Fountain solution in offset printing
Q: Please explain the purpose of fountain solution in offset printing.
A: Fountain solution acts as a weak sacrificial layer and prevents ink from depositing on the non-image area of the plate. The fountain solution's thin film presents a physical barrier to ink, just like adding oil to a pan to prevent food from sticking or using flour to prevent dough from sticking to the table.
During lithographic printing, some of the water is mulled into the ink films. This occurs on the inking rollers and on the plate image. The water inside the ink does not represent a printing problem as long as the ink can absorb it. This depends on the dampener setting. The water droplets must be small and surrounded by ink. The type of ink and fountain solution combination controls this property.
The fountain solution can only replenish the existing water in the microscopic holes in the non-image areas of the lithographic plate. The microscopic holes must be filled with water prior to any exposure to ink.
This step is accomplished during plate processing. If the water in the microscopic holes is allowed to evaporate, it will be filled with droplets of oil from the ink. Tinting and toning will occur. The fountain solution neither can replace the embedded oil in the plate grain nor wet the non-image areas. This problem is commonly known as "plate oxidization." The only solution is to replace the plate.
In the past, pH has been used in the pressroom for metering fountain solution. Today, we use conductivity. However, measuring pH is valuable in determining batch to batch consistency of fountain solution concentrate.
Conductivity is the measurement of a solution's ability to conduct electricity. Understanding the properties of salts is important to how fountain solution is metered. Salts are used as a fountain solution measurement tool by measuring how the salt's positive and negative charges move.
Conductivity meter composition
A conductivity meter is an electrical circuit composed of two wires attached to ordinary battery poles, an electric current measuring meter, and an on/off switch. During conductivity measurements, the two wires are dipped in the fountain solution. When the switch is turned on, the positive moiety of the fountain solution salt moves to the negative pole of the battery, while the negative moiety migrates to the positive pole. As a result of the solution charge motion, an electric current is generated. The measured electric current is converted to conductivity units (micromhos). The strength of this current depends on how much fountain solution concentrate has been added to the water.
What should the conductivity be?
The amount of fountain solution concentration needed to make a press-ready solution should fall within the supplier recommended range. However, the exact concentration must be determined by actual press runs. The conductivity ranges between 800 to 3500 micro mhos/cm.
The concentration that provides the most trouble-free press run is the appropriate concentration for your system.
Too much concentrate?
Higher than necessary concentration produces faster initial clean-up and low initial dampener settings. As the print run progresses, the dampener settings will be gradually increased. In extreme cases, ink and water balance is impossible to control. The range between scum and wash will be very narrow.
Too little concentrate?
If fountain solution concentration is too low, plates are slower to clean during makeready. The need for higher than required initial dampener would be obvious. Generally, the dampener setting will be high during the entire press run. In some cases, ink water balance may become difficult to control.
Fingerprint your concentrate
The best quality control tool is establishing a relationship between the volume of fountain solution and its conductivity. Make a number of mixes, in quarter ounce increments, added to a gallon of purified water. A chart is then created that displays the relationship between the number of ounces per gallon and the press-ready conductivity. Incoming batches can be plotted on these standard curves to evaluate consistency.
The clarity of the concentrate
Fountain solution concentrate is a mixture of competing compounds. Combining all of these compounds in a clear solution is the art of fountain solution formulation. Clouding or settlement indicates that one or more of the fountain solution components has separated. Metering a separated concentrate leads to inconsistent press-ready solution.
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