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Innovations in Coex flat dies.

Cloeren Inc. of Orange, Texas, has quietly come out with some radical new die designs for multilayer cast film, sheet and extrusion coating. All the new designs feature enhancements to the company three-year-old Epoch coextrusion die series, which grew out of Cloeren's original specialty as a maker of feedblocks.

For automatic gauge control, the company has developed a device designed to eliminate any temperature gradients across the die lip. And its dies for extrusion coating can now include a system for edge-bead reduction (EBR) that resides entirely within the die. New film and sheet dies have a similar internal deckling system for changing web width without changing dies. The company has also adopted customized inverted prelands as standard on its Epoch dies.

All the new die features go beyond quality improvements and offer savings in the form of reduced resin costs and faster start-ups, says company president Peter Cloeren.

FLAT-LINING THE LIP

Like their predecessors, Cloeren's new automatic gauging systems still adjust the die lip with thermally actuated bolts to control film or sheet thickness. And, they continue to rely on electric heating and convection cooling. But the two newest auto-gauging systems also make use of an "isothermal lip device" for eliminating temperature variations across the lip.

Typically, auto-die lips can suffer from localized temperature differences caused by heat transfer from the individual adjustment bolts. Cloeren explains that these gradients affect web thickness by changing the melt's surface viscosity as it exits the die. These new Autogauge systems, however, use a heat pipe to isolate the thermally actuated adjustment rods from the lip. The heat pipe distributes temperature variations across the lip to come within as little as 1|degree~ F uniformity from one end to the other. Otherwise, the variation might run as high as 10|degrees~ F.

As a result, says Cloeren, response times are up to 50% faster because the system no longer has to correct for the temperature gradients normally caused by its own diebolts. Faster response times, in turn, allow processors to run more on-spec product or operate closer to their tolerances for quality improvements and resin savings.

Cloeren recommends the isothermal lip control especially for temperature-dependent polymers such as PET and nylon, as well as sensitive compounds like those containing Ti02. The isothermal lip device can be found on the company's Autogauge III system. The Autogauge IV has the device too but with a thermally conductive jacketing around the adjustment rods. The jacketing works to remove excess heat from the lip adjustment assembly, further cutting response times.

DRAWING A BEAD ON WASTE

Tackling the waste problem of edge beads in extrusion coating, Cloeren has introduced its own version of an internal deckling system. Cloeren's deckling assembly consists of an internal rod, plug, and full-length blade. Located near the die lip, the rod establishes web width. At the same time, the plug and blade gradually narrow the melt flow path between the manifold and rod. The unit's positioning, or that of its individual parts, adjusts from outside the die.

According to Cloeren, the system completely eliminates edge bead and its associated scrap for nearly all resins. Only those with a melt-flow index above 13 g/10 min would create some difficulty in completely getting rid of the bead.

Unlike some earlier systems, where shorter deckling components sat on the end of rods, the EBR III system features full-length solid members. That way, Cloeren says, polymer doesn't hang up and degrade behind the deckling components, creating streaking problems during width increases. The company's EBR II, meanwhile, features a full-length blade but has a conventional plug.

Cloeren attributes resin savings to the EBR--mostly because freedom from edge-beads lets processors precisely match extrudate and substrate widths. They can then avoid the need to overcompensate for the beads with a wider web. And by preventing polymer build-up behind the deckle, it makes for easier changeovers and less maintenance, according to Cloeren.

NO MORE UGLY DECKLING

For cast film and sheet dies, the company has also introduced a similar internal deckle system (IDS). It incorporates the same design as the EBR III and addresses two problems associated with web-width changes. It ends the days when changing width means changing dies or putting up with too much edge trim, according to Cloeren.

IDS's primary benefits come from the space it saves when compared with the more common external deckling approach. Because it resides inside the die, the IDS allows for "more flexible positioning" of the die relative to the chill roll, says Cloeren. By contrast, he notes, external deckling often entails a sacrifice in terms of its positioning. Poor positioning can cause neck-in and other physical property problems, such as reducing the amount of orientation. "With IDS there's no negative impact when deckling the die," he says. "It doesn't effect film properties at all."

Cloeren cites PET as a prime candidate for internal deckling because that resin's lack of melt strength means the die must sit in extremely close proximity to the chill roll--often a difficult task with external deckling in the way. Also, he recommends the systems for applications needing good orientation, such as some LLDPE films.

INVERTED PRELAND

A feature that was introduced a year ago on some of Cloeren's Epoch dies, meanwhile, has recently become standard for all. The dies now employ inverted preland areas--which appear in a top view as a triangle pointed away from the manifold rather than nestled into it as with a typical coathanger die.

But the company determines the exact shape of each preland with a computer modeling system that tailors the design to the flow characteristics of a given resin family.

Previously, Cloeren explains, manifold systems and the area downstream from them were designed together using a straight-line concept. Yet polymer flow and viscosity behavior conform to exponential curves. So prelands, as well as manifolds, should fit those curves, he argues.

The result of the tailored prelands: no deviation of flow distribution across the width of the lip, he says. And because the design results in a more uniform wetted surface area, it cuts pressure differential across the die width to reduce "clamshell deflection."
COPYRIGHT 1992 Gardner Publications, Inc.
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Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Ogando, Joseph
Publication:Plastics Technology
Date:Oct 1, 1992
Words:1022
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