## 1 Introduction  
Additive manufacturing or three-dimensional (3D) printing is widely used in several industrial areas such as automotive, aerospace, mechanical, medicine, biological systems, and food supply chains [1]. Based on 3D CAD data, additive manufacturing can be used to fabricate complex geometries economically with a wide variety of materials. Polymers are the most commonly used materials in the 3D printing industry owing to their diversity and ease of adaptation to different 3D printing processes [2]. Typically, polymers are available in the following forms: thermoplastic filaments, reactive monomers, resin, and powder [3]. However, a major disadvantage associated with polymers is the generation of toxic materials, such as volatile organic compounds and ultrafine particles, during 3D printing. [PERSON] et al. [4] studied the toxicity of 3D printing. They investigated four thermoplastics, namely, acrylonitrile-butadiene-styrene (ABS), polylactic acid (PLA), polyethylene terephthalate (PET), and nylon. They reported that PLA is considerably less toxic than ABS. Polylactic acid is a bioplastic that is environment-friendly, bio-based, and biodegradable. Owing to environmental concerns, [PERSON] et al. [5] tried to estimate the biodegradation of a co-extruded starch/poly (lactic acid) polymeric material. [PERSON] et al. [6] studied the mechanical properties of virgin and recycled PLA. Currently, fused filament fabrication (FFF) using PLA is commonly used by the public [7].  
However, PLA is characterized by poor toughness, brittleness, and low elongation at break in commercial applications. [PERSON] et al. [8] measured the mechanical properties of 3D-printed PLA specimens in realistic environmental conditions. [PERSON] et al. [9] suggested a methodology to improve the mechanical, thermal, and degradation properties of PLA by blending it with lignin. [PERSON] and [PERSON] [10] investigated the effects of the color of PLA on its material properties. To broaden the scope of application of PLA, the thermal degradation or aging of PLA must be understood because it is one of the failure mechanisms of polymer products [11].  
Analysis of Fourier-transform infrared (FTIR) spectra is one of the best methods for understanding the degradation of PLA. FTIR spectroscopy is a non-destructive and quick technique to access molecular-level change of materials, so it is easy to generate big data. In order to observe changes in the molecular level, there are several other technologies such as Raman spectroscopy, mass spectrometry, nuclear magnetic resonance spectroscopy (NMR), and X-ray photoelectron spectroscopy [12]. [PERSON] et al. [9] used FTIR and differential scanning calorimetry (DSC) to verify the formation of PLA-TAIC-Lignin cross-linked structures. [PERSON] et al. [13] employed thermogravimetry (TG)-FTIR to investigate the effects of PLA degradation with two additives, namely, starch and wood flour. [PERSON] et al. [14] studied PLA/TiO2 nanocomposites by using FT-NIR spectra. [PERSON] et al. [15] used FTIR to monitor PLA degradation in implantable medical devices. However, interpreting FTIR spectra is difficult. Therefore, this study uses artificial neural networks (ANNs), a deep-learning technique, in this study. [PERSON] [16] reported that the thermal degradation of 3D-printed ABS and PLA could be classified using ANNs based on FTIR. He found that the FTIR spectra of PLA are harder to distinguish than those of ABS. The present study focuses on the thermal degradation of 3D-printed PLA and classifies it by using ANNs based on FTIR. In addition, this study determines the best wavenumber range among the FTIR spectra for classifying the degree of PLA degradation.