Average Work In Process Research Articles

Optimization In a Casing Manufacturing Industry Using Simulation Study

This project describes how Simulation can be applied to optimize a job shop scheduling problem (JSSP) at a precision aerospace components manufacturing company. The product selected for the simulation study is casing. As 8 different casings are manufactured in a single casing manufacturing line, the demand requirements are not met. To identify the bottleneck operations, the existing manufacturing process is simulated using Arena simulation software. The model is simulated for various batch sizes and Scheduling rules and the performance parameters like total casing output, average work in process, average utilization and average flow time were determined. Design of experiments and Interaction plot is used to determine the optimum batch size and scheduling rule. The results showed that for maximum casing output and Resource utilization, the optimum batch size is 5 and optimum scheduling rule is Shortest processing time (SPT) priority rule. To minimize the Average WIP and flow time, the optimum batch size is 2 and optimum scheduling rule is Shortest processing time (SPT) priority rule.

IMPROVING PERFORMANCE IN AN ALUMINUM EXTRUSION PLANT USING DISCRETE EVENT SIMULATION: A CASE STUDY

Simulation has been used in many industrial applications for performance improvement. It excels over other system analysis methods in its high flexibility and ability to model system details with high accuracy. In this study, Discrete Event Simulation (DES) is used to improve the performance of an aluminum extrusion plant. A case study is presented in a local factory in which problems are identified, and their effects on efficiency are monitored. The main problem noticed was high production rates with respect to demand rates which resulted in large amounts of work-in-process (WIP) inventory. It was found that the current base system is unstable and suggestions were made to lower production rates in order to stabilize it. Average WIP was reduced by 324% once the system was stabilized with only 1.77% difference in weekly throughput which improved the system considerably. Next, alternatives were suggested to improve throughput and reduce WIP while maintaining stability. The alternative with optimized batch sizes had the best improvement in throughput of 3.54%. The combined model with optimized batch sizes and an added pool for chemical treatment had the most WIP versus other alternatives.

Enfoque híbrido metaheurístico AG-RS para el problema de asignación del buffer que minimiza el inventario en proceso en líneas de producción abiertas en serie

El problema de asignación del buffer (BAP, por sus siglas en inglés) es clasificado como un problema de optimización combinatorio NP-Duro en el diseño de las líneas de producción. Éste consiste en definir la asignación de lugares de almacenamiento (buffers) dentro de una línea de producción, con el fin de aumentar al máximo la eficiencia del proceso. Los métodos de optimización que han sido reportados con mayor éxito en los últimos años son las técnicas metaheurísticas. En este trabajo, se propone un enfoque híbrido que utiliza las técnicas metaheurísticas de: Algoritmos Genéticos (AG) y Recocido Simulado (RS), con el objetivo de determinar los buffers requeridos que minimicen el promedio de inventario en proceso (<em>WIP</em>, por sus siglas en inglés) en líneas de producción abiertas en serie <em>M/M/1/K</em>. La evaluación se realiza con un método analítico de descomposición. Los resultados obtenidos demuestran la eficiencia computacional del algoritmo híbrido propuesto con respecto a un RS o AG estándar.

Development and evaluation of a Basestock-CONWIP pull production control strategy in balanced assembly systems

In this study, a new pull production control strategy called Basestock-Constant Work-in-Process (B-CONWIP) is proposed. It is used to control the flow of materials and information in balanced assembly production systems. This proposed control strategy uses one type of authorization cards called CONWIP card that limits the work-in-process (WIP) in the whole system. It has been applied in a single-product and a mixed-product assembly system balanced by two efficient Genetic algorithms introduced in literature. The performance of this control strategy is compared with another pull production control strategy called Basestock Kanban CONWIP (BK-CONWIP), which is a very promising production control strategy found in literature. The proposed strategy has two control parameters, CONWIP authorization cards and basestock levels while BK-CONWIP has three control parameters Kanban authorization cards, CONWIP authorization cards and basestock levels. The comparison is based on three performance measures average system WIP, percentage of satisfied customer demand (service level) and WIP variation between workstations. The performance of the proposed strategy B-CONWIP and BK-CONWIP is mainly similar in both types of assembly systems when mean demand rates are low with respect to mean service rates with the proposed strategy being easier to control and optimize. On the other hand, when mean demand rates are high with respect to mean service rates; B-CONWIP is preferable if service level is more important, while BK-CONWIP is preferable if WIP level is more important. Regarding WIP variation, it mainly depends on the efficiency of the balancing approach. The more efficient the balancing approach, the less WIP variation. Treating demand as lost instead of backordered results in decreased average system WIP and does not affect service levels in both PCSs. It is also shown that S-KDP is more flexible in dealing with situations of variable product mixes than d-KDP because control parameters can be used by any product which minimizes the effect of the unbalanced systems.

