3D printing of thermosets with diverse rheological and functional applicabilities

Abstract

Thermosets such as silicone are ubiquitous. However, existing manufacturing of thermosets involves either a prolonged manufacturing cycle (e.g., reaction injection molding), low geometric complexity (e.g., casting), or limited processable materials (e.g., frontal polymerization). Here, we report an in situ dual heating (ISDH) strategy for the rapid 3D printing of thermosets with complex structures and diverse rheological properties by incorporating direct ink writing (DIW) technique and a heating-accelerated in situ gelation mechanism. Enabled by an integrated Joule heater at the printhead, extruded thermosetting inks can quickly cure in situ, allowing for DIW of various thermosets with viscosities spanning five orders of magnitude, printed height over 100 mm, and high resolution of 50 μm. We further demonstrate DIW of a set of heterogenous thermosets using multiple functional materials and present a hybrid printing of a multilayer soft electronic circuit. Our ISDH strategy paves the way for fast manufacturing of thermosets for various emerging fields.

Fig. 1. Overview of in situ dual heating-enabled DIW of thermosets.

a Schematic illustration of the DIW platform in which a Joule heater is integrated into the nozzle. The freshly extruded ink undergoes in situ gelation as being heated bilaterally by the Joule heater and the cured ink base, referred to as in situ dual heating (ISDH). b The gelation time at different temperatures of a representative thermosetting ink (Sylgard 184). c Comparison of viscosity as a function of time between conventional DIW (~25 °C) and our work (150-200 °C). The solid curves are calculated using Eq. (1) by selecting Sylgard 184 and setting the temperature as 25 °C and 190 °C, respectively. d Exemplary printable inks with various rheological properties such as Newtonian, shear-thinning, and yield-stress. e The ISDH strategy enables DIW of thermosets with high geometric complexity and resolution. Functional composites, heterogeneous thermosets, and soft electronics can also be readily printed.

Fig. 2. The mechanism of in situ dual heating.

a Schematic illustration of ISDH. The freshly extruded ink is labeled as ${\left(i+1\right)}^{\mathrm{th}}$ layer and the top layer of the cured ink base is labeled as $i^{\mathrm{th}}$ layer. The temperature of the Joule heater, ${\left(i+1\right)}^{\mathrm{th}}$ layer, and $i^{\mathrm{th}}$ layer is denoted as $T_{\mathrm{heater}}$, $T_{\mathrm{i}+1}$, and $T_{\mathrm{i}}$, respectively. b Simulated degassing time at 50, 100, 190, and 220 °C. c Experimental and simulated temperature of freshly extruded ink when i = 2, 5, 10. d Finite element analysis of the thermal flux on the upper and lower side of the freshly extruded ink. e $T_{\mathrm{i}}$ as a function of $T_{\mathrm{heater}}$ during continuous heating. f Temperature of $i^{\mathrm{th}}$ layer as a function of time after the Joule heater is removed. g The aspect ratio of the filament’s cross-section h/b is plotted as a function of $T_{\mathrm{heater}}$ and $T_{\mathrm{i}}$ using Sylgard 184. The unreachable domain refers to $T_{\mathrm{i}}$ greater than the corresponding value in e under the same $T_{\mathrm{heater}}$. h Degassing time under different temperatures.

Fig. 3. Characterization of ISDH-enabled DIW of low-viscosity Sylgard 184 with base-to-curing agent ratio of 10:1.

a Photograph of printing a cylinder tube. b Optical and scanning electron microscope (SEM) images of a printed cylinder tube with a height of 120 mm and thickness of 1.5 mm. (Left scale bar = 12 mm, right scale bar = 0.8 mm). c Optical image of a printed wrinkled ring with a diameter of 90 mm using a nozzle with 1-mm diameter (scale bar = 2 mm). d Serpentine network printed with a 25 μm nozzle (scale bar = 100 μm). e–g Comparison of the e tensile test; f elastic modulus and ultimate strength; g infrared and h Raman spectroscopy between ISDH-printed and molded samples.

Fig. 4. Demonstration of ISDH-enabled DIW of diverse thermosets.

a, b Viscosity and shear moduli of some typical thermosetting inks with variable rheological properties. c–k ISDH-printed parts using c, d Sylgard 184; e Sylgard 186; f DragonSkin 30; g DragonSkin 10, h DragonSkin 10 + 20%wt SiO₂; i Ecoflex 00-30; j Ecoflex 00−10; k Epoxy. Scale bar = 15 mm for c, d, scale bar = 8 mm for e–g and scale bar = 5 mm for h, k.

Fig. 5. DIW of heterogeneous and functional thermosets.

a Heterogeneous thermosets using Sylgard 184, DragonSkin 10, and DragonSkin 10 + SiO₂ (scale bar = 5 mm). b Heterogeneous thermosets using epoxy and Ecoflex 00-30 (scale bar = 5 mm). c A magnetic strip with five segments in which NdFeB vol% varies from 0 to 20%. The magnetization directions are denoted by black arrows. d A magnetic flower with three petals in which NdFeB vol% are 5, 15, and 20%. The magnetizations are pointing down. e A magnetic stent printed with 20 vol% magnetic composites. The magnetizations are in the hoop direction as highlighted by black arrows. Simulated deformed shapes agree well with experiments when the magnetic field of 50 mT is applied (scale bar = 8 mm).

Fig. 6. Demonstration of hybrid 3D printing of soft electronics.

a Schematic illustration of printing a conductive coil with pick-and-place LED on Ecoflex 00-30 substrate. b Image of the conductive coil with pick-and-place LED on Ecoflex 00-30 substrate. c The LED light can be turned on by an alternating magnetic field. d The LED light keeps on when the Ecoflex substrate is bent. e Schematic design of a multi-layered soft touch sensor. f Image of the soft touch sensor consisting of a capacitive touchpad, LED light, and power indicator (scale bar=8 mm). g The LED light turns on when a finger touches the soft touch sensor (scale bar = 10 mm). h The soft touch sensor reserves high structural integrity under stretching, bending, and twisting deformation.