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FLEXO Magazine : March 2011
Technologies & Techniques the pan, resulting in muddy printing and color striation across the roll. Types of pumps For general printing applications, there are three basic types of pumps: centrifugal, peristaltic and diaphragm. Each has its own benefits and weaknesses. Centrifugal pumps were the standard on most flexographic and gravure presses for many years and many are still in use, especially in gravure. In centrifugal pumps, a motor spins an impeller (a bladed disk) inside a housing called a volute. This generates pressure that pushes the ink up. There are several reasons why centrifugal pumps are popular, as follows: • They are rugged and cheap to operate, with few parts that wear out or require maintenance. • They are available with a range of motors, from electric to variable-speed electric to explosion-proof electric to air for use in solvents, water or lighter UV. • They provide circulation (mixing) for the ink both in the volute (which is preferred) and through bypass systems, keeping viscosity and color density in better control. • They can provide very high flow if needed. • They are relatively easy to clean. Simply put the pump in a bucket of wash-up solution and flush out the pump and deck. • They can be “deadheaded,” where the output is com- pletely shut, without damage. • They are available in electric versions that are economi- cal to operate: Electricity is typically about 1/8 the cost of compressed air. However, centrifugal pumps do have some drawbacks, as follows: • They are heavy, especially the explosion-proof models. However, many manufacturers offer a “dismountable mo- tor” option that allows separation of the motor and pump to make them lighter to carry and easier to clean. • The flow they deliver varies with viscosity, specific gravity and deck height. Centrifugal pumps deliver less ink to upper decks than they do to lower decks. Unlike displacement pumps, centrifugals need to be sized for the upper decks and heaviest fluids. • Variable speed is an expensive option on electric versions (though free on air versions). • They are harder to clean than peristaltic pumps, which is why peristaltics have become the standard on quick-change label presses. • They usually cannot pump high- viscosity inks (e.g., UV ink, EB ink) and coatings. • 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. 36 FLeXO march 2011 www.flexography.org FLX_March11.indd 36 3/18/11 1:32 PM