Expanded porous thermoplastic polymer membranes and device for the production thereof

In an apparatus for making a foamed porous membrane comprising an extruder equipped with a metering apparatus and heating elements to form a polymer melt, said extruder having a cylinder connected via a first melt pump to a die for extrusion of a membrane from said polymer melt, an injection stage before said first melt pump for charging said polymer melt with a cell former, a heat exchanger located between said first melt pump and said die to form a second mixing stage, the improvement comprising a second melt pump located upstream from said die for forcing said polymer melt through said die.