Effect of Press Speed ​​on Flexographic Product Quality

Is speed the most important?

If a printing worker is asked, and how fast he wants to press, he will often ask: “How fast can the press itself be?” The faster the press, the higher the output and the greater the amount of time printed. However, faster press speeds also result in faster production of more scrap! Therefore, we must first understand the speed problem, change the impact of speed on the print quality, in order to optimize the printing press parameters at the possible limit speed.

How does changing the speed of the press affect the final print? This article analyzes the relationship between printing press speed and other components related to printing, including anilox rolls, plates, and substrates, and how it changes the field and reproduction tone.

When the cell on the anilox roller passes through the sealed ink fountain, the cell is filled with ink. When the anilox roller leaves the ink fountain, the doctor blade scrapes ink in the direction of the cell, removing excess ink from the surface of the anilox roller. However, due to the viscosity and surface tension (intramolecular force) of the ink, the squeegee also drags the ink out of the pocket. Towing the ink will bias the ink toward the back of the cell, causing uneven ink distribution. Once the net is separated from the ink fountain, the ink gradually becomes flat in the net.

Filling of anilox roller net in ink fountain

The amount of ink in the cell depends on several parameters, including the speed at which the anilox roll passes through the ink chamber. When the speed is low, the shear force between the inks is small, and the amount of dragged ink is small. As the speed increases, the shear force increases, the amount of dragged ink increases, and, too fast, the time for the ink to reach the printing plate is also reduced. Therefore, if the speed is too fast, the supply of ink to the printing plate will be reduced, and the color density of the final printing product will be reduced, making the image look lighter.

Offset of the ink when the anilox roller leaves the ink fountain

Changing the speed of the press affects the paper and plastic film differently. During the transfer of the ink from the printing plate to the substrate, the ink can be transferred by the absorption speed of the substrate (such as a substrate such as paper), and can also be transferred by ink film separation. Ink film separation between the plate and the substrate is usually non-uniform, leaving more ink on the plate than transfer to the substrate. As the speed of the printing press increases, the position of the separation point of the ink layer will shift toward the substrate, resulting in more ink remaining on the printing plate, resulting in a decrease in the density of printing in the field.
If all the printing density goes up and down together, the color of the print will be lighter. This also happens with halftone copying because the change in the speed of the press has a greater effect on the reproduction of halftones. The effect of printer speed can be determined by using dot gain (net dot enlargement can measure ink transfer relative to solid density).

The slowest speed is first set to 40m/min, and the corresponding minimum speed change is shown on the vertical axis in the figure, to see the case of different dot gains. As the speed increases, the dot gain decreases and the maximum change approaches 4%. As the ink spreads inside the paper structure, the halftone dots increase. With the speed of the printing press, the time that the ink absorbs through the substrate decreases, so the diffusion of the ink in the substrate decreases, and the dot enlargement decreases.

The effect of speed on tone reproduction

Overall, the speed of the press affects the transfer of the ink. As the speed increases, both the field density and dot gains change. As the speed increases, the density drops, and the color tone of the prints fades. These changes in density are so large that they can be identified by the naked eye and therefore have a fatal effect on image quality. Customers will refuse to accept such products. Without understanding the impact of speed, simply increasing speed can only produce more waste products faster.