Companies with high product variety are faced with finding the right material control policy which is a challenge in the workshop. Under such circumstances, Kanban or generic CONWIP system do not help to meet the company’s needs. There is another way known as to use the Paired-cell Overlapping Loops of Cards with Authorization(POLCA) method. POLCA is a strategy of mixed pull and push designing for the environment of production with a high mix low volume. At my paper, I am going to use this method applied to a job shop environment by using the real data from a traditional Chinese company. The production system will be tested by comparing the output from the current system. I am going to use the flow time, throughput and work in process to evaluate the performance of the new production system and current system. The results show the better one is the system of POLCA. The manufacturing environment of the chose company is facing the problem of highly customized product and high demand variability.
The common issues concentrated by manufacturing today is on the how to improve the flow time, work in process and throughput. In this scenario, known as job shop, high product mix, various production volume and the product will be undergone through different kinds of process routes. There is no dominant flow pattern. However, it requires different workstations with different product routes. Beside all of this, one significant issue is the adoption of suitable mechanisms of production and material flow control. An advanced technology combined with good PMFC mechanism can result in the great improvement of performance and quality of manufacturing. Not all the material control strategies works under the environment of job shop. The very famous kanban system does not apply to job shop environment . However, over the years, there have been a lot of types of material control strategies proposed for solving these issues. Among these strategies, a new approach known as Quick Response Manufacturing was invented by professor Rajan Suri in order to response to this highly dynamic market.
The main propose of QRM is to reduce lead time. The principles of are different from lean method which mainly pay attention on how to standardize on the process and products and reduction of waste. QRM focuses on how to reduce lead time. As part of overall strategy, there is a new strategy of material control known as POLCA introduced into the job shop environment.The main difference between POLCA and Kanban system is POLCA is working as a capacity signal but Kanban is working as inventory replenishment signal. Under the circumstance of the QRM Organization, how POLCA is working. POLCA has four key features incorporating aspects of MRP and kanban which enable a company to customize their products while giving it control over congestion and excessive WIP. How does Card-based material control method works. It was used to communicate and control the movement of material between cells. Instead of being specific to the product, the production cards which is called POLCA cards are assigned to certain pairs of cells.
Since the propose of QRM is to reduce lead time to response quickly to customized orders, if there is a routing for orders going from cell A to cell B, then a pair of cell A and cell B is allocated a number of POLCA cards, all of them called A/B cards, how to choose the number of such cards is going to talk later. The planning horizon might be longer than the lead time, so you should know how to forecast the sale by aggregate families to choose which routing might be used. For each pair of cells, they stay with the job during its journey through both cells in the pair. For example, if A/B card is assigned to a job as the job enters into cell A., the A/B card would stay with job until the whole job completed at cell B. When the job moves to the next cell, this A/B card would be returned to cell A. Most of the cells would be assigned to more than one pair, there would be multiple loops of cards that overlap in each cell,
The data in this paper is from real world as a steel company. The simulation model before and after accomplishment of POLCA in this job shop environment. The company is located at Jiangsu.China. The company is a steel roll manufacturer producing roll. The process of this manufacturer includes roughing, heat treatment, machining, quality and delivery. During the process, in other word, each cells would be assigned to one pair, and each pair would be assigned to different loops based on the different products and volume of demand. The routes and process for each product will be different and volume of demand from customers.
There are three types of manufacturing system which are push system, pull system and hybrid push/pull system. The flow control of material have been developed such as material requirement planning, just in time and workload control. These are used to increase the performance of shop floor. There are strategies of traditional pull control like Kanban and CONWIP but with some limitation when they are applied to a low volume and high mix environment. For example, in the kanban system, this system requires a minimum inventory in the each workstation, the signals will be sent to the upstream to begin working as soon as the inventory is taken by the downstream process. However, if company use this concept to the high mix and low volume environment, it will definitely cause high WIP at each stage. In order to deal with this issue, Professor Suri developed POLCA in 1998, as mentioned above, the basic concept of POLCA is based on the cards to show the free capacity between two cells. The shop floor is operated by the the release of authorization and POLCA card. What is authorization release system , it is in POLCA to suppose that every cell as a black box and make plans of material flow between cells.
