Mechanical Response of FDM-Fabricated PEEK and Glass Fiber-Reinforced PEEK Under Varying Process Conditions

Abstract

Polyether Ether Ketone (PEEK) is a high-performance polymer increasingly utilized in additive manufacturing due to its exceptional thermal, chemical, and mechanical properties. Thus, they are used to produce aerospace brackets, fuel system parts, seals, compressor valve plates, etc. This study investigates the mechanical performance of both neat PEEK and glass fiber-reinforced PEEK (PEEK + GF) composites fabricated via fused deposition modeling (FDM). The effects of print speed, print orientation, and post-heat treatment were systematically evaluated. Among the tested orientations, the 0° print direction with post-heat treatment at 250 °C yielded highest tensile strength of ~80 MPa, outperforming the 45° and 90° orientations. Print speeds ranging from 5 to 20 mm/s and annealing temperatures between 250 °C and 300 °C significantly influenced material properties. For neat PEEK, both tensile strength and microhardness improved with increasing print speed and post-heat treatment, peaking at 20 mm/s and 250 °C. However, annealing at 300 °C led to performance degradation, attributing to gas-induced porosity within the material. The PEEK + GF composites achieved a maximum ultimate tensile strength (UTS) of approximately 83 MPa under the same optimal conditions (20 mm/s print speed and 250 °C post-treatment). This enhancement is attributed to improved fiber alignment along the print path, increased crystallinity, and superior interfacial bonding. Notably, the composites did not exhibit the microstructural damage observed in neat PEEK at the higher annealing temperature.

1. Introduction

Composites offer numerous advantages over traditional fully dense materials like steel and aluminum, including lower weight, higher specific strength and stiffness, superior vibration damping, enhanced design flexibility, and greater resistance to corrosion and wear [1]. Among thermoplastic composites, glass fiber-reinforced systems are increasingly favored by design engineers due to their affordability, high specific strength, and elevated modulus. Polyether ether ketone (PEEK) has emerged as a premier thermoplastic matrix material, prized for its outstanding thermal stability, chemical resistance, mechanical durability, and recyclability [2]. PEEK maintains performance in harsh environments and resists degradation from acids, bases, and organic solvents. Its biocompatibility has also enabled its use in biomedical implants, including spinal cages and dental components [3]. Beyond healthcare, PEEK is widely employed in applications such as bearings, seals, gears, bushings, and electronic components [4].

Despite its attractive properties, PEEK poses several manufacturing challenges due to its high melting point and semi-crystalline nature. Conventional molding techniques demand high-temperature, high-strength tooling, resulting in elevated production costs [5,6]. Injection molding can lead to warping or dimensional instability if cooling is not properly controlled [7], while extrusion is hindered by issues such as die wear, high processing temperatures, and the need for precise cooling to maintain dimensional accuracy [8,9]. Subtractive machining is also inefficient, particularly for complex geometries, due to significant tool wear and material waste [10,11].

Additive manufacturing (AM), especially fused deposition modeling (FDM), has emerged as a promising alternative for processing PEEK and its composites. AM enables the fabrication of intricate geometries, lattice structures, and lightweight hollow components that are difficult to achieve through conventional methods [12,13]. Its layer-by-layer approach minimizes material waste, shortens production cycles, adds functionality, and supports near-net-shape manufacturing [14,15,16]. Additionally, AM facilitates rapid prototyping and iterative design, making it particularly attractive for customized biomedical implants [17,18,19]. Cellular architectures such as honeycomb structures, known for their high stiffness-to-weight ratios [20], can also be efficiently produced for aerospace and automotive applications [21,22,23]. As a result, AM of high-performance thermoplastic composites has garnered growing interest in both research and industrial domains.

