Paper is a material composed of variable plant fiber tissues, which has the function of absorbing and releasing moisture. It regulates balance with the air humidity as the moisture in the air keeps changing. When the air humidity is higher than the moisture content in the paper, it absorbs moisture from the air and expands; conversely, it releases moisture and shrinks to become shorter, thereby achieving a relative balance with the environmental humidity.

Flat plate offset printing utilizes the principle of immiscibility between oil and water for printing. Therefore, the printed paper typically tends to elongate rather than shorten. To eliminate such malfunctions, today's article delves into the printing process related to this topic:

1. Opt for horizontal grain during printing whenever possible.

Paper fibers come in two orientations: longitudinal and transverse. During the papermaking process, the pulp (plant fibers) flows vertically onto the moving wire mesh, causing the fibers to accelerate abruptly and typically bond along the direction of the mesh movement, forming the longitudinal and transverse fibers we refer to. The plant fibers in paper are hollow, semi-transparent tubular structures, and we can describe it this way: the length direction of the fibers is what we call the longitudinal fibers, while the gap between fibers is the transverse fibers. Experimental results show that the cohesiveness of the fibers themselves is greater than the bonding force between fibers (mainly hydrogen bonding). When paper gets moist, the elongation in the longitudinal direction is smaller than in the transverse direction, with a ratio of approximately 1:2. Therefore, when printing products with accurate registration, the deformation relationship between the paper's longitudinal and transverse fibers must be considered. Hence, during printing, the longitudinal fiber direction of the paper should be parallel to the cylinder axis, and the direction of the fibers in the same batch of printing paper should be consistent.

2. Offset Water Printing (commonly referred to as Press Water)

In cases where it's challenging to handle printing paper, to prevent the paper from elongating during printing, you can first apply a clear water pass to acclimate the paper to the printing requirements. Subsequently, begin printing colors. This method can somewhat help in addressing misregistration issues.

3. Control the moisture content of printed pages

The moisture content of the printing plate is a crucial factor in controlling the expansion and contraction of paper. The moisture from the plate is transferred to the paper via the rubber blanket, with higher plate moisture leading to greater expansion. Therefore, it is better to control the plate moisture as low as possible, especially when printing on 80g double-sided coated paper.

4. Employed a roller machine to elongate the printing plate

Typically, monochrome presses experience misregistration during the third and fourth color printing processes, while dual-color presses encounter it in the second process. This is due to the elongation and enlargement of images on the paper. The reason lies in the high extensibility of the PS plate material, which can be elongated using a puller to ensure that the image on the printing plate extends evenly with the image on the paper, thereby achieving accurate registration.