The color biting phenomenon is one of the common issues in gravure printing, where the subsequent color dissolves the ink of the previously printed color, and in severe cases, pulls off the ink of the previous color. This phenomenon often occurs in medium to low-speed overprinting with deeper printing plates.
When the color biting phenomenon occurs, it is evident that the ink printed is dissolved by the ink of the subsequent printing unit. On the printing plate of the subsequent unit, there is a noticeable color mixing change, and in severe cases, it leads to color mixing or even color contamination, severely polluting the ink of the subsequent color. Color biting can be divided into three stages: mild, severe, and extremely severe:
Stage One - Mild: Initially, the printing plate of the subsequent color is stained, causing impure colors. The overprinted colors have insufficient color saturation and brightness. In mild cases, shadowing may occur, such as when black is overprinted with white ink, especially in barcode areas where noticeable shadows can appear. In multi-color overprinting, a slight color difference in overprinting may occur.
Stage Two - Severe Contamination: The ink of the subsequent color pulls off part of the softened ink layer of the previous color, and the pulled-off ink layer of the previous color mixes with the ink of the subsequent color. The stained ink, through circulation, gradually contaminates the ink in the ink fountain, causing color contamination and color mixing in the subsequent color's ink fountain. If not handled in time, the ink of the subsequent color will undergo fundamental changes, leading to color seepage at the tail of the printed graphics or noticeable color differences.
From the printing plate and ink fountain, it is clearly visible: for example, if blue ink is bitten by yellow ink, the yellow ink eventually turns into green ink; if blue ink is bitten by red ink, the red ink turns into purple ink. The obvious changes in the ink within the fountain directly lead to an inability to ensure the quality of the printed products.
Stage Three - Extremely Severe Biting: This type of biting is similar to reverse sticking. During overprinting, the previous color ink is directly pulled off by the subsequent color ink, forming a textual and graphic defect similar to reverse sticking. The main cause of this issue is still the insufficient adhesion of the film. When the viscosity of the subsequent color ink is too high, the stripping of the ink film leads to this problem.
There are many factors related to the occurrence of color biting, which are associated with the printing plate, ink, solvent, film, the drying capacity of the machine, and the machine speed, among others. There are many mixed factors, but when analyzed from the root cause, they can be summarized into three situations: ink not thoroughly dried, ink redissolved, and insufficient ink adhesion.
- Ink Not Thoroughly Dried and Redissolved by Subsequent Ink:
- The printing plate's cells are too deep, leading to a large amount of ink, and the ink layer transferred to the film is too thick, preventing the ink from drying completely. When overprinted with the subsequent color, the solvent in the subsequent cells redissolves the ink, causing color biting.
- The printing plate's cells are too deep, and the solvent ink retention in the cells is large, making it relatively easy for the subsequent color's ink to redissolve the previous color's ink, creating conditions for color biting.
- The drying capacity of the printing machine is insufficient, preventing the ink from drying in time; or the drying temperature is too high, causing the ink to dry on the surface but remain wet inside.
- The addition of too much slow-drying solvent slows down the drying speed of the ink, preventing it from drying thoroughly. The presence of internal solvents makes redissolution relatively easy.
- High environmental temperature and humidity, or excessive air pressure, affect the evaporation rate of the ink solvent, reducing the drying performance of the ink. This makes the ink difficult to dry and prone to redissolution by the subsequent overprinting ink, leading to color biting.
- Strong Redissolving Power of Ink, Subsequent Wet Ink Redissolves Dried Previous Ink:
- The printing machine speed is too slow, and the dried ink film takes a long time to pass through the overprinting process. This allows the subsequent color ink to penetrate the previous color's ink film, causing redissolution and contamination, leading to color biting during ink transfer and stripping.
- The ink resin selection is mismatched, and the resin is easily redissolved by the solvent. During printing, the ink film is easily dissolved by the solvent of the subsequent color.
- The printing plate engraving is too deep, leading to a large storage of solvent in the subsequent color's ink, providing conditions for the redissolution of the ink film.
- There is too much true solvent in the ink, or the solvent's dissolving power on the ink resin is too strong, redissolving the previous color's ink and causing color biting.
- The pressure of the impression roller is too high, allowing the printed ink film to fully contact the wet ink in the subsequent color's cells and maintain a certain pressure. This creates conditions for mutual dissolution of the inks, leading to color biting.
