Coextrusion

Coextrusion uses multiple extruders, each feeding a different resin or resin blend, that meet inside a single feedblock or multi-manifold die before being formed into one continuous film. Rather than blending the resins together, coextrusion keeps each material as its own distinct layer within the finished film, stacked together like a microscopic layer cake that can be anywhere from three to eleven or more layers thick on advanced lines.

The purpose of coextrusion is to let each layer do a specific job that a single resin couldn't do as well or as economically on its own. A typical three-layer structure might use a tough LLDPE-based core for strength, an outer layer formulated with slip and anti-block additives for good machinability, and an inner sealant layer like LDPE for reliable heat sealing. More complex structures add barrier layers like EVOH or nylon, tie layers to bond otherwise incompatible resins, and sometimes a recycled or filled layer in the middle to manage cost.

Coextrusion is what makes modern flexible packaging possible at the performance and cost levels customers expect; it lets a converter use small amounts of expensive specialty resins exactly where they're needed, like a thin barrier layer, while filling the bulk of the structure with lower-cost commodity resins like LLDPE or LDPE.

A food packaging film that needs both strength and an oxygen barrier might be built as a five-layer coextruded structure: an outer layer for printability, a tie layer, an EVOH barrier core, another tie layer, and a sealant layer, all extruded simultaneously into one film.

People sometimes assume a coextruded film is automatically a blended or homogenous mix of its component resins, but the layers actually remain physically distinct and visible under cross-section, even though they're permanently bonded together. This is precisely why tie layers exist: many resin pairs, like polyethylene and EVOH, won't bond to each other without a compatibilizing tie layer in between.