Patent Application
20080230978
A vacuum transport system provides shingled sheets across a stack prior to individually registering the sheets onto the stack. Shingling the sheets allows sheet and transport velocity and acceleration levels to be relatively low, and thus not stressful to transport drives and to the sheets. This allows an incoming sheet stream to be reliably stacked at a very high stack rate.
A basic finishing function for a production printer is a high capacity stacker. The purpose of the stacker is to compile printed sheets into a well-formed stack suitable to user end requirements, such as off-line finishing or bulk distribution. Current production printers are equipped with a high capacity stacker that produces a stack in which sheets can be optionally offset to one of two positions in the cross-process direction. This stacker design has proven effective and reliable at speeds of at least 110 ppm.
The conventional VGT transport consists of torso independently driven belt transport assemblies, VGT-1 and VGT-2, each having vacuum ports 240 (
Recent work has demonstrated the feasibility of even higher stacking rates beyond 110 ppm, for the conventional vacuum transport. Given the expected advances in high speed production printers, the requirement to reliably compile a stream of sheets at speeds exceeding 200 ppm will be desirable. Existing high capacity stacker technology is not easily extensible to such rates due to the needed speed of the sheets using the current singular transport of individual sheets through the vacuum transport.
The conventional VGT transport system operates in a stop/start cycle in which the belts are rapidly accelerated from a stop to a transport speed to acquire and transport a sheet. Then the vacuum transport must rapidly decelerate back to a stop position once for each transport cycle. As the processing speeds increase, the time interval for each cycle must be reduced, placing large dynamic forces on the sheets and transport components. These forces and speed increases have the possibility of causing high speed failure modes due to the potential for excessive kinetic energy. For example, excessive transport speed may cause bounce back of the sheet once it is rapidly stopped against the registration wall 260. Additionally, aerodynamic forces acting on the sheet may cause the sheet edge to experience turbulence or flapping. As an example of the forces and speeds experienced, at 270 ppm, it is estimated that the current VGT belts will need to reach speeds of about 2.4 m/s and accelerate at 6.1 g's if current start/stop pitch cycles are maintained. Even if the VGT belts were run continuously whenever possible, the vacuum transport belts (and thus the sheets) would still travel at a speed of about 1.95 m/s, which is over twice the current maximum sheet speed of about 0.95 m/s.
Because kinetic energy of sheets being registered would be quadrupled at such speeds, sheet bounce back and lead edge damage is much more likely to occur. Accordingly, a more robust approach to sheet transport across the stack is desirable to achieve increased processing rates of 200 ppm or more with reduced potential for sheet damage or system malfunction.
Thus, speeds attainable with singularly fed sheets have reached a ceiling that will be difficult to break though without material changes in transport mode. One possible mechanism to improve the speed would be to transport twice the sheets, such as by transporting two sheets at the same time. However, due to limitations of vacuum transport systems that rely on vacuum applied to portions of the top surface of a top sheet, reliable transporting of the lower sheet(s) is difficult.
In accordance with aspects of the disclosure, the existing vacuum gripper transport architecture is modified so that incoming sheets can be transported across the existing stack at a relatively slow speed, which can even be slower than the currently attainable speeds, yet provide registration on top of the stack at very high stacking rates. This makes the modified architecture particularly suited for reliably feeding sheets from a TIPP system feed from multiple imaging machines, such as production printers.
In certain exemplary embodiments, this can be achieved by allowing sheets to overlap each other prior to their acquisition onto a vacuum gripper transport (VGT). The overlapped or shingled sheets can then be serially acquired by vacuum ports on the VGT transport that are spaced the same distance apart as the shingled sheet lead edges. Such a distance is referred to as the shingle distance. Each vacuum transport operates in an intermittent stop/start mode once per pitch. However, each cycle only advances the sheet by one shingle distance. Alternatively, each vacuum transport may advance in unison at an appropriate continuous speed such that each sheet advances by the shingle distance each pitch. Because multiple vacuum transports operate concurrently and because of the shingling distance being shorter than sheet length, multiple sheets are effectively transported simultaneously by the collective vacuum transport system at a set (albeit each is offset by the shingling distance). As a result, the speed and acceleration requirements for the vacuum transport and the sheets being transported are greatly reduced compared to conventional vacuum transports that essentially transport sheets singularly.
In accordance with aspects of the disclosure, a high speed sheet stacker is provided including a plurality of vacuum transport sub-assemblies interdigitated with an adjacent sub-assembly and provided with a spatial pitch that is less than or equal to the shingle distance. The collective vacuum transport assembly can thus acquire shingled sheets and transport the shingled sheets as a set, with each sheet being offset by at least one shingle distance. As a result, speed and acceleration requirements for the vacuum transport and the sheets being transported are greatly reduced compared to conventional vacuum transports that essentially transport sheets singularly.
