ToC Figure. This work introduces a versatile, reconfigurable direct ink write manufacturing method in tandem with a two-stage curing hybrid ink designed to fabricate high-strength, self-supporting parts in unconventional printing spaces, such as underneath a build surface or horizontally.
1. Introduction
Extrusion-based additive manufacturing (AM) such as fused filament
fabrication (FFF) and direct ink writing (DIW) are popular choices for
3D printing due to their low cost, simplicity, open framework, and broad
material versatility. In recent years, researchers have aimed to broaden
potential applications of extrusion-based AM by modifying build
hardware, control schemes, and layer structure [1-4]. Furthermore,
researchers have also utilized DIW material versatility to create
structures with a vast breadth of properties ranging from elastomeric,
viscoelastic, to high-strength rigid [5-9]. As a result, developing
novel AM techniques in tandem with ink feedstock with tuned mechanical
characteristics to create application-specific methods have been of
great interest [10-16].
An emerging application of AM is
to fabricate new components directly onto existing structures, which can
have unstructured surfaces unsuitable for conventional DIW 3D printing.
This includes applications where the existing build surface is either
too large or sensitive to be moved such as in-situ construction repair
and in-vivo bone and tissue repair [17-19]. However, conventional
extrusion-based techniques deposit layer on the XY plane with a
downward-facing extrusion nozzle and build parts upwards [20]. Due
to the limited printing space, 3D printed parts are then removed from
the tray to be assembled with others. Thus,
the ability to create parts in
unstructured manufacturing space by these conventional techniques is
limited. Fabricating on unconventional and nonplanar surfaces has been
explored via both redefining motion of the build plate and/or extrusion
head [21-23]. Yet, while previous studies have successfully created
geometries unattainable with conventional AM methods using a moving
build plate, this approach is unsuitable for printing onto an existing,
fixed build surface [24, 25]. Furthermore, studies utilizing
movement of the extrusion head predominantly used FFF to deposit rapidly
solidifying thermoplastics rather than DIW. This can be attributed to
the sensitivity to gravitational effects on the viscoelastic,
shear-thinning DIW inks requiring the extrusion nozzle to be facing
downwards during deposition, at the cost of impeding formable geometries
[26]. To overcome this barrier, a rapidly solidifying DIW ink must
be utilized. To achieve this, researchers have investigated inks with
polymer networks catalyzed via continuous ultraviolet (UV) irradiation
[27, 28]. Moreover, to further preserve shape integrity and improve
properties, researchers have developed multi-stage curing inks that
typically consists of a photopolymer and a thermal cure resin [29].
However, the secondary thermal curing requires placing printed structure
in an oven, which is impossible if the application is to print an object
on an existing structure.
In this work, we introduce a
novel DIW technique for manufacturing parts in non-traditional
environments whereby the extrusion nozzle is repositioned to facilitate
fabrication of parts in unstructured manufacturing spaces, such as
underneath the build surface or horizontally (Figure 1a). Parts
were created using a two-stage photo-epoxy thermoset resin wherein the
acrylate-based photo resin enabled rapid shape forming while the epoxy
resin developed a high-strength network highly compatible with the photo
cured polymer network at room temperature (Figure 1b, c). We
investigated various ratios of photo cure and epoxy monomer constituents
of the two-stage resin on mechanical characteristics to determine the
ideal balance of strength and rapid polymerization. The tough epoxy
resin constituent of printed ink rose to 50% of degree of conversion
(DoC) after 24 hours, increasing further to 73.2% after 48 hours, and
finally achieving 86.3% after 72 hours in ambient and room-temperature
conditions. Due to the photo-epoxy two-stage curing resin’s ability to
form an interpenetrating polymer network, the fully cured printed
structures demonstrated a high bonding affinity to a variety of
substrates. With a favorable photo-to-epoxy resin ratio, we then
demonstrated the capabilities of this technique via 3D printing of
several structures under a build platform, including a bio-inspired
“beehive”, conical structures with large overhangs, and load-bearing
arches (Figure 1c). Similarly, horizontal parts were printed
via reconfiguring the DIW extrusion nozzle such that the direction ink
deposition was parallel to the XY plane whereby structures could be
fabricated on a vertical surface. As a demonstration, we printed a
zero-support horizontal beam with integrated conductive elements enable
in-situ deformation sensing.
2. Experimental Methods and Materials
2.1 Two-Stage Resin Preparation
The two-stage curing resin consists of varying ratios of photopolymer
resin and epoxy resin (Figure 1a-c). A detailed breakdown of
chemical constituents can be found in Table 1. The photopolymer
resin component consists of 95 wt% ethoxylated trimethylolpropane
triacrylate (TMPTA) monomer (Sigma-Aldrich, St. Louis, MO, USA) and 5
wt% glycidyl methacrylate (GMA) (Sigma-Aldrich) as a reactive diluent.
The epoxy resin contains a 100:32 ratio of Epon 828 (difunctional
bisphenol A/epichlorohydrin (DGEBA); Hexion, Columbus, OH, USA) and
Jeffamine D230 curing agent (O,O′-Bis(2-aminopropyl) polypropylene
glycol-block-polyethylene glycol-blockpolypropylene glycol)
(Sigma-Aldrich). The photo curing and epoxy resins were separately hand
mixed, and then the resins were combined to form the two-stage resin,
followed by 5 minutes of magnetic stirring to further homogenize the
mixture. 1 wt% photo initiator Irgacure 819
(Bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide) (Sigma-Aldrich) and 3
wt% co-curing agent to the two-stage resin mixture to facilitate
photopolymerization and catalyze room-temperature curing, respectively.
The co-curing agent consists of 70 wt% Triethanolamine, 20 wt%
Piperazine, and 10 wt% N-aminoethylpiperazine (Sigma-Aldrich). Finally,
7 wt% fumed silica (Sigma-Aldrich) was mixed into the liquid two-stage
resin to enable shear-thinning behavior required for DIW 3D printing.
Due to the co-curing agent’s ability to catalyze epoxy resin at room
temperature, the working life of the two-stage ink was around four
hours.
Several different compositions of the two-stage curing resin were
created with varying ratios of photopolymer resin and epoxy resin:
70:30, 50:50, 30:70 wt% (denoted as PE30, PE50, and PE70,
respectively). Additionally, inks containing purely photopolymer resin
and purely epoxy resin (100:0, 0:100) were created (denoted as P and E,
respectively).
Once prepared, the two-stage resin was loaded into syringes and
centrifuged for 20 minutes to completely remove air bubbles, then
mounted to a custom built DIW printer [30]. The DIW printer uses
compressed air delivered by an Ultimus V air pressure controller
(Nordson EFD, East Providence, RI, USA) to extrude the created ink
through a tapered deposition nozzle (Figure 1a).
Additionally, a conductive DIW ink was prepared with ME603 conductor
paste (DuPont, Wilmington, DE, USA) combined with 3 wt% Timical Super
C45 carbon black (MTI Corporation, Richmond, CA, USA) to improve
conductivity and aid in shape retention post-extrusion [31].