a continuous method of making dry - stretch microporous membrane battery separators from polypropylene or polyethylene or both benefits to the manufacturers in the production efficiency . the precursor - film extrusion in this invention is accomplished by multiple small film - extrusion lines at a compatible line speed with the followed oven processes . the overall production process starts continuously from film extrusion to annealing and to stretching . the benefits of the inventive continuous process includes a higher product yield , more effective oven - time usage , no need to handle and manage the intermediate products , less need in labor and machine maintenance , and potentially more stable product quality . the dry - stretch membrane separators made with this inventive continuous method include single - ply pp or pe separators having a thickness ranging from 0 . 2 mil to 2 . 0 mil ; pp / pe / pp trilayer microporous membrane separators having a thickness ranging from 0 . 6 mil to 4 . 0 mil . the pp / pe / pp trilayer can be accomplished in the early extrusion via either co - extrusion or extruding separately and then interposing pe layer between two pp layers , continuously , right before annealing / bonding and stretching process .