Method and apparatus for preheating and pressure-fixing a toner image
RELATED APPLICATION 
     This application is related to co-assigned U.S. patent application Ser. No. 07/754,489, filed Sep. 3, 1991, IMAGE FIXING DEVICE HAVING HEAT RECYCLING MEANS, J. P. Swapceinski et al. 
    
    
     TECHNICAL FIELD 
     This invention relates to the fixing of toner images carried on a receiving sheet. It is particularly useful in fixing color toner images carried on a heat softenable layer of a receiving sheet. 
     BACKGROUND ART 
     U.S. patent application Ser. No. 405,258, now U.S. Pat. No. 5,089,363, filed Sep. 11, 1989 in the name of Rimai et al, and U.S. Pat. No. 5,023,038, issued Jun 11, 1991 in the name of Aslam et al disclose a method and apparatus for fixing a multicolor toner image carried on a heat softenable outside layer of a receiving sheet. The receiving sheet is passed across a preheating plate to raise the temperature of the thermoplastic layer to or above its softening point. It is fed into a pressure nip created by a pressure roller and a belt or web backed by a heated roller. The belt or web is of a hard ferrotyping material such as stainless steel, nickel or the like. Relatively high pressure is applied between the belt and pressure roller to embed much or all of the toner image in the thermoplastic layer fixing the image. Some of the toner may be not entirely embedded but may be fused on the top of the layer, but with much of it embedded, the hard ferrotyping belt provides a photographic quality with an absence of relief and a high gloss. The image and heat softenable layer are retained in contact with the belt as it moves away from the pressure nip. The belt and receiving sheet are allowed to cool until the heat softenable layer is below its glass transition temperature. At this point it can be separated without offset. All this is accomplished without the use of offset-preventing liquids which would reduce the photographic quality of the image. 
     In the methods shown in these applications, the back of the receiving sheet is contacted by a heated plate just prior to the receiving sheet entering the pressure nip. The heated plate preheats the receiver up to or approaching the glass transition temperature of the heat softenable layer. This preheating permits the moisture in the receiving sheet to gradually escape while not constrained by a pressure nip and also allows the nip itself to be quite narrow as is generally required in higher pressure systems. 
     This approach to preheating, of course, requires a separate element, and space must be provided for it. To be most effective, it must be positioned as close as possible to the nip. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the invention to preheat receiving sheets that are to be heat-pressure fixed, especially receiving sheets of the type described in the Rimai et al and Aslam et al applications, with a simpler and more efficient construction. 
     This and other objects are accomplished by a method and apparatus which brings the receiving sheet into contact with the heated belt at a position substantially in advance of the pressure nip. The heated belt then directly preheats the heat softenable layer of the receiving sheet as the receiving sheet is transported by the belt into the nip. 
     According to a preferred embodiment, a pressure or squeegee roller is positioned and urged against the heated belt at the initial position of contact of the receiving sheet to assure contact at that position. When the invention is used with a receiving sheet having a heat softenable layer which contacts the belt, additional means are not necessary to hold the receiving sheet in contact with the belt until it reaches the pressure nip. Instead, the tackiness of the heat softenable layer when heated causes it to be sufficiently adhesive to the belt that it follows the belt into the nip. 
     With this structure, a separate preheating device is eliminated. Preheating is done directly to the heat softenable layer rather than through the back of the receiving sheet, which assures that the layer attains the correct temperature for fixing. At no time is it necessary to contact the opposite side of the sheet with a heating element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front schematic of a color electrophotographic apparatus in which the invention is particularly usable. 
     FIG. 2 is a front schematic of a portion of the fixing device forming a component of the apparatus shown in FIG. 1. 
     FIG. 3 is a perspective view of the fixing device shown in FIG. 2. 
    
    
     BEST MODES OF CARRYING OUT THE INVENTION 
     Although not limited thereto, this invention is particularly usable in a color electrophotographic apparatus substantially as shown in FIG. 1. According to FIG. 1, color electrophotographic apparatus 1 includes an image member 10 having an outside or peripheral image surface upon which a series of different color toner images are formed. The image surface can include various photoconductive and other layers making it electrophotosensitive and usable in electrophotography. It is uniformly charged at a charging station 11. The charged surface is imagewise exposed at an exposure station, for example, laser 13 to create a series of electrostatic images. The electrostatic images are each toned by a different one of toning stations 15, 16, 17 and 18 which contain different color toners to create a series of different color toner images. The different color toner images are transferred in registration to a receiving sheet 5 (FIG. 2) to form a multicolor image. The receiving sheet is fed from a receiving sheet supply 21 and secured to the outside surface of a transfer drum 20. 