Robust production capacity planning under uncertain wafer lots transfer probabilities for semiconductor automated material handling systems

In this paper, wafer lots transfer probability (WLTP) is introduced to capture the flowing rate of wafer lots among production bays, which is uncertain due to various wafer types and quantities. We study a new production capacity planning problem for wafer fabrication systems with uncertain WLTP. Based on an open queueing network model, the average work-in-process (WIP) level of the system is evaluated. Because of the uncertain WLTP, the average WIP level fluctuates significantly and sometimes exceeds its upper bound. Therefore, we develop a robust production capacity planning model with two layers: the bottom layer for finding the maximum WIP fluctuation under a given vehicle quantity, and the upper layer for determining the vehicle quantities to minimize the WIP fluctuation and the probability of the average WIP exceeding the upper bound. A method based on the monotonicity of the objective functions is developed to solve such a bi-objective optimization problem.

The Analysis of Performances of an Assembly Line in Strike-Zone Supply Method Managed with Kanban and Conwip

The present paper is part of a larger study of the authors, which consists in the analysis of performances of several production systems. On a multiproduct assembly line supplied in Strike-Zone, are made researches on the influence of the method of production flow management, the method of launching in production and by varying the clients demand size on several performance indicators. The assembly line has mainly manual activities, divided in 5 workstations. The line produces two types of similar products. Using dynamic simulation (Arena software) that mimics the lines functioning mode, different experimental conditions are created, determined by the variation of the above mentioned parameters, following the evolution of the performance indicators: average work in process, cost per unit, systems reactivity to client demand and throughput. The obtained results were interpreted and structured, in a manner to help the process managers to take adequate decisions considering the wanted objectives.

The Analysis of Performances of an Assembly Line in Synchronous Supply Managed with Kanban and Conwip Methods

The present paper is part of a larger study of the authors, which consists in the analysis of performances of several production systems. The researches presented in this paper analyze the influence of the method of production flow management, the method of launching in production and the size of demand from the client on several performance indicators of a multiproduct assembly line supplied on stock. The assembly line is multiproduct type (are assembled 2 types of products that resemble as configuration and technology) and it has 5 workstations with mainly manual activities, that are supplied with parts using a synchronous method. The performance indicators considered were: average work in process, cost per unit, systems reactivity to client demand and throughput, their values being obtained by simulating the functioning of the assembly line in different experimental conditions, determined by the variation of the above mentioned parameters, using ARENA software. The synthesis of the results and their interpretation allow the process managers to take adequate decisions depending on targeted objectives.

Influence of Production Flow Management Methods on an Assembly Line Supplied on Stock

The present paper is a continuation of a larger study of the authors, which consists of an analysis on the performances of several production systems. In this paper it is analyzed the influences of the method of managing the flows, the method of production programing and the clients demand on performance indicators of a multiproduct assembly line supplied on stock. The assembly line is designed to produce two types of similar products and is made of 5 workstations with mainly manual activities, which are supplied with parts on a stock based method. The performance indicators chosen for analysis were: average work in process, cost per unit, systems reactivity to client demand and throughput. The values of these indicators were obtained by simulating the assembly lines functioning in different experimental conditions, determined of the variation of above mentioned variables, by using ARENA software. The obtained results were interpreted and structured, in a manner to help the process managers to take adequate decisions considering the wanted objectives.

Simplification of DES models of M/M/1 tandem queues by approximating WIP-dependent inter-departure times

This paper presents two algorithms to analytically approximate work in process (WIP)-dependent inter-departure times for tandem queues composed of a series of M/M/1 systems. The first algorithm is used for homogeneous tandem queues, the second for such with bottlenecks. Both algorithms are based on the possible combinations of distributing the WIP on the queues. For each combination the time to the next departure is estimated. A weighted average of all estimated times of each WIP level is calculated to get the expected mean inter-departure time. The generated inter-departure times are used in a simple model of the tandem queue. The inter-departure times, the average WIP and average cycle time of the tandem queue and the simple model are compared in several tandem queue parameterizations. Results show only a small error between the simple model and the tandem queue, rendering this approach applicable in many applications.

Optimization of POLCA-controlled production systems with a simulation-driven genetic algorithm

The present work is meant to show the effective capability of optimizing an unbalanced Paired-Cell Overlapping Loops of Cards with Authorization (POLCA)-controlled production system by means of a heuristic algorithm. This objective is suggested by the fact that one of the most significant issues when using card-driven production control systems is represented by the optimized setting of the large number of cards within the control loops. This is particularly true in the case of unbalanced systems, where the number of cards may vary significantly among the different loops. Little law is usually adopted in literature to infer this number from historical data, but the obtained number is usually far from the optimum. Indeed, in real-world applications, the systems to be controlled are designed to process units with very different routings, each with different probability to occur. In all these situations, they result particularly difficult to set correctly. To this aim, in the present work a Genetic Algorithm is used. The objective is that of finding the correct number of cards and to reduce the overall Total Throughput Time and the average Work In Process. The proposed approach may provide a valid support tool to overcome these limitations, making the most of POLCA capabilities in many manufacturing configurations.

