Fuser apparatus and method

Abstract: A heat and pressure fusing apparatus for fixing toner images (20) to substrates (22) such as plain paper, the toner comprising a thermoplastic resin. The apparatus includes two nip forming members (26, 30) which cooperate to form a nip having an asymmetrical pressure profile. More particularly, the pressure profile through the nip, from entrance to exit, is such that toner images on a substrate passing through the nip are first subjected to relatively low pressure which continues until the toner begins to flow as a result of being heated. Once toner flow commences, the images are subjected to pressure high enough to force the toner into the substrate. The pressure profile of the nip can be varied to accommodate different fusing speeds for different processors.

Description: This invention relates generally to copying or printing apparatus and, more especially, it relates to the fusing in such apparatus of toner material. In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, by the application of heat and pressure. In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky, while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of the support member or otherwise upon the surface thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known. One approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated roll thereby to effect heating of the toner image within the nip. With this conventional type of arrangement, the temperature profile through the nip is somewhat exponential while the pressure profile is symmetrical. A plot of nip pressure versus position in the nip yields a somewhat parabolic shape. A symmetrical pressure profile results in the application of the highest pressure to toner which has not yet been heated sufficiently to be in a molten state. This results in wasted mechanical energy. As fuser speeds increase, it becomes more and more difficult to obtain adequate nips using roll fusers because an increased nip width is required. Nip width varies approximately as the square of the roll diameter so that for example, doubling the process speed would require double the nip width which, in turn, would require the fuser and pressure roll diameters to be increased by a factor of four. In addition, larger rolls require higher loads and produce an inferior release geometry. The foregoing drawbacks do not apply to belt fusers which are known in the prior art. Thus, belt fusers of the prior art have been provided with larger nip areas in order to allow faster fusing speeds However, all known prior devices inherently waste mechanical energy due to their symmetrical pressure profiles. U.S. patents Nos 4,563,073 and 4,565,439 each disclose a belt fuser for fixing toner images The fusing apparatus is characterized by the separation of the heat and pressure functions such that the applications of heat and pressure are effected at different locations on a thin flexible belt forming the toner contacting surface A pressure roll cooperates with a stationary mandrel to form a nip through which the belt and copy substrate pass simultaneously The belt is heated such that by the time it passes through the nip its temperature is sufficient together with the applied pressure for fusing the toner images passing therethrough. A release agent management system comprising low mass donor and metering rolls, one of which is in contact with the belt, applies silicone oil to the belt without unacceptably reducing the fusing capability of the belt. There is disclosed in GB-A-2050943 a fusing device for an electrostatic copier which has a flexible thermally conductive web driven between first and second pressure fixing rollers that are further supported by a rotatable third pressure roller bearing on the web and the second pressure fixing roller. The entrapped copy sheet follows a concave path formed by the web and the surface of the first pressure fixing roller. The present invention provides a fuser apparatus, comprising first and second nip forming members; means for applying a load between said members to thereby form a nip therebetween, said nip having a substrate entrance zone and a substrate exit zone; said first nip forming member comprising means engaging the second nip forming member with different degrees of pressure when said load is applied, whereby said nip has an assymetrical pressure profile; characterised in that said first member comprises a stationary mandrel around which a belt passes, the portion of the mandrel adjacent said exit zone of the nip being closer to the other fuser member than the portion of the mandrel adjacent said entrance zone. The nip may be so arranged that the pressure in said entrance zone is lower than the pressure in said exit zone. When the apparatus is used to fuse a toner image to a substrate, the pressure profile through the nip may be such that the image-carrying substrate initially moves through the nip at a low pressure (only large enough to ensure good thermal contact with the toner and minimum image shifting) while the toner is being