# Effect of Bead Geometry and Layer Time on Microstructure and Thermomechanical Properties of Large-Format Polymer Composites

## TL;DR

This study examines how bead geometry and layer time affect the microstructure and thermomechanical properties of large-format polymer composites.

## Contribution

The paper introduces a heat transfer model and reveals how bead width and shear rate influence fiber orientation and material performance in large-format additive manufacturing.

## Key findings

- Increasing bead width causes a thinning effect in the fiber-rich shell, which is reduced at higher shear rates.
- Randomly oriented fibers in the core increase the coefficient of thermal expansion in the x-direction for larger beads at higher shear rates.
- Interlayer strength drops rapidly when layer time exceeds the cooling time below the glass transition temperature.

## Abstract

Large-format additive manufacturing (LFAM) is a manufacturing process in which high volumes of material are extruded in a layer-by-layer fashion to create large structures with often complex geometries. The Loci-One system, operated and developed by Loci Robotics Inc., is an LFAM-type system that was used to print single-bead walls of 20% by weight carbon fiber reinforced acrylonitrile butadiene styrene (CF-ABS) using various print parameter inputs. This study observed the influence of bead width and layer time on thermomechanical performance via material characterization techniques that accounted for the complex microstructure of LFAM parts to develop a better understanding of parameter–structure–property relationships. Printed parts were characterized by measuring the coefficient of thermal expansion (CTE) and interlayer strength. Near the edges of the printed beads, microscopy revealed a “thinning effect” experienced by a shell composed primarily of highly oriented fiber as the bead width was increased; however, this effect was diminished with a higher shear rate. The CTE results demonstrated the influence of mesostructure on the thermomechanical response. Increased shear rates were expected to lower CTE in the x-direction due to a higher ratio of fiber oriented in the print direction, but this relationship was not always observed. For the larger bead widths printed at higher shear rates, the randomly oriented fiber at the core dominated the thermomechanical response and increased CTE overall in the x-direction. A heat transfer model was developed for this work to determine how much time was required for the deposited bead to cool to the glass transition temperature. Interlayer strength results revealed a rapid decrease once the printed layer time exceeded the time required for the extrudate to cool below the glass transition temperature.