     For highest quality work, the receiving sheet has a heat softenable outside layer to which the images are transferred. Such transfer is effected by heating the heat softenable layer, for example, by use of a lamp 23 located inside transfer drum 20. Image member 10 may also be heated to assist in the process by a lamp 25, but should not be heated to a temperature that would cause the toner to stick to the surface of image member 10 or affect the light sensitive properties of its photoconductive layers. Transfer drum 20 is rotated once for each color image to be transferred to cause the receiving sheet to receive the color images in registration, creating a multicolor image partially embedded in the heat softened outside layer. Image member 10 is cleaned by cleaning device 27 for reuse. 
     When all images have been transferred to the receiving sheet, it is stripped from transfer drum 10 by an articulatable skive 28 and transported by a suitable transport 29 to a fixing device 30. The multicolor toner image is fixed by fixing device 30 utilizing a combination of pressure and heat as will be more fully described. The toner image can be further treated by a texturizing or glossing device 50. It may also be cut into smaller images by a slitting, chopping or other cutting device 60 and ultimately deposited in output tray 70. 
     Transport device 29 is shown in FIG. 1 as a relatively short transport belt. If fuser 30 is to operate at a slower speed than transfer drum 20, the distance between transport drum 20 and fuser 30 needs to be longer than the longest sheet or a loop or other provision for taking up slack in the sheet must be provided. 
     Referring to FIGS. 1-3, fixing device 30 includes a ferrotyping belt 31 which is trained primarily around a pair of rollers, relatively large heated roller 32 and a small separation roller 33. Belt 31 can be any of a number of hard materials, including nickel, stainless steel and other metals, polyethylene, polypropylene and other high melting point plastics. It may be covered by a release material such as certain silicons, polyamides or polytetrafluoroethylenes. For highest quality work, no release liquid is used. Heated roller 32 is heated by an internal lamp 35. Roller 42 may also be heated, but preferably is not. 
     As best shown in FIG. 2, release sheet 5 is conveyed by transport device 29 into contact with belt 31 at a contact point 36 partially defined by a pressure or scuff roller 37. Scuff roller 37 is spring urged against belt 31 at contact position 36 where belt 31 is backed by heated roller 32. 
     Belt 31 is driven through an endless path by rotation of heated roller 32. As belt 31 moves in a generally clockwise direction, receiving sheet 5 follows belt 31 into a pressure nip 40 formed between belt 31 and a pressure roller 42. During the transport of the receiving sheet 5 from contact point 36 to nip 40 it is preheated by heated roller 32 through belt 31 to raise the temperature of the heat softenable layer to a temperature above its glass transition temperature. 
     Other rollers can be provided to maintain receiving sheet 5 against belt 31 between contact point 36 and nip 40. However, using a heat softenable layer of a polyester having a melting point of about 60° C. with a belt 31 maintained at about 100° C., the heat softenable layer becomes sufficiently tacky that it adheres to belt 31. It thus passes into nip 40 without the need of additional rollers to guide it. This aspect is effective even though receiving sheet 5 is of relatively stiff stock to give the final print a photographic quality. 
     The preheating step has several advantages. It continues to drive moisture out of receiving sheet 5, commonly a process begun in the transfer step. This is best accomplished when the receiving sheet is not in nip 40, since there is a tendency for the receiving sheet to blister if it still contains moisture as it comes out of nip 40. Further, preheating receiving sheet 5 allows the nip 40 to be relatively short in the intrack direction, which further permits both rollers 32 and 42 to be relatively hard, providing the pressure necessary for embedding the toner in the heat softenable layer and obtaining a high gloss. 
