Squeegee seems simple, in fact, it is a quite complicated part of screen printing. In other methods of printing, the tools for ink transfer are doctor blade, ink squirt, pressure squirt and glue head. Each tool has its own unique function. In screen printing, the function of glue scraping mainly has the following four aspects:
1. Transfer the ink to the substrate through the screen.
2. Scrape off excess ink on the silk screen.
3. Keep the silk screen in contact with the substrate line.
4. Can print substrates of different shapes.
Only when the above four functions of the rubber squeegee are well exerted can high-quality printed matter be printed. The physical properties of the scraper play a decisive role in the above functions. (For example, a rubber squeegee with high hardness can scrape off the excess ink well, but it can not meet the requirements of various substrate shapes well). Therefore, when selecting the squeegee, we must fully consider the influence of the squeegee performance factors on the above functions to avoid adversely affecting the printing effect. When selecting the appropriate performance of the rubber squeegee, the following four factors need to be considered:
1. Hardness: The rubber scraper is usually made of polyurethane, and the hardness of the rubber scraper is measured with a hardness tester. The hardness, size and shape of the squeegee determine the flexibility, flexibility and pressure of the squeegee. Hardness ranges from 55 degrees to 90 degrees (Shore A). The hardness value is measured with a Shore A hardness tester. 55A-65A is a low-hard rubber scraper, 66A-75A is medium-hard, and more than 75A is high-hard.
High-hardness squeegee is very beneficial to the first three functions of squeegee. It presses the ink into the screen and transfers the ink from the fine screen to the substrate, and keeps the screen in contact with the substrate. The only defect of high-hardness rubber squeegee is that it can not meet the printing of substrates of different shapes, and the quality requirements of the rubber squeegee are also very high when printing rough and uneven surfaces.
2. Effective width: refers to the width of the rubber scraper that is inserted into the aluminum scraper handle and extends from the aluminum scraper handle. This width is important and determines the curvature of the rubber scraper under pressure. The curvature is the cubic value of the effective width. If twice the effective width is obtained, the curvature is greater than 23 = 8.
The squeegee bending will cause two changes: the angle between the squeegee and the screen changes; the printing pressure transmitted to the substrate is reduced. For example, the rubber squeegee is compared to a group of springs. When the spring is stressed at a certain angle, the spring begins to bend, and the transmitted force becomes smaller and smaller. The printing angle of the rubber squeegee is reduced, the printing pressure is insufficient, and the printing effect is not ideal. The printing angle is small, the printing pressure is reduced, the ink volume is large, and the ink layer is thickened. (In this case, the printing operator will increase the printing pressure, the curvature of the rubber scraper increases, and the resulting ink layer is still not ideal. The solution is to increase the hardness of the rubber scraper and increase the angle of the rubber scraper.) This is the effective of the rubber scraper The effect of width on the four functions of glue scraping.
Usually the effective width of the rubber scraper is between 20MM and 30MM. The selection of the best effective width is based on the flatness of the substrate, the resolution of the screen image and other printing parameters such as: squeegee angle, speed and ink type. When printing high-resolution images on flat / smooth substrates, the effective width is small (less than 20MM), high hardness, and fast printing with suitable inks. On uneven and rough substrates, the effective width should be increased (sometimes greater than 30MM), the hardness is reduced, and the adaptability of the low-hardness rubber scraping is poor, and it cannot be excessively bent. You can also use a three-layer scraper or two-layer scraper with soft sides in the middle scratch. Generally, as the effective width increases, the printing speed decreases and the printing angle is increased.
Squeegee shape:
The shape of the rubber scraper refers to the cross section of the rubber scraper. Most rubber squeegee shapes are simply square squeegees. Other shapes are used for specific printing conditions. For example, hard glue scraping performance is better than soft glue scraping, but poor adaptability to substrates. Using diagonal scraping, the printing edge is easily deformed (similar to soft rubber scraping).
There are three shapes of rubber scrapers for printing workers to choose: square scrapers (or right-angle scrapers), various diagonal scrapers and round scrapers (horn scrapers). The square squeegee has a wide application range and can provide the maximum printing pressure. The printing angle depends on the effective width. Oblique scraping provides maximum adaptability, the printing pressure is reduced, and the printing angle is smaller than the set angle. The adaptability of the round scraper is the worst, the printing pressure provided is the smallest, and the printing angle can be set freely (often the smallest angle). The above reasons will affect the amount of ink.
blade:
The knife edge is an important part to complete the four functions of the rubber scraper. The sharpness of the squeegee is a factor that initially controls the amount of ink. The sharp doctor blade provides the smallest ink drop and the finest image. The blunt and round edge of the knife increases the amount of ink, and it also affects the color and line fineness.
When printing large areas and simple patterns with transparent oil, the change in color and line fineness is not obvious. When printing fine lines and four-color dots, using translucent (UV) and transparent ink, you will quickly find the deformation of the knife edge. Compare the first and last prints to determine when the glue should be scraped. Polished.
Since the selection of rubber squeegee is an important part of printing quality control, the maintenance of rubber squeegee is also an important part of ensuring the normal use of rubber squeegee. The best way to ensure the quality of the rubber squeegee is to clarify the maintenance before and after printing and how to polish the rubber squeegee through a clear process.
1. Store the rubber scraper flat and do not roll it in a loop, which may cause permanent bending.
2. The aluminum scraper handle should be placed downwards, and the blade should not be placed downwards.
3. Clean the squeegee immediately after printing, at this time it is easy to wash off the ink, otherwise wipe the dry ink will damage the knife edge.
4. Do not immerse the rubber scraper in the solvent. Although the squeegee is solvent resistant, the squeegee becomes brittle due to absorption of the solvent.
5. The rubber scrape just after cleaning is still soft, the solvent is not completely volatilized, and it cannot be polished immediately. After 12 hours to 24 hours of "rest", it can be used again. This can not only improve the polishing effect but also improve the printing effect.
6. According to the specific printing quality requirements, the rubber scraper should be polished frequently. When cleaning and polishing, the loss of the rubber scraper should be reduced as much as possible.
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