Orientation of short fibers in reinforced thermoplastic parts

Polymers reinforced with staple fibers have significantly improved mechanical properties compared with the base resin, since the modulus of elasticity is multiplied by a factor of 3 to 5 compared with the same polymer without fibers. What's more, they can be shaped by conventional means such as injection molding or extrusion.

However, unlike structural composites, fibers cannot be optimally arranged. This is because, during forming, the fibers are transported by the fluid, and undergo stresses that orient them in certain directions. This not only determines the level and anisotropy of mechanical properties, but also the warpage of the parts or the level of residual stresses through anisotropic shrinkage.

In this article, we focus on orientation mechanisms, not on their impact on properties.

The materials concerned are mainly short-fiber-reinforced thermoplastics (average length 200 to 500 µm, diameter 10 to 15 µm), but the main mechanisms also apply to long-fiber-reinforced thermoplastics (long-fiber granules, GFL), or BMC (Bulk Molding Compound) reinforced thermosets, whose fibers are one to two centimeters long.