1 Basis Weight
Quantitatively commonly known as gram weight refers to the weight of paper per unit area, expressed in grams per square meter. Generally, those with a basis weight below 250 g / m2 are called paper; those with a basis weight above 250 g / m2 are called cardboard or cardboard. Quantification is the most basic physical index of paper. Its height and uniformity affect all physical and printing properties of paper.
2 Thickness and tightness
Thickness indicates the thickness of paper. It refers to the thickness of paper or paperboard measured directly under a certain pressure between two measuring boards, in mm or μm. Generally, the thickness directly affects the opacity of the paper, and the greater the thickness, the better the opacity. The uniformity of thickness is especially important for paper. Uneven thickness will affect the decline of 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 by dividing the quantity by the thickness.
Tightness is a physical indicator to measure the degree of density of paper structure. For papers of the same kind of paper fiber, the greater the tightness, the denser the paper quality, otherwise, the looser the paper quality. The tightness of the paper is proportional to the tensile strength.
3 Tensile strength and elongation (Tensile Strength)
Tensile strength is the maximum tensile force that the operator can bear before the unit width of paper or cardboard 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 tensile force is increased so that the spline is broken, the stretch reaches the limit, and the ratio of the length of the stretch to the original length is called the elongation rate Expressed as a percentage.
For web printing, the paper's tensile strength and elongation are an important indicator. Because the paper roll is driven by the rotary press, the paper will be under a lot of tension. If the tensile strength is too small, it will cause paper breakage; if the elongation is too large or too large, it will change the size of the paper and increase the overprint error. Therefore, the use of paper with high tensile strength and large elongation can effectively prevent and reduce paper breakage during printing and reduce overprint errors.
4 folding resistance
The folding endurance of the paper refers to the number of folds that the sample can withstand before being broken in a special instrument under the specified test conditions, expressed as double fold times.
Folding resistance is one of the important indicators that indicate the mechanical strength of paper and cardboard. It is a comprehensive measure of the strength and flexibility of paper. This indicator is necessary for a variety of paper and cardboard, and it is necessary for newspapers, banknote paper, and maps. Paper and cover paper are particularly important because they are subject to repeated folding during use. In order to prolong their service life, they must be required to have such a high number of folding endurance.
5 Surface Strength of Paper
The surface strength of the paper refers to the combination of fibers, fillers, pigments, etc. on the surface of the paper with the paper, and the firmness of the connection, that is, the strength of the combination of the materials on the surface of the paper.
Generally speaking, the surface strength of newsprint is relatively low, while the surface strength of coated paper is relatively high. The phenomenon of dusting and fluffing often occurring in printing is closely related to the surface strength of the paper. In newspaper printing, the surface of the paper is often peeled off by the ink film and left on the ink roller, causing the printed product to not ink, white spots appear, and the small fibers of the filler material falling off can also block the graphic part on the printing plate, making the printing The text of is becoming less and less clear.
6Smoothness
Generally speaking, the smoothness refers to the time required for a certain 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, expressed in seconds.
The smoothness of the paper surface has a direct impact on printing. Paper with high smoothness is easily covered by ink film evenly during printing, so the printing effect of coated paper is better than that of offset paper. Paper with higher smoothness can be used for more precise printing dots to produce higher quality printed matter. We require paper with uniform smoothness, followed by paper with high smoothness. The unit of smoothness is the time of air flow. The test principle is to apply a certain amount of air pressure to the surface of the paper on a surface of about one square inch to calculate the time that air flows along the surface. The longer the time, the higher the smoothness.
7 Receptivity
The absorbency of paper refers to the ability of paper to absorb water or solvents. For the ink, it is the absorption capacity of 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, as well as the printing method and printing pressure.
The paper needs to have certain absorbency to promote the transfer of ink. However, if the absorbency is too high, the ink binder will be excessively penetrated into the paper, and the pigment material will accumulate on the surface of the paper, making it difficult to form a film, resulting in lack of gloss in the printed product, and even the ink layer powdered. Generally, the greater the absorption, the more obvious the dot expansion. When the absorption is too strong, it will also cause ink through printing, which will seriously affect the quality of the printing.
8 Brightness
The whiteness of the paper refers to the whiteness of the paper, which is the total reflection of the uniform diffuse reflection of visible light in all wavelength ranges. The factors affecting whiteness are mainly the lignin content and the degree of bleaching in the paper paddle. Due to the yellow color of the lignin process, it is necessary to reduce the lignin content in the paper as much as possible to obtain high whiteness paper.
The whiteness of the paper has a greater influence on the color range of the final printed product. The higher the whiteness of the paper, the more the true color of the ink can be reproduced. The brighter the final printed product, the wider the color gamut. The whiteness of general newsprint is low and yellow, and the whiteness of offset paper and coated paper is relatively high.
9 Opacity (OPAcity)
This feature means that when printed text and pictures are printed on one side of the paper and text or pictures are printed on the other side, they will not show through. Good paper has high opacity, so that the finished book allows readers to concentrate on the content of their own pages when reading, so as not to be obscured by the two sides of the printed pattern and make the eyes tired. The factors that affect the opacity are: i. The thickness of the paper itself; ii. Whether there is gluing; iii. The smoothness and roughness of the paper surface; iv. The plant fiber components contained. The test method is to measure how much light can penetrate the paper itself, and the unit is percentage ratio.
10 Gloss (Gloss)
Glossiness is the degree to which the surface of the paper is close to the complete mirror-reversing capability in terms of the reverse incident light capability. It is the ratio of the mirror-reversing rate of the paper at a certain angle to that of standard black glass at the same angle Expressed.
Paper mills usually measure the amount of light reflected on the surface of white paper and express the rate of reflection as a percentage. The industry will use this rate to classify paper, and paper with a high rate is a higher-quality finished product. When the gloss of the paper is large, the color of the printed matter appears more vivid. Some posters and magazines use high-brightness paper to highlight their text and pictures to attract attention, but books are made with lower-brightness paper to reduce the effect of too much reflected light on the eyes Tired easily.
It can be seen from the above that the performance of the paper is related to its printability, which is known to all printers. But which quantity has the greatest influence on the printing reproduction, the difference between the quantity and the relationship of the printing reproduction is not clear to everyone. If the influence of each quantity on printing performance can be described by a certain value, it will undoubtedly be of great help to the printing industry to guide production.
11 Surface efficiency
Surface efficiency is a non-optical characteristic that affects the reproduction of the color tone of the paper.
PSE (%) = 100-A (%) + G (%) / 2
PSE-paper surface efficiency (paPEr surface efficiency);
A—The ink absorption capacity of paper;
G—Glossiness of paper.
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