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FLEXO Magazine : February 2009
TECHNOLOGIES & TECHNIQUES Porosity. Porosity is a measure of the resistance to airflow through a sheet of paper under pressure. It is an indicator of ab- sorbency (penetration of oil and water) and hence the amount of ink that penetrates into the surface of the sheet. It can affect ink absorption, drying and adhesion. Thickness: Caliper/Gauge. Caliper is the thickness of a single sheet of paper, paperboard or combined board. Gauge is the term used to reflect the thickness of a single layer of film. Paper is reported in mils or thousandths of an inch, paperboard in points and film in gauge. Thickness is important because wide variations can cause the final print impression to be uneven. For paper sub- strates, caliper and smooth- ness are inversely related. Higher caliper papers tend to be rougher while lower caliper papers tend to be smoother. SURFACE PROPERTIES Brightness. Brightness is the measurement of blue light (457nm) reflectance. Higher numbers on a 0-100 scale indicate a brighter surface. Most white papers have a brightness of 60 to 90 percent. The brightness of a substrate will influence the intensity of printed color, and the perception of print con- trast. High brightness sub- strates can improve bar code contrast and scannability. Color. Whiteness can be defined as a substrate's ability to reflect all colors of light equally. The most brilliant color reproductions are on substrates with high reflectance values balanced across the visible light spectrum (400 - 700nm). Coefficient of Friction/Slide Angle. Static CoF measures the force required to initiate movement between two surfaces. Kinetic CoF measures the force required to sustain uniform movement. Modifiers, like waxes, are added to reduce CoF and therefore, increase the ease with which a substrate will move across itself. Gloss. Gloss reflects light, like a mirror, and gives a substrate a shiny appearance. It increases with surface smoothness. Printed ink gloss is influenced by substrate gloss. Gloss is reported as a percentage; higher values indicate higher gloss. For most paper substrates, gloss is measured using a 75 0 angle. A 20 0 angle is recommended for very high gloss papers. Film substrates typi- cally use a 45 0 angle for gloss measurement, except for high gloss films, which use a 20 0 angle. - Opacity. Opacity obstructs light transmission. Opacity is re- ported as a percentage; higher values indicate higher opacity. Opacity of paper products is influenced by the degree of fiber re- fining. Increased refining results increase opacity. Fillers in paper also increase opacity, or hiding power, of paper products. High opacity minimizes show-through of an image printed on the op- posite side of the substrate or the product within the package. Smoothness. Smoothness is a measure of the finish, or texture, of the substrate's surface. It is arguably the most important sub- strate surface property for print quality because it influences ink lay and ink transfer. For paper products, the fillers, coating, super calendaring and sizing influ- ence it. Surface Strength/Pick Resistance. Pick resistance is a measure of the substrate surface cohesive strength versus the force required to split a wet ink film. If the pick resistance is too low, the ink will pull the coating off of the substrate onto the printing plate instead of transfer- ring and adhering to the substrate. Surface Tension/Treat Level. Surface tension is a measure of the substrate sur- face energy that influences ink transfer and adhesion to a substrate. Substrates typi- cally should be 8 to 10 dynes/ em higher than the ink. Wash boarding. On corru- gated substrate, the liners fol- low the contours of the fluted medium producing alternate ridges and valleys instead of forming a flat, smooth outer surface. The lighter the weight of the linear used, the more likely that wash boarding will occur. Wash boarding compromises print quality by creating an uneven surface. Increasing impression on press to overcome wash boarding leads to poor print quality, loss of caliper and flat crush. CHEMICAL PROPERTIES Aging/Fade. Aging/fade resistance reflects the ability of a substrate to resist changes in its optical, chemical or structural properties over time. It is characterized by a change in the ap- pearance of the substrate, such as yellowing, loss of brightness, and/or fading. Moisture. The moisture content of paper directly influences how much ink the paper will absorb during printing. Ambient air moisture will affect the moisture content of the paper. A very FEBRUARY 2009 www.flexography.org FLEXO
Sustainable Winter 2009