In preparing this review, the relevant literature was identified through a structured search carried out across Scopus, Web of Science, and Google Scholar. These databases were chosen because they cover most of the peer-reviewed work in additive manufacturing and provide good depth in studies related to FDM and polymer processing. The search was performed for publications appearing between 2010 and 2025, using combinations of terms related to FDM printing parameters, nozzle geometry and wear, thermal management, recycled or sustainable feedstocks, post-processing of printed parts, and waste-to-value polymer applications. Only articles written in English and offering experimental data, analytical insight, or substantial review of FDM processes were considered. Work that focused exclusively on non-FDM techniques, patents, or sources lacking technical detail on print behaviour or sustainability aspects was excluded to maintain relevance.

All retrieved papers were screened initially by title and abstract, after which the shortlisted studies were examined in full to determine their contribution to understanding the interplay between printing parameters, nozzle design, post-processing treatments, and sustainable material use in FDM. This structured approach ensured that the discussion presented in this review reflects the most pertinent findings across mechanical performance, melt flow characteristics, energy and material efficiency, and environmental considerations.

In FDM, the orientation of the parts being built is vital as it affects their mechanical properties. Build orientation is the part’s placement concerning the build plate during the modelling process. The mechanical performance of parts built in a flat orientation is consistently better than that of parts produced vertically or horizontally. This is due to the decreased internal tensions and improved layer adhesion. Megersa et al. discovered that the ultimate tensile strength (UTS) of PLA specimens was 98.16% influenced by construct orientation, with flat orientation producing the maximum strength. For instance, the build orientation significantly influences the ultimate tensile strength (UTS), contributing 98.16% to its variance. By using a flat orientation, the predicted UTS and elastic modulus were achieved with error percentages of 4.33% and 2.74%, respectively (Megersa et al., 2024). Ahmad et al. found that the construction orientation is the most important factor affecting the mechanical properties of oil palm fibre composites. This was determined through mechanical tensile and flexural testing, along with fractography analysis of the fractured samples (Ahmad et al., 2022). On the other hand, Huang et al. found that ABS specimens printed horizontally performed the best, but tensile strength and impact resistance were lowest for vertically constructed components (Huang et al., 2019). The material-dependent character of the build’s orientation impact is shown by this variation. The improvement is due to enhanced interlayer adhesion and reduced porosity (Hibbert et al., 2019).

Another important factor influencing the final mechanical characteristics is the layer thickness of each layer deposited during the process. Due to increased interlayer bonding and decreased porosity, thinner layers often result in higher mechanical strength and better surface quality (KAM et al., 2020; Chokshi et al., 2022; Huang et al., 2019; Shafaat and Ashtiani, 2021). Excessively thin layers, however, may result in longer printing times and more material consumption (Omar et al., 2022). When examining PLA, Chokshi et al. (2022) observed that layer height had a considerable impact on flexural strength but less on tensile strength. The necessity of optimising layer thickness for particular materials and desired mechanical qualities is shown by this variance. Furthermore, by strengthening the interfacial contact between rasters and layers, decreasing layer thickness might improve the mechanical performance of FDM-printed products (Jo et al., 2018).

Mechanical characteristics of the FDM-printed part are also influenced by the air gap, which is the distance between neighbouring extrusion paths. The strength of the parts can be increased by reducing porosity and improving layer adhesion through a smaller air gap. On the other hand, smaller air gaps may result in nozzle blockage or inadequate material flow, which could affect the mechanical qualities and print quality (Mohamed et al., 2016a; Vosynek et al., 2018; Elmushyakhi, 2022). The effect of air gap on the dynamic mechanical properties of PC-ABS components was examined by Mohamed et al. (2016a), which has shown that air gap has a major influence on loss modulus and storage compliance. This was analysed through analytic and graphical techniques for estimation and optimization of FDM processing parameters using Q-optimal design and graphical optimization technique. Similarly, Elmushyakhi (Elmushyakhi, 2022) incorporated air gap as a criterion in their study on freeze-thaw stability, demonstrating its wide applicability. In this study, the effects of freeze–thaw cycles on the physical characteristics of FDM objects printed using the ULTEM 9085 filament is assessed. The samples used in this study were printed with an air gap of–0.025 mm and at raster angles of 0°, 45°, and 90°. The results indicate that freeze thaw conditioning reduces the tensile properties of FDM samples.

