Analysis of the Influencing Factors of Color Difference in Plastic Flexible Packaging Printing (I)

Ink is one of the main factors that determine the color consistency of prints. The ink itself also has a color difference problem between the same batch and different batches, which is an important indicator reflecting the quality of the ink.

To solve the problem of the color difference of plastic gravure, the first thing is to select the ink with stable quality and no deviation in hue. It is best to use the same batch of ink for a batch of printed matter (the best one is to administer a spot color ink when formulating a spot color ink) because it is difficult to ensure that there is no difference in the hue of the ink between batches in some batches of ink. If the hue deviation of the ink is too large, it is difficult to control the consistency of the printing ink by other methods, especially when printing a gradation version.

Second, the amount of ink added to the ink tray each time during the printing process is required to be less, so as to keep adding new ink frequently. Especially for some patterns with less ink, if you add too much ink at a time, the ink will become oxidized when exposed to the air for a long time. For example, if the ink is used for a long time, it will be gray, and it will not be black as new ink. , with a long red ink will darken red, not as bright as new ink. In the printing process, the volatilization rate of the mixed solvent used in the ink must be strictly controlled. When the volatilization rate is too fast, the ink transfer rate may be reduced to affect the color of the printed product.

In addition, when using a two-component ink, it must be fully stirred and less equipped. Otherwise, it is difficult to maintain the stability of the viscosity, and it may also cause other printing failures.

One, scraper

The position, angle and pressure of the doctor blade in contact with the printing plate have a great influence on the ink color, especially the ink in the shallow layer, so the ideal state is that the doctor blade is in the same position from beginning to end. However, due to various reasons, the position, angle and pressure of the squeegee had to be adjusted during the printing process. Therefore, we must understand the effect of the change on the color of the printed product and take corresponding measures.

The contact position of the squeegee and the plate roller determines the distance between the plate contact point and the printing point. The distance should be adjusted according to the printing speed, ink viscosity, mesh depth, and plate quality. In general, the longer the distance, the greater the ink drying in the mesh, and the lower the ink transfer rate. As far as its effect on the printing effect is concerned, it is equivalent to the partial withdrawal of the ink. At the same time, the longer distance is also a test of ink printability. On the contrary, when the distance is short, the transfer rate of the ink is relatively high, and at the same time, some fine lines on the plate are also transferred to the printing material.

The blade contact angle is the angle between the tangent line of the blade and the plate roller contact point and the blade. Raising the contact angle makes it easy to scrape off the ink on the plate to make the picture clear, but it may also result in a phenomenon that the ink layer is too thin, the gloss of the printing ink is poor, and the roller is easily damaged. On the other hand, when the contact angle is reduced, the printing ink layer becomes thicker and the gloss of the printed matter is better, but the graphic corner is easily blurred.

The pressure of the squeegee also has a certain influence on the ink color. When the pressure becomes larger, the ink of the printed product becomes lighter; when the pressure becomes smaller, the ink of the printed product tends to become darker.

Second, the printing plate

The effect of printing plates on the consistency of ink color is mainly manifested in:

1. Blocked version

Blocking is a phenomenon in which blockages (clogging of meshes due to solidified substances in ink, insoluble substances of high viscosity substances, or foreign substances, etc.) caused by a certain cause cause a decrease in the transfer rate. The amount of ink transferred at this time is reduced, thereby affecting the hue. In the event of a blockage, clean the plate with solvent or a special cleaning agent (usually using other decontamination methods to cause other troubles). At the same time, take appropriate measures against the cause of blockage, and make it work under normal conditions.

There are many influencing factors of blockage. Except for factors such as the use of two-component inks that may cause chemical reactions that cause chemical changes and cause insolubility, other reasons can be attributed to the fact that the ink is solid on the layout, mixed with impurities, and the quality of the plate is low. And other factors. In addition, the mixing of moisture or other types of inks in the ink during the printing process will hinder the fluidity of the ink, resulting in the reagglomeration of the pigment and the blocking failure.

In addition, a detailed inspection of the printing plate is required before printing. If the printing plate adheres to oil stains or the like, a printing failure similar to that of blocking printing occurs at the time of printing.

2. Plate wear

When there is a large number of prints, the degree of wear is greater, and the printing ink color changes. On the whole, it tends to become lighter (since the cells become lighter and the amount of ink is reduced).
After a certain number of printing, it is necessary to replate. After printing for a long time, the printing plate becomes lighter. Although it is possible to maintain the color stability through other process adjustments, the value of the replating plate will be lost after excessive use, and only the re-platemaking will be performed.
It is also possible for various defects or even damages to occur during the use of the printing plate. Due to the influencing factors in gravure roll platemaking, it is almost impossible to completely repeat the original platemaking process conditions when remaking, even if the same printing conditions are often difficult to obtain the original printing effect. In order to solve this problem, if the printing batch is large and is a frequently printed variety, it can be considered as a set of two or even multiple editions.

3. Plate Design and Color Management

In addition, the effect on the printing process and post-press processing should be fully taken into account when designing the plate. For example, the color of some dot areas is prone to level and level, so it should be handled well during plate making. Many platemaking companies have begun to use color management software and have played a positive role in improving the quality of color reproduction.