The direction of paper fibers is very important, 3 tips to help you distinguish!For book printing, in addition to beautiful binding design and advanced printing equipment, paper is also an important factor that cannot be ignored. When we receive a hardcover book, we may find that why can't the pages be laid out smoothly? Why does the ring lining wrinkle and become uneven? Actually, this is all because the fiber direction of the paper was not taken into account when opening it.
Paper is composed of paper fibers, and during the papermaking process, the fibers are subjected to significant traction in the direction of the paper machine's operation, causing most of the fibers to align along the direction of the machine's operation. This alignment direction is commonly referred to as the fiber direction. So paper has a certain directionality. The longitudinal paper we often refer to refers to paper in which the arrangement direction of plant fibers is parallel to the long side direction of the paper, while the transverse paper refers to paper in which the arrangement direction of plant fibers is perpendicular to the long side direction of the paper.
The Influence of Paper Fiber Direction on Printing and Solutions
The direction of paper fibers can have a certain impact on printing, mainly manifested as inaccurate overprinting and stretching deformation.
1. Inaccurate overprinting
There are two main reasons for inaccurate overprinting during the printing process: firstly, the rules for adjusting the printing machine are not accurate; The second is that the paper undergoes deformation under the pressure of printing and water. The printing process cannot do without water, and paper fibers have the characteristic of absorbing water and expanding. When paper fibers absorb moisture and swell, the amplitude of expansion in the diameter direction of the fibers is greater than in the length direction. Therefore, when paper absorbs moisture, the transverse elongation is generally 2-8 times greater than the longitudinal elongation.
Experimental results have shown that using a single sheet of paper with a specification of 787mm × 1092mm, when the relative humidity increases from 50% to 65%, the longitudinal elongation rate is 0.05% and the transverse elongation rate is 0.15%. If the paper is longitudinal, the elongation of its long side is 0.55mm and the elongation of its short side is 1.18mm; if it is transverse, the elongation of its long side is 1.64mm and the elongation of its short side is 0.40mm. Although both transverse and longitudinal papers will deform to some extent under the influence of humidity, it is obvious that transverse paper deforms more severely than longitudinal paper under the influence of humidity.
Therefore, it is best to use vertical paper when printing books to reduce overprinting errors.
2.Expansion deformation
Paper requires high humidity in the environment during use and storage. If the moisture content of the paper itself is not balanced with the relative humidity of the warehouse, the paper will absorb or lose water. Careful observation of the deformation of paper reveals that different fiber directions of the paper exhibit different stretching and deformation characteristics. Taking longitudinal paper as an example, the expansion and contraction of the long side parallel to the direction of the paper fibers are relatively small, even negligible, while the expansion and contraction of the short side perpendicular to the direction of the paper fibers are relatively obvious.
In the process of writing a book, if the fiber direction of the inner pages is perpendicular to the spine, it may sometimes affect the normal flipping of the book. As shown in Figure 1, it can be seen that the pages of the entire book are upright and cannot be completely flattened. This is mainly because when the paper undergoes stretching and deformation, the spine of the book is fixed by hot melt adhesive, and the size remains basically stable, while other unconstrained parts of the paper in the book core undergo deformation, resulting in one side of the paper remaining the same size and one side changing size, thus causing this phenomenon. If the fiber direction of the paper is parallel to the spine, the paper size is stable in the spine direction, and the expansion and contraction of the inner page size is mainly in the width direction of the book. This direction is not restricted and will not affect the shape of the book. Moreover, the flipping resistance is low, which can better flatten it, as shown in Figure 2.
Figure 1: The fiber direction of the paper is perpendicular to the spine of the book
If two types of paper with different fiber directions are mixed in the same book, after being left for a period of time, there will be uneven cuts on the book mouth, top, and bottom. This is also because different fiber directions on paper can cause different stretching deformations. A type of fiber oriented paper undergoes expansion and contraction in the width direction of the book, while remaining relatively unchanged in the length direction; Another type of fiber oriented paper remains relatively unchanged in the width direction of the book, but expands and contracts in the length direction, resulting in uneven cuts on the edges, top, and bottom of the finished book.
To reduce quality issues caused by expansion deformation, we can take the following measures:
(1) Reduce water loss, such as increasing printing speed appropriately and lowering drying temperature; The pressure of the printing machine should not be too high to avoid excessive deformation of the paper due to stress; Properly increase the relative temperature and humidity of the warehouse to ensure the balance of paper moisture content.
(2) When printing and opening paper, keep the spine direction parallel to the fiber direction of the paper, so that the finished book can be read more easily.
(3) Train the relevant personnel of the printing factory to avoid using paper with different fiber directions for the same book.
Correctly determine the direction of paper fibers
The arrangement direction of paper fibers causes differences in many properties between the longitudinal and transverse directions of paper, such as the longitudinal tensile force of paper being much greater than the transverse tensile force, the transverse expansion rate being greater than the longitudinal expansion rate, and the longitudinal dimensional stability being better than the transverse dimensional stability.
Vertical paper can effectively reduce the elongation and deformation of paper during the printing process, avoid problems such as wrinkling, misalignment, and waste, and also effectively improve the printing speed and product quality of the printing machine.
It can be seen that there are three main methods to correctly distinguish between vertical paper and horizontal paper before printing.
(1) Bending method. Bend the paper horizontally and vertically without folding it, and feel the rebound force of the paper horizontally and vertically. If the rebound force is small, it indicates less resistance, and the fiber direction is parallel to this side.
(2) Tear off paper method. Tear a piece of paper horizontally and vertically, observe the size of the cut surface, and the direction of the smaller cut surface is the direction of the paper fibers.
(3) Brush water method. Draw a line with a brush dipped in water along the horizontal and vertical directions of the paper, and observe if the line is smooth. The smooth direction is the direction of the paper fibers.