Co-extrusion is the process of pressing two or more materials through the same mold to produce a single piece. In this process, two or more orifices are arranged in such a way that the fusion and interlocking of the extrusions takes place and a laminar structure is formed before cooling. In co-extrusion, a separate extruder is used to feed each material to the mold, but the orifices can be arranged such that each extruder supplies two or more layers of the same material.
Each co-extruder has a different laminate layer which gives a required characteristic property such as heat-sealability, rigidity and impermeability, all of which are impossible to achieve using a single material. When multiple plastics are combined, the result can yield properties distinct from those of a single material.
Different are the end results with coextrusion, like film and multilayer film co-extrusion or packaging. Co-extruded film is a multilayer film produced by a compound molding technique that extrudes thermoplastic resin with multiple extruders. The material passes through a single die to build up layers of melted resin both on the inside and outside of a film. The same principles can be used for co-extruded packaging solutions, creating multilayer packaging to create more durable and strong packaging with a mix of materials.
Co-extrusion technology that involves combining two streams of products in the final die, can be used also for Food Extrusion, like snacks and cereals. Food extrusion generally involve direct expanded type of food, these means food that contains cereal grain flour with high starch content. These products have the characteristics of light, crispy, airy, or crunchy texture. In the manufacturing of these products, the phenomenon of direct expansion is utilized, where the starchy material expands as it exits the die of the extruder. In co-extrusion two extruders are used where the direct expansion can take place. Like direct expanded snacks, most of co-extruded products are made with low moisture content. Many coextruded products have the center core, as soft filling and are more often sweet. These include both breakfast cereals and snacks.
In general, Co-Extrusion has many advantages for the plastic and food industry:
- Use of different materials gives unique characteristics like stiffness, heat-seal-ability, impermeability
- Multi-layer production in one single extrusion process
- More complex product extrusion
- Reduce the number of steps required in general extrusion process