Paper and printed paper surface characteristics studied using an optical method.
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
AIMS OF TIllS STUDY.
Conventional measurements of print quality have been made on print
density or print gloss. Few studies have been concerned with the
characteristics of printed surfaces. This study concerns a measure of print
quality in terms of print smoothness by an optical method. The aims of the
1. To improve an instrument so as to be able to measure the reflection
of printed surfaces.
2. To verify the method for reliable determinations of print
smoothness. This requires a large number of samples to be examined and
statistical methods employed to ensure significant data. When this has been
achieved, a printed surface model can be proposed.
3. To investigate the effects on print smoothness, of materials involved
in the offset lithographic process, on a laboratory scale printing press.
Print smoothness is one of the most important print quality factors. It
is hoped that this study will provide a better understanding of ink / paper
interactions; and, for the offset lithgraphic process, 'emulsion ink' / paper
interactions. The method of this study may be useful to both ink makers for
the developments of their ink formulations and printers to be aware of such
printed defects which may occur.