Process introduction
Screen printing, that is, screen printing, refers to the use of screen as the base of the plate, and through the method of photosensitive plate making, the screen printing plate graphic part of the mesh can pass through the ink, and the non-graphic part of the mesh can not pass through the ink. The principle is printed, and a screen printing plate with graphics and text is made.
Screen printing principle
The basic principle of screen printing is to use the basic principle that the mesh of the graphic part of the screen printing plate is transparent to the ink, and the mesh of the non-graphic part is impermeable to the ink. When printing, pour ink on one end of the screen printing plate, apply a certain pressure to the ink part of the screen printing plate with a scraper, and move to the other end of the screen printing plate at the same time. The ink is squeezed onto the substrate by the scraper from the mesh of the graphic part during the movement. Due to the viscosity of the ink, the imprint is fixed within a certain range. During the printing process, the squeegee is always in line contact with the screen printing plate and the substrate, and the contact line moves with the movement of the squeegee. A certain gap is maintained between them, so that the screen printing plate during printing generates a reaction force to the squeegee through its own tension. This reaction force is called rebound force. Due to the effect of resilience, the screen printing plate and the substrate are only in moving line contact, while other parts of the screen printing plate and the substrate are separated. The ink and the screen are broken, which ensures the printing dimensional accuracy and avoids smudging the substrate. When the squeegee scrapes the entire layout and lifts up, the screen printing plate also lifts up, and gently scrapes the ink back to the initial position, so far it is a printing stroke.
Screen printing consists of five elements, screen printing plate, squeegee, ink, printing table and substrate.
The graphic precision obtained by the screen printing method is lower than that of the actinic method, but the process is simple, the production efficiency is high, and the cost is low, and it is suitable for mass production of products with low requirements.
Advantages of silk screen printing
(1) Not limited by the size and shape of the substrate
General printing can only be done on a flat surface, while screen printing can not only print on a flat surface, but also print on a special shape such as a spherical surface.
(2) The layout is soft and the printing pressure is small
The wire mesh is soft and elastic.
(3) The ink layer has strong coverage
It can be printed in pure white on all black paper, with strong three-dimensional effect.
(4) Suitable for various types of ink
(5) Strong optical rotation resistance
The gloss of the printed matter can be kept unchanged. (air temperature and sunlight have no effect). This makes printing some self-adhesives without the need for additional lamination and other processes.
(6) Flexible and diverse printing methods
(7) Plate making is convenient, the price is cheap, and the technology is easy to master
(8) Strong adhesion
(9) It can be screen printed by hand or machine printed
(10) It is suitable for long-term display, and the outdoor advertising is expressive
Process abnormality and analysis
1. The pattern is not clear
Reasons: Improper selection of ink or quality problems in the ink itself; too fast printing of ink, making the printing patterns or lines unclear, resulting in incomplete patterns or broken lines; the viscosity of the ink is too high, resulting in incomplete patterns during printing Or the line breakage phenomenon; the phenomenon of jamming in printing, which affects the normal ink flow of the ink; the pressure of the scraper is insufficient or uneven during printing, resulting in uneven ink flow; the surface of the substrate is uneven.
Solution: replace the appropriate ink; use a slower drying solvent to prepare the ink; dilute the ink or add a thixotropic agent to reduce the viscosity; correctly select the slow drying solvent to dilute the ink, and pay attention to the ink-passing performance of the printing screen; Increase the strength of the scraper, and pay attention to the uniformity of the scraping force; deal with the surface of the substrate (such as printing a rough-grained substrate, you can use varnish or other materials as a primer before printing).
2. Appears obvious reticulation
Reason: the ink is too fast drying; the viscosity of the ink is too high and the fluidity is not enough; the mesh used is too coarse.
Solution: use a slower drying solvent to prepare the ink; dilute the ink or add a thixotropic agent to reduce the viscosity and improve the fluidity of the ink; use a higher mesh yarn.
3. Poor ink drying
Reasons: Improper selection of solvent will corrode the substrate (especially when printing soft PVC or coated nylon cloth); the solvent is too slow to dry or the drying time is insufficient.
Solution: choose a solvent suitable for the substrate; choose a faster drying solvent or prolong the drying time.
4. Poor ink adhesion
Reasons: The selected ink is not suitable for the substrate; the surface of the substrate is not clean, with oil stains or surface treatment agents (such as: preservatives/release agents, etc.); some special materials, due to too low surface tension, will also affect the adhesion of the ink. Insufficient drying time, the ink is not fully cured; too much additives or improperly added additives; too much additives or improperly added inks.
Solution: Correctly select the ink suitable for the substrate; treat the surface of the substrate or replace the substrate; prolong the drying time so that the ink can be completely cured, especially for some inks with chronic cross-linking reaction (such as PET, PP, PE, etc., It must be cured for a long time to have good adhesion; the correct addition of relevant additives, too much defoamer will affect the adhesion.