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CONWIP (CONstant Work In Progress) is a lean methodology for implementing 'Pull System'. A CONWIP card is associated with a certain amount of quantity. Once the part is consumed, the card goes back to the start of the process. On the way back, it checks the backlog items and manages the demand of parts within the process

 

An application-oriented question on the topic along with responses can be seen below. The best answer was provided by Raj Saxena and Selva Mariappan Subramanian

 

Applause for all the respondents - Pradeepan Sekar, Kishan Raval, Raj Saxena, Satyajit Das, Ram Rajagopalan, Selva Mariappan Subramanian, Himanshu Pathak, Ibukun Onifade, Tushar Maradwar

 

Also review the answer provided by Mr Venugopal R, Benchmark Six Sigma's in-house expert.

Question

Q 267. CONWIP (CONstant Work In Progress) - is an approach to implement pull system in a process. Describe CONWIP and explain advantages and disadvantages of using CONWIP against KANBAN with the help of examples

 

Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday.

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CONWIP

 

CONWIP stands CONstant Work In Progress. CONWIP is a one type of pull system which uses a single global set of cards to control Total WIP anywhere in the system or shop floor.

Raw material can enter in CONWIP only when there a demand, it receives through the authorized cards and this same card authorize raw material to move throughout the system till production completion. This cards will only be released when the final product moved out from system and then this card will be used to enter new material in system as per demand.

In CONWIP system, WIP is not controlled at the individual workstation level. In CONWIP Card limit the total number of work that can be anywhere in system.

In CONWIP system, when demand occurs and material enters in the system, material flows freely similar like Push system and WIP accumulates in front of the bottleneck stations.

CONWIP is easier to manage as there is only one set of Global card that needed review and adjustment. Kanban System is more difficult to manage but it support strict WIP Control system as its card control is implemented at workstation level.

CONWIP differs from Kanban in three mains ways:-

1.       Use of backlog to dictate the part number sequence

2.       Cards are associated with all parts produced on a line rather than individual part numbers

3.       Jobs are pushed between workstations in series once they have authorized by a card to start at the beginning of the line.

 

 

Advantages and disadvantages of using CONWIP against KANBAN

 

Advantages of CONWIP against KANBAN

1.       Number of Variants

Kanban works with high quantity but low variety because every card has permanent number associated with the card. And this works when there is a continuous demand for specific product number.

                Example – Made to stock parts that sold in large quantity.

                                                       image.png.03d5dfe933d72e33776df54965f5079b.png image.png.cb4a46603c7e438503ce566abed3c129.png

                Above both the examples show single variant product which is easy to manage even with high quantity. In both examples product is unique then Unique Kanban require for this product. As we know Kanban is assigned to a part number then it makes difficult.

 

 

 

 

Here CONWIP wins as it has no part number assigned by default to CONWIP card hence it can be assigned temporarily to CONWIP part even if the part is produced once. And it’s more suitable for made to order parts. 

Example – Made to order parts that sold in large variety.

             image.png.c5622c2aa3beac3c9afa7113aac08879.png     image.png.00ea3c54d5396d7f22dc40f0f657caa8.png      image.png.133eac1d6dd386a35ba8ad1d59748d3b.png   

 

 

2.       Better Performance than Kanban

In a single line, CONWIP System has better performance than Kanban system with N circulating card.

Lets look at with the below explained example. Here we are comparing two Kanban and CONWIP system applied in an assembly system given the same number of circulating cards within the system. Consider an assembly system fed by two fabrication lines with three stations for each fabrication lines as well as the assembly line in below picture.

We can see the below picture giving us  the simulation results consist of systems throughput and average WIP of both Kanban and CONWIP systems, when the total number of circulating cards within the both systems is identical and equal to 13.

image.png.bf8b7ac189f97a85f638e0483224ee53.png

In above picture, k denotes the number of kanbans at i station of j fabrication line in the Kanban system. That is, 11 k , 12 k and 13 k denote the number of kanbans at stations 1, 2 and 3 of the first fabrication line, respectively. Similarly, 21 k , 22 k and 23 k denote the number of kanbans at stations 1, 2 and 3 of fabrication line 2, respectively. 1 k and 2 k also denote the number of kanbans at stations 1 and 2 of the assembly line, respectively.

