Image forming apparatus with conveyance speed control based in part on loop detection

An image forming apparatus comprising:

• transfer means which transfers to recording materials unfixed images borne by an image bearing member;
• fixing means which fixes the transferred unfixed image on the recording materials;
• loop detecting means which detects a loop of the recording material to be formed between the transfer means and the fixing means;
• recording material detection means which determines that the recording materials have reached the fixing means;
• and
• control means which controls a conveyance speed of the recording material by the fixing means,
• wherein based on detected result of the recording material detection means and the loop detecting means, and length of the recording material, the control means sets the conveyance speed of the fixing means so that when the rear end of the recording material passes through the transfer means after the loop detecting means detects a loop of the recording material, a loop length of the recording materials will be set to a lower limit length, and the control means controls the conveyance speed of the fixing means at the set conveyance speed, and
• wherein when L0 represents loop length of the recording material between the transfer means and the fixing means when the recording material runs into the fixing means, L1 represents loop length of the recording material between the transfer means and the fixing means when a loop is detected, La represents the lower limit loop length of the recording material between the transfer means and the fixing means, Lb represents upper limit loop length of the recording material between the transfer means and the fixing means, relation among the loop length of the recording materials is La<L0<L1<Lb, Vt represents conveyance speed of the recording material of transfer means, Vf represents conveyance speed of the recording material of fixing portion before a loop is detected, S represents length in the conveyance direction of the recording material, and T represents loop formation time after the recording material enters the fixing means till when the loop detecting means detects the loop of the recording material, conveyance speed Vfa of the recording material by the fixing means to be set by the control means is constant conveyance speed that satisfies the following relation:
• Vt+(L1−Lb)×Vt/(S−Vt×T−L0)≦Vfa
• Vfa≦Vt+(L1−La)×Vt/(S−Vt×T−L0).