Nine steps to solve the ink problem in flexible packaging printing

When the ink on the engraved printing cylinder isn't properly cleaned, or when the ink in the ink tray becomes too thick, it can lead to issues like poor coloration, dirt, ink bleeding, knife wire, and knife lines on the printed material. These problems are often linked to the viscosity of the ink. If the ink temporarily separates, it may cause hanging ink. High ink viscosity can result in ink running, but this is also closely related to whether the angle and pressure of the scraper are correctly adjusted. If the ink on the entire printing roller isn’t cleaned, or if the pressure from the finger or new ink isn’t properly managed, there might be an issue with the ink itself. If cleaning improves after replacing the ink, it could mean that the thinner was added incorrectly.

The solution is to replace the ink or adjust the thinner, and then re-adjust the scraper for optimal performance.

2. Check the printed substrate (such as plastic film).

Issues like ink leakage, white spots, orange peel texture, and poor adhesion can occur if the surface tension or static electricity of the printed material is not properly controlled. If the surface tension of the substrate is below 35 dynes, ink adhesion will be weak, leading to problems like orange peel, ink spots, and whiskers. Additionally, if the ink dries too quickly, it may block the printing process, resulting in missing areas or white spots.

The solution involves performing surface treatment on the printed material, installing static elimination equipment on the printing machine, increasing humidity in the work area, and adjusting the drying speed of the ink using a slower-drying solvent. If the ink is diluted, it can improve printing speed and overall print quality.

3. Evaluate the purity and odor of the thinner.

If the thinner used in the ink has low purity, it may leave behind solvent residues, causing an unpleasant odor on the final product.

To check the purity of the solvent, you can perform a simple test: take a strip of white paper about 8 cm wide, dip it into the solvent, and then wave it vigorously. Observe how quickly the solvent dries and whether the paper changes color. Also, smell the paper after it’s dried. If it doesn’t dry quickly, the solvent may have a slow evaporation rate. If the paper turns color, it could indicate impurities in the solvent. If there's a lingering smell, the solvent is likely not pure. You can also test by scraping both the original and diluted ink onto a film, then checking the residue. A larger residue from the original ink and less from the diluted one suggests normal performance, while unusual smells from either sample indicate a problem with the ink or solvent.

The solution is to replace the ink or adjust the solvent formula accordingly.

4. Proper use of slow-drying agents.

If the slow-drying agent or the slow-drying thinner used in the ink is incorrect, it can cause the ink to "bite" or dissolve the print, resulting in dryness or sticking between printed sheets.

The solution is to change the thinner formula, adjust the drying properties of the ink, and increase the printing speed for better results.

5. Adjusting the ratio of thinners.

Using different ratios of ink thinners can help control the drying time, color density, and viscosity of the ink. This adjustment helps increase printing speed and improve print clarity. It also reduces the pressure on the impression roller and the workload of the ink scraper, which supports faster printing. Controlling the ink viscosity is crucial for managing ink application, adapting to printing speed, and adjusting to room temperature conditions.

The solution is to mix fast-drying, medium-drying, slow-drying, or ultra-slow-drying thinners based on the printing speed, the depth of the graphics, the number of lines on the printing cylinder, and the ambient temperature in the printing area.

6. Transitioning from high to low viscosity ink.

If the printing speed is slow and the printing plate is deep, high-viscosity ink should be used. Otherwise, the printed images may develop water ripples due to low viscosity. Conversely, if the printing speed is fast and the graphics are light, low-viscosity ink is more suitable. Using high-viscosity ink in this case could result in insufficient ink coverage, missed areas, or uneven coating, limiting the printing speed.

The solution is to add thinners and diluents to adjust the ink’s viscosity for optimal performance.

7. Use of ink leveling rollers with spot color and metallic inks.

Spot color printing requires consistent and even ink color, and metallic inks containing copper or gold powder tend to settle easily. Therefore, using an ink leveling roller is essential. An ink stirring rod should also be placed in the ink tray. When using spot color inks, it's important to combine the thinner with the diluent rather than using a solvent alone. Metallic inks should not be diluted with solvents; instead, a diluent should be added to adjust the consistency.

8. Use and mixing of non-true solvents.

Although non-true solvents cannot dissolve the resin in the ink, they can be mixed with true solvents to reduce ink viscosity and minimize the risk of ink biting or dissolving during the printing process.

9. Adhesion test of printed products.

To test the adhesion of the ink, apply adhesive tape to the printed material, press it down firmly, and lift it slowly. If very little ink comes off, the adhesion is strong. If more than 40% of the ink sticks to the tape, the adhesion is considered poor and the ink may not be suitable for the application.