We have shown that the binder composition has a significant role in the behaviour of filled materials and that the choice of the binder directly influences shear viscosity of the composite materials as the binder material with lower viscosity will result in overall lower viscosity of composite material. Similarly, other researchers have investigated different thermoplastic binders for highly filled lead zirconate titanate feedstocks [26]. Among the investigated binders, they showed that low-density polyethylene is a preferred choice for the binder since it exhibited low viscosity and low die swell. Multicomponent binder systems are frequently used in SDS processes, where the reduction of viscosity can be achieved by the addition of a low molecular weight component to the binder system (i.e., waxes and stearic acid) [27]. Particle size and size distribution is yet another essential factor influencing rheology of composite materials. Usually, bigger particles result in lower viscosities and viscoelastic moduli (G' and G\"), while smaller particles have a large surface area and tend to agglomerate, which results in higher viscosities and viscoelastic moduli [28]. The influence of particle size is closely related also to powder loading. [PERSON] et al. [29], investigated the influence of powder particle size distribution and solid loading content of 316L feedstocks on rheological properties. They observed that the viscosity increased as the particle size decreased; however, this was the case only for high powder loading, while for powder loadings less than 65 vol.% the viscosity changes were less significant. Effect of filler shape can be explained through aspect ratio, which is the deviation of geometry from an ideal spherical shape. It is known that as the aspect ratio increases, the friction between polymer binder and the fillers increases, which hinders the movement of polymeric chains and results in higher thickness [21]. For example, different viscosity values were shown for PLA (Polylactide) mixed with copper particles with different shapes (aspect ratios) [30]. For stainless steel 17-4 PH powders mixed with a wax-based binder, the researchers [31] reported higher viscosities for PIM feedstock with a higher particle surface area. In the study of [PERSON] et al. [32], the authors studied the effect of shape and size of stainless steel 17-4 PH powders on rheological properties of feedstock material. Their results showed that both the size and shape of the particles affected the rheological behaviour of the feedstock. However, in the case of powders, the influence of the size was more significant compared to the shape of the particles. In another study [33], the researchers investigated the rheology of highly filled Inconel feedstock alloy 718 and observed that shear viscosity decreased with increasing particle sizes at a fixed 50% filler load. Additives that are mixed with polymer and fillers significantly affect the rheological properties of composite materials. They can act as coupling agents between the filler and polymer, wetting agents, or as an internal lubricant [19]. Surface treatment disperses and wets the fillers, which results in reduced viscosity and lower \(G^{\prime}\) values [19; 34; 35]. Some authors have also reported that surface-treated fillers result in reduced magnitudes of steady-state viscosity, decreased apparent yield values, and lower dynamic moduli. They hypothesised that the coupling agent tied to the filler surface and on the other end of the hydrocarbon chain enabled connectivity with the polymer [34; 36]. While the need for additives, especially in highly filled polymers, is generally recognised, the exact mechanisms of how they work have not been clearly understood, yet [21]. In summary, the rheological behaviour of highly filled materials is complex, and as previously noted, several factors affecting rheological behaviour are interrelated.