The objective function is expressed as the sum of the two terms: Model 1 (Allocation Model) The objective function represented by equation (7) is to mini- mise the sum of three costs. The first cost component is the incremental cost of processing. If the mixer is run at the reference speed (usually the lowest speed) this cost is zero. For running at higher speed, an incremental processing cost is incurred. The second cost component is the sequence depen- dence cost owing to the cleaning required between two success- ive batches of compounds of different types. The third cost component is the cost due to speed change. Constraints (8) and (9) ensure that each compound should be assigned to only one position and each position should be filled with only one compound. The sequence and speed of compounds at two successive positions are captured by constraints (10). Constraint (11) ensures that the available capacity of the mixer is not violated. Constraints (12) define the continuous and binary decision variables. Model 2 (Sequencing Model) Table 1. Working volume, available time, changeover times on each mixer, j Table 2. Speed available and cost of running per hour on the mixers. *The mixer can run at this speed, but was never used by the company. Table 4, Mixing time, ¢ at different speeds (r.p.m.) on the four mixers. *Preferred speed, e.g. for compound 1 on mixer 1 the preferred speed is 40 r.p.m.; and the mixing time for compound | on mixer 1 at the preferred speed is tf, = 120. ~ Compound has not been run on this mixer and no data are available. Table 5, Compound allocation on four mixers obtained from allocation model. Table 7. Compound allocation and sequencing on four mixers obtained by the company personnel. *Cleaner was used because of idle time/chemical leakage. +Cleaner was used because of idle time/sheet cutter blade was broken. Total cost = 84279.97. Total cost = 76686.18. Table 6. Compound sequencing on four mixers obtained from sequencing model.
