Sheet handling apparatus with rotary drum

According to the present invention a sheet handling apparatus is provided which comprises a rotary drum with openings at its peripheral wall. A strip with perforations formed therein spirals circumferentially over an outer surface of the drum in a circumferential spiralling direction, such that a screen is formed over the drum. A suction system controls a flow of air through the perforations thereby to attract sheets towards the drum. The strip is biased by means of a tensioning assembly, which exerts a tensioning force on the strip substantially parallel to the circumferential spiralling direction of the strip.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a drying drum assembly for a sheet handling apparatus and a method for the production thereof.

2. Description of Background Art

In high capacity printing systems (>200 sheets per minute) the proper drying of freshly printed sheets is critical. After leaving the print head the ink on the sheets is wet and able to contaminate parts of the printing system or other sheets by contact. These wet sheets cannot be stacked or flipped for duplex printing, but need to dry first. Therefore, after printing, the sheets are transported towards a sheet handling apparatus with a rotatable drying drum, against the outer surface of which the sheets are temporarily adhered. The drum can is heated e.g. by infrared lights, to speed up the drying process. Since the drum rotates the sheet flow is not interrupted, allowing for a continuous printing process. After drying the sheets disengage from the drum and are transported towards for example a stacking unit or redirected to the print head to be duplex printed on their blank sides.

Such a rotary drum has an outer peripheral wall with openings formed therein. A circumferential screen is provided over the outer surface of the drum. The screen comprises perforations. A suction system controls a flow of air through the openings of the drum and the perforations of the screen to attract sheets towards the peripheral wall of the drum, such that the sheets can be removably fixed to the screen.

Due to the heating applied for drying the sheets, the screen and drum also become heated. Since the drum and screen are often formed of different materials, differences in thermal expansion can result in releasing the screen from the drum. To prevent this release such screens are glued securely to the drum.

Drawback of the above described sheet handling apparatus is that the screen cannot be easily replaced. Additionally, the production of such a sheet handling apparatus is relatively complex.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved drying drum assembly for a sheet handling apparatus, which can be easily assembled and maintained.

The object of the present invention is achieved by a drying drum assembly according to claim1. The drying drum assembly according to the present invention comprises a rotary drum having an outer peripheral wall provided with openings. A strip with perforations formed therein is provided, which strip spirals circumferentially over the outer surface of the peripheral wall of the drum in a circumferential spiralling direction, such that a screen is formed over the peripheral wall of the drum. The openings of the drum and the perforations of the strip are positioned with respect to one another for a fluid connection to one another and for a fluid connection to suction system. This suction system may then control a flow of air through the openings of the drum and the perforations of the screen, such that the sheets may be removably fixed on the screen. The strip is biased by means of a tensioning device, which exerts a tensioning force on the strip substantially parallel to the circumferential spiralling direction of the strip.

To ensure proper drying of the sheets and to simultaneously maintain the high through put speed required for high capacity printing, the drum is preferably sufficiently large to allow the sheets to dry while on the drum. The diameter of the drum may be significantly larger than the sheet length of the sheets drying on it. During operation the drum holds a plurality of sheets, for example more than 5 or preferably more than 10. Sheets may then be transported from the image forming unit to the drum by means of a first transport mechanism. The drum may pick up the sheets from the first transport mechanism, where the sheets are held onto screen via vacuum forces working through the perforations. The sheets may then be carried in a rotational motion preferably over the majority of a single turning of the drum (i.e less than 360°), during which time the sheets may be dried by exposure to radiation heaters. When dried, the sheets may leave the drum to be transported via a second transport mechanism either to a finisher unit or back towards the image forming unit for duplex printing.

The strip revolves around the drum, for example, from one end of the drum to the other end in multiple adjoining loops, preferably covering the majority of the outer surface of the peripheral wall. It is the insight of the inventors that any slack in a strip spiralling over the surface of the drum for forming a screen can be compensated by a tensioning force in the circumferential spiralling direction of the strip. The strip is secured to the drum by the tensioning device, which preferably pulls on the strip in either the clockwise or counter clockwise circumferential spiralling direction. Basically the tensioning device pulls on the strip in the direction wherein the strip is wound around the drum. The tensioning device provides a continuously present force on the strip to drive or pull the strip against the peripheral wall of the drum. When differences in the thermal expansion of the drum and screen occur, the strip is kept pressed against the drum by the tensioning device, effectively preventing a (partial) release of the screen from the drum.

Since the screen according to the present invention need not be permanently adhered to the screen production and maintenance are relatively simple. The step of gluing the screen to the drum as known from the prior art is not required in an assembly according to the present invention. Thereby, a sheet handling apparatus comprising a drying drum assembly according to the present invention is easier to produce. Since the screen need not be permanently fixed to the drum by glue, the screen may easily be replaced by removing the strip present on the drum and wrapping a new strip around the drum. As such the drying drum assembly is easy to maintain, since the screen can be easily replaced, if required. Thereby, the object of the present invention has been achieved.

More specific optional features of the invention are indicated in the dependent claims.

