The apparatus is provided for impregnating a melt of polymer with a fluid working medium, such as, an expanding agent or an additive. Impregnation is carried out for the purpose of manufacturing foamed shaped parts using an injection molding machine. The impregnating body includes at least one melt channel formed in a gas permeable segment and a channel system for fluid infeed. The gas permeable material, e.g. sintered metal, separates the fluid from the stream of melt in the melt channel(s). Each melt channel has open pores at its inner surface which enable the impregnation. The melt channels can contain static mixers in the stream of melt to promote the distribution of the diffused fluid in the flow of melt.