The challenges of emulsification: for offset printers, emulsification, or ink/water balance, is a major challenge, and ink companies are offering guidance to help customers run their presses as smoothly as possible.
Unfortunately, though, emulsification, or ink/water balance, is the largest headache for offset printers. It's not just the ink and fountain solutions; plates, substrates, temperature, humidity and pigment strength, are among the many factors that can derail the press run. Avoiding emulsification is a challenge for printers.
"Ink emulsification is the process by which the press ready fountain solution becomes mixed into the ink," said Jim Galloway, INX International Ink Company's Research & Development Corporate Technology Center. "In this process, the fountain solution is distributed evenly throughout the ink in small droplets. This balanced ink/ water mixture forms a stable emulsion that is necessary for offset printing process--anything beyond that creates pressroom print problems."
"There's more art than science to getting the ink and water balance right," noted Andrew Matthews, technical director for Flint Group.
The Basics of Lithography
The basics are a little more complicated than that, of course: a lithographic plate is flat. The image area is receptive to ink, and the non-image area is receptive to water or fountain solution. The image area has low surface energy, so water moves away easily. In the non-image area, surface energy is high, so water creates a continuous film.
"You're feeding ink and water to the plate at the same time and there's no barrier on the plate," said Lisa Fine, president of Flexo Tech. "The only thing that separates the two is that the image area is oil-loving and the non-image area is water-loving."
When the particles of water and oil hold each other together, it is called emulsification. When the ink contains too much water, it becomes over-emulsifted, and you might as well shut the press down.
"Emulsification is an issue that includes so many variables," said Don Sierzega, product manager, publication heatset for Sun Chemical. "The top three are the paper, fountain solution and ink.
"Generally speaking, the ink chemistry has a role as to how much affinity the ink has for the fountain solution and how stable the emulsion is," said Mr. Sierzega. "However, conditions can go out of control, whether it is temperature, process control, feedback in the dampening system, the fountain solution not setting to the image, which leads to pushing more water, and that's how the ink-water balance gets altered."
Fountain solutions play a key role. They lower surface tension, increase and maintain the plate's receptivity to water. For the most part, acid fountain solutions are ideal for heatset and sheetfed, and neutral is used for newspaper printing. Mr. Galloway noted that if an offset ink did not emulsify with the fountain solution, the ink would not evenly transfer to the printing plate.
"The ink/water emulsion produced on an offset printing press is all about transfer," Mr. Galloway added. "It is relatively easy to get the fountain solution and the ink to mix. Getting the ink from the fountain to the substrate greatly depends on the split in the rollers and the adhesion properties of the rollers, plates and blankets during offset lithographic printing. This transfer of the ink and water throughout the roller train is one of the most important factors in the emulsification process on an offset printing press. The amount of ink and water film thicknesses produced is very important to achieve the proper split emulsification film transfer."
"There's always a small amount of fountain solution that emulsifies in the ink due to shearing forces and chemical affinity- for example, an ink loaded with polyesters or alkyds may draw more water than a hydrocarbon resin," Ms. Fine added.
There are a huge variety of problems that occur when the ink-water balance goes awry.
"The problem is that everything affects the ink-water balance," Ms. Fine added. "Higher press speeds increase shear forces. The ink chemistry is critical. Temperature plays a role, as a very hot day lowers the viscosity of the ink and pulls water in more easily. Too much water creates snowflaking, leaving voids and unclear images on the substrate. If there is no emulsification, the ink will mottle and the ink transfer will be poorer. Piling can occur, where the ink hangs back on the plate and doesn't transfer onto the substrate."
"Temperatures and relative humidity are also other major variables in a pressroom that is sometimes not controlled," Mr. Galloway said. "Varying temperatures will have an effect on the level of fountain solution being used. A lot of heat is generated on presses. The only way for these presses to cool down is by the evaporation of the fountain solution. Some presses (typically heatset) have water cooled rollers to aid in this process. Temperature variation will also create different demands on the ink for emulsification because of varying emulsion stability. Uncontrolled humidity in the pressroom will also cause varying evaporation rates."
For ink manufacturers, higher-strength pigmented inks are actually a challenge when it comes to ink-water balance.
"Strength standards tie our hands," Mr. Matthews said. "A higher-strength pigmented ink leaves a thinner ink film, which in turn requires a narrow balance between the ink and water. A less pigmented ink system is laid down as a thicker film, and allows for a wider water window."
"Using a really strong ink has an impact, as the printer will use less ink, which creates a thinner film which can be disrupted by emulsification," Ms. Fine added.
