## 1 Introduction  
3D printing, also known as additive manufacturing (AM), has been used in automotives, aerospace, mechanical systems, medicine, biological systems, food supply chains, and so on [1, 2]. The key advantage of 3D printing is the capability to economically build complex shapes using a wide variety of materials. By using this technology, consumers and industries can rapidly make a prototype in early-stage product design. Seven 3D printing processes have been categorized by ASTM International: material extrusion, powder bed fusion, vat photo-polymerization, material jetting, binder jetting, sheet lamination, and directed energy deposition. Among them, fused deposition modeling (FDM) or fused filament fabrication (FFF), which belongs to the material extrusion process, is becoming the most popular due to its inexpensive and flexible extrusion systems including thermoplastic materials. However, thermal degradation phenomena of the 3D-printed thermoplastics is an inevitable problem for long-term reliability. This study focuses on thermal degradation of the thermoplastics, which is one of the causes of failure. To characterize thermoplastics and polymers with thermal degradation, researchers have used Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and so on. [PERSON] et al. [3] developed a novel system of poly(lactide acid)-d-alpha-tocopherol polyethylene glycol 1000 succinate (PLA-TPGS) nanoparticles(NPs) for quantum dot (QD) formulation to improve imaging effects. They used FTIR and XPS to analyze the surface chemistry of the samples. [PERSON] et al. [4] reported biodegradable lactic-acid-based telechelic pre-polymers incorporating CNTs characterized by FTIR, TGA, DSC, POM, and so on. [PERSON] and [PERSON] [5] reported the production of new biodegradable nanocomposites from polyactide (PLA), tetraethoxysilane (TEOS), and wood flour (WF) using an in-sihisol-gel process and a melt blending method. They used FTIR to characterize SiO\({}_{2}\), PLA, and hybrids. They found that the PLA-g-AA/SiO\({}_{2}\) hybrid could enhance the thermal and mechanical properties of PLA. [PERSON] et al. [6] tried to make 3D-printed structure using maize protein as biopolymers. They used DSC and DMA to obtain glass transitions and the thermos-mechanical properties of Zein-based materials and applied direct beam X-ray scattering (WAXS) and FTIR to characterize the structure of Zein during processing. [PERSON] et al. [7] investigated mechanical properties of PLA resin blended with PCL resin. Samples were fabricated using injection molding. They found that the carbonyl groups of the blend material with DCP (dicumyl peroxide) enhanced the viscous property in the PCL phase and the interfacial adhesion in the dual phase nature of the PLA/PCL blend using FTIR spectroscopy. [PERSON] and [PERSON] [8] studied that the thermal degradation of poly(vinyl alcohol) using thermal analysis-FTIR (TA-FTIR), thermogravimetry (TG) and DSC. [PERSON] et al. [9] reported pyrolysis profiles of plastic blends such as ABS/PVC, ABS, PA5, and ABS/PC using thermogravimetric-FTIR (TG-FTIR). Among several methods, FTIR is a nondestructive technique used to obtain an infrared spectrum of absorption or transmission of the thermoplastics. The molecular bond structure could be observed so that material identification, contamination, degradation, and chemical contact are quantitatively characterized using FTIR. However, the information from FTIR is hard to be understood for novices and a feature extraction of FTIR is not easy without a priori knowledge. To overcome these problems, researchers have adopted machine learning or deep learning techniques with FTIR. [PERSON] et al. [10] used FTIR and the machine learning method to detect microbial spoilage of beef. [PERSON] et al. [11] used FTIR and artificial neural networks (ANNs) to detect meat spoilage.