. An exemplary distance between rollers is 10p. The contra-rotational action (indicated by arrows 108 and 110) of inner and outer rollers, 94 and 96 respectively, causes the ink 105 to be pressed towards the circumference of the printing drum 12.

The flow of ink is schematically shown in Fig. 7. Ink 105 exiting from the outlet 106 of manifold 92 (arrow 112) is directed between inner and outer rollers, 94 and 96 respectively (referenced 114) onto the inner circumference (referenced 116) of the drum 12.

Fig. 8 illustrates the action of the squeegee blade 100 within the inking system 90. Squeegee blade 100, which is pivotably attached (about axis 120) to a support member 122, which is itself connected to stationary shaft 24, is activated by a mechanical rotation device 124, connected to support member 122.

Squeegee blade 100 is positioned on the inner circumference of drum 12 so that its edge is pressed to the perforated wall of the drum.

The angle a of the squeegee blade 100 is selectable to define the thickness of the ink 105 layer and consequently the ink density on the printed substrate, for example, paper 85. The mechanical rotation device 124 can rotate the squeegee blade 100 around its axis 120 to any desired angle a. During rotation, the ink flows in the area, referenced a, causing hydro-dynamic pressure to develop thus pressing the ink through the drum and assisting the capillary forces. It is a feature of the invention that any of the alternatives of the inking system 90, described hereinabove, makes it possible to print regardless of the influence of gravity. Thus, the printing drum 12 can be located underneath the impression drum 14 (for example, drums 12c and 12d of Fig. 1) and ink 105 can be selectively applied from the ink supply 102 within printing drum 12, through master 29, on to paper 85 which passes between the impression cylinder 14 and the printing drum 12. The paper 85 travels along the outside circumference of impression drum 14 held by gripper 88 (indicated by clockwise arrow 124) to be inked in turn by each of the masters around each of the printing drums 12.

Finally, the paper 85 is collected by delivery system 84.

Reference is now also made to Fig. 9, which is a flow chart illustration of the printing process of a single sheet of paper 85. Printing takes place in the "engaged"mode, that is the common impression drum 14 rotates together with the printing drums 12 (step 302).

The feeding system 82 feeds a sheet of paper 85 (step 304) from its tray 86 and the nearest gripper 88 grasps the paper 85, which is then passed underneath the color printing drum 12a (step 306) which changes it position from semi-engaged mode to fully engaged mode. Once the printing drum is engaged, it remains in this mode as long as there is paper fed to the printing drum. If paper 85 is not fed, the semi-engaged mode is automatically activated and the pumping of the ink ceases so that ink does not go on to the drum.

In the engaged mode, the under-blanket 30, which is stretched on the surface of the common impression drum 14, presses the paper 85 against the printing drum 12d. Ink 105 is injected from the inking system 90 through the stencil master 29 of printing drum 12d on to the paper 85 to create an image (step 308).

After inking, the paper 85 passes underneath the UV light system 27, stationed after each printing drum. The UV light system 27 dries the ink (step 310) before the inked paper encounters the next color printing drum 12.

The paper 85, held by the same set of grippers 88, passes in the same manner underneath all printing drums 12 in turn, thereby creating a full color image on it (step 312). The fact that the paper 85 is held by the same set of grippers 88 during the entire printing process dramatically improves the registration accuracy. This is in contrast to prior art systems, where the paper (or substrate) is transferred from gripper to gripper.

The printing steps 306-312 are repeated for each drum (step 314) in turn (12c, 12b, 12a and 12e).

After the paper 85 leaves the last color printing drum 12e, the gripper 88 holding the paper opens by, for example, an eccentric cam system, known in the art, and the paper 85 is transferred to the delivery system 84 where all printed papers 85 are collected (step 316).

It will be appreciated by persons skilled in the art that the present invention is not limited to stencil printing but is suitable for imaging of any photosensitive material.

It will be further appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the invention is defined by the claims which follow :