FIELD OF THE INVENTION

The present invention relates to an endless flexible belt for a printing system. The endless belt of the invention finds particular application as an intermediate transfer member in a printing system for an offset printer in which, instead of ink being applied directly onto a substrate, a mirror reflection of the desired image is formed by ink deposition (e.g. ink jetted droplets) on the intermediate transfer member, the latter then serving to transport the image to an impression station at which the image is impressed on a substrate. In its different aspects, the invention is concerned with a flexible belt for use in a printing system, a belt system that comprises such a belt, and an apparatus for installing such a belt in a belt system.

BACKGROUND

Digital printing techniques have been developed that allow a printer to receive instructions directly from a computer without the need to prepare printing plates. Amongst these are color laser printers that use the xerographic process. Color laser printers using dry toners are suitable for certain applications, but they do not produce images of a photographic quality acceptable for publications, such as magazines.

A process that is better suited for short run high quality digital printing is used in the HP-Indigo printer. In this process, an electrostatic image is produced on an electrically charged image bearing cylinder by exposure to laser light. The electrostatic charge attracts oil-based inks to form a color ink image on the image bearing cylinder. The ink image is then transferred by way of a blanket cylinder onto paper or any other printing medium, the substrate.

Inkjet and bubble jet processes are commonly used in home and office printers. In these processes droplets of ink are sprayed onto a final substrate in an image pattern. In general, the resolution of such processes is limited due to wicking by the inks into paper substrates. Fibrous substrates, such as paper, generally require specific coatings engineered to absorb the liquid ink in a controlled fashion or to prevent its penetration below the surface of the substrate. Using specially coated substrates is, however, a costly option that is unsuitable for certain printing applications. Furthermore, the use of coated substrates creates its own problems in that the surface of the substrate remains wet and additional costly and time consuming steps are needed to dry the ink so that it is not later smeared as the substrate is being handled, for example stacked or wound into a roll. Furthermore, excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed perfecting or duplex printing) difficult, if not impossible. Additionally, direct inkjet printing may result in poor image quality because of variation of the distance between the print head and the surface of the substrate.

Using a printing technique based on an intermediate transfer step overcomes many problems associated with inkjet printing directly onto the substrate. It allows the distance between the surface of the image transfer member and the inkjet print head to be maintained constant, and reduces wetting of the substrate as the ink can be dried on the image transfer surface before being applied to the substrate. Consequently, the final image quality on the substrate is less affected by the physical properties of the substrate.

The use of transfer members which receive ink droplets from an ink or bubble jet apparatus to form an ink image and transfer the image to a final substrate have been reported in the patent literature. Various ones of these systems utilize inks having aqueous carriers, non-aqueous carrier liquids or solid inks that have no carrier liquid at all.

The use of aqueous based inks has a number of distinct advantages. Compared to non-aqueous based liquid inks, the carrier liquid is not toxic and there is no problem in dealing with the liquid that is evaporated as the image dries. As compared with solid inks, the amount of material that remains on the printed image can be controlled, allowing for thinner printed images and more vivid colors.

Generally, a substantial portion or even all the liquid is evaporated from the image on the transfer member before the image is transferred to the final substrate, in order to avoid bleeding of the image into the structure of the final substrate. Various methods are described in the literature for removing the liquid, including heating the image and a combination of coagulation of the image particles on the transfer member, followed by removal of the liquid by heating, air knife or other means.

PCT application No. PCT/IB2013/051716, which entered the U.S. national stage as application Ser. No. 14/382,751 and which claims priority from U.S. Provisional Patent Application No. 61/606,913, (both of which application are herein incorporated by reference in their entirety), teaches a printing process designed to use aqueous inks. The disclosure of the latter application overlaps with disclosure provided herein but it should be made clear that the present invention is not restricted in its application to such a process and may be used in any printing system that uses an intermediate transfer member constructed as a flexible belt regardless of whether or not the ink is water based, hence regardless of the type of release layer suitable to accommodate the ink or printing process being used.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to the construction and installation of a continuous flexible belt, suitable for use as an intermediate transfer member in a printing system, which belt is guided when in use, for instance over rollers. The flexible belt of the invention may however serve other purposes, for example as a substrate carrier and may also be applicable to a belt mounted over a rotatable rigid drum, also referred to as a drum-mounted blanket. The invention seeks in particular to provide a flexible belt that remains in a well defined plane as it travels around an endless path and that is constrained laterally to prevent it from meandering.

