Robust manufacturing line design with controlled moderate robustness in bottleneck buffer time to manage stochastic inter-task times

Task time variations in a manufacturing line can result in a longer time to complete tasks than a given cycle time, leading to line stoppage and loss of production time. In practice, a portion of the cycle time is often allocated as a predefined fixed-size buffer time, determined based on experience, to account for such uncertain variations for a paced line without storage-buffers between stations. However, the size of the required buffer time in each station depends on the variation levels of task times and the desired conservatism level for preventing cycle time violation. In addition, there are uncertainties in other non-value adding activity times in stations, known as inter-task times. Although many studies on stochastic manufacturing line design focused on minimizing the cost incurred when the cycle time is exceeded due to task time variations, they mostly disregarded the inter-task times. Furthermore, it is worth studying the simultaneous effect of the manufacturing time uncertainty level and that of the conservatism level on the cycle time. This paper proposes a new method for moderately robust manufacturing line design that incorporates the conservatism level and uncertainties in the task and inter-task times. A new interpretation of the non-productive times in stations is presented by introducing the concept of the α-fractal buffer time to manage the effect of manufacturing time uncertainties. To overcome the problem of excessive robustness, a moderate robust approach with conservatism-level flexibility is used, focusing on the cycle time in the bottleneck station. The effect of the uncertainties and conservatism levels on the cycle time is analyzed through numerical examples. This study can be used for improving a manufacturing system in which uncertainties in tasks and inter-task times significantly degrade its productivity.

► Created new models for robust manufacturing line design with controlled moderate robustness.
► Introduced a new interpretation of the non-productive times in stations by the stochastic α-fractal buffer time concept.
► Considered the conservatism level in preventing the cycle time violation in the bottleneck station.
► Included the uncertainty in non-value-adding activity times in stations in addition to the stochastic processing times.
► Could help improve the design of manufacturing systems with uncertain task and inter-task times.