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November 6, 2003

10 Min Read
Tooling Corner: Cast film dies: Optimizing productivity

Editor?s note: Karl M. Maurer is regional sales manager of Extrusion Dies Industries, LLC, a supplier of dies for cast film, sheet, and coating.

The innovative tapered shape of the Contour Die is designed to yield uniform die body deflection while incorporating a manifold that maximizes flow streamlining. To offset greater internal pressure at the center of the die, the die body is heavier there and lighter at either end. The die is shown with lips pointing downward. The obliquely oriented module at left is the Autoflex VI-R gauge profiling system. Electrical components are visible at the top.

The optimal design of new, retrofitted, or remanufactured flat-die systems for specific cast film applications can give the film manufacturer a competitive advantage in today?s global marketplace. Multiple technical considerations are involved.

Maintaining Draw Stability

Draw or takeup instability takes the form of fluctuations in the takeup area even when the extrusion process is stable and takeup speed is constant. The phenomenon underlying this cause of defective product is called melt or draw resonance. Early in the process, melt resonance is manifested by an oscillation in the frost line position on the first chill roll. It is especially problematic when the web edges begin to oscillate.

Factors in the development of melt resonance include: 1) draw ratio, or the ratio of the initial cross-sectional area of the extrudate to the final area, 2) draw distance, or the length of the draw zone, 3) line speed, or takeup velocity, and 4) rheological properties of the polymer, including shear viscosity, extensional viscosity, and elastic relaxation time.

Of these factors, only draw ratio relates to die geometry, namely the lip gap. One way of controlling melt resonance is to employ a heavier die, with a high resistance to die-body deflection (discussed below) that provides tight dimensional control over the gap. In general, the greater the lip gap, the greater the tendency toward instability, with the likelihood of instability growing as the line speed increases.

Melt resonance is also caused by a large draw distance. To reduce the air gap, or the distance from where the web exits the die to where it contacts the chill roll, and to provide better stabilization of the web, EDI provides a vacuum box with a dual-chamber design. One chamber removes entrained air between the film and the chill roll. The other develops a higher vacuum that stabilizes the film, reduces neck-in, and controls edge movement.

This dual-deckle cast film die is shown with the lip area pointing downward. The bottom-most component is a bronze-colored external deckle; visible above it is a bronze-colored internal blade. At left is the handwheel used to activate both deckles at once. The obliquely oriented assembly at right, with array of wires, is the Autoflex VI-R gauge profiling system, which automatically adjusts the flexible lip of the die.

A recent development minimizes the distance between die lips and casting roll centers on the movable deckle barriers at both ends of the die, which enable operators to change product width for each job run. The innovation involves a dual deckling system comprised of an external deckle that seals off the die gap from outside, and an internal deckle that blocks flow with blades between the die lips. Because the external and internal deckle are linked to a common drive system, the units function as one, providing the leak-preventing capability of an external deckle plus the flow control of an internal deckle.

The challenge in developing a dual-deckle system for cast film was to incorporate an external deckle without increasing the air gap. In the system described, the clearance between external deckle and casting roll is 10.4 mm and the total air gap is 22.5 mm, which is at the middle of the standard range for cast film.

Tightening Gauge Control

Automated gauge profiling technology provides the tight control over the transverse thickness profile that is essential for downgauging and, in the case of oriented film, for preventing gauge bands and other defects that develop as the film is stretched. As a rule of thumb, autoprofiling reduces gauge variation to half of the minimum achievable with a manual gauge adjusting system.

Autoflex computerized gauge profiling systems center on a series of closely spaced thermally actuated adjuster blocks arrayed along the flexible lip of the die. These blocks operate in response to feedback from a computerized downstream gauge scanner. When a thicker-than-target area is detected in the extrudate, power to the cartridge heaters at corresponding points in the lip is automatically increased. This causes the blocks to thermally expand, which tightens the lip gap in the area. Conversely, thinner-than-target areas are addressed by a reduction in power.

This thermally actuated adjustment system offers multiple improvements.

  • Faster response. A switch from tool steel to beryllium copper has improved the thermal conductivity of the adjuster blocks. This makes the Autoflex VI-R system respond more quickly than the earlier Autoflex IV, which translates into substantially improved sensitivity to feedback from downstream gauge scanners.

    In turn, the new Autoflex VI-L system incorporates a tiny lever for each segment of the flexible lip. These levers multiply the effect of the thermal actuation process, doubling the speed of adjuster block movement in comparison with the Autoflex VI-R system and enabling producers of film to reach target gauge sooner and without operator intervention.

  • Reduced thermal crosstalk. Designed as a self-contained module that can be readily disengaged from the die for maintenance, the Autoflex VI-R system responds more precisely to feedback from gauge monitors because its thermally actuated adjusting mechanism is isolated from the heat of the die body.

  • Higher-resolution flexible lips. For ultra-fine gauge control, EDI supplies the Microflex design. The average distance between the centerlines of the adjuster blocks is only 21 mm instead of the 28.6 mm on standard Autoflex IV and VI-R dies.

