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CONWIP Systems

CONWIP Systems. CONWIP – Constant work-in-process Similar to Kanban system in goal of maintaining constant level of work-in-process. Differs from Kanban in that all WIP is aggregated and treated as a whole. CONWIP Systems. Backlog List. CONWIP Facility. Demand.

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CONWIP Systems

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  1. CONWIP Systems CONWIP – Constant work-in-process Similar to Kanban system in goal of maintaining constant level of work-in-process. Differs from Kanban in that all WIP is aggregated and treated as a whole.

  2. CONWIP Systems Backlog List CONWIP Facility Demand As a batch of jobs is completed and leaves the facility, a signal is sent to release the next order. The order is the next batch of jobs on the backlog list. The backlog list is based on the forecast of customer demand and low finished goods inventory.

  3. CONWIP Systems Insert fig 7.11 The amount of WIP in the system (N) is the control parameter of interest for CONWIP. Too large an N results in long lead times and excess WIP. Too small an N results in idle workstations and reduced output.

  4. CONWIP Systems Insert fig 7.11 • Technique to determine N: • Identify bottleneck station. • Gradually increase N (release faster than withdrawals occur) until input queue before bottleneck is rarely empty.

  5. CONWIP Systems Comparison to Kanban Systems: CONWIP systems immediately react to increases in customer demand. As customers increase the rate at which jobs are withdrawn, the high demand jobs are released into the system. CONWIP systems do not need the setup and maintenance of Kanban cards and containers for each part type. CONWIP systems react well for highly variable part mix demand while maintaining relatively constant levels of inventory.

  6. CONWIP Systems Comparison to Kanban Systems cont: CONWIP systems are more complex in that they require a means for maintaining the backlog list. CONWIP systems require ample storage space before machines because although WIP as a whole is constant, WIP is not controlled within each workcenter. Over time, as demand changes, unwanted WIP may “clog up” the system and count towards the overall WIP, thus making the system less responsive. To resolve, periodically “cleanse” the system of the unused WIP.

  7. CONWIP Systems • Developing/Prioritizing Backlog List: • Earliest due date • Minimum slack time (due date – E(lead time)) • Target finished-goods inventory levels • Insert table 7.6

  8. CONWIP Systems Rough performance measures if processing times are constant: Let: tj = average batch processing time at workstation j. M = number of work centers. If parts visit all work centers, then: is cumulative processing time at all stations. If there are N jobs in the system and M workcenters, then on average there are N/M jobs at each workcenter. Therefore the time, Tj spent at a job center is: And the time, T, spent in the system is:

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