Integration of material handling devices assignment and facility layout problems

Highlights

• Three different facility location problems are considered.

• Material handling device assignment decisions are integrated into the problem.

• A mathematical programming model is proposed for this aim.

• The proposed integrated approach compared with the hierarchical approach.

• The results indicate that the proposed approach affects the facility layout.

Abstract

Facility layout problems focus on the assignment of departments into the facility layout by considering the minimization of total costs. A well-designed facility result in efficient material flows in transportation. For this reason, the problem should be considered with the changes in demands that cause different material flows between departments. While the aim of the static facility layout problem is to obtain the optimal facility layout, the dynamic facility layout problem includes the multi-periods dealing with the changes in material flows between departments. Furthermore, the material handling devices may affect the facility layout decisions although it is omitted in general. In this study, three different facility layout problems concern static, dynamic and flexible bay structure and material handling device assignment problems are considered simultaneously. Three mixed integer programming models are proposed for the addressed problems with integrated decisions. The proposed models are compared with a two-stage approach composed of solving the classical facility layout problems first, then solving material handling devices assignment problems. The comparative results show that the integrated decision affects the facility layout and decreases the total costs. Eleven test instances are considered and the integrated model provides cost efficiency for these test instances.

Introduction

The layout design is one of the decisions that have a direct impact on the production costs of firms in an increasingly competitive environment. The transportation costs associated with the facility arrangement correspond to 20 %–50 % of the operating costs, 2 %–10 % of these transportation costs can be reduced with good plant planning [1]. A well-regulated plant results in efficient material use and reduced transport time [2].

The facility layout problem (FLP) aims to determine the locations of the departments within a supportive for continuous production (or service) [3]. FLPs deal with the physical placement of machines, departments, workstations in a layout for a specific aim (such as minimum transportation amount in the manufacturing sector) [4].

Type of FLPs varies on the produced product, sector, facility structure, demand variability and many other factors. FLP deals with the placement of the N facilities, which are usually given, in N areas, in a way to build the lowest cost layout. Therefore, all alternatives (N!) must be evaluated for the best placement. As the number of facilities increases, the number of alternatives increases exponentially and obtaining the optimal solution becomes difficult. The problem is therefore having computational complexity, defined as NP-hard [5].

Facility layout problems are handled together with different approaches in real life applications. One of these is the flexible bay structure, which limits the continuous space settlement problem. In one of the early studies, Tong [6] proposed a flexible bay structure based on layout, facility base, and the width of the flexible bay that is divided horizontally or vertically depending on the number of sections in the facility layout.

Fig. 1 shows an example representation of the flexible bay structure. While the flexible bay widths in the same line may differ, their heights must be the same [7]. The aim of the flexible bay structure is to divide a specific region into sub-regions, ensuring to minimize the total material handling costs. The areas of the bays may not be the same, but each bay must provide aspect ratio constraints or minimum length constraints [8].

A specific case of the facility layout problems is defined as Dynamic Facility Layout Problem (DFLP). Various definitions are made in many studies on DFLP [[9], [10], [11], [12], [13], [14], [15]]. In general, the dynamic facility layout problem refers to creating a facility design that provides multi-period planning, in which the product demands changes during periods thereby material transportations between departments vary extensively from a period to another period. The aim of the dynamic facility layout problem is to minimize the sum of transportation costs between the departments and to ensure that the facility is organized more effectively by keeping the displacement costs of the departments at the lowest level during the planning horizon.

As predicted in Meller and Gau [16], there is a tendency in the literature to organize layout and production system designs with simultaneous approaches. Another consideration taken into account is that when dealing with the facility layout problem is the assignment of material handling devices. The type of material handling devices affects the layout to be used for the settlement of the machines [17]. This requires considering these two dependent design problems together to find a solution for facility layout [18]. Efficient material handling ensures lower processing times and cycle times in production processes, on-time delivery and better quality [19].

The main contribution of the study is to show the effects of integrated material handling device assignment decisions in the facility layout models. Therefore, three different facility layout problems which are static, static with flexible bays and dynamic with flexible bays are evaluated. The problems are considered as consecutive models (solve facility layout problem first then solve the material handling device assignments problem) and integrated models. In this way, the changes in the minimum cost facility layouts of the problem with different characteristics are presented. Besides, scenarios and their effects on the problem are discussed.