One-step volumetric additive manufacturing of complex polymer structures

Abstract

Two limitations of additive manufacturing methods that arise from layer-based fabrication are slow speed and geometric constraints (which include poor surface quality). Both limitations are overcome in the work reported here, introducing a new volumetric additive fabrication paradigm that produces photopolymer structures with complex nonperiodic three-dimensional geometries on a time scale of seconds. We implement this approach using holographic patterning of light fields, demonstrate the fabrication of a variety of structures, and study the properties of the light patterns and photosensitive resins required for this fabrication approach. The results indicate that low-absorbing resins containing ~0.1% photoinitiator, illuminated at modest powers (~10 to 100 mW), may be successfully used to build full structures in ~1 to 10 s.

Fig. 1. Holographic volumetric 3D fabrication system schematic and example structures.

(A) SLM, liquid crystal on silicon spatial light modulator; FTL, Fourier transform lens; BB, beam block to eliminate undiffracted light; HP, hologram plane; 4fN, telescope lens pairs in the “4-f” configuration used for beam expansion or image relaying [4f2 incorporates a pinhole spatial filter (SF)]. The inset image details the configuration of 45° prism mirrors for directing image subcomponent beams at orthogonal directions into the resin volume. (B to G) Structures fabricated using this system, each from a single exposure of 5- to 10-s duration. Scale bars, 2 mm.

Fig. 2. Induction time and curing dose dependence on key process parameters.

(A) Summary of polymerization induction times t_i before gelation in three-beam regions, as determined by the first appearance of cube edges, showing strut sizes from 0.6 to 1.2 mm. Error bars are estimates of data reproducibility based on N = 3 measurements at typical conditions, given one-sided due to the tendency of cure time measurements to bias upward from gradual resin degradation. Colored dotted lines are power-law fits to the data at each PI concentration. The black dashed line is the equation ${\mathit{t}}_{\mathrm{i}}=\frac{[{\mathrm{O}}_{2,0}]}{{\mathit{R}}_{\text{init}}}$, where the variables on the right-hand side are estimated from measurements of system parameters or similar resin formulations. The insets show a typical cube structure used to generate these data, and an intensity-compensated image that was used for exposure. (B) Comparison of model-predicted and experimentally measured three-beam gel times t_G3, with the dashed line indicating unit slope. Data from three different laser powers between 6 and 40 mW are represented at each PI concentration. (C) Energy doses required to cure cube struts (three-beam regions), plotted for the highest and lowest beam power used at each PI concentration.

Fig. 3. Optical attenuation and three-beam superposition compensation model.

(A) Representative plane at which all three-beam contributions are calculated, shown in (B) as a heat map representing relative intensities. Beams 1 and 2 are incident from the left and bottom as indicated by black arrows, and beam 3 is directed into the page. (C and E) Summed volumetric absorption values from three-beam superposition, without compensation, at the location marked by the dashed line in (B), comparing different [PI]. (D and F) Intensity profiles at the same [PI] but compensated to attain equal peak intensity in three-beam overlap regions.

Fig. 4. A process performance comparison of volumetric fabrication to other polymer-based AM methods.

Resolution is defined as 1/(2δ), where δ is the minimum feature size. The gray dashed boundary oval encloses fabrication results from two scenarios and represents the authors’ speculation regarding the near-term potential of the volumetric fabrication method reported in this work. Plotted data points represent specific published results or system operating parameters known first hand to the authors. PμSL/LAPμSL, projection micro-stereolithography and its large-area variant (8, 25, 31); CLIP (10), continuous liquid interface printing; DIW, direct ink writing (–34); DLW, direct laser writing; SLA, stereolithography; SLS, selective laser sintering. Commercial system performance is based on the manufacturer’s specifications.