MXPA98000678A - Paper conditioner with transfer rollers / articulan backrest - Google Patents

Abstract

The present invention relates to a paper conditioner, for controlling the image-dependent screwing which uses one or more transfer rollers against rotating, are spaced in the space between copies, and which meet as the leading edge enters the knitted area. hold and separate when the rear edge is about to pass. This allows the rollers to have a less demanding stroke tolerance when assembling the rollers when the blade is at the clamping point and then jointing the separate rollers in the inter-copying space.

Description

PAPER CONDITIONER WITH ARTICULATING ROLLERS, TRANSFER / BACKUP This invention relates in general to a substrate conditioning device for an electrophotographic printing machine and more particularly relates to a device for dosing and applying moisture to sheets cut in a machine full-color photomechanical printing (process). In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential to sensitize its surface. The loaded portion of the photoconductive member is exposed to a light image of an original document that is reproduced. The exposure of the charged photoconductor member selectively dissipates the charges in the irradiated areas. This records a latent electrostatic image in the photoconductive member that corresponds to the information areas contained within the original document. After the latent electrostatic image is recorded on the photoconductive member, the latent image is revealed by bringing a developing material into contact. In general, the developer material comprises organic pigment particles that adhere triboelectrically to carrier granules. The organic pigment particles are attracted from the carrier granules to the latent image, forming a powder image of REF: 26390 organic pigment in the photoconductor member. The organic pigment powder image is then transferred from the photoconductive member to a copy sheet. The organic pigment particles are heated to permanently attach the powder image to the copy sheet. The above generally describes a typical black and white electrophotographic printing machine. With the emergence of multi-color eletrophotography, it is convenient to use an architecture comprising a plurality of imaging stations. An example of the architecture of multiple image formation stations, * uses an image-in-image system (101 = image-on-image) where the photoreceptor member is recharged, reshaped in image and revealed for each separation color. This loading, image formation, development and reloading, re-formation of image and development, all followed by transfer to paper, is carried out in a single revolution of the photoreceptor, in so-called one-step machines, while multi-step architectures for each color separation with a single charge, image and development, with separate transfer operations for each color. The one-step architecture offers a potential for high performance. In order to fix or fuse electroscopic organic pigment material on a support member by heat and pressure, it is necessary to apply pressure and raise the temperature of the organic pigment to a point at which the constituents of the organic pigment material become sticky and coalesce. . This action causes the organic pigment to circulate in a certain proportion within the fibers or pores of the support medium (typically paper). Subsequently, as the organic pigment material cools, solidification of the organic pigment material occurs, causing the organic pigment material to bind tightly to the support member. In both the xerographic arts as well as electrophotographic recording, the use of thermal energy and pressure to fix organic pigment images on a support member is old and well known. One approach to pressure and heat fix images of organic electroscopic pigment on a support has been to pass the support containing the organic pigment images between a pair of opposite roller members, at least one of which is heated internally. During operation of such a fixing system, the support member to which the organic pigment images are electrostatically adhered moves through the clamping point formed between the rollers and thus warms under pressure. A large amount of heat is applied to the organic pigment and the copy sheet containing the organic pigment image.