In an exemplary embodiment, the perforations in the screen and the openings of the drum are radially aligned for a fluid connection between one another. The perforations in the screen are then positioned radially outward with respect to the openings of the drum. The perforations for example (partially) overlap the openings when viewed in the radial direction. Thereto, the openings of the drum may be larger in size (e.g. diameter) than the perforations in the screen, such that for example one or more screen perforations may be overlapped or covered by a single drum opening. Air is then able to flow from outside the drum, through the perforations in the screen to and through the openings of the drum towards the suction system. The air may be sucked in via the perforations by means of a suction system.

In a preferred embodiment, the periphery or the inside of the drum comprises one or more hollow chambers or channels connectable to a suction system, such as a pump or fan for creating an underpressure in the chamber. The hollow chamber and thus the pump or fan may be in fluid connection to the outside of the drum via the openings in the peripheral wall of the drum and the perforations in the screen. As such, air can be sucked through the openings into the drum and the perforations in the screen, thereby to attract sheets towards screen. The perforations in the strip are preferably smaller in area or cross-section than the openings in the peripheral wall of the drum. Via the openings in the peripheral wall of the drum and the perforations in the screen in fluid connection therewith sheets can be efficiently held against the drum via suction.

In a preferred embodiment, the drying drum assembly according to the present invention further comprises air channels provided on the peripheral wall of the drum, which air channels are delimited by the screen. Therein, the openings of the drum are formed by said air channels. As such, the peripheral wall of the drum may comprise one or more air channels, which may for example be open in the radially outward direction for forming the openings. Said air channels extend along the inner surface of the screen, preferably in the axial direction. The air channels may extend from one axial end of the drum to the other axial end of the drum. The air channels are arranged for a fluid connection to the suction system. A manifold is preferably provided in or on the drum, for example as a disk-shaped manifold positioned at an axial end of the drum. The manifold thus connects the air channels to the suction system.

In another embodiment, the strip is wrapped around the drum to form a screen preferably over the majority of the peripheral wall of the drum. Preferably, no spacing is present between adjacent loops for forming a relatively smooth surface. In a preferred embodiment, the strip is provided over the air channels, such that the perforations are in fluid connection with the air channels. Air may then pass through the perforations preferably directly into the air channels, which may be connected to the suction system for providing an underpressure in the air channels. Said underpressure in the air channels effectively sucks the air in through the perforations and provides a suction force on the sheets on the screen.

In a preferred embodiment, the tensioning device is positioned at an end of the strip. The tensioning device is arranged to pull on the strip substantially in the direction in which the strip extends around the drum. This allows advantageously for positioning the tensioning device without affecting the strip and thereby interrupting the screen. In one exemplary embodiment, one end of the strip is engaged by a tensioning device, while the other end of the strip is secured to the drum, e.g. by means of a clamp, fastener, or weld. In another example, tensioning devices may be applied to either end of the strip.

In an embodiment the tensioning assembly comprises a tensioning device at each end of the strip. The tensioning devices are oriented in substantially opposite directions to one another in the circumferential spiralling direction of the strip. Basically, one tensioning device pulls on the strip in the substantially clockwise direction, while the other tensioning device at the other end of the strip pulls in the substantially counter clockwise direction. The tensioning devices thus secure and pull on the longitudinal strip, such that their pulling forces are aimed against one another, as seen in the circumferential direction of the drum. Due to the fact that the strip is being pulled at both its ends, the strip is wrapped tightly around the outer surface of the drum. The tensioning devices keep the strip under tension and thus pressed against the drum, even when thermal expansion differences between the drum and strip cause changes in the tension in the strip. For example, when during drying the sheets are heated, the strip and the drum are also heated. The screen formed by the strip might expand more than the drum. To prevent slacking of the strip, the tensioning devices keep the strip under tension, pulling it against the outer surface of the drum. The tensioning devices thereby provide an effective means of holding the screen wrapped around the drum during both heating and cooling.

Preferably the width of the strip is small compared to the diameter of the drum. The strip then revolves a plurality of times around the drum (e.g. 10 revolutions or more). The spiralling direction of the strip substantially corresponds to the circumferential direction of the drum, especially when the strip is narrow. The clockwise and counter clockwise forces working on the strip can as such be defined with respect to the circumferential direction of the drum as well as the circumferential spiralling direction of the strip. It lies within the scope of the present invention to offset the direction of the tensioning forces by a small angle with respect to the circumferential direction of the drum, for example by an offset angle in the range of an angle by which the circumferential spiralling direction of the strip deviates from the circumferential direction of the drum.

In an embodiment the tensioning device comprises a lever pivotably provided on the drum. Preferably the lever is substantially located inside the drum for forming a compact construction and keeping the outer surface of the drum free and/or smooth. The lever is connected to the drum via a spring element. The spring element during operation exerts a force on the lever. The lever transmits this force to the strip, which is connected to the lever. Preferably, the lever comprises a pivoting axis connected to the drum and substantially parallel to a rotation axis of the drum. This construction allows for a compact and durable tensioning assembly.

In an embodiment the spring element during operation is arranged for exerting a continuous pulling force in the circumferential spiralling direction on the end of the strip via the lever. The spring element is biased, such that in the case of thermal expansion differences between the drum and the strip, the spring element is able to supply a tensioning force for holding the strip onto the drum.

