Green Composites in Additive Manufacturing: A Combined Review and Bibliometric Exploration

Abstract

This review provides a comprehensive analysis of recent developments in the additive manufacturing of green composites, with a particular focus on their mechanical behavior. A bibliometric analysis of 482 research articles indexed in the Web of Science Core Collection and published between 2015 and 2025 reveals a sharp increase in publications, with dominant contributions from countries such as China, India, and the United States, as well as strong collaboration networks centered on materials science and polymer engineering. Thematic clustering highlights a growing emphasis on natural fiber reinforcement, biodegradable matrices, and performance optimization. Despite these advances, few studies have combined bibliometric analysis with a technical evaluation of mechanical performance, leaving a gap in understanding the relationship between research trends and material or process optimization. Building on these insights, the review synthesizes current knowledge on material composition, print parameters, infill design, and post-processing, identifying their combined effects on tensile strength, stiffness, and durability. The study concludes that multi-variable optimization—encompassing fiber-matrix compatibility, print architecture, and thermal control—is essential to achieving eco-efficient and high-performance green composites in additive manufacturing.

1. Introduction

Additive manufacturing (AM), widely known as 3D printing, has emerged as a transformative technology, reshaping the landscape of design, prototyping, and manufacturing across numerous sectors, including automotive, aerospace, biomedical, and construction. Among the various AM techniques, fused deposition modeling (FDM) stands out for its simplicity, low cost, and versatility, particularly in the production of thermoplastic components. FDM operates by depositing material layer by layer along a predefined path, offering great potential for material customization and waste minimization [1,2,3]. However, traditional thermoplastic filaments such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyethylene terephthalate glycol (PETG) often face limitations related to mechanical performance, thermal stability, and environmental impact [4,5,6,7,8,9].

The layer-by-layer fabrication inherent to AM processes, while providing unparalleled design freedom, fundamentally dictates the microstructural characteristics of the manufactured components. This is particularly important for composite materials, where the arrangement and orientation of constituent phases, especially fibrous reinforcements, govern the final properties [10]. The interfaces formed between successive layers and the potential for directional alignment of materials can lead to anisotropic behavior, meaning that properties vary depending on the loading direction [11]. Consequently, the AM of composites is not merely a replication of bulk composite material properties; rather, it generates materials with distinct, process-induced microstructural features that profoundly influence their performance. This understanding is vital when considering the application of AM to new classes of materials such as green composites. Furthermore, the pursuit of sustainability extends beyond material choice to encompass manufacturing processes themselves. AM’s potential for decentralized production, often termed “cloud production”, and on-demand manufacturing can significantly reduce transportation-related emissions and minimize overproduction, thereby complementing the environmental benefits offered by the use of green composite materials [12].

“Green composites” are a class of materials typically engineered by embedding natural fibers, derived from plant or animal sources, within a polymer matrix that is bio-based, biodegradable, or both [13]. Their growing significance stems from a range of environmental advantages over traditional synthetic fiber-reinforced polymer composites. These benefits include enhanced biodegradability at the end of product life, derivation from renewable resources, which reduces dependence on fossil fuels, a lower overall carbon footprint associated with their production, and lower embodied energy [7,14,15,16,17,18,19]. The material landscape for green composites is diverse, encompassing a variety of natural fibers such as flax, hemp, jute, wood fibers, and agricultural residues like rice husks. These are combined with bio-polymers, including Polylactic Acid (PLA), which is derived from corn starch or sugarcane, and Polyhydroxyalkanoates (PHA), produced by microbial fermentation. Natural fiber-reinforced polymer composites (NFRPCs) are thus viewed as sustainable biocomposites, offering attractive properties like a low weight-to-strength ratio and good flexural strength [13]. They hold the potential to replace more expensive and non-degradable petroleum-based composites in numerous applications [20].

The “green” attribute of these composites is multifaceted, extending beyond simple biodegradability or renewability. It often includes lower toxicity profiles compared to some synthetic counterparts, reduced energy consumption during the processing of natural fibers, and the potential for carbon sequestration as the plant-based fibers absorb atmospheric CO₂ during their growth phase [21]. However, it is very important to recognize that “green” does not inherently equate to “high-performance” or “problem-free.” Natural fibers present several challenges, including their hydrophilic (water-absorbing) nature, which can lead to dimensional instability and incompatibility with hydrophobic polymer matrices. They also exhibit variability in properties depending on their source and processing, and generally possess lower thermal stability compared to synthetic reinforcing fibers like glass or carbon [13]. These inherent characteristics directly impact their processability using AM techniques and the mechanical integrity of the final composite parts. Therefore, the development and application of green composites, particularly in AM, involve a careful optimization process, balancing the desired sustainability objectives with the necessary engineering performance requirements. The environmental credentials of a green composite should ideally be assessed from a lifecycle perspective. While natural fibers themselves are renewable, the energy consumed and chemicals used in their pre-treatment (to enhance compatibility with matrices) and during the AM process itself can contribute to the overall environmental footprint. This necessitates a complex evaluation to ensure that the final product genuinely meets sustainability goals.

The environmental challenges posed by conventional polymer use—especially petroleum-based plastics—have intensified research efforts toward sustainable alternatives. In this context, green composites have attracted significant attention. These materials promise to reduce environmental burdens through lower carbon footprints, biodegradability, and the valorization of renewable resources [15,22]. The integration of green composites into 3D printing applications presents a dual opportunity: enhancing mechanical performance while advancing sustainability goals.

Recent years have witnessed an increase in studies aiming to reinforce thermoplastic matrices with natural fibers (e.g., flax, hemp, kenaf, jute) [23,24,25,26,27,28], agricultural by-products (e.g., rice husk, wheat straw, and coconut shell) [29,30,31,32,33,34], and biofillers (e.g., lignin, starch, and microcrystalline cellulose) [35,36,37,38,39,40]. These reinforcements improve not only the stiffness and strength of the printed parts but also their biodegradability and environmental compatibility [41]. For instance, one study [42] developed biodegradable green composites using flax, jute, and their hybrid as reinforcements in a PLA matrix via hot press molding. Mechanical tests showed optimal tensile strength at 40 wt% fiber loading. Hybrid composites exhibited enhanced impact resistance due to improved fiber–matrix interaction, confirmed by SEM and FTIR. The materials showed promising biodegradability, making them suitable for eco-friendly applications in packaging and automotive interiors. Similarly, the incorporation of wood flour or lignocellulosic residues into polymer matrices has been shown to improve stiffness [43,44].

Despite these promising developments, the effective implementation of green composites in FDM processes remains a complex challenge. Several critical factors influence the mechanical properties of 3D printed green composites, including material compatibility, interfacial adhesion between matrix and filler, fiber dispersion, aspect ratio, and printing parameters such as nozzle temperature, layer height, infill density, and print orientation [45,46,47]. Poor interfacial bonding or uneven filler distribution can lead to void formation and weak zones, ultimately compromising mechanical strength. Therefore, optimizing composite formulation and print process settings is essential for achieving consistent and robust mechanical performance.

