How to limit WIP (Work-in-process) inventory
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.
Card control policy
In a CONWIP system, a card is shared by all kinds of products. However, Duenyas (1994) proposed a dedicated card control policy in CONWIP and he stated that this policy could perform as a multiple chain closed queuing network.
How to Implement this in PlanetTogether
CONWIP can be implemented in PlanetTogether by using virtual "CONWIP Inventories" to limit the release of work into production at various stages. For each stage where you would like to limit inventory, follow these steps:
1. Create a CONWIP Inventory (I.e. an Inventory named however you wish to denote it) with On-hand Qty equal to the max number of units allowed in the plant at any one point in time.
2. Set the bill-of-material of the operation to be limited from starting (when inventory is too high) to consume the CONWIP Inventory. This will act as a constraint to production when the CONWIP Inventory falls to zero.
3. Set the operation that signifies the WIP is finished being produced (such as the last operation of a Job) to produce the CONWIP Inventory as a Product.
In effect, this will limit the number of units that are in WIP at any point in time.