Understanding Co-Extrusion and Multi-Layer Blow Molding Processes

Co-Extrusion is a specialized processing technology used in blow molding machines, extruders, and blown film machines. This technique involves using two or more plastic materials, which are melted into a liquid state at high temperatures and then introduced into a precise die. Through the internal flow path system design, the molten plastics combine into a single form. Due to the high-temperature melting and extrusion of multiple plastic materials, the final product inherits the properties of the different plastics and forms a unified structure that does not separate over time.

In blow molding machines, co-extrusion is also known as multi-layer extrusion. Multiple screws are used to melt and extrude different raw materials, which then converge and extrude through layers within the die. The end result is a parison with multiple layers of different materials at the same thickness. These parisons are then inflated with compressed air and cooled in a mold, resulting in a bottle or container that combines different material properties while conserving resources.

1. Barrier Properties: For instance, using EVOH in the inner layer prevents the product from contacting oxygen, extending its shelf life.
2. Improved Appearance: Using EVOH on the outer layer enhances the visual appeal, making the product more attractive.
3. Cost Reduction: Using recycled material for the inner layer and virgin material with color masterbatch for the outer layer reduces the cost of new material and colorants.
4. Compliance with Environmental Regulations and ESG Requirements: By using more recycled material in the middle or inner layers while maintaining a new material outer layer, the appearance remains unchanged, ensuring customer experience is not affected.

1. Two Layers: Virgin material on the outer layer, recycled material on the inner layer. This setup maximizes the use of recycled materials for environmental benefits and material savings.
2. Three Layers: Virgin material on the outer and inner layers with recycled material in the middle. This allows for more recycled material usage while ensuring that the recycled material does not contact the internal liquid.
3. Two Layers: Outer layer with color masterbatch and virgin material, inner layer with virgin material. The outer layer can be made thinner to save on expensive color masterbatch.
4. Three Layers: Barrier or special material (such as EVOH) on the outer layer, adhesive (AD) in the middle layer, and virgin material on the inner layer. Since special materials are usually more expensive, making the outer layer thinner helps save on costly materials.

Since recycled materials usually have color and are darker than virgin materials, if the product is white, the color of the middle layer might be visible, affecting the product’s appearance quality.
Solutions:
1. Increase the thickness of the outer layer to make the middle layer color less visible.
2. Add color masterbatch to the middle layer of recycled material to match the outer layer color, minimizing the color difference.

Co-extrusion is worth considering when the product needs barrier performance, recycled middle layers, special color effects, or improved stiffness without using a single expensive material throughout the wall. Jonh Huah evaluates the number of layers, resin compatibility, die head design, scrap handling, and target wall distribution before proposing a machine configuration. For simple containers, a single-layer machine may still be the more practical choice.