1. Introduction

Figure 1. Multiscale and multi-level structures in bead foams and related research topics.

Figure 1. Multiscale and multi-level structures in bead foams and related research topics.

2. Preparation of bead foams

Figure 2. Schematic of autoclave foaming with water as the dispersion medium (A). Reproduced with permission from [1], Copyright 2021 Taylor & Francis Group. Extrusion foaming equipped with an underwater granulator (B). Reproduced with permission from [20]. Extrusion foaming equipped with a winder-cooling pelletizer (C). Reproduced with permission from [11]. Copyright 2022 American Chemical Society.

Figure 2. Schematic of autoclave foaming with water as the dispersion medium (A). Reproduced with permission from [1], Copyright 2021 Taylor & Francis Group. Extrusion foaming equipped with an underwater granulator (B). Reproduced with permission from [20]. Extrusion foaming equipped with a winder-cooling pelletizer (C). Reproduced with permission from [11]. Copyright 2022 American Chemical Society.

2.1. Expandable polystyrene (EPS) beads

EPS foams account for the largest share of the bead foam market. The development of EPS was begun in 1949, when chemist Fritz Stastny at BASF successfully produced the material in laboratory. Generally, blowing agent (usually pentane) was introduced in PS suspension polymerization process to produce EPS beads, which contain 4−7 wt% blowing agent. EPS beads are produced in the form of small grains. Usually, EPS beads could not be used alone, which needed to be pre-expanded and molded into final parts with various geometry [29]. Molded parts produced from EPS beads offer several advantages such as excellent thermal insulation, low density, low moisture absorption, and a relatively low cost. Additionally, EPS production process is simple. Due to these benefits, EPS parts are widely used in various applications such as packing, building insulation [8], transportation and flotation foams. However, the intrinsic flammable features of EPS prevent it from being directly used in specific fields such as building insulation [8,30,31]. Consequently, research on EPS has focused mainly on adding multi-functionality, particularly flame-retardant properties, to EPS using a variety of methods.

The incorporation of various fillers in the polymerization process has been demonstrated as an effective method for manufacturing multifunctional EPS beads. Recently, hydrophobic Fe₃O₄ was incorporated into EPS by means of in situ suspension polymerization of styrene (Figure 3) [7]. The authors systematically studied the effects of different experimental variables, such as Fe3O4 content, stirring speed, and the amount of surfactant, on the size and magnetic properties of the expandable PS/Fe₃O₄ beads. SEM and energy-dispersive spectrometer results showed that Fe₃O₄ was well dispersed in the synthesized colorized and magnetic beads. It was proposed that the bead size and size distribution, as well as the degree of color could be adjusted by altering the Fe3O4 content. Flame retardants − such as aluminum hydroxide, ammonium polyphosphate, magnesium hydroxide − could be coated onto the surface of EPS beads or incorporated into PS chains via suspension polymerization. Hong et al. [32] used methylene diphenyl diisocyanate (MDI) as binding agent to improve the compatibility between intumescent flame retardant and EPS beads. cone calorimeter tests showed that the parts made by MDI-coated flame-retarded EPS exhibited good flame resistance performance. By incorporation the flame retardants into EPS through in situ polymerization, Bai et al. [31] prepared halogen-free flame-retardant EPS beads. The fire retardant, called hexaphenoxycyclotriphosphazene (HPCTP), was uniformly dispersed in the PS matrix at nanoscale. The particles formed by HPCTP were of the order of 33 − 57 nm and the average diameter was 44.86 nm. The mechanism of flame retardancy was also proposed based on the microscale combustion calorimetry results. In addition, fillers could be deposited on the surface of EPS beads. Lelis et al. [33] prepared SiO2 coated EPS beads by physical vapor deposition. The EPS beads were then pre-expanded and molded into EPS parts. The resulting expanded PS coated with SiO2 additives had almost 10% lower moisture adsorption and an 8.4% better deformation resistance. Surprisingly, the molded parts showed enhanced inter-bead bonding strength, which could be due to the SiO₂ acting as a binder.

Figure 3. Schematic of the synthesis route of colorized PS/Fe3O4 beads and EPS/Fe3O4 beads. Reproduced with permission from [7].

Figure 3. Schematic of the synthesis route of colorized PS/Fe3O4 beads and EPS/Fe3O4 beads. Reproduced with permission from [7].