The rheological behavior of filled filaments is another major concern. Many natural fillers increase the viscosity of the melt, leading to extrusion difficulties or nozzle clogging [48,49,50]. Additionally, moisture absorption—commonly observed in natural fibers—can adversely affect dimensional stability and lead to steam-induced voids during printing [51,52,53]. To mitigate such issues, the pre-treatment of fibers (e.g., alkalization and silanization) or compatibilizer additives (e.g., maleic anhydride) are often employed to improve dispersion and interfacial bonding [54,55,56].

Furthermore, post-processing treatments such as annealing or chemical surface modification can enhance the mechanical stability of 3D printed green composites [55,57,58,59,60,61]. For example, thermal annealing has been shown to increase crystallinity in PLA-based composites, thereby improving stiffness and dimensional stability [62].

Applications of 3D printed green composites span a wide range of sectors. In consumer products and furniture, wood-plastic composites offer aesthetic and tactile advantages. In packaging, starch-based blends reinforced with plant fibers are being explored for biodegradable containers and cushioning materials. The automotive sector has also shown interest in lightweight, bio-based panels and components to reduce vehicle weight and emissions.

Despite notable progress, challenges remain in standardizing the characterization of mechanical performance for green composites in 3D printing. Current literature often lacks consistency in testing protocols, making cross-study comparisons difficult. Additionally, long-term performance, including fatigue resistance, creep behavior, and durability under varying environmental conditions (e.g., humidity and UV exposure), remains underexplored. There is also a need to better understand the recyclability and end-of-life scenarios of these materials, ensuring that sustainability goals are met across the entire lifecycle.

The fusion of 3D printing with green composite materials presents a promising avenue toward more sustainable and functional materials in manufacturing. However, achieving high mechanical performance alongside environmental benefits requires a careful balance of material selection, processing optimization, and property characterization. This review aims to provide a comprehensive overview of the current state of research on green composites for FDM 3D printing, with a particular focus on the mechanical properties of printed parts. It highlights recent advancements in material development, reinforcement strategies, processing challenges, and mechanical characterization, offering insights into future directions for sustainable additive manufacturing.

Through the combined use of bibliometric analysis and technical review, this study provides a unique perspective by not only mapping the evolution and intensity of research activity in the field but also linking these trends with the underlying technical advances. This integrated approach highlights that research interest in this area has grown significantly in the past five years, thereby offering both a quantitative overview and a qualitative assessment of the state of the art.

2. Materials and Methods

In this study, we set out to explore how the research landscape has evolved at the intersection of 3D printing and green or sustainable materials and composites. To do so, we combined a bibliometric mapping with a thematic review, following a two-step workflow. First, we collected the data, taking care to build a dataset that genuinely reflects the field; then, we analyzed the records to uncover trends, connections between concepts, and practical directions emerging from recent work.

To keep our process transparent, we followed the PRISMA logic, which is illustrated in Figure 1 as a sequence of selection stages—from the first identification of records to the final inclusion in the dataset. The core search was run in the Web of Science Core Collection using the query: (“3D printing” OR “additive manufacturing”) AND (“green composite*” OR “sustainable composite*” OR “green material*” OR “sustainable material*”).

This combination was chosen deliberately to capture publications that link additive manufacturing with environmentally oriented materials, both at the level of composites and at the broader level of sustainable feedstocks. The terms reflect the main themes in current literature and connect directly with the conceptual framework of our review.

After retrieving the initial set of 544 records, we screened them by publication year to focus on work that mirrors the latest thinking in the field. We focused on the papers published between 2015 and 2025 (up to 21 July 2025), which led to a refined list of records that also included early-access articles. This approach not only allowed us to map the state of the art but also to highlight the latest developments and material solutions that are shaping sustainable additive manufacturing.

The included document types were articles, review articles, proceeding papers, early access items, book chapters, and corrections. Editorial materials and meeting abstracts were excluded from the analysis.

In total, 62 records were removed during the eligibility and screening process: 1 record was excluded because it did not fit the 15-year period range, one record was excluded for language (non-English), 3 records were excluded due to document type (two editorial materials and one meeting abstract), and 57 records were excluded during manual screening because they were out of context:

• Twenty-four papers, because they mention 3D printing and eco-friendly/renewable terms, but without the context of “green/sustainable composites/materials;
• Eleven papers, because they discuss general recycling or CE without a clear link to additive manufacturing of green composites;
• Nine papers, because the use of “sustainable” in the biomedical context (e.g., drug delivery), not about green/sustainable composites;
• Seven papers, because they represent broad surveys with only marginal references to green composites in AM;
• Six papers, because they were on topic, focus on 4D rather than 3D printing and sustainability.

After applying these criteria, the final dataset comprised 482 records.

The bibliometric investigation was carried out with the aid of VOSviewer (version 1.6.20), a platform for mapping and visualizing bibliographic information. Through this software, we generated co-occurrence networks, bibliographic coupling maps, and keyword linkages, which helped define the major research directions, key references, and patterns of collaboration within the field. In parallel, a thematic review was performed to detect recurrent topics, new research frontiers, and areas that remain underexplored.

The resulting visualizations illustrate both the structural connections and the evolution of the domain over time. For example, the generated maps reveal clusters of related themes, while time-based overlays trace how research topics have developed from foundational studies to novel emerging themes.

By combining these quantitative mappings with a qualitative review of the literature, the approach offers an integrated perspective on research into green composites in 3D printing.

3. Additive Manufacturing of Green Composites: Opportunities and Intrinsic Challenges

The convergence of AM and green composites presents a compelling opportunity to leverage AM’s design freedom and material efficiency for the production of sustainable products. AM can facilitate the creation of optimized, lightweight structures using materials with a lower environmental impact, potentially enabling on-demand manufacturing of customized green composite parts with significantly reduced material waste compared to traditional methods [63]. This synergy supports the broader goals of sustainable manufacturing by considering greener input materials with lower embodied energy [63].

Despite this potential, the additive manufacturing of green composites is associated with several intrinsic challenges. Processability issues are prominent, largely due to the inherent characteristics of natural fibers. For instance, the relatively low thermal degradation temperature of many natural fibers (often below 200 °C) can be problematic for AM processes like Fused Filament Fabrication (FFF) or Selective Laser Sintering (SLS) that rely on heat to melt or sinter the matrix material [64]. This thermal sensitivity can lead to fiber degradation, charring, and the release of volatiles, compromising the mechanical properties and dimensional accuracy of the printed part. An illustrative example of mitigating intrinsic challenges in green composites is provided by the use of sisal fibers in high-temperature environments [65]. Tests demonstrated that sisal fibers prevented interlayer adhesion loss up to 400 °C, whereas fiber-free mixtures showed approximately a 37% reduction in interlayer adhesion at the same temperature.

Furthermore, sisal fibers effectively mitigated compressive strength losses across all analyzed temperatures, reducing the strength loss in the Y-direction from 34% in the reference composite (REF-C) to only 0.3% in the composite reinforced with 12 mm fibers (SF05%12) at 600 °C. The presence of sisal fibers also decreased the severity of damage and reduced the impact of thermal exposure on anisotropy induced by the 3D printing process, demonstrating that 3D-printed sisal fiber–reinforced composites (3DP-SFCC) can achieve improved mechanical performance under elevated temperature conditions.

