After printing on plastic film or cellophane paper, the surface of imprinting has the appearance of dryness, but after printing the drum or bag, the back surface will stick dirty during storage. When it is serious, the inks cannot be torn apart, and the ink layer of pictures and texts is transferred and dyed, resulting in the scrapping of the products. This phenomenon is more likely to occur in high temperature season, but high temperature is not the only reason for sticking back. Concave adhesion is also closely related to a series of factors such as plastic, ink, solvent, air volume, light, speed and so on.
1. The solvent did not volatilize completely
① Plastic film printing ink is dried by solvent evaporation. The volatilization rate is an important factor that determines the drying condition of ink layer and affects the printing quality. The slower the solvent volatilization is, the better the reproduction of the plate is, the more beautiful the printing color is, but it is easy to adhere; on the contrary, white imprinting will occur. Therefore, it is the key to choose the solvent with appropriate evaporation speed for gravure printing of plastic film. In continuous register printing, the solvent must be completely volatilized within the running time between two printing stations, otherwise, the roller will stick when printing in the next station.
② After film printing, the organic solvent in the ink is not completely volatilized in the drying system, and there is still residual heat after the film is rolled up, which brings the solvent remaining in the ink layer with the condition of continuous volatilization, thus forming adhesion.
③ In high temperature and humidity season, the film is too tight or compressed after printing.
Countermeasures: the volatile speed of solvent in ink, not only depends on the boiling point of solvent, vapor pressure, vapor latent heat and so on. Also depends on the operating environment temperature, humidity, air volume, solute and ink thickness and other conditions. Therefore, according to the change of conditions, the volatile speed of solvent in the ink should be adjusted at any time. In general temperature, if the ink layer dries too fast, the solvent with slow evaporation (such as Shanghai 74-92 type plastic film slow drying thinner) should be added, otherwise, the solvent with fast evaporation (such as 74-91 plastic film fast drying thinner) should be added. It can also be diluted with mixed solvents of xylene, ethanol and isopropanol (the three solvents are mixed at 1 / 3 ratio, and it is better to use single solvent than mixed solvent). If you feel the drying is too fast, you can use a small amount of butanol to replace part of the ethanol (butanol can increase the ink luster, but you must pay attention to the appropriate amount, so as not to affect the printing drying.
The solvents used in gravure printing of plastic film are mainly alcohol and benzene. Ethanol and isopropanol are used for alcohols, toluene and xylene are used for benzene. Due to the need of nitrocellulose, esters, such as ethyl acetate and butyl acetate, should be added.
Comparison of volatilization rate of common solvents
Solvent name temperature (℃) evaporation rate (s)
Toluene 3031
Xylene 3070
Isopropanol 3055
Ethanol 3047
Butanol 30147
Ethyl acetate 3018
Butyl acetate 3070
In short, we should strive to ink in the organic solvent before winding up to be completely volatile. After the film printing, it is loosely put into the iron wire textile basket, so that the ink layer can continue to ventilate and dry, and then cut and bag after curing. After the film printing bag is made, it is lightly bound and placed in the carton vertically to reduce the pressure between the film surfaces.
2. Poor ink quality
Some plastic gravure inks have low melting point and are easy to adhere when operating at high temperature and high relative humidity.
Countermeasures: (1) when conditions permit, install air conditioner in the operation workshop, control the room temperature between 18 ℃ and 21 ℃, and control the relative humidity below 65%; (2) change the ink with good ink property.
3. Poor film printability
Reasons: 1. Plastic film processed by non packaging resin; 2. Insufficient opening agent in resin. Countermeasures: replace plastic film.
4. Static electricity
Cause: static electricity of plastic film products, forming adhesion between films.
Countermeasure: add antistatic agent to plastics
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