What is co-extrusion?

Co-extrusion is the process of extruding multiple polymers simultaneously through a single die, fusing them into one multilayer film in a single continuous operation. A modern 7-layer line can produce a film that would otherwise require 3-4 separate laminations — saving time, adhesive, and money.

The line at a glance

A multilayer co-extrusion line has four main zones:

  1. Extruders — one for each resin. A 7-layer film needs 7 extruders, each melting and pressurizing its own polymer (EVOH, PA, PE, tie resin, etc.) at temperatures between 180°C and 260°C.
  2. Feedblock + die — channels all 7 melt streams into a precise layered arrangement, then spreads them to the final film width (typically 1.2-2.5 m flat-die, or annular die for blown film).
  3. Cooling & orientation — for cast film, the melt is quenched on a chilled roll; for blown film, it's inflated into a bubble and air-cooled.
  4. Winding & slitting — the finished film is trimmed, slit to customer widths, and wound into rolls.

Layer by layer: a typical 7-layer structure

For high-barrier food packaging, a 7-layer cast film typically looks like this:

  1. PE (outer) — print surface, sealable, printable
  2. PE — bulk layer for cost and stiffness
  3. Tie resin — adhesive bonding layer
  4. EVOH — the oxygen barrier (typically 5-15 μm)
  5. Tie resin — adhesive bonding layer
  6. PA (nylon) — toughness + humidity protection for EVOH
  7. PE (inner) — food contact layer, heat-sealable

Total thickness is typically 60-200 μm, with EVOH representing only 5-10% of the total. The other 90-95% is structural and seal PE/PA — but the package is only as good as its thinnest functional layer.

Why manufacturers use multilayer structures

  • Cost efficiency — use expensive EVOH only where it's needed (5-15 μm), and bulk up with cheaper PE
  • Customized barrier — adjust EVOH thickness to dial in exactly the OTR your product needs
  • Mechanical performance — combine PA's puncture resistance with PE's sealability
  • Single-process production — no need for separate adhesive lamination steps
  • Recyclable mono-material versions — PE / EVOH / PE structures work with PE recycling streams

Cast vs blown — which process?

Cast film is made by extruding through a flat die and quenching on a chilled roll. It produces a stiffer, clearer, higher-gloss film with excellent gauge uniformity. Best for: high-clarity packaging, printing, lamination, food contact.

Blown film is made by extruding through an annular die and inflating into a bubble. It produces a tougher, more impact-resistant film with balanced orientation. Best for: heavy-duty bags, shrink film, applications requiring high tear strength.

Most high-barrier food packaging films are produced by cast co-extrusion. For a deeper look at the differences, see our cast film and blown film product pages.

Quality control: what we test

At FreshFlex, every production run is tested for:

  • Oxygen transmission rate (OTR) per ASTM D3985
  • Moisture vapor transmission rate (MVTR) per ASTM F1249
  • Tensile strength and elongation per ASTM D882
  • Heat seal strength per ASTM F88
  • Puncture and impact resistance
  • Coefficient of friction (COF) for machinability
  • Optical properties (haze, gloss)

For buyers, the key specs to request from any supplier are OTR (the lower the better for sensitive products) and seal strength (consistency matters for high-speed packing lines).