Abstract:
The invention relates to a system and method of eliminating wrinkling, tearing and web-breaks of a web in an offset printing press as the web contacts the chill rollers on start-up. The system operates to eliminate the wrinkling that generates the tearing. During startup, a spreader roller impinges the web, stretching out wrinkles. During acceleration of the press, when thermal contraction of the paper also achieves the desired stretching, the spreader roller is retracted to prevent smearing of ink on the web. Uneven pressure of the spreader is utilized to implement steering of the web to enhance web centering.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system and method for preventing wrinkling of a web in a web-offset press. More particularly, the invention relates to a system and method for preventing wrinkling, tearing, and web-breaks of a web as the web contacts a first chill roller after passage through a dryer, to thereby minimize press down-time. 
     BACKGROUND OF THE INVENTION 
     In a web-fed printing system such as a web-offset press, a series of repeating images are printed on a web of material, typically paper. In the typical process, the web is fed to sequential printing units, wherein each printing unit prints a different color ink on the web to produce a multi-color image. The web is subsequently fed to a thermal dryer which dries the ink and is then routed to a series of chill rollers which operate to cool the web and set the ink. The web is next slit in the longitudinal direction (the direction of web movement) by a slitting mechanism to produce a plurality of continuous ribbons. The ribbons are directed to a folder wherein the ribbons are aligned one on top of the other, folded longitudinally, and then cut laterally to produce a plurality of multi-page, approximately page-length segments, each of which is termed a “signature”. The signatures are subsequently bound together to produce magazines, catalogs, or other printed products. 
     As the art of web-offset high-volume printing has progressed, publishers have migrated to the usage of lighter basis-weight paper for the printing of magazines, catalogs, and coupons. Lightweight paper has a lower cost per impression, as well as lower postal mailing costs. 
     SUMMARY OF THE INVENTION 
     Such lightweight paper has been found to yield an acceptable level of quality in terms of print density and opacity, and is gaining popularity. As schematically illustrated in FIG. 2, however, a serious problem in the usage of this paper is the formation of wrinkles or creases  44  or the like as the web emerges from the dryer and wraps around the first chill roller  33  during start-up of the press. Because the web  14  is unsupported in the long span of the dryer, it tends to form corrugations  48 . These corrugations  48  decrease the effective width of the web. 
     During start-up, when the web  14  moves to contact the first chill roller  33 , these corrugations  48  must flatten out. Because the width of the web as laid down on the chill roller is slightly less than the true width of the web, the excess paper manifests as creases  44  substantially parallel to the long axis  46  of the web  14 . A crease is raised in a direction away from the center of the chill roller  33 . As the paper wraps around the circumference of the chill roller  33 , the raised portion of the paper is now describing a semicircle whose radius is larger than the paper which is lying flat on the surface of the chill roller  33 . The creased paper is consequently stretched, and lighter weight paper is often sufficiently weak such that it will rip. The crease  44  can create a series of small rips  50  in the paper, every few inches along the crease. 
     The numerous small rips  50  in the paper can be the initial point of a web-break, especially if the rip happens to occur in the region of the web which will be longitudinally slit to form ribbons of lesser width. A rip encountering the slitting mechanism can result in a failure of the slitting mechanism with resultant web-break, or a ribbon with a torn edge can be greatly weakened, resulting in a higher probability of breakage under tension as the rip passes through the folder. 
     During normal running of the press, creases in the web present a much lesser problem. When the press is running at full speed, the web travels from the dryer to the first chill roller so quickly that the web is still hot. The web is cooled and resultant thermal shrinkage occurs quickly on the first chill roller. The shrinkage to a large extent eliminates the creases before ripping occurs. 
     Spreader rollers are commonly used on printing press in-feeds to remove wrinkles from the web before the web is fed to the printing units. Spreader rollers are well known in the art; see, for example, U.S. Pat. Nos. 4,566,162, 5,607,039, and 5,729,878. The design of a spreader roller can be as simple as a handmade application of adhesive tape, wound in two separate helical patterns on an existing roller. The helical patterns operate to carry wrinkles toward the edges of the web. 
     Another problem, particularly severe in the case of lightweight paper, is poor lateral stability of the paper under conditions of low web tension. During normal operation of the press, if the web should break, the web is subsequently re-threaded through the press, with a handmade splice at the point of breakage. Such splices have poor strength and would pull apart if subjected to a tension typically used for the paper. Thus, a lower tension is used until the splice has passed through the press. The combination of lightweight paper and lower tension causes poor lateral stability of the web. During slow speed operation of the press, the web may slowly creep off-center to either side of the chill rollers, falling partially off the edge of the roller. The wrinkled paper will generally break, causing additional downtime re-webbing the press. During start-up, a pressman may manually hold the paper and pull it to remove creases and prevent crawling toward either side, but this action requires valuable manpower, and requires skill to avoid accidentally ripping the paper. 
