The release mechanism used comprises both, timing and balancing functions. This means that, on one hand, jobs waiting in the pool are considered for release according to the earliest planned release time, which determines their relative urgency (timing function). Planned release times are determined by backward scheduling from the promised due date, using a constant lead time per operation, which was established through some pilot simulation runs under an immediate release. On the other hand, jobs are only released if they fit workload norms for required capacity groups (balancing function). Capacity groups in our study refer to the smallest production units to be controlled during release : work centres.