In this study, we explored PEEK filaments developed in the laboratory and determined their suitability for 3D printing design-specific structures with favorable mechanical properties. Even though in some cases, the PEEK parts developed with the LD filaments exhibited reduced mechanical properties than the ones developed with CA filaments, overall, the results confirm that favorable quality filaments can be developed in-house, and those can be used in a FDM 3D printer to yield mechanically robust 3D printed parts. One of the primary reasons for the degradation of mechanical properties in the parts printed with LD filaments is the presence of pores or defects in the filament that can be seen in Fig. 7. The cross-section of a LD filament shows the presence of such defects. Furthermore, the slight inconsistency in the filament diameter also degrades the mechanical properties of the printed parts. For instance, when the filament diameter is thicker than 1.75 mm at a fixed extruder flow, excess material flow results in buildups in the print layers, as seen in the compression specimen (yellow arrows in Fig. 8). That being said, a controlled amount of excess material flow can also promote a robust filament-to-filament bonding in the lateral direction. Yet, an extra flow results in dimensional inaccuracy surface finish and even affects interlayer adhesion. On the other hand, when the feedstock filament was thinner than 1.75 mm, at a fixed extruder flow, it created voids in the print layers due to a shortage in the material flow ( red arrows in Fig. 8). These voids are critical in affecting the load distribution between the layers when the specimens are subjected to stress and degrades the mechanical properties.
Indeed, it is critical to employ an optimum set of FDM processing parameters to achieve the parts with the best mechanical properties. It is evident that printing parameters such as nozzle temperature, chamber temperature, layer height, and printing speed significantly affect the mechanical properties of the printed structure [20, 25]. Temperatures of the nozzle, build plate, and chamber mainly affects the crystalline/amorphous ratio of the 3D printed PEEK structures [18, 26]. PEEK, similar to many other polymers, is semicrystalline, meaning that PEEK in the solid-state consists of crystalline and amorphous structures, each of which has different characteristics . On the one hand, a completely crystalline structure is characterized by its high molecular order, close polymer chain packing, and strong intermolecular forces. Therefore, the crystalline region in PEEK contributes to having a more dense and rigid polymer. On the other hand, the amorphous region is characterized by its random and intertwined structure. Thus, the density and mechanical properties of PEEK depend primarily on the crystalline/amorphous ratio [28, 29]. In the present study, all the test specimen parts exhibit a mixture of amorphous and crystalline PEEK, which is indicated by the mix of dark brown and beige-colored regions (Fig. 3). The dark brown indicates the amorphous regions, and the beige region exhibit the crystalline regions. The amorphous and crystalline combination in a single printed part is not appropriate for obtaining excellent mechanical properties. A good way to address this is to anneal the parts post-printing. To analyze the effect of post-heat-treatment, we annealed one set of tensile specimens printed using: Nozzle Temp.: 4100C, Chamber Temp:900C, Print Speed: 50 mm/sec, and layer height: 0.1 mm. We observed an enhancement of 7% in the mechanical properties and achieved a tensile strength of 114.34 MPa in the annealed parts. This is because annealing crystallizes the amorphous regions and forms a completely crystallized part corroborated by the uniformly beige colored PEEK specimen shown in Fig. 9a. Further, with the help of our DSC results, we observed a 3.21% rise in crystallinity in the annealed parts as compared to the non-annealed ones. (Fig. 9b). However, we did not anneal the other sets as we wanted to analyze the effect of the printing conditions on the mechanical properties of the specimens.
Along the same lines, the mechanical tests, including compression, tensile, and bending, showed increased mechanical properties by increasing the nozzle temperature, which can also be explained by the higher crystallinity of 3D printed PEEK samples produced by higher printing temperature. Furthermore, higher nozzle temperatures decrease the melt viscosity of the polymer/PEEK extrudate, resulting in easy flow of the material through the nozzle, resulting in better filament deposition and filament-to-filament bonding. Also, a more viscous extrudate helps diffuse well with the underlying polymer layer and promotes a strong filament-to-filament bonding in the lateral and vertical direction [30–32]. Similar results have been shown in a study conducted by Yang et al. focusing on the thermal processing effect on the crystallinity of 3D printed PEEK structure . In that study, 3D printed PEEK tensile bars were printed with nozzle temperatures ranging from 360 0C to 480 0C, and the results showed an increase in PEEK crystallinity from 16–21%. Also, the results showed that tensile strength and elastic modulus were increased when the nozzle temperature increased from 360 0C to 420 0C, accompanied by an increase in crystallinity. However, the tensile properties decreased from nozzle temperature of 440 0C to 480 0C due to material degradation . The degradation of PEEK during the 3D printing process above the nozzle temperature of 430 0C was also captured by Vaezi et al. . A study by Wang et al.  investigated the effect of different AM parameters (e.g., nozzle diameter, nozzle temperature, and printing speed) on the mechanical performance of FDM PEEK. Nozzle temperatures of 420 0C, 430 0C, and 440 0C were investigated in the latter study, all of which were more than the highest temperature used in the present study. The authors observed that when optimum printing parameters were combined with the nozzle temperature of 4300C, it produced a structure with higher compression and bending properties compared to the nozzle temperature of 420 0C . In another study, Ding et al.  studied the effect of nozzle temperature on the mechanical properties of 3D printed PEEK samples. The nozzle temperature range used in that study was 360 0C to 420 0C with a gradient of 10 0C, similar to the temperature profile explored in the present study. The results showed that the relative density of the printed PEEK samples increased with the increase in nozzle temperature and achieved the highest relative density (92.5%) at the highest nozzle temperature (420 0C). Increasing the nozzle temperature improves the fluidity of the polymer melt through the nozzle and eliminates the air particles, which increase the density of the 3D printed structure and improve the adhesion between the printed layers. In the same study, the results showed that increasing nozzle temperature from 360 0C to 420 0C significantly increased tensile and flexural strengths .
