Extrusion coating is when the melted polymer is extruded on to an existing film before passing through the calender rolls. The existing film can be another polymer, cardboard, woven and non-woven fabrics, metallic foil or paper. Multiple layers may be formed by extrusion coating both sides of the primary film or building a multilayer structure by introducing several extrusion coated layers. Coextrusion can only be used for polymers with similar processing conditions. Where the processing conditions are different, particularly in the case of substrates that cannot be melted with the polymer, such as metallic foil or paper, then extrusion coating is the only choice.