Introduction

Additive manufacturing (AM) constitutes a swiftly expanding market, attaining significance in the shift towards advanced industrial production. Previously, emphasis predominantly centered on the 3D printing of metal [1–5] and polymer materials [6–9]. However, there exists a burgeoning interest in more intricate applications and innovative material types, commonly referred to as modern or advanced materials. These materials hold potential for implementation across five distinct categories [10]: technical ceramics (oxides, carbides) [11; 12], polymers (such as the PAEK family encompassing PEEK and PEKK) [13; 14], metals (refractory metals like tungsten and molybdenum) [15; 16], 4D materials (materials exhibiting shape memory) [17–20], and composites, specifically polymer composite materials (PCMs) featuring continuous fibers [21; 22].

Presently, traditional PCM production, primarily conducted using woven fiber sheets and thermosetting resins, stands as one of the labor-intensive and costly manufacturing processes [23]. Nonetheless, composites persist as one of the swiftest growing and rapidly evolving industrial segments in the global market. The anticipation is that AM technologies will assume a pivotal role in this evolution, given the array of emerging commercially available technologies and processes.

Recently, VoxelMatters (UK), a company specializing in market research and analytics within the AM industry, unveiled a comprehensive map showcasing technologies and existing companies offering commercial systems for implementing 3D printing processes [24]. This map provides users with insights into the spectrum of extant 3D printing technologies and the evolving landscape of AM across various materials, notably polymer composites. Employees at VoxelMatters [25] highlight that among all material families, polymer composites reinforced with fibers-specifically continuous fibers-possess distinctive properties and advantages. They underscore that leveraging 3D printing technology will augment the utilization of these materials, enabling more efficient, cost-effective, and expedited manufacturing of parts with a unique combination of final functional properties.

There are currently companies offering desktop systems designed for 3D printing of continuous fiber PCM. However, utilizing these 3D printers for PCM manufacturing presents several disadvantages, the primary one being the limitation of fiber placement solely within the plane of the construction platform [26]. This limitation significantly impacts product design and creation since the highest mechanical properties are attained when load application aligns with the direction of reinforcement. This drawback restricts the production of a broad range of parts that experience loads not confined to the same plane. To address these limitations and other issues inherent in desktop 3D printers, specialized equipment is being developed based on industrial robotic manipulators. This approach introduces fresh possibilities and sets new benchmarks for PCM manufacturing. Key advantages include a larger working area facilitated by robotic arms compared to desktop 3D printers, along with the capability to fabricate spatially reinforced polymer composite products. This is made feasible due to the increased degrees of freedom afforded by robot manipulators. Detailed insights into the intricacies of producing such products and materials, encompassing aspects like tool path planning, kinematics, robot collision avoidance, and technological constraints encountered during the printing process, are extensively described in papers.

This review focuses on examining the present-day technologies for 3D printing of spatially reinforced PCM with added continuous fibers through industrial robotic manipulators. It highlights leading advanced companies providing readily available commercial systems and assesses the successful utilization of these systems in producing reinforced parts.

Existing technology of 3D printingfor continuously reinforced PCM

The evolution of 3D printing technologies for continuously reinforced PCM has been steady yet notably swift, resulting in the emergence of various terms and methodologies. Researchers at the Technical University of Munich have endeavored to establish a conceptual framework for standardizing 3D printing processes involving continuously reinforced PCM. The intricacy of this endeavor largely stems from the idiosyncrasies inherent in the execution of printing processes adopted by different companies [29]. Nevertheless, contemporary trends indicate a movement toward categorizing these processes based on the methodologies used for supplying reinforcing and matrix materials to the printing head and their deposition during the part-building process. Fig. 1 provides an overview of the current implementation schemes and identifies companies offering commercial systems for PCM 3D printing incorporating continuous fibers.

It is evident that there are presently five distinct approaches employed in the realization of the robotic 3D printing process for continuously reinforced PCMs using commercially available equipment. Below, a summary of the primary accomplishments attained thus far for each of these schemes is outlined.

1. In situ impregnation

The core principle of this technology involves the in situ impregnation of continuous dry fiber within the extrusion head of the printer using specialized thermosetting or thermoplastic polymer materials. This is followed by material extrusion through a nozzle and subsequent curing. Among the key companies employing the in situ impregnation process via industrial robotic manipulators are Continuous Composites (USA) [33], Orbital Composites (USA) [34], and Moi Composites (Italy) [35]. Predominantly, carbon and basalt fibers serve as the primary reinforcing materials, while less commonly used options include glass fiber and natural fibers.

Continuous Composites, established in 2015, has pioneered a patented 3D printing technology termed Continuous Fiber 3D Printing (CF3D). This technique involves the in situ impregnation of continuous dry fiber with a specialized fast-curing thermoset resin. As the material is extruded through the nozzle, it undergoes instant curing facilitated by a UV light source. According to the company’s reports [36], the utilization of an industrial robot enables material deposition in any direction, optimizing the orientation of reinforcing fibers based on the specific design requirements of the manufactured part. Continuous Composites’ patented CF3D technology can be employed with gantry robots or industrial robots, providing flexibility in manufacturing. Utilizing a 6-axis robot from Comau, the company has successfully fabricated intricate parts and components, including a carbon fiber aircraft wing spar element (Fig. 2, b).