Also, in the present study, mechanical properties were enhanced by increasing the chamber temperature. The chamber temperature affects the cooling speed of the printed layers. Thus, it affects the crystallinity of the printed structure. For instance, a 3D printed PEEK structure at a chamber temperature of 25 0C will be cooled faster than a structure printed at 900C, thus increasing the chamber temperature improves the crystallinity of the 3D printed structure. Hu et al. proposed several design structures to improve the mechanical properties of 3D printed PEEK using finite element analysis . The study showed that PEEK structures printed at a chamber temperature of 60 0C were more crystalline and had better mechanical performance than those printed at 25 0C [34, 35]. Moreover, the strength of interlayer bonding improves when the temperature field is more uniform , which can be achieved by increasing the chamber's temperature and printing bed. A study by Wu et al.  also investigated the effect of chamber temperature on the 3D printed PEEK structure quality. In that study, the warp deformation of PEEK structures has been monitored versus chamber temperature of 90 0C, 100 0C, 110 0C, 120 0C, and 130 0C. The results showed that increasing the chamber temperature improved the quality of 3D printed structures in which increasing the temperature from 90 0C to 130 0C reduced the warp deformation from 1.93 mm to 0.65 mm . In the present study, 900C chamber temperature helped in elevating the mechanical properties compared to the 250C; however, it should be noted that 900C was not sufficient to yield completely crystalline parts (Fig. 3) with optimum mechanical properties.
The layer height is another important factor that has an inversely proportional relationship with the quality and mechanical performance of the 3D printed structures. Smaller layer height provides a better surface finish of the 3D printed structure [20, 36]. Additionally, smaller layer height produces denser structures that significantly increase the mechanical properties of the printed structures . Liaw et al. studied the effect of different 3D printing parameters on the interlayer bonding strength of the 3D printed PEEK structures . In that study, the 3D printed PEEK structures were printed in the vertical direction then the strength of the interlayer was evaluated by conducting a three-point flexure test where the bending load was applied perpendicular to the printing direction. It was noticed that among all the 3D printing parameters investigated in that study, the layer height was the only parameter that affected the flexure modulus significantly. The results showed that reducing the layer height increased flexure stress at break and flexure modulus. We also observed a similar trend in the present study, where decreasing the layer height increased the compressive, tensile and flexural strengths of the 3D printed PEEK parts. Thinner layers eliminate microvoids between printed layers by reducing the layer height, which results in a closely stacked layer and better bonding .
Interestingly, printing speed is a complicated factor that can either improve or hamper the mechanical performance of the printed structure when combined with other specific printing parameters. For instance, higher printing speed with a thick layer height and low nozzle temperature reduces the quality and mechanical properties of the printer structure. However, when a higher printing speed is combined with thinner layer height and high nozzle temperature, it improves the quality and mechanical properties of the printed structure, as this parameter combination decreases the interlayer defects and increases interlayer bonding . Geng et al. studied the effect of the FDM extrusion and printing speeds on the accuracy of the extruded PEEK filament . In that study, defects in 3D printed PEEK straight lines were investigated at different extrusion and printing speeds. There were evident defects in the printed line at an extrusion speed of 1 mm/min and printing speed of 6.9 mm/min compared to the printed line at an extrusion speed of 80 mm/min and a printing speed of 335.5 mm/min, which has no apparent defects . This is because the melt flow should have a sufficient pressure drop through the liquefier to move out of the nozzle and be deposited on the printing bed. In the case of slow extrusion speed, therefore, slow printing speed, there will be an insufficient pressure drop in the liquefier zone, creating cavities in the filament . Our observations are similar to the previous studies. Higher printing speeds yields better mechanical properties as it helps in reducing the extent of defects in the print layers, ensures a favorable interlayer adhesion and avoids extrudate build-ups. Indeed, the print speed should be optimized as too high speeds might not give sufficient time to deposit the extrudate and even affect the dimensional accuracy resulting in voids.
Thus, in this study, the most favorable printing conditions identified are Nozzle Temp: 4100C, Chamber Temp: 900C, Print Speed: 50 mm/sec, and layer height: 0.1 mm. We used this printing combination and LD filaments to print a femoral head, as shown in Fig. 10. Indeed, we noticed layers with varying amorphous and crystalline regions but the overall resolution and quality was outstanding, indicating that LD filaments and a combination of optimum 3D printing parameters can yield excellent orthopedic medical devices.