Paper and printed paper surface characteristics studied using an optical method

Sananpanichkul, Wanna (1993). Paper and printed paper surface characteristics studied using an optical method. PhD thesis The Open University.



A non-contact optical method for evaluating surface characteristics is reviewed. The optical reflectance instrument has been improved to be able to evaluate printed surfaces. Experiments were conducted with solid prints prepared on two types of papers printed with a heatset yellow ink. Both paper surfaces and printed surfaces are characterized into two regions: above the surface plane resulting in macrosmoothness (Sm) and below the surface plane resulting in microsmoothness (Su). For a better understanding of such optical print smoothness, a printed surface model is proposed based on Barkas' classical model. It is generally known that the qualities of a print are determined by the materials and their interactions in the process, therefore the formation of printed surface characteristics has been discussed in relation to ink and paper interaction. Print smoothness is influenced by the uncompressed paper roughness and porosity, which determine the degree of ink penetration and ink distribution on the surface.

Offset lithographic printing has been the most widely used printing process, printing onto paper substrates. To achieve good press performance and high quality prints, the ink has to emulsify a certain amount of fountain solution; maintenance of this ink and water balance is, therefore, very important. The effects of fountain solution emulsified in the ink on print smoothness wp-re investigated. A range, of varying amounts of fountain solutions, was emulsified in a heatset yellow ink using a high speed laboratory mixer; these 'emulsion inks' were printed as soon as possible after preparation. It was found that the print smoothness in macro regions, for both uncoated and coated papers, decreased significantly. In addition, an unpigmented ink system was employed to verify the role and the effects of pigment in the emulsification mechanism on print smoothness. The results indicated that pigment is the dominant contributor, to a smoothness decrease; and the pigment effect arises from the amount of fountain solution emulsified in the ink.

It has become important to measure print quality directly and quantitatively in the developments of ink, paper and printing technologies. This method makes it possible to measure print smoothness as a criterion for print quality.

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