1. Field of the Invention
The present invention relates to a fixing apparatus equipped with a belt.
2. Description of the Related Art
On-demand control of a fixing apparatus is one approach to achieve high-speed start-up and energy saving of a fixing apparatus. To realize the on-demand control, some fixing apparatuses use an endless thin belt as a rotatable heating member. The thin belt has a smaller heat capacity as compared to those of conventional thick heating rollers, enabling saving in time for warm-up of the fixing apparatuses and reduction in power consumption at the warm-up.
In the cases where the thin belt consisted of a base layer and a release layer is used as a belt equipped in an on-demand fixing apparatus, sometimes uniform fixability cannot be obtained when the image requires a large amount of toner to be fixed on a recording material. In addition, when a sheet of plain paper having an uneven surface is used in the fixing apparatus, sometimes toner fusion of a toner image is changed in various ways leading to uneven brightness on the surface of the image after fixing.
Japanese Patent Application Laid-Open No. 2003-323965 discusses a fixing belt including an elastic layer, which is made of silicone rubber for example, to be interposed between a base layer and a release layer of the fixing belt.
The elastic layer provided in the belt, however, decreases the heat transfer coefficient from the back surface to the front surface of the belt.
The belt moves a distance l for a period of time t, the distance being given by: l=(t)(v) (mm) where t (sec) is the time that takes the thermal energy output from a heater to be transferred to the outer surface of the belt, and v (mm/sec) is the rotation speed of the belt. Thus, if the heater heats a certain point of the inner surface of the belt, when the heat is completely transferred to the outer surface of the belt, the belt is to move downstream in the rotation direction by a distance l. In other words, a smaller thermal conductivity between the front and back surfaces of a belt and a higher rotation speed of the belt result in a larger distance l. This result causes a problem in that a sufficient thermal flow cannot be supplied to the belt surface on the opposite side to the heater while a recording material passes through a nip portion, and poor fixability is likely to occur on the material.