[22] confirmed the feasibility of using ROM to obtain the Young's modulus and tensile strength of unidirectional carbon-fibre reinforced polymers using 3D printing (CFRP) [22]. However, the ideal ROM assumes the fibres in the composite are aligned unidirectionally and distributed uniformly. That was the reason why in [PERSON] et al. 's study, the original ROM was confirmed to experience discrepancies when the composites consisted of dispersed fibres. The main reason was attributed to the inability of the model to consider stress- and strain-increasing factors due to embedded reinforcements, interface failure, void presence, and so on. To address this issue, the authors developed a modified ROM (MROM) model to capture this nonlinear behaviour of the tensile strength to compensate for the lack of microgeometry analysis [23]. Manufacturing plastic parts using AM, in particular, the FDM process, is drawing a lot of attention recently but despite the benefits they have to offer, some challenges are still preventing the manufacturers from transitioning from conventional methods such as injection moulding and hot pressing to AM methods [24]. The most critical issue when dealing with parts printed by the FDM method is called material anisotropy. The word anisotropy refers to the low layer adhesion and delamination of the deposited layers when the part is under loading. Moreover, the air voids that form between the layers and continue throughout the sample can act as potential pathways for the cracks to propagate. The air gaps, as well as the low interlayer adhesion, can lead to fibre delamination and premature failure of the part when the load direction is perpendicular to the build orientation of the printed parts [9; 25; 26; 27]. The proper selection of the polymer which can show less anisotropy at certain levels, as well as the process parameters can play an important role in mitigating or minimising this anisotropy. In the preliminary study conducted by the authors [28], mechanical anisotropy phenomena in HIPS and ABS FDM-printed parts were preliminarily investigated. The longitudinal and transverse build orientations have much less influence on HIPS samples than in ABS samples. Lower layer adhesion was found in ABS samples compared to in HIPS samples. As a result, the tensile strength of FDM-printed HIPS is much closer to those to those mould-injected samples. However, other key FDM parameters including printing temperature, infill percentage, and layer thickness were not investigated to optimise the FDM processes. Limited research is available to understand the influences of FDM process parameters and material selection on the degree of material anisotropy in FDM-based printed specimens. The FDM process is one of the most popular methods of additive manufacturing methods, but its inherent structural anisotropy is one of the main drawbacks that may keep industries reluctant to replace this method with their conventional and old means of parts production such as injection moulding or hot pressing. Although many studies have attempted to study the influence of process parameters, most of them have not incorporated the analysis of internal structures such as SEM or micro-CT to investigate the process parameters' effects on the degree of anisotropy (DOA). This study has a distinct focus on assessing the structural anisotropy of printed parts. It delves into the impact of altering various printing parameters--such as layer thickness, infill density, nozzle temperature, and raster angle--while maintaining consistency in other factors. Employing this distinctive approach allows for a comprehensive exploration of the influence of each parameter on internal structures through SEM and micro-CT analyses, shedding light on intricate details. Standard tensile samples are printed using the FDM process and tested. The samples are characterised using microstructural analysis including SEM, XRD, TGA, DSC, micro-CT, etc.