Abstract: A system and method for controlling the nucleation level of a stream of material such as a polyol resin for forming a polyurethane foam. Temperature and pressure control devices communicate with a stream of nucleated material flowing in a recirculation loop from and into a supply of nucleated material. Continuous density measurements are made of the nucleated material in the recirculation loop and are fed to a controller which regulates the amount of a gas injected into the recirculation loop. A material feed line extends from the recirculation loop to a material dispenser. A material bypass line extends from the dispenser back to the recirculation loop for returning unused nucleated material.