2.2. Expanded polypropylene (EPP) beads

EPP have attracted much attention since its invention in the 1980s. EPP parts molded from the foamed beads show excellent solvent resistance, high impact resistance, and good heat resistance, high energy absorption capacity. EPP beads are typically fabricated by autoclave foaming. The foaming temperature window for EPP beads is very narrow due to the high crystallinity and low melt strength. The destruction of PP crystals during autoclave foaming results in a significant reduction in the melt strength, making it challenging to prepare EPP beads with desired properties [34]. Therefore, copolymerization with α-olefin or molecular modification to improve the foaming behavior and reduce the processing difficulty are necessary [13,35,36]. Furthermore, the crystalline structures of PP are sensitive to foaming conditions, such as temperature profile, gas type and pressure, and saturation time. As a result, the foaming process should be carefully adjusted to achieve the expected foaming results.

Park et al. [37] studied the foaming behavior of EPP beads using CO₂ as the blowing agent in a lab-scale autoclave and systematically discussed the critical processing variables for EPP beads production. It was found that an optimum stirring speed, which dependent on the shape of stirring propeller and channel sizes, was necessary to allow a uniform distribution of PP pellets in the dispersing media. Meanwhile, it was proposed that die geometry and saturation pressure are the key to pore morphology and thermal properties of EPP beads. When the pressure increased from 3.8 to 5.5 MPa, the cell density increased nearly one order of magnitude and the expansion ratio increased from 3.4 to 16.4 times. In another study conducted by the same group, Nofar et al. [38] explored the effect of various foaming parameters on the formation of double melting peak in EPP beads with the help of a high-pressure DSC. It was demonstrated that the thermal properties of PP samples were strongly affected by the saturation temperature and pressure. An increase in saturation temperature in an appropriate range facilitated the crystals perfection to form a higher melting peak (T_m-high). Meanwhile, the melted crystals formed a lower melting peak (T_m-low) during the cooling stage. Similarly, Guo et al. [39] also demonstrated that the double melting peak temperature in EPP beads shifted to higher value with an increase in saturation temperature and pressure. The formation of double melting peak in EPP beads is helpful to the sintering of foamed beads.

To improve the foamability of linear PP, several methods for increasing expansion ratio or optimizing cellular structure have been employed, including copolymerization, blending and modification (like grafting, cross-linking). The type and content of comonomer in PP copolymers have a significant influence on the foaming behavior of EPP beads [34]. Park et al. [34] used a pilot autoclave to compare the difference in the foaming behavior of a linear PP and PP copolymers containing ethylene comonomer. They reported that the addition of 3.5 wt% ethylene comonomer reduced the crystallinity and melting temperature (T_m) of PP resin, improving the PP foamability and increasing the cell size. By blending 3 ‒ 8 wt% high melting point PP, Hu et al. [40] found that the cellular morphology and melting properties of EPP beads based on low melting point PP could be improved. Guo et al. [36] fabricated high melt strength polypropylene (HMSPP) by direct polymerization and discussed the impact of melt strength on the cellular morphology and properties of EPP beads obtained by autoclave foaming. They found that EPP beads prepared from HMSPP had more uniform and well-distributed cells than that from polypropylene homopolymer. Janani et al. [41] investigated the role of polymorphism in the production of EPP bead foams with double melting point and concluded that the utilization of polymorphism during solid-state foaming is a feasible way to prepare EPP beads with nano-sized cells.

Reducing the energy consumption is one trend in the preparation of bead foams. Strategies such as blending PP with various polyolefin components or copolymerization propylene with other short-chain alkene have been utilized to fabricate EPP beads with moderate foaming conditions. Lyu et al. [13] synthesized various ethylene−propylene−butene-1 ternary copolymer (TPP) and ethylene−propylene copolymer and investigated the autoclave foaming behavior of these copolymers. It was found that the addition of 1-butene could reduce 5 °C for TPP in saturation temperature when the expansion ratio of EPP beads was identical. Huang et al. [42] found that the temperature and pressure could be reduced by blending a certain amount of polyputene-1 (PB-1) during the autoclave foaming of EPP beads without compromising the expansion ratio. Additionally, n-pentane has a significant plasticization effect on PP. Compared with CO₂ as the blowing agent, Zhai et al. [43] have reported that using n-pentane as the foaming agent during PP autoclave foaming could result in an obvious reduction in the foaming temperatures from 150 − 153 °C to 92 − 96 °C, an increase in the expansion from 8 − 20 to 10 − 50 fold, and an increase in the cell density from 10⁵ − 10⁸ to 10⁷ − 10⁹ cells/cm³. Lower foaming temperatures and higher expansion ratios reduce the production energy consumption, which enhances the competitiveness of EPP foams.

