As previously noted, the flooding and printing action on a rotary system is all part of the same continuous motion, and separate strokes for these functions are not required. On a flatbed press, printing is a two-step process that involves a pass over the screen by the floodbar to fill mesh openings with ink, followed by a squeegee pass to bring the stencil and substrate into contact for ink transfer. The situation is a little different with flatbed presses. Because of the way rotary presses operate, their production rates are reported as linear measurements and typically fall within a range of 100-450 ft/min (30-100 m/min), depending on the model and the specific application. The only factor that determines the production rate is the speed of screen rotation/substrate movement (remember, these values are the same), which is limited primarily by the type and rheology of the ink being used. On a rotary press, images are applied to the entire width of the substrate as the material moves continuously through the printing mechanism–substrate size really has no impact on press speed. Productivity Among the biggest attractions of rotary screen printing is its production speed. The virtues of rotary screen printing are most apparent when the process is compared to traditional flat screen printing. The geometry of the screen and the position of the squeegee within the screen combine to provide both the screen-flooding and image-transfer functions in a single smooth operation that repeats with every revolution of the screen. In short, rotary printing is a continuous, stepless image-transfer method. The squeegee then shears the ink as the stencil and substrate come into contact, allowing the ink to transfer cleanly to the material. The motion of the screen causes this bead of ink to roll, which forces ink into stencil openings, essentially flooding the screen without requiring a floodbar. Ink is automatically fed into the center of the screen and collects in a wedge-shaped “well” formed by the leading side of the squeegee and the screen’s interior surface. The squeegee on a rotary press is in a fixed position with its edge making contact with the inside surface of the screen precisely at the point where the screen, substrate, and impression roller come together (Figure 1). (The impression roller serves the same function as the press bed on a flatbed press.) As the web passes through the rotary unit, the screen spins at a rate that identically matches the speed of substrate movement. In rotary printing, the web travels at a consistent speed between the screen and a steel or rubber impression roller immediately below the screen. These machines are designed for roll-to-roll (web) printing on flexible materials ranging from narrow web films to wide-format roll textiles. Rotary presses place the squeegee within the screen. Rotary screen printing is so named because it uses a cylindrical screen that rotates in a fixed position rather than a flat screen that is raised and lowered over the same print location. For many applications requiring efficient, high-volume, high-quality printing, rotary screen printing may be the answer. The good news is that you don’t have to sacrifice the benefits of screen printing to overcome the limitations of flatbed printing technology. Yet throughput continues to be limited because every sheet of substrate still must pause at each printing station to receive the image. Businesses that invest in automatic, multicolor, inline flatbed systems regain some of this productivity by eliminating manual handling from all or most of the sequence. When screen printing is used as a piece-printing process with manual material handling, screen shops sacrifice productivity. Screen printing is also associated with piece-decorating applications, in which individual sheets of substrate are printed one by one, usually on semi- and three-quarter-automatic flatbed presses that require manual loading and/or unloading. Most people think of screen printing as a flat printing process because the substrates are usually flat and decorated in a horizontal position. Join the Screen Printing Brain Squad - and Make Your Voice Heard!.Get Your Free Subscription to Screen Printing magazine.
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