So in above picture, average WIP and throughput in the both systems given the same number of cards

 

image.png.b3f1f45103af86628f317f9b159f59fd.png

 

From the table above, two systems has the same rate of throughput (equal 3.75 parts per hour) and, average WIP equivalent 1.09 and 1.45 parts achieved by Kanban and CONWIP system, respectively.

Henceforth, when the total number of cards in both Kanban and 28 CONWIP systems is equivalent, average WIP can be less in Kanban system than in CONWIP given the same level of throughput. Since final demand is unlimited, the number of Kanban’s at the last station of the assembly line is not relevant in Kanban system. Thus, the last station (station 3) is not shown in the above table.

 

Disadvantages of CONWIP against KANBAN

1.       Doesn’t manage the sequence

Kanban manages production sequence automatically. Kanban system automatically reproduced what is required to produce if this system has enough Kanban for each part type.

Whereas CONWIP needs human input to reproduce the required produce.

 

Assume you have a system with 50 CONWIP cards. If someone messes up your backlog priority and puts 50 times the same product as a priority, then your entire system is full with this one product. You cannot even produce something else unless you sell one of the other products.

image.png.30a67a322be77415a1dd3bd110d0f53b.png

 

Source:-

http://webuser.bus.umich.edu/whopp/Reprint/CONWIPPullAlternative.pdf

https://www.informs-sim.org/wsc01papers/122.PDF

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.453.6307&rep=rep1&type=pdf

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CONWIP

 

CONWIP- Constant Work In Progress is similar to KANBAN for maintaining constant work in progress. In this method WIP is aggregated and treated as a whole. This methodology is used for effective production control. It uses pull principle.

 

Backlog list —>   CONWIP —> demand   

 

Push- product jobs are scheduled.

Pull- product jobs are trigged to start on completion of previous jobs. (Demand based job or backlog list completion).

 

CONWIP works on demand basis. Batch of job is completed and leaves the facility then the signal is sent to release the next batch of jobs from backlog list.CONWIP relies on signal to start working. Backlog list is organised in order to customer demand and low finished goods inventory.

 

N= Amount of WIP in a system

 

If N is larger it indicates long lead times and excess WIP.

If N is smaller it indicates idle workstation and reduced output from system.

 

To determine N first identify bottleneck station then gradually increase N until bottleneck is rarely empty from input queue.

 

Card count (m) = Maximum WIP level for the line

 

Production quota (q) =Target product quantity/Period

 

Maximum work ahead amount = N

 

If q+N is produced during a period then the line is stopped until the start of next period.

 

 

Merits of CONWIP over KANBAN

 

1. CONWIP system immediately react to increase in customer demand because only high demand jobs are released into the system.

 

Example:

Factory produces two products A and B. In case demand for product A increases then WIP constant is increased to product A. Thus results the factory produced more product A on demand basis. So control over demand (input) increased as a result throughput (output) achieved.

 

2. CONWIP system do not need the setup and maintenance of KANBAN cards and containers for each part type.

 

Example:

Automobile industries need large setup to place containers to manufacture all the parts while using KANBAN method.

 

3. CONWIP Systems reacts well for highly variable part mix demand while maintaining constant levels of inventory.

 

Example:

The inventories are constant in CONWIP system only the manufacturing of products are varying on demand.

 

 

Demerits of CONWIP over KANBAN

 

1. KANBAN is very useful in repetitive manufacturing where CONWIP is not effective for such constant demand based manufacturers.

 

Example:

In stores constant refilling of products was easy while using barcode reader. It is one of the application of KANBAN. Here based on demand the products are selling and refilling is a repetitive work.

