How do I solve the problem of the printing ink not scraping clean?

In the printing industry, we often encounter a tricky problem – printing ink that won’t scrape off clean. The reasons behind this problem are complex and intertwined, and each link can have a significant impact on print quality.

First, let’s talk about the factor of ink viscosity. When the viscosity of the ink increases, its internal physical properties change significantly. Just like a tightly knit group, the increase in viscosity increases the interaction between the ink molecules and the surface tension. This change triggers a “power struggle” when the squeegee comes into contact with the ink. The squeegee can ideally remove the ink from the spare part of the plate roller smoothly, but as the viscosity of the ink changes, its impact increases, breaking the original balance. This imbalance is like suddenly adding weight to one end of a scale, which prevents the doctor blade from completely removing the ink, ultimately affecting product quality. To solve this problem, we can add a suitable amount of thinner to reduce the ink viscosity. When adding thinner, it is necessary to add it slowly according to a certain ratio based on the type of ink and the actual viscosity, and stir well to mix thoroughly. This phenomenon can be seen in some classic printing experiments. Under certain conditions, as the ink viscosity gradually increases, the ink residue on the printed matter becomes more and more serious.

Next, look at the squeegee pressure. When the printing press is first started, the squeegee pressure is usually set at a low level to avoid excessive wear of the plate cylinder by the squeegee. However, as the printing press speed increases, the situation becomes complicated. The impact of the ink on the plate cylinder on the squeegee at high speeds will increase rapidly, just like a sudden gust of wind on a calm lake, the original weak balance is instantly broken, resulting in the squeegee being unable to scrape the ink clean. In this case, the operator needs to increase the squeegee pressure in time according to the increase in machine speed. Generally, the pressure adjustment device equipped on the printing machine can be used to increase the pressure in small increments at a time, while observing the printing effect, until the ideal ink-scraping state is achieved. In actual printing production, this situation is not uncommon. For example, in the process of high-speed rotary printing newspapers, if the squeegee pressure is not adjusted in time, the ink on the newspaper will easily become blurred.

Problems with the ink during use should not be ignored. After prolonged use, the composition of the ink will change. On the one hand, the pigments in the ink will gradually precipitate during the circulation process. This is like a mixed solution in which the solutes slowly precipitate after being left for a long time. On the other hand, water vapor in the air will take the opportunity to invade the ink when the solvent evaporates. This is because the evaporation of the solvent causes the balance of the ambient humidity around the ink to be disrupted, and the water vapor will condense and dissolve into the ink. These changes can all affect the overall performance of the ink, greatly reducing its wiping properties and making it difficult for the squeegee to remove the ink. To prevent ink aging, we can regularly replace the ink and ensure that the humidity in the printing environment is stable. Humidity control equipment such as dehumidifiers or humidifiers can be installed in the printing workshop to control the humidity within a suitable range for ink stability, generally recommended to be between 40% and 60%.

The nature of the solvent also has a key impact on the ink-scraping effect. When the solvent in the mixture evaporates too quickly, it not only rapidly increases the viscosity of the ink, but also triggers a series of chain reactions. The ink will adhere tightly to the free parts of the plate roller, and because the solvent evaporates too quickly, this adherent ink will appear to be in a gelatinous state. Imagine the plate cylinder as a high-speed rotating disc. When it passes the squeegee, this gelatinous ink acts like superglue, and it is difficult for the squeegee to completely remove it in one rotation of the plate cylinder. As a result, this ink is carried onto the printing substrate. At this point, we need to adjust the evaporation rate of the solvent. This can be done by replacing the solvent with one that evaporates at a more appropriate rate, or by adding an additive such as a retarder. If the solvent evaporates too slowly, on the other hand, the ink on the unoccupied parts of the plate roller does not dry on the surface of the plate roller in time, and its state is similar to that of the ink on the areas with dots. When the printing substrate passes through the plate roller, this un-dried ink can easily be transferred to the substrate, causing printing quality problems. In this case, you can choose a solvent with a faster evaporation rate, or appropriately increase the temperature of the printing environment, but be careful that the increased temperature does not adversely affect other printing materials and equipment.

The service life of the printing cylinder is also an important factor. As the printing cylinder continues to serve its purpose, its surface will gradually wear away under the repeated action of the squeegee. This is like a pair of shoes that are often worn, and the soles will become smoother and smoother. This change in the surface of the printing cylinder will increase its affinity with the ink, directly increasing the difficulty of the squeegee scraping off the ink. Worn printing rollers can be repaired by processes such as surface grinding and re-chroming, which restore the roughness of the surface and reduce the adhesion of the ink. If the wear on the printing roller is too severe, a new one needs to be replaced in time to ensure printing quality.

