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Abstract:
Additive manufacturing (AM) methods, popularly known as 3D printing technologies, particularly the pioneering laser stereolithography (SLA), have revolutionized the production of complex polymeric components. However, challenges such as anisotropy, resulting from the layer-by-layer construction method, can affect the thermomechanical properties and dimensional stability of 3D-printed objects. Although anisotropy in SLA 3D printing is often overlooked due to the high precision of this technique, its impact on the properties and structural performance of the 3D-printed prototype becomes more significant when printing small devices designed for precise micro-mechanisms. This experimental study investigates the impact of the chosen printing surface – a less explored factor – on the performance of SLA 4D-printed thermo-responsive shape memory epoxy (SMEp) specimens. Two identical dog-bone specimens were printed from two distinct surfaces: edge and flat surface, to examine how variations in surface area and quantity of layers influence the microstructure, thermal behavior, mechanical properties, and shape memory performance. The results of this experimental investigation reveal that specimens printed from the edge, with a higher number of layers and smaller surface area, exhibit superior interlayer bonding, tensile strength, dimensional stability, and shape recovery efficiency compared to those printed from the flat surface. Conversely, specimens with fewer, larger layers demonstrated greater elongation and thermal expansion but reduced structural integrity and shape recovery performance. These results highlight the importance of experimentally investigating how different build orientations affect the properties and performance of SLA 3D-printed materials, especially before designing and employing them in applications demanding high precision and reliability.

Keywords:
Additive manufacturing, Laser stereolithography, Shape memory polymers, Materials processing, Anisotropy, Printing orientation

Abstract:
4D printing is the additive manufacturing (3D printing) of objects that can transform their shape in a controlled and predictable way when subjected to external stimuli. A thermo-responsive shape memory polymer (SMP) is a highly suitable material to 4D print smart devices, due to its actuation function and the capability of recovering its original shape from the deformed one upon heating. This study presents the results of employing an epoxy resin in the additive manufacturing of complex-shaped smart devices with shape-morphing properties using laser stereolithography (SLA). To quantify the shape memory behaviour of the shape memory epoxy (SMEp), we first investigate the thermomechanical properties of the 3D-printed specimens in a tensile testing machine coupled with an environmental thermal chamber. This approach allows us to determine the shape fixity and recovery of SMEp. Next, we propose effective designs of complex-shaped devices, with the aim of promoting shape morphing through micro-actuators and compliant joints acting as active regions in combination with multiplying mechanisms or kinematic chains in each of the devices. We manufacture the complex-shaped prototypes by using SLA directly from the computer-aided designs. The shape memory trials of the 3D-printed prototypes reveal quite precise shape recovery of the devices, illustrating their shape-memory. In fact, the inclusion of micro-actuators and compliant joints within the complex-geometry devices allows for local triggering, deformation and recovery, resulting in a prompt response of the devices to heat. Therefore, innovative designs, along with the suitable smart material and high-quality manufacturing process, lead to 4D printed devices with fast actuation and shape-morphing properties. Overall, this research may contribute to the development of smart materials and 4D printing technology for applications in fields such as biomedical engineering, robotics, transport and aerospace engineering.

Keywords:
Shape memory polymers,Shape memory epoxy,Shape morphing structures,Laser stereolithography,3D and 4D printing