In accordance with aspects of the disclosure, at least two sheets are transported as a set by the vacuum transport system. In certain exemplary embodiments, this is achieved by providing at least two vacuum transport belt sub-assemblies, one for each sheet being transported as a set. However, in certain embodiments, five vacuum transport sub-assemblies have been found to be optimal to achieve sufficient transport speed while not excessively increasing the size and complexity of the stack handler.
In accordance with further aspects of the disclosure, a shingled transport zone is provided upstream of the vacuum transport sub-assemblies that includes a plurality of nips spaced in the process direction to pre-position sheets of two or more lengths for transport to the vacuum transport subassemblies. In certain embodiments, there may be three nips to accommodate small, medium and large length sheets, in which at least two of the three include a nip release mechanism to allow the sheets to readily pass thereby. In other embodiments, at least five nips are provided to accommodate at least two additional intermediate sheet sizes. In this latter embodiment, four of the five nips may be openable by including a nip release mechanism.
In accordance with further aspects of the disclosure, an offsetting function for offsetting the sheets in a cross-process direction is provided upstream of the shingled transport zone. In certain embodiments, this can be provided in a turn baffle.
In an exemplary embodiment, a vacuum transport system for transporting a set of sheets to a registration zone is provided that includes: a shingling loading zone; a shingled transport zone; and a vacuum transport assembly. The shingling loading zone receives singular incoming sheets at a predetermined speed and outputs sequential sheets that are optionally shifted on a sheet-by-sheet basis laterally in a cross-process direction. The shingled transport zone is of a length sufficient to accommodate at least one maximum sheet length, and includes at least one pinch nip that slows down the incoming sheets and shingles the incoming sheets by a predetermined shingle distance, the at least one pinch nip having a transport profile that transports the shingled sheets in unison at a reduced speed in the process direction. The vacuum transport assembly includes at least two vacuum transport belt subassemblies, each sub-assembly including a plurality of belts spatially separated in the cross-process direction, the belts of each sub-assembly being interdigitated with belts of an adjacent sub-assembly, the sub-assemblies being provided with a spatial pitch less than or equal to a shingle distance and defining an overlap region between adjacent sub-assemblies. Belts of each vacuum transport sub-assembly include at least one vacuum port in contact with a vacuum plenum to acquire a leading edge of a sheet, the collective vacuum transport assembly being advanced to transport a shingled set of multiple sheets though the vacuum transport assembly simultaneously. Each sheet is separated by at least one shingle distance, and the last vacuum transport sub-assembly in the process direction transports a single sheet into the registration zone.
The vacuum transport system may be part of a sheet stacker, including a tray for receiving stacked sheets provided in the registration zone.
In an exemplary embodiment, a method for transporting a set of sheets to a registration zone is provide that includes:
receiving singular incoming sheets at a predetermined speed at a loading zone and outputting sequential sheets that are optionally shifted on a sheet-by-sheet basis laterally in a cross-process direction;
receiving the offset sheets in a shingled transport zone of a length sufficient to accommodate at least one maximum sheet length and slowing the sheets to a transport speed;
shingling a set of sheets in the shingled transport zone by a predetermined shingle distance and transporting the set of sheets in the process direction to a vacuum transport assembly including at least two vacuum transport belt subassemblies, each sub-assembly including a plurality of belts spatially separated in the cross-process direction, the belts of each sub-assembly being interdigitated with belts of an adjacent sub-assembly, the sub-assemblies being provided with a spatial pitch less than or equal to a shingle distance and defining an overlap region between adjacent sub-assemblies, wherein belts of each vacuum transport sub-assembly include at least one vacuum port in contact with a vacuum plenum to acquire a leading edge of a sheet;
collectively transporting the set of shingled sheets simultaneously thorough the vacuum transport system; and
advancing each successive shingled sheet of the set into a registration zone singularly.
Exemplary embodiments will be described with reference to the drawings, wherein:
A first embodiment of a shingled vacuum transport system 300 will be described with reference to
Sheets are fed from one or more imaging machines into a sheet stacker 100, such as the one shown in
As the sheet trail edge clears the turn baffle 312, strain energy is released and the trail edge ‘flicks’ downward below the turn baffle. If desired to allow for curl latitude, a low airflow can be maintained oriented vertically downward to encourage curled trail edges to ‘flick’. In general, there will be previous sheets already in the shingled transport zone 320, so the incoming sheet is actually impinging upon and being guided on top of the previously fed sheet.
As sheets enter the shingled transport zone 320, they become shingled such that an upper sheet's lead edge always trails a lower sheet's lead edge by a predetermined distance, referred to as the shingle distance. All shingled sheets travel in unison via a set of pinch nips 320A-E operating with either a stop/start profile or a continuous velocity. Each of pinch nips 320A-E, with the exception of the leftmost nip 320E, has a nip release mechanism that allows the nip to controllably open or close. In the embodiment shown, the nip release is formed by a mechanism that allows at least one of the two nip roller pairs to be displaced relative to the other by a distance that allows the sheet to freely pass therebetween. For example, one or both of the roller pairs may be biased away from the other by a solenoid and a spring used to return a predetermined nip spacing upon release of the solenoid. The nip releases are used to allow different lengths of media to enter the correct distance into the shingled transport zone 320 at high speed before decelerating. That is, the zone 320 is sized to accommodate the longest size sheet so that it is fully received within the zone (i.e., is allowed to exit loading zone 310 and “flick”). Although five nips 320A-E are shown, lesser or greater numbers can be provided depending on the flexibility of the system for accommodating alternative sheet sizes. For example, three nips could be provided to accommodate small, medium and large sheet sizes.