In FFF, high fiber loadings in filaments can lead to increased melt viscosity, causing nozzle clogging and inconsistent extrusion. The incorporation of thermally modified (TM) wood particles in PLA filaments can improve 3D printing performance compared to non-modified wood [66]. TM particles produced filaments with lower surface roughness and porosity, resulting in better extrusion and higher tensile strength in printed parts. However, tensile properties of 3D-printed specimens remained lower than pure PLA, and injection-moulded samples were 18–69% stronger than their printed counterparts. These findings highlight the benefits of particle thermal modification but also indicate the need for further optimization of particle size and extrusion parameters to enhance filament quality and printed part performance.

Furthermore, the hydrophilic nature of natural fibers means they readily absorb moisture from the environment. If not adequately dried prior to AM processing, this moisture can vaporize at elevated temperatures, causing hydrolysis of polymer matrices (especially polyesters like PLA) and creating voids within the printed structure, thereby weakening the material [64]. Environmental humidity significantly affects the mechanical performance of 3D-printed continuous flax fiber/PLA composites [67]. Porosity, influenced by the printing pattern, ranged from 4.32% to 6.2%, and higher moisture conditioning led to exponential decreases in transverse strength and stiffness, longitudinal modulus, and shear properties—up to 64% reductions at 98% relative humidity. Elevated humidity also altered failure modes, increasing yarn–matrix debonding in longitudinal tests and promoting interlaminar fractures in shear specimens, highlighting the sensitivity of natural fiber composites to environmental conditions.

Achieving strong adhesion between the natural fibers and the polymer matrix within the dynamic and rapid thermal cycling environment of AM processes is another significant hurdle. The inherent chemical incompatibility between hydrophilic fibers and often hydrophobic biopolymers necessitates effective fiber surface treatments or the use of coupling agents. However, the efficacy of these treatments must be maintained throughout the AM process. Controlling defects such as porosity, poor interlayer adhesion (delamination), and undesired anisotropy is also critical [68]. The organic nature of natural fibers, which contributes to their “green” credentials, paradoxically presents some of the most substantial obstacles for their successful integration into AM processes. This creates a fundamental materials science and engineering challenge that is distinct from the AM of conventional unfilled polymers or metals. Addressing these challenges may require not only adapting existing AM techniques but also developing novel material formulations—such as fibers with enhanced thermal stability or bio-polymer matrices with lower processing temperatures—along with process modifications specifically tailored for these sensitive but promising sustainable materials.

3.1. Critical Role of Mechanical Properties in Engineering Applications

For green composites fabricated via additive manufacturing to serve as viable alternatives to conventional materials in structural, semi-structural, or functional engineering applications, their mechanical properties are of great importance [64]. Properties such as tensile strength, flexural strength and modulus, impact resistance, and compressive strength dictate the material’s ability to withstand various types of loads, its durability under service conditions, and its overall performance integrity in the intended application. Fiber-reinforced polymer composites, including green variants, are often selected for their potential to offer favorable specific properties, such as high strength-to-weight or stiffness-to-weight ratios [13].

The optimization of mechanical properties in AM green composites is rarely a straightforward endeavor. Often, a trade-off exists between enhancing one property at the expense of another, or between maximizing mechanical performance and maintaining processability or sustainability goals. For example, increasing the fiber content in a composite is a common strategy to improve stiffness and, to a certain extent, strength. However, exceeding an optimal fiber loading can lead to several undesirable consequences. Higher fiber content can increase the viscosity of the polymer melt, making filament extrusion for FFF more challenging and potentially leading to nozzle clogging [2]. It can also result in fiber agglomeration if dispersion is not adequately controlled, creating stress concentration points and reducing overall strength [69]. Furthermore, very high fiber loadings may increase the material’s brittleness, thereby reducing its toughness and impact resistance.

Nguyen et al. [70] similarly concluded that while recycled, bio-based, and blended composites offer environmental advantages, the mechanical performance of FDM-printed parts generally lags behind that of injection-molded counterparts. Their review emphasized the need for solutions such as plasticizers, compatibilizers, additives, and surface modifications to overcome poor interlayer adhesion and weak bonding. Our synthesis corroborates these limitations but extends the discussion by linking them to AM-specific parameters such as raster angle, layer thickness, and infill density, which directly influence stress transfer and anisotropy. However, Nguyen et al. [70] stressed the urgency of standardizing recycled and blended filaments; our combined bibliometric–technical analysis highlights that optimization of both material formulation and printing strategy is equally critical to closing the performance gap.

3.2. Influence of Material Composition on Mechanical Performance

3.2.1. Effect of Fiber Type, Loading, Aspect Ratio, and Size

The choice of natural fiber, its concentration within the matrix, and its physical characteristics (aspect ratio and particle size) significantly influence the mechanical performance of AM green composites. Different fiber types impart distinct properties; for instance, bast fibers like flax or hemp are generally known for higher stiffness and tensile strength compared to wood fibers, which might contribute more to bulk and potentially impact resistance depending on their form.

The fiber loading (volume or weight fraction) is a critical parameter. Generally, increasing fiber content can enhance stiffness (modulus) and, up to an optimal point, strength. However, beyond this optimum, mechanical properties often decline. This reduction can be attributed to several factors, including increased likelihood of fiber agglomeration (poor dispersion), insufficient wetting of fibers by the polymer matrix, leading to weak interfacial bonding, and increased porosity [69]. For example, one study [71] enhances PLA for 3D printing by reinforcing it with rice husk and rice straw. Under identical conditions, it examined the effects of fiber type, particle size (100 vs. 200 mesh), and NaOH treatment. At the same time, high fiber content lowered tensile strength, 200-mesh and treated fibers improved stiffness, flexural strength, and impact resistance. FTIR and SEM confirmed better fiber–matrix bonding. These natural fiber composites offered a sustainable, high-performance alternative for applications such as automotive and construction. Another study [72] aimed to enhance PLA with 6 wt.% micro-nano rice husk fibers treated with silane agents KH550 and KH570. The treatments improve fiber dispersion, bonding, thermal stability, water resistance, and mechanical properties—boosting tensile modulus by up to 98%.

For short fiber composites, performance is highly dependent on both fiber volume fraction (FVF) and the orientation of fibers within the matrix [73].

Fiber aspect ratio (length-to-diameter ratio) and particle size also play essential roles [74]. Longer fibers with higher aspect ratios are generally more effective in transferring stress and providing reinforcement, provided they can be adequately dispersed and oriented [75,76]. However, in AM processes like FFF, long fibers can be challenging to process and may break during filament extrusion or printing [77,78].

3.2.2. Effect of Fiber Orientation and Distribution (Inherent to AM)

Additive manufacturing processes, particularly material extrusion techniques like FFF, inherently influence the orientation and distribution of reinforcing fibers within the printed part. During the extrusion of composite filaments through the printer nozzle and their subsequent deposition onto the build platform, fibers (especially short or particulate ones) tend to align themselves along the direction of melt flow and deposition path [2]. This alignment leads to anisotropic mechanical properties, meaning the material exhibits different strength and stiffness values when loaded in different directions relative to the print orientation [10].