Compared to thermoset composites, thermoplastic composites offer recyclability and a reduced environmental footprint, aligning with global sustainability goals [24,25]. Recent studies emphasize the critical role of process optimization in achieving desirable properties in PEEK-based composites. For instance, Kwon et al. [26] highlighted the influence of fiber distribution in commingled glass fiber/polypropylene yarns on viscoelastic behavior. Yu et al. [27] demonstrated that laser power and compaction pressure enhance the interlaminar shear strength (ILSS) of 3D-printed carbon fiber/PEEK composites, while higher printing speeds negatively impact ILSS. Similarly, Vatandas et al. [28] found that increasing fiber volume fraction in FDM-processed carbon fiber/PEEK composites reduced flexural strength. Zhang et al. [29] reported that glass fiber/PEEK composites fabricated via melt impregnation exhibited improved mechanical and dielectric properties at elevated temperatures, confirming their suitability for high-temperature load-bearing applications. Further research has focused on systematic process optimization. Jiang et al. [13] employed the Taguchi method to identify layer thickness as the most influential factor affecting tensile strength in printed PEEK composites, followed by extrusion width, printing speed, and temperature. Wang and Zou [30] optimized post-printing heat treatment parameters to enhance tensile, flexural, and ILSS properties. Hu et al. [31] observed that small additions (~2 vol.%) of short carbon fibers improved tensile strength, whereas higher loadings (5 vol.%) led to performance degradation due to fiber agglomeration.

Collectively, these findings underscore the importance of process parameter optimization in tailoring the mechanical performance of fiber-reinforced PEEK composites for advanced engineering applications. Based on the literature, FDM has demonstrated strong potential for fabricating PEEK and glass fiber-reinforced PEEK composites for biomedical use. However, achieving superior mechanical properties necessitates precise optimization of FDM process parameters. Although recent studies have advanced this area, considerable opportunities remain for further refinement. In addition, post-processing heat treatments have demonstrated significant potential in enhancing the mechanical integrity of printed composites. In the present study, we aim to optimize printing speed, raster angle (orientation), and heat treatment temperature to improve the mechanical performance of FDM-processed PEEK and glass fiber-reinforced PEEK composites.

2. Materials and Methods

2.1. Materials

The PEEK matrix material (PEEK 150PF) was procured from 3DXTECH (Grand Rapids, MI, USA). The PEEK + GF20 filament was directly procured from 3DXTech^® Additive Manufacturing. The material possesses a density of 1.46 g/cc. Printing of both pure PEEK and PEEK + GF20 was carried out using the same 3D printer, the INTAMSYS FUNMAT HT fused deposition modeling (FDM) system (INTAMSYS Technology Inc., Plymouth, MA, USA make). The key physio-mechanical properties of neat PEEK and PEEK/glass fiber (PEEK/GF) composites are summarized in

Table 1. Comparison of density, thermal, and mechanical properties of neat PEEK and 20 wt.% glass fiber-reinforced PEEK (PEEK/GF-20) [4].

Materials	Density (g/cc)	Melting Point (℃)	Tensile Strength (MPa)	Glass Transition Temperature (℃)	Elastic Modulus (GPa)	Elongation (%)
PEEK	1.2	343	70–80	143	3.8	6
PEEK + GF-20	1.46	363	105–120	143	5.3	2.5

. The detailed printing conditions are presented in

Table 2. Printing parameters used for fabrication of PEEK and PEEK/GF specimens under varying conditions.

Parameters
Structure	Print orientation	0°, 45°, 90°
	Infill density	100%
	Layer thickness	0.3 mm
	Raster width	1.75 mm
Temperature	Nozzle temperature	405 °C
	Platform Temperature	150 °C
	Chamber Temperature	90 °C
Speed	Printing speed	5 mm/s, 10 mm/s, 20 mm/s
	Extrusion speed	10 mm/s

, with key process parameters including an extrusion temperature of 405 °C, a bed temperature of 150 °C, and an infill density of 100%.

The specimens were fabricated using fused deposition modeling (FDM), as schematically illustrated in Figure 1. In this process, the printer follows a G-code file to deposit thermoplastic filament layer by layer, which solidifies upon cooling to form the final structure. Two types of filaments were employed: neat PEEK and glass fiber-reinforced PEEK (PEEK/GF). The choice of filaments significantly influences surface quality, mechanical performance, and thermal stability of the printed parts. To enhance printing resolution and ensure dimensional accuracy, key process parameters, such as layer height/thickness, were carefully optimized.

Print orientation and raster angle have a significant impact on the mechanical performance of FDM-fabricated components [8]. To identify the optimal raster angle, tensile tests were conducted on specimens with varying print orientations. The influence of molecular chain alignment induced during extrusion was also examined by comparing printed samples with those produced via conventional extrusion. The designed sample orientations, printing layout, and representative as-printed specimens are presented in Figure 2.

