Often referred to as 3D printing, additive manufacturing (AM) was initially created in the 1980s to address the extremely specific demands of model creation and rapid prototyping (RP). Since then, it has evolved into a versatile technological platform for rapid prototyping and computer-assisted design (CAD) (Hofmann, 2014). A variety of techniques that quickly and easily convert virtual solid model data into physical model are what constitute additive manufacturing (Rashid, 2019). Without the requirement for molds or machining, which are typical of traditional forming and subtractive manufacturing, it allows custom components to be made from metals (Yang et al., 2022), ceramics (Felzmann et al., 2012; Franchin et al., 2022) and polymers. Polymer-based additive manufacturing can be roughly classified into four categories, each of which has its own technology. Among them, polymers are undoubtedly the most frequently used additive manufacturing material category. Table 1 summarizes the advantages and disadvantages of AM classification methods for polymers. The first additive manufacturing technique is photopolymerization, in which a liquid photopolymer in a vat is selectively cured by photoactivated polymerization (Kaiser and Chalfin, 2017). Stereolithography (SLA) (Credi et al., 2016; Palaganas et al., 2017; Voet et al., 2018), digital light processing (DLP) (Ligon-Auer et al., 2016; Zhu et al., 2017, 2018), continuous liquid interface production (CLIP) (Tumbleston et al., 2015; Janusziewicz et al., 2016) and multiphoton polymerization (MPP) (Nikolić and Stevanović, 2006) etc., All fall into this category. In stereolithography, the light source may be either a laser or a projector in digital processing of the light (Borrello et al., 2018). The second technique is called powder bed fusion technology (PFT), where the polymer is delivered towards the print bed as a powder layer and then portions of the powder bed are thermally fused. Selective laser sintering (SLS) also belongs to this technology, as defined in the standard text of ASTM F2792-12 (Kaiser and Chalfin, 2017).The technique can be applied in cores or molds for manufacturing facilities made of various metal alloys as well as organic materials like sand. The powder bed used in the majority of SLS applications is made of thermoplastic polymers, especially different types of polyamide like PA11 and PA12 (Hofmann, 2014). The third technique is called material jetting, in which UV crosslinking is employed to selectively deposit viscous droplets of photopolymers (Udroiu et al., 2019). Material extrusion is the final significant additive manufacturing technique (Schirmeister et al., 2019). This technique is based on the original fused filament manufacturing (FFF) process and relies on a simple design using thermoplastic materials that appear to be essentially industry standard. (Li et al., 2020). The idea of melting a “typical” plastic product and extruding it via a narrow, heated nozzle seems to be quite straightforward as shown in the schematic of Figure 2 (Hofmann, 2014). This allows the extruded melt to deposit in the X-Y control motion of the anterior layer of the manufactured element (Rashid, 2019).

Additive manufacturing technology plays an important role in meeting the needs of individual production, overcoming many of the long-standing limitations of traditional processes. However, the technology still has certain shortcomings. For example, models printed with ABS or PET have rough details and poor fineness (Fu et al., 2021). Multiple material substitution combinations are required to produce fine print details. In addition, the SLS powder sintering process requires additional curing of the printed object; otherwise, it tends to cause liquid absorption and staining of the porous surface Wang et al. (2020b). Due to the limitations of material properties, additive manufacturing is still unable to accurately replace real products and remains more at the stage of simulating products. On the other hand, additive manufacturing is not very efficient. For the 3D printers commonly used in the market, a three-dimensional plastic model the size of a puck requires 28 h (Stansbury and Idacavage, 2016) of continuous machine work. The development of lightweight thermoplastic polymer foams enables researchers to quickly print small-scale parts before expanding them. An innovative and creative manufacturing process is created by the successful combination of polymer foaming with additive manufacturing, and thermoplastic polymer foam can be produced by SLA (Wirth et al., 2020), FFF and direct ink writing (Chen et al. (2018a), 2019), etc. Of these combined foaming processes, the SLA process is available for fewer materials because it requires a higher cure rate. In addition, SLA relies on the combination of cured polymers with soluble components for post-processing to form honeycomb structures (Wirth et al., 2020). In contrast, the FFF process is well suited to thermoplastic foams due to its low system cost, wide material selection, and improved toughness. The FFF technologies will be highlighted in this review.

Since Scott Crump created fused deposition modeling (FDM) and received a patent for it in 1989, the term “fused filament fabrication” (FFF) is widely used as a replacement (Schmitt et al., 2017). A fusion of thermoplastic material is extruded from the nozzle by heating and then the material is selectively printed on the platform according to the predetermined trajectory of the model slice (Zhang et al., 2020). The raw material, which is targeted at the inside of the hot-end, is bought on reels of filament with a 1.75 or 3 mm diameter (Eltorai et al., 2015). For amorphous or semi-crystalline polymers, the extruder is heated to a polymer-appropriate process temperature, which is either above the glass or melting temperature for semi-crystalline polymers (Nadiyapara and Pande, 2017). For a successful construction procedure, the CAD geometry parameters for a particular filament, build speed, processing temperature, and polymer melt rheology must all be carefully balanced (Pérez et al., 2018). The various elements that affect the performance of FFF printers are graphically represented in Figure 3. On the other hand, there are many challenges in implementing the FFF process to achieve thermoplastic foaming. Since it is printed in one piece, the structure used for support may also be embedded inside the model during the printing process and cannot be removed. At the same time, human manipulation can lead to warping, deformation, etc. Hwang et al. effectively solves the deformation problem due to the thermal expansion of the material by embedding metal particles into the polymer (Hwang et al., 2015). In addition to the control of deformation problems, Kokkinis et al. developed a magnetically assisted 3D printing platform in terms of workpiece anisotropy (Kokkinis et al., 2015). They achieved particle orientation control by adding magnetized aluminum platelets to a polymer matrix. Owing to the arrangement of anisotropic particles, the target performance of the workpiece in a specific direction was improved (Liu et al., 2022). Another problem with the FFF process is that the printed part contains a certain amount of porosity that can adversely affect the final mechanical properties (Kakumanu and Srinivas Sundarram, 2018). Even though the technology can achieve high precision in theory, it is still very difficult to manufacture in practice.