In the example shown, for the largest sheet size, such as a 20.5″ long sheet, all of nips 320A-D may open to allow the sheet to fully enter before decelerating and loading. For a smallest sheet size, such as a 7″ long sheet, all of nips 320A-E can remain closed so that the sheet is initially decelerated and acquired by nip 320A. Intermediate sheet sizes can have a fewer number of nips closed.
From the shingled transport zone 320, the shingled sheets are fed to the vacuum transport zone 340, where they remain shingled as they transport across the stack via a stop/start transport motion once per system pitch cycle (or could use a continuous transport profile). Each sheet's lead edge is acquired by holes 344 on one or more spatially offset belts of the first vacuum transport sub-assembly 340A, whereupon the sheet is transported to overlap region 345 (
The vacuum transport belt sub-assemblies 340A-E can be similar in design to the ones used in conventional
During operation, each sheet's lead edge will be advanced by an upstream vacuum transport belt sub-assembly (one of subassemblies 340A-E) and transferred to the next downstream vacuum transport belt sub-assembly. The speed and acceleration rate for this indexing motion can be modest and still achieve stacking rate equal to or well in excess of conventional stacking rates of the system of
As the sheet exits the left-most vacuum transport belt sub-assembly 340E, its lead edge is no longer tacked by vacuum to the transport belts and the sheet enters the registration zone 350. The registration scuffer belts 250 then cycle on and drive the lead edge up against the stack registration wall 260. Because the sheet speed is relatively low, there are no issues with sheet damage or bounce back. Thus, reliable transport and stacking can be achieved. Testing performed suggests that there is sufficient time to fully register each sheet within the available pitch cycle at even speeds well in excess of 200 ppm (at a pitch cycle of about 0.222 sec).
The basic steady state behavior of the shingling transport 320 and vacuum transport 340 are rather straightforward. However, there are certain use cases that require slightly different behavior. A few of the more typical cases are described below:
The first sheet in a job can be handled normally until its lead edge is ready to be acquired by the rightmost vacuum transport sub-assembly 340A. Since no sheets precede it, the vacuum ports 344 of the other vacuum transport belt sub-assemblies 340B-E will be open and thus proper sealed port pressure may not be achieved for the sheet (if the vacuum transport belt sub-assemblies share a high capacity vacuum blower). In this event, the unused vacuum transport belt sub-assemblies (340B-E) can all be parked in a sealed port condition so that their belt holes 344 do not line up with their plenums 342. That is, both spaced ports 344 formed by holes in the belt (best shown in
The system must also act differently to accommodate the last sheet in a job. In this case, there are no sheets following the last sheet. En an analogous manner to the first sheet example, after each vacuum transport belt sub-assembly passes the sheet lead edge to its left neighbor, it parks at a park position so as to seal off its plenum 342. The last sheet therefore behaves just as any other sheet once it arrives at registration zone 350.
Skipped pitches or photoreceptor seam pitches in a job are other areas that may require special handling. In this case, there is a gap in the incoming stream of sheets due to either a skipped pitch or a photoreceptor seam pitch (a large intercopy gap that occurs once each photoreceptor belt revolution). In either event, the stacker response is made to delay advancing the shingled sheets in both the shingled transport zone 320 and the vacuum transport zone 340 until the next sheet arrives. Once the next sheet arrives, it is stopped at the usual point and normal motion of the shingled sheets can resume.
Mixed length media may also require special handling. If a smaller length sheet follows a larger sheet, the stacker can accommodate this by closing down the shingled transport nips 320A-E as appropriate and parking the next sheet. Depending on its size, the sheet lead edge may be ‘N’ shingle distances behind the previous sheet's lead edge, which the stacker treats as ‘N’ skipped pitches between the sheets. If a larger length sheet follows a smaller sheet, the system will need to schedule an appropriate number of skipped pitches between them so that the prior sheet is allowed to first index far enough into the shingled transport zone so that the larger sheet can be properly parked.
Because of the presence of shingled sheets in the shingled transport zone 320, it is difficult to achieve desirable cross-process offsetting of the sheets. Accordingly, in exemplary, embodiments, the offsetting function is achieved upstream from the shingled transport zone 320. For example, an offsetting transport can be provided at loading zone 310, such as provided at turn baffle 312. The offset function can be achieved using a simple translating nip with a nip release. Therefore, sheets can be optionally offset inboard or outboard prior to arriving at the shingling transport zone 320.
It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, and are also intended to be encompassed by the following claims.