Although there was much research focused on the extrusion foaming of PP, these studies mainly investigated the preparation of foamed strands not foamed beads [18,35,44-47]. Recently, Huang et al. [12] successfully manufactured EPP beads by a commercially available extrusion line equipped with the self-designed wind cutting tool. They studied the cell growth behavior after the foam exited the extruder die using a high-definition camera. As shown in Figure 4, EPP beads with various shapes and sizes could be easily produced by adjusting the pelletizing speeds. The obtained foamed beads had a lowest density of 0.052 g/cm³ and the open cell content of the beads were higher than 50%. In another work on extrusion foaming, Huang et al. [11] improved the cell uniformity of EPP bead by blending 10 wt% TPU resin. A more uniform cell morphology contributes to a better mechanical strength of EPP molded parts. With the help of a foam extrusion line designed by Sulzer, Tammaro et al. [10] produced EPP beads with the bulk density ranging from 20 to 100 g/cm³. They used a long chain branched HMSPP as the feedstock and CO₂ and isobutane as the blowing agent. It was proposed that strain hardening feature of HMSPP made it easier to produce EPP beads with lower open cell content.

Figure 4. (a) Using high-definition camera (set at 3840 frames/s (x128 times)) to record the real-time cell growth of the beads at different pelletizing speeds, and (b) the diameter size curve of the bead foams (before leaving the extruder die) as a function of time. Reproduced with permission from [12]. Copyright 2022 Elsevier.

Figure 4. (a) Using high-definition camera (set at 3840 frames/s (x128 times)) to record the real-time cell growth of the beads at different pelletizing speeds, and (b) the diameter size curve of the bead foams (before leaving the extruder die) as a function of time. Reproduced with permission from [12]. Copyright 2022 Elsevier.

EPP beads produced through autoclave foaming usually display closed and uniform cell structures; however, the beads prepared by the extrusion foaming process exhibit a higher content of open cells and less uniformity. How to achieve high uniform and closed cells in the extrusion foaming process is an important problem need to be solved.

2.3. Expanded thermoplastic polyurethane (ETPU) beads

TPU is a category of TPE, which is composed of soft segments and hard segments. Oligomer diols with low glass transition temperature (T_g) such as poly(tetramethylene glycol) [48] acted as soft segments, which endow TPU with excellent stretchability and elasticity. ETPU beads with pore structures exhibit many advantageous properties including low density, high resilience, and cushioning effect [15,16]. A lot of research efforts has focused on elucidating the relationship between the TPU molecular chains’ structure and foaming behavior, strategies for suppression of the shrinkage of the foamed beads, and the adjustment of the foaming process [49-51].

The foaming behavior of ETPU beads is affected by the chemical structure, composition ratio, and molecular weight of the hard and soft segments in TPU. Nofar et al. [52] investigated the effects of hard segments content on the foaming behaviors of TPU pellets. The hard segment content in the TPU resins were 39 wt%, 49 wt%, and 57 wt%, and the corresponding Shore hardness of these pellets were 29D, 46D and 64D, respectively. Ordered structures with higher perfection formed in TPU with higher hard segments content, thus leading to higher melting temperatures. An increase in the hardness broadened the foaming temperature window and increased the suitable foaming temperature. For TPU with the highest hardness, 64D TPU exhibited lowest shrinkage ratio, but the expansion ratios of foamed samples were lower than 3-times. Furthermore, they compared the foaming behavior of TPUs with similar hard segments but different type of soft segments (polyester and polyether type) [53]. Compared with polyether based TPU, polyester based TPU showed a wider foaming temperature window. When the temperature increased from 85 °C to 115 °C, the polyether based TPU formed a larger and more closed packed microcrystalline under supercritical CO₂ conditions, which might hinder the growth of the nucleated cells and led to a limited expansion ratio of the foamed beads. Additionally, the effects of thermal annealing and pressure drop rate on foaming behavior were also explored. In their another study, Nofar et al. [54] had further studied the impact of different soft segments molecular weight on the foaming performance of TPU pellets. TPU with three soft segment molecular weights of 1000 g/mol, 2000 g/mol and 3500 g/mol were selected. It was found that an increase in soft segment molecular weight reduced the T_g of the soft segments, promoted the formation of wide ordered hard segments accumulation and led to a broader melting range and wider foaming temperature window. It was also observed that increasing the length of soft segments could not only facilitate the expansion of the bead foams, but also enhance the crystallization of hard segments, thereby promoting heterogeneous cell nucleation and reducing the shrinkage of the foamed samples (Figure 5).

