Designing unbalanced paced lines: a conceptual model and an experimental campaign

This study aims to provide a new approach to the effective design of paced lines by focusing on protective capacity. The paper investigates – through an analytical model – the required level of protective capacity and its location along the line; then the proposed model is tested via simulation, by considering a four-station production (or assembly) paced line. The simulated line is plagued by uncertainty coming from four sources of variance, i.e. machine unavailability, processing time variability, products mix imbalance and quality losses (due to defective products). Main results show that: (i) although the balanced line has been almost the only kind of configuration investigated up to now in the literature when paced multi-product lines are considered, it is seldom undesirable, since protective capacity plays a relevant role in determining productivity, mainly when unavailability increases; (ii) the nearer the affected station to the gate of the line, the greater the harmful effects of variance; (iii) a bottleneck's location along the line remarkably affects system performances: in particular to smooth the effects of environmental variance the bottleneck should be located at the end of the line; (iv) the most relevant effect on performances at the shop floor level is given by quality losses, followed by the number of products in the mix.