Mechanical qualities are also influenced by the raster angle (angle of the extrusion path) concerning the build plate (Megersa et al., 2024; Huang et al., 2019). Variations in angle can result in different levels of internal stress distribution and layer adhesion. A 45-degree raster angle is ideal for increasing tensile strength. This occurs due to improved molecular diffusion and enhanced interlayer bonding during the printing process. While other studies have shown that other factors like construct orientation or layer thickness have a greater impact (Abdullah et al., 2018; Christiyan et al., 2014). According to Gao et al. (2021), the highest overall performance for PEEK samples in terms of deformation and tensile strength was obtained using a 0°/90° raster angle. It highlights the fact that the ideal raster angle depends on the material.

The quantity of infill material in a printed component has a major impact on its mechanical qualities. Although a higher infill density often results in greater strength and stiffness, it also uses more material and takes longer to print (Ahmad et al., 2022; Rizwee and Kumar, 2024; Dezaki and Mohd Ariffin, 2020; Do et al., 2022). On the other hand, components with a lower infill density are lighter and weaker, making them appropriate for situations where weight reduction is a top priority. Ahmad et al. (2022) demonstrated that the mechanical strength of oil palm fibre composites is influenced by infill density. This was determined through mechanical tensile and flexural testing of optimised PLA prints. The results show the tensile strength of the printed specimens ranged from 0.95 to 35.38 MPa, the Young’s modulus from 0.11 to 1.88 GPa, and the flexural strength from 2.50 to 31.98 MPa. In addition, build orientation had the largest influence on tensile strength, Young’s modulus, and flexural strength. The optimum printing parameter for FDM using oil palm fiber composite was 0.4 mm layer thickness, flat (0°) of orientation, 50% infill density, and 10 mm/s printing speed. Rizwee and Kumar (Rizwee and Kumar, 2024) found out that the tensile strength of PLA specimens was most significantly impacted by 100% infill density and by optimising the raster angle and increasing extrusion temperature to improve layer fusion. There is a trade-off between material efficiency and strength when choosing the infill density.

A suitable temperature for extrusion guarantees good adhesion of the layers and optimal material flow. Insufficient material flow, deformation, or poor layer adhesion can result from extremely high or low temperatures (Megersa et al., 2024; Kafshgar et al., 2021; Muhamedagic et al., 2022; Deng et al., 2018). Megersa et al. demonstrated the impact of extrusion temperature on the strength of PLA specimens. This study explored how layer thickness, raster angle, build orientation, and extrusion temperature influence the ultimate tensile strength (UTS) and elastic modulus of Polylactic Acid (PLA) specimens. The Taguchi method was employed for optimization, and analysis of variance (ANOVA) was used to assess the significance of these factors (Megersa et al., 2024). The optimal extrusion temperature for improving the toughness and strength of PLA depends on various other printing parameters. These include layer thickness, which influences adhesion between layers; print speed, which affects material deposition and cooling; extrusion temperature, which controls melt flow and bonding; infill density, which contributes to structural integrity; and raster angle, which impacts the way stress is distributed within the printed component.

Additionally, for carbon fibre-reinforced polyamide, tensile strength is less affected by printing temperature compared to other factors. This indicates that aspects such as fibre orientation, interlayer bonding, and the intrinsic properties of the composite material have a more significant role in determining its mechanical performance.

Layer adhesion and internal structure of the printed components are influenced by printing speed, which is the speed at which the filament is extruded and deposited. Although faster printing rates might save time, they may also affect mechanical qualities and layer adhesion because of inadequate cooling and bonding time between layers. While enhanced mechanical characteristics can be achieved by greater layer adhesion and solidification at lower printing rates, this comes at the expense of longer printing times (Ahmad et al., 2022; Huang et al., 2019; Natarajan et al., 2022). Huang et al. (2019) discovered that the quality of ABS components produced using FDM decreased when printing speed increased. Natarajan et al. (2022) found that in PLA/Acacia concinna composites, printing speed and strength were negatively correlated.