In accordance with some embodiments, there is provided a flexible belt for use in a printing system, comprising an endless strip which, in use, travels along a continuous path, wherein formations are provided along the sides of the strip which are capable of engaging with lateral tracks to place the belt under lateral tension, the lateral tracks further serving to constrain the belt to follow the continuous path.

In an embodiment of the invention intended for use as an intermediate transfer member in a printing system, the strip is an initially elongate strip having parallel straight sides of which the ends are releasably or permanently securable to one another to form an endless loop, and, when in use, the belt serves to transport ink images from an image forming station to an impression station of the printing system.

The ends of the elongate strip may be secured to one another in a releasable manner (e.g. zip fastener, hooks or magnets) or permanently by soldering, gluing, or taping (e.g. using Kapton® tape, RTV liquid adhesives or PTFE thermoplastic adhesives with a connective strip overlapping both ends of the strip), or by any other method commonly known. Any previously mentioned method of joining the ends of the belt may cause a discontinuity, referred to herein as a seam, and it is desirable to avoid an increase in the thickness or discontinuity of chemical and/or mechanical properties of the belt at the seam.

In an alternative embodiment, the belt is devoid of a scam and is formed as a continuous belt.

The strip from which the belt is made generally comprises at least a reinforcement layer and a release layer. In some embodiments, the belt additionally includes a compressible layer so that the belt may itself serve in a manner analogous to the blanket of an offset litho press. In other embodiments, a blanket cylinder carrying a compressible blanket, also termed a pressure cylinder, may be provided at the impression station, and the belt, which may then be optionally devoid of a compressible layer, may pass between a pressure cylinder and an impression cylinder in order for the ink image that it carries to be impressed on the substrate.

In some embodiments of the invention, each side of the strip from which the belt is made is provided with spaced formations. Such spaced formations may conveniently be the teeth of one half of a zip fastener that is secured to the belt along the respective side of the strip. The laterally projecting formations need not be evenly spaced and in an embodiment of the invention a predetermined irregular spacing may serve to control parameters associated with the use of the belt in a printing system.

In an alternative embodiment, the formations may comprise two flexible beads, arranged one on each side of the strip, the beads having a diameter larger than the thickness of the belt. In this arrangement, the bead is considered to provide a continuous formation on each side of the strip.

In an alternative embodiment, the formations may be a combination of beads and lateral spaced projections. The combination advantageously permits identification of belt sections corresponding to the lateral projection adjacent to that section, each section having unique projection characteristics (e.g. color, shape, etc.). Additionally or alternatively, each side of the strip may have different formations.

The formations, irrespective of shape, spacing along the edges or lack thereof, can be made of any material having heat resistivity compatible with the operating temperature at which the belt is used. Preferably, the formations may be made of a material having a low friction coefficient to ensure their smooth running within the lateral tracks. Using materials having satisfactory abrasion resistance can advantageously reduce or prevent the formation of debris that may result from the rapid displacement of a belt during the printing process. In one embodiment, the formations are made of a material having or comprising an agent having lubricating properties. Lamellar materials may serve as lubricating agents in the formations positioned at the side of the belt. In one embodiment, the formations used for the lateral guidance of the belt are made of a nylon or polyamide polymer supplemented with molybdenum disulfide or from polyacetal filled with PFTE. Alternatively, or in addition, the formations may have an anti-friction coating, such as PTFE.

