Uteco ensures high-quality printing and operator safety using CFD simulation

Figure 2 – 2D slice showing in red the ink distribution
and the ink spalshing due to the roller
drag and the effect of the doctor blade

When printing at high speed with Rotogravure Presses (Figure 1) the print quality might be affected by air entrainment, that is mainly due to two distinct phenomena. The first type of air entrainment is due to air drag associated with the high rotational speed of the roller. The second type of air entrainment is due to ink splashing produced by the drag of the roller on the ink (Figure 2). Both the phenomena increase their negative effect on the print quality as the production rate increases, and both can be managed by adopting appropriate design solutions. For the air dragged by the rotating roller as it interfaces with the ink, Uteco developed solutions using specific devices located below the roller that reduce the amount of entrained air.
In this case, CFD simulation was used to understand the phenomenon and to design the geometry that reduced the risk of print defects to a minimum.

The cause of the ink splashing was initially less clear and intuitive. While it was clear that these print defects were due to air bubbles inside the ink volume, the question was: “Where do the bubbles come from?”. When looking at the printing machine, one can only see foam at the ink-free surface. Uteco and its customers perceive the foam as something to be avoided and associate it with potential print defects.

Figure 4 – Large air box with multiple nozzles

In reality, the foam was only a symptom of the real source of the ink-splashing print defect. Ink splashing takes place in the rear, hidden area of the machine, where ink is lifted up by the roller and then suddenly halted by the “doctor blade”. The consequence is that the ink tends to fall and splash at high velocity, thus mixing with air and generating foam. The foam generated in this hidden part of the machine then moves, together with the main ink stream, and becomes visible in the front part of the machine. So the foam is produced by the ink splashing, but is not the cause of the print defects. As a matter of fact, ink splashing also produces air entrainment and the formation of air bubbles inside the ink volume just beneath the roller. This must be avoided or limited. By using CFD modeling of the Rotogravure printing process, the engineers were able to understand these phenomena. With the source of the problem was identified, finding the solution was quite easy.

Figure 5 – Air velocity vectors, showing uniform air
flow impinging on the moving film

A new system was implemented to minimize ink splashing, and the mixing of air and ink. Air bubbles in the ink volume are prevented from reaching the printing area and, at the same time, foam generation is limited. The CFD simulations of the Rotogravure Press were developed using both 2D and 3D models. The 2D model was helpful to understand the physical phenomena associated with the ink-air interaction and to verify the efficacy of the new system in reducing air entrainment. The same 2D models also allowed the engineers to understand the importance of disposing of the air bubbles by facilitating air movement away from the roller-area to less risky areas further away from roller. The 3D CFD models permitted the calculation and visualization of the extent of the air bubbles beneath the roller in different geometrical configurations. It was quite evident from the results that the new system reduces the air bubbles’ dimension and pressurizes the ink volume which also helps to decrease air entrainment at the sides of the roller, where most of the printing defects manifest. The 3D model also clearly demonstrated the improvements in ink-flow distribution in the ink recirculation system, located in the visible area of the machine (Figure 3). This, together with the more limited formation of foam, can also be regarded as an “aesthetic improvement” of the machine’s fluid-dynamic behaviour, and is perceived by customers as proof of high-quality printing.

Figure 6 – Vapor concentratuion in the evaporation
and suction areas

After the printing phase, the volatile part of the ink must be forced to evaporate. The volatile part might consist of water or chemical solvents. The first challenge in ink drying is to fully evaporate the volatile part on a fast-moving film. The second challenge is to prevent this vapor from escaping into the external environment, where operators might smell and inhale the chemical solvents. The drying process is achieved through air boxes with multiple nozzles that impinge hot air onto the moving film (Figure 4 and Figure 5). The design of these ink drying systems was heavily supported using CFD with multiple objectives, such as the even distribution of the air flow from the nozzles, ensuring total evaporation, reducing pressure losses in the air boxes, evaluating the minimum suction or flow-rate to prevent solvent escape. In a few words, the aim here is to evaporate the volatile parts of the ink safely and with minimum energy.

The first and most important design objective was to ensure that each nozzle was fed with the same amount of air and that the air velocity along the linear nozzle was uniform (Figure 5). Achieving this target would ensure that the evaporation rate on the moving film would be uniform. But this was not sufficient. Another design objective required guaranteeing the evaporation of the total volatile matter before the film exited the drying area. For this reason, the CFD models, in addition to predicting the velocity distribution, had to explicitly simulate the evaporation, and the transport and diffusion of the vapor.

Figure 7 – Ink evaporation from the moving
film versus operating temperature

Specific models were implemented to predict the evaporation rates for both water and chemical solvents. The same models also allowed estimates of the minimum operating temperature to guarantee total evaporation of each type of solvent (Figure 7). This kind of information is of paramount importance both for printing quality and to minimize energy consumption --two of the reasons that customers purchase Uteco printing machines. After evaporation, vapor suction must be guaranteed for safe operation. This is done by reducing the area from which solvent vapor can escape to the external environment, and by creating a proper suction effect. Also in this case, CFD modelling allowed the prediction of vapor escape and the calculation of the minimum suction level to ensure total vapor capture with minimum energy consumption.