Flexible multi-manned assembly line balancing problem: Model, heuristic procedure, and lower bounds for line length minimization

Assembly lines dedicated to the production of large products often allow multiple workers to perform tasks simultaneously on the product. Previous works on such multi-manned lines define workstations with fixed, discrete, and restrictive frontiers, despite commonly considering continuous paced line control. This paper proposes flexible station frontiers for multi-manned lines and shows that such innovation allows significantly shorter line lengths. A new Mixed Integer Linear Programming model and a novel model-based heuristic procedure are presented to describe and optimize lines. Algorithmic lower bounds are also introduced for the problem. The formulation was compared to a literature benchmark of regular multi-manned solutions. These experiments showed that flexible multi-manned formulations can lead to line length reductions of up to 42%. Such reductions were obtained for most instances (81 out of 88), with an average value of 18%. The relationship between cycle time and minimal line length is also analyzed, demonstrating that efficient solution sets can be continuous or discrete, depending on the instance.