385. Critical WIP
The amount of work-in-process inventory at which a deterministic line just barely achieves its bottleneck throughput.
385.1. Why it’s “critical”
- Below : line is starved — bottleneck idle some of the time, throughput limited by WIP rather than capacity
- At : bottleneck just fully utilized, cycle time =
- Above : bottleneck still at max throughput, but cycle time grows (excess WIP just sits in queues)
For deterministic lines, is the minimum WIP that achieves maximum throughput. Beyond , you’re tying up capital without throughput gain.
385.2. Real lines need more WIP
Variability makes the curves wider — to achieve close-to- throughput on a real (variable) line, you need WIP somewhat above . How much above depends on variability — see best/worst/PWC curves.
PWC throughput at WIP:
For : TH = . So real lines need to approach .
385.3. Usage
- CONWIP control: set WIP cap at , chosen to hit target throughput
- Lean assessment: cycle time / ratio tells you how variability-burdened your line is
- Capacity planning: increasing (de-bottlenecking) lowers → less WIP needed
385.4. Worked example
Line: bottleneck produces 100 units/hour (), raw processing time hours.
Need at least 300 units of WIP in the line to keep the bottleneck fed.
For a real (variable) line, set CONWIP cap at units to hit close to .