(1) Viscosity
Viscosity, also known as internal friction, is the resistance produced when one layer of fluid moves relative to another layer of fluid. It is a property within a fluid that impedes its flow. Ink viscosity is expressed in Pa·s. The viscosity of screen printing ink is about 12Pa.s.
The ink with too high viscosity has poor wettability to the substrate, and it is not easy to transfer to the substrate through the screen. It makes screen printing difficult and imprinting is lacking. If the viscosity is too small, the imprint will expand, and the lines of the screen printing product will merge and become waste products. Viscosity indicators can be measured using a viscometer.
The relationship between viscosity change and printing suitability is: the more stable the viscosity of the ink on the printing plate, the better, but after transferring to the printed piece, the faster the viscosity becomes, the better. Thixotropy is unfavorable to the former and beneficial to the latter, so appropriate thixotropy is desirable, and surface shear thickening is harmful to screen printing. Adding solvents, diluents or plasticizers can reduce the viscosity; adding fillers, pigments and silicon compounds can increase the viscosity.
(2) The influence of ink viscosity on the graphics and texts of Indian dramas
In screen printing, adjusting the proper ink viscosity has a direct impact on printing quality. The viscosity of the oil is different, and the graphic effect after printing is also different. Generally, the ink has three conditions: high viscosity, low viscosity, and moderate viscosity. Different viscosities have different printing effects, as shown in the figure.
①When the viscosity of the ink is too high, the printing leakage is poor, and the ink is difficult to transfer to the substrate, making screen printing very difficult. As a result, the amount of ink received on the graphic part of the substrate is very small and uneven, making the graphic incomplete and not full. Products after screen printing can only be waste products.
②When the viscosity of the ink is too small, although the missing printing property is good, the phenomenon of ink diffusion in the scraping direction of the scraper is serious, which makes the lines of the graphic and text thicker, and the parts with close distances between the lines will also merge, which also fails to meet the printing quality requirements.
③When the viscosity of the ink is appropriate, the surface of the printed ink is flat, the edges of the graphics and text are clean and tidy, and the printing size is accurate.
In order to overcome the adverse results caused by high or low viscosity in printing, it is usually solved by adjusting the viscosity of the ink and using scrapers with different hardness. When the ink viscosity is high, a soft scraper can be used; when the ink viscosity is low, a hard scraper can be used. Or changing the scraping angle can also achieve good results.
Generally, the following basic principles can be used to check whether the viscosity of the ink is appropriate :
The principle of judging the low viscosity of the ink is: put the ink on the screen printing plate, let the ink flow freely in the graphic mesh part without leaking from the mesh part without pressure.
The principle of judging the high viscosity of the ink is : use a scraper to scrape the ink onto the substrate through the screen printing plate. After drying, the surface of the ink layer has no traces of the screen, but a smooth plane.
