Printing Paper Aspects--Physical Properties (I)

Paper physical properties

one. Basis Weight

Quantitatively known as grammage refers to the weight of paper per unit area expressed in grams per square meter. In general, a paper with a basis weight of 250 g/m2 or less is called paper; a paper with a basis weight of 250 g/m2 or more is called cardboard or board paper. Quantification is the most basic physical index of paper. Its level and uniformity affect all the physical and printing properties of paper.

two. Thickness and tightness

The thickness refers to the thickness of the paper, which refers to the thickness of the paper or paperboard directly measured between the two measuring boards under a certain pressure, in mm or μm. In general, the thickness directly affects the opacity of the paper, and the greater the thickness, the better the opacity. Thickness uniformity is particularly important for paper, and uneven thickness can affect the decline in gloss and whiteness and the instability of tone and color reproduction.

Tightness refers to the weight per unit volume of paper, also known as density, which is calculated as the basis weight divided by the thickness.

Tightness is a physical index to measure the degree of structural density of paper. For paper of the same kind of paper fiber, the tighter the paper is, the denser the paper is, and conversely, the looser the paper is. The tightness of paper is directly proportional to the tensile strength.

three. Tensile Strength and Tensile Strength

Tensile strength is the maximum tensile force that the operator can withstand before the unit-width paper or board breaks under the conditions specified by the standard test method. When the paper is stretched by tension, the paper will stretch in the direction of the force. When the tension is increased to make the spline broken, the elongation reaches the limit. The ratio of the length of the elongation to the original length is called elongation. Expressed as a percentage.

For web printing, the tensile strength and elongation of the paper are an important indicator. Because the paper roll is driven by the rotary printing press, the paper is subjected to a great deal of tension. If the tensile strength is too small, it will cause a web break; if the elongation is too large, it will change the size of the paper and increase the overlay error. Therefore, the use of a paper having a small tensile strength and a large elongation can effectively prevent and reduce the occurrence of paper breakage during printing and reduce overprinting errors.

four. Flexibility

The folding resistance of paper refers to the number of folds that can be experienced before the specimen is broken in a special instrument under specified test conditions, expressed in double folds.

The folding endurance is one of the important indicators of the mechanical strength of paper and board, and is a comprehensive measure of the strength and flexibility of the paper. This index is necessary for a variety of paper and paperboard, and for newspapers, banknotes, and maps. Paper and cover paper are particularly important because they are subject to repeated folding during use, and in order to prolong their useful life, they must be required to have such a high number of folding times.

Fives. Paper Surface Strength

The surface strength of paper refers to the combination of fibers, fillers, pigments, etc. on the surface of the paper and the paper, and the firmness of the connection, that is, the strength of the binding of the surface layers of the paper.

In general, the surface strength of newsprint is relatively low, and the surface strength of coated paper is higher. The appearance of powder off and hair loss in printing is closely related to the surface strength of the paper. In the printing of newspapers, the surface of the paper is often peeled off by the ink film and left on the ink roller, causing the ink to remain uninked and white spots to appear. The fine fibers of the fallen filler material also block the graphics on the printing plate, causing the printing. The picture is less and less clear.

six. Smoothness

The so-called smoothness refers to the time in seconds required for a volume of air to pass through the gap between the sample surface and the glass surface under a certain pressure and a certain area under a certain degree of vacuum.

The smoothness of the paper surface has a direct effect on the printing, and paper with high smoothness is easily covered by the ink film during printing, so that the coated paper has better printing effect than the offset paper. The higher the smoothness of the paper, can be used for more precise printing screens to produce higher quality prints. We demand paper that is even and smooth, followed by high-smooth paper. The unit of smoothness is the time of air flow. The test principle is to apply a fixed amount of air pressure to the surface of the paper on an area of ​​about one square inch of paper to calculate the time that the air flows along the surface. The longer the time, the higher the smoothness.

Seven. Receptivity

Absorption of paper refers to the paper's ability to absorb water or solvents. For the ink, it is the ability to absorb the ink, and it can also be said that the ink penetrates the paper. It is not only related to the degree of looseness and capillary state of the paper, but also related to the surface properties of the paper fiber, the content of fillers, pigments, rubber compounds, the composition and characteristics of the ink, and also related to the printing method, printing pressure, and the like.

Paper needs some absorbency to promote the transfer of ink. However, if the absorption is too high, the ink binder will be excessively infiltrated into the interior of the paper, and the pigment material will accumulate on the surface of the paper, and it will be difficult to form a film, resulting in lack of gloss of the print and even powdering of the ink layer. In general, the greater the absorption, the more obvious the dot expansion, and when the absorption is too strong, it will also cause the ink to appear through the printing, seriously affecting the quality of printed products.

Eight. Whiteness (Brightness)

The whiteness of paper refers to the degree of whiteness of the paper, which is the total reflection of visible light diffusely reflected in all wavelength ranges. The factors affecting the whiteness are mainly the lignin content and the degree of bleaching in the paper pulp. Since the lignin process is yellow, it is necessary to minimize the lignin content in the paper in order to obtain high brightness paper.

The whiteness of the paper has a greater influence on the color rendering range of the final print. The higher the whiteness of the paper, the more the true color of the ink can be reproduced. The more vivid the color the print presents, the wider the color gamut is. The whiteness of general newsprint is low and yellow, and the whiteness of offset and coated paper is relatively high.

Source: Graphic Arts Perspective