Plastics Technology

NOV 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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to, so that you've reached the upper capacity limit of even a well- designed and maintained system. Has your material mix changed? A new material may have flow characteristics that make it more difficult to convey and that may be putting a subtle drag on throughput rates. Over time, it's possible that your receiver replenishment cycle times have fallen out of balance. If resin throughput on a given machine has been reduced, but the timing of receiver cycling was not also changed, receiver cycles could be longer than needed. When the receiver becomes overfilled, the pump may continue to try to deliver material or a relief valve may open. In either case, you're not only wasting pump cycles, conveying capacity and time, but also are starving other receivers of needed fill time. You're throwing away capacity instead of putting it where it is needed. So it is important to check all of your receivers periodically; not just the ones that seem to be causing you problems. DISTANCE CHANGES Has the position of a material source been changed? It's not uncommon to see a processor that can convey material from one location, but cannot pull the same amount of material from a different location that appears to be about the same overall distance away. Even small increases in distance can create conveying chal- lenges, because vacuum pumps have to sustain suffi- cient vacuum to keep large amounts of material in sus- pension and sufficient air velocity (cfm) to keep it moving. There can be a lot of factors that go into calcu- lating the actual distance over which material must be moved in a system. For example, the effective distance involved in moving material from a point 200 ft across a facility can add up to a lot more than the distance between point A and point B, due to system design factors that, essentially, make the distance longer. Consider this calculation: • Start with horizontal feet of con veyor line required: In this case, 200 ft. • If the line is mounted overhead, multiply vertical feet x 2 and add to the previous figure: 20 x 2 = 40 ft. • If the line contains 90° bends , multiply the number of bends x 20 ft and add to the previous total: 5 bends x 20 = 100 ft. • Total: 200 + 40 + 100 = 340 ft. Other factors, including the use of flexible hose, may also add to the overall distance that your pump must pull material. SYSTEM DESIGN LIMITATIONS Although a good conveying-system design seeks to minimize fea- tures that add to the distance or difficulty of moving material, such optimal layouts can be difficult to achieve and maintain in a fast- changing production environment. When conveying lines run overhead, you not only need to worry about the limits imposed by vertical rises (see above) but you also need to worry about vertical drops—to a resin selection station, for example. Dips or drops like these must be well designed or they will form "traps" in the system, where material plugs can form. If a conveying cycle ends—or a straining pump triggers a vacuum relief valve—while material is still suspended in the vertical tube, the material will fall to the bottom of the "trap," forming a plug. The remedy is automatic purging, regulated by the conveying control and a valve at the material pickup point. At the appropriate point in a cycle, the valve closes to material and opens to outside air, conveying any material remaining in the line through to the designated receiver before the system begins to supply another destination. Another design issue that can affect conveying performance is upwardly sloping conveying lines (i.e., lines that incline upward at angles of 45° or so). Unlike 90° bends, where materials flow turbulently through Drops often require automatic purge (such as with this Conair auto-flushing common-line valve) or they can trap materials and form plugs in the system. Use of flexible hoses should also be held to a minimum because they reduce conveying throughput. QUESTIONS ABOUT MATERIAL HANDLING? Visit the Resin Conveying Zone. @plastechmag 41 Plastics Technology M AT E R I A L C O N V E Y I N G

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