Figure 5. Evolution of the cellular morphology of ETPU beads with the increase in soft segments molecular weight. Reproduced with permission from [54]. Copyright 2020 Elsevier.

Figure 5. Evolution of the cellular morphology of ETPU beads with the increase in soft segments molecular weight. Reproduced with permission from [54]. Copyright 2020 Elsevier.

Based on the theory that hard domains could act as heterogeneous nucleating agent, it was argued that the adjustment of cellular morphology of TPE foams could be realized by changing the degree of ordering of hard segments [55-58]. Park et al. found that large amount of less ordered hard segments micro-crystallites in TPU induced by glycerol monosterate and high-pressure butane could promote cell nucleation and thus increase the cell density [56]. A similar result was also observed by the same group in PEBA foams [57].

The formation of cellular structures in polymer requires enhanced molecular mobility at elevated temperature. Jiang et al. [4] defined the “high-elastic state” of TPU by softening temperature (T_s) and viscous flow temperature (T_v) and associated the molecular movement with the autoclave foaming behavior of TPU pellets. In the “high-elastic state”, the polymer chain motion was significantly enhanced, and large deformation of the entire molecular chain was possible. Within this state, the low-ordered hard segments disassociated and mixed into the soft segment domains, while part of the hard segments rearranged into more closed packed. The hard segments of different order acted as nucleation sites to promote cell nucleation, and the nucleated cell could expand to a large extent under the "high elastic state". The soft segment chains were significantly stretched, and some of the hard segments dissociated and rearranged, eventually leading to the formation of cell structures. ETPU beads with high expansion ratio were obtained within the temperature range defined by the “high elastic state” (Figure 6). Another study conducted by Jiang et al. [55] demonstrated that the proposed “high elastic state” could also be used to explain the autoclave foaming behavior of EPEBA beads.

Figure 6. Expansion ratios of ETPU beads as a function of foaming temperature. Hollow dots indicate the occurrence of fusion bonding between the ETPU beads. Reproduced with permission from [4]. Copyright 2021 Elsevier.

Figure 6. Expansion ratios of ETPU beads as a function of foaming temperature. Hollow dots indicate the occurrence of fusion bonding between the ETPU beads. Reproduced with permission from [4]. Copyright 2021 Elsevier.

Post-foaming shrinkage is a common phenomenon in TPE physical foaming, and the shrinkage mechanism has been widely discussed [1,59,60]. On one hand, the rate of gas diffusion out of the cells is much faster than the rate of the air diffusion into the cells, creating a negative pressure in the pores. On the other hand, the cell walls of TPE foams have a low modulus and cannot resist the compressive force generated by negative pressure. Figure 7 shows the shrinkage behavior of ETPU beads obtained over a wide range of foaming temperatures. After foaming, the expansion ratios of the ETPU beads were very high, even higher than 25 times. However, these foamed beads shrunk seriously with the gas diffusion out of the cells. When the air fully diffuses into the cells, the shrunk beads will “expand” again to a certain degree. To reduce the shrinkage ratio of TPE foams, strategies such as blending with plastics [61,62], chemical cross-linking [63,64], using gas mixture as blowing agent [49,65], compounding with fillers [66,67] have been proposed. Detailed discussion could be found in the review on physical foaming of TPE [1].

Figure 7. Comparation of the expansion ratio of TPU foamed beads in the initial and stable state (a), evolution of the expansion ratio of TPU foamed beads after foaming (b), digital photos of TPU foamed beads in the initial and stable state (c-d). Reproduced with permission from [25]. Copyright 2023 Elsevier.

Figure 7. Comparation of the expansion ratio of TPU foamed beads in the initial and stable state (a), evolution of the expansion ratio of TPU foamed beads after foaming (b), digital photos of TPU foamed beads in the initial and stable state (c-d). Reproduced with permission from [25]. Copyright 2023 Elsevier.

2.4. Expanded polylactic acid (EPLA) beads

PLA is a biodegradable plastic synthesized from biological resources, which is widely used in food packaging and biomedicine due to its good biodegradability and biocompatibility. At the same time, PLA is also considered an ideal alternative to petroleum-based plastics to address the growing problem of plastic pollution. A great deal of research is devoted to the production of PLA foams by physical foaming. The introduction of cellular structure can make PLA lightweight without affecting the biocompatibility of PLA and accelerate the degradation rate of PLA foams [68].

