Patent Application
20080150229
Transport of cut sheets with wet or molten images on one or both sides requires negligible interaction between the transport means and the images. Interaction between the transport system and the images may result in alteration of the images if the transport system marks the images prior to drying, solidifying, or fusing the image onto the paper. Fully non-contacting transport using air jets requires continuous closed-loop feedback and jet control to achieve sufficient control of sheet transport. Such a system can be prohibitively expensive. It is therefore a benefit of the present embodiments to provide an open loop, in the sense that no sheet position sensing is required, relatively inexpensive and virtually non-contacting means to transport sheets with wet or molten images thereon.
Interaction may also cause transfer of the marking material, such as ink or toner, to the transport system. When the transport system transports a different sheet, the marking material may transfer onto the other sheet, leaving a ghost image of the previous sheet's image on the new sheet.
In addition, cut sheets, such as individual pages of paper, may have issues related to cockling or curling of the sheets as they are transported. Generally, contactless fusing, where the media moves through a fusing process to fix the image onto the media, may involve knives of gases or vapors for heating, drying and cooling the media. For a web medium that comes in large rolls, this may not be as much of a problem because tension in the roll assists in keeping the medium flat. It may become more difficult to keep cut sheets of media flat in a contactless system.
A first embodiment is transport system for cut sheet media having a first and second cylinder to form a nip, a support subsystem to transport edges of cut sheets having at least one image into and out of the nip, and an array of contact points on each cylinder to make contact with the cut sheets without marking the image.
Another embodiment includes a wheel for a print medium transport system having an outer rim having a series of contact points, an inner hub supporting a means to accommodate a drive shaft, and an internal spring connecting the outer rim to the inner hub.
Another embodiment is a method of transporting cut sheets in a printing system. The method forms a nip between at least one pair of cylinders, each cylinder having an array of contact points, guides a first edge of a cut sheet into the nip, and uses the arrays of contact points to transport the cut sheets through one of either a fusing or drying process.
Embodiments of the invention may be best understood by reading the disclosure with reference to the drawings, wherein:
Each pair of cylinders, such as pair 12, has two cylinders arranged adjacent to each other to form a nip. Nip as used here means the region between two cylinders where at least a portion of each cylinder is in contact with the print media. As will be discussed in more detail later, in embodiments disclosed here, a portion of the cylinder consists of contact points and only those come into contact with the print media.
In the transport system of
The first pair of cylinders has a motor 14 for turning the cylinders to allow the print media to move along in the process direction 28. The print media has an image, such as a tacked but unfused toner image, a molten image or a wet image, that undergoes a fusing process as it moves through the transport system. The maximum distance of one cylinder pair from the next pair of cylinders may depend upon a shortest sheet length used in the system. This ensures that sheets do not ‘fall’ out of the transport system during the fusing process.
In addition to each pair of cylinders 12, 6 and 20 optionally having their own motors 14, 18 and 22, the system may also have a motion control 29 to alter the relative motions of the cylinders for tensioning purposes in the system. For example, to tension sheets as they are transported, sequential nips can be driven at slightly higher speed than the upstream nips for a short time to wind up the torsional compliance of the cylinders. Then the nips can be maintained at the same speed for the rest of the time that the sheet is within the grasp of both nips. The cylinders may also all be driven by the same motor, but altering the relative motion of one or the other pairs of cylinders would not be as easily accomplished. In one embodiment, where starwheels having internal springs form the arrays of contact points, speeding up each successive pairs of cylinders to be slightly faster than a previous pair of cylinders tensions the internal springs and produces process direction tensioning of the sheets held by the cylinder pairs.
The arrays of contact points on one cylinder may be offset from the array of contact points on the other cylinder in the pair. The example of
The array of contact points has the characteristic that each point makes light contact with the sheet and image on the sheet in such a manner as not to alter or mark the image. Experiments have determined that the amount of force applied to the print media that will cause visible marking or alteration of the sheet is approximately 80 grams (for typical coated paper media). Using an array of contact points, each point makes contact with the media using much less force than 80 grams, and spreading the light contact out across several points of contact allows sufficient force to be applied to the media to cause it to be controllably transported.
Returning to
Employing the pairs of cylinders such as 12 may also allow better control of the support subsystem. As shown in
The arrays of contact points should have sufficient compliance so that the system can accommodate different thicknesses of print media. Because both sides of the sheet may have unfused toner, molten or wet inks, one cannot use large area resilient contacts on either side. It would be expensive to have compliant shafts for each disk in a series, and alignment of the shafts would be critical. One embodiment has compliancy built into the disks, as will be discussed in more detail further.
If compliant disks are used, some reinforcement of the disks may be necessary in the lateral dimension to ensure that the disks do not shift.
The use of compliant disks allows the arrays of contact points to deflect or offset inward as needed to accommodate thicker media. Generally, the cylinders will be arranged such that the width of the gap at the nip is slightly less than the thinnest media accommodated by the system. The thinnest media accommodated by the system will be referred to as the minimum thickness. The cylinders will be arranged such that the array of contact points will be separated by a distance smaller than the minimum thickness. When the media moves into the gap, the compliant disks will displace to allow passage of the media with minimum contact.
Internal springs, and spiral springs in particular, provide several advantages. The springs allow the outer rim 50 to deflect or offset from thicker media to control the contact force of any one point against the image. In addition, the springs can accommodate small intermittent differential speeds between different starwheel assemblies contacting the same sheet. These speed differentials may result from speed control errors, or from a purposeful adjustment of speeds to tension the sheet. As mentioned previously, the speed control may have each successive pair of cylinders run slightly faster than the previous sheet to tension the springs in the process direction. This may assist in maintaining the flatness of the sheet in printing processes where water content varies and slack sheets may allow fiber realignment to occur.
In addition, no alignment features exist for the disks when they slide onto the shaft, resulting in random azimuthal placement. The combination of pseudo-random teeth placement and random azimuthal placement mitigates the tendency of the human brain to detect patterns in an image or document when viewed at the natural reading distance.
Experiments using stainless steel disks approximately 125 microns thick showed no tendency to leave visible marks on the images. The experiments also did not result in any transfer of marking material to the disks, also referred to as ‘hot offset.’ If hot offset is shown to be an issue under particular conditions such as for certain toners or inks, various methods, such as coating with fluoro-hydrocarbons can be used to alleviate the problem by reducing the surface energy of at least a portion of the wheel, such as the tips. The coatings may also increase wear strength of the wheels
Returning to
In this manner, a virtually ‘contactless’ transport system is provided for a fusing or drying process in a print system employing cut sheets. Arrays of contact points spread the force necessary to move the media, while limiting the amount of force that occurs at any one point, eliminating marking of the image or sheet or transferring of the marking material.
It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
