How to Control Solvent Residual Volume in Gravure Printing Soft Packaging (2)

The ink has good solvent release and the residual solvent is relatively small. The inks of different manufacturers differ in their systems, and the difference in solvent release is mainly due to the difference between ink binders, resins and pigments.

In the same ink system, the solvent releasability of each color ink is also different. Among them, the releasability of magenta ink and black ink solvent is relatively poor. The main reason is that the pigment particles of magenta ink and black ink are very small, and the adsorption capacity is small. Very strong.

Due to stricter solvent residue requirements, many flexible packaging companies have adopted benzene-free inks, benzene-free and ketone-free inks, and alcohol-water white inks to effectively reduce the residual of solvents, especially benzene-based solvents.

2. Volatilization speed and viscosity of working ink

In the printing process, the solvent used in the ink is basically a mixed solvent, and the volatilization rate has the greatest influence on the solvent residue. If the mixed solvent contains more slow-drying solvents, serious solvent residues will result. Therefore, the rational design of mixed solvent formulations is a very important technical problem. In addition, the purity of the solvent will directly affect the volatilization rate. The content of water in the solvent has the greatest effect on the volatilization rate of the solvent. The ink viscosity also has a great influence on the solvent residue.

3. Dry conditions

The drying conditions include the printing price of the structure, length, air volume, and air temperature of the drying box, which determine the drying efficiency of the printing product. Poor drying will increase the residual solvent. Increasing the air volume and air temperature will increase the drying efficiency. However, when the ink layer is thick, it may cause the ink layer surface to dry quickly, which will hinder the escape of the solvent in the ink layer, resulting in an increase in the amount of residual solvent. In addition, the drying device should be regularly maintained and cleaned to avoid clogging.

With the increase of printing speed and the increasingly stringent requirements of the market for the environmental performance of flexible packaging processing equipment, the printing and compounding machine drying system has been continuously improved. For example, the last color group of the gravure printing machine uses two ovens or an extended oven; the guide rollers are added to the last color group of the gravure printer to transfer the prints back to the front of the first color group, thus increasing the drying distance and enabling the solvent to Fully volatilized; effective partitioning in the printing oven (divided into upper and lower parts, all gases with lower solvent content are discharged, and gas with lower solvent content in the upper part is subjected to hot air two cycles), so that high concentrations of solvent gases are directly Exhaust, do not participate in the second return, greatly reducing the amount of solvent residues in printed products. Another example is the use of variable frequency control for drying system fans to achieve reliable control and regulation of air flow and wind speed.

4. Printing speed

The speed of printing determines the length of the drying time of the printed product, and the printing speed should be gradually increased while ensuring that the printing ink layer is sufficiently dry.

5. Drying medium humidity and organic solvent concentration

The humidity of the drying medium (air) has a great influence on the residual solvent. One is that the moisture in the air enters the small ink printer, which will inevitably cause the overall volatility of the solvent to deteriorate. Second, the presence of large amounts of moisture in the drying medium will also inhibit the volatilization of the solvent. Double the humidity, the drying speed of the ink will be almost twice as slow. Therefore, under high humidity conditions, the printing speed should be reduced as much as possible and the contact between ink and air should be reduced to ensure that the ink can be fully dried. At the same time, the solvent formulation should be adjusted immediately to formulate a special formulation that is suitable for this climate. The humidity of the printing shop air should not exceed 70%, of course, it can not be too dry, otherwise it will cause static electricity problems.

6. Selective absorption of solvents by the substrate film

Different substrate films have different sorption capacities and sorption tendencies for the solvents used for printing. PP-based materials can easily adsorb hydrocarbon-based solvents and have a large amount of adsorption to toluene and other substances; PVDC has a stronger ability to adsorb toluene; water-absorptive materials such as PA and PT can easily absorb alcohol-based solvents.