The manufacture of foamed polymer trim parts making use of physical blowing agents requires the diffusion and sorption of the blowing agent in the polymer melt. Said diffusion and sorption are achieved by intensive mixing. Therefore a statical mixing device is enclosed in a pressure chamber, which comprises a hollow porous cylinder, which has a surface suitable for contacting the melt with the blowing agent.The protrusions extending at least partially into the melt flow cause a multiple reorganisation of the polymer melt, which leads to thorough mixing and shortens the diffusion path. The statical mixing device, which may be of modular construction, increases mass transport and mixing by a small modification of a conventional injection molding machine.