The mechanical performance of FDM-printed polymers is governed by the combined effects of thermal history, polymer flow behaviour, and interlayer diffusion during material deposition. When a filament strand is extruded, the degree of molecular interdiffusion across adjacent layers is determined by its temperature relative to the polymer’s glass transition and melt regimes. Higher extrusion temperatures, smaller layer heights, and reduced air gaps promote increased interfacial contact area and longer residence times above the critical diffusion temperature, enabling polymer chains to reptate across the interface and form stronger intermolecular entanglements. This mechanism directly enhances interlayer bonding strength and reduces the likelihood of crack initiation at bead boundaries (Patti, 2024).

Conversely, insufficient thermal energy, rapid cooling, or large internal voids limit chain mobility and produce partially fused interfaces that act as stress concentrators under load. Build orientation and raster angle further influence the alignment of these interfaces relative to the applied stress direction; orientations that place weak interlayer bonds perpendicular to the load path generally exhibit lower tensile and flexural strength. The extent and distribution of voids introduced by air gaps or suboptimal infill patterns also reduce the effective load-bearing cross-section, contributing to earlier failure (Webbe Kerekes et al., 2019).

Post-processing treatments such as annealing modify the internal microstructure by increasing crystallinity, relieving residual stresses, and improving dimensional stability. These thermal modifications improve stiffness and heat resistance but may also affect ductility depending on the crystallisation kinetics of the specific polymer. Surface-based post-processing methods enhance interlayer contact or heal surface defects by increasing local polymer mobility or enabling partial remelting. Collectively, these mechanisms explain the experimentally observed trends in mechanical behaviour under varying process parameters and post-processing strategies (Marabello et al., 2025). The effects of printing parameters on the performance of polymer composites is summarised in Table 2.

The reviewed studies show that parameter settings that increase interlayer contact and reduce voids generally result in higher mechanical strength in FDM parts. Build orientations that align the raster direction with the applied load minimise weak interlayer planes and therefore improve tensile and flexural performance. Smaller layer heights and zero or negative air gaps increase the bonding area and promote polymer chain diffusion between layers. Raster angles aligned closer to the load path reduce crack propagation across bead boundaries. Moderate extrusion temperatures and controlled printing speeds maintain suitable melt flow and thermal conditions for uniform layer fusion. These effects collectively explain why certain parameter levels consistently show better mechanical properties in experimental studies.

The primary limitations of 3D-printed polymers, including fine surface finish, internal stresses, and variable mechanical strength, have been significantly improved through post-processing techniques. Post-processing enhances surface characteristics by employing methods such as polishing and leveling, which promote durability, thermal stability, and environmental resistance through chemical coatings. Additionally, post-processing plays a crucial role in optimizing the mechanical, thermal, and chemical properties of printed parts to meet specific functional requirements.

For polymer composites, post-processing enhances bonding and load distribution, thereby improving performance in demanding applications. It bridges the gap between raw 3D-printed parts and high-quality, reliable products suitable for industrial applications.

Post-processing methods for 3D-printed polymers and their composites can be categorized into mechanical, chemical, thermal, aesthetic, surface finishing, and advanced treatments (Park et al., 2022), as illustrated in Figure 1. Mechanical methods, such as sanding, polishing, and machining, are commonly used to improve surface finish but may compromise the dimensional accuracy of the printed parts. Among chemical methods, techniques like vapour polishing involve exposure to solvents to achieve smoother surfaces. Thermal post-processing methods, such as annealing, enhance the dimensional stability and mechanical properties of 3D-printed parts. However, some post-processing techniques may have unintended negative effects. For instance, certain post-curing conditions might preserve dimensional accuracy but adversely impact the biocompatibility and physical properties of the material. The primary post-processing methods include chemical treatment, annealing, and vapor smoothing.

The post-processing techniques have significant effects on the mechanical properties of 3D-printed polymers and their composites. Annealing has been revealed to enhance the interlayer tensile strength of 3D-printed composites. For amorphous polymers like polyethylene terephthalate-glycol (PETG), the annealing at temperatures above the glass transition temperature can recover the reduced interlayer tensile strength caused by the addition of short carbon fibres. In the case of semi-crystalline polymers such as polylactic acid, the recovery of interlayer tensile strength is observed when the annealing temperature is higher than the glass transition temperature but lower than the cold crystallisation temperature (Stoeffler et al., 2013). Moreover, the incorporation of carbon nanotubes (CNTs) in polyetherimide (PEI) composites has been identified to improve bond strength and reduce porosity which can give enhanced mechanical properties in 3D printed samples. The combination of CNT incorporation and annealing further improves mechanical performance and reduces warping in 3D-printed PEI samples (Vijaya Kumar and Velmurugan, 2022). This highlights the synergistic effect of reinforcement materials and post-processing techniques on the mechanical properties of 3D-printed composites (Ravi and Zagabathuni, 2026). The scale and size of 3D-printed composites also play a crucial role in finding their mechanical properties. Recent studies have shown that elastic moduli and yield strengths decrease as the scale increases. Increasing the sample size directly results in higher elastic modulus and yield strength.

