*5.4. Post-Processing Technique*

Many studies have verified that there are some shortcomings in FDM components that cannot be overcome by only optimizing process parameters. These shortcomings, such as shape distortion, microvoids, uneven fiber distribution, and stairs-stepping effect [179], directly affect the mechanical characteristics of FDM parts. Therefore, post-processing techniques [180], including chemical treatment [181–183], heat treatment [184–186], laser treatment [187,188], and ultrasound treatment [189,190], are often adopted to improve mechanical strength and print quality of parts. However, these treatments may have influences on structural performance as well as process parameters. For instance, heat treatment can enhance the mechanical strength of printed products by improving crystallinity and removing residual stress of polymers [191]. At the same time, this treatment can result in changes in porosity due to annealing temperature as well, which will affect the infill density consequently [185]. Another example is ultrasound treatment. Mohamed et al. [192] used an ultrasonic transducer to improve the surface quality of components with different frequencies, and they observed from the result that the surface roughness was significantly smoother than before, together with a decrease in road width and layer thickness. Therefore, the optimal values obtained from process parameter optimization (classified as pre-processing) may change after post-processing, which needs to be paid more attention to.

### *5.5. Facing Real Parts*

Most studies in the literature focus on "dog bone" samples to analyze the function of process parameters. It should be noted that the conclusion or result obtained from "lab experiment" may not apply to real applications. The review shows that there are only a couple of reports on improving the mechanical performance of a real part. For example, Zaman et al. [128] optimized five process parameters on compressive strength of drilling grid from the aerospace industry using the Taguchi design of experiments. Lee et al. [164] analyzed the relationship between process parameters and elastic performance of a compliant catapult using the Taguchi method. The maximum throwing distance was achieved by setting optimal parameters combination obtained. Since FDM products are ultimately used in practical applications, more research on real objects needs to be carried out, which can be another direction for future research.

### *5.6. Combination with 4D Printing*

4D printed structures can change shape or property by stimulus, showing innovation and smartness, which has attracted unprecedented interest in recent years [193]. With the increasing application of FDM printers for 4D printing, the effect of process parameters on shape memory effect (SME) for smart materials is becoming a research hotspot [194]. For example, Kaˇcergis et al. [105] evaluated the impact of platform temperature, print speed, and number of layers on the behavior of shape-shifting 'hinge' structure. They pointed out the higher print speed and lower platform temperature resulted in a higher deformation angle. In addition, the more active layers, the more time for shape recovery. Rajkumar and Shanmugam [195] analyzed the mechanisms of process parameters, such as infill density, thickness, and print speed, on shape-transformation, based on which they applied the results in manufacturing controllable curved components. In fact, there exist many unknown problems for 4D printing to be investigated, such as material behaviors, shape-shifting effects, and actuation methods [196] for smart and multi-materials obtained through the FDM approach. Therefore, research on the application of the FDM technique in printing 4D structures is exciting and appealing work awaiting further exploration.

**Author Contributions:** Conceptualization, G.G.; methodology, G.G.; software, G.T.; formal analysis, Z.L.; investigation, G.G., F.X. and G.T.; resources, J.X. and Z.L.; writing—original draft preparation, G.G.; writing—review and editing, G.G. and F.X.; supervision, J.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work is supported by Jiangsu Provincial Double-Innovation Doctor Program. The authors wish to express their appreciation to Lecturer Yumei Wu at Jiangsu University of Science and Technology for her valuable comments.

**Conflicts of Interest:** The authors declare no conflict of interest.
