Color difference control of solvent ink

Abstract: For ink producers and users, minimizing the color difference is always the direction. In this paper, the effects of color difference, depth of the cell, doctor blade, viscosity and other factors are introduced in detail. Therefore, some objective solutions are also proposed to control the color difference.

Keywords: ink, color difference, cell depth, doctor blade, viscosity, X-Rite SP instrument, control of color difference, is the biggest problem for solvent-based ink control. The color of the printed product is consistent with two aspects: on the one hand, it means that the same batch of colors is the same; on the other hand, it means that the colors of different batches are the same. For a printed matter, it is very difficult to ensure that it is consistent with the same batch, and the consistency of different batches is even more difficult.

Comparing with the printing process of plain printing, flexo printing, silk printing, etc., gravure printing is a kind of printing process with less variation of product color. The reason for the color difference may be mainly caused by the different ink preparation batches, and it is easier to compare and find out. Therefore, in the process of ink transfer to the substrate, in addition to being affected by the ink volume and ink concentration, the influence of other factors on the gravure color difference can be said to be small.

First, the relationship between intaglio network points and colors

From the principle of intaglio printing, it can be known that the amount of gravure ink is mainly determined by the shape, size, depth and depth of the printing roller, which is determined by the volume of the mesh ink in the unit area, and the printing capacity of the printing plate is accompanied by the printing plate cylinder. The change in the change, so when the large print volume and the need to use multiple sets of plate cylinder to complete the task of the same product (such as smoke packaging), you need to strictly control the process parameters when making the same color plate cylinder, For example, the number of screen lines, screen angles, depths, passage grooves, and even the hardness of the engraved copper layer should be kept consistent so as to minimize the color difference caused by the replacement of the plate cylinder. Once this happens, it has to be adjusted in the ink. When the new version is used, a suitable amount of diluting ink is used to reduce the coloring power of the ink and make it similar to the original color.

With the increase of the number of prints in the same printing plate cylinder, the surface chrome layer gradually wears under the action of the ink squeegee to gradually change the cell site, and the amount of ink on the plate cylinder also becomes smaller, and the color of the print is also changed. It gradually fades, although this is an unavoidable process, but due to the gravure cylinder in the copper layer after the 0.08-0.10mm hard chromium plating layer, this layer of chromium has a good rub resistance, generally The normal service life (in the case of a full version dark version) is between 1.5 and 2 million revolutions. Therefore, the color difference of the gravure product in the small batch printing is not easy to detect, and only in the large-volume printing, the later product will have a perceptible color difference compared with the previous product. Some ink suppliers have a special supply of color-reducing agents, which can compensate for the coloring power of certain spot colors to delay the process of color differences caused by the shallowing of the cell's holes, which in essence extends the service life of the plate cylinder. Of course, Make fine adjustments during operation. For a worn plate cylinder, it is confirmed that if the cell wall is not damaged, it can be re-chromed and then chrome plated, and it can be reused. If the cell wall is damaged, the plate cylinder must be re-created. There is a difference between the depth of the newly made printing plate and the old version, and the printing ink should be adjusted according to the actual printing.

The influence of the ink concentration on the color difference is mainly caused by the change of the ink pigment content per unit volume. The main factor that causes this change in the printing process is that the gravure ink continuously circulates in the ink supply system, causing the solvent in the ink to continuously evaporate, causing the ink to become more and more concentrated, and then adding the solvent to adjust the ink to the initial setting. The process of setting the concentration is often short-lived, and sometimes an excess of solvent is added and the adjustment is repeated. This kind of frequent adjustment often reflects the color difference on the products. Therefore, when formulating the relevant operating regulations, the addition of solvent is more appropriate and less appropriate, and the conditional unit is better to use an automatic solvent adding device on the unit. The ink concentration in the ink tank is detected and adjusted at any time, which is very good for reducing the color difference in batch printing.

Second, the main factors affecting color difference

The first is the influence of ink, which is the main factor that determines the consistency of the print color. The hue of printing ink should be stable. The color difference between the same batch and different batch should be kept as small as possible; new ink should be added frequently during printing; for spot color ink ( In particular, some spot colors formulated with white ink should be stirred frequently to prevent the separation of these composite colors; the ink should preferably remain sticky during printing; the solvent added during printing should not be contaminated by other colors.

In addition, the ink used for printing should be the same as that used for proofing (same manufacturer, same series, same model) so as to ensure uniform color.

The second is the effect of the squeegee. This is because the material of the squeegee, the assembly size, the position of the contact plate, the angle and the pressure have a great influence on the color of the print, especially the light color of the shallow screen. The softer the squeegee (or when the assembly is long), the darker the print. The larger the distance between the contact point of the doctor blade and the printing plate and the printing point, the lighter the color of the printed product. The greater the pressure of the blade, the lighter the color of the print.

The third is the effect of the plate roller, because the plate roller will become light after use for a period of time, and the amount of ink will reduce the color deviation. Therefore, after a certain number of prints, consider replating rolls. In addition, the color will become lighter if blocking occurs.

The fourth is the influence of the light source. It is best to use a standard light source (D65) or daylight as the light source. The light source should be located directly above the light source. Because of the difference in the spectral band of the light source, the hue of some colors will change. The difference in light source angle also affects the hue of the color.

Fifth, the impact of other printing conditions, such as the temperature and humidity of the printing shop, the printing speed and solvent drying speed changes, printing pressure, the hardness of the imprinting roller will affect the color of the print.

In the final analysis, when the color of a printed product deviates, it is necessary to consider several factors, such as the amount of ink applied to the ink, the amount of transfer, the color of the ink itself, and the condition of the light source during observation.