Predicting interfacial layer adhesion strength in 3D printable silicone

Direct ink writing is an excellent method for depositing soft elastomeric silicones into a 3D shape, as shown by several examples in the current literature. However, further investigation into the quality of these 3D printed parts is required to ensure this process can be reliable. Especially for fields like soft robotics, where these printed silicone parts are inflated and the bulk material is in tension, interfacial layer adhesion is integral to the ultimate tensile robustness of a part and preventing premature failure. Interfacial layer adhesion changes based on the cure state of the previously deposited silicone layer, which can change based on amount of cure retarding additives, when the material was last mixed, and the temperature of the printing chamber. Yet, interfacial layer adhesion between printable silicone has not yet been characterized in the literature. This work seeks to rectify that by quantifying the relationship between curing kinetics, tested using differential scanning calorimetry, and peel testing of a printable Ecoflex 00-30 based silicone formulation. These data show a negative correlation between cure percent and interfacial adhesion force with values usable for prediction of printed part tensile strength. These data are then implemented into a G-code visualization tool to display the interfacial adhesion quality based on CAD model geometry and print speed. This work shows that cure percent has to be incorporated into direct ink write path planning to ensure control of the part's final tensile behavior.