Development of 4D printed actuators with integrated temperature-controlled triggering system
Creators: name: Delbart, Robin
Publisher: name: The University of Edinburgh
Year: 2023
Subjects: 4D printing
4D printed actuators
thermo-responsive actuators
printable multifunctional materials
Joule’s effect
temperature-controlled triggering system
Polilactic Acid
Fused Deposition Modeling
plastic deformation

description: 4D printing actuators are additive manufactured components able to transform energy into mechanical motion in response to specific stimuli such as water, heat, magnetic field or combination of heat and light. They are lightweight, low-cost and relatively simple in comparison to conventional pneumatic, electromechanical or electro-magnetic actuators. These characteristics allow them to perform a range of unique tasks, such as self-assembling, self-repairing, and morphing into different shapes. However, as most of the developed 4D printed actuators are triggered thanks to an external stimuli, it is difficult to control the temperature gradient along large structure and make their utilisation in an external environment dependant of the weather, for example. To counter this, 4D printed actuator can now been triggered with an internal source of heat such as joule heating since the availability of conductive printable filaments on the market. Electrical properties are added by adding particular reinforcement such as Carbon Black (CB), Graphene, metal powder or carbon nanotubes to a classic polymer such as polylactic acid (PLA), Acrylonitrile Butadiene Styrene (ABS) or Nylon among others. This thesis presents a comprehensive study on the development of a 4D printed actuator with a temperature-controlled triggering system. Experimental techniques and numerical models have been employed to determine the influence of the printing parameters of the 3D printer on the final mechanical and electrical properties in order to choose the best set of parameters to print the actuator. Fused Deposition Modeling (FDM) is used as it offers a large variety of printable materials, is easy to use, is the cheapest 3D printing technique and offers a large range of modifiable printing parameters that influence the final properties. Tensile tests were perform modifying the printing parameters to determine the anisotropic mechanical properties in terms of stiffness, strength, and strain. Fractographies are employed to shed light into the different failure modes triggered according to the printed configurations. In addition a numerical model was developed to determine the role of the thermal history and predict the crystallinity of each configuration. Results showed that samples were strongly influenced by the printing direction. Furthermore, the nozzle diameter played a significant role in the mesostructure and failure mechanisms, resulting in large differences in ductility for samples printed at raster direction 45º. It was also found that the layer height had a strong influence on the temperature profile and the resulting crystallinity, which can drastically change the final mechanical properties hence the importance of being able to predict the crystallisation rate. Self-heating and oven-heating techniques have been compared, showing the advantages of the self-heating method to maintain electrical conductivity due to the lower thermal inertia. The influence of the printing parameters have also been investigated, showing huge difference in term of electrical properties and response when heated. Small-Angle X-Ray Scattering has been employed to discern the underlying physics, showing the crucial role of crystallinity, filler dispersion and carbon black aggregates to form an effective segregated conductive network. In addition, as the actuator will be triggered more than once, the specimens were subjected to heating cycle showing that the electrical properties as a function of the temperature become repeatable after the second heating cycle Once the influence of the printing parameters on the electrical and mechanical properties have been done, the actuator was printed using the best set of parameters and is composed of one layer of carbon black polylactic acid embedded within two layers of Thermoplastic polyurethane (TPU). The actuator has been programmed (subsequently heated above the glass transition temperature (Tg), stretched, cooled below Tg and released) to be triggered with an electric voltage. Two motion mechanisms were observed : the difference of thermal expansion between the layer of carbon black reinforced PLA and the layers of TPU makes the actuator bend until the glass transition temperature is reached then unbend to recover the initial shape creating a bending cycle. The effect of the programming parameters on the actuation have been investigated and results shows that the voltage used to program the actuator influences the bending cycle including the maximal bending peak, the response time and the recovery. In addition, a numerical model has been created to fully understand the physics involved during the different programming and activation steps and to predict the final shape of the actuator Low velocity impact tests were performed at different energies on panels manufactured in the same configuration as the actuators aiming to investigate the reliability of the actuator after an impact. Two main failure mechanisms can be identified, delamination of the layer printed at the same height and delamination at the interface between the two materials. Once impacted panels were cut to create the actuator and program them to be triggered afterward. The impacted actuators do not show any irreversible damage and actuators heal as soon as the programming is done. However, the impact influence the bending cycle and a good recovery is observed for actuators impacted at 5 J and 10 J, while when impacted at a higher energy, the recover is not total, resulting in a more bent shape than the initial shape.