Balancing and cyclically sequencing synchronous, asynchronous, and hybrid unpaced assembly lines

Mixed-model assembly lines are product-oriented production layouts often employed for large scale manufacturing of similar products. The unpaced variant of these lines employs a conveyor to discretely move pieces between stations either synchronously or asynchronously. Workload balancing and product sequencing are common optimization problems associated with these lines. While many works detail balancing and sequencing separately, few explicitly combine these degrees of freedom. Furthermore, hybrid (i.e. partly synchronous and partly asynchronous) lines are not explicitly described by previous optimization models. This paper presents a mixed-integer linear programming model capable of representing such unpaced lines and explicitly combine balancing, sequencing and cyclical scheduling. The application of the proposed method to a new dataset demonstrates the advantages of simultaneously balancing and sequencing lines, generating more efficient solutions than previously described models for 238 out of 240 instances. These results implied, however, in greater computational costs required to combine the degrees of freedom. Furthermore, a direct performance comparison study between synchronous, asynchronous, and hybrid lines is conducted. This allows line designers and managers to explicitly evaluate economical trade-offs between these line types.