During initial trials, PEEK and PEEK/GF specimens printed at a 0° orientation exhibited significant warpage, primarily due to high printing speeds and insufficient support structures. To mitigate these issues, the build platform and nozzle temperatures were optimized to ensure stable filament extrusion and improved print quality. Using the optimized parameters, a total of 18 specimens were fabricated—nine each of neat PEEK and PEEK/GF—across three raster orientations (0°, 45°, and 90°). Support structures were incorporated to minimize deformation during printing. All specimens were printed directly on the bedplate with 100% infill density, and the Z-axis defined the build height. Raster angle and printing speed were varied as detailed in Table 2, while all other parameters were held constant. For each parameter combination, four identical specimens were produced to ensure reproducibility.

2.2. Physical and Mechanical Characterization

2.2.1. Deformation Due to Heat Treatment

Heat treatment induced warpage in the 3D-printed specimens, as previously reported by Wang and Zou [30]. Deformation was quantified using a dial indicator setup, with the most pronounced warpage observed at a printing speed of 5 mm/s and a heat treatment temperature of 300 °C.

Heat treatment of PEEK and PEEK-GF specimens was performed at 250 °C and 300 °C to enhance crystallinity, relieve internal stresses, and improve mechanical properties. The process was conducted using a Thermo Kendro^® programmable muffle furnace (Thermo Fisher Scientific Inc., Waltham, MA, USA). Samples were heated at a controlled rate of 10 °C/min until the target temperature was reached, followed by a holding period of 2 h, as illustrated in the heat treatment cycle (Figure 3). After the holding period, the specimens were allowed to cool gradually to room temperature inside the furnace. Optical microscope images of the as-printed and heat-treated samples considered in this study are presented in Figure 4. These images highlight the presence of voids in the as-printed samples, which were subsequently microwelded due to polymer flow during heat treatment. As a result, only limited porosity was observed in the heat-treated specimens.

2.2.2. Hardness

Vickers hardness testing was performed in accordance with ASTM E384-99 using a Struers DuraScan 20 G5 hardness tester (Struers, Ballerup, Denmark). A test load of 0.2 N was applied with a dwell time of 15 s. Each specimen was evaluated at three randomly selected locations, namely upper, middle, and lower regions, and the average of these measurements was reported.

2.2.3. Tensile Strength

Tensile testing was conducted in accordance with ASTM D638-01 using an electromechanical universal testing machine (UTM; Jinan-WDW-100, Jinan Fangyuan Testing Instrument Co., Ltd., Jinan, China) equipped with an extensometer. The UTM featured a maximum load capacity of 5 kN and a minimum measurable load of 50 N. Dog-bone-shaped specimens (Figure 2c) were used to evaluate tensile strength and elastic modulus at a constant crosshead speed of 0.1 mm/min. Each test was performed on two identical specimens, and yield strength was determined using the 0.2% offset method.

3. Results and Discussion

3.1. Effect of Print Orientation

Tensile testing revealed that print orientation has a pronounced effect on the mechanical performance of FDM-fabricated specimens. Samples printed at 0° orientation exhibited the highest tensile strength (77 MPa), while those printed at 90° showed the lowest (13 MPa). Figure 5a presents the corresponding stress–strain curves, and Figure 5b compares the ultimate tensile strengths across different orientations for both neat PEEK and PEEK/GF specimens.

The superior mechanical performance of specimens printed at 0° orientation is attributed to strong interlayer bonding aligned with the loading direction, which enhances resistance to both tensile and shear stresses. In contrast, specimens printed at 45° and 90° orientations exhibited reduced tensile strength and elongation, primarily due to weaker inter-filament adhesion. While a 45° orientation may improve surface finish and mitigate stair-stepping effects, it compromises mechanical integrity. When the raster angle aligns with the loading direction, as in 45° and 90° orientations, both tensile strength and elongation tend to decrease. This behavior is attributed to the brittle and weak bonding between adjacent filaments. When the interfacial strength between filaments is lower than the intrinsic strength of the base material, the overall tensile response becomes dominated by inter-filament failure. The slight reduction in elongation at break observed in 0° samples may be linked to crystallization induced by melt shear during extrusion [33].