 

2. CONWIP systems are more complex in means of maintaining backlog list here backlog list is the responsibility of inventory control staff.

 

Example:

Calculation of backlog list is based on customer demand. Tedious process to maintain those backlogs in priority wise.

 

3. CONWIP requires ample storage space before machines because WIP as a whole is constant. WIP is not controlled in each work place. 

 

Example:

By using KANBAN inventory system, Toyota manufactures car parts. Here the parts are delivered to product line when there is a need. So there is no need of ample storage space and that saves space on product line to assemble products by workers.

 

4. Over a time as demand change the unwanted WIP may clog up the system in turns making the system less responsive one. To overcome it periodically cleanse the unused WIP.

 

Example:

One of the software companies fixed WIP constant as 3 developers and 2 testers for a project. But customer demands demo of the product as a first phase before final handover. Here for the first phase necessity for the developer is higher than testers. So, it leads to delay in outcome.

 

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CONWIP

Production system and Inventory control:

 

Production system and inventory control is often classified as two system 1. Push System and 2. Pull System.

 

In Push System, Planning involves forecasting of the customer demands and inventory needs to be maintained to meet the demand. The system will produce the enough product as per requirement based on scheduling and it will be pushed to the customers or the next phase of manufacturing.

 

In Pull System, Production begins with customer order. A trigger is required from the customer. The main advantage of push system is to reduce inventory levels and cost of carrying and holding goods, as the system allows to manufacture enough goods only to meet order. Unlike the Push System, pull system will not allow excess storage of material to push the supply towards customer.

 

The most effective way of implementing Pull system is Via Kanban Cards. The concept of Kanban is so much associated with Pull system, as sometimes even used synonymously. However, some of the other Pull systems are JIT (Just In Time), CONWIP (CONstant Work In Progress).

 

Kanban:

 

Kanban is a lean method aiming towards reducing the inventory and to manage the work by balancing the demands with available capacity. This method was originated in Toyota production system.

 

Kanban refers to Billboard in Japanese. Kanban is both a type of card (Billboard or signboard) and Production or Manufacturing system in Lean Manufacturing. In traditional method of Kanban, a card is used to identify the demand of the particular product. But often with the development of lean concepts in various sectors, Kanban is replaced with any form , even in digital form as per the application.

  

 

How Kanban Works?

 

In Kanban methodology, there will be a limited number of Kanban (signals) either in form of card or any other form and each signal represents certain product type and a certain quantity. Every product will be attached with Kanban (card) and Once the product is consumed and then the card will come back to the initial loop to schedule the production. Now the new product will be attached to the Kanban and the stock will be replenished, once after it is consumed.

As the number of Kanban cards are limited, System will not allow to stock more than a particular quantity. The number of kanbans, way of prioritizing production in the queue, FIFO requirements will be defined in the system. Kanban makes a simplest form of Pull system.

 image.png.ffe10095a24f2cb6eb91b720558cdcbe.png

 

 

 

CONWIP:

 

CONWIP (CONstant Work In Progress) system is very similar to Kanban. CONWIP also associated with signal or card to trigger the manufacturing, processing or servicing of the product, but unlike the Kanban , it is not associated with a particular product.

 

In CONWIP system, if a part leaves the system, CONWIP card will go to initial stage and the card is not associated with any product. If the card is returned from inventory location, next part with backlog and most urgent will be next in line for the production. In CONWIP, it gives signal about capacity available. But the production will be scheduled as per the forecasted demand.

 

image.png.23858bbdfcd29a9a70d45e9016664ffd.png

 

 

 

Hence CONWIP is a kind of single stage Kanban system, except the product type will be assigned only when it way backs to production floor; Product is assigned only after reviewing the backlogs, urgency or priority base, sometimes even by the forecast of demand.

 

 

Advantages of CONWIP:

 

When Variants and Product Varieties are high

 

Kanban works very well with High quantity low variant product. Kanban replenish the stock of the part number which is having continuous demand. Kanban works well with made-to-stock parts (Fast Moving Consumer Goods or the items sold in larger quantity with continuous demand).

