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INTERDISCIPLINARY JOURNAL OF CONTEMPORARY RESEARCH IN BUSINESS
JUNE 2011
VOL 3, NO 2
2.1. Push Systems
A push system supports batch manufacturing operations. In batch manufacturing, the materialplanning
department of an organization typically releases a schedule that is developed by taking
into consideration the lead times of various components and subassemblies (Vollmann et
al.,1997). These lead times consist mainly of the waiting time and the queue time for a particular
component as the actual processing time only constitutes a small part of the total lead time. The
main objective of the push system is to effectively utilize the capacity of each work center. This
type of manufacturing system schedules shop orders by taking into consideration the routing of
every component. The criterion used to allocate the schedule is the due date of the finished
product. As MRP systems have evolved, it has become possible to track the progress of the
components produced throughout the shop floor. In the mid 1970’s, Orlicky introduced the
concept of ‘Material Requirements Planning’ and is recognized by many as the father of the
modern MRP system (Vollmann et al.,1997). Various studies have been conducted to understand
the performance of MRP or push systems. Suri, Krishnamurthy and Vernon (2004) identify the
following key issues in modeling a push system:
• Estimating release lead‐times
• Modeling future requirements for different products
• Determining the safety lead‐times and stock
Buzacott and Shanthikumar (1994) conducted research that studied safety stock vs. safety leadtime
in a MRP-controlled production system. Their research proposes that safety lead time is
preferable than safety stock whenever it is possible to have a good forecast of the future
requirements. But in cases where it is only possible to predict the mean demand then either of the
two can be used. Even Suri et al (2004) conclude that in case of single product systems where
future demand is accurately known it would be more appropriate to implement push strategy
with safety lead times.
2.2. Pull Systems
Pull (kanban) strategies have been the subject of numerous studies by researchers (Uzsoy et al.,
1990; Berkley, 1992; Liberopoulos &Dallery, 2000 ; the references therein) . A kanban system is
a common example of a pull production system that is extensively used in industry. The Toyota
Production System has made the word kanban so synonymous with the pull system they practice
that pull systems are often referred to as kanban systems (Ohno,1988). In Japanese, kanban
means display or a card. A kanban in a pull system is an authorization to produce. It is a signal
from a downstream manufacturing process that prompts the upstream process to begin
production of a component that has been consumed from inventory. The two things that have
become integral to the pull type manufacturing system are:
• kanban cards
• Standardized containers
The containers are used to standardize the production batch sizes, protect the material from
damage and also assist in maintaining visual control of the WIP inventory
(Hyer&Wemmerlov,2002). Though kanban type pull systems are most commonly used, there
exist other pull systems, like POLCA (Paired-cell overlapping loops of cards with authorization),
CONWIP (Constant WIP), and DBR (Drum-buffer-rope) (Hyer and Wemmerlov,2002). POLCA
is a generic card system in which the cards are circulated immediately after a product exits the
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