This heat evaporates a large amount of the moisture contained in the leaf. The amount of heat applied to the front and back sides of the. leaf, it's often different. This causes different evaporation of moisture from both sides of the sheet and contributes to the screwing of the sheet. A solution to this problem is set forth in the U.S. patent application. Serial No. 07 / 695,838; the contents of which are incorporated herein by reference. A second problem associated with loss of moisture on paper is the ripple of the paper. As the sheets pass through the fixation system, moisture is removed and the temperature of the sheet rises. If, after fixing the leaf, it is allowed to rest in a collection area fully exposed to its environment, its moisture content will be compensated by the environment by absorption of moisture through the entire face of at least one side of the leaf. of paper. However, if the copy sheet becomes part of a large compiled set, both sides of all the papers in the compilation (except for the top sheet) will effectively be sealed against moisture within the atmosphere. The only route available to this drying paper for moisture re-absorption is through the edges of the leaves, leaving the moisture content of the central portions of the leaves relatively unchanged. This non-uniform moisture re-absorption pattern results in edge tensions leading to paper curling on the edges of the paper. The resulting wave pattern can typically have an amplitude of 3.175 to 6.35 mm (1/8 to 1/4 of an inch). In addition to being cosmetically unpleasant, the wave at the edge creates a secondary handling problem since pages having this wave pattern on their edges are more difficult to feed than secondary paper handling machines, such as a binding apparatus. For this reason, printers continue to favor the use of rotocalcographic (offset) presses for large compilations. A number of solutions to this problem have been advanced. A proposed solution is to use a dampening system of rotocalcographic (offset) presses to add moisture to each sheet as it leaves the copier. These systems are typically based on the generation of a water reservoir in a roller interface to distribute the water evenly over the rollers. These systems usually operate with a supply of paper webs and their use with a cut sheet feeder system creates certain difficulties that were not previously contemplated or addressed. Normal damping systems are more suitable for use with conventional roto-rotogravure (offset) printers. The patent of the U.S.A. No. 4,375,327 attempts to solve the problem of enroscamiento of leaves, as it arises due to another cause: adherence of paper to a device of fixation with roller and does not address the problem caused by the loss of moisture. The patent of the U.S.A. No. 4,652,110 (the contents of which are hereby incorporated by reference) attempts to replenish moisture lost in the fixing process by collecting moisture, as it is removed from the copy sheet for reapplication to the sheet at a later time. There remains a need for a system to prevent edge ripple caused by the loss of moisture from the copy sheet during the stage of printing fixation or electrostatic reproduction that is practical for use with electrostatic machines. The conditioner described here is located after the melter and before a mechanical unscrewing. The function of the conditioner is to replace the moisture lost in the fusing process and thus reduce the image-dependent twisting. The approach taken to replace moisture consists in directing a sheet between two closely spaced rollers; a roller called the backing roller, is covered with rubber and moves forward the blade. The second roller, called the transfer roller, rotates in the opposite direction and applies a thin film of water to the paper on the side opposite the backing roller. The surface of the transfer roller is "wetted" by passing it through a flooded holding point. The film thickness deposited on the surface of the transfer roller is determined by the pressure between the transfer roller and a dosing roller. Two sets of these rollers are required to wet both sides of the sheet. One of the biggest problems is to ensure immediate proximity between the backing roller and the transfer roller. They should be placed in such a way that they do not touch in the gap space or otherwise the water will be applied to the surface of the backing roll that will be applied to the opposite side of the sheet in an uncontrolled manner. Also, they should be close enough that the thinnest blade can effectively move through the attachment point.

This requires running tolerances in the order of .0127 mm (.0005 inch) which is difficult and expensive to achieve in production. This invention describes an alternate method for mounting rollers that require less demanding tolerances. The following descriptions may be relevant to various aspects of the present invention. US-A-5,434,029 Inventor: Moser Issue Date: July 18, 1995 US-A-5,264,899 Owner: Mandel Issued: November 23, 1993 US-A-3, 705,451 Owner: Dahlgren Issued: December 12, 1972 US-A- 3,647,525 Owner: Dahlgren Issued: March 7, 1972 Portions of the above descriptions can be summarized as follows: The US patent No. 5,434,029 discloses an apparatus and method for preventing the screwing of a substrate having electrostatically adhered organic pigment images between substrates has been subjected to heat for the purpose of fixing the organic pigment images to the substrate. Simultaneous restriction of the copying substrate and the application of moisture, is carried out by passing the substrate through the clamping point formed by two rollers coupled by pressure, one of which is used to apply the water to the back side of the substrate, according to the Substrate passes through the fastening point mentioned later. The patent of the U.S.A. No. 5,264,899 describes a system for adding moisture to a copy sheet. The step of fixing the organic pigment of the electrostatic reproduction drains the paper, which can lead to the formation of a ripple on the edge of the sheet. The invention uses a pair of porous rollers defining a fastening point for transferring additional moisture to the copy sheet, as it passes through the fastening point. The added moisture prevents wave formation at the edge.

