Saifallah Benjaafar

Make-to-order, make-to-stock, or delay product differentiation? A common framework for modeling and analysis

Delaying product differentiation is a hybrid strategy that strives to reconcile the dual needs of high variety and quick response time. A common product platform is built to stock in the first stage of production (called the Make-To-Stock (MTS) stage) which is then differentiated into different products after demand is known in the second stage (known as the Make-To-Order (MTO) stage). In this article, we develop models to compute the costs and benefits of delaying differentiation in series production systems when the order lead times are load dependent. The models are then used to gain insights through analytical and numerical comparisons. For example, we observe the following patterns in a large number of numerical experiments. The effect of congestion in the MTS and MTO stages is asymmetric with tighter capacity at the MTO stage having a greater detrimental effect on the desirability of delaying differentiation. If there is flexibility in choosing the point of differentiation, higher loading is observed to favor later differentiation. Also, if the sequence in which work is performed can be affected, then placing workstations that have a tighter capacity in the MTS stage lowers costs.

Design of flexible plant layouts

In this paper, we present an approach for the design of plant layouts in stochastic environments. We consider systems where the product mix and product demand are subject to variability and where duplicates of the same department type may exist in the same facility. In contrast to a job shop layout, we allow these duplicates to be placed in non-adjacent locations on the plant floor and for flow allocation between pairs of individual departments to be made as a function of the layout and the product demand realization. We present a scenario-based procedure that iteratively solves for layout and flow allocation. We show that having duplicates of the same departments, which can be strategically located in different areas of the plant floor, can significantly reduce material handling cost while effectively hedging against fluctuations in flow patterns and volumes. We show that the effect of duplication is of the diminishing kind, with most of the cost reduction occurring with relatively few duplicates. We also show that the quality of the obtained layouts can be quite insensitive to inaccuracies in estimating demand scenario probabilities.

Modelling, measurement and evaluation of sequencing flexibility in manufacturing systems

Sequencing flexibility refers to the possibility of interchanging the order in which required manufacturing operations are performed. In this paper, we address several issues related to the modelling, measurement and performance evaluation of this flexibility in manufacturing systems. In particular, we introduce several representation and measurement schemes for sequencing flexibility and discuss the usefulness and limitations each. We then propose a new performance-based approach for quantifying the value of flexibility. To this effect, we study the relationship between flexibility and system performance under a variety of design assumptions and operating conditions. These relationships are used to identify key characteristics of a measure of flexibility that is reflective of system performance. This measure is to allow system designers and managers to predict performance based on existing levels of flexibility and/or determine the amount of flexibility required to achieve a certain level of performance....

Modeling and Analysis of Congestion in the Design of Facility Layouts

Reducing manufacturing lead times and minimizing work-in-process (WIP) inventories are the cornerstones of popular manufacturing strategies such as Lean, Quick Response, and Just-in-Time Manufacturing. In this paper, we present a model that captures the relationshipbetween facility layout and congestion-related measures of performance. We use the model to introduce a formulation of the facility layout design problem where the objective is to minimize work-in-process (WIP). In contrast to some recent research, we show that layouts obtained using a WIP-based formulation can be very different from those obtained using the conventional quadratic assignment problem (QAP) formulation. For example, we show that aQAP-optimal layout can be WIP-infeasible. Similarly, we show that two QAP-optimal layouts can have vastly different WIP values. In general, we show that WIP is not monotonic in material-handling travel distances. This leads to a number of surprising results. For instance, we show that it is possible to reduce overall distances between departments but increase WIP. Furthermore, we find that the relative desirability of a layout can be affected by changes in material-handling capacity even when travel distances remain the same. We examine the effect of various system parameters on the difference in WIP between QAP- and WIP-optimal layouts. We find that although there are conditions under which the difference in WIP is significant, there are those under which both layouts are WIP-equivalent.

Machine sharing in manufacturing systems : Total flexibility versus chaining

In this paper, we compare the operational performance of two machine-sharing configurations: total flexibility and chaining. We show that chaining captures most of the benefits of total flexibility while limiting the number of part types processed on any individual machine to only two. We examine the relative desirability of the two configurations under varying buffer sizes, loading conditions, number of machines, and setup times, as well as for different control policies. For nonzero setups times, we show that chained configurations can outperform fully flexible ones. This particularly is the case when either the number of machines or length of setup times is high. We also find that the effect of the system size on performance diminishes with the number of machines. This means that multiple smaller chains can perform almost as well as a single long one. Our results are consistent with the recent findings of Jordan and Graves (1995), who examined the economic benefits of chaining relative to full flexibility.

Models for performance evaluation of flexibility in manufacturing systems

We investigated the relationship between flexibility and performance of manufacturing systems. In particular, the effect of varying flexibility in either part production requirements or the machines capabilities is examined. Conditions under which a positive correlation between flexibility and performance exists are identified and the characteristics of this correlation are described. System management and control mechanisms necessary for realizing the benefits of flexibility are outlined. The results are first illustrated through a series of models of specific manufacturing situations. A unifying and generic framework for flexibility modelling and analysis is then introduced. The framework is used to re-interpret and generalize the result of the previous models. It is also used to argue for the existence of a number of fundamental relationships between flexibility and manufacturing performance.

Performance bounds for the effectiveness of pooling in multi-processing systems

The need for quantifying the effect of resource pooling on performance of multi-processing systems arises frequently in the design of a variety of manufacturing, communication, and service systems. In this paper, we examine the effect of resource pooling and assess its impact on system performance. In particular, we provide performance bounds on the effectiveness of several pooling scenarios and discuss capacity and utilization tradeoffs between independent and pooled systems. We also propose a methodology for making optimal pooling decisions and describe the characteristics of this optimal solution. Limitations to the effectiveness of pooling are identified and conditions under which pooling may degrade performance are discussed.

The strategic value of flexibility in sequential decision making

Abstract This paper formalizes the notion of flexibility in sequential decision making and investigates conditions under which the use of flexibility as an additional criterion may be justified. The correlations between flexibility and value, and flexibility and risk, are studied under various assumptions of uncertainty and information. A number of approaches to constructing a multiple objective decision criterion are discussed. In particular, characteristics of a dual-objective value function, that accounts for both expected value and flexibility, are described. The usefulness of these results is illustrated by applying them to decision processes in discrete part manufacturing. Relationships between flexibility and manufacturing performance are shown and implications to part flow control are discussed.

Scope versus focus: issues of flexibility, capacity, and number of production facilities

How should a multi-product manufacturing firm design production facilities? How many facilities should it have? How many and which products should be assigned to each facility? What batch sizes/scheduling rules are appropriate for facilities making more than one product? These are questions that have become more relevant now as advances in manufacturing technologies offer an increasing array of equipment choices. In this article, we introduce models that can help operations managers answer the above questions. For a specific product mix, these models lead to explicit expressions for the number of facilities, the number of products assigned to each facility and their corresponding capacities. We evaluate the effect of different operating parameters and scheduling policies on the optimality of different configurations. In particular, we show that the choice of the scheduling and batch sizing policies can have a significant effect on the nature of the optimal mix of flexible and d edicated facilities as well the size of these facilities.

On production batches, transfer batches, and lead times

In this paper we extend a model proposed by Karmarkar for studying the relationship between batch sizes and production lead times. In our extended model we differentiate between production and transfer batches and examine the effect of having smaller transfer batches on lead times. The model is used to obtain optimal transfer batch sizes and determine their relationship to the size of production batches. Further extensions are made to include multiple machines and material-handling considerations and to evaluate their effect on batching decisions. Conditions under which either type of batching can be useful are identified and the corresponding optimal batch sizes are described.