Some Issues on Printed Materials (1)

What are the properties of printing paper?

There are many properties of paper, such as mechanical properties, optical properties, chemical properties, etc. The following describes the printing-related properties.

1 specifications. The specification of paper is the first thing printers should be familiar with. Different machines have specific requirements for paper specifications. Paper specifications generally include the following:
a. Form. There are two types of printing paper, flat paper and web. Flatbed paper cuts paper to a certain length,

Fixed width paper. Web is paper that rolls a paper on a paper roll core in a cylindrical shape.
b. Size. The size of the sheet refers to the length and width of the sheet, such as 880 x 1230 mm.
c. Quantification. The weight of paper per unit area is generally expressed in g/m2, such as 120g/m2.

2 thickness. Thickness refers to the thickness of the paper. The thickness of the paper affects the compressibility and opacity of the paper. Because the center distance between the blanket cylinder and the impression cylinder is relatively fixed to a certain extent, if the thickness of the sheet is uneven, the printing pressure will change, and the imprint will be uneven, which will affect the quality of the printed matter. Paper thickness also has an effect on opacity. The thinner the paper, the more likely it is that the opacity will decrease.

3 tightness. Tightness refers to the weight per unit volume of paper, which is density. Tightness affects the optical and physical properties of the paper.
The tightness is also related to the ink absorption performance, and the paper with a large degree of tightness has a relatively lower ink absorption performance. In high-tight paper, the ink is mainly dried by an oxide conjunctiva.

4 mechanical strength. Mechanical strength includes tensile strength, compressibility and surface strength.

a. Tensile strength. Under the action of external force, the paper is pulled and the paper is deformed. When the external force is removed, the ability of the paper to return to its original state is called the tensile strength of the paper. The tensile strength is related to the moisture content of the paper and the fiber arrangement of the paper. Printing paper requires greater tensile strength. When paper is subjected to external forces, it will not break.

b. Compressibility. When the paper is under certain pressure, it will be slightly compressed. After removing the excess force, the degree of restoring the original state of the paper is called compressibility. Compressibility is related to the size of the pressure. The compressibility of paper is particularly important for printing paper. Because when printing, the paper is subjected to a certain pressure so that the ink can be transferred from the plate to the paper. If there is not a certain degree of compressibility, the graphic part of the plate cannot be completely in contact with the surface of the paper, and the ink cannot be well transferred, resulting in blurred prints and reduced print quality. Elastomeric (compressible) paper can reduce the smoothness of the paper and the flatness of the text surface.

c. Surface strength. The surface strength of paper is the bond strength between fiber, filler, and rubber. The surface strength of the paper should be able to withstand the viscosity of the ink on the paper, otherwise the printing process will ink the surface of the paper fibers, fillers from the paper surface.

5 smoothness. Smoothness indicates how smooth and smooth the surface of the paper is. The degree of smoothness determines the degree of contact between the paper and the printing plate, and the clarity of the graphic printed on paper with poor smoothness is affected.

6 two sides. The difference between the positive and negative sides of the paper is called the two sides of the paper. Both sides of the paper have a greater impact on printing. Because the front of the paper has a tight texture, the smoothness, surface strength, and sizing degree are all better than those on the reverse side, resulting in uneven ink on both sides of the printed product.

7 optical properties. Including the paper's opacity, gloss, color (including whiteness) and smoothness.

a. Opacity: refers to the performance of the graphic after printing does not pass through the other side.
b. gloss. The gloss of paper has a lot to do with printing. High-gloss paper can make the color of prints bright, but paper with too strong gloss will make prints glare, which is not conducive to reading.
c. color. The color of the paper will largely change the appearance of the print.
d. Whiteness. The whiteness of paper refers to the ability of the paper to reflect completely after being exposed to light. The high whiteness of the paper enhances the contrast between the ink and the paper, helping to distinguish the graphics. And bright colors.

8 Water content. The amount of moisture in the paper will affect all other characteristics. The most important factor is the change in paper size. Changes in size will cause misprints during printing. The change of water content will also bring other problems to the printing and post process. For example, moisture will slow the drying speed of ink, decrease the tensile strength of paper, and the surface strength. When the moisture content is low, the paper will be hard and brittle, and the static electricity will easily occur. It is prone to double-sheet or empty-sheet phenomenon.

9 dimensional stability. Printing paper has higher requirements for dimensional stability. If the size of the paper changes during printing, the color registration will not be correct. For lithographic printing, every time printing, the paper must absorb a part of the water, the paper's dimensional stability is not good, will make the next color can not be set and produce waste, defective products.

 10 absorption performance. The printing process requires the paper to have a considerable absorption capacity, so that the paper can absorb the ink as soon as possible. The printing paper absorbs the binder in the ink, while the pigment particles stay on the surface of the paper and dries quickly. The gap between the fibers of the paper is generally lower than that of the pigment. The particle diameter is so large that during printing, the ink is pressed into the gaps of the paper, and those small fiber gaps smaller than the diameter of the pigment particles absorb the binder under the action of the fiber capillary. If the gap between the fibers is small, the capillary action of the fiber is damaged and the ink absorption performance is poor. If the space is too large, the capillary tube not only absorbs the binder but also absorbs the pigment, resulting in a print-through phenomenon.

Acidity and alkalinity. Paper's acidity (pH) is more related to printed matter. The pH of the paper is one of the reasons that affect the drying rate of the ink oxidation conjunctiva. Acid paper inhibits drying of the ink oxidation conjunctiva. For lithography, the pH of the paper also affects the pH of the wetting fluid. If the paper is too alkaline, the alkaline substance will be continuously dissolved during printing, and transferred to the wetting fluid, causing its pH value to change. This will lead to a series of problems, such as the expansion of the area of ​​the fat-sensitive graphic on the printing plate. Dirty, ink emulsification and so on. Paper alkali or acid, after a period of time, will also cause ink fade on the print.