The patent of the U.S.A. No. 3,705,451 discloses a roller for material conditioning and fluid transfer and the method for preparing the same, wherein a metallic roller is chrome, the surface of the chrome is ground and polished to provide a very smooth and uninterrupted surface, the surface it is treated with a solution of chromium solvent such as hydrochloric or sulfuric acid, mixed with equal parts of water and gum arabic to remove chromium oxide from its surface and cover the surface with a coating that prevents oxidation to make the roller permanently hydrophilic . The patent of the U.S.A. No. 3,647,525 discloses a method and apparatus for applying a controlled amount of liquid to a moving web of a liquid receiving material, comprising a hydrophilic transfer roller of uniform finish in rotary pressure coupling with the web. The pressure between a smooth surface metering roller and the transfer roller is adjustable to precisely control the thickness of a fluid film having low viscosity which is supplied by the transfer roller to the web. The relative surface velocities of the transfer roller of the weft are adjustable to control the speed at which the dosed film is fed to the weft and to control the hydraulic pressure exerted on the film to move it to the weft.

A backup roller is used to further control the hydraulic fluid pressure that is transferred to the web. The dosing roller and the transfer roller are biased to regulate the moisture profile across the width of the weft. In accordance with one aspect of the present invention, a device for adding moisture to a copy sheet is provided. The device comprises a reservoir for storing a quantity of liquid, a pair of generally cylindrical rollers, each having an outer cylindrical surface, the rollers being aligned with each other on their axes to define a clamping point between the outer cylindrical surfaces, a dosing device in circumferential surface contact with one of the rollers, for controlling the fluid flow from the reservoir at least one of the rollers and a selectively operable support mechanism for separating and coupling the pair of rollers based on the location of a sheet. According to another aspect of the invention, there is provided a system for fixing an organic pigment image to a copy sheet in an electrophotographic system, to avoid the formation of a ripple on the edge of the copy sheet. The system comprises first and second splicing rollers defining a clamping point between them, at least one of the splicing rolls is heated, wherein the splicing rolls serve to fix an organic pigment image on a copy sheet through the application of heat and pressure to the copy sheet; a device for transporting a copy sheet from the fuser rollers to a sheet conditioning system, comprising: a reservoir for storing a quantity of liquid, a pair of generally cylindrical rollers, each having an outer cylindrical surface, the rollers they align with each other on their axes, to define a clamping point between the outer cylindrical surfaces, a dosing device that makes circumferential contact with one of the rollers to control the flow of fluid from the reservoir at least to one of the rollers and a support mechanism selectively operable to separate and couple the pair of rollers based on the location of a sheet. According to a further aspect of the invention, there is provided a method for replenishing the moisture lost by a copy sheet as it is heated in an electrophotographic machine, of the type having a thermal fuser. The method comprises the steps of transporting liquid from a reservoir to one of a pair of rollers that are arranged to form a clamping point therebetween, when they are selectively coupled, transporting the copy sheet from the merger through the clamping point of the rollers, transfer liquid from the rollers to the copy sheet and selectively detach the rollers when a sheet is not in the clamping point. Other features of the present invention will be apparent as discussed in the following description and with reference to the drawings, wherein: Figure 1 is a schematic elevation view of an image-in-picture electrophotographic single-pass printing machine in full color, using the device described here; and Figure 2 is a detailed elevation side view of the paper conditioning device. This invention relates to an image forming system that is used to produce color output in a revolution or single pass of a photoreceptor band. It will be understood, however, that it is not intended to limit the invention to the modality described. On the contrary, it is intended to cover all alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims, including a multi-step color process system, a color system highlighted in Multiple steps or single and a black and white printing system. Turning now to Figure 1, the printing machine of the present invention utilizes a retentive load surface in the form of an Active Matrix photo acceptor (AMAT) band 10 supported for movement in the direction indicated by arrow 12, to progress sequentially through the various xerographic process stations. The web is trapped with respect to an impulse roller 14, tension rollers 16 and fixed roller 18 and the roller 14 is operatively connected as a pulse motor 20 to