Co-extrusion die system enhances PET sheet producer’s capabilities for multiple barrier structures

  • Evertis de México: manufacturer of thermoformable sheet for food and non-food applications
  • Meeting the challenges posed by asymmetrical multi-layer structures

A coextrusion die system from Nordson Corporation has enabled an extrusion processor specializing in PET-based semi-rigid packaging sheet to maintain layer uniformity well within tolerances while avoiding product defects arising from asymmetrical layer structures.

Evertis de México: manufacturer of thermoformable sheet for food and non-food applications

Evertis de México S.A. de C.V. is a manufacturer of thermoformable sheet for food packaging such as poultry, cheese, and processed meat, as well as non-food applications. The company is a subsidiary of the international IMG Group, a leading player in the field of PET film extrusion. In addition to the plant in Mexico, the group has manufacturing facilities in Brazil and Portugal.

Evertis de México S.A. de C.V. recently installed a Nordson EDI coextrusion die system for producing a range of structures. These include standard multi-layer, medium-barrier, and high-barrier sheet with thicknesses from 180 to 1,016 µ (0.180 to 1.016 mm). The barrier sheet structures are often asymmetrical, in that the materials or layer thicknesses above the central layer differ from those below it. The flow control of the Nordson EDI system has made it possible to maintain tight layer tolerances in such structures while preventing “wave,” “zig-zag” and other defects caused by instabilities at the interface between layers.

“At Evertis we produce semi-rigid barrier PET sheet for food and non-food applications, and it is essential that we maintain layer uniformity within tolerances at all times. The Nordson EDI die system allows us to control layer interfaces, and thereby avoid product defects such as waves,” said Jacques Tillet, director of maintenance at Evertis de México.

Meeting the challenges posed by asymmetrical multi-layer structures

“Because critical layer interfaces are shifted into higher shear regions of the flow paths, coextrusion instabilities are more common with structures that are asymmetrical,” said Nordson EDI chief technologist Sam G. Iuliano. “Our die system yields streamlined melt streams and fine-tunes them at the point of confluence. In addition, the ease with which adjustments can be made enables the system to be quite versatile in both layer configuration and product width.”

Main components of the die system used by Evertis de México are the following:

  • Die with manifold and internal deckle: Nordson has designed the manifold, or flow channel, inside the die to reduce shear stress levels at the layer interfaces, resulting in improved layer uniformity. At the same time, the sections of the manifold at each end of the die are sized to accommodate adjustable internal deckles for making changes to product width.
  • Adjustable feedblock: A feedblock combines melt streams from separate extruders into a multi-layer “sandwich” that the extrusion die subsequently distributes to target product width. The so-called Ultraflow V-S feedblock incorporates adjustable “combining planes” located where the melt streams join the central flow channel. These make it possible to balance the velocities of the combining streams. When operating in “free-float” mode, they automatically compensate for the changes in layer thickness ratios that accompany product structure changes. Adjustments can be made without taking the feedblock off-line, increasing up-time. Another adjustment that can be carried out on-line is use of a selector spool that makes it possible to change layer sequences in the structure.
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