Temperature-self regulating fuser member

Abstract

 A heated fuser member (31) for use in the fusing apparatus of a xerographic copying machine, for fixing toner images to a support surface (22), includes a heating element (35) formed of a material which heats the fuser member to the required fusing temperature, and which is temperature-self regulating. The element (35) is formed of a semiconducting ceramic material having a positive temperature coefficient of resistivity, and exhibiting a Curie temperature transition point at which the resistance of the material increases with increasing temperature.

Description

[0001] This invention relates to fusing apparatus as is commonly used in xerographic copying machines and, more particularly, to a heated pressure fusing apparatus including a heated fuser member whose heating element is formed of a material which enables the fuser to be temperature-self regulating.

[0002] In a typical xerographic process a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark, and then exposed by being illuminated in an image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination, but retains its electrostatic charge in the dark areas, whi.ch areas correspond to the graphic material on the original document. The resulting electrostatic latent image is developed by depositing on the photoconductor a finely divided electrostatically attractable developing material (toner). The toner will normally be attracted to those areas on the photoconductor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This visible image of developing material is then transferred to a support surface, such as plain paper or any other suitable substrate, to become the ultimate copy. Any residual developing material remaining on the photoconductor is cleaned and the photoconductor is reused as described above for subsequent copies. The toner image that was transferred to the plain paper is then fixed thereto. Since the developing material is heat fusible, application of sufficient heat to the paper causes the developing material to melt and be fused into the paper so as to be permanently affixed thereto.

[0003] One very basic approach to fusing in a xerographic copying machine is the use of the so-called hot roll pressure fuser apparatus. Typically, in this apparatus, the paper with the toner image thereon is passed between a pair of opposed rollers, at least one of which is heated. Generally, the heated roll is formed of a hollow cylinder having a radiant heater, such as an infrared lamp or a halogen lamp, centrally located within the cylinder to heat the roll, in series with a bimetal thermostat. A typical example of this type of heated fuser roll is illustrated in U.S. Patent No. 3,637,976. During operation of the fusing apparatus, the paper to which the toner images are electrostatically adhered is passed through the nip formed between the rolls with the toner image contacting the fuser roll to effect heating of the toner image within the nip. Fusing is enhanced by the second roll or pressure roll as it is commonly called as the result of a biasing force which forces the rolls into engagement. The thermostat intermittently interrupts the current flow as the roll temperature reaches a predetermined value. The roll then cools to some lower temperature whereupon the thermos restores the current, and the roll heats up again.

[0004] Many of the problems that occur with the use of a hot roll-pressure fusing apparatus are in the heated fus- ing roll. In particular, these problems relate to the means employed for heating the fuser roll and its control. For example, in many of the known hot-roll fusers it is extremely difficult to maintain a constant temperature at the nip of the rollers where the actual fusing of the toner occurs, and where temperature control is critical. Temperature control is difficult because (1) it is difficult to sense the temperature in this region; (2) thermal lag, i.e., the responsiveness of roll temperature under varying demands of thermal output; and (3) there are both different machines modes, i.e., standby, off, continuous operation, and different size papers to contend with. The type of thermostat control as described above is conspicuously oscillatory in nature. The thermostat, by necessity being situated on the circumference of the roll in order to control the temperature of' that surface, is relatively remote from the heater and, thus, the temperature fluctuations are usually significant. Reductions in this aforesaid differential temperature characteristic requires extensive and expensive proportional feed-back control means. In addition to these problems, radiant- type heated fuser rolls generally require very high heating temperatures for the heating element to enable the roll temperature in the nip of the rollers to be high enough to melt the toner. The use of these high temperatures can result in deterioration of the fuser roll.

[0005] It is therefore an object of the present invention to overcome many of the disadvantages of the hot roll fusers described in the prior art and to provide a fuser member having a heating element which is temperature-self regulating and permits relatively simple control of temperature in the critical area where fusing occurs.

[0006] According to the present invention, a heating element for heating a fuser member is formed of a semiconducting ceramic material having a positive temperature coefficient of resistivity, the material exhibiting a Curie temperature transition point at which the resistance of the material increases with increasing temperature.