effect movement of the web through the xerographic stations. With continued reference to Figure 1, a portion of the band 10 passes through the charging station A, where a corona generating device indicated generally by the reference number 22, charges the photoconductive surface of the band 10 to a potential relatively high, substantially uniform preferably negative. Next, the charged portion of the photoconductive surface is advanced through an image forming / exposure station B. At the image / exposure forming station B, a controller, generally indicated by the reference number 90, receives the image signals that represent the desired output image and process these signals to convert them to the various color separations of the image that are transmitted to a scanning device with laser-based output 24 which causes the charge retentive surface to be discharged from the according to the output of the scanning device. Preferably, the scanning device is a Laser Scanning Assembly (ROS = Raster Output Scanner). Alternatively, ROS can be replaced by other xerographic exposure devices such as LED arrays. The photoreceptor, which is initially charged at a voltage V0, is subjected to a loss of charge Vdd, at a level equal to approximately -500 volts. When exposed to exposure station B, Vexpose is discharged equal to approximately -50 volts. In this way, after exposure, the photoreceptor contains a monopolar voltage profile, the voltage is high and low, the last corresponds to charged areas1 and the first corresponds to unloaded or background areas. In a first developing station C, the developing structure, indicated generally by the reference number 42, using a hybrid jumping development system (HJD = Hybrid Jumping Development), the developing roller, better known as the donor roller, is energized by two development fields (potentials through an air space). The first field is the ac jump field that is used to generate the organic pigment cloud. The second field is the development field of, which is used to control the amount of organic pigment mass revealed in the photoreceptor. The cloud of organic pigment causes charged organic pigment particles to attract the latent electrostatic image. Proper developer shunt is achieved by an energy supply. This type of system is non-contact type, in which only organic pigment particles (black, for example) are attracted to the latent image and there is no mechanical contact between the photoreceptor and an organic pigment delivery device to disturb a previously revealed image, but not fixed. A corona charging device 36 having a characteristic slope of high output current against control surface voltage (I / V) is used to raise the voltage level of both the organic pigment areas and the non-organic pigment in the photoreceptor, at a substantially uniform level. The recharging device 36 serves to recharge the photoreceptor to a predetermined level. A second exposure / imaging device 38 comprising a laser-based output structure, is used to selectively discharge the photoreceptor in areas with organic pigment and / or bare areas, according to the image to be revealed with the second organic pigment color. At this point, the photoreceptor contains areas with organic pigment and no organic pigment, at relatively high voltage levels and areas with organic pigment and no organic pigment at relatively low voltage levels. These low-voltage areas represent image areas that are revealed using discharged area development (DAD Discharged Area Development). To this end, a relatively charged developer material 40, comprising colored organic pigment, is employed. The organic pigment which by way of example may be yellow, is contained in a developer housing structure 42 disposed in a second developer station D and is presented to the latent images in the photoreceptor by a second HSD developer system. An energy supply (not shown) serves to electrically derive the developing structure to an effective level to reveal the image areas discharged with negatively charged yellow organic pigment particles 40. f The above procedure is repeated for a third image forming apparatus of a third organic pigment of suitable color such as magenta or solferino and for a fourth image forming apparatus and organic pigment of suitable color such as azure blue. The exposure control scheme described below can be used for those subsequent imaging steps. In this way, an image of organic pigment composed in full color is revealed in the photoreceptor band. In the proportion in which some organic pigment charge is totally neutralized, or the polarity is reversed, thereby causing the composite image developed in the photoreceptor to consist of both positive and negative organic pigment, a dichorotron pre-transfer member Negative 50 is provided to condition the organic pigment for effective transfer to a substrate using positive corona chips. Subsequent to image development, a sheet of support material 52 is moved in contact with the organic pigment images in the transfer station G. The sheet of support material is advanced to the transfer station G by the feeding apparatus. of conventional sheets, not shown. Preferably, the sheet feeding apparatus includes a feed roller that contacts the uppermost