A film casting simulation has been used to demonstrate why the encapsulation extrusion coating process is so effective industrially in enhancing the stability of the extrusion coating of low-melt-strength polymers. In the present study, it is particularly intended to explain theoretically why and how the co-extrusion of high-melt-strength polymers like low-density polyethylene (LDPE)-attached at the periphery using encapsulation dies-dramatically improves the film coating process of low-melt-strength polymers like high-density polyethylene (HDPE). The undesirable neck-in and draw resonance phenomena frequently occurring in the extrusion coating of HDPE are shown in this study to be due to the fact that lower axial tension in the HDPE film rendered by its low melt strength leads to process instabilities. High melt strength LDPE will then increase the film tension when externally-attached, and as a result improves the stability of HDPE extrusion coating. (C) 2004 Elsevier B.V. All rights reserved.