Nordson Extrusion Dies Industries, Chippewa Falls, Wis., has developed enhancements to extrusion coating dies and internal deckles that make it possible for converters to reduce edge bead with low melt strength polymers while maintaining coat weight uniformity—two goals that reportedly have often been mutually exclusive with such resins.
While standard internal deckles with edge profile control do a good job of reducing edge bead with the high melt strength resin LDPE, they have been less effective with linear polymers such as LLDPE and PP and with more expensive special-function copolymers such as ethylene-vinyl acetate, ethylene-acrylic acid, and ethylene-methyl acrylate, the supplier says. Trimming the larger beaded edges developed with these materials is a costly waste of material, since the combination of substrate and polymer coating cannot be reused.
Research by Nordson Extrusion Dies Industries has led to development of a technology package that resolves the issue raised by low melt strength polymers. The trials were carried out at SAM North America’s Extrusion Technology Center in Phoenix, N.Y. Components of the technology are:
Dies with lip lands that are substantially shorter than the industry standard. The lip land is the final portion of the flow channel leading to the die exit. In standard dies, a longer lip land plays an important role in maintaining coat weight uniformity, but it also makes possible more of the lateral movement of the melt that generates edge bead. The research by Nordson Extrusion Dies Industries has shown that the gauge profile of the coating can be effectively controlled if the shorter land has been engineered appropriately.
A suite of secondary internal deckle blades, sized for various polymers. Deckles are used to vary the width of the coating. An internal deckle consists of three independently adjustable components—located at each end of the die—that can be used to fine-tune the edge profile of the polymer as it exits, minimizing bead formation. Tuning is accomplished by varying the positions of the manifold quill and secondary blade with respect to the lip flag and deckle rod (see schematic) In addition to the effects of these differing offset dimensions on the edge profile, the research by Nordson Extrusion Dies Industries has shown that the size of the radius of the secondary blade is another key determinant. While a short radius is effective for LDPE, longer radii do a better job of stabilizing the thickness profile near the edges with polymers having less melt strength.
The smallest-diameter deckle rod in the industry. The “micro-rod” designed by Nordson Extrusion Dies Industries—half the standard diameter—permits less lateral movement of the melt and thus less edge bead formation.
The short lip land and micro-rod are available in new dies specified by converters focused on making substantial reductions in edge bead. The suite of exchangeable secondary deckle blades with varying radii is available from the company on a custom-designed basis.
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