*Article* **A Force-Balanced Fiber Retardation Model to Predict Fiber-Matrix-Separation during Polymer Processing**

#### **Christoph Kuhn \* and Simon Wehler**

Volkswagen Group, 38442 Wolfsburg, Germany; simon.wehler@volkswagen.de **\*** Correspondence: christoph.kuhn@volkswagen.de

Received: 22 September 2020; Accepted: 27 October 2020; Published: 1 November 2020

**Abstract:** The use of discontinuous fiber reinforced composites in injection and compression molding faces a number of challenges regarding process-induced changes in microstructure, which have a significant influence on the mechanical properties of the final component. The changes in final microstructure are caused by complex fiber movements, such as fiber orientation, attrition and accumulation during flow. While there are existing phenomenological prediction models for both fiber orientation and attrition, the prediction of fiber accumulation due to fiber-matrix separation is currently only possible with a complex mechanistic particle simulation, which is not applicable in industrial simulations. A simplified phenomenological model, the fiber retardation model (FRM), for the prediction of fiber-matrix separation in commercially available software tools is presented in this paper. The model applies a force balance onto an interacting two phase flow of polymer melt and fiber phase and applies a retardation factor K to calculate the slowing and accumulation of the fiber phase. The general model is successfully applied to a simple compression molding simulation.

**Keywords:** plastics processing; fiber reinforced plastics; composites; glass fiber; sheet molding compound; long fiber; fiber orientation; fiber content
