NL2010574C2 - SYSTEM FOR APPLYING LIQUID TONER TO AN IMAGE FORMING MEMBER IN A DIGITAL PRINTER. - Google Patents

Description

System for applying liquid toner to an image forming member in a digital printer

The present invention pertains to a system for applying liquid toner to an image forming member in a digital printer. The image forming member is typically a photoconductive roller adapted to sustain a latent image consisting of electrostatic charges, which is developed by proximity with electrically charged toner particles.

In a conventional wet-type developing apparatus, an anilox roller, immersed in a liquid toner tray, provides a constant volume of liquid toner per unit peripheral length to the developing roller. The anilox roller is typically doctored by a blade before contacting the development roller. The development roller brings the liquid toner into contact with a photoconductor containing an electrostatic image in order to develop that image.

Flexography and gravure printing presses use anilox rollers with high screen resolutions in order to create thin ink layers in one pass. This is possible due to the low viscosity of the ink. High viscosity liquid toner (5 - 100 mPa.s) results in a lower volumetric efficiency of the anilox roller, because the required high-volume anilox rollers can only be manufactured with low screen resolutions. This low anilox screen resolution results in a non-uniform toner distribution on developing roller, referred to as an "anilox pattern", which is a limiting factor for printing quality and resolution.

Another non-electric system to provide thin uniform very high viscosity layers is a multiple-roller inking system.

One phenomenon to be dealt with in this context is ribbing.

It is clear that conventional liquid development systems will cause non-uniformities in the toner layer, either by ribbing or by anilox patterns.

It is therefore an object of embodiments of the present invention to provide a system for applying liquid toner to an image forming member in a digital printer, which does not have the aforementioned drawbacks.

According to an aspect of the present invention, there is provided a system for applying liquid toner to an image forming member in a digital printer, the system including a feed roller and a developing roller, the feed roller being arranged to transfer liquid toner from a reservoir to the developing roller, the system comprising pattern deactivating means adapted to substantially eliminate, reduce or prevent a pattern resulting from the transfer of toner from the feed roller to the developing roller.

Embodiments of the present invention enable the creation of a uniform toner layer on the developing roller of 10%-30% solid content and a layer thickness of approximately 10 pm or less. This is realized by smoothening anilox patterns, if existing, or by preventing ribbing patterns.

In an embodiment of the present invention, the feed roller has an anilox surface, and the pattern derives from the surface pattern of the anilox surface. Anilox rollers, typically with a doctor blade, are used to accurately meter the amount of toner supplied to the developing roll, especially high-viscosity toner. However, it has been observed that the pattern of the anilox roller subsists in the successively transferred toner layers, even down to the final printed image. The present invention is therefore particularly useful in situations where Anilox rollers are used.

Anilox patterns can be smoothened by a co-rotating or counter-rotating roller, or a smoothening blade, typically arranged upstream of a point of contact between the developing roller and the imaging roller.

In a particular embodiment, the pattern deactivating means comprises a smoothening roller arranged to run in contact with the developing roller, and a velocity introduction means configured to introduce a velocity gradient in the layer of liquid toner on the developing roller. The velocity introduction means may be configured for moving the smoothening roller in an axial direction back- and forward relative to the developing roller. Alternatively or in addition, the velocity introduction means is configured to introduce a difference in rotation speed and/or direction between the pattern deactivating roller and the developing roller.

This embodiment gives good results with high-viscosity toner, such as toners with a solid content of approximately 15% to 30%, which may initially be present on the developing roller in a layer of approximately 4 to 20 pm. The smoothening roller performs the function of removing the Anilox patterns, by means of its axial displacement and/or peripheral velocity difference. A bias difference is applied between the smoothening roller and the developing roller, to push the toner particles towards the developing roller and/or to further thicken the toner on the developing roller. Any ribbing effects that may occur, will only substantively affect the upper layer, consisting of clear carrier liquid, which does not affect the final image formation.

In a more particular embodiment, the smoothening roller is configured to run in a countersense relative to the developing roll. It is an additional advantage of this embodiment that the occurrence of ribbing in the liquid exiting the contact zone between the smoothening roller and the developing roller is avoided.