According to Land and Gaalman, the performance of shop floor throughput time and on time delivery are impacted by the when to release to the floor and what to release to the floor. So the system of POLCA does not only work at the shop floor but it also decides on at what time to release the orders to the shop floor. The figure2 how the POLCA card works to a particle order. According to the flow AB-C, the order will be operated into this flow. Before starting the first operation in machine A, the order must be allocated to A/B POLCA card. A/B card will be sent together to machine B after the job at A finishes. At B, the order will be waiting for the next card B/C. Until the job at B finishes, the A/B card will be sent back to machine A. Until all the process completed, this process will be carried on. In order to calculate the numbers of cards needed for loop, there is equation(1). Let me briefly explain this equation. La and Lb are lead time of cell A and cell B, Fa/b is the number of jobs going from A to B during certain period. D means planning time.
In the next figure, it has shown the simulation of overall procedure of POLCA. Firstly, the data was collected from shop floor. Then, the conceptual model was developed, After this model was proved, a base model would be built. Meanwhile, the paired loop cards was built. Then you need to predict the parameters in the POLCA model. This process is not very hard to build and all the parameters in this model are mot hard to predict. In order to give readers a clear image, next part is going to show some examples of parameters calculated for POLCA model.
There are a few analysis which have been proposed for the study of POLCA mechanism. At first place, those researches are talking about POLCA at planning and implementation. Krishnamurthy and Suri were talking about the procedure of using POLCA and POLCAâs features and operation4. The results of implementation indicates that POLCA improve the satisfaction of employees while improving operationâs efficiency. In 2006, a newly proposed POLCA material control system which is called Generic POLCA proposed by Fernandas and Silvo . GPOLCA starts to process job when all the desired GPOLCA cards are available. Compared with MRP and POLCA, it is the best production control strategy for make to order and multi-product1. There is another analysis about hoe to increase effectiveness of POLCA strategy. It is called load-Based POLCA developed by Fernandas and Silvo . This system combined with an system of advance resource planning system. This system recognizes that manufacturing systemâs nature and has high-level planning tool.
There are some previous work to compare POLCA with other material control system. One of the studies is to determine the workload balancing capability of CONWIP, POLCA and m-CONWIP . The simulation results showed that CONWIP has no workload balancing capability. The control workload and achieve same utilization level in a CONWIP controlled make-to-order environment has increased the total throughput time of orders. POLCA shows a better workload balancing capability than CONWIP, but they are not perfectly notice signal and sometimes imbalance in workload . This paper shows that m-CONWIP has the best working balancing capability.