EPLA beads are mainly produced by autoclave foaming technology. During autoclave foaming process, crystal structure [69], molecular weight [70] and molecular architecture play a vital role in the foaming behavior of EPLA beads [71]. PLA has the problem of low melt strength, which makes it difficult to foam. Chemical modification has been proven to be effective to improve the foamability of PLA. Altstädt et al. [70] incorporated various chemical modifiers into PLA by reactive extrusion using a twin-screw extruder. It was found that the modified PLA exhibited higher molecular weight and reduced crystallinity. The incorporation of organic peroxide and multifunctional epoxide resulted in the most significant increase in the molecular weight and polydispersity of PLA. Under the foaming conditions of 157 °C/0.5 h/18 MPa, all the modified PLA foams showed a reduced density, with a low density of 107 kg/cm³. An increase in extensional viscosity of modified PLA also led to larger cells because the cells had more time to expand before cell coalescence. Using a long chain branched PLA, Park et al. [69] investigate the formation of double melting peaks in PLA bead foams. They found that high pressure CO₂ saturation would dramatically affect the crystal structure of PLA, thus altering the cellular morphology of PLA foams. On the one hand, during the high pressure CO₂ saturation process, the formation of a small number of perfected crystals of large size would increase the high melting temperature. On the other hand, the foaming and cooling steps would destroy part of the crystals and induce the formation of the low melting peak.

As a biopolymer, PLA is highly susceptible to hydrolytic degradation, especially under the high pressure CO₂ saturation process during autoclave foaming. Under the saturation of 120 °C/5.5 MPa/ 15 min, Park et al. found that the Mw decreased from 360 kg/mol for raw pellets to about 305 kg/mol for EPLA beads [72]. When the saturation temperature was gradually increased to 126 °C, more hydrolysis occurred, and the M_w of the foamed sample was slowly reduced to about 285 kg/mol (Figure 8). The hydrolysis of PLA is very sensitive to the saturation time. Under the saturation of 120 °C/5.5 MPa, it was observed that the M_w was dramatically reduced from 300 kg/mol to 160 kg/mol when the saturation time was extended from 15 min to 60 min. These results suggest that the saturation condition should be properly selected to control the extent of hydrolysis to produce EPLA beads with desirable cellular morphology and expansion ratio. By compounding PLA with a hydrolysis stabilizer via reactive extrusion, Bonten et al. [28] found that the hydrolysis of PLA during autoclave foaming could be markedly suppressed without compromising the foamability of EPLA beads.

Figure 8. The average molecular weight of the EPLA beads saturated at different temperatures (a) and times (b). Reproduced with permission from [72]. Copyright 2015 Elsevier.

Figure 8. The average molecular weight of the EPLA beads saturated at different temperatures (a) and times (b). Reproduced with permission from [72]. Copyright 2015 Elsevier.

3. Molding of bead foams

3.1. Steam-chest molding (SCM)

Steam-chest molding (SCM) is an important bead foam molding technology, which was first used to produce EPS parts, and has the advantages of high production efficiency and the ability to produce large-sized products [4,16,72,80]. The wide adjustable temperature range and high latent heat make saturated steam capable of rapid heating. High pressure saturated steam is used as a heat transfer medium to heat the foamed beads in SCM, so that the beads are softened, and the interfacial diffusion and entanglement of molecular chains between the beads are realized under huge compression force. Single bead foams become molded parts with complex geometry after SCM.

The main equipment involved in SCM includes a steam generator, a feed system, and a SCM machine. The molding process of the bead foams is realized in the SCM machine. The description of SCM could be found in literature [71,81]. As depicted in Figure 9, the whole molding cycle can be divided into several steps.

Ⅰ. The movable mold is shifted to close the mold. At this stage, a certain gap is maintained according to the final thickness of the products. The foamed beads are injected into the mold to fill the cavity, and then the mold is closed to the final position.

Ⅱ. The high-pressure saturated steam enters the mold cavity through the gas nozzles and heats the foamed beads. In order to obtain a uniform temperature profile, the steam is usually introduced in the following order: double-sided preheating, cross- steam from the fixed mold and then from the movable mold, simultaneous steaming from the double molds (Figure 9 c-e). Steam pressure and steaming time are the key parameters during the process. In the heating step, the formation of inter-bead bonding between adjacent beads determines the mechanical properties of the final parts.

