Various PLA resins have been tried with chemical blowing agent and physical blowing agent in extrusion process to study the expansion dependency of amount of blowing agent and cell density, respectively. It has been found that a `dip' exists in both cases; foam density decreases to a minimum, then increases. It suggests competing mechanisms during foaming. Bubble growth model with surface vaporization appears inadequate in describing the counterintuitive foam density increase at increased blowing agent loading. Investigating foam rod diameter suggests premature gas separation from the polymeric melt before foaming occurred at a higher gas loading. It adversely affects foaming efficiency. Also discussed is the foam density variation during aging.