Considerations for Paper Used in Booklet Printing: Four Major Factors Affecting Quality
Booklet printing products are visual commodities and artworks, the quality of which depends on various factors. The characteristics of the paper are undoubtedly one of the key factors. High-quality paper can effectively transfer ink, ensuring that images and text are clearly and fully reproduced, resulting in satisfactory replication. However, due to limitations in materials like paper and technological processes, most color continuous-tone images cannot be faithfully replicated. Instead, the printed images can only approximate the original as closely as possible.
1. The Impact of Paper Characteristics on Print Gloss
During the printing process, the ink film transfers to the paper surface, filling in the unevenness and making the printed surface relatively smooth. The specular reflection of light from the ink film surface determines the gloss of the printed product, and the ink film surface is closely related to the paper characteristics.
1.1 Ink Film Thickness
The thickness of the ink film is a major factor affecting the gloss of printed products. After the paper absorbs the maximum amount of ink binder, the remaining binder stays in the ink film, which can effectively enhance the gloss. The thicker the ink film, the more binder remains, and the better the gloss. Research shows that the trend of gloss improvement with increasing ink film thickness varies with different types of paper. For example, high-gloss coated paper shows a decrease in gloss with increasing ink film thickness when the film is thin, as the ink film masks the paper's inherent gloss. However, once the ink film thickness reaches 1.5μm, the paper's absorption of the binder saturates, and further increases in ink film thickness lead to higher gloss. For coated paperboard, gloss increases rapidly with ink film thickness up to 3.8μm, after which it stabilizes.
1.2 Paper Characteristics Affecting Print Gloss
The capillary network structure formed by paper fibers absorbs ink, and the balance between absorption time and binder setting time determines the extent of binder penetration during gloss formation. Generally, print gloss decreases as the paper's absorption capacity increases.
The smoothness and inherent gloss of the paper also affect print gloss. Higher smoothness facilitates the formation of a uniform ink film, enhancing gloss. Papers with higher inherent gloss improve the reflectivity of the ink film, especially with transparent inks. Additionally, the pH level of the paper surface can influence gloss, with higher pH values promoting ink drying and thus enhancing gloss.
2. The Impact of Paper Characteristics on Tone Reproduction
Ideal tone reproduction is crucial for high-quality printed images, and most poor-quality prints result from issues in tone reproduction.
The primary goal in image reproduction is to match the visual appearance of the original. However, this is often unachievable. The density range of images reproduced on different types of paper varies, and typically, the original's density range exceeds that of the printed image. As paper quality decreases, the reproducible density range narrows, affecting saturation and color.
The solution is to compress the original's tone range to match the reproducible density range of the paper.
3. The Impact of Paper Characteristics on Dot Gain
3.1 Dot Gain
Dot gain significantly affects color reproduction more than any other variable. The amount of ink printed on paper influences dot gain, which in turn affects print contrast. In halftone images, dot gain alters image contrast and leads to loss of detail and sharpness. In multicolor printing, it causes loss of contrast, darkening, dot clogging, and abrupt color changes.
3.2 Paper Characteristics Affecting Dot Gain
The smooth surface of ink produces the best solid density on high-quality paper. As paper quality decreases, solid ink density drops, leading to dot gain and affecting print quality.
Higher screen rulings require smaller dot diameters, and even minor paper imperfections can distort or miss fine dots. Therefore, high-screen ruling prints demand smoother paper. However, adjustments in ink properties or process parameters can sometimes compensate for paper characteristics.
4. The Impact of Paper Characteristics on Registration Accuracy
For fine prints, registration accuracy is critical, typically allowing an error margin of 0.05mm. In multicolor printing, many factors can cause misregistration, but paper deformation due to moisture content changes is significant.
Changes in paper moisture content not only cause expansion or contraction but also lead to other deformations like curling or wrinkling, severely affecting print quality. Printers are particularly concerned with how relative humidity affects paper moisture content.
. At constant room temperature, changes in relative humidity at high or low humidity levels cause more significant moisture content changes than at moderate humidity levels. Thus, printing under moderate humidity conditions is most favorable.
. Paper exhibits a hysteresis loop in moisture absorption and desorption, meaning that after absorbing or losing moisture, the paper's moisture content does not return to its original level when re-exposed to the same relative humidity.
To ensure uniform moisture content across the paper and adapt it to the printing environment, and to reduce the paper's sensitivity to humidity and improve dimensional stability, paper is typically conditioned (pre-humidified) before printing.
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