Ⅲ. Cool water is sprayed onto the mold surface to cool the mold and the molded parts. Cooling can freeze the molecular chains that diffuse and entangle in the welded interface and eliminate the adverse effect of high temperature on the cell structure and the dimensional stability of the product [82]. Compressed air is then used to blow off the condensation water, and most of the remaining water is removed by vacuum.

Ⅳ. In the final step, the molds open and the molded part is ejected.

In addition to conventional EPS [8] and EPP [10,11,13,42,83], SCM can be used to mold ETPU [4,16] and EPLA [72] parts and even to produce engineering plastic molded parts such as EPC and expanded polybutylene terephthalate (EPBT) parts [78,80]. As an amorphous polymer, the T_g of PS is around 100 °C and the temperature of saturated steam used to mold EPS beads is 107 ‒ 120 °C. The molding temperature of EPP beads is usually in the range of 120 ‒ 150 °C, which is dependent on the molecular chain structure and expansion ratio of EPP foamed beads [11,13,42,84]. Zhai et al. molded ETPU beads with different bead densities (e.g., 0.24, 0.29, and 0.35 g/cm³) and investigated the molding behavior of the beads and mechanical performance of ETPU parts. The molding temperature was 135.7 °C. They found that all the beads had good inter-bead bonding and the tensile strength of the molded part was increased from 0.91 MPa to 1.80 MPa. Accompanying this increase in tensile strength was an increase in the elongation at break of the parts from 163.1% to 360.1%. Additionally, ETPU parts showed excellent deformation recovery ability. The ETPU cell structure remained stable after 200 compression cycle at 60% strain. In a more recent study, Jiang et al. prepared EPEBA parts using two types of foamed beads with different hardness [55]. EPEBA parts had a low density of less than 0.15 g/cm³ and a high resilience above 70%.

Many research studies have been conducted by Altstädt et al. to prepare bead foam parts based on engineering materials [19,76,78,80,85,86]. They produced EPBT beads by extrusion foaming equipped with an underwater granulator [85]. It was found that EPBT beads with finer cell morphology and lower density could be obtained by incorporating a chain extender. An SCM machine with a customized steam generator was used to weld the foamed beads. The steam temperature was in the range of 192 ‒ 205 °C. It was found that the pure EPBT beads could not be molded; however, the chemically modified EPBT beads could be welded into final parts. EPBT parts showed a lowest density of 167 kg/m³ and excellent thermal stability under compression. Using similar processing techniques, they also prepared expanded polyamide 12 bead foams [76] and expanded polycarbonate [78].

SCM has been utilized for over 70 years, and how to reduce the energy consumption of the molding process, how to improve the molding efficiency, and how to improve the welding strength of the bead foams are common concerns in both academia and industry. Aiming to reduce the temperature variation among the beads during molding process, Hossieny et al. [87] introduced hot air as a second heat transfer medium in the steam line. Since the hot air has a low Joule−Thompson coefficient, using a of mixture heating media could improve the surface quality and enhance the tensile strength of the molded EPP parts. As a result, the steaming time was reduced by approximately 32% and the molding temperature decreased by 16 °C. The results reveal that the incorporation of a certain amount of hot air in combination with saturated steam during the molding process can increase molding efficiency and reduce energy consumption.

Figure 9. The molding steps of a complete SCM process.

Figure 9. The molding steps of a complete SCM process.

3.2. Adhesive-assisted molding (AAM)

Adhesive-assisted molding (AAM) is a process that involves coating foamed beads with a particular adhesive, filling the coated beads into a mold, and finally solidifying the adhesive to produce bead foam parts. The use of AAM is possible with all types of foamed beads provided the appropriate adhesive and curing process selection. Welding foamed beads in a traditional SCM machine is a challenging task, especially for engineering plastics with high T_m or T_g. Nonetheless, AAM has been demonstrated as a viable method to produce molded parts using foamed beads made of engineering plastic. Zhai et al. [74] firstly fabricated lightweight expanded polyimide (EPI) parts with the assistance of AAM. EPI beads were prepared using solid-state foaming technique, where CO₂ and tetrahydrofuran were utilized as the main and co-blowing agents, respectively. The molding process used a PEI/chloroform solution as the adhesive. First, the solution-coated EPI beads were transferred to a mold, and then the adhesive layers among beads were solidified by heating and compression. EPI parts had the lowest density of 115.5 kg/m³ and a flexural strength of 0.51 MPa. In another study related to AAM, Jiang et al. [22] prepared expanded poly(ether imide) (EPEI) parts using epoxy resin as the inter-bead bonding material. EPEI bead foams with an expansion ratio of 30‒56 times were obtained by temperature rising foaming with CO₂/acetone as the mixed foaming agent. As illustrated in Figure 10, EPEI beads were coated with epoxy resin and then filled into a mold cavity, and finally the bonding layers were cured by heat compression to produce molded EPEI products. EPEI parts had a low density of 80 ‒ 200 kg/m³, high thermal stability at 160 °C and superior flame-retardant abilities. In recent work [23,24], flexible EPEBA parts have been fabricated by AAM.

