- May 01, 1996, Boyle, Edward
Of all the components that can affect the quality and ultimate impact of a finished printed product, the base stock often plays the most critical role. The choice of such variables as adhesives, inks, and coatings depends in large part of the type of substrate being converted.
But when working with paper, how does one choose between the hundreds of base stocks available? Among the most important paper characteristics to be considered when choosing a substrate are paper strength, base sheet uniformity, moisture content, smoothness, gloss, ink receptivity, brilliance, whiteness, and coating type. The importance that each of those characteristics plays can vary from job to job.
"It depends on what you're going to print, what your subject matter is," suggests Bruce M. Riddell, VP of engineering for Spectrum Label Corp., San Carlos, CA. "Based on the job requirements, you might look at what the strength of the paper is going to be, the opacity of the paper, moisture resistance, the surface day coating, and other factors."
The strength of the paper is, of course, critical to the efficiency of any converting operation. Before the label, tag, carton, or form can even reach the customer and perform its function, it must "survive" the converting process. Web-fed jobs, in particular, must be strong enough to withstand the stress of unwind and rewind operations. Therefore, uniform strength is necessary across and along the substrate.
Paper strength, says Riddell, can be affected by a number of variables, including moisture content, tensile strength, the type of coating, the paper's thickness and orientation, among others.
Base Sheet Uniformity
Base sheet uniformity is critical in both cross-machine machine direction and machine direction in areas of caliper, basis weight, moisture, and absorbency. This is essential in order to avoid sheet deformities, such as wrinkles and ridges, and to assure uniform coating pickup.
To achieve uniform moisture, caliper, and basis weight, the paper web is frequently dried to less than 1% moisture at the size press and 3-4% at the reel. This wastes energy, reduces production, reduces strength, and can lead to a (literally) shocking experience from static discharge at the paper machine reel. It is nonetheless standard operating procedure at many mills.
"Moisture, of course, can have a great effect when you're printing paper," says Riddell who serves as narrow web vice chair and board member of the Flexographic Technical Assn. "And moisture content varies tremendously from roll to roll. It doesn't really affect the printing characteristics that much, unless it's really moist, but it has a tremendous impact on curl, and that has a tremendous impact on the way the paper feeds into the converting machines and how it dispenses during application."
Misregistration due to paper can occur if the roll has tight or loose edges or baggy centers or if the edge trim weaves significantly. This can cause so much movement that even the automatic control devices prove ineffective, causing the web to vacillate along the web path between printing units. Another problem that can cause misregistration is the tension variation between rolls of paper. Other factors that contribute to print quality include smoothness, gloss, and uniformity.
The smoothness of the ink film is determined, in part, by the substrate. An ink film is approximately 1 micron thick. To obtain its maximum gloss, an ink film should be thicker than the roughness of the paper. Consequently, a paper roughness of 1 micron or less is best suited for producing high ink glosses.
Of course, says Riddell, a rougher sheet is actually preferable for laminating, since the adhesive will adhere better to a less smooth surface and provide better bond strength, superior hold, and performance.
"What really affects ink receptivity is the clay coating or the way the paper is finished," explains Riddell. "If it is very porous, if the fibers of the paper are not intimate with themselves and their base stock, that will affect die way the ink transfers. If the fibers are well locked together, they're going to provide a nice, smooth coating. If they're sticking up in the air and are kind of random, it makes for an inconsistent coating."
Another paper property that has a significant effect on ink gloss is the gloss of the paper itself. Ink density is a measure of the percent of pigment that is contained in an ink film. This pigment loading accounts for most of the density, with paper absorption contributing a small amount.
However, it is ink film thickness, not ink density, that determines ink gloss. Two ink films printed to the same ink thickness but with different levels of pigment loading will give the same gloss numbers but will not be equal in density. Printers use this knowledge in an attempt to overcome the roughness of the paper. They will reduce the pigment loading for the ink, which forces them to increase the film thickness to achieve the desired density.
"Normally, if it's glossy, it has a clay coat on it," explains Riddell. "And more gloss gives you a better holdout of the inks and a better color. Anything that's glossy will inherently give you a glossier finished product."
Variations in the gloss, of course, can affect the finished product. For example, if you're laying down a red ink on a clay-coated stock that is inconsistent or too light or flaky, the ink won't stand up properly, resulting in poor performance.
One paper property that affects ink gloss is absorption. Ink films transfer from surface to surface by "splitting" which produces a split pattern within the ink film and is present until the ink flows or levels. If the absorption rate of the paper is fast enough to pull the thin oils from the film before the leveling is completed, the ink film retains the patterncausing a roughness within the ink film that scatters the fight and produces a reduction in print ink gloss.
The uniformity of an ink film may be affected in the first printing unit by a phenomenon known as "water interference." The ink, as it moves through the inking system to the paper, comes in contact with water, which is suspended within the ink film and on the film surface. The water then contacts the paper before the ink, and if it is not removed from between the paper and the ink film via absorption or a similar method, interference can occur. This causes the ink film to be spotted with water droplets, causing a drop in density and a disruption the uniformity of the ink film.
The ability of the paper to absorb the water while in the printing nip determines whether or not the ink can uniformly transfer from the blanket to the paper.
Brilliance, Whiteness Uniformity
Brilliance and whiteness are critical characteristics for most process print jobs. Whiteness will affect the visual spectrum that can be achieved with the base substrates. Yet, says Riddell, not only does whiteness vary from supplier to supplier, it also varies across a single roll of stock. And, not surprisingly, consistent whiteness is particularly important for color process jobs.
For example, whiteness can affect the final color, depending on the opacity of the ink. If a printer is using a letterpress ink with greater density, it will hide the substrate and any inherent whiteness. Other type inks may not function as effectively, however.
"You can't reproduce what one printer did with litho on a good white paper with flexo, gravure, or any other process if you don't have the same whiteness," notes Riddell. And, he adds, this will become even more difficult in the years ahead as recycled papers become more popular -- or even required -- for use by many environmentally conscious end users.
"These are the kinds of things that you just learn over time by seeing a lot of different situation and scenarios," says Riddell. "It's kind of a boring issue, but it gets pretty exciting when a customer calls and says, `this material isn't working.' That's when you need to know these things."