Gerard Gaalman

Combined make-to-order and make-to-stock in a food production system

The research into multi-product production/inventory control systems has mainly assumed one of the two strategies: Make-to-Order (MTO) or Make-to-Stock (MTS). In practice, however, many companies cater to an increasing variety of products with varying logistical demands (e.g. short due dates, specific products) and production characteristics (e.g. capacity usage, setup) to different market segments and so they are moving to more MTO-production. As a consequence they operate under a hybrid MTO-MTS strategy. Important issues arising out of such situations are, for example, which products should be manufactured to stock and which ones on order and, how to allocate capacity among various MTO-MTS products. This paper presents the state-of-the-art literature review of the combined MTO-MTS production situations. A variety of production management issues in the context of food processing companies, where combined MTO-MTS production is quite common, are discussed in details. The authors propose a comprehensive hierarchical planning framework that covers the important production management decisions to serve as a starting point for evaluation and further research on the planning system for MTO-MTS situations.

The influence of shop characteristics on workload control

The magic word of the last years is internationalization. Researchers more and more focus on export market information and the relationship between this information and export performance. Just a few studies take into account the impact of firm characteristics on these topics. This study shows that country-of-origin, sector and size class (both individually and simultaneously) influence both the number of information sources consulted, and the type of information provider consulted. Besides, the relationship between the amount of export market information and export sales is not straightforward, but needs to be corrected for these three firm characteristics. These conclusions have implications for governmental export promotion campaigns, for the information behaviour of SME-managers, and for the strategies of information providers.

Workload control concepts in job shops A critical assessment

Abstract In the case of production environments with job shop characteristics, much research has been done on partial control such as priority dispatching. The development of comprehensive control concepts lags behind. However, the principles of workload control (WLC) have been elaborated to more comprehensive production control concepts. WLC concepts buffer the shop floor against external dynamics by creating a pool of unreleased jobs. The use of workload norms should turn the queueing of orders on the shop floor into a stationary process which can be characterised by an equilibrium. This paper compares and discusses the concepts of WLC. Assumptions of stationarity implied in the workload norms are exposed. A subdivision of workload definitions is chosen as a starting-point to trace assumptions of stationarity. The assumptions highlighted relate to the shop floor situation and make demands upon the job release function. An obvious conflict between timing and balancing within the job release function leads to an examination of stationarity requirements on the job pool contents. The analysis of stationarity requirements within existing production control concepts provides guidelines for developing production control concepts for job shops working under dynamic circumstances.

Exploring applicability of the workload control concept

To be successful in companies, a production planning and control (PPC) concept should fit to the production environment. Essential elements of the concept should correspond with the characteristics of the production system. For classical concepts such as MRP these elements have become common sense. For example BOMexplosion and constant lead times make MRP known to perform best in environments with high material and low capacity complexity. For many other concepts the situation is less clear. In this paper the Workload Control (WLC) concept is considered for which the requirements for a successful application have never been investigated. A framework is proposed to explore the applicability of WLC in small- to medium-sized make-to-order (MTO) companies. It supports an initial consideration of WLC in the first phase of a PPC selection and implementation process. As a first step in developing the framework the inherent characteristics of the WLC concept and the relevant MTO production characteristics are identified. Confronting the indicators of the company characteristics with the WLC elements results in bestfit indications for the WLC concept. Contrarily to other PPC evaluation schemes the framework considers variability indicators besides averages. Use of this framework for a medium sized MTO company demonstrates its suitability in getting a systematic and quick impression of the applicability of WLC. Essential elements are treated and assessed.

A basic period approach to the economic lot scheduling problem with shelf life considerations

Almost all the research on the economic lot scheduling problem (ELSP) considering limited shelf life of products has assumed a common cycle approach and an unrealistic assumption of possibility of deliberately reducing the production rate. In many cases, like in food processing industry where limited shelf life for products is common, changing the production rates is not allowed at all because it may result in products with poor quality. In this paper, we allow products to be produced more than once in a cycle and do not allow reducing production rates. We present a modification to the Haesslers basic period procedure to account for the shelf life. Proposed ‘branch-and-bound like’ procedure exploits these extra constraints to efficiently achieve a feasible solution. Numerical examples are presented to show that our approach outperforms common cycle approach with shelf life considerations.

