FIBER LENGTH ATTRITION OF LONG-DISCONTINUOUS FIBER REINFORCED POLYMER PELLETS IN A SINGLE SCREW EXTRUDER

Single screw extrusion is widely used in injection molding, extrusion additive manufacturing, and material pre-compounding. A single screw extruder has three stages – the solid conveying zone, the melt-transition or compression zone, and the melt-conveying zone. As the pellets are processed, pellet rupture and fiber breakage occur in the transition and melt-conveying stages of extrusion. Existing literature focuses on modeling fiber breakage in fully molten stage, and there is a lack of understanding of fiber breakage during the partially molten – transition zone. Moreover, existing theoretical melting models apply to continuous solid melting and cannot be applied to study melting of individual pellets. As fiber length influences the thermo-mechanical properties of the manufactured composites, it is crucial to understand why and how fibers break. The goal of this thesis is to identify the mechanisms of pellet melting and fiber breakage by tracking the motion and heat transfer of an individual pellet. In the first part of this thesis, flow of long discontinuous fiber pellets through a single screw extruder is modeled using discrete element method. Results indicate a translational-conveying motion in the first half of the screw and rotational-conveying motion in the second half. In the second part, a sequentially coupled heat transfer model is developed to capture the melting of a single pellet, occurring mainly through the thermal contacts with the heated screw and barrel surfaces. Partial melting, partial crystallization, and re-melting are captured using melting and crystallization kinetics of semi-crystalline polymers. Heat transfer results indicate that the pellets melt from the outside-in, with a molten shell and a solid core. Based on the average pellet degree of melting, the region of interest for ‘melting zone’ is identified. Finally, some common modes of pellet deformation are identified for closer study.

Once the common pellet deformation modes are identified, analytical models based on three-point bending loading condition are developed to model pellet deformation. For a partially molten pellet with molten shell and a softer core, temperature dependent properties are used to estimate pellet deflection. The surface fibers are studied closely to identify a fiber separation mechanism. For the separated fibers, a Weibull based strength distribution is used to develop a fiber attrition algorithm for varying end loads. Results indicate that fiber attrition starts as soon as the outer layer of the pellet melts and then continues until the end of the screw. Recommendations for validation experiments and future work are provided in the end.