- Insufficient Ink Adhesion, Printing Ink Film is Pulled Off by the Cohesive Force of Subsequent Ink:
- The surface corona value of the film is insufficient, leading to inadequate surface activity of the film. This causes the ink's adhesion to the film to be insufficient. Due to the film's insufficient adhesion to the ink, the ink film is stripped off during subsequent color printing, leading to ink dropout and color biting.
- The ink resin selection does not match the film, preventing the ink film from forming good adhesion on the film's surface. It is stripped off during subsequent color printing.
- The previous color printing ink lacks enough binder (insufficient resin), leading to insufficient adhesion of the ink and making it susceptible to erosion by subsequent colors.
- Improper use of solvents causes changes in the ink resin, such as coagulation, leading to a decrease in the ink's adhesion.
- The viscosity of the subsequent color ink is too high, and the ink's cohesive force is large, pulling the previous color ink off the printed film.
When dealing with color biting issues, pay attention to the characteristics of the problem. The transfer of the previous color's ink is normal; it is only after overprinting that the ink layer is pulled off by the subsequent color. By comparing the state of the previous color's ink layer before and after overprinting, it is clear whether it is a color biting fault, avoiding confusion with similar faults.
In actual operation, to effectively solve the problem of color biting, the following precautions should be taken in advance:
- Choose the appropriate depth of the printing plate. A printing plate that is too deep not only increases the cost of ink but also means an increase in the amount of ink used. This leads to a decrease in the drying performance of the ink, making it difficult to increase the printing speed and affecting product quality (such as the increased probability of solvent residue, reverse sticking, and knife lines). Under the premise of meeting color saturation, the screen lines and engraving depth of the printing plate need to be well controlled, especially for multi-color overprinting of dark colors. It is essential to control the relevant processes before printing to reduce the likelihood of color biting from the source.
- Choose ink that matches the machine speed. The formula design of ink for high-speed printing has a strong ability to redissolve the dried ink film. If used for medium to low-speed printing, the longer contact transfer time can easily lead to color biting. The color biting phenomenon is not entirely an issue of ink quality; it is more important to pay attention to the compatibility with actual application conditions. When the color biting phenomenon is mild, moderately increasing the machine speed can help reduce color biting faults.
- Choose the appropriate solvent. Non-true solvents cannot dissolve the connecting resin in the ink. However, by using their non-dissolving characteristics and mixing them with true solvents to dilute the ink, it is possible to reduce the viscosity of the printing ink and alleviate the color biting phenomenon during printing. Appropriately adjusting the solvent formula for subsequent color overprinting (reducing the amount of true solvent) can reduce the redissolving power of the dried ink film of the previous color, slowing down the color biting phenomenon.
- Pay attention to the addition methods of ink and solvent. For overprinting, it is necessary to accelerate the drying performance of the ink, reduce the use of pure solvents, and add ink with resin to enhance adhesion.
- Moderately reduce the viscosity of the subsequent color ink. For the subsequent color, the ink viscosity should be slightly lower than that of the previous color. Adjust the dilution solvent ratio of the subsequent color ink to reduce the dissolving power of the subsequent color ink on the previous color ink when they come into contact, i.e., when diluting the subsequent color ink, try to use less or no slow-volatile solvents and strong dissolving solvents.
- Appropriately increase the drying temperature and air volume of the previous color, and try to dry it as much as possible before printing to avoid the formation of a dried surface with an undried interior.
- Moderately reduce the pressure of the impression roller for the subsequent color, reducing the contact time and pressure between the ink film and the wet ink to decrease the conditionsfor ink redissolution.
- Check the corona value of the film before mounting it on the machine, ensuring that the corona value of the film is sufficient. This ensures that the adhesion of the ink transferred to the film is strong enough to avoid ink film stripping caused by the cohesive pull of subsequent inks.
- Increase the machine speed to shorten the overprinting time between ink film surfaces, reducing the time for dry ink film and wet ink to redissolve, and lowering the possibility of color biting.
- Provide moderate blowing on the subsequent color plate surface (use with caution for layered printing or small text printing), which also helps to alleviate the color biting phenomenon.
- Reduce the use of expired ink and old ink, especially in the first color printing. Moderately supplement a certain amount of ink modifier to replenish the binder of the ink, enhance the adhesion and gloss of the ink, and reduce the risk of color biting.