heated to a molten state. Thereafter, the molten image may be subjected to a high pressure pulse which forces the molten toner into the substrate. The present invention further provides a method of fusing toner to a substrate, said method including the step of: providing first and second nip forming members,said first nip forming member comprising means engaging the second nip forming member (30) with different degrees of pressure when said load is applied, whereby said nip has an assymetrical pressure profile; applying a load between said members to thereby form a nip therebetween, said nip having a substrate entrance zone and a substrate exit zone; characterised by providing as said first member a stationary mandrel around which a belt passes, and forming said nip with said first and second nip forming members such that the portion of the mandrel adjacent said exit zone of the nip is closer to the second member than the portion of the mandrel (42) adjacent said entrance zone. In another aspect, the method includes the step of: heating a substrate carrying toner, and applying pressure to the substrate by passing the substrate through a nip, whereby the peak nip pressure and the temperature at which the toner becomes molten occur at approximately the same position in the nip as the substrate moves therethrough. Preferably, said nip has an entrance zone where said toner images are subjected to low pressure and exit zone where said toner images are subjected to high pressure thereby providing a delay in the application of the high pressure until the toner forming the images becomes molten A method in accordance with the invention may include the step of varying the pressure profile to which the substrate is subjected, for example by altering the dwell time in the entrance, or low pressure, zone Apparatus and methods in accordance with the invention may be utilized in copying apparatus to fuser a toner image to a substrate. By way of example, apparatus and methods in accordance with the invention will now be described with reference to the accompanying drawings, in which: FIGURE 1 is a plot of temperature and pressure versus time for a typical prior art fuser; FIGURE 2 is a plot of temperature and pressure versus time for the fuser of Figure 3; FIGURE 3 is a side elevational view, partly in cross-section of an embodiment of the present invention; and FIGURE 4 is a plot of the load or pressure in the nip of the fuser of Figure 4 versus position in the nip. FIGURE 1 illustrates temperature and pressure profiles through a conventional roll fuser nip from its entrance to its exit. A plot of pressure versus time (or distance through the nip) represented by reference character 10 shows that the pressure profile through the nip is symmetrical and has a somewhat parabolic shape. It also shows that the maximum pressure P₁ in the nip coincides with a toner temperature T₁. At the temperature T₁ which occurs at the maximum pressure the toner forming the images on a substrate has not been sufficiently heated to cause the toner to flow or become molten. Accordingly, the mechanical energy employed at that time to force unmolten toner into the paper is wasted. Figure 2 illustrates temperature and pressure profiles through a fuser nip constructed in accordance with the present invention, which will be discussed in greater detail hereinafter. A plot of pressure versus time represented by reference character 12 shows that the pressure profile through the nip is asymmetrical. It further shows that the peak pressure P₂ coincides with peak toner temperature T₂, the temperature at which the toner has become molten and commences to flow. FIGURE 3 depicts a heat and pressure fuser apparatus 14 in which the temperature and pressure profiles illustrated in FIGURE 2 are present in the nip thereof. The fuser apparatus 38 disclosed in Figure 3 comprises a relatively thin fuser belt structure 16 comprising a base member preferably fabricated from a metal material such as nickel by a conventional electroforming process which provides a uniform thickness in the order of 2-3 mils (51-76 µm). The outer surface of the base member is coated with a conformable layer which preferably comprises silicone rubber. The inner surface of the base member is preferably coated with a low friction material such as polytetrafluoroethylene, commonly known by the tradename Teflon (registered trademark of E. I. duPont). The thickness of the conformable layer is preferably at least 5 mils (127 µm). The belt 16 is heated to a temperature suitable for fusing toner images 20 carried by copy substrates 22 by means of an internally heated roller 40 containing a radiant lamp or heater 18. The roller structure 40 is fabricated so that it is transparent to the radiant energy from the lamp 18. A stationary mandrel 42 is utilized for cooperating with the roller 40 for operatively supporting the belt structure 16. The mandrel 42 cooperates with a pressure roller 30 to form a nip 28 through which substrates carrying toner images pass with the images contacting the heated belt