In an embodiment a tensioning device is positioned near either end of the drum. The strip is spiralled around the drum, such that one end of the strip is near an edge of a first end of the drum, while the other end of the strip is near an edge of a second end of the drum. Basically the strip spirals from one end of the drum to the other. The tensioning devices engage the ends of the strips near the edges of the drum.

In an embodiment the tensioning device further comprises stop elements adjacent the edges of the screen for limiting the axial movement of the strip over the outer peripheral wall of the drum. As such, any axial movement of the screen over the outer surface of the drum is prevented and the different revolutions of the strip are kept pressed together. This prevents spacing between adjacent edges of subsequent revolutions of the strip. Preferably, the stop elements are positioned near the edges at the ends of the drum.

In an embodiment the stop elements are spaced circumferentially apart from one another. A plurality of stop elements is positioned at a distance from one another along each edge of the drum adjacent the edges of the screen. The stop elements are connected to the drum and in contact with the edges of the screen. Preferably the stop elements are adjustable, so that variations in the width of the screen and/or the strip can be overcome. The stop elements effectively limit the movement of the strip to substantially the circumferential (spiralling) direction. Axial “wandering” of the strip is thereby effectively prevented.

In an embodiment, the strip and drum are formed of materials having different thermal expansion coefficients. A smooth perforated outer surface can be easily and cheaply formed by the strip, whereas the drum can be produced by a different method. This eases the production of a drying drum assembly according to the present invention.

In another aspect, the invention provides a sheet handling apparatus which comprises a drying drum assembly according to the present invention, and a suction system for controlling a flow of air through the openings of the drum and the perforations of the strip, thereby to attract sheets towards the peripheral wall of the drum, such that the sheets are removably fixed on the screen. The suction system may be in fluid connection with the openings of the drum and strip to apply a suction force to the sheets on the drum.

In another aspect, the invention provides a sheet handling apparatus which comprises drying drum assembly according to the present invention and a heating system for heating the sheets on the drum. The heating system accelerates the drying of the sheets, allowing for example the drum dimensions to be reduced. Preferably the heating system comprises radiation heaters positioned substantially circumferentially around the drum to allow for contactless heating of the sheets.

In an embodiment the strip and the outer surface of the drum are arranged for a free, preferably substantially frictionless, sliding motion of the strip over de outer surface of the drum. Low friction allows the strip to slide over the drum and prevent an uneven distribution of the tensioning forces throughout the strip. Substantially frictionless contact ensures a proper holding of the screen against the drum. Thereto the outer surface of the drum and the strip are preferably smooth. Preferably the outer surface of the drum and/or a surface of the strip have been treated to minimize friction between the outer surface of the drum and the strip, preferably by means of polishing, sanding, and/or anodizing.

The present invention further relates to a printing system comprising a sheet handling apparatus according to the present invention.

The present invention further relates to a method for producing a drying drum assembly for a sheet handling apparatus according to the present invention, the method comprising the steps of:attaching a first end of a longitudinal strip formed of a first material to a first tensioning device, preferably at an outer surface of a drum, formed of a second material,wrapping the strip around the outer surface of the drum in a pattern spiralling over the outer surface of the drum, such that a screen is formed over at least part of the outer surface of the drum,attaching a second end of a longitudinal strip to a second tensioning device, preferably at the outer surface of the drum, such that the strip is biased in the circumferential spiralling direction of the strip by means of the tensioning devices.

Preferably the first material is different from the second material, specifically the first and second materials possess different thermal expansion characteristics. The drum can be formed of a metal, such aluminium, whereas the strip and thus the screen may be formed of a different metal, such as (anodized) steel. Additionally the method can include the step of pressing the first revolution of the strip against stop elements provided near one end of the drum, and the step of using adjustable stop elements on the other end of the drum for pressing and holding the revolutions of the strip wrapped around the drum together.

The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate particular embodiments of the invention and together with the description serve to explain the principles of the invention. Other embodiments of the invention and many of the attendant advantages of the invention will be readily appreciated as they become better understood with reference to the following detailed description.

It will be appreciated that common and/or well understood elements that may be useful or necessary in a commercially feasible embodiment are not necessarily depicted in order to facilitate a more abstracted view of the embodiments. The elements of the drawings are not necessarily illustrated to scale relative to each other. It will further be appreciated that certain actions and/or steps in an embodiment of a method may be described or depicted in a particular order of occurrences while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used in the present specification have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study, except where specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIG. 1of the drawings, a portion of an inkjet printing system1according to a preferred embodiment of the invention is shown.FIG. 1illustrates in particular the following parts or steps of the printing process in the inkjet printing system1: media pre-treatment, image formation, drying and fixing and optionally post treatment. Each of these will be discussed briefly below.

FIG. 1shows that a sheet S of a receiving medium or print medium, in particular a machine-coated print medium, is transported or conveyed along a transport path P of the system1with the aid of transport mechanism2in a direction indicated by arrows P. The transport mechanism2may comprise a driven belt system having one or more endless belt3. Alternatively, the belt(s)3may be exchanged for one or more drums. The transport mechanism2may be suitably configured depending on the requirements of the sheet transport in each step of the printing process (e.g. sheet registration accuracy) and may hence comprise multiple driven belts3,3′ and/or multiple drums. For a proper conveyance of the sheets S of the receiving medium or print medium, the sheets S should be fixed to or held by the transport mechanism2. The manner of such fixation is not limited and may, for example, be selected from the group: electrostatic fixation, mechanical fixation (e.g. clamping) and vacuum fixation, of which vacuum fixation is particularly preferred.