Isostatic compaction and annealing significantly enhance the mechanical properties of 3D-printed nylon glass fibre composites. Optimal conditions (0.55 MPa and 200 °C) improved strength by over 50% (printing direction) and 100% (transverse direction) and doubled the modulus in the printing direction. These improvements result from better compaction, reduced voids, enhanced crystallinity, and fibre alignment. Compaction and annealing effectively overcome interfacial and void limitations in 3D-printed composites for improving performance (Žigon et al., 2020). Post-processing at 150 °C significantly enhances 3D printed continuous carbon fibre-reinforced composites by reducing porosity by 87% and improving interlaminar strength by 145%, without altering dimensions. The increase in thermal stability (glass transition temperature from 109 °C to 131 °C) is attributed to drying effects that reduce plasticization for enhancing mechanical performance. Post-processing effectively addresses limitations in 3D-printed CCFRC while progressing their suitability for high-performance structural applications (Ma et al., 2024).

Annealing of graphene-reinforced PLA composites significantly improved thermal stability, electrical conductivity, and mechanical properties such as tensile and flexural strength and hardness due to increased crystallinity. Functional groups remained unchanged but impact and notch sensitivity resistances decreased. Annealing enhances the key properties of graphene-reinforced PLA, highlighting its potential for advanced applications in the automotive, aerospace, electronics, and medical sectors, where the use of PLA is increasingly prevalent (Ye et al., 2022).

Moreover, the build direction of the printed parts also influences the effectiveness of heat treatment. Heat-treated and build-direction composites have shown higher tensile strength and Young’s modulus than flat-build composites due to better fibre penetration and reduced porosity (Ravi and Zagabathuni, 2026). Hence, post-processing techniques such as annealing and heat press processing can enhance the tensile strength and modulus of 3D-printed polymsers and their composites. The effectiveness of these methods depends on factors such as polymer type, reinforcement material, process temperature and build direction. By optimising these parameters, it is possible to achieve mechanical properties compared to conventionally manufactured composites. Therefore, the 3D-printed materials are progressively viable for high-performance applications.

Post-processing techniques can significantly influence the impact resistance of 3D-printed polymers and their composites. The layer-by-layer nature of additive manufacturing results in anisotropic properties which creates 3D printed parts exposed to delamination and reduced impact resistance. However, various post-processing methods have been explored to enhance the impact resistance of these materials. Chemical treatment has shown promising results in improving the surface quality and reducing the porosity of 3D-printed thermosetting polymers. By immersing the printed parts in solvents, it has been observed that smoother surfaces and decreased porosity can significantly lead to improved impact resistance (Muhamedagic et al., 2022). Also, thermal treatments such as annealing have been found to affect the fracture properties of 3D-printed composite structures and enhance their impact resistance. Carbon fibre reinforcements have shown a significant increase in impact resistance compared to unreinforced polymers (Kumar Jain et al., 2022). The orientation and distribution of these fibres play a crucial role in determining the impact resistance of the 3D-printed composites (Pascual-Go et al., 2021). Post-processing techniques can be used to optimize fibre orientation and distribution to enhance the impact resistance. Moreover, the development of shape memory polymers and self-healing polymers for 4D printing applications shows potential for fabricating materials with dynamic impact resistance properties that can recover after impact events (Liesenfeld et al., 2024).

The post-processing techniques have a significant role in the surface hardness of 3D-printed polymers and their composites. The layer-by-layer nature of additive manufacturing results in poor surface quality which can be improved through various finishing methods. These post-processing techniques not only enhance the surface finish but also influence the surface hardness of the printed parts. The annealing significantly enhances the impact resistance of PLA 3D printed components, with up to a threefold increase in impact energy. The infill percentage, especially at 100% plays a critical role in determining impact energy. Annealing is an effective post-processing technique to improve the mechanical performance of PLA 3D-printed parts which makes it suitable for applications requiring high mechanical integrity under load (Khan et al., 2024).

