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FLEXO Magazine : March 2011
Technologies & Techniques • They do not work with most automatic wash-up systems currently on the market. Next is the diaphragm pump, typically seen in the double diaphragm pump (DDP) configuration. Diaphragm pumps work by pushing ink through one chamber via pressure on a flexible diaphragm while sucking ink into the other cham- ber, reversing and repeating this process. Almost all are pneumatic, or air-powered. These pumps offer the following advantages: • They are typically the cheapest pumps available for a given flow rate and can handle higher viscosities. • They are compact, especially for the flow rate. • They are often available in a “2-in, 2-out” configuration, allowing the pump to feed ink to the printing deck and suck it back down. This is especially useful for low cor- rugated decks, especially on bottom printers. • They may be inherently explosion-proof, provided the operator follows the proper instructions. Most major man- ufacturers offer special certified pumps for ATEX areas. Double diaphragm pumps also have drawbacks, as fol- lows: • They create pulsation, which can bend the doctor blades, push the chambers outward or blow out the end seals, resulting in leaks and inconsistent printing. Operators typically compensate by putting too much blade pressure on the anilox, which causes premature wear. • They are difficult to clean because the ink cannot be cleaned from the internal chambers directly. (This is especially important with UV and EB ink.) • They need frequent maintenance that often requires removing the pump from the press. • They are expensive to run. Compressed air costs roughly 8 to 10 times more than electricity. Finally, there is the peristaltic pump. The peristaltic pump works by squeezing a tube with one or more rotating roll- ers. This pushes the ink through a tube much like how you push toothpaste through its tube with your fingers. Peristaltic pumps offer benefits that have made them wildly popular for use in label and narrow-web presses, though some manufac- turers such as Schiavi and PCMC use them in larger presses. The advantages of peristaltic pumps are as follows: • They are easy to clean: Just change the tubing. This leads to incredibly fast turnaround, especially if the heads are removable. • They can handle inks with high viscosities, including screen printing ink. • They create much less pulsation than DDPs. The “pulsa- tion” seen in peristaltic pumps tends to be more of an off/ on low-pressure pulsation, as opposed to the high-pres- sure spikes typical of diaphragm pumps. • Some models are reversible, allowing the operator to suck the ink back from the deck and resulting in massive ink savings. This also makes cleanup easier because there is less ink in the system. • Some models allow the user to control the speed of the pump, which can minimize tube wear. • Models with electric motors are economical to operate, particularly compared to DDP pumps that use compressed air. Peristaltic pumps also have several disadvantages, as follows: • Their tubing wears out, though some models are designed for maximum tube life. Pumps de- signed for process environments, as opposed to those designed for laboratories, will typically provide maximum tube longevity. • They are large for the amount of flow they provide. • They are more expensive than other pumps, especially for explosion- proof models. • The benefit of quick turnaround fades with web width, as runs are longer and tube life more criti- cal. However, they do make good washup system pumps because their reversibility greatly simplifies the inking circuit. Centrifugal pumps, DDPs and peri- staltic pumps are the most commonly used in printing. Less common pumps include the following: • Piston and gear pumps. The abrasiveness of the ink destroys the internal workings of these pumps. • Progressive cavity pumps. Although they work well in theory, they are typically too large and expensive for printing use. • Suction (“self-priming”) or hori- zontal centrifugal pumps. As with piston pumps, the ink typically destroys the seals and internal workings of these pumps. • Multi-stage or turbine pumps. These are designed for higher pressures than those required for printing—unless your top deck is about 300 meters in the air! Creating an Optimized SyStem Once you have chosen a pump, there are several items to consider when put- ting together an optimized system. Pump size. Pump size dictates pres- sure and flow. The variable is usually the impeller size in centrifugal pumps, tube diameter in peristaltic pumps and inlet size in DDPs. If you know the required pressure and flow, size the pump to run at about 70 percent capac- ity during normal operation. However, you might consider running a peristaltic pump even more slowly to reduce tube wear. If you don’t know the flow, contact either your press builder or local pump distributor for advice. Typically, they will want to know the type of ink you are pumping, the viscosity, maximum height, press width and press speed. Motor. The next choice is the type of motor. In printing, there are three general options: air/pneumatic, electric and explosion-proof electric. Pneumatic motors are light and typically variable in speed; most manufacturers can also provide versions suitable for UL or ATEX environments. However, these motors are expensive to run, as a horsepower of compressed air costs approximately 8 to 10 times what similar power from electricity would cost. Electric motors are widely available in a variety of speeds, and typical op- www.flexography.org march 2011 FLeXO 37 800.346.8570 www.colorresolutions.com CRI inks help build strong brands with power ful packaging. Clean designs and bold, distinctive colors are the hallmark of consistent brand identity. Custom formulated flexographic inks and coatings from CRI provide brilliant high-gloss colors and a durable scratch and rub-resistant finish. Combined with CRI’s unique TrueColorBalanceTM color management system for superior process control, your printed labels will become an integral part of your customer’s brand promise. And, you’ll get the benefits of water-based inks that are among the most earth-friendly, with extremely low VOC’s and minimal environmental impact. Contact a CRI technical specialist for fast response to your ink and color application questions. Give your labels a new twist. FLX_March11.indd 37 3/18/11 1:32 PM