During the printing, I noticed that the printed product suddenly appeared dirty (both blank and text), and when I stopped for a while, it turned out that the flying insects flew into the ink roller, resulting in dirty prints. For this kind of dirty traces, we used to use wetting powder in the past. This time, I did a small experiment, that is, no wetting powder, no phosphoric acid and corrosion rods, but soap. I took a soapy block and gently rubbed it back and forth across the dirty area until the dirt faded. After I put it on the pad, I saw it again. There was no stain on it and there was no damage.