Precision injection molding is an important technology for improving productivity and lowering costs in fields such as electronics, photonics, and medical plastics by allowing the substitution of molded plastics for more expensive metals and ceramic parts. This work is part of an effort to develop a molding technology that can deliver the micron-level precision required for these applications. Two engineering thermoplastics with glass and/or mineral fillers, poly(etherimide) (PEI) and poly(benzoate-naphthoate) liquid crystal polymer (LCP), were studied. Precision was defined as the standard deviation of part mass and part dimensions as measured on a coordinate measuring machine. Robust design, or design of experiments (DOE), principles were used to determine optimal injection molding parameters. The results showed that ram position and hydraulic pressure were the most effective injection phase cut-off control methods of those studied. With regard to process parameters, those that increase melt fluidity had the greatest effect on improved precision with PEI. For LCP, the effect of process parameters was far more complicated. The cross-sectional morphology of the liquid crystal parts was investigated in the second phase of the study to assess the influence of morphology on part precision.