The raster angle, which defines the direction of filament deposition within a layer, has a profound impact. For instance, parts are generally stiffer and stronger when loaded parallel to the fiber alignment direction (e.g., a 0° raster angle relative to the tensile load) [79]. Studies on natural fiber reinforced composites (NFRCs), even those fabricated by traditional methods like hand layup, demonstrate the overwhelming influence of fiber orientation: it can contribute as much as 77.8% to flexural strength and nearly 96% to impact strength, often outweighing the effect of fiber type itself [10]. In FFF composites, this effect is well-documented. For PLA/wood composites, a 90° raster angle (fibers perpendicular to the length of a flexural specimen) yielded the highest flexural modulus of 3121 MPa in the study [80].

However, the optimal orientation can vary depending on the specific property and loading condition. For instance, in study [11] on bearing strength of FFF composites, specimens with a ±15° fiber orientation exhibited higher bearing strength than those with a 0° orientation, a trend that differs from some traditional composites. This highlights that the process-induced microstructure in AM parts can lead to unique mechanical responses. Careful consideration of toolpath planning and part orientation during the design phase of AM green composites is, therefore, essential to tailor fiber alignment for optimal performance under expected service loads.

3.2.3. Role of Matrix Material and Fiber-Matrix Adhesion

The properties of the polymer matrix itself (its intrinsic strength, stiffness, toughness, and ductility) form the baseline for the composite’s performance. The fibers are added to enhance these properties, but the effectiveness of this reinforcement is critically dependent on the quality of adhesion at the fiber–matrix interface. As previously emphasized, strong interfacial adhesion is necessary to efficiently transfer stress from the weaker matrix to the stronger and stiffer fibers. If the adhesion is poor, the fibers can debond or pull out from the matrix under the load, leading to premature failure and underutilization of the fiber’s reinforcing potential [13].

In green composites, the challenge of achieving good adhesion is often exacerbated by the chemical incompatibility between hydrophilic natural fibers and more hydrophobic bio-polymer matrices. This is where fiber surface treatments (e.g., alkali and silane) and the use of coupling agents play a vital role [71]. For example, untreated hemp fibers significantly reduced the tensile strength of PLA, but the cationization treatment applied to hemp fibers (using EPTA) improved interfacial adhesion to such an extent that the tensile strength of the composite increased by 615% compared to the untreated hemp composite, reaching values close to that of pure PLA [81]. Similarly, NaOH treatment of rice husk and rice straw fibers improved their adhesion to PLA, as evidenced by SEM observations of reduced voids and better fiber-matrix contact, contributing to enhanced mechanical properties [71].

Scaffaro et al. [82] demonstrated that up to 20% of PLA can be successfully substituted with lignocellulosic fillers derived from agricultural and marine waste, with only minimal compromise in processability and mechanical performance. Their work highlights the feasibility of valorizing such waste streams while also enhancing surface hydrophilicity, which may improve biodegradability after disposal.

3.3. Influence of AM Process Parameters on Mechanical Performance

3.3.1. FFF/FDM Parameters (Layer Thickness, Print Speed, Temperature, Infill Pattern/Density, Raster Angle)

The mechanical properties of green composites fabricated via FFF/FDM are highly sensitive to the selection of printing parameters. Each parameter can influence the microstructure, defect density, and ultimately, the performance of the printed part (see

Table 1. The effect of FDM parameters on the mechanical performance of 3D printed green composites.

FDM Parameter	Reported Results
Layer Thickness	This parameter defines the height of each deposited layer. Generally, thinner layers tend to result in improved interlayer bonding and reduced void content between layers, leading to enhanced tensile and flexural strength [83,84]. For instance, Sultana et al. [85] found a layer thickness of 0.1 mm to be optimal for the tensile properties of PLA/wood composites. In another study [86], 0.1 mm layer thickness was also identified as optimal for the tensile strength of jute-reinforced resin composites made by SLA. However, decreasing layer thickness significantly increases print time. Wang et al. [87] reported optimal flexural strength for continuous flax fiber/polyethylene (CFF/PE) composites with a layer thickness of 0.8 mm (combined with a 1.5 mm layer width), while a 0.6 mm thickness yielded the lowest porosity.
Print Speed	The speed at which the nozzle traverses to deposit material affects the heating/cooling rates, fiber impregnation by the molten matrix, and interlayer fusion. Slower print speeds generally allow more time for the polymer to melt thoroughly, wet the fibers, and fuse with the previously deposited layer, leading to stronger parts [64]. However, excessively slow speeds can increase the risk of material degradation due to prolonged exposure to high temperatures, while also extending printing time [88]. Sultana et al. [85] identified an optimal print speed of 20 mm/s for PLA/wood tensile properties. For CFF/PE composites, a speed of 0.167 mm/s was used for achieving optimal flexural strength, with higher speeds causing fiber tearing and lower speeds affecting print quality [87].
Printing Temperature (Nozzle Temperature)	This is a critical parameter that influences the viscosity of the molten polymer, its flow characteristics, and the degree of fusion between layers. An optimal temperature usually exists for a given material system [64]. If the temperature is too low, the viscosity may be too high, leading to poor flow, weak interlayer bonding, and potential nozzle clogging. If too high, the polymer may degrade, or in the case of green composites, the natural fibers may char or decompose. For PLA/wood, Sultana et al. [85] found 190 °C to be optimal for tensile properties, while Wang et al. [87] reported 200 °C as optimal for the flexural strength of CFF/PE.
Infill Density	This parameter determines the amount of material used to fill the internal volume of a printed part. Higher infill densities generally result in denser, stronger, and stiffer parts, as there is more material to carry loads [2]. However, 100% infill is not always necessary or cost-effective and significantly increases print time and material consumption. Sultana et al. [85] found 75% infill density to be optimal for the tensile properties of their PLA/wood samples.
Infill Pattern	The geometric pattern used to fill the interior of a part (e.g., rectilinear, grid, honeycomb, gyroid, triangular) can influence its mechanical response, particularly its load distribution capabilities, stiffness, and failure mechanisms [53]. Different patterns may be optimal for different loading conditions (tensile, compressive, flexural) [89,90,91].
Raster Angle (Fiber Orientation)	As extensively discussed in Section 3.2.2, the raster angle dictates the primary orientation of deposited filaments (and thus aligned fibers) within a layer. This parameter has a dominant effect on the anisotropic mechanical properties of FFF parts [11]. Optimizing the raster angle according to the expected stress directions in the application is essential for maximizing mechanical performance.

).

Other parameters, such as nozzle diameter, bed temperature (which influences adhesion of the first layer and can affect warpage), and cooling fan speed (which affects solidification rate and interlayer bonding), can also play a role in the final properties of FFF green composites [2]. The complex interplay between these parameters necessitates systematic optimization, often using design of experiments (DOE) approaches, to achieve desired mechanical outcomes for specific green composite systems.

