Discontinuous Fiber Orientation in Thermoplastic Processing

A fluid containing solid particles is commonly referred to as a suspension. These systems are found in various fields of physics and have numerous industrial applications. Among these suspensions, this article focuses on thermoplastic polymers filled with discontinuous fibers. Unlike structural composite materials, where continuous fibers are aligned in thin layers or woven, discontinuous fiber composites offer an attractive combination of mechanical properties, light weight, low cost, and the ability to design parts with complex geometries.

Most of these composite materials contain discontinuous fibers, usually glass or carbon, which are processed in different ways depending on their length. Short fiber material pellets are prepared by extruding chopped fibers (with average lengths of less than 1 mm) and the thermoplastic matrix. Long-fiber materials are most often obtained by impregnating a bundle of continuous fibers with the thermoplastic matrix, usually by extrusion, and then cutting these coated fibers to a length of approximately 12 mm to produce pellets. These different pellets are then shaped using conventional methods such as injection or compression molding, extrusion, or additive manufacturing, particularly using fused filament deposition 3D printers. For long-fiber materials, flow in the tools significantly reduces the length of the fibers, but final lengths greater than 1 mm can be achieved by taking certain precautions.

The addition of discontinuous fibers to a thermoplastic is a common operation because it improves the mechanical properties of the material. This reinforcement depends on the orientation of the fibers in relation to the directions of the mechanical stresses experienced by the final part. This orientation of the fibers in the part is caused by the flow of the composite in the forming tools. The addition of these particles also modifies the rheological behavior of the system. In general, they cause an increase in viscosity but also a change in flow lines, the appearance of a flow threshold, and possibly changes in the melt front. To understand, improve, and control this orientation, it is therefore necessary to model the movements of the fibers when the composite material flows within a forming tool.