In another particular embodiment, the pattern deactivating means comprises a smoothening blade arranged in contact with the developing roll. The blade may be made of a suitable polymer, preferably an elastomer, more preferably polyurethane. The smoothening blade removes the anilox patterns by virtue of its squeezing or crimping action, introducing a velocity gradient across the layer. In an embodiment of the present invention, the feed roller has a metal surface. In such embodiments the ribbing pattern will be at most very weakly present downstream of the nip with the developing roller. This embodiment gives good results with low-viscosity toner, such as toners with a solid content of approximately 1-5%, which may be presented to the developing roller in a layer of approximately 50-400 pm.

According to other embodiments the feed roller is a metal roller, and the pattern deactivating means are adapted to substantially eliminate a ribbing pattern.

In a particular embodiment, the pattern deactivating means comprises a plurality of oil removing rollers, arranged to run in successive contact with the developing roller. This embodiment gives good results with low-viscosity toner, such as toners with a solid content of approximately 5%, which may be present on the developing roller in a layer of approximately 40 pm. Successive passes past an oil removing roller may reduce the layer thickness to approximately 5-10 pm by removal of excess carrier liquid (oil), such that an appropriately concentrated layer is obtained. These successive passes also remove the weak ribbing pattern.

In a preferred embodiment, the pattern deactivating means comprises a plurality of oil removing and thickening rollers, arranged to run in successive contact with the developing roller. Successive passes past an oil removing and thickening roller may reduce the layer thickness to approximately 5-10 pm by removal of excess carrier liquid (oil), whilst at the same time thickening the toner. Any ribbing effects that may occur will only substantially affect the upper layer of carrier liquid without consequences for the final image formation.

In an embodiment, the pattern deactivating means comprises a plurality of oil removing rollers, arranged to successively contact the developing roll in a counter-rotating manner. It is an additional advantage of this embodiment that the occurrence of ribbing in the liquid exiting the contact zone between the oil removing rollers and the developing roller is avoided.

In a more particular embodiment, the developing roller and the feed roller are arranged to present a potential difference having such a polarity and magnitude as to promote electrophoretic transfer of toner particles from the feed roller to the developing roller. This embodiment gives good results with low-viscosity toner, such as toners with a solid content of approximately 2 to 3%, which may be present on the developing roller in a layer of approximately 40 pm.

In a more specific embodiment, the pattern deactivating means comprises a thickening corona arranged to operate on the feed roller upstream from a point of contact with the developing roller.

In another more specific embodiment, the pattern deactivating means further comprises a squeegee arranged to operate on the developing roller upstream from a point of contact with the photoconductor roller. The squeegee is preferably biased with respect to the developing roller, in order to push the charged imaging particles to the developing roller. The speed of the squeegee roller is preferably lower than the speed of the developing roller. Such a set-up allows removing a large portion of the carrier liquid, e.g. more than 80% of the thickness, in order to obtain a thin thickened toner layer.

In an embodiment, the developing roller is a rubber roller, the developing roller and the feed roller being arranged to present a potential difference having such a polarity and magnitude as to promote electrophoretic transfer of toner particles from the feed roller to the developing roller, and the pattern deactivating means comprises a squeegee roller. It is an advantage of this embodiment that the thickening of the toner liquid can be performed in part while it is present on the development roller, which facilitates the pattern elimination at the developing roller to the point where the latter step can be carried out by a squeegee roller .

In a particular embodiment, the developer roller is configured to contact the feed roller in a counter-rotating manner. It is an additional advantage of this embodiment that the occurrence of ribbing in the liquid exiting the contact zone between the feed roller and the developing roller is avoided.

In another particular embodiment, the pattern deactivating means comprises a doctor blade or a flexible pressure blade arranged at a distance of the feed roller, upstream of a point of contact between the feed roller and the developing roller. It is an advantage of this embodiment that the initial solid concentration of the liquid toner can be lower.

In another particular embodiment, the pattern deactivating means comprises a metering roller arranged in contact with the feed roller, upstream of a point of contact between the feed roller and the developing roller. It is an advantage of this embodiment that the initial solid concentration of the liquid toner can be lower.