There are some previous work with other material control system is relatively POLCA. One of the studies was to determine CONWIP, POLCA workload balancing capabilities and inter CONWIP . Simulation results show that that did not work CONWIP load balancing. Control workload , and realized makeup CONWIP control in the same order production utilization levels increase the overall throughput of the order of time . POLCA shows better workload balancing capacity than CONWIP, but they are not completely notification signal , sometimes unbalanced workload [ 5 ] . This paper shows that , M- CONWIP have the best job in balance. As the result of this POLCA cards system, all the cells are attached to their former and next cells. However, how does the organization plan the total and complete process. As I mentioned in the above text, usually there is a planning system called HL/MRP which is the final and essential step to complete the whole POLCA system. This high level MRP has the following functions. The one is to predict the need for and the ordering material fro external suppliers and the other is the coordination of material delivery
across all the cells. POLCA usually does not use the MRP planning system because this system looks over micromanagement of every working station. Instead, the POLCA uses high level planning and coordination of material. The HL/MRP sees each cells in the logic of MRP. In this way, only a few steps will be scheduled. This way will result in a simpler system than regular MRP with traditional departments which are organized by function. HL/MRP is driven by the specific demand such as customer demand, sales forecast or the combination of both. After the release of authorization of schedule from HL/MRP, these schedule are given to each cells. Based on these schedule, each cell will develop each of its own lot sizing and forces work to achieve the desired schedule. If the period when it can not meet the targets or when its lead times is different from the one given by the system of HL/MRP, it gives the feedback to the system of HL/MRPã,Why do we choose HL/MRP? Because we need a simpler planning. There are three reasons. The first reason is fewer steps to schedule and control. The ripple effects can quickly become unmanageable while three steps can easily be under control. The second one is with HL/MRP the cells are more likely to keep their ends of bargain and deliver product on time. The third one is the cells can be given realistic and clear targets. In this paper, the main method of material control strategy is POLCA, but it is necessary to describe another strategy which was developed from POLCA strategy. This strategy is G-POLCA. One studies was done on the analysis of POLCA and GPOLCA material control strategies. By this study, the result of previous studies were confirmed again. For achieving the same throughput on the shop floor, GPOLCA has low WIP than POLCA. However, in order to achieve the same level of service, results reveals that GPOLCA gains a longer response time and higher total inventory than POLCA8. Then, let me briefly describe the mechanism and operation of GPOLCA. GPOLCA is like POLCA controlling the release of order according to the combination of cards pf production authorization allocated to the paired of cells and the release of dates. So GOPLCA is a PMFC mechanism for ARM that adapts ideas from the POLCA mechanism. Instead of using inventory of material strategy to implementing input and output control order release strategy like POLCA, GPOLCA is doing this on the basis of an inventory of production authorization cards. GPOLCA uses cards to control jobâs numbers such as WIP in the system. Similar to the POLCA cards, GPOLCA cards are to designed specific to any part but are to the particular pairs of work centers or cells, especially product oriented cells.. When POLCA are finishing the strategy of pull material flow control, GPOLCA implements the strategy of the push type control aiming to the reserved capacity associated with the cards of production authorization. Moreover,
if these cards are needed to process in the same particular pair of cells, they can be reused and distributed to different particular jobs. Workload levels for each paired of cells will be set by GPOLCA cards for the whole system. The work in process for the pair of working cells will be under the control by setting the number of GPOLCA cards. GPOLCA card flow for a particular job or order in a manufacturing system. Then I will demonstrate a the flow of GPOLCA card for a particular order or job in the system of manufacturing. At first, GPOLVA cards must be available for being distributed to jobs. In the routing of a job offer, beginning from the last cell,successive distribution of cards to orders, to make sure that capacity is reserved to allow the job released into system from the upstream pair of cells. The flow of job and cards must be following the controlling below.
1 The cards which are required from each pair of cells in the job routing are distributed to the job when released in the system. In fact, a job may need more than one card in each pair of cell. It depends on the load involved with GPOLCA card
2 When the job arrive to a cell, job can be started as soon as possible, all necessary material and resources are prepared, scheduling mechanism should be used to decide which job to go first.
3Until the moment the downstream cell in the pair of cells, GPOLVA cards stay with the job when the job is released, completes the processing of the job.
4When the job is completed at customer cell, the allocated card stay available for allocation to another job. GPOLCA releases jobs when the cards necessary by a job at each pair of cells in the job routing become available and distributed to the job. However, on the other hand, POLCA uses job release as long as cards for the first pair of cells in the job routing are standby.
The procedure of the GPOLCA to release an order into a manufacturing system is as follows;
1 A planned release date is decided by scheduling of backward from the due date,using the planning lead time.
2Based on the earliest planned released data, each data is considered for release
3When the workload is below the planned level in the required pairs of cells, each unreleased order waits in the pool.
4.In the routing of the job order, the release of order will happen when the cards which are required from each pair of cells,are all standby to the distributed to the order