Search

4D printing has recently emerged as a new manufacturing paradigm by integrating shape memory materials with 3D printing technology. Distinctively, 4D printed structures exhibit dynamic shape changing capability over time in response to certain stimuli. The emerging shape memory property of 4D printed components has attracted increasing research attention due to its potential applications in soft-robotic, origami, and self-construction structures. Ensuring product durability is key to enhancing the technology diffusion of 4D printing as it significantly affects the product service life. In the current literature, durability issues in 4D printing have been largely overlooked and the relationships between external stimulus conditions and the thermo-mechanical cycle life of 4D printed parts remain unknown. To tackle this challenge, in this study, the design of experiments approach is adopted to comprehensively investigate the impacts of external stimulus conditions on the shape memory cycle life of the 4D printed thermo-responsive parts. The results suggest that increasing the operating temperature within the material-allowable temperature range would negatively affect the shape memory cycle times; meanwhile, varying the shape programming hold time also exhibits noticeable impacts on the durability of 4D printed parts. In addition, the extreme stimulus conditions with respect to the high temperature and prolonged shape programming time would worsen the shape memory cycle life.

4D printing has spurred growing interests since its recent emergence, as it enables the fabrication of dynamic structures with reconfigurability over time when exposed to external stimuli, which is not feasible using 3D printing. The current literature on 4D printing is mainly focused on developing new materials and investigating the time-evolving properties of the printed parts, whereas the influences of process parameters on stimuli-response behaviors of 4D printed parts are not adequately explored, especially under cyclic loadings. In this study, experimental analyses are conducted to investigate the effects of infill strategies and stimulus conditions on the shape memory properties of 4D printed thermo-responsive parts. Specifically, cyclic thermo-mechanical tests are performed under different operating temperatures to investigate the shape programmability and recovery characteristic of specimens printed with various infill patterns. The results indicate that specimens printed with the rectilinear pattern exhibit better shape programmability under cyclic thermo-mechanical loadings than polygonal patterns. In addition, the decrease in shape fixity ratios over multiple cycles is also observed for all considered infill patterns. The comparative studies suggest that the increase in operating temperature within the vicinity of the material’s glass transition temperature can improve the cyclic shape memory property.

The integration of additive manufacturing technologies with stimuli-responsive shape memory materials allows the dynamic self-adaptation of fabricated parts upon exposure to external stimulations. The additional dimension of time inspires the concept of four-dimensional printing technology. However, the emerging feedstock materials and the employment of external stimuli in 4D printing are also associated with new workplace hazards and occupational health concerns. Current evaluation studies on additive manufacturing are mainly focused on the safety and health effects that originated from the part production phase and cannot be directly applied in 4D printing processes. In this study, the emissions of volatile organic compounds from stereolithography-based 4D printing process with thermo-responsive materials are targeted as the potential safety concern. Real-time total volatile organic compound monitoring is conducted during various production phases to align the emission sources with critical operating activities, including both operator interventions and machine operations. Comparative experiments are performed to evaluate the effectiveness of proposed emission control strategies. In particular, alterations in operation procedures such as stirring speed in material mixing and post-printing stimulation method can contribute positively to air emission control during manual operations. In addition, the installation of activated carbon fiber filters inside the machine build chamber can lead to a significant reduction of air emissions during part fabrication with an overall total volatile organic compound concentration reduction of 58.91%.

The emergence of smart materials coupled with additive manufacturing technology has provided competitive advantages over traditional manufacturing systems in terms of manufacturing flexibility, product functionality, and the ability to switch between multiple phases under given external stimuli. Although the fabricability of shape memory materials has been widely explored in stereolithography systems, the shape memory performance of printed smart structures has not been extensively studied. More specifically, in current literature, the printing process is mainly considered independent of material characteristics, and a lack of information is reported on how the printing parameters affect the shape fixity and free recovery performance of the printed parts. Therefore, this work is dedicated to experimentally investigating the influences of parameters from both the stereolithography printing process and thermomechanical process (i.e., shape programming and free recovery) on the shape memory properties. Five parameters, including layer thickness, scan speed, maximum programmed angle, hold time, and recovery time, are experimentally analyzed for their impacts on the shape morphing capabilities. According to the results of this study, a variation of 14.33% on the free recovery ratio can be observed when the scan speed is altered. In addition, the printing process parameters exhibit high levels of dominance in affecting the shape memory performance over parameters involved in the thermomechanical process, such as hold time and maximum programmed angle.