These and other technical effects and advantages of embodiments of the present invention will be explained in more detail with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates the "ribbing" phenomenon;

Figure 2 illustrates a printing assembly according to a first embodiment of the present invention;

Figure 3 illustrates the evolution of an anilox pattern in the printing assembly according to the first embodiment of the present invention;

Figure 4 illustrates a printing assembly according to a second embodiment of the present invention;

Figure 5 illustrates a printing assembly according to a third embodiment of the present invention;

Figure 6 illustrates a printing assembly according to a fourth embodiment of the present invention;

Figure 7 schematically illustrates the layers of liquid present in various nips in the printing assembly according to the fourth embodiment of the present invention;

Figure 8 illustrates a printing assembly according to a fifth embodiment of the present invention;

Figure 9 schematically illustrates the layers of liquid present in various nips in the printing assembly according to the fifth embodiment of the present invention;

Figure 10 illustrates a printing assembly according to a sixth embodiment of the present invention;

Figure 11 illustrates a printing assembly according to a seventh embodiment of the present invention;

Figure 12 illustrates a printing assembly according to a eighth embodiment of the present invention; and

Figure 13 illustrates a printing assembly according to a ninth embodiment of the present invention.

Throughout this application, the terms "upstream" and "downstream" are used to designate relative positions along the path taken by the liquid toner on the respective surfaces of various rollers. Accordingly, the terms must be understood in the context of the direction of movement of the relevant surfaces: the term "upstream" designates a position traversed earlier than the position referred to and the term "downstream" designates a position traversed later than the position referred to. The skilled person shall understand that these terms don't necessarily correlate with the terms "up" and "down".

Figure 1 schematically illustrates the "ribbing" phenomenon. Liquids with a viscosity above a critical viscosity at a certain process speed will create a banding pattern in the cross-process direction at the nip exit of co-rotating rollers. In the context of the present application the term nip refers to a layer existing between two co-rotating rollers having a length 1 seen in the process direction, perpendicular to the axis of the co-rotating rollers, and a width w seen in the axial direction of the co-rotating roller. The pattern at the nip exit may persist as toner liquid is transferred from one roller to the next, and may eventually be visible in the printed image.

Ribbing is a fluid-mechanical instability created by two counter-acting effects: viscous forces which remove liquid from the nip due to the rollers' rotation and the surface tension of the liquid creating a pressure drop at the nip exit. As such the split point at the nip exit will exhibit a sinusoid path in axial (cross-process) direction.

Likewise, the structural pattern of the anilox roller may persist as toner liquid is transferred from one roller to the next, and may eventually be visible in the printed image. Any additional measures taken to smoothen out the anilox pattern, must be carefully designed to avoid introducing a source of ribbing patterns.

It is an object of embodiments of the present invention to avoid these unwanted effects.

Throughout the figures, the same numerals have been used to designate the same or very similar elements. The invention pertains to a system for applying liquid toner to an image forming member 140 in a digital printer, the system including a feed roller 120 and a developing roller 130, the feed roller 120 being arranged to transfer liquid toner from a reservoir 110 to the developing roller 120, wherein the system further comprises pattern deactivating means adapted to substantially eliminate a pattern resulting from the transfer of toner from the feed roller 120 to the developing roller 130.

In the various embodiments, the different rollers arranged around the developer roller 130 are preferably at a different bias voltage than the developer roller 130, in order to charge and thicken the toner particles, and to avoid picking up toner particles from the developer roller 130.

In all of the various embodiments, a thickening corona 160 upstream of the development-photoconductor nip is optional. This corona 160 charges and thickens the toner before the development nip, but does not change the uniformity of the layer .

Specific embodiments pertain to a system wherein the feed roller 120 has an anilox surface, and wherein the pattern derives from the surface pattern of the anilox surface.

A first embodiment of the present invention will now be described with reference to Figure 2. In this embodiment, the pattern deactivating means comprises a smoothening roller 150 arranged to run in contact with the developing roller 130, wherein the pressure between the smoothening roller 150 and the developing roller 130 is preferably between 2 and 400 N/m and more preferably between 20 and 175 N/m. Typically, the smoothening roller 150 has a rubber coating, manufactured e.g. from an ECO-rubber (epichlorohydrin), PU (polyurethane), or NBR (nitrylbutadine).

