The undesirable expansion of a sheet of paper on the press, caused either by moisture absorption or by mechanical stretching of the paper. Fan-out produced mechanically commonly occurs on the impression cylinder, as the pressure exerted in the nip of the impression and blanket cylinders irons the sheet out, causing it to expand at its tail end just prior to being printed. After leaving the impression nip, the sheet returns to nearly its original dimensions, but the image is narrowed at its tail end. When the sheet is then passed through successive printing units, the sheet will fan out again, but the successive images will print with narrower tails than the preceding ones, resulting in poor registration of images.
One means of reducing the effects of fan-out is by bowing the grippers (or adjusting the grippers so that the center gripper is slightly out of alignment with the others) on the impression cylinder or on other mechanisms in the infeed section of the press. Another corrective method uses a gripper-bowing device, which exaggerates the effect of fan-out produced by the first impression cylinder, resulting in a much narrower tail-end of the printed image. Consequently, successive images will align properly on the first. (See Infeed Section and Gripper-Bowing Device.)