At CHINAPLAS 2021, Nordson Corporation is to reveal its capabilities to help packaging film manufacturers make the switch from biaxially oriented polypropylene (BOPP) to biaxially oriented polyethylene (BOPE) due to the intensifying regulatory and marketplace demand for recyclable packaging.

“Nordson is working with a number of customers who are interested in switching from BOPP to BOPE,” said Patrick Meinen, Nordson’s global product line manager for Extrusion Dies Industries dies. “One of our key roles is to design the flow channel, or manifold, inside the die to provide process versatility while maintaining a desirable output rate.”

Nordson custom designs each manifold in accordance with the flow properties of the polymer to be processed. Proper manifold design, combined with a high-accuracy system of gauge control, is essential for preventing web breaks, since the stretching that occurs in the biaxial orientation process magnifies any defects in the film caused by out of-tolerance extrusion.

Nordson’s multi-manifold die for BOPE film.

BOPP film typically is stretched to a width six to eight times greater than the width of the die. Film gauge of BOPP is generally 2.5 to 3.0 mm as extruded, then stretched until thicknesses are in the 15 to 20 µm range. Since BOPE is a stiffer material, it cannot be stretched as much transversely, and is drawn down to the 25 to 40 µm range.

To meet the challenge posed by biaxial stretching, it is essential to design a manifold so that polymer flow and film properties are uniform across the width of the die. The challenge is greater in coextrusion where a balanced layer structure is essential for consistent strength properties, tear resistance, and the capability of easily accepting additional surface coatings or treatments.

In Nordson’s multi-manifold dies, each manifold features a coathanger-shaped flow channel that promotes coextrusion stability. The manifolds are symmetrically designed to yield film with good surface properties and to prevent gels and degradation during film production.

The uniform distribution and precise layer-to-layer structure created by these multi-manifold dies ensures that the film retains its integrity during the biaxial stretching process. In turn this achieves good barrier properties.

In coextrusion it is also possible to produce a multi-layer structure whose outer, or skin layers, are narrower than the core of the structure, producing a so-called “naked edge.” This helps to maintain high line speeds, since it keeps the PP or PE skin layers out of the tenter frame clips. Otherwise, these sticky skin materials can prevent the clips from easily releasing the film after stretching has taken place.

“Until now there has been little use of HDPE in biaxially oriented film for packaging, but that is changing,” added Patrick Meinen. “BOPE is emerging as an attractive alternative to BOPP and BOPET because it makes possible mono-material products.”