The smoothening roller 150 may be arranged to allow an amount of axial displacement relative to said developing roller 130. Alternatively, or in addition the smoothening roller 150 may be configured to rotate at a different speed relative to the developing roller 130.

According to a preferred embodiment there are provided movement means for moving the smoothening roller back and forward in an axial direction, preferably at a speed between 0,01 m/s and 0,36 m/s, more preferably at a speed between 0,03 and 0,12 m/s.

The main function of the smoothening roller 150 is to clear the anilox patterns by introducing velocity gradients in the liquid, without creating ribbing patterns in the exit of the nip .

In an exemplary configuration, the solid content of the liquid toner reservoir 110 is controlled at approximately 25%. The total liquid layer thickness on the developing roller 130 is preferably between 4 pm and 20 pm before the smoothening roller 150, and between 4 pm and 10 pm after the smoothening roller 150.

The surface speed of the various rollers 120, 130, 140, 150 is substantially the same, lies preferably between 0.010 and 2.000 m/s, and is more preferably larger than 0,2 m/s.

The doctor blade 121 opposite to the anilox roller 120 controls the amount of toner transported by the anilox roller 120.

The developer roller 130 preferably comprises a semi-conductive rubber, and runs in contact with the anilox roller 120.

The photoconductor roller 140 runs in contact with the developing roller 130. The pressure between them is typically between 7 N/m and 1200 N/m, preferably between 70 N/m and 900 N/m creating a nip with e.g. a nip length 1 of approximately 5 mm, and a nip width w of approximately 500 mm. For the nip length 1 and width w reference is made to figure 1 and the definition given above. The latent image on the photoconductor roller 140 is developed by the toner applied by the developing roller 130. The background noise is kept to a minimum thanks to the potential difference between the developing roller 130 and the photoconductor roller 140 in the non-image areas, and due to the carrier liquid layer between the toner layer and the photoconductor roller 140. Also, if necessary, the corona 160 may further cause a noise reduction by charging the toner particles to a larger extent.

Figure 3 illustrates the evolution of an anilox pattern in the nip between the developing roller 130 and the smoothening roller 150. Seen in the process direction, the layer will first present an anilox pattern (figure 3A). Due to the velocity introduced by the smoothening roller 150, the pattern will be significantly reduced (figure 3B) .As an additional effect of the smoothening roller 150, which preferably sustains a bias difference relative to the developing roller 130, the liquid layer is thickened (see figure 3C). By way of example, if the total liquid layer thickness on the developing roller 130 is 5 pm, a thickened toner layer of approximately 1.25 pm can be achieved. Any ribbing effects that occur at the exit of the interface between the developer roller 130 and the smoothening roller 150 will occur in the clear top layer, and the effect on the final printed image will be non-existent or significantly reduced.

A second embodiment of the present invention will now be described with reference to Figure 4. In this embodiment, the smoothening roller 150 is configured to run in a countersense relative to the developing roller 130. Apart from the rotational direction, smoothening roller 150 may be identical to the smoothening roller disclosed above in connection with Figure 2. However, the introduction of axial displacement will be less useful compared to the embodiment of Figure 2 and may be omitted.

By changing the rotational direction of the non-feed roller (counter-rotating rollers), the liquid at the nip exit will only be removed by one roller. The split point will be at the interface of the non-feed roller surface and the liquid. As such the surface energy of the non-feed roller will act against the creation of a sinusoid split point path. Due to the countersense rollers the split point in the nip exit between the two rollers will be more defined. Thus the ribbing phenomenon can be reduced or eliminated by using counter-rotating rollers.

As for the embodiment of figure 2, a positive corotron 160 can be used to impart additional electric charge to the toner to obtain an additional thickening of the toner layer on the developing roller 130.

Reference is made to Figures 2 and 3 and their description for a discussion of the interaction of the different rollers and evolution of the anilox pattern in this embodiment.

A third embodiment of the present invention will now be described with reference to Figure 5. In this embodiment, the pattern deactivating means comprises a flexible smoothening blade 131 arranged in contact with said developer roll. The blade is oriented in the "running" direction of the developing roller 130, making a sharp angle with the developer roller at an upstream side of the blade, and is capable of introducing a velocity gradient in the layer of liquid toner.

