Abstract:
In a fusing apparatus, such as used in xerography, two rolls form a nip therebetween. A guide member is positionable to direct a sheet approaching the nip to enter the nip at an angle so that an arc or buckle is crated in the sheet between a marking station and the nip. The arc or buckle is helpful in avoiding the transfer of mechanical energy from the fusing apparatus to the marking station. As the trail edge of the sheet exits the marking station, the guide member is positioned to straighten the sheet.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a fusing apparatus, such as used in electrostatographic printing.  
         BACKGROUND  
         [0002]    In electrostatographic printing, commonly known as xerographic or printing or copying, an important process step is known as “fusing.” In the fusing step of the xerographic process, dry marking material, such as toner, which has been placed in imagewise fashion on an imaging substrate, such as a sheet of paper, is subjected to heat and/or pressure in order to melt or otherwise fuse the toner permanently on the substrate. In this way, durable, non-smudging images are rendered on the substrates.  
           [0003]    Currently, the most common design of a fusing apparatus as used in commercial printers includes two rolls, typically called a fuser roll and a pressure roll, forming a nip therebetween for the passage of the substrate therethrough. Typically, the fuser roll further includes, disposed on the interior thereof, one or more heating elements, which radiate heat in response to a current being passed therethrough. The heat from the heating elements passes through the surface of the fuser roll, which in turn contacts the side of the substrate having the image to be fused, so that a combination of heat and pressure successfully fuses the image.  
           [0004]    One practical problem with certain compact designs of xerographic or other printers relates to the unintended transfer of mechanical energy, such as vibration or a torque transient, originating at the fusing apparatus and traveling through a print sheet while another portion of the print sheet is still receiving marking material (e.g., toner or ink) at the marking station. This vibration or other mechanical energy can cause a print defect such as smearing at the marking station.  
         DESCRIPTION OF THE PRIOR ART  
         [0005]    U.S. Pat. No. 5,822,668 describes a general configuration of a fuser module as used in a xerographic printer.  
         SUMMARY OF THE INVENTION  
         [0006]    According to one aspect of the present invention, there is provided a method of conveying a sheet from a marking station to a nip formed by a first roll and a second roll. A lead edge of the sheet is directed toward the nip at an angle which causes the sheet to form an arc between the marking station and the nip as the lead edge of the sheet enters the nip. As a trail edge of the sheet substantially exits the marking station, the sheet is straightened between the marking station and the nip.  
           [0007]    According to another aspect of the present invention, there is provided a printing apparatus, comprising a marking station; a nip, formed by a first roll and a second roll; means for directing a lead edge of the sheet toward the nip at an angle which causes the sheet to form an arc between the marking station and the nip as the lead edge of the sheet enters the nip; and means for straightening the sheet between the marking station and the nip as a trail edge of the sheet substantially exits the marking station.  
           [0008]    According to another aspect of the present invention, there is provided a fusing apparatus for printing, comprising a first roll and a second roll, forming a nip therebetween; and a guide member, the guide member being positionable to direct a lead edge of a sheet toward the nip at an angle which causes the sheet to form an arc as the lead edge of the sheet enters the nip.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a simplified elevational view showing the essential portions of an electrostatographic printer, such as a xerographic printer or copier, relevant to the present invention.  
         [0010]    [0010]FIGS. 2 and 3 are elevational views of a print sheet passing from a charge receptor to a fusing apparatus.  
         [0011]    [0011]FIG. 4 is a perspective view showing, in isolation, fuser rolls and a pivotably mounted guide member. 
     
    
     DETAILED DESCRIPTION  
       [0012]    [0012]FIG. 1 is a simplified elevational view showing the essential portions of an electrostatographic printer, such as a xerographic printer or copier, relevant to the present invention. A printing apparatus  100 , which can be in the form of a digital or analog copier, “laser printer,” ionographic printer, or other device, includes mechanisms which draw substrates, such as sheets of paper, from a stack  102  and cause each sheet to obtain a toner image from the surface of a charge receptor  104 . The toner image is transferred from the charge receptor  104  to the sheet by a transfer corotron  106 , and the sheet is detached from the surface of the charge receptor  104  by a detack corotron Once a particular sheet obtains marking material from charge receptor  104 , the sheet is caused to pass through a fusing apparatus such as generally indicated as  10 . (Although a charge receptor, as would be used in an electrostatographic printer, is shown, other types of “marking station,” such as including a ink-jet printhead and/or an intermediate transfer member, can be contemplated in conjunction with the claimed invention.) Depending on a particular design of a printing apparatus, fusing apparatus  10  according to the invention may be in the form of a fuser module which can be readily removed and installed, in modular fashion, from the larger apparatus  100 .  
         [0013]    A typical design of a fusing apparatus  10  includes a fuser roll  12  and a pressure roll  14 . Fuser roll  12  and pressure roll  14  cooperate to exert pressure against each other across a nip formed therebetween. When a sheet passes through the nip, the pressure of the fuser roll against the pressure roll contributes to the fusing of the image on a sheet. Fuser roll  12  further includes means for heating the surface of the roll, so that heat can be supplied to the sheet in addition to the pressure, further enhancing the fusing process. Typically, the fuser roll  12 , having the heating means associated therewith, is the roll which contacts the side of the sheet having the image desired to be fused.  