3.2. Deformation of 3D-Printed PEEK, PEEK/GF Samples

The deformation behavior of FDM-printed PEEK (Figure 6a) and PEEK/GF (Figure 6b) was evaluated under three post-processing conditions: as-printed, heat treated at 250 °C, and 300 °C. Specimens were fabricated at printing speeds of 5, 10, and 20 mm/s. The results indicate that while printing speed has a moderate influence, heat treatment temperature plays a dominant role in determining the mechanical response. In the as-printed condition, both PEEK and PEEK/GF samples exhibited the least deformation, consistent with the brittle behavior and weak interlayer bonding typically observed in FDM-processed thermoplastics. Limited interlayer diffusion and residual thermal stresses introduced during deposition contribute to the restricted ductility in this state.

Heat treatment at 250 °C led to a noticeable improvement in the deformation behavior of both PEEK and PEEK/GF specimens. This moderate increase in ductility is attributed to partial relaxation of residual stress and enhanced interlayer adhesion. Supporting this, computed tomography (CT) scans revealed a significant reduction in porosity at this temperature, characterized by fewer interlayer voids and more continuous material interfaces—indicating improved structural cohesion without inducing excessive polymer flow or thermal degradation. At 300 °C, the specimens exhibited the highest degree of deformation, reflecting increased polymer chain mobility and substantial stress relief near PEEK’s melting point. However, CT analysis also revealed a resurgence of porosity compared to the 250 °C condition. This suggests that while elevated temperatures enhance ductility, they may also promote localized melting or thermal degradation, potentially compromising the structural integrity of the printed parts. These findings align with those of Miranda et al. [34], who reported notable mechanical improvements at 300 °C but cautioned against the risk of thermally induced defects due to excessive heat exposure. For PEEK/GF composites, overall deformation remained lower than that of neat PEEK under all conditions, primarily due to the constraining effect of glass fibers, which limit polymer chain mobility. Nevertheless, computed tomography (CT) imaging (Figure 7) confirmed enhanced fiber–matrix bonding and reduced delamination following heat treatment, particularly at 250 °C.

Printing speed also influenced the deformation behavior of FDM-printed specimens. Samples printed at 5 mm/s exhibited greater dimensional stability, attributed to longer deposition times that promote improved interlayer fusion. As printing speed increased, interlayer bonding weakened, leading to greater deformation. Although annealing at 300 °C partially mitigated this effect by enhancing polymer chain mobility, it also introduced higher thermal distortion and uneven porosity distribution, particularly in geometrically complex regions. Overall, the combined evidence from CT imaging and mechanical testing highlights the critical need to balance heat treatment temperature and printing parameters to achieve optimal mechanical performance and microstructural integrity in FDM-printed PEEK materials. Among the tested conditions, heat treatment at 250 °C proved most effective in reducing porosity and enhancing mechanical strength without compromising structural stability.

3.3. Hardness of PEEK and PEEK + GF

Figure 8 illustrates the micro-Vickers hardness trends for neat PEEK and PEEK/GF composites as a function of printing speed and post-processing temperature. In the as-printed condition, neat PEEK samples exhibited increasing hardness with higher printing speeds. At a low speed of 5 mm/s, the minimum hardness (~23 HV) was observed, likely due to insufficient thermal input during deposition, which weakened interlayer bonding. Increasing the speed to 10 mm/s and 20 mm/s improved hardness to approximately 28 HV and 36 HV, respectively. This trend aligns with the extrusion-like nature of FDM, where higher speeds enhance melt pressure and thermal energy at the nozzle, promoting better filament deposition, reduced porosity, and improved surface quality. The enhanced interlayer fusion at higher speeds contributes to superior mechanical integrity and increased hardness [35]. Heat treatment at 250 °C further elevated hardness across all print speeds, attributed to increased polymer chain mobility, enhanced crystallinity, and partial healing of interfacial voids. However, annealing at 300 °C resulted in a slight reduction in hardness, particularly at lower print speeds. This decline may be due to over-annealing or excessive relaxation of internal stresses in samples with higher initial porosity, potentially leading to localized degradation or structural instability.

