Offset Emulsification and Printability of Offset Printing (4)

5 experimental results analysis

5.1 Factors Affecting Emulsification Rate

5.1.1 Ink and fountain solution are different

The result of experiment 1 shows that different inks and different pH values ​​of the fountain solution have a great influence on the emulsification time-emulsification rate curve. When using the same ink, the stability of the alkaline dampening solution is better than that of the acidic dampening solution, either Taiyuan or Shenzhen ink. For the same fountain solution, the stability of Shenzhen ink is better than that of Taiyuan ink, see Figure 1) + alkali + too "curve and slightly increase the curve. "Base + deep" is closest to the ideal situation, after reaching the stable point Almost in a straight line, "acid + deep" fluctuates slightly."Acid + Tai" is extremely unstable along the way and is not allowed for production.

As can be seen from the chart, the same ink with a different pH value of the fountain solution, the use of a weak alkaline fountain solution is better than the use of weak acid fountain solution. The newsprint used for newspaper printing is generally weakly acidic. During the printing process, paper fluffs fall off and is rolled back into the water tank. The pH of the fountain solution tends to decrease during the printing process. If a weakly acidic fountain solution is used, the continuous drop in pH will exacerbate the layout deterioration and shorten the life of the plate. The use of weak alkaline fountain solution will ease the situation. Therefore, in newspaper offset printing, the use of alkaline fountain solution is better than acidic fountain solution in some aspects.

5.1.2 Working time of mechanical force

The ideal ink emulsification rate changes with time as shown in the figure below.

In printing, it is desirable that the time X when the ink reaches the stable emulsification rate y is as short as possible. On the one hand, it can increase efficiency. On the other hand, paper can be saved. We also hope that the performance of the ink in maintaining a stable state is better, which is not only beneficial to the control of the printing process but also stabilizes the quality of the product. From a few better curves, after 25 minutes of stirring, the emulsification rate basically reached a steady state. After that, although the force continues, there is no longer a large change in the emulsification rate.

5.1.3 Ink and water ratio

Offset characteristics determine that the ink must have a certain amount of water-holding capacity. When the proportion of ink changes, the emulsification rate of the ink must change. In actual production, the proportion of ink and water is more difficult to accurately control, which requires that the ink has a certain degree of tolerance for water, that is, the rate of emulsification of the ink is basically stable, and the amount of water is allowed to change within a certain range. Make the printing process easy to control.

The relationship between the emulsification rate and the amount of emulsification obtained in Experiment 2 shows that, when the amount of the liquid to be fed is small, the ink is added to the fountain solution and the emulsification rate rapidly increases. When the ink-to-ink ratio is 50%, the water absorption of the ink is nearly saturated, and the change of the emulsification rate is slowed down. All the curves have a relatively stable portion, and then the fluctuation of the amount of the supplied liquid continues to rise. The stable part of the two curves using the acid dampening solution is 50% to 90% of the ink ratio. When the alkaline fountain solution is used, the stability range is much larger. "Alkaline + deep" from 60% to 140% 1" alkali + too" from 40% to 110%. For actual production, it is desirable that the range of the ratio of the ink and ink required to maintain the stability of the emulsification rate be as large as possible, so that the printing process can be easily controlled. With an acid fountain solution, the emulsification rate can be stabilized when the amount of water supplied changes in a relatively small range. However, from the actual production point of view, the control requirements for water volume are more stringent and therefore more difficult to control.

5.2 Effect of Emulsification Rate on Printability of Ink

5.2.1 Impact on Operational Suitability

The Tack value of the ink can reflect the running of the ink on the printing press. The emulsification of the ink causes the Tack value to drop, causing changes in ink delivery and distribution, and affecting production. For the sake of controlling the printing production process, we hope that the Tack value does not change significantly when the emulsification rate changes within a certain range. In this case, as long as we control the emulsification rate of the ink, we can maintain the relative stability of the Tack value and make the printing run smoothly.

The results of Experiment 3 are shown in Table 2 and Figure 3: The Tack value of the ink decreases very slowly with the increase of the emulsification rate, and there is no abrupt change phenomenon. It is proved that the operating suitability of the Shenzhen ink and the Taiyuan ink is stable and suitable for printing. produce.

5.2.2 Impact on Quality Adaptability

In Experiment 4, field density was chosen as a proxy for ink quality fitness. Emulsified ink, ink dilute is inevitable. For the actual printing, of course, the desired color of the product is uniform and the writing is clear. This imposes two requirements on the emulsification rate of the ink, and the density relationship of the ink. (1) The decrease in the density of the ink after emulsification must not be excessive. (2) When the emulsifying rate is kept within a certain range, the density remains stable. From Table 3 and Figure 4, it can be seen that the emulsification rate of Shenzhen ink and Taiyuan ink is in line with the above requirements. Especially when Shenzhen ink and alkaline fountain solution are combined, the emulsification rate is as high as 40% or more, and there is still no large ups and downs, which is very favorable for actual production.

The emulsification rate of the ink is too high, which is unfavorable for printing, but at the same time, it must be noted that the instability of the ink property caused by the instability of the emulsification rate is also detrimental to printing. For rotary offset printing, the stability of the emulsification rate is particularly important due to its fast running speed. The experimental results show that: low emulsification rate and high stability can not always have both at the same time, regardless of the stability at this time and only consider the low emulsification rate does not meet the actual production requirements. "Acid+Tai" is the lowest emulsification rate among the four schemes in this experiment, but its stability is the worst and is not conducive to production. Therefore, the stability of the ink emulsification should be the same as the emulsification rate of the ink, and it also serves as an index for evaluating the performance of the ink. It is also for fountain solution.

6 Suggestions

6.1 It is suggested that the stability of the ink emulsification rate should also be used as an indicator to evaluate the emulsifying performance of the ink when formulating the standard, which is in parallel with the emulsification rate of the ink.

6.2 The previously accepted ink emulsification rate should be controlled between 100/o - 26%, with 21% being the best value. This standard is based on lithographic sheetfed offset printing. The inks, fountain solution, and paper used in web offset printing are all different, and this standard may not be suitable for use. It is recommended that the optimal emulsion rate control range of offset printing ink be reexamined in combination with the actual conditions.

Source: Print Chemicals Network