Figure 10. Schematic illustration of the compression molding process of the EPEI beads and MPEI specimens. Reproduced with permission from [22]. Copyright 2020 American Chemical Society.

Figure 10. Schematic illustration of the compression molding process of the EPEI beads and MPEI specimens. Reproduced with permission from [22]. Copyright 2020 American Chemical Society.

AAM can be easily scaled up to industrial scale. Molded parts with complex geometry and large size can be produced by AAM. Especially for bead foams that cannot not be welded by conventional SCM, AAM provides another feasible approach.

3.3. In-mold foaming and molding (IMFM)

Recently, some attempts have been made to prepare bead foam parts by synergizing the constrained expansion of foamed beads with simultaneous welding under high temperature conditions [6,25,88-90]. Figure 11 shows the schematic of such in-mold foaming and molding (IMFM) process. Typically, the polymer pellets expand freely upon depressurization and the solid beads become foamed beads or bead foams. If the potential expansion volume of the foamed beads is greater than the mold cavity, the expanded beads will deform at the contacts with adjacent beads, creating an enormous compression force between neighboring beads. High temperature and high contact pressure allow the interdiffusion of polymer chains at the interface between the expanded beads. Meanwhile, the high pressure gas inside the cells maintains the stability of the cell structure. After a certain welding time, the mold is cooled in cool water, then the mold is opened, and bead foam parts are produced.

Figure 11. Schematic of unstrained foaming and in mold foaming and molding (IMFM) process. Reproduced with permission from [25]. Copyright 2023 Elsevier.

Figure 11. Schematic of unstrained foaming and in mold foaming and molding (IMFM) process. Reproduced with permission from [25]. Copyright 2023 Elsevier.

In this method, increasing the foaming temperature enhances the molecular mobility and increases the compression ratio of the expanded beads, both of which contribute to improving the inter-bead bonding strength. As shown in Figure 12 A, the ETPU parts exhibited clear inter-bead interfaces. An increase in foaming temperature from 144 °C to 150 °C resulted in the increase of cell size from 29 μm to 42 μm and reduction of cell wall thickness from 2.65 μm to 1.28 μm. ETPU parts obtained at higher temperature showed improved welding strength. The elongation at break and tensile strength both increased with rising temperature (Figure 12 B). Meanwhile, the failure pattern of the molded parts changed from intra-bead fracture to inter-bead fracture. The good inter-bead bonding strength demonstrated that using IMFM is a feasible way to produce bead foam parts without the need for additional molding processes.

Figure 12. The microcellular structure of the cross-section of ETPU parts (A), tensile stress-strain curves of ETPU parts (B), fracture surface of ETPU parts after tensile test. The loading weight was 23 g, and the sintering time was 120 s. Reproduced with permission from [25]. Copyright 2023 Elsevier.

Figure 12. The microcellular structure of the cross-section of ETPU parts (A), tensile stress-strain curves of ETPU parts (B), fracture surface of ETPU parts after tensile test. The loading weight was 23 g, and the sintering time was 120 s. Reproduced with permission from [25]. Copyright 2023 Elsevier.

This strategy has been used to fabricate bead foam parts using various polymers like PEI [89], polyphenylene oxide/high-impact polystyrene composite [6] and poly(butylene adipate-co-terephthalate) blend [88]. IMFM is a simple and efficient method for bead foam part fabrication; however, it remains challenging to prepare products of large size with complex geometry.

3.4. Microwave selective sintering (MSS)

Microwaves belong to a type of special electromagnetic waves with frequencies ranging from 300 MHz to 300 GHz [91]. Microwave radiation can induce violent molecular vibrations and collisions, resulting in the release of large amounts of heat. In polymer processing, microwave heating can be used to weld different polymer parts (called microwave welding) [92]. Most plastics are “transparent” to microwaves, so microwave absorbers such as carbon nanotubes and carbon black are often used in microwave welding. Figure 13 shows the fabrication process of bead foam parts using microwave selective sintering (MSS). First, expanded/expandable beads are coated with conductive materials by ball milling. Second, the coated beads are filled into a poly(tetrafluoroethylene) (PTFE) mold. Third, the PTFE mold is placed in a microwave oven for molding. Rapid heating induced by microwave irradiation occurs at the interface between the beads. Finally, after sufficient cooling, the mold is opened, and the bead foam parts are obtained. During the molding process, loading weight, the power and sintering time are the key parameters. These parameters should be optimized to prepare final parts with good weld strength and surface quality.

