The moisture content within paper greatly affects the image quality when printing on paper media. Especially in the drying process of inkjet printing or the fusing process of electrophotographic printing, the amount of moisture in the paper changes due to the heating process, and then again due to the cooling process, which results in a dimensional change in paper size that is known to greatly affect the accuracy of image position. Although various types of paper are used in the commercial printing field according to the intended application, it is not clear how changes in paper size occur due to differences in paper structure, and image positioning adjustments are currently made based on empirical rules. In this study, we conducted simultaneous measurements of weight and length across multiple paper types with different layer structures, examining the dynamic changes that occur immediately following variations in temperature and relative humidity. We consider that in the paper expansion phenomenon, in the first step, moisture diffuse into the paper voids, in the second step, moisture is absorbed into the paper fiber, where they diffuse, causing the fiber diameter to expand, and as a result, the paper size expands. Through comparisons between experimental results and numerical calculations, we elucidated the effects of relative humidity on water molecule diffusion within paper fibers, as well as the influence of coating layers and peripheral moisture content of paper fibers on dimensional change.
