## 2 Materials and Methods
### Materials  
The polylactic acid (PLA) polymer in the 3052D grade was delivered as a coarse powder by Plastika Kritis S.A. (Heraklion, Crete, Greece). The samples were prepared using this particular PLA polymer as the matrix material. The glass additives in the form of beads, flakes, and powder were procured from Kremer Pigmente (Kremer Pigmente GmbH & Co. KG, Aichstetten, Germany). According to the manufacturer's specifications, the glass beads have a powdered texture and were white. Their diameter ranges from 0 to 50 \(\upmu\)m, and they are smooth and round with no pores. The glass beads' specific gravity is 2.5 g/cm\({}^{3}\), and their bulk density is between 1.51 and 1.52 g/cm\({}^{3}\). The glass beads' refractive index is 1.51. Glass platelets with a thickness of around 5 \(\upmu\)m were used in this study as glass flakes. These glass flakes, which are composed of borosilicate C glass and have good chemical resistance, can be used as a protective layer in acrylic paints, epoxy, vinyl ester paints, and coatings to protect surfaces from corrosive chemical and moisture attack, according to the manufacturer. The melting point of these glass flakes is 700 \({}^{\circ}\)C, and their density is 2.52 g/cm\({}^{3}\). These flakes have a nominal particle size of 160 \(\upmu\)m and a bulk density of 0.40 g/cm\({}^{3}\). Finally, the glass powder is colorless and has particles that are typically 20 \(\upmu\)m in size. Its refractive index is determined to be 1.47 and its density is 2.25 g/cm\({}^{3}\). The glass powder has an oil absorption capacity of 32 mL per 100 g and a conductivity of 134 S/cm. Before manufacturing the composites, the chemical and shape characteristics of the glass beads, flakes, and powder were examined using scanning electron microscopy (SEM) by employing a field emission SEM JSM-IT700 HR apparatus developed by Jeol Ltd. in Tokyo, Japan. At two magnifications, 1000\(\times\) and 5000\(\times\), SEM images of the glass beads, flakes, and powder were captured. The glass beads are depicted in Figure 2a,b, the glass  
Figure 1: The specific steps taken in the experimental approach and its process flow illustrated in screenshots (**a**) raw materials, (**b**) drying process, (**c**) filament extrusion, (**d**) filament drying, (**e**) filament quality control, (**f**) filament tensile testing, (**g**) samples 3D printing, (**h**) 3D printed samples, (**i**) three-point bending test, (**j**) tensile test, (**k**–**m**) SEM analysis on the fracture surface at various magnifications. flakes are shown in Figure 2d,e, and the glass powder is shown in Figure 2g,h. Through the inspection of the three different glass fillers using SEM, their shape and their size were verified. The glass particles' EDS mappings results are shown in Figure 2c,f,i. The mappings show that the distribution of particles is mostly uniform, with only a few voids or places showing a different concentration, especially in the case of the glass flakes in Figure 2f. In the SEM images in Figure 2, the shape of the flakes and the powder is quite similar. Still, powder particles seem to be shorter in length, while flakes have a higher length-to-width ratio. These observations refer to the specific grades tested in the study. These particles come in different grades, with some of them being finer than others and each grade is made for specific types of applications. Herein, fine grades were used, as they were considered commonly used particles with good value for their cost. Other grades came with higher prices with differences being from marginal to huge ones. Additionally, these different shape additives are made for different types of applications. Flakes are mainly made for use in coatings, while powder is suitable for acrylics, ceramics, cement, frescos, silicate binder, water glass, tempera, etc.