Synchronization in a two-supplier assembly system: Combining a fixed lead-time module with capacitated make-to-order production

A high-tech manufacturer often produces products that consist of many modules. These modules are either sourced from one of its suppliers or produced in-house. In this paper, we study the common case of an assembly system in which one module is sourced from a supplier with a fixed lead-time, while the other module is produced by the manufacturer itself in a make-to-order production system. Since unavailability of one of the modules has costly consequences for the production of the end-product, it is important to coordinate between the ordering policy for one module and the production of the other. We propose an order policy for the lead-time module with base-stock levels depending on the number of outstanding orders in the production system of the in-house produced module. We prove monotonicity properties of this policy and show optimality. Furthermore, we conduct a computational experiment to evaluate how the costs of this policy compare to those of a policy with fixed base-stock levels and show that average savings of up to 17% are attained.