 

CONWIP overcomes the difficulties faced by Kanban when the every product we produce is unique or customized and when the variants are very high. Here any part can be assigned to any CONWIP card as there is no product assigned to CONWIP as default.

 

1.   CONWIP works very well when the product variants is high and the capacity of storage is maintained.

2.   When all the products are unique or customized, CONWIP overcome the Kanban challenges.

3.   Since any product can be assigned to CONWIP, Product which we produced only once can be assigned to CONWIP.

 

Lesser WIP than Kanban:

 

As we don’t need to create a WIP space for the product which is of low volume, So the space can be give n to the higher volume products. Flexibility of not having product associated with the signal cards helps to reduce the number of cards comparatively. If less number of cards, that means lesser WIP.

 

It is also a pull system:

 

As Like any Pull system, CONWIP is also having all the advantages of the pull system to make the production process efficient and smooth, such as

 

1.   Reducing and maintaining Inventory

2.   Lean Manufacturing Application

3.   Prevents overproduction

4.   Reduces inventory holding and carrying cost

 

Hybrid is possible (Kanban and CONWIP)

 

While Kanban maintains the tighter control on system WIP through individual card allocated for each product and will not allow any flexibility.

 

Some system can be hybrid with Both Kanban and CONWIP by assigning Kanban to some stages in the production or to some parts in the production. CONWIP is also described as hybrid push-pull system.

 

The Hybrid system will have Kanban for higher run products and CONWIP for lower volume products. Whenever card returns , both the cards will follow the same queue and the product will be assigned to CONWIP card only before the production based on priority and Backlog. It is quite a good system to have the benefits of Kanban and CONWIP cards.

 

image.png.b4c9529dbedd42842b02706ae5184e77.png 

 

 

   

 

 

Disadvantages of CONWIP:

 

Need Human Inputs to schedule production:

 

Unlike Kanban, CONWIP can’t Produce the parts automatically. If Kanban cards comes without product, system will reproduce that product automatically.

But in CONWIP, Human input based on some prioritization or forecasting model is required. There is an assumption that people organizing backlog or prioritizing the requirements knows what they are doing. If prioritization is done for non-moving products in large quantity it will occupy your inventory space and you will not be able to produce another product which is on demand until we sold out those product we have produced.

 

Extra Effort:

 

CONWIP needs Extra effort as we need to sort the backlogs , prioritize the demand and assign the product to CONWIP Before production.It is not automatic as like Kanban

 

Bullwhip effect:

 

The effect of human decision certainly leads to Bullwhip effect. This effect is described as tendency of increasing the quantity swing. As the human involved in production scheduling overreacts to the demand signal leads to WIP quantity swing in the system.

 

Overloading the production system:

 

Here time is not considered to schedule the production, only the quantity of stock is considered. In a balanced system, if we have products of different Throughput rate. Utilization of the resources will not be constant in CONWIP as we need to maintain the CONWIP quantity. It will leads to overloading or underutilization of the resources.

 

It is important to have balanced workload among products  and balanced system to overcome this particular limitations of CONWIP

 

 

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CONWIP is a  single-stage kanban system and is also a hybrid push-pull system. kanban  maintains tighter control of system work in progress through the individual cards at each workstation.... CONWIP systems are easier to implement and adjust, since There is  one set of system card used to manage system WIP.

 

Kanban works well with high-quantity low-variety parts. Since every card has a part number permanently  associated with the card, the cards always replenish this part number. see  this works only if there is a continuous demand for this particular part number. A good example of this would be any made-to-stock parts that will be sold in larger quantities.


other side if you produce to order, a kanban will have difficulties. in case every product you make is unique, then you would need a unique kanban for this product. but KANBAN... are always assigned a part number, this will be difficult.

