Rajan Suri’s work | Review Practitioner’s Guide to POLCA | Conclusions

POLCA is a card-based production control system for high mix/low volume, and customer-specific manufacturing. It helps to decrease the throughput time on a shop floor with production cells, and products that have to follow their own route during their assembly or manufacturing.

20 years after he invented POLCA, Rajan Suri published a book which reviews the possibilities and experiences: The Practitioner’s Guide to POLCA. This book helps you to decide: POLCA yes, or POLCA no? It is also a guide to implement it. A very comprehensive and well-written book, be it with a little bias towards the advantages of POLCA.

The first part of this article summarizes Rajan Suri’s work in the area of Quick Response Manufacturing. In the second part his latest book is reviewed and some conclusions about the applicability of POLCA are drawn. Finally, Rajan Suri comments on the review of his book.

Rajan Suri, emeritus professor of industrial engineering at the university of Wisconsin-Madison, introduced a new process improvement method called Quick Response Manufacturing (QRM) in 1998. He described this method in his magnum opus Quick Response Manufacturing: A Company-wide Approach to Reducing Lead Times.

Like Lean manufacturing, QRM aims to improve the flow, or throughput. The most important difference is that QRM was developed specifically for high-mix, low-volume and/or customer-specific production.

Waiting time reduction
Besides this, QRM focuses not on the elimination of waste in general, but only on the reduction of waiting times. Therefore it has only one performance indicator:  the Manufacturing Critical-path Time (MCT), which is the time from getting the raw materials until the product delivery. The underlying idea: Reduction of the MCT reduces the overhead, and consequently the non-value adding activities, the most.

Quick Response Cells
Important QRM tools are the formation of Quick Response Cells (QRC’s) and the application of POLCA to control their workload. QRC’s are flexible and multifunctional production cells. These compact cells contain their own machines and/or office tools, and are typically staffed with three to ten cross-trained people. Each QRC can autonomously complete the sequence of tasks needed for all jobs belonging to one Focused Target Market Segment (FTMS).

Mini-companies
By dividing a company into such ‘mini-companies’, customer-specific orders can choose their own route along a selected number of these, depending on the processes needed. This concept, although refined in QRM, was not entirely new when Suri introduced it.  In logistics, it is called a ‘job shop’ environment. There is also an overlap with the idea of autonomous teams, that attracts a lot of renewed attention today. Suri’s main contribution was that he described what the best way is to design the QRC’s, with the idea to reduce not only the lead times inside of them, but also between them.

POLCA-cards regulate the sequence of the jobs in the Quick Response Cells

Traffic
To regulate the traffic – the flow of tasks - between the cells, he introduced an entirely new tool, called POLCA. This is an acronym for Paired-cell (with) Overlapping Loops of (circulating) Cards with Authorization. In his new book, The Practitioner’s Guide to POLCA, Suri explains what POLCA is, how to implement it, and what lessons were learned in six companies that applied it.

Before this book is discussed, it is recalled why POLCA was developed, and how that system works (A similar and far more comprehensive description can be found in the book).

Kanban
In Lean, to reduce (intermediate) stock, fixed production lines for product families are formed, controlled with Kanban-cards. After consumption of a small amount of a certain item, a Kanban-signal is send to the preceding work cell. The message is: “I used this specific item, please send me a new one”. So, Kanban supports pull or just-in-time production.

However, only a limited number of product variants are allowed, because for Kanban it is necessary to keep intermediate stock for all product types. In a job shop environment, with customer-specific products and varying production routes, Kanban does not work. In that case the amount of intermediate stock controlled with Kanban cards becomes infinite.

A POLCA card A/B, with on the left side the supplying cell A and on the right side the receiving cell B, gives cell A permission to work on semi-finished products which will need further processing at cell B .

POLCA
To solve this problem Suri developed a Kanban-variant, called POLCA. Like Kanban, POLCA-cards aim to reduce the amount of intermediate stock between work cells. However in this case only the amount of stock (or workload) is controlled, irrespective of what this material is.

POLCA-cards send the following message to QRC’s upstream: “I just finished one of the jobs you sent me, so you are allowed to send me a new one, since I now have free capacity available to process it”. This way, POLCA regulates that the people in the QRC’s work only on jobs than can continue to flow, because there is free capacity available downstream, instead of piling up and causing traffic jams. Other jobs can better wait a while!

POLCA also limits the amount of work-in-progress on the factory floor as a whole. In addition, it instructs the QRC’s to work only on jobs for which all materials are available, and which have the highest delivery-priority. I will come back to those aspects later.

Pairs
POLCA connects all pairs of production cells that can exchange materials, with circulating POLCA-cards.  A POLCA card A/B, with on one side the supplying cell A, and on the other side the receiving cell B, gives cell A permission to work on semi-finished products which will afterwards need further processing at cell B. See the picture below.

The POLCA-system. The arrows show the route that is travelled by an intermediary product, along Quick Response Cells A, B and C. First, an A/B-card travels with the product, next a B/C-card. Within cell B, both cards travel with the product. (source: Rajan Suri, adapted for this publication).

When the work is ready at cell A, the product is sent to cell B, together with the A/B card (or with multiple A/B cards, if there is much capacity needed at B to process this job)

Now assume that the next destination for this semi-finished product is C. Then cell B needs a B/C (permission B to C) card to work on it. So, two POLCA-cards travel inside cell B with this particular semi-finished product, a card from the cell it came from (A/B) and a card for the next station to send it to (B/C). This explains the word ‘Overlapping’ in POLCA, Paired-cell Overlapping Loops of Cards with Authorization.

When the work is finished at B, the A/B card is returned to cell A. This is the signal for this cell that free capacity has become available at B, so it’s again permitted to execute jobs that need processing at that cell later (then, a new POLCA-cycle starts).

This workload control system is comparable to traffic lights, which give traffic access to quiet motorways, but block the entrance to heavily used roads (a metaphor for QRC’s that at a particular moment do not have the capacity to process jobs).       

Authorization
Now, what about the word ‘authorization’ in POLCA? To prevent a ‘traffic jam’ at the entrance of the shop floor, only a limited number of production orders are permitted to start their journey. Their selection is done by the first cell, the planning cell. Starting from the desired delivery date of a production order, the people in the planning cell calculate back (with help of the ERP-system or with a simple spreadsheet) when this order should be released, to be finished in time. Some buffer time is added.

The result is a list of authorization dates for all orders. On those dates, after checking that all materials to process them are available, those orders are released.

However, the planning cell does more. The people in it also calculate secondary authorization lists for each QRC, which are refreshed for example once a day. Those lists specify the dates at which specific orders should be received and processed further at specific QRC’s, to be ready in time.

Release-and-flow
Most of the early implementations of POLCA did not use the secondary authorization lists, but applied a simplified form of POLCA, which Suri calls release-and-flow POLCA in his new book.  This form of POLCA might be the one that is known to most people. I myself did also not know that it is a simplified form.

So, the in-depth description of ‘standard POLCA’ as Suri now calls it, might already justify the publication of his new book. Standard POLCA and Suri’ s book will be discussed now. Next, some conclusions are drawn regarding the applicabilty of POLCA.

> Continue to read in part 2!