     There is a need to prevent the formation of creases on the web and prevent lateral instability of the web during the period of press start-up when the web is moving too slowly for the paper to remain hot enough at the point that the web first contacts the first chill roller. 
     An object of the present invention is to provide a mechanism for the restoration of web width prior to the first chill roller during start-up, to prevent wrinkling and tearing of the web with resultant web-breaks and press downtime. 
     A further object of the invention is to retract the web-width restoration mechanism during normal running of the press, to avoid marring the hot ink on the web. 
     A yet further object of the invention is to pull the web in a manner that prevents the paper from traveling off-center of the rollers far enough to reach the edges of the chill rollers and the various other rollers in the web path after the dryer. 
     Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following detailed description, claims, and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a heat-set offset-web printing press line; 
     FIG. 2 is a diagram of the formation of a crease or wrinkle in a web as the web encounters a chill roller; 
     FIGS.  3 ( a )- 3 ( e ) illustrate various prior art embodiments of a spreader roller; 
     FIG. 4 is a side view of the actuating mechanism which moves the spreader roller into and out of engagement with a web on a printing press line. 
     FIG. 5 is an end view of a portion of a web on a printing press that includes a chill roller and a spreader roller positioned immediately before the chill roller. 
     FIG. 6 is a schematic of logic elements in the press control system which control the retraction mechanism. 
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrated in FIG. 1 is a portion of a typical multicolor web-fed printing press line  10 . A web of material  14  (e.g. paper) is sequentially driven from a reel stand  12  through a press  16  which includes a plurality of printing units  18 . Each printing unit  18  includes an upper blanket cylinder  20 , an upper plate cylinder  22 , a lower blanket cylinder  24 , and a lower plate cylinder  26 . The printing units  18  cooperate to print multi-color images on the upper and lower surfaces of the web  14 . Each printing unit  18  prints an associated color of ink. The desired image is repeatedly printed on the web  14  as the web  14  travels in a longitudinal direction past the printing units  18 . 
     The web is next suitably guided through a dryer unit  28  and a chill unit  30 . The chill unit  30  includes a plurality of chill rollers  32  as well as a spreader roller  34 . The spreader roller  34  is mounted to be lowered into the web path and retracted therefrom by an actuation mechanism  36 . A controller  38  such as a programmable logic controller controls the positioning of the spreader roller  34  via the actuation mechanism  36 . 
     The web is next routed by idler rollers  39  to other web processing units such as a slitter stand  40  and a folder  42 . The slitter stand  40  includes a slitting mechanism (not shown). As the web travels in a longitudinal direction, the slitting mechanism operates to slit the web in a direction substantially parallel to the direction of web movement into two or more ribbons. The folder  42  includes angle bars (not shown) which operate to align ribbons one on top of the other. The folder  42  also operates to longitudinally fold the ribbons. The folder  42  also includes a cutting mechanism (not shown) which operates to cut the folded ribbons in a direction transverse to the direction of web movement at the appropriate time in order to properly separate the repeating images on the web  14  with respect to the position of ink on the web. 
     In the preferred embodiment, chill roller  33  is the first roller the web encounters after the web  14  exits the dryer  28 . As previously stated, the spreader roller  34  is positioned to be lowered into contact with the web  14  and retracted therefrom. 
     As shown in FIGS.  3 ( a )- 3 ( e ), the spreader roller can take a variety of forms. For example, these figures illustrate spreader rollers having a rigid core and a covering surrounding the core. In FIG.  3 ( a ), the covering is compliant and includes a pair of helical grooves wherein the angle of the groove with the axis of the spreader roller progressively decreases as each groove spirals from the roller center line to one of the roller ends. FIG.  3 ( b ) illustrates a roller wherein the width of the covering portions between the grooves progressively decreases in the direction from the roller center line to one of the roller ends. FIG.  3 ( e ) illustrates a spreader roller having a covering including a pair of helical grooves which spiral outward from the center of the roller and wherein the depth of each helical groove progressively increases with the distance from the roller center line to one of the roller ends. 
     As best seen in FIGS. 4 and 5, the spreader roller  34  is supported by lever arms  52  on either side of the web, and each arm  52  is moved by an actuation mechanism  36  into and out of contact with the web. The actuation mechanism may be, for example, a pneumatic actuator such as part number 2000TR-1013, manufactured by American Cylinders of Chicago, Ill. 