Bi and Huang [92] highlighted that natural fibers, due to their high aspect ratio and thermal stability, are highly suitable for FDM and DIW processing, while nanocellulose derivatives (CNC and CNF) offer unique rheological control for gel-based systems. However, they emphasized that unresolved challenges such as poor interfacial adhesion, rheological complexity, and scale-up limitations continue to hinder practical deployment.

3.3.2. SLS Parameters (Laser Power, Scan Speed, Powder Bed Temperature, Layer Thickness)

For green composites processed via Selective Laser Sintering, the key parameters influencing sintering quality, part density, and mechanical properties are shown in

Table 2. The effect of SLS parameters on mechanical performance of 3D printed green composites.

Parameter	Reported Results
Laser Power	This determines the energy input for melting/sintering the polymer powder particles. Higher laser power can promote better fusion and densification, but excessive power can lead to degradation of the bio-polymer matrix or, more critically, the natural fibers, resulting in charring and reduced mechanical strength [93].
Scan Speed	The speed at which the laser beam traverses the powder bed affects the energy density delivered to the material. Slower scan speeds increase the interaction time and thus the energy input per unit area, which can improve sintering but also raise the risk of degradation if not balanced with laser power [93].
Powder Bed Temperature (Preheating Temperature)	The bulk powder material in the build chamber is typically preheated to a temperature slightly below the polymer’s melting or sintering point. This preheating is important for reducing thermal gradients between sintered and unsintered regions, minimizing part warpage and curling, and making it easier for the laser to raise the temperature of selected regions to the fusion point [93]. An optimal preheating temperature is critical for high part quality.
Layer Thickness	This parameter defines the thickness of each powder layer spread during the process. Research on Prosopis chilensis/polyethersulfone (PCPC) shows that increasing layer thickness tends to reduce tensile strength, bending strength, impact resistance, and density, while improving dimensional accuracy [94]. Conversely, reducing layer thickness enhances fiber alignment during powder recoating and increases modulus of elasticity [95].

.

The optimization of these parameters is essential for achieving well-sintered, dense green composite parts with good mechanical integrity. For instance, in [93], a study of Prosopis chilensis powder (PCP)/polyethersulfone (PES) composites, optimal SLS parameters were identified as a scanning speed of 1.8 m/s, a preheating temperature of 80 °C, and a laser power of 12 W. Using these optimized parameters significantly enhanced the quality and mechanical properties of the PCPC SLS parts. The challenge lies in finding a processing window that allows sufficient matrix fusion for good particle bonding and densification while limiting the thermal degradation of the sensitive natural fiber component.

3.4. Mechanical Performance of Additively Manufactured Green Composites

A wide range of mechanical properties has been reported for green composites processed through additive manufacturing, depending on factors such as fiber type, matrix material, print parameters, and reinforcement architecture.

Table 3. Summary of Reported Mechanical Properties of Additively Manufactured Green Composites.

