Since UV ink printing is done by surface adhesion rather than oxidation or penetration, the amount of surface tension is very important. Since the plastic surface is a non-polar and incompatible interface, even solvent-based inks are difficult to adhere to. In addition, since the additives in the plastic often float to the surface, there may be situations where an actual connection cannot be achieved.
There are three types of plastic surface treatment methods:
1. Corona treatment
It is an electric shock treatment that uses high voltage and high frequency to generate electric shocks from the ground and the induced air nozzle, respectively. No current passes between them until the voltage reaches 3000-5000 volts/square centimeter. The electric shock molecules are ejected from the air nozzle, carrying high-energy free electrons and accelerating to the positive electrode. This is to change the polarity of the material and increase the surface roughness.
The dyne value of the printed material after corona treatment is not the higher the better. The normal dyne value is more suitable when it differs from the ink by about 10. People often ask, why can’t printing be done smoothly when the dyne value is already very high? One of the reasons is that many printed materials contain a lot of additives, and these additives will float to the surface of the material due to high temperature, causing printing failure. The surface tension value of the material treated in this way will gradually decrease with the increase in days, so it is recommended to test it with a dyne pen before printing.
2. Primer treatment
Using a layer of film as a medium between the ink and the printed material, it is normally used for materials that cannot be corona-treated, such as PP. Its treatment methods are divided into online PRIMER and offline PRIMER treatment. The thickness of this film has a great relationship with whether the printing is smooth or not. Too thick will easily cause waste, and too thin will easily cause printing failure.
3. Flame treatment
It is carried out by the flame method, is generally suitable for irregular shapes or high-speed (1000 m/min) film treatment, and is used in bumpers and blow-molding tables.
Usually, the special plastic material treated by these three methods can significantly improve the adhesion of the pattern.