Heater control of fuser roller

Abstract: A control arrangement for the heater of a fuser roll (12) for fixing toner images to copy substrates. A light chopper (36) rotates with the fuser roll and generates signals which actuate a switch (42) operably coupling a power source (46) with the fuser roll heater (30). The heater (which may be in the form of a filament lamp) is energized at maximum power at a duty cycle of 10 %: this provides for a fail-safe arrangement in the event that the switch (42) short circuits because the heater then fails through being overloaded before the fuser roll becomes overheated.

Description: This invention relates generally to xerographic copying apparatus, and more particularly, it relates to the heat and pressure fixing of particulate thermoplastic toner by direct contact with a heated fuser roller. In the process of conventional 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, as by 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 support members or otherwise upon the surfaces 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 (fuser) roll thereby to effect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereat the toner particles in the image areas of the toner liquefy and cause a splitting action in the molten toner resulting in "hot offset." Splitting occurs when the cohesive forces holding the viscous toner mass together are less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll. Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip. Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus. One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting," has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, known by the tradename Teflon to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of approximately 50 to 100 micrometers (several mils) and the thickness of the oil being less than 1 micrometer. Silicone based (polydimethylsiloxane) oils which possesses a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface. A fuser roll construction of the type described above is fabricated by applying in any suitable manner a solid layer of abhesive material to a rigid core or substrate such as the solid Teflon outer surface or covering of the aforementioned arrangement. External fuser roll heating, for example, using a heat lamp, has been recognized to have substantial unit manufacturing cost (UMC) and size benefits over the more conventional internal roll heating methods for high speed applications (about 600 mm per second). However, external fuser roll heating exhibits significantly increased fire risk as well. Thus, the viability of external heating of a fuser roll is dependent on the provision of a fail-safe method of insuring that a fire will not occur. Most fuser roll temperature controllers use either a triac or two silicon controlled rectifiers (SCRs) to switch the external heater on and off. When a low level signal is supplied to these switching devices they become conductive and remain on (latched) until the current drops to zero. In the case of a heat lamp powered by 60 Hz AC power, for example, zero crossing occurs 120 per second. Thus, in normal operation, shutdown occurs within 1/120th of a second of loss of the control signal. The problem with these switching devices is that they often fail by short circuiting. In this case, the over-temperature sensor near the fuser roll would eventually shut down the fuser. However, such sensors take tens of seconds to minutes to activate in the event of a triac or SCR failure, far too long for external heating. In addition, over-temperature sensors provide no protection should the fuser roll stop rotating. In JP-A-58 144 868 an arrangement is described to prevent the burnout of a fixing device. In this arrangement an automatic timer is set in dependence upon the detected temperature of the fixing device thereby enabling a quick interruption of the electrical power when abnormally high temperatures are detected. JP-A-5 767 971 discloses an arrangement in which abnormal rises of temperature in a halogen lamp are detected by monitoring the electrical conduction through the lamp using a timer circuit, the timer circuit operating a relay for disconnecting the power. The present invention provides a contact fuser apparatus having a fuser roll structure rotatable in contact with a backup roll structure and forming a nip therewith through which copy substrates carrying toner images pass with the toner images contacting the fuser roll structure a heat source for elevating the temperature of said fuser roll structure; switching means operably coupling said heat source and a source of alternating power; and actuating means for periodically actuating said switching means, when the temperature of the fuser roll structure falls below a predetermined temperature set point, to couple periodically said heat source to said source of alternating power whereby the duty cycle of the power signal energising the heat source is such as to prevent overloading of said heat source whilst effecting operation of said heat source at approximately full power