Reasons for dirty prints

In the process of printing quality inspection, the phenomenon of dirty prints can often be detected. Printing products from the dirty form is divided into two categories according to the form: one is called oil dirty, the other is called floating dirty.

Dirty oil is mainly due to the fact that the blank part of the printing plate has sensed the ink, rendering it dirty on the printing plate in a greasy state. It often starts from a small part, then gradually expands the scope and the dirty part is not easily wiped off, and it is quickly reflected on the printed sheet.

The reasons for the dirty oil are: 1. The quality of the printing plate (plate base is not good or the regenerated version), the blank part of the printing plate is oxidized; 2. The exposure time is insufficient and the developer concentration is too low when printing; 3. Printing ink Insufficient amount of fat and additives; 4. Printing pressure is too light or too heavy; 5. The plate is not stretched during loading; 6. The pH of the fountain solution is improper; 7. The surface of the blanket is aged.

The cause of floating is mainly due to the emulsification of the ink. A form of floating is the appearance of fine particles of the ink, which is often more uniformly present on the entire printing plate. The other form is that the ink roller is less viscous. The emulsion ink splashes on the printing plate, blanket, or even splashes directly on the paper.

The main cause of floating dirt is the ink emulsification caused by ink imbalance. In addition, it is related to paper quality, water, ink roller pressure and other factors. In multi-color printing, if all inks are filthy, it is likely to be a paper problem. If only one or two colors of ink are dirty, it is an ink or a plate problem. Dirty tends to cause large quantities of printed materials to be discarded. This type of printing quality accident must cause operator attention.