After printing on plastic film or transparent paper, the impression appears to be dry, but the back side is sticky during storage after printing on the reel or bag. In severe cases, the printed parts cannot be torn apart, and the graphic ink layer moves Contamination, causing the product to be scrapped. This phenomenon is more likely to occur in high temperature seasons, but high temperature is not the only cause of back sticking. It is also closely related to a series of factors such as plastic, ink, solvent evaporation rate, operating environment air volume, and light.
1. The solvent in the gravure ink fails to evaporate completely after printing.
(1) Plastic film printing is to dry the ink by solvent evaporation. Its volatilization speed is an important factor that determines the drying condition of the ink layer and affects the printing quality. The slower the solvent evaporates, the better the reproducibility of the plate and the more beautiful the color of the printed product, but it is easy to stick; otherwise, it will produce the impression of whitish imprint. Therefore, choosing a solvent with an appropriate volatilization rate is the key to plastic film gravure printing. In continuous register printing, the solvent must be completely evaporated during the running time of the film between the two printing stations, otherwise, the roller will stick when printing in the next station.
(2) After the film is printed, the organic solvent in the ink is not completely volatilized in the drying system, and there is still heat after the film is rolled up, which brings the conditions of continued volatilization to the solvent remaining in the ink layer and forms adhesion.
(3) After high temperature and humid season, the film is too tight or compressed after printing.
Countermeasure: The volatilization rate of the solvent in the ink depends not only on the boiling point of the solvent, vapor pressure, vapor latent heat, etc., but also on the temperature, humidity, air volume, solute and ink layer thickness of the operating environment. Therefore, the volatilization rate of the solvent in the ink should be adjusted at any time according to the change of conditions. At normal temperature, if the ink layer dries too fast, a solvent that evaporates slowly (such as Shanghai-made 74-92 plastic film slow-drying thinner) should be added; otherwise, a solvent that evaporates quickly (such as 74-91 plastic film should be added) Quick-drying thinner). It can also be diluted with a mixed solvent of xylene, ethanol and isopropanol (the three solvents are mixed in a ratio of 1/3, using a single solvent is not as good as a mixed solvent). If you feel the drying is too fast, you can use a little butanol to replace part of the ethanol (butanol has the effect of increasing the gloss of the ink). But be sure to add the appropriate amount to avoid affecting the drying of the printed parts.
The solvent for plastic film gravure printing is mainly alcohols and benzenes. Alcohols use ethanol and isopropanol, and benzenes use toluene and xylene. Due to the needs of nitrocellulose, esters such as ethyl acetate and butyl acetate should also be added. In short, the organic solvent in the ink should be completely volatilized before winding. Slow down the vehicle speed properly and increase the air volume. After the film is printed, it is loosely stored in a wire braided basket to allow the ink layer to continue to be ventilated and dried. After curing, it is cut and bagged. After the film is printed and bagged, it is wrapped and placed vertically in the carton to reduce the film surface Pressure.
2. The ink of plastic gravure ink is bad.
The bonding material in some plastic gravure inks has a low melting point and is easy to stick when the operating environment temperature is high and the relative humidity is large.
Countermeasures: ①When conditions permit, install air conditioners in the operation workshop, control the room temperature between 18 ℃ ～ 20 ℃, and control the relative humidity below 65%; ②Replace the ink with good ink quality
3. Plastic film thin printing and packaging is not suitable.
Reasons: â‘ Plastic film made of non-packaging resin; â‘¡ Insufficient opening agent in resin.
Countermeasure: Replace the plastic film.
4. Effect of static electricity. Reason: The plastic film generates static electricity, which forms adhesion between the films.
Countermeasure: Add antistatic stab to plastic.
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