Workload control and optimised order release: an assessment by simulation

An important scheduling function of manufacturing systems is controlled order release. While there exists a broad literature on order release, reported release procedures typically use simple sequencing rules and greedy heuristics to determine which jobs to select for release. While this is appealing due to its simplicity, its adequateness has recently been questioned. In response, this study uses an integer linear programming model to select orders for release to the shop floor. Using simulation, we show that optimisation has the potential to improve performance compared to ‘classical’ release based on pool sequencing rules. However, in order to also outperform more powerful pool sequencing rules, load balancing and timing must be considered at release. Existing optimisation-based release methods emphasise load balancing in periods when jobs are on time. In line with recent advances in Workload Control theory, we show that a better percentage tardy performance can be achieved by only emphasising load balancing when many jobs are urgent. However, counterintuitively, emphasising urgency in underload periods leads to higher mean tardiness. Compared to previous literature we further highlight that continuous optimisation-based release outperforms periodic optimisation-based release. This has important implications on how optimised-based release should be designed.     

Continuous workload control order release revisited: an assessment by simulation

Order release is a key component of the Workload Control concept. Jobs do not enter the shop floor directly – they are retained in a pre-shop pool and released in time to meet due dates while keeping work-in-process within limits or norms. There are two important groups of release methods: continuous methods, for which the workload falling to a specified level can trigger a release at any moment in time; and, periodic release methods, for which releases take place at fixed intervals. Continuous release methods in general have been shown to outperform periodic release methods. Yet, there is incongruence in the results presented in the literature on the relative performance of the various continuous release methods. We use a job shop simulation model to examine the performance of continuous release methods from the literature and find that the contradictory results are explained by the different rules applied to sequence jobs in the pool – a factor neglected in previous work. Finally, a new breed of continuous release methods has recently emerged, but these have not been compared with prior approaches. Therefore, we also examine these methods and show that they significantly improve overall performance, although this is to the detriment of jobs with large processing times.     

Job sequencing and selection within workload control order release: an assessment by simulation

Recent research has highlighted the potential impact of pool sequencing on order release performance but it suffered from two shortcomings. First, arguably the best release solution for workload control in practice combines periodic with continuous release. Although the two types of releases serve different functions, recent work assumed the same sequencing rule should be used for both. Here, the use of different sequencing rules for periodic and continuous releases is evaluated. Using a job-shop simulation, we demonstrate that the rule applied during continuous releases has only a negligible impact on performance. Therefore, jobs can be pulled intermediately from the pool by workers using a more straightforward sequencing rule than the one applied for periodic release. Second, it was assumed that all jobs in the pool are sequenced and then a subset is selected for release. But for some load-oriented sequencing rules, the priority value used for sequencing jobs should be updated after each job selection from the pool. Our simulation results show that although this may improve load balancing at release, it does not in fact improve overall shop performance. Therefore, the greedy heuristic of first sequencing and then selecting jobs can be maintained, which allows the release decision-making process to retain its simplicity. The work has important implications for the use of sequencing rules in practice.     

On the combined effect of due date setting,order release,and output control: an assessment by simulation

Workload Control is a production control concept for high-variety shops built on the principle of input/output control. The literature, however, has argued that input/output control overemphasises throughput improvements to the detriment of the timing of individual orders and, consequently, that it needs to be supplemented by a preceding customer enquiry stage where due dates are set. Yet, although there are broad separate literature streams on due date setting, order release, and output control, there is a lack of research on the three functions together. In response, this study uses simulation to assess the combined performance effect of all three functions. Results show that each control function can be related to a specific performance objective. The degree of emphasis that should be placed on each function, therefore, depends on a company’s specific performance needs. Due date setting and capacity adjustments (output control) are shown to support each other as they address different performance objectives. Meanwhile, order release (input control) is effective in reducing work-in-process and can play a role in making throughput improvements when capacity adjustments are not possible. Findings enhance existing literature on the diagnosis of delivery reliability performance in high-variety shops, with important implications for research and practice.     

