Foreword
All printers using water-based inks always encounter the problem of ink blistering. This is a costly failure that can cause equipment downtime and discourage operators. Ink suppliers are making every effort to come up with solutions, but this problem will hardly be completely eradicated. In order to minimize the occurrence of ink blistering, equipment suppliers have invested extra attention in the design of ink metering systems. However, the blistering problem of water-based ink will still exist.
Printers first need to ask themselves: Is ink bubbling really a problem for print buyers? Customers usually do not complain about the poor quality of prints because of ink blistering. They can only complain about color consistency, overprinting, edge definition, inability to recognize barcodes or print defects.
This article introduces the results of product testing on different ink suppliers, as well as the problems of water-based ink blistering and printing color inconsistency of these suppliers under monitoring.
problem
Almost all water-based flexographic inks will foam. Although this phenomenon will not be noticed in most cases, the problem of ink foaming day after day becomes a problem. This phenomenon is inevitable in any printing equipment using water-based ink. Ink blistering is more common on printing equipment equipped with a chambered doctor blade system, but this has nothing to do with the supplier of the doctor blade system. The appearance of foam means that there are two ink phases instead of one. Compared with the original ink phase, the foam phase has different composition components per unit volume.
There are many explanations for ink foaming in the flexo industry, and now some explanations and solutions provided have been confirmed.
Basic elements
The following are the relevant factors that affect ink foaming: the ink itself; the system that controls the ink (ink pump, ink delivery tube, and metering system); the operator.
Ink suppliers can change the ink formulation, so that the ink is not easy to foam.
If air bubbles appear in the ink or air is trapped in the ink, the system that controls the ink will exacerbate the generation of foam.
The operator took the wrong action, causing the ink to foam more and more.
When the ink is foaming, the claim that only the above party is responsible is as good as the fantasy. Each factor has its own responsibility for creating a stable condition to minimize the risk of serious ink blistering.
The blistering of the ink is caused by the air contained in the ink. When foaming occurs, the ink density drops significantly. If the viscosity cup is used for measurement, the viscosity will increase. When the operator adds water to reduce the viscosity, the foam will increase. If the viscosity of the ink is measured while the ink is foaming, it will lead to erroneous corrective actions.
Ink suppliers usually recommend adding an anti-foaming agent when the ink foams, but this is only effective when the ink surface foams. If the bubbles in the ink are evenly distributed (micro-foam), then in most cases the operator or ink supplier is helpless.
It is almost unknown how the ink started to bubble. It is generally believed that this is related to the use of a chambered doctor blade system, and blistering begins at the front doctor blade. Assuming that the air is brought into the chamber by the anilox roller, some people think it is related to the ink pump system and ink flow.
Most materials will foam, such as heating to start the plastic foaming process. As for the bubbles in the liquid, physical knowledge tells us that when a bubble with radius r is formed in the liquid with interfacial tension Y, the vapor pressure it contains is P, then P = 2 × Y / r.
Heat is used to release water vapor in liquids or solids. In solids, bubbles are partially trapped, forming rigid foams; in liquids, bubbles float on the surface and burst.
In order to minimize the risk of foaming of water-based flexo inks, the measures that can be taken are:
· Reduce the ink stirring phenomenon caused by the ink circulation system as much as possible;
· Add as little unnecessary water as possible (water added by the operator or water left in the inking system after cleaning);
· Use ink formulations that are less likely to foam (eg avoid solids in ink formulations);
· Ink suppliers and operators have agreed procedures to maintain ink quality;
· Keep a record of what was done during production. Recording the time and activity of ink production for a single variety will help corrective actions when blistering.
test
The three ink suppliers were invited to provide two types of ink: black ink and green ink for testing. The ink formulation must have time stability and minimum foaming. Before loading the ink into the ink fountain, its density was measured and recorded. The test records are:
Printing color (use spectrophotometer);
Paper color (use spectrophotometer);
Ink density.
More than 5 hours after the end of each measurement, the ink density was measured and recorded again. The printing speed is 4,000 sheets / hour.
Ink density measurement uses a 50 ml Eppendorf Multipet pipette. The pipette is filled with ink and then placed on a balance with a measurement accuracy of 0.01 g; 50 ml of ink is returned to the ink tank and the pipette is placed on the balance again; the balance reading is divided by 50 To get the density value in g / ml (equal to Kg / dm3). [next]
Test Results
After the ink in the inking system circulates for a short time, the viscosity of the ink drops. Among the three inks, the black ink produced by the second ink supplier may be the least affected (see Figure 1).
The adjustment method of the second ink supplier is to add a very small amount of defoamer to the green ink 30 minutes after the start of the test, which will be effective at the 35th minute, but its effect will disappear at the end of the test.
The density of black ink decreases less than that of green ink. Compared with the density measured before the test, the ink recovered its similar density again 5 hours after the test.
The changes in ink and paper colors of the three ink suppliers during the test are shown in Figure 2. The ink color changes of the three suppliers are at an acceptable level.
The second supplier's green ink color changes closely follow the pace of paper. Two types of paper were used in the test. The first supplier used commercial A type paper for printing (except the first sheet), and the third supplier printed on B type paper. The second supplier chose two types of paper.
It can be inferred from the density measurement that there is foaming in the ink, but this will not affect the printing color, and the foaming ink will not have any problems.
(The curve in Figure 1 shows the measurement result of ink density)
(Figure 2 color changes during the test)
The test sequence is to use the same ink within 4 hours, adding 1 cubic decimeter of fresh ink every 30 minutes. During this period, the printed color was stable, and the severity of blistering did not exceed the test process.
in conclusion
The inks of the three suppliers used in this test all had blistering. Ink foaming will not cause a huge color change within 1 hour. The ink started to bubble immediately after the inking system was started, and there was no significant change in the bubble during the test. The test results of the second supplier show that the anti-foaming agent can reduce bubbles, but this is a temporary phenomenon.
The ink density reading recorded more than 5 hours after the end of the test showed that the ink no longer foamed, the density did not change much, and the ink viscosity was not affected.
Suggest
· Add fresh ink within the specified time interval (for example, every 30 minutes), the ink supplier must perfect the ink formula for this as much as possible.
Measure the viscosity of the ink before it is loaded into the inking system, but if the ink foams during printing, the measurement of the viscosity cannot provide useful help for corrective measures.
· Avoid high solid content in the ink formula, especially when using the cavity doctor blade system.
· Ensure that sufficient ink flow passes through the system.
· Ink delivery system (pipe fittings, ink delivery tube, ink pump and chamber) should not have sharp edges and corners.
A benign interaction between the operator, ink supplier, and ink metering system designer, setting positive measurable and achievable goals, that is, maintaining consistent colors, will help overcome the problems caused by foaming ink. Of course, this does not change the nature of the ink that inherently or easily foams.
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