When replacing the smoothening roller by a blade, the split point is geometrically determined by the blade point. Therefore ribbing can be reduced or eliminated by using, if possible, a blade instead of co-rotating rollers. The blade may be a flexible scraper blade, preferably made of polyurethane or PET.

A positive corotron 160 is used to impart electric charge to the toner to obtain the thickening of the toner layer on the developing roller 130.

Specific embodiments pertain to a system wherein the feed roller 120 has a metal surface, and wherein the pattern may be a weak ribbing pattern.

Embodiments of the present invention are based on the insight of the inventors that ribbing can be prevented by thickening the toner layer before the nip exit of two corotating rollers. It is generally considered impossible to meter a layer with a thickness below 10 pm in one pass without using an anilox surface, because the required manufacturing tolerances would have to be smaller than practically achievable. In embodiments of the present invention, the liquid layer is picked up at a thickness of approximately 400 pm or metered at a thickness of 50-200 pm at low solid content, followed by thickening and oil removal stages to reach the desired layer thickness and solid content.

A fourth embodiment of the present invention will now be described with reference to Figure 6. In this embodiment, the pattern deactivating means comprises a plurality of thickening and oil removing rollers 170, 172, arranged to run in successive contact with the developing roller 130.

The pressure used between the thickening and oil removing rollers 170, 172 and the developing roller 130, and the material used for the coating of the oil removing rollers 170, 172 may be the same as specified above in connection with smoothening roller 150.

The liquid toner layer picked up by the metal feed roller 120 may be approximately 400 pm. In this embodiment, it is transferred to the co-rotating developing roller 130 over a gap of approximately 80 pm, to leave a layer of approximately 40 pm on the latter.

The primary function of the thickening and oil removing rollers 170, 172 is to charge the toner particles and pushing them towards the developer roller 130 to create a clear liquid layer at the top of the liquid layer. Any rivulets appearing at the nip exit exist substantially only in de clear liquid layer, leading to an improved uniformity of the particle distribution, and an improved final image.

The thickening and oil removing rollers 170, 172 may be at an exemplary bias tension of 800 V, where the feed roller 120 and the developing roller 130 are at a bias tension of 500 V, such that a voltage difference of 300V is applied.

The thickening and oil removing rollers 170, 172 are preferably equipped with cleaners or scrapers 173, 174 which allow removal of excess toner from the developing roller 130 - mostly clear liquid - which may be recycled or discarded.

A co-rotating roller at the same peripheral speed of the developer roller will typically remove 50% of the clear liquid layer. Multiple rollers are necessary to remove enough carrier liquid, as is illustrated by the following table .

* not yet thickened; corresponds with the thickness of a maximum thickened toner layer.

By way of example, if the total liquid layer thickness on the developing roller 130 is 40 pm at a solid content of 5%, a thickened toner layer of approximately 1.3 pm can be achieved. After a first thickening/oil removal stage, the total layer thickness is reduced to approximately 20 pm, and the condensed toner layer is reduced to 65% of its earlier value. A second thickening/oil removal stage brings the total layer thickness further down to approximately 10 pm. Any ribbing effects that occur at the exit of the interface between the developer roller 130 and the thickening rollers 170, 172 will occur in the clear top layer, and will not affect the final printed image.

Alternatively, the nip pressure may be increased such that the maximum nip flow rate can be limited to the point where only one co-rotating roller is enough to remove sufficient carrier liquid.

Figure 7 schematically illustrates the layers of liquid present in various nips in the printing assembly according to the fourth embodiment of the present invention.

A fifth embodiment of the present invention will now be described with reference to Figure 8. In this embodiment, the developer roller 130 and the feed roller 120 are arranged to present a potential difference having such a polarity and magnitude as to promote electrophoretic transfer of toner particles from the feed roller 120 to the developer roller 130. There is provided a doctor blade 121 arranged to operate on the feed roller 120 upstream from a point of contact with the developing roller 130. The doctor blade 121 is arranged at a distance of the feed roller 120, e.g. a distance of 60 to 80 pm. This will allow a liquid layer on the feed roller of e.g. 400 pm to be reduced to a liquid layer 60 to 80 pm. The gap between the metal feed roller 120 and the rubber developing roller 130 may be e.g. 80 pm, such a split of the layer at the end of the nip will result in a layer thickness of approximately 40 μη. The rubber developing roller 130 may have an outer layer of a semi-conductive rubber.