Thermal degradation mechanisms of Nicalon fibre:a thermodynamic simulation

Thermodynamic calculations for the thermal degradation of the Nicalon fibre in inert gas flow at atmospheric pressure have been performed, based on minimization of the Gibbs energy of the Si-C-O-H chemical system. The calculations are based on a critically selected thermodynamic database of the participating compounds. The results are presented by means of diagrams illustrating the quantities of condensed and gaseous species obtained as a function of treatment temperature. These are compared with recently reported TEM studies of as-received and heat-treated material, which illustrate the sequential morphologies of its structure and nanotexture as a function of treatment temperature. The main step of the observed degradation mechanism is successfully simulated in terms of the temperature, the oxygen content and the weight loss of the material. An endogenous oxidation mechanism is proposed for degradation of the fibre.     

An order release mechanism for a flexible flow system

Order release is where orders are released to the shop floor for processing. An Order Release Mechanism (ORM) can control order release, such that orders are selectively released so as to improve shop management and performance. This paper reports on the development of an ORM for a specific Flexible Flow System (FFS), where the primary criterion in running the system is flow time. The ORM determines what order to release when, and makes the routeing decisions for each order. Using a simulation model, the performance of the FFS with the ORM was compared to present practice without an ORM and an ‘off-the-shelf’ ORM from the literature, namely CONWIP. It is found that the ORM not only improves the mean time that orders spend in the shop under all load conditions, but also reduces the variance of this measure. It would be expected that the total time that orders spend in the system increases with the ORM, at least compared to immediate release, but at high load levels it is found that this measure also decreases. An analysis of variance suggests that the choice of ORM is more important than the choice of the dispatching rule used on the shop floor.     

pH Sensitive graft copolymers for zero order drug release: a mechanistic analysis

Aliphatic polyesters containing pendent unsaturation were synthesized by the polycondensation of a diol, dicarboxylic acid and glycidyl methacrylate. Grafting methacrylic acid (MAA) resulted in graft copolymers containing polyester backbone and MAA grafts. Depending on composition, the polymers swelled extensively and eroded or dissolved at near neutral pH but remained in collapsed state at acidic pH. Three representative drugs differing in solubility, viz., Diltiazem hydrochloride (DH), Indomethacin (IM) and Verapamil hydrochloride (VH) were released at constant rate from tablets made by compressing spray-dried microparticles. The release of DH at constant rate has been attributed to increase in diffusion coefficient of the drug from the swollen layer of matrix. The release of IM and VH at constant rate was governed by erosion and was enhanced in matrices which undergo dissolution. The release rate was enhanced with increasing MAA content and the frequency of grafts along the polyester backbone. Once a day dosage forms for drugs differing in solubility have been developed using a single polymer matrix which is easy to manufacture.     

Alternative control mechanisms for cyclical scheduling systems

This paper investigates the relative performance of alternative mechanisms for controlling the cycle length in a cyclical scheduling system. While all methods investigated are able to maintain a target long run average cycle length, some provide greater inventory efficiency than others, with more informed application of idle time. Simulation results demonstrate that a fixed idle time policy provides worst results, while an aggregate workload heuristic based on earlier dynamic policy characterisation provides superior performance. The importance of the control mechanism is shown to be greatest when the process is subject to high demand intensity, particularly when the demand process is sporadic.     

Scattering by a slab containing randomly located cylinders: comparison between radiative transfer and electromagnetic simulation

This study is devoted to the examination of scattering of waves by a slab containing randomly located cylinders. For the first time to our knowledge, the complete transmission problem has been solved numerically. We have compared the radiative transfer theory with a numerical solution of the wave equation. We discuss the coherent effects, such as forward-scattering dip and backscattering enhancement. It is seen that the radiative transfer equation can be used with great accuracy even for optically thin systems whose geometric thickness is comparable with the wavelength. We have also shown the presence of dependent scattering.     

Integrating load-based order release and priority dispatching

Workload control (WLC) is a well-established production control concept for job shops that put primary emphasis on load-based order release. Recent advances in load-based order release research have led to an improved delivery performance at reduced shop floor workloads. But although order release is the primary focus of WLC research, it must be coupled with priority dispatching on the shop floor if order progress is to be regulated. Prior simulation research suggests that load-based order release methods should only be coupled with simple dispatching rules because other, more powerful rules can conflict with the functioning of the release method. Yet, recent empirical research suggests that powerful priority dispatching rules – such as due date-oriented dispatching rules – are in fact needed for a high level of delivery performance to be obtained in practice. This paper focuses on overcoming the conflict between order release and dispatching, so load-based order release can be combined with due date-oriented dispatching. Preliminary analysis reveals that part of the conflict is because existing due date-oriented dispatching rules overcompensate for schedule deviations that occur when orders are either released earlier or later than planned. Two alternative new dispatching rules based on an improved method of determining operation due dates are then developed to better account for schedule deviations and overcome the conflict with load-based order release. Further improvements in delivery performance are obtained, while the large workload reductions achieved by recently developed load-based order release methods are retained.     