 

CONWIP, on the other hand, has no part number assigned by default to the @CONWIP card. Hence, any part number can be assigned (temporarily) to the @CONWIP part, even if the part is produced only one time. Therefore, @CONWIP is well suited for made-to-order parts. However, for made-to-stock production, you need very good production sequence or else you may end up in hot water with

CONWIP

 

 

 

Below image shows Difference between KANBAN & CONWIP

 

CONWIP.png

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The term Push-Pull was initially originated in logistics and supply chain management but nowadays it is widely used in sales and marketing also. When the production sequence is scheduled and the material pushed into the production line, it is known as push system, but in the pull system it will follow the “supermarket model” where limited inventory are kept and is replaced as soon as possible it is consumed.

 

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Push-Pull System are:-

 

  1. Kanban System

  2. Material Requirement System (MRP) System

  3. CONstant Work In Progress (CONWIP) System

  4. POLCA System

  5. K,Y System

  6. PAC System

 

There are some advantages and disadvantages in using CONWIP at a production system. Some are given below:

 

Advantages:

 

 

Kanban works well with high-quantity low-variety parts and it works very efficiently in a continuous process or where there is a continuous demand for the same type of items. In Kanban every part or item is identified by a card number and these numbers are vice versa. A good example is that a company produces a mass production of a particular part and stockpiles it to sell in large quantities. If you have to produce a unique or distinctive item then the Kanban system will face difficulties as it is always assigned a part number. On the other hand CONWIP has no part number earmark by default to the CONWIP card. Hence any part number can be assigned to the CONWIP part, even if the part is produced only once and therefore it is well matched for customized parts.

 

  1. The life cycle of both Kanban and CONWIP are almost similar, so it's easy to combine both of them into a hybrid system. There are at least two types of hybrid systems possible, where both kanban cards with numbers and CONWIP cards without numbers can circle through the system. The only one difference between them is that before production, CONWIP parts get assigned a part number from the backlog sequenced items.

 

Disadvantages:

 

  1. CONWIP needs extra human intervention to reproduce the correct products items as it can be easily messed up the backlog sequence priorities and therefore it needs additional attention or awareness. These human decisions have a higher likelihood for the bullwhip effect, since in CONWIP people decide the priority sequence. On the other hand Kanban can automatically manage the production system sequence. 

  2. CONWIP and Kanban both use a quantity of parts to prevent overloading at the production line and this system works well, when all parts are consumed at the same time during production. If there is a different production time of the parts then there will be totally different jobs for the production system.

Edited by Satyajit Das
spelling mistake

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CONWIP is a hybrid push-pull system. CONWIP controls the WIP in the system. It extends the Kanban approach but simplieifes the execution, providing more flexibility

 

  • Simpler than Kanban in the number of cards used
  • Have been shown to have lower WIP than Kanban

 

 

The picture below explains the comparison of Kanban and ConWIP

 

In certain cases Kanban has proven better than ConWIP.  Kanban has an additional advantage in that it automatically manages the production sequence. Based on the product mix kanbans, the system can figure out the next ones. However in ConWIP, human input is required to manage the sequence and priority, which may lead to excess/ shortage of inventory. This could lead to the Bull Whip effect.

 

Another factor to be considered is the product type, whether it’s few product with large volumes or large number of special type of products (made to order). ConWIP has shown the handle this better.  separate sorting of the backlog and matching the backlog with the CONWIP cards

 

 

 

References:

https://www.researchgate.net/figure/Simple-CONWIP-system_fig1_301741360

https://www.allaboutlean.com/conwip-comparison/

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Benchmark Six Sigma Expert View by Venugopal R

 

CONWIP and KANBAN are methods in Lean Management, aiming for triggering ‘Pull Systems’.

 

KANBAN originated as early as 1940 in Toyota Production Systems and was developed by Taiichi Ohno. KANBAN, in Japanese translates as ‘Billboard’. The idea of KANBAN was to indicate the ‘available capacity’ and helps to implement Lean and JIT (Just-In-Time) production.

