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I. Basis Weight of Paper
Basis weight is a fundamental physical parameter for paper and paperboard, directly influencing tensile strength, bursting strength, tear resistance, density, thickness, and opacity. Different paper types are designed for specific applications based on these characteristics.
Newsprint features a soft texture, excellent ink absorption, and adequate mechanical strength. It exhibits smoothness and opacity while resisting surface roughening or powder shedding, making it suitable for newspapers, magazines, and books. Letterpress paper has a uniform, smooth surface with high whiteness and moderate sizing, ensuring good ink receptivity and non-transparency without surface degradation-ideal for textbooks and literary publications. Offset paper offers a relatively bright, uniform, and smooth surface with balanced ink absorption and sizing, combined with high mechanical strength and dimensional stability, rendering it appropriate for books, dictionaries, illustrations, inserts, and color images. Coated paper provides superior smoothness, whiteness, gloss, and surface efficiency, making it optimal for high-quality printing such as picture albums, periodicals, inserts, and premium publications.
II. Thickness and Density of Paper
Uniform thickness across a batch is essential to maintain consistent book or publication thickness after binding; otherwise, product quality may be compromised. Additionally, thickness influences opacity and electrical properties. Most paper is single-layered, ranging from 20 to 400 μm in thickness, whereas cardboard is typically multi-layered with a thickness of approximately 0.5 mm.
Density, measured in g/cm³, is a critical index for evaluating and comparing the performance of different paper types. Generally, higher density correlates positively with increased bursting and tensile strength. Moreover, density significantly affects ink absorption and opacity: greater density results in slower ink absorption and reduced opacity. Excessively dense paper may hinder proper ink adhesion due to poor absorbency, potentially causing set-off (ink transfer to adjacent surfaces).
For practical applications, newsprint should not exhibit excessive density. Letterpress paper requires good absorbency, with typical densities around 0.7 g/cm³. In contrast, offset printing demands higher density, usually exceeding 0.75 g/cm³.
III. Strength of Paper
Paper strength-including tensile strength, tear resistance, and folding endurance-is a key indicator of durability. High-strength paper resists environmental stress and mechanical wear, thereby extending the lifespan of printed products. This property is particularly crucial in bookbinding and packaging applications. Insufficient strength can lead to page breakage or detachment in bound documents, impairing readability. In packaging, robust paper ensures effective protection of contents during transport, minimizing damage.
Paper and paperboard must possess sufficient folding endurance and burst resistance. Mechanical strength varies with moisture content. Bursting strength, defined as the maximum uniform pressure per unit area that paper can withstand before rupture, reflects its resistance to internal pressure.
Folding endurance refers to the number of double-fold cycles (180° reciprocating folds) a sheet can endure before failure. This property is sensitive to ambient relative humidity; endurance increases with rising humidity. Overly dry paper becomes brittle, reducing fold resistance. Printing papers must meet minimum folding endurance standards: letterpress paper should withstand at least 3–4 folds, offset paper 3–5 folds, and writing paper 2–4 folds. Banknote paper, subjected to repeated folding in circulation, requires exceptionally high folding endurance. Similarly, drawing papers and boxboard demand elevated fold resistance.
IV. Compressibility and Softness of Paper
Compressibility and softness are vital physical attributes for printing substrates. Due to the inherent plasticity and elasticity of cellulose fibers, paper exhibits measurable softness and compressibility. Typical compressibility for general printing paper ranges between 50% and 60%, with elasticity reaching approximately 85%.
Greater thickness reduction under compression indicates higher softness. Highly compressible paper improves print quality in letterpress and gravure processes by enabling better contact between the printing plate and substrate, resulting in sharper image reproduction and uniform ink distribution.
V. Dimensional Stability and Expansion Rate of Paper
Changes in humidity cause fiber swelling or contraction, leading to paper expansion or shrinkage. The magnitude and rate of dimensional change depend on raw materials and paper type. Lateral expansion generally exceeds longitudinal expansion due to greater transverse fiber swelling upon moisture absorption compared to axial elongation.
Newsprint and letterpress paper typically do not require strict control over expansion rates due to their printing methods. Papers with higher filler content exhibit lower expansion rates. Coated paper generally demonstrates less dimensional variation than offset paper. Both are affected by fluctuations in environmental relative humidity, necessitating climate-controlled printing environments. Maintaining stable humidity levels in printing facilities is essential to ensure registration accuracy and consistent output quality, especially in multicolor printing.
A comprehensive understanding of paper's physical properties enables optimized printing practices and enhances final print quality.