To improve spreading and pinning (i.e. fixation of pigments and water-dispersed polymer particles) of the ink on the print medium, in particular on slow absorbing media, such as machine-coated media, the print medium may be pre-treated, i.e. treated prior to the printing of an image on the medium. The pre-treatment step may comprise one or more of the following:pre-heating of the print medium to enhance spreading of the ink used on the print medium and/or to enhance absorption into the print medium of the ink used;(ii) primer pre-treatment for increasing the surface tension of print medium in order to improve the wettability of the print medium by the ink used and to control the stability of the dispersed solid fraction of the ink composition, i.e. pigments and dispersed polymer particles; (N.B. primer pre-treatment can be performed in a gas phase, e.g. with gaseous acids such as hydrochloric acid, sulphuric acid, acetic acid, phosphoric acid and lactic acid, or in a liquid phase by coating the print medium with a pre-treatment liquid. A pre-treatment liquid may include water as a solvent, one or more co-solvents, additives such as surfactants, and at least one compound selected from a polyvalent metal salt, an acid and a cationic resin); and(iii) corona or plasma treatment.

FIG. 1illustrates that the sheet S of print medium may be conveyed to and passed through a first pre-treatment module4, which module may comprise a preheater, (e.g. a radiation heater), a corona/plasma treatment unit, a gaseous acid treatment unit or a combination of any of these. Subsequently, a predetermined quantity of the pre-treatment liquid may optionally be applied on a surface of the print medium via a pre-treatment liquid applying device5. Specifically, the pre-treatment liquid is provided from a storage tank6to the pre-treatment liquid applying device5, which comprises double rollers7,7′. A surface of the double rollers7,7′ may be covered with a porous material, such as sponge. After providing the pre-treatment liquid to auxiliary roller7′ first, the pre-treatment liquid is transferred to main roller7, and a predetermined quantity is applied onto the surface of the print medium. Thereafter, the coated printing medium (e.g. paper) onto which the pre-treatment liquid was applied may optionally be heated and dried by a dryer device8, which comprises a dryer heater installed at a position downstream of the pre-treatment liquid applying device5in order to reduce the quantity of water content in the pre-treatment liquid to a predetermined range. It is preferable to decrease the water content in an amount of 1.0 weight % to 30 weight % based on the total water content in the pre-treatment liquid provided on the print medium sheet S. To prevent the transport mechanism2from being contaminated with pre-treatment liquid, a cleaning unit (not shown) may be installed and/or the transport mechanism2may include a plurality of belts or drums3,3′, as noted above. The latter measure avoids or prevents contamination of other parts of the printing system1, particularly of the transport mechanism2in the printing region.

It will be appreciated that any conventionally known methods can be used to apply the pre-treatment liquid. Specific examples of an application technique include: roller coating (as shown), ink-jet application, curtain coating and spray coating. There is no specific restriction in the number of times the pre-treatment liquid may be applied. It may be applied just one time, or it may be applied two times or more. An application twice or more may be preferable, as cockling of the coated print medium can be prevented and the film formed by the surface pre-treatment liquid will produce a uniform dry surface with no wrinkles after application twice or more. A coating device5that employs one or more rollers7,7′ is desirable because this technique does not need to take ejection properties into consideration and it can apply the pre-treatment liquid homogeneously to a print medium. In addition, the amount of the pre-treatment liquid applied with a roller or with other means can be suitably adjusted by controlling one or more of: the physical properties of the pre-treatment liquid, the contact pressure of the roller, and the rotational speed of the roller in the coating device. An application area of the pre-treatment liquid may be only that portion of the sheet S to be printed, or an entire surface of a print portion and/or a non-print portion. However, when the pre-treatment liquid is applied only to a print portion, unevenness may occur between the application area and a non-application area caused by swelling of cellulose contained in coated printing paper with water from the pre-treatment liquid followed by drying. From a view-point of uniform drying, it is thus preferable to apply a pre-treatment liquid to the entire surface of a coated printing paper, and roller coating can be preferably used as a coating method to the whole surface. The pre-treatment liquid may be an aqueous liquid.

Corona or plasma treatment may be used as a pre-treatment step by exposing a sheet of a print medium to corona discharge or plasma treatment. In particular, when used on media such as polyethylene (PE) films, polypropylene (PP) films, polyethylene terephthalate (PET) films and machine coated media, the adhesion and spreading of the ink can be improved by increasing the surface energy of the medium. With machine-coated media, the absorption of water can be promoted which may induce faster fixation of the image and less puddling on the print medium. Surface properties of the print medium may be tuned by using different gases or gas mixtures as medium in the corona or plasma treatment. Examples of such gases include: air, oxygen, nitrogen, carbon dioxide, methane, fluorine gas, argon, neon, and mixtures thereof. Corona treatment in air is most preferred.

