(689a) In Situ Nanometrology of Digital Light Processing Additive Manufacturing

Digital light processing (DLP) additive manufacturing (AM) is rapidly realizing its potential as a transformative manufacturing technology that affords unprecedented design flexibility and customization of final parts. With increased interest for deployment in critical applications such as healthcare, the need for fundamental insight into mechanical performance and feature resolution is increasing. In DLP, part formation occurs at millisecond to second times scales and micrometer length scales. Due to species diffusion, local depletion, and light absorption, the resultant parts exhibit inherent variation in conversion that corresponds with anisotropic chemical, thermal, and mechanical properties. This undesired heterogeneity compromises part performance compared to bulk counterparts. With high resolution and the ability to sense material properties, the atomic force microscope (AFM) is well suited to study the resin printing process in various in-situ and ex-situ modalities.¹ However, to date, integrated instruments combining 3D printing and nanometrology have not existed. Here, we introduce a hybrid instrument combining an AFM, an inverted optical microscope, and a 405 nm digitally programmable projection source.² We show that the instrument can be operated in two core modalities. In the first, the nanomechanical properties of patterned regions are spatially mapped as a function of print conditions. Notably, these properties can be compared in the just-printed, monomer-swollen state, and the ethanol-washed, dry state. Voxel modulus is found to vary depending on feature size and the absence/presence of neighboring voxels. In the second modality, the SPM acts as a stationary, local, high speed probe of cure rheology. It is uniquely capable of measuring polymerization at the small length scales and fast time scales of 3D printing. We find that for a thiol-ene resin, conversion is detected 10’s of microns away from the light source. Furthermore, increasing light intensity to speed reaction rate does not fundamentally increase spatial control of cure by a significant amount. Overall, the hybrid instrument provides unique insight into future pathways to improve resolution and homogeneity of DLP parts.

1) Fiedler‐Higgins, C. I., Cox, L. M., DelRio, F. W., Killgore, J. P. Monitoring Fast, Voxel‐Scale Cure Kinetics via Sample‐Coupled‐Resonance Photorheology, Small Methods 2019, 3, 1800275.

2) Higgins, C.I., Brown, T.E., Killgore, J.P. Digital Light Processing in a Hybrid Atomic Force Microscope: In situ, Nanoscale Characterization of the Printing Process (submitted)

Figure 1: In-situ photopolymerization in the hybrid SLA/AFM. a) Optical view of AFM cantilever and projected 405 nm pattern, with composite map of resultant surface stiffness on right. b) Checkerboard 3D print nanomechanically mapped in monomer-swollen environment. Significant effects of inter and intra voxel interaction are observed as stiffness variations.