Question

Describe CONWIP and Kanban and Compare the two of them.which one is better.

Please answer in three parts first includes "Describe CONWIP and Kanban" and second includes their comparision and the third part includes "which one is better".

Answer 1

CONWIP and Kanban :

• CONWIP (CONstant work in process) are pull-oriented production control systems. Such systems can be classified as pull and push systems (Spearman et al. 1990). In a push system, the production order is scheduled and the material is pushed into the production line. In a pull system, the start of each product assembly process is triggered by the completion of another at the end of production line. This pull-variant is known for its ease of implementation.
• CONWIP is a kind of single-stage kanban system and is also a hybrid push-pull system. While Kanban systems maintain tighter control of system WIP through the individual cards at each workstation, CONWIP systems are easier to implement and adjust, since only one set of system cards is used to manage system WIP. CONWIP uses cards to control the number of WIPs. For example, no part is allowed to enter the system without a card (authority).
• After a finished part is completed at the last workstation, a card is transferred to the first workstation and a new part is pushed into the sequential process route. In their paper, Spearman et al. (1990) used a simulation to make a comparison among the CONWIP, kanban and push systems, and found that CONWIP systems can achieve a lower WIP level than kanban systems.

The Kanban System :

Kanban is both a type of card and a control system in manufacturing named after the cards. Both were developed by Toyota in Japan. There are a limited number of cards, with each card representing a certain part type and a certain quantity (at least one). In the picture below, we have cards for green squares, red diamonds, and blue hexagons. The kanban can be attached to a part or be separate. Every part must have a kanban attached with it.

• Assume you have blue hexagons in stock, each with a card attached. If someone takes out a blue hexagon, the card is removed from the part and goes back for reproduction. A card without a part arriving back at the beginning of the loop is an order to reproduce this part. Hence the card eventually comes back to storage again with a newly produced part.
• This is a kanban system in its simplest form. Of course, there are numerous additional details like how many cards to use, various prioritizing and grouping options of the cards for reproduction, and detailed usages of FiFo lines and supermarkets. For detail, see my numerous posts about kanban.

The CONWIP System :

The CONWIP (Constant Work In Progress) system is very similar to a kanban system, except that the CONWIP card is not associated with a certain part type, but only with a certain quantity (also at least one). The illustration below is comparable to the illustration above except it represents a CONWIP system.

• Completed parts all have a CONWIP card attached. If the part leaves the system (e.g., it is sold, used downstream, etc.), the card returns back to the beginning of the loop. Remember, the card is now not associated with a part type, only a constant quantity. On its way to the beginning of the loop, it meets the backlog. The backlog is a list of part types and quantities that need to be produced. The first part in the backlog is the most urgent one. If a CONWIP card comes back from the finished goods inventory, the next part type in line is assigned to the returning card. The card is hence a signal that capacity is available, and the backlog defines what to do with the capacity. The merging of the blank card with the part type is called the system entry. This part then gets reproduced and eventually makes its way back to the finished goods of the loop.
• Hence a CONWIP card is like a kanban card, except the part type gets assigned only on its way back when it meets the most urgent demand in the backlog.

(2)COMPARISON OF CONWIP AND Kanban :

Number of Variants :

• 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. Of course, 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.
• 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 once. Therefore, CONWIP is well suited for made-to-order parts. However, for made-to-stock production, you need a good production sequence or you may end up in hot water with CONWIP.

Hybrid System with Kanban Is Quite Possible :

• The first hybrid system has kanbans assigned to the high-runner part types and CONWIP to the low volume exotic parts. Whenever a kanban card comes along, the part number of this kanban is produced. Whenever a CONWIP card comes along, the most urgent part from the backlog of low-volume exotic parts is produced. Both kanban and CONWIP cards wait in a joint queue. The only difference is that just before production, CONWIP parts get assigned a part number from the backlog.
• CONWIP has one big loop and kanban has smaller loops within the CONWIP system. However, this would mean that you now have TWO cards attached to each part, a CONWIP and a kanban. This is twice the work, and two times the opportunity for mix-ups. Matching CONWIP cards with kanban cards will be quite a challenge

• CONWIP is Less WIP than Kanban

(3) Overall, I like the CONWIP system. It seems to be quite useful, especially for made-to-order products. I can also imagine it to be quite suitable as a hybrid in combination with kanban. This concludes my four-post series on CONWIP. I hope this was insightful for you. It definitely was for me, as I am always learning new things by blogging about them