         [0014]    Generally, the most common means for generating the desired heat within the fuser roll  12  is one or more heating elements within the interior of fuser roll  12 , so that heat generated by the heating elements will cause the outer surface of fuser roll  12  to reach a desired temperature. Basically, the heating elements can comprise any material which outputs a certain amount of heat in response to the application of electrical power thereto: such heat-generating materials are well known in the art.  
         [0015]    As mentioned above, a practical problem with certain compact designs of xerographic or other printers relates to the unintended transfer of mechanical energy, such as vibration, originating at the fusing apparatus  10  and traveling through a print sheet while another portion of the print sheet is still receiving marking material (e.g., toner or ink) at the marking station such as charge receptor  104 . This vibration or other mechanical energy can cause a print defect.  
         [0016]    [0016]FIGS. 2 and 3 are elevational views of a print sheet S passing from a charge receptor  104  to a fusing apparatus  10  showing a method and apparatus which addresses the problem of transfer of mechanical energy through a print sheet. In addition to the elements described above, there is provided, just upstream of the nip formed by rolls  12 ,  14 , a movable guide member, or plate,  20 , which extends substantially the length of the rolls  12 ,  14 . Also adjacent the nip is an upper input guide  22 . In this embodiment, guide plate  20  is operatively associated with a bell crank  24 , which in turn is associated with a solenoid  26  and a tension spring  28 . The solenoid  28 , through the bell crank  24 , operates to selectably position guide plate  20  relative to the nip. Solenoid  28  is controlled via a control system (not shown) which is coordinated with the overall operation of the printing apparatus.  
         [0017]    With reference to FIG. 2, solenoid  26  is operated to position guide plate  20  so that a lead edge of the sheet S emerging from the charge receptor  104  (and still having a portion in contact with charge receptor  104 ) is directed toward the nip at an angle which causes the sheet to form an arc between the charge receptor  104  and the nip as the lead edge of the sheet enters the nip.  
         [0018]    While a sheet is passing through a printing machine with a portion thereof near or in the nip and another portion thereof still in contact with charge receptor  104 , it is possible that vibration or other mechanical energy from the fusing apparatus can travel through the sheet and cause a print defect for the portion of the sheet still in contact with the marking station. More specifically, when the sheet enters the fuser nip a large torque transient is imparted to the fuser nip and subsequent drive system. As a result of this sudden transient the whole system slows down momentarily, and the sheet decelerates as well. If the sheet is straight from the fuser nip back to the transfer zone, this deceleration will be directly seen at transfer, causing a smear as the sheet momentarily is moving backwards. If, as in FIG. 2, the sheet is formed into an arc between the fuser nip and the transfer zone, then this deceleration simply pushes back on the arc, which momentarily makes it higher, but does not affect the image in the transfer zone. This buckle or arc thus serves to dampen the energy due to the torque transient.  
         [0019]    The deliberate creation of an arc, or buckle, in sheet S between the marking station and the nip serves to lessen the ability for such a print defect to occur.  
         [0020]    However, when an arc is created in sheet S, such as shown in FIG. 2, there may be a danger that, once the trail edge of the sheet clears the charge receptor  104  (broadly, when the sheet substantially exits the marking station), the sheet may flick upward and contact the upper input guide  22 . Thus, it is desirable to remove the arc once the sheet has cleared the marking station.  
         [0021]    [0021]FIG. 3 shows the elements of FIG. 2, after the sheet S has begun to travel through the nip and the trail edge of the sheet has cleared the charge receptor  104 . Here, solenoid  26  is operated to position to guide plate  20  so that the arc shown in FIG. 2 is straightened and the balance of sheet S is moved straight through the nip.  
         [0022]    As part of a larger control system governing the entire printing apparatus, the control of the solenoid  26  or other device can be modified for optimal performance. For instance, once the guide plate  20  is positioned to create an arc in the sheet (as in FIG. 2), the guide plate can be withdrawn (toward the position in FIG. 3) to straighten the sheet at any time during the passage of a sheet S through the system, e.g., at some predetermined time before the trail edge of the sheet is expected to exit the marking station; in response to the trail edge being detected as passing a certain point in the sheet path; or in response to some detected physical condition such as a lack of vibration in the apparatus. Also, depending on a specific design, the motion of the guide plate  20  between the FIG. 2 and FIG. 3 positions can be, in various parts of a sheet-feeding cycle, relatively gradual or relatively abrupt.  
         [0023]    Although the illustrated embodiment shows guide plate  20  being positionable via a solenoid, other electromechanical devices for effecting the positioning are readily contemplated, such as a cam mounted on a rotating axle. If the fusing apparatus is in the form of a module (such as  10  in FIG. 1) which is readily removable and installable in a larger apparatus, the electromechanical device can be provided as part of the module, or the device can be part of the larger apparatus.  
         [0024]    [0024]FIG. 4 is a perspective view showing, in isolation, the rollers  12 ,  14  and a mounted guide plate  20 . In this embodiment, the guide plate  20  is pivotably mounted coaxially with roll  14 . In alternate embodiments, the guide plate  20  can be pivotably mounted relative to some other axis, or can be slidably mounted. If the fusing apparatus is in the form of a module (such as  10  in FIG. 1) which is readily removable and installable in a larger apparatus, the guide plate  20  can be provided as part of the module, or the guide plate can be part of the larger apparatus.