Co-Extrusion and Multi-Layer Blow Molding

Co-extrusion uses two or more plastic materials in the same extrusion process. Each material is melted separately and guided through a die so the layers combine into one parison. After molding, the product has a unified wall structure that keeps the selected material properties in different layers.

In multi-layer extrusion blow molding, multiple screws melt and feed different materials into a multi-layer die head. The die forms a layered parison, which is inflated with compressed air in the mold and cooled to form the container. The finished container can be configured for product needs such as barrier performance, recycled material use, or color.

1. Barrier performance: EVOH or another barrier material can reduce oxygen permeation; performance depends on layer structure, thickness, and use conditions.
2. Appearance control: Outer layers can use virgin resin or color masterbatch to meet surface and color requirements.
3. Material cost: Recycled material can be evaluated for a middle layer, with virgin inner and outer layers selected for the product requirements.
4. Recycled material placement: Recycled resin may be considered in an inner or middle layer where the product requirements allow; suitability depends on the contents, resin grade, product structure, and market requirements.

Common structures include two layers with virgin resin outside and recycled material inside, which can increase recycled-material use while meeting material and appearance needs; three layers with virgin inner and outer layers plus recycled material in the middle; two layers with a thin outer layer of color masterbatch blended with virgin resin and a virgin inner layer to reduce use of the more expensive color masterbatch; and three layers with a barrier or specialty material such as EVOH on the outside, adhesive (AD) in the middle, and virgin resin inside. Specialty materials are often more expensive, so reducing that layer's thickness may help limit material use. The location and proportion of recycled resin should be evaluated against the contents, resin grade, product design, and market requirements. Recycled resin in an inner layer contacts the contents. A middle-layer structure alone does not establish contact suitability or acceptance in recycling streams.

Recycled material is often darker than virgin resin, so it may show through on white or light-colored products. Solutions include increasing the outer layer thickness or adding color masterbatch to the recycled layer. Resin compatibility, adhesive needs, scrap handling, and layer thickness stability must also be checked.

Co-extrusion is worth evaluating when a product needs barrier performance or a specific appearance, but it is not necessary for every container. Since 1976, Jonh Huah has manufactured blow molding equipment and can assess a configuration based on the contents, barrier requirements, recycled-material proportion, production speed, and downstream leak-testing requirements, while checking layer count, resin compatibility, die-head design, scrap handling, and target wall distribution. For simple containers, a single-layer machine may still be the more practical choice; suitability depends on the contents, material, product design, and market requirements.