Even if everything is running smoothly, there is no guarantee that it will continue.
"You can reach the sweet spot and everything looks great, but a few hours later, the press may be running hotter, changing the ink and water balance," Mr. Matthews said.
When Less is More
How, then, does one solve emulsification? One key is to begin with low levels of ink and water.
"Less is best," said Andrew Matthews, technical director for Flint Group. ''You should always start low with the ink and water levels, and add to reach the appropriate density."
"There is no universal correct ink/water ratio," Mr. Galloway noted. "As a rule it is usually safe to assume that near-optimal transfer will occur with minimal dampening and ink levels necessary to achieve satisfactory print density targets and to maintain the correct tonal color balance print reproduction. However this does not mean that the ink will not print satisfactory with varying levels of emulsified fountain solution."
There are adjustments that can be made on press, such as adding more water or ink, but while too many changes may work, it is hiding the problem.
"While all lithographic offset inks need to emulsify with the fountain solution, there is a need to control this level of emulsification on the press," Mr. Galloway added. "All offset inks have a designed print window for optimum press performance. On press, this ink/water emulsion should not lose any of the performance characteristics of the ink itself so that the tack, viscosity and proper ink transfer can be maintained. If the ink/water balance is not maintained, then the ink's splitting properties are reduced, and this fluid viscosity transfer is interrupted. As a result, the ink/water emulsion becomes unstable, the ink "builds up" and it emulsifies. This ink is said to become waterlogged. The only remedy for a waterlogged ink that has lost its ability to transfer the ink film emulsion to the ink rollers and the printing plate(s) is to remove the waterlogged ink from the ink pan, wash-up the unit(s), add fresh ink(s) and reset the ink and water balance back to minimum start-up configurations to build back up to target print densities."
"When you push more water, the density becomes wrong, so the next step is to add more ink, which leads to more problems," Mr. Sierzega said.
"You can run more ink and water and it may look good for a while, but eventually problems will arise," Mr. Matthews observed. "Experienced pressmen can see if the ink is waterlogged and if the plates look right. If there's a problem with either one, they can make the appropriate changes to get the balance right."
What to Do About Emulsification
The causes and effects of emulsification are numerous, but the key question is how one can avoid or fix an ink-water imbalance. That, unfortunately, is not an easy question to answer.
"You can make some adjustments, can change the fountain solution to something that has less affinity to ink, or change the ink," Ms. Fine suggested.
"When you formulate the ink, you have to use the best vehicles you can, as well as selecting the right chemistry and the varnish for the end use application," Mr. Sierzega said.
INX recommends these pressroom best practices for controlling optimum ink/water balance operating window:
* Proper fountain solution and alcohol substitute dosage following the fountain solution etch manufacturer's recommendations.
* Maintain proper fountain solution temperatures (approximately 55-65[degrees]F).
* Proper dampener roller settings following press manufacturer's guidelines.
* Keep pre-damp settings at the lowest setting possible.
* On selected presses (Komori, Heidelberg and KBA Planeta) check and maintain water ratio settings (ramp speeds) to press manufacturer's specifications.
* Select an ink system which exhibits stable water take-up characteristics for your printing conditions (press, fount, substrate, plates, etc.)
* Maintain dampener form roller hardness within roller manufacturer's Durometer specifications (usually 23-27)
* Run initial scum line during make ready and adjust water level up slightly to confirm minimal water setting (this is especially important for UV ink systems).
* Once water settings are correct, proceed to adjust inks to proper print densities using a Densitometer.
* Check for press wash contaminates in the fountain solution.
* On forms with light coverage print jobs add ink take-off bars at the tail edge of the sheet to help maintain ink/water balance, to aid in promoting proper ink transfer and to prevent any tendency for over emulsification of the inks.
* Request that the ink manufacturer reformulates the ink(s) for better water resistance properties to reduced emulsification tendencies.
"Commercial offset printing is still a very complex process," Mr. Galloway said. "If one of the components in the printing process is not performing optimally, then it will seriously compromise the print quality and the commercial print reproduction process.
"We need to be constantly alert to subtle revisions that are being made to our printing process by the suppliers of paper, ink and chemistry," Mr. Galloway concluded. "Suppliers are constantly improving and changing their products with the hope of building a better mouse trap. Your technical service team may troubleshoot to find out what has changed by checking all of the press side variables, but sometimes nothing stands out. This can be very frustrating process, but we need to understand that these process changes are constant in our industry."
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|Date:||Aug 1, 2007|
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