The physical properties of paper, including quantification, thickness, tightness, strength, compressibility, flexibility, stretch rate, and dimensional stability, are closely related to the physical properties of the paper and the quality of the print.
First, the quantitative paper
Quantification is the most basic physical indicator of paper and board. It is directly related to the physical properties of the paper such as tensile strength, bursting resistance, tear strength, tightness, thickness, and opacity.
Different papers have different uses. Newsprint paper is soft, ink-absorption is good, it has a certain level of mechanical strength and opacity, the surface does not pull hair, can not afford powder, suitable for printing newspapers, magazines and books. Relief paper is flat and even, with a certain degree of whiteness and mechanical strength, good ink-absorbing opaque, light sizing, surface is not napped, no powder, suitable for printing textbooks, literature and other materials. Offset paper is whiter, even and smooth, with proper ink absorption and sizing, high mechanical strength, and good dimensional stability. It is suitable for printing books, dictionaries, pictures, inserts, and color images. Coated paper has a high degree of smoothness, whiteness, high gloss, high surface efficiency, suitable for printing pictures, inserts, pictorials, pictures, important books and magazines.
Second, the paper thickness and tightness
1. The thickness of the paper should be uniform, and the thickness of at least one batch of paper must not be too different. Otherwise, the thickness of the book will be inconsistent after binding into a book, which will affect the quality of books and periodicals. In addition, the thickness of the paper also has an effect on the opacity and the electrical properties. The paper is generally single-layered and has a thickness of 20-400 μm. The paperboard is multi-layered and the thickness is generally about 0.5 mm.
2. Tightness is the basic physical index of the paper, in units of g/cm3, which is the basis for comparing various paper strengths and other properties. In general, the tightness of paper is directly proportional to its burst strength and tensile strength. In addition, the tightness of the paper directly affects its ink absorption and opacity. The greater the general tightness, the slower the absorption speed and the lower the opacity of the paper. When the tightness is too large, due to poor ink absorption, the ink is not easily fixed on the surface of the paper, and the back-scratching phenomenon often occurs.
The tightness of the newsprint should not be too high; the letterpress paper requires better absorption, and the general tightness is about 0.7g/cm3; and the lithographic printing requires the paper to have a higher degree of tightness, generally above 0.75g/cm3.
Third, the paper strength
Paper and paperboard must have a certain degree of folding and bursting resistance. The physical strength of the paper changes with the amount of moisture in the paper. The paper's bursting resistance, also known as jacking force, is the maximum pressure that the paper can withstand evenly on a unit area.
The folding resistance of paper is the number of times that paper or cardboard can withstand 180° reciprocating folding. The folding endurance and the relative humidity of the air are very large, the relative humidity increases, the folding endurance also increases, the paper is too dry to become brittle, and the folding endurance falls. The printing paper should be subjected to binding processing so that the folding endurance should reach a certain standard. For example, the relief paper undergoes 180° reciprocating folding times of not less than 3-4 times; the offset paper is not less than 3-5 times; and there is a lot of writing paper. In 2-4 times; banknote paper in the circulation multiple times, requiring a high degree of folding; map paper, cardboard paper also have higher requirements for folding degree.
Fourth, the paper's compressibility and flexibility
Paper's compressibility and softness are an important physical indicator of printed paper. Due to the plasticity and elasticity of the fibers, the paper has a certain degree of softness and compressibility. The general printable paper has a compressibility of 50% to 60% and an elasticity of about 85%. The greater the thickness change after the paper is pressed, the softer the paper is. Highly flexible paper is very advantageous for letterpress printing and gravure printing. It can print clear graphics and ensure uniform ink color.
Fifth, the paper expansion rate
Due to the influence of humidity changes, tension or loosening occurs between fibers, so when the temperature changes, the paper will expand and contract. The speed and degree of this change will vary depending on the type of paper and the raw materials.
The transverse expansion of the paper is generally greater than in the longitudinal direction because the fibers expand more laterally than when they are wet.
Due to the print characteristics of newsprint and letterpress paper, their expansion and contraction rates are generally not required. Paper containing more fillers has a lower stretch rate, and the coated paper's stretch rate is usually less than that of offset paper. They increase or decrease with the relative humidity in the environment, so the printing shop needs to install air conditioners to keep the relative humidity stable. Only in this way can we guarantee the production of good-quality color prints.
Therefore, only by understanding the physical characteristics of the paper, printing according to the physical characteristics of the paper can print better quality prints.