From the process of digital printing in book
printing process, the raw materials used in digital printing are mainly paper and
ink, which will directly affect the printing output work and color gamut space
of printing output. Only the stability of raw materials can ensure the
stability of printing output.
01 paper
Digital printing has a wide range of
substrates, but paper is still the main substrate. With the development of
technology, digital printing has higher requirements for paper performance and
printability. The physical and chemical properties, mechanical properties and
optical properties of paper used in digital printing and traditional printing
are different.
(1) Physical and chemical properties
The physical and chemical properties of
paper include the physical and chemical properties of paper.
Among them, the physical properties of
paper include weight, thickness, tightness, ink absorption and smoothness. In
digital printing of book printing , the paper transmission speed greatly
affects the quality of printed matter. The paper weight, stiffness and paper
fiber arrangement direction will affect the paper transportation. If the paper
weight exceeds 100g / m2, if the stiffness is enough, it will not bring
problems to the transmission. When the paper weight is less than 200g / m2, the
paper fiber arrangement direction and the printing machine running direction
should be consistent; when the paper weight exceeds 200g / m2, the paper fiber
arrangement direction and the printing machine running direction should be
mutually perpendicular. The friction free paper transmission system is
completed with the aid of vacuum system, which requires the paper surface as
smooth as possible and less pinholes.
In addition, the thickness of paper should
be uniform in digital printing of book printing, so as not to affect the
uniformity of printing ink. The paper tightness can not be too large, otherwise
it is easy to cause paper embrittlement, and the opacity of paper and ink
absorption performance will also decline; but the paper must have a certain
degree of tightness, so as to have tensile strength. The ink absorption speed of
paper should not be too fast or too slow. If it is too fast, the ink imprint
will be dull, even powdered and transparent, the printing density will be
reduced, and the color phase will be changed. If it is too slow, the fixing
speed of the ink will be reduced, and adhesion or dirt will be caused.
Exquisite printing requires high smoothness of paper, so that ink can be
transferred completely and obtain high gloss printed matter.
The chemical properties of paper include
water content, acidity and alkalinity. In electrostatic dry powder digital
printing, the toner needs to be heated to 120 ~ 150 � to be fixed. The paper will become
brittle due to the loss of moisture, and it is easy to break when folding. To
solve this problem, the paper with 30% relative humidity is prepared in the
digital printing system or the paper is fed in the air-conditioned environment,
and the paper with compact fiber interwoven structure and elastic coating
should be selected. In the acid-base property of paper, the ideal paper should
be neutral. However, due to the complexity of the pulping and papermaking
process, the produced paper may be acidic or alkaline. When the ink is printed
on the acid or alkali paper, the color of the ink will fade after a period of
time, which will make the printed products lose luster.
(2) Mechanical properties
The mechanical properties of paper include
tensile strength, tearing degree and so on. The tensile strength of paper is
very important for book paper, especially for high-speed digital printing press.
If the tensile strength of paper is low, there will be hair and powder
dropping, and the glossiness of printed matter will also be reduced. Digital
printing paper needs to have a high degree of internal tear and edge tear,
which is because in digital printing, paper is easy to break, tear and other
phenomena, which reduces the printing efficiency and book printing quality.
(3) Optical properties
The optical properties of paper include
whiteness, glossiness and opacity. Digital printing has strict requirements on
the glossiness of paper. Whether using fast drying ink or slow drying ink, the
glossiness of printed matter will increase with the increase of paper
glossiness. However, the high glossiness of paper does not mean the high
glossiness of printed matter, which is also affected by the smoothness of paper
printing. The uniformity of paper luster is more important than the average
level of gloss.
Different types of printed matter have
different requirements for paper opacity. In order to prevent people from
seeing the pictures and texts on the back of the paper when reading, the higher
the opacity of the paper used for magazines and books, the better. Labels,
bills of lading, etc. do not require high paper opacity, as long as there is no
through printing.
