Sharing clearances to improve machine layout

This paper considers a double-row layout problem with shared clearances in the context of semiconductor manufacturing. By sharing some clearances, reductions in both layout area and material handling cost of approximately 7–10% are achieved. Along with minimal clearances for separating adjacent machines, clearances that can be shared by adjacent machines are considered. The shared clearances may be located on either or both sides of machines. A mixed integer linear programming formulation of this problem is established, with the objective to minimise both material flow cost and layout area. A hybrid approach combining multi-objective tabu search and heuristic rules is proposed to solve it. Computational results show that the hybrid approach is very effective for this problem and finds machine layouts with reduced areas and handling costs by exploiting shared clearances. 1. Introduction For some industries, such as semiconductor manufacturing, machines may require two types of clearance. The first type, minimum clearance, describes the spacing necessary for ventilation or to minimise vibration effects among adjacent machines. The second type, additional clearance, is necessary to allow technician access to one or both sides of the machine or to store work in process (WIP). These additional clearances are required in addition to the minimum clearances. Considering these two types of clearances as a single clearance may result in excessive spacing between machines, as there is often the possibility of sharing additional clearances among adjacent machines. For example, technicians rarely require access to maintenance panels of two side-by-side machines simultaneously. Thus, layout area can be saved by considering the minimum clearance and additional clearance independently. Since the material flow cost is related to the distance between machines, reducing the footprint of the manufacturing area is likely to reduce the overall material flow distance (and thus, cost) as well. Among the numerous variants of facility layout problems, the single-row layout problem (SRLP) and the multiple-row layout problem (MRLP) are the most widely implemented layout patterns in the literature and in practice (Solimanpur, Vrat, and Shankar 2005). Both the SRLP and MRLP share a common objective of arranging machines to minimise the total cost These are combinatorial problems due to the assumption that all machines are separated by minimum clearances. The SRLP arranges machines in one row, which may be a straight line, semi-circular or U-shape, with each pair of adjacent machines being separated by a minimum allowable clearance (Djellab and Gourgand 2001; Hungerländer and Rendl …