[0007] The present invention relates to the application of ceramic heating elements of a class known as positive temperature coefficient materials (PCT). The preferred ceramic material is described as ferroelectric and has the property of possessing low resistance up to some characteristic temperature known as the Curie temperature. Upon attaining this temperature, the electrical resistance of the ceramic material increases typically from 50 ohms to 5000 ohms or more within a span of less than ten (10) degrees centigrade. It is thus to be appreciated that such a material may be configured to furnish its own thermostat and, furthermore, since the effect is internal, pronounced and confined to a narrow temperature band, the oscillatory variations of temperature may be minimized. Such a system has advantages over the conventional and known methods of control. The use of these ceramic materials leads to superior control, the elimination of a conventional heater or thermostatic control system and therefore a more economical device. This also leads to a more reliable device since thermostats are somewhat prone to contact failure. The self-limiting feature eliminates temperature overshoot and promotes rapid heat up. Moreover, a relatively even temperature gradient may be attained at along the surface of the fuser member, avoiding large temperature fluctuations and eliminating centre to edge temperature differentials. Also, the invention avoids the use of high temperature heating elements, thereby avoiding deterioration of the fuser member.

[0008] In order that the invention may be more readily understood, reference will now be made to the accompanying drawings in which:-

Fig. 1 is a schematic sectional view of a copier; and

Figs. 2-4 are sectional views illustrating alternative embodiments of heating elements for use within a heated fuser member constructed in accordance with the present invention.

[0009] Referrinq now to the drawinqs and particularly to FIG. 1 thereof, there is shown an electrophotographic copying machine employing a fusinq device in which a heated fuser member in accordance with the present invention can be utilized. The various processinq stations shown in FIG. 1 will be represented in part as blocks and the processinq stations will only be briefly described. The particular copying machine illustrated in FIG. 1 is merely exemplary as far as the present invention is concerned for a complete understanding of an xeroqraphic proces and, in particular, how a fusing apparatus is employed in such a process. A fusin^q apparatus employing a heated fuser member in accordance with the present invention may be utilized in a wide variety of devices including coated paper copiers and plain paper copiers and is not necessarily limited to the particular type of copier system shown in FIG. 1.

[0010] In FIG. 1, the reference numeral 10 generally designates an electrophotographic copying machine which includes a rotating drum 1 having a photoconductive surface 12 secured around the outer surface of the drum. Any of numerous inorganic or organic photooonductive materials can be employed such as for example, a selenium alloy. Additionally, the photoconductor can be in the form of a belt instead of a drum. As drum 11 rotates in the direction of arrow 14, it passes through the various processing stations disposed around the periphery of the drum.

[0011] First, drum 11 rotates a portion of photoconductive surface 12 through a charging apparatus which includes a corona generating device 15 that is positioned closely adjacent the surface of the photoconductor. Corona generating device 15 imparts a uniform electrostatic charge to photoconductor surface 12.

[0012] An image of the document to be copied is transmitted to photoconductor surface 12 by the exposure and imaging station generally designated 16. This station could, for example, include a reciprocating carriage that is movably mounted on top of the copying machine cabinet. The carriage would include a transparent platen on which documents are placed faced down for copying. Overlying the platen would be a movable cover connected to one side of the carriage. An operator can raise and lower the cover and thereby place on or remove documents from the platen. A series of lamps would be used to illuminate the original document. By incorporating an optical system comprising mirrors and lenses a light image of the original document to be copied is projected onto the charged portion of photoconductive surface 12. The movement of the carriage and therefore the scanning of the original document is in timed relationship with the movement of rotating drum 11. Thus photoconductive surface 12 is selectively exposed to dissipate the charge thereon and record an electrostatic latent image corresponding to the indicia on the original document.

[0013] As drum 11 rotates, the latent image on photoconductive surface 12 is carried past a developer station 17. The developer material used can, for example, be a two component developer which comprises carrier particles having toner particles adhering thereto. The carrier particles are formed of a magnetic material while the toner particles are usually a heat settable plastic. However, a single component toner can also be used. Preferably a magnetic brush developing unit is used in which a rotating magnetic roll 18 picks up toner from a hopper 19 to form a rotating magnetic brush, and carries that toner into contact with the latent image on photoconductive surface 12. The charged or latent image areas of the photoreceptor electrostatically attracts and holds the toner particles, thus developing the latent image.