Image Formation

When employing an inkjet printer loaded with inkjet inks, the image formation is typically performed in a manner whereby ink droplets are ejected from inkjet heads onto a print medium based on digital signals. Although both single-pass inkjet printing and multi-pass (i.e. scanning) inkjet printing may be used for image formation, single-pass inkjet printing is preferable as it is effective to perform high-speed printing. Single-pass inkjet printing is an inkjet printing method with which ink droplets are deposited onto the print medium to form all pixels of the image in a single passage of the print medium through the image forming device, i.e. beneath an inkjet marking module.

Referring toFIG. 1, after pre-treatment, the sheet S of print medium is conveyed on the transport belt3to an image forming device or inkjet marking module9, where image formation is carried out by ejecting ink from inkjet marking device91,92,93,94arranged so that a whole width of the sheet S is covered. That is, the image forming device9comprises an inkjet marking module having four inkjet marking devices91,92,93,94, each being configured and arranged to eject an ink of a different colour (e.g. Cyan, Magenta, Yellow and Black). Such an inkjet marking device91,92,93,94for use in single-pass inkjet printing typically has a length corresponding to at least a width of a desired printing range R (i.e. indicated by the double-headed arrow on sheet S), with the printing range R being perpendicular to the media transport direction along the transport path P.

Each inkjet marking device91,92,93,94may have a single print head having a length corresponding to the desired printing range R. Alternatively, as shown inFIG. 2, the inkjet marking device91may be constructed by combining two or more inkjet heads or printing heads101-107, such that a combined length of individual inkjet heads covers the entire width of the printing range R. Such a construction of the inkjet marking device91is termed a page wide array (PWA) of print heads. As shown inFIG. 2, the inkjet marking device91(and the others92,93,94may be identical) comprises seven individual inkjet heads101-107arranged in two parallel rows, with a first row having four inkjet heads101-104and a second row having three inkjet heads105-107arranged in a staggered configuration with respect to the inkjet heads101-104of the first row. The staggered arrangement provides a page-wide array of inkjet nozzles90, which nozzles are substantially equidistant in the length direction of the inkjet marking device91. The staggered configuration may also provide a redundancy of nozzles in an area O where the inkjet heads of the first row and the second row overlap. (See inFIG. 3A). The staggering of the nozzles90may further be used to decrease an effective nozzle pitch d (and hence to increase print resolution) in the length direction of the inkjet marking device91. In particular, the inkjet heads are arranged such that positions of the nozzles90of the inkjet heads105-107in the second row are shifted in the length direction of the inkjet marking device91by half the nozzle pitch d, the nozzle pitch d being the distance between adjacent nozzles90in an inkjet head101-107. (SeeFIG. 3B, which shows a detailed view of80inFIG. 3A). The nozzle pitch d of each head is, for example, about 360 dpi, where “dpi” indicates a number of dots per 2.54 cm (i.e. dots per inch). The resolution may be further increased by using more rows of inkjet heads, each of which are arranged such that the positions of the nozzles of each row are shifted in the length direction with respect to the positions of the nozzles of all other rows.

In the process of image formation by ejecting ink, an inkjet head or a printing head employed may be an on-demand type or a continuous type inkjet head. As an ink ejection system, an electrical-mechanical conversion system (e.g. a single-cavity type, a double-cavity type, a bender type, a piston type, a shear mode type, or a shared wall type) or an electrical-thermal conversion system (e.g. a thermal inkjet type, or a Bubble Jet® type) may be employed. Among them, it is preferable to use a piezo type inkjet recording head which has nozzles of a diameter of 30 μm or less in the current image forming method.

The image formation via the inkjet marking module9may optionally be carried out while the sheet S of print medium is temperature controlled. For this purpose, a temperature control device10may be arranged to control the temperature of the surface of the transport mechanism2(e.g. belt or drum3) below the inkjet marking module9. The temperature control device10may be used to control the surface temperature of the sheet S within a predetermined range, for example in the range of 30° C. to 60° C. The temperature control device10may comprise one or more heaters, e.g. radiation heaters, and/or a cooling means, for example a cold blast, in order to control and maintain the surface temperature of the print medium within the desired range. During and/or after printing, the print medium is conveyed or transported downstream through the inkjet marking module9.

Post Treatment

To improve or enhance the robustness of a printed image or other properties, such as gloss level, the sheet S may be post treated, which is an optional step in the printing process. For example, in a preferred embodiment, the printed sheets S may be post-treated by laminating the print image. That is, the post-treatment may include a step of applying (e.g. by jetting) a post-treatment liquid onto a surface of the coating layer, onto which the ink has been applied, so as to form a transparent protective layer over the printed recording medium. In the post-treatment step, the post-treatment liquid may be applied over the entire surface of an image on the print medium or it may be applied only to specific portions of the surface of an image. The method of applying the post-treatment liquid is not particularly limited, and may be selected from various methods depending on the type of the post-treatment liquid. However, the same method as used in coating the pre-treatment liquid or an inkjet printing method is preferable. Of these, an inkjet printing method is particularly preferable in view of: (i) avoiding contact between the printed image and the post-treatment liquid applicator; (ii) the construction of an inkjet recording apparatus used; and (iii) the storage stability of the post-treatment liquid. In the post-treatment step, a post-treatment liquid containing a transparent resin may be applied on the surface of a formed image so that a dry adhesion amount of the post-treatment liquid is 0.5 g/m2to 10 g/m2, preferably 2 g/m2to 8 g/m2, thereby to form a protective layer on the recording medium. If the dry adhesion amount is less than 0.5 g/m2, little or no improvement in image quality (image density, colour saturation, glossiness and fixability) may be obtained. If the dry adhesion amount is greater than 10 g/m2, on the other hand, this can be disadvantageous from the view-point of cost efficiency, because the dryness of the protective layer degrades and the effect of improving the image quality is saturated.

