The paper materials used in the printing process are typically produced using either a long wire machine or a modified rotary screen machine. Both systems play a crucial role in forming the paper sheet, but the way the sheet is formed leads to two distinct surfaces.
During the formation process, one side of the paper comes into contact with the screen, while the other side is in contact with a felt. This interaction results in two different surface characteristics. The side that touches the screen retains the visible mesh pattern and is referred to as the "back" side. The opposite side, which remains smooth and free of mesh marks, is known as the "front" side.
One of the main reasons for this difference lies in the uneven distribution of fillers and short fibers between the two sides. As the pulp, which contains about 1% solids, flows onto the wire section, a significant amount of water is removed. During this process, both water and fine particles—such as fillers and short fibers—are lost. While these materials tend to be evenly distributed on the front side where there's more water, they become less concentrated on the back side, especially in areas where the paper is drier. This leads to a lower concentration of fillers and fibers on the reverse side, contributing to its different texture and properties.
Another factor that contributes to the distinction between the front and back of the paper is the contact with the screen during formation. Due to the high plasticity of the pulp, the screen’s structure leaves visible traces on the paper surface. The pulp that reaches the screen has a low consistency (around 1%), and to enhance water removal, the system uses press rolls and suction boxes beneath the screen. Some of the pulp can get trapped in the mesh, creating a pattern that remains even after pressing and drying. As a result, the back side often shows a faint reticulated texture.
These differences between the two sides affect various properties of the paper, such as smoothness, gloss, absorbency, ink absorption, and curling. Understanding which side is the front or back is important for achieving optimal print quality.
In general, for uncoated paper, you can visually identify the front and back sides. The side with visible mesh marks is the back, while the side with a clearer, smoother appearance is the front. This simple visual check helps in determining the correct side for printing and finishing processes.
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