An ABC-based cost model with inventory and order level costs: a comparison with TOC

Activity-Based Costing (ABC) was developed to address the deficiencies of traditional accounting systems in the modern business environment by helping managers understand product and customer profitability and identify priority areas for process improvements. In this study, activity-based costing (ABC) concepts are integrated with a Mixed Integer Program (MIP) for order management and profitability analysis in the case of a firm facing demand in excess of capacity. The model considers unit-level, batch, and order-related costs within a mixed-integer programming model representing the firm's operating structure. The performance criterion of profit and service levels during the 20-period planning horizon is used for model evaluation in a comparison with the results from a Theory of Constraints (TOC) formulation. The analysis of these competing models will provide guidelines for applications of order management models with simultaneous consideration of production planning and profitability analysis, as well as help managers understand product and customer profitability, and identify priority areas for process improvements. The results indicate that the ABC-based model is more effective in increasing profitability and reducing inventory levels when compared with the TOC-based formulation, making better use of overhead cost information in the selection of orders.     

How to recognize the order of infinitesimal mechanisms: A numerical approach

Structures presenting kinematical inderterminacy are usually called mechanisms. This paper is entirely concerned with assemblies which reveal themselves to be mechanisms at a null value of the load. Among them a first distinction is made between infinitesimal and finite ones, the former being characterized by one or several directions of lower (but not zero) stiffness, whereas the latter show at least one finite admissible displacement for which none of the bars undergoes any elongation. Moreover, there exists the possibility to make a further distinction among the infinitesimal mechanisms, according to which is the order of the stiffness along the direction considered above. The way of evaluating this order is to perform a local analysis of the strain energy of the assembly, once the displacement field is parametrized in terms of a suitable parameter. By means of a finite element technique, this analysis can be easily performed through the numerical approach presented in this report.     

Optimal order release decisions for asynchronous production systems

Asynchronous production systems are classified into systems with fixed, unrestricted, and restricted work-in-process inventory. For each class routing optimization models are presented. Furthermore, extensions are provided in order to include optimal lotsizing and/or part mix decisions. These extensions find optimal solutions for two previously published problems with only heuristic solution procedures.     

Integrating order release control with due-date assignment rules

This study integrates order release control methods with due-date assignment rules and assesses its impact on the accuracy of inter-operation time estimation and performance of due-date assignment. The assessment is made by using an experimental design with three due-date assignment rules, three scheduling rules and three order release models. The three order release models are: (1) Basic model, in which three due-date assignment rules consider the order arrival time as the order release time; (2) Control model, in which three due-date assignment rules integrate with the order release control method developed here; and (3) Adjustment model, in which the control model integrates with the order release control adjustment developed here. Simulation results in this study indicate that integrating the order release control method with due-date assignment rules will significantly enhance not only the accuracy of interoperation time estimation, but also the performance of due-date assignment rules.     

Successful implementation of an order release mechanism based on workload control: a case study of a make-to-stock manufacturer

This paper deals with improving the lead-time performance of a large crystal manufacturer that uses a state-of-the-art commercial Enterprise Resource Planning system. Since the company encountered some limitations of the standard production planning and control (PPC) system it sought for improvements by implementing an order release mechanism based on workload control (WLC). WLC employs certain rules for releasing orders in order to maintain a certain level of work in process to achieve a certain utilisation of the production system and thus control the flow times in order to meet the required due dates of the orders. We describe the successful implementation of an order release mechanism based on the WLC concept in this make-to-stock company. The paper describes the implemented order release mechanism, the implementation process and its impact on the company’s performance. We show that the core function of WLC – the order release mechanism – can be integrated successfully into an existing PPC system. Furthermore, this study highlights the applicability of WLC to a wider range of companies, especially to make-to-stock manufacturers.     