[0014] Transfer station 20 includes a corona transfer charging apparatus 21. In timed relationship with the arrival of the developed image at transfer corona 21, a copy sheet also arrives at transfer station 20. The copy sheet is fed from a supply of sheets 22 stored in removable tray 23. A feed roller 24 feeds the uppermost copy sheet from the supply 22, through paper guide 25 and into the nip of queuing rollers 26. At a predetermined time in the course of a copy cycle, the queuing rollers 26 are actuated to feed the copy sheet along paper guide 27 and into contact with the developed image carried on photoreceptor surface 12. By virtue of the electric charge that is generated by transfer corona 21, toner particles are attracted from photoreceptor surface 12 toward the copy sheet to which they loosely adhere. After transferring the toner powder to the copy sheet, the sheet is stripped away from drum 11 by a suitable apparatus, and advanced by, belt conveyor 28 to fixing station 29.

[0015] The copy sheet then passes into fixing station 29 which includes a fusing apparatus in which the toner material now residing on the copy paper is heated to a temperature at which the toner particles melt and are thereby fused into the copy paper so as to form a permanent copy of the original document. An example of a fusing apparatus including a fuser member that forms the basic subject matter of the present invention is illustrated in its operative position in FIG. 1. As shown, the fuser apparatus includes a heated fuser member or roll 31, and a backup member or roll 32. The copy sheet with the toner powder image thereon is interposed between fuser roll 31 and backup roll 32. A release material, e.g. polytetrafluoroethylene, can be on the fuser roll to prevent offset and allow for easy release of the paper from the roll. After the toner image is permanently affixed to the copy sheet, the sheet is separated from the fuser roll and advanced to a catch tray 33 for subsequent removal from the copier by an operator.

[0016] In order to remove residual toner particles which adhere to photoconductive surface 12 after the transfer of the powder image to the copy sheet, copying machine 10 is provided with a cleaning system generally designated by reference number 34. The cleaning mechanism can, for example, include a corona generating device and a brush which contacts photoconductive surface 12. First, the remaining toner particles are brought under the influence of the corona generating device to neutralize the electrostatic charge remaining on photoconductive surface 12 and that of the residual toner particles. Thereafter, the neutralized particles are removed from surface 12 by the rotatably mounted brush. After the cleaning operation, a discharge lamp can be used to discharge remaining charges on surface 12 prior to the recharging thereof at corona device 15 for the next copying cycle.

[0017] Referring now to the specific subject matter of the present invention a fusing apparatus is provided with a heated fuser member such as, for example, member 31, that includes as the means for heating the member, heating elements formed of a semiconducting ceramic material which has a positive temperature coefficient of resistivity and exhibits a Curie temperature transition point at which the resistance of the material increases with increasing temperature. The preferred semiconducting ceramic materials embodied within the present invention have a Curie temperature or transition temperature such that when the material reaches its particular Curie temperature the crystalline structure changes, i.e. from the tetragonal to the cubic phase. This transition in structure is accompanied by a marked change in electrical properties. In particular, the resistance of these materials increases by several powers of ten when their temperature is raised to their respective Curie temperatures. Many of these and other physical characteristics of these materials are explained in detail in Phillips Technical Review, volume 30, page 170, 1969 in an article entitled "PTC Thermistors as Self-Regulative Heating Elements" by E. Andrich.

[0018] The positive temperature coefficient materials embodied within the present invention when employed as the heating element for a heated fuser roll, imparts to the fuser roll the ability to operate as a self-regulating heat sonrce. At a given voltage the heating element in the fuser will draw a high current. This is because the material is cold andits resistance is low. Within a few seconds the Curie temperature of the material is reached, there is a sharp increase in resistance, e.g. from 50 ohms to 5,000 ohms, and an immediate restriction in the amount of power absorbed. Thereafter a state of equilibrium arises in which the power absorbed adjusts itself such that it is equal to the heat dissipated. Thus, the material tends to keep its temperature substantially in the vicinity of the Curie temperature. The particular ceramic material composition that is chosen for use as the heating element, of course, depends upon the fusing temperature requirements.

[0019] Of the ceramic semiconducting materials available it has been found in accordance with the present invention that ceramic semiconductors like barium titanate or a mixture of barium titanate with strontium titanate and/or lead titanate are suited for use as heating elements for a fusing apparatus because of the Curie temperatures which these materials exhibit. In accordance with the present invention, ceramic semiconducting materials that exhibit Curie temperatures within the range of about 150°C to about 220° are the preferred materials for use as the heating elements.

