Total Flexographic Ink Management
As in most industries, technology, automation and process control are major reasons for the rapid advancement in flexographic print quality and productivity. Automated impression settings and cleaning processes, temperature and tension controls, robotics, color measurement, controlled anilox cell volume, consistent substrate porosity and surface tension, and rapid advancements in plate-making technology are just a few examples.
The most important process variable to control may be what the customer sees. Ink on the substrate. Variation in ink viscosity, temperature, and pH (for water-based ink) can cause color inconsistency and print defects that can be objectionable to the customer. Uncontrolled ink can cost printers a lot of money in returned product and wasted ink and/or solvent. Let’s discuss why it’s important to control each element.
Ink Viscosity – Viscosity is the measurement of resistance to flow. Ink viscosity is the primary control of the density of the ink color. Typically, printers use viscosity measurement cups like Zahn, Shell or Ford cups to measure ink viscosity. This is done by placing the cup in the ink, pulling it out, and recording the amount of time it takes for the ink to drain from the cup. The longer it takes to drain, the higher the viscosity, the more pigment in the ink film, and the denser the ink color. However, inks with higher viscosities can be more difficult to dry, print dirtier and are costlier to use. Inconsistency in viscosity can cause objectionable variation in color and print quality.
Ink Temperature – Most people don’t realize the negative impact of low or elevated temperatures on ink. Low temperature, which is less likely than elevated temperature, can make ink flow poorly, even at low viscosity. It can cause low density, pin-holing and dirty print. Elevated temperature makes solvent, water and amines evaporate more quickly. This can produce increased consumption of ink, solvent, water and amine, as well as inconsistent print quality. Ideal ink temperature is room temperature, 72 degrees Fahrenheit. Uncontrolled, it isn’t unusual for ink temperature to exceed 120 degrees.
Ink pH – Monitoring and controlling the pH level of water-based inks stabilizes the viscosity of the ink and helps the ink flow and re-wet properly. This improves consistency of color density and print cleanliness. It also helps keep the printing plates, anilox cells and doctor blade chambers clean. pH is controlled by ensuring the proper amount of amine is in the ink. Most water-based inks use ammonium hydroxide and volatile amines to control and maintain alkaline pH in the ink. This solubilizes the acidic resins and stabilizes the pigment. Amine allows the ink to flow properly. When amine evaporates, ink viscosity increases. In addition, the ink will foam and dry faster on the printing plate. All this creates dirty print. Ideal pH is typically between 8.5 to 9.5. Distilled water has a pH of 7, so adding water alone to reduce viscosity can increase the pH level, causing the viscosity to rise more quickly.
GAMA International Total Ink Management System – Whether it be ink, coating, or adhesive – water-based, solvent-based, UV, or EB, GAMA has a system that automates monitoring and control of viscosity, temperature and pH.
Viscosity, Temperature and pH Measurement – GAMA’s in-line sensor continually measures viscosity, temperature and pH (water-based only). The ink flows from the pump through the sensor on the way to the print station. In the sensor there is a vibrating disc that measures viscosity by measuring the resistance to the vibration. Torsion/vibration is the most accurate and consistent method of automated viscosity measurement and can be calibrated to any measuring cup. A thermometer in the sensor measures the temperature of the ink. For water-based applications, the sensor is stainless steel and includes a pH probe that continuously measures the pH level of the fluid.
Operator Interface – Viscosity, temperature and pH data is sent from the sensor to control boards and the operator monitor via cables. The measurements are displayed on the monitor. The operator can then adjust/control the ink or coating by inputting the desired viscosity, temperature and pH.
Viscosity and pH Control – When the viscosity increases above tolerance, or the pH drops below tolerance, the controls open air valves to dispense small amounts of solvent, water or amine into the ink bucket or pump. The amount added is determined by the length of time the valve is open. The length of time is determined by how far the viscosity or pH is from the desired setting. When an ink is first added to the system, the viscosity and/or pH may be off from the desired setting by quite a bit. In this case, the system will open the valves for a longer than it would after the ink has already been adjusted to the desired measurements and changes to be slightly out of tolerance.
Temperature Control – After the ink exits the sensor it flows through a temperature control heat exchanger prior to flowing to the print station. The heat exchanger is connected to a chiller that will pump chilled water/glycol through it when the temperature elevates out of tolerance. When the temperature is within tolerance (typically under 75 degrees Fahrenheit), chilled water will not flow, but when the sensor measures the ink or coating out of tolerance, a valve opens that allows the chilled water to flow until the temperature drops into tolerance.
System Calibration – The GAMA system can be calibrated to just about any measuring cup and/or pH meter. The pH meter comes already calibrated, but can be calibrated to an existing pH meter, if desired. Calibration of both viscosity and pH is extremely simple but should only be needed periodically, not every time a new ink is used. Once a week should be sufficient.
System Cleaning – One of best features of the GAMA system is that the sensor is cleaned when the print station is cleaned. Whether the print station is cleaned through the pump, or an off-line automated wash-up system, the wash-up solution flows through the sensor prior to being pumped through the print station. In addition, the pH probe is kept wet due to a valve that closes to maintain cleaning solution in the pH probe cavity.
System Benefits – Ink, solvent and amine savings, consistent ink density and print cleanliness, cleaner anilox rolls, printing plates and chambers, reduced operator involvement, and improved quality of used ink put back in storage.
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Author :John Bingham
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