1. Technical Field
The present invention relates to a fixing device and a control method used therein, and more particularly, to a fixing device for use in an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, and a method for controlling temperature in the fixing device.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of these features, an image is formed by attracting developer or toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
In general, a fixing device employed in electrophotographic image formation includes a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip. As a recording medium bearing a toner image thereupon enters the fixing nip, heat from the fuser member causes the toner particles to fuse and melt, while pressure between the fuser and pressure members causes the molten toner to set onto the recording medium.
To date, some fixing devices employ a fixing member consisting of a thin, flexible belt or film that exhibits an extremely low heat capacity. Using the low-heat capacity material substantially saves energy for heating the fixing member, and consequently allows for shortening a warm-up time required to heat the fixing member from a room temperature to an operational, reload temperature upon power-on, as well as a first-print time required to initiate and execute a user-submitted print request to perform printing on an initial print page, which is completed outputting the resulting print.
During sequential processing of multiple print pages, on which different types of images are printed, the temperature of the fixing member is controlled to a certain setpoint temperature. For example, the setpoint temperature may be fixed to a single value determined optimized for print properties of an initial print page. Processing all the multiple print pages with the single, fixed setpoint temperature is undesirable, however, because it would cause excessive or insufficient heating of the fixing member relative to specific properties of each print page, resulting in undue energy consumption and fixing failure. To prevent undue energy consumption and fixing failure, the fixing device may control temperature using a setpoint temperature that is variable depending on a print page printed on the recording medium.
The inventors have recognized that although effective for its intended purpose, the temperature control based on a variable setpoint temperature also has a drawback. As the setpoint temperature decreases during operation, the amount of heat applied to the fixing member also decreases, while the fixing member constantly loses a substantial amount of heat absorbed by the recording medium passing through the fixing nip, eventually causing the temperature of the fixing member to suddenly decline below the setpoint temperature. The problem is particularly pronounced where the fixing member is of a low-heat capacity material. An excessive reduction in the fixing temperature can cause a fixing failure, known in the art as “cold offset”, in which the toner image partially comes off where the toner particles forming the image fail to fuse properly due to a lack of heat applied to the recording medium.