The oil removing rollers 170, 172 may be at an exemplary bias tension of 800 V, where the developing roller 130 is at a bias tension of 500 V and the feed roller 120 is at a bias of tension of 1000 V.

The feed roller 120 could also be a rubber roller instead of a metal roller.

The operation of the oil removing rollers 170, 172 is illustrated by the following tables.

A first example is summarized in the table above. By way of the first example, if the total liquid layer thickness on the developing roller 130 is 40 pm at a solid content of 2%, a thickened toner layer of approximately 1.28 pm (60% efficiency) can be achieved. After a first oil removal stage, the total layer thickness is reduced to approximately 20 pm. A second oil removal stage brings the total layer thickness further down to approximately 10 pm. Any ribbing effects that occur at the exit of the interface between the developer roller 130 and the oil removing rollers 170, 172 will occur in the clear top layer, leading to an improved final printed image.

A second example is summarized in the table above. By way of the second example, if the total liquid layer thickness on the developing roller 130 is 30 pm at a solid content of 3%, a thickened toner layer of approximately 1.29 pm (72% efficiency) can be achieved. After a first oil removal stage, the total layer thickness is reduced to approximately 15 pm. A second oil removal stage brings the total layer thickness further down to approximately 7.5 pm. Any ribbing effects that occur at the exit of the interface between the developer roller 130 and the oil removal rollers 170, 172 will occur in the clear top layer, leaving the thickened toner layer substantially unaffected.

The fifth embodiment comprises a thickening corona 127 arranged to operate on the feed roller 120 upstream from a point of contact with said developing roller 130. Further, as explained above, the potential difference between the feed roller 120 to the developer roller 130 promotes electrophoretic transfer of toner particles from the feed roller 120 to the developer roller 130. Thus, the rollers 170, 172 will only have an oil removal function, and the potential difference between the rollers 170, 172 will ensure that the liquid toner remains on the developing roller .

Figure 9 schematically illustrates the layers of liquid present in various nips in the printing assembly according to the fifth embodiment of the present invention, according to the second example given above.

A sixth embodiment of the present invention will now be described with reference to Figure 10. In this embodiment, the pattern deactivating means comprises a plurality of thickening and oil removing rollers 170, 172, arranged to successively contact the developer roll 130 in a counterrotating manner. Scrapers 173, 174 are adjusted accordingly.

The thickening and oil removing rollers 170, 172 may be at an exemplary bias tension of 800 V, where the feed roller 120 and the developing roller 130 are at a bias tension of 500 V.

Reference is made to Figure 7 and its description for a discussion of the evolution of the liquid layer thicknesses in this embodiment.

A seventh embodiment of the present invention will now be described with reference to Figure 11. In this embodiment, the developer roller 130 is a rubber roller, the developer roller 130 and the feed roller 120 being arranged to present an optional potential difference having such a polarity and magnitude as to promote electrophoretic transfer of toner particles from the feed roller 120 to the developer roller 130 (using a small gap), and the pattern deactivating means comprises a squeegee roller 180. The developer roller 130 is configured to contact the feed roller 120 in a counterrotating manner and may rotate at the same speed so that the complete toner flux from the feed roller 120 is transferred to the developer roller 130. In an exemplary embodiment, the following set-up may be used: - a toner liquid in the reservoir having a low solid content value, e.g. between 1% and 5%; - a feed roller speed vl of 1 m/s in order to obtain a layer thickness of 100 μπι on the feed roller 120; - a developer roller speed v2 of 1 m/s, so that a layer thickness of 100 μπι is transferred to the developer roller 130; - a squeegee roller speed v3 (v3 < v2) calculated for obtaining a toner layer between 4 and 10 μπι, e.g. 5 μπι. In case of a solid content of 1,25% in the reservoir, this results in a toner layer of 5 μπι with a solid content of 25%.

Thickening is achieved by the corona 160, upstream from the squeegee roller 180. The main function of the squeegee roller 180 is to remove carrier liquid to increase the toner particle concentration of the toner. The squeegee roller 180 may be equipped with a cleaning scraper 181, allowing the excess carrier liquid to be discarded or recycled.

