Figure 3 . Adhesion strength characterization of DIW printed
fully cured two-stage resin structures. (a) Two-stage resin
structures printed onto (i ) acrylic, (ii ) wood,
(iii ) glass, (iv ) aluminum, and (v ) concrete
substrates. (b) Experimental setup showing the tensile grip
clamped to the sample on fixed substrate. (c) Quantified
adhesion strengths of printed structures on the aforementioned
substrates at 0 hour (solid) and 72 hours (patterned).
3.3 Unstructured 3D Printing
To demonstrate the capabilities of our unstructured AM process and
two-stage resin, we 3D printed various inverted structures including a
bio-inspired “beehive”, conical geometries, and load-bearing arches
(Figures 4a-d, Video 1). Using the inverted printing technique,
we were able to create conical structures with large overhangs without
support materials due to a combination of gravitational forces acting
“upwards” relative to the structure as well as rapid
photopolymerization to solidify layers. The synergy of these two
phenomena enabled us to create large structures with comparably small
base layers without sagging. Furthermore, the multi-phase polymerization
mechanism of the printed ink ensured that the large structures preserve
their shape integrity over time, thereby ensuring the larger structures
continue to resist gravitational forces and do not delaminate from the
substrate. To further exemplify the demonstrated overhang printing
capabilities coupled with progressive substrate adhesion strength, we
printed 24 mm tall arches (Video 2). Upon fully cured, the
arches could support a one-kilogram weight, as corroborated by adhesion
strength characterization previously discussed (Figure 4e).
This is of particular interest when considering the application space of
such a structure. To accomplish this feat using conventional DIW
manufacturing, the thin-walled structure would likely need to be printed
with support material [26], then removed from the original print
substrate and repositioned in an inverted configuration, thereby
compromising the bond strength between the structure and end-use
surface. Conversely, the proposed unstructured print technique can
satisfy this load bearing task with an as-printed, high-strength
structure.