sheet of a stack of copy sheets. The feed roller rotates to advance the uppermost sheet from the stack in a ramp which directs the advancing sheet of support material in contact with the photoconductive surface of the band 10, in a synchronized sequence, such that the image of developed organic pigment powder contacts the advancing sheet of support material in the transfer station G. The transfer station G includes a transfer dichorotron 54 which sprays positive ions on the back side of the sheet 52. This attracts the images of organic pigment powder negatively charged from the web 10 to the web 52. A detaching dichorotron 56 is provided to facilitate detaching the webs from the web 10. After transfer, the web continues to move, in the direction of the arrow 58, on a conveyor (not shown) advancing the sheet to the fusing station H. The fusing station H includes a fusing structure, i Generally designated by the reference numeral 60, which permanently fixes the powder image transferred to the sheet 52. Preferably, the fuser structure 60 comprises a heated fusing roll 62 and a backing or pressure roll 64. The sheet 52 passes between the fusing roll 62 and backing roll 64, with the image of organic pigment powder contacting the fusing roll 62. In this manner, the organic pigment powder images are permanently affixed to sheet 52. After fusing, a ramp not shown, guides the advancing sheets 52 to a receiving tray not shown, for subsequent removal of the printing machine by the operator. After the sheet of support material is separated from the photoconductive surface of the strip 10, the residual organic pigment particles transported by the non-image areas on the photoconductive surface are removed. These particles are removed in the cleaning station I, using a cleaning brush structure contained in a housing 66. It is considered that the above description is sufficient for purposes of the present application to illustrate the overall operation of a color printing machine .

As illustrated in Figure 2, the sheet conditioning device, generally referred to with the reference numeral 100, has transfer rollers 102, 103 that articulate in an almost vertical direction, such that when the leading edge of the sheets is in a vertical direction. entrance sheets 52 enters the areas of attachment point 106, 107, transfer rollers 102, 103 move towards the blade 52 to engage the rotatable backing rollers 104, 105, which are in a fixed position. Similarly, when the trailing edge of the sheet is about to leave the clamping points 106, 107, the transfer rollers 102, 103 move away from the sheet 52 to loosen the backing rollers 104, 105. Springs 126, 127 provide the normal force for the transfer rolls 102, 103 against the backing rolls 104, 105. Since the backing rolls 104, 105 are coated with rubber, a thick or thin sheet will deflect the rubber surface and provide strength of necessary impulse. The clamping points of the rollers 106, 107 come off in the space between copies eg .381 mm (.015 inch) and there is no danger that the backing rolls 104, 105 are wet. The wetting agent, in this case water, is distributed to the transfer rollers 102, 103 of the manifolds 110, 111, by means of dosing rollers 108, 109. The contact between the dosing rollers 108, 109 and the transfer rollers 102 103 can be adjusted by using adjustment screws 112, 113, which can be manually adjusted as illustrated, or adjusters 112, 113 can be moved by a motor (not shown) or other device to provide automatic adjustment depending on the desired film thickness in the transfer roller. The manifold 111 must be modified for the upper transfer roll 103 / dosing roller 109 structure, such that the wetting agent prevents it from dripping into the sheet and producing undesirable wetting characteristics. This can be achieved by using a liquid dam in combination with the upper dosing roller 109, to provide a flooded holding point. The amount of moisture added to a sheet is a function of the relative speed between sheet 52 and transfer rolls 102, 103, these transfer rolls 102, 103 are rotated in a direction opposite to the direction of the sheet as indicated by the arrows 99. A detector 130 located upstream of the first dampening holding point 106, detects the leading and trailing edge position of the blade and provides the necessary synchronization to close and open the clamping points 106, 107. For example, if the speed of the blade is known when it is in the detector 130 and the distance of the detector 130 to each wetting grip point 106, 107 and the speed between the fastening points and the speed of the blade at each fastening point is known , then it is a relatively simple algorithm to determine when to attach and detach each attachment point. Alternatively, a second detector 131 may be used between the attachment points 106, 107 to assist in determining the proper sequencing of the attachment / detachment of the attachment point. Only one of the many methods for separating clamping points 106, 107 is illustrated. In Figure 2, two stepped motors 120, 121 are illustrated by displacing two cams 122, 123. As each cam 122, 123 rotates in the direction clockwise separates the respective transfer roller 102, 103 from the respective backup roller 104, 105. In the position illustrated by the cam 122, the attachment point 106 can be separated by .381 mm (. 