Squeegee knife--The characteristics and application of metal knife and rubber knife
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The structure of the squeegee is very simple, which makes it often overlooked in screen printing. But in fact, the squeegee plays an important role in printing. The squeegee is divided into a metal knife, a rubber knife and a combination knife. This article will mainly introduce the characteristics and applications of metal knives and rubber knives.
First, the metal knife
1. Pad printing squeegee
The quality evaluation indexes of the scraper mainly include hardness, thickness, elasticity, and straightness of the knife edge. Generally, a 0.25mm thick flat steel strip with good elasticity is used as the scraper, and the cutting edge should be sharply scraped, smooth and straight, and can be closely matched with the steel plate surface; if the scraper edge is not sharp or defective, the scraping will be dirty. Adhere to the edge of the pattern or create a scraper ink mark, which will severely scratch the streaks. The scraper should be at an angle to the printing plate to prevent the squeegee from falling into the concave portion of the printing plate, and excessively squeegee damage the pattern; if the blade is found to be in uneven contact with the printing plate, it should be adjusted tightly; the length of the squeegee should be greater than The pattern text is close to the width of the steel plate, and the pressure of the blade should be as small as possible under the premise of scraping the ink on the non-image portion of the printing plate, and the wear loss is small.
2. SMT scraper
SMT metal scrapers are also commonly used. With the use of more closely spaced components, the amount of metal scraper is increasing. They are made of stainless steel or brass and have a flat blade shape with a printing angle of 30 to 45°. Some scrapers are coated with a lubricating material. Because they use lower pressure, they don't dig out solder paste from the holes, and because they are metallic, they are not as easy to wear as rubber scrapers, so they don't need sharpness. They are much more expensive than rubber scrapers and can cause stencil wear.
Second, rubber knife
1. SMT rubber and plastic scraping knife
When using a rubber blade, the type of blade selected is different in the printed circuit assembly (PCA) of standard components and dense components. The amount of solder paste is also very different. The fine pitch components require much less solder than the standard surface mount components. The pad area and thickness also control the amount of solder paste. A squeegee having a hardness of 70 to 90 was measured using a rubber hardness meter. When excessive pressure is applied, solder paste that penetrates into the bottom of the stencil may cause a tin bridge that requires frequent bottom rubbing. In order to prevent bottom penetration, the pad opening must have a sealing effect when printed. This depends on the roughness of the aperture wall of the stencil. The degree of wear, pressure and hardness of the scraper determine the quality of the print and should be carefully monitored. To get acceptable print quality, the edges of the blade should be sharp and straight. Low blade pressure can result in missing and rough edges, while a blade with a high or soft blade will produce a speckled print that can even damage the blade and the stencil or screen. Excessive pressure also tends to dig out the solder paste from the wide opening, causing insufficient solder fillet.
2. Printing squeegee
A rubber band of 50% reclaimed rubber is generally used. The thickness, hardness and width of the scraper as well as the elasticity and the pressure withstand have a great influence on the printing quality. The thicker scraper has high hardness, poor elasticity and large blade pressure, suitable for printing fine points and lines; thinner rubber scraper elasticity Large, suitable for printing map texture; too narrow rubber squeegee elastic pressure adjustment room is small, but the thin and wide rubber squeegee is soft and weak in printing, the edge of the pattern is not smooth, the layout color is not clean, affecting printing effect. The angle of the scraper should be appropriate, and it is suitable for the reverse side of the screen to be impervious, and the front and the dry plate are not suitable. Different angles of scraper and squeegee angle should be selected according to different fabrics and different pattern structures. Generally, the angle of the blade of the scraped pattern should not be too large, and the angle of the edge of the scraped pattern can be larger. The hardness of the scraper and the countertop has a great influence on the mesh printing. Dot printing requires high definition, non-textile surface is smooth, texture is crisp, not easy to deform, dot definition is guaranteed; and textiles are both hygroscopic, surface is not smooth, and there are fabric holes, how to print the dots clearly, The proficiency of scratching is very important. The scraper strip should have a straight and sharp edge, and the round mouth should not be used. The 90° blade strip is the most widely used. Polyurethane scrapers are better than rubber scrapers. The blade with the cutting edge is strong, the embossing line is finer and more solid, and the screen is quickly rebounded. After the ink is scraped, the screen is clean and the reproducibility of the dot is good.
3. Container printing squeegee center point setting
When the scraper is installed, the line passing vertex must coincide with the diameter of the substrate. If this condition is not met, even if other conditions are met, a good printing effect cannot be obtained, and an unqualified printed product may be produced. The setting of the center of the blade is an important factor in the surface printing conditions. The printing pressure must be kept to a minimum. Especially in the surface screen printing, if the printing pressure is too large when printing a plastic drum, the blade condition will be deteriorated, which has a bad influence. Generally, the curved surface printing uses a sword-shaped scraper rubber, and the flat printing uses parallel scraper rubber. Sword-shaped scraper rubber is used for curved printing and is determined by the content and purpose of printing. In short, the use of scraper rubber in accordance with the purpose of printing is not necessarily limited to the difference between the surface and the plane.