A multivariate sensor is described for the in-mold measurement of melt temperature, melt pressure, melt velocity, melt viscosity, and mold temperature. Melt pressure and temperature are respectively obtained through the incorporation of a piezo-ceramic element and infrared thermopile within the sensor head. Melt velocity is derived from the initial response of the melt temperature as the polymer melt flows across the sensor window. The apparent melt viscosity is then derived based on the derivative of the increasing melt pressure relative to the melt velocity as the polymer melt propagates in a cavity of known thickness. The process states detected from the described sensor are compared to the data stream from a suite of commercial sensors in an instrumented mold including piezoelectric pressure sensors, infrared pyrometer, and thermocouples. A design of experiments is implemented to assess the sensor’s response relative to changes in barrel temperature, mold coolant temperature, volumetric flow rate, polymer molecular weight, and other factors. Predictive models for quality assurance are developed using multiple regression of data obtained from the design of experiments. The results indicate that the data from the multivariate sensor is highly useful for process control and quality assurance.