Continuous Composites employs both structural fibers (such as carbon, glass-filled, and Kevlar) and functional fibers (including optical and metallic fibers) as reinforcing materials. The choice of the matrix polymer material is predicated on mechanical properties, heat transfer characteristics, and environmental resilience, aligning with the operational requisites of the intended product.

The successful application of CF3D technology has extended to several university research laboratories [37–40]. In one study [40], the researchers showcased their investigations into the mechanical properties of PCMs manufactured using CF3D technology via the Comau industrial robotic arm. They examined samples fabricated from high-temperature thermosetting acrylic polymer GF-2, combined with high-strength carbon fiber T-1100 at a volume fraction of 41.5 %. The resulting samples exhibited a Young’s modulus of 122 GPa and a tensile strength of 1599 MPa, constituting 89 % (137 GPa) and 55 % (2926 MPa) of the theoretical values, respectively. The authors [40] underscored that these results are notably high within the realm of additive manufacturing, highlighting the promising potential of CF3D technology for manufacturing PCM parts.

Established in 2014, Orbital Composites initially carved its niche in 3D printing components tailored for space applications. Presently, the company is pioneering its proprietary 3D printing technology, entailing the in situ impregnation of continuous dry fiber with thermoplastic polymer material, subsequently compacted using a roller. The manufacturer currently offers three distinct types of 3D printers based on industrial robots: “Orbital e-” – a 6-axis robot geared towards educational and research endeavors, featuring a 1.2×1.2 m building platform capable of printing with high-temperature thermoplastics; “Orbital S” is an industrial-class robot with a unique manipulator movement system, enabling flexible attachment points. This facilitates the printing of large parts from multiple angles (Fig. 2, c); “Orbital F” is a container-type 3D printer for producing substantial composite structures of large dimensions.

The 3D printers developed by Orbital Composites possess the capability to heat the nozzle to temperatures surpassing 500 °C. This functionality enables printing with a wide spectrum of matrix materials, encompassing all prevailing low-temperature thermoplastics, in addition to high-temperature materials like PEEK, PEKK, and others.

Founded in 2018 at the Polytechnic University of Milan, Moi Composites has pioneered a 3D printing technology named Continuous Fiber Manufacturing (CFM) for continuously reinforced PCM. This technology involves impregnation utilizing epoxy resin, vinyl ester, and acrylic in conjunction with continuous glass, carbon, basalt, and other fibers (Fig. 2, e). Besides manufacturing 3D printers, the company produces 3D printing tool heads capable of integration with any 4-axis CNC machines, offering a flexible and scalable printing solution.

Moi Composites uses various thermosetting matrix materials, including epoxy resin, complex vinyl esters, and acrylic compositions. Continuous glass, carbon, and basalt fibers are employed as reinforcing components. The company is currently developing aramid and natural fibers. The company emphasises the use of acrylic materials for architectural details due to their transparency and the absence of a need for temperature during curing/post-curing. Materials based on complex vinyl esters are employed for marine components, whereas epoxy resin-based materials are favoured for the oil and gas as well as aerospace sectors.

2. Prepreg coextrusion

The underlying principle of this technology involves the coextrusion of a composite comprising preformed prepreg containing continuous fibers along with the addition of a thermoplastic element to facilitate adhesion to the matrix material. Key companies employing the prepreg coextrusion process utilizing robotic manipulators include Anisoprint (Luxembourg) [41] and CEAD (the Netherlands) [42].

Anisoprint, established in 2015, has innovated its own Continuous Fiber Coextrusion (CFC) 3D printing technology, employing two nozzles for matrix and reinforcing materials. The reinforcement nozzle comprises two distinct spools: one holding a tow of continuous fibers impregnated with thermoset, while the other contains a thermoplastic filament to enhance adhesion between the reinforcement and the matrix. Both are fed into a single extruder. This configuration of the process enables precise control over the volumetric ratio of fibers, while the utilization of a robot permits the establishment of intricate curvilinear trajectories during the 3D printing process (Fig. 3, a). The resulting manufactured parts constitute PCM structures comprising thermoset and thermoplastic polymers, interwoven with continuous fibers.

Several low-temperature thermoplastics like PC, PLA, TPU, PETG, and PA can function effectively as matrix materials. Reinforcement can be achieved using prepregs containing continuous fibers of carbon, glass, aramid, basalt, and boron.

It is noteworthy that, the robotic prepreg coextrusion technology developed by Anisoprint has found extensive application across various industries [43–48]. In a study by the authors of [48], the focus was on investigating the 3D printing of conformal paths using industrial robots to create shell structures composed of PCM through coextrusion technology. The primary stages of the study involved the development of the production system, trajectory planning for conformal paths, and performance testing. The equipment utilized in the study comprised a Universal Robots UR10e robot equipped with a coextrusion head, along with an Anisoprint Composer A4 desktop 3D printer. During the research, three samples were fabricated (Fig. 4): the first sample was produced using 3-axis Composer A4 equipment; in the second sample, the conical part was also crafted on a 3D printer, while the stiffeners were generated by a robotic system; the third sample was manufactured using a conformal method with the robotic system. The part produced conformally using the robotic system exhibited a compressive strength and stiffness that were 258.6 % and 134.9 % higher, respectively, compared to parts created using a 3D printer with three degrees of freedom.