dissipation, and whereby any circuit failure causing continuous energization of said heat source will lead to power interruption and safe shutdown through failure of the heat source before fire or overheating can occur, said actuating means including means responsive to the rotation of one of the roll structures for generating a string of electrical pulses indicative of roll structure rotation, the frequency of operation of said actuating means being low relative to the frequency of the alternating current energizing the heat source to ensure that the heat source is operated at a maximum time average power dissipation that does not exceed the lamp power rating of the heat source. In an embodiment of the present invention, a slotted disk is attached to a fuser roll for use as an optical chopper that, in conjunction with a light sensor, delivers a string of electrical pulses only when the fuser roll is rotating. The pulses are conditioned by the selection of a slot width and/or use of a capacitance filter to insure that the pulses are shorter than the half cycle of the AC supply to the heat source (in the case of a 60 Hz supply that is less than 1/120th of a second). In addition, the pulse frequency is much lower than that of the AC supply so the duty cycle of the heat source is low (e.g. less than 20% and the lamp is energized less than one fifth of the cycles). Zero crossing sensing circuitry could, of course be employed to reduce RF noise generated when a triac or SCR is activated mid cycle by delaying the turn-on command until the start of the next cycle. By way of example, an embodiment of the invention will now be described with reference to the accompanying drawing (one FIGURE only) which is a side elevational schematic view of a heat and pressure fuser. Referring now to the FIGURE, a roll fuser apparatus is generally indicated 10. The fuser apparatus shown comprises an externally heated roll structure 12 cooperating with a non-heated backup or pressure roll structure 14 to form a nip 16 through which a copy substrate 18 passes with toner images 20 formed thereon in a well known manner. However, this invention is not restricted to apparatus having an externally heated roller structure and pertains also to apparatus having an internally heated roll structure. The toner images 20 contact the heated roll structure while a force is applied between the roll structures in a conventional manner to create pressure therebetween resulting in the deformation of one of the roll structures by the other to thereby form the nip 16. As the substrate passes out of the nip 16, it is stripped from the heated roll structure by a stripping device (not shown) after which it is free to move along a predetermined path toward the exit of the machine (not shown) in which the fuser apparatus 10 is to be utilized. A temperature sensor 24 is provided for sensing the surface temperature of the externally heated roll structure 12 and in conjunction with conventional control 26 maintains the surface temperature at a predetermined value, for example, in the order of 190-205 °C (375-400 °F). The heated roll structure 12 comprises a cylinder 28 having an external heat lamp 30 and reflector 31 adjacent the outer surface of said heated roll structure. When suitably energized via the aforementioned control, the heating element radiates heat to an outer surface of the structure 12 which preferably comprises silicone or VITON (trademark of E. du Pont de Nemours & Co.) rubber having a thickness in the range 0.25mm (0.010 in.) to about 2.5mm (0.100 in.). The backup roll structure 14 comprises a solid metal core 32 to which can be adhered a relatively thick layer 34 of deformable material for example an elastomer known as ethylene-propylene terpolymer which is based on stereosperific linear polymers of ethylene, propylene and small amounts of non-conjugated diene which is commonly referred to as EPDM which layer carries a thin overcoat of PFA. PFA is a fluorinated copolymer of perfluoroalkoxy and tetrafluoroethylene Due to the construction of the backup roll structure shown it is deformed by the harder heated roll structure 12 when the required pressure is applied therebetween, the pressure being a function of the desired deformation which corresponds to the desired length of the nip 16. The invention can also be used in a fuser wherein the heated (fuser) roll is deformed by the pressure, or backup, roll. A slotted disk 36 is attached to the fuser roll and serves as an optical chopper that, in conjunction with a light sensor or photocell 38, delivers a string of electrical pulses only when the fuser roll is rotating. The pulses are conditioned by a capacitance filter 40 to insure that the pulses are shorter than the half cycle of the AC supply to the heat lamp 30 (in the case of a 60 Hz supply, that is less than 1/120th of a second). Proper selection of the slot widths of the chopper can also be used to condition the pulses. In addition, the pulse frequency is much lower than that of the AC supply so the lamp duty cycle is low (e.g. less than 20% and the lamp is energized less