The tensile behaviour of weld-line and weld-line-free specimens of a range of high-density polyethylene formulations—modified with surface-coated calcium carbonate, masterbatched carbon black and a stabiliser package—was investigated. Among these formulations were highly filled variants, with up to 60 wt% calcium carbonate, based on a D- and I-optimal experimental design. Scheffé polynomial models were fitted to the results, with and without cross-validation, for the investigation of the primary and interactive effects of the additives on the system. Variable effects were found in the different measures. Calcium carbonate exhibited the strongest effects on the system, given its high loadings – decreasing ultimate tensile strength (UTS), increasing Youngs’ modulus (YM) and severely decreasing elongation at break (EaB). Strong antagonisms between carbon black and calcium carbonate were found in UTS and YM, opposed by a significant synergism in EaB. Weld lines were found to reduce UTS and EaB, with a weakly positive effect on YM, albeit with complex interactions with the additives in many cases. The variable failure mechanics of the specimens were investigated in more detail by tensile tests coupled with digital image correlation and fractography. Rheometric analyses were performed to better understand the processing of the materials.