     Note that by contacting the heat softenable layer and belt 31 at the position of contact 36, the heat softenable layer is heated directly for preheating rather than being heated through the other side of the receiving sheet as in the prior art. This has the substantial advantage of assuring that the heat softenable layer is raised in temperature to the appropriate level for fixing without necessarily utilizing energy necessary to heat the rest of the sheet. It also eliminates a costly preheating device that, by necessity, had to be positioned almost into the nip 40 for greatest effectiveness. 
     Thus, both rollers 32 and 42 are hard rollers. For example, they may both have an aluminum or other metallic surface. Alternatively, pressure roller 42 can have a very thin elastomeric outer layer, for example, 4 mils thick. Such a construction still permits pressures in excess of 100 pounds per square inch but with a small amount of compliance, which compliance helps equalize fixing between the image and non-image areas. 
     After receiving sheet 5 passes through nip 40, it continues on belt 31 for a sufficient distance to allow it to cool until the heat softenable layer of receiving sheet 5 is below its glass transition temperature. At this point the belt 31 passes around small separation roller 33 and the receiving sheet, because of its stiffness, separates from belt 31 and passes on to be further processed. 
     The cooling process can be assisted by removing heat from belt 31 as it moves from nip 40 to separation roller 33. For example, as seen in FIG. 3, a cooling air manifold 43 can force cooling air against the back of web 31 to reduce its temperature. To increase the efficiency of the fixing device 30, heat may be removed from the cooling portion of the path of web 31, i.e., that portion of the path extending away from nip 40, by a roller heat exchanging device 49 and returned to web 31 at its lower portion which returns to heated roller 32. More specifically, a roller 46 having a heat insulating core, such as a thermoset plastic or glass, is coated with a thin layer of heat conductive material, for example, a 5 mil thick layer 45 of copper. Roller 46 is journaled for rotation by frictional engagement with belt 31 and is insulated on both sides by appropriate insulation material 44, which may also be a thermoset plastic. 
     Copper layer 45 contacts the lower surface of belt 31 in the portion of its path leading away from heated roller 32 and nip 40 and absorbs heat from it, lowering the temperature of the belt and raising the temperature of the copper layer 45. Belt 31 goes on to be air cooled by manifold 43 and returns toward roller 32 on the lower portion of its path at a somewhat cooler temperature. The heat absorbed from the upper portion of belt 31 in copper layer 45 is passed on to the lower portion of belt 31 thereby cooling the copper layer of roller 46 and warming belt 31 as it again approaches heated roller 32. 
     Copper layer 45 is thermally matched with belt 31. Its thinness allows its temperature to be substantially reduced by belt 31 as the belt returns to heated roller 32 so that it can then absorb substantial heat from the upper portion of the belt. 
     The following example is illustrative only; the temperature can be varied substantially while retaining the advantages of the devices, especially if different materials are used. A receiving sheet having a toner image on a heat softenable polyester layer having a glass transition temperature of about 60° C. and a toner image also having a glass transition temperature of about 60° C. is fixed with a device constructed according to the FIGS. and including a heated roller 32 that heats belt 31 to a temperature in excess of 100° C., for example, 130° C. Heated roller 32 and pressure roller 42 are urged together with sufficient force to create a pressure between belt 31 and pressure roller 42 in excess of 100 pounds per square inch, preferably in excess of 300 pounds per square inch. This pressure, of course, is partially a function of the hardness of the pressure roller 42, belt 31 being a hard ferrotyping belt. As belt 31 moves away from nip 40 and contacts heat exchanging roller 46, it has a temperature of between 110° and 130° C. That temperature is substantially reduced by heat exchanging roller 46 and cooling air manifold 43 until its temperature is as low as 35° C. as belt 31 passes around separation roller 33. At this temperature, the heat softenable layer on the receiving sheet separates readily from belt 31 without offset of toner or portions of the heat softenable layer onto the belt. This substantially cooled belt then is heated by as much as 30° C. when it contacts heat exchanging roller 46 again. At this point the layer 45 and the belt 31 reach a temperature equilibrium at approximately 60° C., which both warms belt 31 as it approaches heated roller 32 and cools conductive layer 45 for its return to contact of the upper portion of belt 31. This structure adds substantial thermal efficiency to the fixing device without the use of expensive exchange apparatus such as a heat pump or the like. 
     The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.