Split-billet coextrusion

The coextrusion processes are the second group of methods for processing "unprocessable" polymers. They are divided into split billet coextrusion and concentric billet coextrusion. In principle they involve the extrusion of the unprocessable component in a sandwich with a processable polymer, to promote extrusion. A form of substrate drawing is involved. 

Split Billet Coextrusion. During the development of the split billet technique, it was realized that by placing a polymer film between the billet halves, the film would be extruded with the outside halves and ultradrawn thin film could be produced (Figure 4). 0.1 mm thick films of Alathon 7050 draw ratio 24 were produced at 120°C, a draw ratio that was unobtainable using a slit die. The film in the center of the billet is forced to follow the two outside halves because of the compression in the conical die and friction between the components. 

These examples show how versatile the split billet coextrusion process is in producing oriented thin films. 

These examples illustrate the use of the split billet coextrusion process for film extrusion and show some of the factors that are important in optimizing the process. 

In the present extrusion of HDPE ribbons, the deformation patterns were examined by the deformation of parallel ink marks preimprinted on the surface of a HDPE (Figure 1-d). At EDR >12, the low and high MW HDPE exhibited a typical shear parabola and a W-shape deformation profile, respectively, with both characteristics enhanced at higher EDR as shown in Figure 2. These characteristics of the deformation patterns are in well agreement with our previous observations (3) and further confirm the previous conclusion that there is no significant effect of cutting a billet into two halves and/or coextrusion of a film with the split billet halves on the deformation flow patterns. 

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