Because it can print objects made of various material types and then alter the print substance, FFF has the obvious benefit of allowing for more users control over device manufacturing for experimental applications (Weng et al., 2016). FFF is used as a tool for bioengineering tissue, with components ranging from bones or teeth to blood vessels or organ scaffolding. FFF is being used in a multitude of ways in the building industry (Zhang et al., 2021). In contrast to complicated geometries, FFF provides design freedom that aids in the optimization of design models that enable higher functional integration and lower material waste. Biomimetic actuators, such as worm-like and flower-like actuators, are examples of intricate magnetic actuators with complex topologies. FFF has been successfully applied to these actuators (Qi et al., 2020). In conclusion, additive manufacturing is becoming increasingly prevalent outside of specialized industries and is currently used in a variety of fields, including lightweight engineering, energy technology, pharmaceuticals and even food production. It performs significantly more than just its conventional role in rapid prototyping, rapid tooling, and concept modeling (Gross et al., 2014).

The concept of polymer foams was originally proposed in the hope of introducing a large number of voids into the polymer matrix that were smaller than the defects already existed in the polymer. Therefore, the rigidity of the product can be improved while the quality of the product is reduced, and other properties such as strength are not significantly affected. Polymer foam materials, as compared to non-foamed materials, have certain good qualities like high impact strength, extended fatigue life, high toughness (Zhai et al., 2022), strong thermal stability (Wicklein et al., 2015; Hu et al., 2019), low thermal conductivity (Ji et al., 2013) and dielectric constant (Wu et al., 2013), etc. Polymeric foams can be generated using a variety of techniques, including physical blowing agents (PBA) (Sauceau et al., 2011; Zhai et al., 2022) and chemical blowing agents (CBA) (Sadik et al., 2018). Thermally expandable microspheres (TEMs) (Diani and Gall, 2006; Peng et al., 2013; Zhang et al., 2019) have received increasing attention as a type of PBA. The polymer foam molding process generally includes three basic stages: the formation stage of the polymer/gas homogeneous system; the nucleation stage of the bubbles; and the growth and shaping stage of the cells, as shown in Figure 4. At first, the gas diffuses and disperses into the polymer under specific conditions, and after a certain time it reaches thermodynamic equilibrium to form a homogeneous system; changing the temperature or pressure causes the gas solubility in the homogeneous system to drop quickly, resulting in a supersaturated state. Numerous microscopic bubbles are generated in the system as it tends to revert to a low-energy steady state. The system’s pressure differential between the inner and outer the bubbles increases as the system’s gas solubility is further reduced, which causes the bubbles to grow and expand. Afterward, the system is cooled and shaped to take on the final cell shape and structure.

In order to manufacture polymer foam materials, the foam molding process can be separated into batch autoclave foaming (Yeh et al., 2017), continuous extrusion foaming (Lee et al., 2008; Sauceau et al., 2011), and injection molding foaming (Ameli et al., 2014; Villamil Jiménez et al., 2020), depending on the degree of continuous operation. Table 2 lists the various foaming techniques in alphabetical order. Batch autoclave foaming is typically performed in a high-pressure reactor, which provides the sample being foamed with a specific high temperature and high pressure airtight environment, allowing gases like CO₂ and N₂ to gradually diffuse and dissolve into the polymer matrix. Cell nucleation develops and finally takes shape as the thermodynamic conditions change. Anson Wong (Wong et al., 2011) and colleagues examined the foaming behavior of semi-crystalline polymers and discovered that samples using the Wessling method, including HDPE, PP, and PET, showed varying degrees of crystallization. They discovered that the dissolution slope reduced as crystallinity increased. This indicates that the amorphous area of the polymer is the sole place where adsorption takes place. Due to the semi-continuous foaming process, the diffusion of the gas in the polymer matrix can take place at a much higher rate and efficiency (Yu et al., 2015). Kuma et al. (Kumar et al., 2000) proposed a method for producing solid-phase PET foamed sheets using a semi-continuous method in response to the drawbacks of discontinuous methods for producing polymeric micro-foam materials. After the gas had been saturated, the separator layer was removed with a heating device and then foamed. The last foaming technology is continuous foaming, which is currently the most popular polymer foaming technology (Rizvi et al., 2018; Li et al., 2021; Gunasekaran et al., 2022a). Supercritical CO₂ can significantly reduce the time required for polymers to reach saturation, allowing for the production of microporous polymers in an industrial environment (Yeh et al., 2017; Hu et al., 2021). Therefore, it is imperative to provide a continuous preparation technology for micro-foamed materials based on established plastic processing techniques like extrusion (Nofar, 2016; Rizvi et al., 2018) and injection molding (Jahani et al., 2014, 2015; Contreras et al., 2020).