     Referring again to FIG. 1, during web-up of the press, the actuation mechanisms  36  pull the spreader roller  34  away from the chill roller  33 , to avoid impeding the threading operation. When the web reaches a pre-selected speed, the reel stand  12  applies tension to the web  14 . This tension would be sufficient to generate rips in any wrinkles in the web at the chill roller  33 . At the time the reel stand  12  applies tension, the actuation mechanisms lower the spreader roller  34  into contact with the web, so that the spreader roller  34  spreads the web  14  and eliminates the wrinkles. As the press accelerates to normal printing speed, the web  14  is still hot when it exits the dryer unit  28  and reaches the chill unit  30 . The web shrinkage now performs the equivalent of the spreading operation, and the spreader roller  34  is retracted by the actuation mechanism to avoid marring the hot ink printed on the web. 
     FIG. 6 illustrates an example of the logic in the controller  38 . Logic gate  54  acts as an AND gate to control the actuation of the spreader roller  34 . The AND gate has a first input a signal indicative of slitter stand tension, and a second input a signal indicative of whether or not the printing units are applying ink. Thus, for example, engagement of the spreader roller  34  in the web path occurs if the web  14  is at a higher tension than that used when the press is hand-webbed, and the printing units  18  are not applying ink. The engagement of the spreader roller  34  is undesirable during hand-webbing of the press, as it impedes easy threading of the web. The logic gate  54  may be implemented as a logic rung in a Programmable-Logic controller, which is generally already present to control the press. Many alternatives may be used to control the actuation of the spreader roller  34  other than tension and ink application signals. For example, a speed set-point could also be used, wherein the selected set-point is typical of the press speeds at which tension and printing, respectively, occur. The only requirement is that the spreader roller  34  be disengaged while normal printing is occurring. 
     Referring to FIG. 5, a further advantage of using a spreader roller similar to that shown in FIG.  3 ( a ) is realized with the utilization of a pair of web-sensors  56  positioned on respective sides of the spreader roller  34 . The web-sensors  56  may be implemented as using sensing methods such as sonar, infrared, retro-reflecting, or other sensing methods as are known in the art. The sensors  56  operate to track the position of the web. For example, if the web  14  should deviate from the desired center position with respect to the chill roller  33 , sensor  56  measures the deviation. A signal from sensor  56  engages a pressure-reducing valve  58 , reducing the actuation pressure of the actuation mechanism  36  that is on the same side as the sensor  56 . The decreased pressure on one side of the spreader roller  34  causes the angled ridges of the roller  34  to perform less effectively on that side. A net pull of the web  14  is accomplished, effectively re-centering the web. Web-breaks due to poor centering are therefore minimized. 
     The web  14  may be so slack that the angled ridges of the roller depicted in FIG.  3 ( a ) are not compressed sufficiently to create lateral forces on the web. To overcome this difficulty, the spreader roller  34  is positioned at a small distance from the chill roller  33 . With sufficient web tension, the force of the actuation mechanism  36  causes the spreader roller  34  to impinge into the web a distance insufficient to trap the web between the spreader roller  34  and the chill roller  33 . At the area of contact with the spreader roller  34 , the web  14  is unimpeded by friction against the chill roller  33 , so the spreading action is optimal. However, during conditions of slack web, the force of the actuating mechanism causes the spreader roller  34  to “bottom out” against the chill roller  33 . This bottoming out causes pressure on the angled ridges of the spreader roller  34 . A lateral web-centering force is therefore still created, generating sufficient centering motion. 
     It is to be understood that the invention is not confined to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as may come within the scope of the following claims. It will be apparent that many modifications and variations are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced other than is specifically described. 
     Alternative embodiments and variations of the method taught in the present specifications may suggest themselves to those skilled in the art upon reading the above description. In particular, the example of a single-web press has been described for clarity, but the invention pertains to a multi-web press as well. The invention should not be constrained to the types of spreader rollers illustrated and described, but to any roller whose function is to spread a web. Although pneumatic actuators operating the spreader roller with lever arms is described, any of a number of arrangements are known in the art, which are capable of impinging a roller into a web. For example, a direct mechanical actuation of an electrical solenoid, or a chain drive, could also be employed. Also, any of a variety of actuation parameters, such as press speed, web color, or web temperature, could be used as criteria for removal of the spreader roller from the web, upon initiation of printing. 
     Various other features and advantages of the invention are set forth in the following claims.