In contrast to neat PEEK, the PEEK/GF composite exhibited a distinct hardness response. While the as-printed PEEK/GF samples at 5 mm/s demonstrated higher hardness than neat PEEK, the variation in hardness with increasing print speed was less pronounced. The highest hardness (~46 HV) was achieved at a print speed of 20 mm/s following heat treatment at 250 °C, indicating that elevated annealing temperatures enhance both matrix crystallinity and fiber–matrix interfacial bonding. At lower print speeds (e.g., 5 mm/s), heat treatment had a limited effect on improving hardness in PEEK/GF samples. This behavior is attributed to the thermal resistance of the embedded glass fibers, which restrict the extent of heat-induced structural changes. Although the thermal energy during annealing was sufficient to remelt the matrix and reduce porosity, particularly in samples with higher initial void content, the reinforcing fibers constrained polymer mobility, limiting the benefits of heat treatment at lower speeds. Conversely, higher printing speeds facilitated improved layer-to-layer bonding due to shorter deposition intervals, resulting in denser structures with enhanced resistance to indentation. However, samples heat treated at 300 °C exhibited slightly lower hardness compared to those treated at 250 °C. This reduction is likely due to localized thermal degradation or over-annealing at the fiber–matrix interface, which may weaken interfacial bonding. As a result, the strengthening effect from increased crystallinity was offset by microstructural instability, underscoring the importance of optimizing heat treatment conditions to balance crystallinity enhancement and structural integrity.

In contrast to neat PEEK, the PEEK/GF composite exhibited a distinct hardness response. While its as-printed hardness at 5 mm/s was notably higher than that of neat PEEK, the variation in hardness with increasing print speed was comparatively modest. The highest hardness (~46 HV) was achieved at a print speed of 20 mm/s following heat treatment at 250 °C, indicating that elevated annealing temperatures enhance both matrix crystallinity and fiber–matrix interfacial strength. At lower print speeds (e.g., 5 mm/s), heat treatment had a limited effect on improving the hardness of PEEK/GF samples. This is attributed to the thermal resistance of the embedded glass fibers, which restricts the extent of heat-induced structural changes. Although the thermal energy input was sufficient to remelt the matrix and reduce porosity, particularly in samples with higher initial void content, the reinforcing fibers constrained polymer mobility, limiting the benefits of annealing. Higher printing speeds promoted improved interlayer bonding due to shorter deposition intervals, resulting in denser structures with enhanced resistance to indentation. However, samples heat treated at 300 °C exhibited slightly lower hardness than those treated at 250 °C. This reduction is likely due to localized thermal degradation at the fiber–matrix interface, where excessive heat may weaken interfacial bonding. The stress-relieving effects observed at 250 °C appear to optimize interfacial strength, whereas overheating at 300 °C compromises the structural integrity of the composite. Consequently, the benefits of crystallinity-induced strengthening were diminished at the higher annealing temperature. These findings underscore that mechanical performance is governed by the interplay between printing speed and post-processing temperature, with distinct mechanisms influencing neat and composite systems. Both neat PEEK and PEEK/GF achieved optimal hardness at higher print speeds combined with annealing at 250 °C. In contrast, the effectiveness of heat treatment at 300 °C diminished at lower printing speeds, confirming that insufficient interlayer fusion during deposition limits the extent of microstructural recovery achievable through post-processing.

3.4. Tensile Strength of PEEK and PEEK + GF

Tensile data for FDM-printed PEEK specimens fabricated at different printing speeds (5, 10, and 20 mm/s) under various heat treatment conditions are presented in Figure 9. The as-printed tensile response is shown in Figure 9a, while Figure 9b,c illustrates the tensile behavior following heat treatments at 250 °C and 300 °C, respectively. Figure 9d provides a comparative summary of the ultimate tensile strength (UTS) across all conditions. In the as-printed state (Figure 9a), an increase in printing speed led to improved mechanical performance. The sample printed at 20 mm/s exhibited the highest UTS (~90 MPa), which is attributed to enhanced thermal management during deposition and reduced accumulation of residual stresses. These factors contribute to improved interlayer bonding and overall structural integrity.

Heat treatment at 250 °C (Figure 9b), which is slightly above the glass transition temperature of PEEK, facilitates stress relaxation and moderate secondary crystallization. Among all tested conditions, this treatment yielded the best overall mechanical performance. Notably, the sample printed at 20 mm/s exhibited the highest tensile strength and elongation after annealing at 250 °C, suggesting enhanced chain mobility and molecular reorganization at both the microstructural and molecular levels. In contrast, heat treatment at 300 °C (Figure 9c), which approaches the melting point of PEEK, resulted in increased crystallinity but did not improve strength or ductility compared to the 250 °C condition. This outcome is likely due to the introduction of microstructural defects, such as localized degradation or void formation, which offsets the benefits of enhanced crystallinity. The trend in ultimate tensile strength (UTS) is clearly illustrated in the bar chart (Figure 9d), showing a consistent increase in UTS with higher printing speeds. The best performance was observed at 20 mm/s combined with 250 °C heat treatment, reinforcing the importance of balancing optimal post-processing temperature with high-quality initial print conditions. Slower printing speeds, which result in inadequate interlayer fusion, remain a limiting factor for achieving full microstructural recovery through heat treatment.