The CEAD company, established in 2014, specializes in manufacturing large-scale robotic 3D printers and is known for developing its proprietary technology known as “Continuous Fiber Additive Manufacturing” (CFAM). This patented technology involves a print head that integrates continuous fibers with molten thermoplastic granules (Fig. 3, b, c).

3. Prepreg extrusion

The technology involves the extrusion of a composite prepreg impregnated with a thermoplastic polymer and embedded with continuous fibers. One notable commercial entity utilizing prepreg extrusion technology alongside industrial robotic manipulators is Ingersoll Machine Tools (USA) [49].

Founded in 1891, Ingersoll Machine Tools became a part of the Camozzi Group Corporation (Italy) in 2003. The company primarily specializes in the manufacturing of precision machines for metalworking, 3D printing, and automated fiber placement. Since 2015, the company has expanded its presence in the AM domain, currently offering five commercially available solutions, including MasterPrint Linear, MasterPrint 3X, and MasterPrint 5X. These machines are designed as large-scale 3D printers tailored for printing large-format PCM. Furthermore, the company presents two additional solutions grounded on industrial robotic arms specifically dedicated to 3D printing continuously reinforced PCM: “MasterPrint Robotic” and “MasterPrint Continuous Filament” (Fig. 5).

4. In situ consolidation

In situ consolidation technology, also referred to as automated fiber placement, entails the passage of fiber in prepreg form through a nozzle and subsequent heating with an added heat source directly at the outlet. In the realm of equipment manufacturers, there are companies providing desktop solutions, whereas the primary manufacturer utilizing industrial robots in this domain is the prominent company Electroimpact (USA) [50].

Established in 1986, Electroimpact employs robotic systems for 3D printing through Automated Fiber Placement (AFP) technology. The company’s developed technology merges the AFP method with FDM 3D printing [51]. This innovative approach involves printing a mold using the FDM method from a soluble polymer material. Continuous fibers are then laid in the form of a narrow ribbon on the mold’s surface by heating and sealing using a roller pre-impregnated with synthetic resin non-metallic fibers via the AFP method. After fiber placement, the polymer mold is dissolved. Electroimpact’s developments encompass SCRAM (Scalable Composite Robotic Additive Manufacturing), a 6-axis machine that combines AFP technology with FDM 3D printing [52].

Continuous fiber tape serves as the reinforcing material, pressed using a specialized roller during installation. The matrix materials primarily comprise thermoplastic polymers from the PAEK family (such as PEEK, PEKK, etc.), alongside nylon and other low-temperature thermoplastics like PA12, ABS, and others. Besides thermoplastics as matrix components, the printing technology also involves the utilization of water-soluble thermoplastics for producing temporary equipment. Fig. 6 showcases a 3D printer from the SCRAM series and a product exemplifying continuously reinforced PCM.

5. Inline impregnation

Inline impregnation technology represents a hybrid process amalgamating the benefits of both conventional and additive manufacturing. In this method, composite fibers are prepared using conventional impregnation processes and subsequently applied to the build platform through a nozzle. As of now, “Moi Composites” (Italy) the company previously mentioned, stands as one of the representatives pioneering this technology.

Conclusions

This review presents an in-depth analysis of existing additive technologies and equipment utilized in the manufacturing of continuously reinforced PCM employing industrial robotic manipulators. It underscores the exceptional relevance and promise of this research domain, particularly in introducing spatially reinforced PCM imbued with distinctive properties for producing components in aviation, marine, nuclear, and other industrial sectors. The technologies discussed in the review are actively employed in the production of large-scale structural components, lightweight and durable aircraft parts, and composite equipment. Beyond structural applications, the utilization of robotic systems opens doors to creating shape-memory polymer 4D materials for intelligent structures. These structures offer controllable attributes, such as deployable hinge structures for solar panels and mirror antennas in space applications, reconfigurable antenna devices capable of altering directional patterns during operation, and intelligent metamaterial designs with adaptive dynamic characteristics for energy absorption and noise suppression across various frequency bands.

Significantly, the use of industrial robots offers increased degrees of freedom, enabling the fabrication of materials with an ordered arrangement of continuous fibers. This ability facilitates the formation of a directed internal structure in products, accounting for material property anisotropy. The creation of an ordered, directional structure via robotic 3D printing using continuous fibers achieves optimal reinforcing effects, aligning with the operational requirements of the final product. Despite the notable advancements, it’s evident from the literature analysis that the development of spatially reinforced PCMs using industrial robots remains an underexplored yet promising research area. The rapid evolution of the additive technology market and its distinctive capabilities in product shaping highlight the immense potential of this field. A key objective in advancing AM within this research domain is the standardization of manufacturing processes for continuously reinforced PCMs using industrial robotic manipulators, with the ultimate aim of deploying these technologies across diverse industries.