- Dec 10, 2017 -
Metal and rubber ink scraper characteristics and applications - The structure of the doctor blade is very simple, which makes it in the screen printing is often overlooked. But in fact the doctor blade plays an important role in printing. The doctor blade is divided into a metal knife, rubber knife and combination knife. This article will introduce the metal knife, rubber knife features and applications.
First, the metal knife
1. Printing squeegee
Scratch quality evaluation indicators are the hardness, thickness, flexibility, knife-edge straightness and so on. The general use of 0.25mm thick flexible flat strip for scraper, the edge should be front scraping, smooth, straight, with the steel plate surface closely match; scraper knife edge is not sharp or defects will cause scraping ink is not net and Adhesion in the edge of the pattern or scraper ink marks, the serious will be scraping the plate streaks. Scraper should be at a certain angle with the plate, to avoid scraping knife scraper clenched into the plate cavity, excessive ink damage and pattern; if scraper and plate contact with uneven, it is necessary to adjust to close; scraper length should be greater than Pattern text close to the width of the plate, the blade pressure should be scraped in the non-graphic printing plate on the premise of the ink under the pressure as small as possible, wear and tear will be small.
2. SMT shaving knife
SMT metal scraper is also commonly used. With the use of denser pitch components, the amount of metal scraper is increasing. They are made of stainless steel or brass and have a flat blade shape with printing angles of 30-45 °. Some scrapers are coated with a lubricant. Because they use less pressure, they do not dig solder paste out of the openings, but because they are metal, they do not wear as easily as rubber squeegees and do not require sharpness. They are much more expensive than rubber scrapers and can cause membrane wear.
Second, rubber knife
1. SMT rubber scratch knife
When using a squeegee, the type of squeegee that is selected is a difference in the printed circuit assembly (PCA) of standard and pad components. The demand for solder paste volume is also very different, dense pitch components require much less solder than standard surface-mount components, pad area and thickness also control solder paste volume. Using a rubber hardness tester measured 70 ~ 90 scraper. Paste that penetrates into the bottom of the film may cause the tin bridge to require frequent bottom wiping when using excessive pressure. In order to prevent the infiltration of the bottom, pad openings must have a seal on printing. This depends on the roughness of the open-cell wall. Scratch-knife wear, pressure and hardness determine the printing quality, it should be carefully monitored. To get acceptable print quality, the edge of the knife should be sharp and straight. Squeegee pressure can result in missing and rough edges, and squeegee blades that have high or very high squeegee pressure will produce a blob-like print that may even damage the squeegee and film or screen. Excessive pressure also tends to dig solder paste out of wide openings, causing insufficient solder fillet.
2. Printing squeegee
General use of 50% recycled rubber rubber strips. Scraper thickness, hardness and width, as well as the flexibility and pressure on the size of a great impact on the printing quality, thicker blade hardness, poor flexibility, squeegee pressure, suitable for printing fine ideas and lines pattern; thin rubber squeegee flexibility Large, suitable for printing real map pattern; Narrow rubber squeegee Poor flexibility Poor pressure regulation, but thin and wide squeegee in the printing soft and weak, the pattern is not smooth edges, the layout of the color paste income net, affect the printing effect. The angle of the scraper should be appropriate to reverse the screen impermeable, positive and not dry version as appropriate. According to different fabrics and different patterns of the structure of the choice of different angles of scraper and scraping angle. General scraping pattern edge angle should not be too large, coarse scratch pattern edge angle can be larger. Scratch and the hardness of the table mesh mesh printing a great impact. Dot printing requires high definition, non-textile surface smooth, crisp texture, not easy to deformation, network clarity is guaranteed; and textiles both hygroscopic, the surface is not smooth, and there are fabric mesh, how to print a clear dot, Scratch-printing proficiency is very important. Squeegee blade to have a sharp sharp edge, can not use the round mouth, 90 ° edge strip the most widely used. Polyurethane scrapers are better than rubber scrapers. Blade with a blade pressure, pressure line more detailed and more realistic, so that the rapid rebound screen, after scraping the ink version of the net, the network of good reproducibility.
3. Container printing doctor set the center of the knife
Scrapers installed, the line through the apex must be consistent with the diameter of the substrate. If this condition does not have, even if other conditions have been met also can not get a good printing effect, will produce unqualified printing products. Setting the center of the squeegee is an important factor in the conditions of the surface printing. Printing pressure must be kept to a minimum. Especially in the curved screen printing, such as screen printing plastic bucket when the pressure is too large, it will deteriorate the scraper conditions, have a bad influence. General surface printing using sword-shaped scraper rubber, flat printing parallel scraper rubber. Sword-shaped scraper rubber used for surface printing, is determined by the printing content, purpose. In short, to use scraping rubber in line with the purpose of printing, it is not necessary to limit the difference between curved and flat surfaces.