The tensile performance of glass fiber-reinforced PEEK (PEEK/GF) under similar conditions is presented in Figure 10a–d. As with neat PEEK, three printing speeds (5, 10, and 20 mm/s) were evaluated across as-printed, 250 °C, and 300 °C heat-treated states, shown in Figure 10a, Figure 10b, Figure 10c, respectively. The UTS values for all conditions are summarized in Figure 10d. The incorporation of short glass fibers enhances stiffness and strength; however, performance is highly dependent on fiber–matrix interfacial bonding and thermal processing history. In the as-printed state (Figure 10a), distinct tensile responses were observed across print speeds. The 20 mm/s sample achieved the highest UTS (~77 MPa) and the greatest elongation, indicating improved interlayer bonding and reduced internal defects. This trend aligns with the literature suggesting that higher print speeds reduce heat accumulation and prevent over-crystallization, which are important factors in composites, where thermal mismatch between fiber and matrix can lead to interfacial debonding [2,3]. Conversely, the lowest strength (~62 MPa) was recorded for the 5 mm/s sample, likely due to prolonged thermal exposure promoting shrinkage-induced voids and stress concentration sites.

Post-processing at 250 °C (Figure 10b), slightly above the glass transition temperature of PEEK, yielded the highest mechanical performance among all tested conditions. The specimen printed at 20 mm/s achieved the greatest ultimate tensile strength (UTS) (~85 MPa) and the highest strain-to-failure. This outcome suggests that 250 °C provides optimal thermal energy for stress relaxation and promotes sufficient interdiffusion and reorganization of the polymer matrix, enhancing fiber–matrix load transfer without inducing thermal damage. In contrast, heat treatment at 300 °C (Figure 10c), despite its proximity to PEEK’s melting point, resulted in reduced UTS and significantly lower elongation across all print speeds compared to the 250 °C condition. The diminished ductility and premature failure observed at this higher temperature indicate that excessive thermal exposure may be detrimental to the PEEK/GF system. Likely causes include over-crystallization, which reduces matrix flexibility, and exacerbated thermal mismatch between the polymer and glass fibers, leading to interfacial debonding and microcrack formation [6]. Figure 10d presents a comparative summary of UTS values, highlighting that the highest strength (~85 MPa) was achieved by the sample printed at 20 mm/s and heat-treated at 250 °C. These findings confirm that, for PEEK/GF composites, a moderate post-processing temperature (250 °C) is optimal for enhancing fiber–matrix interfacial bonding and mechanical performance. In contrast, the 300 °C treatment induced structural degradation, underscoring that aggressive thermal post-processing can be counterproductive in fiber-reinforced systems due to the complex thermal dynamics between the reinforcing phase and the polymer matrix.

3.5. Fractographic Analysis

SEM fractography of tensile-tested neat PEEK specimens printed at a speed of 20 mm/s reveals fracture surfaces characterized by prominent voids and spherical pores (Figure 11a). These morphological features are indicative of inadequate interlayer bonding and incomplete fusion between deposited layers, likely resulting from insufficient thermal energy at higher deposition rates. The presence of smooth fracture planes across the surface further suggests a predominantly brittle failure mode, with minimal plastic deformation prior to fracture. This behavior implies that the material absorbed limited energy before crack initiation and propagation, highlighting the influence of processing parameters on fracture resistance. Further examination of the SEM fractography of tensile-tested neat PEEK printed at 20 mm/s reveals distinct interlayer delamination features, reinforcing the conclusion that weak adhesion between adjacent layers significantly contributed to premature failure. This is a well-documented issue in FDM-processed materials at elevated print speeds, where rapid cooling inhibits polymer chain diffusion across layers, resulting in poor interfacial bonding [33]. Additionally, the presence of spherical and irregularly shaped voids likely served as stress concentrators and crack initiation sites, accelerating failure under tensile loading.

