Modelling forgetting due to intermittent production in mixed-model line scheduling

Forgetting effects occur whenever production is interrupted and workers lose previously acquired routine. Existing forgetting models primarily focused on the impacts of production breaks in single-product production environments. Consequently, forgetting models neglected times during which a different product type is produced despite being an important cause for forgetting in industrial mixed-model production. Therefore, this work extends existing forgetting models for application in mixed-model line scheduling models by considering times of intermittent production. These are modelled as interruptions in addition to production break interruptions. A routine loss factor is introduced that allows to partially consider times elapsed during which different products are processed. The higher the routine loss factor is, the higher the considered portion of the time elapsed due to intermittent production becomes and the higher becomes the amount of forgetting. Through means of simulation studies it is investigated how the routine loss factor affects the height of learning effects present and the line performance in terms of makespan and flow time. While sequence types with a higher degree of grouping are almost insensitive with respect to the height of the routine loss factor, sequences with no grouping are strongly affected. Depending on the height of the routine loss factor different sequence types are preferred. Hence, not modelling forgetting appropriately can lead to non-optimal schedules.