Maintaining consistent ink color across batches or even multiple batches of printed materials is a fundamental requirement and a significant challenge in printing. For manufacturers, the color of their packaging has become a crucial means of product identification and a symbol (even a company's signature color). Consumers can recognize the red of Coca-Cola, the orange-yellow of Tide laundry detergent, and the colors of many other products. If product packaging appears inconsistent, consumers may perceive unstable product quality or even question its authenticity. Therefore, well-known companies place great importance on the color of their product packaging, demanding consistent ink color.

The consistency of color is relative. Strictly controlling color differences within the same batch is difficult, and strictly controlling color differences between large batches or different batches is even more challenging. Gravure printing of flexible plastic packaging often involves multiple batches and large-volume printing; therefore, controlling color difference is a critical issue for flexible plastic packaging printing companies.

I. Ink

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

To solve the color difference problem in plastic gravure printing, the first step is to select inks with stable quality and no hue deviation. It's best to use the same batch of ink for each batch of prints (and ideally, prepare a full quantity of spot color ink at once), as it's difficult to guarantee consistent hue between different batches. If the ink hue deviation is too large, it becomes difficult to control the ink's color characteristics through other methods, especially when printing tonal gradations.

Secondly, the amount of ink added to the ink tank each time during printing should be small to ensure frequent replenishment. This is particularly important for designs with low ink application rates. Adding too much ink at once will cause the ink to oxidize and become stale after prolonged exposure to air; for example, aged black ink will have a grayish tinge and won't be as black as new ink; aged red ink will turn a dark red and won't be as vibrant as new ink. The evaporation rate of the solvent mixture used in the inks must also be strictly controlled during printing. If the evaporation rate is too fast, the ink transfer rate may decrease, affecting the color of the printed product.

Furthermore, when using two-component inks, thorough stirring is essential, and small batches should be prepared frequently. Otherwise, it's difficult to maintain stable ink viscosity, and other printing defects may occur.

II. Squeegee

The contact position, angle, and pressure of the squeegee with the printing cylinder have a significant impact on the ink color of the printed product, especially in shallow areas. Ideally, the squeegee should remain in the same position throughout the printing process. However, due to various reasons, the position, angle, and pressure of the squeegee must be adjusted during printing. Therefore, we must understand the impact of these changes on the printed color and take corresponding measures.

The contact position between the squeegee and the printing cylinder determines the distance between the squeegee/plate contact point and the impression point. This distance should be adjusted according to printing speed, ink viscosity, cell depth, and plate quality. Generally, the longer this distance, the greater the drying of the ink within the cells, and the lower the ink transfer rate. In terms of its impact on the printing effect, this is equivalent to partial fading of the ink color. A longer distance also tests the ink's adaptability. Conversely, a shorter distance results in a relatively higher ink transfer rate, but it may also cause some fine lines on the printing plate to transfer to the substrate.

The doctor blade angle is the angle between the tangent line at the contact point between the doctor blade and the printing cylinder and the doctor blade itself. Increasing the doctor blade angle makes it easier to scrape the ink off the printing plate, resulting in clear images and text. However, this can also lead to an excessively thin ink layer, poor ink film gloss, and potential damage to the printing cylinder. Conversely, decreasing the doctor blade angle results in a thicker ink layer and better print gloss, but the edges and corners of the images are more prone to blurring.

Doctor blade pressure also affects ink color; higher pressure results in lighter ink, while lower pressure tends to result in darker ink.