As a post-treatment liquid, an aqueous solution comprising components capable of forming a transparent protective layer over the print medium sheet S (e.g. a water-dispersible resin, a surfactant, water, and other additives as required) is preferably used. The water-dispersible resin in the post-treatment liquid preferably has a glass transition temperature (Tg) of −30° C. or higher, and more preferably in the range of −20° C. to 100° C. The minimum film forming temperature (MFT) of the water-dispersible resin is preferably 50° C. or lower, and more preferably 35° C. or lower. The water-dispersible resin is preferably radiation curable to improve the glossiness and fixability of the image. As the water-dispersible resin, for example, any one or more of an acrylic resin, a styrene-acrylic resin, a urethane resin, an acryl-silicone resin, a fluorine resin or the like, is preferably employed. The water-dispersible resin can be suitably selected from the same materials as that used for the inkjet ink. The amount of the water-dispersible resin contained, as a solid content, in the protective layer is preferably 1% by mass to 50% by mass. The surfactant used in the post-treatment liquid is not particularly limited and may be suitably selected from those used in the inkjet ink. Examples of the other components of the post-treatment liquid include antifungal agents, antifoaming agents, and pH adjustors.

Hitherto, the printing process was described such that the image formation step was performed in-line with the pre-treatment step (e.g. application of an (aqueous) pre-treatment liquid) and a drying and fixing step, all performed by the same apparatus, as shown inFIG. 1. However, the printing system1and the associated printing process are not restricted to the above-mentioned embodiment. A system and method are also contemplated in which two or more separate machines are interconnected through a transport mechanism2, such as a belt conveyor3, drum conveyor or a roller, and the step of applying a pre-treatment liquid, the (optional) step of drying a coating solution, the step of ejecting an inkjet ink to form an image and the step or drying an fixing the printed image are performed separately. Nevertheless, it is still preferable to carry out the image formation with the above defined in-line image forming method and printing system1.

With reference now toFIG. 4of the drawings, the inkjet printing system1according to the preferred embodiment of the invention is shown to include an apparatus20for detecting defects in the printing system1, and particularly for identifying and for classifying deformations D in the sheets S of print medium when the sheets S are on the transport path P of the printing system1. In this particular embodiment, the apparatus20comprises a sensing unit21, which processes the sheets S on the transport path P before those sheets S enter the image forming device9. In this regard, it will be noted that the printing system1inFIG. 4has a transport path P which includes both a simplex path PSand a duplex path PDand the sensing unit21of the apparatus20is arranged such that sheets S input on the simplex path PSand also returning on the duplex path PDall pass via the sensing unit21.

At least one first sensor device22in the form of an optical sensor, such as a laser scanner, is provided within the sensing unit21for sensing the surface geometry or topology of the sheets S as they travel on a first pass or a second pass along the transport path P. The laser scanner or optical sensor device22generates digital image data I of the three-dimensional surface geometry or topology of each sheet S sensed or scanned. When performing the sensing or measuring of the surface geometry or topology of the sheets S on the transport path P of printing system1with the first sensor device(s)22, it is highly desirable for the purposes of accuracy and reliability that the sheets S are transported or conveyed in the sensing unit21in substantially the same manner as those sheets S are later transported in the image forming unit or marking module9. To this end, the sensing unit21includes a sheet conveyor mechanism23that simulates the sheet transport conditions provided by the transport mechanism3′ within the image forming unit9. In this regard, both the conveyor mechanism23and the transport mechanism3′ include a belt transport device with vacuum sheet-holding pressure, as seen inFIG. 4.

The sheet topology data from the first sensor device22is then transmitted (e.g. either via a cable connection or wirelessly) to a controller24which includes a processor device25for processing and analysing the digital image data I to detect and to classify any defect or deformation D in the surface geometry or topology of each sheet S sensed or scanned. The sensing unit21is thus arranged to scan the sheets S for detecting and measuring any deformations or defects D before the sheets S enter the image forming device or inkjet marking module9. In this way, if the processor device25determines that a sheet S on the transport path P includes a defect or deformation D that would render the sheet unsuitable for printing, the controller24is configured to prevent the sheet S from progressing to the inkjet marking module9. The sensing unit21comprising the first sensor device(s)22is therefore desirably provided as a separate sentry unit positioned on the transport path P sufficiently upstream of the marking module9. The controller24and processor device25may be integrated within the sentry unit21or they may be separately or remotely located.

Drying and Fixing

After an image has been formed on the print medium, the printed ink must be dried and the image must be fixed on the print medium. Drying comprises evaporation of solvents, and particularly those solvents that have poor absorption characteristics with respect to the selected print medium.

