Theoretical analysis of ink color matching
As mentioned in the previous issue, inks are produced by different companies or different regions, and their color changes and printing characteristics are different. In addition, the printing conditions used by each printing company are also different. In other words, what we do is tailored Customized database. For example: the company has a large number of orders and often has to do the post-process processing of varnish. In the past, masters have to consider 2 factors when producing:
1) Hue changes after the print is dried
2) The color change of the printed matter after oil
In order to balance these factors and control the production quality, the masters often spend a lot of time and waste a lot of materials to adjust the color tone of the printed matter. In fact, when we use computer color matching, these two factors can already be taken into account. When creating a database, enter a data with varnish effect. In this way, whenever a varnish effect is needed, the computer will calculate the formula containing the varnish effect data, and since then Master will not spend a lot of resources to adjust the ink color. If the user or the customer does not have confidence in the direct production, a trial draft can be made in advance, and the IGT printability tester can be used for pre-press testing.
This leads to another question. Since the effect of varnish treatment can be predicted, can the paper color be taken into account? of course can. Calculate the color of the paper and the degree of ink penetration, because the ink / pigment used by the user has been considered in the color matching software. If the printing method depends on the substrate to develop color, then when calculating the formula, it will be Consider the background color of the substrate.
On the other hand, different papers have different absorbency. However, this variable affects our final printed result. Taking book paper as an example, when printing this kind of paper, masters often worry about the color change after the ink is dried, which is often much darker than the printed sheet just produced. Therefore, the color matching system also takes this into consideration, that is, when the database is established, the two types of paper are processed separately (see the figure below). These are the coated paper database and the non-coated paper database.
In conclusion, the database we have established must meet the needs of the plant:
Ink type, substrate type, post-process type
When the entire database is completed, it can be put into production. At the same time of production, of course, it is also necessary to consider a suitable database to prepare the target ink. [next]
produce
Everything is ready, this time with an example to share with you the entire color matching process. This time, we chose to deploy a grass green color (Figure 1).
First input the color data of the sample into the software using a spectrometer, and then select the color matching function. At this point, the computer asks us to select the database (Figure 2, A). According to the paper used in the sample, select the database of uncoated paper (Figure 2, B). Then select the number of colors used (Figure 2, C), enter the formula window, enter the production paper information.
(Measured by spectrometer)
After finishing, click the formula operation again. After a while, the computer displays the results of using different color inks to formulate the target.
If feasible, the computer will calculate up to 50 recipes. Because it is possible to formulate the target color by using different ink combinations, then how can we choose the most suitable formula?
Choose a recipe
For lithography, we can rely on:
1) Delta E (DE)-color tolerance
2) Metamerism Index (MI)-metamerism index;
3) Curve Fit Index (CFI)-curve fit index;
4) Cost-these four factors of cost to consider how to choose the formula.
Color tolerance refers to the difference between the formula calculated by the computer and the target standard. It is calculated under a single light source. The smaller the value, the higher the accuracy. However, it should be noted that it only represents the color comparison under a certain light source, and fails to detect the deviation under different light sources.
The metamerism index represents the color change of two samples under different light sources. Expressed by numbers, the lower the value, the less the color change. Considering the â–³ E color tolerance, it is a very valuable reference for quality verification data.
The curve coincides with the index. Colors are actually like humans, with their own unique index, and the spectral curve is their color index. If the spectral curves of the two samples are exactly the same, they can be said to be the same color, that is, when the spectral curves of the two samples overlap, what is the sum of their surrounding areas.
Cost is a reference index that we can enter at the same time when entering ink data. When the computer calculates the formula, the formula cost will be calculated together, so that the customer can consider a low-cost and satisfactory formula to produce.
In addition to the four reference values ​​mentioned above, there are actually a number of factors worth considering, such as Correctability Index (CRI)-modified index, Opacity-hiding power, etc. Of course, the computer displays multiple indexes at the same time for users' reference, so that users can find the most suitable formula.
White ink and transparent ink
Generally speaking, in traditional color matching methods, masters will add a large amount of white ink to dilute the color density, or it is easier to control the color change when adjusting the ink; another purpose is to increase the hiding power of the ink. It ’s a color, but in fact it does n’t need to be white ink. Another advantage of computer color matching is that it can reduce the amount of white ink used, even without it. It all depends on the color of the substrate. The amount of white ink used for printing is indeed of great benefit to the printing house. Because the white ink pigments are coarser and harder than the particles of ordinary ink pigments, the loss of the printing plate will increase during production, and sometimes the printing suitability of the ink itself will be reduced. Properties, such as abrasion resistance, drying time, etc. On the other hand, white ink is generally more expensive than transparent ink, so the cost will increase, so we recommend using transparent ink instead of white ink for color matching.
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