Filtration devices that include filtration cassettes that feature a plurality of stacked subassemblies, wherein at least one of the subassemblies includes a porous membrane having a front side and a back side and a perimeter; and an overmolded frame that encapsulates the membrane around the perimeter of the front side and the back side of the membrane; wherein the overmolded frame forms at least one retentate port, at least one feed port, at least one filtrate port, and a flow channel proximate a perimeter of the frame; and an overmolded plastic jacket that penetrates the flow channels of the subassemblies to seal the stacked subassemblies; and methods for making the filtration device and cassettes.