Table 2.

Current studies from the literature on FDM thermoplastic materials, rheological characterization at specific temperatures, main results to develop specific requirements.

Thermoplastic-Based Systems	Scope	Testing	Results	Reference
Blends
Polycaprolactone (PCL)/hydrolysed collagen (HC)	To produce biodegradable items for applications in agriculture and plant nurseries.	Capillary rheometer and flow curves at 130, 140, 150 °C	A decrease in the melt viscosity was observed with the addition of HC due to its plasticizing effect	Seggiani et al., 2018 [62]
Polybutylene succinate (PBS)/ Poly (butylene succinateran-adipate) (PBSA)	To develop semi-crystalline biodegradable filaments	Frequency sweep from 150 °C to 230 °C (TTS principle). Continuous flow measurements (Cox–Merz rule).	The viscosity values of samples lie below 104 Pa*s, which allows forecasting a suitable flow in the nozzle	Candal et al., 2020 [49]
Poly(lactide) blends containing low molecular weight polymers of chemically identical but enantiomerically different nature (poly(L-lactide) (PLLA) and poly(D-Lactide) (PDLA).)	To promote interfacial weld and stiffness	Oscillatory frequency sweep tests at 180 °C	Any composition possessing η0 > 5000 Pa*s is too viscous for extrusion. Any composition characterized by η0 < 500 Pa*s, could not be printed due to uncontrolled fluctuations in volume flow rate and consequential loss in print resolution	Srinivas et al., 2020 [26]
Brominated butyl-rubber (BIIR)/polypropylene (PP) thermoplastic vulcanizate (BIIR/PP-TPV)	To flexible FDM	Frequency sweep at 180 °C	The low viscosity of blends prepared by masterbatch procedure and interfacial compatibilization effectively improves the bonding strength between the adjacent layers of the 3D printed product.	Hou et al., 2020 [104]
Polypropylene (PP)/ elastomeric ethylene-octene copolymer (EOC)	To address the deficiencies of PP in melt extrusion processing (warpage and poor layer adhesion)	Frequency sweep at 190 and 210 °C	The EOC addition did not change substantially the complex viscosity of the blends	Ho and Kontopoulou 2022 [30]
Thermoplastic starch (TPS)/polylactic acid (PLA) /poly(butyleneadipate-co-terephthalate) (PBAT) and chain extender (CE)	Highly renewable filaments for 3D printing	Frequency sweep at 180 °C.	Complex viscosity and modulus increment of blends with the addition of chain extender	Ju et al., 2022 [105]
Composites
Wood flour (WF)/thermoplastic polyurethane (TPU) and modification with ethylene-propylene-diene-monomer grafted maleic anhydride (EPDM-g-MAH)	Adding low-cost natural fibre to make a low cost, biodegradable, and ecofriendly material	Frequency sweep at 200 °C.	Moduli and complex viscosity increase in composites after the modification with EPDM-g-MAH	Bi et al., 2018 [106]
Polylactide/hemp hurd	To valuable reinforcement of PLA-based blend	Frequency sweep at 190 °C	The biocomposites showed shear-thinning behaviour. Composites at the highest filler loading (40 wt.%) displayed lower melt flow and lower ease of processability than other biocomposite blends.	Xiao et al., 2019 [107]
Inorganic Fullerene Tungsten Sulphide (IF-WS2) nanoparticles/ poly-ether-ketone-ketone (PEEK)	Nanoparticles addition to enhance the flowability of PEEK	Dynamic shear tests at 400 °C	At low shear rate, the shear viscosity of PEEK was reduced with the addition of 2 wt% IF-WS2. The difference in viscosity of samples becomes less pronounced in the higher-shear rate range.	Golbang et al., 2020 [108]
Carbon fibre (CF)/ polyetherimide (PEI)/oligophenylene sulfone (OPSU)/polycarbonate (PC)	Plasticizing high-performance polymers	Capillary rheometer at a temperature of 380 °C	OPSU and PC decrease the melt viscosity of carbon-filled composite preserving the mechanical properties and heat resistance at a sufficiently high level.	Slonov et al., 2020 [109]
Wood flour (WF)/polyhydroxyalkanoates (PHA)	To cost reduction in PHA uses	Frequency sweep at 190 °C	The fluidity of the composites decreased with the increase in WF content	Tian et al., 2021 [110]
Polylactic acid (PLA)/ maraging steel particles	To develop composite functional filaments with additional properties (magnetic, electrical, optical)	Frequency and flow tests at 160 °C (Cox–Merz rule)	The viscous component predominates over the elastic component for all the samples. Remarkable decrease in the viscoelastic moduli as the particle content increases	Díaz-García et al., 2022 [111]
Boron nitride nanosheets/thermoplastic polyurethane (TPU)	To high-power integrated electronic devices for 5 G system	Rotational rheometer in the small-amplitude shear oscillation mode at 225 °C	G′ of the composites diverged from the behaviour of the pure TPU. However, liquid-to-solid transition not significantly affected the viscosity within the shear rate range of the 3D printing process	Gao et al., 2022 [27]
Polylactic acid (PLA)/ thermoplastic polyurethane (TPU) blend with enzymatically modified lignin (EL)	To enhance mechanical and thermal properties of bio-based polymers	Frequency sweep at 170–200 °C depending on materials	A gradual decrease in complex viscosity at higher EL concentrations	Murillo-Morales et al., 2023 [112]
Polyetherketoneketone (PEKK)/mica platelets	To provide added mechanical strength to the PEKK	Frequency sweep at 360 °C and temperature ramp	Mica doping does not significantly alter the viscoelastic properties inherent to unfilled PEKK	Kennedy et al., 2022 [37]
Poly (L-lactic acid)/cellulose nanocrystals	To produce fully green, high-performance consumables	Capillary rheology measurements and small amplitude oscillatory shear experiments	Composites exhibited shear-thinning behaviour favourable for the stable extrusion at nozzle, and G″ > G′ beneficial to the interfuse adhesion during welding	Wu et al., 2022 [113]