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In gravure printing, uneven ink film thickness occurs from time to time. There are many reasons for uneven ink. In the case of gravure ink, it is poor ink blending, or the ink is accidentally mixed into the ink when the ink is blended. At that time, the ink swells or softens due to the addition of solvent. Blocking the mesh holes that should be ink-permeable causes the effect of the plate film, making the ink unable to pass normally.
In order to prevent this kind of failure, the gravure ink (especially the old ink) after deployment should be filtered once with a net before use. When reusing a used printing plate, the old ink attached to the plate frame must be completely removed. When storing the printing plate after printing, wash it sufficiently (including the squeegee). If you follow the above requirements, the uneven ink will not happen.
If the tip of the front end of the scraper is damaged, there will be traces along the movement direction of the scraper. Especially when printing transparent materials, there will be obvious ink unevenness. Therefore, the front end of the scraper must be well protected so that it will not be damaged. If it is damaged, it must be carefully polished with a grinder.
Water-based ink is made of water-based resin, pigments, solvents and additives through compound processing and grinding. The water solubility mainly plays the role of the binder, so that the pigment particles are evenly dispersed, so that the ink has a certain fluidity and the adhesive force that occurs, so that the ink forms a uniform film after printing.
Matters needing attention before using water-based ink:
1. Water-based inks cannot be mixed with solvent-based inks, or use benzene and ester organic solvents in water-based inks.
2. Water-based inks should be stored indoors at a temperature of 5-40 degrees and avoid direct sunlight.
3. Because of the evaporation of water during the printing process, the viscosity will rise and the PH value will drop. The pH stabilizer should be between 8-9.5.
4. Because of the complexity and diversity of current printing substrates, strict pre-production tests are required before selecting a certain ink for mass production.
5. The ink will be gelatinous when stored for a long time, so it needs to be fully stirred during use.
6. After the ink is dispensed, you need to stand still and defoam before use, otherwise bubbles or pinholes will occur in the ink layer.
7. Static electricity on the screen or printed surface will cause pinholes in the printing ink layer.
8. When printing is stopped for a short time, it should be covered with a layer of ink, otherwise it is easy to cause network blocking.
9. Rinse with water immediately after printing, otherwise it is easy to block the net after being dry, and the thickness of the screen can be increased to reduce the oil blocking the net.
The viscosity change of water-based ink is related to the following points:
1. The ink has a certain viscosity at a certain temperature (25 ℃). The viscosity of the ink decreases with the increase of temperature, and the viscosity increases with the decrease of temperature. Such an inversely proportional property.
2. The change of viscosity has a subtle effect on the migration area of the ink migration or the flow mode of the dots, especially in multi-color printing. If the room temperature changes greatly, the migration state will change, and the color tone will often change. This is because the temperature change affects the change of ink viscosity.
3. The fluidity of the ink
The characteristic of ink flowing under the action of external force and gravity is usually called the fluidity of ink. There is a close relationship between the fluidity of ink and viscosity. When ink increases viscosity, it will reduce its fluidity, and vice versa.
4. Ink with high consistency is not equal to high viscosity, but ink with high viscosity is not necessarily thick. Only the ink with the same viscosity, the greater the consistency, the worse the fluidity.
If the fluidity is too small, the ink will not spread smoothly and uniformly, causing the same printing surface to fade and deepen;
Excessive fluidity causes uneven distribution of ink, unclear printing levels, insufficient ink color, enlarged dots, etc.
Gravure printing uses plastic water-based inks, which are environmentally friendly, and more and more people in the industry are paying attention to. So what should be done before gravure printing uses plastic water-based inks?
1. Printing plate
The printing plate adopts electronic engraving gravure or laser engraving gravure, and the depth of the mesh is 15-25um, which is lighter than solvent ink gravure. This is because the ink is produced with tap water, and the boiling point of water is higher than that of the solvent. Therefore, the ink is not easy to volatilize, and the shallow plate can increase the printing speed and avoid water ripples. If you use solvent-based ink printing, the cell depth of the printing plate is 35---40um, which is easy to produce water ripples; but when using ink printing, the cell depth of the printing plate is more than 25um, there will be water ripples, so choose the right one The printing plate is crucial. Water can be used to adjust the viscosity of the ink when in use, and the viscosity of the coating-4 can be controlled to be 15-25 seconds/25°C. If the viscosity is too high, the transfer performance of the ink will be poor, and it is easy to cause the dirty plate, paste plate and other defects.
2. Good drying equipment
The drying temperature should be determined according to the different substrates and the size of the printing area. For example, for aluminum foil wine label printing, the drying temperature should be above 100℃; for plastic film printing, the drying temperature should be 50~60℃. Wind power should be stronger than using solvent-based inks, because hot air can quickly dry the ink after it is transferred to the substrate.
3. Surface treatment
Except for aluminum foil, vacuum aluminized film, and paper, all plastic films need to be treated with electric spark before ink printing, so that their surface tension can reach 0.04N/m or more. For the ink produced with water as the main solvent, since the surface tension of water is about 0.072N/m, it can be seen that the surface tension of the ink differs from the surface tension of the film, which will affect the adhesion of the ink on the substrate and improve the plastic film. Surface tension can improve the adhesion of printed patterns on the film.
How to measure the viscosity of PET water-based printing ink?
1. Operation steps
①Hold the chronograph stopwatch in one hand, and the 3# Zahn viscosity cup in the other hand, and adjust the stopwatch to the state of timing preparation.
②Put the clean viscosity cup into the ink to be tested, and require the cup to sink in the ink as a whole.
③Lift the viscosity cup up, and press the stopwatch to start timing at the moment when the ink surface is exposed at the mouth of the cup.
④Visually observe the change in the viscosity of the ink flowing out of the outflow port at the bottom of the cup. When the ink flowing out changes from a continuous state to a droplet shape, press the stopwatch to stop timing.
⑤Read the calculation time on the stopwatch, which is the viscosity of the ink to be tested (time: seconds).
2. Matters needing attention
①Before ink viscosity measurement, check whether there is any solid matter adhered or blocked in the inner wall of the viscosity cup and the outflow hole. If it is, it must be cleaned before the measurement. Otherwise, the measured viscosity is not true.
②When measuring the dark ink and then measuring the light ink, the cup body ink should be washed (or wiped) clean to avoid polluting the light ink and causing hue changes.
③When adding solvent, stir while adding, and add slowly. Too fast will easily damage the ink structure and cause the ink to separate and precipitate.
How should gravure printing companies screen thin film gravure inks? This is an urgent issue in the application of ink and wash. The laboratory test of film gravure ink should include the following:
①The adhesion test of ink on different polar films;
②Drying performance characterized by initial drying length;
③Fineness;
④Viscosity;
⑤Color density;
⑥pH stability;
⑦The ink film is wear-resistant and scratch-resistant;
⑧Organic solvent content;
⑨Surface tension.
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