FIG. 1of the drawings schematically shows a drying and fixing unit11, which may comprise one or more heater, for example a radiation heater. After an image has been formed on the print medium sheet S, the sheet S is conveyed to and passed through the drying and fixing unit11. The ink on the sheet S is heated such that any solvent present in the printed image (e.g. to a large extent water) evaporates. The speed of evaporation, and hence the speed of drying, may be enhanced by increasing the air refresh rate in the drying and fixing unit11. Simultaneously, film formation of the ink occurs, because the prints are heated to a temperature above the minimum film formation temperature (MFT). The residence time of the sheet S in the drying and fixing unit11and the temperature at which the drying and fixing unit11operates are optimized, such that when the sheet S leaves the drying and fixing unit11a dry and robust image has been obtained. As described above, the transport mechanism2in the fixing and drying unit11may be separate from the transport mechanism2of the pre-treatment and printing parts or sections of the printing system1.

The drying and fixing unit11comprises a sheet handling apparatus30, schematically shown inFIG. 5a-c.The sheet handling apparatus30comprises a drying drum assembly100. The drying drum assembly comprises a drying drum31. Openings formed by air channels (60inFIG. 8) are provided in the outer surface32of the drum31, such that air is able to pass through the peripheral wall of the drum31. A strip35is wrapped around the drum31in a spiralling pattern. The strip35runs over the air channels in the outer surface32in a circumferential spiralling direction D. The spiralling direction D comprises a circumferential component and a, preferably small, axial component with respect to the drum31. When the axial component is very small, for example when a narrow strip35is used, the spiralling direction D approximates the circumferential direction of the drum31. One end35aof the strip35is positioned near the inFIG. 5aleft end of the drum31. The strip then revolves in adjoining loops around the outer surface32of the drum31to the inFIG. 5aright end of the drum31. As such, the adjoining loops of the strip35form a continuous surface sheet36covering the majority of the outer surface32of the drum31. As such the revolved strip31forms the screen36.

Perforations (not shown) are present in the strip35to allow air to be sucked through the strip35and the outer surface32of the drum31. Air channels60are formed in the peripheral wall of the drum31. The air flow and the suction force through the in the strip35and the openings of the air channels60into said air channels60of the drum31are controlled via a suction system (not shown). Sheets present on the screen36are held onto the screen36via suction. Basically an underpressure in the air channels below the screen results in a vacuum force which pulls the cut sheet media onto the screen36, such that the sheets follow the circumference of the drum31. The underpressure and/or vacuum force is adjustable to allow a user to set the forces attracting the sheets towards the peripheral wall of the drum31, for example for different media types. By removing or reducing the underpressure or vacuum force along a predefined angular of the drum's circumference the sheets can be released from the screen36.

The tensioning assembly40,41exerts a tensioning force Fa, Fbon the strip35. As such the strip35is biased in the spiralling direction D. The tensioning forces Fa, Fbon the strip35run substantially parallel to the circumferential spiralling direction D of the strip35. InFIG. 5athe tensioning assembly40,41comprises a tensioning device40,41at either end35a,35bof the strip35. Each tensioning device40,41secures a respective end35a,35bof the strip35. Additionally, the tensioning devices40,41exert a tensioning force Fa, Fbon the ends35a,35b,which force Fa, Fbpulls on the free ends35a,35bof the strip35. Thereby the strip35is pulled taut around the drum31. Since the tensioning devices40,41are oriented in substantially opposite directions to one another in the circumferential spiralling direction D of the strip35, the tensioning forces Fa, Fbpull the strip35against the outer surface32of the drum31. Thus, by pulling on the ends35a,35bof the strip35the tensioning devices40,41fix the screen in a tight fit around the outer surface32of the peripheral wall of the drum31.

Alternatively a single tensioning device40,41can be provided at one end35a,35bof the strip35, while the other end35a,35bof the strip35is fixed to the drum31in a rigid manner. Basically the strip35is fixed to the drum31at one end35a,35b,while the single tensioning device40,41pulls on the other end35a,35b.

FIG. 5b-cschematically illustrate the workings of the sheet handling apparatus30with the drying drum assembly100according to the present invention. During normal operation the sheet35and in consequence the screen36are wrapped tightly around the outer surface32of the peripheral wall of the drum31. During drying the drum31and the screen36become heated and will therefore expand. InFIG. 5ba situation is shown wherein the screen36has expanded more than the drum31, resulting in the screen36being released from the drum31. However, as shown inFIG. 5c, the tensioning device40compensates for this thermal expansion difference by pulling on the strip35in the direction of the tensioning force Fa. In this manner, the thermal expansion differences are overcome yielding a durable yet easily replaceable fixation of the screen36onto the drum31.

FIG. 6shows a perspective view of an embodiment of a sheet handling apparatus30with a drying drum assembly100according to the present invention. The drum31is preferably formed of a metal, such as aluminium. The outer surface31of the peripheral wall of the drum31is provided with openings through which air is able to flow. Similarly, the strip35has been provided with small perforations for allowing air to pass through them. Via these perforations sheets can be temporarily fixed onto the outer surface of the screen36via a vacuum force.

The strip35inFIG. 6is preferably formed of a metal, such as steel. This metal has preferably been treated by sanding, grinding and/or anodizing to provide a smooth surface. Friction between the strip35and the outer surface32of the drum31is minimized to facilitate an even distribution of the biasing forces through the strip35. The strip35being formed of a different material than the drum31is an underlying cause of the difference in thermal expansion between the screen36and the drum31. Alternatively, a temperature difference between the screen36and the drum31can contribute to the differences in thermal expansion between the drum31and the sheet36.

