Causes of printing ink drying problems!

Causes of printing ink drying problems!

In printing, the problem of poor ink drying or too fast ink drying often occurs. To solve this problem, we must first find out the reason.

① The ink layer is too thick

In the process of large-area solid printing, the ink layer on the surface of the printed product is completely dried and the bottom layer is not completely dried during the drying process due to the large amount of ink and the thick ink layer, that is, the phenomenon of false drying. Except that the ink layer is too thick, too fast paper feeding speed and insufficient UV light irradiation time can also cause false drying of UV ink. Once the phenomenon of false ink drying occurs, it is easy to cause the ink layer on the surface of the printed product to fall off during the subsequent processing. This is because the incompletely dried ink bottom layer has poor bonding fastness to the surface of the substrate, causing the ink layer to be scraped and peeled off under the action of friction.

Solution

For jobs that use spot color UV ink to print on the ground, try to match the color of the spot color UV ink as darker as possible, so that during the printing process, the ink bottom layer can be completely dried by deep ink thin printing to increase its contact The bonding fastness of the substrate surface. At the same time, it is necessary to ensure a suitable paper speed and sufficient UV light irradiation time. If the paper speed is faster, the power of the UV lamp must be increased to ensure the ink drying effect.

② Insufficient UV lamp power

Insufficient UV lamp power is also one of the reasons for the failure of UV ink drying, which is related to the daily production operations of the company, mainly in the following aspects:

1. All UV lamps have an effective service life (800-1000 hours in general). When the use time of the UV lamp exceeds its service life, although the UV lamp can continue to work, its power has been greatly reduced.

2. After replacing the UV lamp, the use timer of the UV lamp was not reset, resulting in the use time of the UV lamp has exceeded the service life without knowing it at all.

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

4. The UV lamp power configuration is unreasonable. For example, the reasonable configuration of the UV lamp power of each group of domestic full-rotation adhesive label printers is basically above 3000 watts. Because of the faster printing speed of foreign equipment, the power of each group of UV lamps is basically More than 7000 watts, and some full-rotary label printers have only 2000 watts per set of UV lamps.

Solution

First of all, a complete and effective UV lamp use management system must be established to ensure that the UV lamps work within the effective service life, promptly replace the UV lamps that have exceeded the service life, and pay attention to the use of UV lamps for the units that have replaced the UV lamps. The timer is reset to zero; secondly, the reflector of the UV lamp should be cleaned regularly to reduce the loss of its reflected energy; furthermore, the power of the UV lamp should be reasonably configured according to the structural characteristics of the equipment. 

③ The ink is beyond the shelf life or the additives are excessive

Under normal circumstances, if the ink shelf life is exceeded, the photoinitiator in the ink may fail to cause poor drying after printing. In addition, in printing production, in order to improve the performance of the ink, it is often necessary to add additives such as varnish and detackifier to the ink. If the proportion of additives is too high, it may cause the ink to be too thin and cause poor drying. UV ink itself has thixotropy, that is, it becomes thinner and thinner under the action of the shearing force of the ink roller. Moreover, after the printing equipment has been running for a period of time, the ink rollers will generate heat due to friction, thereby reducing the viscosity of the ink.

Solution

First of all, ensure that the ink is used within its shelf life; secondly, pay attention to the amount of ink auxiliary and desiccant. It is recommended that the proportion of UV ink additives should not exceed 3% in summer and 8% in winter.

④ UV inks of different colors have different drying degrees

Tests have shown that UV inks of different colors have different spectral absorption, so under the same power UV lamp, the drying degree of different colors of UV inks is different. For example, white UV ink can directly reflect light, black UV ink will diffusely reflect light, and magenta UV ink has a better light absorption effect.

According to experience, the difficulty of drying four primary color UV inks is black, cyan, yellow and magenta in order from difficult to easy, while gold and silver UV inks are more difficult to dry than four primary color UV inks. White UV inks reflect light. The effect is also difficult to dry.

Solution

According to the drying characteristics of different colors of UV inks, the printing color sequence should be arranged reasonably. The inks that are more difficult to dry are printed first, and then the inks that are easier to dry are printed. This can solve the problem of poor ink drying to a certain extent.

⑤ The density of the rubber roller is too low

In actual production, new equipment is more prone to ink drying failure. After careful inspection, it was found that it was because the rubber roller with too low density absorbed the photoinitiator in the UV ink, which resulted in poor drying of the ink.

The drying of UV ink is mainly achieved by the chemical reaction between the photoinitiator and the resin. Among them, the molecular structure of the resin is larger, and the molecular structure of the photoinitiator is smaller. If the rubber roller density is too low, many small holes will appear on its surface. Because the diameter of these small holes is larger than the diameter of the photoinitiator, part of the photoinitiator in the UV ink will be absorbed by the rubber roller, causing the UV ink to dry During the process, the chemical reaction between the resin and the photoinitiator was insufficient, resulting in poor drying of the ink, and at the same time, the phenomenon of ink drying 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 12-24 hours. At this time, the rubber roller will gradually absorb the UV varnish. After being saturated, it is difficult to absorb the photoinitiator in the UV ink.

⑥ Other reasons

The bright resin ink used in lithographic printing, under normal circumstances, the print transferred to the paper by the blanket can usually be dried within a few hours. However, in daily production, it is often encountered that the blot is too slow to dry, so the following reasons need to be investigated:

1. Too much water supply?

2. The pH of the fountain solution is too low?

3. The temperature in the printing workshop is too low or the humidity is too high?

4. Poor absorption of paper?

Solution

When the above problems occur, we must strictly control the size of the water on the page and master the pH value of the fountain solution. When the print is not dry, you should open the print in time to ventilate the paper so that air can enter the paper to speed up the conjunctival time.

⑦ The ink dries too fast

When printing on a single-color offset press, the print of the previous color is often dried too fast, forming "crystallization", making the next color not overprinted. The main reason is that the dry oil is added too much or the semi-finished product is stacked for too long. This phenomenon is again It often occurs when the yellow plate is printed or the light background is full.

Solution

If this problem occurs, if the number of products that cannot be overprinted is small, you can use a clean soft cloth dipped in magnesium carbonate powder, and gently wipe the surface of each printed product to make the surface rough; irradiate with infrared rays to cause the ink layer to expand due to heat Coarse; if the quantity is large, the humidity can be treated to make the ink layer swell and swell, and the surface will be roughened.