شبیه سازی عملکرد یا کار JIT در یک کارگاه چاپخانه SIMULATION OF JIT PERFORMANCE IN A PRINTING SHOP

ABSTRACT A medium sized UK based academic publishers own a subsidiary printing business. Presently the Academic Printers (AP) is experiencing productions line flow problems reducing the efficiency of the operation. Most of the problems are generated by the imbalanced workflow through the system. By implementing a JIT production planning system it is hoped that some of the production problems can be resolved. Using the simulation software a model was created to investigate the performance of the AP under a variety of operating conditions. Results showed that operating the system with JIT control would not produce economic performance improvements due to constraints applied by the printing process. 1 INTRODUCTION Printing operations scheduling is a challenging task because of the many constraints imposed by the processing technology used. The current scheduling practices are mostly driven by the nature of the printing processes resulting in a considerable amount of work-in-processes (WIP), long waiting times and long lead times. Facing severe competition from South East Asian printing companies many western publishing companies have been forced to reconsider their traditional printing operations. Some of the publishing companies turned to subcontractors to eliminate at least some of the operation problems. However, this has created some other unpredictable problems such as quality, style and design. Just in time (JIT) production is generally referred to as a manufacturing system for achieving excellence through continuous improvements in productivity and the elimination of waste. The ultimate goal of JIT is a ‘balanced’ manufacturing system, which is one that achieves a smooth rapid flow of materials through the system. A vast number of JIT-based research activities has been reported and described in various professional journals or conference proceedings. The early studies have concentrated more on philosophy and fundamentals of the JIT system investigating what JIT offers as a new production system and how it could be implemented (White et al, 1999; Yasin et al., 1997; Huson and Nanda, 1995; Fullerton and McWatters, 2001) The design and analysis studies concentrate more on shop floor implementation of JIT to control the production (Savsar and Al-Jawini, 1995; Baykoc and Erol, 1998; Pandey and Khokhajaikiat, 1996) These studies have been extended to Kanban allocation and the number of Kanban that should be used to create a streamlined production system (Upton, 1998; McMullen et al., 1998; Huang and Kusiak, 1998; Thesen, 1999; Savsar and Choueiki, 2000; Grosfield-Nir et al., 2000) The AP is a medium-sized UK based printing company, which specializes in producing high quality academic texts for publishers worldwide. The system at present suffers from high WIP due to unbalanced production. In this paper, we implemented a JIT shop floor control system through a simulation model to improve the performance measures in this printing company. We have created several real life-like scenarios to analyse the performance of the system under different operating conditions. The performance criteria considered are WIP, lead time, production rate, and throughput time. For the purposes of this study the AP is simplified into its core printing activities. Other support roles carried out by AP such as plate manufacture and jacket and cover printing will be assumed to be external activities, which when required in the model will always be available. 2 SHOP FLOOR PERFORMANCE IN PRINTING OPERATIONS Nomenclature Job -An order of a batch of books Batch Size -Number of books required in a Job Cased Job -Hardback book Patterson, Ozbayrak, and Papadopoulou Signature -Bundle of pages (normally 32), which is created by folding and cutting a sheet and gluing a number of signatures together forms a book. Sheet -Large sheet of paper, which is run through the Printing Presses and has a signature printed on it by the Plates. Plate -An aluminium sheet having a printing surface produced by photographic deposition 2.1 The AP Scheduling System AP can be described as operating a job shop facility as it produces a high variety of batch types and sizes in a make to order environment. A book may consist of only four different parts; pages, cover, binding and case but these are unique for every different type of book produced. The machines visited by a book during manufacture depend on the specification of the job, for example if the book is colour it has to be printed on a colour press. This adds to the complexity of the manufacturing process as the process flow varies from job to job. Due to this high variability of both batch sizes and book types scheduling at the AP is a complex and difficult task. The AP Planning Department uses an assembly line balancing approach to scheduling. This technique is used in an attempt to utilize workstations to their full potential by trying to ensure that bottlenecks are not created and workstations are not starved by the workload in the system. The Planning Department tries to balance the workload in the system to maintain machine utilisation and control WIP. It is important to remember that every Job is unique and has different process times on a varying combination of machines. When a new job is sent to a workstation the workstation must be set-up to handle that specific job. Orders for jobs are received by the AP from the Planning Department in the form of a machine loading list. The loading list is broken down by machine name. When a press finishes printing a job it is assigned the preceding Job. The system at present suffers from high WIP due to unbalanced production. A key factor in this is the high variability of the batch sizes and signature numbers in the Jobs. For example an order for 50,000 books would at present be printed in one transfer batch creating high WIP and an inflexible unbalanced environment. Machines become blocked with work and throughput times become unpredictable.