After attaching an end of the strip35to a tensioning device40, the strip35inFIG. 6has been spiralled around the drum31, such that the edges of a first loop of the strip35are in contact with a proceeding and/or following loop of the strip35. This results in a relatively smooth surface of the screen36and a complete coverage of the respective part of the outer surface32of the drum31. The other end35bof the strip is then attached to a second tensioning device41, such that the strip36is pulled taut around the drum31by the tensioning devices40,41.

The tensioning device40inFIG. 6extends from inside the drum31towards the outer surface32of the drum31where it40engages the strip35.FIGS. 7 and 8show a more detailed illustration of the tensioning device40.

The tensioning device40inFIG. 7-8comprises a lever44extending from inside the drum31to the outer surface32. At the outer surface32the lever44is attached to an end35aof the strip35, via fixation means46. InFIG. 7-8the end35aof the strip35is clamped onto the lever44by means of a screw46. Alternatively, clamps or other releasable holding means can be used. Inside the drum31the lever44is provided on a pivoting axis45. This pivoting axis45is connected to the drum31and runs substantially parallel to the rotation axis of the drum31. As such the lever44is able to pivot with respect to the drum31.

While one end of the lever44is connected to the strip35, the other end of the lever44is attached to a spring element43. The spring element43is connected to an adjacent end of the lever to the drum31. The spring element43is able to exert a spring force on the lever44, which spring force is transferred to the strip35. By biasing the spring element43the strip35experiences a continuous tensioning force in the circumferential spiralling direction D. In the embodiment inFIG. 8the spring force is increased when the end35amoves counter clockwise. When slacking of the strip35due to thermal expansion differences between the strip35and the drum31as inFIG. 5boccur, the end35ais moved clockwise by the spring force on the lever44. The strip35is then pulled taut against the outer surface32. When the drum assembly100in the apparatus30cools down, the lever44returns to its initial position. The spring element43can be a spring, leaf spring, or resilient pad. Alternatively, an actuator is provided instead of the spring element43for exerting a force on the lever44.

InFIG. 7the lever44is substantially L-shaped to provide a compact tensioning device40. The spring element43extends substantially in the radial direction and is attached to a first leg of the L-shaped lever44. This first leg runs locally substantially parallel to the circumferential direction of the drum. The second leg of the L-shaped lever44extends substantially in the radial direction of the drum31and comprises the pivoting axis45and the fixation means46.

FIG. 8further illustrates the air channels60extending axially (directly) below the screen35. The radially outward end or side face (top side inFIG. 8) of a channel60is open. The openings of the air channels60form the openings of the drum31. The screen or strip35is provided over the air channels60, such that the perforations in the screen35are in fluid communication with the air channels60. The air channels60in turn are connected to the suction system (not shown) for sucking in air through the perforations and channels. A perforation in the screen35may be positioned directly on a channel60(i.e. on the opening of the air channel60), when viewed in the radial direction. As such, the perforations and an opening of the drum overlap. It will be appreciated that an intermediate chamber may be provided between one or more perforations in the screen and one or more openings of the air channels60in the drum31to form a fluid connection between the one or more perforations in the screen35and the one or more opening of the air channels60of the drum31. The air channels60effectively form an air passage or conduit system which extends from a perforation of the screen35to the suction system. In an alternative and basic embodiment, the drum31with its openings may be formed by a cylinder with perforations in its sidewall.

FIGS. 9 and 10illustrate stop elements50which lie against the outer edges of the screen36. The stop elements extend from the outer surface32of the drum31and confine the movement of the strip35to the circumferential spiralling direction D. This effectively prevents the screen36from “wandering” in the axial direction of the drum31and keeps the different loops of the strip35pressed together. As such a relatively smooth surface of the screen36during operation is ensured. The stop elements50can be fixed, as inFIG. 9or adjustable as inFIG. 10. InFIG. 10the stop element50is able to pivot around a axis substantially radial with respect to the drum and can be secured at any desired pivoting angle. Preferably the drum31comprises fixed stop elements50near the edge of one end of the drum31, while the stop elements50at the other edge of the drum31are adjustable. The screen36extends between the stop elements50at either end of the drum31. After wrapping the strip35around the drum31the adjustable stop elements50can be used to press the strip36together in axial direction of the drum31. The stop elements50can for example be a flange or a plurality of stop elements50spaced circumferentially apart from one another.

It will also be appreciated that in this document the terms “comprise”, “comprising”, “include”, “including”, “contain”, “containing”, “have”, “having”, and any variations thereof, are intended to be understood in an inclusive (i.e. non-exclusive) sense, such that the process, method, device, apparatus or system described herein is not limited to those features or parts or elements or steps recited but may include other elements, features, parts or steps not expressly listed or inherent to such process, method, article, or apparatus. Furthermore, the terms “a” and “an” used herein are intended to be understood as meaning one or more unless explicitly stated otherwise. Moreover, the terms “first”, “second”, “third”, etc. are used merely as labels, and are not intended to impose numerical requirements on or to establish a certain ranking of importance of their objects. The terms radial, axial, tangential, and circumferential in this description are generally defined with respect to the drum31, unless stated otherwise.