The Relationship Between Paper Properties and Print Quality
The quality of printed products is intricately tied to the properties of the paper used. Therefore, in printing operations, the requirements for paper vary depending on the nature of the printed material and the desired quality. Different papers, due to their unique physical and chemical properties, exhibit varying adaptability to different printing processes.
Ink Absorption of Paper
During printing, the transfer of ink relies on pressure and the capillary action created by the gaps between interwoven paper fibers. When paper is subjected to printing pressure, the fiber gaps compress; once the pressure is released, the paper's surface, due to its elasticity, returns to its original state and absorbs the ink, thus forming the printed image.
The ink absorption capacity of paper reflects the characteristics of its fibers, including their interwoven structure, the properties and content of fillers and sizing agents, and the porosity that drives capillary action. Factors such as printing machine pressure, the duration of contact between the paper and the press, the ink viscosity, and the paper's structure influence this absorption. Papers with higher lignin content generally exhibit greater ink absorption. For example, newsprint, which contains minimal fillers and sizing agents, has good ink absorption properties. This allows for reduced pressure, enhancing the durability of printing plates.
However, excessively loose paper with large fiber gaps may cause "picking" when using highly viscous ink. Conversely, low-viscosity ink may seep into the fibers too quickly, resulting in "show-through." Properly adjusting ink viscosity is crucial to prevent binder seepage, loss of surface ink, and diminished gloss, all of which can compromise print quality.
Paper Smoothness
Smoothness is a critical indicator of paper quality, reflecting the uniformity and flatness of its surface. Rougher papers with poor smoothness can lead to inconsistencies in ink distribution, both between the front and back sides of the paper and even within the same sheet. This inconsistency affects the uniformity of ink coverage on the printed page.
The smoothness of paper significantly impacts the reproduction and expansion of halftone dots. For smoother papers, lower printing pressure is required, while rougher papers necessitate increased pressure to meet the requirements for detailed text and graphics. This increased pressure can, however, reduce the durability of the printing plate.
Paper with a smooth surface produces clearer and more vibrant prints, as it allows uniform ink transfer from the plate to the paper. Conversely, rougher papers, with their uneven surfaces, hinder consistent ink transfer under the same printing conditions, affecting print clarity and uniformity.
Paper Elasticity and Plasticity
Paper elasticity refers to its ability to recover its shape after the printing pressure is released, while plasticity denotes the degree of deformation the paper undergoes under pressure. Elasticity affects ink absorption, while good plasticity ensures better contact between the printing plate and the paper, resulting in richer and more saturated prints.
The elasticity and plasticity of different papers vary, necessitating adjustments in printing operations. Papers with higher elasticity require greater pressure but less ink, while less elastic papers require reduced pressure and increased ink supply. During operation, excessive pressure should be avoided to maintain quality and prolong plate life.
Paper Cutting
Paper cutting is a critical initial step in the printing process and has a direct impact on the quality of semi-finished and finished products. Therefore, improving cutting precision is an important challenge in printing technology.
Cutting machines range from mechanical and hydraulic models to computer-controlled systems. Older imported computer-controlled cutters often suffer from outdated systems and frequent malfunctions, complicating cutting operations. Conventional cutting machines lack precision, are inefficient, and produce higher waste rates, negatively impacting product quality. Modernized or new cutting machines, such as SWD systems with computer controls, digital displays, and projection screens, offer enhanced safety, ease of operation, and high precision, addressing these issues effectively.
Properly cut paper should have perfectly square corners, consistent dimensions across all sheets, and a straight edge with minimal error—ideally not exceeding 0.1mm. Cuts should be clean and neat, avoiding defects like tears, streaks, or fuzziness. Lower layers of paper should be fully cut without damage or contamination. Different paper types and materials also have specific cutting requirements and tolerances.
When operating cutting machines, the following precautions should be taken:
. Inspect the machine, motor, blade, switches, and brakes at the start and end of each shift.
. Ensure the pressure bar is free of oil, the paper table is dry and rust-free, and no foreign objects like nails are present.
. Carefully review job orders to verify specifications and quantities.
. Separate and stack different types of paper or printed products to prevent confusion.
. Maintain consistency in grain direction when stacking paper.
. Cut materials to precise dimensions without deviations or skewing.
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