Abstract:
An external heater for a fuser assembly for a reproduction apparatus. The fuser assembly includes a fuser member for fusing a marking particle image to a receiver member. The external heater has a heat transfer surface adapted to be selectively engaged with the fuser member, and a device for heating said heat transfer surface. A mechanism is provided for engaging a variable portion of the heat transfer surface with the fuser member to selectively change the amount of heat transferred from the heating device to the fuser member through the heat transfer surface.

Description:
FIELD OF THE INVENTION 
   This invention relates in general to a fuser assembly for an electrographic reproduction apparatus, and more particularly to a variable power external heater for a fuser assembly to effectively regulate the fuser temperature according to the media type being fused. 
   BACKGROUND OF THE INVENTION 
   In typical commercial reproduction apparatus (electrostatographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon. 
   One type of fuser assembly for typical electrographic reproduction apparatus includes at least one heated roller, having an aluminum core and an elastomeric cover layer, and at least one pressure roller in nip relation with the heated roller. The fuser assembly rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member. 
   A typical roller fuser assembly of the prior art is shown in  FIG. 1 . The roller fuser assembly  10  includes a fuser roller  12 , a pressure roller  14 , and other necessary sub-systems and components (not shown). The roller  12  (or both rollers  12  and  14 ) is heated internally (for example by lamps  16 ,  18 ), and/or externally for example by heat rollers  20  (as shown in  FIG. 2 ), to pre-set temperatures. When fusing prints on receiver members S, the rollers  12  and  14  are pressed together to form a nip, and rotation of the rollers drive prints through the nip. In the nip, heat energy stored in the fuser roller  12  is transferred to the prints, and heats up and melts the toner image carried by the receiver member. Under the temperature and pressure, the toner is fixed on the receiver member. 
   The configuration of the fuser roller  12  can greatly affect the receiver member release characteristics and heat transfer of the fuser. Generally the fuser roller  12  has a metal core  22 , a base cushion  24 , and a thin release topcoat  26 . A thicker base cushion makes release geometry in the nip area more favorable for the receiver member to be released from the fuser roller  12 , but makes the heat more difficult to transfer from the core  22  to the outer surface of the topcoat  26 . 
   To preserve the favorable release geometry and improve the heat transfer characteristics, some kind of external heaters are often used.  FIG. 2  shows a fuser with one or two roller external heaters  20 , with heating lamps  30  inside. The metal external heating rollers  20  have high thermal conductivity and can transfer higher amount of heat than other external heating methodologies, such as radiation heating. They are also simple, less expensive, and present less potential fire hazards. However, since the external heating rollers  20  usually have small diameter, it is difficult to provided a large nip between an external heating roller and a fuser roller. This limits the heat transfer rate between an external heating roller  20  and a fuser roller  12 . Furthermore, a high force between the external heating roller  20  and the fuser roller  12 , may cause wear and damage to the fuser roller topcoat  26 . 
   SUMMARY OF THE INVENTION 
   It is the purpose of the invention to improve the heat transfer rate, and to reduce the force applied to a fuser roller having an external heater. Accordingly, this invention provides an external heater for a fuser assembly for a reproduction apparatus. The fuser assembly includes a fuser member for fusing a marking particle image to a receiver member. The external heater has a heat transfer surface adapted to be selectively engaged with the fuser member, and a device for heating said heat transfer surface. A mechanism is provided for engaging a variable portion of the heat transfer surface with the fuser member to selectively change the amount of heat transferred from the heating device to the fuser member through the heat transfer surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
       FIG. 1  is a side elevational view of a typical internally heated roller fuser assembly of the prior art; 
       FIG. 2  is a side elevational view of a typical externally heated roller fuser assembly of the prior art; 
       FIG. 3  is a side elevational view of a belt external heater according to this invention; 
       FIG. 4   a  is a side elevational view of a belt external heater according to this invention in a stand-by relation with a fusing roller; 
       FIG. 4   b  is a side elevational view of the belt external heater of  FIG. 4   a  in the operative relation with the fusing roller; 
       FIG. 5   a  is a side elevational view of an alternate arrangement for a belt external heater according to this invention in a stand-by relation with a fusing roller; 
       FIG. 5   b  is a side elevational view of the alternate arrangement of the belt external heater of  FIG. 5   a  in the operative relation with the fusing roller; 
       FIG. 6  is a side elevational view of another alternate embodiment of a belt external heater according to this invention; 
       FIG. 7  is a graph depicting heat transfer rate for the belt external heater according to this invention plotted against dwell time; and 
       FIG. 8  is a graph depicting temperature change on the surface of a fuser roller with, or without, engagement with a belt external heater according to this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the accompanying drawings,  FIG. 3  shows a belt external heater, according to this invention, for a roller fuser assembly. The belt external heater, designated generally by the numeral  40 , has a metal belt  42 , entrained about two metal rollers  44 ,  46 . The rollers  44 ,  46  have heater lamps  48 ,  50  coaxially disposed inside the rollers respectively. A linkage  52  is provided to keep the belt  42  in operative contact with the rollers  44 ,  46 . The linkage  52  necessary to keep the belt in proper operative relation with the rollers  44 ,  46  includes a pair of pivotally interconnected arms  54 ,  56  connected at the free ends to the rollers  44 ,  46 . The arms  54 ,  56  have respective seats  58 ,  60  that support a compression spring  62 . The compression spring  62 , or any other suitable mechanism, is used to urge the rollers  44 ,  46  in a direction to maintain the belt  42  under tension. 
   The heat transfer rate for a sample of the belt external heater  40  according to this invention is shown in the graph of  FIG. 7 . Two representative belt thickness values have been included. The metal belt  42 , according to this invention, has a thermal conductivity k in the range of about 1 to 250 Btu/(hr ft F), and preferably &gt;20 Btu/(hr ft F). The thickness of the belt  42  is in the range of about 0.1 to 20 mils, and preferably 5 to 10 mils. The outer surface of the belt  42 , which contacts the fuser roller assembly  10 , preferably has a chrome coating and is polished such that the roughness of the belt is better than 0.1 μm Ra. 
   The desired contact length for the belt  42  to the fuser roller  12  is determined in the following manner. First, the maximum heat transfer rate needed for the fuser roller is determined. The dwell time DT, from for example  FIG. 7 , is then determined. The contact length L is calculated according to the formula:
 