structure. The force necessary to effect nip pressure between the mandrel and pressure roll is provided by means of a rotary cam 44. The mandrel 42 is configured such that when the load is applied via the cam 44 the pressure profile created in the nip is as depicted in Figure 4 To this end, the portion of the mandrel adjacent the exit zone of the nip is closer to the roller 30 than the portion adjacent the entrance zone, producing a low pressure zone at the nip entrance leading to a high pressure zone at the nip exit. As illustrated in Figure 4, the pressure through the nip is asymmetrical so that the peak pressure in the nip does not occur before the temperature of the toner images is sufficiently high to cause the toner to be somewhat molten and able to flow into the substrate when the pressure is applied. The mandrel 42 also has a small radius of curvature at the nip exit zone, resulting in good stripping. To summarize, in the fuser apparatus shown in Figure 3, the fusing nip is configured such that the pressure profile through the nip from its entrance to its exit is asymmetrical. Thus, the toner image initially moves therethrough at a low pressure (only large enough to insure good thermal contact and minimum image shifting) while being heated to a molten state. Then the molten image is subjected to a very high pressure pulse which forces the molten toner into the substrate This provides for several advantages, one being that mechanical energy is not wasted in trying to force unmolten toner into the substrate. Another advantage is that the peak pressure is provided at a more optimum time, e.g. closer to the nip exit. Another advantage is that the length of the low pressure (entrance) zone can be easily adjusted to provide adequate dwell at almost any process speed, thus enabling a very high speed fuser.

Claims: A heat and pressure fuser apparatus, comprising: first (42,16) and second (30) nip forming members; means (44) for applying a load between said members (42,30) to thereby form a nip (28) therebetween, said nip (28) having a substrate entrance zone and a substrate exit zone; said first nip forming member (42,16) comprising means (16) engaging the second nip forming member (30) with different degrees of pressure when said load is applied, whereby said nip (28) has an assymetrical pressure profile; characterised in that said first member comprises a stationary mandrel (42) around which a belt (16) passes, the portion of the mandrel (42) adjacent said exit zone of the nip (28) being closer to the second member (30) than the portion of the mandrel (42) adjacent said entrance zone. Apparatus according to claim 1, wherein said nip (28) is arranged so that the pressure in the entrance zone thereof is lower than the pressure in the exit zone thereof. Apparatus according to claim 1 or 2 for fusing toner material (20) to a substrate (22) as the toner-carrying substrate moves through the nip (28), the apparatus including means (18) operable to heat the toner whereby the toner (20) becomes molten at the location of maximum pressure in the nip. Apparatus according to any of the preceding claims, including heating means (18) arranged to heat the belt (16), the toner (20) being heated by contact with the belt (16) as the substrate moves through the nip (28) A method of fusing toner to a substrate, said method including the step of: providing first (42,16) and second (30) nip forming members,said first nip forming member (42,16) comprising means (16) engaging the second nip forming member (30) with different degrees of pressure when said load is applied, whereby said nip (28) has an assymetrical pressure profile; applying a load between said members (42,30) to thereby form a nip (28) therebetween, said nip (28) having a substrate entrance zone and a substrate exit zone; characterised by providing as said first member a stationary mandrel (42) around which a belt (16) passes, and forming said nip with said first (42,16) and second (30) nip forming members such that the portion of the mandrel (42) adjacent said exit zone of the nip (28) is closer to the second member (30) than the portion of the mandrel (42) adjacent said entrance zone. 6. The method of claim 5, including the steps of: heating a substrate carrying toner, and applying pressure to the substrate by passing the substrate through a nip, whereby the peak nip pressure and the temperature at which the toner becomes molten occur at approximately the same position in the nip as the substrate moves therethrough. 7. The method according to claim 6 wherein said nip has an entrance zone where said toner images are subjected to low pressure and exit zone where said toner images are subjected to high pressure thereby providing a delay in the application of the high pressure until the toner forming the images becomes molten.

Inventors: FIELD JOHN R, KARZ ROBERT S, MATHERS JAMES E, MOSER RABIN, FIELD, JOHN R., KARZ, ROBERT S., MATHERS, JAMES E., MOSER, RABIN
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1989
Main classification: G03G15
IPC codes: G03G15