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].