Nested mandrel

Coextrusion can be performed with flat, tubular, and different shaped dies. The simplest application is to nest mandrels and support them with spiders or supply the plastic through circular manifolds and/or multiple ports. Up to 8-layer spiral mandrel blown film dies have been built that require eight separate spiral flow passages with the attendant problem of structural rigidity, interlayer temperature control, gauge control, and cleaning. Many techniques are available for coextrusion, some of them patented and available under license (Chapter 5). 

There are two main types of coextrusion blown film dies concentric (also called nested mandrel) and stack (also called pancake). Comparisons of these die types were published in two mid-1990s reports [10,11] and are summarized below. 

Stack dies (Fig. 3.7) separate the flow gaps between vertically stacked plates instead of between nested mandrels. Theoretically, any number of plates can be stacked to add one layer for each parting line between plates. This technique yields some potential advantages. Because the various extruders can be located in a fan around the... 

To produce multilayer and multicolor extrudates, for example for cable sheathing, packaging films with barrier layers and/or recycled layers, two to seven extruders can be fitted with multiple-layer blown film fittings with the same number of nesting spiral mandrels to distribute the melt flows (Fig. 20). 

Concentric dies (Fig. 3.6) are comprised of a series of hollow spiral mandrels nested inside one another. A narrow gap remains between each mandrel and the one directly inside it. Inside these gaps is where polymer flows. The different gaps separate the various polymer layers. Just prior to the die exit, the different polymers flow together into a single flow channel. Their laminar flow behavior mainly keeps the polymers in separate layers through the final product. These types of dies have been used for many years and perform very well. A couple of possible concerns to consider when designing... 

Dies, coextrusion n. Dies of nested configuration. Individual polymers flow through separate mandrel passages and are combined in the primary land area. Adjacent polymer layers must exhibit similar flows and adhesion characteristics to form the coextrusion. 

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