The inks themselves offer a wide variety of properties. “In selecting the proper web offset heatset ink formulation for a specific stock, primary considerations are to these physical properties: tack, stability, viscosity, pigmentation level, gloss and scuff resistance,” said Vince Bellini, marketing manager, web offset at Sun Chemical Corporation.
Assessing printing characteristics is critical to determining how ink is likely to perform, and having standard equipment is essential. Without these instruments or procedures, an ink company would have a difficult time ensuring its products perform on press. In addition, an excellent ink laboratory can help when trouble-shooting is required.
“Though ink testing is not a substitute for actual verification on a press, thorough testing can indicate the tendency for an ink to have problems in the printing environment,” said Lisa Hahn, president of Flexo Tech. “The importance of having the right equipment, verifying its functioning, and understanding the limitations of what it can and cannot tell you about press performance is critical.”
Many of these pieces of equipment have been around for decades, although there have been improvements made. “The Inkometer has been around since I started,” Mr. Bellini said. “A lot of these instruments have been updated digitally, but the concepts are the same.”
To continue to provide quality inks, a well-equipped ink laboratory has to be able to test its products. Knowing how their inks will perform in terms of tack, viscosity/yield and on substrates will give the ink manufacturer a basis from which to create the inks their customers require.
Setting up a testing laboratory isn’t confined to ink manufacturers alone. Suppliers and printers alike also have ink laboratories set up to test ink.
Viscosity and Elasticity
The Duke Falling Rod Viscometer and the Laray Viscometer are the two most common viscometers. “They measure a certain number of parameters,” said Ms. Hahn. “They measure viscosity at different shear rates, as well as yield value, which is the minimum amount of shear needed to get the ink to start flowing. If the viscosity of an ink isn’t right, this can lead to transfer problems and emulsification.”
However, while these viscometers can determine viscosity, elasticity is also an important characteristic for ink. “A controlled stress rheometer can measure viscosity and elasticity at progressively increasing shear rates,” Ms. Hahn said. “The viscous and elastic components are essential to understanding an ink’s behavior.”
“We’re now using more sophisticated devices to measure viscosity, such as the Duke viscometer,” said Steve Simpson, senior vice president and chief technical officer at Superior Printing Ink. “There is an interest in controlled stress rheometers. The problem is that our industry and its customers are very familiar with the Laray and Duke and their numbers, and we need to address this in order to use more sophisticated techniques.”
Generally speaking, tack is the force required to split ink at the nip of the roller. As press speeds increase, the tack of the ink should be lower.
There are a number of printing problems caused by excessive tack. If the ink is too tacky, picking occurs, pulling off fibers of paper. Linting is caused when these fibers travel through the roller train and end up on the plate or blanket. Piling is an advanced case of linting, where multiple fibers stick to the image area of the plate. Tack can also lead to trapping problems.
In particular, excessive tack is a major issue with uncoated substrates. “Uncoated stocks, such as newsprint, are not designed to have high hold-out properties, so they do not require the high rub-resistant properties critical in coated stock ink formulations,” Mr. Bellini said. “However, these inks must have low linting and picking properties.”
The Inkometer approximates the tack characteristics of an ink by measuring the restoring force that must be applied to the roller train (to which the ink has been applied) to put it back to its original position. The roller train is essentially deflected from the vertical by the splitting of the ink film. The Inkometer can also provide a stability curve to determine tack over time.
“You need an Inkometer, or in Europe, the Tackometer,” said Mr. Bellini. “The Inkometer gives you the value that is required to determine the tack of the ink. It also provides the stability curve of the ink over a 10-minute period.”
“Tack stability is determined as a function of time,” Ms. Hahn said. “There are a lot of rollers on an offset press, and the ink is being subjected to a lot of spreading and splitting. You want to make sure all this shearing is not making the ink more tacky by driving off the oils or causing the ink to dry out.”
Other related instruments provide ink formulators with important information, including picking, trapping, gloss or other characteristics.
A pick tester can give the ink formulator an idea of how resistant the substrate is to picking.
“The IGT Pick Tester permits the formulator to test an ink on a half-inch strip of the paper stock to be used,” Mr. Bellini said. “It simulates the action of a web press running at a prescribed speed within certain limitations. By printing single colors at designated speeds, it demonstrates how resistant the stock is to picking at a given tack reading, evidenced by the degree of surface fracturing.”
“The Prufbau Printability Tester has become very important,” Mr. Bellini added. “It gives you the ability to look at picking, wet trapping and printability. It’s almost a necessity these days.”
To create the right gloss for a particular substrate, formulators will need to determine the appropriate pigmentation levels, tack and viscosity. Mr. Bellini said that a glossmeter can give an accurate, reproducible reading of the level of gloss.
The reaction of ink and fountain solution is of tremendous importance to offset printers. Excess emulsification can cause snowflaking to occur, creating voids where the emulsified droplets of water are printed. Ink transfer and color strength can also be adversely affected. The two most commonly used pieces of equipment to determine water pickup are the Duke Ink-Water Emulsification Tester and the Kershaw Water Pickup System.
“Water pickup shows the ink’s propensity to mix with water,” said Ms. Hahn. “Some of the additives in offset ink cause excess emulsification to occur. These tests show whether the ink will have a tendency to emulsify water into itself.”
“An area where advancement has been made in testing is in emulsification systems,” said Rich Bradley, president of Carroll Scientific. “Newer testers titrate fountain solution into the ink and measure the relative viscosity of the ink and look for changes...this gives us much more information to help us predict how an ink will perform on press.”
