Image forming apparatus

An image forming apparatus comprising:

• an endless belt configured to carry a toner image transferred at a first transfer position;
• an outer roller contacting an outer peripheral surface of said belt and configured to transfer the toner image from said belt onto a recording material at a secondary transfer position;
• a plurality of stretching rollers configured to stretch said belt, wherein said stretching rollers include an inner roller provided correspondingly to the secondary transfer position and an upstream roller provided on a side downstream of the first transfer position and upstream of said inner roller with respect to a rotational direction of said belt;
• a voltage source configured to apply a transfer bias, to said outer roller or said inner roller, for transferring the toner image from said belt onto the recording material;
• a supporting member contactable to an inner peripheral surface of said belt on a side upstream of said inner roller and downstream of said upstream roller with respect to the rotational direction of said belt;
• a moving mechanism configured to move said supporting member between a first position and a second position different from the first position in a plane substantially perpendicular to a rotational axis direction of said inner roller;
• a sensor configured to detect a current value or a voltage value when a bias is applied from said voltage source to said outer roller or said inner roller;
• and
• a controller capable of executing an operation in a first mode in which an image is formed on the recording material in a state in which said supporting member is disposed at the first position and an operation in a second mode in which the image is formed on the recording material in a state in which said supporting member is disposed at the second position, and capable of executing an operation in a test mode in which, during non-image formation, the transfer bias is adjusted on the basis of a detection result of said sensor acquired under application of a test bias from said voltage source to said outer roller or said inner roller, wherein on the basis of a detection result of said sensor acquired under application of the test bias in the state in which said supporting member is disposed at the first position, said controller determines a first transfer bias to be applied from said voltage source to said outer roller or said inner roller during the operation in the first mode, and determines a second transfer bias to be applied from said voltage source to said outer roller or said inner roller during the operation in the second mode.

An image forming apparatus comprising:

• an image forming portion configured to form a toner image on a photosensitive member;
• an intermediary transfer belt onto which the toner image formed on said photosensitive member is transferred;
• a secondary transfer roller forming a transfer portion in which a recording material is nipped between said secondary transfer roller and said intermediary transfer belt and configured to transfer the toner image from said intermediary transfer belt onto the recording material;
• an inner roller provided in contact with an inner peripheral surface of said intermediary transfer belt at a position where the transfer portion is formed and configured to stretch said intermediary transfer belt;
• a back-up member provided upstream of and adjacent to said inner roller with respect to a rotational direction of said intermediary transfer belt and being contactable with and movable to an inner peripheral surface of said intermediary transfer belt;
• a moving mechanism configured to move said back-up member;
• a voltage source configured to apply a voltage to said secondary transfer roller or said inner roller;
• a detecting portion configured to detect an output voltage or an output current of said voltage source;
• and
• a controller configured to, during a preparatory operation executed at a time from input of a start instruction of an image forming job before the toner image is transferred onto a first recording material to which the image is to be first transferred in an image forming job:
• position said back-up member in a position for the first recording material, then control said voltage source so that a test bias is applied by said voltage source, and then set the voltage to be applied by said voltage source when the toner image is transferred onto the first recording material on the basis of a detection result of said detecting portion when the test bias is applied, wherein depending on a kind of the recording material, said controller positions said back-up member to one of a plurality of positions including a first position where said back-up member and said intermediary transfer belt contact one another and a second position where said back-up member is separated from said intermediary transfer belt.

An image forming apparatus comprising:

• an image forming portion configured to form a toner image on a photosensitive member;
• an intermediary transfer belt onto which the toner image formed on said photosensitive member is transferred;
• a secondary transfer roller forming a transfer portion at which a recording material is nipped between said secondary transfer roller and said intermediary transfer belt and configured to transfer the toner image from said intermediary transfer belt onto the recording material;
• an inner roller provided in contact with an inner peripheral surface of said intermediary transfer belt at a position where the transfer portion is formed and configured to stretch said intermediary transfer belt;
• an upstream roller provided in contact with the inner peripheral surface of said intermediary transfer belt and provided upstream of and adjacent to said inner roller with respect to a rotational direction of said intermediate transfer belt;
• a back-up member provided upstream of and adjacent to said inner roller with respect to the rotational direction of said intermediary transfer belt and being contactable with and movable to the inner peripheral surface of said intermediary transfer belt;
• a moving mechanism configured to move said back-up member;
• a voltage source configured to apply a voltage to said secondary transfer roller or said inner roller;
• a detecting portion configured to detect an output voltage or an output current of said voltage source;
• and
• a controller configured to, during a preparatory operation executed at a time from input of a start instruction of an image forming job until before the toner image is transferred onto a first recording material to which the image is to be first transferred in the image forming job:
• position said back-up member in a position for the first recording material,
• then control said voltage source so that a test bias is applied by said voltage source, and
• then set the voltage to be applied by said voltage source when the toner image is transferred onto the first recording material on the basis of a detection result of said detecting portion when the test bias is applied,
• wherein depending on a kind of the recording material, said controller positions said back-up member at one of a plurality of positions including a first position where a penetration amount of said back-up member through a common tangential line between said inner roller and said upstream roller on a side where said intermediary transfer belt is stretched is a first penetration amount and a second position where the penetration amount is a second penetration amount greater than the first penetration amount.