Production planning and control in SMEs: time for change

This study aims to provide a profound picture of where existing production planning and control (PPC) concepts fail in small to medium sized make-to-order (MTO) companies. While several studies have discussed the specific requirements of small and medium sized enterprises (SMEs) through surveys or observations in order to determine the suitability of existing PPC concepts, in general, in-depth empirical evidence is rare. In this article, seven case study companies are studied with the backbone of the analysis based on order progress data collected from each company. In each case, logistic performance problems are determined and traced back to their root causes. To distinguish between common problems and company specific elements, a comparative analysis across the cases has been performed. The analysis shows that a significant proportion of common PPC problems can be attributed to a limited set of decision points in the flow of orders. Furthermore, we find that most performance losses can actually be predicted before an order is released to the shop floor. Decisions after release are mainly made in order to correct for the delays caused before release. In the pre-production phase, before release, two common problem areas are identified: (1) inadequate capacity planning overviews to support sales decisions, and (2) uncontrolled delays in engineering. Observed problems that appear after release tend to be more diverse and company-specific. A review of available PPC concepts for MTO SMEs indicates that these areas have been neglected in the literature to date, leading to our call for a new focus in PPC research.

On the who-rule in Dual Resource Constrained (DRC) manufacturing systems

The who-rule is a labour allocation rule used in labour and machine-limited dual resource constrained (DRC) systems. A who-rule selects one worker out of several workers to be transferred to a work centre. By means of a practical instance, the paper shows that the who-rule plays a role in the daily practice of worker assignment. Previous simulation studies, however, either have not mentioned the who-rule or have treated it as a fixed factor. The present study will explore the need of including the who-rule in simulation studies. It will describe in detail at what decision moments the who-rule needs to be applied in simulation. Further, it will explore the flow time effects of applying different who-rules in several DRC systems where labour flexibility is limited and workers differ with respect to task proficiencies, the number of skills they possess and the loads of work centres for which they are responsible. As with other labour allocation rules, the impact of the who-rule depends on the specific DRC shop modelled. The paper will show that the average labour utilization, and the types and extent of worker differences, determine the impact of the who-rule on shop performance.

Quasi on-line scheduling procedures for flexible manufacturing systems

Abstract This paper presents three quasi on-line scheduling procedures for FMSs consisting of work stations, transport devices, and operators. In the scheduling, different types of decisions are taken to perform a particular operation, i.e. the selection of (a) a work station, (b) a transport device and (c) an operator. Further, (d) the scheduling sequence of the operations has to be determined. The three developed procedures differ in the way these four decision problems are solved hierarchically. Several dispatching rules (SPT, SPT.TOT, SPT/ TOT and EFTA) are available to solve the last mentioned decision problem. Limited buffer capacities in an FMS may cause deadlock in the procedures as well as in practice. The scheduling procedures involve a buffer handling method to avoid deadlock. A case study is presented to demonstrate the three procedures and to show some of its properties. Based on simulation tests, some conclusions are drawn about the performance of the scheduling procedures and the various di...

Scheduling of packaging lines in the process industry: An empirical investigation

Abstract In the process industry, homogeneous products have to be packed. For an unpacked product (milk, paint), a variety of packaging materials (glass, tin) and packaging sizes ( 1 2 litre, 1 kilo) are available. Generally, the packaging lines are used for various products (milk, buttermilk) in one type of packaging material (glass) and various packaging sizes ( 1 2 litre, 1 litre). These possibilities make the scheduling of the packaging lines rather complex. In recent years, under market pressure, the number of products in the process industry has been increased, the order sizes have been reduced, and the delivery times have been shortened. This has caused augmented scheduling tasks and, usually, the scheduling systems supporting these tasks have not followed these changes sufficiently. We have investigated and compared the origins of the increasing complexity of the scheduling problems of several companies in the process industry producing dairy products, pharmaceutical products, tobacco, paint, chocolate products, and foods, and we have analyzed the ways in which these companies deal with this increasing complexity in different situations. Schedulers have to deal with a wide range of information such as due dates, availability of material, sequence-dependent set-up times, and availability of packaging lines and employees. In most cases the scheduling is still done by hand, which is very time-consuming. We show the need for a more adequate support system which will provide necessary information, and speed up the development and evaluation of production schedules. We also discuss future trends with regard to the packaging process, and their likely effect on the scheduling process.

Influence of capacity- and time-constrained intermediate storage in two-stage food production systems

In food processing, two-stage production systems with a batch processor in the first stage and packaging lines in the second stage are common and mostly separated by capacity- and time-constrained intermediate storage. This combination of constraints is common in practice, but the literature hardly pays any attention to this. In this paper, we show how various capacity and time constraints influence the performance of a specific two-stage system. We study the effects of several basic scheduling and sequencing rules in the presence of these constraints in order to learn the characteristics of systems like this. Contrary to the common sense in operations management, the LPT rule is able to maximize the total production volume per day. Furthermore, we show that adding one tank has considerable effects. Finally, we conclude that the optimal setup frequency for batches in the first stage is dictated by the storage time constraint.