Many industries use linear production lines, with buffers between eachpair of machines for absorbing small breakdowns or other irregularities. Thesebuffers have different thresholds for triggering the possible speeds of the machines.The goal of this study is to tune the values of these thresholds in order to enhancethe productivity of the line. A simulation-optimization approach is proposed andapplied to a case study involving a soft-drink plastic bottle company. We show thatthe production can be increased by a few pallets per day. Suchresults are appealingfor the company, as updating these thresholds does not implyany cost.