An overall mathematical model is developed for estimating the number of lactide bubbles nucleated during the post-treatment step in polylactic acid (PLA) production. After PLA polymerization, residual unreacted lactide must be removed. The proposed model is used to determine the appropriate temperature for the lactide removal and is composed of three sub-models for surface tension, partial vapor pressure, and heterogeneous nucleation rate, which are expressed as functions of temperature. Visual observation experiments on lacttide bubble formation in PLA are conducted to compare the values estimated using the overall mathematical model with the experimental data. The surface tension is confirmed to be proportional to the -3.7th power of the absolute temperature in our experiments and to the -4th power of the temperature in the surface tension sub-model. The partial vapor pressure of the lactide in the molten PLA is proportional to its molar concentration, both in the experiments and the partial vapor pressure sub-model. The numbers of bubbles observed in our experiments are in good agreement with those estimated using the overall mathematical model.