The dimensional change produced by paper soaked in water or air-dried after soaking in water is called the elasticity of paper. The stretchability of paper is expressed as a percentage of the original sample by the growth of the longitudinal or transverse length of the paper sample that has been pre-balanced in standard atmospheric pressure and immersed in water. The stretchability after immersion and air drying is also expressed in this way.
The stretchability of paper is different between horizontal and vertical. The elasticity of paper is inevitably produced by papermaking materials and processes, and the elasticity of paper can be reduced or eliminated through prepress treatment. The general method is to rehydrate and then dry.
The elasticity of paper has a serious impact on the printing process and the quality of printed matter, mainly in the following situations. The first situation is that the overprinting of multi-color printing is not allowed. When the offset printing machine is printed, each printing color will go through the dampening solution once, and it will expand and contract.
The second situation is that the printing of gravure and flexographic printing with solvent or water-based ink will also cause paper stretching for each color group, which will cause inaccurate multi-color overprinting and cause difficulties in post-press processing. The third situation is that after printing, the paper absorbs moisture or loses water to cause expansion and contraction, and then post-processing such as bronzing, die-cutting, box-gluing and other processes will lead to inaccurate alignment and result in a large amount of waste products.
When the moisture absorption of various parts of the paper is not uniform during the printing process and storage process, the shape of the paper will change, such as ruffles, tight edges, curls, wrinkles, waves, etc. Slight deformation makes printing difficult, and paper cannot be used when it is severely deformed.
