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Effects on mechanical properties of user-controlled and user-uncontrolled factors in a 3D printing system were investigated using a flame retardant carbon fiber reinforced nylon filament. Moisture and print orientation, both user controllable elements, dominated mechanical property variation. Dry samples exhibited tensile (flexural) strength 2.4 (2.1) times greater than that of wet samples, while ZX samples were measured to have about half the tensile strength of XY and XZ samples. User-uncontrolled factors including material to material variation and printer to printer variation were found to have minor impact (<5%) on part tensile strength. Reinforcing the part with 21 vol% continuous fiber lowers the moisture induced variance from 13% to 2% in addition to increasing the measured strength by about 10Â (to 451 MPa).

Environmental effects—temperature and moisture—on 3D printed part dimensional accuracy are explored. The coefficient of thermal expansion of four different nylon materials was determined for XY and ZX print orientations, with 0°, 45°/−45°, and 90° infill patterns. Unreinforced nylon exhibited a thermal expansion coefficient of the same order regardless of condition (from 11.4 to 17.5 × 10−5 1/°C), while nylons reinforced with discontinuous carbon fiber were highly anisotropic, for instance exhibiting 2.2 × 10−5 1/°C in the flow direction (0° infill angle) and 24.8 × 10−5 1/°C in the ZX orientation. The temperature profile of a part during printing is shown, demonstrating a build steady state temperature of ~ 35 °C. The effect of moisture uptake by the part was also explored, with dimensional changes of ~0.5–1.5% seen depending on feature, with height expanding the most. The effects of moisture were significantly reduced for large flat parts with the inclusion of continuous fiber reinforcement throughout the part.

Visualization and correct assessment of alveolar volume via intact lung imaging is important to study and assess respiratory mechanics. Optical Coherence Tomography (OCT), a real-time imaging technique based on near-infrared interferometry, can image several layers of distal alveoli in intact, ex vivo lung tissue. However optical effects associated with heterogeneity of lung tissue, including the refraction caused by air-tissue interfaces along alveoli and duct walls, and changes in speed of light as it travels through the tissue, result in inaccurate measurement of alveolar volume. Experimentally such errors have been difficult to analyze because of lack of ’ground truth,’ as the lung has a unique microstructure of liquid-coated thin walls surrounding relatively large airspaces, which is difficult to model with cellular foams. In addition, both lung and foams contain airspaces of highly irregular shape, further complicating quantitative measurement of optical artifacts and correction. To address this we have adapted the Bragg-Nye bubble raft, a crystalline two-dimensional arrangement of elements similar in geometry to alveoli (up to several hundred μm in diameter with thin walls) as an inflated lung phantom in order to understand, analyze and correct these errors. By applying exact optical ray tracing on OCT images of the bubble raft, the errors are predicted and corrected. The results are validated by imaging the bubble raft with OCT from one edge and with a charged coupled device (CCD) camera in transillumination from top, providing ground truth for the OCT.

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