An eighth embodiment of the present invention will now be described with reference to Figure 12. In this embodiment, the developer roller 130 is a rubber roller, the developer roller 130 and the feed roller 120 being arranged to present a potential difference having such a polarity and magnitude as to promote electrophoretic transfer of toner particles from the feed roller 120 to the developer roller 130, and the ribbing pattern deactivating means comprises a squeegee roller 180. To adjust or control the amount of the liquid toner on the feed roller, a doctor blade 121 is arranged at a distance of the feed metal roller 120, upstream of a point of contact between the feed roller 120 and the developer roller 130.

Thickening is achieved by the corona 127, downstream from the doctor blade 121, and upstream from the nip between the feed roller 120 and the developing roller 130. Further, there may be provided a potential difference between the feed roller main 120 and the developer roller 130, e.g. between 500 V and lkV. Also a potential difference may be applied between the squeegee 180 and the developer roller 130, e.g. between 100 V and 500 V. The main function of the squeegee roller 180 is to remove carrier liquid to increase the toner particle concentration of the toner. The squeegee roller 180 may be equipped with a cleaning scraper 181, allowing the excess carrier liquid to be discarded or recycled. The uniformity of the obtained toner layer will not be distorted because of the thickening by the corona 127 and the fact that the squeegee 180 is biased with respect to the developer roller 130, and pushes the thickened toner layer against the developer roller 130.

The operation of the eight embodiment is illustrated by an example which is summarized in the following table.

By way of the example, if the total liquid layer thickness on the feed roller 120, after doctor blade 121, is 100 pm at a solid content of 1,25%, a thickened toner layer of approximately 1.3 pm can be achieved. During the transfer from the feed roller 120 to the developer roller 130, the total layer thickness is reduced to approximately 50 pm. The squeegee 180 brings the total layer thickness further down to approximately 5 pm. Any ribbing effects that occur at the exit of the interface between the developer roller 130 and the squeegee 180 will occur in the clear top layer, leading to an improved final printed image. The considerations above also apply for toner layers with a thickness larger than 100 μπι immediately after the doctor blade 121. Further the skilled person understands that the solid content may vary, and that the values given above are merely exemplary.

A ninth embodiment of the present invention will now be described with reference to Figure 13. In this embodiment, the developer roller 130 is a rubber roller, the developer roller 130 and the feed roller 120 being arranged to present an optional potential difference having such a polarity and magnitude as to promote electrophoretic transfer of toner particles from the feed roller 120 to the developer roller 130 (small gap), and the pattern deactivating means comprises a squeegee roller 180. A metering roller 126 is arranged in contact with the feed roller 120, upstream of a point of contact between the feed roller 120 and the developer roller 130.

Thickening is achieved by the corona 127, downstream from the metering roller/scraper assembly 126, and upstream from the nip between the feed roller 120 and the developing roller 130. The main function of the squeegee roller 180 is to remove carrier liquid to increase the toner particle concentration of the toner. The squeegee roller 180 may be equipped with a cleaning scraper 181, allowing the excess carrier liquid to be discarded or recycled.

The operation of the embodiment of figure 13 is similar to the operation of the embodiment of figure 12, and reference is made to the description given above for figure 12.

The eighth embodiment and the ninth embodiment have the advantage that no anilox pattern and no ribbing pattern are created on the feed roller. To minimize the effect of gap width errors between feed roller 120 and doctor blade 121 or metering roller 126 a thick low-concentrated layer is preferably created on the feed roller 120 (50 pm - 200 pm). To prepare the layer for development, the layer thickness is reduced below 10 pm without removing particles by a combination of thickening corona 127 and oil removal squeegee roller 180.

While the invention has been described hereinabove with reference to embodiments using positively charged toner particles and electric tensions or fields arranged to act on these positively charged toner particles, in particular to electrophoretically move them, a skilled person will immediately appreciate that the invention equally applies to embodiments using negatively charged toner particles. In the latter case, the polarity of the electric fields acting on the toner particles needs to be reversed, leading to a physically equivalent arrangement with the same technical effects. All voltage ranges mentioned in the present description with respect to embodiments operating with positively charged toner particles are hereby stated to also apply to corresponding embodiments operating with negatively charged toner particles, provided that the sign of the voltage values is changed.

