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Key physical properties of paper-including basis weight, thickness, density, strength, compressibility, softness, elongation, and dimensional stability-are closely linked to print quality.
I. Basis Weight
Basis weight is the most fundamental physical parameter of paper and board, influencing tensile strength, burst strength, tear resistance, opacity, and thickness. Different paper types serve distinct purposes:
- Newsprint is soft, opaque, and absorbent, with adequate mechanical strength, suitable for newspapers and magazines.
- Letterpress paper is smooth, moderately white, and sized to resist pilling, ideal for textbooks and literature.
- Offset paper is bright, uniform, and dimensionally stable, with balanced sizing and absorption, suitable for books, catalogs, and color inserts.
- Coated paper offers high smoothness, whiteness, and gloss, delivering excellent image reproduction for high-end publications and albums.
II. Thickness and Density
Paper thickness must be uniform within a batch; otherwise, bound books may exhibit uneven edges, compromising appearance and durability. Thickness also affects opacity and electrical characteristics. Standard paper ranges from 20 to 400 μm; cardboard exceeds 0.5 mm and is typically multi-ply.
Density (g/cm³) is a key indicator for comparing paper strength. Higher density generally correlates with greater burst and tensile strength. However, excessive density reduces porosity, slowing ink absorption and increasing the risk of set-off. Newsprint requires moderate density (~0.7 g/cm³), while offset paper demands higher compactness (>0.75 g/cm³).
III. Mechanical Strength
Strength parameters-tensile, tear, and folding endurance-are vital for product longevity. High-strength paper resists environmental stress and mechanical wear, especially important in bookbinding and packaging. Weak paper may lead to page detachment or package failure during transport.
Folding endurance measures the number of 180° double folds a sheet can withstand before breaking. It increases with humidity but declines in dry conditions. Minimum requirements include:
- Letterpress paper: ≥3–4 folds
- Offset paper: ≥3–5 folds
- Writing paper: ≥2–4 folds
- Banknote paper: very high endurance due to circulation demands
- Boxboard and technical drawings: also require high folding resistance
IV. Compressibility and Softness
Compressibility and elasticity are crucial for printing performance. Due to fiber plasticity, paper exhibits softness and resilience. Typical compressibility ranges from 50% to 60%, with elasticity around 85%. Greater thickness reduction under pressure indicates higher softness, which benefits letterpress and gravure printing by ensuring even ink transfer and sharp image reproduction.
V. Expansion Rate
Paper expands or contracts with humidity changes due to fiber swelling or shrinking. Lateral expansion typically exceeds longitudinal expansion because transverse fiber swelling is more pronounced.
Newsprint and letterpress paper have no strict stretch requirements. Papers with higher filler content exhibit lower expansion rates. Coated paper generally has lower expansion than offset paper. Humidity fluctuations directly affect dimensional stability, necessitating climate-controlled printing environments to maintain registration accuracy and print quality.
In conclusion, a thorough understanding of paper's physical properties is essential for optimizing printing processes and achieving superior print outcomes.