 

KANBAN operates under the principle of using a limited number of cards (KANBAN cards), with each card representing a specific quantity (one or more) of a specific type of part. Thus, every part will have a KANBAN card attached to it. To express KANBAN in simplest terms, if any part is completely processed and consumed by market or for subsequent processing, its card gets released and goes back to the beginning of the process flow and pulls-in another part for being processed.

 

While KANBAN works well for many situations, it faces challenges when we have to accommodate a changing product-mix. It becomes difficult to have ‘product-specific’ cards for each type of product. Further, KANBAN method faces practical issues when we have small orders or infrequent jobs. WIPs may have to wait for long time, since the system would respond only after the KANBAN system authorizations will have to be propagated all the way to the beginning of the process to trigger new release.

 

The CONWIP system (Constant Work-In-Progress) was developed by Mark Spearman and Wallace Hope in 1990. The CONWIP system is similar to KANBAN in many ways, but instead of the card being associated with a specific type of part, it is associated with a certain quantity. So, when CONWIP card gets released upon the part(s) represented by it being consumed, it goes back, just like the KANBAN card does, to the beginning of the process. However, on the way, it checks with the backlog to see which parts are in demand along with its quantity. These details are captured by the card, so that the replenishment will be done for the part in demand for the required quantity.

 

KANBAN was focusing on pulling the required part to keep the inventory low, whereas CONWIP focuses on replenishing the capacity, in line with market requirement, by ensuring constant WIP capacity.

 

One of the ways of ensuring the WIP capacity, is by pre-defining WIP levels for each type of part and the processing time / batch for each of those parts. With that, not only the overall inventory can be expressed in terms of time, but also the breakdown of part specific inventory hours as well.

 

The below table gives an example:

image.png.05c27c9bfe5fb0d59a11be5ae3ba304f.png

It may be seen that the inventory information for various parts have been made comparable by expressing in hours. The part wise comparison to targeted WIP is also available. Usually an MRP system will provide real time data for the CONWIP card on the priority capacity. However, it may be kept in mind that for certain situations, the target WIP could be dynamic and would vary depending on the fluctuating market requirements.

 

While KANBAN system is very suitable for high volume, low variant parts, CONWIP will be a better choice for made-to-order parts with lower volumes, but higher variants.

 

A hybrid system encompassing both KANBAN and CONWIP is also used in certain situations where both high-volume parts and less frequent ones co-exist. One such model is where KANBAN cards are assigned to high volume parts and CONWIP to low volume parts. Depending on the type of card that is received, the part to be produced is chosen as per the KANBAN system of the CONWIP system. If this is practiced successfully, the benefits of both these systems can be availed.

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CONWIP - Constant work in Process Production Control

Origin – CONWIP first defined in 1990 is an alternative system (pull-based) to Kanban. Objective of CONWIP is to maintain the work in process in a production system at a constant level.

Why CONWIP – The prime objective of lean is to produce right product at the right time (With Zero Waste) at a repetitive cost and many Japanese organization saw considerable success in achieving these objectives by using “pull based production planning systems” – often referred as Kanban or Just in Time (JIT) systems. However, as these systems were primarily focussed for repetitive manufacturing, the principles didn’t work in diverse product production industries where the shop-floor is controlled by job-orders.

CONWIP production control system shares the benefits of Kanban while also being very much applicable to production environments with wider varieties. CONWIP is different from Kanban in broadly three ways-

1.       It uses back-log to dictate the part number sequence

2.       Cards are associated with all parts which gets produced in a line rather than just individual part numbers.

3.       Jobs are pushed between work-stations in a series post they are authorized by a card to start at the beginning of the line.

 

A sample of CONWIP is shown below-

 

image.png.9399340a6661808f0270c1c57567391c.png

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CONWIP (Constant Work In Progress) is a method of implementing pull in an operating system. CONWIP is a combination of pull and push systems. The entrance of an item into the system is based on pull, but its movement through the processes towards the end is based on push. The inventory in a CONWIP system is controlled only at the end of the line, there is always a fixed amount of finished goods inventory that can be kept at anytime. This system is maintained by the use of cards, a completed product frees up a card which will be attached to the new entrant into the system. In CONWIP, nothing is allowed to enter the system without the card. The number of cards in they system is equal to the maximum work in progress inventory.