The fit of the doctor blade is also crucial. When the doctor blade is too soft and the angle too flat, air pressure can become a “spoiler”. Under the influence of air pressure, the doctor blade will bend at the point where it contacts the roller surface. The ideal doctoring method is a precise “point” contact, but this becomes a broad “surface” contact, which cannot effectively remove the ink from the spare parts (non-dot parts) of the roller. To solve this problem, the doctor blade assembly needs to be readjusted, a doctor blade material with the right hardness selected, and the doctor blade angle adjusted to a suitable range, generally between 30° and 60°.

The problem of the plate roller assembly being out of alignment should not be underestimated. Generally speaking, when the plate roller runout is within 10, the impact on color registration and product quality may be small. However, the situation changes when the roller is running from the side with the longer oval diameter to the side with the shorter one. This sudden change in the instantaneous distance, coupled with the fact that the squeegee is an external force acting on the roller, and the time difference between the two, will cause the squeegee pressure to tighten and loosen during one plate cycle, which in turn causes partial smudging. This is like a dance with a stable rhythm, but suddenly there is a rhythmical disruption, which affects the overall effect. To solve the problem of the plate roller assembly being out of alignment, a high-precision measuring tool needs to be used when installing the plate roller to ensure that the plate roller is installed concentrically, with the deviation being controlled within a very small range. If the concentricity problem is discovered during the printing process, the machine should be stopped in time to readjust the position of the plate roller.

Solvent itself also has various problems. Water entering the solvent is a serious problem because the solvent is used in combination with the ink. Just as impurities mixed in with food, water entering the solvent will directly damage the overall performance of the ink, resulting in poor ink transfer. Therefore, ensure that the solvent storage container is well sealed to prevent the solvent from coming into contact with moisture. If the solvent is contaminated with water, replace it with a new one in time. In addition, improper solvent mixing can also have adverse consequences. A suitable solvent mixture is like a precise key that can make the ink dry at the right speed. An improper mixture will cause the ink to dry too quickly or too slowly, which will lead to the problem of poor scraping. The solvent should be prepared according to the type of ink and the printing environment, and precise measuring tools should be used during the preparation process. In addition, if the solvent itself contains impurities, such as low purity or high water content, it will also have a negative impact on the performance of the ink. For solvents with impurities, they can be filtered to remove impurities, or simply replaced with high-quality solvents.
The non-circulation of ink is also an easily overlooked cause. Ink that is not circulated is like a pool of stagnant water, and its viscosity is difficult to control. This is because the thixotropic characteristics of the ink mean that its performance will deteriorate when it is not circulated. At the same time, a skin will form on the surface of the ink in the ink tank, increasing the likelihood of dried ink adhering to the plate roller, which in turn leads to unclean scraping. To solve the problem of non-circulation of ink, an ink circulation system can be installed to ensure that the ink is continuously and stably circulated during the printing process, and the ink circulation pipes and ink tanks should be cleaned regularly to prevent ink from accumulating and forming a skin in the pipes.

Finally, the ink agitator may also be the culprit. After use, the raised surface of the ink agitator will more or less adhere to ink. As the number of uses increases, the amount of ink will gradually accumulate. When the solvent has completely evaporated, only dry ink remains. During subsequent use, because the ink in the ink tank cannot completely wet the dry ink on the surface of the ink agitator, this gelatinous ink will be transferred to the surface of the plate roller when the ink agitator rotates with the plate roller. Due to its extremely strong adhesion, it is difficult for the squeegee to remove it after the plate roller has completed one revolution, resulting in the problem of ink not being completely wiped off the printing substrate, which is particularly obvious in the non-dot parts of the plate roller. For the ink stick, the ink residue on its surface should be cleaned regularly. It can be soaked and cleaned with a special solvent to ensure that the surface is clean and residue-free. At the same time, during use, the rotation speed of the ink stick can be increased appropriately to make the ink mix more evenly in the tank and reduce the adhesion of the ink to the ink stick.

We can assume that if an intelligent monitoring system can be developed to monitor the various parameters of the ink, the squeegee pressure, the state of the plate roller, etc., in real time, and automatically adjust the parameters according to a pre-set model, it may be possible to greatly reduce the problem of the ink not being scraped clean. Such a system can draw on the real-time monitoring and control technology for complex systems in the aerospace industry, collect data through sensors, and use algorithms for analysis and decision-making, bringing new quality assurance to the printing industry.