Q&A on Printing Paper (139-149)

139. What is the importance of whiteness for printing paper?
Answer: Since the contrast of printed products increases with the increase of whiteness of the paper, most of the colored inks are transparent or translucent, and the reflected light of the paper surface will be transmitted through the ink layer. Therefore, for color prints, high whiteness paper should be used for printing, and for high-brightness prints that are often read, such as books and periodicals, the whiteness of the paper does not have to be too high.

140. What is transparency and opacity?
A: Both transparency and opacity are physical quantities that describe the light transmission properties of paper. They depend on the amount of light transmitted by the paper. Although the transparency is related to the opacity, the meaning is different. The opacity depends on the total amount of transmitted light. The transparency depends only on the transmission of the mirror. The amount of light. Almost all paper is translucent.

141. What is the transparency of paper? How to measure?
A: Transparency is a very important quality indicator for tracing paper, copy paper and certain wrapping papers. Transparency can be used to truly measure transparency. Transparency is the ratio of specular transmission to total transmission. It is the best way to evaluate paper transparency.

142. What is the opacity of paper?
A: The opacity of white paper indicates the resistance of paper to the transmission of light. It is the main aspect that determines that the paper does not transmit after printing. The so-called opacity, which refers to the opacity of white paper, is measured by the ratio of the intensity of the light through the paper and the intensity of the illuminated light.

143. How to measure the opacity in practice?
A: Although conceptually opacity is measured and expressed in terms of the ratio of transmitted light to irradiated light, in actual production, for ease of testing and consistent with printing, contrast is generally used, ie, backing up with a single sheet of paper. The paper's opacity is described by the ratio of the diffuse reflectance in black and the diffuse reflectance in backing the white body. Instruments that measure reflectivity can measure opacity.

144. What is the representation of paper opacity?
A: There are three common ways of expressing opacity, namely TAPPI opacity, printing opacity and IGT printing opacity.
1 TAPPI opacity, denoted as C0.89, expressed as follows:

R0—the reflectivity of a black backsheet on a single sheet of paper;
R0.89—Reflectance when the backing effective reflectance is 89% white body.
2 printing opacity, expressed as:

R0—reflectivity of single sheet backing with black body;
R ∞ - The reflectivity of a single sheet of paper that is backed by a thick enough layer of the same paper.
3IGT printing opacity expressed as:

Rp0—the reflectivity of the reverse side of the printed area;
Rp∞ - reflectivity when the back side of the unprinted area is backed with a thick paper layer.

145. What are the factors that affect the opacity of paper?
A: There are many factors that affect the opacity of paper. The paper has the ability to reflect light. The paper with high whiteness has less opacity than the paper with the same amount of low whiteness. The wavelength of the measured light mainly affects the absorptive capacity of the paper. Since paper absorbs more blue light, the opacity of white paper measured with blue light is much higher than that measured with red light; the dye, even with a small amount of dye, causes a significant increase in opacity when measuring light wavelengths and dyes The maximum absorption wavelength corresponds to the maximum opacity of the dye; increased quantitation leads to an increase in the scattering capacity, which can increase the opacity of the paper; beating and wet pressing, for most pulps, beating and wet pressing will not decrease Transparency; Apparent density, increased apparent density, usually in the paper to produce more optical contact, and reduce the opacity; fillers, fillers can significantly increase the paper's light scattering capabilities, thereby increasing the paper's opacity; Wax, immersed paraffin or a substance with a refractive index and cellulose in the paper, the opacity of the paper will be greatly reduced; Generally, the lower the reflectivity of the slurry, the higher the opacity of the paper.

146. What is printing opacity? What are the factors that affect printing opacity?
A: Figure 1 shows the difference between white opacity and opacity: Figure A shows the opacity of white paper. If the print density of the image is fixed, the opacity of the paper determines the degree of image transparency; Figure B shows the It is the image on the back of the sheet. The situation that can be seen from the front through the sheet, that is, the printing opacity, depends not only on the opacity of the white paper but also on the degree of penetration of the ink into the sheet, the more easily the ink penetrates. Paper, the worse its print opacity.
Factors that affect print opacity are white opacity and paper resistance to ink penetration. Therefore, for a single-sided printing product, a good white opacity and printing opacity can be achieved at the same time in order to achieve a higher print quality.

147. How does the light scattering of the paper affect the quality of the halftone printed matter?
A: The light scattering property of paper causes the reflectivity to decrease and the reflection density to increase, which reduces the printing density and the reflectance of the blank part between the halftone dots of the mesh printing product, so that the density of the blank part increases and the contrast of the image layer decreases. The light entering the interior of the paper from the space between the dots is scattered through the dots due to scattering. The closer to the space, the stronger the light that passes through, resulting in a low density at the edges of the dots and a high density in the middle, and There is also a certain density in the margins of the edge of the dot, which means that dot gain occurs and image sharpness is reduced.
148. What is the surface efficiency of the paper?
A: The surface efficiency of paper is not the optical properties of paper, but the printability index of paper that affects the quality of prints in relation to the optical properties. It is a physical quantity that combines paper glossiness and ink absorption ability. It can be calculated as follows:
In the formula: PSE—paper surface efficiency;
A—paper ink absorption capacity;
G—glossiness of the paper (specular gloss of paper 75° angle).

149. How does the surface efficiency of the paper affect the ink rendering effect?
A: The hue of the ink on the paper is closely related to the surface efficiency of the paper. The intensity and color efficiency of color increase with the increase of paper surface efficiency, while the color shift and gray scale decrease with the increase of paper surface efficiency.

- "Printing Technology - Equipment and Special Printing"