The flatness of the book block with a circle-binding hard cover directly affects its overall aesthetics and durability. If the book block is uneven, curled, or deformed, it will not only compromise the stability of the binding structure but may also lead to a loose fit between the cover and the book block, impacting the reading experience. Therefore, a comprehensive approach is needed during production, encompassing material selection, process control, equipment debugging, and environmental management. A systematic operational process is essential to ensure the book block achieves an ideal flatness.
Paper, as the core material of the book block, has physical properties that form the basis for controlling flatness. Papers of different weights, fiber orientations, and moisture contents exhibit significant differences in hygroscopicity and elasticity. If mixed or not standardized, inconsistent shrinkage rates can easily lead to book block deformation. For example, when lightweight paper and coated paper are mixed, the former has strong hygroscopicity and high elasticity, while the latter is relatively stable. The combination of the two can easily generate stress when humidity changes, causing the book block to bend. Therefore, when making circle-binding hard covers, it is crucial to strictly select paper types, prioritizing materials from the same batch, with the same weight and consistent fiber direction. Pre-treatment (such as moisture conditioning) is necessary to ensure uniform paper moisture content and reduce deformation caused by environmental changes later.
The folding and collating processes of the book block are critical to flatness. Folding must ensure precise creases and even pressure to avoid localized bulging due to excessively deep or misaligned creases. Traditional manual folding is prone to minor variations due to operational errors, while modern automated folding machines significantly improve folding accuracy through mechanical positioning and pressure control. The collating process requires attention to the page order and orientation. Misaligned, upside-down, or reversed pages not only disrupt the continuity of the content but may also cause the book block to tilt due to uneven page thickness. Therefore, after collating, page numbers and orientation must be checked manually or using a machine vision system to ensure complete consistency in the page order of each book block.
The stability of the perfect binding process is essential for the flatness of the book block. Circle-binding hard covers typically use hot melt adhesive or animal glue to fix the book block. The viscosity, curing time, and coating uniformity of the glue directly affect the bonding effect of the pages. If the glue viscosity is too low, it is easy to flow to the edges of the pages before curing, leading to localized adhesion or increased thickness; if the viscosity is too high, uneven coating may create gaps in the glue, causing the pages to loosen or fall off. Furthermore, the curing temperature and time of the glue must be strictly controlled. Too high a temperature may cause the paper to shrink, while too low a temperature may cause incomplete curing and deformation of the book block. In practice, process parameters need to be adjusted according to the paper type and glue characteristics, and the bonding effect should be verified by producing sample books.
Flattening and drying the book block is the last line of defense to ensure flatness. Newly bound book blocks have internal stress because the glue is not fully cured. If they proceed directly to the next process, the stress release can easily cause the book block to spring back or bend. Therefore, the book block needs to be placed in a specialized flattening machine and flattened using uniform pressure (usually several hundred kilograms) and a suitable temperature (to avoid high-temperature damage to the paper) to allow the glue to fully penetrate and fix the pages. After flattening, it needs to be left to stand in a dry environment for a certain period of time to ensure that the glue is completely cured and to prevent paper expansion and contraction due to humidity changes. The drying environment should be well-ventilated and have stable temperature and humidity to prevent deformation of the book block due to localized dampness or excessively rapid drying.
Equipment precision and operating procedures are the technical support for ensuring the flatness of the book block. The production of circle binding hard covers involves multiple machines, including folding machines, collating machines, perfect binding machines, and flattening machines. Precision deviations in any of these machines can accumulate in the final product. For example, wear on the glue application rollers of the perfect binding machine can lead to uneven glue application; miscalibrated pressure sensors in the flattening machine can cause uneven stress on the book block. Therefore, regular maintenance and precision calibration of the equipment are necessary to ensure the stability of parameters at each stage. Meanwhile, operators must undergo professional training and be familiar with the process flow and quality standards to avoid affecting the flatness of the book block due to human error.
Environmental temperature and humidity management is an easily overlooked but crucial aspect. Paper is extremely sensitive to environmental humidity; when humidity is too high, the paper absorbs moisture and expands, potentially making the book block thicker and softer; when humidity is too low, the paper loses water and shrinks, potentially making the book block thinner and more brittle. The circle binding hard cover production workshop must be equipped with a temperature and humidity control system to maintain environmental humidity within a certain range and avoid direct sunlight or direct drafts from ventilation vents to prevent localized humidity differences from causing book block deformation. Furthermore, raw materials and semi-finished products must also be protected against moisture and dust during storage and transportation to minimize the interference of environmental factors on the flatness of the book block.
