Ravi Tomar1 Furkan Ulu1 Ram Mohan1

1, North Carolina Agricultural and Technical State University, Greensboro, North Carolina, United States

PolyJet printing technology has gained extensive application for additive manufacturing of polymeric materials with print resolution in the order of tens of microns, and capability of building complex multi-material morphologies. Additive manufacturing (AM) process uses tessellated 3D models. Digital models for AM require sub-models for individual materials and geometric descriptions assembled to create complex multi-material morphological and geometric configurations. Tessellation of assembled digital model however results in loss of geometrical congruency and chordal error such as gaps that occur between geometrically non-coplanar surfaces between sub-models. These approximation errors limit manufacturing repeatability of AM process and impact the final part quality. Present work investigates the influences of tessellation, built orientation during AM printing on the mechanical properties, behavior, and part quality in a PolyJet printing process. A design of experiments based on tensile deformation behavior of AM printed material test coupons following ASTM D638 Type 1 specimens was conducted. Digital geometry STL files for tensile testing coupons were obtained using: 1. non-assembled single material configuration, 2. same geometry formed with assembled planar sub-model geometries, 3. same geometry formed with assembled non-planar sub-model geometries. All three digital models were printed with the same polymer in all cases; different built directions for printing of test coupons were considered. Poor tessellation in case of digital print models impacted geometric accuracy that resulted in porosity of the printed part in the interface regions. In case of assembled non-planar geometry, poor tessellation was predominant compared to assembled planar & non-assembled geometries. This is due to more non-congruency between the interfaces that resulted in porosity at the sub-assembly interface regions and poor final part quality. Test specimens were also printed with multiple built orientations to analyze influence of tessellation along multiple built directions and correlating them to tensile mechanical behavior and properties.

Digital models with multi-material configurations allow design and generation of complex morphologies. CAD geometries with geometrical regions of assembled planar and non-planar inclusions of different sizes but with same volume fractions of inclusions are considered, thus digitally creating different material morphologies. Results from tensile behavior from such PolyJet printed ASTM test coupons formed with different morphological configurations, but printed with a single material thus capturing the effect from interfaces will be highlighted. Study findings provide insight on the influence of digital assembly and tessellation artifacts on part quality and tensile behavior. This understanding aids in optimal digital design of complex multi-material morphologies to ensure printed part quality and repeatability in AM.