Multilayer film or sheet prepared by coextrusion is extruded wherein the thickness of the inner layers is varied by varying the flow of one component to the central portion of the sheet while diverting that portion to an external portion of the sheet or film....http://www.google.ca/patents/US4094947?utm_source=gb-gplus-sharePatent US4094947 - Multilayer coextrusion process for producing selective reflectivity
