Also talk about the thickness of the ink layer
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First, the relationship between ink viscosity and black layer thickness
“Ink” said: “The increase in the thickness of the ink layer increases the viscosity of the ink, and the fluidity of the ink is small.” Here, the viscosity and fluidity of the ink are mentioned. So, what is the viscosity of the ink?
Viscosity is a physical quantity that measures ink performance. It is defined as a measure of the ability of the interaction between ink particles to hinder their relative motion, and the viscosity is a manifestation of the strength of the ink cohesion. The unit of viscosity is "poise", "centipoise" or "pa·second", "mPa·second".
The viscosity of the ink is usually measured by a "rotary viscometer" (Amila viscometer), a "drop stick viscometer" (Raleigh viscometer), and a "parallel plate viscometer". It can also be measured by a bubble viscometer, a capillary viscometer, a falling ball viscometer, a cone and plate viscometer. It is worth noting that the amount of sample ink taken when measuring viscosity is strictly limited. Using a rotary viscometer, the ink is placed in a measuring incubator. The ink is placed in a thick walled tube using a drop bar viscometer. The amount of ink dropped on the glass plate was 0, 5 cm 3 using a parallel plate viscometer. That is to say, viscosity is a unitized quantity, which is a metric based on the unit physical quantity, not an arbitrary variable.
When the thickness of the ink layer changes, does the viscosity change for the same ink? No! What changes now is the total adhesion of the ink, not the viscosity. Imagine an ink that has a viscosity when we take it one kilogram; and another viscosity when it takes 0.5 kilograms; when it takes 0.1 kilogram, it has a new one. Viscosity; what is the significance of measuring the viscosity of the public?
Here again involves another physical quantity of ink - viscosity. Macroscopically, "viscosity" and "viscosity" represent the same type of performance. But they are not the same thing. Viscosity is a measure of the resistance to resistance when the ink is broken apart. The viscosity of the ink can also be measured with a special instrument. But it is not dimensioned.
There is also a definitive definition of fluidity: the ability of an ink to expand its diameter under pressure. The fluidity can be measured by a fluidity meter.
The ink is a non-Newtonian fluid. Its motion law is much more complicated than Newtonian fluid. Viscosity is not only related to its structural components, but also to temperature. Fluidity is not only related to its structural composition and temperature, but also to the thixotropy of the ink.
Under normal circumstances, the viscosity and fluidity change in the opposite direction, that is, the greater the viscosity, the smaller the fluidity, the smaller the viscosity and the greater the fluidity. But they are not inversely proportional. For example, adding corn flour to the ink reduces the viscosity and does not increase the fluidity, but decreases.
Second, the relationship between the darkness of ink and the thickness of the ink layer
"Ink" said: "Printing dark ink, the amount of ink can be a little thinner. Because the dark ink has a strong hiding power, it is not easy to produce a flaw in the bottom."
The thickness of the ink layer is both a very important issue and a very sensitive issue. But it has little to do with ink color. For the continuous adjustment of the manuscript is a printed matter, which is clearly stipulated in the People's Republic of China industry standard "CY/5-91 "Quality Requirements and Test Methods for Lithographic Prints". Item 4.1.1 of the standard lists “Table 1 Density Range”:
The solid density here represents the thickness of the ink layer. Because, if there is no determined thickness of the ink layer, there is no accurate neutral gray balance, and there is no correct color reproduction.
In order to achieve standardization and data management, there is a corresponding field density test area in the "GATF color copy test chart" in the United States. In the future, the development of printing to CTP requires a color environment, which is the data of standardized management.
At present, the most advanced multi-color offset presses are already equipped with pre-sets. The thickness of each of the ink layers is determined, or the pre-press set data is input through a plate scanner or through various interfaces. The thickness of the ink layer will never be adjusted to the darkness of the different colors of the ink.
For packaged prints consisting of all-color solid color patches, a thicker ink layer is allowed, regardless of the neutral gray balance problem. However, the appropriate ink layer thickness can still be determined by the "GATF Test Pattern" method. But instead of distinguishing them by visual color, they are based on the physical fitness of the paper.
As for the coverage of the ink, it is not transferred by the depth of the hue, but by the transparency of the raw material of the ink. The four-color offset inks currently on the market are sufficiently transparent. It must have the ability to print both solid and four-color dots.
Third, the relationship between paper quality and ink layer thickness
"Ink" said: "For products with rough surface, soft texture and strong ink absorption, the ink color should be lighter and the ink layer can be thickened appropriately."
The first thing to emphasize is that, as mentioned earlier in this article, the hue and solid density of the ink are specified and should not be adjusted at will. If the printing suitability of the paper is related to the ink, it is the following two aspects:
1. Color reproduction suitability of paper
It is mainly determined by the whiteness and reflectivity of the paper, and its data can be determined in the "GATF Test Chart". In general, the color rendering of paper is proportional to the smoothness of the paper surface and inversely proportional to the ink absorption of the paper. So, some experts designed "paper efficiency"，This concept. It can be calculated using the following formula:
PSE=100-A+PG/2
Where: PSE - paper efficiency
A——Absorbability
PG - paper gloss
From the experimental results, it is consistent with the ash rate, color deviation rate and actual efficiency of the ink printed on the paper.
2. Surface strength of paper
The direct performance of this kind of fitness is the phenomenon of powder removal during printing. It is related to the viscosity of the ink and also to the printing speed. Here, I am still willing to avoid direct ink layering. The adjustment of the thickness of the ink layer is also determined by the color reduction value of the product. What happens when the phenomenon of powder removal and hair loss occurs? The viscosity of the ink can be appropriately reduced by adding the No. 6 varnish. In a low temperature environment, a method of preheating the ink can be employed. As for the relationship with printing speed, there is a formula that can be calculated:
VP=3600V/2π 4.7R (sheets/hour)
Where: VP - printing speed
V——critical pulling speed
R——imprint roller radius
Under the premise that other conditions are unchanged, the limit of printing speed can be calculated by calculation to ensure that no paper fluffing occurs.
There are also some expressions and references in the "Ink" text about "friction coefficient", "ink absorption", "tinting power" and other professional vocabulary, which is not rigorous and accurate. In view of the length and the level of myself, this is not a case.
Generally speaking, the quantity and quality of engineering and technical talents in China's printing industry are not enough. The existing theoretical level of technical knowledge has a great distance from the actual needs. I put forward the above personal opinions on the relevant content in the "Ink" text, so as to convey some basic theoretical knowledge. In China's printing industry, not only must we have practical experience, but also theoretical knowledge. Not only do you need to know the terminology and vocabulary, but you should also understand its true meaning and laws. In fact, theory can guide practice.