As an alternative, or in addition, the track with which the formations engage may be lubricated or anti-friction coated or impregnated with an agent able to reduce friction. In one embodiment, the lateral tracks are made of anodized aluminum or of stainless steel. The porosity of the material may advantageously be used to impregnate the lateral tracks with an anti-friction agent, such as PTFE.

As an alternative or in addition, the track and formations may have opposing magnetic properties thereby creating repulsive forces between each other, thus lessening frictional forces.

In some embodiments, the spaced formations or the flexible beads may be retained in the tracks by rollers that rotate as the belt moves along the track.

In some embodiments, the ends of the strip may be secured to one another to form a continuous loop using end formations similar to the formations projecting laterally from the strip to enable belt tensioning and/or guiding along the tracks. For example, the ends of the strip may each be secured to one half of a zipper.

In a second aspect of the invention, a belt as set out above forms part of a belt system having a support frame having supporting surfaces for guiding and driving the belt, wherein the support frame further includes two lateral tracks, extending one on each side of the belt, each track being of suitable cross-section to slidably retain the formations on the sides of the belt. For example, lateral formations having an approximate circular cross-section may be retained by tracks having a C-shaped cross-section. In some embodiments, the surfaces for guiding and driving the belt comprise rotating rollers but other means of supporting the belt, such as a pneumatic table or linear drive, may alternatively be used. Such support may be provided by indirect or intermittent contact. In one embodiment, a single roller may suffice, a situation corresponding to the belt being mounted on a drum.

In some embodiments, lateral guiding tracks are provided to guide and tension the belt only in the region of the image forming station. In some embodiments, lateral guiding tracks are additionally provided at the impression station at which the image is impressed on the substrate. In some embodiments, lateral guiding tracks are additionally provided at strategic positions such as drying station(s), cooling station(s), conditioning station, etc. In still further embodiments, continuous guide tracks are provided around the full circumference of the support frame of the belt system along the path to be followed by the belt.

In an embodiment, plates are mounted on the support frame having support surfaces contacting the inner side of the belt, the support surfaces lying in a plane offset from a flat plane passing through the two tracks such that lateral tension in the belt, resulting from engagement of the formations in the tracks, serves to flatten the belt against the support surfaces.

In accordance with a further aspect, the invention provides an apparatus for assembling a belt system comprising

a) an elongate strip having parallel straight sides, formations along the length of the sides of the strip, and two ends securable to one another to form an endless loop flexible belt; and

b) a support frame having surfaces for supporting the belt and including two tracks, extending one on each side of the belt, each track having a cross section suitable to retain slidably the formations on the sides of the belt;

said assembling apparatus comprising

i) a rigid body defining two open-ended tracks, arranged one on each side of the body, for receiving the formations on the sides of the strip; a first end permitting introduction into the tracks of formations on the sides of a strip that is to be looped to form a belt, and the second end of each track being engageable with branch entry points provided in a respective one of the endless tracks of the support frame; and

ii) at least one sprocket rotatably mounted on the body to engage with the formations located within one of the open-ended tracks to feed the elongate strip into the endless tracks.

Two sprockets may be mounted on a common shaft to engage with the formations lying within both open-ended tracks and the sprockets may be mounted on a common shaft, driven manually or by an electric motor.

To assist in applying a lateral stress to the belt during its mounting on the support frame, the open-ended tracks may be divergent, being more closely spaced apart from each other at the first one end than at the second end.

An anchoring may furthermore be provided to permit the body of the apparatus to be secured to the support frame of the belt to ensure accurate guidance of the belt as it is fed onto the support frame.

In an alternative embodiment, the belt may be installed by securing the leading edge of the belt strip introduced first in between the lateral tracks to a cable which can be manually or automatically moved to install the belt. For example, one or both lateral ends of the belt leading edge can be reversibly attached to a cable residing within each track. Advancing the cable(s) in turn advances the belt along the tracks. Alternatively or additionally, the edge of the belt in the area ultimately forming the seam when both edges are secured one to the other can have lower flexibility than in the areas other than the seam. This local “rigidity” may ease the insertion of the lateral formations of the belt strip into their respective tracks.