The laser polishing method is known to reduce surface properties and enhance the performance of 3D-printed nylon six polymers. This technique addresses industrial challenges by improving surface roughness, flexural strength, and tensile strength with eco-friendly polishing. Here, the experiments using response surface methodology have examined the effects of varying laser parameters on surface finish and mechanical properties. Post-polishing analysis for mechanical testing and scanning electron microscopy was compared to pre-polished samples. Multi-optimization has revealed a 20.2% reduction in surface roughness, an 8.27% increase in flexural strength and a 1.45% improvement in tensile strength. The optimal laser scanning time for samples was 0.23 min with an energy consumption of 1.58 kWh per experiment. These results prove that laser polishing significantly enhances the mechanical and surface properties of nylon 6 with a sustainable and efficient postprocessing solution. This innovative method is a promising advancement for the 3D printing industry to improve product performance and environmental benefits. Figure 4 represents the effect of laser polishing post-processing technique and their effects on a microscopic level and mechanical strength (Rithika and Sudha, 2024).

The post-processing methods for 3D-printed polymers and composites can face several challenges and limitations. One of the primary issues is the potential for reduced bonding between reinforcement and matrix materials which can result in decreased mechanical properties and structural integrity. This is particularly challenging for fibre-reinforced polymer composites where the interface between fibres and the polymer matrix is crucial for overall performance.

Surface finishing techniques categorized into chemical, thermal and physical methods have their own set of challenges. The chemical methods may change the material properties and undesirable reactions while the thermal treatments can cause dimensional changes. The physical methods such as sanding or polishing may remove too much material or create inconsistencies in the surface texture. Also, the time and cost of post-processing can be significant and compensate for some of the advantages of 3D printing.

While post-processing enhances the properties of 3D-printed polymers and composites, it introduces complexities. The challenge lies in balancing the desired improvements in surface quality, mechanical properties, and functionality with the potential drawbacks of material alteration, increased production time, and additional costs. Future research should focus on developing more efficient post-processing techniques that can maintain the integrity of the printed parts while attaining the desired enhancements. A comparative summary of the post-processing techniques discussed in this section is presented in Table 3.

The performance of an FDM nozzle depends strongly on the flow resistance inside the melt channel, which determines the pressure required to maintain a steady extrusion rate. For most polymer filaments, the melt can be approximated as a non-Newtonian shear-thinning fluid, in which the pressure drop increases sharply as the nozzle diameter decreases. A practical estimate can be made using the simplified Hagen-Poiseuille relationship, which, although derived for Newtonian fluids, provides a useful first-order approximation for design decisions. For example, reducing the nozzle diameter from 0.4 mm to 0.3 mm can nearly double the required pressure for an extrusion speed of 5–10 mm/s, depending on the melt viscosity of PLA or PETG. This is consistent with experimental findings in which small-diameter nozzles exhibit higher flow resistance, greater temperature sensitivity, and a higher likelihood of melt instabilities.

Similarly, the extrusion pressure varies with print speed. When the deposition velocity is increased, the polymer enters the nozzle with less residence time for heat transfer, which increases the apparent viscosity and consequently the pressure drop. In practical terms, a 20%–30% increase in printing speed may require a 40%–50% increase in extrusion force for high-viscosity materials such as fibre-filled composites. These relationships provide practitioners with a clearer basis for choosing nozzle diameters when balancing resolution, flow stability, and required motor torque.

Nozzle wear prediction can also be approached using simplified abrasive wear models. Reinforced filaments containing carbon fibres or mineral particles cause gradual enlargement of the orifice diameter. Using Archard’s wear relationship, the wear volume is proportional to the product of abrasive load, sliding distance, and a material-specific wear coefficient. For a typical carbon-fibre PLA, this means that steel nozzles may show measurable dimensional change after 15–20 h of printing at moderate flow rates, whereas hardened steel or tungsten carbide nozzles exhibit significantly slower wear. Including these estimates helps practitioners select nozzle materials based on anticipated print duration and filament abrasiveness.