CONWIP differs from Kanban in 2 major ways; there is a pull system for every stage of the process in a Kanban system, every stage requires a Kanban signal from the succeeding stage before producing the next lot. CONWIP implements pull for the entire system at the beginning only, the only signal is to pull material into the system. Also, the cards used in Kanban carry specific item tags, a Kanban card can pull only one particular item in the process. This is not the case in CONWIP, the cards are generic, any card can pull any item into the process. The items to be picked are determined by scheduling based on the existing backlog.

ADVANTAGES OF CONWIP OVER KANBAN

1.       Kanban will be very effective only in a high quantity, low variety environment where production is made to stock. Consider a setup where items are made to order and the components of each order are not similar, Kanban cards will need to be made for every new order which will be difficult. This is not required in CONWIP since the cards are generic.

2.       It is easier to control the sequence of production in a CONWIP system than in a Kanban system. Since a backlog system is used in CONWIP, production planning personnel can always rearrange the sequence in case of any change in product requirements since only one pull system is place at the beginning of the process. Rearranging sequence in a Kanban system can be done only on the production floor

DISADVANTAGES OF CONWIP

1.       A CONWIP system is more sensitive to bullwhip effect. Since the sequencing is manually done as against the automatic sequencing in a Kanban system, misjudging the requirements is more likely. The system can end up with many items at the end of the process that are not currently needed.

2.       CONWIP involves sorting of backlogs and matching the backlog to CONWIP cards. This involves more manual work and could also result in errors.

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What is CONWIP?

CONWIP , stands for “constant work in progress”  is one kind of pull-oriented production control systems, it was developed by Mark Spearman and Wallace Hopp in 1990.  It is combination of pull and push systems. This system is very similar to KANBAN system , only difference is that the CONWIP card is not associated with a certain part type (it will change as per Made to order requirement or backlog) ,but only with a certain quantity at least minimum one.

CONWIP go round with different part like roller coaster seats with people, where as per capacity everytime new set of people get filled .

image.png.ea649a67f66a61d5b562f652cc7a0463.png

 

By above table (Refer fig 1)  we can understand this concept very well, all different type of  finished part kept with CONWIP cards attached , once customer or any downstream process withdraw any part then CONWIP card return to the beginning of the loop , now this card is not associated with any part type, it has only constant quantity. At beginning of the loop ,backlog part type assigned to this CONWIP card because this is most urgent one to produce. If any CONWIP card comes back from the finished goods inventory, the next part type from backlog line is assigned to the returning card. The card is just gives signal  that capacity is available, and the backlog defines what to do with the capacity.

 

Advantages:

Number of variants

Compare ot KANBAN system , CONWIP can manage more number of variants, as it has no part number assigned by default to the CONWIP card, therefore any part number can be assigned (temporarily) to the CONWIP part, even if the part is produced only once. Therefore, CONWIP is well suited for made-to-order parts.

 

Less WIP than Kanban:

In CONWIP system it will be less WIP than a kanban system, since “in a kanban system, there will be generally WIP upstream from the bottleneck . In CONWIP system, WIP will tend to collect at the bottleneck.”

 

 

Disadvantages:

Does Not Manage Production Sequence Automatically.

Compare to KANBAN system in CONWIP need  needs human input to reproduce the correct products, so due to human intervention/decisions whole production sequence may mess up.

 
Sensitive to the Bullwhip Effect

in CONWIP humans decide the priority so it may result high demand and high inventory as we  go back in the value chain due to human overreaction to demand signals.

 

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The best answer has been provided by Selva Mariappan Subramanian and Raj Saxena for clearly explaining how CONWIP works along with its advantages and disadvantages using examples. 

 

Also review the answer provided by Mr Venugopal R, Benchmark Six Sigma's in-house expert.

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