Petri Net model of repetitive push manufacturing with Polca to minimise value-added WIP

A common simplifying assumption used in the literature is to minimise average Work In Process (WIP), while achieving maximum production rate, which does not consider the fact that the value of WIP increases down the stream of a production process as labour, time, energy and resources are added to it. This paper aims at minimising queues of parts waiting downstream of production process and in turn minimising value-added WIP by constraining more and more inventory of unfinished products at the earlier stages of production. For this reason, generic black token closed loop Petri Net (PN) model of Flexible Flow Shop (FFS) with Paired-cell Overlapping Loops with Card Authorisation (POLCA) is developed. Tokens in the control loops represent Polca cards to control the flow of material from order release till finished products. Marking of PN is done through Mix Integer Linear Programming after the computation of semi-positive P invariants. Simulation is being done to obtain queues of parts, waiting at each station during demand variation. Results were compared with PN model of FFS without Polca. Results showed the constriction of unnecessary WIP towards the initial stage of production instead of at bottlenecks, with proposed model, and in turn minimisation of value-added WIP.

Parallel kanban-CONWIP for production system with rework entrance

Parallel kanban-constant work in process (WIP) (CONWIP) (PKC) integrates kanban and CONWIP systems to regulate separately production of high- and low-demand items. This paper studies the performance of PKC variants under a production environment that requires the entrance of rework parts. A case study was performed at an electronics assembly plant with a three-stage asynchronous processing. Discrete-event simulations were conducted using an analysis enhanced by multi-factor ANOVA and response surface methodology. The studied performance measures were the total output, average WIP, flow times, and average bottleneck utilisation. A harmonising effect of the high- and low-demand items was observed when item composition was maintained at a certain ratio. The merge rework policy was consistently superior to the original rework policy. The categorisation in loading rule effectively reduced variations in PKC performance.

Performance Optimization of an Imbalanced Flexible Manufacturing System Using Taguchi Approach

Considering the diverse range of manufacturing capabilities of modern manufacturing systems, uniform distribution of load onthe workstations (WSs) is difficult to achieve as some of the WSs are required to process more parts than others or sometimes processing time of a WS can differ from one part to the other. These situations lead to unbalancing of the manufacturing system. The objective of this paper is to study and optimize the performance of an imbalanced Flexible Manufacturing System (FMS) under different operating environments. The FMS under consideration manufactures three types of parts having different processing characteristics. The experimental variables for the study are buffer capacities at the WSs, reduction in processing time of bottleneck WS, WSs processing time distribution, and parts release control. Throughput, average work-in-process (AWIP) and Average Throughput Time (ATT) are taken as performance measures. Taguchi approach is used to analyze the effects of above variables and establish the combinations of best factor levels to get the optimal performance. All variables are found to affect the performance to some extent. Reduction in processing time of the bottleneck WS and processing time distribution affected the performance of the system severely. The best factor level ombinations differ for parts having different processing characteristics. This paper may help industry in analyzing the performance of an imbalanced FMS.

Impact of volume, routing and product mix flexibilities on the performance of a job shop: a simulation study

In this paper, the impact of the factors batch size, setup times, priority dispatching of jobs, machine failures, rework, volume, routing and product flexibility on the performance of a job shop is studied. The performance of the simulation models are measured in four dimensions: mean flow time, average work-in process, percentage throughput of the shop and the percentage of jobs meeting their due dates. The simulation results show that routing flexibility and machine selection rules have significant impact on the system performance. It is also found that the system performance starts to deteriorate when the level of routing flexibility increases.

