We investigated, by visual observation and numerical calculations, the foaming behavior of polypropylene within a foam injection mold cavity with the environmentally benign physical blowing agents nitrogen (N-2) and carbon dioxide (CO2). An 85-ton core-back injection-molding machine with temperature and pressure monitoring systems as well as a high-pressure view cell was used for the investigation. The experiments showed a prominent difference in bubble nucleation and growth between N-2 and CO2 injection foaming. Even when the weight concentration of N-2 dissolved in polymer was one-third that of CO2, N-2 injection foaming provided a bubble number density that was 30 times larger and a bubble size that was one-third smaller compared to CO2 injection foaming. Classical bubble nucleation and growth models developed for batch foaming were employed to analyze these experimental results. The models reasonably explained the differences in injection foaming behavior between N-2 and CO2. It was clearly demonstrated by both experiments and numerical calculations that N-2 provides a higher number of bubbles with a smaller bubble size in foam injection molding compared to CO2 as a result of the lower solubility of N-2 in the polymer and the larger degree of super-saturation. POLYM. ENG. SCI., 52:875-883, 2011. (C) 2011 Society of Plastics Engineers