06/15/2026
By Danielle Fretwell

The Francis College of Engineering, Department of Mechanical Engineering, invites you to attend a Doctoral Dissertation defense by Stiven Kodra titled: "Design, Characterization, And Control Of A Compact Pellet Extruder For Sustainable Fused Granular Fabrication"

Candidate Name: Stiven Kodra
Degree: Doctoral
Defense Date: Monday, June 22, 2026
Time: Noon - 2 p.m.
Location: ETIC 445

Committee:

  • Advisor: David O. Kazmer, Ph.D., PE, Professor, Plastics Engineering, University of Massachusetts Lowell
  • Christopher J. Hansen, Ph.D., Mechanical Engineering, University of Massachusetts Lowell
  • Juan Pablo Trelles, Ph.D., Mechanical Engineering, University of Massachusetts Lowell
  • Davide Masato, Ph.D., Plastics Engineering, University of Massachusetts Lowell

Abstract: This dissertation presents the design, characterization, and control of a compact pellet extruder for fused granular fabrication (FGF), motivated by the need for lightweight, energy-efficient systems capable of processing post-consumer recycled and heterogeneous polymer feedstocks. The work was conducted in support of a U.S. Army SBIR Phase II program in partnership with re:3D Inc., with the applied objective of fabricating functional supplies directly from plastic waste streams. Three interconnected research thrusts — screw design, steady-state characterization, and transient dynamic control — form a vertically integrated framework spanning hardware design through process control. 

The first thrust presents the design, simulation, and experimental validation of a novel mixing screw with an 8:1 length-to-diameter (L/D) ratio, incorporating double flights with variable pitch and counter-rotating mixing slots that promote chaotic advection and melt homogeneity despite the shortened axial length. Validation on a 20 mm pilot-scale extruder processing high-impact polystyrene and recycled polypropylene confirmed linear throughput scaling with screw speed and specific energy consumption of 0.264–0.344 kWh/kg at 31–57% thermodynamic efficiency, outperforming conventional general-purpose and barrier screws at 27:1 L/D. 

The second thrust scales the design to a 31 mm extruder and characterizes its steady-state performance across a full-factorial design of experiments using post-consumer recycled polypropylene. Regression models achieved R² > 0.92 across all responses, and the process gain matrix confirmed intuitive, decoupled control: barrel temperature governed melt temperature while screw speed-controlled mass output at ~1.17 g/min per RPM. A minimum specific energy consumption of 0.254 kWh/kg was achieved at 76% thermodynamic efficiency, a 26% improvement over the prior design. 

The third thrust addresses lag and drool — transient extrusion defects at road starts and ends in screw-driven FGF for which no compensation strategy had been reported for recycled feedstocks. A structured design of experiments across two materials (recycled polypropylene and a wood-fiber-reinforced composite) was used to quantify these defects, and empirical system identification yielded first-order transfer function models relating screw speed to deposited road width. These models establish a direct, physics-based connection between the extruder's melt compressibility time constant and the Linear Advance compensation factor, implemented as a hardware-agnostic G-code post-processor requiring no firmware or hardware modification. Collectively, the three thrusts demonstrate that compact 8:1 L/D mixing screws can process recycled and fiber-filled feedstocks with energy efficiencies competitive with conventional 27:1 L/D designs, while enabling integration into lightweight, deployable printing platforms in support of sustainable additive manufacturing.