Reducing the Quality/Productivity Tradeoffs

The manifold is the heart of any die, as it provides the flow channel that distributes molten polymer to the required end-product width and thickness. Its design plays a critical role in optimizing melt flow and minimizing the deformities in the cross-sectional profile of the film caused by nonuniform die-body deflection. Until recently, however, these two functions have been at cross-purposes.

To achieve streamlined flow, die engineers have typically utilized coat-hanger manifold designs, so-called because the back walls of the manifold, on either side of the melt entry port, are positioned at an angle to the die exit rather than parallel to it, forming two sides of a triangle. By eliminating dead spots or polymer hangups, the coat-hanger design minimizes polymer degradation, allows rapid purges, and enhances throughput. On the other hand, in order to design dies with die-body deflection that is uniform across the width of the die, designers have had to sacrifice some of this streamlining.

Die-body deflection is caused by the pressure of the molten polymer that the extruder continuously charges into the manifold. Multiplied across the entire area of the manifold, this pressure, typically in the range of 1000 to 4000 psi, generates thousands of pounds of force, enough to deflect heavy steel die bodies.

Unlike coat-hanger designs, manifolds for so-called constant-deflection dies have had straight backlines, with all of the body bolts that clamp together the upper and lower halves of the die positioned at the same distance from the die exit. A key advantage of a constant-deflection die is a reduction of the time required to adjust the gauge profiling system. Operators can achieve target gauge several minutes sooner when starting up the line or changing extrusion rates.

The Contour Die has a manifold of standard coat-hanger design but the die body has an unusual tapered shape that provides for a die-body deflection that is uniform across the width of the extrudate. Since there is a pressure gradient across the width of the manifold from the center to the ends, the die designers built in extra die-body thickness where the force was greater and less die-body thickness where there was less force. The result is a sculpted configuration that is smaller and tapered at the ends.

Improving Coextrusion Structures

Manifold design considerations are even more complex for dies used in coextrusion. Advanced flow-channel geometries are engineered to minimize product defects that result from nonuniformities at the interfaces between layers and from the overall distortions caused by the differing flow behaviors of polymers in a coextrusion. EDI initially addressed the problem of interfacial deformation by developing coat-hanger manifolds with elongated-teardrop cross-sectional shapes. These reduce the level of shear stress along the back wall of the manifold compared with a teardrop cross section.

The key advance since these designs (the Multiflow II and IV manifolds) has been the development of the Multiflow V modified coat-hanger manifold, which differs from the earlier elongated-teardrop manifolds in three ways.

  • The backline of the manifold is straight, running parallel to the die lips so that all body bolts are equidistant from the lip exit. Besides making the die a ?constant-deflection? system, this configuration yields a melt distribution that is less sensitive to variations in throughput rate.

  • The Multiflow V manifold has a varying cross section with an aspect ratio (length to height) that increases towards the ends of the die. This minimizes the interfacial deformations caused by the viscous encapsulation that occurs when lower-viscosity layer material is driven toward the ends of the die more readily than higher-viscosity material.m The preland section of the manifold is curved rather than triangular, promoting a more uniform distribution of polymer across the width of the die and thus preventing the formation of M- or W-shaped patterns.

    • While the varying cross-sectional geometry of the Multiflow V manifold precludes use of full-plug internal deckling, the Multiflow VI design provides the advantages of the Multiflow V manifold, while incorporating a constant cross-sectional segment that accommodates such deckling.

      There are multiple ways of determining the best configuration of the system for combining melt-flow layers into a coextruded structure. These include feedblocks, which shape each molten polymer into a layer and combine all layers into a multilayer sandwich, which is then distributed to full product width by the manifold; multimanifold dies, which distribute individual layers to full width before combining them; and combination systems that employ both approaches.

      A growing number of barrier-film producers, for example, use a feedblock to combine the core layer of barrier polymer with two tie layers, then feed this three-layer structure into the central manifold of a three-manifold die, the other two manifolds providing the skin layers. Because the skin-layer manifolds are wider than the central manifold, barrier resin is prevented from contaminating the edge trim, enabling the scrap to be reused.

      Increasing Uptime

      The chief recurring die-related causes of downtime in cast film extrusion are product changeovers and the cleaning and maintenance of dies. Besides the dual-deckle system mentioned earlier, two other new developments address these problems. A motorized die separation device makes it possible to split and clean a die without taking it offline, and a manually actuated lip scraper traverses the width of the die in less than a minute to scrape the buildup from the lips.

      For coextrusion, EDI has increased versatility with two developments: Accuflow feedblocks, which are adjustable online; and I-S series interchangeable layer-sequencing spools, which enable users of both conventional (Ultraflow) and Accuflow feedblocks to change the sequence of materials without blocking flow channels or disassembling the feedblock.

      Contact informationExtrusion Dies Industries LLCChippewa Falls, WIKarl M. Maurer (715) 726-1201www.extrusiondies.com

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