Why is there color difference in plastic flexible packaging printing?

How to maintain consistency in the ink color of a batch or batches of prints in printing is a fundamental requirement and an important challenge. For manufacturers, the color of the packaging used in their products has become an important means and symbol of distinguishing products (even becoming the logo color of the enterprise). If there are differences in the appearance of product packaging, consumers may think that the product quality is unstable, and even suspect the authenticity of the product. So these well-known enterprises attach great importance to the color of product packaging and require that its ink color be stable and consistent.

The consistency of colors can only be said to be relative, and strict control of color differences in the same batch is relatively difficult, while strict control of color differences between large batches or different batches is even more difficult. Plastic flexible packaging intaglio printing is mostly multi batch printing and mass printing, controlling color difference is a very important issue for plastic flexible packaging printing enterprises.

1、 Ink

Ink is one of the main factors determining the color consistency of printed products. Ink itself also has color differences between the same batch and different batches, which is an important indicator of ink quality.

To solve the color difference problem in plastic gravure printing, it is necessary to first select ink with stable quality and no color deviation. It is best to use the same batch of ink for printing a batch of prints (it is best to prepare spot color ink in sufficient quantities at once), as it is difficult to ensure that there is no color difference between different batches of ink. If the color deviation of the ink is too large, it is difficult to control the consistency of the printing ink color through other methods, especially when printing layered plates.

Secondly, it is required to add less ink to the ink tank each time during the printing process to maintain frequent addition of new ink. Especially for patterns with low ink application, if too much ink is added at once, the ink will oxidize and become stale after being exposed to the air for a long time. For example, black ink that has been used for a long time may have a gray color, not as black as new ink; Red ink that has been used for a long time will turn dark red, not as bright as new ink. During the printing process, it is necessary to strictly control the evaporation rate of the mixed solvent used in the ink. When the evaporation rate is too fast, the transfer rate of the ink may decrease, which in turn affects the color of the printed product.

Furthermore, when using two component ink, it is necessary to thoroughly stir and mix less frequently. Otherwise, it is difficult to maintain the stability of ink viscosity and may also cause other printing failures.

2、 Scraper

The contact position, angle, and pressure between the scraper and the printing plate cylinder have a significant impact on the ink color of the printed product, especially in shallow areas. Therefore, the ideal state is that the scraper is always in the same position. However, due to various reasons, the position, angle, and pressure of the scraper have to be adjusted during the printing process. Therefore, we must understand the impact of its changes on the color of the printed product and take corresponding measures.

The contact position between the scraper and the printing plate drum determines the distance between the blade, plate contact point, and the imprinting point. The length of this distance should be adjusted based on printing speed, ink viscosity, mesh depth, layout quality, etc. Generally speaking, the longer the distance, the greater the degree of drying of the ink inside the mesh hole, and the lower the transfer rate of the ink. In terms of its impact on the printing effect, it is equivalent to a partial fading of the ink color. At the same time, when the distance is longer, it is also a test of ink adaptability. On the contrary, if the distance is short, the ink transfer rate is relatively high, and it will also cause some small patterns on the layout to transfer to the printing material.

The angle of the scraper refers to the angle between the tangent line at the contact point between the scraper blade and the printing plate cylinder and the scraper. By increasing the angle of the scraper, it is easy to scrape the ink off the layout, making the graphics and text clear. However, it may also cause phenomena such as thin ink layer and poor ink film gloss, and it is also easy to damage the printing plate drum. On the contrary, reducing the scraper angle can thicken the printing ink layer and improve the gloss of the printed product, but the edges and corners of the image and text are easily blurred.

The pressure of the scraper also has a certain impact on the ink color, and as the pressure increases, the ink color becomes lighter; When the pressure becomes lighter, there is a tendency for the ink color of the print to become darker.

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