Abstract: A method for predicting pressures in an injection molding system for molding plastic parts requires providing a mold that has at least one channel with each additional channel having a constant cross-sectional shape along its length and each channel having different thicknesses with a constant cross-sectional shape along its length. At least one first sensor configured to collect pressure data from each channel is provided. At least three second sensors configured to detect the presence of plastic located at known distances downstream of the at least one first sensor. Molten plastic is injected in each of the channels and sensor data is collected for the molten plastic flowing through each channel. A curve is fitted to progressive measured occurrences of pressure at the first sensor when plastic is first detected at each of the second sensors for each channel. Pressure can be predicted for a given flow rate, temperature, and channel thickness at, between, or beyond the measured occurrences.