Green Composite System	AM/Purpose	Key Finding	Limitations/ Challenges	Source
Prosopis chilensis powder (PCP)/polyethersulfone (PES) composite (PCPC)	SLS/to investigate the effects of scanning speed, preheating temperature, and laser power on the dimensional accuracy, mechanical properties (bending and tensile strengths), and surface roughness of PCPC parts.	The tests showed that optimal SLS parameters—1.8 m/s scanning speed, 80 °C preheating temperature, and 12 W laser power—significantly enhanced the quality of PCPC parts.	Limited availability and variety of sustainable SLS feedstock materials The mechanical properties of wood-composite SLS parts are of low quality Dimensional accuracy remains sensitive to parameter variations Post-processing (e.g., wax infiltration) required to improve surface quality and strength	[93]
PLA-rice husk filaments were produced by manually blending pelletized PLA with 0%, 0.5%, 1.0%, and 2.0% rice husk powder for 5 min to ensure homogeneity.	FDM/to evaluate rice husk-reinforced PLA composites for additive manufacturing and analyze their structure–property relationships.	Tensile strength increased with temperature for all samples, rising due to better melting and fiber impregnation. Strength peaked at 2.0 wt% rice husk but dropped at 177 °C due to defects from excess fibers.	Increasing particle (rice husk) content is still limited, since higher loadings may negatively affect tensile properties. Optimization of processing parameters is still required to balance porosity, strength, and higher filler incorporation.	[96]
Recycled polypropylene reinforced with hemp or harakeke fibers (up to 30 wt%)	FDM; Evaluate mechanical properties and process performance of natural fiber-reinforced PP composites in FDM	Reinforced filaments showed >50% increase in ultimate tensile strength and 143% increase in Young’s modulus vs. neat PP. - Mechanical improvement not fully retained in FDM-printed parts; some composites showed lower strength than unfilled PP. - SEM: good fiber dispersion and alignment, but porosity and fiber pull-out present. - Fiber addition improved dimensional stability during extrusion and printing.	The mechanical improvements observed in filaments were not fully translated to FDM-printed specimens, with some composites performing worse than unfilled polypropylene. SEM analysis revealed porosity and fiber pull-out, indicating insufficient interfacial bonding between fibers and the polypropylene matrix.	[97]
PLA-based wood fiber composite (commercial “wood filament”)	FDM; Study influence of 3D printing parameters on tensile properties using Taguchi DOE approach	Maximum tensile strength (10.15 MPa), modulus (198.57 MPa), and load (243.59 N) achieved at 0.1 mm layer height, 100% infill, 20 mm/s speed, 190 °C nozzle temp. - Maximum elongation (27.48%) occurred at 0.3 mm layer height, 75% infill, 10 mm/s speed, 190 °C temp, but tensile strength was lowest (3.09 MPa). - Regression analysis: layer height significantly affects strength (69.43%), modulus (63.42%), and load (69.43%); no parameter significantly affected elongation. - SEM: lower layer height improves fiber-matrix bonding and mechanical performance.	The mechanical performance of wood–PLA composites is highly sensitive to printing parameters, requiring precise optimization for consistent results. At higher layer thickness, increased gaps between successive layers reduce interlayer bonding and weaken mechanical performance. The study did not include a direct comparison between pure PLA and wood–PLA under identical printing parameters, leaving a key research gap.	[85]
Biobased polyethylene (BioPE) + thermomechanical pulp (TMP) fibers (0–30% w/w)	FDM; Evaluate influence of TMP fibers and maleic anhydride (MAPE) on printability and mechanical performance	TMP fibers increased melt viscosity and enhanced mechanical properties. - Up to 127% increase in tensile strength at 30% TMP + 6% MAPE vs. neat BioPE. - TMP fibers enabled successful 3D printing and improved mechanical behavior of printed parts. - Biocomposites with 10% and 20% TMP were effectively 3D printed via FDM.	Increasing fiber content enhances stiffness and strength but reduces elongation at maximum strength, limiting ductility. Although micromechanics showed a good interface, the interfacial shear strength was relatively low, indicating potential for improved fiber–matrix bonding. Further research is needed to clarify the mechanisms for enhancing interface properties and maximizing the effect of coupling agents.	[98]
Wood fiber-reinforced (bio)polymer biocomposites (hygromorphic biocomposites)	FDM; Investigate effects of printing parameters (orientation, width) on mechanical behavior	Mechanical properties strongly depend on printing orientation (due to fiber anisotropy) and printing width (100%, 200%, 300%). - Printed parts show high porosity (~20%), reducing Young’s modulus and strength compared to compression-molded samples. - Wider filaments increase porosity and reduce interfilament cohesion, leading to lower tensile strength and faster water uptake.	Mechanical properties are highly dependent on printing orientation due to fibre anisotropy, leading to variability in performance. Printed biocomposites exhibit relatively high porosity (~20%), which contributes to reduced stiffness, damage mechanisms, and lower mechanical reliability.	[41]
Wood fiber-reinforced PLA composites (WFRPCs)	FDM; Assess influence of extrusion temperature (200–230 °C) on physical and mechanical properties	Physical properties (moisture content, roughness, water absorption, swelling) were largely unaffected by extrusion temperature. - Density and color difference increased with temperature. - Tensile and flexural strength decreased above 200 °C. - Compressive strength and internal bond strength increased by 15.1% and 24.3% from 200 °C to 230 °C. - SEM showed improved fiber–PLA interface compatibility and inter-filament adhesion at higher temperatures.	Tensile and flexural properties decrease when extrusion temperature exceeds 200 °C, limiting mechanical performance at higher processing temperatures. While SEM showed improved fiber/PLA interface at higher temperatures, achieving consistent adhesion across the entire component remains challenging.	[99]
Reconstituted wood (wood pulp + wood flour + wood fibers; wood-polypropylene composite with 40% wood flour)	Adapted Additive Manufacturing (filament deposition via CNC-based 3D printing) to manufacture objects with low environmental impact	Macroscopic structure reveals fiber strengthening via multilayer intercalation. Microscopic structure shows nonwoven filament composite with fiber orientation in the extrusion direction. Agglomeration of wood flour and adhesion to wood fibers were observed. Mechanical properties: filament tensile strength ≈ 5.45 MPa, strain ≈ 1.25%, Young’s modulus ≈ 600.09 MPa; for wood–polypropylene composite (40% filler): tensile strength ≈ 25 MPa, Young’s modulus ≈ 3.5 GPa. Material performance is limited by heterogeneous wood fiber morphology and sensitivity to humidity and temperature. Humidity uptake is driven by hemicellulose and starch content. Shrinkage occurs during cooling.	Mechanical strength is limited by the heterogeneous morphology of wood fibers, causing variability in filament performance. Sensitivity to ambient conditions, including humidity uptake, leads to dimensional instability and potential shrinkage during cooling.	[100]
Enzymatically modified thermomechanical pulp (TMP) fibers + PLA matrix	FDM/3D printing of bio-based filaments with reduced water uptake	Laccase-assisted grafting of octyl gallate (OG) or lauryl gallate (LG) improved fiber hydrophobicity. OG-treated fibers showed lowest water absorption and best adhesion to PLA, enabling strong, printable filaments. OG-based filaments had highest tensile strength and maximum force among composites, surpassing neat PLA in load-bearing capacity. Lower performance in other samples due to poor fiber–matrix adhesion or fiber agglomeration.	The study focused on OG and LG grafting; further research is needed to explore other modifications or higher fiber loadings for broader mechanical property optimization. Hydrophobic modification adds an extra processing step, which could increase production complexity and time.	[101]
PLA and PLA/wood fiber composites	FDM/3D printing of tensile, flexural, and impact test samples; optimizing parameters via Design of Experiments (DoE)	Shell number is the most significant factor for tensile strength in PLA (followed by layer height and infill density). Material type is the dominant factor affecting all mechanical strengths; PLA consistently outperforms PLA/wood composites. PLA’s higher strength is due to better layer bonding; composites suffer from poor fiber–matrix adhesion and fiber agglomeration. Increasing shell number improves mechanical properties and reduces infill cavities across materials.	The L8 and L6 designs considered only a subset of influential printing factors (infill density, layer height and number of shells), potentially overlooking other variables such as printing speed, nozzle temperature, or fibre distribution. The tendency of wood fibres to form random bundles and weak interfaces reduces the reproducibility and reliability of mechanical strengths compared to neat PLA.	[102]
Wood flour/PLA composite filament (1.75 mm)	FDM/3D printing of specimens with varying layer thicknesses (0.05–0.3 mm)	Thinner layers (0.05–0.1 mm) significantly improved tensile and bending properties. Increased layer thickness led to higher porosity and water absorption due to larger internal gaps. Thinner layers resulted in denser cross-sections, enhancing strength and modulus.	Only layer thickness was varied, while other influential printing parameters (e.g., nozzle temperature, printing speed, infill density) were kept constant, limiting the generalizability of results. Increased porosity from larger layer thickness reduced mechanical performance, posing challenges for consistent quality control in practical applications.	[103]
Coir fiber powder (CFP)/PLA composite (0.1–0.5 wt% CFP)	3D printing + annealing at 90 °C for 120 min; mechanical and thermal property enhancement	At 0.1 wt% CFP + annealing, tensile and flexural strengths increased by 13.5% and 12.7% vs. neat PLA. Annealing improved crystallinity (index 63%, crystal size 6.7 nm) and thermal stability (Tg 256 °C). Higher CFP content (0.5 wt%) decreased mechanical performance; annealing mitigated some strength loss. Annealed composite with 0.1 wt% as reinforcement demonstrated better properties.	Only very low CFP contents (0.1–0.5 wt%) were investigated, limiting insights into the effect of higher fiber loadings or different particle sizes on composite performance. A single annealing temperature (90 °C) and time (120 min) were applied, which may not capture the broader effects of varying thermal post-processing parameters.	[61]
PLA reinforced with rice husk and rice straw	Fused Filament Fabrication (FFF)/Improve mechanical performance of PLA for structural applications	Surface-treated (NaOH), finely milled (200-mesh) rice husk and rice straw significantly improve PLA’s tensile modulus, flexural strength, and impact resistance. FTIR and SEM confirm enhanced fiber–matrix adhesion. Suitable for automotive and construction use.	The reduction in tensile strength at higher CFP contents suggests weak fiber–matrix adhesion; future research should explore coupling agents or fiber surface modifications to improve stress transfer. Long-term behavior under humidity, UV exposure, and thermal cycling remains unaddressed and requires investigation to assess real-world applicability Key 3D printing parameters (e.g., raster orientation, infill density, nozzle temperature) were not optimized, yet could strongly influence mechanical performance.	[71]
PLA reinforced with micro-nano rice husk (MNRH) treated with KH550 and KH570	Fused Filament Fabrication (FFF) via melt blending/Enhance interfacial bonding and mechanical properties	Silane-treated (KH550 and KH570) MNRH fibers improve dispersion, thermal stability, water resistance, and mechanical properties. Tensile modulus increased by up to 98%	Only two silane coupling agents (KH550 and KH570) were tested, leaving other potential chemical modifications unexplored. 3D printing conditions (e.g., layer height, infill density, nozzle temperature) were not systematically varied, which could influence mechanical performance and reproducibility.	[72]

provides a comparative summary of key mechanical properties gathered from recent studies on PLA and other bio-based polymer composites reinforced with natural fibers. This table highlights the variability in performance across different material formulations and processing conditions, offering a useful reference for selecting suitable combinations for structural or functional applications.

