Unpaced serial production lines with two simultaneous sources of imbalance - Their operating characteristics

This paper investigates the behaviour of unpaced serial production lines with two concurrent imbalance sources, namely, lines that are simultaneously unbalanced with respect to either their work time means (MTs) and variability - as depicted by the coefficients of variation of processing times (CVs), their MTs and buffer capacities (BCs), or their CVs and BCs. The lines were simulated under stable operating conditions with different values of line length, buffer capacity, coefficient of variation, degree of imbalance, as well as a variety of imbalance patterns. Making use of a number of statistical methods, idle time and average buffer level output data were analysed and compared to a balanced line counterpart. Conclusions concerning the relationships among the exogenous and endogenous variables were made. For the MT and CV imbalance, the configuration providing the lowest idle time turned out to be an inverted bowl pattern for MT, along with a bowl-shaped arrangement for CV, whereas the best configuration that generally generates low average buffer levels was found to be an MT decreasing order, together with a CV bowl allocation. As for unbalanced MTs and BCs, it was found that in terms of idle time the best unbalanced pattern is an MT bowl configuration, coupled with a distribution of buffer capacity as evenly as possible. On the other hand, for average buffer level the best pattern was a monotone decreasing MT order, together with an ascending buffer size order. As regards CV and BC imbalance, the most superior pattern with respect to idle time was identified as a bowl-shaped CV configuration, combined with a buffer placement whereby more buffer capacity is concentrated at the beginning of the line, but for average buffer level the most favourable pattern was a bowl-shaped CV assignment, together with a buffer arrangement under which the buffers are concentrated towards the end of the line.