Abstract: An apparatus for applying a foamable fluid plastic material including reactant fluids that form a foam when heated and mixed. A reactant fluid dispenser supplies the reactant fluids to a heated applicator gun mixing chamber at a constant flow rate despite any differences in viscosity. A valve needle extends into the mixing chamber and exposes fluid inlet openings when retracted, permitting the reactant fluids to flow into the mixing chamber. When advanced, the needle closes off the fluid inlet openings and dispenses the mixed fluids. A heater prevents the gun from drawing heat energy from initial quantities of the pre-heated reactant fluids which would prevent initial quantities of the fluids from reacting properly. The valve needle is disposed in a sleeve that defines the mixing chamber and is disposed in a sleeve receptacle of the gun.