1. Field of the Invention
The present invention relates to a fixing unit provided with a separating member capable of properly separating a sheet carrying a fixed toner image from a fixing roller as well as to an image forming apparatus and a sheet processing apparatus.
2. Description of the Related Art
Image forming apparatuses, such as copying machines, printers and facsimile machines, generally employ a heat-and-pressure type fixing unit for fixing a toner image onto a sheet. This type of fixing unit is configured so that a pressure roller is pressed against a fixing roller heated by a heater to create a fixing nip portion between the two rollers. As a sheet carrying an unfixed toner image is passed through the fixing nip portion, the fixing unit applies heat and pressure to the toner image to fix the same to the sheet. Since the toner image fixed onto the sheet acts like glue, the sheet carrying the fixed toner image is apt to turn with the fixing roller. To prevent this inconvenience, the fixing unit is provided with a separating member of which one end hooks on a leading edge of the sheet to separate the sheet from a surface of the fixing roller.
Japanese Unexamined Patent Publication No. 2004-102241 describes a technique for separating a sheet from a surface of a fixing roller by means of a separating member (stripping plate). Expressing a distance between a downstream end of a fixing nip portion and a far end of the separating member by r and a minimum width of a top margin of a sheet formed along a leading edge thereof by L, the separating member of this Publication is disposed to satisfy the following relationship:r≦L
Although the aforementioned Patent Publication shows an arrangement in which a gap K between the far end of the separating member and the surface of the fixing roller is as narrow as 0.5 mm, there is no mention of a case where this gap K is smaller than 0.5 mm in the Publication.
There is a growing tendency today to employ a fixing roller having a rubber layer with a reduced thickness for achieving chiefly a reduction in warm-up time of a color image forming apparatus, for example. The reduction in the thickness of the rubber layer tends to cause a reduction in elasticity of the entire fixing roller, resulting in a loss in sheet separating capability of the apparatus. Also, increased processing speed of an image forming apparatus could cause sheet transfer problems or registration errors, thus producing variations in the width of the top margin. Should this situation occur, it might be necessary to set the aforementioned gap K below 0.5 mm.
According to the above-cited Patent Publication, the far end of the separating member must be at a distance of 1 mm or less from the downstream end of the fixing nip portion when the top margin of the sheet is approximately 1 mm, for instance. In such a case, the separating member is located so close to a sheet transfer path that the separating member may interfere with the sheet being transferred, causing a jam. Although it is preferable to locate the separating member at some distance from the sheet transfer path to prevent jams, there arises a problem that the width of the top margin increases if the separating member is so located.