Relating Polymer Reptation to Material Extrusion 3D Printing: Mechanical Testing and Mathematical Modeling

Upper-division undergraduate students are introduced to polymer processing using material extrusion fused filament fabrication 3D printing to make poly(lactic acid) (PLA) mechanical testing specimens. Computer aided design and slicing software packages are used to demonstrate the process of preparing 3D computer models for printing. Following the introduction of the relevant ASTM standards, mechanical testing specimens are created by using a commercially available desktop printer to introduce tensile, compact tension, and impact resistance testing. Using two different polymer processing parameters, the weaker 90 degrees and stronger 0 degrees raster angles, students rationalize the high degree of anisotropy observed in printed thermoplastic objects. In the 90 degrees orientation, stress is transferred across "welds" or the interfaces between tracks of deposited material, whereas in the 0 degrees orientation, stress is transferred along continuous tracks of material. Using MatLab and the results from basic rheological testing, a nonisothermal degree of healing model is coded to understand why printed objects tend to fail at the welds. Thermogravimetric analysis and differential scanning calorimetry are used to obtain the key material properties of PLA that are needed for modeling. Fracture surfaces are visualized using optical microscopy following mechanical testing to better understand the failure mechanism and to visualize void spaces formed between tracks of the material. The students are evaluated individually by using a written technical report to assess their ability to synthesize the data from many lab activities into a logical sequence to convey their results. The modularity of the laboratory makes it amenable to instructors across a wide variety of disciplines and institutions.