Although the invention has been described hereinabove with reference to specific embodiments, this has been done to illustrate and not to limit the invention, the scope of which is to be determined on the basis of the appended claims. For reasons of clarity, certain features have only been disclosed or described in detail in the context of a particular embodiment.

The skilled person will appreciate that these features can be reused with the same technical effects and advantages in systems according to other embodiments of the present invention, and such other combinations are expressly envisaged.

Claims (19)

A system for introducing liquid toner to an imaging element in a digital printer, which system is a feed roller, a developing roller, and an imaging roller mounted to make rotational contact with the developing roller, which feed roller is adapted to transfer a layer of liquid toner from a vessel to the developing roller, characterized in that the system further comprises cartridge deactivating means adapted to substantially eliminate, reduce or avoid a cartridge resulting from the transfer of toner from the feed roller to the developing roller. The system of claim 1, wherein the pattern deactivation means is configured to apply pressure to a surface of the developing roller upstream of a contact surface between the developing roller and the imaging roller. The system of claim 1 or 2, wherein the pattern deactivation means comprises a pattern deactivation roller mounted to make rotational contact with a layer of liquid toner on the developing roller. A system according to any one of the preceding claims, wherein the feed roll has an anilox surface, and wherein the pattern deactivation means are arranged to substantially eliminate a pattern that results from the surface pattern of the anilox surface. The system of any preceding claim, wherein the pattern deactivation means further comprises a speed inducing means configured to cause a speed gradient in the layer of liquid toner on the developing roll. The system of claims 3 and 5, wherein the pattern deactivation roller acts as a flattening roller (150) and the speed inducing means is configured to move the pattern deactivation roller in an axial direction relative to the developing roller. The system of claims 3 and 5 or claim 6, wherein the pattern deactivation roller acts as a flattening roller, and the speed inducing means is configured to cause a difference in rotation speed and / or direction of rotation between the pattern deactivating roller and the developing roller. The system of any one of claims 5-7, wherein the cartridge deactivation roller is further configured to load the toner particles and / or to push the toner particles toward the developing roller. A system according to any one of the preceding claims, wherein the pattern deactivation means comprises a flattening knife which is arranged in contact with the developing roller and is configured to cause a speed gradient over the liquid toner layer. The system of any one of claims 10 to 12, wherein the feed roll has a flat surface, and wherein the pattern deactivation means are arranged to substantially eliminate a ripple pattern. A system according to any one of the preceding claims, wherein the pattern deactivation means comprises a plurality of deactivation rollers arranged to successively contact a developing roll, each deactivating roll being an oil-removing roll configured to push the toner particles to the developing roll, and to push an upper layer of remove the layer of liquid toner on the developing roller. The system of any preceding claim, wherein the pattern deactivation means comprises a plurality of deactivation rollers arranged to successively contact a developing roll, each deactivation roll being an oil-removing and thickening roll configured to load the toner particles and to send the toner particles to the toner particles. the developing roll, and which is configured to remove an upper layer of the liquid toner layer on the developing roll. A system according to claim 11 or 12, wherein the plurality of oil-removal rollers are arranged to make successive contact with the developing roller in a counter-rotating manner. A system according to any one of the preceding claims, further comprising a corona charger opposite a surface of the developing roller and / or opposite a surface of the feed roller, upstream of a contact surface between the developing roller and the imaging roller. The system of any preceding claim, wherein the developing roller is a rubber roller, and wherein the pattern deactivating means comprises a squeegee roller adapted to act on the developing roller upstream of a contact point with an imaging roller. The system of claim 14 or 15, wherein the developing roller is configured to contact the feed roller in a counter-rotating manner. The system of any one of claims 13 to 16, wherein the developing roller and the feeding roller are arranged to exhibit a potential difference for loading the toner particles and / or for pushing the toner particles to the developing roller. The system of any one of claims 10-17, wherein a thickening corona is provided to act on the feed roll, upstream of the contact point with the developing roll. The system of claim 18, wherein a doctor blade or a flexible printing blade is disposed at a distance from the feed roll, upstream of a contact point between the feed roll and the developing roll; and / or a meter roll mounted in contact with the feed roll, upstream of a point of contact between the feed roll and the developing roll.