Figure 13. Schematic showing the microwave-assisted foaming and sintering to prepare EPEI parts. Reproduced with permission from [73]. Copyright 2020 American Chemical Society.

Figure 13. Schematic showing the microwave-assisted foaming and sintering to prepare EPEI parts. Reproduced with permission from [73]. Copyright 2020 American Chemical Society.

Using carbon nanotubes [73,93,94] or graphene nanoplatelets [95] as the conductive materials, Feng et al. have done much work on fabrication EPEI parts using MSS. MSS has also been used to prepare bead foam parts from PS [27,96] and PVA [97]. Using water as the dielectric material, Liu et al. [27] fabricated EPS parts with enhanced weld strength. The tensile strength of the molded parts was 2.3 MPa, two times higher than that of the counterpart prepared by conventional SCM. Selective and rapid heating characteristics make MSS a promising alternative to bead foam parts fabrication.

Table 1 compares the characteristics of various preparation methods. Although AAM, IMFM, and MSS have their unique advantages over SCM, they are mainly dominated by laboratory research. Despite its limitations in moldable materials, SCM remains the optimal choice for the large-scale production of bead foam parts.

Table 1. The characteristics of different molding method.

Table 1. The characteristics of different molding method.

Process	Number of steps	Weld strength	Product size	Product density	Suitable material
SCM	two	high	large	Very low	limited
AAM	two	very high	large	low	wide
IMFM	one	high	small	low	wide
MSS	two	very high	small	low	wide

4. Mechanism of inter-bead bonding

Figure 14. Cross-section of various bead foams parts prepared by different molding process. EPS parts (a), reproduced with permission from [33]. Copyright 2017 Taylor & Francis. EPP parts (b), reproduced with permission from [13]. Copyright 2022 American Chemical Society. EPEBA parts (c), reproduced with permission from [55]. Copyright 2022 Elsevier. EPBT parts (d), reproduced with permission from [98]. E(PPO/HIPS), expanded polyphenylene oxide/high-impact polystyrene composite (e), reproduced with permission from [6]. Copyright 2021 Wiley-VCH GmbH. ETPU parts (f), reproduced with permission from [25]. Copyright 2023 Elsevier. EPEBA parts (g), reproduced with permission from [24]. Copyright 2021 Elsevier. EPP parts (h), reproduced with permission from [26]. Copyright 2022 Elsevier. EPS parts (i), reproduced with permission from [27]. Copyright 2021 American Chemical Society.

Figure 14. Cross-section of various bead foams parts prepared by different molding process. EPS parts (a), reproduced with permission from [33]. Copyright 2017 Taylor & Francis. EPP parts (b), reproduced with permission from [13]. Copyright 2022 American Chemical Society. EPEBA parts (c), reproduced with permission from [55]. Copyright 2022 Elsevier. EPBT parts (d), reproduced with permission from [98]. E(PPO/HIPS), expanded polyphenylene oxide/high-impact polystyrene composite (e), reproduced with permission from [6]. Copyright 2021 Wiley-VCH GmbH. ETPU parts (f), reproduced with permission from [25]. Copyright 2023 Elsevier. EPEBA parts (g), reproduced with permission from [24]. Copyright 2021 Elsevier. EPP parts (h), reproduced with permission from [26]. Copyright 2022 Elsevier. EPS parts (i), reproduced with permission from [27]. Copyright 2021 American Chemical Society.

Figure 15. Schematic of the heat transfer process in SCM process. Reproduced with permission from [55]. Copyright 2022 Elsevier.

Figure 15. Schematic of the heat transfer process in SCM process. Reproduced with permission from [55]. Copyright 2022 Elsevier.

Figure 16. The mechanism of the restricted expansion and in-situ inter-bead bonding in IMFM process. Reproduced with permission from [25]. Copyright 2023 Elsevier.

Figure 16. The mechanism of the restricted expansion and in-situ inter-bead bonding in IMFM process. Reproduced with permission from [25]. Copyright 2023 Elsevier.

5. Conclusion and future perspectives

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