Freezing of silver cluster and nanowire: A comparison study by molecular dynamics simulation

The crystallization of liquid Ag cluster and nanowire, about 2.3 nm in diameter, has been studied by molecular dynamics simulation at three different cooling rates (i.e., 2 × 10¹³ K/s, 2 × 10¹² K/s, 2 × 10¹¹ K/s). It is found that the structure of Ag cluster in the specified size changes from amorphous to crystalline directly during the cooling process, rather than follows the route of amorphous–icosahedra–crystalline. The Ag nanowire in the specified size also changes from amorphous to crystalline directly, rather than follows the route of amorphous–(multi-shelled)–crystalline. All the finial structures of Ag cluster and nanowire after relaxation are FCC despite the different cooling rates, which means that the FCC is the most stable structure. Furthermore, the crystallization temperature of Ag nanowire is higher than that of cluster at the same cooling rate, which suggests that the crystallization temperature is dimensional-dependent.     

Workload control release mechanisms: from practice back to theory building

Much Workload Control research has focussed on the order release stage but failed to address practical considerations that impact practical application. Order release mechanisms have been developed through simulations that neglect job size variation effects while empirical evidence suggests groups of small/large jobs are often found in practice. When job sizes vary, it is difficult to release all jobs effectively—small jobs favour a short period between releases and a tight workload bounding while large jobs require a longer period between releases and a slacker workload bounding. This paper represents a return from a case study setting to theory building. Through simulation, the impact of job sizes on overall performance is explored using all three aggregate load approaches. Options tested include: using distinct load capacities for small/large jobs and prioritising based on job size or routing length. Results suggest the best solution is assigning priority based on routing length; this improved performance, especially for large jobs, and allowed a short release period to be applied, as favoured by small jobs. These ideas have also been applied to a second practical problem: how to handle rush orders. Again, prioritisation, given to rush orders, leads to the best overall shop performance.     

Information sharing and coordination mechanisms for managing uncertainty in supply chains: a simulation study

The study aims to investigate the effectiveness of information sharing and coordination mechanisms in reducing uncertainty. Supply chains are constantly subject to unpredictable events that can adversely influence its ability to achieve performance objectives. This paper primarily aims at managing uncertainties originating from unexpectedly large demand spikes. The supply chain literature is full of effective supply chain uncertainty management practices. This paper reviews the different practices for improving management of uncertainty and proposes several combinations of information sharing and coordination mechanism for reducing the uncertainty in supply chains. Next, the proposed combinations are tested on the make-to-stock supply chain of a paper tissue manufacturer using an agent-based simulation approach to show how the use of different levels of information sharing and coordination can be effective in managing uncertainty under daily operations facing a huge mismatch of actual and forecast demand. The findings of this research suggest that, a centralised information structure without widespread distribution of information and coordination is not effective in managing uncertainty of supply chain networks, even with increased frequency of information flow. Similarly, coordinating material flows without widespread information sharing does not improve supply chain uncertainty management. Central coordination of material flows with supply chain wide information sharing across different members is found to be essential in managing supply chains effectively under uncertainty.     

Multi-unit controlled release systems of nifedipine and nifedipine:pluronic F-68 solid dispersions: characterization of release mechanisms

Nifedipine (N) and nifedipine:Pluronic® F-68 solid dispersion (SD) pellets were developed and characterized for drug release mechanisms from a multi-unit erosion matrix system for controlled release. Nifedipine was micronized using a jet mill. Solid dispersion with Pluronic F-68 was prepared by the fusion method. Nifedipine and SD were characterized by particle size analysis, solubility, differential scanning calorimetry (DSC), and x-ray diffraction (XRD) studies. Samples were subsequently processed into matrix pellets by extrusion/spheronization using Eudragit® L 100-55 and Eudragit® S 100 as release rate-controlling polymers. Drug release mechanisms from pellets were characterized by microscopy and mercury intrusion porosimetry; DSC and XRD studies indicated no polymorphic changes in N after micronization and also confirmed the formation of SD of N with Pluronic F-68. Pellets of N showed a 24-hr drug release profile following zero-order kinetics. Pellets of SD showed a 12-hr release profile following first-order kinetics. Aqueous solubility of N after SD formation was found to be increased 10-fold. Due to increased solubility of N in SD, the drug release mechanism from the multi-unit erosion matrix changed from pure surface erosion to an erosion/diffusion mechanism, thereby altering the release rate and kinetics.