 L=DT·V 
 
where V is the fuser roller surface speed. The whole belt length can then be determined from L by geometry calculations.
 
   The belt external heater  40  can deliver variable power between 0 and a maximum value P max  to the fuser roller. The heat power P transferred from the external heater to the fuser roller has the following relationship with the heat transfer rate R, the belt width W, the fuser roller surface speed V, and the temperature difference ΔT between the belt and the fuser roller:
 
 P=R·W·V·ΔT 
 
The belt width W and the fuser roller surface speed V are design constants, and the temperature difference ΔT between the belt  42  and the fuser roller  12  is a process constant. Therefore, changing R will change P. In this example of the preferred embodiment of this invention, the heat transfer rate R of the proposed external heater can be improved to 300 to 450 J/(m2 K).
 
   Referring to  FIGS. 4   a ,  4   b , and  5   a ,  5   b , two arrangements for the belt external heater  40  according to this invention are shown for variable power delivery to the fusing roller  12 . In  FIG. 4   a , the external heater is placed symmetrically relative to the fuser roller  12 . As shown in  FIG. 4   b , the entire heater  40  is movable towards or away from the fuser roller  12 . The belt contact length can then be changed from 0 to a maximum value, letting DT change from 0 to DT max  and P from 0 to P max . In  FIG. 5   a , one heating roller (for example roller  44 ) is fixed about its longitudinal axis so as to be just touching the fuser roller  12 , and the other roller (for example roller  46 ) can be moved about the longitudinal axis of the first roller, as shown in  FIG. 5   b . Similarly the belt contact length can be changed from 0 to a maximum value resulting in the heat transfer power P changing from 0 to P max . The selection of the actual dwell time DT and heat transfer power P for desired control of the belt external heater  40  can be accomplished based on the information of the receiver member and image content to be fused, based on the fuser roller temperature and like, or a combination of above two. 
   In an alternate embodiment of the belt external heater according to this invention, designated by the numeral  40 ′ in  FIG. 6 , extra lamp(s)  64 ,  66  can be included inside the belt loop to further increase the heat power transfer capability. 
   An additional aspect of this invention is that the high thermal conductivity of the metal belt  42  can also be used to help to reduce the inter-frame temperature unevenness. The space between two successive receiver members in transport is called the “inter-frame”. During the interframe, the fuser roller  12  contacts the pressure roller  14  (see  FIG. 1 ) and transfers less heat out of the fuser assembly  10 . Therefore, the portion of fuser roller surface at the interframe has a higher temperature than that portion of the fusser roller contacting the receiver member. When this higher temperature portion of fuser roller surface contacts the next print receiver member, the higher temperature may cause a higher gloss for the fused toner image. Typically, if this gloss difference is gradual, one will not be able to see it. However, the sudden temperature change at the receiver member&#39;s lead and/or trail edge highlights this gloss difference and makes it a noticeable and objectionable gloss artifact. The high thermal conductivity metal belt  42  helps to dissipate this temperature difference more quickly, thus reducing or altogether eliminating such gloss artifact. The graph of  FIG. 8  compares the temperature diffusing on the fuser roller surface with or without the metal belt after 0.1, 0.5, and 1 second. That is, after 0.5 seconds of dwell time, the temperature difference starts diffusing within 0.5 inches apart, and makes a gradual temperature change on the fuser roller surface. If the belt external heater  40  according to the present invention were to be used solely for reducing the inter-frame gloss artifact, a dwell time &gt;0.1 seconds is needed. 
   Thus, the principle advantages of the belt external heater according to this invention include the transfer of much higher thermal power to the fuser assembly  10  roller, deliverability of variable power to the fuser roller  12 , and the reduction or elimination of the inter-frame gloss artifact. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
   PARTS LIST 
   
       
         10  Roller fuser assembly 
         12  Fuser roller 
       Pressure roller 
         16 ,  18  Lamps 
         20  Heat roller 
         22  Metal core 
         24  Base cushion 
         26  Release topcoat 
         30  Heating lamps 
         40  Belt external heater 
         40 ′ Belt external heater 
         42  Metal belt 
         44 ,  46  Metal rollers 
         48 ,  50  Heater lamps 
         52  Linkage 
         54 ,  56  Arms 
         58 ,  60  Seats 
         62  Compression spring 
         64 ,  66  Lamps 
       S Receiver member