Ten factors that affect the amount of ink transferred to gravure printing cannot be ignored!
Gravure printing is a very important one of the traditional printing methods (lithography, letterpress, gravure, and screen printing), and it plays an important role in aluminum foil printing for pharmaceutical packaging and plastic flexible packaging printing. Although gravure printing has been used in China for many years, its printing process still has many areas that need to be improved. Now some influencing factors of ink transfer in gravure printing are discussed.
1. The principle of gravure printing
In the printing process, the originals to be copied are divided into text line drafts and halftone drafts with varying levels. Other traditional printing methods such as offset printing (offset printing) and letterpress printing are used to print the text line draft (that is, 100% dots), and the hierarchical copy is copied using a combination of dots with different sizes. The gravure printing must use dots to express both the layered draft and the text line draft, which is determined by its layout characteristics. There are countless recesses in the gravure printing plate with different area sizes or different depths (the general number of network lines used is 50 ~ 100L / cm, small text and shallow layer patterns use high network lines, thick lines, large color blocks And the full version uses a low network cable). The graphic part is lower than the blank part, and the blank part is on the same layout. When printing, first ink on all parts of the layout, then use a scraper to scrape off the ink along the surface of the blank part, and then transfer the ink in the mesh directly to the surface of the substrate by the pressure of the rubber pressure roller; the ink follows the printing When the object enters the oven, under the action of hot air, the solvent in the ink volatilizes, the ink changes from liquid to solid, and adheres to the substrate to complete the printing process.
2. Influencing factors of gravure ink transfer volume
From the perspective of ink, printing is actually a transfer process of ink from printing plate to substrate. It is very important for us to discuss the rules in the ink transfer process for our printing design and actual printing operations. From years of printing on-machine operation practice and theoretical exploration, the following factors have been summed up to affect the amount of ink transfer.
1. Net shape
The formation of net holes mainly uses chemical etching or electromechanical engraving. The shape of the net holes is different: the shape of the corroded dots is "U", and the shape of the electromechanical engraved dots is "V". The production method is different, the volume of a single cell is also different, and the ink storage capacity is also different. The former is greater than the latter, so the transfer amount of printing ink is also greater than the latter. The corroded version is generally used for PTP aluminum foil printing with more text lines due to the large transfer amount, and the electromechanical engraved version has more changes in dots and good repeatability, so it is used for color printing of plastic films with more complicated patterns.
2. Fineness of ink particles
The finer the ink particles, the more ink particles enter the printing grid, and the more the amount of transfer. The fineness of general ink particles is 10-20µm.
3. The viscosity of the ink
The viscosity of the ink is also very important in the ink transfer process. In the production of film composite color printing, the ink viscosity is controlled at 13-25S (Zahn Cup No. 3). Within this range, as the viscosity increases, the amount of transfer increases. If the viscosity is too high, it will be difficult to enter the mesh due to the increased surface tension of the ink, which will reduce the amount of transfer. Too large to cause a quality accident.
4. Scraper angle, pressure and distance from the contact point of the scraper to the imprinting point
The greater the angle of the doctor blade, the greater the pressure, and the more ink is scraped off from the cell, thereby reducing the amount of ink transfer. The farther the distance between the contact point of the doctor blade and the printing plate to the printing point (the contact point of the pressing roller and the printing plate cylinder), the time for the ink in the mesh to dry before contacting the substrate increases, which increases the viscosity and increases The transfer is difficult and the transfer volume is reduced.
5. Printing speed
The faster the speed, the greater the amount of ink transfer. There are two reasons: first, the drying time of the ink in the net before transfer is shortened, and the viscosity does not change much; second, when the pressure of the blade is unchanged, the amount of ink in the net increases, so the transfer amount increases. Therefore, increase the printing speed as much as possible while ensuring the printing quality. The printing speed in the flexible packaging compound is generally 80-150m / min.
6. Solvent evaporation rate
There are three kinds of solvent formulations with different volatilization speeds, such as fast drying, standard drying and slow drying, which are generally provided by ink factories. If the solvent volatilizes too fast, the viscosity of the ink in the ink tray will rise, and more importantly, the viscosity of the ink in the mesh before the transfer will rise sharply, drying up in the mesh, causing the transfer amount to drop significantly, and in serious cases, printing failures will occur . Therefore, in order to improve the ink transfer rate in general, slow drying solvents should be used especially when printing shallow gradation patterns.
7. Electrostatic ink absorption auxiliary device
The actual transfer rate of the ink is only about 60%, and the ink in the fine mesh holes is difficult to form because of the capillary phenomenon. With the extension of printing time, the phenomenon of missed printing is more obvious. The electrostatic ink absorption device uses the electric field force to help the ink in the mesh to be transferred to the substrate, which can increase the transfer rate by 10% -20%, especially in the printing of small dots, the increase is more obvious.
8. Press roller pressure
The role of the pressure roller is to help form "capillaries" between the substrate and the plate cylinder, so that the ink is transferred to the substrate along the "vascular". The greater the pressure, the better. The higher the pressure, the more difficult the substrate drive, which affects the accuracy of the overprint. Therefore, in the printing practice, the principle of setting the pressure is as small as possible while ensuring the printing effect.
9. The surface tension, flatness and smoothness of the substrate
The surface tension of the substrate is generally 1.2X10-4N higher than that of the ink, so that the capillary action will be formed with the help of the pressing roller, and the ink will be transferred from the printing plate to the surface of the substrate. The greater the surface tension, the greater the amount of transfer. Under the same printing conditions, plastic film NY, PET and BOPP printing
Brush ink transfer rate is gradually reduced, because the surface tension of their printing surface is also gradually reduced (of course they all meet the printing requirements: surface tension is greater than 0.04N / m). The flatness and smoothness of the film surface are very good, while the flatness and smoothness of the paper surface are different depending on the type of paper. The better the flatness and the smoother the substrate, the greater the amount of ink transfer during printing.
10. Environmental impact: temperature, humidity, granularity
Generally, when the temperature is 20-25 ° C, the ink has good fluidity and good printing effect. The higher the temperature, the faster the volatilization of the solvent in the ink, the higher the viscosity of the ink, and the destruction of the ink solvent balance; if the temperature is very low, it will reduce the fluidity of the ink, thereby affecting the ink transfer.
High humidity, water vapor will condense on the plate roller, causing condensation on the surface of the plate roller, which cannot be produced. At the same time, water vapor condenses into the ink, which will damage the quality of the ink; low humidity, dry air, easy to form static electricity, and easy to form static burrs on the edges of the printed pattern .
There is a lot of dust in the air in the printing workshop, and it is easy to make the guide roller and the pressing roller dirty. The foreign particles on the guide roller pressing roller will gather less and more, which may cause irregular leakage of the printed pattern.
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