[0020] By adjusting the composition of the ceramic semiconducting materials, one can either raise or lower the Curie temperature of the material, and/or also impart semiconducting properties thereto. For example, by mixing barium titanate (BaTiO₃) with strontium titanate (SrTiO₃) or with lead titanate (PbTiO₃) when manufacturing the heating elements, compositions are obtained having a wide range of Curie temperatures, and therefor a correspondingly wide range of positive temperature coefficient of resistivity characteristics. Depending on the proportion of strontium titanate or lead titanate in these compositions, the Curie temperature is either raised or lowered. For example, the Curie temperature for barium titanate is about 120°C. In the formula (Ba_1-ySr_y)TiO₃ as the value of y goes from 0.1 to 0.7, the Curie temperature falls from approximately 70°C to about -110°C. On the other hand, considering the formula (Ba_1-xPb_x)TiO₃ as the value of x goes from 0.05 to 0.6, the Curie temperature of the composition rises from approximately 150°C to about 380°C. By adding both strontium titanate and lead titanate to barium titanate, a relatively wide range of Curie temperatures can be obtained. The addition of metallic ions to these ceramic materials imparts semiconducting properties thereto. For example, adding small amounts of lanthanum in the form of lanthanum titanate (LaTiO ₃), i.e. 0.3 mol %, is sufficient to impart semiconducting properties to barium titanate containing strontium or lead titanate.

[0021] FIGS. 2-4 illustrate some of the various ways that the ceramic semiconducting heating elements in accordance with the present invention can be employed in a heated fuser member. In FIG. 2 the heating element is in the form of a hollow cylinder 35, whereas in FIG. 3 several wafer shaped heating elements 36 are secured together to form a cylinder. FIG. 4 illustrates the use of heating elements in the form of a plurality of rods 37 spaced around the circumference of a core 38. Details of heated fuser members incorporating the heating elements as illustrated in FIGS. 3 and 4 are the subject of two commonly assigned and copending U.S. patent applications both entitled "Hot Roll Fusing Member with Semiconducting Ceramic Heating Elements" by Hugh St. L. Dannatt.

[0022] As an example of the temperature-self regulating properties of a heated roll according to the present invention, a roll was manufactured whose core was constructed of a plurality of 31.75 mm (1t") diameter wafer type heating elements (similar to that shown in FIG_. 3) formed of barium titanate. The heating elements were contained in a sleeve of aluminum which was overcoated with a layer of polytetra-fluoroethylene material, i.e. Teflon. A stretched rubber silicone belt was used with the heated roll instead of the standard pressure roll. Current was applied to the roll and its outer surface temperature heated to 103°C (218°F). After passing sheets of 22 mm by 28 mm (8½" by 11") paper between the fuser roll and the belt, the temperature of the outer surface of the roll was measured by a thermocouple applied to the surface of the roll. The results are shown in the attached table.

[0023] Although a slight amount of cooling of the fuser roll occurred due to the effect of the initial sheets of paper passing through the heated roll, the barium titanate heating elements regulated the temperature of the roll to a relatively even temperature gradient and avoided any large temperature fluctuations.

Claims

1 . A heated fuser member (31) for use in a fusing apparatus for fixing toner images to a support surface (22) and including a heating element for heating the fuser member, characterized in that said heating clement (35,36,37) is formed of a semiconducting ceramic material having a positive temperature coefficient of resistivity, said material exhibiting a Curie temperature transition point at which the resistance of said material increases with increasing temperature.

2. A fuser member as claimed in claim 1, characterized in that said material comprises barium titanate and metallic ions in sufficient quantity to impart the semiconductive properties to the material.

3. A fuser member as claimed in claim 2, characterized in that said metallic ion is lanthanum.

4. A fuser member as claimed in claim 2 or 3, char- acterized in that said material further comprises strontium titanate and/or lead titanate.

5. A fuser member as claimed in any preceding claim, characterized in that said material exhibits a Curie temperature ranging from about 150°C to about 220°C.

6. A fuser member as claimed in any preceding claim, characterized in that the heating element (35,36,37) is a roller structure.

Drawing