015 inch). When the cams are in the position illustrated by the cam 123, the cam surface does not touch the pivot arm 117, but the contact dimension is determined by the adjusting screw 129. A similar screw 128 is provided for the arm 116. This scheme uses two stepped motors 120, 121 which move cams 122, 123 through displacement members 124, 125. Alternate methods may employ solenoids, clutches, cables, etc. Also, alternative methods may articulate the backing rolls 104, 105 instead of the transfer rolls 102., 103. In recapitulating, a paper conditioner for controlling image-dependent screwing is provided, which utilizes one or more counter-rotating transfer rolls, which are initially spaced from their respective backing rolls in the space between copies, and which is They meet as the leading edge enters the area of attachment point and separate when the trailing edge is about to pass. Therefore, it is apparent that a device for packaging paper that fully satisfies the previously established objects and advantages has been provided according to the present invention. While this invention has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to cover all these alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, "property is claimed as contained in the following

Claims (17)

1. CLAIMS 1. A device for adding moisture to a copy sheet, characterized in that it comprises: a deposit for storing a quantity of liquid; a pair of generally cylindrical rollers, each having an outer cylindrical surface, the rollers align with each other on their axes in order to define a clamping point between the outer cylindrical surfaces; a dosing device that makes surface, circumferential contact with one of the rollers to control the flow of fluid from the reservoir to at least one of the rollers; and a support mechanism selectively operable to separate and couple the pair of rollers based on the location of a sheet. An apparatus according to claim 1, characterized in that the pair of rollers comprises: a first roller having an elastomeric coating for moving a sheet in a first direction through the fastening point; and a second roller, for applying liquid to one side of the sheet opposite the side contacting the first roller, the second roller has a smooth outer cylindrical surface, wherein the second roller rotates in a direction opposite to the direction of the first roller. An apparatus according to claim 1, characterized in that the metering device comprises: a metering roller in circumferential contact with one of the pair of cylindrical rollers and furthermore makes contact with the liquid in the tank; and an adjuster, to vary the contact force between the dosing roller and the first of the pair of cylindrical rollers in order to vary the amount of liquid transferred. An apparatus according to claim 1, characterized in that the selectively operable support mechanism comprises: an elongate support for holding the dosing device and one of the pair of cylindrical rollers in a tightly adjusted and pivotally mounted relationship, such that one of the pair of cylindrical rollers is movable from a position in contact with the second pair of cylindrical rollers to a position removed therefrom; an actuator, connected to the elongated support for pivotally moving the elongated support in such a way that one of the pair of cylindrical rollers moves from a position in contact with the second pair of cylindrical rollers to a position thereupon withdrawn. An apparatus according to claim 1, characterized in that it further comprises a detector, located adjacent to the pair of rollers for detecting a leading edge of a sheet and for generating a signal indicative thereof. An apparatus according to claim 5, characterized in that the pair of rollers comprises: a first roller having an elastomeric coating for moving a sheet in a first direction through the fastening point; and a second roller, for applying liquid to one side of the sheet opposite the side contacting the first roller, the second roller has a smooth outer cylindrical surface, wherein the second roller rotates in a direction opposite to the direction of the first roller. 7. An apparatus according to claim 5, characterized in that the dosing device comprises: a metering roller in circumferential contact with one of the pair of cylindrical rollers and further in contact with the liquid in the reservoir; and an adjuster for varying the contact force between the dosing roller and the first of the pair of rollers in order to vary the amount of liquid transferred. 8. An apparatus according to claim 5, characterized in that the selectively operable support mechanism comprises: an elongated support for holding the dosing device and one of the pair of cylindrical rollers in an adjusted and pivotally mounted relationship, such that the first of the pair of cylindrical rollers is mobile from a position in contact with the second of the pair of cylindrical rollers to a position removed from there; an actuator that responds to the signal from the detector, connected to the elongated support to pivotally move the elongated support, such that the first of the pair of cylindrical rollers moves from a position in contact with the second of the pair of cylindrical rollers to a position there withdrawn. 