A new device of German invention, called the Flex Ring sleeve, is letting pipe extruders do what they once only dared dream about—change the gap of the melt flow channel at any location around the die circumference.

Conventional pipe die flow channels can withstand high melt pressure. But they don''t allow easy adjustment if two opposing thick regions occur on the pipe wall. One solution, from German engineer Heinz Gross of Dr.-Ing. H. Gross Kunststoff-Verfahrenstechnik (Rossdorf), enables existing pipe dies to be retrofitted with a device called the Flex Ring sleeve that alters the gap of the melt flow channel at any location around its circumference. This evens out thickness variations while the line is running.

Gross'' Flex Ring sleeve for annular dies incorporates individual, thin, flexible, annular, metal walls that support each other like a leaf spring. These membranes can range in thickness down to .2 mm. The sleeve consists of one piece of metal, partially solid and multiwalled at the tip. Despite being flexible, the construction can be designed to withstand any polymer pressure as it exits the flow channel. Dead zones are not formed and the ring can be used to process a wide variety of resins including corrosive fluoropolymers and polymers with different viscosities.

Thickness is adjustable manually by regulating screw bolts spaced around the rings outer surface. Gross says the adjustment is advantageous if local flow resistance requires fine-tuning, or in cases where there are problems of precise melt flow distribution.

This concept is particularly interesting to processors of coextruded pipe who want to reduce attainable layer thicknesses. A Flex Ring sleeve for coextrusion has better thickness variation control and saves more polymer than conventional equipment, he says.

The Flex Ring is made of a corrosion-resistant alloy with a flange that attaches to the die. The other end includes the multi walls. The number of screws around the perimeter depends on die diameter and the rings deformation behavior.

Gross says so far, this construction has allowed material savings of 250g/m on a 110-mm-diameter foamed-inner-core PVC pipe, made on the first day of installation. A second processor halved thickness tolerances of his 30-mm-diameter nylon pipe and cut startup time by 40%. Gross says that to alter thickness distribution, an operator only needs to adjust the screws in the position where the distortion occurs, while the equipment is running.

Some processors who use Flex Ring in production include Dr. Bohlmann Kunststoffvertrieb (Coswig, Germany) and Pipelife Deutschland (Bad Zwischenahn, Germany). Heinz Hetzenecker, plant director at Pipelife, says the unit has been successfully used for multilayer pipes and was a more cost-effective way to achieve uniform layer tolerances than investing in new equipment. He is awaiting two additional Flex Ring units.

Gross is no stranger to membrane technology. His first membrane-equipped feedblocks for cast film and sheet production were introduced 10 years ago (October 1994 MP/MPI). Gross is also working on using the Flex Ring sleeves on tubular film dies to produce even-thickness sausage skins, as well as with annular dies to process foamed sheet.

Robert Colvin [email protected]

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