than one fifth of the cycles). The conditioned pulses are fed to switching means (comprising a triac 42) via the electronic switch 44 which is part of the temperature control circuitry and which is activated when the roll temperature falls below the predetermined temperature set point. The triac operatively connects the AC power source 46 with the external heat lamp 30. Zero crossing sensing circuitry could, of course be employed to reduce RF noise generated by when the triac is activated mid cycle by delaying the turn-on command until the start of the next cycle. The triac 42 could be replaced by other suitable switching means, for example one or more silicon controlled rectifier. Since a continuous string of pulses is required to keep the lamp energized, this method of control is fail-safe with respect to the fuser roll rotation. There is no way for the system to generate pulses once the fuser roll and its attached disk stall. It has the added advantage that, should the roll slow down, the average power delivered to the roll would be reduced proportionately. In addition, the low duty cycle of the lamp 30 affords fail-safe protection in the event of a triac or SCR short circuit. This results because a fuser lamp filament is essentially a time averaging device for the time frames below a few tenths of seconds. Thus, a 10 volt lamp could be run either continuously at 10 volts or at 100 volts at a 10% duty cycle so long as the cycle times are short in comparison to the lamp filament's thermal time constant. This time constant is the time required for the lamp filament to cool to 1/e of its operating temperature referenced to the ambient temperature once the power is interrupted. Expressed mathematically, this is the time for the temperature to reach: Ta + (To-Ta)/e where Ta is the ambient temperature, To is the filament operating temperature, and e is the natural logarithm. Preferably the lamp is specified for full power dissipation at a 10% duty cycle at 120 VAC so that it would be significantly overloaded should the triac short circuit and cause the duty cycle to effectively increase to 100%. The system would thus fail safely through relatively inexpensive lamp burn-out rather than by a potentially catastrophic fire.

Claims: A contact fuser apparatus (10) having a fuser roll structure (12) rotatable in contact with a backup roll structure (14) and forming a nip (16) therewith through which copy substrates (18) carrying toner images pass with the toner images contacting the fuser roll structure (12): a heat source (30) for elevating the temperature of said fuser roll structure (12); switching means (42) operably coupling said heat source (30) and a source of alternating power (46); and actuating means (36, 38, 40, 44) for periodically actuating said switching means (42), when the temperature of the fuser roll structure (12) falls below a predetermined temperature set point, to couple periodically said heat source (30) to said source of alternating power whereby the duty cycle of the power signal energising the heat source (30) is such as to prevent overloading of said heat source (30) whilst effecting operation of said heat source (30) at approximately full power dissipation, and whereby any circuit failure causing continuous energization of said heat source (30) will lead to power interruption and safe shutdown through failure of the heat source (30) before fire or overheating can occur, said actuating means (36,38,40,44) including means (36,38) responsive to the rotation of one of the roll structures for generating a string of electrical pulses indicative of roll structure rotation, the frequency of operation of said actuating means being low relative to the frequency of the alternating current energizing the heat source (30) to ensure that the heat source (30) is operated at a maximum time average power dissipation that does not exceed the lamp power rating of the heat source (30). Apparatus according to claim 1, wherein said heat source (30) is located externally of said fuser roll structure (12). Apparatus according to claim 1 or claim 2, wherein said means (36,38) responsive to the rotation of one of said roll structures (12,14) comprises a slotted disk (36) carried by one of said roll structures and a photocell (38). Apparatus according to any one of the preceding claims, wherein the heat source (30) is a filament lamp (30). Apparatus according to claim 4, wherein the maximum period between switch activations is short relative to the characteristic thermal relaxation time of the lamp filament so as to minimize filament fatigue due to thermal cycling. Apparatus according to any one of the preceding claims, wherein said switching means (42) comprises a triac, or one or more silicon controlled rectifiers. Apparatus according to any one of the preceding claims, including means for conditioning said signals so that they have a duration of less than the alternating current half cycle period.

Inventors: KARZ ROBERT S, KARZ, ROBERT S.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1989
Main classification: G03G15
IPC codes: G03G15