Quick-change coex film dies give all the flexibility you need.

Two new multi-layer dies for blown film use different approaches to provide the flexibility to produce a range of different structures with short changeover times. Windmoeller & Hoelscher Corp., Lincoln, R.I., recently added its name to the list of blown-film equipment suppliers offering modular, stackable dies. In addition, Battenfeld Gloucester Engineering Co., Inc., Gloucester, Mass., has provided new information on its recently introduced Optiflow LP concentric-mandrel design.

MOVABLE MODULES

At an open house last fall at its Lengerich, Germany, world headquarters, W&H unveiled the Multicone coex die that will be available from 6 to 14 in. diam. It has cone-shaped melt-distribution elements that nest inside each other like drink cups.

Multicone takes aim at the modular stackable or nested dies introduced by most other suppliers since 1991. These dies are built for flexibility because they allow processors to add or subtract layers and tweak structures simply by moving modules around. The Multicone design was also motivated by the fact that W&H's standard five-layer die design cannot accommodate internal bubble cooling (IBC) below 16 in. diam., says blown-film sales manager Andrew Wheeler. The Multicone is IBC compatible in smaller widths. W&H is placing a 14-in. maximum width limit on the new die because the firm contends that wider stackable units are plagued by poor melt distribution and leakage problems.

The side-fed Multicone die is geared for five- to seven-layer structures. It features narrow flow channels to cut residence times and high melt-flow speeds to eliminate hangups and cut changeover times. The new die is expected to be displayed this fall at the K'98 show in Dusseldorf, Germany.

NEW BLOWN-FILM AUXILIARIES

At the Lengerich open house, Windmoeller & Hoelscher rolled out several other new products for processors of surface-protection and other tacky films. One is an oscillating haul-off called Nostic. It has non-contact air-turning bars with a concave shape. According to Wheeler, this design reduces the possibility that air pockets from the bars will cause the film to curve. Nostic can be teamed with a new oscillating sizing cage, which rotates at the same speed as the hauloff. Wheeler says this minimizes bubble twisting inside the cage.

Also for sticky films, Windmoeller & Hoelscher offers a new traversing, beta-type thickness gauge that is mounted at the layflat. The gauge measures the full web every 20 sec and can detect whether thickness variation is on the top or the bottom layer of the collapsed bubble. For most applications, W&H has usually advocated use of on-the-bubble capacitance gauges, but the firm found these units, which require contact with the bubble, to be incompatible with sticky films.

SKINNING THE SAME CAT

Stack dies aren't the only route to coex flexibility. About a year ago, Battenfeld Gloucester launched the Optiflow LP (Low Profile) line of dies, which are built for both greater flexibility and productivity, says Bill Hellmuth, senior product manager for blown-film systems. Flexibility is enhanced by internal features that speed changeovers. And the fact that the unit stands only 40 in. tall from floor to air ring influences output because it effectively adds cooling distance between the die and the nip centerline. The 40-in. height also makes it easier and safer for operators to thread up the bubble.

Compared with typical spiral dies, Optiflow LP reportedly has a more streamlined concentric mandrel design to keep layer-to-layer interfaces short, a key element in reducing interfacial flow instability. (That's when one layer "scuffs" against the other.) A shallower groove at the start of the spiral increases material flow velocity, which results in faster product changeovers while reducing the risk of degrading temperature-sensitive materials. The distance between flow entry and exit is a tight 20 in. Stack dies, by comparison, can have more than 30 in. between entry and exit.

Gloucester displayed the Optiflow LP for the first time at NPE '97 in Chicago, where it was used on a seven-layer structure containing nylon and EVOH - two extremely temperature-sensitive resins. Typically, lines of this type run around the clock, mainly because they can take hours to purge, and hours more before they're running just right again. But Gloucester managed to fire up the complex coex system in less than an hour every day of the five-day show, and had it running virtually gel-free from the start.

Optiflow LP is available from 3 to 90 in. diam. for LLDPE-based structures, and 3.5 to 14 in. for HMW-HDPE lines. It can handle up to nine layers. Gloucester has 50 such dies operating commercially and is making the bottom-fed Optiflow LP design its standard for all blown film dies, says Hellmuth.