This paper describes an approach for modeling product life-cycles to create time-dependent inventories for use in environmental impact assessment. A general process module is defined relating resource inputs and outflows, based upon an embedded mathematical model. Then, a parametric model to represent the average performance of manufacturing processes is proposed and used within modules. Different parameter values may be used to represent a variety of life-cycle processes. Individual modules are combined to form product life-cycle networks. Designers may specify the required system output (product demand) as a function of time and the integrated network calculates the necessary time-dependent resource flows throughout the network.