## 1 Introduction  
Traditionally, conductive polymer composites (CPCs) demonstrate potential applications in the areas of antistatic, electromagnetic interference shielding, thermal management, fuel cells, sensing and so forth [1; 2; 3; 4; 5; 6]. The CPCs can be prepared by melt blending which is compatible with current industrial processing techniques, such as extrusion and/or injection molding [7]. To make a polymer composite conductive, it is paramount for electrically conductive fillers to attain a three-dimensional (3D) network within the host polymer matrix. The critical filler concentration at which the polymer matrix translates from an insulator to a conductor is recognized as the percolation threshold (\(p_{\mathrm{c}}\)) [8]. Multi-walled carbon nanotubes (CNT), which are characteristic of high aspect (length to diameter) ratio [9], intrinsically high thermal and electrical conductivity [10] have been extensively adopted as one of the most important functional fillers to fabricate CPCs. However, commercial CNT is supplied in the form of heavily entangled bundles, which makes it difficult to disperse in a polymer matrix [10]. To achieve higher electrical conductivity, the concept of hybrid fillers, i.e., the utilization of fillers with different morphologies or aspect ratios has been adopted by researchers to prepare CPCs. [PERSON] et al. [1] revealed that the \(p_{\mathrm{c}}\) of polypropylene (PP) composites loaded with hybrid fillers of CNT and carbon black (CB) is much lower than that of only CNT or CB-containing counterparts. For example, they found that the \(p_{\mathrm{c}}\) can be reduced from 2.4 to 0.21 wt % by simply substituting half of the CNT with CB [1]. [PERSON] and coworkers [11] reported that the volume electrical conductivity of high density PE incorporated with hybrid fillers of CB and carbon fiber is nearly three orders of magnitude higher than that of only CB-containing counterparts, at the same total filler concentrations. In addition, [PERSON] et al. [12] reported that the addition of silver particles into PP is advantageous to the formation of conductive pathways in the presence of CNT, leading to a significant increase of electrical conductivity. Recently, there has been increasing demand for micro-components in the areas of electronics, automotive, microelectromechanical systems (MEMS) and microsystems [13; 14]. Microinjection molding (\(\upmu\)IM) is becoming an important technology thanks to its suitability for mass production of microparts with complex shape and high surface quality at relatively low cost [13]. So far, a number of studies have been conducted with respect to the \(\upmu\)IM of unfilled thermoplastics [15; 16; 17; 18]. However, \(\upmu\)IM of filler-containing polymer composites is receiving attention due to their potential multifunctional performance. For instance, [PERSON] et al. [18] studied the properties of microinjection molded polycarbonate (PC)/CNT and PP/CNT nanocomposites. The authors of [18] found that the \(p_{\mathrm{c}}\) for both CNT-containing microparts shifted to higher filler concentrations when compared with their compression molded counterparts, which is attributed to the prevailing high shear rates in \(\upmu\)IM. Similar findings were reported by [PERSON] et al. [19] in different types of carbon fillers loaded PP nanocomposites in \(\upmu\)IM. Additionally, they found [19] that the CNT and high structure CB are more effective than graphite nanoplatelets (GNP) in enhancing the electrical conductivity of PP-based nanocomposites, which is closely related to the development of microstructure in corresponding microparts. For instance, a \"grape-like\" structure was typically observed in PP/CB microparts, and a conductive network could be formed in CNT-containing counterparts thanks to its higher aspect ratio. However, such morphology was hardly detected in either the compression molded PP/GNP composites or their microparts. Nevertheless, the presence of crystal structures in PP could affect the distribution of incorporated fillers [18].