Modeling flow in extrusion dies

The continuous manufacture of semi-finished or finished products of constant cross-section (sheets, films, tubes, profiles, etc.) by extrusion through a die is by far the most widespread plastics processing method. In 2001, for example, the tonnage of materials processed by extrusion in France amounted to 2.1 million tonnes, or 47% of total consumption of thermoplastic products.

In descending order of importance, still in terms of tonnage of material processed, the various products concerned by this process are :

• Blown films: mainly polyethylenes (PE-BD, PE-BDL, PE-HD) and, in particular, for the past ten years, the new metallocene polyethylenes;

• sheets and plates: PVC, polypropylene, polystyrene ;

• pipes: almost exclusively PVC, with some polyethylene (mainly high-density), but also polyamide and silicones;

• Profiles: mainly PVC ;

• finally, insulation for electrical and telephone cables: PVC, low-density polyethylene and specific highly-filled formulations.

An extrusion line generally consists of the following components:

• a single-screw or twin-screw extruder that melts or plasticizes the polymer, pressurizes it, and delivers a regular flow of material at a temperature that is as homogeneous as possible;

• the die, with its more or less complex geometry, which gives the product the required shape.

On leaving the die, the product may undergo further mechanical treatments (forming, stretching, blowing, etc.), while being cooled and then packaged.

The main role of an extrusion die is to provide a product of a given size at the tool outlet, perfectly uniform in terms of flow rates and temperatures. This is particularly important for the phases that follow extrusion itself (drawing, forming, cooling, etc.), where any heterogeneity at the die outlet can only be exacerbated and lead to an incorrect product.

Flow in a die is therefore a distribution problem, and the geometrical design of these tools must enable this distribution to be achieved as effectively as possible, for the highest possible flow conditions. For a long time, this design was based on empiricism and the experience of design offices and users. Thanks to advances in numerical simulation and the development of IT tools, a more scientific approach to the problem now makes it possible to build on the experience of practitioners, and to approach the design of extrusion dies in a rational, high-performance way.

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