AWIP: A simulation-based feedback control algorithm for scalable design of self-regulating production control systems

A new simulation-based feedback control algorithm, called Adaptive Work In Process (AWIP), for the design of Self-regulating Production Control Systems (SPCSs) such as Kanban, CONWIP, base stock control and their generalizations is presented. The problem of minimizing average Work In Process (WIP) subject to a required throughput is solved. The AWIP algorithm is used as a feedback controller to adjust the WIP at various stages in the production system. The algorithm is synthesized based on the structural properties of SPCSs that are established analytically in this paper. In this approach simulation is used to provide the feedback to the controllers, and leads to an iterative numerical computational algorithm. Computational experiments show that the AWIP algorithm is near-optimal, and computationally efficient, which makes it an attractive approach for designing and controlling large production systems. [Supplementary materials are available for this article. Go to the publisher's online edition of IIE Transactions for the following free supplemental resource: Appendix]

Restricted work-in-process: A study of differences between Kanban and CONWIP

This article presents a simulation study over a small supply chain, where the amount of work-in-process (WIP) is restricted. The supply chain consists of five linked machines, or production facilities, with stochastic operation times. A number of test cases are made where the number of jobs in the machines and the buffer areas are restricted. The restrictions are designed both in the Kanban way, linked to every machine, and in the CONWIP way, connected only to the total production line. But no Kanban-cards and -cells are involved in our study, just restricted inventories between the machines. With the same amount of limited WIP, CONWIP-control compared to Kanban-control presents a higher throughput rate, less time between jobs out, but the jobs stay on average longer in the system. The stochastic operation times cause that the upstream machine sometimes consumes the jobs in a rate that the downstream machine does not catch up with, therefore all available storage room temporarily are not used. Kanban- and CONWIP-control presents the same amount of average outflow per time unit with the same variation in operation times and with the same amount of real average WIP. But Kanban-control causes a lower utilisation of present available storage room and storage equipment than CONWIP. The user of Kanban and CONWIP can only control maximum WIP and not average WIP; average WIP is a consequence of existing variations, so the difference is important. The coefficient of variation of the lead-times increases when WIP increases; this is very difficult to handle in practical applications. Restricted WIP that shortens the lead-time and decreases its variation is more important than if it is a “push” or “pull” system. Finally, it is argued that CONWIP-control is to prefer over Kanban-control in theory, but in practice there is a lack of CONWIP installation guidelines.

On the workload and ‘phaseload’ allocation problems of short reliable production lines with finite buffers

Serial flow or production lines are modeled as tandem queueing networks and formulated as continuous-time Markov chains to investigate how to maximize throughput or minimize the average work-in-process (WIP) when the total service time and the total number of service phases among the stations are fixed (these are the workload and 'phaseload' allocation problems, respectively). This paper examines both the effect of the kind of service time distribution on the optimal workload allocation in order to maximize throughput or minimize the average WIP of perfectly reliable production lines. The new approach of this work is the differentiation of the number of service phases of the service time distribution which is assumed to be of phase type at all stations of the flow line and the placement of storage space (buffers) between any two successive stations in order to examine the effect of these factors to the form of the optimal workload vector t and the optimal service phase vector ph.

Decomposition/Aggregation-Based Dynamic Programming Optimization of PartiallyHomogeneous UnreliableTransfer Lines

The general problem of buffers sizing for mean work in process/inventory minimization in a particular class of single part unreliable manufacturing flow lines, subjected to a constant rate of demand for finished parts, is analyzed. Two variants of the problem are considered: buffers sizing for average work in process minimization when there is a fixed requirement on parts availability in the buffer next to last; minimization of an aggregate measure of average work in process and demand backlog when the complete flow line is considered. A fluid model of part production is employed. The production control policies of interest are suboptimal, strictly decentralized, and are unambiguously parameterized by the size of buffer levels. Optimization of policy parameters is based on the analysis of the structural properties of an associated dynamic program. The latter is built around an approximate, flow line decomposition based, buffer levels dependent theoretical expression of the policy performance measure. The nature of the related flow line approximations is discussed and numerical results of the dynamic programming procedure are reported. Scalability of the computations is demonstrated. The numerical results suggest that when parameters are optimal, both a form of flow line balancing and a just in time internal production principle, are in place.

Minimizing WIP inventory in reliable production lines

This work deals with the well-known buffer allocation problem in short reliable production lines. The objective is to find the optimal buffer allocation (OBA) that minimizes the average work-in-process (WIP) inventory, subject to a minimum required throughput. The study leads to some insights concerning the evolution of the throughput and WIP as a function of the ordered buffer allocations. In the case of balanced lines, it is observed that the so-called self-similarity phenomenon prevails. It is also pointed out that the choice of the throughput level is critical for finding the OBA that minimizes the average WIP. Also a heuristic algorithm is proposed to find the OBA, which reduces the search space by over 50% compared to enumeration.

A computer simulation study of manual flow lines

A flow line production system having finite intermediate buffers and random processing times has been simulated to determine the system efficiency and the average work inprocess. From the simulation results, simple and reliable empirical equations for determing the efficiency of the line have been developed. Analysis of the buffer utilization results show that in a balanced system, work in-process remains high in the first few stages of the line, and low towards the end of the line. It has also been observed that the balanced line (equal buffens) is generally the most productive, and that a small imbalance, due to larger buffers in the center of the line, has little effect on its performance.