In the process of using ink, when the ink is transferred from the rubber roller to the anilox roller, the anilox roller to the printing plate, and the printing plate to the cardboard, the amine will volatilize from the ink to reduce the pH value.
When the pH value starts to decrease close to 8.0, more and more resins tend to dry out because of the volatilization of amine water, which will cause the color density to drop and the ink to become a sticky solid.
Therefore, more and more paper scraps and fibers will stick to the printing plate and have to be shut down to clean the printing plate, so that more amine water is evaporated.
Due to the increase in viscosity, people often mistakenly think that the water evaporates and add a lot of water to dilute it, but it also lowers the pH value, which causes insufficient color concentration and damages the resin structure.
In the process of overprint printing, the control of the viscosity and acidity of the ink is even more important.
The resin dissolves in the alkaline state. If the pH value of the latter color ink is greater than the previous color, then when the latter color ink is superimposed on the former color ink, the former color ink will be dissolved by the latter ink and cause "bleeding". (That is, the ink of the previous color will dissolve on the printing plate).
In order to improve this phenomenon, the pH value of the ink of the latter color is usually slightly lower than that of the ink of the previous color by 0.2, and the viscosity is slightly higher than that of the ink of the previous color by 1-2 seconds.
First color pH9.0
Second color pH8.8
Third color pH8.6
Fourth color pH 8.4
When the effect of ink overprint is not good, in addition to adjusting the pH and viscosity of the ink, if the effect is not good, ask the ink supplier to increase the drying speed of the ink of the previous color, increase the surface activity of the ink of the next color, and enlarge the ink The distance between colors can further improve the overprinting effect of ink and wash. This is the water resistance and wettability of ink and wash.
In general, the drying of ink on cardboard by water-based printers is mainly due to its penetration into the fibers of the cardboard, and the volatilization process is relatively slow.
When overprinting, the former color often occupies most of the paper fiber holes, and the penetration of the latter color will be relatively slow during printing.
Moreover, the surface tension of ink and wash (34 dynes) is lower than that of paper (42-50 dynes). After the previous color is printed, the surface tension of the corrugated cardboard will inevitably be reduced, making it difficult to print the next color.
Therefore, it is necessary to reduce the surface tension of the latter color by adding a surfactant to improve the printing effect of the overlapping color.
There are also many types of cups for measuring the viscosity of ink, such as: DIN cup, Zhan cup, Ford cup, Rigosha cup, coated 4 cup, etc. Each cup is divided into 1, 2, 3, and 4 models.
Cups with the same number of seconds, such as DIN # 4, Zhan # 4, and Rigosha # 4 cups have different absolute viscosities. Each cup has its applicable range, but it should be controlled as much as 10-60 no matter which one is used. Between seconds.
However, if the problem of overlapping colors is to be solved simply from ink and wash, I am afraid it is difficult! The methods mentioned above can only be improved. why?
Because the conditions that affect water-based inks are:
1. The ink content of the pattern roller mesh, the scraper form, the middle and high hardness of the rubber roller.
2. Printing plate transfer ability, printing area, printing surface, flatness, hardness.
3. The machine adjusts the contact plate pressure, printing pressure, and printing speed.
4. When the pH value of the paperboard surface after processing changes, it shows acidity or alkalinity, paperboard temperature, and paperboard humidity.
5. Color and ink absorption of tissue paper.
6. Ambient temperature and relative humidity in the workplace.
7. Printing pressure, printing plate and pattern roller pressure.
In addition, if there is a large amount of white material in the background color during overprinting, the latter color overprinting will be difficult.
If it is printed on the whiteboard, the ink supplier can be asked to remove the white material from the ink and achieve the color required by the customer to readjust the ink formula, which will improve. If it is printed on a yellow board, it can only be solved by other methods.
To do better, we need to cooperate in many ways. For example, the strength of the cardboard must be guaranteed. Do not crush the corrugation during the paper feeding process or the printing process.
If conditions permit, printing in two passes is also a solution. If the customer's requirements are very strict, then the background color of the overprinted part can only be cut out and overprinted!
However, it is necessary to prepare a pH meter when printing!