On the trade-offs between scheduling objectives for unpaced mixed-model assembly lines

Scheduling decisions in mixed-model assembly lines are frequently multi-objective. No work till date has jointly examined the impact of the production sequence in unpaced manual mixed-model assembly lines operated as permutation flow shops concerning a set of objectives broad enough to capture the interests of its customers and suppliers as well as the manufacturer's interests related to productivity, production flow and human aspects. This work sets up a simulation study based on a real unpaced mixed-model assembly line with seven products to analyse the impact of five different sequence types on objectives from all these interest fields while modelling human learning and deterioration effects explicitly. The results show that considerable trade-offs exist as different sequence types are preferred for the various scheduling objectives. For makespan and mean flow time as widely used productivity- and flow-related objectives, cases of conflicts and complementarity can be found depending on the actual operational conditions in terms of product and volume mix. A strong dilemma emerges between supplier- and human-related objectives favouring different sequence types. Concerning due date based customer-related objectives, the preferred sequence type varies with the operational conditions. The observed trade-offs confirm the multi-objective nature of the scheduling decision.