“The biggest single problem in lithography is trying to predict lithographic behavior and how the ink will interact with the fountain solution,” Mr. Simpson said. “The Duke Ink-Water Emulsification Tester allows you to manually plot the water pickup rate of ink over time. The Kershaw Water Pickup System uses a torque-sensing stirrer, and a peristaltic pump feeds fountain solution into the ink. The resultant change in flow properties of the ink is automatically plotted. The advantage of the Kershaw is that the emulsion is finer and more representative of how ink and fountain solution interact on the press.”
Evaluating dispersion quality is also essential. If the particle sizes are too large, the ink will perform poorly on the press. The NPIRI Grind Gauge is the most common approach. Ink is drawn down on the gauge, and the point at which scratches or speckle appear is noted. This will show the largest particles that are present in the ink. The limitation is that it cannot show the distribution of smaller particles, which are important in determining color strength, gloss and transparency.
“The NPIRI Grind Gauge is a great example of a simple tool that gives you invaluable information about the particle size and dispersion characteristics of the components of your ink,” Mr. Bradley said.
“We are in the business of making or using pigment dispersions, and if your pigment dispersion isn’t correct from the outset, you can’t expect to have a good ink,” Mr. Simpson said. “The NPIRI Grind Gauge gives you an idea of dispersion quality but can be backed up with other techniques if more information is required.”
“Preparation of dispersions is so critical,” Ms. Hahn said. “A lot of oversized particles will cause poor ink transfer and color density.”
To determine color strength, ink manufacturers use bleach, or tint, tests, in which the ink is diluted with a specific concentration of white ink, then compared to the standard either by spectrophotometer or by eye. However, bleach tests are not standardized, and most companies have their own variations of these tests.
“The bleach test, in essence, measures the pigment dispersion and concentration in the ink and flush,” said Matt McClure, vice president and technical director for Apollo Colors. “You make up the same concentration and either use the spectrophotometer or a drawdown for visual assessment.”
Mr. McClure served on the NPIRI task force on bleaching, which came up with some unexpected conclusions back in 1994.
“There was a NPIRI task force back in the mid-’90s that looked at bleach testing, and came out with a universal bleaching test method that used spectrophotometers. We wanted to create a more stable and reproducible bleaching white.”
Mr. McClure and Sal Moscuzza, senior vice president and ink technologist at Superior Printing Ink, presented the task force’s findings at the 1994 NPIRI Technical Conference in Philadelphia, PA.
“We saw less overall variation using non-NPIRI bleaches,” Mr. McClure said. “We also found little difference between bleaches made with zinc oxide or titanium dioxide. We didn’t find that time lapse between sample preparation and sample measurement was significant, ad the ratio of bleach to flush color as also not significant. The spectrophotometer type and software showed minimal effect on determining the strength and color values.”
Still, there can be discrepancies with bleach tests. “If the bleach is not compatible, you can get a false reading, which happens on occasion,” Mr. McClure said. “There are a variety of different bleaching whites out there, and if I’m using Bleach A and my customer is using Bleach X, there could be problems.”
Offset inks are used for a variety of different packaging, from food to fertilizer. If the product spills onto the ink, it is essential that the ink does not change color or bleed. While Ms. Hahn noted there is no one particular test to make sure a product will not affect the ink, manufacturers need to subject the inks to the product.
“Product resistance testing is important to make sure that ink is not attacked by the products,” Ms. Hahn said.
Rub testing may be critical in coated applications, as different media face unique forces. As a result, testing equipment offers a variety of motions to simulate these forces.
“The Sutherland Rub Tester is somewhat of an industry standard and has a specific rubbing motion while the Gavarti GCat simulates shipping movement,” Mr. Simpson said.
“We are geared toward testing abrasion resistance,” said Mr. Bradley. “The Sutherland Rub Tester uses a back and forth motion moving paper against printed ink in a controlled manner to simulate real world stress, and is the most common tester here in the U.S. The Gavarti GA-CAT (Comprehensive Abrasion Tester) applies a reciprocating motion that mimics in-the-field damage the closest.
“Two less common testers are the Prufbau, which reciprocates back and forth but adds a rotational movement at the end of each stroke, and the PIRA, which uses a unique orbital buffing motion,” Mr. Bradley added. “A secondary system would be the Taber Abrasion Tester, which is very harsh and is used mainly in testing very tough coatings and UV systems.
“For us, coefficient of friction (CoF) is also important,” Mr. Bradley said. “The flatbed CoF Tester measures static and dynamic CoF, and advancement over more simple Slide Angle inclined plane testers, which are limited to mainly static tests.”
Inks also have to be impervious to light. While the most common examples are inks used for outdoor signage, which have to maintain lightfastness against UV, folding cartons, magazines and even maps have their own requirements.
With this in mind, ink manufacturers have to judge the effect of light on substrates using their inks by simulating lighting conditions. A Fadeometer allows the ink formulator to test a sample of the ink by comparing the exposed portion of the sample to the unexposed part.
“Equipment such as the Fadeometer tests the lightfastness of inks using a xenon arc lamp,” Mr. Simpson said. “The results allow you to estimate lightfastness under a number of different conditions.”
“The Fadeometer uses a xenon arc lamp to expose printed samples to light for selected time intervals,” Ms. Hahn said. “This is particularly critical for outdoor signage, in order to establish that the pigments in the ink will not fade during the term of exposure to sunlight.”
The Final Test
Regardless of all the testing equipment that is available, not everything can be assessed prior to leaving the ink factory.
“You can’t duplicate the interactions of ink, fountain solution and the printing plate,” Mr. Simpson concluded. “If the relationship with the plate is not correct, the ink will not necessarily print well. In the lab, we can only fit the pieces of the jigsaw puzzle as best we can. We have to run our ink on the press, under normal commercial conditions, and that is the final test.”
When the ink does go on the printing press, the press runs are the final proof of whether the ink manufacturer has successfully completed their work.