Methods and apparatus for producing high quality closed cell foams for use in applications such as coatings, sealant beads, seam filling and gaskets. In a first embodiment an in-line single pass static mixing device (10) containing a very large number of individual mixing elements (12) is used to homogeneously disperse a gas throughout a highly viscous liquid polymeric material such as a plastisol, silicone, butyl or urethane based material. In a second embodiment both a dynamic mixer (3a) and a single pass static mixer (3b) are used in series to form the closed cell foam.