9. A system for fixing an image of organic pigment to a copy sheet in an electrophotographic system to avoid the formation of a wave on the edge of the copy sheet and a twisting in the body of the sheet, characterized in that it comprises: first and second merging rollers defining a fastening point therebetween, at least one of the merging rollers is heated, wherein the merging rollers serve to fix an image of organic pigment on a copy sheet through the application of heat and pressure to the copy sheet; a device for transporting a copy sheet from the fuser rolls to a sheet conditioning system, comprising: a reservoir for storing a quantity of liquid, a pair of generally cylindrical rollers, each having an outer cylindrical surface, the rollers being align themselves on their axes to define a point of clamping between the outer cylindrical surfaces, a dosing device that makes circumferential surface contact with one of the rollers, to control the flow of fluid from the reservoir to at least one of the rollers, and a support mechanism selectively operable to separate and couple the pair of rollers based on the location of a sheet. A system according to claim 9, characterized in that the pair of rollers comprises: a first roller having an elastomeric coating for moving a sheet in a first direction through the attachment point and a second roller, for applying liquid to one side of the sheet opposite the side that contacts the first roller, the second roller has a smooth outer cylindrical surface, wherein the second roller rotates in a direction opposite to the direction of the first roller. 11. An apparatus according to claim 9, characterized in that the dosing device comprises: a metering roller in circumferential contact with one of the pair of cylindrical rollers and further in contact with the liquid in the reservoir; and an adjuster for varying the contact force between the dosing roller and the first of the pair of cylindrical rollers in order to vary the amount of liquid transferred. 12. An apparatus according to claim 9, characterized in that the selectively operable support mechanism comprises: an elongated support for holding the dosing device and one of the pair of cylindrical rollers in an adjusted and pivotally mounted relationship, such that one of the pair of cylindrical rollers is movable from a position in contact with the second pair of cylindrical rollers to a position thereupon withdrawn, - an actuator, connected to the elongated support for pivotally moving the elongated support in such a way that one of the pair of cylindrical rollers it moves from a position in contact with the second pair of cylindrical rollers to a position thereupon withdrawn. An apparatus according to claim 9, characterized in that it further comprises a detector, located adjacent to the pair of rollers to detect a leading edge of a sheet and generate a signal indicative thereof. An apparatus according to claim 13, characterized in that the pair of rollers comprises: a first roller having an elastomeric coating for moving a sheet in a first direction through the attachment point and a second roller for applying liquid to a side of the blade opposite the side contacting the first roller, the second roller has a smooth outer cylindrical surface, wherein the second roller rotates in a direction opposite to the direction of the first roller. 15. An apparatus in accordance with the claim 13, characterized in that the metering device comprises: a metering roller in circumferential contact with one of the pair of cylindrical rollers and further in contact with the liquid in the reservoir; and an adjuster for varying the contact force between the dosing roller and the first of the pair of cylindrical rollers in order to vary the amount of liquid transferred. 16. An apparatus according to claim 13, characterized in that the selectively operable support mechanism comprises: an elongated support for holding the dosing device and one of the pair of cylindrical rollers in an adjusted and pivotally mounted relationship, such that one of the pair of cylindrical rollers is movable from a position in contact with the second pair of cylindrical rollers to a position thereupon withdrawn; an actuator that responds to the signal of the detector, connected to the elongated support to pivotally move the elongated support, such that the first of the pair of cylindrical rollers moves from a position in contact with the second of the pair of cylindrical rollers to a position withdrawal from there. 17. A method for replenishing the moisture that a copy sheet loses as it is heated in an electrophotographic machine of the type having a thermal fuser, characterized in that it comprises the steps of: transporting liquid from one reservoir to one of the pair of rollers that are arranged for form a point of attachment between them when they are selectively coupled; transport the copy sheet from the merger through the clamping point of the rollers, -transfer liquid from the rollers to the copy sheet; and selectively detaching the rollers when a sheet is not in the attachment point.