The present invention provides membrane cassettes and stacks thereof which are suitable for a use in a variety of electrochemical applications. The invention further provides membrane cassettes which comprise one or more external manifolds which deliver reagents and/or coolant to one or more reactant or coolant flow fields of the membrane cassettes. In particular, the present invention describes the insert molding method, whereby the plenums of the external manifolds are created during the stack encapsulation step. The invention describes several methods for creating the manifold runner geometry via insert-molding, machining, or with separate components.