This complex interplay underscores that achieving desired mechanical performance is not simply a matter of maximizing fiber content. It involves a nuanced understanding of the interactions between fiber type, content, morphology, dispersion, the quality of the fiber-matrix interface, and the capability of the chosen AM process to effectively consolidate the composite material. Consequently, a multi-objective optimization approach is often necessary in the design and fabrication of AM green composites, seeking a balance between mechanical performance, processability, cost-effectiveness, and the overarching environmental benefits.

Billings et al. [104] demonstrated that integrating wood fibers into a PLA matrix yields sustainable FDM-printed composites with enhanced tensile modulus and strength, while leveraging non-contact digital image correlation (DIC) to reveal strain localization in complex structures like honeycombs, woven bowls, and frame bins. This review reinforces these findings by showing that stiffness improvements due to natural fiber reinforcement are consistently reported across different systems. However, persistent challenges such as porosity, the hydrophilic/hydrophobic mismatch, and structural reproducibility remain barriers to industrial scalability. Results of bibliometric analysis.

The processed dataset enabled a focused examination of how research on green composites within additive manufacturing has been structured and developed in recent years.

Using VOSviewer, we generated a map based on bibliographic data, applying keyword co-occurrence with full counting. To ensure that only terms with a consistent presence were included, we set the minimum number of occurrences at 5, for a selection of 125 keywords, full counting. The clustering was performed using the VOSviewer default algorithm (Louvain method, resolution = 1.00), and the layout was based on the LinLog/modularity technique.

Figure 2 illustrates the resulting network. Nodes correspond to frequently used keywords, while the links between them indicate co-occurrence relationships, revealing how concepts are woven together in published studies. The color-coded clusters highlight thematic concentrations, making it possible to see at a glance where established knowledge is dense and where emerging topics are beginning to form. This mapping offers a structural snapshot of the field, providing valuable cues for identifying connections, gaps, and future research opportunities in green composites for additive manufacturing. The terms were automatically organized by the clustering algorithm into four distinct thematic clusters, each represented by a different color on the map.

The green cluster forms the structural core of the field, bringing together terms such as additive manufacturing, sustainable materials, composites, and biomaterials. This cluster captures the general discourse on processes and material choices in 3D printing, highlighting the intersection between technological advances in additive manufacturing and the pursuit of sustainability.

The blue cluster emphasizes material science aspects, linking polymer and green composites, natural fibers, and reinforcement with performance-related terms like biodegradation and tensile properties. This indicates a strong research focus on bio-derived feedstocks and their mechanical behavior in additive manufacturing.

The red cluster gathers concepts associated with process optimization and broader sustainability topics, including recycling, circular economy, behavior, polymerization, and material degradation. This cluster reflects research that examines how operational settings, environmental strategies, and material transformations interact, emphasizing efforts to optimize 3D printing processes while integrating sustainable practices.

The yellow cluster is smaller and more specialized, centering around terms like geopolymer, construction, concrete, and durability. This cluster reflects studies exploring alternative sustainable materials and their potential applications beyond polymer-based systems, extending into the domain of large-scale and construction-related 3D printing.

Overall, the network reveals a highly-interconnected research landscape where material innovation, process engineering, and sustainability considerations converge. The four clusters highlight both mature areas—such as the use of PLA and natural fibers—and emerging niches like geopolymer-based composites, providing a clear map of current trends and future opportunities in green composites for additive manufacturing.

Having identified the main thematic clusters, we next explored how these concepts evolved over time. This temporal dimension was captured in Figure 3. The same software parameters were applied, as per Figure 2.

Figure 3 therefore provides insight into the chronological dynamics of research on green composites in 3D printing, revealing how specific topics have emerged, matured, or declined within the scholarly discourse.

The publication years in the dataset span from 2015 to 2025; however, in this visualization, the legend range was calibrated to accentuate the color differences, making it easier to distinguish between earlier contributions and more recent research trends within the field.

The figure acts as both a map of historical development and a guidepost for identifying trends that are shaping current and future investigations into green composites for 3D printing.

The overlay visualization shows that the most recent growth (yellow nodes) concentrates around terms such as “sustainable composite”, “biopolymer”, “life cycle assessment”, “hydrogel”, “biochar”, and “hybrid composites”, indicating a post-2023 shift toward sustainability metrics, bio-based chemistries, and multifunctional materials. Earlier anchors—“reinforcement”, “natural fibers”, “FDM/FFF”, “rheological properties”, and baseline “mechanical properties”—appear in cooler blue-teal tones, marking the 2015–2022 phase when the agenda consolidated around materials, testing, and process parameters. The central, intermediate-color nodes (“3D printing”, “mechanical properties”, “circular economy”, “PLA/biocomposites”) act as bridges linking foundational sustainability framing to these newer, application-driven niches (e.g., biomedical hydrogels, carbon/biochar additives, and construction-oriented systems). Overall, the time overlay suggests a progression from early materials/process baselining to a current emphasis on environmental performance and added functionality. Furthermore, we developed a bibliographic map based on text data used in titles and abstract fields. It provides a visual representation of how key terms from the full records and cited references are interconnected, outlining clusters of related concepts and illustrating their significance within the overall dataset. In the next step, full counting was applied and a minimum occurrence threshold of ten was set, narrowing the analysis to 150 frequently used terms and ensuring that only relevant keywords shaped the resulting map presented in Figure 4. The clusters were identified with the default Louvain clustering algorithm at a resolution of 1.00, and the layout was produced using the LinLog/modularity attraction–repulsion technique.

The map presented in Figure 4 reveals three well-defined thematic clusters that shape current research directions in this field.

The red cluster is centered on material characterization and mechanical performance, bringing together terms such as polylactic acid, tensile strength, sample, specimen, and density. This cluster reflects studies focused on experimental methods, detailed testing, and the mechanical behavior of bio-based materials used in additive manufacturing.

In contrast, the green cluster emphasizes manufacturing processes and technological development, with frequent terms including manufacturing, technology, innovation, construction, efficiency, and industrial application, capturing investigations into process optimization, large-scale uses, and the integration of sustainable materials into diverse systems. This focus is largely driven by industrial requirements for scalability and efficiency, as well as by sustainability frameworks that promote circular economy practices and low-impact construction materials. This focus is largely driven by industrial requirements for scalability and efficiency, as well as by sustainability frameworks that promote circular economy practices and low-impact construction materials.

The blue cluster points toward functional applications and innovation topics, connecting terms like device, hydrogel, electronic, protein, and tissue engineering, which indicate emerging research on advanced uses of green composites and their potential in next-generation printing technologies. The rise of this cluster can be linked to rapid advances in nanotechnology and bioprinting, which opened opportunities for hydrogels, biomedical devices, and multifunctional electronic applications.

