coextrusion
The extrusion of multiple layers of material simultaneously.
coextrusion: pushing multiple materials through one die at once
Coextrusion forces two or more materials through a single die opening at the same time, bonding them into a composite structure as they emerge. Rather than laminating separate extruded sheets afterward, the materials converge inside the die itself and fuse under pressure and heat, creating a permanently bonded multilayer product in one operation. This happens in the extruder barrel, where separate polymer streams or one polymer with reinforcing material are fed into distinct channels that merge before the die exit.
The most common setup uses a feedblock or multiple screws to control material flow rates independently, ensuring each layer enters the die at the correct proportions and temperature. A two-layer film might pair a tough outer skin (polypropylene) with a flexible core (linear low-density polyethylene), each fed at different speeds to achieve the desired thickness ratio. Three-layer and five-layer coextrusion is routine in food packaging, where barrier materials like ethylene vinyl alcohol sit between structural layers. Tube extrusion can coextrude rigid outer walls with softer inner liners for applications like medical tubing or industrial hose.
Why use it instead of gluing or laminating. The bond is chemical and permanent, not adhesive-dependent, so it survives thermal stress, moisture, and flexing without delamination. Coextruded film cools and consolidates immediately, allowing faster line speeds than postlam operations. The process also eliminates waste from separate extrusion runs and reduces handling steps.
The main control variables are barrel temperature, screw speed, and melt pressure in each channel. If feed rates drift out of balance, one layer becomes too thin or uneven, compromising strength or barrier properties. Material compatibility matters: both polymers must have similar melting ranges and rheological behavior to fuse properly without shearing or separating at the die. Recycled content and virgin resin can coextrude together provided the melt index difference is manageable.
Coextrusion appears in film, sheet, tube, profile, and monofilament processes. Extrusion coating, where a molten polymer layer is applied directly to paper or foil, is a related technique but simpler: it adds one layer to an existing substrate rather than combining multiple feeds into a die. The term originated in the 1950s and refers directly to the simultaneous extrusion of multiple streams.