In contrast, the fracture surface of the PEEK/GF composite printed at the same speed (Figure 11b) displays a markedly rougher texture, characterized by layered tearing and visible glass fiber pull-out. This irregular morphology is indicative of a more energy-dissipative failure mechanism. The alignment of deposition layers and embedded fibers in the longitudinal direction suggests improved load transfer throughout the matrix, contributing to delayed crack initiation and propagation. Crucially, features such as fiber–matrix debonding and voids left by pulled-out fibers point to interfacial failure mechanisms, which are typical in fiber-reinforced thermoplastics and are associated with enhanced toughness. These mechanisms facilitate energy absorption through fiber bridging and crack deflection [31]. Compared to neat PEEK, the incorporation of glass fibers significantly altered the fracture behavior, promoting a quasi-ductile failure mode consistent with the higher UTS and greater strain-to-failure observed in mechanical testing. Moreover, the layered tearing morphology suggests that the fibers played a key role in deflecting and arresting crack propagation. Thus, while higher print speeds introduced some interfacial weaknesses, the presence of glass fibers effectively compensated by providing additional toughening mechanisms, making the PEEK/GF composite more resilient under rapid manufacturing conditions.

Figure 11b shows the fracture surface of the PEEK/GF composite printed at 20 mm/s, revealing a distinctly rougher texture with clear evidence of layered tearing and fiber pull-out. This irregular and rugged morphology is indicative of a more energy-dissipative failure mechanism. The alignment of deposition layers and embedded glass fibers along the longitudinal direction suggests improved load transfer throughout the matrix, which contributed to delaying the onset of catastrophic fracture.

Notably, features such as fiber–matrix debonding and voids left by pulled-out fibers highlight the dominance of interfacial failure mechanisms. These are well-established in fiber-reinforced thermoplastics and are associated with enhanced toughness, as they enable energy absorption through fiber bridging and crack deflection [36]. Compared to neat PEEK, the incorporation of glass fibers significantly altered the fracture mode toward a quasi-ductile behavior, consistent with the higher ultimate tensile strength (UTS) and greater strain-to-failure observed in the stress–strain response.

Moreover, the layered tearing morphology further supports the role of glass fibers in deflecting and arresting crack propagation, thereby enhancing mechanical performance. While higher print speeds may introduce some degree of interfacial weakness, the presence of glass fibers effectively compensates by providing additional toughening mechanisms. As a result, PEEK/GF composites demonstrate superior performance under rapid manufacturing conditions, making them a promising material for high-speed FDM applications.

4. Conclusions

This study systematically investigated the mechanical performance of FDM-printed neat PEEK and PEEK/GF composites, focusing on the effects of printing speed and post-processing conditions. The findings offer a clear framework for optimizing the fabrication of high-performance thermoplastic components:

• Print speed and post-heat treatment (HT) significantly influenced the mechanical and structural behavior of both neat PEEK and PEEK/GF composites fabricated using FDM.
• Among the investigated conditions, heat treatment at 250 °C combined with the highest tested print speed (20 mm/s) showed comparatively improved mechanical performance for both materials.
• For neat PEEK, specimens treated at 250 °C exhibited better tensile properties, whereas treatment at 300 °C resulted in reduced performance, which may be associated with the formation of gas-induced porosity during heat treatment.
• For PEEK/GF composites, heat treatment at 250 °C appeared to enhance fiber–matrix load transfer, whereas a higher treatment temperature of 300 °C showed indications of microstructural degradation.
• The effectiveness of post-heat treatment was also influenced by the degree of interlayer fusion achieved during printing; lower print speeds tended to result in weaker interlayer bonding, thereby limiting the improvements attainable through heat treatment.
• Overall, the results indicate that controlled post-processing temperatures, particularly around 250 °C can positively influence the mechanical response of FDM-printed PEEK-based materials while maintaining structural stability under the investigated conditions.

5. Future Work

Future work will focus on advancing the understanding of structure–property relationships in additively manufactured PEEK and its composites. Detailed thermal and crystallinity analyses will be conducted using techniques such as Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD), complemented by quantitative microstructural evaluations through Scanning Electron Microscopy (SEM) and Computed Tomography (CT) to assess porosity and interlayer bonding. In addition, future studies will explore the optimization of printing parameters, advanced post-heat treatment conditions, and extended mechanical testing—including fatigue and creep—to further enhance the performance and reliability of FDM-printed PEEK components.