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

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. In general, the amount of solvent adsorbed by PA and PET is smaller than that of PP.

7. Other factors

(1) If the newly printed product can adopt an effective treatment method in time, it will also reduce the residual solvent. Therefore, after the printed product is printed, it is advantageous to control the residual solvent by placing it in the ventilated place for one or two days and recombination.

(2) In the printing process, a color group with a deeper color of the printing plate and a color group with poor solvent volatility may not be printed with the ink and may be subjected to secondary drying, which is advantageous for reducing the residual amount of the solvent.

(3) For multi-color printing, do not use too many overprint colors when designing the pattern. Use spot color ink as much as possible to prevent overprinting of the local ink layer.

(4) If the same printing plate has both a solid ground color and a very shallow layered Chinese printing and packaging network, it is extremely difficult to control the amount of residual solvent. When the volatilization rate of the solvent is increased, the shallow mesh will block the plate, and when the volatilization rate of the solvent is reduced, there will be a large amount of solvent residue in the solid. Therefore, this design should be avoided when making plates.

(5) When designing a flexible packaging product structure, the poorest barrier film should be used as a printing layer as much as possible, and a barrier layer and a heat sealing layer with a relatively high barrier property should be compounded inside, so as to avoid contamination of the contents by the residual solvent.

When the amount of residual solvent exceeds the standard, it can also be remedied by ventilation and drying.