  This work explores a spatial printing method to fabricate continuous
fiber-reinforced thermoplastic composites (CFRTPCs), which can achieve
exceptional mechanical performance. For models giving complex 3D stress
distribution under loads, typical planar-layer based fiber placement usually
fails to provide sufficient reinforcement due to their orientations being
constrained to planes. The effectiveness of fiber reinforcement could be
maximized by using multi-axis additive manufacturing (MAAM) to better control
the orientation of continuous fibers in 3D-printed composites. Here, we propose
a computational approach to generate 3D toolpaths that satisfy two major
reinforcement objectives: 1) following the maximal stress directions in
critical regions and 2) connecting multiple load-bearing regions by continuous
fibers. Principal stress lines are first extracted in an input solid model to
identify critical regions. Curved layers aligned with maximal stresses in these
critical regions are generated by computing an optimized scalar field and
extracting its iso-surfaces. Then, topological analysis and operations are
applied to each curved layer to generate a computational domain that preserves
fiber continuity between load-bearing regions. Lastly, continuous fiber
toolpaths aligned with maximal stresses are generated on each surface layer by
computing an optimized scalar field and extracting its iso-curves. A hardware
system with dual robotic arms is employed to conduct the physical MAAM tasks
depositing polymer or fiber reinforced polymer composite materials by applying
a force normal to the extrusion plane to aid consolidation. When comparing to
planar-layer based printing results in tension, up to 644% failure load and
240% stiffness are observed on shapes fabricated by our spatial printing
method.
