Common problems and countermeasures in screen printing (3)

V. Pinhole

The phenomenon of pinholes is the biggest headache for screen printing workers. If it is the printing of opaque objects such as billboards and thick paper, such holes that are not easily observed are generally not a problem. However, when precise printing is performed on aluminum plates, glass, and acrylic plates, post-processing and etching processes are not allowed to generate pinholes. In addition, the reasons for the occurrence of pinholes are also varied. Many of them are reasons that cannot be explained at present, and some are still problems with quality management. Pinholes are one of the most important inspection items in the inspection of printed products.

1 Dust and foreign objects attached to the plate. At the time of plate making, there are some sols mixed into the water wash image. In addition, when the emulsion is applied, dust may be mixed in, and pinholes may be formed on the screen. These tests can be found and repaired promptly if they are checked. If dust and foreign matter adhere to the screen, blocking the screen openings can also cause pinholes. Before the official printing, if the use of strong ink absorption paper, after several printing, you can remove the dust from the plate.

2 substrate surface cleaning. Aluminum plates, glass plates, acrylic plates, etc. should be pre-treated to make their surfaces clean before printing. After the substrate has been pretreated, it should be printed immediately. In multicolor printing, a method of rubbing with alcohol prior to printing is generally used. In addition, semi-automatic and fully automatic ultrasonic cleaners can also be used. After pre-treatment, dirt such as grease can be removed, and dust attached to the surface can also be removed.

Washing machine detergent is often mixed with broken fibers, this detergent is soluble in alcohol, when cleaning the surface of the substrate, it tends to form a thin surfactant film, when the ink is printed on the film pinholes occur. Therefore pay special attention when using cleaning agents and alcohol. When carrying the substrate by hand, the fingerprint of the hand will also adhere to the printed surface, forming pinholes during printing.

Six, bubbles

Substrate will appear bubbles on the ink after printing. The main reasons for bubbles are as follows:

1 The substrate is poorly processed before printing. Substrate attached to the surface of dust and oil traces and other substances.

2 bubbles in the ink. In order to adjust the ink, adding solvents and additives to stir, the ink will be mixed with some bubbles, if placed regardless of the low viscosity of the ink will naturally deaeration, high viscosity inks and some can not be natural deaeration. Some of these bubbles are naturally eliminated in the printing process due to the transfer of ink, while others become larger and larger. In order to remove these bubbles, defoamers should be used. The amount of defoamer added in the ink is generally about 0.1 to 1%. If the amount exceeds the prescribed amount, it will have a foaming effect.

After the ink is transferred, even if it is foamed, as long as the wettability of the substrate and the ink flowability are good, the bubbles on the surface of the printing ink film will be gradually eliminated, and the ink forms a flat printing ink film. If ink bubbles are not eliminated, the ink film will form a ring-shaped uneven film surface. Bubbles in the general ink can be degassed by the action of the screen when passing through the screen. In addition, the use of hot water or boiling water will have a better defoaming effect when the ink is mixed and stirred.

For transparent inks such as red, blue, and green, the linking material of these inks has the feature of easy foaming due to the small proportion of organic pigments in fine particles. If the corresponding thinner, tackifier or release agent is added, the ink can also be converted into a stable ink with good printability.

3 The printing speed is too fast or the printing speed is not uniform. Appropriately reduce the printing speed and maintain the uniformity of printing speed.

If none of the above measures can eliminate air bubbles in prints, consider using other types of ink.

Seven, net marks

Screen marks sometimes appear on the surface of the ink film. The main cause of the screen marks is the poor fluidity of the ink. During the screen printing process, when the printing plate is lifted, the ink transferred to the printing material fills the screen traces with its own flow, making the surface of the ink film smooth. If the ink has poor fluidity, when the screen printing plate is lifted, the ink flow is relatively small, and the screen marks cannot be filled, and a smooth and smooth ink film cannot be obtained. To prevent the appearance of screen marks on prints, the following method can be used.

1 Use a highly fluid ink for printing.

2 can consider the use of slow drying ink printing, increase the ink flow time so that the ink gradually exhibited and cured.

(3) Use silk finer wire mesh when making plates.

(to be continued)