In screen printing, when the screen template bounces off the substrate, the ink in the pattern part of the screen will follow the screen to form a fluffy "spider thread" that is pulled up and floats from the substrate. When the screen continues to be raised, this "spider thread" is scattered on the substrate before and after drifting with the wind, making the surface of the substrate dirty and the edges of the pattern not sharp. This phenomenon is very annoying and is not expected by any printing staff. This is a static phenomenon.
In optical disk printing, the so-called screen printing is a process in which the ink is transferred to the substrate by a relative movement of the screen and the squeegee under a certain squeegee pressure. The screen (screen) and squeegee (polyurethane squeegee) are both insulators. In the screen printing, they make relative scratches on the position and rub each other, especially when the humidity is low, it will generate static electricity and affect the surface charge of the substrate. Distribution status. In particular, insulating materials, such as PC substrates for optical discs, PET sheets for touch-opening, and PVC used in name and signage, PP and PE in plastic bottles, etc. They are basically insulators, and are not noticed during screen printing operations. Static electricity will be generated. For example, when decorating ink for screen-printed membrane switches, the PET film base is coated with a layer of PVC or other self-adhesive film to protect its surface so that it will not be scratched during cutting and packaging. However, the protective film must be removed before the screen printing after blanking. Some operators often remove the protective film very quickly when peeling the protective film. Who knows that at this instant when the film is quickly removed, the protective film is quickly separated from the PET sheet During the process, static electricity is generated especially on the surface of the PET sheet under low humidity. As a result, the ink on the edge of the handwriting on the silk screen has flying hairs, and the plush patterned ink filaments are deposited on the transparent PET sheet. This is a typical example, which caused great confusion for the silk screen workshop of the membrane switch unit.
1. Due to the existence of static electricity, the production difficulties caused by static electricity etc. in the printing process are integrated. The performance in screen printing is as follows:
1. After screen printing, the screen is raised, and the ink on the blot is pulled up together with the screen, and the phenomenon of "flying hair" -shaped ink thread is only broken after the workpiece is taken out;
2. During overprinting, the ink edges of the previous color and the latter color and the edges of the pattern repel each other, and they do not match well, so that the ink cannot be printed between the two color inks;
3. During screen printing, the workpiece is stuck to the screen, especially at the screen position where the scraper moves to the end;
4. The printing materials stick together. 5. During the printing process, the ink is difficult to spread on the screen, and the cake sticks together. 6. The printing material absorbs dust, and the ink marks are damaged, pitted, and the surface is rough Unclean
The above problems are mainly the presence of static electricity, but sometimes the static electricity is also related to the failure of the printing process to meet the requirements. For example: when the humidity is low and the amount of solvent in the ink is insufficient, on the one hand, the humidity in the air is low and the static electricity cannot be Through moisture transfer, at the same time, the ink solvent is low and the viscosity is large. With such a large viscosity ink, it will naturally produce wire drawing; if the printing speed is accelerated during the screen printing process, the squeegee and the screen plate will be strongly rubbed to generate static electricity. Especially under the condition of high tension screen, when the polyurethane rubber scraping pressure on the screen is large and the scraping speed is fast, it can most cause static electricity. It is often said that triboelectricity is the same.
Because of the existence of static electricity in screen printing, it can cause many quality problems and even make printing unable to continue. So how to avoid and eliminate static electricity in screen printing?
First of all, it is controlled from the process conditions of the screen itself. Under the high tension of the screen, the screen distance is reduced as much as possible. As the screen pitch decreases, the pressure of the scraper decreases, and the friction of the scraper against the screen reduces the possibility of static electricity generation. In the silk screen of the optical disc, in order to ensure the accuracy of the printed pattern and reduce the appearance of moire and overprint text on the pattern while avoiding the production of static electricity, the tension of the disc is as high as 26N / cm, and the screen pitch is as small as 1.0mm. If the screen printing may still show small static electricity under the above conditions, you can adjust the scraping speed to avoid it.
Matching the environmental conditions of screen printing, the better printing environment temperature of screen printing is 23 â„ƒ Â± 2 â„ƒ, relative humidity 60% ~ 70%, which is good for the control of the solvent volatilization speed in the solvent ink and the good control of the viscosity of the UV light solid ink , Stabilizing the viscosity of the ink, it can meet the printing suitability of the ink, and the printing suitability of the ink is good, it is impossible because of the ink itself. [next]
2. If the above process conditions are controlled, then the control methods for electricity are:
1. The workpiece itself is live. At this time, you can use an anti-static brush or anti-static cloth to de-electrically treat the surface before screen printing. The method is very simple: brush the workpiece with an anti-static brush or wipe the surface of the workpiece in one direction with an anti-static cloth, and print it after processing.
2. Screen-printed plastic parts are static charged. Antistatic agents can be added to plastic formulations in advance, which requires suppliers to produce plastic raw materials.
3. A method of applying antistatic paint can also be added on the surface of the static-charged workpiece to eliminate static electricity.
4. Reasonable use of ink solvents. Even if the mixing performance of the ink solvent is good, the solvent with good conductivity can adjust the viscosity of the ink.
5. Increase the grounding of the workpiece from the screen printing equipment to discharge its charge and eliminate static electricity.
6. In the screen printing workshop without humidity control measures, during the low humidity season, the surplus increases and splashes water on the ground, and the humidity of the screen printing environment is adjusted during the evaporation of water on the ground. This method is often used in simple production units without humidity control equipment. In addition to humidity control, it also has the role of preventing dust in control.
In short, the generation and elimination of static electricity should be analyzed according to specific conditions and conditions and corresponding measures should be made, so that the printing work can be continued conveniently and smoothly under the guarantee of product quality.
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