Together, these clusters offer a structured view of thematic diversity within the dataset and reveal how methodological standardization, industrial upscaling, and cross-disciplinary innovation act as drivers behind the observed distribution of research topics.

To complement the structural and temporal perspectives presented in the previous figures, we further examined the density and concentration of research topics within the dataset. The same software parameters were applied, as per Figure 4. Figure 5 highlights the concepts that dominate current investigations. The brightest zones, shown in yellow, correspond to terms such as manufacturing, technology, challenge, review, paper, polylactic acid, sample, and tensile strength. These terms appear most frequently and co-occur across numerous studies, indicating the central focus on material testing, mechanical properties, and technological development within green composite research for additive manufacturing. Surrounding green areas represent secondary topics like geopolymer, construction, innovation, and hydrogel, which are less frequent but still relevant to the discourse. These terms illustrate the branching of research toward application-oriented directions, with construction materials responding to sustainability pressures and hydrogels emerging from biomedical innovation. The blue outer regions indicate sparsely discussed concepts with limited connections, suggesting potential opportunities for deeper exploration. Such peripheral zones often mark early-stage or niche ideas that could evolve into future research frontiers once aligned with industrial or environmental priorities. This density view therefore provides a clear picture of where scholarly attention is most concentrated and which themes form the backbone of research in this field.

An additional analysis focused on co-authorship by country, aiming to explore the collaborative research networks within the field of green composites through a temporal overlay.

The visualization presented in Figure 6 illustrates the collaborative landscape in research on green composites within additive manufacturing, showing both the density of collaborations and their temporal evolution. Each node represents a country, with the size of the node indicating the volume of publications, while the connecting lines reflect the intensity of co-authorship links. The color gradient provides a temporal dimension: countries shown in cooler shades such as blue and teal (e.g., the USA, Germany, and China) indicate earlier and sustained contributions, whereas warmer tones like yellow and light green (e.g., Romania, Portugal, Pakistan, and Turkey) reflect more recent or emerging participation in the field. Denser regions with many interlinked nodes, particularly around the USA, China, Germany, and India, highlight established hubs of collaboration, while peripheral nodes signal growing networks and opportunities for broader international cooperation. The concentration of central hubs reflects mature research infrastructures and longstanding international funding frameworks, while the appearance of smaller, newer nodes indicates that policy incentives and sustainability agendas are enabling new entrants to join the field.

The co-authorship network was generated in VOSviewer (version 1.6.20) using full counting, with a minimum threshold of one co-authored publication per country. The clustering was based on the Louvain algorithm with a resolution of 1.00, and the layout followed the LinLog/modularity attraction–repulsion technique.

Together, the map underscores both the global reach of the field and the shifting dynamics of international research partnerships over time. It also shows that collaboration is increasingly shaped by the dual drivers of research capacity (in established hubs) and sustainability-oriented policy frameworks (in emerging contributors).

To expand the perspective from countries to individual contributions, we further analyzed the most cited authors in the field. Figure 7 illustrates a density map of cited authors, where the brightest zones highlight those with the strongest presence and influence in research on green composites for additive manufacturing. For example, names such as Siqueira (2017), Mazzanti (2019), and Tumer (2021) appear in high-density regions, indicating that their works are frequently referenced and have become central to discourse. Similarly, authors like Rajeshkumar (2021) and Ilyas (2021) show strong citation clusters, pointing to impactful studies on biopolymers and natural fibers. The color gradient also integrates a temporal view: more recent influential contributions, such as those by Firoozi (2024) or Tanase (2024), emerge in cooler areas, reflecting their growing visibility. This visualization therefore captures both the intellectual backbone of the field and the evolving pattern of author influence over time.

The citation density map was generated in VOSviewer (version 1.6.20) using full counting, with a minimum citation threshold of 15. Clustering was performed with the Louvain algorithm (resolution 1.00), and the layout was created using the LinLog/modularity attraction–repulsion method.

The series of analyses presented above provides a comprehensive view of how research on green composites in 3D printing has evolved and where it currently stands. By integrating keyword co-occurrence networks, temporal overlays, density visualizations, and collaboration maps, we revealed not only the thematic structure of the field but also the dynamics that drive it over time and across regions.

The four clusters identified in the keyword analysis outline both established research directions—such as PLA-based composites and process optimization—and emerging areas like geopolymers and advanced functional applications. The density and temporal views further emphasize the dominance of certain core topics while pointing toward less explored concepts that could inspire future work.

At the same time, the co-authorship and citation analyses underscore the global and collaborative nature of this research domain, highlighting key contributors and networks that shape its development. Together, these findings provide a solid foundation for guiding subsequent investigations and strategic collaborations in sustainable additive manufacturing.

4. Conclusions

This review has provided a comprehensive overview of the current state of research in the field of additive manufacturing of green composites, with a particular focus on mechanical performance. Through the combined use of bibliometric analysis and technical review, it is evident that research interest in this area has grown significantly in the past five years. The most frequently studied materials include PLA-based composites reinforced with natural fibers such as flax, hemp, kenaf, and wood particles. Mechanical properties such as tensile strength and stiffness are highly influenced by factors like fiber type, infill pattern, layer thickness, and raster orientation, underscoring the complexity involved in optimizing these sustainable systems.

The bibliometric analysis revealed distinct patterns in global research activity. China, India, and the United States lead in publication output, with dense collaboration networks across Europe and Asia. It also demonstrates a clear shift in research priorities, with early studies focusing on foundational concepts such as sustainable composites and the circular economy, while more recent work emphasizes mechanical performance, additive manufacturing processes, and biocomposite optimization. This evolution reflects a deeper integration of environmental considerations into advanced manufacturing research.

Despite promising progress, several technological challenges persist. These include poor interfacial adhesion between hydrophilic fibers and hydrophobic polymer matrices, processing limitations due to fiber size and dispersion, and anisotropic behavior caused by directional deposition during printing. Additionally, while some mechanical enhancements have been achieved through optimized infill structures or post-processing treatments, reproducibility and standardization remain underdeveloped areas. More attention is also needed on process modeling and real-time quality control to scale up green composite printing for structural applications.

The bibliometric mapping highlights underexplored areas, such as durability under real service conditions, recyclability, and the lack of standardized testing protocols. Connected with the outcomes of the technical review, these gaps translate into research opportunities, including the development of reproducible testing methodologies to enable cross-study comparability, systematic investigations into long-term performance, and the integration of life-cycle assessment into material and process selection. In this way, the combined bibliometric–technical perspective provides a strategic roadmap for advancing sustainable additive manufacturing.

This review also has certain limitations. The bibliometric analysis is restricted to publications indexed in the Web of Science Core Collection, which may exclude relevant studies from other databases. In addition, the focus on mechanical performance narrows the scope, leaving aspects such as environmental impacts during large-scale production, cost analyses, and industrial implementation less explored.

To advance the practical implementation of green composites in additive manufacturing, future research should focus on tailoring interface chemistry through surface treatments or compatibilizers, developing printable bio-based matrices with improved rheological properties, and employing machine learning and simulation tools for multi-parameter optimization of print quality and mechanical performance.