Analysis of the causes of poor drying of label printing ink

Reason one: the ink layer is too thick, the paper feeding speed is too fast or the UV light irradiation time is insufficient. In the large-area solid printing process, since the ink amount is too large and the ink layer is too thick, the ink layer on the surface of the printed product will be in the drying process. There is a phenomenon that the surface layer is completely dry and the bottom layer is not completely dried, that is, the phenomenon of dry drying. In addition to the ink layer is too thick, the paper feed speed is too fast, and the UV light irradiation time is insufficient to cause the UV ink to dry. Once the ink dry phenomenon occurs, it is easy to cause the ink layer on the surface of the printed product to fall off during the post-processing. This is because the poorly dried ink underlayer has poor bonding fastness to the surface of the substrate, so that the ink layer is scraped off by the frictional force.

Solution: For the live parts printed with spot color UV ink, try to match the color of the spot color UV ink with some ink, so that the bottom layer of the ink can be thoroughly dried by deep ink printing in the printing process. The combination of its adhesion to the surface of the substrate. At the same time, it is necessary to ensure a proper paper feed speed and sufficient UV light irradiation time. If the paper feed speed is faster, the power of the UV lamp must be increased to ensure the ink drying effect.

Reason 2: Insufficient UV lamp power The lack of UV lamp power is also one of the causes of poor UV ink drying failure. This is related to the daily production operations of self-adhesive label printing companies, mainly in the following aspects.

(1) All UV lamps have an effective service life (normally 800 to 1000 hours). When the UV lamp is used for longer than its service life, although the UV lamp can continue to work, its power has been greatly reduced. . For the cost of label printing companies, the UV lamps that have exceeded their service life have not been replaced in time.

(2) After replacing the UV lamp, the timer of the UV lamp is not reset to zero, so that the use time of the UV lamp has exceeded the service life but is completely unknown.

(3) Do not pay attention to the surface cleaning of the UV lamp. After a long time of use, the reflector of the UV lamp is too dirty, causing the reflection energy loss of the UV lamp (reflected energy can account for about 50% of the power of the UV lamp).

(4) The power configuration of the UV lamp is unreasonable. For example, some of the all-round self-adhesive label printers have a power of only 2000 watts per group of UV lamps. This configuration is obviously off-center. It is understood that at present, the reasonable configuration of each group of UV lamp power in the domestic full-rotation sticker printing press is basically more than 3,000 watts. Because of the faster printing speed of foreign equipment, the power of each group of UV lamps is basically above 7000 watts.

Solution: First of all, self-adhesive label printing enterprises must establish a complete and effective UV lamp usage management system to ensure that UV lamps work within an effective service life, replace UV lamps that have exceeded their service life in time, and replace UVs. The unit of the lamp pays attention to zeroing the use timer of the UV lamp. Secondly, the reflector of the UV lamp is periodically cleaned to reduce the loss of reflected energy; furthermore, the power of the UV lamp is reasonably configured according to the structural characteristics of the device.

Reason three: The ink exceeds the shelf life or the ink additive is excessive

Under normal circumstances, the shelf life of UV inks is about 1 year. If the shelf life is exceeded, the ink may be poorly dried due to the failure of the photoinitiator in the ink after printing. In addition, in the printing production, in order to improve the performance of the UV ink, it is often necessary to add an additive such as a varnish and an viscous agent to the ink. If the proportion of the additive is too high, the UV ink may be too thin to cause poor drying. In fact, the UV ink itself has thixotropic properties, that is, it becomes thinner and thinner under the action of the ink roller shearing force. Moreover, after the printing equipment is operated for a period of time, frictional heat is generated between the ink rollers, thereby reducing the viscosity of the ink.

Solution: First, ensure that the UV ink is used during its shelf life; secondly, it is recommended that the ratio of UV ink additives does not exceed 3% in summer and does not exceed 8% in winter.

Reason 4: UV inks of different colors have different degrees of drying

Tests have shown that different colors of UV inks have different absorption properties for the spectrum, so under different UV lamps, the UV inks of different colors have different degrees of drying. For example, the white UV ink can directly reflect the light, the black UV ink will form a diffuse reflection of the light, and the magenta UV ink absorbs the light better.

According to the author's experience, the difficulty of drying the four primary color UV inks is black, cyan, yellow and magenta in the order of difficulty, while the gold and silver UV inks are more difficult to dry than the four primary color UV inks. It is also difficult to dry with reflection.

Solution: According to the drying characteristics of UV inks of different colors, reasonably arrange the printing color sequence, first print the ink which is difficult to dry, and then print the ink which is easier to dry, so that the problem of poor drying of the ink can be solved to some extent.

Reason 5: The rubber roller density is too low

In actual production, the author found that the new equipment is more prone to poor drying of the ink. After careful inspection, it was found that the rubber roller in the UV ink absorbed the photoinitiator in the UV ink due to the low density, which caused the ink to dry poorly.

The drying of the UV ink is mainly achieved by a chemical reaction between the photoinitiator and the resin, wherein the molecular structure of the resin is large, and the molecular structure of the photoinitiator is small. If the density of the rubber roller is too low, many small holes will appear on the surface. Since the diameter of these small holes is larger than the diameter of the photoinitiator, part of the photoinitiator in the UV ink is absorbed by the rubber roller, causing the UV ink to dry. In the process, the chemical reaction of the resin and the photoinitiator is insufficient, resulting in poor drying of the ink, and accompanied by the phenomenon that the ink is dried on the surface of the rubber roller.

Solution: The easiest way is to replace the rubber roller; if it is too late to replace the rubber roller, you can also remove the rubber roller and soak it in UV varnish for 12 to 24 hours. At this time, the rubber roller will gradually absorb the UV varnish. When it reaches saturation, it is difficult to absorb the photoinitiator in the UV ink.