a very thin inner layer composed of a polymer chosen for its barrier and / or inertness properties is fabricated within a container preform constructed mainly from another polymer , or a structure of polymers . the inner layer on the preform is produced either by a controlled coating method involving coating of the injection mold core rod prior to injection molding or by a coating applied directly to the preform after injection molding . this enables both inner and main layers to be brought together at a time when the interface between them is molten . a tie layer can be employed , when desirable , to enable the layers to be combined without melting the layer interface .