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PREDICTING AND IMPROVING MECHANICAL STRENGTH OF THERMOPLASTIC POLYMER PARTS PRODUCED BY MATERIAL EXTRUSION ADDITIVE MANUFACTURING
Restricted (Penn State Only)
Doctor of Philosophy
Date of Defense:
November 16, 2018
Timothy William Simpson, Dissertation Advisor
Timothy William Simpson, Committee Chair
Charles E Bakis, Committee Member
Michael Anthony Hickner, Committee Member
Nicholas Alexander Meisel, Outside Member
Material Extrusion Additive Manufacturing (MEAM) is an additive manufacturing technology where parts are built by selectively depositing extrudate in a layer-by-layer process. Thermoplastic polymers are the most commonly used class of materials to produce MEAM parts. Strength is developed in these thermoplastic polymer MEAM parts when polymer molecules diffuse across the interface between adjacent roads and layers of deposited extrudate and become entangled with molecules on both sides of the interface. This interfacial diffusion and entanglement is known as polymer welding. Determining the strength of these intra-road and intra-layer weld interfaces is key to determining MEAM part strength. A theory for determining the strength of thermoplastic polymer MEAM parts is presented. The novel equation to calculate the strength of polymer weld interfaces within MEAM parts is derived. Part strength is then calculated, with proper consideration given to the internal structure and possible failure modes of MEAM parts. Part strength prediction calculations are then validated experimentally using two different materials and eight different build strategies. Predicted part strengths fall within 5% of the experimental mean for each material and build strategy combination tested. Effects of build discontinuities on MEAM part strength and deformation of MEAM parts under tensile load are also explored. Changes in build strategy are shown to change strength of the MEAM parts by changing the thermal history at the weld interfaces within the MEAM parts. Using the knowledge of how build strategy effects themal history, a revised build strategy for complex geometry parts is presented. The revised build strategy is shown to increase strength of the complex geometry part by 45%. Deformation of MEAM parts is studied using Digital Image Correlation (DIC), a full-field strain measurement technique. Parts with solid infill are shown to respond to tensile deformation in a manner similar to conventionally manufactured parts. Deformation of sparse infill geometry parts are also explored. Using information from these experiments, a novel sparse infill geometry is presented and shown to outperform conventional sparse infill geometries.
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