The structure of the doctor blade is very simple, which makes it often overlooked in screen printing. But in fact, the doctor blade plays an important role in printing. The doctor blade is divided into metal knife, rubber knife and combination knife. This article will mainly introduce the characteristics and applications of metal knives and rubber knives. 1. Metal Knife 1. Pad scraper The quality evaluation indicators of the doctor blade mainly include hardness, thickness, elasticity, straightness of the blade edge and so on. Generally, a 0.25mm thick flat steel belt with good elasticity is used as the scraper. Its blade should be sharp, smooth, straight, and can closely match the steel plate surface; if the blade is not sharp or damaged, it will cause scraping ink. It sticks to the edges of the pattern or produces ink marks from the blade, which will seriously scratch the printing plate. The scraper should be at a certain angle to the printing plate to avoid the scraper falling down into the recess of the printing plate, excessive ink scraping and patterning; if it is found that the scraper is not in contact with the printing plate, it should be adjusted tightly; The pattern text is close to the width of the steel plate. The pressure of the scraper should be as low as possible under the premise of scraping off the non-graphic part of the ink on the printing plate, and the wear and tear will be smaller. SMT metal scrapers are also commonly used. With the use of more closely spaced components, the amount of metal scrapers is increasing. They are made of stainless steel or brass, have a flat blade shape, and use a printing angle of 30 to 45 °. Some scrapers are coated with lubricating materials. Because they use lower pressure, they don't dig out solder paste from the openings, and because they are metallic, they don't wear as easily as rubber scrapers, so they don't need sharpness. They are much more expensive than rubber blades and may cause film wear. Second, rubber knife 1. SMT rubber scraper When using a rubber scraper, the type of scraper used is different between standard components and dense-foot components in printed circuit assembly (PCA). Demand for solder paste volume is also very different. Close-pitch components require much less solder volume than standard surface-mount components. Pad area and thickness also control the amount of solder paste. The scraper with a hardness of 70 to 90 was measured using a rubber durometer. When excessive pressure is used, the solder paste that has penetrated into the bottom of the diaphragm may cause a solder bridge, requiring frequent bottom wiping. To prevent bottom penetration, the pad opening must have a sealing effect during printing. This depends on the roughness of the aperture wall of the membrane. The degree of wear, pressure and hardness of the squeegee determine the printing quality and should be carefully monitored. To achieve acceptable print quality, the blade edge should be sharp and straight. Low doctor blade pressure will cause omissions and rough edges, while doctor blades with high doctor blade pressure or very soft will produce a spotty printing effect, and may even damage the doctor blade and the membrane or screen. Excessive pressure also tends to dig out solder paste from wide openings, causing insufficient solder fillet. 2. Printing squeegee Generally, 50% recycled rubber rubber strips are used. The thickness, hardness and width of the scraper, as well as the elasticity and the amount of pressure have a great influence on the printing quality. The thicker scraper has greater hardness, poor elasticity and large scraper pressure, suitable for printing fine ideas and line patterns; the thinner rubber scraper elasticity Large, suitable for printing maps; too narrow rubber squeegee has little room for pressure adjustment, but the thin and wide rubber squeegee is soft and weak when printing, the edges of the pattern are not smooth, and the printing paste is not clean, which affects the printing effect. The angle of the squeegee should be appropriate. It is appropriate that the screen does not penetrate the reverse side and the front side does not dry. According to different fabrics and different pattern structures, different angle scrapers and scraping angles should be selected. Generally, the blade angle of fine pattern should not be too large. The blade angle of coarse pattern can be larger. The hardness of the squeegee and table top has a great influence on the mesh tone printing. Dot printing requires high definition. The surface of non-textiles is smooth, the texture is firm, and it is not easy to deform. The definition of dots is guaranteed; while the textiles are hygroscopic, the surface is not smooth, and there are fabric meshes. How to print the dots clearly, The proficiency of scratch printing is very important. Squeegee strips must have straight and sharp edges, round edges cannot be used, and 90 ° edge strips are the most widely used. Polyurethane scrapers are better than rubber scrapers. The blade with a strong blade pressure is strong, and the imprint line is thinner and more solid, so that the silk screen rebounds quickly. When the doctor blade is installed, the apex of its neutral line must coincide with the diameter of the substrate. If this condition is not met, even if all other conditions are met, a good printing effect cannot be obtained, and unqualified printed products will be produced. The setting of the blade center is an important factor in the surface printing conditions. Imprint must be controlled to a minimum. Especially in curved screen printing, if the printing pressure is too large when printing a plastic barrel, it will deteriorate the doctor blade condition and have a very bad effect. Generally, curved blade rubber is used for curved printing, and parallel blade rubber is used for flat printing. The sword-shaped scraper rubber is used for curved surface printing and is determined by the content and purpose of the printing. In short, it is necessary to use a scraper rubber that is consistent with the purpose of printing, not necessarily limited to the difference between curved surface and flat surface. Our
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2. SMT scraper
3. Setting of the center point of the container printing squeegee