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Screenchanger valve stops unwanted flow.

A new check valve from Extek Inc. of Marblehead, Mass., keeps backpressure-driven polymer from flowing where it was never meant to go--the wrong way up an idle coextruder. Called the CoExChek valve, this simple device enables the company's hydraulic screenchangers to stop backflow before it reaches, and potentially damages, screenpacks and even secondary extruders themselves.

Located in the bushing downstream of the breaker plate, this ball valve closes off the melt channel to isolate any equipment farther upstream. Reverse flow itself activates the device by pushing the ball into its seat and blocking off the channel until forward flow starts.

Extek president Bart Jones explains that the need for the valve is rooted in a problem that has long plagued coextrusion lines lacking melt pumps. With at least one extruder running, backpressure in the system can force the melt the wrong way into coextruders during startups or periods of improper adjustment, Jones explains.

This "backfiring" flow would then dislodge the screens from the holders, sometimes causing bits of metal to enter the system. And in the worst cases, the reverse flow would even make its way up into extruder barrels and feed sections, causing maintenance headaches at the least or even more extensive damage. "Without a melt pump, standard breaker plates just weren't acceptable," Jones says.

So in the past, the company addressed this reverse-flow problem by modifying screenpacks. On the lowtech side, a simple retaining ring could hold the screens in place during less rigorous applications. Next, special locking covers were developed to encase the screens. But this arrangement involved some cumbersome operator involvement during screen changes because the hot cover had to be unlocked manually. The CoExChek valve, by contrast, is completely automatic.

Since this mechanical valve operates without any human or electronic intervention, it has uses unrelated to startups. The valve also stops flow to any unnecessary extruders that have been shut off intentionally.

The valve is available as an option on new screenchangers, which range in size from 2.5 to 8 in., or it can be retrofitted to existing units in that size range. Jones says pricing on the brand-new valve has not been set yet, but he expects it to cost significantly less than locking coex cover plates. That earlier solution cost roughly $1000 for a 4.5-in. screenchanger.


Also new from Extek is the addition of slide-plate cartridge heaters on its two largest hydraulic screenchangers. The heaters augment the integral heat pipes that distribute heat from an Extek screenchanger body to its slide plate. The bulk of the heat, meanwhile, has always been--and will continue to be--provided by four cartridge heaters in the body.

The new slide-plate heaters have been added as a response to growing high-temperature applications in the 300-600 F range, such as PET and nylon. Positioned on the end of the slide plate, the new heaters work to speed the temperature-distributing action of the heat-pipes, Jones says. The heaters do not require removal for low-temperature applications and are now standard on the 6- and 8-in. models.
COPYRIGHT 1993 Gardner Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Ogando, Joseph
Publication:Plastics Technology
Article Type:Evaluation
Date:Feb 1, 1993
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